Chiral monolayer-protected Au-Pd bimetallic nanoclusters: effect of palladium doping on their chiroptical responses

We present an effect of palladium doping on the chiroptical responses of optically active gold nanoclusters. Monolayer-protected Au-Pd bimetallic nanoclusters of 1.5-2.0 nm in diameter are synthesized by the reduction in mixtures of Au and Pd salts in the presence of chiral thiol, glutathione (GSH), and isolated by electrophoretic separation. For synthetic feed mole ratios of Au/Pd=3/1 and 1/1, the isolated nanocluster compounds have much higher Au/Pd ratios of ∼8.4 (7.9-8.8) and ∼5.1 (4.7-5.4), respectively. We find that circular dichroism (CD) offers a greater advantage in detecting a foreign atom doping in the nanocluster as compared to absorption spectroscopy; namely, there is a significant difference in Cotton effects between the Au-Pd bimetallic nanoclusters and the monometallic counterparts (Au or Pd nanoclusters), whereas almost no difference can be seen between them in the UV-vis absorption spectra. In the bimetallic nanoclusters, interestingly, their CD profiles are almost independent on the Pd dopant content. The X-ray photoelectron spectroscopy (XPS) reveals that Au atoms are predominated in the surface (including staples) layer of these Au-Pd nanoclusters, indicating a Pd(core)-Au(shell) (inverted core-shell) structure, so the similarity in the CD responses observed would be due both to the predominance of Au in the outermost chiral surface structure and possession of their similar metal core configuration.