Enantioselective Separation on Chiral Au Nanoparticles

Abstract

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 during exposure to enantiomerically pure and 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 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 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.

Polarimetric Detection of Enantioselective Adsorption by Chiral Au Nanoparticles — Effects of Temperature, Wavelength and Size

“…Following the procedures developed and described in our previous studies of optical rotation by solutions of PO and chiral cys/Au NPs, [32], [33] [32] The goal of this work has been an empirical study of the effects of light wavelength, temperature and NP size on the degree of optical rotation observed in these types of experiments. This will guide the choice of conditions used for future quantitative measurements of the enantiospecific equilibrium constants for adsorption of chiral probe molecules on chiral NPs.…”

“…[32] M trisodium citrate dihydrate (C 6 H 5 Na 3 O 7 2H 2 O) was prepared in a three-neck round bottom flask. After 10 minutes of vigo-rous mixing using a magnetic stir plate, 0.6 mL of freshly prepared 0.1 M sodium borohydride (NaBH 4 ) was injected into the flask.…”

“…[28] For the most part, previous work on chiral NPs has focused on their structure and on their optical properties, [11], [12], [28]- [31] however, attention has been focussed more recently on their enantiospecific interactions with chiral probe molecules. [32], [33] Ultimately, this effort will lead to enantioselective NP functionality in processes such as separations and catalysis.…”

“…[32], [33] The Au NPs are rendered chiral by modification of their surfaces with either D-or L-cysteine (cys). The key observation that reveals their enantioselectivity is that during the addition of racemic propylene oxide (rac-PO) to solutions containing either D-or L-cys/Au NPs there is a change in optical rotation of linearly polarized light.…”

The impacts of nanotechnology on catalysis by precious metal nanoparticles

“…With regard to chiral nanoparticles, there are generally two types. Type I nanoparticles pertain to chiral ligand-protected nanoparticles, which mimic chiral ligandmodifi ed complexes [125] . For example, Li and coworkers recently attained enantioselective hydrogenation (96 % ee) of α -ketoesters using chirally modifi ed Pt nanoparticles encapsulated inside CNTs, in which the high activity and enantioselectivity were attributed to the unique 1D nanochannels of the CNTs as the nanochannels readily enrich both the chiral modifi er and the reactants [126] .…”