ARXPS characterisation of plasma polymerised surface chemical gradients

The aim of this paper is twofold: first to report on the lateral and vertical characterisation of a surface chemical gradient of carboxylic-acid functionality and second, to demonstrate the use of said gradient to probe the passive adsorption of immunoglobulin G (IgG) as a function of the density of surface carboxylic-acid groups. A surface chemical gradient of carboxylic-acid functionality was fabricated by the plasma copolymerisation of octadiene (OD) and acrylic acid (AA). The plasma-polymerised gradient was over 12 mm, with 2 mm of plasma-polymerised OD at one end and 2 mm of plasma-polymerised AA at the other. By means of linescan angle resolved x-ray photoelectron spectroscopy (ARXPS) it is shown precisely how acid functionality varies from the 2 mm position (OD end) on the gradient to the 10 mm position (AA end). By recording data from 16 angles at each of the 25 sampling points along the gradient, it is shown that the surface gradient also changes vertically, most notably in the thickness of the plasma polymer. At the OD end the plasma-polymerised layer is 6.3 nm thick, while at the AA end the plasma-polymerised layer is 5 run. More subtle changes in chemistry through the plasma-polymerised layer are shown at the 7.5 and 10 mm points. An identical gradient is used to probe IgG adsorption along the length of the gradient. ARXPS is used to monitor the nitrogen 1s (N1s) signal at 25 points, the N1s signal being unique to adsorbed IgG. It is demonstrated that IgG adsorbs in far greater amount (IgG per unit area) at the OD end, and the amount of adsorbed IgG decreases along the length of the gradient. It is estimated that >200 ng/cm(2) adsorbed at the OD, while at the AA the amount adsorbed was <20 ng/cm(2). Copyright (C) 2006 John Wiley & Sons, Ltd.