Hydration of sulphobetaine and tetra(ethylene glycol)-terminated self-assembled monolayers studied by sum frequency generation vibrational spectroscopy.

Sum-frequency generation (SFG) vibrational spectroscopy is used to study the surface and the underlying substrate of both homogeneous and mixed self-assembled monolayers (SAMs) of 11-mercaptoundecyl-1-sulphobetainethiol (HS(CH(2))(11)N(+)(CH(3))(2)(CH(2))(3)SO(3)(-)) and 1-mercapto-11-undecyl tetra(ethylene glycol) (HS(CH(2))(11)O(CH(2)CH(2)O)(4)OH) with an 11-mercapto-1-undecanol (HS(CH(2))(11)OH) diluent. SFG results on the C-H region of the dry and hydrated SAMs gave an in situ look into the molecular orientation and suggested an approach to maximize signal-to-noise ratio on these difficult to analyze hydrophilic SAMs. Vibrational fingerprint studies in the 3000-3600 cm(-1) spectral range for the SAMs exposed serially to air, water, and deuterated water revealed that a layer of tightly bound structured water was associated with the surface of a nonfouling monolayer but was not present on a hydrophobic N-undecylmercaptan (HS(CH(2))(10)CH(3)) control. The percentage of water retained upon submersion in D(2)O correlated well with the relative amount of protein that was previously shown to absorb onto the monolayers. These results provide evidence supporting the current theory regarding the role of a tightly bound vicinal water layer in the protein resistance of a nonfouling group.