Electrochemistry on Self-assembled Monolayers

The redox reactions of metal proteins such as cytochrome c (cyt. c) are known to be promoted by modifying electrodes with SAMs of nitrogen-containing heterocyclic molecules. SEIRAS studies of the modified electrodes directed toward the understanding the promotion mechanism have been reported. The systems reported so far are 2-pyridinethiol, 4-pyridinethiol (4-PySH), bis(2-pyridyl)disul-fide, bis(4-pyridyl)disulfide, 2-pyrimidinethiol, 6-amino-8-purinethiol, mercapto-propionic acid, mercaptoethanol, 4,4 -dithiodipyridine, and L-cysteine [95-99]. 

0 V (vs Ag/AgCI) and (b) potential dependence at the fixed pH of 5.3. The Intensity data were taken from time-resolved SEIRA spectra collected every 5 s and are given as relative intensities with respect to that 20 min after the injection of the molecule for each measurement. The concentration of 4-PySH was fixed at ca. 100 mM [96]. 

On the basis of the SEIRAS experiments, the difference in the promotion activities of the SAMs prepared under different conditions was explained as follows the sulfide is strongly adsorbed on Au and prevents the adsorption of 4-PySH. However, since the sulfide concentration is very low, it takes some period of time for its adsorption. Accordingly, by employing the conditions under which 4-PySH can be adsorbed quickly, the electrode surface can be covered with 4-PySH with less interference from sulfide. 

The promotion activity also depends on the structure of the precursor molecule. The orientation analyses of the SAMs of 2- and 4-PySH and their disulfides suggest that the N ends of the molecules must be directed toward the solution side for promoting the electrochemistry of cyt c [95]. It was also found that the ring vibrations of the pyridyl moieties of the SAMs are sensitive to the protonation at the N atom on the ring. From the pH dependence of a ring vibration mode, the pfC value for a 4-PySH SAM on an Au electrode was determined [100]. 

The spectrum of cyt. c greatly depends on the terminal groups (-COOH, -COH, -SH, -NH2) of the SAMs, reflecting different interactions with cyt c [99]. The potential difference tactic used in this measurement is favorable in examin- 

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