Adsorption of β-lactoglobulin and β-casein to metal surfaces and their removal by a non-ionic surfactant, as monitored via a quartz crystal microbalance

The use of a quartz crystal microbalance apparatus has been explored as a means of monitoring protein adsorption at the solid-liquid interface in real time. Gold-water and hydrophobic gold-water interfaces were studied, with adsorbed films of pure samples of the milk proteins beta-casein and beta-lactoglobulin. Adsorbed protein surface concentrations calculated from the observed frequency shifts and the Sauerbrey equation were higher by factor of 2 to 3 compared with results from other common techniques. This suggested that the frequency shifts were not only due to the adsorbed protein amount, but also to changes in protein hydrodynamic layer thickness. At neutral pH, for both beta-casein and beta-lactoglobulin, the results could be sensibly modelled in terms of a protein-rich inner layer close to the surface and a more diffuse outer layer extending into the bulk aqueous phase. The water-soluble surfactant octaethylene glycol n-dodecylether (C12E8) was added to pre-adsorbed protein films and then the systems were rinsed with buffer to try to remove adsorbed protein. Proteins adsorbed on the gold surface appeared to be more difficult to remove by this procedure-particularly for beta-lactoglobulin, possibly due to the formation of covalent bonding between protein sulphydryl groups and the gold surface. On hydrophobic gold surfaces, proteins appeared to be removed more easily, though not necessarily completely, owing to the greater affinity of the surfactant for the hydrophobic gold surface. Adsorption of a cationic gelatin onto the hydrophobic gold surface at pH7 indicated the formation of a relatively thick adsorbed "surface gel" layer, with large amounts of entrapped water. Addition of beta-lactoglobulin to a 24-hour-old gelatin film indicated the formation of an even thicker film--probably due to the formation of an electrostatic complex between the oppositely charged proteins at this pH. (C) 1997 Elsevier Science B.V.