Nonlinear Optical Methods for Characterization of Molecular Structure and Surface Chemistry

Abstract

The principles, strengths and limitations of several nonlinear optical (NLO) methods for characterizing biological systems are reviewed. NLO methods encompass a wide range of approaches that can be used for real-time, in-situ characterization of biological systems, typically in a label-free mode. Multiphoton excitation fluorescence (MPEF) is widely used for high-quality imaging based on electronic transitions, but lacks interface specificity. Second harmonic generation (SHG) is a parametric process that has all the virtues of the two-photon version of MPEF, yielding a signal at twice the frequency of the excitation light, which provides interface specificity. Both SHG and MPEF can provide images with high structural contrast, but they typically lack molecular or chemical specificity. Other NLO methods such as coherent anti-Stokes Raman scattering (CARS) and stimulated Raman scattering (SRS) can provide high-sensitivity imaging with chemical information since Raman active vibrations are probed. However, CARS and SRS lack interface and surface specificity. A NLO method that provides both interface/surface specificity as well as molecular specificity is vibrational sum frequency generation (SFG) spectroscopy. Vibration modes that are both Raman and IR active are probed in the SFG process, providing the molecular specificity. SFG, like SHG, is a parametric process, which provides the interface and surface specificity. SFG is typically done in the reflection mode from planar samples. This has yielded rich and detailed information about the molecular structure of biomaterial interfaces and biomolecules interacting with their surfaces. However, 2-D systems have limitations for understanding the interactions of biomolecules and interfaces in the 3-D biological environment. The recent advances made in instrumentation and analysis methods for sum frequency scattering (SFS) now present the opportunity for SFS to be used to directly study biological solutions. By detecting the scattering at angles away from the phase-matched direction even centrosymmetric structures that are isotropic (e.g., spherical nanoparticles functionalized with self-assembled monolayers or biomolecules) can be probed. Often a combination of multiple NLO methods or a combination of a NLO method with other spectroscopic methods is required to obtain a full understanding of the molecular structure and surface chemistry of biomaterials and the biomolecules that interact with them. Using the right combination methods provides a powerful approach for characterizing biological materials.

Second Harmonic Generation for Moisture Monitoring in Dimethoxyethane at a Gold-Solvent Interface Using Plasmonic Structures

“…Such configurations can lead to spectroscopic information that is not accessible by way of the classical one-photon linear spectroscopy tools that are governed by restricting dipolar selection rules. SFG thus, is an intrinsically surface-selective vibrational spectroscopy method [12][13][14][15][16]. A specific and simplified form of this is the use of two photons with the same fundamental frequency ω combined to generate a single photon of frequency 2ω, as in SHG [11].…”

Surface analysis tools for characterizing biological materials

“…A group of techniques that have the potential to provide molecular-level information about complex biomaterials, without the necessity to place these materials under UHV conditions, are nonlinear optical methods. 32 The different modalities of nonlinear optical methods have their strengths and weaknesses. For example, coherent anti-Stokes Raman scattering (CARS) provides high resolution images that allow the user to map chemical information within a biomaterial, but lacks surface specificity.…”

Prediction of the Linear and Nonlinear Optical Properties of a Schiff Base Derivatives via DFT

“…Studies of nonlinear optical processes contribute significantly to the development of photonics [2,3], spectroscopy [4,5], fiber optic lines [4], optical switches [6], frequency converters [7], electrooptic modulators [8], and data transmission network, among others [9] and still in numerous applications in the medical and pharmacological sectors [10][11][12]. The compounds that have a high nonlinearity are of great interest for the field of nonlinear optics, since they make up the manufacture of devices that operate with high speed [9,13].…”