One-Step Synthesis of Nitrogen/Fluorine Co-Doped Carbon Dots for Use in Ferric Ions and Ascorbic Acid Detection

Carbon dots (CDs) have caught enormous attention owing to their distinctive properties, such as their high water solubility, tunable optical properties, and easy surface modification, which can be generally used for the detection of heavy metals and organic pollutants. Herein, nitrogen and fluorine co-doped carbon dots (NFCDs) were designed via a rapid, low-cost, and one-step microwave-assisted technique using DL-malic acid and levofloxacin. The NFCDs emitted intense green fluorescence under UV lighting, and the optical emission peak at 490 nm was observed upon a 280 nm excitation, with a high quantum yield of 21.03%. Interestingly, the spectral measurements illustrated excitation-independent and concentration-independent single-color fluorescence owing to the presence of nitrogen and fluorine elements in the surface functional groups. Additionally, the NFCDs were applied for the selective detection of Fe3+ and ascorbic acid based on the “turn-off” mode. The detection limits were determined as 1.03 and 4.22 µM, respectively. The quenching mechanisms were explored using the static quenching mechanism and the inner filter effect. Therefore, a NFCDs fluorescent probe with single color emission was successfully developed for the convenient and rapid detection of Fe3+ and ascorbic acid in environments.


Introduction
With the development of chemical industries, water pollution caused by heavy metal ions and organic pollutants has become a major environmental and public health issue. Ferric ion (Fe 3+ ) is a transition metal that plays a vital role in biogeochemical processes [1,2]. As the fourth abundant element in the Earth's crust, iron can be found in both surface and underground waters. Moreover, the various applications of Fe-related industrial processes, such as sewage treatment systems and a Fenton reagent, would further increase the amount of iron in environments and thus cause potential hazards to human health [3,4]. It has been widely recognized that a high concentration of Fe 3+ is associated with various health implications, including cancers of different organs, cellular homeostasis, or metabolism [4][5][6][7]. Additionally, the permissible amount of ferric ions in drinking waters was set at 300 ppb by the Environmental Protection Agency [1,8]. Therefore, the ferric ion level is one important property to consider when evaluating the quality of water. 280 nm excitation, independent of the excitation wavelengths and concentration. The green fluorescence can be quenched by both Fe 3+ and AA, respectively. The as-prepared NFCDs were further used for the detection of Fe 3+ and AA, demonstrating LODs of 1.03 and 4.22 µM, respectively. The LODs are comparable with recent research [35][36][37][38][39]. Therefore, the N and F co-doped CDs with green fluorescence were successfully synthesized with an excitation-independent emission, high QY, and high selectivity and sensitivity for the detection of Fe 3+ and AA. Nanomaterials 2022, 12, x FOR PEER REVIEW 3 of 14 and surface functional groups. In addition, the NFCDs were found to emit a green fluorescence under the UV lamp, with a photoluminescence quantum yield (QY) detected as 21.03%. The optimal emission wavelengths of the NFCDs were observed at 490 nm upon a 280 nm excitation, independent of the excitation wavelengths and concentration. The green fluorescence can be quenched by both Fe 3+ and AA, respectively. The as-prepared NFCDs were further used for the detection of Fe 3+ and AA, demonstrating LODs of 1.03 and 4.22 µM, respectively. The LODs are comparable with recent research [35][36][37][38][39]. Therefore, the N and F co-doped CDs with green fluorescence were successfully synthesized with an excitation-independent emission, high QY, and high selectivity and sensitivity for the detection of Fe 3+ and AA. Scheme 1. The preparation approach of NFCDs from Lev and DL-MA.

Synthesis of NFCDs
Firstly, the desired amounts of DL-malic acid and levofloxacin were dissolved in 10 mL of ultrapure water, which was then heated in a microwave oven (600 W, M1-L213B, Media, China) for 5 min. The resultant solids were dissolved in water with an ultrasound treatment (40 KHz, F-040, Fuyang Technology group Co., Ltd, Shenzhen, China) for around 24 h and then filtered with a filtration membrane (0.22 µm). The obtained solution was further purified with dialysis bags (MDCO 34-1000 Da, Biomed Instrument Inc., USA) for 24 h. Eventually, the purified NFCDs solution was obtained and further freeze-dried at −70 °C for further characterization.

Characterization of NFCDs
The morphology of the NFCDs was analyzed by transmission electron microscopy (TEM, JEM-2100, JEOL Corp., Japan). A FIR8900 Fourier transform infrared (FT-IR) spectrometer and X-ray photoelectron spectroscopy (XPS, Escalab 250Xi, Thermo Scientific, Waltham, MA, USA) were used to investigate the surface groups of the NFCDs. The Xray diffraction (XRD) pattern of the NFCDs was obtained from the Neotoku Corporation (Japan) with CuKα1 radiation operated at 40 kV and 40 mA. For the optical properties

Synthesis of NFCDs
Firstly, the desired amounts of DL-malic acid and levofloxacin were dissolved in 10 mL of ultrapure water, which was then heated in a microwave oven (600 W, M1-L213B, Media, China) for 5 min. The resultant solids were dissolved in water with an ultrasound treatment (40 KHz, F-040, Fuyang Technology group Co., Ltd., Shenzhen, China) for around 24 h and then filtered with a filtration membrane (0.22 µm). The obtained solution was further purified with dialysis bags (MDCO 34-1000 Da, Biomed Instrument Inc., USA) for 24 h. Eventually, the purified NFCDs solution was obtained and further freeze-dried at −70 • C for further characterization.

Characterization of NFCDs
The morphology of the NFCDs was analyzed by transmission electron microscopy (TEM, JEM-2100, JEOL Corp., Japan). A FIR8900 Fourier transform infrared (FT-IR) spectrometer and X-ray photoelectron spectroscopy (XPS, Escalab 250Xi, Thermo Scientific, Waltham, MA, USA) were used to investigate the surface groups of the NFCDs. The X-ray diffraction (XRD) pattern of the NFCDs was obtained from the Neotoku Corporation (Japan) with CuKα1 radiation operated at 40 kV and 40 mA. For the optical properties analysis, the UV-vis absorbance spectra and fluorescence light (PL) spectra were collected on a U-4100 spectrophotometer (Hitachi, Tokyo, Japan) and Hitachi F-7000 fluorescence spectrometer (150 W xenon lamp), respectively. The PL decay lifetimes were measured and estimated based on an FLS920 spectrophotometer supplied by Edinburgh Instruments. The absolute quantum yield (QY) was measured in the calibration sphere with a HORIBA FLS920.

Stability of NFCDs Solution
In order to examine the photostability of the NFCDs, the solution was continuously exposed to UV light (365 nm) for 2 h. Upon 280 nm excitation, the PL spectra were recorded every 10 min. Moreover, the concentration-dependent PL property of the NFCDs-1 was explored by collecting the PL spectra of solutions with different concentrations, ranging from 0.005 to 0.5 mg/mL. Additionally, the effects of the acidic or basic environments on the emission spectra (upon 280 nm excitation) were further investigated with different pH values (ranging from 2.2 to 13.1), which were tuned using a NaOH solution (0.1 mol/L), HCl solution (0.1 mol/L), and NaH 2 PO 3 solution (0.01 mol/L).

Fluorescence Quenching Study of Fe 3+ and AA
For the detection of Fe 3+ and AA, based on the "turn-off" mode, the stock solution of the NFCDs was mixed with different amounts of FeCl 3 solution and AA solution. For the detection of Fe 3+ , solutions with different Fe 3+ concentrations ranging from 5 to 500 µM were prepared in 0.02 mg/mL NFCDs. On the other hand, the concentration of AA was set from 5 to 900 µM. After mixing for 5 min, the PL spectra of different solutions were recorded upon a 280 nm excitation. All the experiments were conducted in triplicate.

Morphology and Structure Characterization of NFCDs
In order to explore the morphology and structure of the NFCDs particles, the HR-TEM and XRD instruments were applied. The nanoparticles were found to be quasi-spherical (seen in Figure 1a), and the average size was estimated to be 5.5 ± 0.1 nm, based on 83 particles using the Gaussian distribution calculation (Figure 1b). The lattice spacing was further measured as 0.32 nm, corresponding to the (002) diffraction plane of graphite [43]. from this, the splitting of the N1s peak in Figure 2d indicates the presence of pyridinic N (399.25 eV) and graphitic N (401.24 eV) [49,50]. Lastly, the F1s spectra displayed a single characteristic peak at 687.09 eV, assigned to the covalent C-F, respectively [40]. Thus, the XPS results of all the existing functional groups demonstrated a close match with the FT-IR analysis, confirming the presence of common luminescent centers [33].  In addition, the morphology of the solid, detected by XRD in Figure 1c, demonstrated a broad peak centered at 23 • , which was assigned to the disordered carbon in the NFCDs. In order to obtain a full understanding of the structure of NFCDs, the surface function groups were further detected by FT-IR spectroscopy, as seen in Figure 1d. A characteristic peak at around 3400 cm −1 of the FT-IR spectrum was observed, attributed to the stretching vibrations of the O-H and N-H functional groups. The peaks displayed in the region of 2850-2950 cm −1 were ascribed to the typical sp 3 C-H stretching vibrations [44]. The peaks located at 1735, 1616, and 1404 cm −1 were related to the stretching vibrations of COOH, C=O, and C-N, respectively [40,45]. Moreover, the characteristic absorption peak of the C-F bond, together with the C=C stretching vibration, was observed at 1274 and 1175 cm −1 [40]. Thus, the as-prepared NFCDs possessed different functional groups, such as the hydroxyl, carboxyl, and amide groups, which are attributed to the fluorescence of the NFCDs [33].
Furthermore, the XPS analysis was performed to estimate the chemical compositions. Figure 2a displayed the survey spectrum of the NFCDs, revealing four typical peaks at 284.7, 401.1, 532.3, and 687.6 eV. These binding energies were ascribed to C1s, N1s, O1s, and F1s, respectively [34,35]. The high-resolution spectra of C1s, as seen in Figure 2b, was deconvoluted into four typical peaks: C=C/C-C (284.49 eV) [45,46], C-N/C-O (285.76 eV) [17,47], O=C-O (288.62 eV) [35,48], and C-F (290.3 eV) [41]. Three peaks, corresponding to the binding energies of C-O (531.59 eV), C-O (532.97 eV), and O-C=O (534.16 eV), were obtained from the deconvolution of the O1s spectra, as shown in Figure 2c [29,35]. Apart from this, the splitting of the N1s peak in Figure 2d indicates the presence of pyridinic N (399.25 eV) and graphitic N (401.24 eV) [49,50]. Lastly, the F1s spectra displayed a single characteristic peak at 687.09 eV, assigned to the covalent C-F, respectively [40]. Thus, the XPS results of all the existing functional groups demonstrated a close match with the FT-IR analysis, confirming the presence of common luminescent centers [33].
from this, the splitting of the N1s peak in Figure 2d indicates the presence of pyridinic N (399.25 eV) and graphitic N (401.24 eV) [49,50]. Lastly, the F1s spectra displayed a single characteristic peak at 687.09 eV, assigned to the covalent C-F, respectively [40]. Thus, the XPS results of all the existing functional groups demonstrated a close match with the FT-IR analysis, confirming the presence of common luminescent centers [33].

Optical Property Characterization of NFCDs
The optical properties of the NFCDs solution were characterized. The photophysical properties of the as-synthesized NFCDs were examined through the ultraviolet-visible (UV-visible) and photoluminescence (PL) spectroscopies. As seen in Figure 3a, the UV-vis spectrum demonstrated three typical absorption bands. A strong absorption peak was observed at 210 nm, falling in the region of 210-250 nm, assigned to the aromatic π-π* transition of the C=C bond, while the second strong absorption peak, located at 290 nm, was related to the π-π* transitions of the C-N/C=N or C-F bonds [51,52]. Moreover, a Nanomaterials 2022, 12, 2377 6 of 14 distinct peak, observed at 330 nm, was attributed to the n-π* transitions of the C=O/C=N bonds in NFCDs [27,53]. The inset displayed a colorless solution under daylight and the one with an intense green fluorescence under UV light. Thus, the carbon cores and surface functional groups of the NFCDs absorbed the energies provided by UV light first, for which the excited state, in turn, drops to the ground state by emitting green fluorescence. Moreover, an optimal emission at around 490 nm (λ ex = 280 nm) was displayed with the corresponding excitation spectra in Figure 3a, demonstrating two broad excitation peaks at 280 nm and 330 nm, respectively.
The optical properties of the NFCDs solution were characterized. The photophysical properties of the as-synthesized NFCDs were examined through the ultraviolet-visible (UV-visible) and photoluminescence (PL) spectroscopies. As seen in Figure 3a, the UV-vis spectrum demonstrated three typical absorption bands. A strong absorption peak was observed at 210 nm, falling in the region of 210-250 nm, assigned to the aromatic π-π* transition of the C=C bond, while the second strong absorption peak, located at 290 nm, was related to the π-π* transitions of the C-N/C=N or C-F bonds [51,52]. Moreover, a distinct peak, observed at 330 nm, was attributed to the n-π* transitions of the C=O/C=N bonds in NFCDs [27,53]. The inset displayed a colorless solution under daylight and the one with an intense green fluorescence under UV light. Thus, the carbon cores and surface functional groups of the NFCDs absorbed the energies provided by UV light first, for which the excited state, in turn, drops to the ground state by emitting green fluorescence. Moreover, an optimal emission at around 490 nm (λex = 280 nm) was displayed with the corresponding excitation spectra in Figure 3a, demonstrating two broad excitation peaks at 280 nm and 330 nm, respectively.  Figure 3b illustrates the PL spectra of the NFCDs solution upon different excitation wavelengths. The optimal emission spectra with a maximum intensity were obtained at around 490 nm under an excitation wavelength of 280 nm, which corresponded to the green fluorescence seen under the UV lamp. It was found that the emission spectra centered at 490 nm hardly shifted with the change of the excitation wavelengths (less than 10 nm), which might be a result of the monotonous surface defeats from the uniform preparation conditions and heteroatom dopant [3,38,44,54,55]. The QY was measured as 21.03%, which was higher than the QYs of most DL-MA-derived CDs [37,56,57].
Furthermore, the effect of concentration on the PL properties of the NFCDs was examined with different concentrations (0.005, 0.01, 0.02, 0.04, 0.06, 0.08, 0.1, 0.3, and 0.5 mg/mL). As presented in Figure 3c, the emission spectra were centered at 490 nm for all the concentrations considered, indicating a concentration-independent optical property. Interestingly, the PL intensity was enhanced when the concentration increased from 0.005 mg/mL until 0.06 mg/mL, after which it started decreasing from 0.06 to 0.5 mg/mL.
To further understand the PL behavior, the photoluminescence excitation (PLE) spectra of the solutions ranging from 0.005 to 0.5 mg/mL and the 3D spectra with concentrations of 0.04 mg/mL and 0.5 mg/mL were collected and displayed in Figure S1. Along with the increase in the NFCDs concentration from 0.005 mg/mL to 0.06 mg/mL, the intensities of the excitation peaks at around 280 nm and 330 nm were enhanced accordingly. At the higher concentrations from 0.08 mg/mL to 0.5 mg/mL, the excitation peaks at 280 and 330 nm gradually decreased, and an increase in the excitation centers at 360 nm was observed, indicating a change in the luminescent centers. The phenomenon might be that the n-π*  Figure 3b illustrates the PL spectra of the NFCDs solution upon different excitation wavelengths. The optimal emission spectra with a maximum intensity were obtained at around 490 nm under an excitation wavelength of 280 nm, which corresponded to the green fluorescence seen under the UV lamp. It was found that the emission spectra centered at 490 nm hardly shifted with the change of the excitation wavelengths (less than 10 nm), which might be a result of the monotonous surface defeats from the uniform preparation conditions and heteroatom dopant [3,38,44,54,55]. The QY was measured as 21.03%, which was higher than the QYs of most DL-MA-derived CDs [37,56,57].
Furthermore, the effect of concentration on the PL properties of the NFCDs was examined with different concentrations (0.005, 0.01, 0.02, 0.04, 0.06, 0.08, 0.1, 0.3, and 0.5 mg/mL). As presented in Figure 3c, the emission spectra were centered at 490 nm for all the concentrations considered, indicating a concentration-independent optical property. Interestingly, the PL intensity was enhanced when the concentration increased from 0.005 mg/mL until 0.06 mg/mL, after which it started decreasing from 0.06 to 0.5 mg/mL.
To further understand the PL behavior, the photoluminescence excitation (PLE) spectra of the solutions ranging from 0.005 to 0.5 mg/mL and the 3D spectra with concentrations of 0.04 mg/mL and 0.5 mg/mL were collected and displayed in Figure S1. Along with the increase in the NFCDs concentration from 0.005 mg/mL to 0.06 mg/mL, the intensities of the excitation peaks at around 280 nm and 330 nm were enhanced accordingly. At the higher concentrations from 0.08 mg/mL to 0.5 mg/mL, the excitation peaks at 280 and 330 nm gradually decreased, and an increase in the excitation centers at 360 nm was observed, indicating a change in the luminescent centers. The phenomenon might be that the n-π* transition of the aggregated hydroxyl groups absorbed 360 nm UV light, which contributes to the fluorescence in the concentrated solutions [34]. The intensity of the excitation peak at 280 nm became lower than that at 360 nm when the concentration was over 0.3 mg/mL. As NFCDs possess hydroxyl and carboxyl groups, at high concentrations, the surface functional groups are more likely to aggregate than that at smaller concentrations, thus leading to a slight change in the excitation peak heights. Therefore, the excitation peaks at 280 nm and 330nm became lower than that at 360 nm, resulting in a decrease in emission intensities [34,58].

Stability of NFCDs
The stability of the NFCDs was investigated under different experimental conditions, including varying UV light exposure times, ionic strengths, and pH values. As seen in Figure S2, the photostability of the NFCDs at 0.02 mg/mL was explored under continuous UV irradiation at 365 nm. After 120 min, the PL intensity remained comparable with that before any treatment, indicating an excellent photostability of NFCDs. Meanwhile, the NFCDs possessed almost 80% of the original PL intensity when the NaCl concentration reached 2 mol/L, presenting excellent stability with different ionic strengths, as seen in Figure S3.
Additionally, the optical properties of the NFCDs in different acidic and basic environments were also measured and displayed in Figure 4. The results illustrated that the PL intensity in an acidic medium decreased mildly when the pH values increased from 2.2 to 6.1, which reached a maximum intensity at 7.0. In the basic solutions, a sharp decrease in the PL intensities was observed for the pH values ranging from 8.0 to 13.1 as well as a blue shift of the emission peak from around 490 to 470 nm. The effects of the solution environments on the PL properties might be attributed to the protonation or deprotonation between the oxygen/nitrogen-containing groups and H + or OH − ions in the acidic or basic conditions, thus leading to a weaker fluorescence compared to the neutral environment [59,60].
peaks at 280 nm and 330nm became lower than that at 360 nm, resulting in a decrease in emission intensities [34,58].

Stability of NFCDs
The stability of the NFCDs was investigated under different experimental conditions, including varying UV light exposure times, ionic strengths, and pH values. As seen in Figure S2, the photostability of the NFCDs at 0.02 mg/mL was explored under continuous UV irradiation at 365 nm. After 120 min, the PL intensity remained comparable with that before any treatment, indicating an excellent photostability of NFCDs. Meanwhile, the NFCDs possessed almost 80% of the original PL intensity when the NaCl concentration reached 2 mol/L, presenting excellent stability with different ionic strengths, as seen in Figure S3.
Additionally, the optical properties of the NFCDs in different acidic and basic environments were also measured and displayed in Figure 4. The results illustrated that the PL intensity in an acidic medium decreased mildly when the pH values increased from 2.2 to 6.1, which reached a maximum intensity at 7.0. In the basic solutions, a sharp decrease in the PL intensities was observed for the pH values ranging from 8.0 to 13.1 as well as a blue shift of the emission peak from around 490 to 470 nm. The effects of the solution environments on the PL properties might be attributed to the protonation or deprotonation between the oxygen/nitrogen-containing groups and H + or OHions in the acidic or basic conditions, thus leading to a weaker fluorescence compared to the neutral environment [59,60].

Sensitivity and Selectivity Study of Fe 3+ Ion
To assess the ability of the as-prepared NFCDs as a fluorescent probe for detecting metal ions, the fluorescence of the NFCDs at 0.02 mg/mL with various metal ions, including Na + , K + , Al 3+ , Fe 3+ , Mg 2+ , Ni 2+ , Mn 2+ , Co 2+ , Ca 2+ , Sn 2+ , Pb 2+ , and Ba 2+ , were measured and displayed in Figure 5a (as dark green columns, also seen in Table S1). The presence of Fe 3+ showed the strongest quenching effects on the fluorescence, while the other ions presented limited quenching abilities. This phenomenon might be related to the intense

Sensitivity and Selectivity Study of Fe 3+ Ion
To assess the ability of the as-prepared NFCDs as a fluorescent probe for detecting metal ions, the fluorescence of the NFCDs at 0.02 mg/mL with various metal ions, including Na + , K + , Al 3+ , Fe 3+ , Mg 2+ , Ni 2+ , Mn 2+ , Co 2+ , Ca 2+ , Sn 2+ , Pb 2+ , and Ba 2+ , were measured and displayed in Figure 5a (as dark green columns, also seen in Table S1). The presence of Fe 3+ showed the strongest quenching effects on the fluorescence, while the other ions presented limited quenching abilities. This phenomenon might be related to the intense affinity of the oxygen/nitrogen-contained groups for Fe 3+ , resulting in strong interactions between the NFCDs and Fe 3+ , thus leading to a quenched fluorescence [26]. Thus, the NFCDs can be utilized as a fluorescent probe for the detection of Fe 3+ ions in a solution.
affinity of the oxygen/nitrogen-contained groups for Fe 3+ , resulting in strong interactions between the NFCDs and Fe 3+ , thus leading to a quenched fluorescence [26]. Thus, the NFCDs can be utilized as a fluorescent probe for the detection of Fe 3+ ions in a solution. The sensitivity to detect Fe 3+ with an NFCDs probe was evaluated with various Fe 3+ concentrations from 5 µM to 300 µM in a 0.02 mg/mL NFCDs solution. As shown in Figure  5b, the PL fluorescence intensity was found to decrease sharply with the increased concentrations of Fe 3+ . While the concentration reached 300 µM, around 90% of the PL fluorescence was quenched, indicating a strong detection ability provided by the NFCDs. A blue shift of the emission spectra was observed with increasing Fe 3+ concentrations, suggesting the formation of new fluorescence centers. The phenomenon might be explained by the chelation reaction between the Fe 3+ and -COOH, -OH, and other groups on the surface of carbon cores [5]. Moreover, as shown in the inset of Figure 5c, a linear relationship between the ferric ion concentrations and PL intensities was obtained (R 2 = 0.9962) when the concentration was from 5 to 30 µM. Additionally, the detection limit (LOD) was estimated to be 1.03 µM, which is comparable with the values obtained from the other systems (Table S2).
Furthermore, the selectivity of the NFCDs for Fe 3+ was also investigated with various interference ions, including Na + , K + , Al 3+ , Mg 2+ , Ni 2+ , Mn 2+ , Co 2+ , Ca 2+ , Sn 2+ , Pb 2+ , and Ba 2+ . Figure 5a (yellow column) shows the PL intensity ratio (F/F0) with 500 µM interference ions with and without the addition of Fe 3+ . Furthermore, the presence of other metal ions had a negligible influence on the quenched fluorescence caused by Fe 3+ . Therefore, the NFCDs solution presented high sensitivity and excellent selectivity for Fe 3+ detection.

Sensitivity and Selectivity Study of AA
The detection capabilities of the NFCDs were explored with the presence of different organic molecules (i.e., L-ascorbic acid (AA), D-asparagine (D-Asn), L-asparagine (L-Asn), L-glutamic acid (L-GA), S-mandelic acid (S-MA), L-leucine (L-Leu), L-glutamine (L-Glu), L-methionine (L-Met), and L-valine (L-Val)). As shown in Figure 6a and Table S3, the presence of organic molecules had almost no effect on the fluorescence of the NFCDs solution, except for the AA. It was observed that 500 µM of AA in 0.02 mg/mL NFCDs has caused an 83% reduction of the original PL intensity. The quenching phenomenon might be due to the complex interactions between the hydroxyl functional groups of AA with the hydrophilic functional groups of NFCDs [61]. The sensitivity to detect Fe 3+ with an NFCDs probe was evaluated with various Fe 3+ concentrations from 5 µM to 300 µM in a 0.02 mg/mL NFCDs solution. As shown in Figure 5b, the PL fluorescence intensity was found to decrease sharply with the increased concentrations of Fe 3+ . While the concentration reached 300 µM, around 90% of the PL fluorescence was quenched, indicating a strong detection ability provided by the NFCDs. A blue shift of the emission spectra was observed with increasing Fe 3+ concentrations, suggesting the formation of new fluorescence centers. The phenomenon might be explained by the chelation reaction between the Fe 3+ and -COOH, -OH, and other groups on the surface of carbon cores [5]. Moreover, as shown in the inset of Figure 5c, a linear relationship between the ferric ion concentrations and PL intensities was obtained (R 2 = 0.9962) when the concentration was from 5 to 30 µM. Additionally, the detection limit (LOD) was estimated to be 1.03 µM, which is comparable with the values obtained from the other systems (Table S2).
Furthermore, the selectivity of the NFCDs for Fe 3+ was also investigated with various interference ions, including Na + , K + , Al 3+ , Mg 2+ , Ni 2+ , Mn 2+ , Co 2+ , Ca 2+ , Sn 2+ , Pb 2+ , and Ba 2+ . Figure 5a (yellow column) shows the PL intensity ratio (F/F 0 ) with 500 µM interference ions with and without the addition of Fe 3+ . Furthermore, the presence of other metal ions had a negligible influence on the quenched fluorescence caused by Fe 3+ . Therefore, the NFCDs solution presented high sensitivity and excellent selectivity for Fe 3+ detection.

Sensitivity and Selectivity Study of AA
The detection capabilities of the NFCDs were explored with the presence of different organic molecules (i.e., L-ascorbic acid (AA), D-asparagine (D-Asn), L-asparagine (L-Asn), L-glutamic acid (L-GA), S-mandelic acid (S-MA), L-leucine (L-Leu), L-glutamine (L-Glu), L-methionine (L-Met), and L-valine (L-Val)). As shown in Figure 6a and Table S3, the presence of organic molecules had almost no effect on the fluorescence of the NFCDs solution, except for the AA. It was observed that 500 µM of AA in 0.02 mg/mL NFCDs has caused an 83% reduction of the original PL intensity. The quenching phenomenon might be due to the complex interactions between the hydroxyl functional groups of AA with the hydrophilic functional groups of NFCDs [61]. To further explore the detection sensitivity of the NFCDs, the fluorescence spectra of NFCDs solutions with various concentrations of AA (0 to 900 µM) were examined and displayed in Figure 6b. As the AA concentration increased, the PL intensity at 490 nm decreased gradually, indicating increased quenching effects. A good linear relationship between the quenching ratio (F/F0) and AA concentration was exhibited from 5 to 100 µM (inset of Figure 6c), with the slope and LOD estimated as −0.0039 µM −1 and 4.22 µM, respectively. Thus, the NFCDs can be used as a "turn-off" sensor for AA detection, which is beneficial due to the easy sample preparation and one-step detection procedure. Table S4 presents the comparison of the current AA detection modes, linear ranges, and LODs from recent reports. Thus, the NFCDs prepared in the present work showed comparable LODs and linear ranges when compared with previous research [9,10,26,37].

Possible Quenching Mechanism of Fe 3+ and AA
To further explore the quenching mechanism, the Stern-Volmer equation (Equation (1)) was used to model the relation between the F0/F versus Fe 3+ and AA concentrations [62].
where F represents the PL intensity of the quenched solution; F0 is the original PL intensity without the presence of Fe 3+ ; ksv stands for the quenching constant in the Stern-Volmer equation; kq presents the quenching constant, and τ is the PL lifetime of the NFCDs. As demonstrated in Figure S4a, the slope of the PL intensity ratio versus Fe 3+ concentration was determined as 0.0318 µM −1 , based on which the quenching rate constant was estimated to be 4.14 × 10 12 M −1 ·s −1 . Additionally, the Stern-Volmer equation, modeling the relation between the F0/F versus AA concentrations, was displayed in Figure S6a, with the quenching rate constant, kq, determined as 7.72 × 10 11 M −1 ·s −1 . Both the quenching constants for the Fe 3+ and AA were one or two orders of magnitude higher than the general considered dynamic quenching constant (DQM), 1.0 × 10 10 M −1 ·s −1 . Thus, the static quenching mechanism (SQM), referring to the reactions between the NFCDs and Fe 3+ or AA, might be responsible for the quenching phenomenon caused by Fe 3+ and AA [63,64]. This assumption was further supported by the UV-vis spectra and PL lifetime analysis. Figure S4b shows the UV absorption curves of the NFCDs and NFCDs/Fe 3+ solutions. To further explore the detection sensitivity of the NFCDs, the fluorescence spectra of NFCDs solutions with various concentrations of AA (0 to 900 µM) were examined and displayed in Figure 6b. As the AA concentration increased, the PL intensity at 490 nm decreased gradually, indicating increased quenching effects. A good linear relationship between the quenching ratio (F/F 0 ) and AA concentration was exhibited from 5 to 100 µM (inset of Figure 6c), with the slope and LOD estimated as −0.0039 µM −1 and 4.22 µM, respectively. Thus, the NFCDs can be used as a "turn-off" sensor for AA detection, which is beneficial due to the easy sample preparation and one-step detection procedure. Table S4 presents the comparison of the current AA detection modes, linear ranges, and LODs from recent reports. Thus, the NFCDs prepared in the present work showed comparable LODs and linear ranges when compared with previous research [9,10,26,37].

Possible Quenching Mechanism of Fe 3+ and AA
To further explore the quenching mechanism, the Stern-Volmer equation (Equation (1)) was used to model the relation between the F 0 /F versus Fe 3+ and AA concentrations [62].
where F represents the PL intensity of the quenched solution; F 0 is the original PL intensity without the presence of Fe 3+ ; k sv stands for the quenching constant in the Stern-Volmer equation; k q presents the quenching constant, and τ is the PL lifetime of the NFCDs. As demonstrated in Figure S4a, the slope of the PL intensity ratio versus Fe 3+ concentration was determined as 0.0318 µM −1 , based on which the quenching rate constant was estimated to be 4.14 × 10 12 M −1 ·s −1 . Additionally, the Stern-Volmer equation, modeling the relation between the F 0 /F versus AA concentrations, was displayed in Figure S6a, with the quenching rate constant, k q , determined as 7.72 × 10 11 M −1 ·s −1 . Both the quenching constants for the Fe 3+ and AA were one or two orders of magnitude higher than the general considered dynamic quenching constant (DQM), 1.0 × 10 10 M −1 ·s −1 . Thus, the static quenching mechanism (SQM), referring to the reactions between the NFCDs and Fe 3+ or AA, might be responsible for the quenching phenomenon caused by Fe 3+ and AA [63,64]. This assumption was further supported by the UV-vis spectra and PL lifetime analysis. Figure S4b shows the UV absorption curves of the NFCDs and NFCDs/Fe 3+ solutions. The absorption band of the NFCDs at 330 nm was gradually reduced along with the addition of Fe 3+ , confirming the formation of a non-fluorescent NFCDs-Fe 3+ chelating complex (SQM). Additional evidence was supplied by the PL lifetime analysis. The fluorescence lifetime of the NFCDs with 500 µM Fe 3+ was estimated as 8.29 ns (Figure S5a), showing a slight increase rather than a decrease compared to 7.69 ns for the NFCDs. Thus, it is reasonable to eliminate the possibility of DQM [25]. Moreover, Figure S5b displays the UV-vis spectrum of the Fe 3+ solution as well as the excitation and emission spectra of the NFCDs. A broad absorption band at 280 nm was seen in the UV-vis spectrum, which overlaps with the excitation peaks of the NFCDs at 280 nm. Thus, the possible quenching mechanism of Fe 3+ against the NFCDs might be a combination of the inner filter effect (IFE) and SQE [65]. For the NFCDs/AA system, Figure S6b shows the UV-vis spectra of the NFCDs and NFCDs/AA solutions, where an absorption peak centered at 250 nm appeared and became stronger with an increased AA concentration. This observation confirmed the formation of the NFCDs/AA complex, which contributed to the quenched fluorescence of NFCDs and thus supported SQM.
Furthermore, the UV-vis spectrum of AA is supplied in Figure S7b, together with the excitation and emission fluorescence spectra. The characteristic absorption of AA was in a range from 210 to 300 nm, overlapping with the excitation centers of NFCDs. Thus, the AA might shield the excitation light of the NFCDs, resulting in decreased PL intensities. Additionally, the PL decay measurements illustrated that the lifetime for the NFCDs/AA solution (8.04 ns, as seen in Figure S7a) showed a negligible difference from the NFCDs' lifetime. Thus, an inner filter effect (IFE) was presented and contributed to the quenching effects in the presence of AA. Therefore, NFCDs can be potentially used as a "turn-off" sensor for the detection of AA based on the SQM and IFE processes. The possible quenching mechanism for both Fe 3+ and AA is presented in Figure 7, providing different states of NFCDs particles and the corresponding UV-vis spectra with the excitation and emission spectra. Thus, both the IFE and SQM contributed to the quenching phenomenon of NFCDs with the presence of Fe 3+ and AA, respectively. overlaps with the excitation peaks of the NFCDs at 280 nm. Thus, the possible quenching mechanism of Fe 3+ against the NFCDs might be a combination of the inner filter effect (IFE) and SQE [65]. For the NFCDs/AA system, Figure S6b shows the UV-vis spectra of the NFCDs and NFCDs/AA solutions, where an absorption peak centered at 250 nm appeared and became stronger with an increased AA concentration. This observation confirmed the formation of the NFCDs/AA complex, which contributed to the quenched fluorescence of NFCDs and thus supported SQM. Furthermore, the UV-vis spectrum of AA is supplied in Figure S7b, together with the excitation and emission fluorescence spectra. The characteristic absorption of AA was in a range from 210 to 300 nm, overlapping with the excitation centers of NFCDs. Thus, the AA might shield the excitation light of the NFCDs, resulting in decreased PL intensities. Additionally, the PL decay measurements illustrated that the lifetime for the NFCDs/AA solution (8.04 ns, as seen in Figure S7a) showed a negligible difference from the NFCDs' lifetime. Thus, an inner filter effect (IFE) was presented and contributed to the quenching effects in the presence of AA. Therefore, NFCDs can be potentially used as a "turn-off" sensor for the detection of AA based on the SQM and IFE processes. The possible quenching mechanism for both Fe 3+ and AA is presented in Figure 7, providing different states of NFCDs particles and the corresponding UV-vis spectra with the excitation and emission spectra. Thus, both the IFE and SQM contributed to the quenching phenomenon of NFCDs with the presence of Fe 3+ and AA, respectively.

Conclusions
In conclusion, a fluorescent carbon-based probe with a high quantum yield of 21.03% was directly developed using DL-malic acid and levofloxacin through a rapid and lowcost microwave treatment. The as-prepared NFCDs exhibited excellent water dispersibility with an average size of 5.49 nm. Additionally, the excitation-independent and concentration-independent fluorescence behaviors of the NFCDs were observed with a single emission of 490 nm. Moreover, the green fluorescence can be quenched by both Fe 3+ and AA as "turn-off" probes, with LODs of 1.03 and 4.22 µM. The quenching mechanisms were further determined as a static quenching mechanism and inner filter effect, based on the lifetime and spectral analysis. This methodology provided a rapid and low-cost

Conclusions
In conclusion, a fluorescent carbon-based probe with a high quantum yield of 21.03% was directly developed using DL-malic acid and levofloxacin through a rapid and low-cost microwave treatment. The as-prepared NFCDs exhibited excellent water dispersibility with an average size of 5.49 nm. Additionally, the excitation-independent and concentrationindependent fluorescence behaviors of the NFCDs were observed with a single emission of 490 nm. Moreover, the green fluorescence can be quenched by both Fe 3+ and AA as "turn-off" probes, with LODs of 1.03 and 4.22 µM. The quenching mechanisms were further determined as a static quenching mechanism and inner filter effect, based on the lifetime and spectral analysis. This methodology provided a rapid and low-cost approach to the synthesis of nitrogen and fluorine co-doped CDs with a high quantum yield, rapid response, and high sensitivity and selectivity as fluorescent probes to detect Fe 3+ and AA in biological and environmental systems.