Correlating Molecular Orientation Distributions and Electrochemical Kinetics in Subpopulations of an Immobilized Protein Film

Abstract

Charge transfer kinetics in immobilized, redox-active films have been studied using both cyclic voltammetry (CV) and electroreflectance (ER) techniques by other groups. 1,2 In these studies, rate constants obtained using ER techniques were higher than those determined from CV data, as observed here. This systematic difference can be explained by considering the following: Adsorption of a monolayer of molecules on a chemically and structurally heterogeneous electrode (e.g., indium-tin oxide) will generate a film that is chemically, structurally and energetically heterogeneous. This heterogeneity will be reflected in a distribution of thermodynamic formal potentials (E 0 ') and apparent rate constants for electron transfer. 3,4 In CV, the potential is scanned over a relatively large range (here it was -250 mV to 250 mV vs Ag/AgCl). At potentials well away from E 0 ' (i.e., at the extremes of the scan range), the large overpotential (E app -E 0 ') provides a sufficient driving force to oxidize and reduce all of the electroactive molecules in the film. In PM-ATR, a modulated potential (E ac ) is applied about the mid-point potential (E dc ) between the anodic and cathodic peaks (E dc ≈ E 0 '). The modulation range, E dc ± E ac , is typically less than 50% of the scan range used in CV (here E dc ± E ac was 44 mV). Thus in PM-ATR, only the subpopulation of electroactive molecules that are oxidized and reduced within E dc ± E ac are probed. This subpopulation should equilibrate more rapidly with the modulated electrode potential (i.e., have a higher k 0 ) relative to the remainder of the film, and thus the k 0 measured for this subpopulation by PM-ATR is expected to be greater than that measured for the entire film by CV.

Characterization of Charge-Transfer Kinetics at Organic/Electrode Interfaces Using Potential-modulated Attenuated Total Reflectance (PM-ATR) Spectroscopy

“…The sensitivity can be enhanced by using an ATR geometry, wherein the probe beam is internally reflected multiple times through a planar waveguide electrode. 13,15,18,20 Compared with transmission and external reflection ER spectroscopy, PM-ATR spectroscopy provides a sensitivity enhancement of ten to several thousand, depending on the geometry of the waveguide electrode. The ATR geometry also provides spatial selectivity because the evanescent wave penetrates only a fraction of the wavelength (λ) into the medium adjacent to the waveguide.…”

“…1C). 15,18 This allows for the selective probing of molecules having absorbance transition dipoles aligned with the electric field vector of the evanescent wave. In an ensemble of molecules with a distribution of molecular orientations, the contribution of each molecule to the ER response is weighted by a factor of cos 2 α, where α is the angle between the electric field vector and the transition dipole of the molecule.…”

“…2,4 Combining potential-modulation with the ATR geometry enables the measurement of charge-transfer kinetics of monolayer to submonolayer thin films on waveguide electrodes, with selectivity to molecular subpopulations in the film provided by appropriate choice of the applied potential and the wavelength and polarization of light, as described further below. This method has been applied to study a wide variety of electrode-supported materials, including small molecule and polymeric semiconductors, [13][14][15][16][17] proteins, 18 inorganic complexes, 19 and semiconductor nanocrystals.…”

Electron-Transfer Rate in Potential-Modulated Redox Reactions with Electro-Active Optical Waveguides

“…Spectroelectrochemical techniques are particularly useful to mitigate those difficulties by focusing solely on the faradaic process and avoiding the strong non-faradaic electrical background. [10][11][12][13][14][15][16][17][18][19][20][21][22] Based on spectroscopic optical changes that typically accompany a redox event, it is usually possible to select the wavelength of a probing light beam to uniquely capture the optical changes associated with the faradaic process and is immune to the presence and motion of background ions in the electric double layer.…”

Orientational switching of protein conformation as a function of nanoparticle curvature and their geometrical fitting

“…16 A common general trend from previous research suggested that the lower curvature of nanoparticle causes more perturbations of the protein structure, however that factor may not be universally true. 10,17,18 For instance, 2D self-assembled monolayers (SAM) of a polypepa) Author to whom correspondence should be addressed. Electronic mail:…”