Chiral Monolayer-Protected Bimetallic Au-Ag Nanoclusters: Alloying Effect on Their Electronic Structure and Chiroptical Activity

摘要

Chiral glutathione (GSH)-protected bimetallic Au-Ag nanoclusters (AuAg(SG)) are synthesized by the reduction of mixtures of Au and Ag salts at the feed mole ratio of Au/Ag = 3/1 in the presence of GSH and isolated by gel electrophoretic separation, yielding a family of magic-numbered bimetallic Au-Ag nanoclusters. The PAGE pattern similarity between Au(SG) and AuAg(SG) as well as the analyses based on small-angle X-ray scattering and X-ray photoelectron spectroscopy (XPS) allow comparisons of their chemical compositions for each family of nanoclusters, and the fractioned nanocluster compounds (three samples) are identified as Au_(12.2)Ag_(2.8)(SG)_(13), Au_(14.4)Ag_(3.6)(SG)_(14), and Au_(17.6)Ag_(7.4)(SG)_(18). The XPS study also suggests that (i) incorporation of Ag heteroatoms into smaller-sized Au nanoclusters is less favorable and (ii) Ag heteroatoms preferentially occupy sites on the cluster's core (including the core-surface and center) rather than the peripheral semiring shell. Optical spectroscopy shows that the electronic structures of Au nanoclusters are significantly modulated by incorporation of Ag atoms. Interestingly, the bimetallic Au-Ag nanocluster compounds exhibit weaker circular dichroism (CD) responses than those of the corresponding Au counterparts. Such a decrease in chiroptical responses can be explained in terms of the increased geometrical isomers that are formed by statistical distribution of Ag heteroatoms in the nanocluster because an increased number of possible configurations is expected to give the average in the CD response with positive and negative bands of different optical isomers.