Res. Chem. Intermed.
, Vol. 32, No. 3–4, pp. 341–355 (2006)
Also available online - www.vsppub.com
Motion of redox molecules in solution monitored
by the highly-sensitive EQCM technique
NORIYUKI ASAKURA, TOSHIAKI KAMACHI and ICHIRO OKURA
Department of Bioengineering, Tokyo Institute of Technology, 4259 Nagatsuta-cho,
Yokohama 226-8501, Japan
Received 28 December 2005; accepted 25 February 2005
Abstract—The behavior of redox molecules in solution that was not detected by electrochemical
techniques was measured by a highly-sensitive electrochemical quartz crystal microbalance (EQCM)
technique that has been improved in this study to obtain a high sensitivity of EQCM measurement
in solution. The improved EQCM technique allowed to monitor the motion of a redox molecule,
that is an access of the molecule to an electrode surface and repulsion from the surface during redox.
An EQCM technique currently in use has measured adsorption of redox molecules on an electrode
surface or polymerization on the surface caused by a chemical reaction following redox, which
exhibits an enough large mass change response to detect with an EQCM measurement. However,
access and repulsion of redox molecule, which is a slight motion of the molecule near on electrode
surface, has not been detected and investigated by an EQCM technique, because the mass change
response seems to be very small. In this study, the redox behavior of methyl viologen on a bare gold
surface, pyridinethiol surface and methylpyridinethiol surface was investigated. Although the three
electrodes give the same cyclic voltammogram of methyl viologen, the three are different in QCM
response recorded at the same time as the voltammetry. Access/repulsion of methyl viologen within
an electrical double layer was monitored by the highly-sensitive EQCM technique.
Keywords: EQCM; viologen; electron transfer.
The behavior of redox molecules during a redox reaction has been a topic of great
interest in the study of intermolecular electron transfer. Electrochemical techniques
have been used to investigate and analyzed a behavior of redox molecules. These
investigations impose high demands on application of the electrochemical theory in
calculation because an electrochemical response involves an electrical double layer
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