Green Synthesis of Nitrogen-Doped Carbon Quantum Dots from Medium-Molecular-Weight Chitosan for Ultrasensitive Fe³⁺ Detection and Responsive Probe 

Green Synthesis of Nitrogen-Doped Carbon Quantum Dots from Medium-Molecular-Weight Chitosan for Ultrasensitive Fe³⁺ Detection and Responsive Probe | 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 Nitrogen-Doped Carbon Quantum Dots from Medium-Molecular-Weight Chitosan for Ultrasensitive Fe³⁺ Detection and Responsive Probe Lulu Feng, Hainan Li, Zishuai Zhang, Shuang Pang, Zhiyu Liu, Jiahao Lin, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8120500/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 28 Feb, 2026 Read the published version in Journal of Fluorescence → Version 1 posted 9 You are reading this latest preprint version Abstract Carbon quantum dots (CQDs), as zero-dimensional carbon-based fluorescent materials, have garnered significant attention in environmental and biological monitoring due to their low toxicity, high stability, and tunable synthesis. This study pioneers a green synthesis of nitrogen-doped carbon quantum dots (N-CQDs) using medium-molecular-weight chitosan as a single precursor via one-step hydrothermal optimization (190 ℃, 24 h). The resulting N-CQDs exhibit uniform spherical morphology (typical diameter: 3 nm) with amorphous carbon cores and abundant surface moieties including amino, hydroxyl, and pyrrolic groups (C/N/O = 64:4:32). These structural attributes enable strong blue fluorescence (λ ex /λ em = 330nm/402nm) and selective Fe 3+ detection through a dual-mechanism: (i) Coordination binding between Fe 3+ and surface functional groups, (ii) Photoinduced electron transfer (PET) facilitated by the high redox potential of Fe 3+ (E⁰ = +0.77 V vs. SHE). The N-CQDs achieve a 2.72 µM detection limit (S/N = 3) for Fe 3+ with excellent selectivity against 15 interfering ions (Al 3+ , Cu 2+ , etc.) and validated performance in tap/lake water samples (99.98-100.04% recoveries). Furthermore, the chitosan-derived N-CQDs were successfully applied as a fluorescent probe for cell imaging in onion epidermis, where they predominantly accumulated on the cell membrane with excellent color rendering performance. Notably, the fluorescence was effectively quenched upon the introduction of Fe 3+ ions, demonstrating their possibility as a responsive probe in biological systems. This work demonstrates the significant potential of biopolysaccharide-derived nanomaterials, highlighting their dual promise for both precise environmental sensing and responsive bioimaging applications. Chitosan Carbon quantum dots Fe3+ detection Fluorescence quenching Environmental monitoring Cell Imaging Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 1 Introduction Carbon quantum dots, as representative zero-dimensional nanomaterials, have demonstrated significant application value in fluorescence labeling [1] , biomedical imaging [2] , and photocatalytic energy conversion [3] owing to their unique photoluminescence properties, excellent water solubility, and low environmental toxicity [4] . Since their discovery, various synthesis techniques have been developed, including arc discharge [5] , laser ablation [6] , electrochemical synthesis [7] , and microwave heating [8] . Among these, hydrothermal synthesis is regarded as the most industrially promising route due to its operational simplicity, tunable process, and low equipment costs [9] . Notably, green synthesis strategies employing natural biomass and polysaccharide materials as precursors have emerged as a research hotspot, significantly reducing material cytotoxicity while achieving dual goals of resource recycling and cost control [10–12] . Iron, the fourth most abundant metal in the Earth’s crust, exhibits dual environmental behavior: Fe 3+ serves as an essential component for oxygen metabolism in organisms [13–14] , yet also acts as a hazardous heavy metal pollutant from industrial emissions [15] . Rapid and highly sensitive detection of Fe 3+ is crucial for monitoring its concentration in natural environments, industrial processes, and biological samples. Traditional techniques, such as inductively coupled plasma mass spectrometry [16] , spectrophotometry [17] , atomic absorption spectroscopy [18] , and electrochemical methods [19] , are limited by complex sample pretreatment and high equipment costs [17, 20] . In contrast, fluorescence spectroscopy has become an emerging technology for Fe 3+ detection due to its operational simplicity, rapid response (< 5 min), and high sensitivity [21] . This study innovatively employs medium-molecular-weight chitosan (200 kDa, deacetylation degree ≥ 90%) as a single carbon source. This natural polysaccharide, derived from partial deacetylation of chitin (An abundant agricultural waste from seafood processing), consists of D-glucosamine and N-acetyl-D-glucosamine units linked by β-(1–4) glycosidic bonds [22–23] . The dense distribution of amino (-NH 2 ) and hydroxyl (-OH) groups [24] within chitosan molecules confers exceptional bioactivity and heavy metal chelation capabilities, leading to widespread applications in biomedicine [25–27] and environmental remediation [28–29] . Considering molecular weight effects on carbonization behavior, this precursor demonstrates three advantages over high-MW (> 50 kDa) counterparts (limited solubility) and low-MW (< 5 kDa) materials (excessive fragmentation): Moderate chain length (~ 1000 sugar units) ensures sufficient dispersibility in acidic solutions. High deacetylation degree provides ≥ 0.9 free amino groups per sugar unit, enabling dense nitrogen doping. Optimized hydrothermal conditions (190°C, 24 h) facilitate controlled cleavage of β-(1–4) glycosidic bonds with concurrent formation of pyrrolic heterocycles. Experimental results confirm that selective quenching occurs via a synergistic mechanism involving coordination effect between Fe 3+ and surface functional groups (amino/hydroxyl/pyrrolic) combined with photoinduced electron transfer, enabling highly specific Fe 3+ detection. The detection limit reached 2.72 µM (S/N = 3) with significant interference immunity against 15 coexisting ions (e.g., Al 3+ , Cu 2+ ). Furthermore, successful application to real water samples demonstrated recovery rates of 99.98–100.04%, validating the method’s reliability. Additionally, the carbon quantum dots were successfully applied as fluorescent probes for cellular imaging in onion epidermis, exhibiting clear Fe 3+ -induced fluorescence quenching, which underscores their potential for bioimaging and intracellular ion detection. This work provides new strategies for high-value utilization of biopolysaccharides [30] in rapid environmental detection and cell imaging. 2 Materials and Methods The medium-molecular-weight chitosan (200 kDa) was prepared using the typical acid degradation method [ 31 ] . All chemical reagents were of analytical reagent (AR) grade, with details as follows: Aladdin Biochemical Technology Co., Ltd.: Ferric chloride hexahydrate (FeCl 3 ·6H 2 O), barium chloride dihydrate (BaCl 2 ·2H 2 O), mercury(II) nitrate monohydrate (Hg(NO 3 ) 2 ·H 2 O), magnesium chloride (MgCl 2 ), aluminum chloride hexahydrate (AlCl 3 ·6H 2 O), cadmium chloride monohydrate (CdCl 2 ·H 2 O), sodium chloride (NaCl). Sinopharm Chemical Reagent Co., Ltd.: Ferrous chloride (FeCl 2 ), nickel chloride hexahydrate (NiCl 2 ·6H 2 O), lead chloride (PbCl 2 ), calcium chloride dihydrate (CaCl 2 ·2H 2 O). Macklin Biochemical Technology Co., Ltd.: Copper(II) chloride dihydrate (CuCl 2 ·2H 2 O), zinc chloride (ZnCl 2 ), cesium chloride (CsCl). 2.1 Preparation of Chitosan Quantum Dots Chitosan-derived carbon quantum dots (CS-CQDs) were synthesized via a hydrothermal method, and the detailed preparation process is illustrated in Fig. 1. First, 0.4 grams of chitosan powder and 0.8 grams of glacial acetic acid were weighed and placed in a beaker containing 40 milliliters of deionized water, followed by thorough stirring to ensure complete mixing. The homogeneous solution was then transferred into a hydrothermal reaction vessel, which was subsequently placed in a blast drying oven set at 190°C for 24 hours. After the reaction, the product was centrifuged for 15 minutes at a speed of 5,000 rpm. To remove macromolecular impurities, the supernatant was carefully extracted using a syringe equipped with a 0.45 µm syringe filter and then transferred into a dialysis bag. The solution in the dialysis bag underwent purification for 48 hours, with the dialysate being replaced every 6 hours. Next, the dialyzed solution was aspirated again using a syringe fitted with a 0.22 µm syringe filter and dispensed into a culture dish. The solution in the dish was then subjected to freeze-drying and placed in a freeze dryer for 48 hours. Upon completion of drying, the sample was retrieved, yielding a light-yellow powder identified as chitosan carbon quantum dots. 2.2 Process Optimization Studies 2.2.1 Optimal Reaction Temperature Study To precisely determine the optimal synthesis temperature for CS-CQDs, a gradient heating method was employed. Specifically, hydrothermal reaction temperatures were sequentially set from 150°C to 210°C at 10°C intervals (150°C, 160°C, 170°C, 180°C, 190°C, 200°C, 210°C), while maintaining a constant reaction time of 12 hours. After completion, the purified solutions were transferred to 5 mL quartz cuvettes, and their fluorescence emission intensities were measured using a fluorescence spectrophotometer (λ ex = 330 nm). 2.2.2 Optimal Reaction Time Study After identifying 190°C as the optimal reaction temperature, the effect of reaction time on CS-CQDs performance was further investigated. Different time gradients were tested (3, 6, 12, 18, 24, 36, and 48 hours) while keeping other parameters unchanged. The purified solutions were analyzed under identical conditions (5 mL cuvette, λ ex = 330 nm) to measure fluorescence emission intensity. 2.3 Response of CS-CQDs to Different Ions To validate the selective recognition capability of CS-CQDs toward Fe 3+ , a comparative analysis was conducted with 15 common metal ions (including Fe 3+ , Fe 2+ , Cu 2+ , Ba 2+ , Hg 2+ , etc.). Stock solutions (1000 µM) of each metal salt were prepared in deionized water. Then, 0.5 mL of each metal ion solution was added to 3 mL of CS-CQDs solution, incubated in the dark for 5 minutes, and analyzed via fluorescence spectrophotometry (λ ex = 330 nm). 2.4 Stability Study The pH-dependent fluorescence stability of CS-CQDs was evaluated by adjusting the pH (1–14) of the dispersion using 0.1 M HCl/NaOH. Fluorescence emission intensity was measured at each pH under standardized conditions (λ ex = 330 nm) 2.5 Fe 3+ Detection in Real Water Samples To assess the practicality of CS-CQDs as Fe 3+ probes, tap water and lake water samples were analyzed. Before detection, samples were centrifuged to remove large insoluble impurities and filtered through 0.45 µm membranes. Fe 3+ concentrations were determined using the standard addition method. 2.6 Application of CS-CQDs as a Fe 3+ Responsive Probe in Cell Imaging To verify the fluorescent labeling capability of CS-CQDs and the responsiveness to Fe 3+ in a biological environment, this study conducted fluorescence imaging experiments using onion epidermal cells as a model. First, freshly peeled onion epidermis was co-incubated with CS-CQDs, and its fluorescence labeling effect was observed under UV excitation light. Subsequently, 0.5 mL of 500 µM FeCl 3 solution was added to the sample using the lateral permeation method. After a reaction period of 10 minutes, fluorescence images were reacquired under the same microscope parameters. 3 Results and Discussion 3.1 Synthesis Optimization Figure 2(a) presents the fluorescence emission intensity of CS-CQDs synthesized at different reaction temperatures. The results indicate that fluorescence intensity peaks at 190°C, significantly exceeding values at other temperatures. Below 190°C, insufficient carbonization yields weakly fluorescent CS-CQDs, while temperatures above 190°C cause fluorescence reduction due to excessive carbonization. Hence, 190°C is optimal for CS-CQDs synthesis. Figure 2(b) shows the fluorescence intensity of CS-CQDs prepared with varying reaction durations. The fluorescence intensity of CS-CQDs progressively increases up to 24 hours, beyond which it declines. Shorter reaction durations (< 24 h) lead to incomplete carbonization, producing undersized quantum dots with inadequate surface functional groups. At 24 h, complete carbonization generates uniformly-sized CS-CQDs with abundant surface groups, achieving peak fluorescence. Extended reaction durations (> 24 h) induce particle aggregation and surface oxidation through over-carbonization, diminishing fluorescence. Thus, 24 hours is established as the optimal reaction time. 3.2 Characterization 3.2.1 Morphological and structural analysis The morphology and size of the particles were established by the TEM. Figure 3(a) contains the HRTEM micrographs at different magnifications, which show low-dimensional carbon nanostructures. Figure 3(b) presents the HRTEM micrograph of CCDs. ImageJ analysis has revealed the interplanar lattice spacing to be 0.24 nm. This is in good agreement with the reported lattice spacing of graphite-related structures in the literature [ 32 ] . X-ray diffraction (XRD) analysis revealed a broad peak at 22.5° (Fig. 3c), characteristic of amorphous carbon, verifying the predominantly amorphous structure of the CS-CQDs. 3.2.2 Functional groups and elemental analysis Fourier-transform infrared (FTIR) spectroscopy analysis of chitosan and CS-CQDs (Fig. 3d) revealed critical structural transformations: The chitosan spectrum (black line) exhibited characteristic peaks at 3434.5 cm⁻¹ (O-H/N-H stretching), 1646.9 cm⁻¹ (C = O stretching, amide I), 1550.3 cm⁻¹ (coupled N-H bending/C-N stretching, amide II), and 1081.8 cm⁻¹ (β-(1–4)-glycosidic bonds, C-O-C), while the CS-CQDs spectrum (red line) showed retention of O-H/N-H groups (3434.5 cm⁻¹), attenuated amide I/II peaks (1646.9/1550.3 cm⁻¹) indicating amide bond cleavage, emergence of a new peak at 1510 cm⁻¹ suggesting aromatic/N-heterocyclic moieties (e.g., pyrrole), and disappearance of the 1081.8 cm⁻¹ peak confirming glycosidic bond scission. Collectively, these changes demonstrate partial preservation of original hydroxyl/amino groups alongside fundamental restructuring—via cleavage of glycosidic/amide bonds and nitrogen redistribution—driving the transformation from polysaccharide to carbon quantum dot architecture. X-ray photoelectron spectroscopy (XPS) analysis of CS-CQDs revealed their elemental composition and chemical bonding states (Fig. 4a), showing three characteristic peaks at 282.1 eV (C 1s), 397.2 eV (N 1s), and 529.4 eV (O 1s) with atomic percentages of 64% C, 4% N, and 32% O. High-resolution spectra demonstrated: (i) C 1s deconvolution into sp² carbon, C-OH, and C = O bonds [ 33 ] ; (ii) N 1s speciation as amino (-NH₂) and pyrrolic [ 34 ] ; and (iii) O 1s components of C-O and C = O bonds. These results indicate substantial sp³-to-sp² carbon conversion, partial nitrogen transformation into pyrrolic structures, and additional C = O formation via carbonization/oxidation, while retaining amino/hydroxyl groups in the CS-CQDs. The differences between chitosan and CS-CQDs are shown in Table 1. Table 1 The differences between chitosan and chitosan carbon quantum dots Critical Structures Glycosidic Bond (C-O-C) Peptide Bond (-CO-NH-) Amino Group (-NH2) Oxhydryl Group (-OH) Carbon Structure Pyrrolic Structure Chitosan exist exist exist exist sp 3 not exist CS-CQDs not exist not exist retain retain sp 2 exist 3.3 Optical properties The optical properties of CS-CQDs in aqueous solution were characterized by UV-Vis spectroscopy (Fig. 5a). Two distinct absorption peaks were observed at 239 nm and 272 nm. The peak at 239 nm is assigned to π→π* transitions of localized π-conjugation domains (e.g., small aromatic clusters or isolated C = C bonds), while the peak at 272 nm arises from n→π* transitions associated with hydroxyl/amino/ pyrrolic functional groups. The absolute quantum yield of the prepared CS-CQDs, measured using an integrating sphere, was 3.96%. Furthermore, the fluorescence characteristics are presented in Fig. 5b. The excitation spectrum (orange curve) exhibits an optimal excitation wavelength at 330 nm, while the corresponding emission spectrum (blue curve) displays a maximum emission peak at 402 nm. Figure 5c displays the fluorescence spectra under excitation wavelengths ranging from 300 to 390 nm, revealing a progressive red shift of the maximum emission wavelength from approximately 400 nm to 480 nm. This phenomenon may originate from the size heterogeneity and distribution of multiple surface emission sites in N-CQDs [ 32 , 35 ] . Furthermore, previous studies suggest that the optical properties of carbon dots correlate strongly with their surface functional groups [ 36 – 38 ] . Specifically in this work, the dominant 402 nm emission peak (Fig. 5b) provides direct evidence for pyrrolic moieties—confirmed by their 400.2 eV XPS signature (Fig. 4c)—serving as critical fluorophores. This emission profile corresponds to an optical bandgap of 3.1 eV ( ), which aligns with the characteristic blue emission of pyrrolic configurations in N-CQDs [ 39 ] . Critically, the pyrrolic moieties serve as electron donors that inject photoexcited electrons into the LUMO of sp 2 -carbon via π-conjugated channels, thereby localizing electron-hole pairs. This process is structurally enabled by the 3.8 Å spatial alignment between C2-N and C6-O groups from chitosan pyrolysis, which directs efficient pyrrole cyclization (∼42%) [ 40 ] . The fluorescence intensity is optimized by introducing pyrrolic, which is achieved through the specific hydrothermal synthesis process (190°C, 24h) for the medium-molecular-weight Chitosan. The pyrrolic structures act as dual functional units: (i) bandgap modulators that tune emission energy through ΔE control, and (ii) electron-transfer pathway. This synergistic design establishes CS-CQDs as efficient blue emitters. 3.4 Fe 3+ Sensing Performance 3.4.1 Specificity The fluorescence images of CS-CQDs solutions under 310 nm excitation (Fig. 6a,b) revealed that while the addition of various metal ions (Fe 2+ , Cu 2+ , Ba 2+ , Hg 2+ , Zn 2+ , Ni 2+ , Mg 2+ , Co 2+ , Pb 2+ , Ca 2+ , Al 3+ , Cd 2+ , Na + , Cs + ) induced only minor changes in fluorescence intensity compared to the pure CS-CQDs control, a remarkably strong fluorescence quenching effect was specifically observed upon Fe 3+ addition. 3.4.2 Interference Test and pH Stability Analysis The fluorescence spectra of CS-CQDs solutions were measured following the addition of either: (i) 1000 µM Fe 3+ alone or (ii) 1000 µM Fe 3+ co-existing with 1000 µM of other metal ions (Fig. 6d). The results revealed minimal interference (< 5% variation) from co-existing ions on the Fe 3+ -induced fluorescence quenching effect (grey bars), demonstrating the CS-CQDs' remarkable selectivity for Fe 3+ detection in mixed-ion environments. pH-dependent studies (Fig. 6e) reveal that CS-CQDs maintain stable fluorescence intensity within the pH 2–10 range, while severe intensity loss occurs at pH 10 due to protonation/deprotonation effects. Remarkably, Fe 3+ -induced quenching remains pH-independent across this pH 2–10 range (Fig. 6f), with consistent quenching efficiency observed upon adding 1000 µM Fe 3+ at pH 2–10. This underscores the robustness of CS-CQDs for Fe 3+ sensing in environmentally relevant conditions. 3.4.3 Limit of detection The sensitivity of the CS-CQDs solution toward Fe 3+ was evaluated at pH 7. As shown in Fig. 7a and b, under 330 nm excitation, the fluorescence intensity of CS-CQDs exhibited a pronounced concentration-dependent quenching effect with increasing Fe 3+ concentrations from 0 to 1500 µM. Figure 7c presents the Stern-Volmer quenching curve of CS-CQDs solutions with Fe 3+ concentrations ranging from 0 to 1500 µM. The inset displays the linear portion of the Stern-Volmer plot between 100–700 µM. Within this concentration range, the fluorescence quenching efficiency (F₀-F)/F₀ shows an excellent linear correlation with Fe 3+ concentration, described by the regression equation: (F₀-F)/F₀ = 0.00032X + 0.113 (R² = 0.9955), where F₀ and F represent the fluorescence intensities of CS-CQDs solutions before and after Fe 3+ addition, respectively. Furthermore, the detection limit was determined to be 2.72 µM at a signal-to-noise ratio (S/N) of 3. Table 2 demonstrates that the CQDs synthesized in this study exhibit significantly lower detection limits for Fe³⁺ compared to many previously reported values. Notably, our detection limit is substantially lower than the World Health Organization (WHO) guideline value for Fe³⁺ concentration in drinking water (5.36 µM) [ 41 ] . This remarkably low limit of detection highlights the significant potential of the prepared CQDs for trace Fe³⁺ sensing applications. Table 2 Comparison of various quantum dots fluorescent sensors for detecting Fe 3+ detection Type of CQDs Precursors Method of synthesis Linear range (µM) LOD (µM) Ref N-CQDs Matrimony vine Hydrothermal 5–60 2.22 [42] CQDs Cat feed stocks Hydrothermal 0–500 32 [43] N-CQDs Clementine peel Hydrothermal 7–50 4.57 [44] N-GQDs L-Glutamic acid One pot 0–50 4.67 [45] GQDs Citric acid Pyrolysis 10–200 10.00 [46] GQDs Waste tea Microwave 0–50 2.5 [47] PEG-GQDs cane molasses Hydrothermal 0–60 5.77 [48] CQDs Camphor waste leaves One-step hydrothermal 0–100 2.03 [49] N-GQDs Chitosan Hydrothermal 100–700 2.73 This work 3.5 Quenching Mechanism Gu et al. [ 50 ] synthesized N-CQDs from lotus roots for Hg(II) detection and cellular imaging. UV-Vis absorption spectra revealed that N-CQDs exhibited an absorption peak at 280 nm, which disappeared upon Hg 2+ addition to the solution, indicating formation of N-CQDs-Hg 2+ complexes. This study similarly investigated the interaction between N-CQDs and Fe³⁺ using UV-Vis spectroscopy (Fig. 8a). Significant spectral changes occurred after Fe 3+ introduction: the 272 nm absorption peak redshifted to 294 nm with markedly enhanced intensity. This phenomenon suggests Fe 3+ binding altered the electronic structure of N-CQDs. We propose that Fe 3+ coordinates with surface functional groups (pyrrolic/amino/hydroxyl) of N-CQDs, forming N-CQDs-Fe 3+ complexes. FT-IR spectroscopy (Fig. 8b) further confirmed the coordination between Fe³⁺ and surface functional groups on N-CQDs. Comparative analysis revealed significant alterations in characteristic vibrational modes: the O–H/N–H stretching band (~ 3400 cm⁻¹) broadened and shifted, while the amide I and II bands (~ 1600 cm⁻¹) showed reduced intensity, indicating Fe³⁺ binding to amino, hydroxyl, and carbonyl groups. Additionally, new peaks appeared in the 500–700 cm⁻¹ region, attributable to Fe–O/Fe–N bonds. These results unequivocally demonstrate the formation of N-CQDs–Fe³⁺ complexes, which facilitate efficient PET by shortening the donor-acceptor distance and enhancing electron transfer efficiency. The fluorescence quenching mechanism primarily stems from the synergistic effects between photoinduced electron transfer (PET) [ 51 ] and coordination bonding. When Fe³⁺ coordinates with N-CQDs' surface functional groups, photoexcited N-CQDs transfer electrons to Fe³⁺'s vacant orbitals, driven by its strong oxidizing capability (E⁰ = +0.77 V vs. SHE), thereby suppressing radiative recombination and causing fluorescence quenching. This PET-dominated process is unequivocally confirmed by the notable decrease in the average fluorescence lifetime (τ int ) from 6.94 ns to 3.90 ns upon Fe³⁺, which provides direct evidence for a dynamic quenching pathway through electron transfer (Fig. 8c, Fig. 8d). Notably, UV-Vis spectral evidence confirms that ground-state N-CQDs-Fe³⁺ complex formation contributes to quenching by shortening donor-acceptor distances through tight coordination, further enhancing PET efficiency. The superior Fe³⁺ detection capability of N-CQDs arises from two key factors: firstly, the specific coordination of Fe 3+ with surface functional groups; and secondly, the coordination-enhanced photoinduced electron transfer (PET) that efficiently quenches fluorescence. These combined mechanisms enable highly selective Fe 3+ detection with a low detection limit of 2.72 µM. 3.6 Stability CS-CQDs solutions in 5 mL cuvettes were stored under controlled conditions (25°C, 25% RH) to evaluate stability: one sample sealed with parafilm under simulated illumination(Fig. 9a), and another sample sealed with parafilm in complete darkness(Fig. 9b). Fluorescence emission spectra were monitored for 5 days. The illuminated sample exhibited progressive intensity loss—30% reduction after 3 days and 60% after 5 days. Conversely, the dark-stored sample showed minimal degradation (3% at day 3, 5% at day 5). These results demonstrate that continuous illumination causes significant fluorescence decay, while dark storage effectively preserves photostability (> 95% retention). This finding provides critical guidance for practical storage protocols. Future studies could explore surface modification or stabilizers to enhance anti-photobleaching properties. 3.7 Applications 3.7.1 Fe 3+ detection in real water samples To evaluate the practical utility of CS-CQDs as fluorescent probes, as shown in Fig. 8c, this study applied them to determine Fe 3+ concentrations in tap water and Dingxiang Lake(41.8544638, 123.3352718) water samples. Before analysis, collected water samples underwent centrifugation to remove large insoluble particulates, followed by filtration through 0.45 µm membranes to eliminate suspended particles. Fe 3+ concentrations were quantified using the standard addition method, with recovery rates calculated across different spiking levels. As presented in Table 3, recovery rates ranged from 99.98% to 100.04%, demonstrating the probe's reliability for real-sample analysis. Table 3 Determination of Fe 3+ in Spiked Tap Water and Lake Water Samples Samples Concentration of Fe 3+ (µM) Recovery (%) Spiked Measured Tap water 1 0 0.33 - 2 100 100.35 100.02 3 150 150.32 99.99 4 200 200.39 100.03 Lake water 1 0 0.32 - 2 100 100.30 99.98 3 150 150.39 100.04 4 200 200.37 100.03 Recovery = [(C 2 − C 1 )/C 0 ] × 100% where C₀ represents the concentration of Fe 3+ spiked into real samples, while C₁ and C₂ denote the background and measured Fe 3+ concentrations in real samples before and after spiking, respectively. 3.7.2 Application of CS-CQDs as a Fe 3+ Responsive Probe in Cell Imaging To assess the cell imaging potential and sensing capability of the synthesized CQDs, fluorescence imaging was performed on onion epidermal cells as an in vitro plant model. Figure 10 (a-c) presents the corresponding microscopic images. Under white light illumination, the typical cellular architecture of the onion epidermis is clearly observed (Fig. 10a, Fig. 10b). Subsequent to staining with CQDs, the cells exhibited intense blue fluorescence under UV light excitation (Fig. 10d, Fig. 10e), indicating successful and homogeneous labeling of the cell membrane. The bright fluorescence suggests that the CQDs possess excellent biocompatibility and a strong affinity for cellular structures. This effective staining is likely facilitated by the coordination between functional groups (e.g., hydroxyl and amino groups) from the CQDs and various binding sites present on the plant cell membrane. The feasibility of employing CQDs as a fluorescent probe for intracellular ion sensing was further demonstrated. Upon introduction of Fe 3+ ions via a lateral permeation method, a significant quenching of the blue fluorescence was observed, with the fluorescence intensity diminishing to a negligible level (Fig. 10f). This turn-off response confirms the efficient diffusion of Fe 3+ ions into the cellular environment and their specific interaction with the CQDs, leading to fluorescence quenching, thereby validating their potential as a sensitive probe for detecting metal ions within a biological context. It is noteworthy that the propagation of excitation and emission light through plant tissues can be influenced by absorption, internal reflection, and scattering [ 52 ] . For instance, the limited penetration depth of UV excitation and the presence of soluble UV-absorbing compounds can affect the detected fluorescence signal. Nevertheless, the clear contrast achieved here underscores the efficacy of CQDs. The findings align with the growing interest in non-destructive optical techniques for agricultural monitoring. For example, fluorescence imaging and spectroscopic methods have been successfully employed to distinguish diseased onions from healthy ones [ 53 ] . Consequently, the developed CQD-based imaging system holds significant promise as a versatile tool. It can be further developed for automated quality inspection, classification in horticulture, and even remote sensing applications, positioning fluorescence imaging as a primary tool for quality control in agriculture, forestry, and environmental research. 4. Conclusion This study innovatively synthesized N-CQDs using medium-molecular-weight chitosan as a single precursor through optimized hydrothermal conditions (190°C, 24 h). This green strategy leverages intrinsic amino/hydroxyl groups to drive in situ pyrrolic ring formation (via C2-N/C6-O cyclization) for autonomous nitrogen self-doping, eliminating complex precursors and toxic reagents. The pyrrole moiety acts as both an electron transfer channel and an emission center, endowing the synthesized N-CQDs with blue fluorescence emission characteristics (λ ~ ex~/λₑₘ = 330nm/402nm). Based on the synergistic mechanisms of the coordination effect and photoinduced electron transfer effect, the developed N-CQDs exhibit exceptional Fe 3+ -sensing capabilities: selective fluorescence quenching toward Fe 3+ among 15 coexisting ions (e.g., Al 3+ , Cu 2+ ); ultrahigh sensitivity (LOD = 2.72 µM, S/N = 3) with linear response (100–700 µM, R²=0.9955); and stable performance across pH 2–10. Validation in environmental waters (tap/lake water) achieved near-quantitative recoveries (99.98–100.04%), confirming practical reliability. Additionally, the N-CQDs were successfully applied as fluorescent probes for cellular imaging in onion epidermis, exhibiting clear Fe 3+ -induced fluorescence quenching, which underscores their potential for bioimaging and intracellular ion detection. This work demonstrates the significant potential of biopolysaccharide-derived nanomaterials in environmental and biological sensing. Declarations Funding: This work was supported by the Natural Science Foundation of Liaoning Province (d802487015) and the Liaoning University Scientific Research Project (d295000059). Author Contribution Lulu Feng: Drafting the manuscript, software implementation, methodology, data collection and analysis, conceptualisation. Hainan Li: Software implementation, methodology, conceptualisation. Zishuai Zhang:Reviewing the manuscript, editing, conceptualisation. Shuang Pang:Reviewing the manuscript, editing, conceptualisation. Zhiyu Liu:Reviewing the manuscript, editing, conceptualisation. Jiahao Lin:Reviewing the manuscript, editing, conceptualisation. Zhenning Lou:Guidance with experimental technique and supervision. Shiyu Wang: Reviewing the manuscript, validation and supervision. References Atchudan R, Edison I J N T, Aseer RK et al Highly fluorescent nitrogen-doped carbon dots derived from Phyllanthus acidus utilized as a fluorescent probe for label-free selective detection of Fe 3+ ions, live cell imaging and fluorescent ink[J]. Biosens Bioelectron ,2018,99303–99311. https://doi.org/10.1016/j.bios.2017.07.076 Liu W, Diao H, ,Chang H et al Green synthesis of carbon dots from rose-heart radish and application for Fe 3+ detection and cell imaging[J]. Sens Actuators: B Chem ,2017,241190–241198. https://doi.org/10.1016/j.snb.2016.10.068 Xin G, Cai-Feng W, ,Zi-Yi Y et al Facile access to versatile fluorescent carbon dots toward light-emitting diodes.[J].Chemical communications (Cambridge, England),2012,48(21):2692–2694. https://doi.org/10.1039/c2cc17769b Bingbing H, Bingping L, ,Tao C et al One-Step Synthesis of Fluorescent Boron Nitride Quantum Dots via a Hydrothermal Strategy Using Melamine as Nitrogen Source for the Detection of Ferric Ions.[J].Langmuir : the ACS journal of surfaces and colloids,2017,33(40):10673–10678. https://doi.org/10.1021/acs.langmuir.7b01699 N S B,A G B. Luminescent carbon nanodots: emergent nanolights.[J].Angewandte Chemie (International ed. in English),2010,49(38):6726–6744. https://doi.org/10.1002/anie.200906623 Cao L, ,Wang X, ,Meziani MJ et al Carbon Dots for Multiphoton Bioimaging.[J]. J Am Chem Soc 2007, 129(37):11318–11319. https://doi.org/10.1021/ja073527l Zhang Y, Wang L, ,Zhang H et al Graphitic carbon quantum dots as a fluorescent sensing platform for highly efficient detection of Fe 3+ ions[J].RSC Advances,2013,3(11):3733–3738. https://doi.org/10.1039/c3ra23410j Sun S, Zhang L, Jiang K et al Toward High-Efficient Red Emissive Carbon Dots: Facile Preparation, Unique Properties, and Applications as Multifunctional Theranostic Agents.[J].Chemistry of Materials,2016,28(23):8659–8668. https://doi.org/10.1021/acs.chemmater.6b03695 Swagatika S, Birendra B, K T M et al Simple one-step synthesis of highly luminescent carbon dots from orange juice: application as excellent bio-imaging agents.[J].Chemical communications (Cambridge, England),2012,48(70):8835–8837. https://doi.org/10.1039/c2cc33796g Vaid S, Kaur V, ,Arya KR et al Transforming agro-waste: sustainable adsorption of Novacron Red FN-2BL on surfactant-modified bamboo fibers[J].Biomass Conversion and Biorefinery,2025,(prepublish):1–25. https://doi.org/10.1007/s13399-025-06717-z Li ZYL, ,Zhenzhen H et al Carbon nanodots as fluorescence probes for rapid, sensitive, and label-free detection of Hg 2+ and biothiols in complex matrices.[J].Chemical communications (Cambridge, England),2012,48(8):1147–1149. https://doi.org/10.1039/c2cc16791c Zhang R, Chen W Nitrogen-doped carbon quantum dots: Facile synthesis and application as a turn-off fluorescent probe for detection of Hg 2+ ions[J]. Biosens Bioelectron ,2014,5583–5590. https://doi.org/10.1016/j.bios.2013.11.074 Rong M, Feng Y, ,Wang Y et al One-pot solid phase pyrolysis synthesis of nitrogen-doped carbon dots for Fe 3+ sensing and bioimaging[J].Sensors & Actuators: B. Chemical,2017,245868–245874. https://doi.org/10.1016/j.snb.2017.02.014 Reza A, Mohammad H, ,Javad K Fluorescent organic nanoparticles with enhanced fluorescence by self-aggregation and their application for detection of Fe 3+ ions[J]. New J Chem 2018, 42(8):5929–5936. https://doi.org/10.1039/c7nj04343k Kapoor A, Yadav S, ,Arya KR et al Punica granatum and Citrus limetta biowaste-derived sorbents for the removal of heavy metals and organic compounds: sustainability perspectives[J].Biomass Conversion and Biorefinery,2024,(prepublish):1–21. https://doi.org/10.1007/s13399-024-05726-8 Biller VD, Bruland WK Analysis of Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb in seawater using the Nobias-chelate PA1 resin and magnetic sector inductively coupled plasma mass spectrometry (ICP-MS)[J].Marine Chemistry,2012,130-13112-20. https://doi.org/10.1016/j.marchem.2011.12.001 Lunvongsa S, Oshima M, ,Motomizu S Determination of total and dissolved amount of iron in water samples using catalytic spectrophotometric flow injection analysis[J].Talanta,2005,68(3):969–973. https://doi.org/10.1016/j.talanta.2005.06.067 Ghaedi M, Shokrollahi A, ,Kianfar A et al The determination of some heavy metals in food samples by flame atomic absorption spectrometry after their separation-preconcentration on bis salicyl aldehyde, 1,3 propan diimine (BSPDI) loaded on activated carbon[J]. J Hazard Mater 2007, 154(1):128–134. https://doi.org/10.1016/j.jhazmat.2007.10.003 Zhu Y, Pan D, ,Hu X et al An electrochemical sensor based on reduced graphene oxide/gold nanoparticles modified electrode for determination of iron in coastal waters[J]. Sens Actuators: B Chem ,2017,2431–2437. https://doi.org/10.1016/j.snb.2016.11.108 Shuhua L, Yunchao L, ,Jun C et al Sulfur-doped graphene quantum dots as a novel fluorescent probe for highly selective and sensitive detection of Fe(3+).[J].Analytical chemistry,2014,86(20):10201–10207. https://doi.org/10.1021/ac503183y Zhang H, Huang Y, ,Lin X et al Lanthanum loaded graphitic carbon nitride nanosheets for highly sensitive and selective fluorescent detection of iron ions[J].Sensors & Actuators: B. Chemical,2018,2552218–2552222. https://doi.org/10.1016/j.snb.2017.09.026 Sun L, Li J, ,Cai J et al One pot synthesis of gold nanoparticles using chitosan with varying degree of deacetylation and molecular weight[J].Carbohydrate Polymers,2017,178105–178114. https://doi.org/10.1016/j.carbpol.2017.09.032 Manesh M, ,Sukumaran S, Nandini S Synthesis and Characterization of Gold-Chitosan Nanocomposite and Application of Resultant Nanocomposite in Sensors.[J].Colloids and Surfaces B: Biointerfaces,2012,93(1):143–147. https://doi.org/10.1016/j.colsurfb.2011.12.028 Zhou S, Han X, ,Liu Y SWASV performance toward heavy metal ions based on a high-activity and simple magnetic chitosan sensing nanomaterials[J]. J Alloys Compd ,2016,6841–6847. https://doi.org/10.1016/j.jallcom.2016.05.152 Gracia M, Anna R, ,Vanesa A et al Bactericidal Effect of Gold-Chitosan Nanocomposites in Coculture Models of Pathogenic Bacteria and Human Macrophages.[J].ACS applied materials & interfaces,2017,9(21):17693–17701. https://doi.org/10.1021/acsami.6b15123 Jing B, Changjun H, ,Mei C et al Plant Esterase-Chitosan/Gold Nanoparticles-Graphene Nanosheet Composite-Based Biosensor for the Ultrasensitive Detection of Organophosphate Pesticides.[J].Journal of agricultural and food chemistry,2015,63(47):10319–10326. https://doi.org/10.1021/acs.jafc.5b03971 Oscar RSebastiánB, ,Cristian S et al Biobased Chitosan Nanocomposite Films Containing Gold Nanoparticles: Obtainment, Characterization, and Catalytic Activity Assessment.[J].ACS Applied Materials & Interfaces,2017,9(19):16561–16570. https://doi.org/10.1021/acsami.7b04422 Abdullah S, Azeman HN, ,Mobarak NN et al Sensitivity enhancement of localized SPR sensor towards Pb(II) ion detection using natural bio-polymer based carrageenan[J].Optik,2018,168784–168793. https://doi.org/10.1016/j.ijleo.2018.05.016 Zhang Z, Fu X, ,Li K et al One-step fabrication of electrochemical biosensor based on DNA-modified three-dimensional reduced graphene oxide and chitosan nanocomposite for highly sensitive detection of Hg(II)[J].Sensors & Actuators: B. Chemical,2016,225453–225462. https://doi.org/10.1016/j.snb.2015.11.091 Sen S, Tiwari NO, ,Arya KR et al New insights on microbial extracellular polysaccharides: production, biological activity, and applications[J].Biomass Conversion and Biorefinery,2025,(prepublish):1–30. https://doi.org/10.1007/s13399-025-06802-3 Dolgopyatova VN, Novikov YV, ,Konovalova NI Influence of the degree of deacetylation on the rate of acid degradation of chitin and chitosan and on the yield of D(+)-glucosamine hydrochloride[J]. Russian J Appl Chem 2011, 84(10):1826–1829. https://doi.org/10.1134/s1070427211100259 Thota PS, Thota MS, ,Bhagavatham SS et al Facile one-pot hydrothermal synthesis of stable and biocompatible fluorescent carbon dots from lemon grass herb[J].IET Nanobiotechnology,2018,12(2):127–132. https://doi.org/10.1049/iet-nbt.2017.0038 Dong Y, Wang R, ,Li H et al Polyamine-functionalized carbon quantum dots for chemical sensing[J].Carbon,2012,50(8):2810–2815. https://doi.org/10.1016/j.carbon.2012.02.046 Murugan N, Prakash M, ,Jayakumar M et al (2019) Green synthesis of fluorescent carbon quantum dots from Eleusine coracana and their application as a fluorescence ‘turn-off’ sensor probe for selective detection of Cu 2+ [J]. Appl Surf Sci 476468–476480. https://doi.org/10.1016/j.apsusc.2019.01.090 Song Z, Quan F, ,Xu Y et al Multifunctional N,S co-doped carbon quantum dots with pH- and thermo-dependent switchable fluorescent properties and highly selective detection of glutathione[J].Carbon,2016,104169–104178. https://doi.org/10.1016/j.carbon.2016.04.003 Dong Y, Shao J, ,Chen C et al Blue luminescent graphene quantum dots and graphene oxide prepared by tuning the carbonization degree of citric acid[J].Carbon,2012,50(12):4738–4743. https://doi.org/10.1016/j.carbon.2012.06.002 Gao Z, Lin Z, ,Chen X et al Carbon dots-based fluorescent probe for trace Hg 2+ detection in water sample[J]. Sens Actuators: B Chem ,2016,222965–222971. https://doi.org/10.1016/j.snb.2015.09.032 Liu Y, Zhao Y, ,Zhang Y One-step green synthesized fluorescent carbon nanodots from bamboo leaves for copper(II) ion detection[J]. Sens Actuators: B Chem ,2014,196647–196652. https://doi.org/10.1016/j.snb.2014.02.053 Huibo W, Mengling Z, ,Kaiqiang W et al Pyrrolic nitrogen dominated the carbon dot mimic oxidase activity[J].Carbon,2021,179692–179700. https://doi.org/10.1016/j.carbon.2021.04.061 Kou CL, Yi MC, ,Xin XL et al Catalyst- and oxidant-free electrochemicalpara-selective hydroxylation ofN-arylamides in batch and continuous-flow[J].GREEN CHEMISTRY,2020,22(19):6437–6443. https://doi.org/10.1039/d0gc02294b Wang YJ, Wei XF, Su Y, Xu R, Song DQ, Ding L, Chen YH (2024) Highly sensitive fluoroprobe for detecting Sudan dyes in paprika utilizing carbon dot-embedded zeolitic imidazolate framework-8. Food Chem 438:137975. https://doi.org/10.1016/j.foodchem.2023.137975 LIU R, ZHANG Y, PIAO Y et al (2021) Development of nitrogen-doped carbon quantum dots as fluorescent probes for highly selective and sensitive detection of the heavy-ion Fe 3+ [J]. Carbon Lett 31:821–829. https://doi.org/10.1007/s42823-020-00222-1 AHN J, SONG Y, KWON JE et al (2019) Food waste-driven N-doped carbon dots: applications for Fe 3+ sensing and cell imaging[J]. Mater Sci Engineering: C 102:106–112. https://doi.org/10.1016/j.msec.2019.04.019 ŠAFRANKO S, STANKOVIĆ A, HAJRA S et al (2021) Preparation of multifunctional N-doped carbon quantum dots from citrus clementina peel: investigating targeted pharmacological activities and the potential application for Fe 3+ sensing[J]. Pharmaceuticals 14(8):857. https://doi.org/10.3390/ph14090857 YU J, XU C X, TIAN Z S et al (2016) Facilely synthesized N-doped carbon quantum dots with high fluorescent yield for sensing Fe 3+ [J]. New J Chem 40(3):2083–2088. https://doi.org/10.1039/c5nj03252k XU F Z, SHI H, HE X X et al (2015) Masking agent-free and channel-switch-mode simultaneous sensing of Fe 3+ and Hg 2+ using dual-excitation graphene quantum dots[J]. Analyst 140(11):3925–3928. https://doi.org/10.1039/c5an00468c ABBAS A, TABISH T A, BULL S J et al (2020) High yield synthesis of graphene quantum dots from biomass waste as a highly selective probe for Fe 3+ sensing[J]. Sci Rep 10:21262. https://doi.org/10.1038/s41598-020-78070-2 LOU Y, JI J (2020) Cane molasses graphene quantum dots passivated by PEG functionalization for detection of metal ions[J]. ACS Omega 5(12):6763–6772. https://doi.org/10.1021/acsomega.0c00098 Li SY, Wang N, ,Wang L et al Carbon quantum dots derived from camphor tree leaves biomass as a highly selective probe for Fe 3+ sensing[J].Biomass Conversion and Biorefinery,2024,(prepublish):1–11. https://doi.org/10.1007/s13399-024-05997-1 Gu D, Shang S, ,Yu Q et al Green synthesis of nitrogen-doped carbon dots from lotus root for Hg(II) ions detection and cell imaging[J]. Appl Surf Sci ,2016,39038–39042. https://doi.org/10.1016/j.apsusc.2016.08.012 Aleksandra FANB Quantum Chemical Characterization and Design of Quantum Dots for Sensing Applications.[J].The journal of physical chemistry. A,2022,126(19):2899–2908. https://doi.org/10.1021/acs.jpca.2c00947 Venkatachalam V, Ganapathy S, ,Nanthagopal M et al Fluorescence imaging of onion epidermal cell utilizing highly luminescent water-soluble CdTe colloidal quantum dots[J].Inorganic Chemistry Communications,2023,149. https://doi.org/10.1016/J.INOCHE.2022.110352 Wang W, Li C Measurement of the light absorption and scattering properties of onion skin and flesh at 633nm[J]. Postharvest Biol Technol ,2013,86494-501. https://doi.org/10.1016/j.postharvbio.2013.07.032 Additional Declarations No competing interests reported. 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09:39:29","extension":"xml","order_by":26,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":142680,"visible":true,"origin":"","legend":"","description":"","filename":"1b838f44bcdb43f78c35e5e46785f5c31structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-8120500/v1/ca807984d78c81f5df5da85d.xml"},{"id":97674691,"identity":"e3b8aab3-7711-4b69-9605-7cc072c02bde","added_by":"auto","created_at":"2025-12-08 09:43:50","extension":"html","order_by":27,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":151676,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8120500/v1/34e9622a08607e8c0c4d9da7.html"},{"id":97674829,"identity":"2d4b7edb-caea-46cb-8ad6-4ad628606ab0","added_by":"auto","created_at":"2025-12-08 09:44:22","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":225803,"visible":true,"origin":"","legend":"\u003cp\u003ePreparation process of chitosan-derived carbon quantum dots.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8120500/v1/7f15f665ff302702fb723a84.png"},{"id":97674987,"identity":"551d444a-38c7-4557-8c67-622fea6b6211","added_by":"auto","created_at":"2025-12-08 09:45:06","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":616484,"visible":true,"origin":"","legend":"\u003cp\u003e(a) CS-CQDs emission spectra at 12h for various temperatures;(b) CS-CQDs emission spectra at 190°C for different reaction durations;(c) \u0026amp; (d) Peak intensity histograms for (a) \u0026amp; (b), respectively.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8120500/v1/be14d13749f70fed54995a49.png"},{"id":97672937,"identity":"2daf95be-594f-490c-ba72-da049f902ac4","added_by":"auto","created_at":"2025-12-08 09:39:06","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":700789,"visible":true,"origin":"","legend":"\u003cp\u003e(a) TEM micrographs at 68000× magnification of CS-CQDs and 680,000×magnification;(b)HRTEM micrographs at 1.25 M×magnification;(c) XRD pattern of CS-CQDs;(d) FTIR spectra of chitosan and CS-CQDs (red line: chitosan quantum dots; black line: chitosan).\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8120500/v1/9efa80d1aaca6f5e6f860483.png"},{"id":97654945,"identity":"730acc2f-ccb5-47c1-8021-f86156f22895","added_by":"auto","created_at":"2025-12-08 07:07:20","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":897138,"visible":true,"origin":"","legend":"\u003cp\u003e(a) XPS spectrum of CS-CQDs;(b) High-resolution C 1s spectrum;(c) High-resolution N 1s spectrum;(d) High-resolution O 1s spectrum.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8120500/v1/241753820fd649574eb9c376.png"},{"id":97654931,"identity":"4747824b-578c-4c49-8d82-dd86b045ee9d","added_by":"auto","created_at":"2025-12-08 07:07:19","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1032619,"visible":true,"origin":"","legend":"\u003cp\u003e(a) UV-Vis spectrum of CS-CQDs;(b) Excitation and emission spectra of CS-CQDs;(c) Emission spectra of CS-CQDs under 300-390 nm excitation;(d) Bar chart of peak intensities derived from Fig.5 (c).\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8120500/v1/1cc5d70eb16438160b66749f.png"},{"id":97654950,"identity":"e83471e3-c9bb-4c6e-b2a1-b31487f9db7e","added_by":"auto","created_at":"2025-12-08 07:07:20","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":490170,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Fluorescence emission spectra of CS-CQDs in the presence of different metal ions;(b) Bar chart of peak intensities corresponding to the curves in (a);(c) Fluorescence images of CS-CQDs solutions with different metal ions;(d) Interference study of CS-CQDs solutions with 1000 µM Fe\u003csup\u003e3+\u003c/sup\u003e (blue bars) and other metal ions (gray bars);(e) Fluorescence intensity of CS-CQDs solutions at different pH values;(f) Fluorescence intensity of CS-CQDs solutions with 1000 µM Fe\u003csup\u003e3+\u003c/sup\u003e at different pH values.\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-8120500/v1/cbff7f4ed026a2fd0e9ec0c0.png"},{"id":97674957,"identity":"dd03c28d-713a-4abc-964a-03aa30dadec8","added_by":"auto","created_at":"2025-12-08 09:44:59","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":911030,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Fluorescence emission spectra of CS-CQDs with increasing Fe\u003csup\u003e3+\u003c/sup\u003e concentration (0-1500 µM);(b) Bar chart of maximum peak intensities derived from panel (a);(c) Nonlinear Stern-Volmer quenching plot of CS-CQDs solutions with Fe\u003csup\u003e3+\u003c/sup\u003e concentrations (0-1500 µM). Inset: Linear region of the Stern-Volmer plot (100-700 µM Fe\u003csup\u003e3+\u003c/sup\u003e).\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-8120500/v1/7a4e6169fad5be81dbaea6e8.png"},{"id":97654943,"identity":"ce2aff7f-a2cb-4aaf-a826-1ff9ad139e6b","added_by":"auto","created_at":"2025-12-08 07:07:20","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":643728,"visible":true,"origin":"","legend":"\u003cp\u003e(a) UV-Vis absorption spectra of CS-CQDs solution before and after Fe\u003csup\u003e3+\u003c/sup\u003e addition.(b) Fourier transform infrared spectra of CS-CQDs before and after binding with Fe\u003csup\u003e3+\u003c/sup\u003e.(c) Fluorescence lifetime decay curve of CS-CQDs without Fe\u003csup\u003e3+\u003c/sup\u003e.(d) Fluorescence lifetime decay curve of CS-NQDs after the addition of Fe\u003csup\u003e3+\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-8120500/v1/e9f1f304ee6e806ce81d8309.png"},{"id":97654952,"identity":"a57fe7dd-c61d-4f02-8250-739d7c977462","added_by":"auto","created_at":"2025-12-08 07:07:20","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":942451,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Fluorescence emission intensity of CS-CQDs solution under light exposure for different durations;(b) Fluorescence emission intensity of CS-CQDs solution under dark conditions for different durations;(c) Decay curves derived from peak intensities of (a) and (b).\u003c/p\u003e","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-8120500/v1/8976132b7deb24db41394663.png"},{"id":97654948,"identity":"594b06e1-b2aa-462b-aa24-801817a0171f","added_by":"auto","created_at":"2025-12-08 07:07:20","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":240228,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFluorescence imaging of onion epidermal cells using chitosan carbon quantum dots (CS-CQDs) and their response to Fe\u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e3+\u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003e.\u003c/strong\u003e\u0026nbsp;(a, d) Low-magnification and (b, e) high-magnification images of cells after staining with CS-CQDs solution. (a-c) were captured under bright-field illumination, while (d-f) were acquired under UV light excitation. (c, f) show the corresponding fields after treatment with FeCl\u003csub\u003e3\u003c/sub\u003e solution, demonstrating significant fluorescence quenching under UV light excitation (f).\u003c/p\u003e","description":"","filename":"floatimage10.png","url":"https://assets-eu.researchsquare.com/files/rs-8120500/v1/995cd4c558c13e72ec9b6d94.png"},{"id":103765421,"identity":"93183d4c-efbf-4187-8d66-a972e8e6b07b","added_by":"auto","created_at":"2026-03-02 16:00:51","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":7998763,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8120500/v1/5fd86c0f-c4a4-4aea-9289-736264c93522.pdf"},{"id":97674860,"identity":"998c88d6-5625-4d46-b6ae-7088eba5103f","added_by":"auto","created_at":"2025-12-08 09:44:32","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":231562,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryMaterial1114.docx","url":"https://assets-eu.researchsquare.com/files/rs-8120500/v1/e35617325484fe344c22e489.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Green Synthesis of Nitrogen-Doped Carbon Quantum Dots from Medium-Molecular-Weight Chitosan for Ultrasensitive Fe³⁺ Detection and Responsive Probe ","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eCarbon quantum dots, as representative zero-dimensional nanomaterials, have demonstrated significant application value in fluorescence labeling\u003csup\u003e[1]\u003c/sup\u003e, biomedical imaging\u003csup\u003e[2]\u003c/sup\u003e, and photocatalytic energy conversion\u003csup\u003e[3]\u003c/sup\u003e owing to their unique photoluminescence properties, excellent water solubility, and low environmental toxicity\u003csup\u003e[4]\u003c/sup\u003e. Since their discovery, various synthesis techniques have been developed, including arc discharge\u003csup\u003e[5]\u003c/sup\u003e, laser ablation\u003csup\u003e[6]\u003c/sup\u003e, electrochemical synthesis\u003csup\u003e[7]\u003c/sup\u003e, and microwave heating\u003csup\u003e[8]\u003c/sup\u003e. Among these, hydrothermal synthesis is regarded as the most industrially promising route due to its operational simplicity, tunable process, and low equipment costs\u003csup\u003e[9]\u003c/sup\u003e. Notably, green synthesis strategies employing natural biomass and polysaccharide materials as precursors have emerged as a research hotspot, significantly reducing material cytotoxicity while achieving dual goals of resource recycling and cost control\u003csup\u003e[10\u0026ndash;12]\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eIron, the fourth most abundant metal in the Earth\u0026rsquo;s crust, exhibits dual environmental behavior: Fe\u003csup\u003e3+\u003c/sup\u003e serves as an essential component for oxygen metabolism in organisms\u003csup\u003e[13\u0026ndash;14]\u003c/sup\u003e, yet also acts as a hazardous heavy metal pollutant from industrial emissions\u003csup\u003e[15]\u003c/sup\u003e. Rapid and highly sensitive detection of Fe\u003csup\u003e3+\u003c/sup\u003e is crucial for monitoring its concentration in natural environments, industrial processes, and biological samples. Traditional techniques, such as inductively coupled plasma mass spectrometry\u003csup\u003e[16]\u003c/sup\u003e, spectrophotometry\u003csup\u003e[17]\u003c/sup\u003e, atomic absorption spectroscopy\u003csup\u003e[18]\u003c/sup\u003e, and electrochemical methods\u003csup\u003e[19]\u003c/sup\u003e, are limited by complex sample pretreatment and high equipment costs\u003csup\u003e[17, 20]\u003c/sup\u003e. In contrast, fluorescence spectroscopy has become an emerging technology for Fe\u003csup\u003e3+\u003c/sup\u003e detection due to its operational simplicity, rapid response (\u0026lt;\u0026thinsp;5 min), and high sensitivity\u003csup\u003e[21]\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eThis study innovatively employs medium-molecular-weight chitosan (200 kDa, deacetylation degree\u0026thinsp;\u0026ge;\u0026thinsp;90%) as a single carbon source. This natural polysaccharide, derived from partial deacetylation of chitin (An abundant agricultural waste from seafood processing), consists of D-glucosamine and N-acetyl-D-glucosamine units linked by \u0026beta;-(1\u0026ndash;4) glycosidic bonds\u003csup\u003e[22\u0026ndash;23]\u003c/sup\u003e. The dense distribution of amino (-NH\u003csub\u003e2\u003c/sub\u003e) and hydroxyl (-OH) groups\u003csup\u003e[24]\u003c/sup\u003e within chitosan molecules confers exceptional bioactivity and heavy metal chelation capabilities, leading to widespread applications in biomedicine\u003csup\u003e[25\u0026ndash;27]\u003c/sup\u003e and environmental remediation\u003csup\u003e[28\u0026ndash;29]\u003c/sup\u003e. Considering molecular weight effects on carbonization behavior, this precursor demonstrates three advantages over high-MW (\u0026gt;\u0026thinsp;50 kDa) counterparts (limited solubility) and low-MW (\u0026lt;\u0026thinsp;5 kDa) materials (excessive fragmentation):\u003c/p\u003e\n\u003col\u003e\n \u003cli\u003e\u003cstrong\u003eModerate chain length (~\u0026thinsp;1000 sugar units) ensures sufficient dispersibility in acidic solutions.\u003c/strong\u003e\u003c/li\u003e\n \u003cli\u003e\u003cstrong\u003eHigh deacetylation degree provides\u0026thinsp;\u0026ge;\u0026thinsp;0.9 free amino groups per sugar unit, enabling dense nitrogen doping.\u003c/strong\u003e\u003c/li\u003e\n \u003cli\u003e\u003cstrong\u003eOptimized hydrothermal conditions (190\u0026deg;C, 24 h) facilitate controlled cleavage of \u0026beta;-(1\u0026ndash;4) glycosidic bonds with concurrent formation of pyrrolic heterocycles.\u003c/strong\u003e\u003c/li\u003e\n\u003c/ol\u003e\n\u003cp\u003eExperimental results confirm that selective quenching occurs via a synergistic mechanism involving coordination effect between Fe\u003csup\u003e3+\u003c/sup\u003e and surface functional groups (amino/hydroxyl/pyrrolic) combined with photoinduced electron transfer, enabling highly specific Fe\u003csup\u003e3+\u003c/sup\u003e detection. The detection limit reached 2.72 \u0026micro;M (S/N\u0026thinsp;=\u0026thinsp;3) with significant interference immunity against 15 coexisting ions (e.g., Al\u003csup\u003e3+\u003c/sup\u003e, Cu\u003csup\u003e2+\u003c/sup\u003e). Furthermore, successful application to real water samples demonstrated recovery rates of 99.98\u0026ndash;100.04%, validating the method\u0026rsquo;s reliability. Additionally, the carbon quantum dots were successfully applied as fluorescent probes for cellular imaging in onion epidermis, exhibiting clear Fe\u003csup\u003e3+\u003c/sup\u003e-induced fluorescence quenching, which underscores their potential for bioimaging and intracellular ion detection. This work provides new strategies for high-value utilization of biopolysaccharides\u003csup\u003e[30]\u003c/sup\u003e in rapid environmental detection and cell imaging.\u003c/p\u003e"},{"header":"2 Materials and Methods","content":"\u003cp\u003eThe medium-molecular-weight chitosan (200 kDa) was prepared using the typical acid degradation method\u003csup\u003e[\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]\u003c/sup\u003e. All chemical reagents were of analytical reagent (AR) grade, with details as follows: Aladdin Biochemical Technology Co., Ltd.: Ferric chloride hexahydrate (FeCl\u003csub\u003e3\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO), barium chloride dihydrate (BaCl\u003csub\u003e2\u003c/sub\u003e\u0026middot;2H\u003csub\u003e2\u003c/sub\u003eO), mercury(II) nitrate monohydrate (Hg(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u0026middot;H\u003csub\u003e2\u003c/sub\u003eO), magnesium chloride (MgCl\u003csub\u003e2\u003c/sub\u003e), aluminum chloride hexahydrate (AlCl\u003csub\u003e3\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO), cadmium chloride monohydrate (CdCl\u003csub\u003e2\u003c/sub\u003e\u0026middot;H\u003csub\u003e2\u003c/sub\u003eO), sodium chloride (NaCl). Sinopharm Chemical Reagent Co., Ltd.: Ferrous chloride (FeCl\u003csub\u003e2\u003c/sub\u003e), nickel chloride hexahydrate (NiCl\u003csub\u003e2\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO), lead chloride (PbCl\u003csub\u003e2\u003c/sub\u003e), calcium chloride dihydrate (CaCl\u003csub\u003e2\u003c/sub\u003e\u0026middot;2H\u003csub\u003e2\u003c/sub\u003eO). Macklin Biochemical Technology Co., Ltd.: Copper(II) chloride dihydrate (CuCl\u003csub\u003e2\u003c/sub\u003e\u0026middot;2H\u003csub\u003e2\u003c/sub\u003eO), zinc chloride (ZnCl\u003csub\u003e2\u003c/sub\u003e), cesium chloride (CsCl).\u003c/p\u003e\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Preparation of Chitosan Quantum Dots\u003c/h2\u003e\u003cp\u003eChitosan-derived carbon quantum dots (CS-CQDs) were synthesized via a hydrothermal method, and the detailed preparation process is illustrated in Fig.\u0026nbsp;1. First, 0.4 grams of chitosan powder and 0.8 grams of glacial acetic acid were weighed and placed in a beaker containing 40 milliliters of deionized water, followed by thorough stirring to ensure complete mixing. The homogeneous solution was then transferred into a hydrothermal reaction vessel, which was subsequently placed in a blast drying oven set at 190\u0026deg;C for 24 hours. After the reaction, the product was centrifuged for 15 minutes at a speed of 5,000 rpm. To remove macromolecular impurities, the supernatant was carefully extracted using a syringe equipped with a 0.45 \u0026micro;m syringe filter and then transferred into a dialysis bag. The solution in the dialysis bag underwent purification for 48 hours, with the dialysate being replaced every 6 hours. Next, the dialyzed solution was aspirated again using a syringe fitted with a 0.22 \u0026micro;m syringe filter and dispensed into a culture dish. The solution in the dish was then subjected to freeze-drying and placed in a freeze dryer for 48 hours. Upon completion of drying, the sample was retrieved, yielding a light-yellow powder identified as chitosan carbon quantum dots.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2 Process Optimization Studies\u003c/h2\u003e\u003cdiv id=\"Sec5\" class=\"Section3\"\u003e\u003ch2\u003e2.2.1 Optimal Reaction Temperature Study\u003c/h2\u003e\u003cp\u003eTo precisely determine the optimal synthesis temperature for CS-CQDs, a gradient heating method was employed. Specifically, hydrothermal reaction temperatures were sequentially set from 150\u0026deg;C to 210\u0026deg;C at 10\u0026deg;C intervals (150\u0026deg;C, 160\u0026deg;C, 170\u0026deg;C, 180\u0026deg;C, 190\u0026deg;C, 200\u0026deg;C, 210\u0026deg;C), while maintaining a constant reaction time of 12 hours. After completion, the purified solutions were transferred to 5 mL quartz cuvettes, and their fluorescence emission intensities were measured using a fluorescence spectrophotometer (λ\u003csub\u003eex\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;330 nm).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section3\"\u003e\u003ch2\u003e2.2.2 Optimal Reaction Time Study\u003c/h2\u003e\u003cp\u003eAfter identifying 190\u0026deg;C as the optimal reaction temperature, the effect of reaction time on CS-CQDs performance was further investigated. Different time gradients were tested (3, 6, 12, 18, 24, 36, and 48 hours) while keeping other parameters unchanged. The purified solutions were analyzed under identical conditions (5 mL cuvette, λ\u003csub\u003eex\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;330 nm) to measure fluorescence emission intensity.\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e2.3 Response of CS-CQDs to Different Ions\u003c/h2\u003e\u003cp\u003eTo validate the selective recognition capability of CS-CQDs toward Fe\u003csup\u003e3+\u003c/sup\u003e, a comparative analysis was conducted with 15 common metal ions (including Fe\u003csup\u003e3+\u003c/sup\u003e, Fe\u003csup\u003e2+\u003c/sup\u003e, Cu\u003csup\u003e2+\u003c/sup\u003e, Ba\u003csup\u003e2+\u003c/sup\u003e, Hg\u003csup\u003e2+\u003c/sup\u003e, etc.). Stock solutions (1000 \u0026micro;M) of each metal salt were prepared in deionized water. Then, 0.5 mL of each metal ion solution was added to 3 mL of CS-CQDs solution, incubated in the dark for 5 minutes, and analyzed via fluorescence spectrophotometry (λ\u003csub\u003eex\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;330 nm).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e2.4 Stability Study\u003c/h2\u003e\u003cp\u003eThe pH-dependent fluorescence stability of CS-CQDs was evaluated by adjusting the pH (1\u0026ndash;14) of the dispersion using 0.1 M HCl/NaOH. Fluorescence emission intensity was measured at each pH under standardized conditions (λ\u003csub\u003eex\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;330 nm)\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e2.5 Fe\u003csup\u003e3+\u003c/sup\u003e Detection in Real Water Samples\u003c/h2\u003e\u003cp\u003eTo assess the practicality of CS-CQDs as Fe\u003csup\u003e3+\u003c/sup\u003e probes, tap water and lake water samples were analyzed. Before detection, samples were centrifuged to remove large insoluble impurities and filtered through 0.45 \u0026micro;m membranes. Fe\u003csup\u003e3+\u003c/sup\u003e concentrations were determined using the standard addition method.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\u003ch2\u003e2.6 Application of CS-CQDs as a Fe\u003csup\u003e3+\u003c/sup\u003e Responsive Probe in Cell Imaging\u003c/h2\u003e\u003cp\u003eTo verify the fluorescent labeling capability of CS-CQDs and the responsiveness to Fe\u003csup\u003e3+\u003c/sup\u003e in a biological environment, this study conducted fluorescence imaging experiments using onion epidermal cells as a model. First, freshly peeled onion epidermis was co-incubated with CS-CQDs, and its fluorescence labeling effect was observed under UV excitation light. Subsequently, 0.5 mL of 500 \u0026micro;M FeCl\u003csub\u003e3\u003c/sub\u003e solution was added to the sample using the lateral permeation method. After a reaction period of 10 minutes, fluorescence images were reacquired under the same microscope parameters.\u003c/p\u003e\u003c/div\u003e"},{"header":"3 Results and Discussion","content":"\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Synthesis Optimization\u003c/h2\u003e\u003cp\u003e\u003cul\u003e\u003cli\u003e\u003cp\u003eFigure\u0026nbsp;2(a) presents the fluorescence emission intensity of CS-CQDs synthesized at different reaction temperatures. The results indicate that fluorescence intensity peaks at 190\u0026deg;C, significantly exceeding values at other temperatures. Below 190\u0026deg;C, insufficient carbonization yields weakly fluorescent CS-CQDs, while temperatures above 190\u0026deg;C cause fluorescence reduction due to excessive carbonization. Hence, 190\u0026deg;C is optimal for CS-CQDs synthesis.\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eFigure\u0026nbsp;2(b) shows the fluorescence intensity of CS-CQDs prepared with varying reaction durations. The fluorescence intensity of CS-CQDs progressively increases up to 24 hours, beyond which it declines. Shorter reaction durations (\u0026lt;\u0026thinsp;24 h) lead to incomplete carbonization, producing undersized quantum dots with inadequate surface functional groups. At 24 h, complete carbonization generates uniformly-sized CS-CQDs with abundant surface groups, achieving peak fluorescence. Extended reaction durations (\u0026gt;\u0026thinsp;24 h) induce particle aggregation and surface oxidation through over-carbonization, diminishing fluorescence. Thus, 24 hours is established as the optimal reaction time.\u003c/p\u003e\u003c/li\u003e\u003c/ul\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Characterization\u003c/h2\u003e\u003cdiv id=\"Sec14\" class=\"Section3\"\u003e\u003ch2\u003e3.2.1 Morphological and structural analysis\u003c/h2\u003e\u003cp\u003e\u003cul\u003e\u003cli\u003e\u003cp\u003eThe morphology and size of the particles were established by the TEM. Figure\u0026nbsp;3(a) contains the HRTEM micrographs at different magnifications, which show low-dimensional carbon nanostructures. Figure\u0026nbsp;3(b) presents the HRTEM micrograph of CCDs. ImageJ analysis has revealed the interplanar lattice spacing to be 0.24 nm. This is in good agreement with the reported lattice spacing of graphite-related structures in the literature\u003csup\u003e[\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]\u003c/sup\u003e. X-ray diffraction (XRD) analysis revealed a broad peak at 22.5\u0026deg; (Fig.\u0026nbsp;3c), characteristic of amorphous carbon, verifying the predominantly amorphous structure of the CS-CQDs.\u003c/p\u003e\u003c/li\u003e\u003c/ul\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec15\" class=\"Section3\"\u003e\u003ch2\u003e3.2.2 Functional groups and elemental analysis\u003c/h2\u003e\u003cp\u003e\u003cul\u003e\u003cli\u003e\u003cp\u003eFourier-transform infrared (FTIR) spectroscopy analysis of chitosan and CS-CQDs (Fig.\u0026nbsp;3d) revealed critical structural transformations: The chitosan spectrum (black line) exhibited characteristic peaks at 3434.5 cm⁻\u0026sup1; (O-H/N-H stretching), 1646.9 cm⁻\u0026sup1; (C\u0026thinsp;=\u0026thinsp;O stretching, amide I), 1550.3 cm⁻\u0026sup1; (coupled N-H bending/C-N stretching, amide II), and 1081.8 cm⁻\u0026sup1; (β-(1\u0026ndash;4)-glycosidic bonds, C-O-C), while the CS-CQDs spectrum (red line) showed retention of O-H/N-H groups (3434.5 cm⁻\u0026sup1;), attenuated amide I/II peaks (1646.9/1550.3 cm⁻\u0026sup1;) indicating amide bond cleavage, emergence of a new peak at 1510 cm⁻\u0026sup1; suggesting aromatic/N-heterocyclic moieties (e.g., pyrrole), and disappearance of the 1081.8 cm⁻\u0026sup1; peak confirming glycosidic bond scission. Collectively, these changes demonstrate partial preservation of original hydroxyl/amino groups alongside fundamental restructuring\u0026mdash;via cleavage of glycosidic/amide bonds and nitrogen redistribution\u0026mdash;driving the transformation from polysaccharide to carbon quantum dot architecture.\u003c/p\u003e\u003c/li\u003e\u003c/ul\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eX-ray photoelectron spectroscopy (XPS) analysis of CS-CQDs revealed their elemental composition and chemical bonding states (Fig.\u0026nbsp;4a), showing three characteristic peaks at 282.1 eV (C 1s), 397.2 eV (N 1s), and 529.4 eV (O 1s) with atomic percentages of 64% C, 4% N, and 32% O. High-resolution spectra demonstrated: (i) C 1s deconvolution into sp\u0026sup2; carbon, C-OH, and C\u0026thinsp;=\u0026thinsp;O bonds\u003csup\u003e[\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]\u003c/sup\u003e; (ii) N 1s speciation as amino (-NH₂) and pyrrolic\u003csup\u003e[\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]\u003c/sup\u003e; and (iii) O 1s components of C-O and C\u0026thinsp;=\u0026thinsp;O bonds. These results indicate substantial sp\u0026sup3;-to-sp\u0026sup2; carbon conversion, partial nitrogen transformation into pyrrolic structures, and additional C\u0026thinsp;=\u0026thinsp;O formation via carbonization/oxidation, while retaining amino/hydroxyl groups in the CS-CQDs. The differences between chitosan and CS-CQDs are shown in Table\u0026nbsp;1.\u003c/p\u003e\u003cp\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 differences between chitosan and chitosan carbon quantum dots\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"7\"\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\u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCritical Structures\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eGlycosidic Bond\u003c/p\u003e\u003cp\u003e(C-O-C)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePeptide Bond\u003c/p\u003e\u003cp\u003e(-CO-NH-)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eAmino\u003c/p\u003e\u003cp\u003eGroup (-NH2)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eOxhydryl\u003c/p\u003e\u003cp\u003eGroup\u003c/p\u003e\u003cp\u003e(-OH)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eCarbon Structure\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003ePyrrolic Structure\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eChitosan\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eexist\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eexist\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eexist\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eexist\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003esp\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003enot exist\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCS-CQDs\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003enot exist\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003enot exist\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eretain\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eretain\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003esp\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003eexist\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Sec16\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Optical properties\u003c/h2\u003e\u003cp\u003eThe optical properties of CS-CQDs in aqueous solution were characterized by UV-Vis spectroscopy (Fig.\u0026nbsp;5a). Two distinct absorption peaks were observed at 239 nm and 272 nm. The peak at 239 nm is assigned to π\u0026rarr;π* transitions of localized π-conjugation domains (e.g., small aromatic clusters or isolated C\u0026thinsp;=\u0026thinsp;C bonds), while the peak at 272 nm arises from n\u0026rarr;π* transitions associated with hydroxyl/amino/ pyrrolic functional groups.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe absolute quantum yield of the prepared CS-CQDs, measured using an integrating sphere, was 3.96%. Furthermore, the fluorescence characteristics are presented in Fig.\u0026nbsp;5b. The excitation spectrum (orange curve) exhibits an optimal excitation wavelength at 330 nm, while the corresponding emission spectrum (blue curve) displays a maximum emission peak at 402 nm. Figure\u0026nbsp;5c displays the fluorescence spectra under excitation wavelengths ranging from 300 to 390 nm, revealing a progressive red shift of the maximum emission wavelength from approximately 400 nm to 480 nm. This phenomenon may originate from the size heterogeneity and distribution of multiple surface emission sites in N-CQDs\u003csup\u003e[\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eFurthermore, previous studies suggest that the optical properties of carbon dots correlate strongly with their surface functional groups\u003csup\u003e[\u003cspan additionalcitationids=\"CR37\" citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]\u003c/sup\u003e. Specifically in this work, the dominant 402 nm emission peak (Fig.\u0026nbsp;5b) provides direct evidence for pyrrolic moieties\u0026mdash;confirmed by their 400.2 eV XPS signature (Fig.\u0026nbsp;4c)\u0026mdash;serving as critical fluorophores. This emission profile corresponds to an optical bandgap of 3.1 eV (\u003cspan class=\"InlineEquation\"\u003e\u003c/span\u003e), which aligns with the characteristic blue emission of pyrrolic configurations in N-CQDs\u003csup\u003e[\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eCritically, the pyrrolic moieties serve as electron donors that inject photoexcited electrons into the LUMO of sp\u003csup\u003e2\u003c/sup\u003e-carbon via π-conjugated channels, thereby localizing electron-hole pairs. This process is structurally enabled by the 3.8 \u0026Aring; spatial alignment between C2-N and C6-O groups from chitosan pyrolysis, which directs efficient pyrrole cyclization (\u0026sim;42%)\u003csup\u003e[\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eThe fluorescence intensity is optimized by introducing pyrrolic, which is achieved through the specific hydrothermal synthesis process (190\u0026deg;C, 24h) for the medium-molecular-weight Chitosan. The pyrrolic structures act as dual functional units: (i) bandgap modulators that tune emission energy through ΔE control, and (ii) electron-transfer pathway. This synergistic design establishes CS-CQDs as efficient blue emitters.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec17\" class=\"Section2\"\u003e\u003ch2\u003e3.4 Fe\u003csup\u003e3+\u003c/sup\u003e Sensing Performance\u003c/h2\u003e\u003cp\u003e\u003cb\u003e3.4.1 Specificity\u003c/b\u003e\u003c/p\u003e\u003cp\u003eThe fluorescence images of CS-CQDs solutions under 310 nm excitation (Fig.\u0026nbsp;6a,b) revealed that while the addition of various metal ions (Fe\u003csup\u003e2+\u003c/sup\u003e, Cu\u003csup\u003e2+\u003c/sup\u003e, Ba\u003csup\u003e2+\u003c/sup\u003e, Hg\u003csup\u003e2+\u003c/sup\u003e, Zn\u003csup\u003e2+\u003c/sup\u003e, Ni\u003csup\u003e2+\u003c/sup\u003e, Mg\u003csup\u003e2+\u003c/sup\u003e, Co\u003csup\u003e2+\u003c/sup\u003e, Pb\u003csup\u003e2+\u003c/sup\u003e, Ca\u003csup\u003e2+\u003c/sup\u003e, Al\u003csup\u003e3+\u003c/sup\u003e, Cd\u003csup\u003e2+\u003c/sup\u003e, Na\u003csup\u003e+\u003c/sup\u003e, Cs\u003csup\u003e+\u003c/sup\u003e) induced only minor changes in fluorescence intensity compared to the pure CS-CQDs control, a remarkably strong fluorescence quenching effect was specifically observed upon Fe\u003csup\u003e3+\u003c/sup\u003e addition.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cdiv id=\"Sec18\" class=\"Section3\"\u003e\u003ch2\u003e3.4.2 Interference Test and pH Stability Analysis\u003c/h2\u003e\u003cp\u003eThe fluorescence spectra of CS-CQDs solutions were measured following the addition of either: (i) 1000 \u0026micro;M Fe\u003csup\u003e3+\u003c/sup\u003e alone or (ii) 1000 \u0026micro;M Fe\u003csup\u003e3+\u003c/sup\u003e co-existing with 1000 \u0026micro;M of other metal ions (Fig.\u0026nbsp;6d). The results revealed minimal interference (\u0026lt;\u0026thinsp;5% variation) from co-existing ions on the Fe\u003csup\u003e3+\u003c/sup\u003e-induced fluorescence quenching effect (grey bars), demonstrating the CS-CQDs' remarkable selectivity for Fe\u003csup\u003e3+\u003c/sup\u003e detection in mixed-ion environments.\u003c/p\u003e\u003cp\u003epH-dependent studies (Fig.\u0026nbsp;6e) reveal that CS-CQDs maintain stable fluorescence intensity within the pH 2\u0026ndash;10 range, while severe intensity loss occurs at pH\u0026thinsp;\u0026lt;\u0026thinsp;2 or \u0026gt;\u0026thinsp;10 due to protonation/deprotonation effects. Remarkably, Fe\u003csup\u003e3+\u003c/sup\u003e-induced quenching remains pH-independent across this pH 2\u0026ndash;10 range (Fig.\u0026nbsp;6f), with consistent quenching efficiency observed upon adding 1000 \u0026micro;M Fe\u003csup\u003e3+\u003c/sup\u003e at pH 2\u0026ndash;10. This underscores the robustness of CS-CQDs for Fe\u003csup\u003e3+\u003c/sup\u003e sensing in environmentally relevant conditions.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec19\" class=\"Section3\"\u003e\u003ch2\u003e3.4.3 Limit of detection\u003c/h2\u003e\u003cp\u003eThe sensitivity of the CS-CQDs solution toward Fe\u003csup\u003e3+\u003c/sup\u003e was evaluated at pH 7. As shown in Fig.\u0026nbsp;7a and b, under 330 nm excitation, the fluorescence intensity of CS-CQDs exhibited a pronounced concentration-dependent quenching effect with increasing Fe\u003csup\u003e3+\u003c/sup\u003e concentrations from 0 to 1500 \u0026micro;M.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;7c presents the Stern-Volmer quenching curve of CS-CQDs solutions with Fe\u003csup\u003e3+\u003c/sup\u003e concentrations ranging from 0 to 1500 \u0026micro;M. The inset displays the linear portion of the Stern-Volmer plot between 100\u0026ndash;700 \u0026micro;M. Within this concentration range, the fluorescence quenching efficiency (F₀-F)/F₀ shows an excellent linear correlation with Fe\u003csup\u003e3+\u003c/sup\u003e concentration, described by the regression equation: (F₀-F)/F₀ = 0.00032X\u0026thinsp;+\u0026thinsp;0.113 (R\u0026sup2; = 0.9955), where F₀ and F represent the fluorescence intensities of CS-CQDs solutions before and after Fe\u003csup\u003e3+\u003c/sup\u003e addition, respectively. Furthermore, the detection limit was determined to be 2.72 \u0026micro;M at a signal-to-noise ratio (S/N) of 3.\u003c/p\u003e\u003cp\u003eTable\u0026nbsp;2 demonstrates that the CQDs synthesized in this study exhibit significantly lower detection limits for Fe\u0026sup3;⁺ compared to many previously reported values. Notably, our detection limit is substantially lower than the World Health Organization (WHO) guideline value for Fe\u0026sup3;⁺ concentration in drinking water (5.36 \u0026micro;M)\u003csup\u003e[\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]\u003c/sup\u003e. This remarkably low limit of detection highlights the significant potential of the prepared CQDs for trace Fe\u0026sup3;⁺ sensing applications.\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\u003eComparison of various quantum dots fluorescent sensors for detecting 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\u003eType of CQDs\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003ePrecursors\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eMethod of synthesis\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eLinear range (\u0026micro;M)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eLOD (\u0026micro;M)\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\u003eN-CQDs\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eMatrimony vine\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eHydrothermal\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e5\u0026ndash;60\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e2.22\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e[42]\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCQDs\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCat feed stocks\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eHydrothermal\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0\u0026ndash;500\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e32\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e[43]\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eN-CQDs\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eClementine peel\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eHydrothermal\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e7\u0026ndash;50\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e4.57\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e[44]\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eN-GQDs\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eL-Glutamic acid\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eOne pot\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0\u0026ndash;50\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e4.67\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e[45]\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eGQDs\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCitric acid\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePyrolysis\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e10\u0026ndash;200\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e10.00\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e[46]\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eGQDs\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eWaste tea\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eMicrowave\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0\u0026ndash;50\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e2.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e[47]\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePEG-GQDs\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003ecane molasses\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eHydrothermal\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0\u0026ndash;60\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e5.77\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e[48]\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCQDs\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCamphor waste leaves\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eOne-step hydrothermal\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0\u0026ndash;100\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e2.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e[49]\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eN-GQDs\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eChitosan\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eHydrothermal\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e100\u0026ndash;700\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e2.73\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\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\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Sec20\" class=\"Section2\"\u003e\u003ch2\u003e3.5 Quenching Mechanism\u003c/h2\u003e\u003cp\u003eGu et al. \u003csup\u003e[\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e]\u003c/sup\u003e synthesized N-CQDs from lotus roots for Hg(II) detection and cellular imaging. UV-Vis absorption spectra revealed that N-CQDs exhibited an absorption peak at 280 nm, which disappeared upon Hg\u003csup\u003e2+\u003c/sup\u003e addition to the solution, indicating formation of N-CQDs-Hg\u003csup\u003e2+\u003c/sup\u003e complexes.\u003c/p\u003e\u003cp\u003eThis study similarly investigated the interaction between N-CQDs and Fe\u0026sup3;⁺ using UV-Vis spectroscopy (Fig.\u0026nbsp;8a). Significant spectral changes occurred after Fe\u003csup\u003e3+\u003c/sup\u003e introduction: the 272 nm absorption peak redshifted to 294 nm with markedly enhanced intensity. This phenomenon suggests Fe\u003csup\u003e3+\u003c/sup\u003e binding altered the electronic structure of N-CQDs. We propose that Fe\u003csup\u003e3+\u003c/sup\u003e coordinates with surface functional groups (pyrrolic/amino/hydroxyl) of N-CQDs, forming N-CQDs-Fe\u003csup\u003e3+\u003c/sup\u003e complexes.\u003c/p\u003e\u003cp\u003eFT-IR spectroscopy (Fig.\u0026nbsp;8b) further confirmed the coordination between Fe\u0026sup3;⁺ and surface functional groups on N-CQDs. Comparative analysis revealed significant alterations in characteristic vibrational modes: the O\u0026ndash;H/N\u0026ndash;H stretching band (~\u0026thinsp;3400 cm⁻\u0026sup1;) broadened and shifted, while the amide I and II bands (~\u0026thinsp;1600 cm⁻\u0026sup1;) showed reduced intensity, indicating Fe\u0026sup3;⁺ binding to amino, hydroxyl, and carbonyl groups. Additionally, new peaks appeared in the 500\u0026ndash;700 cm⁻\u0026sup1; region, attributable to Fe\u0026ndash;O/Fe\u0026ndash;N bonds. These results unequivocally demonstrate the formation of N-CQDs\u0026ndash;Fe\u0026sup3;⁺ complexes, which facilitate efficient PET by shortening the donor-acceptor distance and enhancing electron transfer efficiency.\u003c/p\u003e\u003cp\u003eThe fluorescence quenching mechanism primarily stems from the synergistic effects between photoinduced electron transfer (PET)\u003csup\u003e[\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e]\u003c/sup\u003e and coordination bonding. When Fe\u0026sup3;⁺ coordinates with N-CQDs' surface functional groups, photoexcited N-CQDs transfer electrons to Fe\u0026sup3;⁺'s vacant orbitals, driven by its strong oxidizing capability (E⁰ = +0.77 V vs. SHE), thereby suppressing radiative recombination and causing fluorescence quenching. This PET-dominated process is unequivocally confirmed by the notable decrease in the average fluorescence lifetime (τ\u003csub\u003eint\u003c/sub\u003e) from 6.94 ns to 3.90 ns upon Fe\u0026sup3;⁺, which provides direct evidence for a dynamic quenching pathway through electron transfer (Fig.\u0026nbsp;8c, Fig.\u0026nbsp;8d). Notably, UV-Vis spectral evidence confirms that ground-state N-CQDs-Fe\u0026sup3;⁺ complex formation contributes to quenching by shortening donor-acceptor distances through tight coordination, further enhancing PET efficiency.\u003c/p\u003e\u003cp\u003eThe superior Fe\u0026sup3;⁺ detection capability of N-CQDs arises from two key factors: firstly, the specific coordination of Fe\u003csup\u003e3+\u003c/sup\u003e with surface functional groups; and secondly, the coordination-enhanced photoinduced electron transfer (PET) that efficiently quenches fluorescence. These combined mechanisms enable highly selective Fe\u003csup\u003e3+\u003c/sup\u003e detection with a low detection limit of 2.72 \u0026micro;M.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec21\" class=\"Section2\"\u003e\u003ch2\u003e3.6 Stability\u003c/h2\u003e\u003cp\u003e\u003cul\u003e\u003cli\u003e\u003cp\u003eCS-CQDs solutions in 5 mL cuvettes were stored under controlled conditions (25\u0026deg;C, 25% RH) to evaluate stability: one sample sealed with parafilm under simulated illumination(Fig.\u0026nbsp;9a), and another sample sealed with parafilm in complete darkness(Fig.\u0026nbsp;9b). Fluorescence emission spectra were monitored for 5 days. The illuminated sample exhibited progressive intensity loss\u0026mdash;30% reduction after 3 days and 60% after 5 days. Conversely, the dark-stored sample showed minimal degradation (3% at day 3, 5% at day 5). These results demonstrate that continuous illumination causes significant fluorescence decay, while dark storage effectively preserves photostability (\u0026gt;\u0026thinsp;95% retention). This finding provides critical guidance for practical storage protocols. Future studies could explore surface modification or stabilizers to enhance anti-photobleaching properties.\u003c/p\u003e\u003c/li\u003e\u003c/ul\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec22\" class=\"Section2\"\u003e\u003ch2\u003e3.7 Applications\u003c/h2\u003e\u003cdiv id=\"Sec23\" class=\"Section3\"\u003e\u003ch2\u003e3.7.1 Fe\u003csup\u003e3+\u003c/sup\u003e detection in real water samples\u003c/h2\u003e\u003cp\u003eTo evaluate the practical utility of CS-CQDs as fluorescent probes, as shown in Fig.\u0026nbsp;8c, this study applied them to determine Fe\u003csup\u003e3+\u003c/sup\u003e concentrations in tap water and Dingxiang Lake(41.8544638, 123.3352718) water samples. Before analysis, collected water samples underwent centrifugation to remove large insoluble particulates, followed by filtration through 0.45 \u0026micro;m membranes to eliminate suspended particles. Fe\u003csup\u003e3+\u003c/sup\u003e concentrations were quantified using the standard addition method, with recovery rates calculated across different spiking levels. As presented in Table\u0026nbsp;3, recovery rates ranged from 99.98% to 100.04%, demonstrating the probe's reliability for real-sample analysis.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eDetermination of Fe\u003csup\u003e3+\u003c/sup\u003e in Spiked Tap Water and Lake Water Samples\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=\"char\" char=\".\" 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\u003eSamples\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e\u003cp\u003eConcentration of Fe\u003csup\u003e3+\u003c/sup\u003e (\u0026micro;M)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eRecovery (%)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eSpiked\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eMeasured\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTap water\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.33\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e100\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e100.35\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e100.02\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e150\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e150.32\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e99.99\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e200\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e200.39\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e100.03\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eLake water\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.32\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e100\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e100.30\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e99.98\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e150\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e150.39\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e100.04\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e200\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e200.37\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e100.03\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003ctfoot\u003e\u003ctr\u003e\u003ctd colspan=\"5\"\u003eRecovery = [(C\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;\u0026minus;\u0026thinsp;C\u003csub\u003e1\u003c/sub\u003e)/C\u003csub\u003e0\u003c/sub\u003e] \u0026times; 100%\u003c/td\u003e\u003c/tr\u003e\u003c/tfoot\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003ewhere C₀ represents the concentration of Fe\u003csup\u003e3+\u003c/sup\u003e spiked into real samples, while C₁ and C₂ denote the background and measured Fe\u003csup\u003e3+\u003c/sup\u003e concentrations in real samples before and after spiking, respectively.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec24\" class=\"Section3\"\u003e\u003ch2\u003e3.7.2 Application of CS-CQDs as a Fe\u003csup\u003e3+\u003c/sup\u003e Responsive Probe in Cell Imaging\u003c/h2\u003e\u003cp\u003eTo assess the cell imaging potential and sensing capability of the synthesized CQDs, fluorescence imaging was performed on onion epidermal cells as an in vitro plant model. Figure\u0026nbsp;10 (a-c) presents the corresponding microscopic images. Under white light illumination, the typical cellular architecture of the onion epidermis is clearly observed (Fig.\u0026nbsp;10a, Fig.\u0026nbsp;10b). Subsequent to staining with CQDs, the cells exhibited intense blue fluorescence under UV light excitation (Fig.\u0026nbsp;10d, Fig.\u0026nbsp;10e), indicating successful and homogeneous labeling of the cell membrane. The bright fluorescence suggests that the CQDs possess excellent biocompatibility and a strong affinity for cellular structures. This effective staining is likely facilitated by the coordination between functional groups (e.g., hydroxyl and amino groups) from the CQDs and various binding sites present on the plant cell membrane.\u003c/p\u003e\u003cp\u003eThe feasibility of employing CQDs as a fluorescent probe for intracellular ion sensing was further demonstrated. Upon introduction of Fe\u003csup\u003e3+\u003c/sup\u003e ions via a lateral permeation method, a significant quenching of the blue fluorescence was observed, with the fluorescence intensity diminishing to a negligible level (Fig.\u0026nbsp;10f). This turn-off response confirms the efficient diffusion of Fe\u003csup\u003e3+\u003c/sup\u003e ions into the cellular environment and their specific interaction with the CQDs, leading to fluorescence quenching, thereby validating their potential as a sensitive probe for detecting metal ions within a biological context.\u003c/p\u003e\u003cp\u003eIt is noteworthy that the propagation of excitation and emission light through plant tissues can be influenced by absorption, internal reflection, and scattering\u003csup\u003e[\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e]\u003c/sup\u003e. For instance, the limited penetration depth of UV excitation and the presence of soluble UV-absorbing compounds can affect the detected fluorescence signal. Nevertheless, the clear contrast achieved here underscores the efficacy of CQDs. The findings align with the growing interest in non-destructive optical techniques for agricultural monitoring. For example, fluorescence imaging and spectroscopic methods have been successfully employed to distinguish diseased onions from healthy ones\u003csup\u003e[\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e]\u003c/sup\u003e. Consequently, the developed CQD-based imaging system holds significant promise as a versatile tool. It can be further developed for automated quality inspection, classification in horticulture, and even remote sensing applications, positioning fluorescence imaging as a primary tool for quality control in agriculture, forestry, and environmental research.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eThis study innovatively synthesized N-CQDs using medium-molecular-weight chitosan as a single precursor through optimized hydrothermal conditions (190\u0026deg;C, 24 h). This green strategy leverages intrinsic amino/hydroxyl groups to drive in situ pyrrolic ring formation (via C2-N/C6-O cyclization) for autonomous nitrogen self-doping, eliminating complex precursors and toxic reagents. The pyrrole moiety acts as both an electron transfer channel and an emission center, endowing the synthesized N-CQDs with blue fluorescence emission characteristics (\u0026lambda;\u0026thinsp;~\u0026thinsp;ex~/\u0026lambda;ₑₘ = 330nm/402nm).\u003c/p\u003e\n\u003cp\u003eBased on the synergistic mechanisms of the coordination effect and photoinduced electron transfer effect, the developed N-CQDs exhibit exceptional Fe\u003csup\u003e3+\u003c/sup\u003e-sensing capabilities: selective fluorescence quenching toward Fe\u003csup\u003e3+\u003c/sup\u003e among 15 coexisting ions (e.g., Al\u003csup\u003e3+\u003c/sup\u003e, Cu\u003csup\u003e2+\u003c/sup\u003e); ultrahigh sensitivity (LOD\u0026thinsp;=\u0026thinsp;2.72 \u0026micro;M, S/N\u0026thinsp;=\u0026thinsp;3) with linear response (100\u0026ndash;700 \u0026micro;M, R\u0026sup2;=0.9955); and stable performance across pH 2\u0026ndash;10.\u003c/p\u003e\n\u003cp\u003eValidation in environmental waters (tap/lake water) achieved near-quantitative recoveries (99.98\u0026ndash;100.04%), confirming practical reliability. Additionally, the N-CQDs were successfully applied as fluorescent probes for cellular imaging in onion epidermis, exhibiting clear Fe\u003csup\u003e3+\u003c/sup\u003e-induced fluorescence quenching, which underscores their potential for bioimaging and intracellular ion detection. This work demonstrates the significant potential of biopolysaccharide-derived nanomaterials in environmental and biological sensing.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eFunding:\u003c/h2\u003e\u003cp\u003eThis work was supported by the Natural Science Foundation of Liaoning Province (d802487015) and the Liaoning University Scientific Research Project (d295000059).\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eLulu Feng: Drafting the manuscript, software implementation, methodology, data collection and analysis, conceptualisation. Hainan Li: Software implementation, methodology, conceptualisation. Zishuai Zhang:Reviewing the manuscript, editing, conceptualisation. Shuang Pang:Reviewing the manuscript, editing, conceptualisation. Zhiyu Liu:Reviewing the manuscript, editing, conceptualisation. Jiahao Lin:Reviewing the manuscript, editing, conceptualisation. Zhenning Lou:Guidance with experimental technique and supervision. Shiyu Wang: Reviewing the manuscript, validation and supervision.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAtchudan R, Edison I J N T, Aseer RK et al Highly fluorescent nitrogen-doped carbon dots derived from Phyllanthus acidus utilized as a fluorescent probe for label-free selective detection of Fe\u003csup\u003e3+\u003c/sup\u003e ions, live cell imaging and fluorescent ink[J]. Biosens Bioelectron ,2018,99303\u0026ndash;99311. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.bios.2017.07.076\u003c/span\u003e\u003cspan address=\"10.1016/j.bios.2017.07.076\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLiu W, Diao H, ,Chang H et al Green synthesis of carbon dots from rose-heart radish and application for Fe\u003csup\u003e3+\u003c/sup\u003e detection and cell imaging[J]. Sens Actuators: B Chem ,2017,241190\u0026ndash;241198. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.snb.2016.10.068\u003c/span\u003e\u003cspan address=\"10.1016/j.snb.2016.10.068\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eXin G, Cai-Feng W, ,Zi-Yi Y et al Facile access to versatile fluorescent carbon dots toward light-emitting diodes.[J].Chemical communications (Cambridge, England),2012,48(21):2692\u0026ndash;2694. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1039/c2cc17769b\u003c/span\u003e\u003cspan address=\"10.1039/c2cc17769b\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eBingbing H, Bingping L, ,Tao C et al One-Step Synthesis of Fluorescent Boron Nitride Quantum Dots via a Hydrothermal Strategy Using Melamine as Nitrogen Source for the Detection of Ferric Ions.[J].Langmuir : the ACS journal of surfaces and colloids,2017,33(40):10673\u0026ndash;10678. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1021/acs.langmuir.7b01699\u003c/span\u003e\u003cspan address=\"10.1021/acs.langmuir.7b01699\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eN S B,A G B. Luminescent carbon nanodots: emergent nanolights.[J].Angewandte Chemie (International ed. in English),2010,49(38):6726\u0026ndash;6744. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1002/anie.200906623\u003c/span\u003e\u003cspan address=\"10.1002/anie.200906623\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eCao L, ,Wang X, ,Meziani MJ et al Carbon Dots for Multiphoton Bioimaging.[J]. J Am Chem Soc 2007, 129(37):11318\u0026ndash;11319. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1021/ja073527l\u003c/span\u003e\u003cspan address=\"10.1021/ja073527l\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhang Y, Wang L, ,Zhang H et al Graphitic carbon quantum dots as a fluorescent sensing platform for highly efficient detection of Fe\u003csup\u003e3+\u003c/sup\u003e ions[J].RSC Advances,2013,3(11):3733\u0026ndash;3738. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1039/c3ra23410j\u003c/span\u003e\u003cspan address=\"10.1039/c3ra23410j\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSun S, Zhang L, Jiang K et al Toward High-Efficient Red Emissive Carbon Dots: Facile Preparation, Unique Properties, and Applications as Multifunctional Theranostic Agents.[J].Chemistry of Materials,2016,28(23):8659\u0026ndash;8668. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1021/acs.chemmater.6b03695\u003c/span\u003e\u003cspan address=\"10.1021/acs.chemmater.6b03695\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSwagatika S, Birendra B, K T M et al Simple one-step synthesis of highly luminescent carbon dots from orange juice: application as excellent bio-imaging agents.[J].Chemical communications (Cambridge, England),2012,48(70):8835\u0026ndash;8837. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1039/c2cc33796g\u003c/span\u003e\u003cspan address=\"10.1039/c2cc33796g\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eVaid S, Kaur V, ,Arya KR et al Transforming agro-waste: sustainable adsorption of Novacron Red FN-2BL on surfactant-modified bamboo fibers[J].Biomass Conversion and Biorefinery,2025,(prepublish):1\u0026ndash;25. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s13399-025-06717-z\u003c/span\u003e\u003cspan address=\"10.1007/s13399-025-06717-z\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLi ZYL, ,Zhenzhen H et al Carbon nanodots as fluorescence probes for rapid, sensitive, and label-free detection of Hg\u003csup\u003e2+\u003c/sup\u003e and biothiols in complex matrices.[J].Chemical communications (Cambridge, England),2012,48(8):1147\u0026ndash;1149. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1039/c2cc16791c\u003c/span\u003e\u003cspan address=\"10.1039/c2cc16791c\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhang R, Chen W Nitrogen-doped carbon quantum dots: Facile synthesis and application as a turn-off fluorescent probe for detection of Hg\u003csup\u003e2+\u003c/sup\u003e ions[J]. Biosens Bioelectron ,2014,5583\u0026ndash;5590. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.bios.2013.11.074\u003c/span\u003e\u003cspan address=\"10.1016/j.bios.2013.11.074\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eRong M, Feng Y, ,Wang Y et al One-pot solid phase pyrolysis synthesis of nitrogen-doped carbon dots for Fe\u003csup\u003e3+\u003c/sup\u003e sensing and bioimaging[J].Sensors \u0026amp; Actuators: B. Chemical,2017,245868\u0026ndash;245874. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.snb.2017.02.014\u003c/span\u003e\u003cspan address=\"10.1016/j.snb.2017.02.014\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eReza A, Mohammad H, ,Javad K Fluorescent organic nanoparticles with enhanced fluorescence by self-aggregation and their application for detection of Fe\u003csup\u003e3+\u003c/sup\u003e ions[J]. New J Chem 2018, 42(8):5929\u0026ndash;5936. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1039/c7nj04343k\u003c/span\u003e\u003cspan address=\"10.1039/c7nj04343k\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKapoor A, Yadav S, ,Arya KR et al Punica granatum and Citrus limetta biowaste-derived sorbents for the removal of heavy metals and organic compounds: sustainability perspectives[J].Biomass Conversion and Biorefinery,2024,(prepublish):1\u0026ndash;21. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s13399-024-05726-8\u003c/span\u003e\u003cspan address=\"10.1007/s13399-024-05726-8\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eBiller VD, Bruland WK Analysis of Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb in seawater using the Nobias-chelate PA1 resin and magnetic sector inductively coupled plasma mass spectrometry (ICP-MS)[J].Marine Chemistry,2012,130-13112-20. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.marchem.2011.12.001\u003c/span\u003e\u003cspan address=\"10.1016/j.marchem.2011.12.001\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLunvongsa S, Oshima M, ,Motomizu S Determination of total and dissolved amount of iron in water samples using catalytic spectrophotometric flow injection analysis[J].Talanta,2005,68(3):969\u0026ndash;973. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.talanta.2005.06.067\u003c/span\u003e\u003cspan address=\"10.1016/j.talanta.2005.06.067\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eGhaedi M, Shokrollahi A, ,Kianfar A et al The determination of some heavy metals in food samples by flame atomic absorption spectrometry after their separation-preconcentration on bis salicyl aldehyde, 1,3 propan diimine (BSPDI) loaded on activated carbon[J]. J Hazard Mater 2007, 154(1):128\u0026ndash;134. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jhazmat.2007.10.003\u003c/span\u003e\u003cspan address=\"10.1016/j.jhazmat.2007.10.003\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhu Y, Pan D, ,Hu X et al An electrochemical sensor based on reduced graphene oxide/gold nanoparticles modified electrode for determination of iron in coastal waters[J]. Sens Actuators: B Chem ,2017,2431\u0026ndash;2437. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.snb.2016.11.108\u003c/span\u003e\u003cspan address=\"10.1016/j.snb.2016.11.108\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eShuhua L, Yunchao L, ,Jun C et al Sulfur-doped graphene quantum dots as a novel fluorescent probe for highly selective and sensitive detection of Fe(3+).[J].Analytical chemistry,2014,86(20):10201\u0026ndash;10207. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1021/ac503183y\u003c/span\u003e\u003cspan address=\"10.1021/ac503183y\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhang H, Huang Y, ,Lin X et al Lanthanum loaded graphitic carbon nitride nanosheets for highly sensitive and selective fluorescent detection of iron ions[J].Sensors \u0026amp; Actuators: B. Chemical,2018,2552218\u0026ndash;2552222. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.snb.2017.09.026\u003c/span\u003e\u003cspan address=\"10.1016/j.snb.2017.09.026\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSun L, Li J, ,Cai J et al One pot synthesis of gold nanoparticles using chitosan with varying degree of deacetylation and molecular weight[J].Carbohydrate Polymers,2017,178105\u0026ndash;178114. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.carbpol.2017.09.032\u003c/span\u003e\u003cspan address=\"10.1016/j.carbpol.2017.09.032\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eManesh M, ,Sukumaran S, Nandini S Synthesis and Characterization of Gold-Chitosan Nanocomposite and Application of Resultant Nanocomposite in Sensors.[J].Colloids and Surfaces B: Biointerfaces,2012,93(1):143\u0026ndash;147. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.colsurfb.2011.12.028\u003c/span\u003e\u003cspan address=\"10.1016/j.colsurfb.2011.12.028\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhou S, Han X, ,Liu Y SWASV performance toward heavy metal ions based on a high-activity and simple magnetic chitosan sensing nanomaterials[J]. J Alloys Compd ,2016,6841\u0026ndash;6847. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jallcom.2016.05.152\u003c/span\u003e\u003cspan address=\"10.1016/j.jallcom.2016.05.152\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eGracia M, Anna R, ,Vanesa A et al Bactericidal Effect of Gold-Chitosan Nanocomposites in Coculture Models of Pathogenic Bacteria and Human Macrophages.[J].ACS applied materials \u0026amp; interfaces,2017,9(21):17693\u0026ndash;17701. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1021/acsami.6b15123\u003c/span\u003e\u003cspan address=\"10.1021/acsami.6b15123\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eJing B, Changjun H, ,Mei C et al Plant Esterase-Chitosan/Gold Nanoparticles-Graphene Nanosheet Composite-Based Biosensor for the Ultrasensitive Detection of Organophosphate Pesticides.[J].Journal of agricultural and food chemistry,2015,63(47):10319\u0026ndash;10326. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1021/acs.jafc.5b03971\u003c/span\u003e\u003cspan address=\"10.1021/acs.jafc.5b03971\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eOscar RSebasti\u0026aacute;nB, ,Cristian S et al Biobased Chitosan Nanocomposite Films Containing Gold Nanoparticles: Obtainment, Characterization, and Catalytic Activity Assessment.[J].ACS Applied Materials \u0026amp; Interfaces,2017,9(19):16561\u0026ndash;16570. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1021/acsami.7b04422\u003c/span\u003e\u003cspan address=\"10.1021/acsami.7b04422\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eAbdullah S, Azeman HN, ,Mobarak NN et al Sensitivity enhancement of localized SPR sensor towards Pb(II) ion detection using natural bio-polymer based carrageenan[J].Optik,2018,168784\u0026ndash;168793.\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.ijleo.2018.05.016\u003c/span\u003e\u003cspan address=\"10.1016/j.ijleo.2018.05.016\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhang Z, Fu X, ,Li K et al One-step fabrication of electrochemical biosensor based on DNA-modified three-dimensional reduced graphene oxide and chitosan nanocomposite for highly sensitive detection of Hg(II)[J].Sensors \u0026amp; Actuators: B. Chemical,2016,225453\u0026ndash;225462. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.snb.2015.11.091\u003c/span\u003e\u003cspan address=\"10.1016/j.snb.2015.11.091\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSen S, Tiwari NO, ,Arya KR et al New insights on microbial extracellular polysaccharides: production, biological activity, and applications[J].Biomass Conversion and Biorefinery,2025,(prepublish):1\u0026ndash;30. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s13399-025-06802-3\u003c/span\u003e\u003cspan address=\"10.1007/s13399-025-06802-3\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eDolgopyatova VN, Novikov YV, ,Konovalova NI Influence of the degree of deacetylation on the rate of acid degradation of chitin and chitosan and on the yield of D(+)-glucosamine hydrochloride[J]. Russian J Appl Chem 2011, 84(10):1826\u0026ndash;1829. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1134/s1070427211100259\u003c/span\u003e\u003cspan address=\"10.1134/s1070427211100259\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eThota PS, Thota MS, ,Bhagavatham SS et al Facile one-pot hydrothermal synthesis of stable and biocompatible fluorescent carbon dots from lemon grass herb[J].IET Nanobiotechnology,2018,12(2):127\u0026ndash;132. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1049/iet-nbt.2017.0038\u003c/span\u003e\u003cspan address=\"10.1049/iet-nbt.2017.0038\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eDong Y, Wang R, ,Li H et al Polyamine-functionalized carbon quantum dots for chemical sensing[J].Carbon,2012,50(8):2810\u0026ndash;2815. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.carbon.2012.02.046\u003c/span\u003e\u003cspan address=\"10.1016/j.carbon.2012.02.046\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eMurugan N, Prakash M, ,Jayakumar M et al (2019) Green synthesis of fluorescent carbon quantum dots from Eleusine coracana and their application as a fluorescence \u0026lsquo;turn-off\u0026rsquo; sensor probe for selective detection of Cu\u003csup\u003e2+\u003c/sup\u003e[J]. Appl Surf Sci 476468\u0026ndash;476480. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.apsusc.2019.01.090\u003c/span\u003e\u003cspan address=\"10.1016/j.apsusc.2019.01.090\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSong Z, Quan F, ,Xu Y et al Multifunctional N,S co-doped carbon quantum dots with pH- and thermo-dependent switchable fluorescent properties and highly selective detection of glutathione[J].Carbon,2016,104169\u0026ndash;104178. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.carbon.2016.04.003\u003c/span\u003e\u003cspan address=\"10.1016/j.carbon.2016.04.003\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eDong Y, Shao J, ,Chen C et al Blue luminescent graphene quantum dots and graphene oxide prepared by tuning the carbonization degree of citric acid[J].Carbon,2012,50(12):4738\u0026ndash;4743. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.carbon.2012.06.002\u003c/span\u003e\u003cspan address=\"10.1016/j.carbon.2012.06.002\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eGao Z, Lin Z, ,Chen X et al Carbon dots-based fluorescent probe for trace Hg\u003csup\u003e2+\u003c/sup\u003e detection in water sample[J]. Sens Actuators: B Chem ,2016,222965\u0026ndash;222971. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.snb.2015.09.032\u003c/span\u003e\u003cspan address=\"10.1016/j.snb.2015.09.032\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLiu Y, Zhao Y, ,Zhang Y One-step green synthesized fluorescent carbon nanodots from bamboo leaves for copper(II) ion detection[J]. Sens Actuators: B Chem ,2014,196647\u0026ndash;196652. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.snb.2014.02.053\u003c/span\u003e\u003cspan address=\"10.1016/j.snb.2014.02.053\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eHuibo W, Mengling Z, ,Kaiqiang W et al Pyrrolic nitrogen dominated the carbon dot mimic oxidase activity[J].Carbon,2021,179692\u0026ndash;179700. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.carbon.2021.04.061\u003c/span\u003e\u003cspan address=\"10.1016/j.carbon.2021.04.061\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKou CL, Yi MC, ,Xin XL et al Catalyst- and oxidant-free electrochemicalpara-selective hydroxylation ofN-arylamides in batch and continuous-flow[J].GREEN CHEMISTRY,2020,22(19):6437\u0026ndash;6443. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1039/d0gc02294b\u003c/span\u003e\u003cspan address=\"10.1039/d0gc02294b\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWang YJ, Wei XF, Su Y, Xu R, Song DQ, Ding L, Chen YH (2024) Highly sensitive fluoroprobe for detecting Sudan dyes in paprika utilizing carbon dot-embedded zeolitic imidazolate framework-8. Food Chem 438:137975. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.foodchem.2023.137975\u003c/span\u003e\u003cspan address=\"10.1016/j.foodchem.2023.137975\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLIU R, ZHANG Y, PIAO Y et al (2021) Development of nitrogen-doped carbon quantum dots as fluorescent probes for highly selective and sensitive detection of the heavy-ion Fe\u003csup\u003e3+\u003c/sup\u003e[J]. Carbon Lett 31:821\u0026ndash;829. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s42823-020-00222-1\u003c/span\u003e\u003cspan address=\"10.1007/s42823-020-00222-1\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eAHN J, SONG Y, KWON JE et al (2019) Food waste-driven N-doped carbon dots: applications for Fe\u003csup\u003e3+\u003c/sup\u003e sensing and cell imaging[J]. Mater Sci Engineering: C 102:106\u0026ndash;112. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.msec.2019.04.019\u003c/span\u003e\u003cspan address=\"10.1016/j.msec.2019.04.019\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eŠAFRANKO S, STANKOVIĆ A, HAJRA S et al (2021) Preparation of multifunctional N-doped carbon quantum dots from citrus clementina peel: investigating targeted pharmacological activities and the potential application for Fe\u003csup\u003e3+\u003c/sup\u003e sensing[J]. Pharmaceuticals 14(8):857. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3390/ph14090857\u003c/span\u003e\u003cspan address=\"10.3390/ph14090857\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eYU J, XU C X, TIAN Z S et al (2016) Facilely synthesized N-doped carbon quantum dots with high fluorescent yield for sensing Fe\u003csup\u003e3+\u003c/sup\u003e[J]. New J Chem 40(3):2083\u0026ndash;2088. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1039/c5nj03252k\u003c/span\u003e\u003cspan address=\"10.1039/c5nj03252k\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eXU F Z, SHI H, HE X X et al (2015) Masking agent-free and channel-switch-mode simultaneous sensing of Fe\u003csup\u003e3+\u003c/sup\u003e and Hg\u003csup\u003e2+\u003c/sup\u003e using dual-excitation graphene quantum dots[J]. Analyst 140(11):3925\u0026ndash;3928. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1039/c5an00468c\u003c/span\u003e\u003cspan address=\"10.1039/c5an00468c\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eABBAS A, TABISH T A, BULL S J et al (2020) High yield synthesis of graphene quantum dots from biomass waste as a highly selective probe for Fe\u003csup\u003e3+\u003c/sup\u003e sensing[J]. Sci Rep 10:21262. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1038/s41598-020-78070-2\u003c/span\u003e\u003cspan address=\"10.1038/s41598-020-78070-2\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLOU Y, JI J (2020) Cane molasses graphene quantum dots passivated by PEG functionalization for detection of metal ions[J]. ACS Omega 5(12):6763\u0026ndash;6772. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1021/acsomega.0c00098\u003c/span\u003e\u003cspan address=\"10.1021/acsomega.0c00098\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLi SY, Wang N, ,Wang L et al Carbon quantum dots derived from camphor tree leaves biomass as a highly selective probe for Fe\u003csup\u003e3+\u003c/sup\u003e sensing[J].Biomass Conversion and Biorefinery,2024,(prepublish):1\u0026ndash;11. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s13399-024-05997-1\u003c/span\u003e\u003cspan address=\"10.1007/s13399-024-05997-1\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eGu D, Shang S, ,Yu Q et al Green synthesis of nitrogen-doped carbon dots from lotus root for Hg(II) ions detection and cell imaging[J]. Appl Surf Sci ,2016,39038\u0026ndash;39042. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.apsusc.2016.08.012\u003c/span\u003e\u003cspan address=\"10.1016/j.apsusc.2016.08.012\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eAleksandra FANB Quantum Chemical Characterization and Design of Quantum Dots for Sensing Applications.[J].The journal of physical chemistry. A,2022,126(19):2899\u0026ndash;2908. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1021/acs.jpca.2c00947\u003c/span\u003e\u003cspan address=\"10.1021/acs.jpca.2c00947\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eVenkatachalam V, Ganapathy S, ,Nanthagopal M et al Fluorescence imaging of onion epidermal cell utilizing highly luminescent water-soluble CdTe colloidal quantum dots[J].Inorganic Chemistry Communications,2023,149. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/J.INOCHE.2022.110352\u003c/span\u003e\u003cspan address=\"10.1016/J.INOCHE.2022.110352\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWang W, Li C Measurement of the light absorption and scattering properties of onion skin and flesh at 633nm[J]. Postharvest Biol Technol ,2013,86494-501. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.postharvbio.2013.07.032\u003c/span\u003e\u003cspan address=\"10.1016/j.postharvbio.2013.07.032\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\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":"Chitosan, Carbon quantum dots, Fe3+ detection, Fluorescence quenching, Environmental monitoring, Cell Imaging","lastPublishedDoi":"10.21203/rs.3.rs-8120500/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8120500/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eCarbon quantum dots (CQDs), as zero-dimensional carbon-based fluorescent materials, have garnered significant attention in environmental and biological monitoring due to their low toxicity, high stability, and tunable synthesis. This study pioneers a green synthesis of nitrogen-doped carbon quantum dots (N-CQDs) using medium-molecular-weight chitosan as a single precursor via one-step hydrothermal optimization (190 ℃, 24 h). The resulting N-CQDs exhibit uniform spherical morphology (typical diameter: 3 nm) with amorphous carbon cores and abundant surface moieties including amino, hydroxyl, and pyrrolic groups (C/N/O\u0026thinsp;=\u0026thinsp;64:4:32). These structural attributes enable strong blue fluorescence (λ\u003csub\u003eex\u003c/sub\u003e/λ\u003csub\u003eem\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;330nm/402nm) and selective Fe\u003csup\u003e3+\u003c/sup\u003e detection through a dual-mechanism: (i) Coordination binding between Fe\u003csup\u003e3+\u003c/sup\u003e and surface functional groups, (ii) Photoinduced electron transfer (PET) facilitated by the high redox potential of Fe\u003csup\u003e3+\u003c/sup\u003e (E⁰ = +0.77 V vs. SHE). The N-CQDs achieve a 2.72 \u0026micro;M detection limit (S/N\u0026thinsp;=\u0026thinsp;3) for Fe\u003csup\u003e3+\u003c/sup\u003e with excellent selectivity against 15 interfering ions (Al\u003csup\u003e3+\u003c/sup\u003e, Cu\u003csup\u003e2+\u003c/sup\u003e, etc.) and validated performance in tap/lake water samples (99.98-100.04% recoveries). Furthermore, the chitosan-derived N-CQDs were successfully applied as a fluorescent probe for cell imaging in onion epidermis, where they predominantly accumulated on the cell membrane with excellent color rendering performance. Notably, the fluorescence was effectively quenched upon the introduction of Fe\u003csup\u003e3+\u003c/sup\u003e ions, demonstrating their possibility as a responsive probe in biological systems. This work demonstrates the significant potential of biopolysaccharide-derived nanomaterials, highlighting their dual promise for both precise environmental sensing and responsive bioimaging applications.\u003c/p\u003e","manuscriptTitle":"Green Synthesis of Nitrogen-Doped Carbon Quantum Dots from Medium-Molecular-Weight Chitosan for Ultrasensitive Fe³⁺ Detection and Responsive Probe ","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-08 07:07:14","doi":"10.21203/rs.3.rs-8120500/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-12-24T20:16:35+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-12-23T09:52:48+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-12-09T10:51:12+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"151188098821983170333582165222533472811","date":"2025-12-03T03:24:14+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"75950504088126321306786609883464846811","date":"2025-12-02T17:58:44+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-12-02T17:54:15+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-11-25T09:58:21+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-11-25T09:54:38+00:00","index":"","fulltext":""},{"type":"submitted","content":"Journal of Fluorescence","date":"2025-11-15T08:11:51+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":"073b2fed-b764-4050-9ca4-553e6df4a124","owner":[],"postedDate":"December 8th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-03-02T16:00:11+00:00","versionOfRecord":{"articleIdentity":"rs-8120500","link":"https://doi.org/10.1007/s10895-026-04712-9","journal":{"identity":"journal-of-fluorescence","isVorOnly":false,"title":"Journal of Fluorescence"},"publishedOn":"2026-02-28 15:57:17","publishedOnDateReadable":"February 28th, 2026"},"versionCreatedAt":"2025-12-08 07:07:14","video":"","vorDoi":"10.1007/s10895-026-04712-9","vorDoiUrl":"https://doi.org/10.1007/s10895-026-04712-9","workflowStages":[]},"version":"v1","identity":"rs-8120500","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8120500","identity":"rs-8120500","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}