Paper SS1-MoA10
Vicinal and Interfacial Water Structure of Non-Fouling Poly(ethylene glycol) and Sulphobetaine Self Assembled Monolayers

Monday, October 15, 2007, 5:00 pm, Room 608

Poly(ethylene glycol) (PEG) and sulphobetaine (SB) are currently used in the formation of non-fouling surfaces. As yet, an understanding of the water phase effects in the vicinal realm of these surfaces and their impact on protein resistance remains imperfect. In this work, we present analysis of mixed self-assembled monolayers (SAMs) with either the PEG or SB functionality and the impact of hydration on both monolayer and vicinal water structure using multiple surface analytical techniques. The degree of protein resistance in the SAMs was modified through successive addition of diluent hydrophobic thiols to achieve a broader spectrum of protein resistant surfaces as measured through radio-labeled protein adsorption. The hydrophilic diluents yielded lower amounts of protein adsorption overall for both PEG and SB surfaces. However, the overall trends in protein adsorption differed for the two non-fouling thiols. SB yielded lowest adsorbed fg, alb, and lys when assembled in a 50:50V concentration with 11-mercaptoundecan-1-ol; whereas, pure PEG SAMs yielded the lowest levels of adsorbed protein. The packing density and chemical composition of the SAMs were examined by XPS, as well as ToF-SIMS. The orientation of the SB head groups was confirmed through angular XPS revealing a nearly horizontal head group. This contradicted ellipsometric thickness measurements. PEG data showed no similar contradictions. The terminal hydrophilicity of the groups was characterized through contact angle measurements and followed protein adsorption trends. Unpolarized infrared spectroscopy (FTIR) showed that the stretching frequencies, νCH_2,asym and νCH_2,sym, of the ultra-nonfouling SB and PEG SAMs decreased and approached 2918 and 2850 cm^-1, indicative of a crystalline phase, when hydrated. To study hydration effects, each SAM was exposed to a series of timed D₂O soaks. Band shapes of the composite ν_OH band of H₂O obtained were fitted to individual peak components and a ratio of the component band areas from the 3400 and 3200 cm^-1 regions was utilized to cross-compare samples. A single linear trend between the water peak ratio minima and protein adsorption was obtained for both the PEG and SB SAMs with lower ratios corresponding to higher levels of protein resistance. Using this method, FTIR has been used for the first time to demonstrate a correlation between strongly-bound water structure and protein adsorption.