Vibrational Sum Frequency Generation Spectroscopy of Interfaces

Vibrational Sum Frequency Generation (VSFG) is a second-order non-linear optical spectroscopy that has recently gained much attention for its capability to obtain detailed information on interfacial structures. Due to the selection rules, this process is forbidden in centrosymmetric media, making it highly surface specific. To obtain vibrational spectra of interfaces, two laser beams, one of fixed visible frequency (532 nm) and the other of variable infrared frequency (tunable from 1000 to 4300 cm^-1) are overlapped spatially and temporally at the sample surface of interest（Fig. 1）. This can generate a third beam whose frequency is at the sum of the two incident beams: ω_SFG = ω_vis + ω_IR

Fig. 1. Schematic of sum frequency generation process at the air/water interface.

We have used this technique to investigate the molecular structure and hydrogen bonding of water at the aqueous/air interface (J. Phys. Chem. B 113 (2009) 11672-11679 pdf) and concluded that only the upper most two layers of water molecules are ordered in this system.

Moreover, we have utilized SFG to explore specific ion effects on proteins-water, lipids-water and polymer-water interfaces at different surface charges. (J. Am. Chem. Soc. 129 (2007) 12272-12279 pdf ; Langmuir 26 (2010) 16447-16454 pdf ), and the relative affinities of Hofmeister anion series and cation series to the aqueous interfaces of fused quartz titanium dioxide and hydrophobic OTS monolayers at various acidities (Fig. 2) (J. Phys. Chem. C. 116 (2012) 14408-14413 pdf ; J. Phys. Chem. C 116 (2012) 5730-5734 pdf). In these studies the adsorption of ion to a particular interface is studied by observing the increase or decrease in the intensity of interfacial water structure which reflects the change in the interfacial potential (surface charge), therefore one can determine the relative affinity of an ion to the surface by comparing the change in the magnitude of interfacial water structure typically in the range of OH stretch vibration (3000 to 3800 cm-1). Since SFG is sensitive to molecular orientation (polarized spectroscopy), it has been used to study urea re-orientation at protein surfaces by varying surface potentials of the protein (J. Am. Chem. Soc. 129 (2007) 15104-15105 pdf).

Fig. 2. The scheme depicts the relative affinity of ion to the TiO₂ aqueous interface (Left).  VSFS interfacial water spectra of TiO₂ (pI of TiO₂ is 5.5) aqueous interface at pH 10.0 in contact with 1.0 mM monovalent and 0.33mM divalent chloride salt solution. (Right)

More recently, we have explored the binding of alkali cations to the head groups of long-chain fatty acid monolayers. Our results reveal that Li⁺ binds tightest to the negatively charged carboxylate groups, followed by Na⁺, and then K^+.  This indicates that lithium ions have a stronger preference to bind directly to carboxylate groups compared with the larger monovalent cations.

In contrast to urea’s denaturing effect, trimethylamine N-oxide performs as a protein stabilizer and can counteract urea’s denaturation effect on protein structure. Its amphiphilic structure makes it moderately surface active and its strong dipole causes significant influences on water hydrogen bonding network. Classically, surface tension at the air-water interface is conveniently used to explain the denaturation/stabilization effects, which is a surface active molecule such as surfactant causes denaturation (expand the surface area in the solution by reducing gibbs free energy for cavity creation). However, this is certainly not true for both urea and trimethylamine N-oxide cases if and only if air-water interface can represent the interface of a surface neutral protein in aqueous solution. SFG can be used to study how trimethylamnie N-oxide affects the interfacial water structure. Surprising, the intensity in interfacial water structure is observed increasing gradually over hours for trimethylamine N-oxide aqueous solution. This phenome is also observed for small aromatic amino acid-phenylalanine by the other research group. This slow kinetic process at the interface seems not only a special case for TMAO but is a rather general process. Further studies for chemical composition, molecular ordering and concentration change over time are ongoing.

Currently we utilize SFG to explore the interaction of divalent cations with phospholipid monolayers at the air/water interface. Changes in interfacial water structure and headgroup orientation upon ion binding are under investigation.

Raman-MCR Spectroscopy: Detecting Hydration Shell of Molecules

High resolution Raman spectroscopy used in tandem with Multivariate Curve Resolution offers a novel method to investigate the solvation shell of molecules in aqueous solution. Raman spectra in aqueous solution are comprised of two primary components including pure solvent and solute-correlated features. Solute-correlated contributions encompass intramolecular solute vibrations in addition to solvent molecules whose vibrational modes are significantly perturbed by the presence of solute. Thus solute-correlated spectra can report on the interface of solvation, the hydration shell.

We have used Raman-MCR for the solute-correlated “hydration shell” spectra of small molecules to investigate the effect of common biological functional groups on interfacial water structure. A demonstration of this technique for the carbonyl group of acetone is provided below (Fig. 3).

Fig. 3. Demonstration of Raman-MCR analysis of acetone-aqueous solution. Input Raman spectra of H₂O solvent and solution are displayed in blue and red respectively. The resulting Solute-Correlated spectrum after MCR analysis is displayed in black and has been magnified for the purpose of visualization.

Our group intends to expand the application of Raman-MCR to answer fundamental questions concerning the nature of interfacial water at larger and more biologically relevant interfaces, such as the surface of proteins and lipid membranes.The “in solution” interfacial specificity of Raman-MCR offers a promising alternative to non-linear optic techniques, which require orientation of molecules at the air-water interface.