High‐Pressure Probing of a Changeover in the Charge‐Transfer Mechanism for Intact Cytochrome c at Gold/Self‐Assembled Monolayer Junctions

Abstract

Free trade: The electron exchange of cytochrome c freely diffusing to gold electrodes modified by hydroxy‐terminated alkanethiol self‐assembled monolayers (SAMs; see picture) is examined by high‐pressure kinetic studies. The SAM thickness, solution viscosity, and hydrostatic pressure influence a changeover in the charge‐transfer mechanism and the corresponding kinetic response.

Charge-Transfer Patterns for [Ru(NH3)6]3+/2+ at SAM Modified Gold Electrodes: Impact of the Permeability of a Redox Probe

“…As one can see from Fig. (3), the viscosity dependence for the bare Au electrode [11] exhibits the "normal" behavior corresponding to the adiabatic charge-transfer mechanism well-documented in the previous work for different markers [11,[14][15]. The curves indicated as n = 2 (the independent results obtained for three different electrodes), and n = 4 represent the viscosity-sensitive dependencies, exhibiting "anomalous" behavior.…”

“…As it can be expected on the basis of general theoretical notions [3][4][5][6][7][8][9] and the previously obtained results [13][14][15][16][17]27], penetration of redox couple into the SAM interior (that is marked by much higher local viscosity compared to the electrolyte solution) must show up in a different manner for the cases of short-and long-range CT, respectively (where two different intrinsic charge transfer mechanisms are operative), in the absence, as well as in the presence of viscose additives. Namely, for the thicker SAMs the essentially nonadiabatic behavior is expected (Eqs.…”

“…(2), in the absence of glucose, the experimental points obtained both in the present work and by other authors [18][19][20] display remarkable scatter, which has not been detected for the case of other, previously studied markers [13][14][15]. In the presence of viscous additive (glucose) the rate constants for short-chained SAMs (n = 2, 4) also exhibit some systematic downward deviation of experimental points.…”

“…For the case of shortest charge-transfer distances (the thinnest SAMs), the changeover to the adiabatic (solventfriction) regime of electron transfer is expected, exhibiting the essential viscosity dependence and pronounced plateau regions regarding the charge-transfer distance [13][14][15][16][17]27]. In this case the penetration of [Ru(NH3) 6] 3+/2+ species inside of these SAMs (n = 2, 4) will lead to the essential control of the charge-transfer event by the fluctuations of the SAM interior, not the solvent.…”

“…According to contemporary theoretical models [3][4][5][6][7][8][9] and recent experimental studies (performed in molecular and/or ionic liquids exploiting chemical redox markers or redoxactive proteins) [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27], heterogeneous charge transfer mechanism may display two different intrinsic mechanisms: adiabatic (solvent friction) -at short distances, and nonadiabatic (tunneling) -at long distances, as well as intermediate regime. In all cases the unimolecular rate constant (k o et ) of charge transfer processes, according to the resent theoretical update, can be presented by the eq.…”