(701b) Chiral Nanoparticles for Enantioslective Adsorption and Separations

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 and pharmaceuticals
has been probed by optical rotation measurements during exposure to mixtures with
varying enantiomeric excess of chiral compounds such as R- and S-propylene
oxide, R- and S-2-butanol, or R- and S-propranolol. 
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 with 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 the probe molecules such as propylene
oxide from a racemic solution, 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 adsorption model and
accompanying experimental protocol have been developed to enable optical
rotation measurements to be analyzed for quantitative determination of the
ratios of the enantiospecific adsorption equilibrium constants of chiral
species on the surfaces of chiral nanoparticles, K_L^S
/ K_D^S = K_D^R / K_L^R. 
This analysis is robust in the sense that it obviates the need to measure the
absolute surface area of the absorbent nanoparticles, a quantity that is
somewhat difficult to obtain accurately.  This analysis has been applied to
optical rotation data obtained from solutions of R- and S-propylene
oxide, R- and S-2-butanol, or R- and S-propranolol,
in varying concentration ratios, with D- and L-cysteine coated Au
nanoparticles, in varying concentration ratios.

Recently, tetrahexahedral
(THH, 24-sided) Au nanoparticles modified with D- or L-cysteine (Cys)
have been used as enantioselective separators of the chiral pharmaceutical propranolol
in solution phase.  Polarimetry has been used to measure the rotation of
linearly polarized light by solutions containing mixtures of propranalol and Cys/THH-Au
NPs with varying enantiomeric excesses of each.  Polarimetry yields clear
evidence of enantiospecific adsorption of propranalol onto the Cys/THH-Au NPs. 
This extends prior work using propylene oxide as a test chiral probe, by using
the crystalline THH Au NPs with well-defined, crystal facets to separate a real
pharmaceutical.  This work suggests that chiral nanoparticles, coupled with a
density separation method such as centrifugation, could be used for
enantiomeric purification of real pharmaceuticals.  The model developed earlier
has also been used to extract the enantiospecific equilibrium constants for R-
and S-propranalol adsorption onto the D- and L-Cys/THH-Au
NPs.

The
figure illustrates the reversible equilibrium adsorption of R- and S-propylene
oxide (PO) on Au nanoparticles modified with L- or D-cysteine.   The equilibrium constants are enantiospecific, K_L^S
= K_D^R ¹
K_D^S = K_L^R.