Bipolar Electrodes with 100% Current Efficiency for Sensors

Feb 28, 2017 - electrolyte solution without the direct connection with the external power source (driving electrode). When the sufficient voltage was ...
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Bipolar Electrodes with 100% Current Efficiency for Sensors Xiaowei Zhang, Qingfeng Zhai, Huanhuan Xing, Jing Li, and Erkang Wang ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.7b00031 • Publication Date (Web): 28 Feb 2017 Downloaded from http://pubs.acs.org on March 1, 2017

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Bipolar Electrodes with 100% Current Efficiency for Sensors Xiaowei Zhang, a,b Qingfeng Zhai, a,b Huanhuan Xing, a,b Jing Li, a,b,* and Erkang Wang a,b,*

a. State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin, 130022, China.

b. Graduate School of the Chinese Academy of Sciences, Beijing, 100039, P. R. China.

*Corresponding author: Assoc. Prof. Jing Li and Prof. Erkang Wang, Tel: +86-431-85262003, Email: [email protected] and [email protected]

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ABSTRACT

A bipolar electrode (BPE) is an electron conductor that is embedded in the electrolyte solution without the direct connection with the external power source (driving electrode). When the sufficient voltage was provided, the two poles of BPE promote different oxidation and reduction reactions. During the past few years, BPEs with wireless feature and easy integration showed great promise in the various fields including asymmetric modification/synthesis, motion control, targets enrichment/separation, and chemical sensing/biosensing combined with the quantitative relationship between two poles of BPE. In this perspective paper, we first describe the concept and history of the BPE for analytical chemistry and then review the recent developments in the application of BPEs for sensing with ultrahigh current efficiency (ηc=iBPE/ichannel) including the open and closed bipolar system. Finally, we offer the guide for possible challenge faced and solution in the future.

Key words: bipolar electrode, chemical sensing/biosensing, high current efficiency, open bipolar system, closed bipolar system

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In 1969, Fleischmann and co-workers described the concept of fluidized bed electrodes, where a sufficient voltage is applied between two driving electrodes enables electrochemical reactions at conductive particles.1 This fluidized bed electrodes can be regarded as the original embryo of bipolar electrodes (BPEs). Different from the traditional three-electrode system, the set-up of bipolar electrochemistry is simpler and the BPE is not directly contacted with the power source, which offers an easy approach to control large scale of BPE arrays through an electric field across the solution.2 Due to the unique structure and property, BPE has attracted increasing interest in both understanding numerical simulations and various applications such as the electrochemical synthesis, motion control and monitoring, targets separation/enrichment, and chemical sensing/biosensing (including the screening of electrocatalyst).3-14 Notably, although BPE has been known since 1969,1 the first report on analytical application based on BPE was published till 2001 by Manz and co-workers.15 This is principally because the wireless property of BPE also made it difficult to record electrochemical signal (i.e., the Faradaic current) produced on BPE. The marriage between electrochemiluminescence (ECL) and BPE offered a new and useful way to perform analytical detection since 2001. After then, Crooks and co-workers demonstrated that the reaction at the reporting pole just reflects the electrochemical activity occurred at the opposite pole, which is the basis for the quantitative determination of the targets and laid the foundation of the bipolar analytical chemistry.16 Therefore, by combining the BPE reaction with optical readout, the reactions at two poles can be indicated. And various sensors with different detection techniques were developed. Here, we summarize the analytical development of bipolar electrochemistry since 2001, especially the contribution of sensing applications with ultrahigh current efficiency (ηc is critical for the highsensitivity analysis, where ηc=iBPE/ichannel, iBPE is the current through the BPE, ichannel is the current through the channel). The two main bipolar systems including open bipolar system and closed bipolar system were discussed and the two bipolar systems both can achieve 100% current efficiency. The possible challenge and solution in the future were discussed in the end. Open Bipolar Electrode System. Open bipolar electrode system (referred as O-BPS), can be easily constructed by placing a conducting electrode in an electrolyte solution without the direct connection with the external power supply. The ACS Paragon Plus Environment

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channel current (ichannel) flows through two possible routes including the ionic current through the electrolyte (is) and Faradaic current through the BPE (iBPE). The ηc was dependent on the relative resistance value of BPE (RBPE) and electrolyte solution (Rs) because they are in parallel mode. When Rs is much lower than the RBPE, substantial current flows through the solution. While Rs > RBPE, (in the case of lower concentration of electrolyte solution), most of the current passes through the BPE. Arora and Manz first embedded a Pt plate (wire) in a weighing boat as BPE and used a pH indicator solution to demonstrate the reaction at the two poles under a dc power supply of 30 V.15 The polarity of the BPE was opposite to the driving eletrodes. As shown in Figure 1, due to water oxidation and reduction, the pH of solution was changed, accompanied with the color change at two poles. Finally, they coupled the BPE reaction with ECL as readout to successfully achieve the detection and separation of ECL-related targets such as the ECL probe, co-reactant and quencher. However, the overwhelming background produced on the large area of the driving electrode is the main weakness of the O-BPS. Later, Crooks and co-workers demonstrated that the relationship between polarization voltage and electrode length and the potential gradient, which is important to activate the wireless electrochemical reactions (Figure 2). Moreover, they proved that there exists a strict quantitative relationship between the two reactions occurred at both ends of the BPE using ECL as readout.16 They demonstrated one important result that the targets which does not participate directly in the ECL process can also be anlyzed due to the electrically coupled reaction between two poles. The abovementioned work open up a new way for various applications covering molecule recognition and quantification, redox imaging and catalyst screening.17-32 Notably, in the beginning, people focused exclusively on the single-channel O-BPS mode. The targets preferred to react on the driving electrode, which produced huge background and influenced the detection sensitivity. On the other hand, the analytes and reporting probe were often mixed in the same channel, which can influence each other. To resolve the huge background from the driving electrode in the O-BPS, two main strategies were often adopted. One is based on the immobilization of the reporting probe, which is often used in the ECL analysis. For example, Xu’s group reported an ultrahigh sensitivity wireless ECL protocol for the detection of a tumor cells based on O-BPS.33 As shown in Figure 3, recognized element of an antisense DNA labeled with Ru(bpy)32+-conjugated silica nanoparticles was pre-modified on the anodic pole of the BPE and acted as signal readout, which can capture the reporter DNA. When used for the intracellular c-Myc mRNA detection, tumor cells were ACS Paragon Plus Environment

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transfected with antisense DNA/reporter DNA functionalized CdSe@ZnS quantum dot-conjugates and released the reporter DNA, which can hybridize with the antisense DNA on the anodic pole and induced the decreased ECL. Since there is no ECL reagent existed in the anode pole of driving electrode, the background form driving electrode was alleviated. The fabricated sensor exhibited excellent ECL signals with a good linear range over 2×10−16 to 1×10−11 M and provided great promise in the point of care testing. Another alternative was based on the metallic BPE as the sacrifice pole for visual reporting signal. For example, Crook’s group utilized the oxidation and dissolution reaction of Ag at the anodic pole to record the sensing reaction at the cathode.34 Moreover, they also extended this strategy to the BPE arrays for the rapid selection of catalyst for hydrogen evolution reaction (HER) and oxygen reduction reaction.35-37 The visual readout using the naked eyes combined with the wireless character of BPE achieved the high-throughput screening of the catalyst. While to avoid the influence induced from the mixture in the same channel, two-channel design was the efficient strategy and often adopted. Xu’s group designed a unique two-channel O-BPS38 and achieved the sensitive determination of prostate-specific antigen (PSA) without the interference from the bank background. This smart device comprised two ITO bands with a gap of 200 µm as BPEs in two microfluidic channels (one for sensing, one for reporting). In the absence of PSA, the ITO bands with a gap of 200 µm as two BPEs and the driving voltage should be high enough (ca. 12 V) to trigger the two ECL signal at the anodic poles of both BPEs in the reporting channels. If the driving voltage was set lower than 12 V (such as 7 V), there is no ECL signal observed, indicating the low bank background. However, when PSA was introduced to the sensing channel, the depositions of silver particles occurred, which induced the formation of an electronic circuit by making the two adjacent BPEs connected together to behave like one H-shaped BPE and thus only one ECL signal was obsereved at low driving voltage. Although this seperated design provided a sensitive visual analysis of cancer biomarkers without the blank background signal on BPE, the chemical interference from two poles was not resolved. Recently, Crooks’ group employed the dual-channel mode to achieve the physical separation of the ECL reporting cocktail and the solution containing the target.39 Different from the design of Xu’s group, they exposed the two poles of BPE in two separated channels and hence the sensing and reporting functions are chemically decoupled. Although this configuration can eliminate the chemical interference, the current efficiency similar to the Xu’s work is not up to 100% in this open mode and the background from the driving voltage was not resolved. To further improve the ECL efficiency, Sojic’s group made ACS Paragon Plus Environment

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an attempt to transfer the ECL reaction from the conventional surface-confined 2D process to 3D bulk emission using a dispersion of conductive carbon nanotube or carbon microbead as BPEs.40-42 However, the relative low current efficiency and the involvement of the high-driving voltage also limited the further application due to the intrinsic limitation of O-BPS. Can we achieve the 100% current efficiency with the O-BPS? Luckily, our group achieved it using the chemical energy as the driving force of BPE to activate the reactions at two poles for fabricating the self-powered electrochromic imaging platform.43 The basic operation principle of this self-powered BPE was based on the matched potential. As shown in Figure 4, taking the cathodic reporting pole as an example, once the potential of the anodic pole of BPE (Ered) was more negative than that needed for cathodic reaction (Ec-ox), the reactions at two poles can be powered each other. Here, the reduction of Prussian blue (with the onset potential of 0.35V vs Ag/AgCl) was employed as the indicator to reflect the catalytic oxidation reaction of the other pole. The onset oxidation potentials of these reactions are more negative than 0.35 V in the presence of catalysts. And thus the occurrence of the reactions at two poles was free from the extra power source, accompanied by the discoloring of Prussian blue. The discoloring rate of Prussian blue was associated with the performance of the catalyst, which provided a portable and visual platform for screening catalysts or enzyme activity via the electrochromic reaction. Moreover, this unique design without any extra power source eliminated the driving background signal, BPE background signal, uneven reporting signals and the influence of electrolysis from origin, which offered a more direct, easier and reliable target reporting tool, especially for the non-professional people to access the complicated screening of catalyst or enzyme, fast analysis of catalyst or enzyme activity. Closed Bipolar Electrode System. Compared with the O-BPS, the research on the closed bipolar electrode system (C-BPS) appeared later. Figure 5 shows the simplified experimental configuration of C-BPS and the electrolyte and BPE are in series mode. Therefore, the channel current is smaller than that in O-BPS under the same driving voltage and the polarization voltage cross the BPE is larger than that of O-BPS. Since there is only one current path through BPE between the two half cells, and thus the current efficiency will be up to 100% in theory. An important contribution in C-BPS was made in 2012 by Zhang and his colleagues,44 who demonstrated the coupled electrochemical reactions using bipolar microelectrodes and nanoelectrodes in C-BPS and confirmed the different voltammetric responses of BPE from the conventional system. Moreover, they also confirmed that this new method could be used for transient electrochemical ACS Paragon Plus Environment

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imaging, such as rapid extracellular secretion events between individual or neuronal populations and high throughput screening for novel electrocatalysts. It opens up a novel approach for various applications covering sensing, catalyst screening and so on.44-63 Our group for the first time introduced the microfluidic system into the C-BPS to develop ECL sensing microchip using integrated dual-channel mode. And the 100% ηc was proposed due to the serials connection (Figure 6). Moreover, the enormous background ECL signal from the driving anode was eliminated completely with C-BPS by simply placing the anodic poles of driving electrode and BPE in separated channels. Similar to the O-BPS, the reduction reaction at the BPE cathode can also be monitored by the anodic ECL and thus the oxidant such as K3Fe(CN)6 and ECL-related targets including co-reactants and quenchers can be monitored (Figure 6a).64 Combing the biorecognition elements, various biomolecules can be detected using this kind of the configuration, such as the determination of blood sugar coupled with the enzyme reaction. Besides to indicate the electrochemical reactions at the two poles, this C-BPS design can also be used for the analysis of non-electroactive chemicals in the sample cell, such as the conductivity sensing.65 Figure 6b demonstrates the basic principle for the conductivity sensing with the paper-based BPE chip. The voltage dropping across the ECL reporting cell (cell1) can be modulated by the sample conductivity in the analytes cell (cell2) due to the serials connection. Higher conductivity leads to the higher voltage dropping across cell2 due to the serials connection. While the ECL signals were dependent on the voltage dropping, therefore it can be used for indicating the sample conductivity. Moreover, this paper-based BPE design is also a universal sensing platform, that is, all the electrolytes including non-electroactive, electroactive and ECL-related chemicals would be indicated using ECL reaction, which made the qualitative analysis portable. Subsequently, to achieve the multifunction of C-BPS in one chip, an improved configuration with three channels and two BPEs were designed and developed.66 The serial structure allows the ECL as readout for the determination of all the oxidants, reductants or ECL-related chemicals in different reservoirs with a single device (Figure 6c). Moreover, this unique design can also be used for the set of molecular keypad lock and the fabrication of ratiometric sensing platform.67,68 For example, Xu’s group 68 utilized this design and introduced the ECL probes of luminol and Ru(bpy)32+ in two different reservoirs to develop a visual color-switch ECL and the ratiometric detecting principle for the determination of HL60 cancer cells based on the different response of ECL probes to H2O2. The visual color-switch ECL made the results easier to evaluate and more accurate. To simplify the process of the BPEs, our group ACS Paragon Plus Environment

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also employed the nanopores of a poly(ethylene terephthalate) membrane as the template of BPE to develope BPE arrays in C-BPS (Figure 6d). 69 Here, Au nanofibers were pre-deposited in the inner of the channel as BPE arrays and determination of alpha-fetoprotein (AFP) and carcino-embryonic antigen (CEA) in human serum with satisfying results was obtained with this BPE arrays. As mentioned above, most of the BPE sensors needed an optical readout to record the Faradaic current due to the wireless feature of BPE. Exploring new and visual tools for BPE will make the BPE more versatile. LED with the good stability was employed and integrated to the BPE chips as the signal readout for the portable analysis in recent work (Figure 6e) to replace the expensive luminescent reagent and instruments.70 This split BPE design can be by derived by inserting LED in dual-channel design or replacing the reporting cell of the three-channel with LED. Due to the ultrahigh current efficiency of the C-BPS, the reaction occurred at both poles of the BPE can be sensitively reported by the intensity of LED and recognized by the naked eye, which provided a low-cost analytical tool with high throughput and is highly desirable in environmental monitoring and daily diagnosis. In addition, we also explored the performance of electrochromic materials with good redox activity as reporting probe in the C-BPS.71 For example, a displaying platform with Prussian blue (PB) as the reporting probe was proposed for the high-throughput screening of electrocatalysts. The PB is electrodeposited at the cathodic pole of BPE, while the other end is used for the loading of catalyst (Figure 6f). Owing to the quantitative relation between the two reactions occurring at both ends of BPE, the discoloration speeds (PB to Prussian white, PW) are related to the amount or catalytic activity of the catalysts under the sufficient driving voltage. Due to the reversibility of the PB-PW reactions, the established platform can be easily regenerated by applying the opposite driving voltage. With this strategy, we opened a new way for the high-throughput characterization of the performances of catalysts. Meanwhile, by decorating the biorecongnization elements in the sensing poles, we extended this strategy for signaling the concentration of the biomarker-CEA in the blood sample based on targeted induced steric hindrance effects.72 The presence of CEA can effectively retard the interfacial electron-transfer kinetics of the redox probes and thus inhabited the deposition of PB. Conclusion and Perspective The combination of bipolar electrochemistry and ECL in 2001 led to an emergency of new branch named the bipolar analytical chemistry. The simple instruments and low cost with their wireless-contact principle made the bipolar electrochemistry more popular in the field of chemical and biological sensing. ACS Paragon Plus Environment

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Especially, the unique structure of C-BPS with high current efficiency made it possible to spatially separate the sensing pole from the reporting one, which made the sensing more versatile compared with the conventional three-electrode system. To get the high sensitivity, the 100% current efficiency, based on closed or open bipolar electrochemistry will still be the future trends. Although in the foregoing discussion, we have demonstrated the successful analysis of all chemicals with the bipolar electrochemistry, how to extinguish the chemicals (that is the selectivity) need be resolved by combing the analytical tools. In addition, the application of self-powered O-BPS with non-directional power source (chemical energy) as driving force is in the early stage. By rational design the reactions at the two poles, extended analytical platform needs to be explored utilizing this extremely high current efficiency and simplicity. With the requirement and development of the miniaturization, integrated BPE chip with microfluidic channels, driving electrode and BPE combined with different detection technique should be developed for the various applications in the future. For example, the construction of regenerated electrical stimulus-response switching system combined with BPE by simply changing the polarity of the driving voltage need to be explored. Moreover, multichannel BPE system needs to be constructed, which can contain many electrodes and achieve the detection of different analytes in one BPE chip. Accompanying the development of the analytical strategy, BPE will mature in the industrial manufacture, scientific research, daily medical diagnosis, especially in the point-of-care applications.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] and [email protected]. Tel: +86-431-85262003. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant No. 21427811), MOST, China (No. 2016YFA0203200 and 2016YFA0201300) and Youth Innovation Promotion Association CAS (No.2016208) and Jilin province science and technology development plan project 20170101194JC. ACS Paragon Plus Environment

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chemiluminescence: from urface-con fined reactions to bulk emission, Chem . Sci., 2015, 6, 4433-4437. (41) Poulpiquet, A.; Diez-Buitrago, B.; ; Milutinovic, M.; Sentic, M.; Bouffier, L.; Arbault, S.; D.; Kuhn, A.; Sojic, N. Dual enzymatic detection by bulk electrogenerated chemiluminescence. Anal. Chem., 2016, 88, 6585-6592. (42) Poulpiquet, A.; Diez-Buitrago, B.; Milutinovic, M.; Goudea, B.; Bouffier, L.; Arbault, S.; D.; Kuhn, A.; Sojic, N. Dual-color electrogenerated chemiluminescence from dispersions of conductive microbeads addressed by bipolar electrochemistry, ChemElectroChem, 2016, 3, 404-409. (43) Zhang, X. W.; Zhang, L. L.; Zhai, Q. F.; Gu, W. L.; Li, J.; Wang, E. K. Self-powered bipolar electrochromic electrode arrays for direct displaying applications. Anal. Chem. 2016, 88, 2543-2547. (44) Guerrette, J. P.; Oja, S. M.; Zhang, B. Coupled electrochemical reactions at bipolar microelectrodes and nanoelectrodes. Anal. Chem. 2012, 84, 1609-1616. (45) Cox, J. T.; Guerrette, J. P.; Zhang, B. Steady-state voltammetry of a microelectrode in a closed bipolar cell. Anal. Chem. 2012, 84, 8797-8804. (46) Guerrette, J. P.; Percival, S. J.; Zhang, B. Fluorescence coupling for direct imaging of electrocatalytic heterogeneity. J. Am. Chem. Soc. 2013, 135, 855-861. (47) Oja, S. M.; Guerrette, J. P.; David, M. R.; Zhang, B. Fluorescence-enabled electrochemical microscopy with dihydroresorufin as a fluorogenic indicator. Anal. Chem. 2014, 86, 6040-6048. (48) Oja, S. M.; Zhang, B. Imaging transient formation of diffusion layers with fluorescence-enabled electrochemical microscopy. Anal. Chem. 2014, 86, 12299-12307. (49) Sun, A. L.; Zheng, X. W. Electrochemiluminescence behavior of luminol at closed bipolar electrode and its analytical application. Chin. J. Anal. Chem. 2014, 42, 1220-1224. (50) Wu, S. Z.; Zhou, Z. Y.; Xu, L. R.; Su, B.; Fang, Q. Integrating bipolar electrochemistry and electrochemiluminescence imaging with microdroplets for chemical analysis. Biosens. Bioelectron. 2014, 53, 148-153. (51) Wu, M. S.; Liu, Z.; Shi, H. W.; Chen, H. Y.; Xu, J. J. Visual electrochemiluminescence detection of cancer biomarkers on a closed bipolar electrode array chip. Anal. Chem. 2015, 87, 530-537. (52) Essmann, V.; Barwe, S.; Masa, J.; Schuhmann, W. Bipolar electrochemistry for concurrently evaluating the stability of anode and cathode electrocatalysts and the overall cell performance during long-term water electrolysis. Anal. Chem. 2016, 88, 8835-8840. (53) Liu, M.; Liu, R.; Wang, D.; Liu, C. L.; Zhang, C. S. A low-cost, ultraflexible cloth-based ACS Paragon Plus Environment

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microfluidic device for wireless electrochemiluminescence application. Lab on a Chip 2016, 16, 28602870. (54) Lu, W. X.; Bao, N.; Ding, S. N. A bipolar electrochemiluminescence sensing platform based on pencil core and paper reservoirs. Rsc Advances 2016, 6, 25388-25392. (55) Oja, S. M.; Zhang, B. Electrogenerated chemiluminescence reporting on closed bipolar microelectrodes and the influence of electrode size. Chemelectrochem 2016, 3, 457-464. (56) Ongaro, M.; Gambirasi, A.; Ugo, P. Closed bipolar electrochemistry for the low-potential asymmetrical functionalization of micro- and nanowires. Chemelectrochem 2016, 3, 450-456. (57) Shi, H.W.; Zhao, W.; Liu, Z.; Liu, X. C.; Wu, M. S.; Xu, J. J.; Chen, H. Y. Joint enhancement strategy applied in ECL biosensor based on closed bipolar electrodes for the detection of PSA. Talanta 2016, 154, 169-174. (58) Shi, H. W.; Zhao, W.; Liu, Z.; Liu, X. C.; Xu, J. J.; Chen, H. Y. Temporal sensing platform based on bipolar electrode for the ultrasensitive detection of cancer cells. Anal. Chem. 2016, 88, 8795-8801. (59) Takano, S.; Inoue, K. Y.; Ikegawa, M.; Takahashi, Y.; Ino, K.; Shiku, H.; Matsue, T. Liquidjunction-free system for substitutional stripping voltammetry using a closed bipolar electrode system. Electrochem. Commun. 2016, 66, 34-37. (60) Wang, L.; Lian, W. J.; Liu, H. Y. A resettable keypad lock with visible readout based on closed bipolar electrochemistry and electrochromic poly(3-methylthiophene) films. Chem.-Eur. J. 2016, 22, 4825-4832. (61) Wu, M. S.; Liu, Z.; Xu, J. J.; Chen, H. Y. Highly specific electrochemiluminescence detection of cancer cells with a closed bipolar electrode. Chemelectrochem 2016, 3, 429-435. (62) Xu, W.; Fu, K. Y.; Ma, C. X.; Bohn, P. W. Closed bipolar electrode-enabled dual-cell electrochromic detectors for chemical sensing. Analyst 2016, 141, 6018-6024. (63) Xu, W.; Ma, C. X.; Bohn, P. W. Coupling of independent electrochemical reactions and fluorescence at closed bipolar interdigitated electrode arrays. Chemelectrochem 2016, 3, 422-428. (64) Zhang, X. W.; Chen, C. G.; Li, J.; Zhang, L. B.; Wang, E. K. New insight into a microfluidic-based bipolar system for an electrochemiluminescence sensing platform. Anal. Chem. 2013, 85, 5335-5339. (65) Zhang, X. W; Zhai, Q. F.; Xu, L.; Li, J.; Wang, E. K. Paper-based electrochemiluminescence bipolar conductivity sensing mechanism: a critical supplement for the bipolar system. J. Electroanal. Chem. 2016, 781, 15-19. ACS Paragon Plus Environment

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(66) Zhang, X. W.; Li, J.; Jia, X. F.; Li, D. Y.; Wang, E. K. Full-featured electrochemiluminescence sensing platform based on the multichannel closed bipolar system. Anal. Chem. 2014, 86, 5595-5599. (67) Wang, Y. Z.; Zhao, W.; Dai, P. P.; Lu, H. J.; Xu, J. J.; Pan, J.; Chen, H. Y. Spatial-resolved electrochemiluminescence ratiometry based on bipolar electrode for bioanalysis. Biosens. Bioelectron. 2016, 86, 683-689. (68) Zhang, H. R.; Wang, Y. Z.; Zhao, W.; Xu, J. J.; Chen, H. Y. Visual color-switch electrochemiluminescence biosensing of cancer cell based on multichannel bipolar electrode chip. Anal. Chem. 2016, 88, 2884-2890. (69) Zhai, Q. F.; Zhang, X. W.; Han, Y. C.; Zhai, J. F.; Li, J.; Wang, E. K. A nanoscale multichannel closed bipolar electrode array for electrochemiluminescence sensing platform. Anal. Chem. 2016, 88, 945-951. (70) Zhang, X. W.; Chen, C. G.; Yin, J. Y.; Han, Y. C.; Li, J.; Wang, E. K. Portable and visual electrochemical sensor based on the bipolar light emitting diode electrode. Anal. Chem. 2015, 87, 46124616. (71) Zhang, X. W.; Shang, C. S.; Gu, W. L.; Xia, Y.; Li, J.; Wang, E. K. A renewable display platform based on the bipolar electrochromic electrode. ChemElectroChem 2016, 3, 383-386. (72) Zhai, Q. F.; Zhang, X. W.; Xia, Y.; Li, J.; Wang, E. K. Electrochromic sensing platform based on steric hindrance effects for CEA detection. Analyst 2016, 141, 3985-3988.

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FIGURE CAPTIONS

Figure 1. A Pt plate (wire) in a weighing boat to demonstrate the open bipolar system. Reprinted (Adapted) with permission from

Reference 15, Analytical Chemistry. Copyright 2001, American

Chemical Society.

Figure 2. The relationship between polarization voltage and electrode length and the potential gradient, dE/dx, along the fluidic channel. Reprinted (Adapted) with permission Reference 16, from Analytical Chemistry. Copyright 2002, American Chemical Society. Figure 3. Intracellular c-Myc mRNA detection based on the wireless ECL biosensor. Reprinted (Adapted) with permission from Reference 33, Analytical Chemistry. Copyright 2012, American Chemical Society. Figure 4. Self-powered bipolar electrochromic strategy for displaying analysis of redox reaction. Reprinted (Adapted) with permission from Reference 43, Analytical Chemistry. Copyright 2016 , American Chemical Society. Figure 5. A simple experimental configuration of closed bipolar electrode cell. Reprinted (Adapted) with permission from Reference 44, Analytical Chemistry. Copyright 2012, American Chemical Society. Figure 6. (a) Configuration of dual-channel C-BPS for ECL sensors with a two-direction driving electrode. Reprinted (Adapted) with permission from Reference 64, Analytical Chemistry. Copyright 2013, American Chemical Society. (b) The paper-based bipolar conductivity sensing platform using ECL as readout. Reprinted (Adapted) with permission from Reference 65, Journal of Electroanalytical Chemistry. Copyright 2016, Elsevier. (c) Configuration of three-channel BPE sensing platform for the determination of all electroactive chemicals. Reprinted (Adapted) with permission from Reference 66, Analytical Chemistry. Copyright 2014, American Chemical Society. (d) The BPE arrays fabricated based on the nanopores of PET for analytes detection with ECL as output. Reprinted (Adapted) with permission from Reference 69, Analytical Chemistry. Copyright 2016, American Chemical Society. (e) ACS Paragon Plus Environment

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Mechanism and configuration of the split BPE with inserted LED as signal reporting. Reprinted (Adapted) with permission from Reference 70, Analytical Chemistry. Copyright 2015, American Chemical Society. (f) The schematic illustration on electrochromic strategy for the screening of redox reactions using C-BPS. Reprinted (Adapted) with permission from Reference 71, ChemElectroChem. Copyright 2015, John Wiley and Sons.

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Figure 1.

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Figure 2.

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