(735b) Examining the Optical Effects of Chiral Carboxylic Acids Bound to the Surface of CdSe Nanoparticles

Chiral quantum dots (QDs) have recently emerged as an attractive platform for chiral fluorescent sensors, as well as tunable chiral emitters for nanophotonic applications. However, a fundamental understanding of the chiroptical response of these materials is still in its infancy. For example, a number of mechanisms have been proposed to explain the observed chirality in these nanoparticles, likely originating from the surface capping ligands. One such mechanism has suggested that electronic coupling between the chiral surface ligands and achiral QDs may play an important role. In order to probe this mechanism, it is necessary to compare the chiroptical response of QDs bound by chiral ligands with a range of different electronic energies. However, the vast majority of ligands utilized for the synthesis of chiral QDs have been derived from the amino acid, cysteine, where binding likely occurs through the thiolate sulfur atom. In contrast, there has been much less work carried out on chiral ligands where primary binding occurs through other atoms, which would yield different electronic energies and should thus provide noticeable differences in the chiroptical properties of the resulting QDs.

To investigate this, we have synthesized and characterized a new family of chiral CdSe QDs based on monocarboxylic and dicarboxylic acids, including lactic acid, malic acid and tartaric acid, where binding to the CdSe surface occurs through the carboxylate oxygen atom. We have examined the chiroptical response of these materials through circular dichroism (CD) experiments and compared the CD signals to those associated with CdSe QDs bound by cysteine ligands. The results suggest that the intensity and broadness of the CD peaks may be tuned by slight changes to the structure of the chiral carboxylic acid chosen. In addition, the commercial availability of a wide number of these chiral carboxylic acids has allowed for the first time a detailed and systematic study in which the number of stereocenters on the bound chiral ligand can be varied and the resulting CdSe QDs examined by CD spectroscopy. Our initial results suggest an increase in the intensity of CD signals for CdSe QDs bound by chiral ligands containing two stereocenters versus one stereocenter. We have also carried out a detailed study of the binding of these ligands to the surface of the CdSe QDs, suggesting the presence of an extended hydrogen-bonded network of ligands surrounding and stabilizing the nanoparticles in polar solutions. This model is supported by ¹H NMR and FTIR spectroscopies, dynamic light scattering and transmission electron microscopy experiments.

Our results establish a new family of chiral CdSe QDs based on chiral carboxylic acids, as well as a novel method of stabilizing these QDs in polar solutions. These results will not only provide an increased understanding of ligand binding to CdSe surfaces by small monocarboxylic and dicarboxylic acids but will also be critical for the development of both chiral fluorescent sensors and chiral emitters for nanophotonic applications such as security and anti-counterfeiting technologies.