Abstract
Ligand-induced chirality in semiconducting nanocrystals has been the subject of extensive study in the past few years and shows potential applications in optics and biology. Yet, the origin of the chiroptical effect in semiconductor nanoparticles is still not fully understood. Here, we examine the effect of the interaction with amino acids on both the fluorescence and the optical activity of chiral semiconductor quantum dots (QDs). A significant fluorescence enhancement is observed for l/d-Cys-CdTe QDs upon interaction with all the tested amino acids, indicating suppression of nonradiative pathways as well as the passivation of surface trap sites brought via the interaction of the amino group with the CdTe QDs’ surface. Heterochiral amino acids are shown to weaken the circular dichroism (CD) signal, which may be attributed to a different binding configuration of cysteine molecules on the QDs’ surface. Furthermore, a red shift of both CD and fluorescence signals in l/d-Cys-CdTe QDs is only observed upon adding cysteine, while other tested amino acids do not exhibit such an effect. We speculate that the thiol group induces orbital hybridization of the highest occupied molecular orbital (HOMOs) of cysteine and the valence band of CdTe QDs, leading to the decrease of the energy band gap and a concomitant red shift of CD and fluorescence spectra. This is further verified by density functional theory calculations. Both the experimental and theoretical findings indicate that the addition of ligands that do not “directly” interact with the valence band (VB) of the QD (noncysteine moieties) changes the QD photophysical properties, as it probably modifies the way cysteine is bound to the surface. Hence, we conclude that it is not only the chemistry of the amino acid ligand that affects both CD and PL but also the exact geometry of binding that modifies these properties. Understanding the relationship between the QD’s surface and chiral amino acid thus provides an additional perspective on the fundamental origin of induced chiroptical effects in semiconductor nanoparticles, potentially enabling us to optimize the design of chiral semiconductor QDs for chiroptic applications.
Highlights
Ligand-induced chirality in semiconducting nanocrystals has been the subject of extensive study in the past few years and shows potential applications in optics and biology
We show that a significant fluorescence enhancement is observed for L/D-Cys-CdTe quantum dots (QDs) after interacting with all the tested amino acids, and reduced circular dichroism (CD) signals are shown only when heterochiral amino acids are added
Transmission electron microscopy (TEM) images are displayed in Figure S1, with an average diameter of about 3.2 nm, and the interplanar distance between adjacent lattice planes is determined as ca. 0.37 nm, matching the d (111) value of CdTe
Summary
Ligand-induced chirality in semiconducting nanocrystals has been the subject of extensive study in the past few years and shows potential applications in optics and biology. In plasmonic metal nanosystems this interaction likely originates from a dipolar interaction, as reported by Govorov et al.[12] Yet, for QDs, the dipolar interaction is expected to be much weaker due to the difference in dielectric constant, and it is believed that the interaction originates from the hybridization of the molecular orbital between the ligand and QDs.[13] Kotov and co-workers have reported the chiral interaction between the L/D-cysteine and graphene quantum dots (GQDs), whereby the molecular orbital of chiral edge-ligands likely introduces a symmetrybreaking perturbation to the electronic states of GQDs.[14] Markovich and co-workers reported that all the features of CD response can be accounted for as a sum of the chiral responses of individual excitonic states.[13] Balaz et al studying postsynthesis ligand-exchanged colloidal CdSe QDs, concluded that the induced chiroptical activity originated from the hybridization of the HOMO level of the ligand molecular orbital and valence band (VB) level of CdSe QDs.[7] despite these advances, the understanding of the origin of the induced chiroptical effect in QDs is still lacking This is partly since almost all past reports focus on the interaction between originally achiral QDs and their chiral ligands, whereas only a few reports until now study the interaction between the surface of already chiral semiconductor QDs and other chiral moieties (such as amino acids).
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
