Probing the Viscoelasticity and Mass of a Surface-Bound Protein Layer with an Acoustic Waveguide Device

The aim of this work is to quantify the viscoelastic properties and calculate the density of a protein layer deposited on a solid/liquid interface. Devices, which use shear-horizontal surface acoustic waves propagating in a waveguide configuration at 108 and 155 MHz, were employed in the detection of specific binding and adsorption of IgG antibodies to protein A modified surfaces and gold, respectively. To obtain both viscoelastic and mass information about the biological layers, each device was first calibrated to viscosity changes by introducing a series of glycerol-water solutions to the sensing surface. Via this calibration, an equivalent viscosity of 1.00-1.30 cP for IgG layers in solution was obtained. By modeling the proteins as solid noninteracting particles, the quantitative viscoelastic information was then used to calculate protein surface coverages of 10-300 ng/cm 2 , which was in good agreement with surface plasmon resonance measured surface coverages. Modeling results yield information about the mechanical and chemical interactions between proteins in surface layers. Furthermore, for the first time, simultaneous quantitative, real-time mass and viscoelastic information of protein layers at a solid/ liquid interface was achieved by this technique.