Accounting for the size of molecules in determination of adsorption isotherms by XPS; exemplified by adsorption of chicken egg albumin on titanium

Abstract

The adsorption of chicken egg albumin on commercially pure titanium has been studied as a function of protein concentration, using X-ray photoelectron spectroscopy (XPS). The adsorption isotherm has been plotted using the increase in N Is intensity and also by measurement in the decrease in the Ti 2p intensity as the adsorbed film reaches full coverage. It is shown that both sets of data are a good fit to the Temkin isotherm. The influence of the large size of the biomolecule is discussed and the isotherm is modified to take account of the molecular dimension according to the model proposed by Ratner and Paynter. The thicknesses of the adsorbed molecules are measured using atomic force microscopy (AFM) and it is shown that it is only when monolayer coverage has been reached that the molecules begin to take up the characteristic globular shape. Albumin reaches a coverage of 25% of a monolayer in solutions of only 10 ppb by volume, suggesting that it is easily bound to the TiO2 surface. A complete monolayer is formed at a solution concentration of 100 ppm. The carbon Is signal is used to estimate the surface free energy at different surface coverages using the model developed by Kinloch, Kodokian and Watts. The transformation from the initial coverage of hydrophobic contamination molecules to the hydrophilic surface presented by the adsorbed albumin film takes place over a range similar to that required to form the monolayer. Copyright © 2005 John Wiley & Sons, Ltd.