Multidimensional spectroscopic techniques for monitoring adsorption and binding at liquid/solid interfaces.

摘要

New approaches have been developed that utilize multidimensional analysis techniques to model molecular adsorption and binding at liquid/solid interfaces. One-dimensional measurements along the spectroscopic axis lack the informing power to resolve complex systems. However, the acquisition of multiple spectra along a chemical reaction dimension dramatically increases the informing power. The correlation from the successive spectra allows multidimensional analysis to extract the pure component spectra and concentration profiles. A fiber-optic system was developed for in-situ excitation and collection of Raman scattering signals from metal ion-complexation with silica-immobilized 8-hydroxyquinoline (8HQ) in real time under flow-rate and pressure conditions typical of chromatographic separations. Reactions of this reagent were studied by using flow injection methods, and Raman spectra obtained under different solution conditions were used to determine the structures of the various forms of the immobilized ligand. A multivariable least-squares (MVLS) approach was also employed to provide chemical insight into the chemical interactions that affect the interfacial binding equilibrium behavior. These results showed that changes in the surface potential, induced by specific ion adsorption, altered the equilibria of the metal chelate. In order to understand the interfacial reactivity of the immobilized ligand, a water soluble form of 8HQ was synthesized and determination of the stepwise formation constants was obtained with MVLS analysis. For complex systems, analysis of multidimensional data can be challenging without a priori information. For this type of problem, a multivariate technique called self modeling curve resolution (SMCR) was used to reduce the two-component spectra are not known a priori. A SMCR approach is employed to analyze in-situ surface enhanced Raman spectroscopy obtained during potential dependent oxidative adsorption of n-hexanethiolate onto silver. This technique distinguished that there were three species during monolayer formation, and was also able to resolve reaction pathways and their associated pure component Raman spectra for each of the intermediate species.