b State Key Laboratory of Heavy Oil Processing, China University of Petroleum-Beijing, Beijing 102249, China;
c Research Institute for New Materials Technology, Chongqing University of Arts and Sciences, Chongqing 402160, China;
d School of Pharmaceutical Sciences, Guangdong Provincial Key Laboratory of New Drug Screening, Southern Medical University, Guangzhou 510515, China
Since Xu's group firstly reported with fluorescence effect in 2004 , CDs have been attracting great attention for their excellent properties including high stability, bio-compatibility, low photo bleaching and toxicity and so on. It has shown promising applications in various fields such as bioimaging [2-5], drug delivery [6, 7], photocatalysis [8-11], electrocatalysis [12-14], energy conversion [15, 16], etc. To the best of our knowledge, it was testified that the properties of CDs can be significantly improved through heteroatom doping. Heteroatom doping and surface passivation are considered to be two main methods to enhance the photoluminescence of CDs [17-20]. Since heteroatom doping owns the benefit of easy fabrication, costly effective and mass production capability, it has been rapidly developed in these years. For example, Yang et al. used citric acid and ethylenediamine as precursors to synthesize nitrogen-doped C-dots with a quantum yield (QY) reached 80% . Yu et al. have successfully in synthesizing nitrogen doped CDs (N-CDs) via a one-pot hydrothermal treatment of L-glutamic acid (PLQY of 17.8%) . Yu and his group produced nitrogen and sulfur co-doped CDs, which exhibited QY up to 73% . Using hydrothermal treatment of garlic and alfalfa (PLQYof 10%) , Guo and his colleague reported sulfur and nitrogen co-doped CDs (S, N-CDs). Xu et al. synthesized sulfur doped CDs (S-CDs) via a simple and straight forward hydrothermal treatment of sodium citrate and sodium thiosulfate (PLQY of 67%) . The P, N-CDs prepared by a single step hydrothermal method has been reported recently (PLQY of 53.8%) . In addition, the synthesis of P-CDs  and B-CDs  have also be reported through hydrothermal treatment. Researchers used some typical methodology to bedeck the CDs in order to gain particular characteristic of CDs [29, 30]. However, the synthesis of metal-doped carbon dots was still a challenge for researchers. Xu et al. reported a facile and economic approach to synthesize copper doped carbon dots (Cu-CDs) via a one-step hydrothermal approach using sodium citrate and cuprous chloride (PLQY of 9.81%) . Zinc doped carbon dots (Zn-CDs) were synthesized by using zinc ions and sodium citrate as the precursor via a one-pot hydrothermal method (PLQY of 32.3%) . Xu  also produced Zn-doped CDs, regrettably the quantum yield was not ideal. There are fewer reports about doping Mn2+ ions to carbon dots by hydrothermal method.
Herein, Mn2+ ion was directly doped into CDs (Mn-CDs) with blue photoluminescence via a simple strategy for the first time. The lifetime of prepared CDs can reach up to 8.56 ns. Mn-CDs can be applied for the detection of Fe3+, which is of great importance in our daily health. As we all know, it is not only an essential compound to synthesize enzyme in the human body, but also a necessary catalyst in the chemical production. Therefore, these carbon dots have practical significance and will create enormous benefits. In addition, we used different characterization techniques including transmission electron microscopy (TEM), fluorescence spectroscopy, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) to study the chemical and physical properties of the as prepared Mn-CDs.
Water-soluble Mn-CDs were prepared by a modified hydrothermal treatment . The optimization of reaction conditions was discussed. Fig. 1a shows the process of preparing Mn-CDs, with image of upper set showing the different molar ratio of sodium citrate and MnCl2 to synthesis the Mn-CDs of hydrothermal conditions at 200 ℃ for 4 h (from left to right are 1:0.1; 1:0.2; 1:0.25; 1:0.5 and 1:0.7, respectively). It is evident from the graph that the molar ratio of 1:0.5 emitted the brightest light under the ultraviolet radiation. In order to obtain the optimized carbon dots, while keeping the same molar ratio and reaction time, the reaction temperature varied from 180 ℃ to 235 ℃ (180, 190, 200, 205, 215, 235 ℃) was studied and shown in Fig. 1b. From Fig. 1b, the photoluminescence intensity increased as the temperature increased from 180 ℃ to 200 ℃, but significantly decreased when the reaction temperature further increased. The PL intensity was measured in the range of 370–560 nm with an excitation wavelength of 360 nm. It showed that reaction temperature played a crucial role for the synthesis and the optimum reaction temperature was 200 ℃.
|Fig. 1. (a) Schematic image of the synthesis process of blue luminescent Mn-CDs. (b) Photoluminescence spectrum of Mn-CDs at different reaction temperature. (c) Photoluminescence spectra and (d) PLQY of Mn-CDs at different reaction time.|
After researching the reaction temperature (200 ℃) and molar ratio (1:0.5), the effect of hydrothermal reaction time was then investigated varying from 1 h to 10 h (1, 3, 6, 7, 8, 10 h). Fig. 1c showed that the spectra of PL intensity at the reaction time of 6 h reach up to maximum 7.23%. The PL spectra and PLQY of the MnCDs with different molar ratio was shown in Fig. S2 in Supporting information. Therefore, the optimum experimental conditions were 1:0.5 precursor molar ratio (0. 5 mmol sodium citrate and 0.25 mmol MnCl2), reaction temperature at 200 ℃ and reaction time of 6 h. Under these conditions, robust Mn-doped CDs with a superior PLQY (7.23%) were obtained.
The AFM image of the as prepared Mn-CDs is shown in Fig. 2a, indicating that the Mn-CDs morphology possess a uniform and spherical particle property, with heights homogeneously distributed in the range of 4.5–5.5 nm, which is further confirmed by the nanoscale transmission electron microscopy (TEM) shown in Fig. 2b. Fig. 2c shows the high-resolution transmission electron microscopy (HRTEM) of Mn-CDs, it shows Mn-CDs with notable crystalline characteristics with a spacing of 0.21 nm. The result indicated the presence of (100) lattice planes of graphitic carbon. The diameter distribution of Mn-CDs was illustrated in Fig. 2d, confirming that the Mn-CDs possessed a good dispersion around 3–5 nm.
|Fig. 2. (a) AFM image of Mn-CDs under 100 nm. (b) Large area TEM image of MnCDs under 20 nm. (c) HRTEM image of the Mn-CDs. (d) The diameter distribution of Mn-CDs calculated from TEM image.|
The PL spectra (Fig. 3a) showed excitation wavelength independence in the range of 300 nm–390 nm with a 10 nm increment. Inset is the Mn-CDs optical photograph which indicates it emits high blue light in the presence of UV light (365 nm) irradiation. The highest emission peak appears at 440 nm with an excitation of 360 nm. The Mn-CDs shows well absorption peak around 300 nm, which is consistent well with the n-π* transition of C═O . The Fig. 3b illustrates the curve of Mn-CDs' optimal fluorescence excitation and excitation spectrums, fitting well with the PL spectrum. The counter plot of Mn-CDs (Fig. S2) given the deepest red spots appears in the emission of 440 nm with an excitation of 360 nm. The Raman spectrum (Fig. 3c) for Mn-CDs exhibited both G band at 1580 cm-1 and D band at 1376 cm-1. The ratio of ID/IG is 0.84, suggesting that there is a considerable graphitization in the Mn-CDs as prepared. The prepared Mn-CDs illustrates well water solubility and a wide range of pH values. The curve of pH effect (Fig. S3 in Supporting information) indicated that the fluorescence intensity declined in the pH range of 4–10. However, when the pH is lower than 4, or higher than 10, significant decline of fluorescence intensity was observed, inffering that the strong acid or alkali will affect the carbon quantum dots surface emitting groups, which is further proved by the FTIR analysis (Fig. 3d). The Mn-CDs shows a broad band at 3000- 3500 cm-1 corresponding to the O—H vibration. The other is that C═C and C–O vibration bands at approximately 1530 cm-1 and 1300 cm-1 respectivety. This is also proven in following XPS spectrum.
|Fig. 3. (a) PL emission spectra, (b) Fluorescence excitation and excitation spectrum, (c) Raman spectrum and (d) FTIR spectrum of as prepared Mn-CDs.|
To confirm the structure and elemental composition of the MnCDs, X-ray photoelectron spectroscopy (XPS) was used and the results indicate that the Mn-CDs have a major component of carbon, oxygen and manganese (Ⅱ) (Fig. 4a). The high resolution C1s spectrum of prepared Mn-CDs was given in Fig. 4b, which had two peaks at the binding energy of 288.18 and 284.78 eV, attributing to C—C and O—C═. The high resolution O1s spectrum (Fig. 4c) illustrates that the C–O states appear at binding energy of 531.08 and approximate 534.0 eV belonging to the O—C═O, which is consistent with C1s spectrum, simultaneously indicating the presence of a carboxyl group. It is attributed to Mn ion-assisted oxidation. The high-resolution Mn 2p spectrum of different ratio is displayed in Fig. 4d, indicated that it has two peaks Mn 2p3/2 and Mn 2p1/2 at the binding energy of 653.28 eV and 640.18 eV, respectively. The result shows that Mn2+ state exists in the Mn-CDs which can be further demonstrated by the ICP analysis (Table S1 in Supporting information). The ICP data indicated the Mn content is nearly 7.2% at the optimal ratio of around 1:0.5, in accordance with the result shown in Fig. 1a. The maximum PLQY of this doping ratio shows that the current doping process is highly efficient.
|Fig. 4. (a) XPS survey spectrum, high-resolution (b) C1s and (c) O1s of Mn-CDs. (d) Mn2p XPS spectra for Mn-CDs of different molar ratio of precursor.|
It was the first time to use sodium citrate and manganese (Ⅱ) chloride to synthesize manganese (Ⅱ) carbon dots (Mn-CDs). In this contribution, we applied the Mn-CDs to detect Fe3+ [33-35]. To be a practical sensor, the Mn-CDs have to be highly specific toward target ions. In Fig. 5a, 100 μmol/L concentration of various ions including Ca2+, Ce3+, Cu2+, Fe3+, K+, La2+, Mg2+, Mn2+, Na+, Ni2+, Pb2+, Zn2+ were introduced into the Mn-CDs solution and the fluorescence quenching at 440 nm was examined. It's obvious that only Fe3+ showed a significant quenching on Mn-CDs, while other tested ions have no appreciable change. The consequence proves that the prepared Mn-CDs are ideal candidates for the selective and sensitive detection of Fe3+, which can be easily adapted for important sensing application in biology and various other fields. In Fig. 5b, the time-dependent fluorescence quenching of Mn-CDs in the presence of Fe3+ (100 μmol/L) is displayed. The fluorescence response is fast since there is no apparent change in the intensity pointing to the quick response time after 1 min. In Fig. 5c, different concentrations of Fe3+ (0–500 μmol/L) were used to test the fluorescence of Mn-CDs. It is reasonable to conclude that more MnCDs show a concurrent decrease of fluorescence intensity with increasing concentration of Fe3+. In Fig. 5d, an obvious linear curve can be easily observed, which shows that change in fluorescence intensity of the solution of the Mn-CDs (△F) is nearly linearly dependent on concentrations of Fe3+, and linear regression can be defined as △F = 0.9112 + 0.0125C (R2 = 9943) with the limit detection of 2.1 nm, which was calculated based on the three times standard deviation rule [36-38]. It also attested that the prepared Mn-CDs can as an ideal material to produce the probe monitor for Fe3+. The most acceptable PL quenching mechanism was reported. The efficient quenching of fluorescence by Fe3+ ions is the CDs ability to facilitate the electron/hole recombination annihilation through an efficient and reversible electron/hole transfer process. It also attested that the prepared Mn-CDs can as an ideal material to produce the probe monitor for Fe3+. We also studied the UV–vis absorption spectra (Fig. 6a) and PL decay spectra (Fig. 6b) for Mn-CDs before and after quenching process.The stability of the As can be clearly seen from the figure, with the CDs quenching degree increasing, the absorption intensity increases and the lifetime decreases.
|Fig. 5. (a) The change of fluorescence intensity at 440 nm for Mn-CDs in the presence of various metal ions. (b) Time-dependent fluorescence changes of MnCDs in the presence of Fe3+ (100 μmol/L). (c) Emission spectra of the CDs solution with different concentrations of Fe3+ (0, 50, 100, 200, 300, 400, and 500 μmol/L). (d) The change of fluorescence intensity of Mn-CDs solution versus the concentration of Fe3+.|
|Fig. 6. (a) UV–vis absorption spectra and (b) PL decay spectra for Mn-CDs before and after quenching process.|
In summary, highly luminescent manganese(Ⅱ) doped CDs was firstly synthesized via a hydrothermal method, which takes sodium citrate as carbon source and manganese(Ⅱ) chloride as precursor. Mn-CDs have a robust selectivity of Fe3+ while not sensitive to other metal ions. In addition, the fluorescence intensity of Mn-CDs has a good linear relation with the concentration of Fe3+. The as prepared Mn-CDs can be considered as a new turn-off fluorescence probe sensor for Fe3+ ion in tap water. The initial synthesis of Mn-CDs has made a breakthrough in the research of metal ions doping carbon dots, providing researchers a new direction that may help to unriddle the photoluminescence mechanism of metal-doped carbon dots.Acknowledgments
The authors would like to thank the financial support from the National Natural Science Foundation of China (No. 81773642), Hunan Provincial Natural Science Foundation (No. 2018JJ2363), Guangdong-Hong Kong Technology Cooperation Fund (No. 2017A050506016), the Research Program of Yongchuan Science and Technology Commission (Ycstc, No. 2018nb1402). We thank Military Youth Innovation Training Program (No. 16QNP145), Translational Medicine Program (No. GHPLA 2016TM-019) for the support.Appendix A. Supplementary data
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cclet.2019.01.014.
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