Evaluation of amine-functionalized coatings for liquid-phase QCM applications

A selection of compounds was examined for their potential as stable and reactive liquid-phase acoustic wave sensor coatings with the primary amine group as the reactive functional group of interest. Using a 10-MHz thickness shear mode (TSM) sensor with gold electrodes, the following commercially available molecules were examined: 4-aminopyridine, 4-aminothiophenol, 2-aminoethanethiol, 3-amino-5-mercapto-1,2,3-triazole, 3-aminopropyl-triethoxysilane, and poly(ethyleneimine). Based on dry TSM sensor frequency changes, all six candidates exhibited some degree of stable coating formation after incubation and rinsing. Reactivity of the coatings was then assessed via reaction with succinic anhydride. With pyridine- and thiol-based coatings, the frequency change is negligible or is positive, which may indicate some loss of coating material upon reaction of the amino group. The silane coating was stable but exhibited only small frequency decreases for the reaction step. Ln terms of stability, reproducibility and magnitude of the frequency changes observed, the present study indicates that poly(ethyleneimine) is by far the best choice for providing analytically useful signals where covalent reaction with surface-confined amino groups in the liquid phase is desired. (C) 1998 Elsevier Science S.A. All rights reserved.