Article pubs.acs.org/ac
Solid-Phase Synthesis of Highly Fluorescent Nitrogen-Doped Carbon Dots for Sensitive and Selective Probing Ferric Ions in Living Cells Haijuan Zhang,†,‡ Yonglei Chen,†,‡ Meijuan Liang,†,‡ Laifang Xu,†,‡ Shengda Qi,†,‡ Hongli Chen,†,‡ and Xingguo Chen*,†,‡,§ †
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, China Department of Chemistry, Lanzhou University, Lanzhou 730000, China § Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, Lanzhou 730000, China ‡
S Supporting Information *
ABSTRACT: Carbon quantum dots (C-Dots) have drawn extensive attention in recent years due to their stable physicochemical and photochemical properties. However, the development of nitrogen-doped carbon quantum dots (N-doped C-Dots) is still on its early stage. In this paper, a facile and high-output solid-phase synthesis approach was proposed for the fabrication of N-doped, highly fluorescent carbon quantum dots. The obtained N-doped C-Dots exhibited a strong blue emission with an absolute quantum yield (QY) of up to 31%, owing to fluorescence enhancement effect of introduced N atoms into carbon dots. The strong coordination of oxygen-rich groups on N-doped C-Dots to Fe3+ caused fluorescence quenching via nonradiative electron-transfer, leading to the quantitative detection of Fe3+. The probe exhibited a wide linear response concentration range (0.01−500 μM) to Fe3+ with a detection limit of 2.5 nM. Significantly, the N-doped C-Dots possess negligible cytotoxicity, excellent biocompatibility, and high photostability. All these features are favorable for label-free monitoring of Fe3+ in complex biological samples. It was then successfully applied for the fluorescence imaging of intracellular Fe3+. As an efficient chemosensor, the N-doped C-Dots hold great promise to broaden applications in biological systems.
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product yield, and the use of an organic solvent and strong acid/alkali/oxidant. Thus, a facile synthetic route to synthesize high-quality N-doped C-Dots is still imminently desired. Solid-phase synthesis is a new tool in nanoscience to overcome bottlenecks in the production and characterization of nanomaterials. Its procedure is simple and can easily be repeated. Since solid-phase synthesis was initially introduced by Merrifield,18 this protocol has been the standard for preparing peptides and oligonucleotides for a number of years.19 The introduction of this method allowed for the synthesis and purification of complex molecules with relative ease; meanwhile, high product yield was obtained. In the most recent years, the application in nanoparticle research expanded this concept and extended the synthetic utility of such process. Some nanostructured materials have been prepared by solidphase synthesis, such as carbon nanocoils,20 gold nanoparticles,21,22 CdTiO3 submicrocrystals,23 and molecularly imprinted polymer nanoparticles.24 These recent progresses have motivated our curiosity to examine its capability in the preparation of high-quality N-doped C-Dots. It is expected that
s newcomers to the carbon nanomaterials family, carbon dots (C-Dots) have attracted tremendous attention, owing to their remarkable advances in high resistance to photobleaching, robust chemical inertness, low toxicity, and good biocompatibility.1,2 Because of these attractive merits, C-Dots have been widely used in bioimaging, drug delivery, fluorescence sensors, optoelectronic devices, etc.2−9 However, in most cases, the quantum yield (QY) of the as-synthesized CDots is less than 10%, limiting the range of their practical applications. Therefore, improving the QY of C-Dots is still a big challenge for their wide applications. Doping carbon nanomaterials with heteroatoms can effectively tune their intrinsic properties, including optical characteristics, surface and local chemical features. The N atom, having a comparable atomic size and five valence electrons for bonding with carbon atoms, has been widely used for chemical doping of carbon nanomaterials.10−13 In view of the remarkable quantum-confinement and edge effects, doping C-Dots with electron-rich N atoms could drastically alter their electronic characteristics and offer more active sites, thus producing new phenomena and unexpected properties. In the past few years, several methods have been developed for the advanced synthesis of N-doped C-Dots such as electrochemical,8 hydrothermal,14,15 organic synthesis,16 microwave,17 and so on. However, these approaches may be limited by low QY and © XXXX American Chemical Society
Received: July 3, 2014 Accepted: September 11, 2014
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Figure 1. TEM image (A) and the corresponding size distribution histograms (B) of the synthesized N-doped C−Dots.
Hg(NO3)2 and all amino acids were bought from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). 3-(4,5Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) was obtained from Sangon (Shanghai, China). All reagents were of analytical grade and used as received without further purification. Ultrapure water was used throughout the experiment from a 18202V AXL water purification system (Chongqing, China). Apparatus and Characterization. Fourier transform infrared spectrometry (FT-IR) was conducted on a Nicolet Nexus 670 spectrometer using KBr pellets. The morphologies and sizes of N-doped C-Dots were characterized by transmission electron microscopy (TEM, Hitachi-600, Hitachi, Japan). X-ray photoelectron spectroscopy (XPS) was vonducted with a PHI 5702 spectrometer equipped with an Al Kα exciting source. Time-resolved fluorescence spectra were carried out in a time-correlated-single-photon-counting (TCSPC) system from FL920P spectrometer (Edinburgh Instruments, U.K.) with λex = 365 nm. The redox properties of the N-doped C-Dots were examined by cyclic voltammetry (CV, EA161 eDAQ) using a standard three-electrode system, which consists of a platinum sheet as the working electrode, a platinum wire as the counter electrode, and a Ag/Ag+ as the reference electrode. The electrolyte solution employed was 0.10 M (Bu)4NBF4 in freshly dried dimethylformamide. All luminescence spectra were surveyed on an RF-5301PC fluorescence (FL) spectrophotometer using 5/5 nm slit width, and equipped with a 1 cm quartz cell (Shimadzu, Kyoto, Japan). The ultraviolet−visible (UV−vis) absorption spectra were acquired on a TU-1901 UV−vis Spectrophotometer with a 1 cm quartz cell (Beijing Purkinje General Instrument Co., Ltd., Beijing, China). Preparation of Fluorescent N-doped C-Dots. In a typical synthesis, 0.2 g of PEG-diamine and 0.4 g of citric acid were mixed in an agate mortar and ground to a uniform powder. Then the mixture was transferred into a 50 mL Teflonlined autoclave and heated at 180 °C for 1 h. The resultant dark brown mixture was dissolved with 10 mL of water. The supernatant was collected by removing the large dots through centrifugation at 12000 rpm for 10 min and then dialyzing against ultrapure water through a dialysis membrane for 24 h. The as-prepared N-doped C-Dots were stored at 4 °C for
the synthetic strategy could accurate control of lateral dimensions and the surface chemistry of fluorescent N-doped C-Dots, as well as with high QY and product yield. As the most abundant transition metal in cellular systems, ferric ions (Fe3+) play important roles in biological systems by complexation with various regulatory proteins.25 Iron deficiency and overload can both disturb the cellular homeostasis and induce various biological disorders.26 Therefore, determination of intracellular Fe3+ is of great significance in health monitoring. At present, many chemosensors have been developed by using fluorescent metal nanoclusters,27 organic dyes,28 semiconductor quantum dots (QD),29 fluorescent metal organic frameworks,30 etc. These probes often provide admirable selectivity; however, their preparation usually requires specialized synthetic skills and complicated purification procedures. Chen et al. proposed a facile synthesis approach for graphene quantum dots from 3D graphene and subsequently used for Fe3+ sensing.25 However, as far as we know, the development of N-doped C-Dots probes for Fe3+ is still inchoate, which might be attributed to the limitation in synthesis methods and the difficulties in construction of an effective C-Dots-based sensing platform. Herein, a facile and high-output solid-phase thermal strategy for the fabrication of highly fluorescent N-doped C-Dots was discussed. The as-synthesized fluorescent N-doped C-Dots were completely water-soluble and remarkably stable under extreme pH, ionic strengths, and light illumination. Hence, a label-free sensing platform for Fe3+ detection was developed based on the nonradiative electron-transfer induced fluorescent quenching of N-doped C-Dots. We further demonstrated that the probe showed negligible cytotoxicity, excellent biocompatibility and superiority in resistance to photobleaching, and was successfully utilized for intracellular Fe3+ imaging.
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EXPERIMENTAL SECTION Chemicals. Citric acid was purchased from Tianjin Guangfu Chemical Reagents Co. Ltd. (Tianjin, China). Polyoxyethylene bis(amine) (PEG-diamine, MW 2000) was obtained from Aladdin Chemistry Co. Ltd. (Shanghai, China). Tetrabutylammonium tetrafluoroborate ((Bu)4NBF4) was purchased from Sahn Chemical Technology Co. Ltd. (Shanghai, China). Polyethylene glycol (PEG, MW 2000), FeCl3, FeCl2, AlCl3, CoCl2, Zn(NO3)2, AgNO3, Ni(NO3)2, CdCl2, CrCl3, CuCl2, B
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Figure 2. (A) Survey XPS spectra of N-doped C-Dots. High-resolution N 1s (B), C 1s (C), and O 1s (D) spectra of the N-doped C-Dots.
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RESULTS AND DISCUSSION Characterization of N-doped C-Dots. The morphology and structure of the as-prepared N-doped C-Dots were showed in Figure 1. As displayed in Figure 1A, the as-synthesized Ndoped C-Dots were well monodispersed and uniform in size. As estimated from the TEM image, the diameters of N-doped CDots were mainly distributed in the narrow range of 1.3−2.1 nm with an average size of 1.7 nm (Figure 1B). The inset in Figure 1A shows the representative image of an individual nanoparticle, indicating the high crystallinity with a lattice parameter of 0.20 nm, which corresponded to the (102) diffraction plane of graphitic (sp2) carbon.31 The morphology of N-free C-Dots was similar to N-doped C-Dots (Figure S1, Supporting Information). These results indicated that the onepot solid-phase method was efficient for the fabrication of CDots. FT-IR spectra were used to identify the surface functional groups present on the as-prepared N-doped C-Dots (Figure S2, Supporting Information). The absorption bands at 3000−3500 cm−1 were assigned to stretching vibrations of OH and N H. The peak at 1400 cm−1 was from the bending vibration of CNH and indicated the successful adulteration of nitrogen atoms into the C-Dots. The band at 1730 cm−1 was attributed to the vibration of CO, and the peaks at 1000, 1100, and 1200 cm−1 related to the COH stretching vibrations, implying the oxygen-rich property of the N-doped C-Dots. These functional groups improved the hydrophilicity and stability of the N-doped C-Dots in an aqueous system, which accounted for the great potential of the material for sensing in aqueous samples. The surface composition and elemental analysis for the resultant nanoparticles were determined by XPS. The full range XPS analysis (Figure 2A) of the resultant N-doped C-Dots sample clearly showed three peaks at 284.3, 399.9, and 531.4
further use. In the control experiments, 0.2 g of PEG and 0.4 g of citric acid were placed in an autoclave and heated to 180 °C for 1 h in a drying oven for preparation of N-free C-Dots. Other procedures were the same as that of N-doped C-Dots. Cytotoxicity Assays. The cell viability was measured using the MTT assay. Briefly, 5 × 103 cells were incubated with Ndoped C-Dots in triplicate in a 96-well plate for 24 h at 37 °C in a final volume of 100 μL. Cells treated with dimethyl sulfoxide alone were used as controls. At the end of the treatment, 10 μL of MTT (5 mg/mL) was added to each well and incubated for an additional 4 h at 37 °C. An extraction buffer (100 μL, 10% SDS, 5% isobutanol, 0.1% HCl) was added, and the cells were incubated overnight at 37 °C. The absorbance was measured at 570 nm using a microplate reader (Thermo Scientific Multiskan GO, Finland). The cell viability was defined as the ratio of the absorbance in the presence of Ndoped C-Dots to that in the absence of N-doped C-Dots (control; Isample/Icontrol). In Vivo Fluorescence Imaging. The HeLa cells were seeded in a 12-well plate and cultured in Dulbecco’s modified Eagle’s medium (DMEM) supplemented with 10% fetal bovine serum (FBS), 2 mM glutamine, penicillin (100 units/mL), and streptomycin (100 units/mL) at 37 °C in a humidified atmosphere of 5% CO2 overnight. Experiments to assess Fe3+ uptake were performed over 1 h in the same medium supplemented with 100 μM FeCl3. The N-doped C-Dots (0.4 mg·mL−1) were added to the cell culture, and the cells were incubated for another 6 h at 37 °C. The cells were rinsed with the medium for three times to remove the remaining N-doped C-Dots. After the cells were washed with fresh phosphate buffer solution (PBS, 10 mM, pH = 7.4) for three times, the fluorescence images were acquired by a fluorescent microscope (Leica DM 4000B microscope). C
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535 nm with the excitation wavelength ranging from 350 to 480 nm. The excitation-dependent FL behavior of N-doped C-Dots reflected effect from particles of different sizes and a distribution of different surface states,33 as in the case of most of the luminescent carbon dots and graphene quantum dots.34−36 The obtained N-doped C-Dots displayed narrow and symmetrical fluorescence spectra profiles, resulting from their narrower size distribution. Figure S4 (Supporting Information) presents the FL emission spectra of the aqueous solutions of Nfree C-Dots and N-doped C-Dots with excitation at 360 nm; the N-free C-Dots and N-doped C-Dots showed emissions at 442 and 435 nm, respectively. The 7 nm blue shift of the FL emission was thought to be from the strong electron affinity of N atoms doped in the C-Dots.37 Moreover, the FL intensity of N-doped C-Dots was considerably greater than that of N-free C-Dots. Notably, the fluorescence decay of N-doped C-Dots was deconvoluted using a monoexponential decay function to yield a lifetime of 11.7 ns, which was longer than that of N-free C-Dots (Figure S5, Supporting Information). The results of fluorescence spectra and decay of N-doped C-Dots further confirmed that both the size and the surface state of N-doped C-Dots were uniform.15,38 The absolute QY of N-doped CDots was measured to be 31%, which was competent for acting as a highly sensitive chemosensor, whereas the QY of N-free CDots was calculated to be 9%. The highly efficient FL emission possibly resulted from the N-doping-induced modulation of the chemical and electronic characteristics of the C-Dots.8 These results indicated that the solid-phase method was a promising approach to fabricate N-doped C-Dots with high performance. When the N-doped C-Dots are used for practical sensing applications, the material must be water-soluble and stable toward ambient environments. The N-doped C-Dots were inherently water-soluble because of the oxygen-rich functional groups in the structures. To investigate the stability, FL intensity of the N-doped C-Dots toward extreme pH or high ionic strengths in solution were measured (Figure 4). Figure 4A presented the FL intensities of N-doped C-Dots at different pH values. As could be seen that the material had strong FL activities in the range of pH 3 to 11, and the N-doped C-Dots displayed stable fluorescent even under extreme pH conditions. The stable FL intensities could be understood in terms of the change in surface charge owing to protonation−deprotonation.33 We also investigated the effect of ionic strength on the stability of N-doped C-Dots in a solution of 10 mM PBS (pH 7.4) containing different concentrations of NaCl. As shown in Figure 4B, the FL intensities remained constant with the
eV, which were attributed to C 1s, N 1s, and O 1s, respectively. A high-resolution XPS spectrum of C 1s (Figure 2B) confirmed the presence of CC (sp3, 284.6 eV), CN (sp3, 285.9 eV), CO (sp2, 287.7 eV), and CO (sp2, 288.8 eV) bonds.1 This indicated that the as-prepared N-doped C-Dots were rich in hydrophilic groups on the surfaces, which was consistent with the corresponding FT-IR spectrum. The N 1s peaks at 400.0 and 401.5 eV shown in Figure 2C indicated that nitrogen existed mostly in the form of N(C) 3 and NH, respectively,32 implying that N have been partly doped into the C-Dots. The two fitted peaks at 531.8 and 533.0 eV in O 1s spectrum (Figure 2D) were assigned to CO and COH/ COC groups, respectively.31 The surface components of the N-doped C-Dots determined by the XPS were in good agreement with FT-IR results. The remarkable optical properties of the as-obtained Ndoped C-Dots were confirmed by the UV−vis absorption and the fluorescent spectra of the aqueous dispersion of the nanoparticles under different excitation wavelengths, as shown in Figure 3. The resultant N-doped C-Dots displayed a broad
Figure 3. UV−vis absorption and fluorescent emission spectra (with progressively longer excitation wavelengths from 350 to 480 nm) of Ndoped C-Dots.
UV−vis absorption, which was ascribed to the n−π* transition of N-doped C-Dots.1 In addition, the nanodots exhibited strong blue luminescence under excitation at 365 nm (Figure S3, Supporting Information). As displayed in Figure 3, the emission wavelength of N-doped C-Dots was red-shifted from 430 to
Figure 4. (A) FL responses of N-doped C−Dots at different pH values. (B) FL intensities of N-doped C-Dots in 10 mM pH 7.4 PBS after adding various concentrations of NaCl solutions. D
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increase of ionic strength. The reason is that there was almost no ionization of the groups located on the surfaces of N-doped C-Dots, i.e., the N-doped C-Dots were uncharged. Furthermore, the FL intensity had no obvious decrease after 1 h of light illumination (Figure S6, Supporting Information), suggesting excellent photostability of the N-doped C-Dots. Taking the results above, these findings suggested that the Ndoped C-Dots were highly stable even under extreme conditions. Fluorescence Response of N-Doped C-Dots Toward Fe3+. Initially, we explored the feasibility of using such N-doped C-Dots for Fe3+ detection. Figure 5 showed the FL spectra of
Figure 5. Fluorescent emission spectra of N-doped C−Dots upon addition of various concentrations of Fe3+ (from top to bottom, 0, 0.01, 0.25, 5, 25, 50, 75, 100, 200, 300, 500, and 1000 μM). Insets showd the dependence of F0/F on the concentration of Fe3+ within the range of 0.01−500 μM and the photographs of the as-prepared Ndoped C-Dots solutions in the absence and presence of 500 μM Fe3+ under UV light (365 nm).
Figure 6. Selectivity of the N-doped C-Dots-based FL sensor toward Fe3+ over other ions. The concentration of metal ions were 500 μM.
(Figure S8, Supporting Information). Taking into account that amino acids in the biological system were capable of interacting with a lot of metal cations, several typical amino acids were also examined as potential interferences. As shown in Figure 6B, the fluorescence quenching effects of these amines were negligible. All these results clearly demonstrated that the N-doped CDots-based FL sensor was highly selective for Fe3+ over other biological related species and could meet the selective requirements for bioimaging applications. Possible Mechanism of the FL Response of N-Doped C-Dots to Fe3+. Enlightened by previous studies that hydroxyl groups exhibited good binding affinity for Fe3+,39 we proposed a working mechanism for the Fe3+-mediated FL quenching of N-doped C-Dots by nonradiative electron-transfer, as illustrated in Scheme 1. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of N-doped C-Dots could be estimated according to the empirical formula:40,41
the N-doped C-Dots dispersion after adding various concentrations of Fe3+. Upon the addition of Fe3+, a gradual decrease in FL intensity at 435 nm was observed, revealing that the FL intensity of the mixture was sensitive to Fe3+ concentration. The quenching efficiency (F0/F) displayed a good linear relationship versus the concentration of Fe3+ in the range of 0.01−500 μM (Figure 5 inset), where F0 and F are FL intensities at 435 nm in the absence and presence of Fe3+, respectively. The limit of detection (calculated according to a signal-to-noise ratio of S/N = 3) was estimated to be 2.5 nM, which was better than or comparable to those obtained by other fluorescent probes for Fe3+.14,25−27 In addition, the reaction was complete within 1 min (Figure S7, Supporting Information). This observation suggested that the N-doped CDots with quick response could be employed as fluorescent probe for real-time tracking of Fe3+ ions in the biological system. The complexity of the intracellular system poses a great challenge to the analytical methods for metal ion detection not only in sensitivity but more importantly in selectivity. Thus, the selectivity and competition experiments were carried out. As shown in Figure 6A, except for Fe2+, no obvious interferences were observed for other representative cations, including Al3+, Co2+, Zn2+, Ag+, Ni2+, Cd2+, Cr3+, Cu2+, and Hg2+. Furthermore, the selectivity of this sensing system in the presence of possible interference ions was evaluated considering the cross reactivity
E HOMO = −e(Eox + 4.4)
(1)
E LUMO = −e(Ered + 4.4)
(2)
Where Eox and Ered are the onset of oxidation and reduction potential for N-doped C-Dots, respectively. The Ered was determined to be −0.63 eV (Figure S9, Supporting Information). The corresponding ELUMO was calculated to be −3.77 eV. However, the HOMO energy could not be obtained due to the irreversible of the oxidation behavior (data not E
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Scheme 1. Sensing Principle of the N-Doped C-Dots based probe for Fe3+
Figure 7. Cell viability of Hela cells in the presence of different concentrations of N-doped C-Dots.
given). To determine the HOMO levels, we combined the Ered with the optical energy band gap (Eg, resulting from the absorption edge in the absorption spectrum): E HOMO = E LUMO − Eg 40
(3)
Eg was estimated to be 3.25 eV. So, the EHOMO was calculated to be −7.02 eV. In the absence of metal ions, the nanodots exhibited high fluorescence. Previous reports showed that hydroxyl exhibited good binding affinity for Fe3+.39 Thus, when Fe3+ was added, complexation between Fe3+ and phenolic hydroxyl of N-doped C-Dots occurred, leading to the splitting of d orbital of Fe3+ (As previously reported, the crystal field stabilization energy (CFSE) of the complex was 2.70 eV.4). Therefore, electrons in the excited state of N-doped C-Dots were partially transferred to the d orbital of Fe3+; Electron transition in the radiation forms (FL emission) was consequently restrained, leading to fluorescence quenching (Scheme 1), as in the case of other carbon dots.4 It was worth noting that there was a similar response for N-free C-Dots (Figure S10, Supporting Information), confirming the working mechanism proposed above. Compared with the N-free CDots, the N-doped C-Dots showed a higher sensitivity toward Fe3+ quenching, which could be ascribed to the N-dopinginduced modulation of the chemical and electronic characteristics and the easy formation of complexes between phenolic hydroxyl of N-doped C-Dots and Fe3+. Intracellular Imaging of Fe3+. For further biological applications, MTT assays were carried out to evaluate the cytotoxicity of the FL probes to Hela cells. As expected, the cell viability was estimated to be greater than 85% upon addition of the N-doped C-Dots over a wide concentration range of 50− 400 μg·mL−1 (Figure 7). High cell viability confirmed the low toxic, excellent biocompatibility, and the great potential of the as-prepared N-doped C-Dots for monitoring Fe3+ in living cells. Taken together, the N-doped C-Dots gave advantages of small size, high selectivity, good photostability, and especially biocompatibility, substantially making them superior in potential bioimaging applications. Hence, experiments were carried out to further demonstrate the availability of the prepared N-doped C-Dots for imaging intracellular Fe3+ in Hela cells. As shown in Figure 8, by incubating the Hela cells with N-doped C-Dots for 6 h at 37 °C, a significant blue emission from the intracellular region could be observed
Figure 8. Fluorescence microscopy images (A, B) and their corresponding bright-field transmission images (C, D) of Hela cells: (A, C) incubated with 0.4 mg·mL−1 N-doped C-Dots for 6 h at 37 °C. (B, D) first incubated with 0.4 mg·mL−1 N-doped C-Dots for 6 h and then incubated with 100 μM FeCl3 for 1 h at 37 °C.
(Figure 8A). To further investigate if our probe could be applied for semiquantitative monitoring of the intracellular Fe3+ level, exogenous Fe3+ was introduced into the N-doped CDots-pretreated HeLa cells. One can find in Figure 8B that upon supplementing cells with 100 μM of Fe3+ in the growth medium for 1 h at 37 °C, as expected, microscope images showed very weak intracellular fluorescence (Figure 8B), indicating that our probe could be applied for semiquantitative imaging intracellular Fe3+. Furthermore, the cytotoxicity of Ndoped C-Dots was negligible as confirmed by the facts that no significant changes in cell morphology (Figure 8C, D) and cell viability (Figure 7) were observed after the treatments of Hela cells with N-doped C-Dots. All these results indicated that the as-synthesized N-doped C-Dots were membrane permeable and could serve as an effective intracellular Fe3+ imaging probe. Detection of Fe3+ in Biological Samples. On the basis of the observation that the fluorescence of N-doped C-Dots was efficiently quenched by Fe3+, the proposed chemosensor was additionally applied for detecting Fe3+ in biological samples. When the urine and human serum samples were spiked with F
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different Fe3+ concentrations and measured with the current methods, high analytical precision and good average total recoveries were obtained (Table S1, Supporting Information). These results further confirmed its reliability and feasibility for monitoring Fe3+ in biological samples.
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CONCLUSION In summary, the solid-phase thermal reaction has been proven to be an effective strategy for producing highly fluorescent Ndoped C-Dots. Doping of N atoms substantially improved the emission efficiency of carbon dots, yielding a QY of 31%. The N-doped C-Dots were completely water-soluble and remarkably stable against extreme pH, ionic strengths, and light illumination, and could serve as a new fluorescence probe for label free Fe3+ sensing. It is postulated that the Fe3+-mediated FL quenching is attributed to nonradiative electron-transfer that involves partial transfer of an electron in the excited state to the d orbital of Fe3+, thus resulting in a great decrease in the FL intensity. More notably, the developed FL probe gives good biocompatibility, high selectivity, and negligible cytotoxicity, being promising as an efficient platform for quantitatively monitoring the intracellular Fe3+.
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ASSOCIATED CONTENT
S Supporting Information *
Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*X.-g. Chen. E-mail:
[email protected]. Tel: 86-931-8912763. Fax: 86-931-8912582. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful for financial support from the National Natural Science Foundation of China (No. 21375053) and Special Doctorial Program Fund from the Ministry of Education of China (No. 20130211110039).
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REFERENCES
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