Gas-phase selective adsorption on functional monolayers immobilized on a highly sensitive quartz-crystal microbalance

Self-assembled monolayers of alkanethiols having functional groups (HS(CH2)(10)X; X = -H, -COOH, -CONH2, -NH2) were immobilized on an Au electrode of a quartz-crystal microbalance (QCM), and binding kinetics of acetic acid molecules from the gas phase were studied from time courses of frequency decreases (mass increases) of the QCM. A highly sensitive, 63 MHz overtone frequency of a conventional 9 MHz AT-cut QCM was developed to detect monolayer adsorption of small molecules such as acetic acid. Acetic acid molecules are adsorbed onto the -CONH2 membrane as a Langmuir-type monolayer; however, they tend to be adsorbed as multilayers onto the -COOH and -NH2 membranes. Acetic acid was hardly adsorbed onto the simple alkane membrane (-H membrane). The binding ability of the -COOH membrane to acetic acid molecules was reduced compared with that of the -CONH2 membrane due to the intramembranous hydrogen bond in the former.