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Excited Oxidized-Carbon Nanodots Induced by Ozone from Low-Temperature Plasma to Initiate Strong Chemiluminescence for Fast Discrimination of Metal Ions Zi Long, De-Cai Fang, Hong Ren, Jin Ouyang, Lixin He, and Na Na Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b01499 • Publication Date (Web): 04 Jul 2016 Downloaded from http://pubs.acs.org on July 4, 2016

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Analytical Chemistry

Excited Oxidized-Carbon Nanodots Induced by Ozone from LowTemperature Plasma to Initiate Strong Chemiluminescence for Fast Discrimination of Metal Ions Zi Long, Decai Fang, Hong Ren, Jin Ouyang, Lixin He and Na Na* Key Laboratory of Theoretical and Computational Photochemistry, College of Chemistry, Beijing Normal University, Beijing 100875, China

*Tel: +86-10-58805373; Fax: +86-10-58802075; E-mail: [email protected] ABSTRACT: Carbon nanodots (C-dots) are recently well examined due to the emissions with color-tuning and non-blinking properties, while more studies are still needed for the appropriate understanding and application of distinct emissions. In this work, we found the emission of chemiluminescence (CL) by introducing low-temperature plasma (LTP) into C-dots solutions without any reagent added, whose intensity was affected by the presence of different metal ions. Based on both experimental data and theoretical calculations, we found with the ozonation by ozone from LTP, excited oxidized-C-dots would be generated with the addition of ozone onto the conjugated double bonds of C-dots, and these excited species could directly initiate strong CL combining with the deactivation of excited species to the ground state. Significantly, the cross-reactive CL signals were obtained from different kinds of C-dots with the presence of different metal ions. Therefore, a new sensor array (electronic tongue) composed of five different Cdots was designed for fast discrimination of metal ions, which achieved the accurate discrimination of 13 kinds of metal ions in pure water and real samples. It exhibited good reproducibility and sensitivity, which can be used for the quantitative analysis of metal ions such as showing a linear range from 4×10-7 to 6×10-5 mol·L-1 (R2 >0.99) for Fe3+ with a detection limit of 2.5×10-7 mol·L1 . This work not only provides a novel finding of CL from C-dots revealing explicit relationship between structures and CL properties, but also realizes the fast discrimination of metal ions, showing potentials in environmental monitoring and quality identifications.

CL is an emission during reactions without any excited light, showing advantages of simple device, high sensitivity, and low background interference.10 Recently, C-dots have been reported to show significant CL during the oxidation processes, with the presence of strong oxidants such as KMnO4-Ce(IV),9 H2O2-NaHSO3,11 H2O2-HNO2,12 ONOOH– carbonate,13 K2S2O8-Triethylamine14 and even in strong alkaline solutions.15 Some possible mechanisms, such as radiation recombination of oxidant-injected holes and electrons, electron-transfer annihilation, C-dots’ acting as energy acceptors or reductants, have been preliminary deduced for individual processes.16 Due to the lack of examinations on the explicit relationship between the structures and the CL properties of Cdots, the more detailed studies are still crucial important for enlarging the applications of CL-based techniques. Low-temperature plasma (LTP) was a kind of chemically active media consisting of excited species and active particles, which can be initiated by dielectric barrier discharge of air.17 We recently reported that with the assistance of LTP, the significant cataluminescence (CTL, another kind of CL emission) could be obtained during the catalytic oxidation of analytes on nanomaterials surfaces. More interestingly, this kind of emission showed the cross-reactive properties with the distinct

INTRODUCTION Carbon nanodots (C-dots) are discrete, quasi-spherical nanoparticles with sizes below 10 nm, which can be synthesized by many low-cost and simple methods.1 These kinds of nanoparticles have been found to have fascinating photoluminescence behaviors, attracting considerable attention as nascent quantum dots.2,3 However, due to the ambiguous chemical structures, the complex and diverse preparing methods as well as the nonstoichiometric reactants,4 people usually pay more attention to the photoluminescence phenomena and their applications instead of the deeper mechanism studies, which would limit the applications of C-dots in the long run. Currently, there are some explanations on the fluorescence (FL) emission of C-dots prepared by different methods, based on the experimental data from Fourier transform-infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), Raman or FL spectrometry, as well as on some theoretical calculations.5 These models6-9 interpreted a part of individual FL emissions of C-dots; while more studies are still needed for the appropriate understanding of other numbers of experimental phenomena, especially chemiluminescence (CL) of C-dots.

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luminescent efficiencies, which have been used for the fabrication of sensor array for the fast discriminations.17-19 Additionally, fast detection and discrimination of metal ions in our surroundings is significant due to their important roles in industry, life science and food chemistry,20 and therefore many methods have been used for the detection of metal ions.21-23 However the previously reported CL-based techniques normally required the addition of strong oxidants and liquid CL reagents into the system.24,25 Therefore, more simple CL-based methods are still encouraged for the detection of metal ions. Here, we found the strong CL emission of C-dots initiated by LTP, and reported a new insight into the relationship between structures and CL emission properties of C-dots. Based on this CL emission, a new sensor array (electronic tongue) composed of five different C-dots was designed for fast discrimination of metal ions, which showed potentials in environmental monitoring and quality identifications.

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system for 4 min (300 W). After the natural cooling to room temperature, the dark brown solution was obtained. Furthermore, for the fabrication of sensor array, another four kinds of similar C-dots were prepared according to the previous reports.26-29 The synthesis methods of five different kinds of Cdots are listed in Table S1 (Supporting Information). The obtained C-dots were in the dilution of 1:40 (C-dots: deionized water in volume) for the obtainment of FL spectra, UV-Vis absorption spectra, FT-IR spectra and DLS results. The sample for transmission electron microscope (TEM) characterization was prepared by placing a drop of diluted solution on carboncoated copper grid and dried at room temperature. Fabrication of LTP-assisted C-dots CL system. The LTP was generated by the dielectric barrier discharge between a stick electrode inserted in a glass tube (with the gas passing through) and a sheet electrode wrapping the outside of the tube.17 Then, LTP was introduced into C-dots solutions through a polytetrafluoroethylene (PTFE) tube, the CL signals were recorded by BPCL ultra-weak CL analyzer at the same time. Theoretical calculations of structures and reaction process. A simple reactant R1 has been designed for density functional theory (DFT) calculations, in which B3LYP-SCRF/631G (d) method has been employed to characterize the energies, structures, and frequencies.

EXPERIMENTAL SECTION Chemical reagents. All chemicals were of analytical reagent grades. Polyacrylic acid sodium salt (PAAS), ethylene diamine tetraacetic acid (EDTA), D-(+)-glucose, sucrose and guanidine hydrochloride were purchased from Sigma-Aldrich (USA). AgNO3, Pb(NO3)2, CuCl2·2H2O, CdCl2·2.5H2O, ZnCl2, FeCl3, CrCl3·6H2O, Co(NO3)2·6H2O, Ni(NO3)2·6H2O, Al(NO3)3·9H2O, MnCl2·4H2O, Hg(NO3)2·0.5H2O, CaCl2, isopropanol, ortho-phosphoric acid (88%), NaOH were obtained from Sinopharm Chemical Reagent Beijing Co., Ltd. The gas of Ar (99.99%), He (99.99%), and N2 (99.99%) were provided by Beijing Haipu Gas Co. Ltd. All experiments were carried out with deionized water purified by Milli-Q water purification system (Millipore, Bedford, MA). Apparatus and Software. CL signals were acquired by using a BPCL ultra-weak CL analyzer (Biophysics Institute of the Chinese Academy of Science in China) equipped with a CR-105 photomultiplier tube (PMT) (Hamamatsu, Tokyo, Japan). The continuous air acted as discharge was provided by a GA-2000A air generator pump (Beijing Zhongxinghuili Science and Technology Development Co., Ltd), whose flow rate was measured by a flowmeter. The matrix of data was processed using classical linear discriminant analysis (LDA) in SPSS for the discrimination (Version 20.0). The partial structure of C-dots was drawn by Gaussian09. Transmission Electron Microscopy (TEM) was performed on a JEOL 2010 transmission electron microscopy with an accelerating voltage of 200 kV. Dynamic light scattering (DLS) was carried out on a Nano-ZS Zetzsozer ZEN3600 (Malvern Instruments Ltd., U.K.). UV–Vis absorption spectra were carried out on a UV2450 spectrophotometer (Shimadzu, Japan). FL emission spectra were recorded on a Cary Eclipse spectrofluorometer (VARIAN, American). FT-IR spectra were performed on an IR Affinity-1 spectrometer (Shimadzu, Japan). X-ray photoelectron spectroscopy (XPS) analysis was carried out on an ESCALAB 250Xi X-ray photoelectron spectrometer. Synthesis and characterization of C-dots. C-dots were prepared by the microwave-assisted methods. Typically, we simply placed the transparent solution which contained 10.0 mL deionized water, 1.0 g polyacrylic acid sodium salt (PAAS) and 2.0 g glucose (Glu.) into microwave digestion

RESULTS AND DISCUSSION Synthesis and characterization of C-dots. As shown in Figure 1A, the obtained C-dots are near spherical and homogeneously dispersed in water with the size distribution of 2-4 nm as demonstrated by TEM and DLS data. The lattice fringes with an average lattice spacing of about 0.252 nm are assigned to the 102 crystal planes of graphitic carbon (sp2) (the inset of Figure 1A).30 Figure 1B demonstrates the excitation-dependent fluorescence emission of the C-dots, which shows the strongest emission at 565 nm (excited at 450 nm). In addition, the UV-Vis absorption peak of C-dots at about 280 nm (Figure S1) is attributed to the π-π* transition of aromatic sp2 domains, and the absorption at 450 nm might be the origins of the strongest FL emission.31 These have demonstrated the presence of C=C in the structures, which is in accordance with the report.2

Figure 1. (A) TEM image of C-dots. Insets are high resolution TEM image of an individual particle (left) and the DLS result of C-dots (right); (B) FL emission spectra of C-dots at various excitation wavelengths from 350 to 510 nm in 20 nm increments of Cdots.

CL signals of LTP-treated C-dots. Based on the function of LTP for activation,18 we introduced LTP into C-dots solutions

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for the treatment. Here, LTP was introduced into the simply prepared C-dots solutions for obtaining CL signals, which needn’t any purification because there was no significant difference between the signals from the original and the purified C-dots (Figure S2). The crucial roles of both C-dots and LTP for CL emissions have been confirmed by the experiments. As demonstrated by Figure 2A, strong and stable CL signals were recorded during the LTP treatment, while no signals were obtained without the LTP treatment or treating other blank species. Moreover, we obtained higher CL signals from larger amount of C-dots in the same volume (Figure 2B). However, the reaction time will be prolonged at high concentration of Cdots solutions. Therefore, taking the CL efficiency and reaction time into consideration, we diluted the C-dots to 40 times in the subsequent experiments. These experimental data strongly proved that both LTP and C-dots are crucial in the CL emissions.

gas has been preliminarily demonstrated to play a vital role to induce CL emission.

Figure 3. (A) CL signals of C-dots treated by different gas-ignited LTP. (B) CL signals of C-dots induced by LTP ignited by different discharge gases with different oxygen contents. (C) The changes of signals as a function of air flow rates.

Considering the main function of oxygen in this process might be oxidation, the other commonly used strong oxidants, including H2O2, NaClO4, NaOH-H2O2, KMnO4, NaHSO3H2O2, were selected to induce CL for the comparison with LTP-induced CL. As resulted (Figure S3), the quite low CL intensities were obtained induced by these oxidants. This might attribute to the higher amount of oxidants from the gas flow of LTP into the C-dots solutions, without the introducing of other ions or the changing of solution conditions (such as pH, ionic strength or C-dots concentration), which might have negative effects on CL emissions. Furthermore, the LTP device is low-cost, simple, and easy to be controlled for obtaining the stable CL emissions, and therefore it would be beneficial for enlarging the applications.

Figure 2. (A) CL signals of LTP-treated, air-treated C-dots, as well as other species treated by LTP (three parallel experiments). (B) CL signals of C-dots in different concentrations.

Effect of gas flow. As reported, the gas flow played an important role in the dielectric barrier discharge procedures, which will affect the properties of the generated LTP,32 and subsequently would have effect on the CL emissions. To examine the effect of gas flow on the CL emission, different gasignited LTP was applied for the treatment of C-dots solutions. As resulted in Figure 3A, we obtained the significant high CL signals by the air-ignited LTP, while almost no signals were obtained when treated by the Ar, He and N2-ignited LTP. Considering that there is about 21% of oxygen in the air, the oxygen might play an important role in the CL emissions. To examine the role of oxygen in the CL emissions of LTPtreated C-dots, different kinds of discharge gases with different oxygen proportions were used for the generation of CL signals. These gases included O2-N2 mixed gases (O2 proportions: 9.99%, 20%, 30%, 49.99% and 69.99%), air, as well as the pure O2 gas (99.99%). As shown in Figure 3B, the CL intensity is in proportion to the oxygen content in discharge gas, and pure oxygen-ignited LTP induced the strongest CL signals, confirmed the vital role of oxygen in this CL emission process. Although we obtained the highest CL signals by O2-ingnited LTP, we adopted air as the discharge gas in the subsequent studies to achieve the easy operation as well as the low-cost detection for the better applications. Furthermore, we recorded the higher CL intensity by the air flow at the relative higher flow rate (Figure 3C). However, the too high gas flow rate would lead to the unstable reaction due to the vigorous agitation of liquid. Therefore, we selected 180 mL·min-1 air as the only gas flow for the reaction. As a result, oxygen in discharge

Characterization of the LTP-treated C-dots. To examine the mechanism of this CL emission, characterizations of Cdots and LTP-treated C-dots were employed. As demonstrated, the gradually decreased signals with blue shifts on both UVVis absorption (Figure 4A) and FL emission (Figure 4B) of Cdots were recorded after LTP treatment, which combined with the gradual color fading and FL quenching of C-dots solution under visible light and 450 nm-excited light (the inset of Figure 4B). Compared to the FL emission spectra before being treated by LTP (Figure 1B), Figure 4C reveals that the maximum excitation wavelength of the LTP-treated C-dots is distinctly blue-shifted from 450 to 410 nm. These might generate from the changes of surface structure and the decreased sp2 conjugation degree of C-dots15 after the introduction of LTP. However, it cannot be vaguely attributed to the changes of surface structure of C-dots, and the more detailed examinations are still required.1

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kcal·mol-1 energy barrier, indicating the possibility of the ozonation process. It can also be observed that changing from ground state to excited state of R1 is mainly on C3=C4 and C2=O1 bonds, with the bond length changes of 1.395→1.461 and 1.263→1.281Å, respectively. The calculated absorption wavelength of R1 is mainly on 477 nm, and the corresponding FL is on 558 nm. Once the ozonation of R1 is complete, the calculated FL is shifted to 476 nm, which is in accordance with experimental observations. Such shift is also in consistence with the difference of bond length of C3=C4 between ground state and excited state, herein the 0.066 and 0.041 Å have been observed for R1 and R1-O3, respectively (Figure 5).

Figure 4. (A) UV-Vis absorption and (B) FL spectra (λex = 450 nm) of C-dots after LTP treatment for 0, 50, 100, 150, 200, 250 s. Inset: photos of C-dots treated by LTP for different time under visible light (upper) and 450 nm-excited light (below). (C) FL emission spectra of C-dots at various excitation wavelengths from 350 to 510 nm in 20 nm increments. The C-dots were treated by LTP for 150 s.

The crucial role of oxygen for this proposed CL emission has been further demonstrated by more characterizations. As demonstrated, FT-IR spectrum (Figure S4A) proves the presence of condensed aromatic carbon structures in C-dots based on stretching vibration of -OH (3380 cm-1), C=O (1710 cm-1) and C-O (1275 cm-1), stretching vibration (2910 cm-1), bending vibration of C-H (1382 cm-1), and the stretching vibration of aromatic C=C (1577 cm-1).1 Significantly, the transmittance ratios of ν(C=O) to ν(C=C) and ν(C-O) to ν(C=C) increase with the longer LTP treatment time (Figure S4B-E). Similarly, XPS data also shows the increasing ratio of O to C contents with the increase of LTP treatment time (Figure S5A). Therefore, the addition of oxygen onto the conjugated double bonds of C-dots can be demonstrated. Moreover, XPS analysis exhibits the constant nitrogen contents in C-dots, which reveals the less effect of NOx generated by LTP on CL emission process (Figure S5B, C).

Figure 5. The main structure parameters (Å) and relative energies (kcal·mol-1) for R1 and R1-O3 (ground and excited states).

During the reaction, C-dots (Scheme 1A) can be oxidized by LTP with the addition of O3 on conjugated double bonds to form the excited oxidized-C-dots (Scheme 1B). The excited oxidized-C-dots can initiate significant CL, and finally form stable oxidized-C-dots (Scheme 1C) combing with deactivation of excited species to the ground state. Therefore, treated by LTP, the excited oxidized-C-dots will be generated with the addition of O3 onto the conjugated carbon double bonds. Then, CL generated by energy releasing during the deactivation of excited species to the ground state of C-dots. This model specifically revealed the relationship between inner structures and CL properties of C-dots.

Theoretical calculations for mechanism studies. Inspired by the above analysis, we concluded that ozone generated from LTP may be the most important species to this CL emission. Combining with the experimental and theoretical results as well as the reports,8,33 we speculated the optimized partial structure of C-dots mainly consisted of carbonyl, hydroxyl and C=C. Therefore, in order to mimic the main possible pathway to react with O3, a simple reactant R1 has been designed for our DFT calculations, in which B3LYP-SCRF/6-31G (d) method has been employed to characterize the energies, structures, and frequencies (Table S2, S3, Figure S6). The main structural parameters and relative energy are schematically described in Figure 5, from which one can observe that the ozonation product of R1 (R1-O3) is ca. 23.0 kcal·mol-1 stable than that of reactants via a transition state (TS) with only 4.7

Scheme 1. The proposed mechanism for the CL emission by LTP treatment: (A) Partial structural before reaction; (B) The excited oxidized-C-dots. (C) The oxidized-C-dots after LTP treatment. Fast discrimination of metal ions by sensor array. As reported, the cross-reactive signals are crucial important for the fabrication of sensor array.34-39 As resulted, we obtained different CL signals with the presence of different metal ions in C-dots solutions. For example, we recorded the enhanced CL signals with the presence of Zn2+, the decreased signals for Fe3+, and the almost similar CL signals for Cr3+ compared with the C-dots solution without any metal ions added (Figure 6A).

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On the other hand, we observed different CL signals in different kinds of C-dots solutions with the presence of a certain metal ion, such as Zn2+ in five different kinds of C-dots solutions (Figure 6B). Therefore, this system was designed for the fabrication of sensor array to form a “fingerprint pattern” for the fast discrimination of metal ions. The set-up diagram of the C-dots-LTP-based sensor array for metal ions discrimination is shown in Scheme 2.

shows the good reproducibility and can also be used for the fast quantitative analysis of metal ions. Taking the detection of Fe3+ as an example, a good linearity range from 4×10-7 to 6×10-5 mol·L-1 (R2 >0.99), and the limit of detection of about 2.5×10-7 mol·L-1 was demonstrated (Figure S7), which exhibited a wider linear range than some fluorescence-based detection methods.40 For real applications, the detections of the tap water from the Chemistry Building of Beijing Normal University, as well as the tap water spiked with 13 kinds of metal ions have been employed using this sensor array. As shown in Figure 7C, 14 samples are clustered into 14 different groups by LDA. The first three canonical factors contain 58.9, 21.9 and 11.5% of the variation, occupying 92.3% of total variation, which demonstrates that this proposed sensor array has potentials to discriminate metal ions in real samples.

Figure 6. The cross-reactive CL signals for LTP-treated C-dots solutions. (A) CL signals of C-dots solutions (1.0 mL) with Fe3+, Cr3+, and Zn2+ added (1 µM), respectively. (B) CL signals of five different C-dots solutions (1.0 mL) with Zn2+ added (1 µM), respectively.

To clear the mechanism of metal ions discrimination by the present sensor array, FL spectra of C-dots with the presence of different metal ions were measured. As shown in Figure S8, the FL intensity changes differently with the addition of different metal ions. Typically, dramatically quenched FL was obtained with the presence of Fe3+, remarkably enhanced FL was recorded with the addition of Zn2+, and no obvious change appeared after adding Cr3+ into C-dots solution. Compared with Figure 6A, it can be demonstrated that changes of FL signals are in accordance with CL intensity changes. As reported, changes of signals might attribute to the electron, charge or energy transfer caused by the selective interaction between metal ions and C-dots.24,40-42 Specifically, it is likely to form complexes between Fe3+ and phenolic hydroxyls at Cdots which could facilitate charge transfer and thus constrain excitation recombination,43 resulting in significant decrease of CL signals. As for Zn2+, a rigid structure may be formed through the complexation between Zn2+ and C-dots, which would lead to the enhancement of CL due to the energytransfer interaction of them.44 However, Cr3+ may have weak interaction with C-dots, and thus no obvious CL signal change was observed.

Scheme 2. Set-up diagram of the C-dots-LTP-based sensor array. The obtained C-dots were diluted 40 times with water for metal ions sensing.

CONCLUSION In summary, the present work demonstrates the CL emission initiated by the excited oxidized-C-dots, which were formed with the addition of LTP-generated ozone onto the conjugated double bonds of C-dots. Based on the crossreactive CL signals of LTP treated C-dots with the presence of different metal ions, the new sensor array composed of five kinds of carbon dots acted as an artificial tongue for the fast and accurate discrimination of thirteen metal ions. The good reproducibility and linearity have been demonstrated, which has successfully realized the discrimination of metal ions in real samples. In addition, the mechanism based on the experimental data and theoretical calculations has been put forward, which reveals the explicit relationship between the structures and the CL properties. This work has given us a deeper understanding in optical properties of C-dots and also has enlarged the potential applications in environmental monitoring and quality identifications.

To verify the discrimination ability of the present sensor array, thirteen metal ions, including Pb2+, Ag+, Cu2+, Cd2+, Zn2+, Fe3+, Co2+, Ni2+, Ca2+, Al3+, Mn2+, Hg2+ and Cr3+, were selected as models for the test. As demonstrated in Figure 7A, CL signals are different for a given C-dots with the presence of different metal ions, and the same metal ion has different effects on CL properties of different C-dots, which have confirmed the cross-reactive property of signals from this system. The CL intensity patterns of the training matrix (5 C-dots × 13metal ions × 3 replicates) were subjected to linear discriminant analysis (LDA). As resulted, the first three canonical factors contain 62.5%, 22.2% and 10.8% of the variations (95.5% of total variation), showing the well-clustered three-dimensional discriminant score plots (Figure 7B). Thus, the present C-dotsLTP system can be used for the fabrication of a sensor array for metal ions discrimination. Moreover, the present technique

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Figure 7. Discrimination of metal ions. (A) CL responses of sensor array for 13 metal ions with three parallel measurements. (B) Canonical score plots for the first three factors of CL patterns analyzed by LDA (three parallel measurements, the concentration of metal ions is about 1×10-6 mol·L-1). (C) Canonical score plots for the discrimination of tap water and tap water spiked with different metal ions (three parallel measurements).

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Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Table of approaches to synthesize 5 carbon dots, CL signals of LTP-treated original C-dots and the purified C-dots, CL signals induced by the addition of different strong oxidants, UV-Vis absorption spectrum, FT-IR and XPS data of C-dots, details of energies and frequencies of optimized structure in theoretical calculations, linearity and additional figures.

AUTHOR INFORMATION Corresponding Author * [email protected]

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENT The authors gratefully acknowledge the support from the National Natural Science Foundation of China (21422503, 21373030, 21475011), A Foundation for the Author of National Excellent Doctoral Dissertation of PR China (201221), and the Fundamental Research Funds for the Central Universities.

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