Bipolar Electrode Based Reversible Fluorescence Switch Using

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Bipolar electrode-based reversible fluorescence switch using Prussian blue/Au nanoclusters nanocomposite film Huanhuan Xing, Xiaowei Zhang, Qingfeng Zhai, Jing Li, and Erkang Wang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b00246 • Publication Date (Web): 13 Mar 2017 Downloaded from http://pubs.acs.org on March 13, 2017

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Analytical Chemistry

Bipolar electrode-based reversible fluorescence switch using Prussian blue/Au nanoclusters nanocomposite film

Huanhuan Xing,a,b Xiaowei Zhang,a Qingfeng Zhai,a Jing Li,a,* Erkang Wang a,*

a. State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin, 130022, China. b. University of Science and Technology of China, Hefei, Anhui, 230029, P. R. China. Corresponding author: Tel: +86-431-85262003, Fax: +86-431-85689711, E-mail: [email protected]; [email protected]

ABSTRACT A highly efficient fluorescence switch system based on closed bipolar electrode (C-BPE) system was proposed for the first time. Here, Au nanoclusters (Au NCs) were pre-modified on one pole of the BPE and acted as the fluorescent donor. Based on the spectral overlap between the absorbance of electrochromic material-Prussian blue (PB) and fluorescence spectrum of Au NCs, fluorescence quenching (“off” state) induced by inner filter effect was observed. Due to the electrochemical reversible redox reaction between PB and Prussian white, switching the polarity of driving voltage could easily achieve the fluorescence recovery of the Au NCs, corresponding to the “on” state. Through the reasonable design of C-BPE and optimization of driving voltage, the on-off ratio of the integrated fluorescence switch was up to 2.7 1

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and a good fatigue resistance while performing 10 on-off cycles was obtained owing to the good stability of Au NCs and the reversible redox feature of PB. The introduction of BPE made the fluorescence switch more simple and controllable compared with the traditional three-electrode system, which will provide a new route for the design of the electrical-stimuli responsive fluorescence switch, especially for the integration of miniaturization device. INTRODUCTION Bipolar electrode (BPE) generates the electrochemical reactions at two poles in the wireless mode under a sufficient power supply1, which provided a powerful tool for the control of high-throughput BPE arrays. Thanks to the simple configuration and easy miniaturization of BPE, a variety of interesting applications have been explored.2-12 For example, Kuhn’s group utilized bipolar electrodeposition to develop a site-selective synthesis of MOF Janus composites13, which promoted the application in drug delivery, catalysis and sensing. For transforming the Faradic reaction at the two poles of the BPE to the recognized optical signal, electrochemiluminescence(ECL) was first introduced to the BPE system due to the good temporal and spatial control14. Subsequently, the electrically coupled quantitative relationship between the sensing poles and reporting poles was proposed due to the electroneutrality across the BPE15. Since then, a series of sensors were fabricated based on different optical/visible signal as readout such as LED16, ECL17, fluorescence18,19, the deposition/dissolution of metal12 and electrochromic material20,21. Among them, the closed BPE (C-BPE) system with high current efficiency has drawn more interests since the sensing cell 2


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and reporting cell are separated physically and the only current flow path is through BPE22. And various targets have been detected based on the C-BPE including small molecules16, biomolecular23 and cancer cells24. For example, our group integrated C-BPE with the microfluidic system to achieve the 100% current efficiency and eliminated the background from the anode of the driving electrode by pumping the ECL reagent in the reporting channel25. Notably, at the current stage, most of the C-BPE based analysis including sensing and screening is mainly carried out with the ECL as the readout to record the Faradic reaction at the two poles. While the fluorescence analysis using BPE is relatively rare and the principle was mainly based on the change of fluorophore probe signal in the solution induced by bipolar reaction18,19. Moreover, the study on C-BPE based solid-state fluorescence switch with a good fatigue resistance has not been published. Design of stimuli-responsive reversible fluorescence switch has been paid more and more attention especially in the application of memory material, display, logic gates, and sensor26-29. And a variety of fluorescence switches have been constructed by exerting external stimuli such as pH30, biomolecule31 and electrical32. Caused by the easy control, electrical-stimuli fluorescence switch attracted particular notice. For example, Dong’s group smartly utilized the spectral overlap between fluorescent probe (eg. nanoclusters, quantum dots) and electrochromic substance to construct a series of reversible fluorescence switch32-35. Although good performances had been obtained, these works were performed in a traditional three-electrode system, which further limited the integration and miniaturization. 3



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Inspired by unique feature of C-BPE, a highly efficient fluorescence switch based on C-BPE system was proposed for the first time. Here, Au nanoclusters (Au NCs) were pre-immobilized on one pole of the BPE and acted as the donor (Scheme 1). Based on the spectral overlap between the absorbance of electrochromic material-Prussian blue (PB) and fluorescence spectrum of Au NCs, fluorescence quenching (“off” state) induced by inner filter effect was observed. Owing to the electrochemical reversible redox reaction of PB to Prussian white (PW), switching the polarity of driving voltage could easily achieve the fluorescence recovery of the Au NCs (“on” state). Through the reasonable design of C-BPE and optimization of driving voltage, the on-off ratio of the integrated fluorescence switch was up to 2.7 and a good fatigue resistance while performing on-off cycles was obtained due to the good stability of Au NCs and the reversible redox feature of PB. The simple and controllable operation of the fluorescence switch in the system of BPE made it more fascinating than the traditional three-electrode system. Thus we believe it will provide a new route for the design of the integrated electrical-stimuli responsive fluorescence switch. EXPERIMENTAL SECTION

Chemicals and Reagents. All the chemicals were used as received without any further purification. Bovine serum albumin (BSA) and powdery chitosan (CS) were purchased from Sigma-Aldrich Chemical Co.(Milwaukee, WI) HAuCl4·3H2O, KCl, HCl, K3[Fe(CN)6], K4[Fe(CN)6], FeCl3·6H2O, NaOH, KH2PO4, NaHCO3, and 4


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EDTA were bought from Beijing Chemical Reagent Company (Beijing, China). ITO-coated (sheet resistance:∼6 Ω/square) aluminosilicate glass slides were purchased from CSG (Shenzhen, China). Polydimethylsiloxane (PDMS) and curing agent were both obtained from GE Toshiba Sillicones Co., Ltd. The water used in the whole experiment was deionized water (18 MΩ·cm−1) purified by a Milli-Q system (Millipore, Bedford, MA). Instrumentation. Spectroelectrochemical measurements (in-situ fluorescence) were carried out in a re-designed fluorescence cell according to the previous report at room temperature33. For principle demonstration, platinum wires were employed as the driving electrodes in the BPE system, while the PB was pre-deposited on one pole of BPE using three-electrode system, where Ag/AgCl and platinum wire were acted as the reference electrode and counter electrode. All the electrochemical experiments were performed with CHI660D electrochemical work station (Shanghai Chenhua Instrument Corporation, China). Fluorescence spectrum was collected on a Fluoromax-4 spectrofluorometer (Horiba JobinYvon Inc., France) with excitation and emission slit widths of 10 nm and 10 nm, respectively. Absorption curves were recorded on a Cary 500 Scan UV-Vis-NIR spectrometer (Varian). The morphology of Au NCs was characterized by the JEOL 2100F transmission electron microscope (TEM) with an accelerating voltage of 200 kV. Fluorescence visual switch photos were acquired by a hand-held UV lamp (365 nm, 8.0 W) at regular intervals during the on-off operation of the nanocomposite film surface.

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Device Fabrication. Integrated electrodes with designed pattern were prepared using standard photolithographic techniques according to a mature procedure on an ITO substrate (10 cm×8.6 cm) 36. After being etched by the mixture (FeCl3: HCl: H2O = 2:1:1), a series of patterned ITO electrodes were obtained and divided into BPE snippets. Two reservoirs were punched in a PDMS membrane (4×1.5×4 mm and 14×1.5×7 mm) in accordance with the size of the patterned photomask. Finally, the patterned BPE was bonded with the PDMS cells reversibly for the fabrication of the fluorescence switch.

1.1 Preparation PB/Au NCs Nanocomposite Film Modified Electrode. 1.1.1 Synthesis of Au NCs. All glassware employed in the experiment was treated with freshly prepared aqua regia and rinsed completely in water before use. Au NCs were obtained according to previous method37. Generally, 10 mL BSA solution (50 mg/mL) was added into10 mL HAuCl4 solution (10 mM) and the mixture was stirred vigorously for 2 min at 37 °C. Then, 1 mL 1 M NaOH solution was dropped into the above mixture and further stirred for 24 h at 37 °C. To remove unreacted HAuCl4 or NaOH, the solution was dialyzed in deionized water for 48 h. Then the solution was stored at 4°C before use. 1.1.2 Electrodeposition of PB onto ITO. Before modification, the ITO chips were washed with acetone, ethanol, and water in ultrasonic bath sequentially. After rinsed with pure water and dried under N2 flow, the ITO electrodes were electrodeposited with PB in a freshly prepared solution 6


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containing 0.1 M KCl, 0.1 M HCl, 2.5 mM K3[Fe(CN)6] and 2.5 mM FeCl3 by applying a potential of 0.4 V (vs. Ag/AgCl) for 480 s in a traditional three-electrode system. In our previous work38, we had demonstrated that the ITO as driving electrode would be reduced in the more negative voltage and led to the instable signal, therefore the integrated ITO driving electrodes were pre-treated with Au using BPE system. Then, driving voltage (2.4 V 480 s) was applied for the growth of PB film. The obvious color and absorbance change confirmed the successful preparation of the PB film. The modified electrode was then thoroughly rinsed with water to remove the physically adsorbed species. 1.1.3 PB/ Au NCs Layer on ITO Electrode. 1% CS solution was prepared by adding 0.1 g CS in 10 mL 1% acetic acid solution. Then the prepared Au NCs solutions with different volume (1 mL, 2 mL and 3 mL) were mixed with 1 mL CS to obtain the homogeneous solution with different concentration of Au NCs. The above mixture (50 µL) was dropped onto the PB film coated ITO electrode and dried in room temperature to obtain the PB/Au NCs modified surface. RESULT AND DISCUSSION

Characterization of As-prepared Au NCs.

Here, the synthesis of fluorescent Au NCs was conducted in a protection atmosphere of BSA according to previous literature37. The typical fluorescence spectrum was presented in Figure1A, the obtained Au NCs were brown under day 7



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light and exhibited red emission under the UV light (Inset). The fluorescence emission peak was at 650 nm with excitation of 500 nm, which was almost consistent with the report. Transmission electron microscopy (TEM) image of Au NCs was recorded and revealed that the as-prepared Au NCs were ultrasmall with a size of 1.5 nm (see Figure1B), suggesting the successful preparation of the fluorescent Au NCs. Demonstration of Principle. Based on previous reports32-35, the spectral overlap between the absorption band of receptor and the emission band of luminescent materials were of great importance for the electrochemically controlled fluorescence switch, which could result in inner filter effect and resonance energy transfer. Here, PB was employed as receptor due to the excellent electrochromic feature and the corresponding absorption spectrum was collected. As depicted in Figure 2, a broad absorption band covering the range of 500-800 nm was observed with the electrodeposited PB and overlapped well with the emission spectrum of Au NCs, indicating the efficient energy transfer between PB and Au NCs. As we know, PB film could be reduced to Prussian white (PW) accompanied with the decreased absorbance of PB. This excellent reversible electrochromic behavior of PB made it possible for the operation of reversible switch by applying the opposite potentials. Due to the easy control and integration, the transferring between PB to PW was achieved using bipolar system by shifting the polarity of driving voltage in this work (see Scheme 1). To demonstrate the feasibility of reversible fluorescence switch on BPE, a simple 8


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preliminary experiment was carried out in two separated reservoirs using two Pt wires as driving electrodes (See Scheme S1). The PB/Au NCs modified electrode as one pole of the BPE, was placed in the reporting reservoir filled with 0.1 M KH2PO4 solution containing 0.1 M KCl (pH=6.0). In the supporting reservoir, a bare ITO was employed as another pole of the BPE and placed in 2.5 mM K3[Fe(CN)6]/K4[Fe(CN)6] solution containing 0.1 M KCl. The two poles were connected by conducting wire. As a control, the effect of driving voltage on the fluorescence emission of Au NCs was investigated. As shown in Figure S1, the emission intensity remained almost constant under the applied voltage and polarity, indicating the good stability of Au NCs (here, the polarity of reduction voltage was termed as positive, +). However, when PB was introduced, fluorescence switch based on BPE was achieved, see Figure S2a. The emission of Au NCs was relatively weak with the PB/Au NCs nanocomposite film caused by the energy transfer between Au NCs and PB in the open circuit. When the driving voltage was set as 2.8 V, enhanced fluorescence emission of Au NCs was observed due to the reduction of PB to PW. Moreover, when we switched the polarity of driving voltage to -1.8 V, the fluorescence emission of Au NCs could be decreased to the original intensity owing to the reversible formation of PB (See photograph of Figure S2b). Different experiment parameters which influenced the response were optimized including the ratio between Au NCs and CS (see Figure S3), the driving voltage and the response time used for transformation between PB and PW under driving voltage (See Figure S4). Under the optimal condition, the contrast luminescence on-off ratio could be up to 3 (See Figure S4D). 9



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As we know, the BPE system can be easily miniaturized by integrating all the channels and electrodes together. Thus, we utilized the lithography technique to pattern all the electrodes on a chip. As shown in Scheme1, ITO was patterned and used as the BPE and one pole of BPE was pre-modified with PB (2.4 V, 480 s) and Au NCs following the above step for the construction of reversible fluorescence switch. Notably, the driving voltage should be high enough to drive all the reactions due to the serial structure of C-BPE. Therefore, to avoid the reduction of ITO driving cathode in the more negative potential, the driving cathode of the chip was modified with Au layer in advance. As depicted in Figure 3, which was similar to the results obtained in the preliminary experiment, in the original state, the emission of Au NCs was absorbed by PB as a consequence of the spectral overlap of the PB and Au NCs thus the observed emission was very weak. When reduction voltage was applied, obvious fluorescence emission was obtained. When we switched the polarity of the driving voltage, the fluorescence emission was recovered to its original state, indicating that the integrated BPE chip could be used for the reversible fluorescence switch. Optimization of Driving Voltage.

The driving voltage played an important role for the transformation between PB and PW, therefore the influence of driving voltage was investigated to obtain the maximum on-off ratio. As shown in Figure 4A, under the reduction voltage, the PB was reduced to PW accompanied with the decrease of absorbance. With the increase 10


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of the voltage, the absorbance at 700 nm decreased rapidly. Reduction voltage of 3.0 V for 200 s was used for the subsequent experiment. To achieve the reversibility of fluorescence switch, the oxidation voltage from -1.4 V to -2.2 V was also optimized with the response time of 200 s. For the good stability, the original absorbance of PB was firstly measured (ca. 0.750) and chosen as the criterion. As seen in the Figure 4B, when the driving voltage was reversed to oxidation voltage, the absorbance of PB increased and could be recovered to the original value (0.750) at -2.2 V revealing the oxidation of PW film into PB film again. Based on the above results, 3.0 V and -2.2 V were chosen as the optimal “on” and “off” operating voltage for fluorescence switch. The Reversibility and Reproducibility of Switch.

Figure 5A depicted the reversibility of as-prepared fluorescence switch under the optimal condition, no obvious change was observed after 10 repeated cycles in both ‘‘on’’ and ‘‘off’’ states and the on-off ratio could be up to 2.7, indicating the good stability of the nanocomposite film. Moreover, the reproducibility of BPE chips from batch to batch was also investigated. As shown in Figure 5B, three different BPE chips were employed, the relative standard deviation (RSD) obtained was less than 10%, indicating the good reproducibility and reversibility. Such a good fatigue resistance of the fluorescence switch operation and miniaturization of BPE chip might hold a potential in the future potable photoelectric field. CONCLUSIONS

A highly efficient fluorescence switch system combined with closed bipolar 11



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electrode system was successfully constructed. Due to the easy integration and control, all the electrodes and reaction cells were integrated on one BPE chip. The performance of fluorescence switch based on the BPE chip was investigated. Owing to the good stability of nanocomposite film and the reversibility of the Prussian blue to Prussian white, the BPE chip based fluorescence switch system displayed high on-off ratio with good reproducibility and reversibility. Compared with the traditional three-electrode system, the fluorescence switch operated in the BPE mode could be easily

fabricated

and

miniaturized,

which

could

be

extended

to

other

electrical-stimulus switch. Moreover, the present study may provide a new route for the fabrication of the stimulus-response switch in memory material, logic gate, photoelectron science and electrochemical sensors based on BPE. 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). SUPPORTING INFORMATION Additional information about the principle demonstration and the parameters optimization including the driving voltage, the ratio between Au NCs and CS, the response time used for transformation between PB and PW under driving voltage using the non-integrated bipolar electrochemistry system.

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Scheme 1. Schematic illustration of fluorescence switch based the PB/Au NCs nanocomposite film with C-BPE microchip.

Figure 1. (A) Excitation (black line) and emission (red line) fluorescence spectra of as-prepared Au NCs. Shown in the inset are photos of Au NCs (left) under the visible light and (right) UV lamp at 365 nm; (B) The TEM image of Au NCs and the size distribution of Au NCs (inset). 16


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Figure 2. Absorption spectra of the PB layer in the open circuit (blue line) and PW layer after a reduction voltage applied (black line); fluorescence spectrum of Au NCs (red line).

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Figure 3. A: The fluorescence spectrum of PB/Au NCs nanocomposite film in open circuit (blue line), after a reduction voltage applied (red line), after a oxidation voltage applied (black line); B: The photos of fluorescence switch on BPE microchip excited by a hand-held UV lamp (365 nm). (a) in open circuit, (b) with a reduction voltage (3.0 V, 200 s), (c) with a oxidation voltage (-2.2 V, 200 s ).

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Figure 4. The influence of reduction driving voltage (A) and oxidation driving voltage (B) on the absorbance of PB at 700 nm with a response time of 200 s.

Figure 5. (A) The fluorescence intensity at 650 nm of PB/Au NCs nanocomposite film switching driving voltage between 3.0 V and -2.2 V with 200 s for ten cycles; (B) The on-off ratio of fluorescence switch with three different BPE microchips for five cycles.

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Bipolar Electrode Based Reversible Fluorescence Switch Using

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