ATOMIC-FORCE MICROSCOPY AND X-RAY REFLECTIVITY STUDIES OF ALBUMIN ADSORBED ONTO SELF-ASSEMBLED MONOLAYERS OF HEXADECYLTRICHLOROSILANE

Atomic force microscopy (AFM) and X-ray reflectivity (XR) have been us ed together to provide a detailed and direct look at the structure of human serum albumin protein adsorbed onto well-characterized self-asse mbled monolayer (SAM) surfaces at several protein concentrations. The duration of SAM deposition was also varied to investigate the influenc e of the density of hydrocarbon chains in the SAM on protein binding t enacity. Concurrent study of adsorption to bare silicon wafers with na tive oxide surfaces provided a comparison with a hydrophilic surface s imilar to widely studied glass and quartz surfaces. Both AFM and XR me asurements showed that after adsorption, rinsing, and drying, the surf aces of all substrates were covered with no more than a single layer o f adsorbed protein. Thin dense protein layers were seen for the substr ates exposed to protein concentrations of 0.1 and 0.5 mg/mL. Partial s urface coverage by protein aggregates having larger thicknesses was se en for substrates exposed to lower concentrations. The tenacity of the protein adsorption on different substrates was tested by eluting the adsorbed protein with a 1% solution of sodium dodecyl sulfate surfacta nt. This treatment removed almost all protein from the bare silicon su rface and from the fully formed, dense SAMs. A significant amount of a dsorbed protein remained on the surface of the less dense, ''incomplet e'' monolayers, suggesting that protein adsorbed more tenaciously on t hat surface.