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Wireless Electrochemiluminescence with Disposable Minidevice Wenjing Qi, Jianping Lai, Wenyue Gao, Suping Li, Saima Hanif, and Guobao Xu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/ac501833a • Publication Date (Web): 26 Aug 2014 Downloaded from http://pubs.acs.org on September 2, 2014
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Analytical Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
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Wireless Electrochemiluminescence with Disposable Minidevice Wenjing Qi,a,b Jianping Lai,a,b Wenyue Gao,a,b Suping Li,a,b Saima Hanif a,b and Guobao Xua,* a
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, PR China. b University of the Chinese Academy of Sciences, Chinese Academy of Sciences, No. 19A Yuquanlu, Beijing 100049, PR China.
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ABSTRACT: Wireless electrochemiluminescence system based on wireless energy transmission technique has been demonstrated for the first time. It has a disposable transmitter and a coiled energy receptor. The coiled energy receptor is smartly used as the electrodes. The wireless electrochemiluminescence system has been used to detect hydrogen peroxide with good sensitivity, featuring advantages of easy manipulation, low cost, and small size. The handy and cheap wireless electrochemiluminescence device can use laptops as a power supply. It is promising for the development of portable or disposable electrochemiluminescence devices for various applications (e.g. such as point of care testing, field analysis, scientific research and chemical education). These advantages enable to integrate many wireless electrochemiluminescence minidevices with screen printing coiled electrode arrays in microwell plates and CCD cameras to develop electrochemiluminescence high-throughput screening systems broad applications in clinical analysis, drug screening and biomolecular interaction studies.
Wireless energy transmission (wireless power transmission) is the transmission of electrical energy from a power source to an electrical load without conductors.1-3 Common examples of wireless power transmission methods include magnetic induction and electromagnetic radiation in the form of microwaves or lasers. It is useful in cases where interconnecting wires are inconvenient, hazardous, or impossible. These types of transmission have recently attracted a great deal of interest. One of the most important and popular applications of wireless power transmission is wireless charging (also known as inductive charging). The wireless charging technology possesses several advantages of overcoming the limit of distance owing to its wireless nature, charging for multiple devices simultaneously, supplying for invisible wireless magnetic induction designs, no risk of electric shock and easy manipulation. In the light of these advantages, the wireless charging technology has been broadly used in many products from electric vehicles, embedded medical devices and electric toothbrush to mobile phones, lamp and other electric devices. However, wireless energy transmission has never been used in electrochemiluminescence (ECL). ECL, also called electrogenerated chemiluminescence, is a technique resulting from the combination of electrochemistry chemiluminescence.4-8 It has been extensively utilized for clinical analysis with hundreds of million dollars in sales per year, being one of the most important in vitro diagnosis methods. All ECL detection methods require a direct electrical contact of electrochemical instruments with electrodes, either sensing electrodes in conventional ECL methods 9-12 or driving electrodes in ECL methods based on bipolar electrochemistry.13-18 In comparison with conventional ECL methods, ECL methods based on bipolar electrochemistry exert potential control over the electrolyte solution with driving electrodes rather than over individual sensing electrode. This enables simultaneous control of arbitrarily large electrode arrays with only two driving electrodes and simultaneous detection of multianalytes in the same sample, providing a very attractive possibility for high throughput analysis of many analytes in the same sample.13-18 Although the ECL methods based on bipolar electrochemistry eliminate the need of a direct contact of electrochemical instruments with sensing electrodes, they still requires a direct contact of electrochemical instruments with driving electrodes in solutions. It is difficult to individually tune the potential of each sensing electrodes and simultaneously measure many samples. It also has the background noise from driving electrodes. New ECL methods are therefore required to overcome these important limitations.
Scheme 1. Schematic representation of the wireless ECL system (A), the photo (B) and ECL image (C) of the wireless ECL minidevice using two short gold wires with small lacquered copper coils; c(H2O2): 50 mM; c(luminol): 1 mM; 50 mM Na2CO3−NaHCO3 buffer: pH 10.1.
In this study, wireless energy transmission technique has been explored for ECL for the first time (Scheme 1). Simple coils have been used not only as energy receptors but also as ECL electrodes. The use of wireless energy transmission technique provides a facile way to avoid direct electrical contact of electrochemical instruments with any electrodes. A cheap wireless charging accessory of electric toothbrush or wireless power transmission module was utilized to demonstrate the present strategy.
EXPERIMENTAL SECTION Chemicals and Materials. Luminol and hydrogen peroxide (H2O2) were obtained from Beijing Chemical Reagent Company. Sodium carbonate (Na2CO3), sodium bicarbonate (NaHCO3), ethylenediaminetetraacetic acid disodium salt (EDTA) and potassium chloride (KCl) were purchased from Sinopharm Chemical Reagent Company Ltd. (Beijing, China). 50 mM Na2CO3−NaHCO3 buffer solutions (pH 10.1) were used in ECL experiments of luminol and H2O2. All solutions were prepared with doubly distilled water. A copper wire with the diameter of 0.03 mm was used to form copper wire coils with a diameter of ca. 1 cm. A green light emitting diode (LED) was used to check the intensity of the induced current brought by the proposed wireless ECL system. Apparatus. ECL intensities were monitored from the bottom of the ECL cell with a BPCL ultraweak luminescence analyzer purchased from the Institute of Biophysics, Chinese Academic of Sciences. The photo-multiplier tube voltage was set at 800 V. A common CCD camera (Nikon Coolpix S3300) was used to take photos. Electrochemical potentials were supplied by a wireless charging accessory of electric toothbrush purchased from Weihai EMAG electronic company or wireless energy transmission module purchased from Xinketai electronic company. Alternating
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potential-time profile measured via two-channel digital storage oscilloscope (Tektronix TDS 2022 B). Glassy carbon electrodes (GCE) and gold electrodes were firstly polished with 0.3 µm alumina and then cleaned by distilled water in an ultrasonic bath prior to each use. Procedure of H2O2 detection. 1 mL of 0.1 M Na2CO3−NaHCO3 buffer solution (pH 10.1) containing 1 mM luminol, 1 mM EDTA, and different concentrations of H2O2 were pipetted into a microwell and then ECL detection was performed with the BPCL ultraweak luminescence analyzer by measuring total ECL intensity of different wavelengths and wireless ECL minidevice.
tively turning on and turning off the wireless electrochemical minisystem. As shown in Figure 3, ECL is induced upon turning on the wireless electrochemical minisystem, and ECL disappears upon turning off the wireless electrochemical minisystem. The proposed wireless ECL system shows good reproducibility with a relative standard deviation of ± 2.5 % for eight consecutive measurements.
RESULTS AND DISCUSSION To test the feasibility of the homemade wireless charging system, a wireless charging accessory of an electric toothbrush was combined with a homemade copper coil to induce light emission from a green light emitting diode (LED). As shown in Figure 1, a green light can be clearly visualized with eyes when the copper coil number is four. As the copper coil numbers increase, the green light emitted from LED becomes gradually more intense, indicating that the maximum output voltage is readily tunable by changing the copper coil numbers. It thus provides a simple way to initiate ECL by using wireless energy transmission devices and suitable coil electrodes. As a proof of concept, two short gold wires were connected with a small lacquered copper coils via spot welding to build a coil electrode (Scheme 1). An intense blue ECL of luminol and H2O2 generated in the vicinity of gold electrodes using a wireless charging accessory of electric toothbrush is clearly observed with both naked eyes and CCD camera (Scheme 1B and C). According to literature reports, luminol is first electrochemically oxidized and then reacts with hydrogen peroxide to form an aminophthalate ion in an excited state which emits light upon returning to the ground state.4f, 5d, 19 Counter reactions (e.g. the reduction of hydrogen peroxide) and/or capacitive current at counter electrode20 may play important roles in the compensation of electron flow for ECL reaction.
Figure 1. Photos of LED with copper coils as the receiver of proposed wireless energy transmission device before switching on the device (A) and after switching on the device (B-C). The coil numbers are 4 (B) and 5 (C).
Since the wireless power transmission module with a thickness of 0.3 cm, a width of 2 cm, and a length of 2 cm (Inset of Figure 2) is smaller, cheaper, and competent for ECL detection, the detection of H2O2 using the wireless power transmission module was subsequently attempted. As shown in Figure 2, ECL intensities increase linearly with increasing H2O2 concentrations from 5 × 10−6 M to 1 × 10−3 M with a correlation coefficient (r) of 0.9964. The regression equation is log (I-I0) = 1.54 + 0.76 log (c, µM) and the limit of detection (LOD) is 0.9 µM. I0 and I represent the ECL intensities before and after the addition of H2O2, respectively. The reproducibility of the proposed wireless ECL system was investigated by consecu-
Figure 2. ECL intensities of the proposed wireless ECL system at different concentrations of H2O2; Inset: image of the wireless ECL minidevice (right) with a thickness of 0.3 cm, a width of 2 cm, and a length of 2 cm, copper coil electrode; c(H2O2, µM): 0, 5, 10, 50, 100, 200, 500, 1000; c(luminol): 1 mM; c(EDTA): 1 mM; 50 mM Na2CO3−NaHCO3 buffer: pH 10.1. Photomultiplier tube voltage: 700V.
Figure 3. ECL profile with eight switches using the proposed wireless ECL system in 50 mM pH 10.1 Na2CO3−NaHCO3 buffer solution containing 1 mM luminol and 0.5 mM H2O2 and 1 mM EDTA. Photomultiplier tube voltage: 700V; “Turn-on” means that the wireless electrochemical minisystem is turned on to induce ECL; “Turn-off” means that the wireless electrochemical minisystem is turned off.
In contrast to other common ECL methods using commercial wired electrochemistry instruments, the present method features a cheap wireless electrochemical minidevice which only costs a few dollars. This wireless electrochemical minisystem has the advantages of no direct electrical connections, easy manipulation, low cost, small size, and no risk of electric
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shock. These advantages make it highly promising for the development of portable electrochemical devices and disposable electrochemical devices for point of care testing, field analysis, scientific study, and chemical education. Importantly, the wireless ECL holds great promise for the development of ECL high-throughput analysis systems by integrating screen printing coil electrode arrays in microwell plates, multiple wireless electrochemical minidevices, and CCD camera. For ECL experiments, each end of the receiver coil is connected with one electrode. Wireless transmission of electricity leads to an alternating current. Both electrodes are therefore not anode or cathode all the time, but alternating between anode and cathode. In other words, ECL may alternatively occur on both electrodes. Since it is more convenient to check whether wireless transmission of electricity induces an alternating current using conventional electrodes, we have connected two conventional electrodes and one resistor with the receiver coil, and measure potential-time profiles with a twochannel digital storage oscilloscope (Figure 4). The wireless electrochemical minisystem provides alternating potential with a frequency of 32.048 kHz, clearly indicating that wireless transmission of electricity leads to an alternating current. The generation of alternating current may hinder the application of wireless system in the application of ECL as a photonic reporter of electrochemical reaction at the other electrode that have been demonstrated in bipolar electrochemistry.18 Fortunately, it provides an opportunity to use ECL on both electrodes for measurements.
Figure 4. Schematic of the wireless ECL system using conventional GCE and gold wire electrode (A) and corresponding alternating potential-time profile (B) measured via two-channel digital storage oscilloscope (Tektronix TDS 2022 B). c(H2O2): 50 mM; c(luminol): 1 mM; 50 mM Na2CO3−NaHCO3 buffer: pH 10.1.
To check whether ECL occurs on both electrodes, we have connected two GCEs or two gold electrodes with the receiver coil, and subsequently monitor ECL on the electrodes with a CCD camera and the BPCL ultraweak luminescence analyzer. As shown in Figure 5, the photos taken with the CCD camera clearly show that ECL occurs on both gold electrodes or both GCEs connected to ends of the receiver coil. The inhomogenous light distribution in ECL images (Figure 5B) can be ascribed to the inhomogenous distribution of electric field on the electrode surfaces. Moreover, the ECL intensities measured via BPCL ultraweak luminescence analyzer decrease if one of the electrodes connected to ends of the receiver coil is shaded, further confirming that ECL occurs at both ends. The observation of ECL on both electrodes is attributed to the high frequency of induced alternating current and relatively low timeresolved capability of CCD camera and the BPCL ultraweak luminescence analyzer. The BPCL ultraweak luminescence analyzer measures ECL intensity 100 times per second. As mentioned above, the wireless electrochemical minisystem gives alternating current with a frequency of 32.048 kHz. The intensity shown in each dot of the intensity-time profile is
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consequently the integral intensity of about 320 times of ECL generated on both electrodes, resulting in the observation of ECL on both electrodes and enabling the detection using ECL generated on both electrodes. This phenomenon is similar to video which uses the integral of many slides to show a photo.
Figure 6. ECL profile with ten continuous switches at conventional electrodes via the proposed wireless ECL system in 50 mM pH 10.1 Na2CO3−NaHCO3 buffer solution containing 1 mM luminol, 0.2 mM H2O2, and 1 mM EDTA. Photomultiplier tube voltage: 800V. Figure 5. Photos (A) and ECL color images (B) taken with the CCD camera and ECL intensity-time profiles (C) for GCE electrodes (I) and gold electrodes (II) via the proposed wireless ECL system. Inset: the schematic of the wireless ECL system using GCEs and gold electrodes. c(H2O2): 50 mM; c(luminol): 1 mM; 50 mM Na2CO3−NaHCO3 buffer: pH 10.1; Photomultiplier tube voltage: 400V; Black curves (a) in ECL intensity-time profiles represent total ECL intensities at both two GCE or gold electrodes; Red (b) and green (c) curves represent ECL intensity at one GCE (Ф = 3 mm) or gold (Ф = 2 mm) electrode when the other electrode is shaded.
Since conventional electrodes are frequently used for scientific research and ECL detection, we have also tested the feasibility of low cost wireless electrochemical minisystem for ECL detection with conventional GCE and gold wire electrode by connecting the end of copper coils with conventional GCE and a gold wire electrode (Figure 4A). An intense ECL is clearly observed upon switching on the wireless ECL device. The wireless ECL system shows fine reproducibility with a relative standard deviation of ± 3.1 % for ten consecutive measurements (Figure 6), indicating that the proposed wireless ECL system is reliable for ECL measurements with conventional electrodes. The wireless ECL system using conventional electrodes was then utilized in the detection of hydrogen peroxide. The ECL intensities linearly increase with increasing H2O2 concentrations from 2 × 10−6 M to 5 × 10−4 M with a correlation coefficient (r) of 0.9990. The regression equation is I = 36.33 + 19.47 c (µM) and the limit of detection (LOD) is 0.8 µM, which is comparable to that of ECL methods using glassy carbon electrodes with conventional wired electrochemical instruments. These results indicate that the wireless ECL device can be potentially employed as low cost device for scientific research. It is also very appealing in chemical education practicals to demonstrate ECL phenomena.
CONCLUSIONS In conclusion, the wireless energy transmission technique have been explored for ECL applications. The adoption of wireless energy transmission eliminates the need of direct electrical connections and the use of large and expensive electrochemical instruments. Since the handy and cheap wireless electrochemical mini-device can use laptops or batteries as a power supply, it is very promising for the development of portable devices and disposable devices that may find broad applications in areas including point-of-care testing and field analysis21 as well as scientific research and chemical education. Moreover, multiple wireless electrochemical mini-devices can be easily integrated with screen printing helical electrode arrays in microwell plates and CCD camera to develop ECL high-throughput screening systems. The wireless ECL highthroughput screening system have three advantages over the ECL high-throughput screening system based on bipolar electrochemistry. First, the potentials of individual sensing electrode are tunable separately with wireless ECL system. Second, the former can measure many samples simultaneously while the later can measure only one sample each time. Third, the former does not have the background noise from driving electrodes. Since ECL currently has a huge sale per year in clinical analysis currently, the wireless ECL systems may find broad applications, such as clinical analysis, drug screening, and biomolecular interaction studies.
AUTHOR INFORMATION Corresponding Author [email protected].
ACKNOWLEDGMENT This project was supported by the National Natural Science Foundation of China (No. 21175126), the Academy of Sciences for the Developing World (TWAS), and Chinese Academy of Sciences.
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Wireless Electrochemiluminescence with Disposable Minidevice Wenjing Qi, Jianping Lai, Wenyue Gao, Suping Li, Saima Hanif, and Guobao Xu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/ac501833a • Publication Date (Web): 26 Aug 2014 Downloaded from http://pubs.acs.org on September 2, 2014
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Analytical Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
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Analytical Chemistry
Wireless Electrochemiluminescence with Disposable Minidevice Wenjing Qi,a,b Jianping Lai,a,b Wenyue Gao,a,b Suping Li,a,b Saima Hanif a,b and Guobao Xua,* a
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, PR China. b University of the Chinese Academy of Sciences, Chinese Academy of Sciences, No. 19A Yuquanlu, Beijing 100049, PR China.
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ABSTRACT: Wireless electrochemiluminescence system based on wireless energy transmission technique has been demonstrated for the first time. It has a disposable transmitter and a coiled energy receptor. The coiled energy receptor is smartly used as the electrodes. The wireless electrochemiluminescence system has been used to detect hydrogen peroxide with good sensitivity, featuring advantages of easy manipulation, low cost, and small size. The handy and cheap wireless electrochemiluminescence device can use laptops as a power supply. It is promising for the development of portable or disposable electrochemiluminescence devices for various applications (e.g. such as point of care testing, field analysis, scientific research and chemical education). These advantages enable to integrate many wireless electrochemiluminescence minidevices with screen printing coiled electrode arrays in microwell plates and CCD cameras to develop electrochemiluminescence high-throughput screening systems broad applications in clinical analysis, drug screening and biomolecular interaction studies.
Wireless energy transmission (wireless power transmission) is the transmission of electrical energy from a power source to an electrical load without conductors.1-3 Common examples of wireless power transmission methods include magnetic induction and electromagnetic radiation in the form of microwaves or lasers. It is useful in cases where interconnecting wires are inconvenient, hazardous, or impossible. These types of transmission have recently attracted a great deal of interest. One of the most important and popular applications of wireless power transmission is wireless charging (also known as inductive charging). The wireless charging technology possesses several advantages of overcoming the limit of distance owing to its wireless nature, charging for multiple devices simultaneously, supplying for invisible wireless magnetic induction designs, no risk of electric shock and easy manipulation. In the light of these advantages, the wireless charging technology has been broadly used in many products from electric vehicles, embedded medical devices and electric toothbrush to mobile phones, lamp and other electric devices. However, wireless energy transmission has never been used in electrochemiluminescence (ECL). ECL, also called electrogenerated chemiluminescence, is a technique resulting from the combination of electrochemistry chemiluminescence.4-8 It has been extensively utilized for clinical analysis with hundreds of million dollars in sales per year, being one of the most important in vitro diagnosis methods. All ECL detection methods require a direct electrical contact of electrochemical instruments with electrodes, either sensing electrodes in conventional ECL methods 9-12 or driving electrodes in ECL methods based on bipolar electrochemistry.13-18 In comparison with conventional ECL methods, ECL methods based on bipolar electrochemistry exert potential control over the electrolyte solution with driving electrodes rather than over individual sensing electrode. This enables simultaneous control of arbitrarily large electrode arrays with only two driving electrodes and simultaneous detection of multianalytes in the same sample, providing a very attractive possibility for high throughput analysis of many analytes in the same sample.13-18 Although the ECL methods based on bipolar electrochemistry eliminate the need of a direct contact of electrochemical instruments with sensing electrodes, they still requires a direct contact of electrochemical instruments with driving electrodes in solutions. It is difficult to individually tune the potential of each sensing electrodes and simultaneously measure many samples. It also has the background noise from driving electrodes. New ECL methods are therefore required to overcome these important limitations.
Scheme 1. Schematic representation of the wireless ECL system (A), the photo (B) and ECL image (C) of the wireless ECL minidevice using two short gold wires with small lacquered copper coils; c(H2O2): 50 mM; c(luminol): 1 mM; 50 mM Na2CO3−NaHCO3 buffer: pH 10.1.
In this study, wireless energy transmission technique has been explored for ECL for the first time (Scheme 1). Simple coils have been used not only as energy receptors but also as ECL electrodes. The use of wireless energy transmission technique provides a facile way to avoid direct electrical contact of electrochemical instruments with any electrodes. A cheap wireless charging accessory of electric toothbrush or wireless power transmission module was utilized to demonstrate the present strategy.
EXPERIMENTAL SECTION Chemicals and Materials. Luminol and hydrogen peroxide (H2O2) were obtained from Beijing Chemical Reagent Company. Sodium carbonate (Na2CO3), sodium bicarbonate (NaHCO3), ethylenediaminetetraacetic acid disodium salt (EDTA) and potassium chloride (KCl) were purchased from Sinopharm Chemical Reagent Company Ltd. (Beijing, China). 50 mM Na2CO3−NaHCO3 buffer solutions (pH 10.1) were used in ECL experiments of luminol and H2O2. All solutions were prepared with doubly distilled water. A copper wire with the diameter of 0.03 mm was used to form copper wire coils with a diameter of ca. 1 cm. A green light emitting diode (LED) was used to check the intensity of the induced current brought by the proposed wireless ECL system. Apparatus. ECL intensities were monitored from the bottom of the ECL cell with a BPCL ultraweak luminescence analyzer purchased from the Institute of Biophysics, Chinese Academic of Sciences. The photo-multiplier tube voltage was set at 800 V. A common CCD camera (Nikon Coolpix S3300) was used to take photos. Electrochemical potentials were supplied by a wireless charging accessory of electric toothbrush purchased from Weihai EMAG electronic company or wireless energy transmission module purchased from Xinketai electronic company. Alternating
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potential-time profile measured via two-channel digital storage oscilloscope (Tektronix TDS 2022 B). Glassy carbon electrodes (GCE) and gold electrodes were firstly polished with 0.3 µm alumina and then cleaned by distilled water in an ultrasonic bath prior to each use. Procedure of H2O2 detection. 1 mL of 0.1 M Na2CO3−NaHCO3 buffer solution (pH 10.1) containing 1 mM luminol, 1 mM EDTA, and different concentrations of H2O2 were pipetted into a microwell and then ECL detection was performed with the BPCL ultraweak luminescence analyzer by measuring total ECL intensity of different wavelengths and wireless ECL minidevice.
tively turning on and turning off the wireless electrochemical minisystem. As shown in Figure 3, ECL is induced upon turning on the wireless electrochemical minisystem, and ECL disappears upon turning off the wireless electrochemical minisystem. The proposed wireless ECL system shows good reproducibility with a relative standard deviation of ± 2.5 % for eight consecutive measurements.
RESULTS AND DISCUSSION To test the feasibility of the homemade wireless charging system, a wireless charging accessory of an electric toothbrush was combined with a homemade copper coil to induce light emission from a green light emitting diode (LED). As shown in Figure 1, a green light can be clearly visualized with eyes when the copper coil number is four. As the copper coil numbers increase, the green light emitted from LED becomes gradually more intense, indicating that the maximum output voltage is readily tunable by changing the copper coil numbers. It thus provides a simple way to initiate ECL by using wireless energy transmission devices and suitable coil electrodes. As a proof of concept, two short gold wires were connected with a small lacquered copper coils via spot welding to build a coil electrode (Scheme 1). An intense blue ECL of luminol and H2O2 generated in the vicinity of gold electrodes using a wireless charging accessory of electric toothbrush is clearly observed with both naked eyes and CCD camera (Scheme 1B and C). According to literature reports, luminol is first electrochemically oxidized and then reacts with hydrogen peroxide to form an aminophthalate ion in an excited state which emits light upon returning to the ground state.4f, 5d, 19 Counter reactions (e.g. the reduction of hydrogen peroxide) and/or capacitive current at counter electrode20 may play important roles in the compensation of electron flow for ECL reaction.
Figure 1. Photos of LED with copper coils as the receiver of proposed wireless energy transmission device before switching on the device (A) and after switching on the device (B-C). The coil numbers are 4 (B) and 5 (C).
Since the wireless power transmission module with a thickness of 0.3 cm, a width of 2 cm, and a length of 2 cm (Inset of Figure 2) is smaller, cheaper, and competent for ECL detection, the detection of H2O2 using the wireless power transmission module was subsequently attempted. As shown in Figure 2, ECL intensities increase linearly with increasing H2O2 concentrations from 5 × 10−6 M to 1 × 10−3 M with a correlation coefficient (r) of 0.9964. The regression equation is log (I-I0) = 1.54 + 0.76 log (c, µM) and the limit of detection (LOD) is 0.9 µM. I0 and I represent the ECL intensities before and after the addition of H2O2, respectively. The reproducibility of the proposed wireless ECL system was investigated by consecu-
Figure 2. ECL intensities of the proposed wireless ECL system at different concentrations of H2O2; Inset: image of the wireless ECL minidevice (right) with a thickness of 0.3 cm, a width of 2 cm, and a length of 2 cm, copper coil electrode; c(H2O2, µM): 0, 5, 10, 50, 100, 200, 500, 1000; c(luminol): 1 mM; c(EDTA): 1 mM; 50 mM Na2CO3−NaHCO3 buffer: pH 10.1. Photomultiplier tube voltage: 700V.
Figure 3. ECL profile with eight switches using the proposed wireless ECL system in 50 mM pH 10.1 Na2CO3−NaHCO3 buffer solution containing 1 mM luminol and 0.5 mM H2O2 and 1 mM EDTA. Photomultiplier tube voltage: 700V; “Turn-on” means that the wireless electrochemical minisystem is turned on to induce ECL; “Turn-off” means that the wireless electrochemical minisystem is turned off.
In contrast to other common ECL methods using commercial wired electrochemistry instruments, the present method features a cheap wireless electrochemical minidevice which only costs a few dollars. This wireless electrochemical minisystem has the advantages of no direct electrical connections, easy manipulation, low cost, small size, and no risk of electric
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shock. These advantages make it highly promising for the development of portable electrochemical devices and disposable electrochemical devices for point of care testing, field analysis, scientific study, and chemical education. Importantly, the wireless ECL holds great promise for the development of ECL high-throughput analysis systems by integrating screen printing coil electrode arrays in microwell plates, multiple wireless electrochemical minidevices, and CCD camera. For ECL experiments, each end of the receiver coil is connected with one electrode. Wireless transmission of electricity leads to an alternating current. Both electrodes are therefore not anode or cathode all the time, but alternating between anode and cathode. In other words, ECL may alternatively occur on both electrodes. Since it is more convenient to check whether wireless transmission of electricity induces an alternating current using conventional electrodes, we have connected two conventional electrodes and one resistor with the receiver coil, and measure potential-time profiles with a twochannel digital storage oscilloscope (Figure 4). The wireless electrochemical minisystem provides alternating potential with a frequency of 32.048 kHz, clearly indicating that wireless transmission of electricity leads to an alternating current. The generation of alternating current may hinder the application of wireless system in the application of ECL as a photonic reporter of electrochemical reaction at the other electrode that have been demonstrated in bipolar electrochemistry.18 Fortunately, it provides an opportunity to use ECL on both electrodes for measurements.
Figure 4. Schematic of the wireless ECL system using conventional GCE and gold wire electrode (A) and corresponding alternating potential-time profile (B) measured via two-channel digital storage oscilloscope (Tektronix TDS 2022 B). c(H2O2): 50 mM; c(luminol): 1 mM; 50 mM Na2CO3−NaHCO3 buffer: pH 10.1.
To check whether ECL occurs on both electrodes, we have connected two GCEs or two gold electrodes with the receiver coil, and subsequently monitor ECL on the electrodes with a CCD camera and the BPCL ultraweak luminescence analyzer. As shown in Figure 5, the photos taken with the CCD camera clearly show that ECL occurs on both gold electrodes or both GCEs connected to ends of the receiver coil. The inhomogenous light distribution in ECL images (Figure 5B) can be ascribed to the inhomogenous distribution of electric field on the electrode surfaces. Moreover, the ECL intensities measured via BPCL ultraweak luminescence analyzer decrease if one of the electrodes connected to ends of the receiver coil is shaded, further confirming that ECL occurs at both ends. The observation of ECL on both electrodes is attributed to the high frequency of induced alternating current and relatively low timeresolved capability of CCD camera and the BPCL ultraweak luminescence analyzer. The BPCL ultraweak luminescence analyzer measures ECL intensity 100 times per second. As mentioned above, the wireless electrochemical minisystem gives alternating current with a frequency of 32.048 kHz. The intensity shown in each dot of the intensity-time profile is
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consequently the integral intensity of about 320 times of ECL generated on both electrodes, resulting in the observation of ECL on both electrodes and enabling the detection using ECL generated on both electrodes. This phenomenon is similar to video which uses the integral of many slides to show a photo.
Figure 6. ECL profile with ten continuous switches at conventional electrodes via the proposed wireless ECL system in 50 mM pH 10.1 Na2CO3−NaHCO3 buffer solution containing 1 mM luminol, 0.2 mM H2O2, and 1 mM EDTA. Photomultiplier tube voltage: 800V. Figure 5. Photos (A) and ECL color images (B) taken with the CCD camera and ECL intensity-time profiles (C) for GCE electrodes (I) and gold electrodes (II) via the proposed wireless ECL system. Inset: the schematic of the wireless ECL system using GCEs and gold electrodes. c(H2O2): 50 mM; c(luminol): 1 mM; 50 mM Na2CO3−NaHCO3 buffer: pH 10.1; Photomultiplier tube voltage: 400V; Black curves (a) in ECL intensity-time profiles represent total ECL intensities at both two GCE or gold electrodes; Red (b) and green (c) curves represent ECL intensity at one GCE (Ф = 3 mm) or gold (Ф = 2 mm) electrode when the other electrode is shaded.
Since conventional electrodes are frequently used for scientific research and ECL detection, we have also tested the feasibility of low cost wireless electrochemical minisystem for ECL detection with conventional GCE and gold wire electrode by connecting the end of copper coils with conventional GCE and a gold wire electrode (Figure 4A). An intense ECL is clearly observed upon switching on the wireless ECL device. The wireless ECL system shows fine reproducibility with a relative standard deviation of ± 3.1 % for ten consecutive measurements (Figure 6), indicating that the proposed wireless ECL system is reliable for ECL measurements with conventional electrodes. The wireless ECL system using conventional electrodes was then utilized in the detection of hydrogen peroxide. The ECL intensities linearly increase with increasing H2O2 concentrations from 2 × 10−6 M to 5 × 10−4 M with a correlation coefficient (r) of 0.9990. The regression equation is I = 36.33 + 19.47 c (µM) and the limit of detection (LOD) is 0.8 µM, which is comparable to that of ECL methods using glassy carbon electrodes with conventional wired electrochemical instruments. These results indicate that the wireless ECL device can be potentially employed as low cost device for scientific research. It is also very appealing in chemical education practicals to demonstrate ECL phenomena.
CONCLUSIONS In conclusion, the wireless energy transmission technique have been explored for ECL applications. The adoption of wireless energy transmission eliminates the need of direct electrical connections and the use of large and expensive electrochemical instruments. Since the handy and cheap wireless electrochemical mini-device can use laptops or batteries as a power supply, it is very promising for the development of portable devices and disposable devices that may find broad applications in areas including point-of-care testing and field analysis21 as well as scientific research and chemical education. Moreover, multiple wireless electrochemical mini-devices can be easily integrated with screen printing helical electrode arrays in microwell plates and CCD camera to develop ECL high-throughput screening systems. The wireless ECL highthroughput screening system have three advantages over the ECL high-throughput screening system based on bipolar electrochemistry. First, the potentials of individual sensing electrode are tunable separately with wireless ECL system. Second, the former can measure many samples simultaneously while the later can measure only one sample each time. Third, the former does not have the background noise from driving electrodes. Since ECL currently has a huge sale per year in clinical analysis currently, the wireless ECL systems may find broad applications, such as clinical analysis, drug screening, and biomolecular interaction studies.
AUTHOR INFORMATION Corresponding Author [email protected].
ACKNOWLEDGMENT This project was supported by the National Natural Science Foundation of China (No. 21175126), the Academy of Sciences for the Developing World (TWAS), and Chinese Academy of Sciences.
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