Prev. Article Next Article Table of Contents

Research Article

Mediated Oxidation of Ascorbic Acid on a Homologous Series of Ferrocene-Terminated Self-Assembled Monolayers

Birutė Kazakevi

ienė,^† Gintaras Valincius,*^† Gediminas Niaura,^† Zita Talaikytė,^† Marytė Kažemėkaitė,^† Valdemaras Razumas,^† Deivis Plaušinaitis,^‡ Aušra Teišerskienė,^‡ and Vaclovas Lisauskas^§

Department of Bioelectrochemistry and Biospectroscopy, Institute of Biochemistry, Mokslininku 12, LT-08662 Vilnius, Department of Physical Chemistry, Faculty of Chemistry, Vilnius University, Naugarduko 24, LT-03225 Vilnius, and Semiconductor Physics Institute, A. Gotauto 11, LT-01108 Vilnius, Lithuania

Langmuir, 2007, 23 (9), pp 4965–4971

DOI: 10.1021/la0632169

Publication Date (Web): March 23, 2007

†

Institute of Biochemistry.

To whom correspondence should be addressed. Phone: +370-5-2729186. Fax: +370-5-2729196. E-mail: gintaras@bchi.lt.

‡

Vilnius University.

Semiconductor Physics Institute.

Abstract

The kinetics of electrocatalytic oxidation of ascorbate was studied on a series of redox self-assembled monolayers (SAMs) of the general formula Fc(CH₂)₄COO(CH₂)_nSH as electron-transfer mediators, where Fc is the ferrocenyl group and n = 3, 6, 9, and 11. We show that the rate of electron transfer from ascorbate to the surface-confined Fc⁺ decreases with increasing n. The rationale for the dependence of the rate of electrocatalytic activity and n, in the presence of ClO₄, is obtained from Fourier-transform surface-enhanced Raman spectroscopy (FT-SERS), cyclic voltammetry, and electrochemical quartz crystal microbalance (EQCM) data. In particular, FT-SERS shows decreasing amounts of surface-bound ClO₄^- upon oxidation of the ferrocene with decreasing n, while EQCM data show the effective electrode mass increase was consistently higher on the shorter chain SAMs. This mass increase is likely due to increasing ferricinium cation hydration. As n decreases, the SAMs become less ordered (FT-SERS data), as is widely known from previous literature. Disorder favors water penetration into the SAM, which, in turn, increases the hydration of the Fc⁺ (EQCM data). Increased hydration of the Fc⁺ impedes the formation of Fc⁺−ClO₄^- ion pairs (EQCM and FT-SERS data), which, consequently, accelerates the electrocatalytic electron transfer from the solution-dissolved ascorbate.