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• Malachite green is an artificially synthesized organic compound and a toxic triphenylmethane chemical substance^[1-2]. Its molecular formula is C₂₃H₂₅ClN₂. Malachite green can be used as a dye for silk, leather, and paper^[3]. It can also be applied as a biological staining agent to stain cells or cellular tissues blue-green (bacteria, spores, red blood cells, and roundworm eggs), making them easier to study under a microscope^[4-5]. However, research has found that malachite green holds carcinogenic effects. Thus, it has been listed in the list of carcinogens and banned from use. In this instance, the sensing of malachite green is actually of great significance in our production and daily life^[6-7]. To date, various platforms have been successfully employed for the determination of malachite green, such as surface-enhanced Raman scattering (SERS)^[8-9], high-performance liquid chromatography (HPLC)^[10-11], high-performance liquid chromatography-tandem mass spectrometers (HPLC-MS/MS)^[12], electrochemical method^[13-15], and fluorescence method^[16-18]. Among them, the fluorescence method holds some superiorities, including facile equipment, simple process, significant sensitivity, low cost, and excellent rapidity. For example, Hu et al.^[16] developed fluorescence carbon dots-based fluorescence microspheres (CDs@FMs) for ultrasensitively probing malachite green through static quenching and inner filter effect (IFE). Two good linear fluorescence responses for malachite green were obtained in the range of 0.01-0.1 μmol·L^-1 and 0.1-10.0 μmol·L^-1. Gu′s group^[18] prepared a fluorescent probe based on polyacrylic acid (PAA) functionalized CdSe/Cd_xZn_1-xS quantum dots (QDs@PAA), which were applied to detect malachite green due to fluorescence resonance energy transfer (FRET). The fluorescence could be quenched by malachite green in the linear range of 0.05-2 μmol·L^-1, and the detection limit (LOD) was 0.011 μmol·L^-1. Hu et al.^[17] developed red-emissive Se, N, and Cl-CDs for the detection of malachite green. The linear range was 0.07-2.50 μmol·L^-1, with a low LOD of 21 nmol·L^-1. Qiu et al.^[19] established N, S-graphene quantum dots (N, S-GQDs) mixed with CdTe quantum dots for ratiometric fluorescence visual detection of malachite green with a limit detection of 0.459 7 nmol·L^-1.

  Fluorescence sensors are prospective substitutes for the detection of malachite green because of their facile handle, visual observation, high uniqueness and sensing sensitivity, and quick sensing. However, the reported fluorescence sensing sensors were focused on the carbon quantum dots and semiconductor quantum dots. These sensors involved harsh preparation conditions (high temperature and high pressure) and highly toxic metals. Thus, it is still necessary to develop facile fluorescence probes.

  In recent years, fluorescence sensors have been widely applied for chemical detection and biological imaging owing to their high uniqueness and sensing sensitivity. To materialize the determination purpose, fluorescence sensors are in the light of the special combination of identification module and analyte. Thus, various identification modules and identification strategies can be applied for various determination purposes. Metal nanoclusters, as fluorescence models, have been used in the sensing fields^[20]. Metal nanoclusters are composed of only a few atoms with very small sizes, which determines their electronic and optical properties (quantum confinement effect, strong luminescent properties, and catalytic properties). These special properties make metal nanoclusters widely used in some fields, such as environmental, energy, electronics, immune detection, and biopharmaceuticals^[21]. Due to their unique optical and electronic properties, ease of synthesis and surface modification, good biocompatibility, and catalytic activity, silver nanoclusters Ag NCs) are widely used in various fields. The size, morphology, and surface coordination of Ag NCs directly affect their performances and are related to the preparation methods and protectants. The preparation methods mainly include the ligand exchange method, etching method, microemulsion method, electrochemical method, chemical reduction method, and ultrasonic and microwave reduction method^[22]. Among them, the chemical reduction method is widely used due to its simple operation and low cost. To date, the protective agents for Ag NCs are mainly divided into organic (polymer, small molecule, and DNA) and inorganic (zeolite and inorganic glass)^[23]. To our knowledge, folic acid (FA) is a water-soluble vitamin with the molecular formula C₁₉H₁₉N₇O₆. There are several forms in nature, and the parent compound is composed of pteridine, p-aminobenzoic acid, and glutamic acid. FA is rich in many active functional groups (—COOH, —OH, —NH), which can closely coordinate with the Ag atom to ensure the high stability of Ag NCs.

  In this work, a sensitive fluorescence sensor was prepared according to FA-stabilized silver nanoclusters (FA@Ag NCs). FA had many active functional groups to ensure excellent stability and water-solubility of FA@Ag NCs. FA@Ag NCs were applied as targeted platforms, and malachite green was used as a quencher. The quenching mechanism was investigated in detail (Scheme 1). Based on the related experimental data, the linear range and LOD were developed. The selectivity and feasibility of this probe were carried out under optimal detection conditions.

  Scheme 1

  

  Scheme 1.  Schematic illustration of the preparation of FA@Ag NCs and sensing of malachite green

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  1.   Experimental

  1.1   Chemicals

  Sodium chloride, potassium chloride, magnesium chloride, calcium chloride, silver nitrate, sodium carbonate, and sodium bicarbonate were obtained from Chengdu Cologne Chemical Co., Ltd. Sodium sulfate, sodium oxalate, citric acid, glutathione, hydrazine hydrate, glucose, FA, and ascorbic acid were procured from Shanghai Aladdin Biochemical Technology Co., Ltd. Glycine, glutamic acid, histidine, and malachite green were acquired from Shanghai Macklin Biochemical Technology Co., Ltd. All the reagents used were of analytical grade.

  1.2   Preparation of FA@Ag NCs

  Blue-emitting FA@Ag NCs were prepared based on the method published by Chandran et al. with minor modifications^[24]. Typically, 2 mL of AgNO₃ solution (0.01 mol·L^-1) and 0.1 g of FA were added into 10 mL of ultrapure water with 15 min of stirring at room temperature. Next, 150 μL of hydrazine hydrate was transferred into the above mixture at 35 ℃ for 8 h. After that, the reaction flask was left to cool down at normal temperature, and the colorless solution was acquired. Then, the product solution was dialyzed through a dialysis bag (3 000 Da) for 12 h. Finally, the dialyzed solution was stored at 4 ℃ for further use.

  1.3   Apparatus

  Transmission electron microscopy (TEM, a JEM-2100, JEOL, Japan) was employed to study the morphological characteristics and size distribution of FA@Ag NCs) at an accelerating voltage of 200 kV. The crystal form of FA@Ag NCs was obtained on a D/max-2500V/PC X-ray diffraction spectroscopy (XRD, Rigaku, Tokyo, Japan) operated at 40 kV and 40 mA, utilizing Cu Kα radiation (λ=0.154 06 nm) and a scanning range of 10°-80°. An F-7000 fluorescence spectrometer (Hitachi, Japan) was used to study the fluorescence excitation and emission spectra with the excitation/emission slits of 10/10 nm and voltage of 400 V. The ultraviolet-visible (UV-Vis) spectra were investigated through a UV-4500 spectrometer (Hitachi, Japan) with a 200-600 nm recording range and scanning speed of 1 200 nm·min^-1. The X-ray photoelectron (XPS) spectra of FA@Ag NCs were obtained on an ESCALAB 250 X-ray photoelectron spectrometer (Thermo Fisher Scientific, USA) with a monochromatic Al Kα X-ray source (1 486.6 eV) operating at 150 W. The types of functional groups on FA@Ag NCs were recorded by using a PerkinElmer Fourier transfer infrared spectrometer (FTIR, Perkin-Elmer, USA). The FTIR spectrum was recorded at a resolution of 4 cm^-1 in the wavenumber range of 500-4 000 cm^-1. Nexera UHPLC/HPLC system ultrafast liquid chromatograph (Shanghai, China) was applied to determine the malachite green concentration in real samples to compare the results of fluorescence determination.

  1.4   Detection process

  The detection features of FA@Ag NCs for the measurement of malachite green were discussed. The standard solutions of malachite green were prepared by using N, N-dimethylformamide. Under the optimized conditions, a certain concentration of malachite green solution was dropped into FA@Ag NCs-based sensing solution [1 mL FA@Ag NCs solution+1 mL phosphate buffer (pH=6)], and the corresponding emission data were obtained through fluorescence spectrometer. The (I₀-I)/I₀ value was calculated to establish a linear model (I₀ and I were the fluorescence intensities of the sensing solution in the absence and presence of malachite green). In the linear equation, (I₀-I)/I₀ was the dependent variable, and malachite green concentration was the independent variable. The selectivity was also studied under the same conditions. The reproducibility of this probe in actual samples was discussed by using added recovery sensing. For that, the river water samples were obtained from the rivers on the campus. The normal saline samples were acquired from the pharmacy. The Mengniu milk samples were purchased from a convenience store. These samples with pretreatment were carried out in the light of the published article^[25]. Various amounts of malachite green solutions were added to the samples. Then, the corresponding fluorescence data were recorded through the F-7000 fluorescence spectrometer. Recovery rates were processed through the developed linear equation.

  2.   Results and discussion

  2.1   Characterization of FA@Ag NCs

  A series of techniques were carried out to discuss the performances of FA@Ag NCs, including TEM, FTIR, XRD, and XPS. The morphological characteristics of FA@Ag NCs were discussed by using TEM technology. FA@Ag NCs displayed excellently dispersed and globular (Fig. 1A). The average particle size was 2.8 nm (Fig. 1B). The lattice fringe of FA@Ag NCs was studied through a high-resolution TEM (HRTEM) image. As shown in Fig. 1C, the lattice stripes could be observed, and the lattice spacing was 0.23 nm, which indicated the existence of the (111) plane of Ag^{0 [26]}. The crystal type of FA@Ag NCs was investigated through XRD characterization. As indicated in Fig.S1 (Supporting information), a broad characteristic peak around 21.8° could be observed, implying the amorphous property of FA@Ag NCs. It indicated that FA@Ag NCs had the coexistence of local microcrystalline and overall amorphous^[27].

  Figure 1

  

  Figure 1.  (A) TEM image, (B) size distribution histogram, and (C) HRTEM image of FA@Ag NCs

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  Next, in the UV-Vis absorption spectrum of FA@Ag NCs (Fig. 2A), a faint peak around 360 nm was ascribed to the quantum confinement effect in nanomaterials. Furthermore, the absence of peaks between 500 and 600 nm suggested the absence of larger nanoparticles. From Fig. 2A, the maximum excitation and emission wavelengths were 370 and 447 nm, respectively. At the same time, the FA@Ag NCs solution was observed under sunlight and UV light. The colorless solution of FA@Ag NCs displayed blue fluorescence under UV light (inset Fig. 2A). Moreover, the influence of excitation wavelength on the emission wavelength was carried out. As suggested in Fig. 2B, the emission wavelength did not change with the excitation wavelength varying from 340 to 380 nm. This change phenomenon indicated that FA@Ag NCs had excitation-independent characteristics.

  Figure 2

  

  Figure 2.  (A) UV-Vis absorption (black line), excitation (green line), and emission (red line) spectra of FA@Ag NCs; (B) Emission spectra of FA@Ag NCs under various excitation wavelengths

  Inset: Photos of FA@Ag NCs solution under daylight (left) and UV light (right).

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  XPS technology was employed to investigate the types and oxidation states of elements in FA@Ag NCs. XPS full spectrum (Fig.S2) suggested four feature peaks, which are assigned to be C1s (285.0 eV), N1s (400.9 eV), O1s (532.0 eV), and Ag3d (373.4 eV). The atomic fractions were obtained, such as C (66.36%), N (12.2%), O (17.5%), and Ag (0.94%). The fitting feature peaks around 283.8, 284.8, 286.8, and 288.7 eV are attributed to the C—C, C—C/C=C, C—O/C—N, and O—C=O bonds, respectively (Fig. 3A). The single spectrum of N1s was fitted, and the initial peak was divided into two peaks at 398.9 and 399.9 eV, corresponded to N—H and C—N—C bonds, respectively (Fig. 3B). In single O1s spectrum (Fig. 3C), the feature peaks at 531.2, 532.0, and 533.2 eV correspond to C—OH, C=O, and C—O bonds, respectively. The Ag3d spectrum implied two obvious peaks at 367.8 and 373.9 eV, which correspond to Ag⁰3d_3/2 and Ag⁰3d_5/2, respectively (Fig. 3D)^[28]. The FTIR measurement was employed to verify the types of functional groups in FA@Ag NCs. The FTIR spectrum of FA@Ag NCs suggested a broad absorption peak around 3 428 cm^-1, which is assigned to the stretching vibration of —OH. The two weak peaks at 2 964 and 1 111 cm^-1 are attributed to the stretching vibration of C—H and C—O bonds, respectively. A strong feature peak at 1 634 cm^-1 is the stretching vibration of the C=O bond. The weak band at 1 388 cm^-1 corresponds to the deformation vibration of the C—H bond (Fig.S3)^[29]. The above results implied that FA@Ag NCs were successfully prepared.

  Figure 3

  

  Figure 3.  XPS spectra of C1s (A), N1s (B), O1s (C), and Ag3d (D) in FA@Ag NCs

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  2.2   Stability of FA@Ag NCs

  The stability investigation of nanomaterials was essential for the measurement of analytes. The influence of ultraviolet light on the fluorescence intensity of FA@Ag NCs is displayed in Fig.S4A. The test implied that the fluorescence intensity of FA@Ag NCs suggested insignificant variation after the UV radiation of 2.5 min. At the same time, the long-term stability of FA@Ag NCs was studied under 25 ℃. It could be seen from Fig.S4B that after the storage of 20 d, FA@Ag NCs maintained a strong fluorescence intensity with negligible variation. Moreover, the influence of ionic strength on the fluorescence intensity of FA@Ag NCs was discussed under various NaCl amounts. When the NaCl amount increased to 0.05 mol·L^-1, there was no decrease in fluorescence intensity (Fig.S4C). The stability tests suggested that FA@Ag NCs could meet the basic usage requirements as fluorescent sensors.

  2.3   Optimization of determination

  To obtain the optimal test determination conditions, the effects of detection time and buffer solution pH on the fluorescence intensity variation were carried out. The fluorescence intensities of FA@Ag NCs in the absence and presence of malachite green were recorded, and the relative values were calculated (ΔI=I₀/I, where I₀ and I represented the fluorescence intensities of the FA@Ag NCs sensing system in the absence and presence of malachite green, respectively). After a reaction of 150 s, Fig.S5A displayed the change of ΔI value under different pH values (changing from 6 to 8). The ΔI revealed the topmost value at 6. As shown in Fig.S5B, the change of the ΔI value was not significant, with the determination time varying from 0 to 150 s under the pH of 6. Noticeably, the degree of fluorescence quenching quickly reached its maximum. Thus, we selected 60 s as the determination time.

  2.4   Fluorescence determination of FA@Ag NCs to malachite green

  The fluorescence performance of the nanoprobes was assessed through a spectrofluorometric strategy. A succinctly prepared FA@Ag NCs emitted at 447 nm under the excitation wavelength of 370 nm. As suggested in Fig. 4A, the incipient fluorescence intensity of FA@Ag NCs displayed a decrease with the malachite green amount increasing from 0 to 200 μmol·L^-1. In this paper, the "turn-off" strategy represented the decrease in fluorescence intensity of the sensing platform. Significantly, this developed nanosensor provided a wide concentration linear response range changing from 0.5 to 200 μmol·L^-1 (Fig. 4B). The linear curve was carried out with malachite green concentration as the horizontal axis and (I₀-I)/I₀ as the vertical axis. The equation of the fitting curve was developed to be (I₀-I)/I₀=0.002 7c+0.037, where c was the malachite green concentration. The fitting coefficient was 0.984 7, with a linear range of 0.5-200 μmol·L^-1 and a LOD of 0.084 μmol·L^-1. In general, the as-developed nanoprobe implied significant sensitivity to malachite green. It could be ascribed to the specific structures of FA@Ag NCs.

  Figure 4

  

  Figure 4.  (A) Fluorescence responses of FA@Ag NCs upon the addition of different concentrations of malachite green; (B) Corresponding linear relationship between (I₀-I)/I₀ and malachite green concentration in the range of 0.5-200 μmol·L^-1

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  2.5   Selectivity investigation

  The selectivity and peculiarity of this developed sensor were fundamental for the trustworthy use of the as-prepared FA@Ag NCs in the determination of malachite green. Kinds of cations, anions and organic substances were applied to investigate the selectivity and peculiarity for the malachite green sensing, such as Na⁺, K⁺, Mg²⁺, Ca²⁺, CO₃^2-, HCO₃^-, SO₄^2-, C₂O₄^2-, citric acid, glucose, glutathione, ascorbic acid, glycine, glutamic acid, and histidine (the concentration was all 200 μmol·L^-1). The test results are suggested in Fig.S6. These controls indicated a weak impact (fluorescence enhancement or attenuation) on the fluorescence of FA@Ag NCs. The significant selectivity combined with excellent sensitivity and rapid response of FA@Ag NCs to malachite green displayed that the as-prepared platform might be directly employed for the determination of malachite green in actual samples.

  2.6   Principle of determination

  Some strategies have been carried out to suggest the fluorescence quenching principle, such as aggregation-caused quenching, inner filter effect, Förster resonance energy transfer, static quenching, and dynamic quenching. Based on our continuous exploration, the determination of malachite green by FA@Ag NCs should be ascribed to one of the above mechanisms. Different characterizations were applied to discuss the quenching principle of FA@Ag NCs by malachite green. The UV-Vis absorption spectrum of malachite green possessed two broadbands of 300-340 nm and 350-462 nm (Fig. 5A). In fluorescence excitation and emission spectra of FA@Ag NCs, the emission peak was located at 447 nm under the excitation wavelength of 370 nm. Compared with the UV-Vis absorption spectrum of malachite green and fluorescence excitation or emission spectra of FA@Ag NCs, an evident overlap phenomenon was noticed. This factor was a fundamental precondition for the existence of the inner filter effect or Förster resonance energy transfer^[30]. To further discuss the specific mechanism of FA@Ag NCs for the determination of malachite green, the fluorescence lifetime of FA@Ag NCs in the absence and presence of malachite green was studied (Fig. 5B). Strikingly, slight changes between lifetime of FA@Ag NCs (1 mL FA@Ag NCs stock solution+1 mL H₂O, 1.69 ns) and that of FA@Ag NCs (1 mL FA@Ag NCs stock solution+1 mL H₂O)+malachite green (200 μmol·L^-1, 1.65 ns) could be observed^[31]. To our best knowledge, the occurrence of Förster resonance energy transfer involved an overlap between the absorption spectrum of the measured object and the excitation or emission spectrum of the sensor. Although the overlap suggested that Förster resonance energy transfer might exist, slight changes in fluorescence lifetime could exclude it. This result excluded both Förster resonance energy transfer and dynamic quenching. Therefore, the inner filter effect and static quenching might be the main quenching mechanisms.

  Figure 5

  

  Figure 5.  (A) UV-Vis absorption spectrum of malachite green (black line), excitation (green line), and emission (red line)spectra of FA@Ag NCs; (B) Fluorescence lifetime curves of FA@Ag NCs in the absence and presence of malachite green (200 μmol·L^-1); (C) Fluorescence quenching efficiencies before and after considering innerfilter effect; (D) Linear relationship between I₀/I and malachite green concentration in the range of 0.5-100 μmol·L^-1

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  To check the above inference, the observed fluorescence intensity was corrected by using the following Eq.1^[32].

  $ I_{\text {cor }}=I_{\text {obs }} \times 10^{\left(A_{\mathrm{ex}}+A_{\mathrm{em}}\right) / 2} $

  (1)

  where, I_cor and I_obs refer to the corrected and observed fluorescence intensities, A_ex and A_em represent the absorbance at the excitation (370 nm) and emission (447 nm) wavelengths, respectively (Table S1). From Fig.S7, the measured and corrected relative fluorescence intensity following the removal of the inner filter effect was observed, suggesting that the fluorescence weakening might be assigned to the interaction between FA@Ag NCs and malachite green. Meanwhile, the fluorescence quenching efficiency (E=1-I/I₀) was also studied. As shown in Fig. 5C, the difference between E_obs and E_cor was not significant, indicating the existence of the inner filter effect, but it was not the primary quenching mechanism (E_obs and E_cor refer to the observed and corrected quenching efficiency through the inner filter effect)^[33]. Furthermore, static quenching might also be the main mechanism. The Stern-Volmer equation was used to discuss the possibility of static quenching^[34].

  $ I_0 / I=1+K_{\mathrm{SV}} c=1+k_{\mathrm{q}} \tau_0 c $

  (2)

  where K_SV is the Stern-Volmer quenching constant, c refers to the malachite green concentration, τ₀ refers to the lifetime of FA@Ag NCs (1.69 ns), and k_q is the quenching rate coefficient. Based on the Stern-Volmer equation and linear fitting curve (Fig. 5D), the K_SV and k_q were calculated to be 4.9×10³ L·mol^-1 and 2.9×10¹² L·mol^-1·s^-1, respectively. The k_q value was larger than 2×10¹⁰ L·mol^-1·s^-1 (maximum scatter collision quenching constant), implying that the main principle was static quenching^[35].

  2.7   Application investigation in actual samples

  FA@Ag NCs were employed to determine malachite green in river water, normal saline, and Mengniu milk samples. The results are displayed in Table 1. The measurement results suggested that excellent recovery rates were acquired, changing from 95.56% to 108.56%. Meanwhile, the relative standard deviations (RSDs) were less than 3.94%. FA@Ag NCs were greener, simpler, and quicker. In this study, malachite green was also quantified by an HPLC analysis method, and the findings were compared with sensor results. The recovery percentages for HPLC were found to vary from 99.33% to 102.89%. The RSDs were lower than 2.02%. The effective recovery of the malachite green demonstrated that the probe had a promising application for the determination of malachite green in real samples. The tests further suggested that the FA@Ag NCs-based fluorescence nanoprobe held excellent exactitude, repeatability, and feasibility and could be contentedly applied for the determination of malachite green in real samples. Compared with other reported articles, this platform was discussed by its convenience, low consumption, wider linear range, and green preparation procedure (Table S2). These superiorities would attract more researchers to apply it in more fields.

  Table 1

  

  Table 1.  Malachite green measurement concentration in real samples

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  Sample	Added concentration / (μmol·L-1)	Total found concentration / (μmol·L-1)			Recovery / %			RSD / % (n=3)
  		Probe	HPLC		Probe	HPLC		Probe	HPLC
  River water	30.00	28.67	29.80		95.56	99.33		2.24	1.21
  	70.00	75.27	70.60		107.52	100.86		1.54	0.62
  Normal saline	30.00	32.57	30.10		108.56	100.33		3.09	2.02
  	70.00	69.77	69.67		99.67	99.52		3.63	0.22
  Mengniu milk	30.00	30.63	30.87		102.11	102.89		3.94	1.14
  	70.00	69.47	69.77		99.24	99.67		2.86	1.22

  3.   Conclusions

  Herein, FA@Ag NCs were carried out successfully by using a facile, rapid, and green one-pot restoration strategy. FA@Ag NCs indicated convincing water solubility, stability, salt resistance, and strong fluorescence. The fluorescence quantum yield was suggested to be 7.6%. It was implied that the blue fluorescence of FA@Ag NCs could be quenched by malachite green, given the static quenching. Therefore, FA@Ag NCs were efficient fluorescence sensors for the measurement of malachite green. The excellent selectivity of FA@Ag NCs for the malachite green sensing was displayed with potential metal ions, non-metal ions, and biomolecules as control substances. This platform was successfully used to measure malachite green in river water, normal saline, and Mengniu milk samples with excellent reliability and good accuracy. Thus, the as-developed FA@Ag NCs hold a possible application in people's daily production and life. This method has the advantages of simple operation, low cost, good reproducibility, and high sensitivity, providing a promising approach for constructing fluorescent sensors for detecting other analytes.

  Supporting information is available at http://www.wjhxxb.cn

  
  
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  Scheme 1  Schematic illustration of the preparation of FA@Ag NCs and sensing of malachite green

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  Figure 1  (A) TEM image, (B) size distribution histogram, and (C) HRTEM image of FA@Ag NCs

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  Figure 2  (A) UV-Vis absorption (black line), excitation (green line), and emission (red line) spectra of FA@Ag NCs; (B) Emission spectra of FA@Ag NCs under various excitation wavelengths

  Inset: Photos of FA@Ag NCs solution under daylight (left) and UV light (right).

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  Figure 3  XPS spectra of C1s (A), N1s (B), O1s (C), and Ag3d (D) in FA@Ag NCs

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  Figure 4  (A) Fluorescence responses of FA@Ag NCs upon the addition of different concentrations of malachite green; (B) Corresponding linear relationship between (I₀-I)/I₀ and malachite green concentration in the range of 0.5-200 μmol·L^-1

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  Figure 5  (A) UV-Vis absorption spectrum of malachite green (black line), excitation (green line), and emission (red line)spectra of FA@Ag NCs; (B) Fluorescence lifetime curves of FA@Ag NCs in the absence and presence of malachite green (200 μmol·L^-1); (C) Fluorescence quenching efficiencies before and after considering innerfilter effect; (D) Linear relationship between I₀/I and malachite green concentration in the range of 0.5-100 μmol·L^-1

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  Table 1.  Malachite green measurement concentration in real samples

  Sample	Added concentration / (μmol·L-1)	Total found concentration / (μmol·L-1)			Recovery / %			RSD / % (n=3)
  		Probe	HPLC		Probe	HPLC		Probe	HPLC
  River water	30.00	28.67	29.80		95.56	99.33		2.24	1.21
  	70.00	75.27	70.60		107.52	100.86		1.54	0.62
  Normal saline	30.00	32.57	30.10		108.56	100.33		3.09	2.02
  	70.00	69.77	69.67		99.67	99.52		3.63	0.22
  Mengniu milk	30.00	30.63	30.87		102.11	102.89		3.94	1.14
  	70.00	69.47	69.77		99.24	99.67		2.86	1.22

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