Article pubs.acs.org/ac
Sulfur-Doped Graphene Quantum Dots as a Novel Fluorescent Probe for Highly Selective and Sensitive Detection of Fe3+ Shuhua Li, Yunchao Li, Jun Cao, Jia Zhu,* Louzhen Fan, and Xiaohong Li* Department of Chemistry, Beijing Normal University, Beijing 100875, People’s Republic of China S Supporting Information *
ABSTRACT: Sulfur-doped graphene quantum dots (SGQDs) with stable blue-green fluorescence were synthesized by one-step electrolysis of graphite in sodium p-toluenesulfonate aqueous solution. Compared with GQDs, the S-GQDs drastically improved the electronic properties and surface chemical reactivities, which exhibited a sensitive response to Fe3+. Therefore, the S-GQDs were used as an efficient fluorescent probe for highly selective detection of Fe3+. Upon increasing of Fe3+ concentration ranging from 0.01 to 0.70 μM, the fluorescence intensity of S-GQDs gradually decreased and reached a plateau at 0.90 μM. The difference in the fluorescence intensity of S-GQDs before and after adding Fe3+ was proportional to the concentration of Fe3+, and the calibration curve displayed linear regions over the range of 0−0.70 μM. The detection limit was 4.2 nM. Finally, this novel fluorescent probe was successfully applied to the direct analysis of Fe3+ in human serum, which presents potential applications in clinical diagnosis and may open a new way to the design of effective fluorescence probes for other biologically related targets.
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fluorescent probes for the detection of metal ions (Fe3+, Cu2+, or Pb2+),24−26 small organic molecules (2,4,6-trinitrotoluene or paranitrophenol),27,28 and biomaterials (pyrocatechol, human immunoglobulin G, or protein kinase)29−31 have been explored. The major disadvantage of GQDs-based fluorescent probes is that the sensitivity and selectivity are limited due to the low quantum yields (QYs) and nonspecificity of GQDs.22,32,33 Doping GQDs with heteroatoms could effectively modulate their band gap and electronic density, enhancing chemical activity, and QYs of GQDs for practical applications, which were proven through theoretical calculations and detailed experiments.34−38 For example, nitrogen-doped GQDs (NGQDs) showed efficient electrocatalytic activities for the oxygen reduction reaction34 and higher fluorescence quantum yields (QY) for cellular and deep-tissue imaging.35,36 Borondoped GQDs (B-GQDs) gave rise to rich fluorescence owing to their peculiar interaction with the surrounding media.37 To the best of our knowledge, sulfur-doped graphene quantum dots (S-GQDs) have rarely been reported,38 and their unique optoelectronic properties are almost completely unknown. S atom is much larger than carbon (C) atom, and the difference of electronegativity between S (2.58) and C (2.55) appears to be too small to offer significant charge transfer in CS composites,39 thus chemical doping of S into the framework of GQDs would seem to be quite difficult.40 In this paper, we report an electrochemical approach for the facile preparation of S-GQDs by electrolysis of graphite in
INTRODUCTION 3+ Fe , one of the most essential metal ions in biological systems, plays crucial roles in many physiological and pathological processes including cellular metabolism, enzyme catalysis, and oxygen transport, as well as DNA and RNA synthesis.1−6 Abnormal Fe3+ fluctuations are the hallmarks of diseases, such as anemia, intelligence decline, arthritis, heart failure, diabetes and cancer.7,8 Thus, the determination of Fe3+ is of fundamental importance for the early identification and diagnosis of these diseases. Current detection assays including voltammetry,9 spectrophotometry,10−12 atomic absorption spectrometry,13 and inductively coupled plasma mass spectrometry (ICP-MS),14 require sophisticated instrumentation and tedious sample preparation procedures, which limits their practical applications. Recently, the fluorescence assays display unique advantages of high sensitivity, great simplicity, easy monitoring, and rapid response, providing a better choice for the detection of Fe3+. Nevertheless, most of these reported fluorescent probes (organic chromophores, fluorescent conjugated polymers, semiconductor quantum dots, and so forth)15−17 are poisonous, biologically incompatible, and water-insoluble, which greatly limited the detection of Fe3+ in biological systems. Therefore, the development of a novel fluorenscent probe with low cytotoxicity, excellent biocompatibility, and high water solubility has become increasingly important and urgent. In recent years, graphene quantum dots (GQDs) have ignited increasing research interest, owing to their chemical inertness, low cytotoxicity, excellent biocompatibility, good dispersibility in water, and stable photoluminescence (PL).18−23 On the basis of these unique properties, various GQDs-based © XXXX American Chemical Society
Received: June 20, 2014 Accepted: September 23, 2014
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Figure 1. (a) TEM image of the as-prepared S-GQDs. The inset is the HRTEM image. (b) Size distribution of S-GQDs.
Figure 2. (a) XPS, (b) S 2p, and (c) C 1s spectra of S-GQDs. (d) Structure illustration of S-GQDs (not drawn to scale). were collected by filtering the resulting solution by 0.22 μm filter membrane to remove the precipitated graphite oxide and graphite particles. Then the obtained dark-brown solution was dialyzed over deionized water in a dialysis bag (retain molecular weight 3500 Da) for 1 day to remove the electrolyte and finally dried under vacuum for 24 h. The obtained S-GQDs were redispersed in distilled water for further characterization and use. When the sodium p-toluenesulfonate was changed to sodium hydroxide (2M) under otherwise identical conditions, undoped GQDs were prepared. Characterization. Transmission electron micrographs (TEM) were taken on a JEOL JEM 2100 transmission electron microscope (FEI). Atomic force microscopic (AFM) images were obtained by MultiMode V SPM (VEECO). X-ray diffraction (XRD) patterns were carried out with an X-ray diffraction using CuKα radiation (XRD, PANalytical X’Pert Pro MPD). The Raman spectra were measured using Laser Confocal Micro-Raman Spectroscopy (LabRAM Aramis). X-ray photoelectron spectroscopy (XPS) was performed with an ESCALab 250Xi electron spectrometer from VG Scientific using 300 W Al Kα radiation. UV−vis absorption and fluorescence spectra were recorded on UV-2450 spectrophotometry and a PerkinElmer-LS55 fluorescence spectrometer, respectively. The Fourier transform infrared (FT-IR) spectra were measured using a Nicolet 380 spectrograph. Nanosecond fluorescence lifetime experiments were performed with a time-correlated single-photon counting (TCSPC) system under right-angle sample geometry. A 380 nm picosecond diode laser (Edinburgh Instruments EPL375, repetition rate 2 MHz) was used to excite the samples. The fluorescence was collected by a
sodium p-toluenesulfonate aqueous solution. The as-prepared S-GQDs showed a sensitive response to Fe 3+ in the concentration range of 0.01−0.70 μM, and the detection limit was as low as 4.2 nM. This reported S-GQDs-based fluorescent assay was further successfully applied to detect Fe3+ in human serum, which presents potential applications in clinical medicine examination and disease diagnosis.
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EXPERIMENTAL SECTION
Materials and Reagents. High purity graphite rods were purchased from Shanghai Carbon Co., Ltd. All reagents used in this work were of analytical reagent grade, purchased from J&K and used without further purification. The water used throughout all the experiments was purified through a Millipore system (ULUPURE, Chengdu, China). Solutions of K+, Na+, Ca2+, Mg2+, Al3+, Cu2+, Zn2+, Ni2+, Co2+, Ag+, Cd2+, Mn2+, Hg2+, Pb2+, Fe2+, and Fe3+ were prepared from KCl, NaCl, CaCl2, MgCl2, AlCl3, CuCl2, ZnCl2, NiCl2, CoCl2, AgNO3, CdCl2, MnCl2, Hg(NO3)2, Pb(Ac)2, FeSO4, and FeCl3, respectively. The human serum samples were directly obtained from the Affiliated Hospital of the Beijing Normal University. Preparation of S-GQDs and Undoped GQDs. The S-GQDs were prepared through a facile electrochemical approach performed on CHI 705 with a constant electrolysis voltage of 5 V. The graphite rod as a working electrode was inserted into 8 mL 0.1 M sodium ptoluenesulfonate aqueous solution, and a parallel Pt foil was used as counter electrode. After electrochemical reaction for 3 h, the S-GQDs B
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photomultiplier tube (Hamamatsu H5783p) connected to a TCSPC board (Becker & Hickl SPC-130). Detection of Fe3+ Using S-GQDs. The detection of Fe3+ was performed at room temperature in HEPES (10 mM, pH = 7.0) buffer solution. In a typical run, 25 μg S-GQDs were dispersed in 4 mL HEPES buffer, followed by the addition of a calculated amount of Fe3+, and then diluted the solution to 5 mL with 10 mM HEPES buffer (pH = 7.0). The resulting solution was shaken well and incubated for 10 min before recording the fluorescence emission spectra. The sensitivity and selectivity measurements were conducted in triplicate. For determination of Fe3+ in human serum, the serum was first treated with trichloroacetic acid (TCA) to release Fe3+ from protein.41 Equal volumes of human serum and 20% (w/v) TCA were mixed and then heated at 90 °C for 15 min. After centrifuging and removing the protein deposit, the supernatant was subjected to the analysis. In the test solutions, the human serum was diluted 100-fold. Aliquots of the above deproteinized human serum sample (0, 0.1, 0.2, 0.4, 0.6, 0.8, or 1.0 mL) were added to 4 mL S-GQD solution in a 5 mL volumetric flask, and then diluting the solution to 5 mL with 10 mM HEPES buffer (pH = 7.0). The final concentration of S-GQDs was 5 μg/mL. After the resulting solution incubated at room temperature for 10 min, the fluorescence emission spectra at 480 nm were recorded. The unknown amount of Fe3+ in the human serum sample was estimated by using the standard addition method with FeCl3 as the standard. The deproteinized human serum samples were spiked with Fe3+ at different concentration levels and then analyzed with the proposed method.
the SI). As a comparison, XPS measurements of the sodium ptoluenesulfonate were also performed (Figure S3 in the SI). The S 2p spectrum of sodium p-toluenesulfonate (Figure S3b in the SI) is composed of a double peak centered at 167.8 and 169.1 eV, which are assigned to the S 2p3/2 of sulfonate. The CS peak (Figure S3c in the SI) appears at a higher energy (290.9 eV) than that of S-GQDs (286.6 eV), indicating that C atoms are bonded with sulfonic acid group. The significant differences in S 2p and C 1s spectra between S-GQDs and sodium p-toluenesulfonate indicate that S atoms are indeed covalently bonded to the GQDs, and the likely structure of the S-GQDs based on the above analysis is shown in Figure 2d. As shown in Figure S4a (SI), S-GQDs show a broad and weak (002) diffraction peak centered at around 26°, indicating the disordered stacking of S-GQDs and introduction of more active sites along the surfaces of the S-GQD during electrochemical process.44 The Raman spectrum of the SGQDs (Figure S4b in the SI) shows the characteristic D band at 1330 cm−1 and G band at 1609 cm−1, which are related to the disordered C atoms at the edges of the S-GQDs and sp2bonded C atoms, respectively. It is noteworthy that the SGQDs have a higher ID/IG ratio (ca. 1.27) than those of S-free GQDs (ca. 0.91)23 prepared by almost the same methods as SGQDs, suggesting that the intercalation of S atoms into the conjugated carbon backbone has led to numerous defects on the surface of GQDs.46,47 We deduce that the introduction of active sites and surface defects may improve the electronic properties and surface chemical reactivities of GQDs. The optical properties of the S-GQDs were investigated. The UV−vis spectrum of the S-GQDs (Figure S5 in the SI) shows two typical absorption peaks centered at 230 and 310 nm, which are assigned to π → π* transition of CC and n → π* transition of CO, respectively.48 The light yellow S-GQDs aqueous solution emitted strong blue-green fluorescence upon being irradiated by a 365 nm lamp (Figure 3 inset), and the
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RESULTS AND DISCUSSION Characterization of the S-GQDs. S-GQDs were prepared by electrolysis of graphite in sodium p-toluenesulfonate aqueous solution with a constant voltage of 5 V, which was high enough to oxidize the CC bonds and drived the electrolyte ions to intercalate into the framework of GQDs.42 TEM shows that the S-GQDs are well-dispersed (Figure 1a) and have a relatively narrow size distribution ranging from 2 to 4 nm with an average diameter of 3 nm (Figure 1b). The highresolution TEM (HRTEM) image (inset of Figure 1a) indicates the high crystallinity of the S-GQDs, and the lattice spacing of 0.242 nm, (1120) lattice fringes of graphene,43 is corresponding to the lattice constant in the plane of graphite. The AFM images (Figure S1a-b in the Supporting Information, SI) reveal a typical topographic height of 0.3−1.2 nm with an average height of 0.7 nm, suggesting that most of the S-GQDs are single or bilayered.32 XPS measurements were performed to investigate the composition of the as-prepared S-GQDs. The XPS survey spectrum of the S-GQDs (Figure 2a) shows the presence of S, C, and O with atomic percentages of 4.25%, 70.91%, and 24.84%, and the corresponding S 2p, C 1s, and O 1s peaks are located at ca. 167, 284, and 531 eV, respectively. The S 2p spectrum of S-GQDs (Figure 2b) is composed of two peaks centered at 164.7 and 169.1 eV, suggesting that S exists in two forms. The former peak can be deconvoluted into two distinct components at 163.8 and 165.5 eV, which agree with the reported 2p3/2 and 2p1/2 positions of the CSC covalent bond of the thiophene-like S, owing to their spin− orbit couplings.40,44,45 The latter peak can be fit with two components at 168.6 and 169.9 eV, which is consistent with CS(O)2C Sulphone bridges.40,44,45 The Sulphone bridges represent 17.8% of the total sulfur content, indicating that most of S atoms exist as CSC covalent bond of the thiophene-like S. The C 1s spectrum of S-GQDs (Figure 2c) reveals the presence of CC (284.5 eV), CS (286.1 eV), CO (286.6 eV), and CO (288.3 eV), which is consistent with the corresponding FT-IR spectra (Figure S2 in
Figure 3. Fluorescence emission spectra of S-GQD aqueous solution. The inset are the photographs of S-GQD aqueous solution under visible light (left) and a UV beam of 365 nm (right).
emission wavelength is nearly excitation-independent, with the maximum excitation wavelength and the maximum emission wavelength at 380 and 480 nm, respectively (Figure 3). The QY of the S-GQDs was as high as 10.6% when excited at 380 nm (Table S1 in the SI), comparable with those of reported GQDs (Table S2 in the SI).22,32,33,49−51 Furthermore, the SGQDs show excellent photostability as the fluorescence intensity did not change even after continuous excitation under a Xe lamp for 24 h (Figure S6 in the SI). C
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Scheme 1. Fluorescence Quenching Mechanism of the S-GQDs in the Presence of Fe3+ (a) and the Electron Transfer Process from S-GQDs to Fe3+ (b)
Figure 4. (a) Fluorescence spectra of S-GQDs (5 μg/mL) with different concentration of Fe3+ (from top to bottom: 0, 0.01, 0.02, 0.04, 0.08, 0.12, 0.16, 0.20, 0.24, 0.28, 0.32, 0.36, 0.40, 0.50, 0.60, 0.70, 0.80, 0.90, 1.00, 1.20, 1.40, and 1.60 μM, respectively). The excitation wavelength was fixed at 380 nm for the fluorescence spectra. The inset in (a) shows photographs of S-GQD aqueous solutions with different concentration of Fe3+ under UV irradiation (365 nm). (b) The curve of the fluorescence quenching values ΔF vs Fe3+ concentration ranging from 0 to 1.60 μM. The inset in (b) is the linear calibration plot for Fe3+ detection.
Mechanism for the Fluorescence Response of S-GQDs to Fe3+. On the basis of the special coordination interaction between Fe3+ and phenolic hydroxyl groups in traditional organic chemistry52 and enlightened by previous studies of the fluorescence quenching effect of graphene oxide (GO) nanosheets to Fe3+,53 we speculate that S-GQDs can also exhibit a sensitive response to Fe3+. As shown in Figure S7a (SI), the fluorescence intensity of S-GQDs at 480 nm obviously decreased in the presence of 0.7 μM Fe3+. In contrast, no significant reduction of fluorescence intensity was observed upon adding the same amount of Fe3+ to undoped GQDs, NGQDs34 and B-GQDs37 (Figure S7b−d in the SI). It is evident that the incorporation of S atoms in GQDs should be of crucial importance for tuning the electronic local density of GQDs and promoting the coordination interaction between Fe3+ and phenolic hydroxyl groups on the edge of S-GQDs. S (electronegativity of S: 2.58) is an electron donor, but fundamentally different from N (electronegativity of N: 3.04) because of its smaller electronegativity and larger atomic radius. Therefore, the valence electrons of S in the third shell tend to lose easily, and the higher surface electron density of O atoms further promotes the coordination interaction. The speculation has been confirmed by density functional theory (DFT) calculations (B3LYP/6-31G*), in which chemical doping of S atoms into the conjugated carbon skeleton of GQDs effectively modulates the electronic density of GQDs (Figure S8 and Table S3 in the SI). Therefore, we deduce that when Fe3+ ions are added into the S-GQDs solution, they can coordinate with phenolic hydroxyl groups on the edge of S-GQDs (Scheme 1a), and the electrons in the excited state of S-GQDs will transfer to the half-filled 3d orbits of Fe3+ (Scheme 1b), facilitating nonradiative electron/hole recombination annihilation and leading to significant fluorescence quenching.54−56
The time-correlated single-photon counting (TCSPC) experiments were applied to further test the charge transfer and exciton recombination process of S-GQDs in the presence and absence of Fe3+. As shown in Figure S9 (SI), the average fluorescence lifetime of pure S-GQDs (black line) was 5.8 ns after fitting, reflecting a fast exciton recombination process. After the addition of Fe3+ (the red line), the lifetime of S-GQDs decreased to 1.6 ns. The significantly reduced fluorescence lifetime indicated a dynamics quenching occurred, and further confirmed that there was a fast electron transfer process in the S-GQDs-Fe3+ system.57 Detection of Fe3+ Using S-GQDs. On the basis of the discussion above, we explored the feasibility of using S-GQDs as fluorescent probes for the detection of Fe3+. When Fe3+ with concentration ranged from 0 to 1.6 μM was added to the asprepared S-GQDs (10 mM HEPES buffer, pH = 7.0) respectively, the fluorescence intensity of S-GQDs at 480 nm gradually decreased (Figure 4a). Accordingly, the color of SGQDs changed from bright blue-green to faint blue-green under UV irradiation (the inset of Figure 4a). Figure 4b presents the relationship of the fluorescent quenching value ΔF = F0 − F (F0 and F are the fluorescence intensities of S-GQDs at 480 nm in the absence and presence of Fe3+, respectively) with the concentration of Fe3+. The inset of Figure 4b shows that the ΔF has a good linear relationship with the concentration of Fe3+ in the range of 0−0.7 μM, and the linear regression equation is ΔF = 5.9998+ 629.4216C with a correlation coefficient of 0.9988 (n = 6), where C is the concentration of Fe3+ (μM). The detection limit of 4.2 nM was obtained based on a 3δ/S (δ is the standard deviation of the blank signal and S is the slope of the linear calibration plot), which was much lower than most of the previous reported assays for Fe3+ detection.53,58−63 Table1 shows the comparison D
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of different fluorescent probes for Fe3+ detection, suggesting our sensing system exhibits higher sensitivity.
also performed to show anti-interference abilities of S-GQDs for detecting Fe3+ (Figure 5). It is very encouraging that the present method can detect Fe3+ in the presence of all possible interference ions, underlining the high selectivity of S-GQDs for detecting Fe3+ and the great potential of S-GQDs for real sample application. The reversibility of the fluorescent probe is very important in practical application. It was observed that S-GQDs aqueous solution without Fe3+ exhibited a strong fluorescence emission peak at 480 nm (Figure 6a, curve a), while the fluorescence intensity of S-GQDs decreased obviously upon adding 0.7 μM Fe3+ (Figure 6a, curve b). Then, when 0.7 μM EDTA was added into the S-GQDs-Fe3+ mixture, the fluorescence intensity of S-GQDs was almost completely restored (Figure 6a, curve c). The results demonstrated that EDTA could readily attack Fe3+ from the S-GQDs-Fe3+ to form more stable Fe-EDTA, and the S-GQDs were released. This process could be repeated at least five times without signal lost (Figure 6b), which indicated that the S-GQDs-based fluorescent probe could be regenerated. In order to explore the practical application, the fluorescence assay was applied to detect Fe3+ in human serum sample. As shown in Figure S10 (SI), upon increasing the amount of deproteinized human serum sample, the fluorescence intensity of S-GQDs (5 μg/mL) gradually decreased (Figure S10a in the SI), and the fluorescent quenching value ΔF had a linear relation with the volume of human serum sample (Figure S10b in the SI). The results indicated that the S-GQDs-based fluorescence probe was capable of detecting Fe3+ in the human serum sample. For quantitative measurement, a standard addition method with FeCl3 as the standard was employed to estimate the unknown concentration of Fe3+ in human serum (Figure 7). The concentration of Fe3+ in a selected human serum was found to be 25.4 ± 0.3 μM by this assay, which was well consistent with the value (26.5 ± 0.5 μM) obtained by ICP-MS.
Table 1. Comparison of Different Fluorescent Probes for Fe3+ Detection fluorescent probes GO nanosheets anthracene-appended amino acids pyrazoline derivative 2,5-diphenylfuran and 8-hydroxyquinoline aminoantipyrine rhodamine B Schiff-base phosphonic acid-functionalized fluorene derivatives S-GQDs
detection limit (μM) 17.9 10 1.4 0.97 0.211 0.11; 1.6 0.02; 0.01 0.0042
ref 53 58 59 60 61 62 63 this work
The selectivity of the assay was also explored: the fluorescence intensity ratios (F/F0) of S-GQDs (5 μg/mL) were analyzed upon adding different metal ions (such as Fe3+, K+, Na+, Ca2+, Mg2+, Al3+, Ag+, Cd2+, Co2+, Cu2+, Hg2+, Mn2+, Ni2+, Pb2+, Zn2+, Fe2+) to the sensing system. As shown in Figure 5, only Fe3+ gave significant quenching effect on the
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Figure 5. Fluorescence intensity ratios (F/F0) of the S-GQDs (5 μg/ mL) in the presence and absence of different metal ions (0.70 μM) listed from left to right: blank, Fe3+, K+, Na+, Ca2+, Mg2+, Al3+, Ag+, Cd2+, Co2+, Cu2+, Hg2+, Mn2+, Ni2+, Pb2+, Zn2+, and Fe2+. The mixed solution contains all of the metal ions mentioned above. The error bars represent the standard deviations based on the independent measurements.
CONCLUSIONS
In summary, S-GQDs were synthesized by one-step electrolysis of graphite in sodium p-toluenesulfonate aqueous solution. Doping with S atoms improved the electronic properties and surface chemical reactivities of GQDs, which was beneficial for chemically binding with Fe3+ forming S-GQDs-Fe3+. The SGQDs emitting strong blue-green fluorescence were used as a novel fluorescence probe for the highly sensitive and selective detection of Fe3+. The detection limit was as low as 4.2 nM. The generation of the fluorescence probe could be achieved by using EDTA to capture Fe3+ from S-GQDs-Fe3+. Finally, the
fluorescence of S-GQDs upon addition of Fe3+, indicating the high selectivity of S-GQDs for Fe3+ in aqueous solution. Further experiments for the effect of coexisting metal ions on the quenched fluorescence intensity of S-GQDs by Fe3+ were
Figure 6. (a) Fluorescence spectra of the original 5 μg/mL S-GQDs (curve a), S-GQDs-Fe3+ mixture (curve b), and S-GQDs-Fe3+-EDTA mixture (curve c). The concentrations of Fe3+ and EDTA are 0.7 μM. (b) Reversible investigation of S-GQDs (5 μg/mL) for Fe3+ (0.7 μM) with addition of EDTA (0.7 μM). E
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Figure 7. (a) Fluorescence spectra of S-GQDs (5 μg/mL) aqueous solution in the presence of different concentration of Fe3+ (from top to bottom: 0, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5 μM) and 0.5 mL deproteined human serum. (b) The regression curve of the fluorescence quenching values ΔF vs Fe3+ concentration ranging from 0 to 0.5 μM. The absolute value of the intercept of the line at x-axis was used to calculate the concentration of unknown Fe3+ in human serum.
fluorescence assay was successfully applied for the detection of Fe3+ in human serum samples, which presents its potential applications in biological detection and clinical diagnosis.
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ASSOCIATED CONTENT
S Supporting Information *
. AFM image of the S-GQDs on a Si substrate (Figure S1); FTIR spectrum of S-GQDs (Figure S2); (a) XPS, (b) S 2p, and (c) C 1s spectra of sodium p-toluenesulfonate (Figure S3); (a) XRD patterns of the pristine graphite and S-GQDs; (b) Raman spectrum of S-GQDs (Figure S4); the UV-vis absorption spectroscopy of S-GQDs (Figure S5); quantum yield measurements; quantum yield of the S-GQDs (Table S1); comparison of the fluorescence quantum yields of GQDs prepared by different methods (Table S2); photostability test of the SGQDs in a fluorescence spectrophotometer with a 150 W Xe lamp under 380 nm excitation (Figure S6); fluorescence spectra of the S-GQDs (Figure S7); the theoretical models of GQDs and S-GQDs (Figure S8); the optimized Cartesian coordinates (Å) of the theoretical models of GQDs and S-GQDs (Table S3); fluorescence quenching mechanism; fluorescence decays (385 nm excitation) of the S-GQDs by TCSPC in the presence (red) and absence (black) of Fe 3+ (Figure S9); and fluorescence spectra of S-GQDs aqueous solution (5 μg/mL) in the presence of different volume of deproteinized human serum (Figure S10). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by NSFC (21073018) and the Major Research Plan of NSFC (21233003). REFERENCES
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