College of Science, Tianjin Chengjian University, Tianjin 300384, China.
Hebei Key Lab of Optic-Electronic Information and Materials, College of Physics Science and Technology, Hebei University, Baoding 071002, China.
ACS Nano. 2020 Apr 28;14(4):4196-4205. doi: 10.1021/acsnano.9b09101. Epub 2020 Apr 20.
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 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.
配体诱导的半导体纳米晶体手性在过去几年中一直是广泛研究的主题，在光学和生物学领域具有潜在的应用。然而，半导体纳米粒子中手性光学效应的起源仍不完全清楚。在这里，我们研究了与氨基酸相互作用对手性半导体量子点（QD）的荧光和旋光性的影响。当与所有测试的氨基酸相互作用时，l/d-Cys-CdTe QD 的荧光强度显著增强，这表明非辐射途径被抑制，以及表面陷阱位被氨基酸与 CdTe QD 表面的相互作用所钝化。杂手性氨基酸被证明会削弱圆二色性（CD）信号，这可能归因于半胱氨酸分子在 QD 表面的不同结合构型。此外，只有在加入半胱氨酸时，l/d-Cys-CdTe QD 的 CD 和荧光信号才会发生红移，而其他测试的氨基酸则没有这种效果。我们推测，巯基基团诱导半胱氨酸的最高占据分子轨道（HOMO）和 CdTe QD 的价带之间的轨道杂化，导致能带隙减小，同时伴随 CD 和荧光光谱的红移。这进一步通过密度泛函理论计算得到验证。实验和理论研究结果均表明，添加不与 QD 的价带（VB）“直接”相互作用的配体（非半胱氨酸部分）会改变 QD 的光物理性质，因为它可能改变半胱氨酸与表面结合的方式。因此，我们得出结论，不仅氨基酸配体的化学性质影响 CD 和 PL，而且结合的精确几何形状也会改变这些性质。因此，了解 QD 表面与手性氨基酸之间的关系为理解半导体纳米粒子中手性光学效应的基本起源提供了一个额外的视角，有可能使我们能够优化手性半导体 QD 的设计，以用于手性应用。
