Paper NS-ThM1
Enantioselective Separation on Chiral Au Nanoparticles

Thursday, October 21, 2010, 8:00 am, Room La Cienega

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. The enhancement of the specific optical rotation of polarized light by R- and S-propylene oxide is sensitive to excitation wavelength. Longer the excitation wavelength, smaller is the specific rotation of polarized light. 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 enantioselectively 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-propylene oxide adsorption on the chiral Au nanoparticles.