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• Sulfur ions (S^2-) are an important form of the sulfur element with diverse roles in nature and industry^[1]. In biological systems, S^2- serves as a key component of cysteine and methionine in proteins, participating in enzyme activity and metabolic processes, thereby sustaining vital physiological functions^[2]. In agriculture, S^2- acts as an essential nutrient for plant growth and is widely incorporated into fertilizers to enhance crop yield and development^[3]. Additionally, S^2- enhances the elasticity and wear resistance of rubber during vulcanization in industrial applications. However, excessive S^2- concentration poses significant environmental and health risks. The oxidation products of S^2- can lead to acid rain, resulting in soil acidification, water pollution, and vegetation destruction. Moreover, sulfuric acid mist, generated by the reaction of sulfur oxides with atmospheric particulate matter, further exacerbates environmental damage. Meanwhile, acute exposure to high concentrations of sulfide (such as hydrogen sulfide) may induce difficulty breathing and impaired consciousness^[4-5]. Given these dual impacts, accurate determination of S^2- in real samples is imperative for environmental monitoring and public health protection.

  To date, common sensing technologies for S^2- have chromatography^[6], colorimetry^[7-9], electrochemistry^[10-11], and fluorescence^[12-16]. However, the traditional methods are limited by complex sample processing, high costs, the need for professional operators, and prolonged incubation periods, highlighting the demand for a simple, low-cost, and highly accurate alternative^[17]. Among emerging approaches, fluorescence-based sensing methods have got hug interest in analytical fields due to their significant sensing speed, high accuracy, and good repeatability^[18]. Compared with other fluorescence nanomaterials, carbon dots (CDs), as promising candidates, have gained apparent interest due to their distinctive optical performance, good water-solubility, biocompatibility, and low toxicity^[19]. Additionally, CDs display adjustable emission wavelengths, significant stability, and can be functionalized with diverse groups to enhance specificity toward analytes^[20]. CD-based probes have been effectively utilized for detecting anions by leveraging the fluorescence enhancement or attenuation phenomenon. For example, Sun et al. constructed red-emission CDs for fluorescent and colorimetric hypochlorite detection via static quenching effect. Notably, the red-emission CDs were also developed as test paper-based sensing platforms due to the changes in fluorescence color. Moreover, the imaging tests were further studied in HeLa cells with satisfactory results^[21]. Wu et al.^[22] constructed a fluorescent probe by assembling phenylenediamine-based CDs with rhodamine B, enabling rapid nitrite sensing under acidic conditions through diazotization-induced quenching. Meanwhile, this platform was successfully applied for nitrite sensing in food samples with recoveries of 96.8%-101.8%. Furthermore, the 1, 3-phenylenediamine and citric acid were applied to synthesize water-soluble CDs with blue fluorescence^[23]. The CDs could be used for fluorescence determination of Cr₂O₇^2- via the inner filter effect (IFE). Finally, the amounts of Cr₂O₇^2- in real samples were determined with recoveries of 95%-105%. Zou et al.^[24] prepared a novel nanoprobe by combining CDs with histidine-stabilized gold nanoclusters (CDs/His-Au NC), exhibiting strong emission at 447 nm under the excitation wavelength of 347 nm. The fluorescence was quenched by I^- via the IFE, yielding a wide linear range of 0.1-60.0 μmol·L^-1 and a low detection limit (LOD) of 34 nmol·L^-1. Next, the detection system was successfully utilized to determine I^- in actual samples. To our knowledge, some studies about CDs for the determination of S^2- have been successfully developed. For example, Pang et al. developed near-infrared-CDs for the fluorescence sensing of H₂S with an LOD of 56 nmol·L^{-1 [25]}, also demonstrating potential for endogenous H₂S. Zeng et al.^[26] established novel N, F-co-doped CDs (N, F-CDs) with high quantum yield and good photostability, which were successfully used for the measurement of S^2- with an LOD of 168 nmol·L^-1. The fluorescence of N, F-CDs@S^2- could be recovered by Cd²⁺ because of the formation of CdS. Barati et al. prepared dual sensing systems for S^2- using fluorescence enhancement (CDs/Hg²⁺) and quenching (CDs/Ag⁺)^[27]. However, most existing CDs-based S^2- probes relied on single-emission strategies, which were susceptible to interference from concentration fluctuations and instrument sensitivity variations^[28].

  To our knowledge, ratiometric fluorescence probes operate by quantifying the ratio of fluorescence intensities at two different wavelengths, thereby mitigating the interference of environmental factors and enhancing their sensitivity and accuracy^[29-32]. Therefore, the development of ratiometric fluorescent probes has attracted widespread attention from researchers.

  Herein, dual-emission fluorescence carbon dots (N-CDs) for the sensing of S^2- were developed by using L-tryptophan and benzimidazole as precursors via hydrothermal methods (Scheme 1). N-CDs exhibited remarkable stability, water-solubility, and strong fluorescence intensity. Two emission peaks were observed at 356 and 442 nm under the excitation wavelength of 303 nm. Upon addition of S^2-, the fluorescence intensity at 442 nm decreased, while that increased at 356 nm, demonstrating a ratiometric response. The detection linear range and LOD for the S^2- were studied. Moreover, the recoveries of S^2- in real samples were evaluated through the standard addition method.

  Scheme 1

  

  Scheme 1.  Schematic illustration of the development of dual-emission N-CDs and sensing of S^2-

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  1.   Experimental

  1.1   Reagents

  Sodium chloride (≥99.8%), potassium chloride (≥99.8%), calcium chloride (≥99%), magnesium chloride (≥99%), zinc chloride (≥98%), aluminum chloride (≥99%), copper chloride (≥98%), potassium dichromate (≥99%), silver nitrate (≥99%), cobalt chloride (≥98%), cadmium chloride (≥99%), lead chloride (AR), ferric chloride (≥98%), sodium fluoride (≥99%), sodium bromide (≥99%), sodium iodide (≥99.5%), sodium carbonate (≥99.5%), sodium bicarbonate (≥99.8%), sodium oxalate (≥99%), sodium phosphate (≥99%), sodium sulfate (≥99%), sodium sulfite (≥98%), sodium thiosulfate (≥97%), sodium sulfide (≥98%), L-tryptophan (99%), and benzimidazole (≥98%) were all got from Shanghai Aladdin Reagent Co., Ltd (China).

  1.2   Apparatus

  A U-4500 spectrophotometer (Hitachi, Japan) was used to record ultraviolet-visible (UV-Vis) data for fluorescence probes and analytes, with a wavelength range of 200-800 nm. The fluorescence data were obtained from an F-7000 spectrophotometer (Hitachi, Japan) with the excitation and emission slits of 5 nm and a voltage of 400 V. A FLS1000 spectrophotometer (Edinburgh, UK) was applied for the fluorescence lifetime measurement of N-CDs and N-CDs/S^2- system (N-CDs: 10 mg·mL^-1, S^2-: 200.0 μmol·L^-1) under Edinburgh Instruments Picosecond Pulsed Laser (EPL) light source (excitation wavelength of 303 nm). The types of functional groups on the surface of N-CDs were determined by using a Bruker Tensor Ⅱ Fourier transform infrared spectroscopy (FTIR, Bremen, Germany). The testing method was investigated through potassium bromide compression technology at a resolution of 4 cm^-1 with a recording range of 4 000-500 cm^-1. The morphology of N-CDs was studied on a JEOL JEM-2100F (Japan) transmission electron microscope (TEM) at an accelerating voltage of 200 kV. Meanwhile, the particle size was counted by using Nano Measurer 1.2.5. The valence states of elements were discussed through an X-ray photoelectron spectroscopy (XPS, Thermo Kalpha, Thermo Fisher, USA) with a monochromatic Al Kα X-ray source (1 486.6 eV) operating at 150 W.

  1.3   Preparation of N-CDs

  N-CDs were established by using a hydrothermal method based on published literature with minor modifications^[33]. In brief, 180 mg of L-tryptophan and 130 mg of benzimidazole were dissolved in 30 mL of ultrapure water at room temperature. Next, the resulting transparent solution was transferred into a hydrothermal kettle (50 mL) and heated at 200 ℃ for 6 h. After completion, the reaction mixture was centrifuged at 8 000 r·min^-1 for 15 min. Subsequently, a dialysis bag (molecular weight cut-off: 500 Da) was utilized to purify the centrifuge solution. Finally, the N-CDs powders were obtained through a freeze dryer for future testing.

  1.4   Development of probes

  The N-CD powders were dissolved in phosphate-buffered saline (PBS) buffer solution (pH=6.0) under stirring at room temperature for 90 s, and the corresponding mass concentration was 10 mg·mL^-1. After the addition of S^2- (0-200.0 μmol·L^-1), the fluorescence emission spectra were recorded at 25 ℃. A working curve was established by discussing the relationship between F₄₄₂/F₃₅₆ (F₃₅₆ and F₄₄₂ refer to the fluorescence intensity at 356 and 442 nm, respectively) and S^2- concentration. The linear range and LOD were obtained based on the above data. For selectivity, some interfering materials (Na⁺, K⁺, Ca²⁺, Mg²⁺, Zn²⁺, Cr⁶⁺, Ag⁺, Al³⁺, Cu²⁺, Co²⁺, Cd²⁺, Pb²⁺, and Fe³⁺; F^-, Cl^-, Br^-, I^-, CO₃^2-, HCO₃^-, C₂O₄^2-, PO₄^3-, SO₄^2-, SO₃^2-, and S₂O₃^2-, their concentrations were all 200.0 μmol·L^-1) were added separately into a series of parallel probe solutions to record the variation of fluorescence signals. For anti-interference evaluation, 200.0 μmol·L^-1 interfering substance and 200.0 μmol·L^-1 S^2- solution were simultaneously added to the probe solution.

  1.5   Actual sample determination

  The tap and river water samples were collected from our laboratory and the Fenhe River, respectively. Orange juice samples (Minute Maid) were purchased from the Meitehao supermarket. All samples were centrifuged at 8 000 r·min^-1 for 15 min to remove insoluble substances, and the supernatants were applied for recovery tests. In the sensing process, standard S^2- solutions were added to these supernatants under gentle stirring at room temperature for 5 min. Subsequently, the spiked solutions were dropped into the above sensing system (the final concentrations of S^2- were 15.0, 25.0, and 35.0 μmol·L^-1), which was transferred into a fluorescent colorimetric dish (1 cm×1 cm). The fluorescent data were obtained under the excitation wavelength of 303 nm. Finally, the recovery rate and relative standard deviation (RSD) were calculated to evaluate the feasibility of the sensing platform.

  2.   Results and discussion

  2.1   Characterization

  TEM was employed to study the morphology and particle size of N-CDs. As displayed in Fig.1A, some uniform spherical particles could be observed without any obvious aggregation. The inset in Fig.1A indicates the particle size distribution histogram. The particle size distribution was between 2.3 and 2.9 nm with an average diameter of 2.6 nm. The elemental analysis and surface composition of N-CDs were performed through XPS and FTIR characterization. The full XPS spectrum exhibited three main peaks at 285.1, 400.4, and 530.9 eV, which matched C1s, N1s, and O1s, respectively (Fig.1B)^[20]. The single XPS spectrum of C1s was decomposed into three peaks at 283.9, 284.9, and 287.7 eV, corresponding to the C—C, C—N, and C=O bonds, respectively (Fig.1C)^[24]. Similarly, the single XPS spectrum of N1s displayed two feature peaks at 399.2 and 400.5 eV, which were assigned to pyridinic N and pyrrolic N (Fig.1D)^[34]. The high-resolution XPS spectrum of O1s in Fig.1E displayed some characteristic peaks, such as 530.7 eV (C—O) and 532.1 eV (C=O)^[35]. FTIR technology indicated that some functional groups could be observed on the surface of N-CDs (Fig.1F). The feature peaks at 3 371, 2 935, and 2 899 cm^-1 are ascribed to the stretching vibration of O—H, C—H (asymmetric), and C—H (symmetric), respectively^[36]. Moreover, a strong peak at 1 658 cm^-1 is attributed to the stretching vibration of the C=O bond, implying the existence of O-containing functional groups^[37]. Some bending vibrations were also observed, such as 1 433 cm^-1 (N—H), 1 262 cm^-1 (O—H), and 875 cm^-1 (=CH)^[28]. Collectively, FTIR and XPS results confirmed the presence of amino, hydroxyl, and carboxyl groups on the N-CDs surface, which contributed to their excellent water-solubility and dispersibility. In addition, the ζ-potential value of N-CDs was detected to be -15.2 mV (Fig.S1, Supporting information), indicating the presence of some negative functional groups on the surface of N-CDs.

  Figure 1

  

  Figure 1.  (A) TEM image of N-CDs (inset: histogram of particle size distribution); (B) Full XPS spectrum of N-CDs; XPS spectra of (C) C1s, (D) N1s, and (E) O1s on N-CDs; (F) FTIR spectrum of N-CDs

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  2.2   Optical feature

  The optical features of N-CDs were investigated by using a UV-Vis absorption and fluorescence spectrophotometer. In the UV-Vis absorption spectrum (Fig.2A), the strong characteristic peak at 275 nm is ascribed to the π-π* transition of the C=C bond, while the weak peak around 350 nm is attributed to the n-π* transition of the C=O bond^[30]. Under ultraviolet irradiation, the N-CDs solution (10 mg·mL^-1) emitted blue fluorescence. Meanwhile, the color was light yellow under sunlight (inset in Fig.2A). Fluorescence spectra displayed dual emission peaks at 356 and 442 nm under the optimum excitation wavelength of 303 nm, confirming their blue fluorescence emission. Furthermore, the excitation-dependent feature was discussed under various excitation wavelengths. As shown in Fig.2B, both the emission wavelength and intensity varied significantly when the excitation wavelength was tuned from 280 to 330 nm, suggesting the excitation-dependent characteristic, which was often associated with particle non-uniformity and surface defect^[38].

  Figure 2

  

  Figure 2.  (A) UV-Vis absorption, excitation, and emission spectra of N-CDs; (B) Emission spectra of N-CDs under different excitation wavelengths

  Inset: images of N-CDs solution under sunlight (left) and UV light (right).

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  To our knowledge, the stability of fluorescent probes is one of the core factors affecting their imaging quality and application effectiveness. Thus, the stability of N-CDs was studied under various storage times, light exposure times, and sodium chloride solutions. As displayed in Fig.S2A, the emission intensity remained almost constant after storage at room temperature for 30 d, demonstrating excellent long-term stability. Subsequently, the N-CDs also suggested prominent photostability under ultraviolet irradiation for 15 min (Fig.S2B). In addition, they showed strong anti-interference capability against ionic strength, as the fluorescence signal experienced only minor fluctuations even in the presence of a high NaCl concentration (0.05 mol·L^-1) (Fig.S2C). The above tests demonstrated that the N-CDs had outstanding stability, highlighting great promise in a complex analytical environment.

  2.3   N-CDs for S^2- determination

  In the determination process of N-CDs to S^2-, the detection conditions were optimized, including pH value and incubation time. From Fig.S3A, the quenching rate gradually decreased when the pH value increased from 6.0 to 8.0. Obviously, 6.0 was selected as the best detection pH value. Subsequently, after 90 s of incubation time, the quenching rate reached its maximum value, suggesting that this probe had remarkable sensitivity for S^2- (Fig.S3B). Under the optimal measurement conditions (pH: 6.0, incubation time: 90 s), the determination behavior of the N-CDs-based probe for S^2- sensing was carried out. As shown in Fig.3A, the emission intensity ratio at 356 and 442 nm (F₄₄₂/F₃₅₆) gradually decreased as the S^2- concentration (c_S) varied from 0 to 200.0 μmol·L^-1. The F₄₄₂/F₃₅₆ value displayed a prominent linear relationship with S^2- amount in the range of 0-60.0 μmol·L^-1 (Fig.3B). The linear equation was fitted to be F₄₄₂/F₃₅₆=-0.004 4c_S+0.62 with a regression coefficient of 0.982 9. Based on the LOD=3b/k (b is the standard deviation of blank samples, k is the slope of the fitted curve), the LOD was determined to be 0.076 μmol·L^-1. Meanwhile, the fluorescence color of the sensing system was recorded in the presence of various S^2- concentrations. The blue fluorescence showed a slight decrease with S^2- concentration increasing from 0 to 200.0 μmol·L^-1 (Fig.3B). Compared with other reported measurement methods^{[6-16, 25-27]}, the as-fabricated fluorescence platform suggested an equivalent linear range and LOD value, as summarized in Table 1.

  Figure 3

  

  Figure 3.  (A) Fluorescence emission spectra of N-CDs in the presence of various S^2- concentrations; (B) Linear relationship between F₄₄₂/F₃₅₆ and S^2- concentration

  Inset: fluorescence colors of sensing solution in the presence of various S^2- concentrations.

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  Table 1

  

  Table 1.  Comparison of different methods for S^2- determination

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  Method	Probe	Linear range / (μmol·L-1)	LOD / (μmol·L-1)	Ref.
  Chromatography	Not described	10-100	2.5	[6]
  Colorimetry	EHL@AgNPRs-3	0.2-20	0.041 3	[7]
  Colorimetry	SnTe	0.3-900	7×10-3	[8]
  Colorimetry	Ti-MOF@0.3Cu	0.5-30	0.16	[9]
  Electrochemistry	CoNi@NGs	0.5-1 000	0.47	[10]
  Electrochemistry	CVD-CNT	1.25-112.5	0.3	[11]
  Fluorescence	GSH-AuNCs-CDs	1-50	0.35	[12]
  Fluorescence	Tb-2I	0-1 000	0.47	[13]
  Fluorescence	DBA-NBD-Cl	0-11.7	7×10-5	[14]
  Fluorescence	CuBDC-FAM-DNA	0.002-0.12	1.5×10-3	[15]
  Fluorescence	BSA-Au NCs	0-30	0.395	[16]
  Fluorescence	NIR-CDs	0.5-20	0.056	[25]
  Fluorescence	N, F-CDs	0-30	0.16	[26]
  Fluorescence	CD/Hg2+	2-10	0.32	[27]
  Fluorescence	N-CDs	0-60.0	0.076	This work

  2.4   Selectivity investigation

  Selectivity was a pivotal factor in evaluating the practicability of N-CDs. To evaluate the selectivity of this ratiometric fluorescent probe for S^2- detection, several potential interfering substances were added into sensing system, including metal ions (Na⁺, K⁺, Ca²⁺, Mg²⁺, Zn²⁺, Cr⁶⁺, Ag⁺, Al³⁺, Cu²⁺, Co²⁺, Cd²⁺, Pb²⁺, and Fe³⁺) and non-metal ions (F^-, Cl^-, Br^-, I^-, CO₃^2-, HCO₃^-, C₂O₄^2-, PO₄^3-, SO₄^2-, SO₃^2-, and S₂O₃^2-). As shown in Fig.S4A, none of the control substances except S^2- significantly altered the fluorescence signal of N-CDs, indicating its prime selectivity towards S^2-. Moreover, the interference tests were also studied by discussing the intensity of the sensing probe when S^2- and controls coexisted simultaneously. As indicated in Fig.S4B, the controls displayed a weak impact on the S^2- measurement based on the N-CDs-based fluorescence probe. In addition, the pictures of the sensing solution with different controls and S^2- under an ultraviolet lamp were obtained by using a smartphone. As hinted in Fig.S4C, weak quenching of blue fluorescence was found with the addition of S^2-. Briefly, the newly prepared N-CDs, which originated from amino acids, exhibited excellent selectivity and strong anti-interference capability for S^2- sensing.

  2.5   Sensing mechanism for S^2-

  To study the detection mechanism of N-CDs for S^2-, some related analysis technologies were conducted, including UV-Vis absorption, fluorescence, and lifetime. Firstly, the UV-Vis absorption of S^2- and the fluorescence spectrum of N-CDs were recorded. As indicated in Fig.4A, the UV-Vis absorption spectrum of S^2- (200.0 μmol·L^-1) did not overlap with the fluorescence spectrum of N-CDs, indicating the absence of IFE or Förster resonance energy transfer^[18-19]. In addition, the formula [τ=(B₁t₁²+B₂t₂²)/(B₁t₁+B₂t₂)] (τ refers to the average fluorescence lifetime; B₁ and B₂ refer to the amplitude or pre exponential factor of two components; t₁ and t₂ are the fluorescence lifetimes of two components) was utilized to calculate the fluorescence lifetime. The data and spectra were listed in Table S1 and Fig.4B, respectively. The lifetime of N-CDs solution (10 mg·mL^-1) was carried out to be 2.56 ns. After the addition of S^2- (200.0 μmol·L^-1), slight changes in lifetime were observed (2.54 ns), demonstrating that the quenching reason might be ascribed to the static quenching effect^[20]. Moreover, the UV-Vis absorption data were applied to confirm the above judgment. As hinted in Fig.4C, a new feature peak was observed in the UV-Vis absorption spectrum of N-CDs/S^2- system, indicating the presence of a ground-state complex between N-CDs and S^2-. In addition, as S^2- concentration changed to 200.0 μmol·L^-1, the emission intensity of the analytical system (442 nm) decreased, and the corresponding peak altered from 442 to 432 nm, hinting at the existence of an interaction between N-CDs and S^2- (Fig.3A)^{[26, 39]}. Next, the Stern-Volmer equation (F₀/F=1+K_svc_S=K_qτc_S) was further utilized to investigate the static quenching effect^[19], where F₀ and F refer to the emission intensities of the probe solution without and with the addition of S^2-, K_SV is the Stern-Volmer constant (L·mol^-1), K_q refers to the quenching constant (L·mol^-1·s^-1), and τ is the lifetime of N-CDs (2.56 ns). From Fig.4D, K_q was determined to be 3.91×10¹² L·mol^-1·s^-1 [higher than the maximum scatter collision quenching constant (2×10¹⁰ L·mol^-1·s^-1)]^[19]. Therefore, the static quenching effect was the main cause of fluorescence quenching.

  Figure 4

  

  Figure 4.  (A) UV-Vis absorption spectrum of S^2-, and excitation and emission spectra of N-CDs; (B) Fluorescence lifetime spectra of N-CDs and N-CDs/S²; (C) UV-Vis absorption spectra of N-CDs, S^2-, and N-CDs/S^2-; (D) Linear relationship between F₀/F at 442 nm and S^2- concentration

  Inset: the corresponding magnified spectra.

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  2.6   Determination of S^2- in actual samples

  To estimate the accuracy of the as-prepared sensing platform, ratiometric fluorescent N-CDs were employed to measure S^2- concentrations in orange juice, tap, and river water samples. Different concentrations of S^2- (15.0, 25.0, and 35.0 μmol·L^-1) were spiked into these real samples, and the measurement results are shown in Table 2. After spiking with definite S^2- concentration, the recovery rates changed from 94.2% to 105.5%, and the RSD values varied from 2.1% to 4.5%. These results indicated that the fluorescence-based sensing method had higher feasibility and exactitude. Additionally, the probe synthesized here provided some advantages: (a) utilization of one-pot hydrothermal synthesis technology; (b) choice of environmentally friendly amino acids as precursors; (c) the development of ratiometric fluorescent probes with excellent properties; (d) significant sensitivity and specificity for S^2- determination.

  Table 2

  

  Table 2.  Determination of S^2- in actual samples through this fluorescence system

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  Sample	Added concentration / (μmol·L-1)	Detected concentration / (μmol·L-1)	Recovery rate / %	RSD / %
  Orange juice	15.0	14.2	94.7	2.8
  	25.0	26.4	105.5	2.1
  	35.0	35.1	100.4	3.7
  Tap water	15.0	15.4	102.9	3.6
  	25.0	25.1	100.5	4.5
  	35.0	35.3	101.0	3.9
  River water	15.0	14.1	94.2	4.0
  	25.0	24.7	98.8	3.1
  	35.0	34.9	99.6	2.9

  2.7   Assessment of sustainability

  The effect of chemical procedures on the environment has always been a highly concerning topic. Thus, it is necessary to ensure the environmental safety of assay methods. To evaluate the greenness and blueness of the assay, the analytical greenness metric for sample preparation (AGREEprep) tool and the blueness assessment through blue applicability grade index (BAGI) framework were used.

  AGREEprep tool involved 10 key aspects: sample preparation, hazardous materials, material renewability, waste, sample size, sample throughput, automation, energy, analytical instrument, and safety. Each aspect was rated on a scale from 0 (non-sustainable) to 1 (highly sustainable). Based on the analysis of the AGREEprep tool, the score was carried out to be 0.80, demonstrating that this sensing platform was safe and environmentally friendly (Fig.5A).

  Figure 5

  

  Figure 5.  Assessment of greenness (A) and blueness (B)

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  The blueness assessment was performed by using the BAGI tool. In this tool, some features were listed as follows: type of analysis, number of analytes, analytical instrumentation, number of processed samples, sample preparation, number of samples per hour, type of reagents and materials, requirement for preconcentration, automation degree, and sample amount. Each feature was assigned four fixed scores, such as 10.0 (dark blue), 7.5 (blue), 5.0 (light blue), or 2.5 (white) points. The score of the BAGI analysis was 82.5, indicating the significant cost-effectiveness and practicality of the detection system (Fig.5B).

  3.   Conclusions

  In conclusion, the as-synthesized N-CDs exhibited significant stability, water-solubility, and strong fluorescence, making them an excellent fluorescent probe candidate. Due to the static quenching effect between N-CDs and S^2-, this sensing system was successfully utilized as a novel ratiometric probe towards the sensitive determination of S^2- through recording the variation of the F₄₄₂/F₃₅₆ ratio. In detail, the linear range and LOD were 0-60.0 μmol·L^-1 and 0.076 μmol·L^-1, respectively. Furthermore, the novel platform displayed selective S^2- measurement in real samples, while the AGREEprep and BAGI tools indicated high sustainability, highlighting its significant potential for application in foodstuff analysis.

  
  Supporting information is available at http://www.wjhxxb.cn
  
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  Scheme 1  Schematic illustration of the development of dual-emission N-CDs and sensing of S^2-

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  Figure 1  (A) TEM image of N-CDs (inset: histogram of particle size distribution); (B) Full XPS spectrum of N-CDs; XPS spectra of (C) C1s, (D) N1s, and (E) O1s on N-CDs; (F) FTIR spectrum of N-CDs

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  Figure 2  (A) UV-Vis absorption, excitation, and emission spectra of N-CDs; (B) Emission spectra of N-CDs under different excitation wavelengths

  Inset: images of N-CDs solution under sunlight (left) and UV light (right).

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  Figure 3  (A) Fluorescence emission spectra of N-CDs in the presence of various S^2- concentrations; (B) Linear relationship between F₄₄₂/F₃₅₆ and S^2- concentration

  Inset: fluorescence colors of sensing solution in the presence of various S^2- concentrations.

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  Figure 4  (A) UV-Vis absorption spectrum of S^2-, and excitation and emission spectra of N-CDs; (B) Fluorescence lifetime spectra of N-CDs and N-CDs/S²; (C) UV-Vis absorption spectra of N-CDs, S^2-, and N-CDs/S^2-; (D) Linear relationship between F₀/F at 442 nm and S^2- concentration

  Inset: the corresponding magnified spectra.

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  Figure 5  Assessment of greenness (A) and blueness (B)

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  Table 1.  Comparison of different methods for S^2- determination

  Method	Probe	Linear range / (μmol·L-1)	LOD / (μmol·L-1)	Ref.
  Chromatography	Not described	10-100	2.5	[6]
  Colorimetry	EHL@AgNPRs-3	0.2-20	0.041 3	[7]
  Colorimetry	SnTe	0.3-900	7×10-3	[8]
  Colorimetry	Ti-MOF@0.3Cu	0.5-30	0.16	[9]
  Electrochemistry	CoNi@NGs	0.5-1 000	0.47	[10]
  Electrochemistry	CVD-CNT	1.25-112.5	0.3	[11]
  Fluorescence	GSH-AuNCs-CDs	1-50	0.35	[12]
  Fluorescence	Tb-2I	0-1 000	0.47	[13]
  Fluorescence	DBA-NBD-Cl	0-11.7	7×10-5	[14]
  Fluorescence	CuBDC-FAM-DNA	0.002-0.12	1.5×10-3	[15]
  Fluorescence	BSA-Au NCs	0-30	0.395	[16]
  Fluorescence	NIR-CDs	0.5-20	0.056	[25]
  Fluorescence	N, F-CDs	0-30	0.16	[26]
  Fluorescence	CD/Hg2+	2-10	0.32	[27]
  Fluorescence	N-CDs	0-60.0	0.076	This work

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  Table 2.  Determination of S^2- in actual samples through this fluorescence system

  Sample	Added concentration / (μmol·L-1)	Detected concentration / (μmol·L-1)	Recovery rate / %	RSD / %
  Orange juice	15.0	14.2	94.7	2.8
  	25.0	26.4	105.5	2.1
  	35.0	35.1	100.4	3.7
  Tap water	15.0	15.4	102.9	3.6
  	25.0	25.1	100.5	4.5
  	35.0	35.3	101.0	3.9
  River water	15.0	14.1	94.2	4.0
  	25.0	24.7	98.8	3.1
  	35.0	34.9	99.6	2.9

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