Synthesis of Pyridinic-Rich N, S Co-doped Carbon Quantum Dots as Effective Enzyme Mimics
© The Author(s). 2017
Received: 14 March 2017
Accepted: 17 May 2017
Published: 25 May 2017
N and S co-doped carbon quantum dots (N, S-CQDs) with high N- and S-doping level were synthesized by microwave solid-phase pyrolysis within 50 s. Owing to the dominant pyridinic N injection into the conjugated framework, both high enzyme mimics catalytic activity and photoluminescence quantum yield are achieved simultaneously.
The carbon quantum dots (CQDs) which emerged as a novel zero-dimensional carbon material have received tremendous attention because of their high chemical stability, low cytotoxicity, and unique electronic nature and optical behaviors [1–3]. With active surface groups such as –OH and –CO2H, CQDs can be recombined with other organics or inorganics for various fantastic applications including bio-imaging [4, 5], optoelectronic devices, and photocatalysts for degradation of organic dyes or production of hydrogen from water splitting [6–8]. Very recently, both experimental and theoretical results confirmed that heteroatom doping was an effective method to improve the electronic and optical properties of CQDs [9, 10]. Among the novel composites, N-doped CQDs or nitrogen/sulfur co-doped CQDs (N, S-CQDs) demonstrated much high fluorescence quantum efficiency or photocatalytic activity than that of pristine one [11, 12]. Also, the enhancement in performance of N-doped CQDs had shown a positive correlation to the nitrogen doping amount [13, 14]. Although these studies convincingly certify that N-doping strikingly influences the properties of CQDs, however, there are scarce reports on the effective hetero-doping methods for CQDs. Suffered from the high solubility of inorganic precursor of the dopant, conventional hydrothermal carbonization routes would lead to large amount of dopants remaining in the reaction solution and thus quite low N-doping amount in the final CQDs.
Herein, we reported synthesis of nitrogen-rich N, S co-doped carbon quantum dots (N, S-CQDs) by microwave-assisted approach within only 50 s. Citric acid (CA) was chosen as carbon source, and thiourea was used as not only a nitrogen and sulfur source but also a weak base. The nitrogen and sulfur concentration of N, S-CQDs reaches 12.8 and 7.2 wt%, respectively, which was about five and three times higher than reported for N-CQDs and N, S co-doped CQDs [11, 14].
The N, S-CQDs were obtained in the following ways: the mixture of 0.42 g (2 mmol) of citric acid monohydrate and 0.46 g of (6 mmol) thiourea was put in a porcelain crucible and heated for 50 s in a microwave reactor (445 W). The obtained brownish yellow product was added into 30 mL of deionized water to form a yellow suspension and centrifuged at 9000 rpm for 20 min. Then, the supernatant was purified with a 0.22-μm filter membrane and dialyzed with deionized water through a dialysis membrane (retained molecular weight,1000 Da) for 24 h. Finally, the dialysate was further freeze-dried under vacuum. The pristine CQDs were synthesized from neat citric acid monohydrate, and the subsequent treatment process was the same with that of N, S CQDs.
The enzyme-mimic activity of N, S-CQDs for the decomposition of H2O2 was measured in a 30-mL buffer solution of citric acid–disodium hydrogen phosphate (pH ≈ 3.5, 35 °C) containing 1 μg mL−1 of N, S-CQDs and 8 × 10−4 M of tetramethylbenzidine (TMB) substrate. After 160 μL of H2O2 (30%) solution was added to the colorless buffer solution, the reaction starts and then takes the solution to measure the absorbance of the blue oxidation product of TMB at 652 nm each 2 min. Finally, the reaction rates of oxidation TMB were calculated. The reusability test of N, S-CQDs was performed in the reaction system containing 60-mL buffer solution of citric acid–disodium hydrogen phosphate and 2 μg ml−1 of N, S-CQDs as well as 5 × 10−3 M of TMB substrate. The reaction started as the addition of H2O2 solution (0.3%, 320 μL) into the mixed solution and took a small amount of solution to measure the absorbance at 652 nm after 1 h and the first cycle was finished. Then, 320 μL of fresh H2O2 (0.3%) solution was added to the reaction system for the next cycle. Other three-time cycle reactions were repeated in the same condition. The corresponding absorbance was calculated by subtracting the last absorbance.
The transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) images were obtained on a JEM-2100 electron microscope with a high voltage (200 kV). The selected-area electron diffraction (SAED) was measured by FEITF20 (FEI high-resolution field-emission transmission electron microscopy) with a condition of 200 kV. The UV/vis absorption spectra were carried out with UV-3600 (Shimadzu UV-VIS-NIR Spectrophotometer). The fluorescence spectra were recorded on F-7000 (Hitachi Fluorescence Spectrometer) with the condition of 700 V. The fluorescence lifetime and FLQY were measured by the FM-4P-TCSPC (Horiba Jobin Yvon). The excitation and emission wavelengths are 358 and 436 nm, respectively. The X-ray powder diffractometer (XRD) were characterized by D8 Advance (Germany Bruker AXS Ltd.) using Cu Kα with the condition of 40 kV and 40 mA. The Fourier transform infrared (FT-IR) spectra were carried out with Nicolet iS10 (Thermo Fisher Infrared Spectrometer). X-ray photoelectron spectrometer (XPS) was obtained on PHI 5000 Versa (UIVAC-PHI). TG-MS (thermogravimetric-mass spectrometry) is measured by Netzsch STA 449C with a heating rate of 10 K min−1 from 35 °C up to a final temperature of 450 °C under the N2 air (10%, air) flow.
Results and Discussions
The photoluminescence (PL) spectra (Fig. 2b) illustrate that the N, S-CQDs have broad distribution of excitations. The maximum excitation wavelength is at 358 (emission wavelength 436 nm) owing to the 340 nm of absorption peak. It can be seen from the inset image in Fig. 2b that the colorless and transparent N, S-CQDs aqueous solution becomes bright blue under 365 nm UV irradiation. The solution of N, S-CQDs remained clear for 10 months without precipitations; this high stability of N, S-CQD particles is ascribe to their much small and uniform size as well as hydrophilic groups on the surface.
Figure 2c illustrates the emission spectra of N, S-CQDs with different excitation wavelengths. When the excitation wavelength changes from 290 to 370 nm, the peaks of emission at 440 nm show nearly no shift. The emission components are fairly constant in energy and most probably originate from absorption of n-π* transition at 340 nm. The excitation-independent emission property of CQDs has been studied by fitting the complex emission peaks to multiple Gaussian functions and deduced similar conclusion . While when the excitation wavelength is varied from 390 to 490 nm, the PL emission spectrum exhibited a redshift as the increase of excitation wavelength, characterizing an excitation wavelength-dependent property. This can be ascribed to various surface states of C=O or amide group’s role as discrete exciton trapping centers to affect the emission energy in PL process [11, 19, 28]. Polydispersity and surface heterogeneity is the origin of excitation wavelength-dependent PL behavior [28, 29]. The broad absorption peak at around 430 nm is an ensemble of various surface states, including carboxyl and amide, which enables excitation wavelength-dependent PL behavior of N, S-CQDs. The fluorescence lifetime of N, S-CQDs was determined to evaluate its optical property (Fig. 2d). The PL decay curves of N, S-CQD sample can be fitted by a double-exponential formula, where τ 1 is 3.48 ns, τ 2 is 11.05 ns, and the average lifetime is 6.72 ns. Compared to the average lifetime of 2.42 ns of pristine CQDs , dramatic longer fluorescence lifetimes of both τ 1 and τ 2 were obtained on our sample. It has been reported that the τ 2 proportion and average lifetime become longer with the N-doping amount increasing and concluded that the longer τ 2 stemmed from the surface states [11, 31].
The reusability of N, S-CQDs was investigated by consecutive four times usage for catalase-mimic reaction (Fig. 5b). On the four-cycle usage, no obvious decrease in the activity of N, S-CQDs was observed. The high stability of the intrinsic catalysis activity of N, S-CQDs is ascribed to the dominant pyridinic N-doping in the C=C framework because the pyridinic N can play a role as effective enzyme mimic catalytic sites for H2O2 decomposition.
In summary, we synthesized a pyridinic-rich N, S-CQDs with high N- and S-doping level by microwave solid polymerization method within mere 50 s. Thiourea roles not only as S source but also as weak base to accelerate the intermolecular dehydration at low temperature and multistep carbonization, which enables the high N- and S-doping level in N, S-CQDs and dominant pyridinic N to inject into the conjugated framework as the enzyme mimic active sites. Our work provides an effective method to synthesize pyridinic-rich N, S-CQDs possessing both high PLQY and enzyme mimics activity.
This work was supported by the National Natural Science Foundation of China (Grant Nos. 21273106, 21273111, and 51272101) and the Jiangsu Provincial Natural Science Foundation (No.BK20130053, BK20151265).The authors thank the Analysis Center and High Performance Computing Center of Nanjing University for the sample characterization and theoretical calculations.
TL contributed to the experiment and manuscript preparation. ZC contributed to the manuscript preparation. JZ contributed to the manuscript preparation. YW contributed to the idea of the study and manuscript preparation. ZZ contributed to the manuscript preparation. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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