Metal Charge Transfer Doped Carbon Dots with Reversibly

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Metal Charge Transfer Doped Carbon Dots with Reversibly Switchable, Ultra-High Quantum Yield Photoluminscence Quan Xu, Rigu Su, Yusheng Chen, Sreeprasad Theruvakkattil Sreenivasan, Neng Li, XuSheng Zheng, Junfa Zhu, Haibin Pan, Weijun Li, Chunming Xu, Zhenhai Xia, and Liming Dai ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00277 • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 6, 2018

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ACS Applied Nano Materials

Metal Charge Transfer Doped Carbon Dots with Reversibly Switchable, Ultra-High Quantum Yield Photoluminscence Quan Xu,a Rigu Su,a Yusheng Chen,b Sreeprasad Theruvakkattil Sreenivasan,c Neng Li,d* Xusheng Zheng,e* Junfa Zhu,e Haibin Pan,e Weijun Li,a Chunming Xu,a Zhenhai Xia,f Liming Dai,g* AUTHOR ADDRESS: a.

State Key Laboratory of Heavy Oil Processing, Beijing, 102249, China

b.

Department of Chemical Science, University of Akron, 44325, USA

c.

Polymer Institute and Center for Material and Sensor Characterization, University of Toledo, OH, 43606, USA

d.

Department of Materials Science & Metallurgy, University of Cambridge, Cambridge, CB3 0FS, UK, Email: [email protected]

e.

Hefei National Laboratory for Physical Sciences at the Microscale, Key Laboratory of Strongly-Coupled Quantum Matter Physics of Chinese Academy of Sciences, Hefei Science Center & National Synchrotron Radiation Laboratory, Department of Chemical Physics, University of Science and Technology of China, 230026, China Email: [email protected]

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ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

f.

Department of Materials Science and Engineering and Department of Chemistry, University of North Texas, Denton, Texas, 76203, USA

g.

Center of Advanced Science and Engineering for Carbon (Case4carbon), Department of Macromolecular Science Center of Advanced Science and Engineering, Case Western Reserve University, Cleveland, 44106, USA, Email: [email protected]

KEYWORDS: Carbon dots; Photoluminescence; Mechanism; Nano Sensor; Density Functional Theory


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ABSTRACT: As a class of the heteroatom-doped carbon materials, metal charge-transfer doped carbon dots (CDs) exhibited an excellent optical performance and were widely used as fluorescent probes. To improve fluorescence quantum yield (QY) remains one of the fundamental and challenging issues in the carbon dots field. Herein, we prepared a novel manganese doped CDs (Mn-CDs), which exhibited an ultra-high quantum yield of 54% - the highest quantum yield for metal-doped CDs. Various spectroscopic measurements revealed an in-situ change of dopant oxidation state during the synthesis. Our further study indicated the presence of metal-carbonate, which served as an important component for high quantum yield. We have also studied the reversibly switchable fluorescence property of Mn-CDs by adding Hg2+/S2-, as well as elucidating the underlying mechanism of this switching fluorescence phenomenon. By using the Mn-CDs as fluorescent probes, we developed an extremely sensitive detection method for heavy metal Hg2+ detection at a nM detection limit level.

1

INTRODUCTION

Carbon dots (CDs) with a diameter less than 10 nm and consequent intense quantum confinement emerged as a new class of carbon-based platform of quantum dot (QD)-like fluorescent

nanomaterials.1

Their

highly

tunable

surface

chemistry,

excellent

biocompatibility, and low photo-bleaching make CDs ideal for bioimaging, drug delivery, photocatalysis, electrocatalysis, energy conversion and optical sensing.2-20 Unlike other QDs, the optical and fluorescence properties of CDs were proposed to be due to the π plasmons21-22 and radiative recombination of the surface-confined electrons and holes.23 Diverse pathways, such as surface passivation, functionalization, and heteroatom doping, have demonstrated to be of significant potential in enhancing the optical properties of CDs.24 For doping with various heteroatoms, nitrogen(N) and sulfur(S) were demonstrated to lead to the highest


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enhancement in quantum yield (QY). Single atom doping of CDs with N and S was reported to enhance the QY up to 80%25 and 67%,15 respectively. Subsequently, doping with P,26 B,27 as well as co-doping with a combination of N, S, and P has also been revealed to lead to enhanced luminescence of CDs.28-29 Compared to doping with nonmetals, metal atom doping in CDs is rarely explored. However, the band structure of CDs can be intimately engineered by introducing metal atoms or metal carbonite into the CD matrix. The metal dopant, such as Zn, Mn and Gd, could help the radiative recombination of electrons and hole on the CD surface.30 Moreover, the presence of valence electrons in the dopant metal atoms or metal carbonite could facilitate the charge electron transfer, eventually leading to an enhanced QY. Indeed, metal doping or metal carbonite doped of CDs has been reported to enhance the QY (up to >30%), as well as photocatalysis and hydrogen production efficiency. Our group reported the first exclusively metal charge transfer doped CDs with a QY of ~20% for CDs doped with Cu.31 Later, we reported a superior QY of 32.3% for CDs doped with Zn, which was applied for the sensitive detection of hydrogen peroxide and glucose.30 However, the low QY for Mn-CDs remains as a challenge, which limited the wide application of Mn-CDs. Recent studies indicated that the influence of valence electrons presented in the dopant metal on the electron transfer process could be utilized to enhance QY in metal doped CDs. However, a systematic evaluation of this effect has not been explored, and the importance of unpaired valence electrons to the enhancement of electron transfer has not been recognized. Herein, we report, for the first time, a simple method for the preparation of water-soluble CDs doped with Mn(II) with superior fluorescence properties and a QY >50% - the record high QY for metal-doped CDs. Mn charge transfer doped CDs (Mn-CDs) prepared through a facile, one-step hydrothermal route using sodium citrate and manganese carbonate as the


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precursors and citric acid as the auxiliary agents showed bright blue fluorescence (QY~54%) and ability to reversibly switch the luminescence on or off in the presence of compatible ions. Leveraging the superior reversible fluorescence properties of the Mn-CDs, we have developed a reusable UFP for the nanomolar level detection of a mercury ion from simulated polluted water samples. By using characterization techniques, including transmission electron microscopy (TEM), fluorescence spectroscopy, X-ray photoelectron spectroscopy (XPS), Xray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy, and frontier orbital calculations using density functional theory (DFT), we have also taken a combined experimental and theoretical approach to the study of the chemical and physical properties of the Mn-CDs, the effects of Mn charge transfer doping, and the mechanism behind the enhanced QY and reversible luminescence. 2

EXPERIMENTAL SECTION 2.1

Materials

The sodium citrate and citric acid monohydrate used were obtained from Aladdin Chemistry Co. Ltd. (Shanghai, China). Manganese(II) carbonate and mercury(II) chloride were purchased from Tianjin Guangfu Chemical Reagents Co. Ltd. (Tianjin, China). All chemicals were analytical grade and used without further purification. Deionized water used in this study was provided by Dongguanshi Qianjing Environmental Equipment Co., Ltd. (Dongguan, China). Tap water was collected directly from the lab, and the lake water was collected from Dongsha Lake in Beijing. 2.2

Synthesis of Mn-CDs

A bottom-up hydrothermal treatment was strategically employed to prepare the CDs in this work. For a typical procedure, a certain amount of sodium citrate, citric acid monohydrate


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and various amounts of manganese(II) carbonate was dissolved in ultrapure water (25 mL) under vigorous stirring; then, the solution was transferred into a 50 mL Teflon-lined autoclave and heated at 195 °C for 2 h. The products were filtered with a 0.22 µm cylinder filtration membrane filter, and dialyzed through a dialysis membrane for 8 h against ultrapure water. The prepared Mn-CDs were identified as CDs-1, CDs-2, CDs-3, and CDs-4, respectively (the optimization of synthetic conditions can be seen in the supporting information in Table S1 that describes the amount of each raw material which was used to create each type of Mn-CDs). The solid products were obtained for further characterization via freeze-drying. 2.3

Characterization

The surface morphology and microstructure of Mn-CDs were examined with high-resolution transmission electron microscopy (HRTEM; Model JEM-2100 and JEM-2100F, JEOL) and atomic force microscopy (AFM; Bruker, USA). The Fourier transform infrared (FTIR) spectra of Mn-CDs were recorded from the infrared absorption spectroscopy Bruker Vertex 70v. The X-ray photoelectron spectra(XPS) were recorded using an ESCALAB 250 spectrometer with a monochromatic X-ray source with Al Ka excitation (1486.6 eV), using C 1s as reference energy (EC1s=284.eV). Fluorescence measurements were performed with a fluorescence spectrophotometer (FS5 from Techcomp (China) LTD), and the quantum yield (QY) was measured using a FS-30 quantum yield accessory with an integrating sphere. Each measurement was conducted three times to obtain an average value. The Raman spectra were recorded using a Raman spectrometer (Horiba) at an excitation wavelength of 325 nm. Photoemission spectroscopy and extended X-ray fine structure (EXAFS) spectra experiments were performed at the photoemission end-station at beamline BL10B in the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China.


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2.4

Detection of Hg2+

In a typical procedure, a volume of 30 µL CDs were diluted into 2 mL of deionized water to guarantee homogeneous dispersion. After 1 min, the emission scan of the CDs was measured using a FS5 with an excitation light of 340 nm; the exciting slit and the emission slit were set as 2.0 nm and 1.8 nm, respectively. The peak value of the emission scan was defined as the initial fluorescence intensity (designated as F0). Based on the above process, the 2 mL deionized water was replaced by various Hg2+ solutions, then the fluorescence intensity was measured after 1 min, and the fluorescence intensity was designated as F1. 3

RESULTS AND DISCUSSION 3.1

Optimization of synthesis conditions

The synthesis of Mn-CDs was prepared with a facile hydrothermal method by using manganese(II) carbonate as the dopant precursor and sodium citrate as the carbon source (Figure 1A). Due to the poor water solubility of manganese(II) carbonate, citric acid was added to enhance the solubility. The effect of reaction conditions (e.g., concentration of citric acid and/or manganese(II) carbonate, reaction temperature and/or time) on the fluorescence characteristics of the as-synthesized Mn-CDs were evaluated to optimize the optical features of the Mn-CDs. The concentration of sodium citrate was fixed at 0.1M (0.735 g) in all experiments. Initially, the mass of manganese(II) carbonate was fixed to be 1.0 g, the hydrothermal reaction time and temperature were kept at 1 h and 200 ℃, respectively while the amount of citric acid used was varied from 0 to 0.5 g. As shown in Figure 1B, S1A, the maximum fluorescent QY and intensity were attained when 0.2 g citric acid was present in the reaction mixture. Comparison of the pH value for solutions before and after the hydrothermal reaction (Figure S1B) indicates that citric acid played an important role in


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regulating the pH of the reaction mixture. It was also found that more manganese(II) was doped into the CDs at a higher concentration of citric acid, leading to an increased QY for the resultant Mn-CDs. In the presence of excess citric acid, however, a reduction of the QY was observed. To produce high-performance Mn-CDs, therefore, it is critical to adjust the concentration of citric acid in the reaction mixture to optimize the pH value between 5 and 6, which was necessary to derive excellent optical properties for the resultant Mn-CDs. By keeping the citric acid concentration at 0.2 g under the same reaction condition, the reaction temperature was systematically varied from 160 to 220 ℃ to optimize the reaction temperature. As illustrated in Figure S1C, D, an optimal reaction temperature of 195 ℃ led to the highest QY of 51%. As shown in Figure S1E, the concentration of manganese(II) carbonate also had a strong influence on the observed QY for the Mn-CDs, and a manganese(II) carbonate mass of 2.0 g was found to be most favorable for a maximized QY of the doped CDs. As can be seen in Figure S1F, a reaction time of 2 h led to an excellent PL behavior with a QY as high as 54.4%. Thus, the optimum reaction conditions for the highest QY of the Mn-CDs were determined to be as following: 0.1 M sodium citrate; 0.20 g citric acid; 2.0 g manganese(II) carbonate; 195 ℃ and 2 h.


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Figure 1. (A) Schematic of the synthetic process. (B) Plot depicting the effect of variation in citric acid concentration in the reaction mixture on the QY of Min-CDs. (C) Large area HRTEM image of Mn-CDs demonstrating the comparatively narrow size distribution (scale bar 10 nm) and the lattice resolved HRTEM image of a single Mn-CD in the inset (scale bar 2nm). (D) PL intensity and (E) UV-Vis, as well as excitation and emission spectrum of MnCDs, prepared under ideal hydrothermal reaction condition. 3.2

Characterization

High-resolution transmission electron microscopy (HRTEM) (Figure 1C) revealed that the Mn-CDs were well-dispersed and comparatively monodispersed with a diameter of ~ 3-6 nm (Figure S2A). The lattice resolved HRTEM image in the inset of Figure 1C confirms the graphite-like structure within the prepared CDs with an interplanar distance of ca. 0.21 nm characteristic of the (010) plane of graphite lattice.28, 32-33 The corresponding atomic force microscopy (AFM) image reveals that topographic height of the CDs is primarily distributed in the range of 4 to 5 nm (Figure S2B). The structural features of the Mn-CDs were explored


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using X-ray diffraction (XRD). The XRD spectrum indicates the presence of graphitic structure in the prepared CDs (Figure S3A), confirming the observation by HRTEM. Unlike the typical graphitic lattice that demonstrated the (002) plane centered at 26.5o, however, a upshifted broad and weak peak centered around 32° was observed for Mn-CDs. The observed upward shift in the XRD peak was attributed to a defective-carbon structure derived from the ordered sp2 layers as reported previously. 34-35 As shown in Figure 1D, the as-synthesized Mn-CDs exhibited excellent luminescence of an intense emission with a λ(em) at 440 nm. The excitation (λ(ex) from 280 nm to 400 nm) dependence of the emission is illustrated in Figure 1D. However, the position of λ(em) was independent of the λ(ex), which was a phenomenon commonly observed in carbonaceous nanomaterials, attributed to the presence of various emissive sites on the surface structure36-37 of particles with a narrow size distribution. The UV-vis spectrum of the Mn-CDs (Figure 1E) reveals a clear absorption band at 340 nm originated from the electronic transitions in oxygen-containing CDs.38-40 The absorption band indicated the wrapping of excited state energy by the surface states and corresponding to n-п* of C=O in the CDs. A monoexponential PL decay with a photoluminescence lifetime of 6.972 ns (Figure S3B) was observed for Mn-CDs. The relatively short PL lifetime indicates the radiative recombination of the surface-trapped electrons and holes in Mn-CDs.41 Moreover, the PL of the Mn-CDs was highly stable even under various detrimental conditions, such as variation in pH, in the presence of salt (NaCl) and under H2O2 (Figure S3C,D,E,F). When the pH ranges from 3 to 11, there is no obvious change in the PL intensity of Mn-CDs. However, the stable fluorescence intensity is disturbed by harsher conditions. It revealed that under extreme acid (pH12) conditions, the Mn-CDs become unstable and would easily decompose. This could lead plausibly due to the leaching of Mn from CDs which leads to the decrease in QY. The Mn-CDs also showed stable PL over an extended period (2 h) in the


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presence of 0 to 2 M NaCl, indicating an enhanced utility of Mn-CDs under high salt conditions. The chemical composition of Mn-CDs was studied by XPS, XANES, and FTIR. The different Mn-CDs obtained by varying the concentration of manganese(II) carbonate (labeled as CDs-1, CDs-2, CDs-3, CDs-4 with the highest photoluminescence efficiency) (see Table S1) were analyzed by XPS. The XPS data confirmed the presence of carbon, oxygen and manganese (Figure S4) in all the Mn-CDs. The C 1S spectrum indicated that carbon atoms in CDs are primarily existing as C-C and C=O (Figures 2A, S5). As more Mn charge transfer doped, the intensity of C=O in the C 1S XPS spectrum increased (Table S2), which was confirmed by O 1S spectra (Figure 2B, S6). The XPS scan of Mn 2p region gave two predominant features at 652.6 and 640.3 eV corresponding to Mn 2p1/2 and Mn 2p3/2, respectively (Figure 2C,S7). 42-43 The observed peak position for Mn was very similar to that of Mn2+ in MnO (Mn 2p3/2: 640.7 eV). 44 The shift of Mn2+ towards lower energy in Mn-CDs indicates that Mn changed to a low oxidation state, suggesting a state transfer from metal oxide to metal carbonate during the formation of the Mn-CDs.45 The existence of oxygenated functionalities on the CDs was reconfirmed through FTIR spectroscopy (Figure 2D). All Mn-CDs showed a broad and weak band at 3000-3500 cm-1, implying the presence of O-H vibration. The weak band indicates the minuscule small amount of –OH groups, which could be due to the trace adsorbed water or surface carboxylic acid groups. The strong feature centered around 1650 cm-1 pointed to the abundance of C=O on the CDs surface, the intensity of which was proportional to the Mn doping. A higher PL was observed for CDs doped with more Mn; the phenomenon could be explained based on the surface passivation from the defects on the CD surface. The Raman spectrum (Figure 2E) of all Mn-CDs exhibited a G (1580 cm-1) and relatively weak D (1376 cm-1) band, suggesting considerable graphitization


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in CDs. Thus, surface defects or passivation may not be the major mechanism for the observed high PL. This is also in agreement with the results of the C 1S XPS (Figure 2A). To gain an in-depth understanding about the chemical nature of the dopant and hence unearth the PL mechanism, we examined Mn-CDs using NEXAFS and XANES. Due to the spin-obit interaction, the Mn L2, L3-edge NEXAFS can be divided into two groups, L3 around ~641 eV and L2 around ~652 eV (Figure 2F).46 The different peaks in the L3-edge of Mn represent different chemical and valence states of Mn, namely Mn2+ (~640 eV) and Mn3+/Mn4+ (~644 eV). From the NEXAFs results, the Mn2+ is the dominant chemical state of Mn in the Mn-CDs. The Mn K-edge XAFS supports this argument (Figure S8). As the Mn content increased from CDs-1 to CDs-4, the Mn L3-edge showed a shift towards lower energy region, indicating a change in Mn oxidation state to become slightly lower than that of Mn2+. Owing to the high chemical energy barrier, however, the formation of Mn-C bonds in Mn-CDs is not favorable. Thus, Mn in doped CDs could exist in the form of the Mn carbonate or Mn-C bonding (i.e., MnCO3). The distance between Mn 2p3/2 and Mn 2p1/2 can be used to qualitatively identify the oxidation state of Mn.47 A gradual increase in the energy distance (∆E) between Mn 2p3/2 and Mn 2p1/2 from CDs-1 to CDs-4 suggests a chemical metamorphosis of Mn towards lower chemical state,48 in agreement with the L3/L2 edge NEXAFS results.


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Figure 2. (A) C1s, (B) O 1S, (C) Mn 2p XPS spectra for CDs. (D) FTIR spectra of CDs. (E) Raman spectra of CDs. (F) Mn L3/L2 edge NEXAFS spectra. 3.3

Simulations

To further understand the effect of Mn doping on CDs and the mechanism leading to embellish PL properties, theoretical calculations were employed to investigate the orbital energy levels in pristine CDs (p-CDs) and Mn-CDs. As shown in Figure 3A,B, we started with the calculation of the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) of the p-CDs with un-ionized MnCO3. Both HOMO and LUMO are dominated by the p-CDs part, indicating that the electrons in the HOMO can be excited to the LUMO if sufficient energy is provided. However, in partially ionized form (Figure 3C), a net charge localized at the -CO3H which dominates the HOMO leading to the energy level to be higher compared to the occupied states of the Mn-CDs. Similarly, in the partially ionized state, the LUMO (Figure 3D) remains dominated by the Mn-CDs. Based on the above understanding of the orbital positions, the possible modes of electron excitations can be summarized as Figure 3E. At the equilibrium, the -(CO3)H groups are partially


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ionized, and as a result, both -CO3- and -(CO3)H are likely to be co-existing on the CDs. Upon light irradiation, additional protons get released due to the ionization of -(CO3)H, and as a result, a stronger acidic environment is offered for the reaction. The production of an acidic environment and the release of one proton (H+) leaves one net charge localized at the CO3-, which then dominates the HOMO of the CDs. The presence of this charge which dominates the HOMO leads to the higher QY than that without -CO3-. Through a similar mechanism, the acidic environment enabled by citric acid in our experimental results in the high PLQY of Mn-CDs. An accurate theoretical calculation of the XANES of Mn-CDs was carried out to accurately identify the Mn-impurity contents and interfacial local atomic coordination of heteroatom-doped CDs. We calculated the XANES spectra (C and Mn

L-edge)

K-edge,

O K-edge,

(see Figure S9A, B) of Mn-CDs and in agreement with the experimental

results, identified that the primary peaks in the total C 1S and O 1S spectrum were different due to the different bonding environments of C and O atoms, indicating the presence of MnCO3 in the doped CDs.

Figure 3. Calculated frontier orbitals for CDs, with C, H, O, and Mn indicated by gray, white, red, and blue spheres. (A) HOMO and (B) LUMO are for a CD with un-ionized


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_(CO3)Mn, (C) and (D) are for CDs with partially ionized _(CO3)Mn. (E) The mechanism of photoexcitation and charge transfer of Mn-CDs under light irradiation. 3.4

Detection of Hg2+

Due to excellent photoluminescence properties, carbon dots have been widely applied in many chemical diagnosis fields, especially in the rapid detection of mercury ion. Inductively coupled plasma mass spectrometry (ICP-MS) and ultraviolet visible spectroscopy (UV-Vis) are two conventional technologies to detect Hg2+, but the procedures are complex and require costly equipment and professional operators. The approach using CD’s fluorescence can be employed for Hg2+ detection with the benefit of fast response, robust and high selectivity, easy operation, and low cost. Based on the quenching effect of mercury ions on the fluorescence of CDs, our group previously used N-doped CDs for highly efficient and selective sensing of Hg2+ in an aqueous phase with a limit of detection (LOD) of 0.23 µM.49 Zhang et al. reported a “turn-on” fluorescence nanosensor for selective determination of Hg2+ with the LOD of several ppb.50 Figure 4B illustrates that compared to other similar sensors, Mn-CDs have higher efficiency and lower LOD. In addition, the selectivity of MnCDs was much better than UV-Vis and ICP-MS due to the selective quenching by Hg2+. The prepared Mn-CDs demonstrated highly stable and intense PL properties, which we leveraged for the luminescence-based detection of metal ion. Before starting the detection of different metal ions, the Mn-CDs interference with the anions taken into consideration (Figure S10). The process is as follows: the same cation was determined namely Na+; then the different anionic solutions corresponding to Na+ were selected (Cl-, SO42-, CO32-, NO3-, HCO3-, SO32-, ClO-, PO43-, CH3COO-, Br-, S2-), when the addition of Mn-CDs in mentioned above 50 µM of various Na+ ionic salt solutions, the fluorescent intensity have no decrease, so the influence of anions was excluded. As shown in Figure 4A, S11A, the Mn-CDs had a high chemical


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affinity towards mercury ions compared to various other metal ion, opening a possibility for highly selective sensing of mercury ions. One possible reason that quenching of fluorescence by Hg2+ ion is the CDs ability to facilitate the electron/hole recombination annihilation through an efficient and reversible electron/hole transfer process.51 An ideal detection time of 1 min was experimentally identified by adjusting the concentration of Hg2+ from 0.1 µM to 50.0 µM and (Figure S11B, C). To determine the detection limit of Hg2+ in aqueous by the Mn-CDs, 2 mL solutions of mercury ions at various concentrations (0, 0.01, 0.05, 0.1, 0.5, 1.0, 5.0, 10.0, 20.0, 50.0, 100.0 µM, respectively) were gradually added to 30 µL of carbon dots. The emission spectrum (from 350 to 600 nm) was obtained with an excitation of 340 nm as shown in Figure S11D. The highly intense fluorescence of Mn-CDs was progressively quenched by increasing the concentration of the mercury ion. It is plausible that the functional groups on the Mn-CDs interact with mercury ions and formed aggregates of CDs, leading to the fluorescence quenching. Quantitatively, the lowest detection limit for the CDs was calculated to be 1.6 nM (Figure 4B). Then, this rapid fluorescence-based sensing method for Hg2+ detection was investigated in actual tap water and lake water (Table S4) as well. The recoveries were 99.6-105% for the two samples, indicates that to some extent, Mn-CDs can be reliably employed for the detection of Hg2+ in real water systems for practical application.” To better understand the fluorescence mechanism and quenching process, we conducted an experiment using three kinds of CDs namely; as-synthesized Mn-CDs with high fluorescence (the pristine Mn-CDs), Mn-CDs with partially quenched fluorescence using small amount of Hg2+ so that fluorescence intensity was reduced by half and completely quenched Mn-CDs with no fluorescence. XPS analysis of these three samples indicated an increase in C=O intensity proportional to the amount of introduced Hg2+ concentration after normalizing the C-C peak intensity (Figure 4C). The observation was consistent with the O 1S spectrum


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(Figure 4D). Since an increase in C=O content resulted in the reduction of fluorescence, it indirectly suggested that the C=O was not the major reason behind the enhanced fluorescence, as discussed earlier. Hence, the Mn carbonate present in the CD lattice might be functioned as the fluorescence active center, leading to the enhanced luminescence. XPS investigations revealed a shift of C atoms to the higher oxidation state during the quenching process. During quenching, Hg2+ plausibly played an important role as catalyst to promote the oxidation of C. The involvement of Hg2+ during the quenching was reconfirmed by comparing the Hg region of the XPS spectrum of partially and completely quenched CDs (Figure 4E). A broadening of Hg features in the completely quenched CD sample indicates a disproportionation reaction of Hg2+ during the quenching process. The bound Hg can act as a “chemical bridge” facilitates the electron flow from C to Mn. In addition, the Hg2+ can be competitively amalgamated with carbonate (CO3) group to decrease the Mn carbonate centers (which are responsible for the enhanced fluorescence) leading to the decrease in the fluorescence. The possible Hg-MnCO3 combination and the destruction of active centers were indicated by the changes in the Mn 2p XPS spectrum. A slight increase in Mn bonding energy indicated the oxidation of Mn, implying the possibility of structural change from Mn carbonate to Mn oxide (Figure 4F). In Figure S12, the adsorption peaks demonstrated a significant decline in intensity, further indicating the change in functional groups of Mn-CDs. From the TEM results (Figure S13), it can also clearly seen that before addition of Hg2+ (Figure S13A), Mn-CDs were well dispersed, however, after addition Hg2+ (Figure S13B) the Mn-CDs are aggregated to large complex form, and because of the size effect, the fluorescence of CDs were quenched. If the quenching is through the formation of Hg-C or Hg-MnCO3 complexes, the addition of a species with higher affinity to Hg should release the complex to reinstate the fluorescence. We investigated this possibility by the addition of Na2S, with S2- having high affinity to Hg. As illustrated in Figure S14A, B, the addition of


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Na2S into the quenched Mn-CDs resulted in the gradual restoration of the fluorescence intensity. As mentioned earlier, S2- selectively interacted with the Hg2+ ion on the surface of carbon dots due to the comparatively higher affinity forms complex releasing the Mn-CDs, leading to the fluorescence restoration. It is expected that there is a chemical equilibrium between S2- and Mn-CDs to combine with Hg2+.

Figure 4. (A) The florescence intensity of Mn-CDs in the presence of 50 µM of various metal ions. (B) The variation of ﬂuorescence intensity of Mn-CDs as a function of the concentration of Hg2+ (100 nM to 1 µM). The inset shows the same relationship in the concentration range from 100 nM to 100 µM. (C-F) High-resolution XPS spectra for MnCDs before and after quenching process demonstrating the change in chemical nature; C 1S(C), O 1S(D), Hg 4f (E), Mn 2p (F). 4

CONCLUSIONS


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In summary, highly photo-stable Mn-CDs with reversibly switchable blue fluorescence were successfully synthesized, for the first time, from sodium citrate and manganese carbonate using citric acid as the auxiliary dispersant. Mn-CDs thus prepared exhibited a high fluorescence quantum yield (>54.4%), a value which, to the best of our knowledge, is the highest for metal-doped CDs. The mechanism of the enhanced photoluminescence quantum yield (PLQY) of Mn-CDs was elucidated via employing a suite of advanced spectroscopic techniques and complimentary frontier orbital calculations. The existence of a transfer stage from metal oxide to metal carbonate during the formation of the Mn-CDs was identified for the first time, which is believed to be the critical factor determining the photoluminescence and PLQY of Mn-CDs. In addition, the Mn-CDs with stable fluorescence over a broad range of pH and various metal ions exhibited a highly beneficial capability to reversibly switch its luminescence in the presence of specific ions. The fundamental mechanism of the photoluminescence switching was elucidated using the high-resolution XPS analysis. The superior luminescence properties of Mn-CDs were utilized for the reversible nanomolar detection of Hg2+, showing great promise for Mn-CDs to be used as reusable UFPs. The indepth understanding of luminescence mechanism for metal-doped CDs with reversible fluorescence and high QY is expected to guide us to explore advanced CD-based UFPs with overall size profiles comparable to prevailing coded fluorescent tags. Further, the multifunctionality of the metal-doped CDs is also useful for other research fields, including energy harvesting and generation. Supporting Information. More characterization data on the optimized conditions, characterization, detection of MnCDs (Figures S1-14). This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author * Address correspondence to: [email protected], [email protected], [email protected] Conflict of Interest The authors declare no conflict of interest. ACKNOWLEDGMENT This research received financial support from National Key Research and Development Plan(No. 2016YFC0303701), Beijing Nova Program (Z171100001117058), Beijing Nova Program Cross Discipline Cooperation Project (Z181100006218138), Science Foundation of China University of Petroleum(Beijing) (No. 2462018BJC004). REFERENCES: (1) Xu, X.; Ray, R.; Gu, Y.; Ploehn, H. J.; Gearheart, L.; Raker, K.; Scrivens, W. A., Electrophoretic Analysis and Purification of Fluorescent Single-Walled Carbon Nanotube Fragments. J. Am. Chem. Soc. 2004, 126, 12736-12737. (2) Pierrat, P.; Wang, R.; Kereselidze, D.; Lux, M.; Didier, P.; Kichler, A.; Pons, F.; Lebeau, L., Efficient Invitro and In vivo Pulmonary Delivery of Nucleic Acid by Carbon Dot-Based Nanocarriers. Biomaterials 2015, 51, 290-302. (3) Yang, Y.; Ji, X.; Jing, M.; Hou, H.; Zhu, Y.; Fang, L.; Yang, X.; Chen, Q.; Banks, C., Carbon Dots Supported Upon N-Doped TiO2 Nanorods Applied into Sodium and Lithium Ion Batteries. J. Mater. Chem. A 2015, 3, 5648-5655. (4) Ding, H.; Yu, S. B.; Wei, J. S.; Xiong, H. M., Full-Color Light-Emitting Carbon Dots with a SurfaceState-Controlled Luminescence Mechanism. Acs Nano 2015, 10, 484-491. (5) Wang, Y.; Kalytchuk, S.; Wang, L.; Zhovtiuk, O.; Cepe, K.; Zboril, R.; Rogach, A. L., Carbon Dot Hybrids with Oligomeric Silsesquioxane: Solid-State Luminophores with High Photoluminescence


20

Page 21 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Quantum Yield and Applicability in White Light Emitting Devices. Chem. Commun. 2015, 51, 29502953. (6) Li, S.; Wang, L.; Chusuei, C. C.; Suarez, V. M.; Blackwelder, P. L.; Micic, M.; Orbulescu, J.; Leblanc, R. M., Nontoxic Carbon Dots Potently Inhibit Human Insulin Fibrillation. Chem. Mater. 2015, 27, 1764-1771. (7) Bourlinos, A. B.; Trivizas, G.; Karakassides, M. A.; Baikousi, M.; Kouloumpis, A.; Gournis, D.; Bakandritsos, A.; Hola, K.; Kozak, O.; Zboril, R.; Papagiannouli, I.; Aloukos, P.; Couris, S., Green and Simple Route toward Boron Doped Carbon Dots with Significantly Enhanced Non-Linear Optical Properties. Carbon 2015, 83, 173-179. (8) Zhou, L.; Fu, P.; Wang, Y.; Sun, L.; Yuan, Y., Microbe-Engaged Synthesis of Carbon Dot-Decorated Reduced Graphene Oxide as High-Performance Oxygen Reduction Catalysts. J. Mater. Chem. A 2016, 4, 7222-7229. (9) Zheng, D. W.; Li, B.; Li, C. X.; Fan, J. X.; Lei, Q.; Li, C.; Xu, Z.; Zhang, X. Z., Carbon-DotDecorated Carbon Nitride Nanoparticles for Enhanced Photodynamic Therapy against Hypoxic Tumor Via Water Splitting. Acs Nano 2016, 10, 8715-8722. (10) Wang, J.; Zhang, W.; Yue, X.; Yang, Q.; Liu, F.; Wang, Y.; Zhang, D.; Li, Z.; Wang, J., One-Pot Synthesis of Multifunctional Magnetic Ferrite–MoS2–Carbon Dot Nanohybrid Adsorbent for Efficient Pb(II) Removal. J. Mater. Chem. A 2016, 4, 3893-3900. (11) Maaoui, H.; Kumar, P.; Kumar, A.; Pan, G. H.; Chtourou, R.; Szunerits, S.; Boukherroub, R.; Jain, S. L., A Prussian Blue/Carbon Dot Nanocomposite as an Efficient Visible Light Active Photocatalyst for C–H Activation of Amines. Photochem. Photobiol. Sci. 2016, 15, 1282-1288. (12) B. Essner, J.; N. McCay, R.; II, C. J. S.; M. Cobb, S.; H. Laber, C.; A. Baker, G., A Switchable Peroxidase Mimic Derived from the Reversible Co-Assembly of Cytochrome C and Carbon Dots. J. Mater. Chem. B 2016, 4, 2163-2170. (13) Yang, L.; Wang, Z.; Wang, J.; Jiang, W.; Jiang, X.; Bai, Z.; He, Y.; Jiang, J.; Wang, D.; Yang, L., Doxorubicin Conjugated Functionalizable Carbon Dots for Nucleus Targeted Delivery and Enhanced Therapeutic Efficacy. Nanoscale 2016, 8, 6801-6809. (14) Miao, H.; Wang, L.; Zhuo, Y.; Zhou, Z.; Yang, X., Label-Free Fluorimetric Detection of Cea Using Carbon Dots Derived from Tomato Juice. Biosens. Bioelectron. 2016, 86, 83-89.


21


Page 22 of 26

(15) Huang, J. J.; Zhong, Z. F.; Rong, M. Z.; Zhou, X.; Chen, X. D.; Zhang, M. Q., An Easy Approach of Preparing Strongly Luminescent Carbon Dots and Their Polymer Based Composites for Enhancing Solar Cell Efficiency. Carbon 2014, 70, 190-198. (16) Li, X.; Rui, M.; Song, J.; Shen, Z.; Zeng, H., Carbon and Graphene Quantum Dots for Optoelectronic and Energy Devices: A Review. Adv. Funct. Mater. 2015, 25, 4929-4947. (17) Feng, T.; Ai, X.; An, G.; Yang, P.; Zhao, Y., Charge-Convertible Carbon Dots for Imaging-Guided Drug Delivery with Enhanced in Vivo Cancer Therapeutic Efficiency. Acs Nano 2016, 10, 44104420. (18) Gao, W.; Ma, Y.; Zhou, Y.; Song, H.; Li, L.; Liu, S.; Liu, X.; Gao, B.; Liu, C.; Zhang, K., High Photoluminescent Nitrogen-Doped Carbon Dots with Unique Double Wavelength Fluorescence Emission for Cell Imaging. Mater. Lett. 2018. (19) Gao, W.; Song, H.; Wang, X.; Liu, X.; Pang, X.; Zhou, Y.; Gao, B.; Peng, X., Carbon Dots with Red Emission for Sensing of Pt2+, Au3+, and Pd2+ and Their Bioapplications in Vitro and in Vivo. ACS Appl. Mater. Interfaces 2018, 10,1147-1154. (20) Liu, X. Y.; Chen, H.; Wang, R.; Shang, Y.; Zhang, Q.; Li, W.; Zhang, G.; Su, J.; Dinh, C. T.; de Arquer, F., 0D–2D Quantum Dot: Metal Dichalcogenide Nanocomposite Photocatalyst Achieves Efficient Hydrogen Generation. Adv. Mater. 2017, 29, 1605646. (21) Luo, P. G.; Sahu, S.; Yang, S.-T.; Sonkar, S. K.; Wang, J.; Wang, H.; LeCroy, G. E.; Cao, L.; Sun, Y.-P., Carbon "Quantum" Dots for Optical Bioimaging. J. Mater. Chem. B 2013, 1, 2116-2127. (22) Cao, L.; Meziani, M. J.; Sahu, S.; Sun, Y. P., Photoluminescence Properties of Graphene Versus Other Carbon Nanomaterials. Acc. Chem. Res. 2013, 46, 171-180. (23) Pinaud, F.; Clarke, S.; Sittner, A.; Dahan, M., Probing Cellular Events, One Quantum Dot at a Time. Nat. Methods 2010, 7, 275-285. (24) Zhang, J. J.; Cheng, F. F.; Li, J. J.; Zhu, J. J.; Lu, Y., Fluorescent Nanoprobes for Sensing and Imaging of Metal Ions: Recent Advances and Future Perspectives. Nano Today 2016, 11, 309-329. (25) Zhao, P.; Zhou, L.; Nie, Z.; Xu, X.; Li, W.; Huang, Y.; He, K.; Yao, S., Versatile Electrochemiluminescent Biosensor for Protein-Nucleic Acid Interaction Based on the Unique Quenching Effect of Deoxyguanosine-5′-Phosphate on Electrochemi-luminescence of Cdte/Zns Quantum Dots. Anal. Chem. 2013, 85, 6279-6286.


22

Page 23 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(26) Shi, D.; Yan, F.; Zheng, T.; Wang, Y.; Zhou, X.; Chen, L., P-Doped Carbon Dots Act as a Nanosensor for Trace 2,4,6-Trinitrophenol Detection and a Fluorescent Reagent for Biological Imaging. RSC Adv. 2015, 5, 98492-98499. (27) Ye, Q.; Yan, F.; Shi, D.; Zheng, T.; Wang, Y.; Zhou, X.; Chen, L., N, B-Doped Carbon Dots as a Sensitive Fluorescence Probe for Hg2+ Ions and 2,4,6-Trinitrophenol Detection for Bioimaging. J. Photochem. Photobiol., B 2016, 162, 1-13. (28) Dong, Y.; Pang, H.; Yang, H. B.; Guo, C.; Shao, J.; Chi, Y.; Li, C. M.; Yu, T., Carbon-Based Dots Co-Doped with Nitrogen and Sulfur for High Quantum Yield and Excitation‐Independent Emission. Angew. Chem. Int. Ed. 2013, 52, 7800-7804. (29) Gong, Y.; Yu, B.; Yang, W.; Zhang, X., Phosphorus, and Nitrogen Co-Doped Carbon Dots as a Fluorescent Probe for Real-Time Measurement of Reactive Oxygen and Nitrogen Species inside Macrophages. Biosen. Bioelectron. 2016, 79, 822-828. (30) Xu, Q.; Liu, Y.; Su, R.; Cai, L.; Li, B.; Zhang, Y.; Zhang, L.; Wang, Y.; Wang, Y.; Li, N., Highly Fluorescent Zn-Doped Carbon Dots as Fenton Reaction-Based Bio-Sensors: An Integrative Experimental-Theoretical Consideration. Nanoscale 2016, 8, 17919-17927. (31) Xu, Q.; Wei, J.; Wang, J.; Liu, Y.; Li, N.; Chen, Y.; Gao, C.; Zhang, W., Facile Synthesis of Copper Doped Carbon Dots and Their Application as a “Turn-Off” Fluorescent Probe in the Detection of Fe3+ Ions. RSC Adv. 2016, 6, 28745-28750. (32) Qu, S.; Zhou, D.; Li, D.; Ji, W.; Jing, P.; Han, D.; Liu, L.; Zeng, H.; Shen, D., Toward Efficient Orange Emissive Carbon Nanodots through Conjugated Sp(2) -Domain Controlling and Surface Charges Engineering. Adv. Mater. 2016, 28, 3516-3521. (33) Yeh, T. F.; Teng, C. Y.; Chen, S. J.; Teng, H., Nitrogen-Doped Graphene Oxide Quantum Dots as Photocatalysts for Overall Water-Splitting under Visible Light Illumination. Adv. Mater. 2014, 26, 3297-3303. (34) Zhou, J.; Booker, C.; Li, R.; Zhou, X.; Sham, T. K.; Sun, X.; Ding, Z., An Electrochemical Avenue to Blue Luminescent Nanocrystals from Multiwalled Carbon Nanotubes. J. Am. Chem. Soc. 2007, 129, 744-745.


23


Page 24 of 26

(35) Yuan, F.; Wang, Z.; Li, X.; Li, Y.; Tan, Z. A.; Fan, L.; Yang, S., Bright Multicolor Bandgap Fluorescent Carbon Quantum Dots for Electroluminescent Light-Emitting Diodes. Adv. Mater. 2017, 29, 1604436. (36) Sahu, S.; Behera, B.; Maiti, T. K.; Mohapatra, S., Simple One-Step Synthesis of Highly Luminescent Carbon Dots from Orange Juice: Application as Excellent Bio-Imaging Agents. Chem. Commun. 2012, 48, 8835-8837. (37) Tang, L.; Ji, R.; Cao, X.; Lin, J.; Jiang, H.; Li, X.; Teng, K. S.; Chi, M. L.; Zeng, S.; Hao, J., Deep Ultraviolet Photoluminescence of Water-Soluble Self-Passivated Graphene Quantum Dots. Acs Nano 2012, 6, 5102-5110. (38) Luo, Z.; Lu, Y.; Somers, L. A.; Johnson, A. T., High Yield Preparation of Macroscopic Graphene Oxide Membranes. J. Am. Chem. Soc. 2009, 131, 898-899. (39) Eda, G.; Lin, Y. Y.; Mattevi, C.; Yamaguchi, H.; Chen, H. A.; Chen, I. S.; Chen, C. W.; Chhowalla, M., Blue Photoluminescence from Chemically Derived Graphene Oxide. Adv. Mater. 2010, 22, 505509. (40) Lai, I. P.-J.; Harroun, S. G.; Chen, S.-Y.; Unnikrishnan, B.; Li, Y.-J.; Huang, C.-C., Solid-State Synthesis of Self-Functional Carbon Quantum Dots for Detection of Bacteria and Tumor Cells. Sens. Actuators, B 2016, 228, 465-470. (41) Lim, S. Y.; Shen, W.; Gao, Z., Carbon Quantum Dots and Their Applications. Chem. Soc. Rev. 2014, 44, 362. (42) Le, T. H.; Ying, Y.; Liu, Y.; Huang, Z.; Kang, F., In-Situ Growth of Mno2 Crystals under NanoporeConstraint in Carbon Nanofibers and Their Electrochemical Performance. Sci. Rep. 2016, 6, 37368. (43) Mathieu Toupin; Thierry Brousse; Daniel Bélanger, Charge Storage Mechanism of MnO2 Electrode Used in Aqueous Electrochemical Capacitor. Chem. Mater. 2004, 16, 3184-3190. (44) Guo, Y.; Zhang, L.; Liu, X.; Li, B.; Tang, D.; Liu, W.; Qin, W., Synthesis of Magnetic Core-Shell Carbon Dots@Mfe2o4 (M = Mn, Zn and Cu) Hybrid Materials and Their Catalytic Properties. J. Mater. Chem. A 2016, 4, 4044-4055. (45) Xi, L.; Schwanke, C.; Xiao, J.; Abdi, F. F.; Zaharieva, I.; Lange, K. M., In Situ L-Edge Xas Study of a Manganese Oxide Water Oxidation Catalyst. J. Phys. Chem. C 2017, 121, 12003-12009.


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Page 25 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(46) Anjum, G.; Ravi, K.; Mollah, S.; Thakur, P.; Gautam, S.; Chae, K. H., Nexafs Studies of La0.8 Bi0.2 Fe1−X Mn X O3 (0.0 ≤ X ≤ 0.4) Multiferroic System Using X-Ray Absorption Spectroscopy. J. Phys. D: Appl. Phys. 2011, 44, 075403. (47) Luo, X.; Lee, W.-T.; Xing, G.; Bao, N.; Yonis, A.; Chu, D.; Lee, J.; Ding, J.; Li, S.; Yi, J., Ferromagnetic Ordering in Mn-Doped Zno Nanoparticles. Nanoscale Res. Lett. 2014, 9, 625-625. (48) De Groot, F. M. F.; Fuggle, J. C.; Thole, B. T.; Sawatzky, G. A., 2p X-Ray Absorption of 3d Transition-Metal Compounds: An Atomic Multiplet Description Including the Crystal Field. Phys. Rev. B 1990, 42, 5459-5468. (49) Xu, Q.; Liu, Y.; Gao, C.; Wei, J.; Zhou, H.; Chen, Y.; Dong, C.; Li, N.; Xia, Z., Synthesis, Mechanistic Investigation, and Application of Photoluminescent Sulfur and Nitrogen Co-Doped Carbon Dots. J. Mater. Chem. C 2015, 3, 9885-9893. (50) Zhang, R.; Chen, W., Nitrogen-Doped Carbon Quantum Dots: Facile Synthesis and Application as a "Turn-Off" Fluorescent Probe for Detection of Hg2+ Ions. Biosens. Bioelectron. 2014, 55, 83-90. (51) Xavier, S. S. J.; Siva, G.; Annaraj, J.; Kim, A. R.; Yoo, D. J.; kumar, G. G., Sensitive and Selective Turn-Off-on Fluorescence Detection of Hg2+ and Cysteine Using Nitrogen Doped Carbon Nanodots Derived from Citron and Urine. Sens. Actuators, B 2018, 259, 1133-1143.


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Table of Contents Graphic

A novel manganese doped CDs (Mn-CDs) was fabricated which exhibited an ultra-high quantum yield of 54% and used as mercury ion detection


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Metal Charge Transfer Doped Carbon Dots with Reversibly

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