Characterizing Chiral Molecules Deposited onto a Silicon Surface

Abstract

We report here on the characterization of two types of chiral molecules deposited onto a silicon surface. Chiral molecules are non­superimposable mirror images of each other. Other than the way they interact with biological systems, chiral molecules have the same physical properties which make them hard to separate. Since many important drug molecules are chiral, effective separation methods are required by industry. We are building a model system to study one separation method called chiral chromatography. In chiral chromatography, separation is achieved by immobilizing a chiral compound along a column and passing the desired chiral mixture through. One of the mirror image molecules of the mixture has a higher attraction to the immobilized phase which causes it to exit the column at a later time. In the model being studied, propranolol is the sample drug molecule and phenylethylpropylurea (PEPU) is the selector molecule. Derivatives of these compounds were deposited onto a flat silicon surface. The resulting samples were studied in order to gain insight into the surface morphology and characteristics of the assembled layers. Using a combination of infra red (IR) spectroscopy and computational analysis it was possible to infer the average bulk molecular orientation of the deposited propranolol molecules. Atomic force microscopy was used to ensure a uniform deposition as well as to quantify the surface roughness. Through X­ray photoelectron spectroscopy (XPS) analysis it was shown that an average layer thickness of four molecules was deposited onto the silicon