QUARTZ-CRYSTAL MICROBALANCES FOR QUANTITATIVE BIOSENSING AND CHARACTERIZING PROTEIN MULTILAYERS

The use of quartz crystal microbalances (QCMs) for quantitative biosen sing and characterization of protein multilayers is demonstrated in th ree case studies. Monolayers of QCM-based affinity biosensors were inv estigated first. Layers of a thiol-containing synthetic peptide consti tuting an epitope of the foot-and-mouse-disease virus were formed on g old electrodes via self-assembly. The binding of specific antibodies t o epitope-modified gold electrodes was detected for different concentr ations of antibody solutions. Oligolayers were studied in a second set of experiments. Dextran hydrogels were modified by thrombin inhibitor s. The QCM response was used in a competitive binding assay to identif y inhibitors for thrombin at different concentrations. Multilayers of proteins formed by self-assembly of a biotin-conjugate and streptavidi n were investigated next. The QCM frequency response was monitored as a function of layer thickness up to 20 protein layers. A linear freque ncy decay was observed with increasing thickness. The decay per layer remained constant, thus indicating perfect mass coupling to the substr ate. Frequency changes a factor of four higher were obtained in buffer solution as compared to measurements in dry air. This indicates a sig nificant incorporation of water (75% weight) in the protein layers. Th is water behaves like a solid concerning the shear mode coupling to th e substrate. The outlook discusses briefly the need for controlled mol ecular engineering of overlayers for subsequent QCM analysis, and the importance of an additional multiparameter analysis with other transdu cer principles and with additional techniques of interface analysis to characterize the mechanical coupling of overlayers as biosensor coati ngs. A promising trend concerns the use of QCM-arrays for screening ex periments. (C) 1997 Elsevier Science Limited.