Aoki, H.; Buhlmann, P.; Umezawa, Y. J. Electroanal. Chem. 1999, 473, 105-112
Abstract
The receptor benzene-1,3,5-tris(N-glycino-thiocarboxamide) was used for the first demonstration of thioamide self-assembly on gold and for the detection of protonated amines. To ascertain the formation of such an unusual type of self-assembled monolayers (SAMs), the self-assembly of thioacetamide and laurylthioamide was investigated beforehand. The linear scan rate dependence of the peak current and the peak separation in cyclic voltammograms (CVs) of thioacetamide confirms thioacetamide binding to gold. The dependence of the reduction potential of thioacetamide SAMs on the pH shows that one electron is required to desorb one thioamide group from the gold electrode. Reductive desorption CVs of laurylthioamide monolayers showed that laurylthioamide occupied an area of 21.4 A2 per molecule, which is close to the corresponding value for alkanethiol SAMs and indicates a high monolayer packing density. Formation of benzene-1,3,5-tris(N-glycino-thiocarboxamide) SAMs was confirmed by reductive desorption, showing that the receptor occupied an area of 150 A2 per molecule. The reversible protonation/deprotonation behavior of SAMs of this receptor was investigated with [Fe(CN)6]4- as electroactive marker. At low pH, [Fe(CN)6]4- was more easily oxidized than at high pH. This reflects the charge density and thereby the degree of protonation of the receptor monolayers; the marker can easily access the electrode surface at low pH, where the receptor SAM is protonated, but is repulsed at high pH, where the receptor SAM is deprotonated. On the other hand, in the presence of amines, [Fe(CN)6]4- oxidation occurs easily even at high pH, indicating that protonated amines bind to the deprotonated receptor SAMs. This phenomenon allowed the determination of amine concentrations in aqueous solutions. The receptor-modified electrodes responded to 1-hexylamine, triethylamine, 1,4,7-triazacyclononane and 1,4,7,10,13,16-hexaazacyclooctadecane at concentrations above 10-2 M, 10-2 M, 10-4 M and 10-7 M, respectively. The selectivity of the electrode can be explained by the strength of the charge-charge interactions and the number of possible hydrogen bonds between the receptor and the protonated amines.