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Comment on “Impact of Adsorption on Scanning Electrochemical Microscopy Voltammetry and Implications for Nanogap Measurements”: Assessment of Heterogeneous Self-Exchange Reaction at Conductor and Insulator Shigeru Amemiya Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 09 Jun 2017 Downloaded from http://pubs.acs.org on June 10, 2017
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Analytical Chemistry
Comment on “Impact of Adsorption on Scanning Electrochemical Microscopy Voltammetry and Implications for Nanogap Measurements”: Assessment of Heterogeneous Self-Exchange Reaction at Conductor and Insulator Shigeru Amemiya* Department of Chemistry, University of Pittsburgh, 219 Parkman Avenue, Pittsburgh, Pennsylvania 15260, United States Herein, we assess general impact of heterogeneous selfexchange reaction (HSER) on electrochemical measurement to comment on a recent report by Unwin and coworkers, particularly about their interpretations of our nanogap voltammograms.1 In ref. 1, they proposed a model to imply that a HSER caused anomalous asymmetry of nanogap voltammograms based on scanning electrochemical microscopy (SECM), which we originally attributed to surface contamination of highly oriented pyrolytic graphite (HOPG).2 Specifically, we obtained asymmetric pairs of voltammograms based on oxidative (Figure 1) and reductive redox cycling of the (ferrocenylmethyl)trimethylammonium (FcTMA+ or R) couple at HOPG across a nanometer-wide gap under a Pt SECM tip to determine extremely high standard electron-transfer (ET) rate constants, k0, of ≥13 cm/s.2 More recently, we reported symmetric pairs of nanogap voltammograms with consistently high k0 values of ≥12 cm/s for the FcTMA2+/+ couple at cleaner HOPG surfaces3 protected with a water adlayer from airborne contamination.4 By contrast, Unwin and co-workers referred to our original work2 to claim that an actual k0 value of ~5 cm/s was overestimated because neither HSER nor adsorption was considered in analysis of asymmetric nanogap voltammograms.1 Most recently, ref. 1 was cited by others to question reliability of nanoscale SECM measurement.5
Figure 1. HSERs at glass and HOPG surfaces in SECM-based nanogap voltammetry. Red arrows indicate the forward reaction.
In ref. 1, Unwin and co-workers were unable to quantitatively simulate asymmetry of our nanogap voltammograms without considering a HSER. First, they measured paired voltammograms at micrometer-wide gaps (≥2.6 µm in Figures 3 and 4 of ref. 1) using 25 µm-diameter Pt tips and attributed large asymmetry and hysteresis of their transient voltammograms to diffusion-limited adsorption of FcTMA+ on HOPG to determine a surface coverage of 22%. The verified model was employed to simulate voltammograms at nanometer-wide gaps (50 nm in Figure 5a of ref. 1). Asymmetry of simulated limiting currents, however, was enhanced only by 2%–6% even when the surface coverage was increased from 22% to 50%. An asymmetry of up to 25% was observed in our experimental nanogap voltammograms at contaminated HOPG (Figure 5 of ref. 2). Moreover, only oxidative branches of simulated nanogap voltammograms showed small hysteresis, which was seen in both branches of our experimental nanogap voltammograms at contaminated and cleaner (Figure 5 of ref. 3) HOPG. By contrast, our simulation based on a similar model ensured that small hysteresis is expected for both of symmetric pairs of nanogap voltammograms (Figures 2 and 6B of ref. 3), which are measured at quasisteady states6 and minimally affected by transient adsorption of FcTMA+ on the local HOPG surface under a 1 µmdiameter Pt tip. Subsequently, Unwin and co-workers added the irreversible HSER at the glass sheath of a Pt tip (eq S-7 of ref. 1) to their verified model based on diffusion-limited adsorption of FcTMA+ on HOPG. The modified model showed a more enhanced asymmetry of ~17% in simulated limiting currents as well as small hysteresis in both branches (Figures 5b and 7 of ref. 1). These features were similar to those of our experimental nanogap voltammograms at contaminated HOPG (Figure 5 of ref. 2) and can be attributed to the quasi-steady-state character of the HSER at glass (Figure 6(ii) of ref. 1). In addition, asymmetric nanogap voltammograms were simulated with a k0 value of 5 cm/s and analyzed without considering HSER and adsorption to estimate k0 values of ≥14 cm/s and
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Analytical Chemistry
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down to 3 cm/s from oxidative and reductive branches, respectively (Tables S-1–S-3 of ref. 1). It, however, should be noted that differences between these misestimated k0 values are significantly larger than differences of up to 30% between minimum k0 values determined from oxidative and reductive branches of reversible nanogap voltammograms at contaminated HOPG (Table 2 of ref. 2). In the following, we assess kinetic requirements for electrochemical observation of a HSER at conductor and insulator to complement our previous thermodynamic argument,3 which manifested that the HSERs considered by Unwin and co-workers1,7 (Figure 1) were unlikely to occur. In general, a HSER between adsorbed and nonadsorbed states of a redox couple is defined as3 Oads + R
Rads + O
imental nanogap voltammograms at contaminated HOPG.1 In fact, a HSER was not considered previously when adsorption of a redox couple on insulator was studied using ionomer-modified electrodes11 as well as thinlayer cells with the face-to-face configuration (Figure 2) based on SECM12 and nanolithography.13 Our kinetic analysis indicates that the rate of the HSER at insulator is always zero in either side-by-side (Figure 1) or face-to-face configuration when adsorption equilibrium is achieved for both forms of a redox couple, O and R. Specifically, their competitive adsorption is represented by Langmuir isotherms as14 (4)
(1) (5)
with an equilibrium constant, Kex, given by (2) where Γi,s is the saturated surface concentration of a species i and βi is its equilibrium constant in the Langmuir isotherm. Specifically, the HSER based on oxidation of FcTMA+ in solution by FcTMA2+ adsorbed on the glass sheath of a Pt tip (eq S-7 of ref. 1) was prevented, because adsorption of FcTMA+ on the glass surface was undetectably weak,1 where ΓR,sβR
