Model bacterial extracellular polysaccharide adsorption onto silica and alumina:: Quartz crystal microbalance with dissipation monitoring of dextran adsorption

Quartz crystal microbalance with dissipation monitoring (QCM-D) was used to investigate dextran adsorption to alumina and silica. Sensitive adsorption measurements combined with determination of nanometer-scale polymer conformations demonstrate the utility of this technique for studying biopolymer adsorption. The adsorbed amounts and polymeric structures of dextran were determined on Al2O3 and SiO2 by real-time monitoring of resonance frequency and energy dissipation changes (Delta f and Delta D). After the sample was rinsed, the apparent mass of retained dextran was 83 ng/cm(2) on the alumina surface and 9 ng/cm(2) on the silica surface based on the frequency and energy dissipation changes. The Delta D/Delta f ratios were significantly different on the two surfaces, indicating different conformations of the polymers. On alumina, the ratio changed as adsorption proceeded indicating changes of dextran conformation from the initial to latter adsorption steps. On silica, the ratio did not change during the experiments. Therefore, the dissipation and frequency data suggest significantly different mechanisms of dextran adsorption on alumina and silica surfaces. Molecular dynamics simulations of 12 monomeric units of dextran on a silica slab illustrated that H2O molecules lead to loosely bound dextran structure onto the SiO2 surface, consistent with the observed high-energy dissipation in the QCM-D experiments.