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GG8.00 - Intrinsic and Induced Chirality of CdSe Nanocrystals 
April 10, 2015   8:15am - 8:30am

Colloidal semiconductor nanocrystals are under intense development for a range of applications, including light-emitting devices for energy-efficient displays and light sources, light-harvesting elements for exploitable and low-cost photovoltaics, logical elements for quantum computing, as well as for many biomedical applications such as imaging, drug delivery, and sensors. In particular, the key advantage of using of artificial nanocrystals in biology, medicine and pharmacology is an ability to manipulate the units of matter at the scale close to molecular one. At this scale many biological processes involve mechanism of molecular recognition, and it is well known that chirality plays a key role in this mechanism. Potentially any nanocrystal can be intrinsically chiral due to their low symmetry and presence of bulk and surface defects. Chirality of the semiconductor nanocrystals makes possible their �lock and key� interaction with biological objects since nanocrystals� sizes are comparable to the sizes of biomolecules and the pores of cell membranes. Here we investigate intrinsic chirality of CdSe nanocrystals using a technique of circular dichroism (CD). We develop an experiment under assumption that as-prepared ensemble is a mixture of achiral nanocrystals and equal amounts of nanocrystals with opposite chirality, and possesses no optical activity. Consequently, to obtain an experimental evidence of intrinsic chirality of the semiconductor nanocrystals, it is necessary to separate their enantiomers. We show that optically active the quantum dots and quantum rods samples can be obtained by preferentially extracting either L- or D-enantiomers of nanocrystals from optically inactive sample. We use an original method of the enantioselective phase transfer of the nanocrystals from organic solvent to water assisted by the chiral molecules of ligand. This method allows us to isolate L- and D-enantiomers of the nanocrystals into different organic and aqueous phases and investigate their CD behaviour. The aqueous phase is enriched with nanocrystals with the same chirality as the ligand molecules used for the phase transfer, for example, L-nanocrystals in the case of using L-cysteine. In the contrast, the organic phase is enriched with the nanocrystals, whose interaction with chiral ligands was ineffective. These nanocrystals are still capped by achiral ligands. The aqueous and organic fractions possess almost mirror image CD signals, but the value of intrinsic CD signal in organic solvent is less. The reason for this difference may be in induced increasing of CD after adsorption of chiral ligands. The experiments with other types of quantum-confined objects such as nanoplatelets, nanotetrapods, and dot-in-rods also show increasing of CD signal after capping with chiral ligands. Importantly, CD increasing differs for different types of nanocrystals. In particular, CD increasing up to 30 times is observed for the nanoplatelets.

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