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 detecti on of protonated amines. To ascertain the formation of such an unusual type of self-assembled monolayers (SAMs), the self-assembly of thioacetamide an d laurylthioamide was investigated beforehand. The linear scan rate depende nce of the peak current and the peak separation in cyclic voltammograms (CV s) of thioacetamide confirms thioacetamide binding to gold. The dependence of the reduction potential of thioacetamide SAMs on the pH shows that one e lectron is required to desorb one thioamide group from the gold electrode. Reductive desorption CVs of laurylthioamide monolayers showed that laurylth ioamide occupied an area of 21.4 Angstrom(2) per molecule, which is close t o the corresponding value for alkanethiol SAMs and indicates a high monolay er packing density. Formation of benzene-1,3,5-tris(N-glycino-thiocarboxami de) SAMs was confirmed by reductive desorption, showing that the receptor o ccupied an area of 150 Angstrom(2) 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 oxi dized more easily than at high pH. This reflects the degree of protonation of the receptor monolayers and thereby the charge density; the marker can a ccess the electrode surface easily at low pH, where the receptor SAM is pro tonated, but it is repelled at high pH, where the receptor SAM is deprotona ted. On the other hand, in the presence of amines, [Fe(CN)(6)](4-) oxidatio n occurs easily even at high pH, indicating that protonated amines bind to the deprotonated receptor SAMs. This phenomenon allowed the determination o f amine concentrations in aqueous solutions. The receptor-modified electrod es responded to 1,4,7-triazacyclononane and 1,4,7,10,13,16-hexaazacycloocta decane 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 intera ctions and the number of possible hydrogen bonds between the receptor and t he protonated amines. (C) 1999 Elsevier Science S.A. All rights reserved.