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Analysis of aqueous systems using all-inorganic perovskite CsPbBr3 quantum dots with stable electrochemiluminescence performance using a closed bipolar electrode
Electrochemistry Communications 108 (2019) 106559
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Analysis of aqueous systems using all-inorganic perovskite CsPbBr3 quantum dots with stable electrochemiluminescence performance using a closed bipolar electrode Nan Haoa, Jinwen Lua, Zhen Daia, Jing Qiana, Jiadong Zhangb, , Yingshu Guod, , Kun Wanga,c, ⁎
⁎
T
⁎
a
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China National & Local Joint Engineering Research Center for Deep Utilization Technology of Rock-salt Resource, Huaiyin Institute of Technology, Huai'an 223003, PR China c Key Laboratory of Sensor Analysis of Tumor Marker, Ministry of Education, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, PR China d Collaborative Innovation Center of Tumor Marker Detection Technology, Equipment and Diagnosis-Therapy Integration in Universities of Shandong, Shandong Province Key Laboratory of Detection Technology for Tumor Markers, School of Chemistry and Chemical Engineering, Linyi University, Linyi 276005, PR China b
ARTICLE INFO
ABSTRACT
Keywords: Electrochemiluminescence Bipolar electrode Perovskite CsPbBr3 QDs
All-inorganic perovskite CsPbX3 (X = Cl, Br, I) quantum dots (QDs) have emerged as a new class of semiconductor nanocrystals, but the stability of CsPbX3 QDs in polar solvents is still a significant challenge. Since most targets in analytical chemistry, especially for biological detection, exist in an aqueous medium, this weakness seriously hampers practical analytical applications of CsPbX3 QDs. In this work, we introduce a closed bipolar electrode (BPE) to extend the application of perovskite QDs to aqueous systems. Based on the principle of conservation of charge in the electrode reactions at opposite ends of the BPE, the concentration of H2O2 in an aqueous medium can be detected by measuring the ECL intensity of CsPbBr3 QDs in an organic solution. Thus, for the first time, H2O2 in an aqueous system has been successfully analyzed using all-inorganic perovskite CsPbBr3 QDs with stable electrochemiluminescence performance combined with a closed bipolar electrode chip.
1. Introduction In recent years, all-inorganic cesium lead halide (CsPbX3, X = Cl, Br, I) perovskite quantum dots (QDs) have attracted great attention due to their superior optical/electronic performance, including their high fluorescence quantum yield (QY), narrow spectral width, compositiondependent tunable bandgap and defect-resistance photophysical properties [1–4]. They have been widely applied in optoelectronic and photovoltaic devices. However, despite these remarkable properties, all-inorganic perovskite QDs are also highly hygroscopic in a polar solvent or a high-humidity environment due to their ionic crystal characteristics. This weakness severely limits their practical applications in analytical chemistry because the majority of targets, especially for biological detection, exist in an aqueous medium [5–7]. Up to now, the main approach to improving the stability has been to encapsulate CsPbX3 QDs within matrices for better water resistance. For instance, Wang et al. [8] constructed a CsPbBr3 QD electrospun fiber membrane sensor, which uses the fluorescence resonance energy transfer (FRET) method for sensitive detection of rhodamine 6G (R6G). Li’s group encapsulated CsPbBr3 QDs within poly(methyl methacrylate) (PMMA) ⁎
polymer nanospheres, and this material was successfully used in live cell imaging [9]. The application of CsPbBr3 QDs in the analysis field is still in its infancy, although there have been some research efforts in this area [8,10–14]. Electrochemiluminescence (ECL), which has high sensitivity, good selectivity, wide linear range and other characteristics, has become a subject of interest to analytical chemists [15–18]. The ECL performance of perovskite nanocrystals (NCs) as novel emitters has been explored. Previous research has studied the co-reactant and annihilation ECL approaches and shown that the ECL spectra of perovskite NCs was nearly the same as their photoluminescence spectra [12,19]. Zhu’s group proposed that high-quality CsPbBr3 QD film produced intense and stable ECL [14], with an ECL efficiency five times higher than the classical Ru(bpy)32+/tri-n-propylamine (TPA) system. However, because CsPbBr3 QDs are unstable in polar solvents due to their intrinsically ionic nature, ECL detection was conducted in organic reagents, which means that the practical application of CsPbBr3 QDs to the analysis of aqueous systems remains a challenge. Bipolar electrodes (BPE) are an effective tool in ECL sensing, as they convert the electrochemical reaction signal into a measurable optical signal, and
Corresponding authors. E-mail addresses: [email protected] (J. Zhang), [email protected] (Y. Guo), [email protected] (K. Wang).
https://doi.org/10.1016/j.elecom.2019.106559 Received 1 September 2019; Received in revised form 18 September 2019; Accepted 18 September 2019 Available online 17 October 2019 1388-2481/ © 2019 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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overcome the problem of contact between the electroactive material and the ECL indicator [20–22]. When a certain driving potential is applied, an overpotential is produced at the interface between the solution and the BPE, which leads to oxidation and reduction reactions at opposite ends of the BPE [23]. According to the principle of charge conservation, the currents generated by oxidation and reduction are equal, so researchers often associate the reduction reaction occurring at the cathode with the ECL intensity generated at the anode, which is then used for quantitative detection [24,25]. It is therefore reasonable to experiment with a closed BPE in conjunction with perovskite QDs, in order to extend their analytical use to aqueous systems. In this work, we constructed a novel closed BPE ECL sensing platform to detect a target in aqueous solution using the excellent luminescence performance of perovskite QDs. Based on the principle of charge conservation, the concentration of hydrogen peroxide in an aqueous solution in the cathode reaction cell was successfully determined by recording the ECL intensity of CsPbBr3/ethyl acetate (EA) at the anode. This BPE-CsPbBr3 ECL system avoids interference between the polar solvents and an organic ECL reporting reagent, and achieves the goal of using perovskite QDs in the analysis of aqueous systems. This simple strategy overcomes the limitation imposed by the intrinsic instability of perovskite QDs in an aqueous medium, and may open a new direction for the application of all-inorganic perovskite QDs for ECL-based detection of targets in polar solvents.
at 600 V with a triple amplification stage. For greater accuracy, each reaction cell was rinsed several times using a syringe before measurement. 3. Results and discussion 3.1. Design principle of the experiment As a proof of concept experiment, hydrogen peroxide was selected as the target molecule [30]. Various concentrations of H2O2 solution were added to the sensing (cathode) cell and an ECL luminescence reagent (CsPbBr3 QD/EA) was added to the reporting (anode) cell (Scheme 1a). EA containing a certain concentration of tetra-n-butylammonium hexafluorophosphate (TBAPF6) was necessary as both electrolyte solution and co-reactant. According to the work of Zhu and Bard, EA participates in the luminescence process of CsPbBr3 as a coreactant. When a voltage was applied to the anode of the BPE, intermediate radicals CH3CO• and [CsPbBr3]+• were produced by the oxidation of EA and CsPbBr3, respectively. Then the unstable excited substances, [CsPbBr3]*, generated from the combination of CH3CO• and [CsPbBr3]+•, finally emitted photons back to the ground state to achieve luminescence [14,31]. The above ECL mechanism can be summarized by the following four reactions: CsPbBr3 − e− → [CsPbBr3]+•
2. Experimental
−
(1) •
CH3COOC2H5 − e → CH3CO + products [CsPbBr3]
2.1. Preparation of CsPbBr3 QDs
+•
•
+ CH3CO → [CsPbBr3]* + products
[CsPbBr3]* → CsPbBr3 + hν
Details of materials and reagents are provided in the Supporting Information. Samples of CsPbBr3 QDs were synthesized following the method reported previously [1,14,26]. As shown in Fig. S1, the precursors of Cs oleate and Pb oleate must be prepared before the synthesis of the CsPbBr3 QDs. Briefly, 1 mL oleic acid (OA) and 1 mL oleylamine (OLA) were added to a mixture containing 10 mL octadecene (ODE) and 0.069 g of PbBr2 after degassing for one hour and heated to 120° C under N2. The solid was completely dissolved and heated to 160 °C. Then, 0.8 mL Cs oleate solution (formed by heating 0.407 g Cs2CO3, 20 mL ODE and 1250 μL OA to 160 °C under N2) was added and the reaction vessel transferred to an ice water bath after 5 s to stop the reaction.
(2) (3) (4)
When a sufficiently high voltage is applied to the BPE, the potential difference between the BPE and the solution drives the oxidation and reduction reactions. The oxidation of CsPbBr3 QDs/EA at the anode and the reduction of H2O2 at the cathode occur simultaneously (Scheme 1b). According to the principle of charge conservation, the currents generated by oxidation and reduction are equal. The concentration of H2O2 may therefore be indirectly detected by measuring the ECL intensity of CsPbBr3 QD/EA at the anode. 3.2. Characterization of CsPbBr3 QDs To demonstrate the successful preparation of CsPbBr3 QDs, TEM, XRD, UV–vis and PL spectra were recorded. The CsPbBr3 QDs dispersed in n-hexane appeared yellow-green under visible light (Fig. 1A), and emitted a bright green light when irradiated by ultraviolet light (365 nm), indicating that the CsPbBr3 QDs have a high photoluminescence quantum yield (PLQY). Previous reports have shown that the PLQY of CsPbBr3 QDs is as high as 90% [32]. The UV–visible and PL spectra of the CsPbBr3 QDs are shown in Fig. 1B. It is clear that the CsPbBr3 QDs exhibit a maximum absorption peak at around 515 nm and a sharp emission peak at around 519 nm (excitation wavelength 350 nm), which is consistent with a previous report [11]. Uniform cubic CsPbBr3 nanocrystals about 9 nm in size could be observed in the TEM image (Fig. 1C) [33,34]. The XRD pattern (Fig. 1D) indicates that welldefined diffraction peaks can be assigned to the orthorhombic phase of CsPbBr3 [35,36]. Measurement and scanning XPS data for the CsPbBr3 QDs are shown in Fig. S2a, and the results are in line with expectations.
2.2. Fabrication of the ITO-based closed BPE device The closed bipolar electrode used in this experiment was fabricated in two main steps. Firstly, ITO glass with a striped pattern was fabricated by pattern etching [27]. The ITO surface was first cleaned using acetone, ethanol, and high purity water in turn. The striped pattern screen was then held on the printing table through the use of a vacuum, and a special ITO printing ink was used to print the pattern on the surface of the ITO glass. Then, the printed ITO glass was air-dried, immersed in an acidic etching solution and etched in a shaker at 37 °C for 15 min. Finally, 10% NaOH was used to clean the remaining ink from the ITO glass. The second step was simply to form a pair of microreaction cells (length 4 mm, width 2 mm) with poly(ethylene terephthalate) (PET) membrane, following the etched pattern [28,29]. 2.3. Quantitative determination of H2O2
3.3. ECL behavior of CsPbBr3 QDs in an organic medium
As shown in Scheme 1a, a solution of H2O2 (0.1 M PBS, pH 7.4) was added to the sensing microcell. N-hexane solution containing 0.05 M CsPbBr3 QDs was added to the reporting microcell and evaporated to form a CsPbBr3 QD film, with the continuous addition of 0.05 M tetra-nbutylammonium hexafluorophosphate (TBAPF6) in EA. The CV scanning potential window was chosen to be 0 to 1.25 V, and the scanning rate was 0.1 V/s. The ECL signals were collected using a PMT operated
After successfully synthesizing CsPbBr3 QDs, we first explored the ECL properties of CsPbBr3 QDs in the organic phase. The experiment was conducted with a traditional three-electrode system. A glassy carbon electrode (GCE) was modified with a CsPbBr3 QDs film and used as the working electrode. It can be seen from Fig. S3 that a CsPbBr3 QD film scraped and coated in EA several times maintained a dense 2
Electrochemistry Communications 108 (2019) 106559
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Scheme 1. Schematic diagram of the CsPbBr3 QD-based BPE-H2O2 sensing system.
shown in Fig. 2A, a GCE modified with CsPbBr3 QDs displayed significant and steady anodic ECL, which is consistent with previous work by Wang and Zhang [12,14]. As shown in Fig. S4, a significant ECL
accumulation and showed a significantly reduced grain size. EA containing 0.05 M TPABF6 was used as the electrolyte solution, and the potential window was 0 to 1.25 V, with a scanning rate of 0.1 V/s. As
Fig. 1. (A) Photographs of CsPbBr3 QDs. (B) UV–vis and PL spectra of CsPbBr3 QDs. (C) TEM image of CsPbBr3 QDs. (D) XRD patterns of CsPbBr3 QDs. 3
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Fig. 2. ECL vs time curve of a GCE modified with CsPbBr3 QD film in (A) EA containing 0.05 M TBAPF6; (B) N2-saturated 0.1 M PBS; (C) ECL vs time curve of CsPbBr3 QDs in BPE containing 10 mM H2O2 (red line: N2-saturated 0.1 M PBS) in the cathode cell. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
signal was detected around 1.2 V. The cyclic voltammograms (CV) showed no obvious peak. It can be seen from Fig. 2B that when the CsPbBr3 QD GCE was immersed in the aqueous solution under the same conditions, an ECL signal could again be observed, but this decreased very rapidly due to the extreme instability of the CsPbBr3 QDs in the polar solvent [5].
were analyzed using the same ITO-based closed BPE device. The linear relationship between the concentration of H2O2 and the ECL intensity of CsPbBr3 QDs/EA is shown in Fig. 3A. As the concentration of H2O2 increased, the electron transfer on the BPE increased, producing a great increase in ECL intensity. As observed, a higher concentration of H2O2 causes a higher ECL intensity, as increased H2O2 reduction in the sensing cell induces increased oxidation of CsPbBr3 QDs/EA due to the need for charge balance [20,29]. There was a linear relationship between ECL intensity and H2O2 concentration within the range 1–200 mM, with a detection limit of 0.05 mM (Fig. 3B).
3.4. BPE-H2O2 ECL sensing performance of CsPbBr3 QDs Optimizing the driving voltage (Etot) played an important role in the BPE-ECL measurement. 3.1 V was selected as Etot to obtain a strong ECL signal (see Fig. S5). Before any further experiments were conducted, the ECL performance of CsPbBr3 QDs on the ITO-based closed BPE device should be investigated. A 0.1 M PBS solution containing 10 mM H2O2 was added to the sensing microcell and an EA solution containing 0.05 M TBAPF6 was added to the reporting microcell modified with CsPbBr3 QD film. When the driving voltage was applied, a strong and stable ECL signal was observed (Fig. 2C), which was attributed to the oxidation of CsPbBr3 QDs/EA at the anode electrode surface. Compared with PBS (red line), the ECL intensity increased about five-fold after adding H2O2 to the cathode cell, which indicated that H2O2 had participated in and promoted the oxidation reaction of CsPbBr3 QDs. This result indicated that the BPE-H2O2 ECL system using CsPbBr3 QDs as the emitter in this work could be used in further experiments.
4. Conclusion In summary, a novel BPE-ECL sensor based on all-inorganic perovskite CsPbBr3 QDs has been successfully constructed. CsPbBr3 QDs were synthesized according to a well-established method and served as the electrochemiluminescent material. A home-made closed bipolar electrode chip was successfully fabricated, and the concentration of H2O2 was indirectly detected by measuring the ECL intensity of CsPbBr3 QDs/EA at the anode. This BPE-ECL device successfully used the allinorganic perovskite CsPbBr3 QDs for the quantitative determination of H2O2 in an aqueous system. In addition, this first application of the electrochemiluminescence of all-inorganic perovskite QDs in a bipolar electrode may open a new direction for us to explore and overcome the problem that has limited the application of perovskite halide materials.
3.5. Sample detection Under the optimum conditions, different concentrations of H2O2
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Fig. 3. (A) ECL intensity of CsPbBr3 QDs in BPE containing different concentrations of H2O2: (a) 1 mM, (b) 5 mM, (c) 10 mM, (d) 20 mM, (e) 50 mM, (f) 100 mM, (g) 200 mM. (B) Corresponding linear relationship between the concentration of H2O2 and the ECL intensity of the CsPbBr3 QDs.
Acknowledgments
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This work was supported by National Natural Science Foundation of China (Nos. 21876068, 21605055 and 21675066), the Open Fund Program of Jiangsu Provincial Engineering Laboratory for Advanced Materials of Salt Chemical Industry (SF201703), and the Foundation of Key Laboratory of Sensor Analysis of Tumor Marker, Ministry of Education, Qingdao University of Science and Technology (No. SATM201807). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.elecom.2019.106559. References [1] L. Protesescu, S. Yakunin, M.I. Bodnarchuk, F. Krieg, R. Caputo, C.H. Hendon, R.X. Yang, A. Walsh, M.V. Kovalenko, Nanocrystals of cesium lead halide perovskites (CsPbBr 3, X = Cl, Br, and I): novel optoelectronic materials showing bright emission with wide color gamut, Nano Lett. 15 (2015) 3692–3696. [2] D.N. Dirin, L. Protesescu, D. Trummer, I.V. Kochetygov, S. Yakunin, F. Krumeich, N.P. Stadie, M.V. Kovalenko, Harnessing defect-tolerance at the nanoscale: highly luminescent lead halide perovskite nanocrystals in mesoporous silica matrixes, Nano Lett. 16 (2016) 5866–5874. [3] S. Gonzalez-Carrero, R.E. Galian, J. Pérez-Prieto, Organic-inorganic and all-inorganic lead halide nanoparticles, Opt. Express 24 (2016) A285–A301. [4] A. Swarnkar, V.K. Ravi, A. Nag, Beyond colloidal cesium lead halide perovskite nanocrystals: analogous metal halides and doping, ACS Energy Lett. 2 (2017) 1089–1098. [5] F. Bella, G. Griffini, J.-P. Correa-Baena, G. Saracco, M. Grätzel, A. Hagfeldt, S. Turri, C. Gerbaldi, Improving efficiency and stability of perovskite solar cells with photocurable fluoropolymers, Science 354 (2016) 203–206. [6] F. Palazon, Q.A. Akkerman, M. Prato, L. Manna, X-ray lithography on perovskite nanocrystals films: from patterning with anion-exchange reactions to enhanced stability in air and water, ACS Nano 10 (2016) 1224–1230. [7] J. De Roo, M. Ibáñez, P. Geiregat, G. Nedelcu, W. Walravens, J. Maes, J.C. Martins, I. Van Driessche, M.V. Kovalenko, Z. Hens, Highly dynamic ligand binding and light absorption coefficient of cesium lead bromide perovskite nanocrystals, ACS Nano 10 (2016) 2071–2081. [8] Y. Wang, Y. Zhu, J. Huang, J. Cai, J. Zhu, X. Yang, J. Shen, H. Jiang, C. Li, CsPbBr 3 perovskite quantum dots-based monolithic electrospun fiber membrane as an ultrastable and ultrasensitive fluorescent sensor in aqueous medium, J. Phys. Chem. Lett. 7 (2016) 4253–4258. [9] Y. Wang, L. Varadi, A. Trinchi, J. Shen, Y. Zhu, G. Wei, C. Li, Spray-assisted coil–globule transition for scalable preparation of water-resistant CsPbBr 3@PMMA perovskite nanospheres with application in live cell imaging, Small 14 (2018) 1803156. [10] T.L. Doane, K.L. Ryan, L. Pathade, K.J. Cruz, H. Zang, M. Cotlet, M.M. Maye, Using perovskite nanoparticles as halide reservoirs in catalysis and as spectrochemical probes of ions in solution, ACS Nano 10 (2016) 5864–5872. [11] Z. Cai, F. Li, W. Xu, S. Xia, J. Zeng, S. He, X. Chen, Colloidal CsPbBr 3 perovskite nanocrystal films as electrochemiluminescence emitters in aqueous solutions, Nano Res. 11 (2018) 1447–1455.
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Analysis of aqueous systems using all-inorganic perovskite CsPbBr3 quantum dots with stable electrochemiluminescence performance using a closed bipolar electrode Nan Haoa, Jinwen Lua, Zhen Daia, Jing Qiana, Jiadong Zhangb, , Yingshu Guod, , Kun Wanga,c, ⁎
⁎
T
⁎
a
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China National & Local Joint Engineering Research Center for Deep Utilization Technology of Rock-salt Resource, Huaiyin Institute of Technology, Huai'an 223003, PR China c Key Laboratory of Sensor Analysis of Tumor Marker, Ministry of Education, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, PR China d Collaborative Innovation Center of Tumor Marker Detection Technology, Equipment and Diagnosis-Therapy Integration in Universities of Shandong, Shandong Province Key Laboratory of Detection Technology for Tumor Markers, School of Chemistry and Chemical Engineering, Linyi University, Linyi 276005, PR China b
ARTICLE INFO
ABSTRACT
Keywords: Electrochemiluminescence Bipolar electrode Perovskite CsPbBr3 QDs
All-inorganic perovskite CsPbX3 (X = Cl, Br, I) quantum dots (QDs) have emerged as a new class of semiconductor nanocrystals, but the stability of CsPbX3 QDs in polar solvents is still a significant challenge. Since most targets in analytical chemistry, especially for biological detection, exist in an aqueous medium, this weakness seriously hampers practical analytical applications of CsPbX3 QDs. In this work, we introduce a closed bipolar electrode (BPE) to extend the application of perovskite QDs to aqueous systems. Based on the principle of conservation of charge in the electrode reactions at opposite ends of the BPE, the concentration of H2O2 in an aqueous medium can be detected by measuring the ECL intensity of CsPbBr3 QDs in an organic solution. Thus, for the first time, H2O2 in an aqueous system has been successfully analyzed using all-inorganic perovskite CsPbBr3 QDs with stable electrochemiluminescence performance combined with a closed bipolar electrode chip.
1. Introduction In recent years, all-inorganic cesium lead halide (CsPbX3, X = Cl, Br, I) perovskite quantum dots (QDs) have attracted great attention due to their superior optical/electronic performance, including their high fluorescence quantum yield (QY), narrow spectral width, compositiondependent tunable bandgap and defect-resistance photophysical properties [1–4]. They have been widely applied in optoelectronic and photovoltaic devices. However, despite these remarkable properties, all-inorganic perovskite QDs are also highly hygroscopic in a polar solvent or a high-humidity environment due to their ionic crystal characteristics. This weakness severely limits their practical applications in analytical chemistry because the majority of targets, especially for biological detection, exist in an aqueous medium [5–7]. Up to now, the main approach to improving the stability has been to encapsulate CsPbX3 QDs within matrices for better water resistance. For instance, Wang et al. [8] constructed a CsPbBr3 QD electrospun fiber membrane sensor, which uses the fluorescence resonance energy transfer (FRET) method for sensitive detection of rhodamine 6G (R6G). Li’s group encapsulated CsPbBr3 QDs within poly(methyl methacrylate) (PMMA) ⁎
polymer nanospheres, and this material was successfully used in live cell imaging [9]. The application of CsPbBr3 QDs in the analysis field is still in its infancy, although there have been some research efforts in this area [8,10–14]. Electrochemiluminescence (ECL), which has high sensitivity, good selectivity, wide linear range and other characteristics, has become a subject of interest to analytical chemists [15–18]. The ECL performance of perovskite nanocrystals (NCs) as novel emitters has been explored. Previous research has studied the co-reactant and annihilation ECL approaches and shown that the ECL spectra of perovskite NCs was nearly the same as their photoluminescence spectra [12,19]. Zhu’s group proposed that high-quality CsPbBr3 QD film produced intense and stable ECL [14], with an ECL efficiency five times higher than the classical Ru(bpy)32+/tri-n-propylamine (TPA) system. However, because CsPbBr3 QDs are unstable in polar solvents due to their intrinsically ionic nature, ECL detection was conducted in organic reagents, which means that the practical application of CsPbBr3 QDs to the analysis of aqueous systems remains a challenge. Bipolar electrodes (BPE) are an effective tool in ECL sensing, as they convert the electrochemical reaction signal into a measurable optical signal, and
Corresponding authors. E-mail addresses: [email protected] (J. Zhang), [email protected] (Y. Guo), [email protected] (K. Wang).
https://doi.org/10.1016/j.elecom.2019.106559 Received 1 September 2019; Received in revised form 18 September 2019; Accepted 18 September 2019 Available online 17 October 2019 1388-2481/ © 2019 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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overcome the problem of contact between the electroactive material and the ECL indicator [20–22]. When a certain driving potential is applied, an overpotential is produced at the interface between the solution and the BPE, which leads to oxidation and reduction reactions at opposite ends of the BPE [23]. According to the principle of charge conservation, the currents generated by oxidation and reduction are equal, so researchers often associate the reduction reaction occurring at the cathode with the ECL intensity generated at the anode, which is then used for quantitative detection [24,25]. It is therefore reasonable to experiment with a closed BPE in conjunction with perovskite QDs, in order to extend their analytical use to aqueous systems. In this work, we constructed a novel closed BPE ECL sensing platform to detect a target in aqueous solution using the excellent luminescence performance of perovskite QDs. Based on the principle of charge conservation, the concentration of hydrogen peroxide in an aqueous solution in the cathode reaction cell was successfully determined by recording the ECL intensity of CsPbBr3/ethyl acetate (EA) at the anode. This BPE-CsPbBr3 ECL system avoids interference between the polar solvents and an organic ECL reporting reagent, and achieves the goal of using perovskite QDs in the analysis of aqueous systems. This simple strategy overcomes the limitation imposed by the intrinsic instability of perovskite QDs in an aqueous medium, and may open a new direction for the application of all-inorganic perovskite QDs for ECL-based detection of targets in polar solvents.
at 600 V with a triple amplification stage. For greater accuracy, each reaction cell was rinsed several times using a syringe before measurement. 3. Results and discussion 3.1. Design principle of the experiment As a proof of concept experiment, hydrogen peroxide was selected as the target molecule [30]. Various concentrations of H2O2 solution were added to the sensing (cathode) cell and an ECL luminescence reagent (CsPbBr3 QD/EA) was added to the reporting (anode) cell (Scheme 1a). EA containing a certain concentration of tetra-n-butylammonium hexafluorophosphate (TBAPF6) was necessary as both electrolyte solution and co-reactant. According to the work of Zhu and Bard, EA participates in the luminescence process of CsPbBr3 as a coreactant. When a voltage was applied to the anode of the BPE, intermediate radicals CH3CO• and [CsPbBr3]+• were produced by the oxidation of EA and CsPbBr3, respectively. Then the unstable excited substances, [CsPbBr3]*, generated from the combination of CH3CO• and [CsPbBr3]+•, finally emitted photons back to the ground state to achieve luminescence [14,31]. The above ECL mechanism can be summarized by the following four reactions: CsPbBr3 − e− → [CsPbBr3]+•
2. Experimental
−
(1) •
CH3COOC2H5 − e → CH3CO + products [CsPbBr3]
2.1. Preparation of CsPbBr3 QDs
+•
•
+ CH3CO → [CsPbBr3]* + products
[CsPbBr3]* → CsPbBr3 + hν
Details of materials and reagents are provided in the Supporting Information. Samples of CsPbBr3 QDs were synthesized following the method reported previously [1,14,26]. As shown in Fig. S1, the precursors of Cs oleate and Pb oleate must be prepared before the synthesis of the CsPbBr3 QDs. Briefly, 1 mL oleic acid (OA) and 1 mL oleylamine (OLA) were added to a mixture containing 10 mL octadecene (ODE) and 0.069 g of PbBr2 after degassing for one hour and heated to 120° C under N2. The solid was completely dissolved and heated to 160 °C. Then, 0.8 mL Cs oleate solution (formed by heating 0.407 g Cs2CO3, 20 mL ODE and 1250 μL OA to 160 °C under N2) was added and the reaction vessel transferred to an ice water bath after 5 s to stop the reaction.
(2) (3) (4)
When a sufficiently high voltage is applied to the BPE, the potential difference between the BPE and the solution drives the oxidation and reduction reactions. The oxidation of CsPbBr3 QDs/EA at the anode and the reduction of H2O2 at the cathode occur simultaneously (Scheme 1b). According to the principle of charge conservation, the currents generated by oxidation and reduction are equal. The concentration of H2O2 may therefore be indirectly detected by measuring the ECL intensity of CsPbBr3 QD/EA at the anode. 3.2. Characterization of CsPbBr3 QDs To demonstrate the successful preparation of CsPbBr3 QDs, TEM, XRD, UV–vis and PL spectra were recorded. The CsPbBr3 QDs dispersed in n-hexane appeared yellow-green under visible light (Fig. 1A), and emitted a bright green light when irradiated by ultraviolet light (365 nm), indicating that the CsPbBr3 QDs have a high photoluminescence quantum yield (PLQY). Previous reports have shown that the PLQY of CsPbBr3 QDs is as high as 90% [32]. The UV–visible and PL spectra of the CsPbBr3 QDs are shown in Fig. 1B. It is clear that the CsPbBr3 QDs exhibit a maximum absorption peak at around 515 nm and a sharp emission peak at around 519 nm (excitation wavelength 350 nm), which is consistent with a previous report [11]. Uniform cubic CsPbBr3 nanocrystals about 9 nm in size could be observed in the TEM image (Fig. 1C) [33,34]. The XRD pattern (Fig. 1D) indicates that welldefined diffraction peaks can be assigned to the orthorhombic phase of CsPbBr3 [35,36]. Measurement and scanning XPS data for the CsPbBr3 QDs are shown in Fig. S2a, and the results are in line with expectations.
2.2. Fabrication of the ITO-based closed BPE device The closed bipolar electrode used in this experiment was fabricated in two main steps. Firstly, ITO glass with a striped pattern was fabricated by pattern etching [27]. The ITO surface was first cleaned using acetone, ethanol, and high purity water in turn. The striped pattern screen was then held on the printing table through the use of a vacuum, and a special ITO printing ink was used to print the pattern on the surface of the ITO glass. Then, the printed ITO glass was air-dried, immersed in an acidic etching solution and etched in a shaker at 37 °C for 15 min. Finally, 10% NaOH was used to clean the remaining ink from the ITO glass. The second step was simply to form a pair of microreaction cells (length 4 mm, width 2 mm) with poly(ethylene terephthalate) (PET) membrane, following the etched pattern [28,29]. 2.3. Quantitative determination of H2O2
3.3. ECL behavior of CsPbBr3 QDs in an organic medium
As shown in Scheme 1a, a solution of H2O2 (0.1 M PBS, pH 7.4) was added to the sensing microcell. N-hexane solution containing 0.05 M CsPbBr3 QDs was added to the reporting microcell and evaporated to form a CsPbBr3 QD film, with the continuous addition of 0.05 M tetra-nbutylammonium hexafluorophosphate (TBAPF6) in EA. The CV scanning potential window was chosen to be 0 to 1.25 V, and the scanning rate was 0.1 V/s. The ECL signals were collected using a PMT operated
After successfully synthesizing CsPbBr3 QDs, we first explored the ECL properties of CsPbBr3 QDs in the organic phase. The experiment was conducted with a traditional three-electrode system. A glassy carbon electrode (GCE) was modified with a CsPbBr3 QDs film and used as the working electrode. It can be seen from Fig. S3 that a CsPbBr3 QD film scraped and coated in EA several times maintained a dense 2
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Scheme 1. Schematic diagram of the CsPbBr3 QD-based BPE-H2O2 sensing system.
shown in Fig. 2A, a GCE modified with CsPbBr3 QDs displayed significant and steady anodic ECL, which is consistent with previous work by Wang and Zhang [12,14]. As shown in Fig. S4, a significant ECL
accumulation and showed a significantly reduced grain size. EA containing 0.05 M TPABF6 was used as the electrolyte solution, and the potential window was 0 to 1.25 V, with a scanning rate of 0.1 V/s. As
Fig. 1. (A) Photographs of CsPbBr3 QDs. (B) UV–vis and PL spectra of CsPbBr3 QDs. (C) TEM image of CsPbBr3 QDs. (D) XRD patterns of CsPbBr3 QDs. 3
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Fig. 2. ECL vs time curve of a GCE modified with CsPbBr3 QD film in (A) EA containing 0.05 M TBAPF6; (B) N2-saturated 0.1 M PBS; (C) ECL vs time curve of CsPbBr3 QDs in BPE containing 10 mM H2O2 (red line: N2-saturated 0.1 M PBS) in the cathode cell. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
signal was detected around 1.2 V. The cyclic voltammograms (CV) showed no obvious peak. It can be seen from Fig. 2B that when the CsPbBr3 QD GCE was immersed in the aqueous solution under the same conditions, an ECL signal could again be observed, but this decreased very rapidly due to the extreme instability of the CsPbBr3 QDs in the polar solvent [5].
were analyzed using the same ITO-based closed BPE device. The linear relationship between the concentration of H2O2 and the ECL intensity of CsPbBr3 QDs/EA is shown in Fig. 3A. As the concentration of H2O2 increased, the electron transfer on the BPE increased, producing a great increase in ECL intensity. As observed, a higher concentration of H2O2 causes a higher ECL intensity, as increased H2O2 reduction in the sensing cell induces increased oxidation of CsPbBr3 QDs/EA due to the need for charge balance [20,29]. There was a linear relationship between ECL intensity and H2O2 concentration within the range 1–200 mM, with a detection limit of 0.05 mM (Fig. 3B).
3.4. BPE-H2O2 ECL sensing performance of CsPbBr3 QDs Optimizing the driving voltage (Etot) played an important role in the BPE-ECL measurement. 3.1 V was selected as Etot to obtain a strong ECL signal (see Fig. S5). Before any further experiments were conducted, the ECL performance of CsPbBr3 QDs on the ITO-based closed BPE device should be investigated. A 0.1 M PBS solution containing 10 mM H2O2 was added to the sensing microcell and an EA solution containing 0.05 M TBAPF6 was added to the reporting microcell modified with CsPbBr3 QD film. When the driving voltage was applied, a strong and stable ECL signal was observed (Fig. 2C), which was attributed to the oxidation of CsPbBr3 QDs/EA at the anode electrode surface. Compared with PBS (red line), the ECL intensity increased about five-fold after adding H2O2 to the cathode cell, which indicated that H2O2 had participated in and promoted the oxidation reaction of CsPbBr3 QDs. This result indicated that the BPE-H2O2 ECL system using CsPbBr3 QDs as the emitter in this work could be used in further experiments.
4. Conclusion In summary, a novel BPE-ECL sensor based on all-inorganic perovskite CsPbBr3 QDs has been successfully constructed. CsPbBr3 QDs were synthesized according to a well-established method and served as the electrochemiluminescent material. A home-made closed bipolar electrode chip was successfully fabricated, and the concentration of H2O2 was indirectly detected by measuring the ECL intensity of CsPbBr3 QDs/EA at the anode. This BPE-ECL device successfully used the allinorganic perovskite CsPbBr3 QDs for the quantitative determination of H2O2 in an aqueous system. In addition, this first application of the electrochemiluminescence of all-inorganic perovskite QDs in a bipolar electrode may open a new direction for us to explore and overcome the problem that has limited the application of perovskite halide materials.
3.5. Sample detection Under the optimum conditions, different concentrations of H2O2
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Fig. 3. (A) ECL intensity of CsPbBr3 QDs in BPE containing different concentrations of H2O2: (a) 1 mM, (b) 5 mM, (c) 10 mM, (d) 20 mM, (e) 50 mM, (f) 100 mM, (g) 200 mM. (B) Corresponding linear relationship between the concentration of H2O2 and the ECL intensity of the CsPbBr3 QDs.
Acknowledgments
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