Sum frequency generation (SFG) spectroscopy is one of the most unique methods to investigate molecular structure and dynamics at interfaces. It is applicable to any interfaces as far as they can be accessed by visible and IR light. It provides a vibrational spectrum that allows for determining molecular structure and identifying chemical species at interfaces. Furthermore, the spectrum indicates absolute orientation of interfacial molecules. For example, Figure 1 shows the vibrational spectra of lipid/water interfaces measured by my group using a newly-developed heterodyne-detected SFG spectrometer. The vibrational bands in 3000 ~ 3800 cm–1 are all due to the OH stretch of water at the interfaces. The water molecules below and above the lipid head groups are identified in 3000 ~ 3500 cm–1 and in 3500 ~ 3800 cm–1, respectively, based on the relation between the OH-stretch frequency and hydrogen-bond strength. The positive and negative bands correspond to water orienting upward to the air and downward to the bulk, respectively. The spectra in Figure 1 clearly indicate that the orientation of water below and above the head groups is governed by the static electric field created by the charges of the cationic and anionic lipids.

Heterodyne-detected sum frequency generation spectroscopy of aqueous interfaces

253rd National Meeting (2017)

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