Fluorescence and Optical Activity of Chiral CdTe Quantum Dots in Their Interaction with Amino Acids

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.

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

“…Similar results were also obtained by Martynenko et al [68]. This model is often used to explain a wide range of observed phenomena including influence of QD size [62], shell thickness [62,69], nature and binding mode of ligands [70][71][72][73][74] on the position, shape and intensity of CD peaks, as well as the influence of QDs on the properties of chiral molecules [75], which will be discussed in the present review. In addition, this method allows the investigation of the QD energy structure using CD spectra [68].…”

“…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].…”

“…L-and D-Cys was added to the solutions of L-/D-Cys functionalized CdTe QDs to create hetero-or homochiral complexes [73]. Formation of heterocomplexes resulted in the large decrease of CD intensity and PL enhancement, unlike the formation of homocomplexes which resulted in values renaming almost at the same level.…”

Biological applications of chiral inorganic nanomaterials

“…During the recent 10 years, great attention has been paid to chiral inorganic materials (CNs), which showed charming optical properties and fancy functions 4 . Up to now, various chiral inorganic building blocks, including chiral semiconductors, 5 chiral metal materials, 6 chiral metal oxides, 7 chiral carbon materials, 8 and chiral ceramics 9 have been constructed with various methods. Interestingly, these novel chiral materials showed tunable optical activities or potential biomedical properties and even showed some similarities with the organic building blocks of organisms 10 .…”

A DFT Study of Interaction of (CdSe)3 Quantum Dots with Nucleobases

“…Highly luminous CdTe/ZnSe core/shell QDs have also been explored to distinguish purines from pyrimidines [49]. Recently, Li and coworkers have investigated the effect of the interaction of amino acids and quantum dots by examining the effect of interaction on the fluorescence and the optical activity of QDs where a significant fluorescence enhancement for L/D-Cys-CdTe QDs upon interaction with various amino acids was observed [50]. In another study, mechanism of DNA sensing was developed based on optical properties of graphene oxide (GO) and molybdenum disulphide (MoS2) nanopores, where GO and MoS2 were employed as quantum dot nanopore and DNA molecule translocate through the nanopore [51].…”