Portable and Visual Electrochemical Sensor Based on the Bipolar

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Portable and Visual Electrochemical Sensor Based on the Bipolar Light Emitting Diode Electrode Xiaowei Zhang, Chaogui Chen, Jian-Yuan Yin, Yanchao Han, Jing Li, and Erkang Wang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b01018 • Publication Date (Web): 15 Apr 2015 Downloaded from http://pubs.acs.org on April 21, 2015

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Analytical Chemistry

Portable and Visual Electrochemical Sensor Based on the Bipolar Light Emitting Diode Electrode Xiaowei Zhang, a,b Chaogui Chen, a,b Jianyuan Yin, c Yanchao Han, a,b Jing Li a,b,* and Erkang Wang a,b,

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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. c. Department of Traditional Chinese Medicinal Chemistry, Pharmacy College, Jilin University, Changchun 130021, PR China Corresponding author: Prof. Erkang Wang and Assoc. Prof. Jing Li, Tel: +86-431-85262003, Email: [email protected]; [email protected]

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ABSTRACT Here we report a novel sensing strategy based on the closed bipolar system, in which we utilize a light emitting diode (LED) to connect a split bipolar electrode (BPE) and generate the luminescent signal in the presence of the target. With this design, we have constructed a BPE array for the quick and high-throughput determination of various electroactive substances with naked eyes. Due to the ultrahigh current efficiency of the closed bipolar system, the sample concentration can be reported by the luminous intensity of the inserted LED without the expensive luminescent agent and instruments. Besides, the stability of the signal is improved because of the electroluminescent property of the LED. To demonstrate the promising applications of the bipolar LED electrode (BP-LED-E), the rapid quantification of four model targets (H2O2, ascorbic acid (AA), glucose and the blood sugar) have been achieved based on different principles.


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INTRODUCTION Since the invention of electrochemiluminescence (ECL) 1 by A. J. Bard, this electrochemical method has been developed into an important microanalytical tool and applied in many fields.

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In 2001,

Manz’s group introduced the ECL technique into the bipolar electrochemistry, 9 and then an important branch of the analytical chemistry named bipolar analytical chemistry has been emerged in the past decade.

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Generally, a bipolar electrode (BPE) or BPE array was employed as the transducer to

transform the chemical or electrochemical signal to the optical signal. At the very beginning, only the ECL related analytes (co-reactant or quencher) could be detected in the anodic pole of a BPE. 9 Later, Chang et al. demonstrated that any electroactive analytes can be detected based on the quantitative relationship between the reactions occurring at both poles of the BPE. 12 However, the presence of some oxidants (for example, O2 and H2O2) would quench the ECL of the Ru(bpy)32+/TPrA system with such single channel designs. 3 Moreover, the current efficiency of the mentioned designs is quite low, which makes the redox strategy impractical or insensitive in most cases. 13 To break the mentioned limitation, a variety of different approaches have been explored to improve the analytical performance of BPE. For example, our group developed a dual-channel closed bipolar system to separate the oxidant and ECL reagent in two channels and achieved almost 100% current efficiency. 13 Recently, Chow et. al. made a new attempt utilizing the electrodissolution of Ag-based BPE to record a sensing/recognition event permanently.14 Based on these principles, a lot of wonderful works have been reported on molecule recognition and quantification,

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redox imaging

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and screening of electrocatalysts.

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Nevertheless, the dependence on the expensive luminescent reagent and instruments, or the use of complex but disposal Ag/Cr-based BPE makes these strategies uneconomic and impractical in routine use. Nyholm and coworkers have ever proposed that an ammeter can be integrated in a split BPE to monitor the current in the open bipolar system, which can avoid the use of the expensive luminescent reagent and instruments.

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However, due to the low current efficiency of the open bipolar system (a


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quite large driving voltage may be needed in this mode) and the quite big volume of the ammeter, the practical application of such design was seriously limited. 10 Inspired by the unique character of the split BPE and 100% current efficiency of the closed bipolar system 13,21,29, a novel style of closed bipolar sensing system by integrating a light emitting diode (LED) into a split BPE (termed BP-LED-E ) was developed (Scheme 1a and c). With the BP-LED-E, all the electroactive analytes can be easily detected with the naked eye using different principles. In addition, the use of LED eliminated the dependence on the expensive luminescent agent and instruments that are commonly used in BPE sensors. As shown in Scheme 1a, if a LED is inserted into the BPE of the dualchannel design or the reporting cell of the three-channel design is replaced by a LED (and c), both of them can be converted to the BP-LED-E sensor. Scheme 1b illustrates the structure and operating mechanism of the BP-LED-E. For running this device, only an external dc power supply is required. Due to the ultrahigh current efficiency of the closed bipolar system, the LED can be easily triggered by the reactions occurring at both poles of the BP-LED-E. More importantly, the LED luminous intensity is quantitatively related to the reaction rate at the ends of the BP-LED-E. With this design, we have constructed a BP-LED-E array and achieved the rapid and sensitive quantification of H2O2, ascorbic acid (AA), glucose and the blood sugar with naked eyes. Thus, the BP-LED-E holds great promise in designing the portable and low-cost sensors for point-of-care and daily application. EXPERIMENTAL SECTION Chemicals and Reagents. All the chemicals were of analytical reagent grade and used as received without any further purification. AA was purchased from Sigma-Aldrich Chemical Co. (Milwaukee, WI). The standard stock solutions of H2O2 was prepared in the 0.1 M phosphate buffer (PB, pH 6.0), and AA was prepared in 0.1 M PB (pH 7.4). Maltose, β-D-glucose, R-lactose, and D-fructose were purchased from Beijing Chemical Reagent Company (Beijing, China), and their solutions (in 0.1 M PB, pH 6.0) were left at room temperature for 24 h before use. Two human serum samples (one from normal and another from diabetes patient) were obtained from the Hospital of Jilin ACS Paragon Plus Environment

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University and diluted 10 times with 0.1 M PB (pH 6.0) for blood sugar analysis. Super thin chip LED (NICHIA) was purchased from local electronic market. Device Fabrication. Au (100 nm)/Ti (10 nm)/glass slide was prepared by physical vapor deposition as the conducting substrate, and the patterned driving electrodes and split BPEs were obtained using the standard photolithographic technique. Then a LED was adhered to the split BPE with silver-epoxy adhesive to construct the BP-LED-E. Note that all the electrical parameters of the selected LEDs (in a chip) should be equal (e.g. driving voltage and current-voltage curve). Finally, two PDMS membranes with a series of reservoirs according to the layout of the electrodes were bonded to the substrate by exposing to air plasma (60 w, air flow: 800 µL/min) for 100 s and then heating at 80 °C for 1 hour. It should also be noted that the anode of the integrated LED should be connected to the cathode of the BPE in the anodic cell (the cell with driving anode). Operation Principle. A Sony wx170 camera was set to the timed mode with 3 second delay and used to record the optical signal from the BP-LED-E. The power supply was turned on 2 seconds after the camera being triggered. RESULT AND DISCUSSION Demonstration of Principle. A signal reporter should be stable enough to provide reliable signal related to the target information. However, the most widely used probe in BPE-based device is the Ru(bpy)32+/TPrA ECL system, which would be easily influenced due to the complex electrochemical reaction and multitudinous influence factors of the light emitting process. In this work, we developed the so-called BP-LED-E, which employed a LED as the signal reporter. Although the ECL related targets cannot be detected using this new sensing mode, the BP-LED-E can be used in the determination of the electroactive analytes due to the quantitative relationship between the reactions occurring at both poles of the BPE. Interestingly, the


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use of LED avoided the dependence on the expensive luminescent agent and instruments commonly used in the BPE sensors. Prior to use, the stability and reproducibility of the LEDs were characterized by the cyclic voltammetry (CV). As shown in Figure 1a, four selected LEDs exhibited almost the same CV curve. In addition, after running 10 cycles, no change of the CV curve was observed (inset of Figure 1a), indicating the ultra-high stability of the selected LEDs. The lowest recognizable luminous intensity emitted from LED with the naked eye was generated at ca. 1.4 V. After reaching the lowest driving voltage, a slight increase in voltage (current indeed) was enough to induce a distinguishable enhancement of the LED brightness. Then, two selected LEDs were integrated into a chip with two parallel BPE and long BPE leads. As depicted in Figure 1c, after adding the testing solution (PB, 0.1 M, pH 7.4) into all the cells, the two LEDs showed identical light intensity when the driving voltage was set at 5.5 V, indicating the same electrochemical performance of the two constructed BP-LED-Es. However, when another LED was connected in parallel into one of the BP-LED-Es (referred as BP-multi-LED-E), the luminous intensity observed from the BP-multi-LED-E was significantly lower than that observed from the BP- LED-E (Figure 1d). This phenomenon proved that the rate-determining step of the whole process was not the excitation of LED but the reaction in the sample cells, which indeed meant that the LED luminous intensity was quantitatively related to the concentration of the reactant. This is the basis of the BP-LED-E sensing strategy. It should also be noted that the LED can offer more genuine information of the reaction occurring at the BPE due to much more stable and simpler electroluminescence process than the ECL reaction. Optimization of the Driving Voltage. Based on the above results, four BP-LED-Es were integrated into a single chip for the fast and sensitive quantification of different analytes (Figure 2a, b and c). During the experiment, it was found that the driving voltage had great influence on the BP-LED-Es performances. Thus, the driving voltage was optimized prior to the sample determination. In this experiment, all the cathodic cells and the first and third anodic cells were filled with PB (0.1 M, pH 7.4). 4 µL of 0.1 mM and 1 mM H2O2 were added ACS Paragon Plus Environment

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into the second and forth anodic cells, respectively. Then the driving voltage was shifted from 4.9 V to 5.9 V (step increment: 0.2 V). As shown in Figure 2d, only the BP-LED-E with 1 mM H2O2 in its anodic cell was lighted when the driving voltage was 4.9 V. The BP-LED-E with 0.1 mM H2O2 was activated at the voltage 5.1 V (Figure 2e). When the voltage was higher than 5.1 V, the first and third BP-LED-Es would also be activated due to the water electrolysis, which was regarded as the background signal (Figure 2f and g). Moreover, the overwhelming background signal would mask the target signal if the driving voltage was higher than 5.5 V (Figure 2h and i). From all the above results, it can be concluded that low driving voltage produced weak target signal with high signal to noise ratio (S/N), and high driving voltage induced strong target signal but low S/N. Hence, the driving voltage of 5.0-5.5 V was selected and regulated according to practical circumstances in the following experiments. Analytes Detection. To demonstrate the potential application of the BP-LED-E, H2O2, AA, glucose and blood sugar were selected as model targets and detected using different principles. First of all, H2O2 was detected in the anodic cell. Briefly, 4 µL of 0.01 mM, 0.1 mM, 1 mM and 10 mM H2O2 were added into the four anodic cells, respectively. All the cathodic cells were filled with PB (0.1 M, pH 7.4), and the driving voltage for H2O2 analysis was 5.1 V. As shown in Figure 3a, the H2O2 concentration-induced differences of the LED light intensity from four BP-LED-Es can be easily recognized by the naked eye. With the aid of PhotoShop histogram, it was found that the LED light intensity was linearly related to the logarithm values of H2O2 concentration. The background signal coming from the white light around the environment was relatively high (histogram value: ca. 60, Figure 3d and e), which can be completely eliminated by using a cassette. 30,31 With the similar method, AA (0.01 mM, 0.1 mM, 1 mM and 10 mM) was detected at the cathodic poles (Figure 3b). These two experiments confirmed that the proposed BPLED-E has the same function as the conventional BPE that can report the electroactive analytes by making full use of both poles of the BPE. Besides, it was also inferred that the fabricated device can be


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applied to the redox imaging, owing to the different performances in the determination of H2O2 and AA under the same driving voltage (Figure 3a and b). With this ingenious design, the selective analysis of glucose and blood sugar were also achieved by introducing an incubation process with glucose oxidase (GOD). Samples (pH 6.0) containing 1 mg/mL GOD and different concentrations of glucose (0.1 mM, 1 mM, 5 mM and 10 mM) were incubated in 37 °C water bath for 30 min to generate H2O2. After incubation, the sample solution was added to the anodic cell and detected under the same conditions as H2O2. As displayed in Figure 3c, the identification and quantification of four glucose samples can be achieved in a single experiment with naked eyes and the histogram. Meanwhile, 10 mM lactose, maltose, fructose and 1 mM glucose were employed to characterize the selectivity of this glucose sensing strategy under the same conditions. Results showed that the selectivity of BP-LED-E strategy toward glucose was very good (Figure 3d, 1st: glucose; 2nd: maltose; 3th: R-lactose; 4th: D-fructose). Prior to the determination of blood sugar, the noise caused by the diluted serum was tested. The second and third (anodic) cells were filled with serum sample and the first and forth cells were filled with PB (0.1 M, pH 6.0), and the driving voltage was set to 5.5 V. Figure 3e shows that the diluted serum doesn’t cause any extra noise compared to the PB. Based on that, the rapid identification of sugar level of two blood samples has been achieved. In this experiment, 0.1 mM and 0.6 mM (one-tenth of the diabetes threshold level: 6 mM) incubated glucose solutions were added to the first and forth anodic cells as the reference points. The second and third cells were filled with incubated blood samples (one from normal people and another from the people with diabetes), and the driving voltage was set to 5.1 V. As shown in Figure 3f, the light from the second BP-LED-E was stronger than that from the first BP-LED-E, indicating this blood sample was from a person with diabetes. The histogram result of the blood sugar level was 12.3 mM, which was in accordance with the result from the glucose meter (13.0 mM). While the luminous intensity of the third BP-LED-E was a little weaker than that of the first BP-LED-E, which meant this blood sample was from a normal person without diabetes (histogram result: 5.1 mM; glucose meter result: 4.7 mM). ACS Paragon Plus Environment

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Compared with the traditional methods, it is very easy to achieve the high-throughput analysis (one of the most important advantages of the BPE) of blood sugar (or other substances) using the device with BP-LED-E array. In addition, the sensitivity of BP-LED-E can also be greatly improved by employing a photomultiplier as the detector.

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From the above results, it can be concluded that the proposed BP-

LED-E design is very suitable for constructing electrochemical sensing platform and redox imaging sensor for various electroactive molecules with high throughput. CONCLUSIONS In conclusion, we have developed a novel style of closed bipolar system, in which a LED is integrated inside the BPE as the signal reporter. Due to the ultrahigh current efficiency of the closed bipolar system, the reaction occurred at both poles of the BPE can be sensitively reported by the LED and recognized by the naked eye. More importantly, there is no need of the expensive ECL reagent and instruments in the BP-LED-E sensing strategy. With this design, we have constructed a visual sensor with a BP-LED-E array for the quantification of various analytes with high throughput. Using the developed chip, we have achieved the rapid determination of four model targets (H2O2, AA, glucose and blood sugar) with good performance, which preliminarily proves the potential of the BP-LED-E in the redox imaging and sensing. Therefore, it is believable that the developed BP-LED-E holds bright prospect in designing low-cost electrochemical devices with high throughput, which is highly desirable in environmental monitoring and daily diagnosis. ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China with Grant No. 21427811, 21190040 and 21305078, Jilin Province Science and Technology Development Plan Project 20130522130JH, the Program of Chinese Academy of Sciences YZ201203. REFERENCES


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FIGURE CAPTIONS Scheme 1. Fundamental structure and mechanism of the dual-channel bipolar system (a), the BP-LEDE sensor (b) and the three-channel bipolar system (c) and their relationships. Figure 1. (a) Uniformity of four selected super thin chip LEDs; inset: cycle stability of the LED; (b) Electrodes configuration of the BP-multi-LED-E device; (c) The uniformity of two selected LED and the constructed BP-LED-Es; (d) The comparison between the BP-LED-E and the BP-multi-LED-E. Figure 2. Fabrication process of the device with a BP-LED-E array (a, b and c) and the optimization of the driving voltage for this device: (d) 4.9 V; (e) 5.1 V; (f) 5.3 V; (g) 5.5 V; (h) 5.7 V; (i) 5.9 V. Figure 3. Determination of different analytes: naked-eye sensing images and the red light intensity data obtained from the PhotoShop histogram. (a) H2O2; (b) AA; (c) Glucose; (d) Selectivity toward the glucose; (e) Background of the diluted serum; (f) Blood sugar test.


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Scheme 1.


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Figure 1.


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Figure 2.


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Figure 3.


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Portable and Visual Electrochemical Sensor Based on the Bipolar

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