Paper-Based Bipolar Electrochemistry
Paper-Based Bipolar Electrochemistry
Journal of Electrochemical Science and Technology. 2013. Oct, 4(4): 146-152
Copyright © 2013, The Korean Electrochemical Society
  • Received : December 09, 2013
  • Accepted : December 27, 2013
  • Published : October 30, 2013
Export by style
Cited by
About the Authors
Christophe Renault
Karen Scida
Kyle N. Knust
Stephen E. Fosdick
Richard M. Crooks

We demonstrate that carbon electrodes screen-printed directly on cellulose paper can be employed to perform bipolar electrochemistry. In addition, an array of 18 screen-printed bipolar electrodes (BPEs) can be simultaneously controlled using a single pair of driving electrodes. The electrochemical state of the BPEs is read-out using electrogenerated chemiluminescence. These results are important because they demonstrate the feasibility of coupling bipolar electrochemistry to microfluidic paperbased analytical devices (μPADs) to perform highly multiplexed, low-cost measurements.
1. Introduction
Here, we demonstrate that bipolar electrochemistry can be implemented on paper substrates and read out by optical signal transduction. Specifically, we show that simple and inexpensive screen-printed carbon electrodes can be used as bipolar electrodes (BPEs) for microfluidic paper-based analytical devices (μPADs). In fact, using just a pair of driving electrodes and a simple power supply, electrogenerated chemiluminescence (ECL) can be simultaneously activated at a μPAD containing an array of 18 BPEs. This result is important because it enables highly multiplexed electrochemical measurements on μPADs.
Following the introduction of μPADs by the Whitesides group, there has been considerable interest in the use of such devices for point-of-care (POC) diagnostics. 1-5 μPADs are typically prepared by patterning hydrophobic walls (using materials like photoresist and wax) on or within materials such as cellulose paper, 2 thereby forming hydrophilic channels that direct the flow of a liquid sample to specific reaction zones. These zones are loaded with reagents that in the presence of an analyte induce either an optical 6-7 or electrochemical 8-10 output. In the case of electrochemical detection, electrodes can be directly integrated in the μPAD by inkjet, stencil- or screen-printing. 8,11-12 Due to their ease of fabrication, screen-printed electrodes (SPEs) are commonly used to perform amperometric and potentiometric measurements on μPADs. 13- 16 SPEs have also been used to produce optical signals via ECL. 17
The time required to fabricate a single SPE is no different than the time necessary to fabricate an array; therefore, multiple SPEs can be easily integrated on a single μPAD platform to perform multiplexed assays. However, traditional three-electrode electrochemical approaches require a direct electrical connection to each SPE for read-out, and this can be difficult to implement on a simple POC sensor. This problem can be solved using bipolar electrochemistry, because faradaic processes are driven on the SPE without a direct electrical connection to a power supply. 18 Consequently, an array of BPEs can be operated using a single pair of driving electrodes and a simple power supply. For example, our group has shown that a dense array of 1000 BPEs can be actuated simultaneously using only two driving electrodes. 19-20
The principles of bipolar electrochemistry and BPEbased sensing are discussed in several recent reviews. 18,21 As shown in Scheme 1a , an externally applied potential bias ( E tot ) of sufficient magnitude can result in enough potential dropped over a BPE (Δ E elec ) to drive oxidation and reduction reactions simultaneously at opposite ends of the BPE. For example, Scheme 1b shows the case in which a carbon SPE, exposed to an aqueous electrolyte solution, produces hydroxide ions at the BPE cathode and protons at the BPE anode as a result of water reduction and oxidation, respectively. A very early application of bipolar electrochemistry to paper substrates was described by Bradley et al. in 1999. 22 Specifically, they used graphite microbeads trapped within the pores of a paper substrate as individual BPEs to reduce a palladium molecular precursor and thereby obtain a metal catalyst network uniformly dispersed onto the paper structure. Several examples of sensors using BPEs microfabricated on glass substrates have also been reported in the literature. 23-26 For example, our group used ECL produced by the co-oxidation of tris(bipyridine) ruthenium(II), Ru(bpy) 3 2+ , and tri-n-propylamine at the BPE anode to indirectly report sensing events at the cathodic poles of BPEs. 27-28
PPT Slide
Lager Image
In the present article, we demonstrate that carbon SPEs are suitable for performing bipolar electrochemistry on μPADs and, therefore, such a platform can host a dense array of many carbon SPEs. Moreover, we show that 18 carbon SPEs can be used as BPEs and simultaneously generate ECL. The ECL signal is stable and reproducible and can be used as an optical readout. We believe that these results open new possibilities for the creation of highly multiplexed sensing systems on μPADs.
2. Experimental Section
- 2.1 Chemicals.
NaH 2 PO 4 , Na 2 HPO 4 , and 2-(dibutylamino)ethanol (DBAE) were purchased from Sigma-Aldrich (St Louis, MO). The pH indicator solution was purchased from Acros (Cat # 61231-5000, Morris Plains, NJ). Ru(bpy) 3 Cl 2 ·6 H 2 O was purchased from Strem Chemicals (Newburyport, MA). NaCl and Whatman grade 1 chromatography paper were purchased from Fisher Scientific (Waltham, MA). Carbon ink (CI-2042) was purchased from Engineered Conductive Materials (Delaware, OH). All the chemicals were used without further purification. Deionized (DI) water having a resistivity >18 MΩ·cm was used for all experiments (Milli-Q gradient system, Millipore, Bedford, MA).
- 2.2 Device fabrication.
The patterns for the single-BPE and BPE array μPADs are provided in Fig. 1 . The devices were fabricated as follows. First, the paper channels were printed on Whatman grade 1 chromatography paper (20 × 20 cm sheet) using a Xerox 8570DN inkjet wax printer, and then the wax/paper assembly was heated in an oven at 120℃ for 90 s to form the hydrophobic wax barriers. 29,30 Second, the patterns of the electrodes were photolithographically defined onto 305 thread/ in 2 mesh (Ryonet W2024305) using a screen printing kit (Ryonet, SKSTARTERKIT). Third, carbon electrodes were manually screen-printed onto the wax patterned paper using the patterned mesh as a mask. Fourth, the electrodes were dried for 15 min at 23℃ and then for 30 additional minutes at 65℃ to stabilize the electronic properties of the ink. In the present article, all the electrodes were fabricated by screen-printing and are therefore referred to as SPEs. It is important to note that these SPEs are also referred to as BPEs and driving electrodes according to their function in the device. Finally, the driving electrodes, which are only used to power the device, were connected to an external power supply using copper tape ( Fig. 1 ). Note that the BPEs were always located between the two driving electrodes but were not connected to any external power source. During the ECL measurements, the μPADs were sandwiched between two glass slides.
PPT Slide
Lager Image
Electrode and wax patterns used to fabricate a single-BPE μPAD and a BPE array μPAD. The 3D views in the bottom row show the devices after screen printing the electrodes on the wax patterned paper. The black, blue, and white colors represent the carbon screen-printed electrodes, the wax, and the paper, respectively. The orange lines represent the copper tape used to connect the driving electrodes to a power supply.
- 2.3 Colorimetric measurements.
The paper channel was soaked in a solution containing 10.0 mM phosphate buffer (pH 7.6) and dried at 65℃ for 1 min. Then 8.0 μL of a colorimetric pH indicator solution mixed with DI water (30% v/v) and NaCl (10.0 mM) was placed on the channel. The color of the channel instantly turned yellow-green indicating a pH of about 7-8. A DC power supply (TDKLAMBDA LLS9120, Tokyo, Japan) was used to power the device at 25.0 V for 90 s.
- 2.4 ECL measurements.
The ECL intensity was recorded using a Nikon AZ100 microscope (0.5X Nikon AZ Plan Apo objective, no filter) equipped with a black and white Quantem 5125C camera. V++ Precision Digital Imaging software (Digital Optics, Auckland, New Zealand) was used to acquire the micrographs. An exposure time of 1 s was used for all micrographs. The light intensity emitted at the anodic poles of the BPEs was averaged over the surface of the BPE poles and background subtracted using ImageJ (NIH, Bethesda, MD). For the experiments shown in Fig. 5a , the time between measurements was 1 min to allow the rehomogenization of the ECL reagents in the diffusion layer. A DC power supply was used to power the paper device.
3. Results and Discussion
- 3.1 Bipolar electrochemistry on a μPAD
The device shown in Fig. 2 was used for a proof-ofconcept experiment to demonstrate that bipolar electrochemistry can be carried out on a μPAD using carbon SPEs. The figure shows two carbon driving electrodes screen-printed at the two opposing edges of a wax-patterned paper channel (12.0 mm long by 4.5 mm wide). A 2.0 mm long carbon SPE is defined at the center of the paper channel and between the driving electrodes. To demonstrate that faradaic processes occur simultaneously at the poles of the BPE, an elegant colorimetric experiment, initially reported by Manz and co-workers, was carried out. 18 The principles of this experiment are summarized in Scheme 1b . In the presence of a sufficiently large value of E tot , water is oxidized and reduced at the anodic and cathodic poles of a BPE generating protons and hydroxide ions, respectively. A colorimetric pH indicator solution is used to show the resulting changes in the local pH of the electrolyte solution. The initial green coloration in the paper channel ( Fig. 2 , top) arises from the colorimetric pH indicator solution filling the channel. However, when a potential bias of E tot = 25.0 V is applied between the two driving electrodes for 90 s ( Fig. 2 , bottom), the color of the dye changes near the edge of each electrode. At the positive driving electrode the color changes from green to red, indicating that the pH decreases (generation of H + ), and near the negative driving electrode the color changes from green to purple, indicating an increase of the pH (generation of OH ). Most importantly, however, the solution-saturated paper near the cathodic and anodic poles of the central BPE turned from green to purple and green to red, corresponding to the evolution of OH and H + , respectively, even in the absence of a direct electrical connection. This result correlates to the case depicted in Scheme 1b and clearly demonstrates that bipolar electrochemistry can be performed on a μPAD.
PPT Slide
Lager Image
Photographs of a single-BPE μPAD. The top and bottom photographs were taken before and after application of 25.0 V for 90 s, respectively. The channel was filled with a colorimetric pH indicator solution, which gives the yellowgreen coloration to the paper. The red and blue colors in the bottom photograph correspond to pH changes resulting from electrochemical reactions at the driving electrodes and the BPE.
- 3.2 ECL detection on a BPE array μPAD
As mentioned earlier, bipolar electrochemistry is well suited for multiplexed analyses, a functionality often desired in POC applications, and ECL provides a convenient and sensitive readout for large arrays of BPEs. 19,31 To test the feasibility of using ECL-based readout for an array of paper-supported BPEs, a μPAD composed of 18 individual BPEs was fabricated. The BPE array μPAD was filled with 40.0 μL of 0.10 M phosphate buffer solution (pH 6.9) containing 5.0 mM Ru(bpy) 3 2+ and 25.0 mM DBAE. 32 Optical micrographs of the device are shown in Figs. 3a and 3b . Figs. 3c and 3d show top and bottom luminescence micrographs, respectively, of the device after applying E tot = 35.0 V to the driving electrodes. A strong ECL signal is apparent at the anodic pole of each of the 18 BPEs, as well as at the anodic driving electrode (only shown in Fig. 3c ). Because wetted paper is semitransparent, and ECL is produced at the interface between the paper matrix and the electrodes, the strongest ECL signal is collected from the top of the device (that is, on the side of the paper opposite to the electrodes). However, lower intensity ECL can also be observed from the bottom of the μPAD ( Fig. 3d ).
PPT Slide
Lager Image
(a, b) Optical and (c, d) luminescence micrographs of a BPE array μPAD. The luminescence micrographs were obtained using Etot = 35.0 V. The positions of the driving electrodes are indicated by the orange dashed line. [Ru(bpy)3 2+] = 5.0 mM and [DBAE] = 25.0 mM in 0.10M phosphate buffer (pH 6.9).
Luminescence micrographs of a BPE array μPAD powered at different values of E tot are shown in Fig. 4a . The ECL intensity above the anodic driving electrode (left edge of the micrographs in Fig. 4a ) does not vary significantly for values of E tot ranging from 20.0 to 35.0 V. However, the ECL intensity observed at the BPEs increases dramatically as E tot increases. The average, normalized ECL intensity produced at the BPEs in the array,
PPT Slide
Lager Image
, is plotted in Fig. 4b as a function of E tot . These results show that a minimum E tot of ~25.0 V is required to produce ECL. That is, at values of E tot > 25.0 V, Δ E elec , which is approximately linearly related to E tot , 21 is sufficient to simultaneously drive the oxidation of DBAE and Ru(bpy) 3 2+ (as well as the reduction of oxygen at the cathodic pole of the BPE) and hence ECL is observed. As shown in Fig. 4b , higher values of Δ E elec lead to increased ECL intensity due to increased kinetics of the relevant faradaic reactions. 33
PPT Slide
Lager Image
(a) Luminescence micrographs of a BPE array μPAD operating at the indicated values of Etot. Note that a strong ECL signal is produced at the anodic driving electrode (on the left side of 3 of the micrographs), indicating that the device is operating correctly. The brightness and contrast of the micrographs have been adjusted to distinguish the ECL signal from the background. (b) Normalized ECL intensity averaged over the 18 BPEs present in a single device as a function of Etot. The different colors correspond to measurements for three independently prepared devices. The error bars correspond to the SD of the ECL intensity between BPEs on the same array.
In our experiments, E tot = 35.0 V led to intense and reproducible ECL signals, and therefore this value was used for a quantitative study of ECL generated by the BPE array μPADs. Figure 5a shows the ECL intensity, I ECL , obtained from 6 consecutive measurements of 6 of the 18 BPEs present on a single device. The coefficient of variation (CV, defined as the ratio of the standard deviation, SD, divided by the average intensity) of I ECL for the six consecutive measurements is 4%. This small variation in I ECL means that a single device can be read out several times without loss of accuracy or reproducibility.
PPT Slide
Lager Image
(a) Six successive measurements of the ECL intensity (normalized) of 6 of the 18 BPEs present on a single BPE array μPAD. An equilibration time of 1 min was allowed between each measurement. (b) Normalized ECL intensity at each BPE of a single BPE array μPAD as a function of the position of the BPE in the array. The number and color coding of the BPEs is the same as in Fig. 3. The error bars represent the SD for three successive measurements. (c) Average ECL intensities obtained using five independently prepared BPE array μPADs. The error bars represent the SD of .
The variation of I ECL for BPEs on the same array (intra-array variability) can be estimated from Fig. 5b . Here, the value of I ECL is plotted as a function of the position of the BPE in the array. The numbering of the BPEs follows the color and letter code shown in Fig. 3 . The CV (defined in this case as the ratio of the SD of three successive measurements divided by
PPT Slide
Lager Image
) of the intra-array ECL intensity is 16%. This value is adequate for manually-produced SPEs but would likely be improved by more reproducible fabrication processes.
To compare reproducibility between devices (interarray variability), the average intensity per array was measured using 5 different BPE array μPADs ( Fig. 5c ). The CV was calculated to be 6%. This low CV value indicates good reproducibility between μPADs, a crucial point for sensing applications. Note that although the inter-array CV is lower than the intraarray CV, this is likely a mathematical artifact caused by averaging over a larger number of samples (18 BPEs/device) for the inter-array CV.
4. Conclusion
To summarize, we have shown that carbon electrodes screen-printed directly onto cellulose paper can be used as BPEs. By using an array consisting of 18 individual BPEs, we also showed that ECL can be generated simultaneously at each BPE. Importantly, an E tot of 35.0 V was sufficient to reproducibly drive the ECL reaction. The reproducibility between individual BPEs and between devices is relatively good and demonstrates that ECL can be used as an optical reporting method on paper substrates. More importantly, because each BPE in the array does not require a direct electrical connection, a single power supply connected to a single pair of driving electrodes is sufficient to power a BPE array of arbitrary size. Accordingly, arrays such as these, fabricated on paper, could be used to simultaneously detect a large number of analytes, thereby opening the way to densely multiplexed μPADs.
This project is sponsored by the Department of the Defense, Defense Threat Reduction Agency (contract number HDTRA-1-13-1-0031). RMC thanks the Robert A. Welch Foundation (Grant F-0032) for sustained research support.
Martinez A. W. , Phillips S. T. , Butte M. J. , Whitesides G. M. 2007 Patterned Paper as a Platform for Inexpensive, Low-Volume, Portable Bioassays Angew. Chem., Int. Ed. 46 1318 -    DOI : 10.1002/anie.200603817
Martinez A. W. , Phillips S. T. , Whitesides G. M. , Carrilho E. 2010 Diagnostics for the Developing World: Microfluidic Paper-Based Analytical Devices Anal. Chem. 82 3 -    DOI : 10.1021/ac9013989
Parolo C. , Merkoci A. 2013 A. Paper-based nanobiosensors for diagnostics Chem. Soc. Rev. 42 450 -    DOI : 10.1039/c2cs35255a
Yetisen A. K. , Akram M. S. , Lowe C. R. 2013 Paper-based microfluidic point-of-care diagnostic devices Lab Chip 13 2210 -    DOI : 10.1039/c3lc50169h
Li X. , Ballerini D. R. , Shen W. A 2012 A perspective on paperbased microfluidics: Current status and future trends Biomicrofluidics 6 11301 -    DOI : 10.1063/1.3687398
Lewis G. G. , DiTucci M. J. , Phillips S. T. 2012 Quantifying Analytes in Paper-Based Microfluidic Devices Without Using External Electronic Readers Angew. Chem., Int. Ed. 51 12707 -    DOI : 10.1002/anie.201207239
Scida K. , Li B. , Ellington A. D. , Crooks R. M. 2013 DNA Detection Using Origami Paper Analytical Devices Anal. Chem. 85 9713 -    DOI : 10.1021/ac402118a
Dungchai W. , Chailapakul O. , Henry C. S. 2009 Electrochemical Detection for Paper-Based Microfluidics Anal. Chem. 81 5821 -    DOI : 10.1021/ac9007573
Liu H. , Xiang Y. , Lu Y. , Crooks R. M. 2012 Aptamer-Based Origami Paper Analytical Device for Electrochemical Detection of Adenosine Angew. Chem., Int. Ed. 51 6925 -    DOI : 10.1002/anie.201202929
Santhiago M. , Wydallis J. B. , Kubota L. T. , Henry C. S. 2013 Construction and Electrochemical Characterization of Microelectrodes for Improved Sensitivity in Paper-Based Analytical Devices Anal. Chem. 85 5233 -    DOI : 10.1021/ac400728y
Glatzel S. , Schnepp Z. , Giordano C. 2013 From Paper to Structured Carbon Electrodes by Inkjet Printing Angew. Chem., Int. Ed. 52 2355 -    DOI : 10.1002/anie.201207693
Tobjörk D. , Österbacka R. 2011 Paper electronics Adv Mater 23 1935 -    DOI : 10.1002/adma.201004692
Maxwell J. E. , Mazzeo A. D. , Whitesides G. M. 2013 Paperbased electroanalytical devices for accessible diagnostic testing MRS Bull 38 309 -    DOI : 10.1557/mrs.2013.56
Nie Z. , Deiss F. , Liu X. , Akbulut O. , Whitesides G. M. 2010 Integration of paper-based microfluidic devices with commercial electrochemical readers Lab Chip 10 3163 -    DOI : 10.1039/c0lc00237b
Wang P. , Ge L. , Yan M. , Song X. , Ge S. , Yu J. 2012 Paperbased three-dimensional electrochemical immunodevice based on multi-walled carbon nanotubes functionalized paper for sensitive point-of-care testing Biosens. Bioelectron. 32 238 -    DOI : 10.1016/j.bios.2011.12.021
Wu Y. , Xue P. , Kang Y. , Hui K. M. 2013 Paper-Based Microfluidic Electrochemical Immunodevice Integrated with Nanobioprobes onto Graphene Film for Ultrasensitive Multiplexed Detection of Cancer Biomarkers Anal. Chem. 85 8661 -    DOI : 10.1021/ac401445a
Delaney J. L. , Hogan C. F. , Tian J. , Shen W. 2011 Electrogenerated Chemiluminescence Detection in Paper-Based Microfluidic Sensors Anal. Chem. 83 1300 -    DOI : 10.1021/ac102392t
Fosdick S. E. , Knust K. N. , Scida K. , Crooks R. M. 2013 Bipolar Electrochemistry Angew. Chem., Int. Ed. 52 10438 -    DOI : 10.1002/anie.201300947
Chow K.-F. , Mavre F. , Crooks J. A. , Chang B.-Y. , Crooks R. M. 2009 A Large-Scale, Wireless Electrochemical Bipolar Electrode Microarray J. Am. Chem. Soc. 131 8364 -    DOI : 10.1021/ja902683f
Fosdick S. E. , Berglund S. P. , Mullins C. B. , Crooks R. M. 2013 Parallel Screening of Electrocatalyst Candidates Using Bipolar Electrochemistry Anal. Chem. 85 2493 -    DOI : 10.1021/ac303581b
Mavre F. , Anand R. K. , Laws D. R. , Chow K.-F. , Chang B.-Y. , Crooks J. A. , Crooks R. M. 2010 Bipolar Electrodes: A Useful Tool for Concentration, Separation, and Detection of Analytes in Microelectrochemical Systems Anal. Chem. 82 8766 -    DOI : 10.1021/ac101262v
Bradley J.-C. , Ma Z. 1999 Contactless Electrodeposition of Palladium Catalysts Angew. Chem., Int. Ed. 38 1663 -    DOI : 10.1002/(SICI)1521-3773(19990601)38:11<1663::AID-ANIE1663>3.0.CO;2-C
Klett O. , Nyholm L. 2003 Separation High Voltage Field Driven On-Chip Amperometric Detection in Capillary Electrophoresis Anal. Chem. 75 1245 -    DOI : 10.1021/ac020660h
Ordeig O. , Godino N. , del Campo J. , Munoz F. X. , Nikolajeff F. , Nyholm L. 2008 On-Chip Electric Field Driven Electrochemical Detection Using a Poly(dimethylsiloxane) Microchannel with Gold Microband Electrodes Anal. Chem. 80 3622 -    DOI : 10.1021/ac702570p
Chow K.-F. , Chang B.-Y. , Zaccheo B. A. , Mavré F. , Crooks R. M. A 2010 A Sensing Platform Based on Electrodissolution of a Ag Bipolar Electrode J. Am. Chem. Soc. 132 9228 -    DOI : 10.1021/ja103715u
Fosdick S. E. , Crooks R. M. 2011 Bipolar Electrodes for Rapid Screening of Electrocatalysts J. Am. Chem. Soc. 134 863 -
Zhan W. , Crooks R. M. 2003 Microelectrochemical Logic Circuits J. Am. Chem. Soc. 125 9934 -    DOI : 10.1021/ja0366585
Chow K.-F. , Mavré F. , Crooks R. M. 2008 Wireless Electrochemical DNA Microarray Sensor J. Am. Chem. Soc. 130 7544 -    DOI : 10.1021/ja802013q
Lu Y. , Shi W. , Jiang L. , Qin J. , Lin B. 2009 Rapid prototyping of paper-based microfluidics with wax for low-cost, portable bioassay Electrophoresis 30 1497 -    DOI : 10.1002/elps.200800563
Carrilho E. , Martinez A. W. , Whitesides G. M. 2009 Understanding Wax Printing: A Simple Micropatterning Process for Paper-Based Microfluidics Anal. Chem. 81 7091 -    DOI : 10.1021/ac901071p
Deiss F. d. r. , LaFratta C. N. , Symer M. , Blicharz T. M. , Sojic N. , Walt D. R. 2009 Multiplexed Sandwich Immunoassays Using Electrochemiluminescence Imaging Resolved at the Single Bead Level J. Am. Chem. Soc. 131 6088 -    DOI : 10.1021/ja901876z
Sun S. , Yang Y. , Liu F. , Pang Y. , Fan J. , Sun L. , Peng X. 2009 Study of Highly Efficient Bimetallic Ruthenium Trisbipyridyl ECL Labels for Coreactant System Anal. Chem. 81 10227 -    DOI : 10.1021/ac9020903
Mavré F. , Chow K.-F. , Sheridan E. , Chang B.-Y. , Crooks J. A. , Crooks R. M. A 2009 A Theoretical and Experimental Framework for Understanding Electrogenerated Chemiluminescence (ECL) Emission at Bipolar Electrodes Anal. Chem. 81 6218 -    DOI : 10.1021/ac900744p