Ultrasonic-Assisted Synthesis of N-Doped, Multicolor Carbon Dots toward Fluorescent Inks, Fluorescence Sensors, and Logic Gate Operations

Over past decades, the multicolor carbon dots (M-CDs) have attracted enormous attentions due to their tunable photoluminescence and versatile applications. Herein, the nitrogen-doped (N-doped) M-CDs including green, chartreuse, and pink emissive CDs are successfully synthesized by ultrasonic treatment of kiwifruit juice with different additive reagents such as ethanol, ethylenediamine, and acetone. Owing to their strong fluorescence upon irradiation with 365 nm UV light, the highly water-soluble M-CDs present great potential in the anticounterfeit field as fluorescent inks. Particularly, the resulting green emission CDs (G-CDs) with excellent fluorescence and stability are applied as a label-free probe model for “on–off” detection of Fe3+. The fluorescence of G-CDs is significantly quenched by Fe3+ through static quenching. The nanoprobe demonstrates good selectivity and sensitivity toward Fe3+ with a detection limit of ~0.11 μM. Besides, the quenched fluorescence of G-CDs by Fe3+ can be recovered by the addition of PO43− or ascorbic acid (AA) into the CDs/Fe3+ system to realize the “off–on” fluorescent process. Furthermore, NOT and IMPLICATION logic gates are constructed based on the selection of Fe3+ and PO43− or AA as the inputs, which makes the G-CD-based sensors utilized as various logic gates at molecular level. Therefore, the N-doped M-CDs hold promising prospects as competitive candidates in monitoring the trace species, applications in food chemistry, anticounterfeit uses, and beyond.


Introduction
Carbon dots (CDs) have received increasing attention owing to their excellent biocompatibility, good water solubility, low cytotoxicity, robust chemical inertness, high photo/chemical stability, and strong resistance to photobleaching [1,2]. They have especially demonstrated huge potential applications in a wide range of fields, such as sensing, bioimaging, drug delivery, photocatalysis, energy storage, anticounterfeiting, light-emitting diodes (LED), and so on [3][4][5][6][7][8]. Among these applications, great efforts have been devoted to synthesizing the efficient CDs via surface modification and/or heteroatom doping [9][10][11][12][13]. There are two methods for synthesizing CDs, classified as "top-down" and "bottom-up", respectively. The "top-down" approaches are to cleave the size of large carbon materials, such as laser ablation, chemical oxidation, and arc-discharge, often involving complex reactions [14,15]. Regarding the "bottom-up" methods, the CDs are synthesized via polymerization and carbonization of molecular precursors under specific conditions such as hydrothermal, solvothermal, pyrolysis, microwave, and ultrasonic synthesis, etc. [16,17]. The ultrasound has attracted more attention in recent years due to its high efficiency and the avoidance of using hazardous chemicals and incorporating impurities [18]. The acoustic cavitation effect of the ultrasound is responsible for the formation of CDs in organic by virtue of their strong and stable photoluminescence (PL) properties, as well as low toxicity.

Preparation of M-CDs
M-CDs were synthesized by a simple ultrasonic-assisted treatment of kiwifruit juice with different additives including EA, EDA, and ACTA, as exhibited in Scheme 1. The G-CDs were synthesized by using the following procedures. Typically, the fresh kiwifruit was purchased from a local supermarket, peeled, and then squeezed into a juice through a juicing machine. After filtration, 20 mL of the obtained clear juice and 2 mL of ethanol were mixed well together. Subsequently, the mixture was exposed to high-intensity ultrasonic irradiation (320 W, 80 kHz) at room temperature (RT) in ambient air for 3 h. The resulting solution was centrifuged at 10,000 rpm for 15 min. Afterwards, the supernatant solution was collected, filtered through a filter membrane (0.22 μm), and further purified through a dialysis membrane (1000 MWCO) for 24 h to eliminate the overreacted residues. Finally, the obtained product was dried under vacuum and stored at 4 °C for further use. The C-CDs were prepared with a similar procedure with the G-CDs just by replacing 2 mL of EA with the same volume of EDA. The P-CDs were synthesized in a similar manner with G-CDs through substituting juice (20 mL) and EA (2 mL) with the juice (2 mL) and ACTN (20 mL), respectively. Scheme 1. Schematic illustration for facile synthesis of the G-CDs, C-CDs, and P-CDs from kiwi fruit.

Fluorescent Ink Evaluation
The as-prepared G-CDs, C-CDs, and P-CDs solutions were further concentrated to obtain fluorescent inks. After cutting a piece from filter paper, the piece was immersed in the G-CDs solution, pulled out, and then dried at RT. Thus, the paper impregnated with G-CDs was gained. Simultaneously, the Chinese characters meaning "Jinan" were written on filter paper with one fountain pen loaded with the solution of C-CDs. Additionally, a Scheme 1. Schematic illustration for facile synthesis of the G-CDs, C-CDs, and P-CDs from kiwi fruit.

Fluorescent Ink Evaluation
The as-prepared G-CDs, C-CDs, and P-CDs solutions were further concentrated to obtain fluorescent inks. After cutting a piece from filter paper, the piece was immersed in the G-CDs solution, pulled out, and then dried at RT. Thus, the paper impregnated with G-CDs was gained. Simultaneously, the Chinese characters meaning "Jinan" were written on filter paper with one fountain pen loaded with the solution of C-CDs. Additionally, a picture was drawn on the filter paper by using P-CDs as the pigment. Photos of these filter papers were taken under daylight and UV light (365 nm), respectively, for comparison.

Ion Detection
The detection of ions was performed in aqueous solution at RT with the obtained G-CDs solution (0.5 mg mL −1 ), and the G-CDs were used as a model throughout the entire experiments. Briefly, 1 mL of 1 mM Na + , Mg 2+ , Zn 2+ , Cu 2+ , Ca 2+ , Fe 3+ , Ni 2+ , Co 2+ , Ba 2+ , Cd 2+ , Mn 2+ , Cr 3+ , K + , Fe 2+ , L-Cys, AA, or PO 4 3− was added to the G-CD solution. After a 5 min reaction at RT, the fluorescence spectra were recorded. The selectivity of the G-CDs was explored under the identical conditions. In a typical procedure, 1 mL of the above solutions was mixed separately with the G-CDs solution (3 mL) by gently shaking. All the experiments were carried out under the same conditions, and the corresponding fluorescence spectra were recorded after a 5 min reaction at RT. Furthermore, the sensitivity of the G-CDs for Fe 3+ was examined via the same procedure by adding aqueous solutions (3 mL) with different concentrations of Fe 3+ . The excited wavelength was set at 360 nm.

Material Characterization
The size and morphology of the as-synthesized CDs were analyzed using transmissionelectron microscopy (TEM, JEOL JEM-2100, Tokyo, Japan). X-ray diffraction (XRD) patterns were recorded on an X-ray diffractometer (Rigaku-TTRIII, Tokyo, Japan). The X-ray photoelectron spectroscopy (XPS) was performed by using an X-ray photoelectron spectrometer (Thermo Escalab 250Xi, Waltham, MA, USA). All fluorescence spectra were performed by a Shimadzu RF-6000 spectrofluorometer equipped with a Xenon arc lamp, and the excitation and emission slits were set at 5 nm (Shimadzu Corporation, Tokyo, Japan). The UV-Vis absorption spectra were recorded with a Shimadzu UV-2600 UV-Vis spectrophotometer (Shimadzu Corporation, Tokyo, Japan). The quantum yield (QY) of CDs was determined with a FS5 fluorescence spectrometer equipped with an SC-30 Integrating Sphere Module (Edinburgh, UK). The fluorescence lifetime was conducted on a DeltaFlex (Horiba Jobin Yvon IBH Ltd., Paris, France).

Physicochemical and Structural Characterizations
Kiwifruit, a fruit with high nutritional value, was used as the precursor for facile fabrication of the CDs. In addition, through a regular adjustment in the added reagents, varying from EA, EDA, and ACTN, the expected M-CDs were successfully prepared. Kiwifruit, as is well known, connately contains carbohydrates, amino acids, vitamin C, and minerals (Ca, K, Se, Zn and Ge) as well as an assortment of phytochemicals (carotenoids), which endows it with an abundance of the elemental C, N, and O. The high nutritional value and unique composition signifies that kiwifruit may be an ideal precursor for developing the N-doped CDs. In this study, G-CDs, C-CDs, and P-CDs with different optical features and surface functional groups were reasonably designed and synthesized.
The X-ray diffraction (XRD) measurement is commonly applied to figure out the structural crystallinity of CDs. Figure 1 shows the XRD patterns of the as-prepared M-CDs. Obviously, all samples demonstrate broad (002) diffraction peaks centered at around 23 • , suggesting that the disordered carbon characteristics are probably due to the heteroatom doping [48]. However, the (002) peak shifts to the small angle region by changing the added reagents from EA and EDA to ACTN. This phenomenon may be associated with the doped nitrogen in the carbon core as well as the oxygen-containing functional groups on the surface of the CDs, which is consistent with the findings in the reported literature [25,49]. heteroatom doping [48]. However, the (002) peak shifts to the small angle region by changing the added reagents from EA and EDA to ACTN. This phenomenon may be associated with the doped nitrogen in the carbon core as well as the oxygen-containing functional groups on the surface of the CDs, which is consistent with the findings in the reported literature [25,49].  (Figure 2i), indicating the limited influences of the added reagents on the particle sizes of the synthesized samples. Obviously, the sizes of CDs do not increase from G-CDs and C-CDs to P-CDs as the fluorescence emission shifts toward pink, indicating they do not follow the traditional size effect at all. The phenomenon probably arises from the hybridizations of the carbon core and surface states and the conjugation effect of the carbon surface [50,51]. Meanwhile, the formation process of CDs from kiwifruit juice with different additives including EA, EDA, and ACTA was tentatively speculated. Upon ultrasonication, the assembling, crosslinking, and dehydration processes may occur between some organic molecules in kiwifruit juice and the additives to produce some intermediates. Generally, the ultrasonic waves can induce the formation and violent collapse of small vacuum bubbles, from which strong hydrodynamic shear-forces and high-speed impinging liquid jets as well as deagglomeration are produced [19]. Thus, these intermediates can be further carbonized, and N-doped CDs are reasonably prepared due to the presence of the N-containing molecules in kiwifruit juice.  (Figure 2i), indicating the limited influences of the added reagents on the particle sizes of the synthesized samples. Obviously, the sizes of CDs do not increase from G-CDs and C-CDs to P-CDs as the fluorescence emission shifts toward pink, indicating they do not follow the traditional size effect at all. The phenomenon probably arises from the hybridizations of the carbon core and surface states and the conjugation effect of the carbon surface [50,51]. Meanwhile, the formation process of CDs from kiwifruit juice with different additives including EA, EDA, and ACTA was tentatively speculated. Upon ultrasonication, the assembling, crosslinking, and dehydration processes may occur between some organic molecules in kiwifruit juice and the additives to produce some intermediates. Generally, the ultrasonic waves can induce the formation and violent collapse of small vacuum bubbles, from which strong hydrodynamic shear-forces and high-speed impinging liquid jets as well as deagglomeration are produced [19]. Thus, these intermediates can be further carbonized, and N-doped CDs are reasonably prepared due to the presence of the N-containing molecules in kiwifruit juice.
X-ray photoelectron spectroscopy (XPS) measurements were performed to characterize the surface state and elemental composition of the as-prepared CDs. The detailed results are displayed in Figure 3. As can be seen from Figure 3a, the survey spectra of the three samples exhibits three typical peaks of C 1s (~285.1 eV), N 1s (~399.6 eV), and O 1s (~532.3 eV), suggesting their same element compositions, i.e., C, O, and N. However, the three-type CDs present different percentages of the atomic contents: C 1s (~64.1%), N 1s (~2.7%), and O 1s (~33.2%) for G-CDs, C 1s (~79.2%), N 1s (~5.7%), and O 1s (~15.1%) for C-CDs, and C 1s (~76.9%), N 1s (~1.7%), and O 1s (~21.0%) for P-CDs (Table S1, ESI). The highresolution C 1s spectra show the presence of sp 2 C (C=C), sp 3 C (C-C/C-N/C-O) and C=O/O=C-OH groups in these CDs (Figure 3b). The deconvoluted five peaks of the C 1 spectra are appropriately located at~284.6,~286.1,~287.5, and~288.7 eV and are ascribe to the C-C/C=C, C-OH/C-N/C-O, C=O, and O=C-OH groups, respectively [52,53]. Three types of N doping can be affirmed in these CDs by the high-resolution N 1 spectra ( Figure 3c). Likely, N 1s can also be divided into three peaks at~399.4,~400.2, and~401.5 eV, corresponding to C-N-C (pyridinic N), N-H (pyrrolic N), and (C) 3 -N (graphitic N), respectively [54]. In addition, the O 1 spectra described in Figure 3d are mainly concentrated at~531.6,~532.4, and~533.5 eV, which are consistent with C=O, C-OH/C-O, and O=C-OH, respectively. The presence of -OH and -COOH groups endows these CDs with excellent water solubility and high selectivity for metal-ion detection [55].
According to the XPS data, the contents of chemical bonds are calculated and summarized (Table S2, ESI). Obviously, all these CDs have the same chemical compositions but different contents of various functional groups, which may be responsible for their unique PL performances [50,51]. X-ray photoelectron spectroscopy (XPS) measurements were performed to characterize the surface state and elemental composition of the as-prepared CDs. The detailed results are displayed in Figure 3. As can be seen from Figure 3a, the survey spectra of the three samples exhibits three typical peaks of C 1s (~285.1 eV), N 1s (~399.6 eV), and O 1s (~532.3 eV), suggesting their same element compositions, i.e., C, O, and N. However, the three-type CDs present different percentages of the atomic contents: C 1s (~64.1%), N 1s (~2.7%), and O 1s (~33.2%) for G-CDs, C 1s (~79.2%), N 1s (~5.7%), and O 1s (~15.1%) for C-CDs, and C 1s (~76.9%), N 1s (~1.7%), and O 1s (~21.0%) for P-CDs (Table S1, ESI). The high-resolution C 1s spectra show the presence of sp 2 C (C=C), sp 3 C (C-C/C-N/C-O) and C=O/O=C-OH groups in these CDs (Figure 3b). The deconvoluted five peaks of the C 1 spectra are appropriately located at ~284.6, ~286.1, ~287.5, and ~288.7 eV and are ascribe to the C-C/C=C, C-OH/C-N/C-O, C=O, and O=C-OH groups, respectively [52,53]. Three types of N doping can be affirmed in these CDs by the high-resolution N 1 spectra ( Figure  3c). Likely, N 1s can also be divided into three peaks at ~399.4, ~400.2, and ~401.5 eV, corresponding to C-N-C (pyridinic N), N-H (pyrrolic N), and (C)3-N (graphitic N), respectively [54]. In addition, the O 1 spectra described in Figure 3d are mainly concentrated at ~531.6, ~532.4, and ~533.5 eV, which are consistent with C=O, C-OH/C-O, and O=C-OH, respectively. The presence of -OH and -COOH groups endows these CDs with excellent water solubility and high selectivity for metal-ion detection [55]. According to the XPS data, the contents of chemical bonds are calculated and summarized (Table S2, ESI). Obviously, all these CDs have the same chemical compositions but different contents of various functional groups, which may be responsible for their unique PL performances [50,51].

Optical Studies
The optical properties of G-CDs, C-CDs, and P-CDs are further verified by UV-Vis absorption and PL spectra, and the corresponding results are exhibited in Figure 4. As displayed in Figure 4a-c, the UV-Vis absorption spectra of the three CDs display relatively

Optical Studies
The optical properties of G-CDs, C-CDs, and P-CDs are further verified by UV-Vis absorption and PL spectra, and the corresponding results are exhibited in Figure 4. As displayed in Figure 4a-c, the UV-Vis absorption spectra of the three CDs display relatively analogous characteristics. Specifically, the peaks at 214 nm (G-CDs), 216 nm (C-CDs), and 238 nm (P-CDs) are ascribed to the π-π* transitions of the aromatic C=C bonds, and the bands at 278 nm (G-CDs), 336 nm (C-CDs), and 275 nm (P-CDs) correspond to the n-π* transition of the conjugated C=O bonds originating from the surface states of the CDs [56]. Likewise, the excitation and emission spectra of the three types of CDs were also optimally conducted (Figure 4a-c). As obtained from the excitation spectra of the three CDs, some differences are apparent. Compared to C-CDs and P-CDs, G-CDs exhibit a relatively narrow excitation spectrum, mainly concentered in the UV region, while those of C-CDs and P-CDs are extended from the UV to the visible region. The maximum excitation/emission peaks of G-CDs are centered at 360/473 nm (Figure 4a), while centered at 460/547 nm for C-CDs ( Figure 4b) and 665/673 nm for P-CDs (Figure 4c). Thus, the optimal fluorescence measurements of G-CDs, C-CDs, and P-CDs are examined under the maximum excitation wavelengths of 360, 460, and 665 nm, respectively. The insets exhibit the respective digital photographs of these CD solutions under the irradiation of daylight (left) and 365 nm UV light (right). Apparently, these CD solutions are transparent, light yellow, and clear under the daylight irradiation. These CDs can be well dispersed in water, which is probably attributed to their small particle diameter and abundant surface hydrophilic groups (carbonyl, carboxylic, and hydroxy) [57]. However, a sharp contrast is observed when the UV light of 365 nm is applied to radiate the three aqueous solutions of CDs. The solutions of G-CDs, C-CDs, and P-CDs separately emit green, chartreuse, and pink. The normalized emission spectra of three CDs are displayed in Figure 4a-c. The full width at half maximum (FWHM) values from G-CDs, C-CDs, and P-CDs are 74, 100, and 22 nm, respectively. The narrower the emission peaks of the CDs are, the stronger the color purities become, which is consistent with the prior report [25]. The absolute fluorescence QYs of G-CDs, C-CDs, and P-CDs are simultaneously determined as~3.7%,~2.4%, and 10.0%, respectively. Furthermore, the PL spectra of the three CDs excited with different wavelengths were also examined (Figure 4d-f). By adjusting the wavelength of the excitation light, G-CDs ( Figure 4d) and C-CDs (Figure 4e) both exhibit homologous excitation-dependent fluorescence behaviors, which is similar with the fluorescent CDs reported earlier [57,58]. The excitation wavelengths of the G-CDs ( Figure 4a) and C-CDs ( Figure 4b) are optimized at 360 nm and 460 nm, respectively. The excitation-dependent fluorescence behavior may result from the optical selection of differently sized nanoparticles or the different energy levels originating from the surface/defects states, which are created by surface groups like C-O, C=O, and -COOH [58]. Interestingly, the fluorescence emission of P-CDs exhibits almost no shift and has only one peak under different excitation wavelengths, revealing the surface state as probably the main reason behind the excitation-independent emission behavior [59,60].
To explore the fluorescence origin of CDs, a universal time-correlated single-photon counting (TCSPC) technique is used to examine the lifetime fluorescence of CDs. The decay curves of the three CDs are shown separately in Figure 4g-i. The fluorescence decay curves of the G-CDs and C-CDs can be well fitted to three kinetic components with a triexponential function (Figure 4g,h, Table S3, ESI), suggesting the presence of three different emissive sites in these two CDs. Besides the two short-lived components (τ 1 = 0.21 ns and τ 2 = 1.62 ns for G-CDs, τ 1 = 0.33 ns and τ 2 = 1.65 ns for C-CDs) resulting from the radiative recombination of the excitons, the two decay processes also contain a long-lived component (τ 3 = 4.67 ns for G-CDs, τ 3 = 5.99 ns for C-CDs). The average lifetimes of the G-CDs and C-CDs are calculated to be 4.12 and 3.84 ns, respectively. The multiexponential fitting proves that the multicomponent of CDs, including the graphite center (or conjugated structures) and surface states [58], contribute to the fluorescence emission. The decay curve of P-CDs is well-fitted monoexponentially (Figure 4i). Compared to those of G-CDs and C-CDs, the decay curve of P-CDs only consists of a long-lived component of τ = 6.18 ns (Table S1, ESI). The relatively longer fluorescence lifetime of P-CDs indicates that the fluorescence mainly arises from the surface trap recombination [61]. become, which is consistent with the prior report [25]. The absolute fluorescence QYs of G-CDs, C-CDs, and P-CDs are simultaneously determined as ~3.7%, ~2.4%, and ~10.0%, respectively. Furthermore, the PL spectra of the three CDs excited with different wavelengths were also examined (Figure 4d-f). By adjusting the wavelength of the excitation light, G-CDs ( Figure 4d) and C-CDs (Figure 4e) both exhibit homologous excitation-dependent fluorescence behaviors, which is similar with the fluorescent CDs reported earlier [57,58]. The excitation wavelengths of the G-CDs (Figure 4a) and C-CDs (Figure 4b) are optimized at 360 nm and 460 nm, respectively. The excitation-dependent fluorescence behavior may result from the optical selection of differently sized nanoparticles or the different energy levels originating from the surface/defects states, which are created by surface groups like C-O, C=O, and -COOH [58]. Interestingly, the fluorescence emission of P-CDs exhibits almost no shift and has only one peak under different excitation wavelengths, revealing the surface state as probably the main reason behind the excitation-independent emission behavior [59,60].
To explore the fluorescence origin of CDs, a universal time-correlated single-photon counting (TCSPC) technique is used to examine the lifetime fluorescence of CDs. The decay curves of the three CDs are shown separately in Figure 4g-i. The fluorescence decay curves of the G-CDs and C-CDs can be well fitted to three kinetic components with a triexponential function (Figure 4g,h, Table S3, ESI), suggesting the presence of three different emissive sites in these two CDs. Besides the two short-lived components (τ1 = 0.21 ns and τ2 = 1.62 ns for G-CDs, τ1 = 0.33 ns and τ2 = 1.65 ns for C-CDs) resulting from the radiative recombination of the excitons, the two decay processes also contain a long-lived component (τ3 = 4.67 ns for G-CDs, τ3 = 5.99 ns for C-CDs). The average lifetimes of the G- Generally, the fluorescence stability is considered as an especially significant factor for their potential applications. Hence, the PL performances of the CD solution encountering different conditions are examined in detail by using the G-CDs as a model, and the related measurement results are displayed in Figure 5. The influences of ionic strength and UV exposure on the stability of the PL are firstly investigated (Figure 5a-d). Obviously, there are no significant changes in the peak characteristics (Figure 5a,c). As can be seen from the PL data (Figure 5a,b), the PL intensity shows almost no obvious differences with the increase in the surrounding ionic strengths by regulating the concentrations of NaCl from 0 to 1 mol L −1 . Likewise, no serious PL bleaching is observed after continuous UV-light irradiation for 180 min (Figure 5c,d). As for the effects of pH, the fluorescence response of CDs becomes even stronger with the increase in the surrounding pH values in the acidic environment, but shows little change in a neutral or alkaline environment (Figure 5e,f). These results highlight that these CDs demonstrate excellent light stability and photobleaching resistance, which are beneficial for their potential applications in fluorescent labels, fluorescent inks, and other fields.
UV-light irradiation for 180 min (Figure 5c,d). As for the effects of pH, the fluorescenc response of CDs becomes even stronger with the increase in the surrounding pH value in the acidic environment, but shows little change in a neutral or alkaline environmen (Figure 5e,f). These results highlight that these CDs demonstrate excellent light stabilit and photobleaching resistance, which are beneficial for their potential applications in flu orescent labels, fluorescent inks, and other fields.

Fluorescent Ink Properties
In view of the unique fluorescence properties including good dispersibility and strong fluorescence and stability, the as-prepared M-CDs are further used as invisible fluorescent inks. Three representation patterns of cutting, writing, and drawing with filter paper are adopted to make them feasible candidates for fluorescent ink applications, and the corresponding results are shown in Figure 6a-f. As displayed in Figure 6a,c,e, there are no visible patterns appearing on the filter paper under the irradiation of daylight, although these filter papers are treated with the as-prepared M-CDs. More interestingly, when excited by the UV light of 365 nm, the filter paper coated with the G-CD solution exhibited green emission (Figure 6b). Visually, the chartreuse Chinese characters of jinan (" Nanomaterials 2022, 12, x FOR PEER REVIEW

Fluorescent In
In view of strong fluorescen orescent inks. Th paper are adopte the correspondin are no visible pa hough these filt when excited by exhibited green e ("济南") appear clear, bright pink a pigment (Figur feit marks, which ") appear in the writing marks with C-CDs as inks (Figure 6d). Simultaneously, a clear, bright pink-emitting picture also emerges on the filter paper drawn with P-CDs as a pigment (Figure 6f). Therefore, these M-CDs can be widely used as various anticounterfeit marks, which have a promising application in the security field. hough these filter papers are treated with the as-prepared M-CDs. More interestingly, when excited by the UV light of 365 nm, the filter paper coated with the G-CD solution exhibited green emission (Figure 6b). Visually, the chartreuse Chinese characters of jinan ("济南") appear in the writing marks with C-CDs as inks (Figure 6d). Simultaneously, a clear, bright pink-emitting picture also emerges on the filter paper drawn with P-CDs as a pigment (Figure 6f). Therefore, these M-CDs can be widely used as various anticounterfeit marks, which have a promising application in the security field.

Sensing Applications
In general, the surface of the biomass-derived CDs reserve the oxygen-containing functional groups, including hydroxyl, carboxyl, and amino groups, and these functional groups can coordinate with the detection object to cause corresponding changes in fluorescence [52,62]. In view of these characteristics, the CDs can serve as a potential sensing candidate, implying their future application in the sensor field. With regards to this, the G-CDs are applied as a model to investigate the fluorescence responses to the interferents including Na + , Mg 2+ , Zn 2+ , Cu 2+ , Ca 2+ , Fe 3+ , Ni 2+ , Co 2+ , Ba 2+ , Cd 2+ , Mn 2+ , Cr 3+ , K + , Fe 2+ , L-Cys, AA, and PO4 3− under the same conditions ( Figure 7). Accordingly, the PL spectra of CDs after the addition of 1 mM interferents as mentioned above (Figure 7a), the obtained F/F0 bar diagram in the absence and presence of different interferents are shown in Figure 7b. Obviously, Fe 3+ has the greatest fluorescence-quenching effect among the ions, suggesting that the CDs are more sensitive and reliable for the detection of Fe 3+ ion than other ions.

Sensing Applications
In general, the surface of the biomass-derived CDs reserve the oxygen-containing functional groups, including hydroxyl, carboxyl, and amino groups, and these functional groups can coordinate with the detection object to cause corresponding changes in fluorescence [52,62]. In view of these characteristics, the CDs can serve as a potential sensing candidate, implying their future application in the sensor field. With regards to this, the G-CDs are applied as a model to investigate the fluorescence responses to the interferents including Na + , Mg 2+ , Zn 2+ , Cu 2+ , Ca 2+ , Fe 3+ , Ni 2+ , Co 2+ , Ba 2+ , Cd 2+ , Mn 2+ , Cr 3+ , K + , Fe 2+ , L-Cys, AA, and PO 4 3− under the same conditions ( Figure 7). Accordingly, the PL spectra of CDs after the addition of 1 mM interferents as mentioned above (Figure 7a), the obtained F/F 0 bar diagram in the absence and presence of different interferents are shown in Figure 7b. Obviously, Fe 3+ has the greatest fluorescence-quenching effect among the ions, suggesting that the CDs are more sensitive and reliable for the detection of Fe 3+ ion than other ions. Excellent fluorescent probes should not only have specific selectivity but also possess high sensitivity. Figure 8a shows the changes in the fluorescence spectra of the G-CD aqueous solution in the presence of different concentrations of Fe 3+ (0, 50, 100, 200, 300,  400, 500, 600, and 700 μM). It clearly demonstrates that the fluorescence can be quenched gradually along with the increase in the concentration of Fe 3+ , suggesting the feasibility of Fe 3+ detection. The quenching efficiency, i.e., (F0 − F)/F0, especially exhibits a good linearity with the concentrations of Fe 3+ within the concentration range of 50 to 700 μM (R 2 = 0.9715) Excellent fluorescent probes should not only have specific selectivity but also possess high sensitivity. Figure 8a shows the changes in the fluorescence spectra of the G-CD aqueous solution in the presence of different concentrations of Fe 3+ (0, 50, 100, 200, 300, 400, 500, 600, and 700 µM). It clearly demonstrates that the fluorescence can be quenched gradually along with the increase in the concentration of Fe 3+ , suggesting the feasibility of Fe 3+ detection. The quenching efficiency, i.e., (F 0 − F)/F 0 , especially exhibits a good linearity with the concentrations of Fe 3+ within the concentration range of 50 to 700 µM (R 2 = 0.9715) (Figure 8b). Here, F 0 and F present the highest PL intensity, excited at 360 nm in the absence and presence of Fe 3+ , respectively. Iron is an especially important element for life; however, once Fe 3+ is out of the effective concentration range it causes diseases. The lowest limit of detection (LOD) of the Fe 3+ is also extremely significant for Fe 3+ sensor. The LOD of G-CDs toward Fe 3+ is estimated to be 0.11 µM, which is calculated according to the equation LOD = 3δ/S (a signal-to-noise ratio of S/N = 3), where δ and S are the standard deviation and the slope of linear range, respectively [63]. Overall, the G-CD fluorescence-sensing probe manifests excellent performances including the wide-detection range, and high selectivity/sensitivity toward Fe 3+ in the aqueous solution. These results reveal that G-CDs can be applied as an efficient fluorescence sensor for trace amounts of Fe 3+ detection in biomedical and environmental systems. Excellent fluorescent probes should not only have specific selectivity but also possess high sensitivity. Figure 8a shows the changes in the fluorescence spectra of the G-CD aqueous solution in the presence of different concentrations of Fe 3+ (0, 50, 100, 200, 300, 400, 500, 600, and 700 μM). It clearly demonstrates that the fluorescence can be quenched gradually along with the increase in the concentration of Fe 3+ , suggesting the feasibility of Fe 3+ detection. The quenching efficiency, i.e., (F0 − F)/F0, especially exhibits a good linearity with the concentrations of Fe 3+ within the concentration range of 50 to 700 μM (R 2 = 0.9715) (Figure 8b). Here, F0 and F present the highest PL intensity, excited at 360 nm in the absence and presence of Fe 3+ , respectively. Iron is an especially important element for life; however, once Fe 3+ is out of the effective concentration range it causes diseases. The lowest limit of detection (LOD) of the Fe 3+ is also extremely significant for Fe 3+ sensor. The LOD of G-CDs toward Fe 3+ is estimated to be 0.11 μM, which is calculated according to the equation LOD = 3δ/S (a signal-to-noise ratio of S/N = 3), where δ and S are the standard deviation and the slope of linear range, respectively [63]. Overall, the G-CD fluorescencesensing probe manifests excellent performances including the wide-detection range, and high selectivity/sensitivity toward Fe 3+ in the aqueous solution. These results reveal that G-CDs can be applied as an efficient fluorescence sensor for trace amounts of Fe 3+ detection in biomedical and environmental systems.

Quenching Mechanism
The fluorescence-quenching process involves the interaction between the quencher molecules and fluorescent molecules. Normally, two quenching forms including dynamic and static quenching mechanisms are widely acknowledged, and the type of quenching mechanism can be determined by the quenching constant from the standard Stern-Volmer equation, i.e., F 0 /F = 1 + K sv [Q], where F 0 and F represent the fluorescence intensities in the absence and presence of quenchers, respectively, [Q] is the concentration of the quencher, and K sv is the quenching constant [64]. The dynamic quenching is mainly related to diffusion, and the K sv of the fluorescent materials may increase along with the rise in temperature, where the maximum scatter-collision-quenching constant by all kinds of quenchers is 2.0 × 10 10 L mol −1 s −1 [65]. As for the static quenching mechanism, the K sv values follow the descending trend with the increase in the temperature. To pursue the quenching mechanism of G-CDs, the quenching constant is calculated by the standard Stern-Volmer equation. As shown in Figure 9, the Stern-Volmer plot (F 0 /F) exhibits a good linear relationship with the concentration of Fe 3+ . The K sv is calculated as 2.2 × 10 3 M −1 , which is larger than the limited dynamic-quenching constant (2.0 × 10 3 M −1 ). Thus, a staticquenching mechanism is reasonably recognized in the system, revealing the formation of nonfluorescent ground-state complexes between CDs and the quencher. The Fe 3+ can chelate or coordinate with the oxygenated and nitrogenous functional groups on the surface of the G-CDs to form the complexes. Thus, it changes the distribution of the energy states and enhances the nonradiative recombination of electron/hole, subsequently resulting in the PL quenching [62][63][64][65][66].
which is larger than the limited dynamic-quenching constant (2.0 × 10 3 M −1 ). Thus, a staticquenching mechanism is reasonably recognized in the system, revealing the formation of nonfluorescent ground-state complexes between CDs and the quencher. The Fe 3+ can chelate or coordinate with the oxygenated and nitrogenous functional groups on the surface of the G-CDs to form the complexes. Thus, it changes the distribution of the energy states and enhances the nonradiative recombination of electron/hole, subsequently resulting in the PL quenching [62][63][64][65][66]. In addition to the sensing of metal ions, the capable sensing of the G-CDs/Fe 3+ system toward anions or molecules was also designed based on the "on-off" and "off-on" characters of the fluorescence ( Figure 10). As displayed in Figure 10, the PL intensity of the G-CDs shows an obvious decrease with the addition of Fe 3+ , which presents the typical "onoff" fluorescence performance ( Figure 11). The fluorescence quenching may originate from the formation of a nonfluorescent complex between the surface oxygen-(or nitrogen-) containing groups in the CDs and Fe 3+ [67], which further confirms the static fluorescence mechanism of CDs ( Figure 11). Notably, a significant PL recovery can be achieved with the subsequent addition of PO4 3− (1 mM) into the G-CD solution containing 700 μM Fe 3+ , thus exhibiting an "off-on" fluorescence character. The stripping of Fe 3+ by the added PO4 3− from the surface of CDs is probably responsible for such fluorescence recovery, and the related off-on fluorescence process is schematically shown in Figure 11. Additionally, the molecule of AA also has the same fluorescence-recovery function as that of the PO4 3− (Figure 10). Namely, the quenched fluorescence of CDs by Fe 3+ can be recovered by the In addition to the sensing of metal ions, the capable sensing of the G-CDs/Fe 3+ system toward anions or molecules was also designed based on the "on-off" and "off-on" characters of the fluorescence ( Figure 10). As displayed in Figure 10, the PL intensity of the G-CDs shows an obvious decrease with the addition of Fe 3+ , which presents the typical "on-off" fluorescence performance ( Figure 11). The fluorescence quenching may originate from the formation of a nonfluorescent complex between the surface oxygen-(or nitrogen-) containing groups in the CDs and Fe 3+ [67], which further confirms the static fluorescence mechanism of CDs ( Figure 11). Notably, a significant PL recovery can be achieved with the subsequent addition of PO 4 3− (1 mM) into the G-CD solution containing 700 µM Fe 3+ , thus exhibiting an "off-on" fluorescence character. The stripping of Fe 3+ by the added PO 4 3− from the surface of CDs is probably responsible for such fluorescence recovery, and the related off-on fluorescence process is schematically shown in Figure 11. Additionally, the molecule of AA also has the same fluorescence-recovery function as that of the PO 4 3− ( Figure 10). Namely, the quenched fluorescence of CDs by Fe 3+ can be recovered by the addition of AA. This phenomenon is rationally ascribed to the transformation from Fe 3+ to Fe 2+ , resulting from the efficient reduction of AA (Figure 11), while the CDs show much higher selectivity toward Fe 3+ than Fe 2+ [68]. The results here feature that the G-CDs can be utilized towards the detection of Fe 3+ , PO 4 3− , and AA. addition of AA. This phenomenon is rationally ascribed to the transformation from Fe 3+ to Fe 2+ , resulting from the efficient reduction of AA ( Figure 11), while the CDs show much higher selectivity toward Fe 3+ than Fe 2+ [68]. The results here feature that the G-CDs can be utilized towards the detection of Fe 3+ , PO4 3− , and AA.

Logic Gate Operation
In view of the sensitive response of the as-prepared CDs towards Fe 3+ , PO4 3− , and AA, a fluorometric logic system with a function of executing a single-input logic operation (NOT) and dual-input logic operation (IMP) is reasonably designed by using the G-CDs as a model ( Figure 12). In this devised logic system, the G-CDs bear the responsibility of logic gates, while the ions (Fe 3+ and PO4 3− ) and molecule (AA) are applied as the chemical inputs, and the fluorescence signals are regarded as the output. To perform the logic gates even better, the absence and presence of these inputs are marked as Boolean logic functions of "0" and "1" states, respectively. Simultaneously, the outputs defined as "0" and "1" separately correspond to the quenched fluorescence and the maximum fluorescence. The corresponding parameters and results of each logic operation are encoded in QR codes with unique spreadsheets, and the related QR codes are wholly exhibited in truth tables (Movie S1, ESI). As for the single-input logic operation, the G-CDs serve as the gate, while Fe 3+ is used as the chemical input. The addition of Fe 3+ toward the G-CD system (input = 1) results in their obvious quenching of fluorescence (output = 0), which is in accord with the single-input-driven logic operation of NOT. Figure 11. Schematic illustration for the "off-on" fluorescence-sensing mechanisms towards Fe 3+ , PO 4 3− , and AA based on G-CDs.

Logic Gate Operation
In view of the sensitive response of the as-prepared CDs towards Fe 3+ , PO 4 3− , and AA, a fluorometric logic system with a function of executing a single-input logic operation (NOT) and dual-input logic operation (IMP) is reasonably designed by using the G-CDs as a model ( Figure 12). In this devised logic system, the G-CDs bear the responsibility of logic gates, while the ions (Fe 3+ and PO 4 3− ) and molecule (AA) are applied as the chemical inputs, and the fluorescence signals are regarded as the output. To perform the logic gates even better, the absence and presence of these inputs are marked as Boolean logic functions of "0" and "1" states, respectively. Simultaneously, the outputs defined as "0" and "1" separately correspond to the quenched fluorescence and the maximum fluorescence. The corresponding parameters and results of each logic operation are encoded in QR codes with unique spreadsheets, and the related QR codes are wholly exhibited in truth tables (Movie S1, ESI). As for the single-input logic operation, the G-CDs serve as the gate, while Fe 3+ is used as the chemical input. The addition of Fe 3+ toward the G-CD system (input = 1) results in their obvious quenching of fluorescence (output = 0), which is in accord with the single-input-driven logic operation of NOT. Further, the IMP logic gate with Fe 3+ and PO4 3− as inputs 1 and 2, respectively, to G-CDs is constructed. The absence of both inputs (input = 0, 0) and even the individual input of PO4 3− (input = 0, 1) cannot cause the significant change of fluorescence (output = 1). However, the fluorescence intensity is substantially quenched (output = 0) only in the Further, the IMP logic gate with Fe 3+ and PO 4 3− as inputs 1 and 2, respectively, to G-CDs is constructed. The absence of both inputs (input = 0, 0) and even the individual input of PO 4 3− (input = 0, 1) cannot cause the significant change of fluorescence (output = 1). However, the fluorescence intensity is substantially quenched (output = 0) only in the presence of Fe 3+ (input = 1, 0). Interestingly, the continuous addition of PO 4 3− (input = 1, 1) can trigger a remarkable recovery of fluorescence (output = 1). Similarly, the multiple input logic gate IMP is also achieved by using Fe 3+ and AA as inputs 1 and 2, respectively. Obviously, only in the presence of Fe 3+ and absence of AA (input = 1, 0), the prominent fluorescence quenching is observed (output = 0), while in the case of other inputs (0/0, 0/1, 1/1), the CD system still demonstrates strong fluorescence performance (output = 1). The responses agree well with the dual-input-driven logic operation, IMP.

Conclusions
In conclusion, in the work, we developed a facile yet effective ultrasonic-assisted synthesis methodology to directly fabricate small-size and strong fluorescent N-doped M-CDs from the nutritious and naturally renewable kiwifruit. The emission colors of M-CDs changed from green and chartreuse to pink just through a regular adjustment in the added reagents varying from ethanol and ethylenediamine to acetone. The as-prepared N-doped CDs exhibited excellent chemical and optical stabilities. It is these appealing properties and superiorities that make the resulting M-CDs smart platforms for fluorescent inks, as well as sequential fluorescent probes for the determination of Fe 3+ and PO 4 3− /or AA. Furthermore, the fluorescent nanoprobers were successfully utilized for the construction of molecular logic gates and for imaging Fe 3+ and PO 4 3− /AA. More significantly, we strongly believe that the ultrasonic-assisted synthesis concept here can be extended to other biomasses for advanced heteroatom-doped CDs towards fluorescent inks/sensors, logic gates, and beyond.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/nano12030312/s1, Table S1: The atomic contents from the XPS data; Table S2: XPS data analysis of the C 1s spectra of three samples; Table S3: Fitting parameters and average PL lifetimes (τ) of the G-CDs, C-CDs, and P-CDs; Video S1: Movie S1.