Rapid Detection of Cr(VI) Using CuO/CN/LDH as an Oxidase Mimetic and the Construction of a Paper-Based Devices

Rapid Detection of Cr(VI) Using CuO/CN/LDH as an Oxidase Mimetic and the Construction of a Paper-Based Devices | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Rapid Detection of Cr(VI) Using CuO/CN/LDH as an Oxidase Mimetic and the Construction of a Paper-Based Devices Shaohui Li, Sijia Hao, Wen Li, Ran Meng, Qiang Wang, Yuqing Wang, and 6 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8252032/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 03 Mar, 2026 Read the published version in Microchimica Acta → Version 1 posted 14 You are reading this latest preprint version Abstract In this study, a ternary structure (CuO/CN/LDH) composed of LDH-anchored g-C 3 N 4 nanosheets loaded with CuO nanoparticles was successfully constructed via hydrothermal and co-precipitation methods, and a colorimetric sensing platform for Cr(VI) detection was proposed. The engineered nanomaterial demonstrates significantly enhanced oxidase-mimetic activity in the presence of Cr(VI), enabling rapid catalytic oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) with distinct chromogenic response. Mechanistic investigations reveal that Cr(VI) facilitates the conversion of dissolved oxygen into superoxide anions via redox cycling, thereby accelerating TMB oxidation. The sensor exhibits exceptional analytical performance with high accuracy (recovery rates of 97.3-103.8%) and sensitivity (detection limit of 25 nM) in aqueous sample analysis. Notably, an innovative paper-based analytical device was developed in conjunction with smartphone-assisted colorimetric analysis, achieving on-site visual quantification of Cr(VI) within 15 min. This integrated system provides a field-deployable solution for environmental monitoring, combining operational simplicity with reliable detection capabilities. The proposed methodology advances current sensing technologies by merging nanomaterial engineering with portable detection platforms, demonstrating significant potential for heavy metal surveillance in water quality management. Based on this, this study also constructed a paper-based colorimetric detection sensor. g-C3N4 CuO Cr(Ⅵ) paper-based sensor Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 1. Introduction With the rapid development of industry, heavy metal ion pollution has increasingly become a focal point of global concern [1] . Among the various heavy metal ions, chromium ions are widely utilized in various commercial applications, including chrome plating, pigment production, leather tanning, paper manufacturing, and chemical production. Chromium is the seventh most abundant element on Earth and is commonly found in nature. While it exists in multiple forms, it primarily occurs in two stable oxidation states: Cr(III) and Cr(VI) [2] . In the presence of excess environmental oxygen, Cr(Ⅲ) can oxidize to form hexavalent chromium (Cr (VI)), which is extremely toxic and highly water-soluble [3, 4] . Cr(VI) is a concern due to its high mobility, strong toxicity, and carcinogenic properties. Long-term exposure may result in respiratory issues, lung cancer, nerve tissue damage, and even death in large dose [5, 6] . In the past decades, a range of techniques have been employed for the sensitive and selective detection of Cr(VI), including ion chromatography (IC) [7] , atomic absorption spectrometry (AAS )[8] , inductively coupled plasma mass spectrometry (ICP-MS) [9] , and electrochemistry [10] , etc. But they usually suffer from disadvantages such as sophisticated pretreatment, the requirement of professional operation, high cost, large instruments, and long pretreatment times, which render them unsuitable for real-time field monitoring [11, 12] . This limits the rapid and real-time detection of Cr(VI). In recent years, nanozyme-based colorimetric detection methods have gained widespread attention due to their rapidity, simplicity, low cost, excellent sensitivity, and ease of detection by the naked eye. Compared to the previously methods, colorimetric assays do not require complex and expensive instruments, and they allow for the detection of pollutant concentrations in a short time using UV-visible spectroscopy [13] .The existing nano enzyme colorimetric sensors mainly include hydrogel and paper-based. Compared with hydrogel, paper-based has the advantages of low price, easy access, easy operation and wide environmental adaptability [14] . In recent years, various nanoparticles, including metals and metal oxides, have been demonstrated to exhibit enzyme-like activity [15] . As a stable oxide of Cu, CuO is an important p-type semiconductor that can form heterojunctions with other semiconductors to enhance catalytic performance [16] . It is regarded as a promising catalytic material due to its abundant active sites, chemical stability, low cost, and environmental benignity [17] . And CuO has drawn significant attention from researchers due to its low toxicity and natural abundance in the Earth's crust. Additionally, copper metal, which is cost-effective and has relatively high natural reserves, is easy to synthesize [18] . In addition, it has been widely used in the sensor field as a mimic of peroxidase. For example, Lu et al [19] . synthesized tubular CuO/NiO nanozymes with significantly enhanced peroxidase-like activity. In the presence of hydrogen peroxide, these nanozymes can rapidly catalyze the conversion of colorless TMB to blue oxTMB, thereby establishing a colorimetric detection method for isoniazid. However, due to the instability and decomposition of H 2 O 2 in practical applications, exploring the oxidase activity of CuO is of significant importance. However, most Cu-based catalysts are unstable in long-term reactions, with the main reasons being the potential-induced surface reconstruction, and catalyst aggregation, among others [20] . Owing to well matched band structure between g-C 3 N 4 and CuO, high-quality p-n heterojunction could be formed [21] . N-type and p-type semiconductors can be developed from internal characteristics by doping ions with furnish supererogatory electron or hole [22] . In addition, graphitic carbon nitride (g-C 3 N 4 ), a polymeric-metal-free photocatalyst of n-type semiconducting behavior, has garnered significant attention from researchers as a polymer semiconductor. This material can be synthesized through the extensive condensation of low-cost nitrogen-rich precursors, such as urea [23] and melamine [24] . It is composed of two earth-abundant elements, carbon and nitrogen, which enable its facile synthesis at a low cost. Due to its simple and cost-effective synthesis process, good thermal stability, non-toxicity, and unique layered structure [25, 26] , g-C 3 N 4 has become a popular research topic in fields such as catalysis [27] and sensing [28] . However, the bulk g-C 3 N 4 has the problem of low specific surface area and low electron mobility [29, 30] . To overcome this drawback, researchers have attempted to construct heterojunction structures by combining g-C 3 N 4 with other p-type semiconductors. Accordingly, the fabrication of heterostructure-based of g-C 3 N 4 has emerged as a beneficial strategy to improve its activity. One of the most effective heterogeneous photocatalysts is the combination of graphite-like carbon nitride (g-C 3 N 4 ) and layered double hydroxides (LDHs). LDHs are typical two-dimensional layered structural materials. They are generally defined by the formula [M 2 + 1−x M 3 + x (OH) 2 ] Z+ (A n– ) x/n ·mH 2 O, where M 2+ represents divalent cations (such as Ca 2+ , Co 2+ ), M 3+ indicates trivalent cations (such as Al 3+ , Co 3+ ), A n− are exchangeable anions in the interlayer region compensating for the positive charge on the layers. LDHs possess characteristics such as openness, ease of metal ion adjustment, abundant redox-active sites, high biocompatibility, low toxicity, customizable size and composition, and good stability [31, 32] . Furthermore, LDHs are facile to synthesize, chemically modifiable, cost-effective, and environmentally friendly, making them well-suited to meet the requirements of sustainable development [33] . It has therefore received a lot of attention from researchers. Currently, LDH has been widely applied in photocatalysis, CO 2 reduction, catalysis, pharmaceuticals, and other applications [34, 35] . However, standalone LDH exhibits relatively low quantum efficiency due to low charge carrier mobility and a limited number of electron-hole pairs [36] . Moreover, their poor semiconductor properties make it challenging to fully utilize their abundant active sites [37] . Copper-based LDHs have been proven to possess good catalytic activity due to the presence of Cu 2+ active sites and excellent dispersion of Cu 2+ ions [38] . According to existing research, the combination of LDHs and carbon materials can enhance the separation of photogenerated charge carriers and provide useful substances for reactions [29] . For example, Faria et al [26] . successfully constructed a 2D/2D CuAl-LDH/GCNN heterostructure by combining CuAl-LDH with g-C 3 N 4 . This heterostructure significantly enhanced the photocatalytic ability for water splitting to produce H 2 and O 2 , demonstrating excellent photocatalytic performance. In this study, CuO/CN/LDH nanocomposites were successfully synthesized through a simple hydrothermal method and co-precipitation technique. The construction of the CN/LDH heterojunction effectively promotes electron transfer and provides more active sites for the loading of CuO. As shown in Scheme 1 , under the presence of Cr(VI), CuO/CN/LDH can rapidly catalyze the color development reaction of colorless TMB, demonstrating enhanced oxidase-like activity. Based on this, we developed a colorimetric method for detecting Cr(VI). Experimental results indicate that this method exhibits good accuracy and specificity, showing great potential for application in environmental detection. This study successfully developed a portable paper-based Cr(VI) detection system by modifying the paper substrate with CuO/CN/LDH nanocomposites, creating a sensor with a concentration-dependent colorimetric response that exhibits excellent linearity across the target concentration range. Leveraging this chromogenic property, a standardized colorimetric reference card was designed to enable both quantitative and semi-quantitative detection of Cr(VI). 2. Experimental section 2.1 Materials Melamine, potassium dichromate (K 2 Cr 2 O 7 ), and anhydrous ethanol (C 2 H 5 OH) were provided by National Pharmaceutical Chemical Reagents. Cerium nitrate hexahydrate (Ce(NO 3 ) 3 ·6H 2 O), copper nitrate trihydrate (Cu(NO 3 ) 2 ·3H 2 O), urea, sodium acetate (CH 3 COONa), acetic acid (CH 3 COONa), and 3,3’,5,5’-tetramethylbenzidine (TMB) were supplied by Aladdin Chemical Company. All reagents, including additional metal ion solutions, were analytically pure and used without further purification. Ultrapure water was employed throughout the experimental process. 2.2 Preparation of g-C 3 N 4 Plate-like g-C 3 N 4 (CN) was prepared by a secondary calcination method [39] . Specifically, melamine was placed in a tubular furnace and heated to 550°C at a rate of 5°C/min under a nitrogen atmosphere, and then maintained for 5 h. After natural cooling, the product was taken out and thoroughly ground to obtain a pale-yellow powder. The pale-yellow powder was then subjected to a second high-temperature calcination under the same conditions, with a holding time of 4 h. After natural cooling, it was thoroughly ground again. The final product obtained was a white powder. 2.3 Preparation of CN/LDH This study is similar to previous preparation processes, but with some modifications [26] . First, 0.2 g of g-C 3 N 4 was mixed with 40 ml of a solution (water: ethanol = 1:1) and subjected to ultrasonic treatment for 2 h. Then, 1 mM Ce(NO 3 ) 3 ·6H 2 O and 1 mM Cu(NO 3 ) 2 ·3H 2 O were dissolved in the above solution, and ultrasonication was continued for 1 h. Next, 20 mM urea was added, and the mixture was stirred for 30 min. The mixture was then placed in a reaction kettle and heated at 120°C for 12 h. After cooling, the mixture was centrifuged at 8000 rpm for 10 min and washed several times with water and ethanol, then dried for later use. 2.4 Preparation of CuO/CN/LDH The synthesis method of CuO/CN/LDH followed previously reported procedures with certain modifications [40] . Specifically, weigh 0.02 g of CN/LDH and dissolve it in 30 ml of a solution (water: ethanol = 1:1), then use ultrasonic treatment to disperse it evenly. Next, weigh 0.03 g of Cu(NO 3 ) 2 ·3H 2 O and place it in an oil bath, stirring until uniform, then heat to 60°C. At this temperature, add 1 mL of dilute ammonia (1:9 dilution) and continue stirring for 4 h. Afterward, centrifuge at 13,000 rpm for 5 min, and wash three times with both water and ethanol. 2.5 Characterizations The morphology and elemental composition of the materials were investigated using a scanning electron microscope (SEM, Hitachi SU8600, Japan). The SEM is equipped with an Oxford Ultim Max 40 energy dispersive X-ray spectroscopy (EDX) system for compositional analysis. The chemical composition and surface chemical states of the samples were analyzed using X-ray photoelectron spectroscopy (XPS) with a Thermo Fisher Scientific K-Alpha instrument (USA). UV-vis absorption spectra were measured using a Mapada UV spectrophotometer (UV-1800, Shanghai, China). Electrochemical measurements were conducted using a CHI660i electrochemical workstation (Chenhua Instrument, Shanghai, China). Fourier transform infrared (FT-IR) spectra were analyzed using FTS3000 FT-IR spectrometer (DigI-Lab, USA). The pH of the buffer was adjusted using a Ray's pH-3c pH meter. X-ray diffraction (XRD) studies were performed using a Rigaku D/max-2400 (Rigaku, Japan). Free radicals in the samples were detected by electron paramagnetic resonance (EPR, BRUKER EMXPLUS, DE). 3. Results and discussion 3.1 Physicochemical properties of the materials The morphological characteristics of the materials were analyzed using scanning electron microscopy (SEM), as shown in Fig. 1 . Figure 1 a shows pristine g-C 3 N 4 , revealing a distinct lamellar structure. The pristine CuCe-LDH (Fig. 1 b) displays spherical morphology, likely formed through hydrothermal self-assembly of lamellar subunits. Compared to CuCe-LDH, the surface of CN/LDH appears relatively rough, with a fluffy appearance (Fig. 1 c). The inset micrograph provides an enlarged view, confirming the successful integration of g-C 3 N 4 nanosheets within the LDH matrix. Following copper oxide incorporation (Fig. 1 d), the composite surface exhibits a continuous particulate coating that reduces surface roughness compared to CN/LDH. High-magnification imaging reveals well-dispersed CuO nanoparticles intercalated between g-C 3 N 4 layers. Figure 1 e presents the element distribution map of the CuO/CN/LDH material. It can be observed that the Ce element is only present in the central spherical part, while the N element is distributed in the outer encapsulating layer. Copper and oxygen exhibit homogeneous distribution throughout both structural domains, showing strong spatial correlation that verifies CuO formation. These analytical findings correlate well with the morphological observations from SEM characterization. The crystal structure was analyzed via X-ray powder diffraction (XRD), with results presented in Fig. 2 a The red curve in the figure represents the XRD pattern of g-C 3 N 4 , in which the peaks at 12.7° and 27.6° correspond to the (100) and (002) planes of g-C 3 N 4 , respectively. The pink curve in the figure is the XRD pattern of LDH, in which the peak(11.97°) originally attributed to the (003) plane of LDH has disappeared. This phenomenon may result from self-assembly under high-temperature and high-pressure conditions, leading to the collapse of the layered structure [41] . This observation is similar to the earlier conclusions drawn from SEM analysis. The presence of the (006) plane reflection at 24.5° confirms retention of the LDH phase (Fig. S1 ). The remaining diffraction peaks at 35.7°, 39.3°, and 47.7° correspond to the (012), (015), and (018) planes of LDH, respectively, and these peaks are attributed to the LDH phase [26] . The remaining peaks are mainly attributed to the diffraction peaks of CeO 2 and CuO, with sharper CeO 2 reflections indicating superior crystallinity relative to CuO. The cyan Curve represents the XRD spectrum of the CN/LDH composite material, where all peaks of CN and LDH appear, further proving the successful synthesis of CN/LDH. The CuO/CN/LDH spectrum (purple curve) exhibits superposition of g-C 3 N 4 and LDH reflections, confirming preservation of both phases during CuO integration. Marked intensification of CuO diffraction peaks evidences effective heterostructuring with the CN/LDH matrix. Surface functional groups were analyzed via FT-IR spectroscopy (Fig. 2 b). The CN spectrum exhibits peaks between 1750 − 1000 cm⁻¹ corresponding to the stretching vibrations of nitrogen-containing heterocycles, alongside a distinct triazine ring breathing mode at 808 cm − 1[42] . In the FT-IR spectrum of LDH, peaks below 800 cm⁻¹ are primarily associated with metal-oxygen (M-O) vibrations [43] . Bands at 1400 − 1060 cm⁻¹ arise from CO 3 2− asymmetric stretching, while the 1051 cm⁻¹ feature corresponds to C-N bending modes in LDH [44, 45] . Broad absorption in the 3400-3000cm⁻¹ region indicates hydroxyl (-OH) stretching vibrations. The CuO/CN/LDH spectrum retains all constituent phase signatures, with an additional Cu-O stretching vibration at 518 cm⁻¹. The strong Cu-O peak observed in CuO/CN/LDH further confirms the successful synthesis of CuO [46] . The chemical states and oxidation environments of constituent elements were probed through XPS. Figure 2 c presents the wide-scan XPS spectrum of CuO/CN/LDH, confirming the coexistence of relevant elements within the synthesized materials. Figure 2 d displays the high-resolution XPS spectrum of C, which is deconvoluted into three distinct peaks at 284.55 eV, 286.07 eV, and 287.98 eV, corresponding to adventitious carbon, C-O bonds, and the N = C–N coordination in g-C 3 N 4 , respectively [47] . Figure 2 e shows the N 1s XPS spectrum of the composite material, which can be deconvoluted into three peaks at 398.48 eV, 400.36 eV, and 404.3 eV. The peak at 398.48 eV is attributed to the sp 2 hybridized nitrogen atoms (C–N = C) in the triazine rings, which are aromatic ring structures containing nitrogen and carbon, where the nitrogen atom is part of the ring by forming double and single bonds with two carbon atoms. The peak at 400.36 eV is attributed to the bridging nitrogen atoms (N-(C) 3 ). The small peak at 404.3 eV is attributed to C–NH X [48] . Figure 2 f presents the XPS spectrum of O 1s, which can be deconvoluted into three peaks. Among them, the main peak at 529.47 eV is attributed to lattice oxygen (O 2− ), which is primarily associated with Cu 2+[49] . This peak at this energy position is a characteristic signal of oxygen in the CuO structure, indicating that these oxygen ions are part of the lattice, thus confirming the successful synthesis of CuO. The peak at 531.16 eV is attributed to oxygen vacancies, while the peak at 533.31 eV is likely related to chemisorbed oxygen in substances such as surface-adsorbed H 2 O. The typical Ce 3d spectrum of the CuO/CN/LDH catalyst, as shown in Fig. 2 g, can be divided into eight peaks. Specifically, the peaks at 882.25 eV, 889.25 eV, and 898.21 eV are attributed to Ce 3d 3/2 , while the peaks at 900.79 eV, 907.42 eV, and 916.57 eV are attributed to Ce 3d 5/2 . These peaks are mainly associated with Ce 4+ . Meanwhile, the peaks at 885.69 eV and 903.62 eV correspond to Ce 3+ . Therefore, Ce in this catalyst exists in a mixed valence state. As shows in Fig. 2 h, the high-resolution XPS spectrum of Cu can be deconvoluted into seven peaks, where the characteristic peaks at 933.0 eV and 953.0 eV are assigned to Cu + , while those at 934.85 eV and 952.9 eV correspond to Cu 2+[50] . In addition, satellite peaks are observed at 941.28 eV, 943.78 eV, and 962.01 eV, which are characteristic signals of Cu 2+ ions, suggesting that their 3d orbitals are partially filled. 3.2 Exploration of Oxidase Activity of CuO/CN/LDH Figure S2 displays UV-Vis absorption spectra of LDH/CuO, CN/CuO, and CuO/CN/LDH before and after Cr(VI) introduction. Comparative analysis reveals CuO/CN/LDH undergoes the most pronounced absorption modulation at 652 nm. Although the spectra of CuO/LDH and CN/CuO also show some enhancement after the addition of Cr(VI), the degree of enhancement is less pronounced compared to CuO/CN/LDH. As shown in Fig. 3 a, in the HAC-NaAC buffer solution, when only TMB is added, the UV-vis spectrum does not show any significant signal change. Similarly, no significant signal change is observed when only Cr(VI) + TMB or CuO/CN/LDH + TMB is added. However, a noticeable signal change in the UV-vis spectrum occurs only when CuO/CN/LDH, Cr(VI), and TMB are added simultaneously. By comparing the above curves, it is evident that the constructed Cr(VI) detection sensor exhibits excellent response characteristics. Systematic optimization of critical parameters (pH, reaction duration, and catalyst loading) was conducted to maximize catalytic efficiency. As shown in Fig. S3, within the pH range of 3.0 to 5.5, the absorbance initially increases and then decreases, with the highest catalytic activity observed at pH 3.5. The oxidase activity of nanoenzymes is usually prominent at acidic pH [51] . In the optimization of TMB concentration, the absorbance (where A represents the absorbance value at 652 nm in the presence of Cr(VI), and A 0 enotes the absorbance without Cr(VI)) gradually increases with the addition of TMB, indicating enhanced catalytic performance. However, beyond a TMB concentration of 0.35 mM, changes in absorbance become negligible, suggesting that the reaction system stabilizes. A TMB concentration of 0.35 mM was adopted to balance signal intensity and economic feasibility. We also investigated the effect of the CuO/CN/LDH amount added, as illustrated in the figure. The absorbance reached its maximum when the material volume was 25 µL, which was therefore chosen as the final added volume. Additionally, the optimization of reaction time revealed minimal absorbance changes around 10 min, leading to the selection of 10 min as the final reaction duration. All experiments were conducted at room temperature to facilitate practical application. In summary, the optimal experimental conditions are as follows: pH 3.5, TMB concentration of 0.35 mM, material volume of 25 µL, and reaction time of 10 min. 3.3 Kinetic analysis of CuO/CN/LDH as an oxidase mimetic The oxidase-mimicking activity of CuO/CN/LDH was evaluated using Cr(VI) as a catalytic substrate. As shown in Fig. 3 (b and c), the apparent enzyme kinetics for the entities were determined using the Michaelis-Menten equation to deduce the maximum reaction rate (V max ) and the Michaelis constant (K m ). The K m value is an important characteristic constant of the enzyme, reflecting the affinity of the catalyst for the substrate. A lower K m value indicates a stronger interaction between CuO/CN/LDH and the substrate, thereby resulting in higher catalytic activity. To investigate the oxidase-like activity of CuO/CN/LDH nanomaterials, we evaluated the catalytic enhancement induced by Cr(VI) and performed kinetic analyses under optimized conditions. The typical Michaelis-Menten curve is presented in Fig. 3 b, from which the Lineweaver-Burk plot was derived (Fig. 3 c). Furthermore, the catalytic performance of CuO/CN/LDH was benchmarked against other nanozymes, as summarized in Table S1 . Kinetic analyses revealed that the CuO/CN/LDH nanocomposite demonstrates a Michaelis constant (K m ) of 0.173 and a maximum reaction rate (V max ) of 4.702 × 10 − 8 . Notably, as shown in Table S1 , its K m value is significantly lower than that of horseradish peroxidase (HRP) and other types of nanozymes, while its Vmax is markedly higher than that of HRP and other nanozymes. 3.4 Ultrasensitive and selective detection of Cr(VI) Based on the experimental results under optimal conditions, as shown in Fig. 4 , a colorimetric method was developed using CuO/CN/LDH oxidase mimetics as a sensor for detecting Cr(VI). Figure 4 a illustrates the response of the CuO/CN/LDH colorimetric sensor to different concentrations of Cr(VI). Within the range of 0 to 9.1 µM, the absorbance at 652 nm in the UV-visible absorption spectra gradually increases with the increasing concentration of Cr(VI). The inset visually documents the progressive color transition of the solution as Cr(VI) concentrations rise. Figure 4 b depicts the linear relationship between Cr(VI) concentration and absorbance intensity at 652 nm. Within the range of 0 to 9.1 µM, there is a good linear relationship between absorbance and Cr(VI) concentration, with a correlation coefficient as high as 0.995. The method achieved a detection limit (LOD) of 25 nM for Cr(VI), demonstrating high sensitivity. As shown in Table S2 , this study compares the proposed method with existing Cr(VI) detection methods. From the table, it can be seen that the constructed CuO colorimetric detection method not only has significant advantages in terms of the environmental friendliness of the materials used but also exhibits comparable or superior performance in terms of detection limits and detection range. To assess selectivity, we tested the method against common interfering cations (e.g., Cd²⁺, Cu²⁺, K⁺, Ba²⁺) under identical experimental conditions. As shown in Fig. S4, the results indicate that these interfering substances, even at concentrations ten times that of Cr(VI), do not affect the colorimetric system. The rapid oxidation of TMB to a blue product occurred exclusively in Cr(VI)-containing solutions, validating the platform’s specificity for Cr(VI) detection in aqueous environments. We further evaluated the nanocomposite’s long-term stability. As illustrated in Fig. S5, its enzymatic activity remains above 95% even after being stored for 8 weeks, indicating that the material possesses excellent storage stability. 3.5 Oxidase-like catalytic mechanism of CuO/CN/LDH To identify the active species governing the enzyme-catalyzed reaction, we employed electron paramagnetic resonance (EPR) spectroscopy to characterize radical generation during the catalytic process. EPR spectra (Fig. 5 ) reveal distinct radical signatures: a 1:2:2:1 quartet in Fig. 5 a confirms hydroxyl radical (·OH) formation, while a 1:1:1 triplet in Fig. 5 b indicates singlet oxygen ( 1 O 2 ) production. Six hyperfine-split peaks in Fig. 5 c further confirm the participation of superoxide radicals (·O 2 − ). To further identify the dominant radicals in the reaction, isopropanol, L-histidine, and chloroform were utilized as scavengers for hydroxyl radicals, singlet oxygen, and superoxide anions, respectively. As shown in Fig. 5 d, absorbance gradually decreases with the increasing concentration of radical scavengers. Notably, the experimental group containing chloroform exhibited the most significant change in absorbance compared to the control group. Therefore, we conclude that superoxide anions play a major role in this enzyme-catalyzed reaction. Furthermore, as depicted in Fig. 5 e, the UV-visible spectra in both air and N 2 environments were compared. A marked decrease in absorbance intensity under N 2 confirms dissolved oxygen’s critical role in sustaining the catalytic cycle. To elucidate the reaction mechanism, XPS was performed on the CuO/CN/LDH nanocomposite post-reaction. Figure 5 f displays the high-resolution XPS results of Cu elements in the nanocomposite after the reaction. Compared with the pre-reaction Cu XPS spectrum, the disappearance of the Cu⁺ characteristic peak in the Cu XPS spectra after reaction demonstrates that the introduction of Cr(VI) induces the oxidation of Cu⁺ to Cu²⁺, which is attributed to the redox reactions occurring in the system. We propose that this process involves the following cascade reactions: The redox interaction between Cu⁺ and Cr(VI) drives electron transfer, while dissolved oxygen in the solution is concurrently reduced to generate superoxide anion radicals (·O 2 ⁻). These reactive oxygen species subsequently oxidize colorless TMB, yielding the characteristic blue oxidized product (oxTMB). The self redox properties of copper atoms induced by O 2 are crucial for the sustained generation of dissolved oxygen [52] . 3.6 Actual sample testing To validate the feasibility and practicality of the developed method, the CuO/CN/LDH composite was applied to detect Cr(VI) in real environmental samples, including tap water and Yellow River water. The accuracy of the method was assessed via recovery experiments. As summarized in Table 1 , the recovery rates for Cr(VI) in tap water ranged from 97.7% to 103.7%, with relative standard deviations (RSDs) of 0.95 to 3.3%. Similarly, in Yellow River water, recoveries spanned 99.0-104.4%, accompanied by RSDs of 1.4–3.1%. These results demonstrate that the proposed method exhibits high reliability, sensitivity, and accuracy, indicating its applicability to diverse aqueous matrices. Table 1 Determination of Cr(VI) in test samples. Sample Added(µM) Found(µM) Recovery(%) RSD(%) Tap water 6 6.22 103.7 2.7 5 5.04 100.8 3.3 3 2.9 97.7 0.95 Yellow River water 6 6.27 104.4 1.4 5 4.95 99.0 3.1 3 3.05 102.0 1.7 3.7 Preparation of CuO/CN/LDH Paper-Based Sensor To meet the growing need for portable environmental monitoring tools, paper-based analytical devices (PADs) have gained prominence owing to their cost-effectiveness, user-friendly operation, and field adaptability. This study developed a paper-based colorimetric sensor for rapid, on-site Cr(VI) detection. The Whatman filter paper was first cut into discs using a mold, followed by complete immersion of the punched filter paper discs in a diluted solution. To ensure thorough solution penetration, the samples were transferred to a thermostatic shaker incubator for constant-speed oscillation. Upon reaction completion, the specimens were oven-dried to obtain the colorimetric detection test strips (see Scheme 2 for detailed fabrication workflow). The detection system innovatively integrates smartphone-based image acquisition with RGB analysis using ImageJ software, establishing an instrument-free quantitative detection methodology. Through systematic optimization experiments (Fig. S6), the optimal detection parameters were determined as follows: reaction system pH 3.5, nanomaterial dilution factor of 20×, TMB chromogenic agent volume of 200 µL, and coloration time of 10 min. Experimental results demonstrate that this integrated detection platform maintains satisfactory sensitivity while significantly reducing detection costs, providing reliable technical support for field screening of heavy metal contamination. To evaluate the accuracy and practical applicability of the paper-based colorimetric device for Cr(Ⅵ) detection, we performed selectivity tests by introducing common metal cations (Al³⁺, Mn²⁺, Mg²⁺, and Pb²⁺) at concentrations tenfold higher than Cr(Ⅵ). As shown in Fig. S7, visible color changes occurred exclusively in the Cr(Ⅵ) group compared to the control. Crucially, none of the interfering ions induced significant color variations, even at 10-times higher concentrations. These results demonstrate the excellent specificity of the developed device for Cr(Ⅵ) detection. As illustrated in Fig. 6 , under optimized experimental conditions, the colorimetric paper-based sensor exhibited a concentration-dependent enhancement of ΔG (The difference in gray values before and after the reaction) values upon addition of Cr(VI). A linear response relationship was observed within the concentration range of 0.45 ~ 8.6 µM, with the calibration curve described by the equation y = 5.15x + 0.331 (R 2 = 0.99), demonstrating robust quantitative capability. The inset in Fig. 6 clearly demonstrates that the color intensity of the test strips increases progressively with rising Cr(VI) concentrations. This visual gradient enables rapid instrument-free assessment, making the method suitable for on-site semi-quantitative analysis. This dual-mode detection strategy effectively bridges laboratory-grade quantification and field-applicable screening, particularly suited for resource-limited settings. 4. Conclusions In summary, this study developed a novel colorimetric detection method for Cr(VI) based on CuO/CN/LDH composites. Experimental results demonstrated that the presence of Cr(VI) in the CuO/CN/LDH-TMB mixed system rapidly catalyzed the oxidation of colorless TMB to blue oxTMB. Through systematic optimization of the reaction system, optimal detection performance was achieved, showing a linear detection range of 0-9.1 μM for Cr(VI) with a low detection limit of 25 nM. Combined XPS and electron paramagnetic resonance (EPR) analyses revealed the catalytic mechanism where Cr(VI) promotes the conversion of dissolved oxygen into superoxide radicals (·O 2 - ) through redox reactions. The method exhibited excellent selectivity and accuracy. To enable portable and low-cost Cr(VI) detection, we successfully implemented this approach in paper-based analytical devices, which maintained good linear response characteristics. This work provides new insights for subsequent research on portable detection technologies. Declarations Declaration of competing interest COI The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Shaohui Li : Conceptualization, Methodology, Software, Investigation, Formal Analysis, Writing-Original Draft. Sijia Hao : Data Curation, Supplementary Experiment. Wen li : Visualization. Ran Meng : Supervision & Visualization. Qiang Wang : Validation & Visualization. Yuqing Wang : Visualization. Lei Wang : Visualization; Xinjing Liu : Visualization; Yuanyuan Zhou : Validation; Haitong Wang : Validation; Dongxia Zhang : Writing - review & editing. Xibin Zhou : Conceptualization, Funding Acquisition, Resources, Supervision, Writing - review & editing. Acknowledgment Funding: This work was supported by the Key Laboratory of Resource Environment and Sustainable Development of Oasis, Gansu Province (GORS202301); References Z. Chen, Z. Zhang, J. Qi, J. You, J. Ma, L. Chen, Colorimetric detection of heavy metal ions with various chromogenic materials: Strategies and applications, Journal of Hazardous Materials, 441 (2023) 129889. C. Wang, M. Shang, H. Wei, M. Zhang, W. Zou, X. Meng, W. Chen, H. Shao, Y. Lai, Specific and sensitive on-site detection of Cr(VI) by surface-enhanced Raman spectroscopy, Sensors and Actuators B: Chemical, 346 (2021). R. Kanagaraj, Y.-S. Nam, S.J. Pai, S.S. Han, K.-B. Lee, Highly selective and sensitive detection of Cr 6+ ions using size-specific label-free gold nanoparticles, Sensors and Actuators B: Chemical, 251 (2017) 683-691. M. Rangel, S. Saluja, V. Barba, J.S. Pérez-Huerta, V. Agarwal, Dual-emissive waste oil based S-doped carbon dots for acetone detection and Cr(VI) detection/reduction/removal, Journal of Environmental Chemical Engineering, 11 (2023) 109438. V.I. Mikhaylov, E.F. Krivoshapkina, A.L. Trigub, V.V. Stalugin, P.V. Krivoshapkin, Detection and Adsorption of Cr(VI) ions by Mesoporous Fe–Alumina Films, ACS Sustainable Chemistry & Engineering, 6 (2018) 9283-9292. Y. Mao, S. Gao, L. Yao, L. Wang, H. Qu, Y. Wu, Y. Chen, L. Zheng, Single-atom nanozyme enabled fast and highly sensitive colorimetric detection of Cr(VI), Journal of Hazardous Materials, 408 (2021) 124898. M. Derbyshire, A. Lamberty, P.H.E. Gardiner, Optimization of the Simultaneous Determination of Cr(III) and Cr(VI) by Ion Chromatography with Chemiluminescence Detection, Analytical Chemistry, 71 (1999) 4203-4207. L. Goodarzi, M.R. Bayatloo, S. Chalavi, S. Nojavan, T. Rahmani, S.B. Azimi, Selective extraction and determination of Cr(VI) in food samples based on tandem electromembrane extraction followed by electrothermal atomic absorption spectrometry, Food Chemistry, 373 (2022) 131442. F. Cubadda, S. Giovannangeli, F. Iosi, A. Raggi, P. Stacchini, Chromium determination in foods by quadrupole inductively coupled plasma–mass spectrometry with ultrasonic nebulization, Food Chemistry, 81 (2003) 463-468. K. Zhao, L. Ge, T.I. Wong, X. Zhou, G. Lisak, Gold-silver nanoparticles modified electrochemical sensor array for simultaneous determination of chromium(III) and chromium(VI) in wastewater samples, Chemosphere, 281 (2021) 130880. J.-f. Guo, D.-q. Huo, M. Yang, C.-j. Hou, J.-j. Li, H.-b. Fa, H.-b. Luo, P. Yang, Colorimetric detection of Cr (VI) based on the leaching of gold nanoparticles using a paper-based sensor, Talanta, 161 (2016) 819-825. C. Wang, N. Liu, X. Zhao, Y. Tian, X. Chen, Y. Zhang, L. Fan, B. Hou, C-doped BiOCl/Bi 2 S 3 heterojunction for highly efficient photoelectrochemical detection and photocatalytic reduction of Cr(VI), Journal of Materials Science & Technology, 164 (2023) 188-197. X. Zheng, Q. Lian, L. Zhou, Y. Jiang, J. Gao, Peroxidase Mimicking of Binary Polyacrylonitrile-CuO Nanoflowers and the Application in Colorimetric Detection of H 2 O 2 and Ascorbic Acid, ACS Sustainable Chemistry & Engineering, 9 (2021) 7030-7043. Z Wang, J Zhang, L Liu, X Wu, H Kuang, C Xu, L Xu. A colorimetric paper-based sensor for toltrazuril and its metabolites in feed, chicken, and egg samples. Food Chemistry. 276(2019) 707-713. J. Xi, G. Wei, L. An, Z. Xu, Z. Xu, L. Fan, L. Gao, Copper/Carbon Hybrid Nanozyme: Tuning Catalytic Activity by the Copper State for Antibacterial Therapy, Nano Letters, 19 (2019) 7645-7654. B. Yang, X. Shao, X. Gu, K. Wang, X. Ning, J. Xia, M. Xie, Y. Tang, Q. Li, S. Tian, CuO@AgNPs nanozyme cavity arrays on screen-printed electrodes for ultrasensitive and on-site SERS detection, Chemical Engineering Journal, 471 (2023) 144522. Q. Wu, L. He, Z.W. Jiang, Y. Li, Z.M. Cao, C.Z. Huang, Y.F. Li, CuO nanoparticles derived from metal-organic gel with excellent electrocatalytic and peroxidase-mimicking activities for glucose and cholesterol detection, Biosensors and Bioelectronics, 145 (2019). Y. Xu, Q. Yin, N. Du, Y. Yi, G. Zhu, An innovative homogeneous electrochemistry coupled with colorimetry dual-model sensing strategy for perfluorooctane sulfonate based on Cu@CuO aerogel nanozyme, Microchimica Acta, 191 (2024) 693. W. Zhu, Y. Cheng, C. Wang, X. Lu, Fabrication of a Tubular CuO/NiO Biomimetic Nanozyme with Synergistically Promoted Peroxidase-like Performance for Isoniazid Sensing, Inorganic Chemistry, 61 (2022) 16239-16247. C.A. Obasanjo, A.S. Zeraati, H.S. Shiran, T.N. Nguyen, S.M. Sadaf, M.G. Kibria, C.-T. Dinh, In situ regeneration of copper catalysts for long-term electrochemical CO 2 reduction to multiple carbon products, Journal of Materials Chemistry A, 10 (2022) 20059-20070. J. Zhang, X. Zhang, Y. Gao, J. Yan, W. Song, Integrating CuO/g-C 3 N 4 p-n heterojunctioned photocathode with MoS 2 QDs@Cu NWs multifunctional signal amplifier for ultrasensitive detection of AβO, Biosensors and Bioelectronics, 176 (2021) 112945. M. Zhang, C. Lai, B. Li, F. Xu, D. Huang, S. Liu, L. Qin, X. Liu, H. Yi, Y. Fu, L. Li, N. An, L. Chen, Insightful understanding of charge carrier transfer in 2D/2D heterojunction photocatalyst: Ni-Co layered double hydroxides deposited on ornamental g-C3N4 ultrathin nanosheet with boosted molecular oxygen activation, Chemical Engineering Journal, 422 (2021). Y. Yang, B. Mao, G. Gong, D. Li, Y. Liu, W. Cao, L. Xing, J. Zeng, W. Shi, S. Yuan, In-situ growth of Zn–AgIn5S8 quantum dots on g-C3N4 towards 0D/2D heterostructured photocatalysts with enhanced hydrogen production, International Journal of Hydrogen Energy, 44 (2019) 15882-15891. X. Wu, X. Wang, F. Wang, H. Yu, Soluble g-C3N4 nanosheets: Facile synthesis and application in photocatalytic hydrogen evolution, Applied Catalysis B: Environmental, 247 (2019) 70-77. X. Zou, M. Li, S. Zhou, C. Chen, J. Zhong, A. Xue, Y. Zhang, Y. Zhao, Diffusion behaviors of ethanol and water through g–C 3 N 4 –based membranes: Insights from molecular dynamics simulation, Journal of Membrane Science, 585 (2019) 81-89. H. Boumeriame, A. Cherevan, D. Eder, D.H. Apaydin, T. Chafik, E.S. Da Silva, J.L. Faria, Engineering g-C 3 N 4 with CuAl-layered double hydroxide in 2D/2D heterostructures for visible-light water splitting, Journal of Colloid and Interface Science, 652 (2023) 2147-2158. N. Wu, Y.-T. Wang, X.-Y. Wang, F.-N. Guo, H. Wen, T. Yang, J.-H. Wang, Enhanced peroxidase-like activity of AuNPs loaded graphitic carbon nitride nanosheets for colorimetric biosensing, Analytica Chimica Acta, 1091 (2019) 69-75. C. Zhang, P. Ni, B. Wang, W. Liu, Y. Jiang, C. Chen, J. Sun, Y. Lu, Enhanced oxidase-like activity of g-C 3 N 4 nanosheets supported Pd nanosheets for ratiometric fluorescence detection of acetylcholinesterase activity and its inhibitor, Chinese Chemical Letters, 33 (2022) 757-761. Q. Gu, C. Feng, C. Liu, J. Rong, Y. Zhang, X. Zheng, Z. Li, S. Xu, Constructing direct Z-scheme heterojunction of NiCo-LDH coated with g-C 3 N 4 for boosting photocatalytic H 2 evolution, Fuel, 371 (2024) 131982. Y. Yang, C. Yin, K. Li, H. Tang, Y. Wang, Z. Wu, Cu Doped Crystalline Carbon-Conjugated g-C 3 N 4 , a Promising Oxygen Reduction Catalyst by Theoretical Study, Journal of The Electrochemical Society, 166 (2019) F755-F759. Q. Sun, K. Yao, Y. Zhang, MnO 2 -directed synthesis of NiFe-LDH@FeOOH nanosheeet arrays for supercapacitor negative electrode, Chinese Chemical Letters, 31 (2020) 2343-2346. B. Wang, Y. Fang, X. Han, R. Jiang, L. Zhao, X. Yang, J. Jin, A. Han, J. Liu, Atomization-Induced High Intrinsic Activity of a Biocompatible MgAl-LDH Supported Ru Single-Atom Nanozyme for Efficient Radicals Scavenging, Angewandte Chemie International Edition, 62 (2023) e202307133. X. Gao, X. Liu, D. Wu, B. Qian, Z. Kou, Z. Pan, Y. Pang, L. Miao, J. Wang, Significant Role of Al in Ternary Layered Double Hydroxides for Enhancing Electrochemical Performance of Flexible Asymmetric Supercapacitor, Advanced Functional Materials, 29 (2019) 1903879. W. Liu, Q. Wang, Z. Liu, G. Ding, Bridging between NiAl-LDH and g-C3N4 by using carbon quantum dots for highly enhanced photoreduction of CO2 into CO, Journal of Colloid and Interface Science, 622 (2022) 21-30. J. Zheng, C. Fan, X. Li, Q. Yang, D. Wang, A. Duan, S. Pan, B. Zhang, J. Ding, S. Rong, H. Yin, Effective mineralization and detoxification of tetracycline hydrochloride enabled by oxygen vacancies in g-C 3 N 4 / LDH composites, Separation and Purification Technology, 305 (2023). H. Sheikhpoor, A. Saljooqi, T. Shamspur, A. Mostafavi, Co-Al Layered double hydroxides decorated with CoFe 2 O 4 nanoparticles and g-C 3 N 4 nanosheets for efficient photocatalytic pesticide degradation, Environmental Technology & Innovation, 23 (2021) 101649. A.-Q. Zhou, J.-M. Yang, X.-W. Zhu, X.-L. Zhu, J.-Y. Liu, K. Zhong, H.-X. Chen, J.-Y. Chu, Y.-S. Du, Y.-H. Song, J.-C. Qian, H.-M. Li, H. Xu, Self-assembly construction of NiCo LDH/ultrathin g-C 3 N 4 nanosheets photocatalyst for enhanced CO 2 reduction and charge separation mechanism study, Rare Metals, 41 (2022) 2118-2128. S. Guru, S. Kumar, S. Bellamkonda, R.R. Gangavarapu, Synthesis of CuTi-LDH supported on g-C 3 N 4 for electrochemical and photoelectrochemical oxygen evolution reactions, International Journal of Hydrogen Energy, 46 (2021) 16414-16430. Q. Shi, K. Fang, W. Chen, Y. Tan, C. Zhang, Designing a superhydrophobic cotton fiber coating exploiting TiO 2 @g-C 3 N 4 layered structure for augmented photocatalysis and efficient water-oil separation, International Journal of Biological Macromolecules, 264 (2024) 130596. M. Qiu, P. Guo, X. Lei, Y. Li, S. Li, R. Meng, X. Zhou, Core-shell spherical cerium oxide, N-doped carbon dots and copper oxide nanocomposites with oxidase-like activity for colorimetric detection of hexavalent chromiums, Journal of Environmental Chemical Engineering, 12 (2024) 112334. A. Elhalil, R. Elmoubarki, A. Machrouhi, M. Sadiq, M. Abdennouri, S. Qourzal, N. Barka, Photocatalytic degradation of caffeine by ZnO-ZnAl 2 O 4 nanoparticles derived from LDH structure, Journal of Environmental Chemical Engineering, 5 (2017) 3719-3726. Y. Li, Y. Li, S. Ma, P. Wang, Q. Hou, J. Han, S. Zhan, Efficient water disinfection with Ag 2 WO 4 -doped mesoporous g-C 3 N 4 under visible light, Journal of Hazardous Materials, 338 (2017) 33-46. S. Sanati, Z. Rezvani, g-C 3 N 4 nanosheet@CoAl-layered double hydroxide composites for electrochemical energy storage in supercapacitors, Chemical Engineering Journal, 362 (2019) 743-757. A.F.d. Silva, J.L.d.S. Duarte, L. Meili, Different routes for MgFe/LDH synthesis and application to remove pollutants of emerging concern, Separation and Purification Technology, 264 (2021) 118353. H. Huang, Z. Li, H. Wang, C. Xia, P. Yan, Q. Zhang, Z. Meng, Adsorption performance of layered double hydroxides for heavy metals removal in soil with the presence of microplastics, Journal of Environmental Chemical Engineering, 10 (2022) 108733. H. Almasi, P. Jafarzadeh, L. Mehryar, Fabrication of novel nanohybrids by impregnation of CuO nanoparticles into bacterial cellulose and chitosan nanofibers: Characterization, antimicrobial and release properties, Carbohydrate Polymers, 186 (2018) 273-281. S. Wang, C. Yuan, W. Chen, Y. Niu, Y. Yan, F. Li, H. Jiang, 3D spherical CuO@g-C 3 N 4 composites activating peroxymonosulfate for high efficient degradation of 2,4,6-trichlorophenol: The mechanism of 1 O 2 generation, Chemical Engineering Journal, 480 (2024) 148050. P. Zhang, J. Zhang, D. Wang, F. Zhang, Y. Zhao, M. Yan, C. Zheng, Q. Wang, M. Long, C. Chen, Modification of g-C 3 N 4 with hydroxyethyl cellulose as solid proton donor via hydrogen bond to enhance H 2 O 2 production, Applied Catalysis B: Environmental, 318 (2022) 121749. H. Wang, T. Zhu, Y. Qiao, S. Dong, Z. Qu, Investigation of the promotion effect of Mo doped CuO catalysts for the low-temperature performance of NH3-SCR reaction, Chinese Chemical Letters, 33 (2022) 5223-5227. Z. Liu, X. Dai, J. He, W. Chen, Y. Wei, Q. Zhou, D. Ma, X. Zheng, Unraveling the thallium immobilization in CuO/PMS system, Chemical Engineering Journal, 472 (2023) 144869. K Feng, G Wang, S Wang, J Ma, H Wu, M Ma, Y Zhang. Breaking the pH Limitation of Nanozymes: Mechanisms, Methods, and Applications. Adv Mater. 2024 Aug;36(31) 2401619. Z Chenghui, C Chuanxia, Z Dan, K Ge, L Fangning, Y Fan, L Yizhong, S Jian. Multienzyme Cascades Based on Highly Efficient Metal–Nitrogen–Carbon Nanozymes for Construction of Versatile Bioassays [J]. Analytical Chemistry, 2022, 94(8) 3485-93. Schemes Schemes 1 and 2 are available in the Supplementary Files section. Additional Declarations No competing interests reported. Supplementary Files SupplementaryMaterials.docx Schemes.docx Cite Share Download PDF Status: Published Journal Publication published 03 Mar, 2026 Read the published version in Microchimica Acta → Version 1 posted Editorial decision: Revision requested 14 Dec, 2025 Reviewers agreed at journal 08 Dec, 2025 Reviews received at journal 08 Dec, 2025 Reviews received at journal 07 Dec, 2025 Reviewers agreed at journal 07 Dec, 2025 Reviews received at journal 07 Dec, 2025 Reviews received at journal 07 Dec, 2025 Reviewers agreed at journal 07 Dec, 2025 Reviewers agreed at journal 07 Dec, 2025 Reviewers agreed at journal 06 Dec, 2025 Reviewers invited by journal 06 Dec, 2025 Editor assigned by journal 03 Dec, 2025 Submission checks completed at journal 03 Dec, 2025 First submitted to journal 01 Dec, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8252032","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":557310945,"identity":"2d2af9f4-1c69-4b57-b216-1b914e29e2ac","order_by":0,"name":"Shaohui Li","email":"","orcid":"","institution":"Northwest Normal University","correspondingAuthor":false,"prefix":"","firstName":"Shaohui","middleName":"","lastName":"Li","suffix":""},{"id":557310946,"identity":"dcd1f472-91ad-4fc5-ae43-cfd4066329a7","order_by":1,"name":"Sijia Hao","email":"","orcid":"","institution":"Northwest Normal University","correspondingAuthor":false,"prefix":"","firstName":"Sijia","middleName":"","lastName":"Hao","suffix":""},{"id":557310947,"identity":"8c8f4f02-3338-4bac-8d0c-c9632ef3a1a7","order_by":2,"name":"Wen Li","email":"","orcid":"","institution":"Northwest Normal University","correspondingAuthor":false,"prefix":"","firstName":"Wen","middleName":"","lastName":"Li","suffix":""},{"id":557310948,"identity":"d2a0a11c-d276-49e9-9997-fc88d046a3a0","order_by":3,"name":"Ran Meng","email":"","orcid":"","institution":"Northwest Normal University","correspondingAuthor":false,"prefix":"","firstName":"Ran","middleName":"","lastName":"Meng","suffix":""},{"id":557310949,"identity":"dcca09b9-06f8-4efc-8be5-3a5e4baf54a9","order_by":4,"name":"Qiang Wang","email":"","orcid":"","institution":"Northwest Normal University","correspondingAuthor":false,"prefix":"","firstName":"Qiang","middleName":"","lastName":"Wang","suffix":""},{"id":557310950,"identity":"4ff1058e-4024-43fc-87c7-8f6fc56b8fd5","order_by":5,"name":"Yuqing Wang","email":"","orcid":"","institution":"Northwest Normal University","correspondingAuthor":false,"prefix":"","firstName":"Yuqing","middleName":"","lastName":"Wang","suffix":""},{"id":557310951,"identity":"66886b7d-f786-4a3d-b7ee-fa0876d58187","order_by":6,"name":"Lei Wang","email":"","orcid":"","institution":"Northwest Normal University","correspondingAuthor":false,"prefix":"","firstName":"Lei","middleName":"","lastName":"Wang","suffix":""},{"id":557310952,"identity":"c31bc533-9976-4e19-8035-ae8b3451b4bb","order_by":7,"name":"Xinjing Liu","email":"","orcid":"","institution":"Northwest Normal University","correspondingAuthor":false,"prefix":"","firstName":"Xinjing","middleName":"","lastName":"Liu","suffix":""},{"id":557310953,"identity":"ca70679f-a959-4c1e-9e80-95e59745b946","order_by":8,"name":"Yuanyuan Zhou","email":"","orcid":"","institution":"Northwest Normal University","correspondingAuthor":false,"prefix":"","firstName":"Yuanyuan","middleName":"","lastName":"Zhou","suffix":""},{"id":557310954,"identity":"686bab85-7503-4ea7-8903-35a378f808c0","order_by":9,"name":"Haitong Wang","email":"","orcid":"","institution":"Northwest Normal University","correspondingAuthor":false,"prefix":"","firstName":"Haitong","middleName":"","lastName":"Wang","suffix":""},{"id":557310955,"identity":"bf67c6b3-faba-4fcd-bac3-595e46c751ff","order_by":10,"name":"Xibin Zhou","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAv0lEQVRIiWNgGAWjYDACCSBmbLCB8tiI15JGupbDJGiRn918TJp3x/k8g9s9Bgwfyg4z8M9uwK+Fcc6xNMmZZ24XG9w5Y8A449xhBok7B/BrYZbIMZP42HY7cduNHANm3rbDDAYSCfi1sIG0JLadg2j5S4wWHogtByBaGInRIiGRlmw580xy4v4baQUHe86l80jcIKBFfkbywdu8O+wSZ85I3vjgR5m1HP8MAlpQwAGQS0lQPwpGwSgYBaMAFwAAy8lCyL81LPEAAAAASUVORK5CYII=","orcid":"","institution":"Northwest Normal University","correspondingAuthor":true,"prefix":"","firstName":"Xibin","middleName":"","lastName":"Zhou","suffix":""},{"id":557310956,"identity":"7216a795-d35a-4a30-8477-467dddda048b","order_by":11,"name":"Dongxia Zhang","email":"","orcid":"","institution":"Northwest Normal University","correspondingAuthor":false,"prefix":"","firstName":"Dongxia","middleName":"","lastName":"Zhang","suffix":""}],"badges":[],"createdAt":"2025-12-01 15:23:38","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8252032/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8252032/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s00604-026-07890-2","type":"published","date":"2026-03-03T15:57:18+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":98428584,"identity":"0a6e1173-9cea-4128-a6e6-75e4a796ae29","added_by":"auto","created_at":"2025-12-17 16:42:10","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":2980334,"visible":true,"origin":"","legend":"","description":"","filename":"Manuscript.docx","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/b8403c4088e256ecb7d16be2.docx"},{"id":98428236,"identity":"fa1b83ce-c155-429a-8122-42b23990798d","added_by":"auto","created_at":"2025-12-17 16:41:50","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":4253991,"visible":true,"origin":"","legend":"","description":"","filename":"Figures.docx","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/70922921e8d9771254d1bbf0.docx"},{"id":98427596,"identity":"db015c92-9e7c-4d9f-9e66-3e686f0c9f79","added_by":"auto","created_at":"2025-12-17 16:40:48","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":4253991,"visible":true,"origin":"","legend":"","description":"","filename":"Figuresd1.docx","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/95bde56464a176559220176c.docx"},{"id":98428326,"identity":"dfb6c8bc-47b0-41de-9a90-080769434666","added_by":"auto","created_at":"2025-12-17 16:41:53","extension":"docx","order_by":3,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":21746,"visible":true,"origin":"","legend":"","description":"","filename":"Tables.docx","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/a76c890df0d0459e04aa0e9e.docx"},{"id":98053751,"identity":"ca9c6c47-8ff8-434a-8694-346af74e641f","added_by":"auto","created_at":"2025-12-12 09:37:08","extension":"docx","order_by":4,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":21746,"visible":true,"origin":"","legend":"","description":"","filename":"Tablesd1.docx","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/b7df00be85d114e8f14cc1bf.docx"},{"id":98053759,"identity":"3dd700a3-3919-460a-a723-18c00b3e66c6","added_by":"auto","created_at":"2025-12-12 09:37:08","extension":"json","order_by":5,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":11886,"visible":true,"origin":"","legend":"","description":"","filename":"94b9c2a91a6b4c458d6c675781c8e675.json","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/698442fc69c8da3f60e9505e.json"},{"id":98053764,"identity":"6fb3baa6-d7c0-47d1-8619-21c4bdb7ba1f","added_by":"auto","created_at":"2025-12-12 09:37:08","extension":"docx","order_by":6,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":1888100,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryMaterials.docx","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/7bbc8a4bdf7acc717b35f5c3.docx"},{"id":98428226,"identity":"16cb9301-c579-45b1-808f-f3e6bac6eaa6","added_by":"auto","created_at":"2025-12-17 16:41:49","extension":"docx","order_by":7,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":1888100,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryMaterialsd1.docx","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/e060981738ef34bbd3e85e49.docx"},{"id":98426216,"identity":"a0b34527-330e-4ef3-a41b-18d0108a7b48","added_by":"auto","created_at":"2025-12-17 16:35:52","extension":"xml","order_by":8,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":125392,"visible":true,"origin":"","legend":"","description":"","filename":"94b9c2a91a6b4c458d6c675781c8e6751enriched.xml","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/c821cfa8c50a5cb1c2e08ac2.xml"},{"id":98428514,"identity":"81272404-3bfb-4026-ba8a-169c71501647","added_by":"auto","created_at":"2025-12-17 16:42:06","extension":"jpeg","order_by":9,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":233148,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/3b9ce4c0021aa6a879661aaf.jpeg"},{"id":98053762,"identity":"11b5a771-d08d-42e6-828c-1978f9565873","added_by":"auto","created_at":"2025-12-12 09:37:08","extension":"jpeg","order_by":10,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":764658,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage10.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/750090af0adfb10177578991.jpeg"},{"id":98053756,"identity":"a0f70fbc-7808-4136-adbf-62a1dc6292a1","added_by":"auto","created_at":"2025-12-12 09:37:08","extension":"jpeg","order_by":11,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":126696,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage11.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/41556b1369e96cac7ab82f61.jpeg"},{"id":98053796,"identity":"d8ee634c-a63e-41c4-85ba-b8e71f225bf3","added_by":"auto","created_at":"2025-12-12 09:37:09","extension":"emf","order_by":12,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":46292944,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage12.emf","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/525cb91bed9bcd2a0e86c537.emf"},{"id":98053772,"identity":"fd73c1ae-3eab-4424-855e-e2b622faa20d","added_by":"auto","created_at":"2025-12-12 09:37:08","extension":"jpeg","order_by":13,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":235662,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage13.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/1d673b85743237e03a6515d0.jpeg"},{"id":98053778,"identity":"05ed940e-f03b-4f3a-a413-6501eee6b31a","added_by":"auto","created_at":"2025-12-12 09:37:08","extension":"emf","order_by":14,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":3704848,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage14.emf","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/9ed4f7106c2327e7fe2f5ce4.emf"},{"id":98428058,"identity":"a160a978-4598-4d3a-8a38-098b54a64563","added_by":"auto","created_at":"2025-12-17 16:41:33","extension":"jpeg","order_by":15,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":91820,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage15.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/10825f2c8fcf558c43307797.jpeg"},{"id":98427605,"identity":"31a5a30d-4c6e-48af-9510-909a54a0adc6","added_by":"auto","created_at":"2025-12-17 16:40:50","extension":"jpeg","order_by":16,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":99408,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage16.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/fbb08d0c401a9a3dfb0fb906.jpeg"},{"id":98426137,"identity":"fb932d6f-aa8b-41b6-b590-35fca92ffdf9","added_by":"auto","created_at":"2025-12-17 16:35:45","extension":"jpeg","order_by":17,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":40818,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage17.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/948b11526f471c7addcee1ce.jpeg"},{"id":98428575,"identity":"deaeab81-3964-48ac-ad55-501327fc7244","added_by":"auto","created_at":"2025-12-17 16:42:09","extension":"jpeg","order_by":18,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":683474,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage18.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/efea20fd0d38f1d1581abf64.jpeg"},{"id":98428141,"identity":"b30a35a2-fcbf-4bad-9365-a7027db03c7d","added_by":"auto","created_at":"2025-12-17 16:41:39","extension":"jpeg","order_by":19,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":103084,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage19.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/d8dc42839d4011ff95b5e190.jpeg"},{"id":98427631,"identity":"de0c46c2-5cca-49bd-bbc1-d37c0fb62a03","added_by":"auto","created_at":"2025-12-17 16:40:53","extension":"jpeg","order_by":20,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":764658,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage10.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/bdae65245561d59c0f60141a.jpeg"},{"id":98428353,"identity":"6ebba1c4-3fd5-4630-a3b4-475f91ee52bf","added_by":"auto","created_at":"2025-12-17 16:41:56","extension":"jpeg","order_by":21,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":274462,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage20.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/006e77757d14828cb02eaa44.jpeg"},{"id":98053774,"identity":"631dbaaa-67f3-4746-8b60-89ce2c808c63","added_by":"auto","created_at":"2025-12-12 09:37:08","extension":"jpeg","order_by":22,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":329434,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage21.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/2df955595bdf2384e927a871.jpeg"},{"id":98427971,"identity":"7ca0a50d-5b1b-423b-ad43-67c0c95bb827","added_by":"auto","created_at":"2025-12-17 16:41:27","extension":"png","order_by":23,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":253598,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage22.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/7f020f4e4458e9342bed447e.png"},{"id":98053776,"identity":"5ea0a5ae-3c2e-4995-aee3-f7f1a3393d63","added_by":"auto","created_at":"2025-12-12 09:37:08","extension":"jpeg","order_by":24,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":233148,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/6ea6cc42a42109870b25514e.jpeg"},{"id":98428126,"identity":"c029feca-5b5e-4200-8264-f87ff7d3bb07","added_by":"auto","created_at":"2025-12-17 16:41:37","extension":"jpeg","order_by":25,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":764658,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage10.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/da17f71c31133f182b409876.jpeg"},{"id":98427350,"identity":"fc9af285-0a3c-4906-a6c2-d9f1cdd6ae77","added_by":"auto","created_at":"2025-12-17 16:40:12","extension":"jpeg","order_by":26,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":126696,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage11.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/01c72955905dfda8c8e80f6d.jpeg"},{"id":98053826,"identity":"bb49e3b9-160e-4fff-9af2-fcbc344f9b6e","added_by":"auto","created_at":"2025-12-12 09:37:09","extension":"emf","order_by":27,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":46292944,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage12.emf","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/b898a6aed7963cd27654afbb.emf"},{"id":98426150,"identity":"4d8aafb5-8a6d-4846-9c58-eff4e8250f69","added_by":"auto","created_at":"2025-12-17 16:35:47","extension":"jpeg","order_by":28,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":235662,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage13.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/f40e8c2084d6495072fd9baf.jpeg"},{"id":98053788,"identity":"f313faa0-e580-44dc-bc8b-8855b9edddb2","added_by":"auto","created_at":"2025-12-12 09:37:08","extension":"emf","order_by":29,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":3704848,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage14.emf","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/906b37e4d34e39b10c9b7c07.emf"},{"id":98053780,"identity":"2abb38aa-4173-4e16-bbc1-84fe6844459e","added_by":"auto","created_at":"2025-12-12 09:37:08","extension":"jpeg","order_by":30,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":91820,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage15.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/0591dd6b16bad1b042bd331d.jpeg"},{"id":98428458,"identity":"8f9fa451-198b-4ed1-9874-6f497e27c4b4","added_by":"auto","created_at":"2025-12-17 16:42:02","extension":"jpeg","order_by":31,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":126696,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage11.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/07350d112fd6c6cff533db00.jpeg"},{"id":98053784,"identity":"94782d80-1cd1-43b3-a58d-e218f139b6e2","added_by":"auto","created_at":"2025-12-12 09:37:08","extension":"jpeg","order_by":32,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":99408,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage16.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/3cb539f60a7a096eb656fde2.jpeg"},{"id":98428645,"identity":"364cae20-762b-40ac-8407-f8ff626fbcdd","added_by":"auto","created_at":"2025-12-17 16:42:13","extension":"jpeg","order_by":33,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":40818,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage17.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/64202e3f5e800b2611918878.jpeg"},{"id":98428252,"identity":"1390415f-d231-4831-bbb5-2ddc1c6e47f1","added_by":"auto","created_at":"2025-12-17 16:41:51","extension":"jpeg","order_by":34,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":683474,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage18.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/519f625b713400e230d59df1.jpeg"},{"id":98428352,"identity":"eebc71af-19c2-477e-8b52-ee87b761e31d","added_by":"auto","created_at":"2025-12-17 16:41:56","extension":"jpeg","order_by":35,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":103084,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage19.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/bb5168442401a5835790e7f9.jpeg"},{"id":98053782,"identity":"a36ca3e8-72fa-4e90-b80a-2765f0277f7c","added_by":"auto","created_at":"2025-12-12 09:37:08","extension":"jpeg","order_by":36,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":274462,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage20.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/162ef01d4ae224499dcec01b.jpeg"},{"id":98427891,"identity":"1a004e2f-9dcf-4ae9-96e5-da697552a85d","added_by":"auto","created_at":"2025-12-17 16:41:19","extension":"jpeg","order_by":37,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":329434,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage21.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/b2da341b4a5c9fd4632d4f74.jpeg"},{"id":98053785,"identity":"718e0d2d-f998-4b02-929e-8e9629d00982","added_by":"auto","created_at":"2025-12-12 09:37:08","extension":"png","order_by":38,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":253598,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage22.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/d55834268bef6f286cecaa48.png"},{"id":98053809,"identity":"d96bf2ce-ddbf-4e04-b2c6-92aaab4f3ef7","added_by":"auto","created_at":"2025-12-12 09:37:09","extension":"emf","order_by":39,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":46292944,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage12.emf","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/18198d5789b7d5024e68dfcc.emf"},{"id":98427760,"identity":"5ae9eb3c-058e-461a-a288-347da5942733","added_by":"auto","created_at":"2025-12-17 16:41:06","extension":"jpeg","order_by":40,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":235662,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage13.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/b5f346e64108ee3f13df824f.jpeg"},{"id":98428612,"identity":"5ac8900f-88ff-4edb-937f-de0217fa90b6","added_by":"auto","created_at":"2025-12-17 16:42:11","extension":"emf","order_by":41,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":33158100,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage6.emf","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/12c07612be01efccb84e9ebc.emf"},{"id":98426187,"identity":"a8c9b107-f664-49a8-811e-a08b59029cc6","added_by":"auto","created_at":"2025-12-17 16:35:49","extension":"jpeg","order_by":42,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":91820,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage15.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/000442c2cce4c8b9136dd678.jpeg"},{"id":98053783,"identity":"a89dfec3-84d8-4710-9c47-420e43f53d8f","added_by":"auto","created_at":"2025-12-12 09:37:08","extension":"jpeg","order_by":43,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":99408,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage16.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/8374229f643aaac35dd34b8d.jpeg"},{"id":98427332,"identity":"1166aa3f-942a-4888-97a6-b66206890e30","added_by":"auto","created_at":"2025-12-17 16:40:09","extension":"jpeg","order_by":44,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":233148,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/21c25d0e439ba206523de048.jpeg"},{"id":98428187,"identity":"8a7cc803-469e-4383-b3de-3c5b842b812a","added_by":"auto","created_at":"2025-12-17 16:41:43","extension":"png","order_by":45,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":37245,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/7ceb1479ef605b25d3787903.png"},{"id":98427739,"identity":"c2549566-f2b3-4528-b7fa-ab9e34b1f264","added_by":"auto","created_at":"2025-12-17 16:41:03","extension":"png","order_by":46,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":125357,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage10.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/9877e2b6f47f352f5c0a4c08.png"},{"id":98053790,"identity":"12ec6d72-03a7-4cde-ab7f-8a74a6b8718a","added_by":"auto","created_at":"2025-12-12 09:37:08","extension":"png","order_by":47,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":56046,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage11.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/55b3a7eb69ba9dafb5ab31ca.png"},{"id":98427900,"identity":"3be4c46a-b121-40fa-bb67-972c09fe4b4f","added_by":"auto","created_at":"2025-12-17 16:41:20","extension":"png","order_by":48,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":121385,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage12.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/8079a118deba84c705485e5e.png"},{"id":98428557,"identity":"f9d29507-916a-4e29-a7bd-2df583239a01","added_by":"auto","created_at":"2025-12-17 16:42:08","extension":"png","order_by":49,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":36533,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage13.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/409f6549179df043d654ccaa.png"},{"id":98428442,"identity":"844bf3a7-4922-46a5-95da-f882fab3d787","added_by":"auto","created_at":"2025-12-17 16:42:01","extension":"png","order_by":50,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":309256,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage14.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/535a661c1f40b55d62537217.png"},{"id":98428815,"identity":"84dba1fd-46a8-4a24-9fdd-36cf28da944e","added_by":"auto","created_at":"2025-12-17 16:42:25","extension":"png","order_by":51,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":12302,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage15.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/7979550f3d58487272be797c.png"},{"id":98427740,"identity":"744de5e6-edda-4625-967b-da3d918d49bc","added_by":"auto","created_at":"2025-12-17 16:41:03","extension":"png","order_by":52,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":18012,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage16.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/7a37cddd016e43749bfd2c7a.png"},{"id":98053799,"identity":"bd20f8b0-a68a-4037-9a81-384da5f8fce6","added_by":"auto","created_at":"2025-12-12 09:37:09","extension":"png","order_by":53,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":11329,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage17.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/4bef74aa6fa7bfc3f921f544.png"},{"id":98427725,"identity":"3c5a14e7-0eb8-4cf2-abc0-2730dab229cd","added_by":"auto","created_at":"2025-12-17 16:41:02","extension":"png","order_by":54,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":24480,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage18.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/8dfcaa314dac1a09e8588f2e.png"},{"id":98053792,"identity":"54ebeb93-e0f9-4a41-aa97-30cf2cbb5ea6","added_by":"auto","created_at":"2025-12-12 09:37:08","extension":"png","order_by":55,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":8973,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage19.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/1c5f2225d01893fb0bde25a8.png"},{"id":98427669,"identity":"7e1028b5-3691-4770-ab52-4167fe33f628","added_by":"auto","created_at":"2025-12-17 16:40:58","extension":"png","order_by":56,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":125357,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/f3628aea396e7cf3c46ddf1f.png"},{"id":98053808,"identity":"efb9f38c-c401-4642-b30c-dc955514fad1","added_by":"auto","created_at":"2025-12-12 09:37:09","extension":"png","order_by":57,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":8858,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage20.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/5e9977c642dbcdf80ce9e2f9.png"},{"id":98053807,"identity":"4c28c971-046d-4bf1-a580-120243d6c898","added_by":"auto","created_at":"2025-12-12 09:37:09","extension":"png","order_by":58,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":60104,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage21.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/f0c8241712861eb4460498cd.png"},{"id":98428386,"identity":"caf2cfe3-284d-44b1-8f7b-9f60dbed2063","added_by":"auto","created_at":"2025-12-17 16:41:58","extension":"png","order_by":59,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":53177,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage22.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/84317f68167aef4612949041.png"},{"id":98427782,"identity":"d1c71000-e1f2-47cc-9e72-2854950557d4","added_by":"auto","created_at":"2025-12-17 16:41:12","extension":"png","order_by":60,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":37245,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage23.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/4ca48431620983f112f6aa6c.png"},{"id":98427776,"identity":"d2c6cd01-77e8-47cc-9d4f-7e788164c927","added_by":"auto","created_at":"2025-12-17 16:41:10","extension":"png","order_by":61,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":125357,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage24.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/c7f22b8164204b960801b8ac.png"},{"id":98428298,"identity":"a88e4259-e106-494f-a198-9827bc9a0317","added_by":"auto","created_at":"2025-12-17 16:41:52","extension":"png","order_by":62,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":56046,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage25.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/b7f916eac2a0975c2715bff9.png"},{"id":98053834,"identity":"cf0944ab-6d7a-4db0-a8ac-31bca9ad2a29","added_by":"auto","created_at":"2025-12-12 09:37:09","extension":"png","order_by":63,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":121385,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage12.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/897e86d94a03641a1d881f2c.png"},{"id":98427908,"identity":"8be5c956-2ecb-4638-b953-9cc71b7ff7d4","added_by":"auto","created_at":"2025-12-17 16:41:20","extension":"png","order_by":64,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":36533,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage27.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/3db25bbcbe1922c8a9038538.png"},{"id":98053831,"identity":"ed2a612e-38f2-4267-b3f5-8c30f6bcc74e","added_by":"auto","created_at":"2025-12-12 09:37:09","extension":"png","order_by":65,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":309256,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage14.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/ff8b6bf86bc8c1b3240d1f17.png"},{"id":98053816,"identity":"0ceab17e-c1a2-4796-b249-ca8c771044ad","added_by":"auto","created_at":"2025-12-12 09:37:09","extension":"png","order_by":66,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":12302,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage29.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/25f305b7a28f8978f8df6ab8.png"},{"id":98426434,"identity":"db845824-3103-4d79-afcd-ef278938c197","added_by":"auto","created_at":"2025-12-17 16:36:18","extension":"png","order_by":67,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":56046,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/f17e0e58b96503d394dfe8e6.png"},{"id":98427791,"identity":"cbbae0e8-066f-4152-91b1-43cc924ed35e","added_by":"auto","created_at":"2025-12-17 16:41:12","extension":"png","order_by":68,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":18012,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage30.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/fdbf76667d543bc2d61ee6fa.png"},{"id":98428653,"identity":"dc0f9735-963f-4db2-844a-fa6f3ff27cf3","added_by":"auto","created_at":"2025-12-17 16:42:14","extension":"png","order_by":69,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":11329,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage31.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/caa229886a9efa7454ec736e.png"},{"id":98053812,"identity":"c5430aa6-d5c7-4f2f-a495-63d86fb68c80","added_by":"auto","created_at":"2025-12-12 09:37:09","extension":"png","order_by":70,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":24480,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage32.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/f1a6a8e2c085c01851028295.png"},{"id":98427694,"identity":"47b72021-fa0a-4aea-97a6-a828f9034f59","added_by":"auto","created_at":"2025-12-17 16:41:00","extension":"png","order_by":71,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":8973,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage33.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/9d70ed13b0169bc3144ca59c.png"},{"id":98428872,"identity":"dcf1c6ca-c163-40c1-8fa6-c16ae3ba646f","added_by":"auto","created_at":"2025-12-17 16:42:29","extension":"png","order_by":72,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":8858,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage34.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/560f7d0317309ba82e3f8394.png"},{"id":98053823,"identity":"2a02da34-b92d-4fb2-963a-66f4ed1f34d2","added_by":"auto","created_at":"2025-12-12 09:37:09","extension":"png","order_by":73,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":60104,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage35.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/6e08af18c697462e52f28d1c.png"},{"id":98426467,"identity":"21a0cce1-4c68-434d-9300-47a7b8b32b84","added_by":"auto","created_at":"2025-12-17 16:36:25","extension":"png","order_by":74,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":53177,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage36.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/b4cc1dac651c3cb07a4e499f.png"},{"id":98428467,"identity":"b63001c0-9ff3-4684-a7c7-f2806d585299","added_by":"auto","created_at":"2025-12-17 16:42:02","extension":"png","order_by":75,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":121385,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage12.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/3177ab88a402167009f6a367.png"},{"id":98053837,"identity":"84f9c75b-7b12-4783-82e7-ef92bbaf81ae","added_by":"auto","created_at":"2025-12-12 09:37:10","extension":"png","order_by":76,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":36533,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/ebecde6d159f39cd415f8afd.png"},{"id":98053820,"identity":"4fdd8b72-c420-4455-afc3-e2ef03f4e675","added_by":"auto","created_at":"2025-12-12 09:37:09","extension":"png","order_by":77,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":285099,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/0a6270dba85fe82d038b21e8.png"},{"id":98427763,"identity":"8d2bb7ca-1344-4c13-b71f-ca341775a604","added_by":"auto","created_at":"2025-12-17 16:41:08","extension":"png","order_by":78,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":12302,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/58efdaaed8b90a2aa9bf1785.png"},{"id":98426156,"identity":"714681a0-68d3-43f5-bc41-25f342f207e4","added_by":"auto","created_at":"2025-12-17 16:35:47","extension":"png","order_by":79,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":18012,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/1468bd4227d1c31f129d5fda.png"},{"id":98426067,"identity":"b9c06541-d69e-43eb-9319-22c09c9006a5","added_by":"auto","created_at":"2025-12-17 16:35:33","extension":"png","order_by":80,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":37245,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/0e86908a4e4697ece029ac1b.png"},{"id":98053828,"identity":"e77950c1-4caa-4372-8823-3042dc5551d6","added_by":"auto","created_at":"2025-12-12 09:37:09","extension":"xml","order_by":81,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":125918,"visible":true,"origin":"","legend":"","description":"","filename":"94b9c2a91a6b4c458d6c675781c8e6751structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/0f216ea01954393cc9372ea1.xml"},{"id":98053836,"identity":"fe56d05d-6de5-4b1e-9304-d926b34d92e9","added_by":"auto","created_at":"2025-12-12 09:37:09","extension":"html","order_by":82,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":131007,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/4545410f72b39f5d294480b9.html"},{"id":98053746,"identity":"1a2d638c-ce3c-405e-81ca-9f8d274d7e9f","added_by":"auto","created_at":"2025-12-12 09:37:07","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":246488,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of (a) g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e; (b) CuCe-LDH; (c) CN/LDH; (d) CuO/CN/LDH; (e) EDX mapping of CuO/CN/LDH.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/60c20b95b46b09161b4d2aa9.png"},{"id":98053747,"identity":"6d2cfce6-7247-4601-ab9f-be77fa9e7a0d","added_by":"auto","created_at":"2025-12-12 09:37:08","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":152759,"visible":true,"origin":"","legend":"\u003cp\u003e(a) XRD image of CN(red), LDH(pink), CN/LDH(cyan), and CuO/CN/LDH (purple); (b) FT-IR image of CN(blank), LDH(red), and CuO/CN/LDH(blue); (c) The full XPS spectrum of CuO/CN/LDH; CuO/CN/LDH XPS highresolution images of CuO/CN/LDH (d) C1s; (e) N 1s; (f) O 1s; (g) Ce 3d; (h) Cu 2p.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/f73d67fca66bbb96d18f1c40.png"},{"id":98428307,"identity":"1bdff54e-22b5-444d-a5cb-df380f4a4ed0","added_by":"auto","created_at":"2025-12-17 16:41:52","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":34828,"visible":true,"origin":"","legend":"\u003cp\u003e(a) CuO/CN/LDH nanocomposites to mimic enzyme activity; (b and c) Kinetics of enzyme-catalyzed reaction of CuO/CN/LDH on TMB after addition of Cr(Ⅵ).\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/5b43d57a666fc04ad3a30117.png"},{"id":98428320,"identity":"ed91ed9b-94c2-4706-9840-7d103281b65e","added_by":"auto","created_at":"2025-12-17 16:41:52","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":83661,"visible":true,"origin":"","legend":"\u003cp\u003e(a) UV–vis spectra of CuO/CN/LDH system in the presence of different concentrations of Cr(Ⅵ); (b) Linearity between absorbance difference at 652 nm and Cr(Ⅵ) concentration for CuO/CN/LDH system.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/96d00bc8db8cc8d44b6107d2.png"},{"id":98427989,"identity":"48592f9d-f1dc-4914-be01-6d31ecb2e548","added_by":"auto","created_at":"2025-12-17 16:41:28","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":70915,"visible":true,"origin":"","legend":"\u003cp\u003e(a) EPR plots of CuO/CN/LDH system ⋅OH; (b) EPR plots of CuO/CN/LDH system \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e; (c) EPR plots of CuO/CN/LDH system O\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e ; (d) Relationships between Abs and different radical scavenger concentrations; (e) Spectrogram of CuO/CN/LDH reaction in air and nitrogen; (f) High-resolution XPS image of Cu in CuO/CN/LDH after the reaction.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/8dead57bc5e9de71585594fb.png"},{"id":98428301,"identity":"0a650de9-36c3-4788-8747-9bbbd32f4220","added_by":"auto","created_at":"2025-12-17 16:41:52","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":40750,"visible":true,"origin":"","legend":"\u003cp\u003eStandard curve for paper based detection of Cr(Ⅵ).\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/3fa38c83a8a60d2a46bb79cb.png"},{"id":104250664,"identity":"afd56e49-5f1a-4f05-bf15-75f7eaa9102f","added_by":"auto","created_at":"2026-03-09 16:04:59","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1483534,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/c84223e7-52b6-4be1-87b3-cad630fa69f1.pdf"},{"id":98428313,"identity":"ccadfd36-64bc-4dca-84dd-025f95049666","added_by":"auto","created_at":"2025-12-17 16:41:52","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1888100,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryMaterials.docx","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/1c613ba91f499abf69c079a6.docx"},{"id":98053749,"identity":"a332d065-1452-42c1-bee9-da71a5552c05","added_by":"auto","created_at":"2025-12-12 09:37:08","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":339892,"visible":true,"origin":"","legend":"","description":"","filename":"Schemes.docx","url":"https://assets-eu.researchsquare.com/files/rs-8252032/v1/24f6264494c30c48d6f395bd.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Rapid Detection of Cr(VI) Using CuO/CN/LDH as an Oxidase Mimetic and the Construction of a Paper-Based Devices","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eWith the rapid development of industry, heavy metal ion pollution has increasingly become a focal point of global concern\u003csup\u003e[1]\u003c/sup\u003e. Among the various heavy metal ions, chromium ions are widely utilized in various commercial applications, including chrome plating, pigment production, leather tanning, paper manufacturing, and chemical production. Chromium is the seventh most abundant element on Earth and is commonly found in nature. While it exists in multiple forms, it primarily occurs in two stable oxidation states: Cr(III) and Cr(VI)\u003csup\u003e[2]\u003c/sup\u003e. In the presence of excess environmental oxygen, Cr(Ⅲ) can oxidize to form hexavalent chromium (Cr (VI)), which is extremely toxic and highly water-soluble\u003csup\u003e[3, 4]\u003c/sup\u003e. Cr(VI) is a concern due to its high mobility, strong toxicity, and carcinogenic properties. Long-term exposure may result in respiratory issues, lung cancer, nerve tissue damage, and even death in large dose\u003csup\u003e[5, 6]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eIn the past decades, a range of techniques have been employed for the sensitive and selective detection of Cr(VI), including ion chromatography (IC)\u003csup\u003e[7]\u003c/sup\u003e, atomic absorption spectrometry (AAS\u003csup\u003e)[8]\u003c/sup\u003e, inductively coupled plasma mass spectrometry (ICP-MS)\u003csup\u003e[9]\u003c/sup\u003e, and electrochemistry\u003csup\u003e[10]\u003c/sup\u003e, etc. But they usually suffer from disadvantages such as sophisticated pretreatment, the requirement of professional operation, high cost, large instruments, and long pretreatment times, which render them unsuitable for real-time field monitoring\u003csup\u003e[11, 12]\u003c/sup\u003e. This limits the rapid and real-time detection of Cr(VI). In recent years, nanozyme-based colorimetric detection methods have gained widespread attention due to their rapidity, simplicity, low cost, excellent sensitivity, and ease of detection by the naked eye. Compared to the previously methods, colorimetric assays do not require complex and expensive instruments, and they allow for the detection of pollutant concentrations in a short time using UV-visible spectroscopy\u003csup\u003e[13]\u003c/sup\u003e .The existing nano enzyme colorimetric sensors mainly include hydrogel and paper-based. Compared with hydrogel, paper-based has the advantages of low price, easy access, easy operation and wide environmental adaptability\u003csup\u003e[14]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eIn recent years, various nanoparticles, including metals and metal oxides, have been demonstrated to exhibit enzyme-like activity\u003csup\u003e[15]\u003c/sup\u003e. As a stable oxide of Cu, CuO is an important p-type semiconductor that can form heterojunctions with other semiconductors to enhance catalytic performance\u003csup\u003e[16]\u003c/sup\u003e. It is regarded as a promising catalytic material due to its abundant active sites, chemical stability, low cost, and environmental benignity\u003csup\u003e[17]\u003c/sup\u003e. And CuO has drawn significant attention from researchers due to its low toxicity and natural abundance in the Earth's crust. Additionally, copper metal, which is cost-effective and has relatively high natural reserves, is easy to synthesize\u003csup\u003e[18]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eIn addition, it has been widely used in the sensor field as a mimic of peroxidase. For example, Lu et al\u003csup\u003e[19]\u003c/sup\u003e. synthesized tubular CuO/NiO nanozymes with significantly enhanced peroxidase-like activity. In the presence of hydrogen peroxide, these nanozymes can rapidly catalyze the conversion of colorless TMB to blue oxTMB, thereby establishing a colorimetric detection method for isoniazid. However, due to the instability and decomposition of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e in practical applications, exploring the oxidase activity of CuO is of significant importance. However, most Cu-based catalysts are unstable in long-term reactions, with the main reasons being the potential-induced surface reconstruction, and catalyst aggregation, among others\u003csup\u003e[20]\u003c/sup\u003e. Owing to well matched band structure between g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e and CuO, high-quality p-n heterojunction could be formed\u003csup\u003e[21]\u003c/sup\u003e. N-type and p-type semiconductors can be developed from internal characteristics by doping ions with furnish supererogatory electron or hole\u003csup\u003e[22]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eIn addition, graphitic carbon nitride (g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e), a polymeric-metal-free photocatalyst of n-type semiconducting behavior, has garnered significant attention from researchers as a polymer semiconductor. This material can be synthesized through the extensive condensation of low-cost nitrogen-rich precursors, such as urea\u003csup\u003e[23]\u003c/sup\u003e and melamine\u003csup\u003e[24]\u003c/sup\u003e. It is composed of two earth-abundant elements, carbon and nitrogen, which enable its facile synthesis at a low cost. Due to its simple and cost-effective synthesis process, good thermal stability, non-toxicity, and unique layered structure\u003csup\u003e[25, 26]\u003c/sup\u003e, g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e has become a popular research topic in fields such as catalysis\u003csup\u003e[27]\u003c/sup\u003e and sensing\u003csup\u003e[28]\u003c/sup\u003e. However, the bulk g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e has the problem of low specific surface area and low electron mobility\u003csup\u003e[29, 30]\u003c/sup\u003e. To overcome this drawback, researchers have attempted to construct heterojunction structures by combining g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e with other p-type semiconductors. Accordingly, the fabrication of heterostructure-based of g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e has emerged as a beneficial strategy to improve its activity. One of the most effective heterogeneous photocatalysts is the combination of graphite-like carbon nitride (g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e) and layered double hydroxides (LDHs).\u003c/p\u003e\u003cp\u003eLDHs are typical two-dimensional layered structural materials. They are generally defined by the formula [M\u003csup\u003e2\u0026thinsp;+\u003c/sup\u003e\u0026thinsp;\u003csub\u003e1\u0026minus;x\u003c/sub\u003eM\u003csup\u003e3\u0026thinsp;+\u003c/sup\u003e\u0026thinsp;\u003csub\u003ex\u003c/sub\u003e(OH)\u003csub\u003e2\u003c/sub\u003e]\u003csup\u003eZ+\u003c/sup\u003e (A\u003csup\u003en\u0026ndash;\u003c/sup\u003e)\u003csub\u003ex/n\u003c/sub\u003e\u0026middot;mH\u003csub\u003e2\u003c/sub\u003eO, where M\u003csup\u003e2+\u003c/sup\u003e represents divalent cations (such as Ca\u003csup\u003e2+\u003c/sup\u003e, Co\u003csup\u003e2+\u003c/sup\u003e), M\u003csup\u003e3+\u003c/sup\u003e indicates trivalent cations (such as Al\u003csup\u003e3+\u003c/sup\u003e, Co\u003csup\u003e3+\u003c/sup\u003e), A\u003csup\u003en\u0026minus;\u003c/sup\u003e are exchangeable anions in the interlayer region compensating for the positive charge on the layers. LDHs possess characteristics such as openness, ease of metal ion adjustment, abundant redox-active sites, high biocompatibility, low toxicity, customizable size and composition, and good stability\u003csup\u003e[31, 32]\u003c/sup\u003e. Furthermore, LDHs are facile to synthesize, chemically modifiable, cost-effective, and environmentally friendly, making them well-suited to meet the requirements of sustainable development\u003csup\u003e[33]\u003c/sup\u003e. It has therefore received a lot of attention from researchers. Currently, LDH has been widely applied in photocatalysis, CO\u003csub\u003e2\u003c/sub\u003e reduction, catalysis, pharmaceuticals, and other applications\u003csup\u003e[34, 35]\u003c/sup\u003e. However, standalone LDH exhibits relatively low quantum efficiency due to low charge carrier mobility and a limited number of electron-hole pairs\u003csup\u003e[36]\u003c/sup\u003e. Moreover, their poor semiconductor properties make it challenging to fully utilize their abundant active sites\u003csup\u003e[37]\u003c/sup\u003e. Copper-based LDHs have been proven to possess good catalytic activity due to the presence of Cu\u003csup\u003e2+\u003c/sup\u003e active sites and excellent dispersion of Cu\u003csup\u003e2+\u003c/sup\u003e ions\u003csup\u003e[38]\u003c/sup\u003e. According to existing research, the combination of LDHs and carbon materials can enhance the separation of photogenerated charge carriers and provide useful substances for reactions\u003csup\u003e[29]\u003c/sup\u003e. For example, Faria et al\u003csup\u003e[26]\u003c/sup\u003e. successfully constructed a 2D/2D CuAl-LDH/GCNN heterostructure by combining CuAl-LDH with g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e. This heterostructure significantly enhanced the photocatalytic ability for water splitting to produce H\u003csub\u003e2\u003c/sub\u003e and O\u003csub\u003e2\u003c/sub\u003e, demonstrating excellent photocatalytic performance.\u003c/p\u003e\u003cp\u003eIn this study, CuO/CN/LDH nanocomposites were successfully synthesized through a simple hydrothermal method and co-precipitation technique. The construction of the CN/LDH heterojunction effectively promotes electron transfer and provides more active sites for the loading of CuO. As shown in Scheme \u003cspan refid=\"Sch1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, under the presence of Cr(VI), CuO/CN/LDH can rapidly catalyze the color development reaction of colorless TMB, demonstrating enhanced oxidase-like activity. Based on this, we developed a colorimetric method for detecting Cr(VI). Experimental results indicate that this method exhibits good accuracy and specificity, showing great potential for application in environmental detection. This study successfully developed a portable paper-based Cr(VI) detection system by modifying the paper substrate with CuO/CN/LDH nanocomposites, creating a sensor with a concentration-dependent colorimetric response that exhibits excellent linearity across the target concentration range. Leveraging this chromogenic property, a standardized colorimetric reference card was designed to enable both quantitative and semi-quantitative detection of Cr(VI).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"2. Experimental section","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Materials\u003c/h2\u003e\u003cp\u003eMelamine, potassium dichromate (K\u003csub\u003e2\u003c/sub\u003eCr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e), and anhydrous ethanol (C\u003csub\u003e2\u003c/sub\u003eH\u003csub\u003e5\u003c/sub\u003eOH) were provided by National Pharmaceutical Chemical Reagents. Cerium nitrate hexahydrate (Ce(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO), copper nitrate trihydrate (Cu(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u0026middot;3H\u003csub\u003e2\u003c/sub\u003eO), urea, sodium acetate (CH\u003csub\u003e3\u003c/sub\u003eCOONa), acetic acid (CH\u003csub\u003e3\u003c/sub\u003eCOONa), and 3,3\u0026rsquo;,5,5\u0026rsquo;-tetramethylbenzidine (TMB) were supplied by Aladdin Chemical Company. All reagents, including additional metal ion solutions, were analytically pure and used without further purification. Ultrapure water was employed throughout the experimental process.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2 Preparation of g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e\u003c/h2\u003e\u003cp\u003ePlate-like g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e (CN) was prepared by a secondary calcination method\u003csup\u003e[39]\u003c/sup\u003e. Specifically, melamine was placed in a tubular furnace and heated to 550\u0026deg;C at a rate of 5\u0026deg;C/min under a nitrogen atmosphere, and then maintained for 5 h. After natural cooling, the product was taken out and thoroughly ground to obtain a pale-yellow powder. The pale-yellow powder was then subjected to a second high-temperature calcination under the same conditions, with a holding time of 4 h. After natural cooling, it was thoroughly ground again. The final product obtained was a white powder.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3 Preparation of CN/LDH\u003c/h2\u003e\u003cp\u003eThis study is similar to previous preparation processes, but with some modifications\u003csup\u003e[26]\u003c/sup\u003e. First, 0.2 g of g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e was mixed with 40 ml of a solution (water: ethanol\u0026thinsp;=\u0026thinsp;1:1) and subjected to ultrasonic treatment for 2 h. Then, 1 mM Ce(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO and 1 mM Cu(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u0026middot;3H\u003csub\u003e2\u003c/sub\u003eO were dissolved in the above solution, and ultrasonication was continued for 1 h. Next, 20 mM urea was added, and the mixture was stirred for 30 min. The mixture was then placed in a reaction kettle and heated at 120\u0026deg;C for 12 h. After cooling, the mixture was centrifuged at 8000 rpm for 10 min and washed several times with water and ethanol, then dried for later use.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e2.4 Preparation of CuO/CN/LDH\u003c/h2\u003e\u003cp\u003eThe synthesis method of CuO/CN/LDH followed previously reported procedures with certain modifications\u003csup\u003e[40]\u003c/sup\u003e. Specifically, weigh 0.02 g of CN/LDH and dissolve it in 30 ml of a solution (water: ethanol\u0026thinsp;=\u0026thinsp;1:1), then use ultrasonic treatment to disperse it evenly. Next, weigh 0.03 g of Cu(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u0026middot;3H\u003csub\u003e2\u003c/sub\u003eO and place it in an oil bath, stirring until uniform, then heat to 60\u0026deg;C. At this temperature, add 1 mL of dilute ammonia (1:9 dilution) and continue stirring for 4 h. Afterward, centrifuge at 13,000 rpm for 5 min, and wash three times with both water and ethanol.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e2.5 Characterizations\u003c/h2\u003e\u003cp\u003eThe morphology and elemental composition of the materials were investigated using a scanning electron microscope (SEM, Hitachi SU8600, Japan). The SEM is equipped with an Oxford Ultim Max 40 energy dispersive X-ray spectroscopy (EDX) system for compositional analysis. The chemical composition and surface chemical states of the samples were analyzed using X-ray photoelectron spectroscopy (XPS) with a Thermo Fisher Scientific K-Alpha instrument (USA). UV-vis absorption spectra were measured using a Mapada UV spectrophotometer (UV-1800, Shanghai, China). Electrochemical measurements were conducted using a CHI660i electrochemical workstation (Chenhua Instrument, Shanghai, China). Fourier transform infrared (FT-IR) spectra were analyzed using FTS3000 FT-IR spectrometer (DigI-Lab, USA). The pH of the buffer was adjusted using a Ray's pH-3c pH meter. X-ray diffraction (XRD) studies were performed using a Rigaku D/max-2400 (Rigaku, Japan). Free radicals in the samples were detected by electron paramagnetic resonance (EPR, BRUKER EMXPLUS, DE).\u003c/p\u003e\u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\n \u003ch2\u003e3.1 Physicochemical properties of the materials\u003c/h2\u003e\n \u003cp\u003eThe morphological characteristics of the materials were analyzed using scanning electron microscopy (SEM), as shown in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e. Figure \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ea shows pristine g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e, revealing a distinct lamellar structure. The pristine CuCe-LDH (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eb) displays spherical morphology, likely formed through hydrothermal self-assembly of lamellar subunits. Compared to CuCe-LDH, the surface of CN/LDH appears relatively rough, with a fluffy appearance (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ec). The inset micrograph provides an enlarged view, confirming the successful integration of g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e nanosheets within the LDH matrix. Following copper oxide incorporation (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ed), the composite surface exhibits a continuous particulate coating that reduces surface roughness compared to CN/LDH. High-magnification imaging reveals well-dispersed CuO nanoparticles intercalated between g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e layers. Figure \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ee presents the element distribution map of the CuO/CN/LDH material. It can be observed that the Ce element is only present in the central spherical part, while the N element is distributed in the outer encapsulating layer. Copper and oxygen exhibit homogeneous distribution throughout both structural domains, showing strong spatial correlation that verifies CuO formation. These analytical findings correlate well with the morphological observations from SEM characterization.\u003c/p\u003e\n \u003cp\u003eThe crystal structure was analyzed via X-ray powder diffraction (XRD), with results presented in Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ea The red curve in the figure represents the XRD pattern of g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e, in which the peaks at 12.7\u0026deg; and 27.6\u0026deg; correspond to the (100) and (002) planes of g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e, respectively. The pink curve in the figure is the XRD pattern of LDH, in which the peak(11.97\u0026deg;) originally attributed to the (003) plane of LDH has disappeared. This phenomenon may result from self-assembly under high-temperature and high-pressure conditions, leading to the collapse of the layered structure\u003csup\u003e[41]\u003c/sup\u003e. This observation is similar to the earlier conclusions drawn from SEM analysis. The presence of the (006) plane reflection at 24.5\u0026deg; confirms retention of the LDH phase (Fig. \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e). The remaining diffraction peaks at 35.7\u0026deg;, 39.3\u0026deg;, and 47.7\u0026deg; correspond to the (012), (015), and (018) planes of LDH, respectively, and these peaks are attributed to the LDH phase\u003csup\u003e[26]\u003c/sup\u003e. The remaining peaks are mainly attributed to the diffraction peaks of CeO\u003csub\u003e2\u003c/sub\u003e and CuO, with sharper CeO\u003csub\u003e2\u003c/sub\u003e reflections indicating superior crystallinity relative to CuO. The cyan Curve represents the XRD spectrum of the CN/LDH composite material, where all peaks of CN and LDH appear, further proving the successful synthesis of CN/LDH. The CuO/CN/LDH spectrum (purple curve) exhibits superposition of g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e and LDH reflections, confirming preservation of both phases during CuO integration. Marked intensification of CuO diffraction peaks evidences effective heterostructuring with the CN/LDH matrix.\u003c/p\u003e\n \u003cp\u003eSurface functional groups were analyzed via FT-IR spectroscopy (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eb). The CN spectrum exhibits peaks between 1750\u0026thinsp;\u0026minus;\u0026thinsp;1000 cm⁻\u0026sup1; corresponding to the stretching vibrations of nitrogen-containing heterocycles, alongside a distinct triazine ring breathing mode at 808 cm\u003csup\u003e\u0026minus;\u0026thinsp;1[42]\u003c/sup\u003e. In the FT-IR spectrum of LDH, peaks below 800 cm⁻\u0026sup1; are primarily associated with metal-oxygen (M-O) vibrations\u003csup\u003e[43]\u003c/sup\u003e. Bands at 1400\u0026thinsp;\u0026minus;\u0026thinsp;1060 cm⁻\u0026sup1; arise from CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e asymmetric stretching, while the 1051 cm⁻\u0026sup1; feature corresponds to C-N bending modes in LDH\u003csup\u003e[44, 45]\u003c/sup\u003e. Broad absorption in the 3400-3000cm⁻\u0026sup1; region indicates hydroxyl (-OH) stretching vibrations. The CuO/CN/LDH spectrum retains all constituent phase signatures, with an additional Cu-O stretching vibration at 518 cm⁻\u0026sup1;.\u003c/p\u003e\n \u003cp\u003eThe strong Cu-O peak observed in CuO/CN/LDH further confirms the successful synthesis of CuO\u003csup\u003e[46]\u003c/sup\u003e.\u003c/p\u003e\n \u003cp\u003eThe chemical states and oxidation environments of constituent elements were probed through XPS. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ec presents the wide-scan XPS spectrum of CuO/CN/LDH, confirming the coexistence of relevant elements within the synthesized materials. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ed displays the high-resolution XPS spectrum of C, which is deconvoluted into three distinct peaks at 284.55 eV, 286.07 eV, and 287.98 eV, corresponding to adventitious carbon, C-O bonds, and the N\u0026thinsp;=\u0026thinsp;C\u0026ndash;N coordination in g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e, respectively\u003csup\u003e[47]\u003c/sup\u003e. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ee shows the N 1s XPS spectrum of the composite material, which can be deconvoluted into three peaks at 398.48 eV, 400.36 eV, and 404.3 eV. The peak at 398.48 eV is attributed to the sp\u003csup\u003e2\u003c/sup\u003e hybridized nitrogen atoms (C\u0026ndash;N\u0026thinsp;=\u0026thinsp;C) in the triazine rings, which are aromatic ring structures containing nitrogen and carbon, where the nitrogen atom is part of the ring by forming double and single bonds with two carbon atoms. The peak at 400.36 eV is attributed to the bridging nitrogen atoms (N-(C)\u003csub\u003e3\u003c/sub\u003e). The small peak at 404.3 eV is attributed to C\u0026ndash;NH\u003csub\u003eX\u003c/sub\u003e\u003csup\u003e[48]\u003c/sup\u003e. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ef presents the XPS spectrum of O 1s, which can be deconvoluted into three peaks. Among them, the main peak at 529.47 eV is attributed to lattice oxygen (O\u003csup\u003e2\u0026minus;\u003c/sup\u003e), which is primarily associated with Cu\u003csup\u003e2+[49]\u003c/sup\u003e. This peak at this energy position is a characteristic signal of oxygen in the CuO structure, indicating that these oxygen ions are part of the lattice, thus confirming the successful synthesis of CuO. The peak at 531.16 eV is attributed to oxygen vacancies, while the peak at 533.31 eV is likely related to chemisorbed oxygen in substances such as surface-adsorbed H\u003csub\u003e2\u003c/sub\u003eO. The typical Ce 3d spectrum of the CuO/CN/LDH catalyst, as shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eg, can be divided into eight peaks. Specifically, the peaks at 882.25 eV, 889.25 eV, and 898.21 eV are attributed to Ce 3d\u003csub\u003e3/2\u003c/sub\u003e, while the peaks at 900.79 eV, 907.42 eV, and 916.57 eV are attributed to Ce 3d\u003csub\u003e5/2\u003c/sub\u003e. These peaks are mainly associated with Ce\u003csup\u003e4+\u003c/sup\u003e. Meanwhile, the peaks at 885.69 eV and 903.62 eV correspond to Ce\u003csup\u003e3+\u003c/sup\u003e. Therefore, Ce in this catalyst exists in a mixed valence state. As shows in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eh, the high-resolution XPS spectrum of Cu can be deconvoluted into seven peaks, where the characteristic peaks at 933.0 eV and 953.0 eV are assigned to Cu\u003csup\u003e+\u003c/sup\u003e, while those at 934.85 eV and 952.9 eV correspond to Cu\u003csup\u003e2+[50]\u003c/sup\u003e. In addition, satellite peaks are observed at 941.28 eV, 943.78 eV, and 962.01 eV, which are characteristic signals of Cu\u003csup\u003e2+\u003c/sup\u003e ions, suggesting that their 3d orbitals are partially filled.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\n \u003ch2\u003e3.2 Exploration of Oxidase Activity of CuO/CN/LDH\u003c/h2\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003eS2\u003c/span\u003e displays UV-Vis absorption spectra of LDH/CuO, CN/CuO, and CuO/CN/LDH before and after Cr(VI) introduction. Comparative analysis reveals CuO/CN/LDH undergoes the most pronounced absorption modulation at 652 nm. Although the spectra of CuO/LDH and CN/CuO also show some enhancement after the addition of Cr(VI), the degree of enhancement is less pronounced compared to CuO/CN/LDH. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea, in the HAC-NaAC buffer solution, when only TMB is added, the UV-vis spectrum does not show any significant signal change. Similarly, no significant signal change is observed when only Cr(VI)\u0026thinsp;+\u0026thinsp;TMB or CuO/CN/LDH\u0026thinsp;+\u0026thinsp;TMB is added. However, a noticeable signal change in the UV-vis spectrum occurs only when CuO/CN/LDH, Cr(VI), and TMB are added simultaneously. By comparing the above curves, it is evident that the constructed Cr(VI) detection sensor exhibits excellent response characteristics.\u003c/p\u003e\n \u003cp\u003eSystematic optimization of critical parameters (pH, reaction duration, and catalyst loading) was conducted to maximize catalytic efficiency. As shown in Fig. S3, within the pH range of 3.0 to 5.5, the absorbance initially increases and then decreases, with the highest catalytic activity observed at pH 3.5. The oxidase activity of nanoenzymes is usually prominent at acidic pH\u003csup\u003e[51]\u003c/sup\u003e. In the optimization of TMB concentration, the absorbance (where A represents the absorbance value at 652 nm in the presence of Cr(VI), and A\u003csub\u003e0\u003c/sub\u003e enotes the absorbance without Cr(VI)) gradually increases with the addition of TMB, indicating enhanced catalytic performance. However, beyond a TMB concentration of 0.35 mM, changes in absorbance become negligible, suggesting that the reaction system stabilizes. A TMB concentration of 0.35 mM was adopted to balance signal intensity and economic feasibility. We also investigated the effect of the CuO/CN/LDH amount added, as illustrated in the figure. The absorbance reached its maximum when the material volume was 25 \u0026micro;L, which was therefore chosen as the final added volume. Additionally, the optimization of reaction time revealed minimal absorbance changes around 10 min, leading to the selection of 10 min as the final reaction duration. All experiments were conducted at room temperature to facilitate practical application. In summary, the optimal experimental conditions are as follows: pH 3.5, TMB concentration of 0.35 mM, material volume of 25 \u0026micro;L, and reaction time of 10 min.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\n \u003ch2\u003e3.3 Kinetic analysis of CuO/CN/LDH as an oxidase mimetic\u003c/h2\u003e\n \u003cp\u003eThe oxidase-mimicking activity of CuO/CN/LDH was evaluated using Cr(VI) as a catalytic substrate. As shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e(b and c), the apparent enzyme kinetics for the entities were determined using the Michaelis-Menten equation to deduce the maximum reaction rate (V\u003csub\u003emax\u003c/sub\u003e) and the Michaelis constant (K\u003csub\u003em\u003c/sub\u003e). The K\u003csub\u003em\u003c/sub\u003e value is an important characteristic constant of the enzyme, reflecting the affinity of the catalyst for the substrate. A lower K\u003csub\u003em\u003c/sub\u003e value indicates a stronger interaction between CuO/CN/LDH and the substrate, thereby resulting in higher catalytic activity. To investigate the oxidase-like activity of CuO/CN/LDH nanomaterials, we evaluated the catalytic enhancement induced by Cr(VI) and performed kinetic analyses under optimized conditions. The typical Michaelis-Menten curve is presented in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb, from which the Lineweaver-Burk plot was derived (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ec). Furthermore, the catalytic performance of CuO/CN/LDH was benchmarked against other nanozymes, as summarized in Table \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e. Kinetic analyses revealed that the CuO/CN/LDH nanocomposite demonstrates a Michaelis constant (K\u003csub\u003em\u003c/sub\u003e) of 0.173 and a maximum reaction rate (V\u003csub\u003emax\u003c/sub\u003e) of 4.702 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;8\u003c/sup\u003e. Notably, as shown in Table \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e, its K\u003csub\u003em\u003c/sub\u003e value is significantly lower than that of horseradish peroxidase (HRP) and other types of nanozymes, while its Vmax is markedly higher than that of HRP and other nanozymes.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\n \u003ch2\u003e3.4 Ultrasensitive and selective detection of Cr(VI)\u003c/h2\u003e\n \u003cp\u003eBased on the experimental results under optimal conditions, as shown in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e, a colorimetric method was developed using CuO/CN/LDH oxidase mimetics as a sensor for detecting Cr(VI). Figure \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea illustrates the response of the CuO/CN/LDH colorimetric sensor to different concentrations of Cr(VI). Within the range of 0 to 9.1 \u0026micro;M, the absorbance at 652 nm in the UV-visible absorption spectra gradually increases with the increasing concentration of Cr(VI). The inset visually documents the progressive color transition of the solution as Cr(VI) concentrations rise. Figure \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eb depicts the linear relationship between Cr(VI) concentration and absorbance intensity at 652 nm. Within the range of 0 to 9.1 \u0026micro;M, there is a good linear relationship between absorbance and Cr(VI) concentration, with a correlation coefficient as high as 0.995. The method achieved a detection limit (LOD) of 25 nM for Cr(VI), demonstrating high sensitivity. As shown in Table \u003cspan class=\"InternalRef\"\u003eS2\u003c/span\u003e, this study compares the proposed method with existing Cr(VI) detection methods. From the table, it can be seen that the constructed CuO colorimetric detection method not only has significant advantages in terms of the environmental friendliness of the materials used but also exhibits comparable or superior performance in terms of detection limits and detection range.\u003c/p\u003e\n \u003cp\u003eTo assess selectivity, we tested the method against common interfering cations (e.g., Cd\u0026sup2;⁺, Cu\u0026sup2;⁺, K⁺, Ba\u0026sup2;⁺) under identical experimental conditions. As shown in Fig. S4, the results indicate that these interfering substances, even at concentrations ten times that of Cr(VI), do not affect the colorimetric system. The rapid oxidation of TMB to a blue product occurred exclusively in Cr(VI)-containing solutions, validating the platform\u0026rsquo;s specificity for Cr(VI) detection in aqueous environments. We further evaluated the nanocomposite\u0026rsquo;s long-term stability. As illustrated in Fig. S5, its enzymatic activity remains above 95% even after being stored for 8 weeks, indicating that the material possesses excellent storage stability.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\n \u003ch2\u003e3.5 Oxidase-like catalytic mechanism of CuO/CN/LDH\u003c/h2\u003e\n \u003cp\u003eTo identify the active species governing the enzyme-catalyzed reaction, we employed electron paramagnetic resonance (EPR) spectroscopy to characterize radical generation during the catalytic process. EPR spectra (Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e) reveal distinct radical signatures: a 1:2:2:1 quartet in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ea confirms hydroxyl radical (\u0026middot;OH) formation, while a 1:1:1 triplet in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003eb indicates singlet oxygen (\u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e) production. Six hyperfine-split peaks in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ec further confirm the participation of superoxide radicals (\u0026middot;O\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e). To further identify the dominant radicals in the reaction, isopropanol, L-histidine, and chloroform were utilized as scavengers for hydroxyl radicals, singlet oxygen, and superoxide anions, respectively. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ed, absorbance gradually decreases with the increasing concentration of radical scavengers. Notably, the experimental group containing chloroform exhibited the most significant change in absorbance compared to the control group. Therefore, we conclude that superoxide anions play a major role in this enzyme-catalyzed reaction. Furthermore, as depicted in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ee, the UV-visible spectra in both air and N\u003csub\u003e2\u003c/sub\u003e environments were compared. A marked decrease in absorbance intensity under N\u003csub\u003e2\u003c/sub\u003e confirms dissolved oxygen\u0026rsquo;s critical role in sustaining the catalytic cycle.\u003c/p\u003e\n \u003cp\u003eTo elucidate the reaction mechanism, XPS was performed on the CuO/CN/LDH nanocomposite post-reaction. Figure \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ef displays the high-resolution XPS results of Cu elements in the nanocomposite after the reaction. Compared with the pre-reaction Cu XPS spectrum, the disappearance of the Cu⁺ characteristic peak in the Cu XPS spectra after reaction demonstrates that the introduction of Cr(VI) induces the oxidation of Cu⁺ to Cu\u0026sup2;⁺, which is attributed to the redox reactions occurring in the system. We propose that this process involves the following cascade reactions: The redox interaction between Cu⁺ and Cr(VI) drives electron transfer, while dissolved oxygen in the solution is concurrently reduced to generate superoxide anion radicals (\u0026middot;O\u003csub\u003e2\u003c/sub\u003e⁻). These reactive oxygen species subsequently oxidize colorless TMB, yielding the characteristic blue oxidized product (oxTMB). The self redox properties of copper atoms induced by O\u003csub\u003e2\u003c/sub\u003e are crucial for the sustained generation of dissolved oxygen\u003csup\u003e[52]\u003c/sup\u003e.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\n \u003ch2\u003e3.6 Actual sample testing\u003c/h2\u003e\n \u003cp\u003eTo validate the feasibility and practicality of the developed method, the CuO/CN/LDH composite was applied to detect Cr(VI) in real environmental samples, including tap water and Yellow River water. The accuracy of the method was assessed via recovery experiments. As summarized in Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e, the recovery rates for Cr(VI) in tap water ranged from 97.7% to 103.7%, with relative standard deviations (RSDs) of 0.95 to 3.3%. Similarly, in Yellow River water, recoveries spanned 99.0-104.4%, accompanied by RSDs of 1.4\u0026ndash;3.1%. These results demonstrate that the proposed method exhibits high reliability, sensitivity, and accuracy, indicating its applicability to diverse aqueous matrices.\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\n \u003ctable id=\"Tab1\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eDetermination of Cr(VI) in test samples.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eSample\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eAdded(\u0026micro;M)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eFound(\u0026micro;M)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eRecovery(%)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eRSD(%)\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" rowspan=\"3\"\u003e\n \u003cp\u003eTap water\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e6.22\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e103.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.7\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e5.04\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e100.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.3\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e97.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.95\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" rowspan=\"3\"\u003e\n \u003cp\u003eYellow River water\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e6.27\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e104.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1.4\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4.95\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e99.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.1\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.05\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e102.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1.7\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n \u003cdiv class=\"gridtable\"\u003e\n \u003cdiv align=\"left\" class=\"colspec\"\u003e\u003cbr\u003e\u003c/div\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e\n \u003ch2\u003e3.7 Preparation of CuO/CN/LDH Paper-Based Sensor\u003c/h2\u003e\n \u003cp\u003eTo meet the growing need for portable environmental monitoring tools, paper-based analytical devices (PADs) have gained prominence owing to their cost-effectiveness, user-friendly operation, and field adaptability. This study developed a paper-based colorimetric sensor for rapid, on-site Cr(VI) detection. The Whatman filter paper was first cut into discs using a mold, followed by complete immersion of the punched filter paper discs in a diluted solution. To ensure thorough solution penetration, the samples were transferred to a thermostatic shaker incubator for constant-speed oscillation. Upon reaction completion, the specimens were oven-dried to obtain the colorimetric detection test strips (see Scheme \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e for detailed fabrication workflow). The detection system innovatively integrates smartphone-based image acquisition with RGB analysis using ImageJ software, establishing an instrument-free quantitative detection methodology. Through systematic optimization experiments (Fig. S6), the optimal detection parameters were determined as follows: reaction system pH 3.5, nanomaterial dilution factor of 20\u0026times;, TMB chromogenic agent volume of 200 \u0026micro;L, and coloration time of 10 min. Experimental results demonstrate that this integrated detection platform maintains satisfactory sensitivity while significantly reducing detection costs, providing reliable technical support for field screening of heavy metal contamination.\u003c/p\u003e\n \u003cp\u003eTo evaluate the accuracy and practical applicability of the paper-based colorimetric device for Cr(Ⅵ) detection, we performed selectivity tests by introducing common metal cations (Al\u0026sup3;⁺, Mn\u0026sup2;⁺, Mg\u0026sup2;⁺, and Pb\u0026sup2;⁺) at concentrations tenfold higher than Cr(Ⅵ). As shown in Fig. S7, visible color changes occurred exclusively in the Cr(Ⅵ) group compared to the control. Crucially, none of the interfering ions induced significant color variations, even at 10-times higher concentrations. These results demonstrate the excellent specificity of the developed device for Cr(Ⅵ) detection.\u003c/p\u003e\n \u003cp\u003eAs illustrated in Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e, under optimized experimental conditions, the colorimetric paper-based sensor exhibited a concentration-dependent enhancement of \u0026Delta;G (The difference in gray values before and after the reaction) values upon addition of Cr(VI). A linear response relationship was observed within the concentration range of 0.45\u0026thinsp;~\u0026thinsp;8.6 \u0026micro;M, with the calibration curve described by the equation y\u0026thinsp;=\u0026thinsp;5.15x\u0026thinsp;+\u0026thinsp;0.331 (R\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.99), demonstrating robust quantitative capability. The inset in Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e clearly demonstrates that the color intensity of the test strips increases progressively with rising Cr(VI) concentrations. This visual gradient enables rapid instrument-free assessment, making the method suitable for on-site semi-quantitative analysis. This dual-mode detection strategy effectively bridges laboratory-grade quantification and field-applicable screening, particularly suited for resource-limited settings.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eIn summary, this study developed a novel colorimetric detection method for Cr(VI) based on CuO/CN/LDH composites. Experimental results demonstrated that the presence of Cr(VI) in the CuO/CN/LDH-TMB mixed system rapidly catalyzed the oxidation of colorless TMB to blue oxTMB. Through systematic optimization of the reaction system, optimal detection performance was achieved, showing a linear detection range of 0-9.1 μM for Cr(VI) with a low detection limit of 25 nM. Combined XPS and electron paramagnetic resonance (EPR) analyses revealed the catalytic mechanism where Cr(VI) promotes the conversion of dissolved oxygen into superoxide radicals (·O\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e) through redox reactions. The method exhibited excellent selectivity and accuracy. To enable portable and low-cost Cr(VI) detection, we successfully implemented this approach in paper-based analytical devices, which maintained good linear response characteristics. This work provides new insights for subsequent research on portable detection technologies.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eDeclaration of competing interest COI\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCRediT authorship contribution statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eShaohui Li\u003c/strong\u003e: Conceptualization, Methodology, Software, Investigation, Formal Analysis, Writing-Original Draft. \u003cstrong\u003eSijia Hao\u003c/strong\u003e: Data Curation, Supplementary Experiment.\u003cstrong\u003e\u0026nbsp;Wen li\u003c/strong\u003e: Visualization. \u003cstrong\u003eRan Meng\u003c/strong\u003e: Supervision \u0026amp; Visualization. \u003cstrong\u003eQiang Wang\u003c/strong\u003e: Validation \u0026amp; Visualization. \u003cstrong\u003eYuqing Wang\u003c/strong\u003e: Visualization. \u003cstrong\u003eLei Wang\u003c/strong\u003e: Visualization;\u0026nbsp;\u003cstrong\u003eXinjing Liu\u003c/strong\u003e:\u0026nbsp;Visualization;\u0026nbsp;\u003cstrong\u003eYuanyuan Zhou\u003c/strong\u003e: Validation;\u0026nbsp;\u003cstrong\u003eHaitong Wang\u003c/strong\u003e: Validation;\u0026nbsp;\u003cstrong\u003eDongxia Zhang\u003c/strong\u003e: Writing - review \u0026amp; editing. \u003cstrong\u003eXibin Zhou\u003c/strong\u003e: Conceptualization, Funding Acquisition, Resources, Supervision, Writing - review \u0026amp; editing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgment\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFunding: This work was supported by the Key Laboratory of Resource Environment and Sustainable Development of Oasis, Gansu Province (GORS202301);\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eZ. Chen, Z. Zhang, J. Qi, J. You, J. Ma, L. Chen, Colorimetric detection of heavy metal ions with various chromogenic materials: Strategies and applications, Journal of Hazardous Materials, 441 (2023) 129889.\u003c/li\u003e\n\u003cli\u003eC. Wang, M. Shang, H. Wei, M. Zhang, W. Zou, X. Meng, W. Chen, H. Shao, Y. Lai, Specific and sensitive on-site detection of Cr(VI) by surface-enhanced Raman spectroscopy, Sensors and Actuators B: Chemical, 346 (2021).\u003c/li\u003e\n\u003cli\u003eR. Kanagaraj, Y.-S. Nam, S.J. Pai, S.S. Han, K.-B. Lee, Highly selective and sensitive detection of Cr\u003csup\u003e6+\u003c/sup\u003e ions using size-specific label-free gold nanoparticles, Sensors and Actuators B: Chemical, 251 (2017) 683-691.\u003c/li\u003e\n\u003cli\u003eM. Rangel, S. Saluja, V. Barba, J.S. P\u0026eacute;rez-Huerta, V. Agarwal, Dual-emissive waste oil based S-doped carbon dots for acetone detection and Cr(VI) detection/reduction/removal, Journal of Environmental Chemical Engineering, 11 (2023) 109438.\u003c/li\u003e\n\u003cli\u003eV.I. Mikhaylov, E.F. Krivoshapkina, A.L. Trigub, V.V. Stalugin, P.V. Krivoshapkin, Detection and Adsorption of Cr(VI) ions by Mesoporous Fe\u0026ndash;Alumina Films, ACS Sustainable Chemistry \u0026amp; Engineering, 6 (2018) 9283-9292.\u003c/li\u003e\n\u003cli\u003eY. Mao, S. Gao, L. Yao, L. Wang, H. Qu, Y. Wu, Y. Chen, L. Zheng, Single-atom nanozyme enabled fast and highly sensitive colorimetric detection of Cr(VI), Journal of Hazardous Materials, 408 (2021) 124898.\u003c/li\u003e\n\u003cli\u003eM. Derbyshire, A. Lamberty, P.H.E. Gardiner, Optimization of the Simultaneous Determination of Cr(III) and Cr(VI) by Ion Chromatography with Chemiluminescence Detection, Analytical Chemistry, 71 (1999) 4203-4207.\u003c/li\u003e\n\u003cli\u003eL. Goodarzi, M.R. Bayatloo, S. Chalavi, S. Nojavan, T. Rahmani, S.B. Azimi, Selective extraction and determination of Cr(VI) in food samples based on tandem electromembrane extraction followed by electrothermal atomic absorption spectrometry, Food Chemistry, 373 (2022) 131442.\u003c/li\u003e\n\u003cli\u003eF. Cubadda, S. Giovannangeli, F. Iosi, A. Raggi, P. Stacchini, Chromium determination in foods by quadrupole inductively coupled plasma\u0026ndash;mass spectrometry with ultrasonic nebulization, Food Chemistry, 81 (2003) 463-468.\u003c/li\u003e\n\u003cli\u003eK. Zhao, L. Ge, T.I. Wong, X. Zhou, G. Lisak, Gold-silver nanoparticles modified electrochemical sensor array for simultaneous determination of chromium(III) and chromium(VI) in wastewater samples, Chemosphere, 281 (2021) 130880.\u003c/li\u003e\n\u003cli\u003eJ.-f. Guo, D.-q. Huo, M. Yang, C.-j. Hou, J.-j. Li, H.-b. Fa, H.-b. Luo, P. Yang, Colorimetric detection of Cr (VI) based on the leaching of gold nanoparticles using a paper-based sensor, Talanta, 161 (2016) 819-825.\u003c/li\u003e\n\u003cli\u003eC. Wang, N. Liu, X. Zhao, Y. Tian, X. Chen, Y. Zhang, L. Fan, B. Hou, C-doped BiOCl/Bi\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e3\u003c/sub\u003e heterojunction for highly efficient photoelectrochemical detection and photocatalytic reduction of Cr(VI), Journal of Materials Science \u0026amp; Technology, 164 (2023) 188-197.\u003c/li\u003e\n\u003cli\u003eX. Zheng, Q. Lian, L. Zhou, Y. Jiang, J. Gao, Peroxidase Mimicking of Binary Polyacrylonitrile-CuO Nanoflowers and the Application in Colorimetric Detection of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e and Ascorbic Acid, ACS Sustainable Chemistry \u0026amp; Engineering, 9 (2021) 7030-7043.\u003c/li\u003e\n\u003cli\u003eZ Wang, J Zhang, L Liu, X Wu, H Kuang, C Xu, L Xu. A colorimetric paper-based sensor for toltrazuril and its metabolites in feed, chicken, and egg samples. Food Chemistry. 276(2019) 707-713.\u003c/li\u003e\n\u003cli\u003eJ. Xi, G. Wei, L. An, Z. Xu, Z. Xu, L. Fan, L. Gao, Copper/Carbon Hybrid Nanozyme: Tuning Catalytic Activity by the Copper State for Antibacterial Therapy, Nano Letters, 19 (2019) 7645-7654.\u003c/li\u003e\n\u003cli\u003eB. Yang, X. Shao, X. Gu, K. Wang, X. Ning, J. Xia, M. Xie, Y. Tang, Q. Li, S. Tian, CuO@AgNPs nanozyme cavity arrays on screen-printed electrodes for ultrasensitive and on-site SERS detection, Chemical Engineering Journal, 471 (2023) 144522.\u003c/li\u003e\n\u003cli\u003eQ. Wu, L. He, Z.W. Jiang, Y. Li, Z.M. Cao, C.Z. Huang, Y.F. Li, CuO nanoparticles derived from metal-organic gel with excellent electrocatalytic and peroxidase-mimicking activities for glucose and cholesterol detection, Biosensors and Bioelectronics, 145 (2019).\u003c/li\u003e\n\u003cli\u003eY. Xu, Q. Yin, N. Du, Y. Yi, G. Zhu, An innovative homogeneous electrochemistry coupled with colorimetry dual-model sensing strategy for perfluorooctane sulfonate based on Cu@CuO aerogel nanozyme, Microchimica Acta, 191 (2024) 693.\u003c/li\u003e\n\u003cli\u003eW. Zhu, Y. Cheng, C. Wang, X. Lu, Fabrication of a Tubular CuO/NiO Biomimetic Nanozyme with Synergistically Promoted Peroxidase-like Performance for Isoniazid Sensing, Inorganic Chemistry, 61 (2022) 16239-16247.\u003c/li\u003e\n\u003cli\u003eC.A. Obasanjo, A.S. Zeraati, H.S. Shiran, T.N. Nguyen, S.M. Sadaf, M.G. Kibria, C.-T. Dinh, In situ regeneration of copper catalysts for long-term electrochemical CO\u003csub\u003e2\u003c/sub\u003e reduction to multiple carbon products, Journal of Materials Chemistry A, 10 (2022) 20059-20070.\u003c/li\u003e\n\u003cli\u003eJ. Zhang, X. Zhang, Y. Gao, J. Yan, W. Song, Integrating CuO/g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e p-n heterojunctioned photocathode with MoS\u003csub\u003e2\u003c/sub\u003e QDs@Cu NWs multifunctional signal amplifier for ultrasensitive detection of A\u0026beta;O, Biosensors and Bioelectronics, 176 (2021) 112945.\u003c/li\u003e\n\u003cli\u003eM. Zhang, C. Lai, B. Li, F. Xu, D. Huang, S. Liu, L. Qin, X. Liu, H. Yi, Y. Fu, L. Li, N. An, L. Chen, Insightful understanding of charge carrier transfer in 2D/2D heterojunction photocatalyst: Ni-Co layered double hydroxides deposited on ornamental g-C3N4 ultrathin nanosheet with boosted molecular oxygen activation, Chemical Engineering Journal, 422 (2021).\u003c/li\u003e\n\u003cli\u003eY. Yang, B. Mao, G. Gong, D. Li, Y. Liu, W. Cao, L. Xing, J. Zeng, W. Shi, S. Yuan, In-situ growth of Zn\u0026ndash;AgIn5S8 quantum dots on g-C3N4 towards 0D/2D heterostructured photocatalysts with enhanced hydrogen production, International Journal of Hydrogen Energy, 44 (2019) 15882-15891.\u003c/li\u003e\n\u003cli\u003eX. Wu, X. Wang, F. Wang, H. Yu, Soluble g-C3N4 nanosheets: Facile synthesis and application in photocatalytic hydrogen evolution, Applied Catalysis B: Environmental, 247 (2019) 70-77.\u003c/li\u003e\n\u003cli\u003eX. Zou, M. Li, S. Zhou, C. Chen, J. Zhong, A. Xue, Y. Zhang, Y. Zhao, Diffusion behaviors of ethanol and water through g\u0026ndash;C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e\u0026ndash;based membranes: Insights from molecular dynamics simulation, Journal of Membrane Science, 585 (2019) 81-89.\u003c/li\u003e\n\u003cli\u003eH. Boumeriame, A. Cherevan, D. Eder, D.H. Apaydin, T. Chafik, E.S. Da Silva, J.L. Faria, Engineering g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e with CuAl-layered double hydroxide in 2D/2D heterostructures for visible-light water splitting, Journal of Colloid and Interface Science, 652 (2023) 2147-2158.\u003c/li\u003e\n\u003cli\u003eN. Wu, Y.-T. Wang, X.-Y. Wang, F.-N. Guo, H. Wen, T. Yang, J.-H. Wang, Enhanced peroxidase-like activity of AuNPs loaded graphitic carbon nitride nanosheets for colorimetric biosensing, Analytica Chimica Acta, 1091 (2019) 69-75.\u003c/li\u003e\n\u003cli\u003eC. Zhang, P. Ni, B. Wang, W. Liu, Y. Jiang, C. Chen, J. Sun, Y. Lu, Enhanced oxidase-like activity of g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e nanosheets supported Pd nanosheets for ratiometric fluorescence detection of acetylcholinesterase activity and its inhibitor, Chinese Chemical Letters, 33 (2022) 757-761.\u003c/li\u003e\n\u003cli\u003eQ. Gu, C. Feng, C. Liu, J. Rong, Y. Zhang, X. Zheng, Z. Li, S. Xu, Constructing direct Z-scheme heterojunction of NiCo-LDH coated with g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e for boosting photocatalytic H\u003csub\u003e2\u003c/sub\u003e evolution, Fuel, 371 (2024) 131982.\u003c/li\u003e\n\u003cli\u003eY. Yang, C. Yin, K. Li, H. Tang, Y. Wang, Z. Wu, Cu Doped Crystalline Carbon-Conjugated g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e, a Promising Oxygen Reduction Catalyst by Theoretical Study, Journal of The Electrochemical Society, 166 (2019) F755-F759.\u003c/li\u003e\n\u003cli\u003eQ. Sun, K. Yao, Y. Zhang, MnO\u003csub\u003e2\u003c/sub\u003e-directed synthesis of NiFe-LDH@FeOOH nanosheeet arrays for supercapacitor negative electrode, Chinese Chemical Letters, 31 (2020) 2343-2346.\u003c/li\u003e\n\u003cli\u003eB. Wang, Y. Fang, X. Han, R. Jiang, L. Zhao, X. Yang, J. Jin, A. Han, J. Liu, Atomization-Induced High Intrinsic Activity of a Biocompatible MgAl-LDH Supported Ru Single-Atom Nanozyme for Efficient Radicals Scavenging, Angewandte Chemie International Edition, 62 (2023) e202307133.\u003c/li\u003e\n\u003cli\u003eX. Gao, X. Liu, D. Wu, B. Qian, Z. Kou, Z. Pan, Y. Pang, L. Miao, J. Wang, Significant Role of Al in Ternary Layered Double Hydroxides for Enhancing Electrochemical Performance of Flexible Asymmetric Supercapacitor, Advanced Functional Materials, 29 (2019) 1903879.\u003c/li\u003e\n\u003cli\u003eW. Liu, Q. Wang, Z. Liu, G. Ding, Bridging between NiAl-LDH and g-C3N4 by using carbon quantum dots for highly enhanced photoreduction of CO2 into CO, Journal of Colloid and Interface Science, 622 (2022) 21-30.\u003c/li\u003e\n\u003cli\u003eJ. Zheng, C. Fan, X. Li, Q. Yang, D. Wang, A. Duan, S. Pan, B. Zhang, J. Ding, S. Rong, H. Yin, Effective mineralization and detoxification of tetracycline hydrochloride enabled by oxygen vacancies in g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e/ LDH composites, Separation and Purification Technology, 305 (2023).\u003c/li\u003e\n\u003cli\u003eH. Sheikhpoor, A. Saljooqi, T. Shamspur, A. Mostafavi, Co-Al Layered double hydroxides decorated with CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanoparticles and g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e nanosheets for efficient photocatalytic pesticide degradation, Environmental Technology \u0026amp; Innovation, 23 (2021) 101649.\u003c/li\u003e\n\u003cli\u003eA.-Q. Zhou, J.-M. Yang, X.-W. Zhu, X.-L. Zhu, J.-Y. Liu, K. Zhong, H.-X. Chen, J.-Y. Chu, Y.-S. Du, Y.-H. Song, J.-C. Qian, H.-M. Li, H. Xu, Self-assembly construction of NiCo LDH/ultrathin g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e nanosheets photocatalyst for enhanced CO\u003csub\u003e2\u003c/sub\u003e reduction and charge separation mechanism study, Rare Metals, 41 (2022) 2118-2128.\u003c/li\u003e\n\u003cli\u003eS. Guru, S. Kumar, S. Bellamkonda, R.R. Gangavarapu, Synthesis of CuTi-LDH supported on g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e for electrochemical and photoelectrochemical oxygen evolution reactions, International Journal of Hydrogen Energy, 46 (2021) 16414-16430.\u003c/li\u003e\n\u003cli\u003eQ. Shi, K. Fang, W. Chen, Y. Tan, C. Zhang, Designing a superhydrophobic cotton fiber coating exploiting TiO\u003csub\u003e2\u003c/sub\u003e@g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e layered structure for augmented photocatalysis and efficient water-oil separation, International Journal of Biological Macromolecules, 264 (2024) 130596.\u003c/li\u003e\n\u003cli\u003eM. Qiu, P. Guo, X. Lei, Y. Li, S. Li, R. Meng, X. Zhou, Core-shell spherical cerium oxide, N-doped carbon dots and copper oxide nanocomposites with oxidase-like activity for colorimetric detection of hexavalent chromiums, Journal of Environmental Chemical Engineering, 12 (2024) 112334.\u003c/li\u003e\n\u003cli\u003eA. Elhalil, R. Elmoubarki, A. Machrouhi, M. Sadiq, M. Abdennouri, S. Qourzal, N. Barka, Photocatalytic degradation of caffeine by ZnO-ZnAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanoparticles derived from LDH structure, Journal of Environmental Chemical Engineering, 5 (2017) 3719-3726.\u003c/li\u003e\n\u003cli\u003eY. Li, Y. Li, S. Ma, P. Wang, Q. Hou, J. Han, S. Zhan, Efficient water disinfection with Ag\u003csub\u003e2\u003c/sub\u003eWO\u003csub\u003e4\u003c/sub\u003e-doped mesoporous g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e under visible light, Journal of Hazardous Materials, 338 (2017) 33-46.\u003c/li\u003e\n\u003cli\u003eS. Sanati, Z. Rezvani, g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e nanosheet@CoAl-layered double hydroxide composites for electrochemical energy storage in supercapacitors, Chemical Engineering Journal, 362 (2019) 743-757.\u003c/li\u003e\n\u003cli\u003eA.F.d. Silva, J.L.d.S. Duarte, L. Meili, Different routes for MgFe/LDH synthesis and application to remove pollutants of emerging concern, Separation and Purification Technology, 264 (2021) 118353.\u003c/li\u003e\n\u003cli\u003eH. Huang, Z. Li, H. Wang, C. Xia, P. Yan, Q. Zhang, Z. Meng, Adsorption performance of layered double hydroxides for heavy metals removal in soil with the presence of microplastics, Journal of Environmental Chemical Engineering, 10 (2022) 108733.\u003c/li\u003e\n\u003cli\u003eH. Almasi, P. Jafarzadeh, L. Mehryar, Fabrication of novel nanohybrids by impregnation of CuO nanoparticles into bacterial cellulose and chitosan nanofibers: Characterization, antimicrobial and release properties, Carbohydrate Polymers, 186 (2018) 273-281.\u003c/li\u003e\n\u003cli\u003eS. Wang, C. Yuan, W. Chen, Y. Niu, Y. Yan, F. Li, H. Jiang, 3D spherical CuO@g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e composites activating peroxymonosulfate for high efficient degradation of 2,4,6-trichlorophenol: The mechanism of \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e generation, Chemical Engineering Journal, 480 (2024) 148050.\u003c/li\u003e\n\u003cli\u003eP. Zhang, J. Zhang, D. Wang, F. Zhang, Y. Zhao, M. Yan, C. Zheng, Q. Wang, M. Long, C. Chen, Modification of g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e with hydroxyethyl cellulose as solid proton donor via hydrogen bond to enhance H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production, Applied Catalysis B: Environmental, 318 (2022) 121749.\u003c/li\u003e\n\u003cli\u003eH. Wang, T. Zhu, Y. Qiao, S. Dong, Z. Qu, Investigation of the promotion effect of Mo doped CuO catalysts for the low-temperature performance of NH3-SCR reaction, Chinese Chemical Letters, 33 (2022) 5223-5227.\u003c/li\u003e\n\u003cli\u003eZ. Liu, X. Dai, J. He, W. Chen, Y. Wei, Q. Zhou, D. Ma, X. Zheng, Unraveling the thallium immobilization in CuO/PMS system, Chemical Engineering Journal, 472 (2023) 144869.\u003c/li\u003e\n\u003cli\u003eK Feng, G Wang, S Wang, J Ma, H Wu, M Ma, Y Zhang. Breaking the pH Limitation of Nanozymes: Mechanisms, Methods, and Applications. Adv Mater. 2024 Aug;36(31) 2401619.\u003c/li\u003e\n\u003cli\u003eZ Chenghui, C Chuanxia, Z Dan, K Ge, L Fangning, Y Fan, L Yizhong, S Jian. Multienzyme Cascades Based on Highly Efficient Metal\u0026ndash;Nitrogen\u0026ndash;Carbon Nanozymes for Construction of Versatile Bioassays [J]. Analytical Chemistry, 2022, 94(8) 3485-93.\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Schemes","content":"\u003cp\u003eSchemes 1 and 2 are available in the Supplementary Files section.\u0026nbsp;\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"microchimica-acta","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"miac","sideBox":"Learn more about [Microchimica Acta](https://link.springer.com/journal/604)","snPcode":"604","submissionUrl":"https://submission.springernature.com/new-submission/604/3","title":"Microchimica Acta","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"g-C3N4, CuO, Cr(Ⅵ), paper-based sensor","lastPublishedDoi":"10.21203/rs.3.rs-8252032/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8252032/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIn this study, a ternary structure (CuO/CN/LDH) composed of LDH-anchored g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e nanosheets loaded with CuO nanoparticles was successfully constructed via hydrothermal and co-precipitation methods, and a colorimetric sensing platform for Cr(VI) detection was proposed. The engineered nanomaterial demonstrates significantly enhanced oxidase-mimetic activity in the presence of Cr(VI), enabling rapid catalytic oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) with distinct chromogenic response. Mechanistic investigations reveal that Cr(VI) facilitates the conversion of dissolved oxygen into superoxide anions via redox cycling, thereby accelerating TMB oxidation. The sensor exhibits exceptional analytical performance with high accuracy (recovery rates of 97.3-103.8%) and sensitivity (detection limit of 25 nM) in aqueous sample analysis. Notably, an innovative paper-based analytical device was developed in conjunction with smartphone-assisted colorimetric analysis, achieving on-site visual quantification of Cr(VI) within 15 min. This integrated system provides a field-deployable solution for environmental monitoring, combining operational simplicity with reliable detection capabilities. The proposed methodology advances current sensing technologies by merging nanomaterial engineering with portable detection platforms, demonstrating significant potential for heavy metal surveillance in water quality management. Based on this, this study also constructed a paper-based colorimetric detection sensor.\u003c/p\u003e","manuscriptTitle":"Rapid Detection of Cr(VI) Using CuO/CN/LDH as an Oxidase Mimetic and the Construction of a Paper-Based Devices","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-12 09:36:57","doi":"10.21203/rs.3.rs-8252032/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-12-14T18:29:22+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"110555915104743686438584008048092308788","date":"2025-12-09T02:33:36+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-12-08T15:38:28+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-12-08T03:53:42+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"74127011745889232120600477541171116732","date":"2025-12-07T14:45:48+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-12-07T07:48:12+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-12-07T07:16:37+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"120989024678825450377590453011221922701","date":"2025-12-07T06:48:19+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"303892892760703769862160048820902843045","date":"2025-12-07T06:26:34+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"23380993750632441274057925040276626945","date":"2025-12-07T01:43:00+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-12-06T22:44:18+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-12-03T10:09:08+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-12-03T10:02:47+00:00","index":"","fulltext":""},{"type":"submitted","content":"Microchimica Acta","date":"2025-12-01T15:04:30+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"microchimica-acta","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"miac","sideBox":"Learn more about [Microchimica Acta](https://link.springer.com/journal/604)","snPcode":"604","submissionUrl":"https://submission.springernature.com/new-submission/604/3","title":"Microchimica Acta","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"36bfbe21-894b-4aac-8ea9-9f76f7ecae17","owner":[],"postedDate":"December 12th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-03-09T16:01:56+00:00","versionOfRecord":{"articleIdentity":"rs-8252032","link":"https://doi.org/10.1007/s00604-026-07890-2","journal":{"identity":"microchimica-acta","isVorOnly":false,"title":"Microchimica Acta"},"publishedOn":"2026-03-03 15:57:18","publishedOnDateReadable":"March 3rd, 2026"},"versionCreatedAt":"2025-12-12 09:36:57","video":"","vorDoi":"10.1007/s00604-026-07890-2","vorDoiUrl":"https://doi.org/10.1007/s00604-026-07890-2","workflowStages":[]},"version":"v1","identity":"rs-8252032","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8252032","identity":"rs-8252032","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}