Adsorption of chiral compounds on chiral surfaces is the initial step in enantioselective processes such as separations and catalysis.  There has been a significant effort over the past decade aimed at the preparation of chiral nanoparticles based on metallic cores modified by chiral ligands.  In principle, these can serve as the basis for enantioselective chemical processing. In this work we demonstrate a simple measurement of enantioselective adsorption on chiral metal nanoparticles using a method that can yield quantitative measures of the enantiospecific adsorption equilibrium constants [1]. The surfaces of chemically synthesized Au nanoparticles have been modified with D- or L-cysteine to render them chiral and enantioselective for adsorption of chiral molecules.  Their enantioselective interaction with chiral compounds has been probed by optical rotation measurements when exposed to racemic propylene oxide.  The ability of optical rotation to detect enantiospecific adsorption arises from the fact that the specific rotation of polarized light by R- and S-propylene oxide is enhanced by interaction Au nanoparticles.  This effect is related to previous observations of enhanced circular dichroism by Au nanoparticles modified by chiral adsorbates.  More importantly, chiral Au nanoparticles modified with either D- or L-cysteine selectively adsorb one enantiomer of propylene oxide from a solution of racemic propylene oxide, thus leaving an enantiomeric excess in the solution phase.  Au nanoparticles modified with L-cysteine (D-cysteine) selectively adsorb the R-propylene oxide (S-propylene oxide).  A simple model has been developed that allows extraction of the enantiospecific equilibrium constants for R- and S-PO adsorption on the chiral Au nanoparticles.[1] N. Shukla, M.A. Bartel, A.J. Gellman �Enantioselective separation on chiral Au nanoparticles� Journal of the American Chemical Society, 132(25), (2010),  8575�8580

Optical rotation measurements of enantioselective separation on chiral Au nanoparticles

247th National Meeting (2014)

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