Citrate-capped gold nanoparticles as well as planar gold surfaces can be efficiently grafted with a covalently attached polymer monolayer a few nanometers thick, by simple contact of the metal surface with dilute aqueous solutions of hydrophilic polymers that are end-capped with disulfide moieties, as shown by UV/vis absorption, dynamic light scattering, and surface plasmon resonance studies. The hydrophilic polymer-coated gold colloids can be freeze-dried and stored as powders that can be subsequently dissolved to yield stable aqueous dispersions, even at very large concentrations. They allow for applying filtrations, gel permeation chromatography, or centrifugation. They do not suffer from undesirable nonspecific adsorption of proteins while allowing the diffusion of small species within the hydrogel surface coating. In addition, specific properties of the original hydrophilic polymers are retained such as a lower critical solution temperature. The latter feature could be useful to enhance optical responses of functionalized gold surfaces toward interaction with various substrates.

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The covalent attachment of Fab' fragments of polyclonal anti-human IgG to a lipid with a terminal linker group was examined by means of quartz crystal microbalance and surface plasmon resonance measurements. The linker lipid was embedded in binary or ternary monolayers of dipalmitoylphosphatidylcholine (DPPC) and cholesterol. Atomic force microscopy images of the films deposited on silanised SiO(2) substrates showed that Fab' fragments take a standing position, thus giving site-directed immobilisation. Human IgG forms a network on interaction with the antibodies. Non-specific binding of bovine serum albumin was found to be very low when DPPC was used as the host matrix. At an optimal Fab' fragment concentration a binding capacity above 60% was obtained. However, if the surface concentration of the immobilised antibodies was too high, the binding capacity decreased due to steric hindrance. The results demonstrate that the covalent coupling of Fab' fragments to N-(epsilon-maleimidocaproyl)-dipalmitoylphosphatidylethanolamine (DPPE-EMC) embedded in a host monolayer matrix of DPPC is a promising approach to achieve a site-directed immobilisation of antibodies with high antigen-binding efficiency.