Figure 3.

Sum-frequency generation (SFG) spectroscopy of bacterial INPs at the surface of aqueous solutions. (A) The incident IR and VIS beams generate a surface-specific SFG signal from the vibrational resonances. The illustration shows the alignment of interfacial water molecules in the case of a negative net charge as found at the natural pH of ∼6.2, in the case of zero net charge at the isoelectric point (IEP) ∼ 4.2, and the opposite alignment in the case of a positive net charge at pH values below the IEP. (B) Corresponding SFG spectra. The O–H band intensity is close to zero at the IEP and increases with the charge-induced alignment of the water molecules. The flip of the molecules’ orientations causes a frequency shift of the O–H stretch band. (C) Temperature-dependent SFG spectra of the O–H stretch band of interfacial H₂O molecules and the C–H stretch vibrations. The intensity of the O–H stretch band, and therefore the interfacial water alignment, is significantly higher at low temperatures.^42,44