Green Synthesis of a Reversible “ON-OFF-ON” Fluorescent sensor for Fe3+ Using Licorice-Derived N-Doped Carbon Dots

Green Synthesis of a Reversible “ON-OFF-ON” Fluorescent sensor for Fe3+ Using Licorice-Derived N-Doped Carbon Dots | 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 Green Synthesis of a Reversible “ON-OFF-ON” Fluorescent sensor for Fe 3+ Using Licorice-Derived N-Doped Carbon Dots Zubair Akram, Anam Arshad, Mohsin Tehseen, Muhammad Mehdi, Ali Raza, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8105416/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 13 Jan, 2026 Read the published version in Journal of Fluorescence → Version 1 posted 12 You are reading this latest preprint version Abstract The detection of ferric ions (Fe 3+ ) is of crucial importance in environmental monitoring and biomedical diagnostics. However, developing highly selective, sensitive, and environmentally friendly detection methods remains an important challenge. In this study, nitrogen-doped carbon quantum dots (N-CQDs) were green-synthesized as a novel and sustainable fluorescent sensor for Fe 3+ detection. The synthesis was achieved via a one-pot hydrothermal method using licorice powder as a renewable carbon source and p -phenylenediamine as a nitrogen dopant. The synthesized N-CQDs display bright blue fluorescence, with a maximum emission at 436 nm when excited at 320 nm. They serve as a highly selective and sensitive fluorescent probe for Fe 3+ , showing a distinct fluorescence “turn-OFF” response. The sensor offers a linear range of 0 to 50 µM for Fe 3+ detection, with a calculated limit of detection (LOD) of 0.346 µM. A notable aspect of this work is the demonstration of an “ON-OFF-ON” sensing paradigm. The fluorescence quenched by Fe 3+ can be effectively restored (“turn-ON”) by adding ascorbic acid, which reduces Fe 3+ . This “ON-OFF-ON” behavior emphasizes the specificity of N-CQDs towards Fe 3+ to Fe 2+ . The practical applicability of the sensor was confirmed through successful detection of Fe³⁺ in complex real-world samples, including beer and human blood serum, achieving excellent recovery percentages (97.6% - 101.7%) and low relative standard deviations (RSD, 0.9% - 1.8%). Overall, this research presents an environmentally friendly N-CQD-based fluorescent sensor with a unique reversible fluorescence response “ON-OFF-ON” for Fe 3+ , holding great potential applications in environmental monitoring and biomedical diagnostics. Nitrogen-Doped Carbon Quantum Dots Ferric Ion Sensing ON-OFF-ON Fluorescence Mechanism Licorice biomass-derived precursors Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1. Introduction Ferric ions (Fe 3+ ) are essential transition metal species, universally distributed in biological systems and environmental matrices, where they support numerous critical processes, such as oxygen transport via hemoglobin, cellular respiration, electron transport reactions, and DNA and RNA synthesis [ 1 – 3 ]. Maintaining an appropriate balance of iron is essential, as both deficiency and excess can cause severe health issues: iron deficiency leads to anemia while increased concentration induces oxidative damage to vital organs (e.g., liver, kidneys) and causes neurodegenerative disorders (Alzheimer’s and Parkinson’s diseases) and carcinogenesis [ 4 – 6 ]. According to U.S Environmental Protection Agency (EPA), the maximum permissible limit of iron in drinking water is 5.36 µM, underscoring the need for careful concentration monitoring [ 7 ]. Elevated concentration of Fe 3+ in environment, originating from industrial discharges, pharmaceutical waste, and corrosion of iron containing materials, is toxic to aquatic ecosystem, disrupt ecological balance and contaminate water resources [ 8 – 10 ]. Iron has dual a nature as being an essential micronutrient at physiologically normal concentration and toxic at elevated level, demands the development of selective and hypersensitive detection techniques capable of quantifying Fe 3+ concentration in biological and environmental samples across a broad range. Conventional techniques for detection of Fe 3+ , including inductively coupled plasma-mass spectrometry (ICP-MS), inductively coupled plasma-optical emission spectrometry (ICP-OES) and atomic absorption spectroscopy (AAS), offer high accuracy but have many critical limitations: difficult and complex sample pretreatment, reliance on sophisticated and costly instruments, prolonged analysis times, the need for highly trained personnel, and limited applicability for rapid real time monitoring [ 11 – 13 ]. These practical constraints have inspired to the development ofsensing platforms with simpler, faster, cost effective, portable, and real time sensing technologies. Fluorescent nanomaterials have emerged as a highly promising platform due to their distinct advantages, including exceptional sensitivity, high selectivity, rapid response time, simplicity, and potential for real time analysis [ 14 , 15 ]. Among the various fluorescent nanosensors, carbon quantum dots (CQDs) have gathered much attention. CQDs are carbon-based nanomaterials, less than 10 nm in diameter are distinguished by their excellent photostability, low toxicity, bright photoluminescence, high water solubility, biocompatibility, facile synthesis, ease of functionalization, and tunable optical properties [ 16 , 17 ]. These characteristics make CQDs ideal candidates for the development of advanced chemo sensors and biosensors, particularly for metal ion detection. A significant trend in the synthesis of CQDs emphasizes the development of sustainable, “green” methods prioritize the use of natural, low-cost, readily available, and biomass-derived precursors. This approach aligns with principles of sustainable chemistry, yielding CQDs with unique functional groups inherited the complex composition of biomass material, enhancing their sensing performance and biocompatibility [ 18 , 19 ]. Additionally, nitrogen doping (N-doping) is a widely implemented and effective approach to alter the electronic structure and enhance the PL quantum yield (QY) of CQDs. The incorporation of nitrogen into lattice of CQDs creates active sites, improves charge carrier mobility and passivates surface defects, often leading to enhanced optical and sensing properties [ 20 – 22 ]. The use of aromatic amines for N-doping can lead to the formation of distinct N-configurations (e.g., pyridinic, pyrrolic nitrogen) within the CQDs framework, which can influence their photophysical properties and their interaction with analytes [ 23 , 24 ]. These types of N-configurations may produce specific coordination sites for metallic ions or alter the electronic properties to make it favorable for sensing mechanism [ 25 , 26 ]. The “ON-OFF-ON” sensing strategy provides an advanced platform for analyte detection, offering enhanced selectivity and reliability through a reversible response mechanism [ 27 , 28 ]. In this scheme, the intrinsic fluorescence of the probe (ON state) is quenched by the target analyte (OFF state). Subsequently, the introduction of a specific reagent reverses the quenching effect, restoring the fluorescence (second “ON” state). While many “ON-OFF-ON” systems are designed for the sequential detection of two different analytes, the present work uniquely employs ascorbic acid as a chemical stimulus (reducing agent) to convert Fe 3+ to Fe 2+ , thereby demonstrating the reversibility and specificity of the Fe 3+ interaction, rather than being used for the detection of ascorbic acid itself. This research focuses on the development of a novel N-doped CQDs (N-CQDs) via a facile, one-pot green hydrothermal synthesis, employing licorice powder (a novel biomass precursor) as the carbon source and p -phenylenediamine as the nitrogen dopant. Then the synthesized N-CQDs were comprehensively characterized to elucidate the structural, morphological, and optical properties. The core objective is to explore their application as a highly selective and sensitive fluorescent probe for Fe 3+ . A distinctive aspect of this work is the demonstration of an “ON-OFF-ON” fluorescence switching mechanism: Fe 3+ ions quench the N-CQDs fluorescence (“OFF” state), and the subsequent addition of ascorbic acid (AA) effectively restores the fluorescence (“ON” state) by reducing Fe 3+ to Fe 2+ . The practical viability of this N-CQD sensor is thoroughly assessed by quantifying Fe 3+ levels in complex real-world matrices, namely beer and human blood serum. This study presents a unique combination of sustainable precursors for N-CQDs synthesis, applied within a specific “ON-OFF-ON” Fe 3+ sensing strategy, offering potential advancements in terms of environmental benignity, cost effectiveness, and analytical efficacy for Fe 3+ detection. 2. Experimental Section Licorice powder of food grade was procured from Xinjiang, China. p -Phenylenediamine (PPD, Sigma-Aldrich, purity ≥ 99%), ferric chloride hexahydrate (FeCl 3 ·6H 2 O), Sinopharm, purity ≥ 98%), ascorbic acid (AA, Sinopharm, purity ≥ 99%), and various metal ion salts including KCl, NaCl, CaCl 2 , MgCl 2 , FeCl 2 , CuCl 2 , ZnCl 2 , MnCl 2 , NiSO 4 , Co(NO 3 ) 2 , PbCl 2 , CdCl 2 , HgCl 2 and AlCl 3 (purity ≥ 98%) were obtained from Sigma-Aldrich. Additionally, glucose (C 6 H 12 O 6 , Millipore Sigma, purity ≥ 99.5%), urea (NH 2 CONH 2 , Merck, purity ≥ 99%), trichloroacetic acid (TCA, purity ≥ 99%) and nitric acid (ACS reagent, 70%) were sourced from the same supplier. All solutions were prepared using ultrapure water generated by a Milli-Q (Hyper PureX) purification system. Buffer solutions, such as Tris-HCl and phosphate buffer, were formulated using analytical grade reagents. Beer sample was purchased from a local supermarket. Human blood serum samples were obtained with prior ethical approval. Dialysis membranes Spectra/Por with a molecular weight cut-off (MWCO) of 500–1000 Da were procured from Spectrum Labs. N-CQDs were synthesized via a one-pot hydrothermal method. In a typical synthesis procedure, 1.0 g of licorice powder and 0.05 g of p -phenylenediamine were dispersed in 30 mL ultrapure water. The licorice (1.0g) and PPD (0.05g) ratio was optimized by testing three different mass ratios (1.0:0.02, 1.0:0.05, 1.0:0.1 g/g) and evaluating the key metrics: relative fluorescence quenching of N-CQDs with Fe 3+ . To achieve a homogeneous dispersion, the mixture was ultrasonicated for 10 minutes. Subsequently, the resulting suspension was transferred into a 50 mL Teflon-lined stainless-steel autoclave. The autoclave was sealed and heated in an oven at 180°C for 10 hours. After the reaction was completed, the autoclave was cooled naturally to room temperature. The crude product, a dark brown solution, underwent a multi-step purification process. First, the solution was centrifuged at 10,000 rpm for 20 minutes to remove large unreacted particles. The supernatant was collected and then filtered through a 0.22 µm syringe filter to eliminate finer particulate matter. Subsequently, the filtered solution was dialyzed against ultrapure water using a dialysis membrane (1000 Da MWCO) for 24 hours. The dialysis water was changed every 6 hours to ensure the efficient removal of unreacted precursors and small molecular byproducts. The purified N-CQDs aqueous solution was dried at 60°C to obtain solid N-CQDs. The synthesized N-CQDs were comprehensively characterized using a range of analytical techniques, each providing unique insights into the structure, properties, and performance of the materials. UV-Visible Absorption spectra of the N-CQDs were recorded on a (UV-2600) spectrophotometer within a wavelength range of 200–800 nm, with ultrapure water serving as the blank reference. Fluorescence excitation and emission spectra were recorded using (PerkinElmer-400 spectrum) spectrofluorometer. The slit widths for excitation and emission were set to 3 nm. All measurements were conducted at room temperature (25 ± 2°C) in ultrapure water. Fourier transform infrared FTIR spectra were recorded in the range of 400–4000 cm − 1 , enabling the identification of surface functional groups on the N-CQDs. X-ray diffraction XRD patterns were collected on a diffractometer over a 2θ range of 10–80° with a step size of 0.02° to determine the crystallinity of the N-CQDs. X-ray Photoeclectron Spectroscopy XPS measurements were performed on a Thermo K-Alpha XPS spectrometer to obtain survey scans and high-resolution spectra for the C1s, N1s, and O1s regions. Binding energies were calibrated using the adventitious C1s peak at 284.8 eV to determine the elemental composition and elemental chemical states in the N-CQDs. The morphology, size, and microstructure of the N-CQDs were examined using a transmission electron microscope (TEM) JEOL JEM-F200. Samples were prepared by drop-casting a dilute aqueous dispersion of N-CQDs onto a carbon-coated copper grid, followed by drying under ambient conditions. The particle size distribution was determined by measuring at least 100 particles from multiple Transmission electron microscopy TEM images using ImageJ 1.54g software. High-resolution TEM (HRTEM) images were acquired to observe lattice fringes, and selected area electron diffraction (SAED) patterns were recorded to assess the crystallinity of the N-CQDs. All fluorescence measurements for Fe 3+ sensing were carried out at room temperature (25 ± 2°C). To a fixed volume of N-CQD solution 3 mL, incremental aliquots of a standard Fe 3+ stock solution was added, resulting in final Fe 3+ concentrations of 0, 2.5, 5, 10, 20, 50, 100, 200, and 250 µM. After each addition, the solution was gently mixed and allowed to incubate for 3 minutes to ensure complete interaction between the N-CQDs and Fe 3+ before fluorescence measurement. Fluorescence emission spectra were recorded with excitation at 320 nm, and the emission intensity at 436 nm was monitored. The fluorescence quenching efficiency was typically expressed as F/F 0 , where F 0 is the initial fluorescence intensity of N-CQDs in the absence of Fe 3+ , and F is the fluorescence intensity in the presence of Fe 3+ . The linear range for Fe³⁺ detection was determined from the linear portion of the calibration curve, which plotted F/F 0 against Fe 3+ concentration. The LOD was calculated using the formula LOD = 3σ/S, where σ is the standard deviation of the fluorescence intensity of blank N-CQD solutions, and S is the slope of the linear calibration curve in the low concentration range. The selectivity of the N-CQD probe for Fe 3+ was evaluated by comparing its fluorescence response to Fe 3+ with that to a range of other potentially interfering metal ions. These ions included alkali metals (K + , Na⁺), alkaline earth metals (Ca 2+ , Mg 2+ ), transition metals (Fe 2+ , Cu 2+ , Zn 2+ , Mn 2+ , Ni 2+ , Co 2+ , Pb 2+ , Cd 2+ , Hg 2+ ), other common cation (Al 3+ ), common anions (SO 4 2− and NO 3 − ) and common interfering substances like glucose, urea and ascorbic acid. The concentration of interfering ions was typically set at 10-fold excess relative to the Fe 3+ concentration used within the linear range. The fluorescence intensity of the N-CQD solution was measured after the addition of each ion. The effect of pH on the fluorescence intensity of N-CQDs was investigated over a pH range of 1–14. Solutions were incubated for 3 minutes at each pH before fluorescence measurement. The photostability of the N-CQDs was assessed by continuously putting the N-CQD solution in the visible light for an extended period. This ensures reliable signal acquisition during fluorescence measurements. Fluorescence intensity at 436 nm was monitored at regular time intervals. The influence of ionic strength on the N-CQD fluorescence was studied by measuring the fluorescence intensity in the presence of varying concentrations of NaCl 10–200 mM. The “ON-OFF-ON” fluorescence switching behavior was investigated through the following steps: The fluorescence emission spectrum of the N-CQDs solution was recorded with excitation at 320 nm, and the emission was monitored at 436 nm. A specific concentration of Fe 3+ solution was added to the N-CQDs solution. The mixture was incubated for 5 minutes, and then the fluorescence spectrum was recorded. A solution of ascorbic acid (AA) with a final concentration of 10 mM was added to the N-CQDs-Fe 3+ mixture. After a 5-minute incubation, the fluorescence spectrum was recorded again to observe the fluorescence recovery. The practical applicability of the N-CQD sensor for Fe 3+ detection was evaluated in beer and human blood serum samples using the standard addition method. The beer sample was first degassed by sonication for 15 minutes to remove dissolved CO₂. Subsequently, the degassed beer was centrifuged at 5000 rpm for 10 minutes and filtered through a 0.22 µm syringe filter to eliminate any particulate matter [ 29 ]. The clear filtrate was then diluted 10-fold with ultrapure water for analysis. Deproteinization of blood serum samples was by vertexing the mixture for 1 minute and then centrifuged at 12,000 rpm for 15 minutes to precipitate proteins. The clear supernatant was carefully collected, and its pH was adjusted to the optimal condition for sensing. The supernatant was then diluted with ultrapure water prior to Fe 3+ determination [ 5 ]. The prepared real sample solutions were first analyzed for their natural Fe 3+ content using the N-CQDs sensor. Subsequently, these samples were spiked with known concentrations of Fe 3+ standard solution. The spiked samples were then analyzed again using the N-CQDs sensor. The recovery was calculated using the formula: Recovery (%) = [(Fe 3+ detected in spiked sample– Fe 3+ detected in unspiked sample)/Fe 3+ added] × 100%. Measurements were performed in triplicate (n = 3) to determine the Relative Standard Deviation (RSD, %). 3. Results and Discussion The N-CQDs were successfully synthesized via a one-pot hydrothermal method, utilizing licorice powder as a green carbon source and p -phenylenediamine for nitrogen doping as illustrated in Fig. 1 (a) . This synthesis route offers advantages in terms of simplicity, cost-effectiveness, and environmental friendliness. The XRD pattern of the synthesized N-CQDs ( Fig. 1 (b)) exhibited a broad peak centered approximately at 2θ values within the range of 20–26°. This peak corresponds to the (002) graphitic plane, typically associated with amorphous carbonaceous materials [ 30 ]. The broad peak indicates the presence of small graphitic domains and a high degree of disorder in the N-CQDs structure. These features are consistent with carbon quantum dots synthesized from biomass and amine precursors through hydrothermal carbonization [ 31 ]. The calculated interlayer d-spacing was larger than that of pristine graphite (0.34 nm), suggesting that the incorporation of heteroatoms (nitrogen and oxygen) and the presence of functional groups have disrupted the regular graphitic stacking. This unique structural feature, characterized by its amorphous nature and abundant surface defects, likely contributes significantly to the photoluminescence and sensing capabilities of the N-CQDs. Then the FTIR spectroscopy was employed to characterize the surface functional groups of the N-CQDs [ 32 , 33 ]. The FTIR spectrum ( Fig. 1 (c)) displayed several absorption bands. For example, a prominent broad band centered around at 3396 cm − 1 was attributed to the stretching vibrations of O-H and N-H. Peaks in the region of 2928 cm − 1 corresponded to the C-H stretching vibrations of aliphatic groups. Strong absorptions at 1648 and 1602 cm − 1 were indicative of C = O stretching vibrations, likely originating from carboxyl functional groups. The presence of C = C and/or C = N stretching vibrations from aromatic rings and imine structures was evident around 1600 − 1500 cm − 1 . C-N stretching vibrations from amines were observed in the 1398 cm − 1 range, confirming the successful incorporation of nitrogen from p -PDA. Additionally, C-O stretching vibrations from hydroxyl groups were detected between 1214 and 1069 cm − 1 . These diverse functional groups on the N-CQD surface are crucial for its aqueous dispersibility and play a key role in the interaction and binding with Fe 3+ ions, which is essential for the sensing application. The transmission electron microscopy (TEM) characterization of the as-prepared N-CQDsprovides valuable insights into their morphological and structural features. As depicted in Fig. 1 (d) , the representative TEM image clearly shows that the CQDs are uniformly dispersed in the sample, without any significant agglomeration. The discrete nature of the CQDs, as evidenced by the clear separation between individual particles, indicates a high-quality synthesis process that effectively controls the growth and assembly of the quantum dots, minimizing intermolecular forces that could lead to aggregation. The inset of Fig. 1 (d) showcases a HRTEM image of a single CQD, offering a magnified and detailed view of its internal structure. The presence of distinct lattice fringes is a key observation, as it provides conclusive evidence for the crystalline nature of the CQD. Precise measurement of the interplanar spacing (d-spacing) reveals a value of approximately 0.21 nm. This specific d-spacing value is characteristic of the (102) lattice planes of graphitic carbon [ 34 ]. Figure 1 (e) illustrates the size distribution histogram of the synthesized CQDs, obtained from statistical analysis of a large number of particles observed in multiple TEM images. The histogram reveals a relatively narrow size distribution, indicating precise control over synthesis conditions. The size of the majority of the CQDs falls within the range of 2.5 nm to 3.5 nm, demonstrating a high level of consistency in the synthesis yield. Upon fitting a distribution curve to the histogram, a peak intensity is observed, centered approximately 2.7–2.8 nm. This peak represents the most frequently occurring diameter among the synthesized CQDs, indicating that the majority of the quantum dots have a diameter within this range. The surface elemental composition of N-CQDs was characterized by XPS, confirming the successful synthesis of a carbonaceous framework with intentional heteroatom doping [ 35 ]. The XPS spectrum ( Fig. 2 (a)) exhibited distinct peaks for C1s (284.8 eV), O1s (532.0 eV), and N1s (399.5 eV), with quantitative analysis revealing atomic percentages of 70.89% C, 25.41% O and 3.71% N. The nitrogen content demonstrating controlled incorporation via the hydrothermal synthesis route. The high oxygen content (25.41%) indicates extensive surface functionalization with hydrophilic groups. The deconvolution of the C1s spectrum ( Fig. 2 (b)) into three Gaussian-Lorentzian components revealed a complex bonding environment. The dominant peak at 284.8 eV corresponds to sp 2 -hybridized carbon in graphitic domains (C = C), with a shoulder at 285.2 eV indicating sp 3 -hybridized carbon (C-C) from lattice defects. A peak at 286.2 eV is assigned to C-O bonds in hydroxyl groups, while 287.3 eV corresponds to C = O (carbonyl). The N1s spectrum ( Fig. 2 (c)) was deconvoluted into three components, each representing distinct doping configurations. A dominant peak at 399.8 eV is assigned to pyridinic-N (N-6), where nitrogen replaces carbon in the graphene lattice, contributing lone-pair electrons that enhance electron transfer. A shoulder at 400.1 eV corresponds to pyrrolic-N (N-5), embedded in rings and responsible for pH-dependent fluorescence. A minor peak at 402.1 eV is attributed to graphitic-N (N-Q), which improves electrical conductivity by donating π-electrons. The O1s spectrum ( Fig. 2 (d)) resolved into two peaks, corroborating C1s findings. A major peak at 531.6 eV arises from C = O (carbonyl), while 533.0 eV corresponds to C-O (hydroxyl). The atomic O/C ratio (0.36) exceeds that of pristine graphene (0.05), confirming extensive oxidation. The optical characteristics of the N-CQDs were investigated using UV-Vis absorption and fluorescence spectroscopy. The UV-Vis absorption spectra of the synthesized N-CQDs in the absence and presence of ferric ions Fe 3+ were investigated, as depicted in Fig. 3 (a) . A distinct and intense absorption peak at approximately 210–220 nm is attributed to π-π * electronic transitions. This peak originates from the sp 2 -hybridized carbon atoms that form the conjugated graphitic core of N-CQDs. Additionally, a broader and less intense absorption feature appears between 260 and 280 nm, corresponding to n-π * transitions [ 36 ]. These transitions involve non-bonding electrons localized on heteroatoms and surface functional groups within N-CQDs. For N-CQDs, nitrogen doping introduces various nitrogen-containing functional groups, such as pyridinic-N, pyrrolic-N, and graphitic-N, in addition to oxygen-based functional groups like carbonyl (C = O) and hydroxyl (O-H). The presence of these surface-associated transitions indicates that the N-CQDs possess a rich array of reactive sites, which are essential for their interaction with external analytes, such as ferric ions. Upon the addition of ferric ions Fe 3+ , the UV-Vis absorption spectrum of N-CQDs (red curve in Fig. 3 (a) ) undergoes a profound transformation, which is complicatedly linked to the observed fluorescence quenching phenomenon. The significant increase in absorption intensity, particularly in the 250–350 nm region, with a notable enhancement around 280 nm, serves as a direct spectroscopic signature of the interaction between N-CQDs and Fe 3+ ions. A systematic study was conducted ( Fig. 3 (b)) on the photoluminescence properties of synthesized carbon quantum dots (N-CQDs) by measuring their emission spectra within the excitation wavelength range of 300–450 nm. The results revealed a distinct excitation-dependent luminescence characteristic of the CQDs [ 37 , 38 ]. When the excitation wavelength was 320 nm, the CQDs exhibited the best luminescence performance, presenting a sharp and intense emission peak at 436 nm. The width of this peak, measured to be 25 nm, indicated high spectral purity and excellent luminescence efficiency. As the excitation wavelength deviated from 320 nm, the brightness of the emission peak decreased, and its position shifted towards longer wavelengths. For example, at 300 nm excitation, the peak brightness decreased by 28% compared to that at 320 nm excitation; at 450 nm excitation, the peak brightness decreased by 42%. This spectral change was attributed to the presence of multiple luminescent centers on the surface of CQDs, the influence of their size on luminescence, and the alteration of the internal electronic state by the surface chemical structure. Compared with previously synthesized CQDs using similar or different methods, our CQDs show superior luminescence performance [ 39 ]. This enhanced performance was due to the unique synthesis method, which led to more uniform CQD sizes, more regular surface structures, and higher internal crystallinity, reducing non-luminescent energy loss processes. These CQDs emit intense and tunable blue light under 320 nm excitation, showing great potential in advanced optical applications such as bioimaging, analyte detection, optoelectronic device fabrication, and anti-counterfeiting fluorescent inks. The quantitative sensing capability of N-CQDs toward Fe 3+ ions were rigorously evaluated by monitoring changes in their fluorescence intensity as a function of increasing Fe 3+ concentration. The photoluminescent property of N-CQDs was visually confirmed by photographs of their aqueous solution under UV light in the absence and presence of Fe 3+ and other analytes in Figure S1 . The quantitative sensing performance of N-CQDs toward Fe 3+ was systematically evaluated by monitoring fluorescence intensity changes as a function of Fe 3+ concentration, establishing their potential for analytical applications [ 40 ]. Figure 3 (c) illustrates the PL emission spectra of N-CQDs upon stepwise addition of Fe 3+ ions, excited at the optimal wavelength of 320 nm. The initial spectrum exhibits a robust blue emission peak centered at 436 nm, characteristic of the N-CQDs’ intrinsic photoluminescence. As Fe 3+ concentration was increased from 0 to 250 µM, a monotonic decrease in fluorescence intensity was observed without discernible shifts in the emission peak position. This behavior indicates that Fe 3+ primarily quenches the radiative recombination processes of N-CQDs rather than altering their electronic structure or chromophoric environment. Further illustrating this trend, Figure shows the fluorescence intensity (at 436 nm) as a function of Fe 3+ concentrations (0-250 µM). The plot depicted a concentration dependent decrease in N-CQDs’ fluorescence, with quenching occurring rapidly at lower concentrations and tending to plateau at higher levels. This behavior suggested potential saturation of binding sites or a shift in the dominant quenching mechanism at elevated analyte concentrations. Subsequently, the selectivity and stability of the N-CQD sensor were tested. As illustrated in Fig. 3 (d) , the N-CQDs exhibited exceptional discrimination toward Fe 3+ : The blank control displayed an F/F₀ ratio of 1.0, reflecting the unquenched PL of N-CQDs. In contrast, the addition of Fe 3+ induced a profound fluorescence quenching, with the F/F₀ ratio dropping to 0.07 (corresponding to a quenching efficiency of 93%). This drastic reduction stems from the synergistic effects of ground-state complexation (via N/O-rich surface ligands) and photoinduced electron transfer (PET), as corroborated by UV-Vis absorption studies ( Fig. 3 (a)) . The high quenching efficiency underscores the strong affinity between Fe 3+ and N-CQDs, attributed to the Lewis acid-base interaction between Fe 3+ and electron-donating groups (e.g., pyridinic-N, carboxyl) on the CQD surface. Alkali (K + , Na + ), alkaline earth (Ca 2+ , Mg 2+ ), transition (Zn 2+ , Ni 2+ , Co 2+ , Cd 2+ , Mn 2+ , Cu 2+ , Pb 2+ , Hg 2+ ), and trivalent (Al 3+ ) ions induced minimal PL changes, with F/F₀ ratios ranging from 0.96 to 1.03. This insensitivity arises from the weaker coordination ability of these ions compared to Fe 3+ . Divalent cations (e.g., Cu 2+ ) showed < 5% quenching, while trivalent Al 3+ (often a competitive interferent) exhibited an F/F₀ of 0.99, confirming the N-CQDs’ preference for Fe 3+ . Common anions (SO 4 2− , NO 3 − ) and biologically relevant molecules (urea, glucose, ascorbic acid) at 250 µM induced F/F₀ ratios of 0.97 to 1.02, indicating minimal interaction with N-CQDs. This stability against anionic and biomolecular interference is critical for real-world applications, as it ensures reliable Fe 3+ detection in complex matrices. A linear relationship between analyte concentration and sensor response was demonstrated for practical analytical applications. Figure 5 (e and f) presents the calibration curve derived from fluorescence intensity versus Fe 3+ concentration, focusing on the lower concentration range for optimal sensitivity and linearity. In the range of 0 to 50 µM Fe 3+ , N-CQDs exhibited a good linear relationship between fluorescence intensity and Fe 3+ concentration [ 41 ] with a calculated limit of detection (LOD) of 0.346 µM better than the LODs of other synthesized CQDs for ferric ions, as summarized in Table 1 . Table 1 The Comparison of N-CQDs with other materials for Fe 3+ detection. Methods of Synthesis Synthesized Materials Chemical Precursors Used Analytes Detected LOD Ref. Hydrothermal Synthesis Hydrothermal Synthesis Hydrothermal Synthesis Hydrothermal Synthesis Hydrothermal Synthesis Solvothermal Synthesis Microwave Synthesis Microwave Synthesis Hydrothermal Synthesis C-dots CDs N-CDs C-dots N-CDs B@HRCDs NCDs N-CDs N-CQDs Zinc gluconate Citrus reticulata PEI and Melamine Adenosine and PPi C. Acid and Melamine Chinese herbal residues Malic acid and Urea Citric acid and Urea Licorice and PPD Fe 3+ Fe 3+ Fe 3+ Fe 3+ Fe 3+ Fe 3+ Fe 3+ Fe 3+ Fe 3+ 1.9 µM 1.11 µM 0.88 µM 0.9 µM 3.18 µM 1.08 µM 1.9 µM 1.0 µM 0.346 µM [ 42 ] [ 43 ] [ 44 ] [ 45 ] [ 46 ] [ 47 ] [ 48 ] [ 49 ] This Work The linear regression analysis yielded the Eq. 1 : $$\:\text{y}\:=\:221.64718-1.91913\text{x}$$ 1 where y is the fluorescence intensity (a.u.) and x are the Fe 3+ concentration (µM). The high coefficient of determination (R 2 = 0.99697) confirmed exceptional model fit to the experimental data. Due to the quenching mechanism is a multi-faceted process, involving ground-state complexation, electron transfer, and potentially the inner filter effect, so its mechanism is detailed as follows. As Lewis acids, Fe 3+ ions have a strong affinity for electron-rich sites on the N-CQDs surface. The abundance of oxygen-containing functional groups and nitrogen heteroatoms on N-CQDs serves as an ideal platform for Fe 3+ chelation. These coordination interactions lead to the formation of a stable N-CQD-Fe 3+ complex in the ground state. This complexation event is spectroscopically evidenced by the red shift and intensity increase in the UV-Vis spectrum, as shown in Fig. 3 (a) . The formation of the complex disturbs the electronic environment of N-CQDs, leading to an altered energy levels and transition probabilities. From a quantum-mechanical perspective, complex formation modifies the electronic wave functions of N-CQDs, thereby facilitating non-radiative decay pathways. The excited-state energy of N-CQDs is dissipated through vibrational relaxation or intersystem crossing, resulting in static quenching. This process effectively removes the emissive N-CQDs from the solution, as they are converted into non-emissive N-CQD-Fe 3+ complexes [ 50 , 51 ]. In addition to static quenching, photoinduced electron transfer (PET) contributes to the observed fluorescence quenching. PET occurs when electron transfer takes place between the excited-state N-CQDs and Fe 3+ ions, a process driven by the favorable redox potential difference between the two species. Upon excitation, N-CQDs populate their excited state, where they can act as electron donors due to their relatively low oxidation potential. Fe 3+ , with its high reduction potential, serves as an efficient electron acceptor. This electron transfer from excited N-CQDs to Fe 3+ diverts energy away from radiative decay channels, leading to fluorescence quenching [ 52 – 54 ]. The enhanced absorbance in the UV-Vis spectrum, particularly in regions overlapping with the excitation wavelength of N-CQDs, supports the involvement of PET by indicating strengthened electronic interactions between N-CQDs and Fe 3+ upon complexation interactions that promote the electron transfer process. This dynamic process reduces the number of excited N-CQDs undergoing radiative emission, thereby contributing to the observed decrease in fluorescence intensity. In addition, the robustness of the N-CQD sensor was assessed under various environmental conditions. N-CQDs exhibited stable fluorescence across a wide pH range, with optimal performance under neutral to slightly alkaline conditions (pH 7.0–9.0), peaking between pH 7.0 and 8.0 ( Fig. 4 a ) . Fluorescence intensity increased gradually from pH 1 to 3, then decreased in strongly acidic (pH 11) conditions, yet retained approximately 60–70% of its maximum intensity even at pH 14. This behavior can be attributed to the protonation and deprotonation of surface functional groups (e.g., carboxyl, hydroxyl, nitrogen containing groups), which modulate surface charge, energy levels, and radiative recombination pathways [ 55 , 56 ]. The broad fluorescence stability within physiological (pH 6.5-8.0) and environmental (pH 5.0–9.0) ranges supports the potential applications of N-CQDs in biological and aqueous systems. N-CQDs exhibited outstanding resistance to photobleaching and long-term degradation ( Fig. 4 b ). Fluorescence intensity remained nearly unchanged for the first 15 days, followed by a gradual decline over 180 days; after 6 months, approximately 90–92% of initial intensity was retained [ 57 , 58 ]. The N-CQDs also exhibited excellent photostability under prolonged UV irradiation (365 nm, 10 W): after 120 mins, they retained 91% of initial fluorescence intensity (Figure S2) . This high stability is attributed to the presence of graphitic core and effective surface passivation, which mitigate photo-oxidation and contribute to a prolonged shelf-life. N-CQDs exhibited exceptional stability under high salt concentrations ( Fig. 4 c ) , a key characteristic for applications in biological or saline environments. Fluorescence intensity decreased only marginally with increasing NaCl concentration: at 200 mM, approximately 85–90% of initial intensity (measured at 10 mM NaCl) was retained [ 59 , 60 ]. This resistance to ionic strength-induced aggregation is attributed to robust surface passivation and the presence of hydrophilic functional groups, maintaining colloidal dispersion. Beside the above, the fluorescence switching kinetics and mechanism were studied. Pristine N-CQDs exhibited intense fluorescence emission at 436 nm, with a peak intensity shown by the blue curve in Fig. 5 , defining the initial “ON” state. Upon addition of Fe 3+ ions, rapid and efficient fluorescence quenching occurred, with the emission intensity shown by the green curve, corresponding to a quenching efficiency of 93%. This abrupt transition to the “OFF” state underscored the strong interaction between N-CQDs and Fe 3+ , likely mediated by photoinduced electron transfer (PET) and the formation of a non-emissive N-CQD-Fe 3+ complex as mentioned before. Subsequent introduction of ascorbic acid to the N-CQDs-Fe 3+ system triggered a remarkable fluorescence recovery, with the emission intensity rebounding to 79% of the original level, shown by the red curve. Notably, the emission peak remained at 436 nm throughout the switching cycle, indicating that the N-CQD chromophore structure was preserved, and the mechanism involved reversible quantum yield modulation rather than structural degradation [ 61 , 62 ]. The observed “On-Off-On” behavior was attributed to a redox-driven mechanism (as shown in Fig. 8): ( i ) quenching mechanism (ON→OFF) : Fe 3+ ions, as strong electron acceptors, facilitated photoinduced electron transfer from excited N-CQDs and formed non-emissive N-CQD-Fe 3+ complexes, leading to efficient fluorescence quenching. The high electron affinity of Fe 3+ destabilizes the excited state of N-CQDs, suppressing radiative recombination. ( ii ) recovery mechanism (OFF→ON) : Ascorbic acid, a potent reducing agent, selectively reduced Fe 3+ to Fe 2+ , which exhibits weaker electron-accepting capability and forms less-stable complexes with N-CQDs. The reduction of Fe 3+ disrupted the quenching interaction, releasing N-CQDs from the non-emissive state and restoring their intrinsic fluorescence. Spiked beer samples demonstrated high recovery rates for Fe 3+ : 98.4% (5 µM), 101.5% (10 µM), and 98.8% (15 µM), indicating minimal matrix interference. RSD values (0.9–1.2%) were exceptionally low, comparable to those of commercial kits (2–3% RSD), due to uniform surface chemistry of N-CQDs and controlled sample preparation. Spiked serum samples demonstrated high analytical accuracy: Fe 3+ recoveries were 97.6% (5 µM), 98.5% (10 µM), and 101.7% (15 µM), with RSD values of 1.3–1.8%, which are considered acceptable for biological analysis. Optimized sample-processing and detection protocols ensured high precision, outperforming protein-based assays that are prone to serum cross-reactivity. Table 2 Detection of Fe³⁺ in Real Samples Using N-CQDs Fluorescent Sensor Real Samples Added Fe 3+ (µM) Sensed by N-CQDs (µM) ( n = 3) Recovery (%) RSD (%) Beer Blood Serum 5 10 15 5 10 15 4.92 ± 0.06 10.15 ± 0.09 14.82 ± 0.16 4.88 ± 0.09 9.85 ± 0.15 15.25 ± 0.20 98.4 ± 1.2 101.5 ± 0.9 98.8 ± 1.1 97.6 ± 1.8 98.5 ± 1.5 101.7 ± 1.3 1.2 0.9 1.1 1.8 1.5 1.3 4. Conclusion In this study, a novel, environmentally friendly fluorescent sensor based on nitrogen-doped carbon quantum dots was successfully synthesized, using a one-pot green hydrothermal method with licorice powder as a renewable carbon source, addressing the critical need for sustainable Fe 3+ detection with high selectivity and sensitivity. The N-CQDs demonstrated excellent photoluminescence and a reversible “ON-OFF-ON” fluorescence switching behavior, in which Fe 3+ -quenched fluorescence was restored by ascorbic acid (via Fe 3+ reduction to Fe 2+ ), further confirming the specificity of N-CQDs for Fe 3+ . Significantly, the sensor performed reliably in complex samples, including beer and human blood serum, as evidenced by high recovery rates and low relative standard deviations, underscoring its practical robustness. Beyond showcasing analytical performance, this work highlights the broader value of leveraging green synthetic routes and renewable carbon sources for designing advanced nanomaterial-based sensors. Future efforts may focus on optimizing the synthesis process to enhance quantum yield and reduce detection limits, as well as integrating the sensor into portable platforms for real-time, on-site Fe 3+ detection across diverse scenarios. Declarations Author Contributions: Zubair Akram and Anam Arshad were involved in investigation, conceptualization, methodology, data curation and writing the original draft. Muhammad Mehdi, Mohsin Tehseen, Ali Raza and Yulan Shi contributed to the methodology and analysis. Nan Wang was involved in visualization, reviewing, editing, validation and supervision. Sajida Noureen and Feng Yu were involved in visualization, validation and supervision. Data Availability: Data presented in this manuscript will be available from corresponding author upon reasonable request. Funding: This work was supported by Xinjiang Science and Technology Program (2023TSYCCX0118). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Conflict of interest: The authors declare no conflict of interest. References Shahbaz M et al (2025) Fluorescent/Photoluminescent Carbon Dots as a Sensor for the Selective and Sensitive Detection of Fe 3+ /Fe 2+ Metal Ions. A Review of the Last Decade . J Fluoresc, : p. 1–24 Yadav R et al (2025) Fluorometric sensing and nanomolar level detection of heavy metal ions using nitrogen doped carbon dots. 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Supplementary Files SupplementaryFile.docx GA.png Graphical Abstract Cite Share Download PDF Status: Published Journal Publication published 13 Jan, 2026 Read the published version in Journal of Fluorescence → Version 1 posted Editorial decision: Revision requested 28 Nov, 2025 Reviews received at journal 25 Nov, 2025 Reviews received at journal 19 Nov, 2025 Reviewers agreed at journal 19 Nov, 2025 Reviewers agreed at journal 17 Nov, 2025 Reviewers agreed at journal 16 Nov, 2025 Reviewers agreed at journal 14 Nov, 2025 Reviewers agreed at journal 14 Nov, 2025 Reviewers invited by journal 14 Nov, 2025 Editor assigned by journal 13 Nov, 2025 Submission checks completed at journal 13 Nov, 2025 First submitted to journal 13 Nov, 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. 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14:05:56","extension":"png","order_by":13,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":534129,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8105416/v1/e63ea0bd7efcd938870f8a14.png"},{"id":96746138,"identity":"070e26b9-5930-4c19-8cf5-c6832dd68319","added_by":"auto","created_at":"2025-11-25 16:01:59","extension":"png","order_by":14,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":308285,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8105416/v1/58a62c448d941da9f19db950.png"},{"id":96746132,"identity":"ec13e674-75d1-4b85-85f3-bed36159d6b3","added_by":"auto","created_at":"2025-11-25 16:01:59","extension":"png","order_by":15,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":425012,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-8105416/v1/ecdf21245fb4a4ce1406316f.png"},{"id":96746139,"identity":"bf287346-351a-402f-879f-42c25ddf5e11","added_by":"auto","created_at":"2025-11-25 16:01:59","extension":"xml","order_by":16,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":145200,"visible":true,"origin":"","legend":"","description":"","filename":"075f112d42e94535b1e86259a446638f1structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-8105416/v1/8f1f9b23314700cfa76cc997.xml"},{"id":96914825,"identity":"95a6d309-29ad-4944-93b6-8b49f10a60b1","added_by":"auto","created_at":"2025-11-27 14:06:27","extension":"html","order_by":17,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":155022,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8105416/v1/bae8807ce120e687a0e7bea2.html"},{"id":96746122,"identity":"7b17e1b5-57e4-48c8-a48d-975f0e4ffd53","added_by":"auto","created_at":"2025-11-25 16:01:59","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":950855,"visible":true,"origin":"","legend":"\u003cp\u003eSynthesis of N-CQDs via Hydrothermal method (a); FTIR spectrum (b); and XRD pattern (c); TEM and HR-TEM (inset) images of N-CQDs (d); Particle size distribution of N-CQDs (e) of N-CQDs.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8105416/v1/d8596cb6ea8ad2c0b5a63b75.png"},{"id":96915283,"identity":"55b79a9e-b6c1-4227-bd49-f0a8a9e3d864","added_by":"auto","created_at":"2025-11-27 14:07:04","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":673954,"visible":true,"origin":"","legend":"\u003cp\u003eComposition of N-CQDs: XPS spectrum of N-CQDs (a); C1s spectrum of N-CQDs (b); N1s spectrum of N-CQDs (c); O1s spectrum (d) of N-CQDs.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8105416/v1/cbfee07f10dc198c5e6e18ea.png"},{"id":96746123,"identity":"11a3b1ee-b602-4805-b47a-a76cff373f2b","added_by":"auto","created_at":"2025-11-25 16:01:59","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":569194,"visible":true,"origin":"","legend":"\u003cp\u003eUV absorption spectra of N-CQDs and N-CQDs with Fe\u003csup\u003e3+\u003c/sup\u003e(a); Changes in emission of N-CQDs with excitation wavelength (b); The effect of various concentrations of Fe\u003csup\u003e3+\u003c/sup\u003e ions on the intensity of N-CQDs (c); The interference of different metal ions, anions and some common interfering species (d); The curve of F/F\u003csub\u003e0\u003c/sub\u003e with various concentration of Fe\u003csup\u003e3+\u003c/sup\u003e ions (e); The linear relationship between F/F\u003csub\u003e0\u003c/sub\u003e and various concentration of Fe\u003csup\u003e3+\u003c/sup\u003e ions (f).\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8105416/v1/e2fe1edcfe10ddec48b3f4e2.png"},{"id":96915010,"identity":"c91c6f06-dc51-48a0-b34b-0b6091549463","added_by":"auto","created_at":"2025-11-27 14:06:43","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":371300,"visible":true,"origin":"","legend":"\u003cp\u003eThe Stability tests of N-CQDs: Emission variation of N-CQDs at different pH (a); Emission variation of N-CQDs at different time intervals (b); Emission variation of N-CQDs at different concentration of salt solution (c); ON-OFF-ON Mechanism of CQDs on adding Fe\u003csup\u003e3+\u003c/sup\u003e and Ascorbic Acid (d).\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8105416/v1/85d565bde85e552df40f1b6e.png"},{"id":96746128,"identity":"800d0127-7ded-44a8-99c1-4511b1d9056c","added_by":"auto","created_at":"2025-11-25 16:01:59","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":494921,"visible":true,"origin":"","legend":"\u003cp\u003eMechanistic Insights into Reversible Switching.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-8105416/v1/090f1d329521ca70aa6b7034.png"},{"id":100614523,"identity":"06c288a4-a629-4df9-bcab-902d28457601","added_by":"auto","created_at":"2026-01-19 17:21:32","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4292371,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8105416/v1/0686bc6e-bd7e-4629-8415-a80c49ce3eda.pdf"},{"id":96746124,"identity":"0945123a-a41c-49fd-a1ff-a2db61ad7e28","added_by":"auto","created_at":"2025-11-25 16:01:59","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":828901,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryFile.docx","url":"https://assets-eu.researchsquare.com/files/rs-8105416/v1/a82c31fa910d387f76bfb490.docx"},{"id":96914245,"identity":"22bd4b4a-e5ae-49fb-8df9-a6eba09d3349","added_by":"auto","created_at":"2025-11-27 14:05:39","extension":"png","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":211768,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eGraphical Abstract\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"GA.png","url":"https://assets-eu.researchsquare.com/files/rs-8105416/v1/15e9abd1d32d6c77d19dc835.png"}],"financialInterests":"No competing interests reported.","formattedTitle":"\u003cp\u003eGreen Synthesis of a Reversible “ON-OFF-ON” Fluorescent sensor for Fe\u003csup\u003e3+\u003c/sup\u003e Using Licorice-Derived N-Doped Carbon Dots\u003c/p\u003e","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eFerric ions (Fe\u003csup\u003e3+\u003c/sup\u003e) are essential transition metal species, universally distributed in biological systems and environmental matrices, where they support numerous critical processes, such as oxygen transport via hemoglobin, cellular respiration, electron transport reactions, and DNA and RNA synthesis [\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Maintaining an appropriate balance of iron is essential, as both deficiency and excess can cause severe health issues: iron deficiency leads to anemia while increased concentration induces oxidative damage to vital organs (e.g., liver, kidneys) and causes neurodegenerative disorders (Alzheimer\u0026rsquo;s and Parkinson\u0026rsquo;s diseases) and carcinogenesis [\u003cspan additionalcitationids=\"CR5\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. According to U.S Environmental Protection Agency (EPA), the maximum permissible limit of iron in drinking water is 5.36 \u0026micro;M, underscoring the need for careful concentration monitoring [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Elevated concentration of Fe\u003csup\u003e3+\u003c/sup\u003e in environment, originating from industrial discharges, pharmaceutical waste, and corrosion of iron containing materials, is toxic to aquatic ecosystem, disrupt ecological balance and contaminate water resources [\u003cspan additionalcitationids=\"CR9\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Iron has dual a nature as being an essential micronutrient at physiologically normal concentration and toxic at elevated level, demands the development of selective and hypersensitive detection techniques capable of quantifying Fe\u003csup\u003e3+\u003c/sup\u003e concentration in biological and environmental samples across a broad range.\u003c/p\u003e\u003cp\u003eConventional techniques for detection of Fe\u003csup\u003e3+\u003c/sup\u003e, including inductively coupled plasma-mass spectrometry (ICP-MS), inductively coupled plasma-optical emission spectrometry (ICP-OES) and atomic absorption spectroscopy (AAS), offer high accuracy but have many critical limitations: difficult and complex sample pretreatment, reliance on sophisticated and costly instruments, prolonged analysis times, the need for highly trained personnel, and limited applicability for rapid real time monitoring [\u003cspan additionalcitationids=\"CR12\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. These practical constraints have inspired to the development ofsensing platforms with simpler, faster, cost effective, portable, and real time sensing technologies.\u003c/p\u003e\u003cp\u003eFluorescent nanomaterials have emerged as a highly promising platform due to their distinct advantages, including exceptional sensitivity, high selectivity, rapid response time, simplicity, and potential for real time analysis [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Among the various fluorescent nanosensors, carbon quantum dots (CQDs) have gathered much attention. CQDs are carbon-based nanomaterials, less than 10 nm in diameter are distinguished by their excellent photostability, low toxicity, bright photoluminescence, high water solubility, biocompatibility, facile synthesis, ease of functionalization, and tunable optical properties [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. These characteristics make CQDs ideal candidates for the development of advanced chemo sensors and biosensors, particularly for metal ion detection.\u003c/p\u003e\u003cp\u003eA significant trend in the synthesis of CQDs emphasizes the development of sustainable, \u0026ldquo;green\u0026rdquo; methods prioritize the use of natural, low-cost, readily available, and biomass-derived precursors. This approach aligns with principles of sustainable chemistry, yielding CQDs with unique functional groups inherited the complex composition of biomass material, enhancing their sensing performance and biocompatibility [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. Additionally, nitrogen doping (N-doping) is a widely implemented and effective approach to alter the electronic structure and enhance the PL quantum yield (QY) of CQDs. The incorporation of nitrogen into lattice of CQDs creates active sites, improves charge carrier mobility and passivates surface defects, often leading to enhanced optical and sensing properties [\u003cspan additionalcitationids=\"CR21\" citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. The use of aromatic amines for N-doping can lead to the formation of distinct N-configurations (e.g., pyridinic, pyrrolic nitrogen) within the CQDs framework, which can influence their photophysical properties and their interaction with analytes [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. These types of N-configurations may produce specific coordination sites for metallic ions or alter the electronic properties to make it favorable for sensing mechanism [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThe \u0026ldquo;ON-OFF-ON\u0026rdquo; sensing strategy provides an advanced platform for analyte detection, offering enhanced selectivity and reliability through a reversible response mechanism [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. In this scheme, the intrinsic fluorescence of the probe (ON state) is quenched by the target analyte (OFF state). Subsequently, the introduction of a specific reagent reverses the quenching effect, restoring the fluorescence (second \u0026ldquo;ON\u0026rdquo; state). While many \u0026ldquo;ON-OFF-ON\u0026rdquo; systems are designed for the sequential detection of two different analytes, the present work uniquely employs ascorbic acid as a chemical stimulus (reducing agent) to convert Fe\u003csup\u003e3+\u003c/sup\u003e to Fe\u003csup\u003e2+\u003c/sup\u003e, thereby demonstrating the reversibility and specificity of the Fe\u003csup\u003e3+\u003c/sup\u003e interaction, rather than being used for the detection of ascorbic acid itself.\u003c/p\u003e\u003cp\u003eThis research focuses on the development of a novel N-doped CQDs (N-CQDs) via a facile, one-pot green hydrothermal synthesis, employing licorice powder (a novel biomass precursor) as the carbon source and \u003cem\u003ep\u003c/em\u003e-phenylenediamine as the nitrogen dopant. Then the synthesized N-CQDs were comprehensively characterized to elucidate the structural, morphological, and optical properties. The core objective is to explore their application as a highly selective and sensitive fluorescent probe for Fe\u003csup\u003e3+\u003c/sup\u003e. A distinctive aspect of this work is the demonstration of an \u0026ldquo;ON-OFF-ON\u0026rdquo; fluorescence switching mechanism: Fe\u003csup\u003e3+\u003c/sup\u003e ions quench the N-CQDs fluorescence (\u0026ldquo;OFF\u0026rdquo; state), and the subsequent addition of ascorbic acid (AA) effectively restores the fluorescence (\u0026ldquo;ON\u0026rdquo; state) by reducing Fe\u003csup\u003e3+\u003c/sup\u003e to Fe\u003csup\u003e2+\u003c/sup\u003e. The practical viability of this N-CQD sensor is thoroughly assessed by quantifying Fe\u003csup\u003e3+\u003c/sup\u003e levels in complex real-world matrices, namely beer and human blood serum. This study presents a unique combination of sustainable precursors for N-CQDs synthesis, applied within a specific \u0026ldquo;ON-OFF-ON\u0026rdquo; Fe\u003csup\u003e3+\u003c/sup\u003e sensing strategy, offering potential advancements in terms of environmental benignity, cost effectiveness, and analytical efficacy for Fe\u003csup\u003e3+\u003c/sup\u003e detection.\u003c/p\u003e"},{"header":"2. Experimental Section","content":"\u003cp\u003eLicorice powder of food grade was procured from Xinjiang, China. \u003cem\u003ep\u003c/em\u003e-Phenylenediamine (PPD, Sigma-Aldrich, purity\u0026thinsp;\u0026ge;\u0026thinsp;99%), ferric chloride hexahydrate (FeCl\u003csub\u003e3\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO), Sinopharm, purity\u0026thinsp;\u0026ge;\u0026thinsp;98%), ascorbic acid (AA, Sinopharm, purity\u0026thinsp;\u0026ge;\u0026thinsp;99%), and various metal ion salts including KCl, NaCl, CaCl\u003csub\u003e2\u003c/sub\u003e, MgCl\u003csub\u003e2\u003c/sub\u003e, FeCl\u003csub\u003e2\u003c/sub\u003e, CuCl\u003csub\u003e2\u003c/sub\u003e, ZnCl\u003csub\u003e2\u003c/sub\u003e, MnCl\u003csub\u003e2\u003c/sub\u003e, NiSO\u003csub\u003e4\u003c/sub\u003e, Co(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e, PbCl\u003csub\u003e2\u003c/sub\u003e, CdCl\u003csub\u003e2\u003c/sub\u003e, HgCl\u003csub\u003e2\u003c/sub\u003e and AlCl\u003csub\u003e3\u003c/sub\u003e (purity\u0026thinsp;\u0026ge;\u0026thinsp;98%) were obtained from Sigma-Aldrich. Additionally, glucose (C\u003csub\u003e6\u003c/sub\u003eH\u003csub\u003e12\u003c/sub\u003eO\u003csub\u003e6\u003c/sub\u003e, Millipore Sigma, purity\u0026thinsp;\u0026ge;\u0026thinsp;99.5%), urea (NH\u003csub\u003e2\u003c/sub\u003eCONH\u003csub\u003e2\u003c/sub\u003e, Merck, purity\u0026thinsp;\u0026ge;\u0026thinsp;99%), trichloroacetic acid (TCA, purity\u0026thinsp;\u0026ge;\u0026thinsp;99%) and nitric acid (ACS reagent, 70%) were sourced from the same supplier.\u003c/p\u003e\u003cp\u003eAll solutions were prepared using ultrapure water generated by a Milli-Q (Hyper PureX) purification system. Buffer solutions, such as Tris-HCl and phosphate buffer, were formulated using analytical grade reagents. Beer sample was purchased from a local supermarket. Human blood serum samples were obtained with prior ethical approval. Dialysis membranes Spectra/Por with a molecular weight cut-off (MWCO) of 500\u0026ndash;1000 Da were procured from Spectrum Labs.\u003c/p\u003e\u003cp\u003eN-CQDs were synthesized via a one-pot hydrothermal method. In a typical synthesis procedure, 1.0 g of licorice powder and 0.05 g of \u003cem\u003ep\u003c/em\u003e-phenylenediamine were dispersed in 30 mL ultrapure water. The licorice (1.0g) and PPD (0.05g) ratio was optimized by testing three different mass ratios (1.0:0.02, 1.0:0.05, 1.0:0.1 g/g) and evaluating the key metrics: relative fluorescence quenching of N-CQDs with Fe\u003csup\u003e3+\u003c/sup\u003e. To achieve a homogeneous dispersion, the mixture was ultrasonicated for 10 minutes. Subsequently, the resulting suspension was transferred into a 50 mL Teflon-lined stainless-steel autoclave. The autoclave was sealed and heated in an oven at 180\u0026deg;C for 10 hours. After the reaction was completed, the autoclave was cooled naturally to room temperature.\u003c/p\u003e\u003cp\u003eThe crude product, a dark brown solution, underwent a multi-step purification process. First, the solution was centrifuged at 10,000 rpm for 20 minutes to remove large unreacted particles. The supernatant was collected and then filtered through a 0.22 \u0026micro;m syringe filter to eliminate finer particulate matter. Subsequently, the filtered solution was dialyzed against ultrapure water using a dialysis membrane (1000 Da MWCO) for 24 hours. The dialysis water was changed every 6 hours to ensure the efficient removal of unreacted precursors and small molecular byproducts. The purified N-CQDs aqueous solution was dried at 60\u0026deg;C to obtain solid N-CQDs.\u003c/p\u003e\u003cp\u003eThe synthesized N-CQDs were comprehensively characterized using a range of analytical techniques, each providing unique insights into the structure, properties, and performance of the materials. UV-Visible Absorption spectra of the N-CQDs were recorded on a (UV-2600) spectrophotometer within a wavelength range of 200\u0026ndash;800 nm, with ultrapure water serving as the blank reference. Fluorescence excitation and emission spectra were recorded using (PerkinElmer-400 spectrum) spectrofluorometer. The slit widths for excitation and emission were set to 3 nm. All measurements were conducted at room temperature (25\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u0026deg;C) in ultrapure water. Fourier transform infrared FTIR spectra were recorded in the range of 400\u0026ndash;4000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, enabling the identification of surface functional groups on the N-CQDs. X-ray diffraction XRD patterns were collected on a diffractometer over a 2θ range of 10\u0026ndash;80\u0026deg; with a step size of 0.02\u0026deg; to determine the crystallinity of the N-CQDs.\u003c/p\u003e\u003cp\u003eX-ray Photoeclectron Spectroscopy XPS measurements were performed on a Thermo K-Alpha XPS spectrometer to obtain survey scans and high-resolution spectra for the C1s, N1s, and O1s regions. Binding energies were calibrated using the adventitious C1s peak at 284.8 eV to determine the elemental composition and elemental chemical states in the N-CQDs. The morphology, size, and microstructure of the N-CQDs were examined using a transmission electron microscope (TEM) JEOL JEM-F200. Samples were prepared by drop-casting a dilute aqueous dispersion of N-CQDs onto a carbon-coated copper grid, followed by drying under ambient conditions. The particle size distribution was determined by measuring at least 100 particles from multiple Transmission electron microscopy TEM images using ImageJ 1.54g software. High-resolution TEM (HRTEM) images were acquired to observe lattice fringes, and selected area electron diffraction (SAED) patterns were recorded to assess the crystallinity of the N-CQDs.\u003c/p\u003e\u003cp\u003eAll fluorescence measurements for Fe\u003csup\u003e3+\u003c/sup\u003e sensing were carried out at room temperature (25\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u0026deg;C). To a fixed volume of N-CQD solution 3 mL, incremental aliquots of a standard Fe\u003csup\u003e3+\u003c/sup\u003e stock solution was added, resulting in final Fe\u003csup\u003e3+\u003c/sup\u003e concentrations of 0, 2.5, 5, 10, 20, 50, 100, 200, and 250 \u0026micro;M. After each addition, the solution was gently mixed and allowed to incubate for 3 minutes to ensure complete interaction between the N-CQDs and Fe\u003csup\u003e3+\u003c/sup\u003e before fluorescence measurement. Fluorescence emission spectra were recorded with excitation at 320 nm, and the emission intensity at 436 nm was monitored. The fluorescence quenching efficiency was typically expressed as F/F\u003csub\u003e0\u003c/sub\u003e, where F\u003csub\u003e0\u003c/sub\u003e is the initial fluorescence intensity of N-CQDs in the absence of Fe\u003csup\u003e3+\u003c/sup\u003e, and F is the fluorescence intensity in the presence of Fe\u003csup\u003e3+\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eThe linear range for Fe\u0026sup3;⁺ detection was determined from the linear portion of the calibration curve, which plotted F/F\u003csub\u003e0\u003c/sub\u003e against Fe\u003csup\u003e3+\u003c/sup\u003e concentration. The LOD was calculated using the formula LOD\u0026thinsp;=\u0026thinsp;3σ/S, where σ is the standard deviation of the fluorescence intensity of blank N-CQD solutions, and S is the slope of the linear calibration curve in the low concentration range.\u003c/p\u003e\u003cp\u003eThe selectivity of the N-CQD probe for Fe\u003csup\u003e3+\u003c/sup\u003e was evaluated by comparing its fluorescence response to Fe\u003csup\u003e3+\u003c/sup\u003e with that to a range of other potentially interfering metal ions. These ions included alkali metals (K\u003csup\u003e+\u003c/sup\u003e, Na⁺), alkaline earth metals (Ca\u003csup\u003e2+\u003c/sup\u003e, Mg\u003csup\u003e2+\u003c/sup\u003e), transition metals (Fe\u003csup\u003e2+\u003c/sup\u003e, Cu\u003csup\u003e2+\u003c/sup\u003e, Zn\u003csup\u003e2+\u003c/sup\u003e, Mn\u003csup\u003e2+\u003c/sup\u003e, Ni\u003csup\u003e2+\u003c/sup\u003e, Co\u003csup\u003e2+\u003c/sup\u003e, Pb\u003csup\u003e2+\u003c/sup\u003e, Cd\u003csup\u003e2+\u003c/sup\u003e, Hg\u003csup\u003e2+\u003c/sup\u003e), other common cation (Al\u003csup\u003e3+\u003c/sup\u003e), common anions (SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e and NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e) and common interfering substances like glucose, urea and ascorbic acid. The concentration of interfering ions was typically set at 10-fold excess relative to the Fe\u003csup\u003e3+\u003c/sup\u003e concentration used within the linear range. The fluorescence intensity of the N-CQD solution was measured after the addition of each ion.\u003c/p\u003e\u003cp\u003eThe effect of pH on the fluorescence intensity of N-CQDs was investigated over a pH range of 1\u0026ndash;14. Solutions were incubated for 3 minutes at each pH before fluorescence measurement. The photostability of the N-CQDs was assessed by continuously putting the N-CQD solution in the visible light for an extended period. This ensures reliable signal acquisition during fluorescence measurements. Fluorescence intensity at 436 nm was monitored at regular time intervals. The influence of ionic strength on the N-CQD fluorescence was studied by measuring the fluorescence intensity in the presence of varying concentrations of NaCl 10\u0026ndash;200 mM.\u003c/p\u003e\u003cp\u003eThe \u0026ldquo;ON-OFF-ON\u0026rdquo; fluorescence switching behavior was investigated through the following steps: The fluorescence emission spectrum of the N-CQDs solution was recorded with excitation at 320 nm, and the emission was monitored at 436 nm. A specific concentration of Fe\u003csup\u003e3+\u003c/sup\u003e solution was added to the N-CQDs solution. The mixture was incubated for 5 minutes, and then the fluorescence spectrum was recorded. A solution of ascorbic acid (AA) with a final concentration of 10 mM was added to the N-CQDs-Fe\u003csup\u003e3+\u003c/sup\u003e mixture. After a 5-minute incubation, the fluorescence spectrum was recorded again to observe the fluorescence recovery.\u003c/p\u003e\u003cp\u003eThe practical applicability of the N-CQD sensor for Fe\u003csup\u003e3+\u003c/sup\u003e detection was evaluated in beer and human blood serum samples using the standard addition method. The beer sample was first degassed by sonication for 15 minutes to remove dissolved CO₂. Subsequently, the degassed beer was centrifuged at 5000 rpm for 10 minutes and filtered through a 0.22 \u0026micro;m syringe filter to eliminate any particulate matter [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. The clear filtrate was then diluted 10-fold with ultrapure water for analysis. Deproteinization of blood serum samples was by vertexing the mixture for 1 minute and then centrifuged at 12,000 rpm for 15 minutes to precipitate proteins. The clear supernatant was carefully collected, and its pH was adjusted to the optimal condition for sensing. The supernatant was then diluted with ultrapure water prior to Fe\u003csup\u003e3+\u003c/sup\u003e determination [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThe prepared real sample solutions were first analyzed for their natural Fe\u003csup\u003e3+\u003c/sup\u003e content using the N-CQDs sensor. Subsequently, these samples were spiked with known concentrations of Fe\u003csup\u003e3+\u003c/sup\u003e standard solution. The spiked samples were then analyzed again using the N-CQDs sensor. The recovery was calculated using the formula: Recovery (%) = [(Fe\u003csup\u003e3+\u003c/sup\u003e detected in spiked sample\u0026ndash; Fe\u003csup\u003e3+\u003c/sup\u003edetected in unspiked sample)/Fe\u003csup\u003e3+\u003c/sup\u003e added] \u0026times; 100%. Measurements were performed in triplicate (n\u0026thinsp;=\u0026thinsp;3) to determine the Relative Standard Deviation (RSD, %).\u003c/p\u003e"},{"header":"3. Results and Discussion","content":"\u003cp\u003eThe N-CQDs were successfully synthesized via a one-pot hydrothermal method, utilizing licorice powder as a green carbon source and \u003cem\u003ep\u003c/em\u003e-phenylenediamine for nitrogen doping as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e\u003cb\u003e(a)\u003c/b\u003e. This synthesis route offers advantages in terms of simplicity, cost-effectiveness, and environmental friendliness.\u003c/p\u003e\u003cp\u003eThe XRD pattern of the synthesized N-CQDs \u003cb\u003e(\u003c/b\u003eFig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e\u003cb\u003e(b))\u003c/b\u003e exhibited a broad peak centered approximately at 2θ values within the range of 20\u0026ndash;26\u0026deg;. This peak corresponds to the (002) graphitic plane, typically associated with amorphous carbonaceous materials [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. The broad peak indicates the presence of small graphitic domains and a high degree of disorder in the N-CQDs structure. These features are consistent with carbon quantum dots synthesized from biomass and amine precursors through hydrothermal carbonization [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. The calculated interlayer d-spacing was larger than that of pristine graphite (0.34 nm), suggesting that the incorporation of heteroatoms (nitrogen and oxygen) and the presence of functional groups have disrupted the regular graphitic stacking. This unique structural feature, characterized by its amorphous nature and abundant surface defects, likely contributes significantly to the photoluminescence and sensing capabilities of the N-CQDs.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThen the FTIR spectroscopy was employed to characterize the surface functional groups of the N-CQDs [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. The FTIR spectrum \u003cb\u003e(\u003c/b\u003eFig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e\u003cb\u003e(c))\u003c/b\u003e displayed several absorption bands. For example, a prominent broad band centered around at 3396 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e was attributed to the stretching vibrations of O-H and N-H. Peaks in the region of 2928 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e corresponded to the C-H stretching vibrations of aliphatic groups. Strong absorptions at 1648 and 1602 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e were indicative of C\u0026thinsp;=\u0026thinsp;O stretching vibrations, likely originating from carboxyl functional groups. The presence of C\u0026thinsp;=\u0026thinsp;C and/or C\u0026thinsp;=\u0026thinsp;N stretching vibrations from aromatic rings and imine structures was evident around 1600\u0026thinsp;\u0026minus;\u0026thinsp;1500 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. C-N stretching vibrations from amines were observed in the 1398 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e range, confirming the successful incorporation of nitrogen from \u003cem\u003ep\u003c/em\u003e-PDA. Additionally, C-O stretching vibrations from hydroxyl groups were detected between 1214 and 1069 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. These diverse functional groups on the N-CQD surface are crucial for its aqueous dispersibility and play a key role in the interaction and binding with Fe\u003csup\u003e3+\u003c/sup\u003e ions, which is essential for the sensing application.\u003c/p\u003e\u003cp\u003eThe transmission electron microscopy (TEM) characterization of the as-prepared N-CQDsprovides valuable insights into their morphological and structural features. As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e\u003cb\u003e(d)\u003c/b\u003e, the representative TEM image clearly shows that the CQDs are uniformly dispersed in the sample, without any significant agglomeration. The discrete nature of the CQDs, as evidenced by the clear separation between individual particles, indicates a high-quality synthesis process that effectively controls the growth and assembly of the quantum dots, minimizing intermolecular forces that could lead to aggregation. The inset of Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e\u003cb\u003e(d)\u003c/b\u003e showcases a HRTEM image of a single CQD, offering a magnified and detailed view of its internal structure. The presence of distinct lattice fringes is a key observation, as it provides conclusive evidence for the crystalline nature of the CQD. Precise measurement of the interplanar spacing (d-spacing) reveals a value of approximately 0.21 nm. This specific d-spacing value is characteristic of the (102) lattice planes of graphitic carbon [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e\u003cb\u003e(e)\u003c/b\u003e illustrates the size distribution histogram of the synthesized CQDs, obtained from statistical analysis of a large number of particles observed in multiple TEM images. The histogram reveals a relatively narrow size distribution, indicating precise control over synthesis conditions. The size of the majority of the CQDs falls within the range of 2.5 nm to 3.5 nm, demonstrating a high level of consistency in the synthesis yield. Upon fitting a distribution curve to the histogram, a peak intensity is observed, centered approximately 2.7\u0026ndash;2.8 nm. This peak represents the most frequently occurring diameter among the synthesized CQDs, indicating that the majority of the quantum dots have a diameter within this range.\u003c/p\u003e\u003cp\u003eThe surface elemental composition of N-CQDs was characterized by XPS, confirming the successful synthesis of a carbonaceous framework with intentional heteroatom doping [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. The XPS spectrum \u003cb\u003e(\u003c/b\u003eFig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e\u003cb\u003e(a))\u003c/b\u003e exhibited distinct peaks for C1s (284.8 eV), O1s (532.0 eV), and N1s (399.5 eV), with quantitative analysis revealing atomic percentages of 70.89% C, 25.41% O and 3.71% N. The nitrogen content demonstrating controlled incorporation via the hydrothermal synthesis route. The high oxygen content (25.41%) indicates extensive surface functionalization with hydrophilic groups.\u003c/p\u003e\u003cp\u003eThe deconvolution of the C1s spectrum \u003cb\u003e(\u003c/b\u003eFig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e\u003cb\u003e(b))\u003c/b\u003e into three Gaussian-Lorentzian components revealed a complex bonding environment. The dominant peak at 284.8 eV corresponds to sp\u003csup\u003e2\u003c/sup\u003e-hybridized carbon in graphitic domains (C\u0026thinsp;=\u0026thinsp;C), with a shoulder at 285.2 eV indicating sp\u003csup\u003e3\u003c/sup\u003e-hybridized carbon (C-C) from lattice defects. A peak at 286.2 eV is assigned to C-O bonds in hydroxyl groups, while 287.3 eV corresponds to C\u0026thinsp;=\u0026thinsp;O (carbonyl). The N1s spectrum \u003cb\u003e(\u003c/b\u003eFig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e\u003cb\u003e(c))\u003c/b\u003e was deconvoluted into three components, each representing distinct doping configurations. A dominant peak at 399.8 eV is assigned to pyridinic-N (N-6), where nitrogen replaces carbon in the graphene lattice, contributing lone-pair electrons that enhance electron transfer. A shoulder at 400.1 eV corresponds to pyrrolic-N (N-5), embedded in rings and responsible for pH-dependent fluorescence. A minor peak at 402.1 eV is attributed to graphitic-N (N-Q), which improves electrical conductivity by donating π-electrons. The O1s spectrum \u003cb\u003e(\u003c/b\u003eFig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e\u003cb\u003e(d))\u003c/b\u003e resolved into two peaks, corroborating C1s findings. A major peak at 531.6 eV arises from C\u0026thinsp;=\u0026thinsp;O (carbonyl), while 533.0 eV corresponds to C-O (hydroxyl). The atomic O/C ratio (0.36) exceeds that of pristine graphene (0.05), confirming extensive oxidation.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe optical characteristics of the N-CQDs were investigated using UV-Vis absorption and fluorescence spectroscopy. The UV-Vis absorption spectra of the synthesized N-CQDs in the absence and presence of ferric ions Fe\u003csup\u003e3+\u003c/sup\u003e were investigated, as depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e\u003cb\u003e(a)\u003c/b\u003e. A distinct and intense absorption peak at approximately 210\u0026ndash;220 nm is attributed to π-π\u003csup\u003e*\u003c/sup\u003e electronic transitions. This peak originates from the sp\u003csup\u003e2\u003c/sup\u003e-hybridized carbon atoms that form the conjugated graphitic core of N-CQDs. Additionally, a broader and less intense absorption feature appears between 260 and 280 nm, corresponding to n-π\u003csup\u003e*\u003c/sup\u003e transitions [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. These transitions involve non-bonding electrons localized on heteroatoms and surface functional groups within N-CQDs. For N-CQDs, nitrogen doping introduces various nitrogen-containing functional groups, such as pyridinic-N, pyrrolic-N, and graphitic-N, in addition to oxygen-based functional groups like carbonyl (C\u0026thinsp;=\u0026thinsp;O) and hydroxyl (O-H). The presence of these surface-associated transitions indicates that the N-CQDs possess a rich array of reactive sites, which are essential for their interaction with external analytes, such as ferric ions.\u003c/p\u003e\u003cp\u003eUpon the addition of ferric ions Fe\u003csup\u003e3+\u003c/sup\u003e, the UV-Vis absorption spectrum of N-CQDs (red curve in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e\u003cb\u003e(a)\u003c/b\u003e) undergoes a profound transformation, which is complicatedly linked to the observed fluorescence quenching phenomenon. The significant increase in absorption intensity, particularly in the 250\u0026ndash;350 nm region, with a notable enhancement around 280 nm, serves as a direct spectroscopic signature of the interaction between N-CQDs and Fe\u003csup\u003e3+\u003c/sup\u003e ions.\u003c/p\u003e\u003cp\u003eA systematic study was conducted \u003cb\u003e(\u003c/b\u003eFig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e\u003cb\u003e(b))\u003c/b\u003e on the photoluminescence properties of synthesized carbon quantum dots (N-CQDs) by measuring their emission spectra within the excitation wavelength range of 300\u0026ndash;450 nm. The results revealed a distinct excitation-dependent luminescence characteristic of the CQDs [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. When the excitation wavelength was 320 nm, the CQDs exhibited the best luminescence performance, presenting a sharp and intense emission peak at 436 nm. The width of this peak, measured to be 25 nm, indicated high spectral purity and excellent luminescence efficiency. As the excitation wavelength deviated from 320 nm, the brightness of the emission peak decreased, and its position shifted towards longer wavelengths. For example, at 300 nm excitation, the peak brightness decreased by 28% compared to that at 320 nm excitation; at 450 nm excitation, the peak brightness decreased by 42%. This spectral change was attributed to the presence of multiple luminescent centers on the surface of CQDs, the influence of their size on luminescence, and the alteration of the internal electronic state by the surface chemical structure.\u003c/p\u003e\u003cp\u003eCompared with previously synthesized CQDs using similar or different methods, our CQDs show superior luminescence performance [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. This enhanced performance was due to the unique synthesis method, which led to more uniform CQD sizes, more regular surface structures, and higher internal crystallinity, reducing non-luminescent energy loss processes. These CQDs emit intense and tunable blue light under 320 nm excitation, showing great potential in advanced optical applications such as bioimaging, analyte detection, optoelectronic device fabrication, and anti-counterfeiting fluorescent inks.\u003c/p\u003e\u003cp\u003eThe quantitative sensing capability of N-CQDs toward Fe\u003csup\u003e3+\u003c/sup\u003e ions were rigorously evaluated by monitoring changes in their fluorescence intensity as a function of increasing Fe\u003csup\u003e3+\u003c/sup\u003e concentration. The photoluminescent property of N-CQDs was visually confirmed by photographs of their aqueous solution under UV light in the absence and presence of Fe\u003csup\u003e3+\u003c/sup\u003e and other analytes in \u003cb\u003eFigure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e\u003c/b\u003e.\u003c/p\u003e\u003cp\u003eThe quantitative sensing performance of N-CQDs toward Fe\u003csup\u003e3+\u003c/sup\u003e was systematically evaluated by monitoring fluorescence intensity changes as a function of Fe\u003csup\u003e3+\u003c/sup\u003e concentration, establishing their potential for analytical applications [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e\u003cb\u003e(c)\u003c/b\u003e illustrates the PL emission spectra of N-CQDs upon stepwise addition of Fe\u003csup\u003e3+\u003c/sup\u003e ions, excited at the optimal wavelength of 320 nm. The initial spectrum exhibits a robust blue emission peak centered at 436 nm, characteristic of the N-CQDs\u0026rsquo; intrinsic photoluminescence. As Fe\u003csup\u003e3+\u003c/sup\u003e concentration was increased from 0 to 250 \u0026micro;M, a monotonic decrease in fluorescence intensity was observed without discernible shifts in the emission peak position. This behavior indicates that Fe\u003csup\u003e3+\u003c/sup\u003e primarily quenches the radiative recombination processes of N-CQDs rather than altering their electronic structure or chromophoric environment. Further illustrating this trend, Figure shows the fluorescence intensity (at 436 nm) as a function of Fe\u003csup\u003e3+\u003c/sup\u003e concentrations (0-250 \u0026micro;M). The plot depicted a concentration dependent decrease in N-CQDs\u0026rsquo; fluorescence, with quenching occurring rapidly at lower concentrations and tending to plateau at higher levels. This behavior suggested potential saturation of binding sites or a shift in the dominant quenching mechanism at elevated analyte concentrations.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eSubsequently, the selectivity and stability of the N-CQD sensor were tested. As illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e\u003cb\u003e(d)\u003c/b\u003e, the N-CQDs exhibited exceptional discrimination toward Fe\u003csup\u003e3+\u003c/sup\u003e: The blank control displayed an F/F₀ ratio of 1.0, reflecting the unquenched PL of N-CQDs. In contrast, the addition of Fe\u003csup\u003e3+\u003c/sup\u003e induced a profound fluorescence quenching, with the F/F₀ ratio dropping to 0.07 (corresponding to a quenching efficiency of 93%). This drastic reduction stems from the synergistic effects of ground-state complexation (via N/O-rich surface ligands) and photoinduced electron transfer (PET), as corroborated by UV-Vis absorption studies \u003cb\u003e(\u003c/b\u003eFig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e\u003cb\u003e(a))\u003c/b\u003e. The high quenching efficiency underscores the strong affinity between Fe\u003csup\u003e3+\u003c/sup\u003e and N-CQDs, attributed to the Lewis acid-base interaction between Fe\u003csup\u003e3+\u003c/sup\u003e and electron-donating groups (e.g., pyridinic-N, carboxyl) on the CQD surface.\u003c/p\u003e\u003cp\u003eAlkali (K\u003csup\u003e+\u003c/sup\u003e, Na\u003csup\u003e+\u003c/sup\u003e), alkaline earth (Ca\u003csup\u003e2+\u003c/sup\u003e, Mg\u003csup\u003e2+\u003c/sup\u003e), transition (Zn\u003csup\u003e2+\u003c/sup\u003e, Ni\u003csup\u003e2+\u003c/sup\u003e, Co\u003csup\u003e2+\u003c/sup\u003e, Cd\u003csup\u003e2+\u003c/sup\u003e, Mn\u003csup\u003e2+\u003c/sup\u003e, Cu\u003csup\u003e2+\u003c/sup\u003e, Pb\u003csup\u003e2+\u003c/sup\u003e, Hg\u003csup\u003e2+\u003c/sup\u003e), and trivalent (Al\u003csup\u003e3+\u003c/sup\u003e) ions induced minimal PL changes, with F/F₀ ratios ranging from 0.96 to 1.03. This insensitivity arises from the weaker coordination ability of these ions compared to Fe\u003csup\u003e3+\u003c/sup\u003e. Divalent cations (e.g., Cu\u003csup\u003e2+\u003c/sup\u003e) showed\u0026thinsp;\u0026lt;\u0026thinsp;5% quenching, while trivalent Al\u003csup\u003e3+\u003c/sup\u003e (often a competitive interferent) exhibited an F/F₀ of 0.99, confirming the N-CQDs\u0026rsquo; preference for Fe\u003csup\u003e3+\u003c/sup\u003e. Common anions (SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e, NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e) and biologically relevant molecules (urea, glucose, ascorbic acid) at 250 \u0026micro;M induced F/F₀ ratios of 0.97 to 1.02, indicating minimal interaction with N-CQDs. This stability against anionic and biomolecular interference is critical for real-world applications, as it ensures reliable Fe\u003csup\u003e3+\u003c/sup\u003e detection in complex matrices.\u003c/p\u003e\u003cp\u003eA linear relationship between analyte concentration and sensor response was demonstrated for practical analytical applications. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e\u003cb\u003e(e and f)\u003c/b\u003e presents the calibration curve derived from fluorescence intensity versus Fe\u003csup\u003e3+\u003c/sup\u003e concentration, focusing on the lower concentration range for optimal sensitivity and linearity. In the range of 0 to 50 \u0026micro;M Fe\u003csup\u003e3+\u003c/sup\u003e, N-CQDs exhibited a good linear relationship between fluorescence intensity and Fe\u003csup\u003e3+\u003c/sup\u003e concentration [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e] with a calculated limit of detection (LOD) of 0.346 \u0026micro;M better than the LODs of other synthesized CQDs for ferric ions, as summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eThe Comparison of N-CQDs with other materials for Fe\u003csup\u003e3+\u003c/sup\u003e detection.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"6\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMethods of\u003c/p\u003e\u003cp\u003eSynthesis\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eSynthesized\u003c/p\u003e\u003cp\u003eMaterials\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eChemical Precursors\u003c/p\u003e\u003cp\u003eUsed\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eAnalytes Detected\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eLOD\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eRef.\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eHydrothermal Synthesis\u003c/p\u003e\u003cp\u003eHydrothermal Synthesis\u003c/p\u003e\u003cp\u003eHydrothermal Synthesis\u003c/p\u003e\u003cp\u003eHydrothermal Synthesis\u003c/p\u003e\u003cp\u003eHydrothermal Synthesis\u003c/p\u003e\u003cp\u003eSolvothermal Synthesis\u003c/p\u003e\u003cp\u003eMicrowave Synthesis\u003c/p\u003e\u003cp\u003eMicrowave Synthesis\u003c/p\u003e\u003cp\u003eHydrothermal Synthesis\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eC-dots\u003c/p\u003e\u003cp\u003eCDs\u003c/p\u003e\u003cp\u003eN-CDs\u003c/p\u003e\u003cp\u003eC-dots\u003c/p\u003e\u003cp\u003eN-CDs\u003c/p\u003e\u003cp\u003eB@HRCDs\u003c/p\u003e\u003cp\u003eNCDs\u003c/p\u003e\u003cp\u003eN-CDs\u003c/p\u003e\u003cp\u003eN-CQDs\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eZinc gluconate\u003c/p\u003e\u003cp\u003e\u003cem\u003eCitrus reticulata\u003c/em\u003e\u003c/p\u003e\u003cp\u003ePEI and Melamine\u003c/p\u003e\u003cp\u003eAdenosine and PPi\u003c/p\u003e\u003cp\u003eC. Acid and Melamine\u003c/p\u003e\u003cp\u003eChinese herbal residues\u003c/p\u003e\u003cp\u003eMalic acid and Urea\u003c/p\u003e\u003cp\u003eCitric acid and Urea\u003c/p\u003e\u003cp\u003eLicorice and PPD\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eFe\u003csup\u003e3+\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eFe\u003csup\u003e3+\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eFe\u003csup\u003e3+\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eFe\u003csup\u003e3+\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eFe\u003csup\u003e3+\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eFe\u003csup\u003e3+\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eFe\u003csup\u003e3+\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eFe\u003csup\u003e3+\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eFe\u003csup\u003e3+\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1.9 \u0026micro;M\u003c/p\u003e\u003cp\u003e1.11 \u0026micro;M\u003c/p\u003e\u003cp\u003e0.88 \u0026micro;M\u003c/p\u003e\u003cp\u003e0.9 \u0026micro;M\u003c/p\u003e\u003cp\u003e3.18\u0026nbsp;\u0026micro;M\u003c/p\u003e\u003cp\u003e1.08 \u0026micro;M\u003c/p\u003e\u003cp\u003e1.9 \u0026micro;M\u003c/p\u003e\u003cp\u003e1.0 \u0026micro;M\u003c/p\u003e\u003cp\u003e0.346 \u0026micro;M\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e[\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]\u003c/p\u003e\u003cp\u003e[\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]\u003c/p\u003e\u003cp\u003e[\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]\u003c/p\u003e\u003cp\u003e[\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]\u003c/p\u003e\u003cp\u003e[\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e]\u003c/p\u003e\u003cp\u003e[\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e]\u003c/p\u003e\u003cp\u003e[\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e]\u003c/p\u003e\u003cp\u003e[\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e]\u003c/p\u003e\u003cp\u003eThis Work\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eThe linear regression analysis yielded the Eq.\u0026nbsp;\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:\\text{y}\\:=\\:221.64718-1.91913\\text{x}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003ewhere \u003cem\u003ey\u003c/em\u003e is the fluorescence intensity (a.u.) and x are the Fe\u003csup\u003e3+\u003c/sup\u003e concentration (\u0026micro;M). The high coefficient of determination (R\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.99697) confirmed exceptional model fit to the experimental data.\u003c/p\u003e\u003cp\u003eDue to the quenching mechanism is a multi-faceted process, involving ground-state complexation, electron transfer, and potentially the inner filter effect, so its mechanism is detailed as follows.\u003c/p\u003e\u003cp\u003eAs Lewis acids, Fe\u003csup\u003e3+\u003c/sup\u003e ions have a strong affinity for electron-rich sites on the N-CQDs surface. The abundance of oxygen-containing functional groups and nitrogen heteroatoms on N-CQDs serves as an ideal platform for Fe\u003csup\u003e3+\u003c/sup\u003e chelation. These coordination interactions lead to the formation of a stable N-CQD-Fe\u003csup\u003e3+\u003c/sup\u003e complex in the ground state. This complexation event is spectroscopically evidenced by the red shift and intensity increase in the UV-Vis spectrum, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e\u003cb\u003e(a)\u003c/b\u003e. The formation of the complex disturbs the electronic environment of N-CQDs, leading to an altered energy levels and transition probabilities. From a quantum-mechanical perspective, complex formation modifies the electronic wave functions of N-CQDs, thereby facilitating non-radiative decay pathways. The excited-state energy of N-CQDs is dissipated through vibrational relaxation or intersystem crossing, resulting in static quenching. This process effectively removes the emissive N-CQDs from the solution, as they are converted into non-emissive N-CQD-Fe\u003csup\u003e3+\u003c/sup\u003e complexes [\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e, \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eIn addition to static quenching, photoinduced electron transfer (PET) contributes to the observed fluorescence quenching. PET occurs when electron transfer takes place between the excited-state N-CQDs and Fe\u003csup\u003e3+\u003c/sup\u003e ions, a process driven by the favorable redox potential difference between the two species. Upon excitation, N-CQDs populate their excited state, where they can act as electron donors due to their relatively low oxidation potential. Fe\u003csup\u003e3+\u003c/sup\u003e, with its high reduction potential, serves as an efficient electron acceptor. This electron transfer from excited N-CQDs to Fe\u003csup\u003e3+\u003c/sup\u003e diverts energy away from radiative decay channels, leading to fluorescence quenching [\u003cspan additionalcitationids=\"CR53\" citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e]. The enhanced absorbance in the UV-Vis spectrum, particularly in regions overlapping with the excitation wavelength of N-CQDs, supports the involvement of PET by indicating strengthened electronic interactions between N-CQDs and Fe\u003csup\u003e3+\u003c/sup\u003e upon complexation interactions that promote the electron transfer process. This dynamic process reduces the number of excited N-CQDs undergoing radiative emission, thereby contributing to the observed decrease in fluorescence intensity.\u003c/p\u003e\u003cp\u003eIn addition, the robustness of the N-CQD sensor was assessed under various environmental conditions.\u003c/p\u003e\u003cp\u003eN-CQDs exhibited stable fluorescence across a wide pH range, with optimal performance under neutral to slightly alkaline conditions (pH 7.0\u0026ndash;9.0), peaking between pH 7.0 and 8.0 \u003cb\u003e(\u003c/b\u003eFig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea\u003cb\u003e)\u003c/b\u003e. Fluorescence intensity increased gradually from pH 1 to 3, then decreased in strongly acidic (pH\u0026thinsp;\u0026lt;\u0026thinsp;3) or highly alkaline (pH\u0026thinsp;\u0026gt;\u0026thinsp;11) conditions, yet retained approximately 60\u0026ndash;70% of its maximum intensity even at pH 14. This behavior can be attributed to the protonation and deprotonation of surface functional groups (e.g., carboxyl, hydroxyl, nitrogen containing groups), which modulate surface charge, energy levels, and radiative recombination pathways [\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e, \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e]. The broad fluorescence stability within physiological (pH 6.5-8.0) and environmental (pH 5.0\u0026ndash;9.0) ranges supports the potential applications of N-CQDs in biological and aqueous systems.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eN-CQDs exhibited outstanding resistance to photobleaching and long-term degradation \u003cb\u003e(\u003c/b\u003eFig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb\u003cb\u003e).\u003c/b\u003e Fluorescence intensity remained nearly unchanged for the first 15 days, followed by a gradual decline over 180 days; after 6 months, approximately 90\u0026ndash;92% of initial intensity was retained [\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e, \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e]. The N-CQDs also exhibited excellent photostability under prolonged UV irradiation (365 nm, 10 W): after 120 mins, they retained 91% of initial fluorescence intensity \u003cb\u003e(Figure S2)\u003c/b\u003e. This high stability is attributed to the presence of graphitic core and effective surface passivation, which mitigate photo-oxidation and contribute to a prolonged shelf-life.\u003c/p\u003e\u003cp\u003eN-CQDs exhibited exceptional stability under high salt concentrations \u003cb\u003e(\u003c/b\u003eFig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec\u003cb\u003e)\u003c/b\u003e, a key characteristic for applications in biological or saline environments. Fluorescence intensity decreased only marginally with increasing NaCl concentration: at 200 mM, approximately 85\u0026ndash;90% of initial intensity (measured at 10 mM NaCl) was retained [\u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e, \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e]. This resistance to ionic strength-induced aggregation is attributed to robust surface passivation and the presence of hydrophilic functional groups, maintaining colloidal dispersion.\u003c/p\u003e\u003cp\u003eBeside the above, the fluorescence switching kinetics and mechanism were studied. Pristine N-CQDs exhibited intense fluorescence emission at 436 nm, with a peak intensity shown by the blue curve in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, defining the initial \u0026ldquo;ON\u0026rdquo; state. Upon addition of Fe\u003csup\u003e3+\u003c/sup\u003e ions, rapid and efficient fluorescence quenching occurred, with the emission intensity shown by the green curve, corresponding to a quenching efficiency of 93%. This abrupt transition to the \u0026ldquo;OFF\u0026rdquo; state underscored the strong interaction between N-CQDs and Fe\u003csup\u003e3+\u003c/sup\u003e, likely mediated by photoinduced electron transfer (PET) and the formation of a non-emissive N-CQD-Fe\u003csup\u003e3+\u003c/sup\u003e complex as mentioned before.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eSubsequent introduction of ascorbic acid to the N-CQDs-Fe\u003csup\u003e3+\u003c/sup\u003e system triggered a remarkable fluorescence recovery, with the emission intensity rebounding to 79% of the original level, shown by the red curve. Notably, the emission peak remained at 436 nm throughout the switching cycle, indicating that the N-CQD chromophore structure was preserved, and the mechanism involved reversible quantum yield modulation rather than structural degradation [\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e, \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e62\u003c/span\u003e]. The observed \u0026ldquo;On-Off-On\u0026rdquo; behavior was attributed to a redox-driven mechanism (as shown in Fig.\u0026nbsp;8): \u003cb\u003e(\u003c/b\u003e\u003cb\u003ei\u003c/b\u003e\u003cb\u003e) quenching mechanism (ON\u0026rarr;OFF)\u003c/b\u003e: Fe\u003csup\u003e3+\u003c/sup\u003e ions, as strong electron acceptors, facilitated photoinduced electron transfer from excited N-CQDs and formed non-emissive N-CQD-Fe\u003csup\u003e3+\u003c/sup\u003e complexes, leading to efficient fluorescence quenching. The high electron affinity of Fe\u003csup\u003e3+\u003c/sup\u003e destabilizes the excited state of N-CQDs, suppressing radiative recombination. \u003cb\u003e(\u003c/b\u003e\u003cb\u003eii\u003c/b\u003e\u003cb\u003e) recovery mechanism (OFF\u0026rarr;ON)\u003c/b\u003e: Ascorbic acid, a potent reducing agent, selectively reduced Fe\u003csup\u003e3+\u003c/sup\u003e to Fe\u003csup\u003e2+\u003c/sup\u003e, which exhibits weaker electron-accepting capability and forms less-stable complexes with N-CQDs. The reduction of Fe\u003csup\u003e3+\u003c/sup\u003e disrupted the quenching interaction, releasing N-CQDs from the non-emissive state and restoring their intrinsic fluorescence.\u003c/p\u003e\u003cp\u003eSpiked beer samples demonstrated high recovery rates for Fe\u003csup\u003e3+\u003c/sup\u003e: 98.4% (5 \u0026micro;M), 101.5% (10 \u0026micro;M), and 98.8% (15 \u0026micro;M), indicating minimal matrix interference. RSD values (0.9\u0026ndash;1.2%) were exceptionally low, comparable to those of commercial kits (2\u0026ndash;3% RSD), due to uniform surface chemistry of N-CQDs and controlled sample preparation. Spiked serum samples demonstrated high analytical accuracy: Fe\u003csup\u003e3+\u003c/sup\u003e recoveries were 97.6% (5 \u0026micro;M), 98.5% (10 \u0026micro;M), and 101.7% (15 \u0026micro;M), with RSD values of 1.3\u0026ndash;1.8%, which are considered acceptable for biological analysis. Optimized sample-processing and detection protocols ensured high precision, outperforming protein-based assays that are prone to serum cross-reactivity.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eDetection of Fe\u0026sup3;⁺ in Real Samples Using N-CQDs Fluorescent Sensor\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eReal Samples\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eAdded Fe\u003csup\u003e3+\u003c/sup\u003e (\u0026micro;M)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eSensed by N-CQDs (\u0026micro;M) (\u003cem\u003en\u003c/em\u003e\u0026thinsp;=\u0026thinsp;3)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eRecovery (%)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eRSD (%)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eBeer\u003c/p\u003e\u003cp\u003eBlood Serum\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e5\u003c/p\u003e\u003cp\u003e10\u003c/p\u003e\u003cp\u003e15\u003c/p\u003e\u003cp\u003e5\u003c/p\u003e\u003cp\u003e10\u003c/p\u003e\u003cp\u003e15\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e4.92\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06\u003c/p\u003e\u003cp\u003e10.15\u0026thinsp;\u0026plusmn;\u0026thinsp;0.09\u003c/p\u003e\u003cp\u003e14.82\u0026thinsp;\u0026plusmn;\u0026thinsp;0.16\u003c/p\u003e\u003cp\u003e4.88\u0026thinsp;\u0026plusmn;\u0026thinsp;0.09\u003c/p\u003e\u003cp\u003e9.85\u0026thinsp;\u0026plusmn;\u0026thinsp;0.15\u003c/p\u003e\u003cp\u003e15.25\u0026thinsp;\u0026plusmn;\u0026thinsp;0.20\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e98.4\u0026thinsp;\u0026plusmn;\u0026thinsp;1.2\u003c/p\u003e\u003cp\u003e101.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.9\u003c/p\u003e\u003cp\u003e98.8\u0026thinsp;\u0026plusmn;\u0026thinsp;1.1\u003c/p\u003e\u003cp\u003e97.6\u0026thinsp;\u0026plusmn;\u0026thinsp;1.8\u003c/p\u003e\u003cp\u003e98.5\u0026thinsp;\u0026plusmn;\u0026thinsp;1.5\u003c/p\u003e\u003cp\u003e101.7\u0026thinsp;\u0026plusmn;\u0026thinsp;1.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1.2\u003c/p\u003e\u003cp\u003e0.9\u003c/p\u003e\u003cp\u003e1.1\u003c/p\u003e\u003cp\u003e1.8\u003c/p\u003e\u003cp\u003e1.5\u003c/p\u003e\u003cp\u003e1.3\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eIn this study, a novel, environmentally friendly fluorescent sensor based on nitrogen-doped carbon quantum dots was successfully synthesized, using a one-pot green hydrothermal method with licorice powder as a renewable carbon source, addressing the critical need for sustainable Fe\u003csup\u003e3+\u003c/sup\u003e detection with high selectivity and sensitivity. The N-CQDs demonstrated excellent photoluminescence and a reversible \u0026ldquo;ON-OFF-ON\u0026rdquo; fluorescence switching behavior, in which Fe\u003csup\u003e3+\u003c/sup\u003e-quenched fluorescence was restored by ascorbic acid (via Fe\u003csup\u003e3+\u003c/sup\u003e reduction to Fe\u003csup\u003e2+\u003c/sup\u003e), further confirming the specificity of N-CQDs for Fe\u003csup\u003e3+\u003c/sup\u003e. Significantly, the sensor performed reliably in complex samples, including beer and human blood serum, as evidenced by high recovery rates and low relative standard deviations, underscoring its practical robustness. Beyond showcasing analytical performance, this work highlights the broader value of leveraging green synthetic routes and renewable carbon sources for designing advanced nanomaterial-based sensors. Future efforts may focus on optimizing the synthesis process to enhance quantum yield and reduce detection limits, as well as integrating the sensor into portable platforms for real-time, on-site Fe\u003csup\u003e3+\u003c/sup\u003e detection across diverse scenarios.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAuthor Contributions:\u0026nbsp;\u003c/strong\u003eZubair Akram and Anam Arshad were involved in investigation, conceptualization, methodology, data curation and writing the original draft. Muhammad Mehdi, Mohsin Tehseen, Ali Raza and Yulan Shi contributed to the methodology and analysis. Nan Wang was involved in visualization, reviewing, editing, validation and supervision. Sajida Noureen and Feng Yu were involved in visualization, validation and supervision.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability:\u003c/strong\u003e Data presented in this manuscript will be available from corresponding author upon reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding:\u003c/strong\u003e This work was supported by Xinjiang Science and Technology Program (2023TSYCCX0118).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eInstitutional Review Board Statement:\u0026nbsp;\u003c/strong\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eInformed Consent Statement:\u003c/strong\u003e Not applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflict of interest:\u003c/strong\u003e The authors declare no conflict of interest.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eShahbaz M et al (2025) \u003cem\u003eFluorescent/Photoluminescent Carbon Dots as a Sensor for the Selective and Sensitive Detection of Fe\u003c/em\u003e\u003csup\u003e\u003cem\u003e3+\u003c/em\u003e\u003c/sup\u003e\u003cem\u003e/Fe\u003c/em\u003e\u003csup\u003e\u003cem\u003e2+\u003c/em\u003e\u003c/sup\u003e \u003cem\u003eMetal Ions. 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Sci Total Environ 811:152389\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLi Y-X et al (2021) \u003cem\u003eHighly fluorescent nitrogen-doped carbon dots for selective and sensitive detection of Hg\u003c/em\u003e\u003csup\u003e\u003cem\u003e2+\u003c/em\u003e\u003c/sup\u003e \u003cem\u003eand ClO\u003c/em\u003e\u003csup\u003e\u0026ndash;\u003c/sup\u003e \u003cem\u003eions and fluorescent ink\u003c/em\u003e. J Photochem Photobiol A 405:112931\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSahu G et al (2024) \u003cem\u003eNitrogen Doped Carbon Quantum Dots as Fluorescence Turn-Off-On Sensor for Detection of Fe\u003c/em\u003e\u003csup\u003e\u003cem\u003e3+\u003c/em\u003e\u003c/sup\u003e \u003cem\u003eIons and Ascorbic Acid in Moringa oleifera and Citrus Lemon\u003c/em\u003e. J Fluoresc, : p. 1\u0026ndash;13\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eXu H et al (2018) \u003cem\u003eNanospace-confined preparation of uniform nitrogen-doped graphene quantum dots for highly selective fluorescence dual-function determination of Fe\u003c/em\u003e\u003csup\u003e\u003cem\u003e3+\u003c/em\u003e\u003c/sup\u003e \u003cem\u003eand ascorbic acid\u003c/em\u003e. RSC Adv 8(10):5500\u0026ndash;5508\u003c/span\u003e\u003c/li\u003e\u003c/ol\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":"journal-of-fluorescence","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jofl","sideBox":"Learn more about [Journal of Fluorescence](https://www.springer.com/journal/10895)","snPcode":"10895","submissionUrl":"https://submission.nature.com/new-submission/10895/3","title":"Journal of Fluorescence","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Nitrogen-Doped Carbon Quantum Dots, Ferric Ion Sensing, ON-OFF-ON Fluorescence Mechanism, Licorice biomass-derived precursors","lastPublishedDoi":"10.21203/rs.3.rs-8105416/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8105416/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe detection of ferric ions (Fe\u003csup\u003e3+\u003c/sup\u003e) is of crucial importance in environmental monitoring and biomedical diagnostics. However, developing highly selective, sensitive, and environmentally friendly detection methods remains an important challenge. In this study, nitrogen-doped carbon quantum dots (N-CQDs) were green-synthesized as a novel and sustainable fluorescent sensor for Fe\u003csup\u003e3+\u003c/sup\u003e detection. The synthesis was achieved via a one-pot hydrothermal method using licorice powder as a renewable carbon source and \u003cem\u003ep\u003c/em\u003e-phenylenediamine as a nitrogen dopant. The synthesized N-CQDs display bright blue fluorescence, with a maximum emission at 436 nm when excited at 320 nm. They serve as a highly selective and sensitive fluorescent probe for Fe\u003csup\u003e3+\u003c/sup\u003e, showing a distinct fluorescence “turn-OFF” response. The sensor offers a linear range of 0 to 50 µM for Fe\u003csup\u003e3+\u003c/sup\u003e detection, with a calculated limit of detection (LOD) of 0.346 µM. A notable aspect of this work is the demonstration of an “ON-OFF-ON” sensing paradigm. The fluorescence quenched by Fe\u003csup\u003e3+\u003c/sup\u003e can be effectively restored (“turn-ON”) by adding ascorbic acid, which reduces Fe\u003csup\u003e3+\u003c/sup\u003e. This “ON-OFF-ON” behavior emphasizes the specificity of N-CQDs towards Fe\u003csup\u003e3+ \u003c/sup\u003eto Fe\u003csup\u003e2+\u003c/sup\u003e. The practical applicability of the sensor was confirmed through successful detection of Fe³⁺ in complex real-world samples, including beer and human blood serum, achieving excellent recovery percentages (97.6% - 101.7%) and low relative standard deviations (RSD, 0.9% - 1.8%). Overall, this research presents an environmentally friendly N-CQD-based fluorescent sensor with a unique reversible fluorescence response “ON-OFF-ON” for Fe\u003csup\u003e3+\u003c/sup\u003e, holding great potential applications in environmental monitoring and biomedical diagnostics.\u003c/p\u003e","manuscriptTitle":"Green Synthesis of a Reversible “ON-OFF-ON” Fluorescent sensor for Fe3+ Using Licorice-Derived N-Doped Carbon Dots","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-25 16:01:54","doi":"10.21203/rs.3.rs-8105416/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-11-28T18:42:33+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-11-25T05:46:13+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-11-20T04:53:13+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"252402308747293955723488047260238128769","date":"2025-11-19T15:29:23+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"86863730237698053074200872102139992858","date":"2025-11-17T15:44:54+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"120849258684052015314779709349981007454","date":"2025-11-16T14:50:17+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"41195012998344160628873656771273486381","date":"2025-11-14T17:04:06+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"303892892760703769862160048820902843045","date":"2025-11-14T14:07:14+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-11-14T13:49:41+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-11-14T01:14:33+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-11-14T01:14:24+00:00","index":"","fulltext":""},{"type":"submitted","content":"Journal of Fluorescence","date":"2025-11-13T11:41:59+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"journal-of-fluorescence","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jofl","sideBox":"Learn more about [Journal of Fluorescence](https://www.springer.com/journal/10895)","snPcode":"10895","submissionUrl":"https://submission.nature.com/new-submission/10895/3","title":"Journal of Fluorescence","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"d69f6f3c-63d5-402c-b47c-79927700c13a","owner":[],"postedDate":"November 25th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-01-19T16:45:02+00:00","versionOfRecord":{"articleIdentity":"rs-8105416","link":"https://doi.org/10.1007/s10895-025-04700-5","journal":{"identity":"journal-of-fluorescence","isVorOnly":false,"title":"Journal of Fluorescence"},"publishedOn":"2026-01-13 16:28:26","publishedOnDateReadable":"January 13th, 2026"},"versionCreatedAt":"2025-11-25 16:01:54","video":"","vorDoi":"10.1007/s10895-025-04700-5","vorDoiUrl":"https://doi.org/10.1007/s10895-025-04700-5","workflowStages":[]},"version":"v1","identity":"rs-8105416","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8105416","identity":"rs-8105416","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}