VSFG is a unique technique that is capable of recording the vibrational spectrum of specifically the surface of a bulk material. Its surface specificity arises due to the selection rules of the undergoing second-order nonlinear process. For such a process the symmetry must be broken. Most of the bulk media do not generate any VSFG signal since molecules are isotropically oriented in the bulk (with the exception of some crystals). However, only at the surface, where the symmetry is broken, VSFG signal will be generated.
VSFG LASER SYSTEM
To generate VSFG two ultrashort laser pulses, one of a visible light and another in the infrared region, are overlapped onto the sample surface. VSFG is enhanced when the infrared is in a resonance with a vibration of a molecule at the surface. We use a commercially available picosecond scanning VSFG system from EKSPLA (UAB, Lithuania). It consists of a Nd:YAG picosecond pump laser and an optical parametric generator (OPG) with a difference frequency generation (DFG) module. Part of the fundamental laser radiation at 1064 nm (pulse length approx. 28 ps at 20 kHz repetition rate) is used to pump the OPG and the DFG module to produce infrared pulses, which can be tuned in a wide range (2.3 – 16 μm). A small part of the laser output is used to create a double frequency (532 nm) of the fundamental beam which later serves as the VIS beam for VSFG signal generation. The VSFG detection system consists of a monochromator with a high stray light rejection and a gated photomultiplier detector. The main advantage of such a detection system is the ability to perform measurements in the daylight.
ADVANCEMENT OF VSFG SPECTROSCOPY
VSFG spectroscopy is a relatively new technique. Since 1987 when the first VSFG spectrum was recorded by Shen et al., it has rapidly developed and new advanced techniques have emerged. First, a broad-band VSFG based on a femtosecond laser system was developed, which enabled the measurement of a wide spectrum with a single laser pulse. Later, a phase-sensitive, also called heterodyne-detected VSFG, has emerged that allows the measurement of the absolute orientation of the molecules at the surface. Further developments include time-resolved, two-dimensional, and high-resolution VSFG spectroscopy.
VSFG is a very versatile technique that can be used to study many different surfaces and interfaces such as solid/air, solid/liquid, liquid/air. The greatest potential of VSFG lies in its ability to measure liquid surfaces with a sensitivity of a few molecular layers. Recently, VSFG has been applied to study protein adsorption and conformation at air/water and lipid/water interfaces. Moreover, it was shown that VSFG has an intrinsic specificity to chiral molecules. It can distinguish α-helix, parallel and antiparallel β-sheet structures without any ambiguity [E. Yan]. VSFG spectroscopy on various biological systems is a rapidly expanding area of research.
SHELL-ISOLATED SURFACE ENHANCED
SHINERS is a novel Raman spectroscopy technique invented in 2010. It was developed in order to overcome the limitations of a substrate used in surface enhanced Raman spectroscopy (SERS). It is well established that when a molecule is adsorbed or located nearby (1 – 2 nm) a noble metal surface, Raman signal gets drastically amplified. For SERS, corrugated surfaces of certain metals (Au, Ag, and Cu) are used, which limits its applicability. The group of Z.Q. Tian came up with a completely new approach - to use shell-isolated nanoparticles to enhance the Raman signal.
Nanoparticles for surface enhanced Raman measurements are made out of an inner Au or Ag core which amplifies the Raman signal and an outer layer of silicon dioxide. The shell layer plays two very important roles: it protects the nanoparticles from degradation and prevents unwanted interactions between nanoparticles and probe molecules. In principle, such nanoparticles can be simply spread on any surface to be studied. In the group of prof. G. Niaura coated metal nanoparticles are produced by chemical specialists Agne Zdaniauskiene and Dr. Tatjana Charkova.
APPLICATIONS OF SHINERS
Although SHINERS is a very new technique, it has already been shown that it can be successfully applied to study various systems even such as biological tissues and yeast cells. Recently, our group used SHINERS to study potential-induced changes in the molecular structure of SAM [A. Zdaniauskiene et al. JPC C, 2017]. However, SHINERS technique has not been applied to study the secondary structure of biomolecules yet. Our group is aiming to use SHINERS technique to study the formation of protein aggregates at lipid membranes.
We use a commercially available inVia confocal microscope from Renishaw with a wide range of lasers, optical components, and accessories to achieve the best parameters for a particular sample. Our Raman microscope is equipped with a set of lasers at different excitation wavelengths (325, 442, 532, 633, 785 nm) and a CCD detector with a high sensitivity and an ultra-low noise level. All kind of samples (liquid, monolayer, powder, solid and others) can be measured with our Raman microscope. LiveTracking technology developed by Renishaw allows maintaining an optimal focusing even on rough or curved samples during the mapping scan. A set of special NEXT filters make it possible to measure spectra with a resolution down to 10 cm-1. With an additional temperature controlled cell, the temperature of the sample can be varied from -150 up to 400°C.