Rapid Screening of Oxygen States in Carbon Quantum Dots by

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Rapid Screening of Oxygen-States in Carbon Quantum Dots by Chemiluminescence Probe Shaoqing Dong, Zhiqin Yuan, Lijuan Zhang, Yanjun Lin, and Chao Lu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b03711 • Publication Date (Web): 31 Oct 2017 Downloaded from http://pubs.acs.org on November 1, 2017

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Rapid

Analytical Chemistry

Screening

of

Oxygen-States

in

Carbon

Quantum

Dots

by

Chemiluminescence Probe

Shaoqing Dong, Zhiqin Yuan, Lijuan Zhang, Yanjun Lin and Chao Lu*

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China

*E-mail: [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT: Oxygen-states (O-states) of carbon quantum dots (CDs) play an important role on their optical properties and analytical applications. However, the rapid screening of O-states in CDs is still a great challenge because of the complicated surface composition. In this study, it is found that chemiluminescence (CL) intensity of prepared CDs in the presence of peroxynitrite (ONOO−) is proportional to the content of C–O group-related O-states. The related mechanism discloses that the O-state dependent CL is assigned to the reason that abundant C–O functional groups in CDs with high O-states could facilitate the electron transfer of the produced smaller energy gaps for strong CL emission. Hence, ONOO−-induced CL can be utilized as a facile probe for the rapid screening of O-states in CDs with some advantages, such as rapid response, low cost and easy operation. Its practicability is verified by detecting the CL of phosphorus-doped CDs with variable phosphorus doping contents. The content of C–O group-related O-states in sulfur/phosphorus-doped CDs measured by the proposed CL probe is consistent with by X-ray photoelectron spectroscopy (XPS) characterization. This strategy can also be extended to distinguish O-states in different types of nanoparticles via tuning the CL probe molecules.

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INTRODUCTION: The distinct properties and applications of luminescent carbon quantum dots (CDs) are highly dependent on their surface states.1-4 For instance, CDs with high quantum yield or photocatalytic activity can be easily prepared through altering their oxygen-states (O-states, a kind of surface states related to oxygenous functional groups on the surface of CDs, such as C–O, C=O, O–C=O and etc.).5-7 Additionally, O-state mediated multicolor CDs have also been successfully applied for bio-imaging,8-10 and a versatile platform for multiplex high-throughput screening or multicolor coding using these CD probes is promised. Therefore, it is of great importance to investigate the O-states of CDs for understanding and even regulating their photophysical and chemical properties. Currently, a number of sophisticated techniques, including electron energy loss spectroscopy,11,12 X-ray photoelectron spectroscopy (XPS),13 Near-Edge X-ray Absorption Fine Structure,14 have been performed to measure O-states of nanoparticles. These reported methods show acceptable capacities on discriminating O-states, but usually require harsh working conditions (e.g., high vacuum) or professionals to manipulate these advanced devices. In addition, high cost and time-consuming characters of these methods are also appeared. Thereby, it is of great significance to exploit a simple and rapid approach to preferably characterize O-states of CDs. Chemiluminescence (CL) in aqueous phase systems is concerning the emission of luminophore, which undergoes radiative relaxation to the ground state upon redox reactions and electron transfer processes.15 On the basis of its excellent characters, including absent excitation light source, low background, simple instrumentation and fast response, CL has attracted growing interests in sensitive detection of analytes in recent decades.16-18 On the other hand, CL can also act as one of the rapid spectroscopic techniques for nanomaterial characterization.19-21 For example, a rapid evaluation method for commonly heterogeneous base catalysts in biodiesel production was successfully realized using acetone cataluminescence as an indicator.19 In 3



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addition, it is reported that the cataluminescence intensity of diethyl ether oxidation reaction on the surface of TiO2 nanoparticles shows direct connection to the content of oxygen vacancies.20 Therefore, diethyl ether cataluminescence can act as an optical probe for the rapid and sensitive detection of oxygen vacancies in TiO2 nanoparticles. Meanwhile, Sun et al. achieved the detection of sulphur vacancy in CdS nanoparticles by utilizing electrogenerated CL.21 These interesting results indicate that CL possesses large potential in nanomaterial characterization. It has been found that the ONOO−-induced CL of CDs was closely related to surface states (especially oxygenous functional groups) of CDs.22 Herein, we intentionally controlled the content of O-states through doping sulfur into CDs and the prepared CDs showed similar morphologies and photoluminescence (PL) characteristics but different surface properties, as supported by PL, UV−vis absorption, Fourier-transform infrared spectroscopy and high-resolution transmission electron microscope (HRTEM). Interestingly, we found that the ONOO−-induced CL intensity is proportional to the content of C–O group-related O-states in sulfur-doped CDs (S-CDs). This O-state indication of CL emission is attributed to electron-hole recombination of CDs in the presence of the oxidizing (•OH/OH−) and reducing (O2•−/O2) radical pair from the decomposition of ONOO− (Scheme 1). The content of C–O group-related O-states in S-CDs obtained by the proposed CL probe were well matched with those obtained by XPS technique. Therefore, the ONOO−-induced CL can be employed as a simple probe for screening O-states in CDs. Finally, the universality of this proposed method was validated by detecting the O-states of phosphorus-doped CDs (P-CDs). The correlation between CL and C–O group-related O-states in P-CDs was consistent with that in S-CDs. With the outstanding properties of the CL technique, the proposed method could become an alternative to the already-developed techniques for evaluating O-states in different nanomaterials.

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Scheme 1. Schematic illustration for CL emission induced by electron-hole injection of the S- and P-doped CDs from ONOO− decomposed radical pair.

EXPERIMENTAL SECTION Chemicals and Materials. Citric acid, urea, thiourea, ascorbic acid (AA), sodium hydroxide (NaOH), sodium sulfate (Na2SO4), sodium phosphate (Na3PO4), sodium hypochlorite (NaClO), dimethylsulfoxide (DMSO), potassium superoxide (KO2), ferrous sulfate (Fe(SO4)2), sodium nitroferricyanide (SNP), hydrogen peroxide (H2O2, 30 wt%), concentrated sulphuric acid (H2SO4, 95%, v/v), phosphoric acid (H3PO4, 85%, v/v), nitric acid (HNO3, 65%, v/v), and hydrochloric acid (HCl, 36%, v/v) were supplied by Beijing Chemical Reagent Company (Beijing, China). p-Benzoquinone (p-BQ) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). Sodium nitrite (NaNO2) and sodium azide (NaN3) were obtained from Tianjin Fuchen Chemical Reagent Co., Ltd. (Tianjin, China). Perfluorinated resin solution (nafion, 5 wt%) were obtained from USA Sigma-Aldrich Reagent Co., Ltd. (Milwaukee, USA). All reagents were analytical grade and used without further purification. All solutions throughout the experiments were prepared with ultrapure water (18.2 MΩcm, Milli Q, Millipore, Barnstead, CA, USA). All glassware was cleaned by fresh aqua regia. Singlet oxygen (1O2) was generated from the mixing of H2O2 and NaClO (H2O2: NaClO = 1: 1). Hydroxyl radical (•OH) was produced by Fenton reaction, mixing H2O2 and 10-exceed Fe(SO4)2. Superoxide anion 5



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(O2•−) was from KO2 into anhydrous DMSO. Nitric oxide (NO) was prepared by SNP. ONOO− solution was freshly produced by reacting of NaNO2 and H2O2-HCl solution, followed by adding NaOH solution. Synthesis of Sulfur or Phosphorus-Doped CDs. CDs were prepared with a facile microwave route according to previously reported procedures with slight modification.23 Briefly, 3.0 g citric acid and 6.0 g urea were dissolved in 10 mL ultrapure water to form a transparent solution and was then heated in the microwave oven (750 W) for 5.0 min. The colorless solution changed into brown and finally turned to dark solid. This solid was then heated at 65 ˚C for 1 h in a vacuum oven to remove any volatile small molecules. After that, 20 mL ultrapure water was added to disperse the solid. The CD solution was purified through centrifugation (5000 r/min, 20 min) to remove insoluble particles, and the residual solution was then dialyzed using a 1000 Da MWCO dialysis bag in water for 48 h to remove the large particles. The obtained dialysate was then freeze-dried for the next applications. S-CDs with different doping contents (0–2.7 wt %) were synthesized with similar strategies. First, 3.0 g citric acid and 6.0 g urea were dissolved in 10 mL ultrapure water to form a transparent solution. And then, a certain amount of H2SO4 was added into the solution. The resulted solution was then heated in the microwave oven (750 W) for 5.0 min. P-CDs with different doping contents (0–2.5 wt%) were obtained by replacing H2SO4 with H3PO4. The prepared S-CDs and P-CDs were purified with the same procedure as referred above. Through simple doping, S-CDs (S doping content, 0.3, 1.5 and 2.7 wt%) and P-CDs (P doping content, 0.9, 1.7 and 2.5 wt%) were prepared, respectively. Instruments and Methods. The PL spectra were performed using a F−7000 fluorescence spectrophotometer (Hitachi, Japan). The UV−vis absorption spectra were collected with UV−3600 spectrophotometer (Shimadzu, Japan). Fourier-transform infrared (FT−IR) spectra were examined by a Nicolet 6700 FT−IR spectrometer (Thermo, U.S.A.). Morphology and size of CDs was performed on a JEM-3010 HRTEM (Hitachi, Japan). XPS measurements were performed using an ESCALAB D250 instrument with a monochromatic Al Kα X-ray source 6


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(Thermo, U.S.A.). The CL detection was conducted on a biophysics chemiluminescence (BPCL) analyzer (Institute of Biophysics, Chinese Academy of Science, China). CL spectra were obtained with F−7000 fluorescence spectrophotometer by turning off the xenon lamp. The cyclic voltammetry electrochemical measurements were supported by a CHI660E electrochemical workstation (Shanghai Chenhua Instrument, China). CL Measurements. Typically, 200 μL NaNO2 (400 μM) was added into 200 μL the mixing solution of H2O2 (4.0 mM) and HCl (20 mM). And then, 10 μL NaOH (0.5 M) was added. Next, 100 μL ONOO− solution was rapidly mixed with 100 μL CD solution (1.0 mg/mL) in a small quartz vial by using microsyringe. As shown in Figure S1, the on-line ONOO− concentration by NaNO2 and H2O2-HCl solution was quantified by UV–vis absorption measurements.24,25 According to absorbance at 302 nm, the concentration of ONOO− from 2.0 mM NaNO2 was calculated to be 100 μM based on Lambert-Beer and Kubelk-Munk theories (A= εbC, ε = 1670 M-1 cm-1). All of CD solutions were prepared by dissolving the CD powder in PBS buffer (50 mM, pH = 7.4). To test the CL response dynamics and reproducibility, the CL intensity of 1.0 mg/mL P-CDs in the present 100 μM ONOO− was measured by a photomultiplier tube (PMT). The work voltage of the BPCL analyzer was set at –1000 V and a data integration time of 0.1 s was used. Cyclic Voltammetry Measurements. A conventional three-electrode system was used for the determination of cyclic voltammetry. The saturated reference electrode was Ag/AgCl electrode, platinum wire as the counter electrode and the working electrode was glassy carbon electrode (GCE). The electrolyte was prepared with 50 mM PBS. The modified GCE was prepared by dropping the mixture of CDs and nafion on the surface of electrode, then the surface of electrode was dried via ultrapure N2 blow. The scanning voltage ranged from –1.8 V to 2.0 V.

RESULTS AND DISCUSSION O-State Dependent CL. Heteroatom doping was usually used to modulate the O-states of 7



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CDs.6 In situ generation of O-states could be accurately controlled based on heteroatom doping, avoiding over surface hybridigation compared to redox method.26 The C–O group-related O-states of CDs often lead to the production of smaller energy separation and affect the CL performance.27 Hence, the CDs with different S doping contents were synthesized by microwave to investigate the impact of O-states on the ONOO−-induced CL. The FT−IR spectra of the as-prepared S-CDs showed visible broad absorption bands around 3000–3500 cm-1 that were stretching vibrations of O–H and N–H (Figure 1A).28 The appearance of these bands indicated that lots of amino and hydroxyl groups existed on the surface of S-CDs. In addition, C=C stretching vibration, C=O stretching vibration and C–O stretching vibration absorption bands were also observed around 1605, 1710 and 1060 cm-1, respectively.29,30 Moreover, it was seen that two peaks around 1640 and 1443 cm-1 belonged to typical stretching vibration of C=N and C–N,31,32 suggesting the formation of carbon-nitrogen bonds on the surface of S-CDs. Note that peak intensity at 1083 cm-1 that attributed to assigned to the C=S increased with the increase of S contents, demonstrating the successfulness of S doping in CDs.28 To further prove the S doping, XPS characterizations of CDs and S-CDs were performed. The full scan XPS spectra of the CDs and S-CDs were displayed in Figure S2A. It can be seen that the weak binding energy peaks assigned to S 2p appeared around 164 eV.33 As for the S 2p XPS spectra, the peaks located at 163.8 eV and ~165.0 eV belonged to 2p3/2 and 2p1/2. These results were agreed with those of –C–S–C– covalent bond from thiophene. In addition to C–S bond, S=O bond with S 2p binding energy of 168.2 eV was also observed (Figure S2B).34 The XPS results, as well as FT−IR spectra demonstrated that S was successfully incorporated into CDs via the formation of thiophene units. From the UV–vis spectra (Figure 1B), the prepared CDs exhibited a broad absorption band centered at 410 nm, which might come from electron transition of surface states.35 Furthermore, the absorbance at 410 nm gradually decreased with the increased S content in CDs, suggesting that the absorption peak of 410 nm is not originated 8


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from the n-π* transition of S=O bonds. Thus, it is concluded that the absorption peak of S-CDs at 410 nm depended on O-states, and S doping could lead to the decreasing of O-states.6 As shown in Figure 1C, the HRTEM images showed that S-CDs possessed the similar size distributions around 5 nm, implying that S doping content did not affect the size of prepared CDs. The synthesized S-CDs were transferred into the round bottom quartz bottle for CL measurements by injecting ONOO− (Figure S3). Interestingly, a clear decrease in the CL emission from S-CDs–ONOO− system appeared upon increasing S doping contents from 0 to 2.7 wt% (Figure 1D). The influence of SO42- on the CL intensity of CDs was further investigated. As shown in Figure S4A, no change in the CL intensity of CDs was observed in the presence of SO42-. It is generally accepted that both surface states and quantum sizes may contribute to the CL properties of quantum dots.27,36 Taking the similar size into consideration, we concluded that the decreased CL emission of S-CDs may be attributed to the decreasing of C–O group-related O-states. On the other hand, we investigated the CL responses of the CDs and S-CDs towards different reactive oxygen species (ROS) and nitrogen species (RNS). As shown in Figure S5, the CL signals of ONOO−-triggered CDs were significantly stronger than those triggered by other ROS and RNS, including O2•−, H2O2, ClO−, and NO. Although 1O2 and •OH were able to induce weak CL emission of CDs, there were no obvious differences in the CL signals towards the variable O-states in CDs. Therefore, ONOO− was chosen as an indicator for screening the O-states in CDs.

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Figure 1. (A) FT–IR spectra, (B) UV–vis absorption spectra and (C) HRTEM images of S-CDs with different S doping contents. (D) CL intensity of S-CDs (1.0 mg/mL) with different S doping contents in the presence of 100 μM ONOO−.

To verify this presumption, O 1s energy level XPS spectra of S-CDs with the variable S doping contents (0, 0.3, 1.5, and 2.7 wt%) were also investigated (Figure 2). After the deconvolution of O 1s spectra, two binding energy peaks around 531.6 and 533.1 eV ascribed to C=O and C–O appeared.13,37 This result demonstrated the existence of oxygenous functional groups on the surface of the as-prepared S-CDs. On the other hand, the relative content of C–O functional groups in S-CDs decreased from 37.4% to 7.7% when increasing S doping contents from 0 to 2.7 wt% (Table S1). Such a phenomenon indicated that the introduction of S was able to diminish the C–O group-related O-state content in CDs. Therefore, the variation trend of CL signals (Figure S6) was coincided with the change of C–O group-related O-states of CDs.

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Figure 2. High-resolution XPS spectra of the O 1s peaks of S-CDs with various S doping contents: (A) 0%, (B) 0.3%, (C) 1.5% and (D) 2.7%, respectively.

Emitting Species. To gain insight into the CL mechanism of the as-prepared S-CDs, the CL measurements of carbon sources and nitrogen sources were also performed. Only ultra-weak CL emissions in the presence of citric acid or urea appeared, showing that carbon sources and nitrogen sources were not responsible for the CL signals (Figure S7) and the S-CDs were the main source of CL emissions. In addition, the CL spectra of ONOO− system in the presence of the CDs and S-CDs showed a maximum emission wavelength at 520 nm (Figure 3A), which was in accordance with the PL spectra of the CDs and S-CDs (Figure S8). The overlap of the CL and PL spectra demonstrated that the major emitting species were excited-state CDs, which were generated by the reaction between the CDs and ONOO−. It is reported that the emission of CDs–ONOO− system may be attributed to the electron-hole 11



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recombination of CDs or energy transfer between excited peroxynitrous acid (ONOOH*) and CDs.38 To figure out the CL emission mechanism in the present system, the scavengers experiments for ROS were conducted. In this study, four scavengers, including AA, NaN3, p-BQ and thiourea were tested. As showed in Figure 3B, the CL signals were completely inhibited upon adding AA into the proposed CL system, suggesting the presence of ROS took a key role in the generation of the emitting species.22 While NaN3 had no effect to the CL signals, indicating that the observed CL was not originated from 1O2.38 Interestingly, with the addition of p-BQ and thiourea (a typical trapping agent for O2•− and •OH, respectively), the CL signals were greatly quenched, demonstrating that O2•− and •OH contributed to the CL emission of this system. Therefore, the emission of CDs–ONOO− system might come from the electron-transfer annihilation of CDs•− and CDs•+ in the presence of oxidizing (•OH) and reducing (O2•−) radical. Mechanism of O-State Dependent CL. To further understand the mechanism of O-state dependent CL in this CL system, the cyclic voltammetry was applied to monitor the changes in the energy levels of the S-CDs. Cathodic peaks at −0.62 ~ −0.68V were observed from S-CDs with different S doping contents (Figure 3C). The results might be due to the formation of CD radical anions (CDs•−).39,40 The onset potential of CDs was more positive than that of S-CDs with higher S doping content, which was ascribed to the reduced O-states in S-CDs upon increasing S doping contents. It was reported that abundant C–O group-related O-states in CDs can produce more of smaller energy gaps between LUMO and HOMO levels.41 Meanwhile, more positive reduction potential and higher reduction current induced by smaller energy gaps may accelerate electron transfer between the electron/hole donors and CDs.42 On the contrary, the low C–O group-related O-states content can reduce the formation of smaller energy gaps, leading to more negative potential and lower reduction current. The LUMO energy level (ELUMO) was calculated to be −4.28 ~ −4.25 eV by the following equation: ELUMO = − (Ered + 4.71) eV,43 where the onset reduction potential (Ered) was determined to be −0.43 ~ −0.46 V. The HOMO energy level (EHOMO) of CDs was calculated to be −6.80 ~ −6.75 eV according to the following equation: EHOMO = 12


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ELUMO − Eg (the band structure parameters were showed in Table S2).44 The optical bandgap (Eg) was calculated using the equation: Eg = 1240/λg (where λg is absorption edge of the CDs).45 Thus, the low O-states can lead to the decreased CL intensity because of the less smaller energy gaps. On the other hand, the partial ONOO− could be protonated to form ONOOH in the physiological conditions, and the homolysis of both ONOOH and ONOO− can generate •OH and O2•− radicals simultaneously.46 The energy level position of CDs relative to the standard redox potentials of radicals in ONOO− system was showed in Figure 3D. Among various radicals, the most positive redox potential from •OH (E0 ≈ (2.84 – 0.06 pH) V vs. SHE) was showed, which is sufficient for hole injection into the HOMO of CDs to produce an oxidizing radical (CDs•+).40 Additionally, the oxidative potential of O2•− (−0.33 V) is adequate for the electron injection into LUMO of CDs to generate a reducing radical (CDs•−).47 The recombination of the CDs•+ and CDs•− results in the formation of excited CDs (CDs*), which can generate strong luminescence through radiative transition from the excited state to the ground state. On the basis of the above discussion, the possible mechanism of O-state dependent CL of S-CDs was described as follows: abundant C–O group-related O-states facilitated the generation of new smaller energy gaps; these numerous smaller energy gaps benefited the hole/electron injection with the generated •OH/O2•− radicals, which could promote the emission rate of the excited CDs for the improved CL emissions.

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Figure 3. (A) CL spectra of ONOO− system with CDs and S-CDs. (B) Effects of ROS scavengers on the CL response of CDs and S-CDs (S doping content: 0.3%) towards ONOO−. (C) Cyclic voltammograms for the S-CDs with different S doping in 50 mM PBS (pH = 7.4). The scan rate was 0.1 V/s. (D) Energy level position of CDs relative to the standard redox potentials of radicals in ONOO− system.

Screening of O-States in CDs Using CL Probe. Doping via microwave or hydrothermal methods can effectively introduce O-states into CDs.23,48 The variety of O-states in CDs could be easily altered by doping other heteroatom, e.g., phosphorus.49 In this work, to validate the feasibility of the proposed CL probe for screening of O-states, variable O-states in P-CDs with different P doping contents were prepared via a microwave method. As expected, P doping could gradually decrease the content of C–O functional groups in CDs, which was supported by the result of XPS (Figure S9) and FT−IR (Figure S10). And the variation trend towards the contents of C–O functional groups was reversely proportional to the P doping contents (Figure S11 and Table S3). Despite of the similar particle size, the electrochemical 14


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signal and UV–vis absorbance of these P-CDs were decreased with the increasing P contents (Figure S12 and Table S4). In other words, electrochemical and photophysical properties of P-CDs were related to their O-states. The as-prepared three P-CDs were applied to the CL measurements. Figure 4 showed that a clear trend of CL intensity ranked as 0.9% > 1.7% > 2.5% P-CDs was observed, which is consistent with S-CDs. It can be seen that the CL intensity of P-CDs remained constant after adding PO43-, indicating the independence between CL intensity and the residue PO43- (Figure S4B). Such a phenomenon indicated that the CL emission of CDs was indeed related to their relative content of O-states. Undoubtedly, these results further illustrated that the proposed CL probe could be a promising alternative to the conventional methods for screening of O-states in CDs. To further demonstrate the rapid detection characters and stability of proposed probe, the CL response dynamics and reproducibility studies of the CDs–ONOO− CL system were investigated. After ONOO− injection, the CL intensity showed a rapid increase to the maximum value within one second (Figure S13), indicating the reaction of proposed CL probe was quick. Meanwhile, twenty repeated measurements of the CL system were utilized to verify the stability of proposed method. As shown in Figure S14, the low relative standard deviation (2.1%) of CL signal suggested that this designed CL probe is highly reproducible.

Figure 4. (A) Relative CL intensity of P-CDs (P doping contents, a 0.9%, b 1.7% and c 2.5%) in the presence of 100 μM ONOO−. (B) Corresponding relative content of C–O functional groups of these P-CDs. 15



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CONCLUSIONS To summarize, a rapid and effective CL probe was first proposed to supply a facile platform for screening of O-states in CDs via the correlation between CL intensity of CDs–ONOO− system and the content of C–O group-related O-states in CDs. The feasibility of proposed strategy was validated by detecting the variable CL signals of P-CDs with the change of C–O group-related O-state content. A connection between the proposed CL probe and XPS technique was established. In comparison with the current characterization techniques for O-states, this new CL approach showed several advantages, including rapid response, low cost and easy operation. Moreover, our study also provided a deep understanding of the relationship between O-states in CDs and CL behaviors, and thus, new avenues for O-state analysis in different nanomaterials using the present CL strategy could be achieved.

ASSOCIATED CONTENT Supporting Information UV−vis absorption spectra of ONOO− generated from the reaction of NaNO2 with H2O2-HCl in different concentrations; The full survey and S 2p XPS of spectra in CDs and S-CDs; Schematic diagram of static injection CL setup; CL intensity of CDs with different SO42- and PO43- contents; CL responses of CDs, S-CDs and P-CDs towards various ROS and RNS; Relative CL intensity and content of C–O functional groups of S-CDs; CL signals of ONOO− system in the presence of different sources including H2O, citric acid, urea, and S-CDs; PL spectra of CDs and S-CDs; The full survey and P 2p of XPS spectra in CDs and P-CDs; FT−IR spectra of the P-CDs with different P doping contents; High-resolution O 1s XPS spectra of P-CDs with various P doping contents; Cyclic voltammograms, UV-vis absorption spectra, HRTEM images of P-CDs and corresponding CL intensity signals; CL response dynamics curve of ONOO− on the surface of P-CDs; CL signals of P-CDs for 20 repeated measurements in the presence of ONOO−; XPS 16


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results of different chemical states of surface O elements of S-CDs with different S doping contents; The band structure parameters of S-CDs with different S doping contents; XPS results of different chemical states of surface O elements of P-CDs with different P doping contents; The band structure parameters of P-CDs with different P doping contents. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Fax/Tel.: +86 10 64411957. ORCID Chao Lu: 0000-0002-7841-7477 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by National Basic Research Program of China (973 Program, 2014CB932103), the National Natural Science Foundation of China (21521005, 21656001, 21575010,

21375006

and

21605003),

the

China

Postdoctoral

Science Foundation

(2016M600899), and the Fundamental Research Funds for the Central Universities (buctrc201619 and ZY1625).

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Rapid Screening of Oxygen States in Carbon Quantum Dots by

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