Combined QCMD and AFM studies of lysozyme and poly-l-lysine–poly-galacturonic acid multilayers

A quartz crystal microbalance with dissipation monitoring (QCMD) has been used to monitor the adsorption and structure of lysozyme monolayers and multilayers, and poly-l-lysine (PLL)–polygalacturonic acid (PGalA) multilayers at a solid–liquid interface using freshly-cleaved mica as a substrate. QCMD measurements were complemented with atomic force microscopy (AFM). AFM images revealed that lysozyme formed incomplete monolayers and provided a basis for calculation of the thickness of the protein film. Comparative studies of adsorption onto standard and mica-coated quartz crystals showed higher areal mass adsorption and a longer-time adsorption process for mica-coated quartz crystals. Simultaneous AFM images and QCMD data were obtained for lysozyme, linear PLL–PGalA and 7 nm PLL dendrimer-PGalA multilayers. The layer-by-layer deposited multilayer films exhibited viscoelastic properties and their growth followed a non-linear regime, associated with the PLL diffusion in and out of the film formation for linear PLL–PGalA films. For the PLL 7 nm dendrimer-PGalA films the AFM images revealed marked changes in surface roughness during layer by layer deposition: these changes influence the interpretation of the QCMD data and provide additional information on the growth and structure of the multilayers.