Guest selective adsorption from the gas phase onto a functional self-assembled monolayer immobilized on a super-sensitive quartz-crystal microbalance

Abstract

A self-assembled monolayer of alkanethiols having functional groups ( HS-(CH 2) 10− X; X = −H, −COOH, −CONH 2 ) was immobilized on Au electrodes of a quartz-crystal microbalance (QCM), and the adsorption behavior of gaseous organic molecules, such as aliphatic acids, amines, alcohols, and alkanes, was observed from frequency decreases (mass increases) of the QCM plate in gas-phase. We used a super-sensitive, seventh overtone frequency of a 9 MHz AT-cut QCM (63 MHz) in order to detect a monolayer adsorption of small organic molecules. Association constant ( K a ), adsorption ( K 1), and desorption rate constants ( K −1) could be obtained from curve fitting of time courses of frequency changes (mass changes). All organic molecules hardly adsorbed onto the alkane ( X = − H) membrane. To the -CONH 2 membrane, only acetic acid can be selectively adsorbed compared with the -H membrane, but not caproic and nonanoic acids, which explains that hydrogen-bond interactions are important but hydrophobic interactions are not. To the -COOH membrane, both acetic acid and ethyle-nediamine can largely adsorb compared with the -H membrane, but not monoamines. The adsorption of ethylenediamine can be explained by acid-base interactions.