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 nm. 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 .

Near edge X-ray absorption fine structure (NEXAFS) has been employed to provide insight into the chemical nature of nitrogen in deposits formed from plasmas of allylamine and propylamine. The nitrogen K-edge spectra of these materials unambiguously demonstrate the presence of significant quantities of sp or sp 2 hybridized nitrogen. This finding, in conjunction with carbon K-edge spectra, strongly indicates that there is a substantial level of dehydrogenation during the plasma polymerization process resulting in the formation of imine groups and, at high power, nitrile groups in addition to sp 3 hybridized amines. Comparison with standard polymers indicates that amide formation (following a few days exposure to atmosphere) is negligible. These findings suggest that the hydrolysis of aminated plasma polymers may be important in their long-term aging.

Thin organic films were deposited from plasmas of allyl alcohol, acrylic acid, allylamine, and octa-1,7-diene onto aluminum substrates. These plasma polymerized films were analyzed by X-ray photoelectron spectroscopy (XPS) before and after prolonged periods of storage under laboratory atmosphere to attempt to quantify changes in surface chemistry as a result of aging over 400 days. Plasma polymers produced from octa-1,7-diene and allylamine showed a distinct uptake of oxygen during the first 30 days post-polymerization, gradually leveling off but not appearing to reach a stable level. In addition, the allylamine plasma polymers showed a loss of nitrogen. Those polymers produced from allyl alcohol and acrylic acid plasmas did not show any changes with time in their oxygen content, or in the composition of their C 1s core levels. While the aging seen in the hydrocarbon plasma polymer and the nitrogen-containing plasma polymer is broadly consistent with what has been previously reported in the literature for similar materials, the aging of plasma-polymerized allyl alcohol and acrylic acid is quite different. The data highlight the differences in aging between different monomers plasma polymerized under the same reactor conditions, and "alike" plasma polymers deposited using very different experimental setups (reactor geometry, power coupling, and frequency).

Plasma polymerisation is a technologically important surface engineering process capable of depositing ultra-thin functionalised films for a variety of purposes. It has many advantages over other surface engineering processes, including that it is completely dry, can be used for complex geometries, and the physico-chemical properties of the film can be tailored through judicious choice of processing conditions. Despite this, the mechanisms of film growth are largely unknown, and current models are based on purely chemical arguments. Consideration of some basic plasma physics shows that some species can arrive at surfaces with energies greater than 1000 kJ mol 21 (.10 eV), and thus open a range of surface reactions that have not been considered previously. This review aims to close the gap between the physics and chemistry of reactive plasma systems.