Circular Dichroism Spectroscopy as a Powerful Tool for Unraveling Assembly of Chiral Nonluminescent Aggregates of Photosensitizer Molecules on Nanoparticle Surfaces.

Recent developments in nanoscience and nanotechnology significantly help improve the properties of traditional materials. A striking example of this is the formation of hybrid nanostructures based on nanoparticles and photosensitizer molecules, the potential range of applications of which extends from photovoltaics to biomedicine. However, the creation of new and effective hybrid nanomaterials of this form inevitably entails new challenges, one of which is a common and critical problem of aggregation of both nanoparticles and photosensitizer molecules. Therefore, a fundamental challenge is to determine the presence of these aggregates, which will produce a significant step toward creating a new generation of materials and devices of broad-spectrum applicability. Here we report on the key role of circular dichroism spectroscopy as a tool to detect the formation of nonluminescent aggregates of chlorin e6, a second-generation photosensitizer, in a hybrid nanostructure with ZnS:Mn quantum dots. These aggregates are active acceptors of photoexcitation energy from quantum dots and limit the photophysical properties of the whole nanostructure. It has been established that circular dichroism spectroscopy reveals the presence of nonluminescent molecule aggregates at chlorin e6 concentrations of ∼10-6 mol/L, which compares very favorably to absorption spectroscopy which does not show any direct indications of aggregation up to ∼10-5 mol/L. This result demonstrates the promise and importance of using circular dichroism spectroscopy in the study of organic/inorganic hybrid nanostructures.

Effect of Chiral Ligand Concentration and Binding Mode on Chiroptical Activity of CdSe/CdS Quantum Dots.

Chiroptically active fluorescent semiconductor nanocrystals, quantum dots (QDs), are of high interest from a theoretical and technological point of view, because they are promising candidates for a range of potential applications. Optical activity can be induced in QDs by capping them with chiral molecules, resulting in circular dichroism (CD) signals in the range of the QD ultraviolet-visible (UV-vis) absorption. However, the effects of the chiral ligand concentration and binding modes on the chiroptical properties of QDs are still poorly understood. In the present study, we report the strong influence of the concentration of a chiral amino acid (cysteine) on its binding modes upon the surface of CdSe/CdS QDs, resulting in varying QD chiroptical activity and corresponding CD signals. Importantly, we demonstrate that the increase of cysteine concentration is accompanied by the growth of the QD CD intensity, reaching a certain critical point, after which it starts to decrease. The intensity of the CD signal varies by almost an order of magnitude across this range. Nuclear magnetic resonance and Fourier transform infrared data, supported by density functional theory calculations, reveal a change in the binding mode of cysteine molecules from tridentate to bidentate when going from low to high concentrations, which results in a change in the CD intensity. Hence, we conclude that the chiroptical properties of QDs are dependent on the concentration and binding modes of the capping chiral ligands. These findings are very important for understanding chiroptical phenomena at the nanoscale and for the design of advanced optically active nanomaterials.

Impact of Shell Thickness on Photoluminescence and Optical Activity in Chiral CdSe/CdS Core/Shell Quantum Dots.

Core/shell quantum dots (QDs) are of high scientific and technological importance as these nanomaterials have found a number of valuable applications. In this paper, we have investigated the dependence of optical activity and photoluminescence upon CdS shell thickness in a range of core-shell structured CdSe/CdS QDs capped with chiral ligands. For our study, five samples of CdSe/CdS were synthesized utilizing successive ion layer adsorption and reaction to vary the thickness of the CdS shell from 0.5 to 2 nm, upon a 2.8 nm diameter CdSe core. Following this, a ligand exchange of the original aliphatic ligands with l- and d-cysteine was carried out, inducing a chiroptical response in these nanostructures. The samples were then characterized using circular dichroism, photoluminescent spectroscopy, and fluorescence lifetime spectroscopy. It has been found that the induced chiroptical response was inversely proportional to the CdS shell thickness and showed a distinct evolution in signal, whereas the photoluminescence of our samples showed a direct relationship to shell thickness. In addition, a detailed study of the influence of annealing time on the optical activity and photoluminescence quantum yield was performed. From our work, we have been able to clearly illustrate the approach and strategies that must be used when designing optimal photoluminescent optically active CdSe/CdS core-shell QDs.