Self-assembly of a tricarboxylate receptor through thioamide groups and its use for electrochemical detection of protonated amines

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 angstroms² 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 angstroms² per molecule. The reversible protonation/deprotonation behavior of SAMs of this receptor was investigated with [Fe(CN)₆]^4- as the electroactive marker. At low pH, [Fe(CN)₆]^4- was oxidized more easily than at high pH. This reflects the degree of protonation of the receptor monolayers and thereby the charge density; the marker can access the electrode surface easily at low pH, where the receptor SAM is protonated, but it is repelled 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 1,4,7-triazacyclononane and 1,4,7,10,13,16-hexaazacyclooctadecane at concentrations above 10^-6 M and 10^-8 M, respectively, but no response to 1-hexylamine and triethylamine was observed. 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.