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

Abstract

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.

Ligand-induced chirality and optical activity in semiconductor nanocrystals: theory and applications

“…Similar results were reported in another paper by Kuznetsova et al [84], showing that cysteine has different binding modes to the surface of CdSe/CdS QDs, dependent on ligand concentration. At low concentrations, cysteine bound to the QD through three functional groups, thiol-, amino-, and carboxyl, but with increasing ligand concentration, more cysteine molecules attached via the thiol-and amino groups, while the carboxyl group remained free.…”

“…The molar ratio of cysteine to the dot-in-rods also influences the CPL intensity. Similarly it was reported [84] that the excess of cysteine molecules in aqueous solutions decreases the intensity of both the CD and CPL signals ( Figure 12D).…”

“…These configurations of the ligands were shown to be diastereoisomers in relation to the ligand surface ( Figure 9). Importance of ligand binding mode was confirmed further in a number of other papers [71,73,84]. For example, CdSe QDs functionalized with thiol-free chiral carboxylic acids containing two carboxyl groups with a similar structure but with different substituents were studied [71].…”

“…An increase of the shell thickness reduces the interactions between holes and ligands exponentially. The deviations from the exponential trend can be explained by the influence of other factors, e.g., dependence of CD signal on ligand concentration [84], discussed in Section 2.3.4. The addition of shell layers can also reduce the confinement energy of the hole and pushes it close to the molecule level, increasing interaction between them [62].…”

“…In some reports it was observed that the CD signal of chiral molecules altered after binding with the QDs compared to that of free molecules [25,55]. In many publications, it was shown that the conformation of chiral ligands is altered on the surface of the QDs [70][71][72]84], which can possibly affect the molecule optical activity. Moreover, it has been theoretically predicted by Govorov et al [88][89][90] that a nonchiral planar molecule can become optically active when it forms a complex and couples via dipole and multipole Coulomb interactions with achiral NCs.…”