Enhancing the Bipolar Redox Cycling Efficiency of Plane-Recessed

Aug 4, 2016 - The individual electrochemical anodic responses of dopamine (DA), epinephrine (EP), and pyrocatechol (CT) were investigated at arrays of...
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Enhancing the Bipolar Redox Cycling Efficiency of Plane-Recessed Microelectrode Arrays by Adding a Chemically Irreversible Interferent Dingwen He,† Jiawei Yan,*,† Feng Zhu,† Yongliang Zhou,† Bingwei Mao,† Alexander Oleinick,‡ Irina Svir,‡ and Christian Amatore*,‡ †

State Key Laboratory for Physical Chemistry of Solid Surfaces, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian 361005, PR China ‡ CNRS UMR 8640 “PASTEUR”, Sorbonne Universités - UPMC Univ Paris 06, Ecole Normale Supérieure − PSL Research University, Département de Chimie, 24 rue Lhomond, Paris 75005, France S Supporting Information *

ABSTRACT: The individual electrochemical anodic responses of dopamine (DA), epinephrine (EP), and pyrocatechol (CT) were investigated at arrays of recessed gold diskmicroelectrodes arrays (MEAs) covered by a gold plane electrode and compared to those of their binary mixture (CT and EP) when the top-plane electrode was operated as a bipolar electrode or as a collector. The interferent species (EP) displays a chemically irreversible wave over the same potential range as the chemically reversible ones of DA or CT. As expected, in the generator-collector (GC) mode, EP did not contribute to the redox cycling amplification that occurred only for DA or CT. Conversely, in the bipolar mode, the presence of EP drastically increased the bipolar redox cycling efficiency of DA and CT. This evidenced that the chemically irreversible oxidation of EP at the anodic poles of the top plane floating electrode provided additional electron fluxes that were used to more efficiently reduce the oxidized DA or CT species at the cathodic poles. This suggests an easy experimental strategy for enhancing the bipolar efficiency of MEAs up to reach a performance identical to that achieved when the same MEAs are operated in a GC mode.

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Indeed, as assemblies of microelectrodes, GC-operated MEAs retain the advantages from radial diffusion while overcoming the disadvantage of low current signal of individual microelectrodes.16−20 Therefore, GC-MEA systems exhibit excellent performance for selective and sensitive electrochemical detection of analytes with chemically reversible redox waves. Interestingly, such MEAs can also be successfully operated in bipolar mode.21−23 Bipolar behavior is generally supposed to require the presence of a high external electrical field parallel to the surface of an electrically isolated conductor (film, band, etc.) immersed into a solution. Coupled electrochemical oxidation and reduction may then occur at opposite ends of the equipotential conductor because the potential of the solution immediately adjacent to the floating conductor varies along its surface.24,25 Microsystems relying on such a strategy have been applied in separating,26 concentrating,26 sensing,27−29 electrogenerated chemiluminescence,30 and electrodeposition,31,32 etc. However, this is not the only way of creating a continuous variation of the solution−electrode

ue to their broad applications in analytical electrochemistry, arrays involving paired-electrode systems have attracted extensive interest in recent years.1 These arrays are generally operated in a generator-collector (GC) mode. In this mode, one electrode (generator) potential is biased to oxidize (or reduce, accordingly) an electroactive species, while its paired electrode (collector) is set at a potential appropriate for converting back the oxidized (or reduced, accordingly) species into the initial one.2,3 When the electrochemical wave of the analyte presents some chemical reversibility, this sets up a redox cycling loop that leads to an amplification of the detection current. Conversely, for species without any chemical reversibility, the device operates as if the generator electrode was alone. The amplification thus depends on the geometry of the paired-electrode device and on the lifetime of the species formed at the generator.2,3 Lithographic techniques have allowed microfabricating microelectrode arrays (MEAs) with various configurations and dimensions of paired-electrode elements. These exhibit versatile functions from simple redox cycling amplification,4−9 detection of intermediates with short lifetimes,10,11 reduction of interference by species giving rise to chemically irreversible waves,12,13 as well as measurements of diffusion coefficient.14,15 © 2016 American Chemical Society

Received: April 13, 2016 Accepted: August 4, 2016 Published: August 4, 2016 8535

DOI: 10.1021/acs.analchem.6b01454 Anal. Chem. 2016, 88, 8535−8541

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Analytical Chemistry potential along the surface of a floating electrode. For example, this can be achieved by imposing a diffusional concentration gradient of redox species next to the floating conductor through operating a nearby working electrode. This occurs, for example, during the SECM examination of unbiased conducting substrates33 in which the SECM tip acts as the working electrode while the floating substrate acts as the bipolar one.34,35 The same occurs in MEAs whose unit cells comprise one working electrode close to a floating electrode.21−23,36−39 Compared to operating in a GC mode, using such MEA in a bipolar one is certainly advantageous for analytical applications that cannot be performed in a laboratory with bipotentiostats, e.g., when implanted in micro- and nanodevices or for in-field measurements. Indeed, in bipolar mode, only the common working electrode potential needs to be controlled. Yet, redox cycling amplification in bipolar mode is always smaller than in the GC mode.23 It may reach the amplification efficiency of the GC mode only when the floating electrode surface area is sufficient to allow the pole located farthest from the working electrode to deliver a current flux of sufficient value at the pole closest to the working electrode.21−23 This presents a significant disadvantage, since this geometrical constraint is synonymous with having a low density of the unit cells in the array. This may be detrimental when the overall array surface area is limited. Indeed, the overall MEA current intensity increases with the number of unit cells.16 Beyond simply enhancing the detected analytical current, important redox cycling may also be extremely useful to nearly eliminate the contribution of an interferent whose wave is chemically irreversible and occurs before that of the species of interest.21,22 This is indeed a direct consequence of redox cycling properties,2,3 since only currents pertaining to at least partially chemically reversible redox couples may be amplified by redox cycling. In this work we wish to investigate another frequent analytical situation, in which the interferent, still having a chemically irreversible wave, has now the same (or a similar) half-wave potential as the analyte while the latter is supposed to give a chemically reversible wave. This is, e.g., the case when some catecholamines need to be detected in the presence of another one.4,5 Interestingly, this case study will serve to establish that, rather than being negative, the presence of such an interferent may be extremely beneficial. In fact, such interferent may be used for increasing the bipolar analytical efficiency of a MEA, up to reach that achieved by operating the same device in a GC mode while keeping the advantage of a bipolar operation.

Silicon wafers were purchased from Jingzhe Electronic Materials Co. Ltd. Chromium and gold targets for magnetron sputtering were bought from Zhongnuo Advanced Materials Technology Co. Ltd. Photoresist AZ5214E was obtained from AZ Electronic Materials, and photoresist S1805 was bought from Shipley Co. Ltd. MEAS Microfabrication. Plane-recessed MEAs were microfabricated using photolithography according to the general design sketched in Figure 1a. The recessed microdisks

Figure 1. (a) Geometrical characteristics of the plane-recessed MEAs used in this study; (b) schematic illustration of the diffusional cross talk between two adjacent microwells when a microwell-MEA performs in a bipolar mode initiated by oxidation of species Red at the poised disks recessed electrodes while the overlaid plane electrode is left floating.

radius, r, was 6 μm. The depth, h, of the microwells was ca. 2.5 μm, and their center−center distance, d, was ca. 8r. The overall area of the top plane electrode of the plane-recessed MEAs was fixed as 2 × 2 mm2. The arrays used in this study contained 256 microwell elements. Note that the aspect ratio h/r ∼ 0.42 of the microwells as well as the d/r ∼ 8 value were purposely fixed at rather low values; that is, they were not optimized for a best analytical performance of the present MEAs (compared to previous MEAs with similar geometries that we reported previously)13,21,22 in order to give redox cycling amplifications in the bipolar mode less than in a GC one. This was indeed required to demonstrate experimentally the validity of the concept investigated in this study (see main text). The microfabrication procedures for plane-recessed MEAs were disclosed in detail in our previous work.13 Yet, they are fully described again in the Supporting Information of this work. Briefly, a layer of silica was first implemented on a 4 in. diameter silicon wafer by thermal oxidation. Then a layer of Cr/Au was sputtered on the wafer and patterned through a first mask. Next, the exposed Au was etched by KI/I2 etchant and the underlying Cr film was etched with a chromium etchant consisting of a mixed solution of Ce(NH4)2(NO3)6 and HClO4. A layer of polyimide precursor was spin-coated onto the wafer and cured to perform as the insulating layer between the top

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EXPERIMENTAL SECTION Materials and Reagents. All solutions were prepared using Milli-Q water (18.2 MΩ·cm, Millipore). Dopamine (DA), pyrocatechol (CT), epinephrine (EP), and Ru(NH3)6Cl3 (see Supporting Information) were bought from Sigma-Aldrich. KNO3, I2, KCl, KI, Ce(NH4)2(NO3)6, and HClO4 were obtained from Sinopharm Chemical Reagents Co. Agar was purchased from Quanzhou city Quangang chemical plant. All chemical reagents were of analytical grade and used without any further purification. Phosphate buffer solution (PBS) was made of 0.1 M K2HPO4 and 0.1 M NaH2PO4 without NaCl. The bulk solution pH was adjusted to 3 with H3PO4 (85%) and was controlled with a pH meter. 8536

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Figure 2. (a) Confocal optical microscopy image of a fraction of the plane-recessed MEA used in this study; (b) SEM image of a fraction of the same MEA. The MEAS used in this study contained 256 unit cells.

Figure 3. Electrochemical responses when the potential of the microdisks electrodes of the plane-recessed MEA used in this study was scanned at 10 mV/s. Solution composition: PBS buffer (pH 3) containing 0.1 mM CT (black); 0.1 mM DA (red); or 0.1 mM EP (blue). (a) Bipolar mode: the top plane electrode was left floating; (b) GC mode: the potential of the top plane electrode was set to 0.2 V vs SCE. See Experimental Section for the PBS buffer composition.

presence of an interferent whose chemically irreversible electrochemical wave closely overlaps with that of the target analyte. Indeed, under such conditions the strategy that we reported previously when the interferent wave occurs much before that of the analyte becomes inoperative because the interferent diffusion to the microdisks of the MEAs cannot be prevented anymore by its capture by the top plane electrode.13,21,22 The second objective that emerged during the course of this initial study led us to investigate how the presence of the interferent could be used favorably in order to enhance the bipolar redox cycling efficiency of the microwellsbased MEAs used in this work to reach its maximum value, i.e., that achieved when using the same MEA in a GC mode.23 For this reason the MEAs used in this study followed the same general design reported before (Figure 1a) but were not optimized for their best analytical performance as we did previously.13,21,22 Indeed, their geometrical characteristics were purposely selected23 to afford an intrinsic bipolar redox cycling efficiency markedly less than that achieved when the same MEA is used in a GC mode. A three-dimensional confocal image of the plane-recessed MEAs used in this work is shown in Figure 2a. The microwells of radius r = 6 and 2.5 μm depth were positioned at the mesh points of a squared ordered network, with microdisk electrodes being located at their bottom. SEM imaging confirmed the adequate geometry of the microwells and their quadratic arrangement (Figure 2b). The center-to-center distance, d ∼ 8r,

plane electrode and the microdisks electrodes at the bottom of the microwells. After that, the top layer of Cr/Au was patterned through lift-off by using a second mask. The exposed insulating layer was etched by inductively coupled plasma (ICP) through a mask. The morphology of plane-recessed MEAs was characterized by confocal microscopy (OLS1200, OLYMPUS) and SEM (scanning electronic microscopy) (HITACHI S-4800); see Figure 2. Electrochemical Experiments. All electrochemical experiments were carried out using a CHI 814A (Chenhua Corp., China). In bipolar mode, the top plane electrode was left floating, and in generator-collector mode, its potential was controlled using a bipotentiostatic control. A saturated calomel electrode (SCE) and a gold wire were used as reference and counter electrodes, respectively. The electrochemical behavior of the present MEAs has been characterized using the chemically reversible outer sphere redox couple Ru(NH3)62+/3+ (see Supporting Information) and found entirely coherent with our former published experimental results21,22 and theoretical analyses,23 considering that the present MEAs were purposely not optimized for the best analytical performance (see main text).

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RESULTS AND DISCUSSION As stated in the Introduction, this work had two objectives. One was to examine the analytical difficulties introduced by the 8537

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Analytical Chemistry between microwells was selected to ensure a significant density of unit cells in the array. However, as shown theoretically in a previous work,23 this necessarily limited the redox cycling efficiency when the MEAs were operated in a bipolar mode compared to their operation in a GC mode (see below). Pyrocatechol (CT), dopamine (DA), and epinephrine (EP) were selected as a typical series of catechols giving rise to electrochemical waves with similar half-wave potentials. Considering the time scale of diffusion across the microwells 2.5 μm height, DA and CT were behaving as chemically reversible redox couples (viz., their −2e − 2H+ oxidation products,40,41 DAox and CTox, could almost be quantitatively reduced back to DA and CT, respectively) while EP behaved as a chemically irreversible one due to the fast cyclization of its −2e − 2H+ product EPox.4,40 Figure 3a evidence that in bipolar mode, i.e., when the top plane electrode was unbiased and the common potential of the recessed disks was scanned, the three species displayed quasisteady-state waves with nearly identical half-wave potentials. Interestingly, the limiting currents of DA and CT were nearly equal while that of EP was comparatively smaller albeit, the diffusion coefficients for the three species were similar, as expected. This is because the DA and CT current intensities were significantly amplified through redox cycling owing to the reducibility of their stable oxidation products (see Figure 1b).21−23 This was impossible for EP, since its oxidation product was not reducible. Thus, its current intensity reflected its simple oxidation at the microdisks working electrodes. Figure 3b shows the same qualitative outcome when the MEAs were operated in the GC mode with the potential of the plane electrode being held at 0.2 V vs SCE. Indeed DA and CT again gave rise to higher current intensities than EP, in agreement with the fact that redox cycling may occur for DA and CT but not for EP. Since the EP wave involved only its electroactivity at the recessed microdisks, its current intensity was nearly identical (Table 1) to that monitored in the bipolar

used here.23 This stems from the fact that the bipolar performance of a MEA is limited by the surface area of the anodic poles. Indeed, that governs the magnitude of the current flux that can be shuttled toward its cathodic poles, where it can be used to promote a redox cycling process. Conversely, in a GC mode, the cathodic pole occupies the whole surface area of the top electrode and its potential is biased on the reduction plateau of DAox or CTox, so that it may perform with the maximal efficiency allowed by the MEA geometrical characteristics. Figure 4a compares the waves observed for bipolar and GC operations for the same equimolar mixture of two chemically reversible couples, viz., DA, 0.1 mM, and CT, 0.1 mM. Qualitatively, the quasi-steady-state current measured for the mixture under each condition was close to the expected addition of the individual currents in Figure 3a (bipolar mode) or Figure 3b (GC mode). More quantitatively, it is noted (Table 1) that in each mode the currents for the mixture were smaller than the exact sums of the individual components. Furthermore, the voltammograms of the mixture exhibited a more pronounced peak shape than those of DA or CT under either operating modes. Such changes in shapes and intensities cannot be ascribed to a change in the diffusional regimes that regulate the redox cycling turnovers, since the geometry of the device, the diffusion coefficients, and the electron transfer kinetics could not vary when each species is oxidized alone or in a mixture. Similarly, this cannot be ascribed to a change of chemical kinetics due to the increased release of proton (2 H+ per couple), since the solution pH was set at a constant value, pH = 3, by the phosphate buffer. We are thus inclined to consider that these effects may only stem from a kinetic coupling between the two redox systems. A likely possibility is the involvement of a cross-electron-transfer reaction occurring between the reduced and oxidized forms of the two species in the solution above the microwells:12,42−44

Table 1. Comparison of Current Responses and Amplification Factors of Binary Mixtures (0.1 mM each) and Individual Components (0.1 mM)a

Indeed, the global equilibrium in eq 1 is necessarily involved when a DA-CT mixture is analyzed. It is displaced thermodynamically, owing to the difference in standard potentials of the two redox couples (these may differ even if their E1/2 values are close) and is continuously pulled toward the side involving the less stable oxidized species, leading to an EC′ mechanism.42−44 Such EC′ sequences have indeed already been reported to perturb electrochemical measurements of mixtures such as those considered here.42−44 We established previously that the effect of such EC′ sequences is drastically reduced in microwell-MEAs when their geometry is optimized to allow an extremely efficient electrochemical redox cycling,12 since this decreases considerably the concentration of the product(s) diffusing out of the microwells after their generation at the microdisk electrodes. In this respect, our decision of selecting geometrical characteristics that did not optimize redox cycling amplification may well be reflected by a non-negligible involvement of an EC′ mechanism. This interpretation is qualitatively coherent with the observation of a decrease of the effect (Table 1) when increasing the redox cycling efficiency upon operating the same MEA in a GC mode rather than in a bipolar one. Nevertheless, whatever is the exact origin of the discrepancies, it is clear that the data in Figure 4a confirm that no benefit may be expected concerning the redox cycling efficiency when the interferent is a chemically stable redox couple (viz., DA) added to the analyte (viz., CT).

Molecule

ibipolar (nA)

iGC (nA)

Af

CT DA EP CT + DA CT + EP

26.8 27.0 11.3 44.2 46.6

36.4 41.4 11.9 67.0 50.0

1.36 1.53 1.05 1.52 1.07

DA ox + CT ⇌ DA + CTox

a

ibipolar represents the cumulative current intensity measured by the recessed disk electrodes of the MEA operated in a bipolar mode; iGC represents the same when the same MEA is operated in a GC mode; see the experimental conditions in the Figure 3 and 4 captions. Af = iGC/ibipolar is the ratio of the currents measured for the same solution in the two modes.

mode. Conversely, the redox cycling efficiencies for DA and CT were much larger than those recorded using a bipolar mode, as was expected based on the geometrical characteristics of the MEAs used in this study.23 This confirms that, unless its design is optimized for maximizing the bipolar redox cycling efficiency, a given MEA operated in a GC mode leads to larger redox cycling amplification than when used under bipolar conditions. The difference in performance may be quite large for arrays with sufficiently large density of element cells, such as those 8538

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Figure 4. Electrochemical responses of plane-recessed MEAs in a PBS buffer (pH 3) containing binary mixtures. (a) 0.1 mM CT + 0.1 mM DA; (b) 0.1 mM CT + 0.1 mM EP. The potential of recessed disks electrodes was scanned at 10 mV/s. The top plane electrode was left floating in the bipolar mode, while its potential was set to 0.2 V vs SCE in GC mode. See the Experimental Section for the PBS buffer composition.

Figure 5. Schematic illustration of differences in redox cycling amplifications in the bipolar (a, b) and GC (c) operating modes. For the bipolar mode are shown the behaviors when the system operates on a solution containing a single chemically reversible redox couple (a) or a mixture of a reversible and an irreversible couple (b). In (c) only the case of a mixture of CT and EP is illustrated. CT: pyrocatechol; EP: epinephrine.

plane electrode,23 its oxidation at the anodic poles that occur at the same potential as that of CT contributes to enhance the current flux that may be delivered to the cathodic poles, as sketched in Figure 5b. This increases the efficiency of the CT bipolar redox cycling compared to the case when EP is absent (Figure 5a) so as to reach a situation that becomes extremely close to that installed when the same MEAs performs under a GC mode (Figure 5c). It is impossible to determine precisely the bipolar or GC amplification, since this would amount to using different MEAs for which the microfabrication process had been thoroughly modified to replace the metallic top plane electrode by a fully insulating plane21 (see the Supporting Information). In the present context where the redox cycling is voluntarily made not very high, this would not be achieved with a sufficient precision to ensure a total reproducibility of the MEAs geometries except for this modification. So, we preferred to rely on the fact that EP and CT have presumably similar diffusion coefficients, and that their oxidations involve an identical electron stoichiometry (2F/mol). Within this approximation, the average value, 11.6 nA, of the current intensities measured for EP alone (Table 1) may be used as a tentative reference value for that of CT in the absence of any redox cycling. From Table 1, this allowed estimating that when CT is alone, the redox cycling induced by the bipolar operation contributed an added current intensity of ca. 26.8−11.6 = 15.2 nA. Hence, the added contribution due to the bipolar redox cycling during CT oxidation alone may be estimated at ca. 130% (viz., 100 × 15.2/11.6) of the current

Of more significant interest for our main purpose in this study is the case in which the interferent gives rise to a chemically irreversible wave (viz., EP). The corresponding results for an equimolar mixture of CT and EP are shown in Figure 4b for each operating mode. It is first observed that the current intensities for the CT-EP mixture in the GC mode or in the bipolar one are extremely similar (they are equal within 7%; Table 1). In the GC mode, the current intensity (50.0 nA; Table 1) compares extremely well with the sum (48.3 nA; Table 1) of those of the individual components in Figure 3b. However, this is not the case when the same device is operated in a bipolar mode, since the current intensity for the mixture exceeds the sum of the individual bipolar currents by 22%. In other words, though EP cannot participate in the redox cycling amplification (Figure 3), its presence in the mixture drastically enhances the efficiency of CT redox cycling during bipolar operation. This enhancement cannot be ascribed to the involvement of any kinetic EC′ sequence equivalent to that in eq 1, since this would reduce the redox cycling efficiency, as evidenced for the DA-CT mixture,12,42−44 nor to a pH effect, since this was buffered at pH = 3. This establishes that the additional current contribution for the mixture under bipolar operation stems from an increased efficiency of the redox cycling mechanism. In other words, CTox is more efficiently reduced at the cathodic poles of the bipolar electrode when EP is present. While EP cannot participate in the redox cycling between the microdisks electrodes and the cathodic poles located around the microwell edges of the top 8539

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Analytical Chemistry intensity in the absence of redox cycling. The added contribution became 46.6 − (2 × 11.6) = 23.4 nA for the equimolar CT-EP mixture, i.e., ca. 200% (viz., 100 × 23.4/11.6) of the current intensity in the absence of redox cycling. When CT is oxidized alone in a GC mode, one obtains (Table 1) 36.4 − 11.6 = 24.8 nA for the added contribution due to its redox cycling, viz., ca. 215% (viz., 100 × 24.8/11.6) of the current intensity estimated in the absence of redox cycling. It is then concluded that within the precision of the above approximations and measurements, the addition of EP to CT in equimolar amounts increased the bipolar redox cycling efficiency to the point that it reached that of the GC one, viz., that of its maximum. Evidently, one could achieve a same maximum efficiency through increasing the center-to-center distance between the microwells in order of increasing the surface areas of the anodic poles.23 However, this would imply reducing the density of microwells and, hence, would decrease the overall analytical current for a given total surface area of the MEA. Furthermore, this optimization should be performed specifically for each targeted analyte.23 Therefore, the present strategy consisting of adding an adequately known amount of another species whose redox wave is chemically irreversible and with a half-wave potential close to that of the analyte offers a very versatile approach for reaching a maximal bipolar efficiency without sacrificing the density of the MEAs microwells.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS In Xiamen, this work was supported by the Natural Science Foundation of China (Nos. 21373174, 21321062), and the Natural Science Foundation of Fujian Province of China (No. 2016J01075). In Paris, this work was supported in parts by ENS, UPMC, and CNRS (UMR 8640). Both groups also thank the CNRS and the four universities (ENS-Paris/Rennes/ Xiamen/Wuhan) joint laboratory LIA NanoBioCatChem for financial support.

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REFERENCES

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CONCLUSIONS This work illustrates a specific property that is associated with the use of bipolar MEAs of the type investigated here when the analyte of interest (chemically reversible redox couple) is analyzed in the presence of interferent, giving rise to a chemically irreversible wave. Indeed, while the current of the interferent cannot be amplified owing to the chemical irreversibility of its electrochemical wave, it may contribute significantly to enhance the efficiency of the redox cycling of the analyte. This occurs because the interferent oxidation (or reduction accordingly) provides an excess of current flux that is used at the cathodic poles of the MEA to increase the redox cycling efficiency of the analyte. Provided that the added interferent concentration is sufficient, the bipolar and GC redox cycling efficiencies become equivalent and the unknown concentrations of the analyte and of the interferent in a mixture may be determined by the classical method of added aliquots (see Supporting Information). However, in a general case, one must ensure this is effectively the case. Otherwise, the redox cycling amplification of the analyte current depends on the interferent excess. Hence, the method of added aliquots must be used with care (see Supporting Information). Conversely, a too large excess would decrease the experimental precision of the analyte current by becoming the main contribution detected at the microdisks of the MEA. Yet, this difficulty is not as severe as it seems, since the correct excess may be determined through precalibrations performed using a GC mode, while using the advantageous bipolar mode for applications.

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Microfabrication detailed procedures; electrochemical characterization of plane-recessed MEAs using the 1eouter sphere chemically reversible Ru(NH3)62+/3+ redox system; principle of analytical determination of composition of mixtures with MEAs operated in the bipolar mode (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.6b01454. 8540

DOI: 10.1021/acs.analchem.6b01454 Anal. Chem. 2016, 88, 8535−8541

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DOI: 10.1021/acs.analchem.6b01454 Anal. Chem. 2016, 88, 8535−8541