Functionalized poly(ethylene glycol)-grafted polysiloxane monolayers for control of protein binding

Two new grafted polysiloxane polymers, a copolysiloxane and a terpolysiloxane, have been synthesized. Both polysiloxane backbones are grafted with dialkyl disulfide chains and 600 MW methoxy-terminated poly(ethylene glycol) (PEG) chains, while the terpolysiloxane also has 3400 MW PEG side chains terminated with N-hydroxysuccinimide (NHS) reactive ester groups. The two polymers spontaneously form monolayers on gold surfaces with estimated thicknesses of 23 and 31 A, respectively. Combined analysis with angle-dependent X-ray photoelectron spectroscopy and static time-of-flight secondary ion mass spectrometry showed that most grafted dialkyl disulfide side chains were bound to the gold surface, forming multiple molecular linkages at the monolayer/gold interface. The PEG-grafted chains were concentrated in the outermost exposed region of the monolayers. Protein adsorption on the two immobilized polymer monolayers was examined with surface plasmon resonance (SPR). The copolymer-covered surface resisted protein adsorption from buffer, while the terpolymer monolayer bound significant amounts of protein. The NHS headgroup in the terpolymer was the primary site for protein binding. These two polymers have potential applications for SPR biosensor surface modification as protein-resistant and protein-immobilizing surfaces, respectively.