Water structure at the interfaces between a zwitterionic self-assembled monolayer/liquid water evaluated by sum-frequency generation spectroscopy

• Sum frequency generation analysis of interfacial water at the zwitterionic SAM surface.
• Weak orientation of water molecules at the surface of zwitterionic SAM.
• Strong orientation of water molecules at the surfaces of lopsidedly charged SAMs and bare prism.
• Importance of charge balance to afford bio-inert surface.

A silane coupling agent having a zwitterionic end group was covalently bound to a semi-cylindrical fused silica prism for sum-frequency generation (SFG) analyses and to a flat glass for estimating biological affinity. It was found that total intensity of the OH stretching region (3000–3600 cm−1) of the SFG spectrum derived from water in contact with a zwitterionic SAM-modified surface was smaller than that of SFG spectra derived from water in contact with a positively or negatively charged SAM-modified surface and a bare silica prism. These results indicated that water molecules in the vicinity of the zwitterionic SAM-modified surface are not strongly oriented in comparison with those of lopsidedly charged SAMs and bare silica. Moreover, the zwitterionic SAM surface suppressed non-specific adsorption of bovine serum albumin in contrast to the significant adsorption to lopsidedly charged SAMs and the bare cover glass. On the other hand, fibroblasts gradually adhered to the SAM surfaces and extended regardless of the electrical charge of the SAM, though the number of cells that adhered to the zwitterionic SAM was the smallest. The results strongly suggested that the charge neutralization of a solid material surface is very important for anti-biofouling properties.

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