Probing Water and Biomolecules at the Air—Water Interface with a Broad Bandwidth Vibrational Sum Frequency Generation Spectrometer from 3800 to 900 cm −1

Abstract

We have built a broad bandwidth vibrational sum frequency generation (VSFG) spectrometer that can provide high-quality spectra over the range of 3800 to 900 cm⁻¹. The spectrometer contains a commercial Ti:sapphire based 6 W regenerative amplifier as the master light source, a home-built pulse shaper to produce a narrow bandwidth 800 nm beam, a commercial optical parametric amplifier to generate a broad bandwidth femtosecond infrared (IR) pulse, and a detection system with a monochromator and a charge-coupled device (CCD). We applied this spectrometer to obtain VSFG spectra of a lipid monolayer at the air–water interface in the O–H stretching region (3800–3000 cm⁻¹), the C–H stretching region (3100–2700 cm⁻¹), the C–D stretching region (2300–2000 cm⁻¹), the C=O stretching region (1800–1700 cm⁻¹), and the PO₂⁻ symmetric stretching region (1200–1000 cm⁻¹). We also obtained the VSFG spectrum of neat water in the O–H stretching region (3800–3000 cm⁻¹) and the VSFG spectrum of a protein, α-synuclein, in the amide I region (1700–1600 cm⁻¹) at the air–water interface. The spectrometer can provide a VSFG spectrum in the O–H stretching region (3800–3000 cm⁻¹) without scanning the IR frequency. This feature will be useful in probing water dynamics at interfaces because the free OH and H-bonded OH can be investigated simultaneously. We have also provided instrumental details and discussed further improvements that should be beneficial to other researchers interested in setting up VSFG instrumentation.