Nonlinear Optical Properties of Thiolate-Protected Gold Clusters

Abstract

Thiolate-protected gold clusters are promising candidates for imaging applications due to their interesting, size-dependent properties. Their high stability and the ability to functionalize the clusters with biocompatible ligands render the clusters interesting for various imaging techniques such as fluorescence microscopy or second-harmonic generation microscopy. The latter nonlinear optical effect has not yet been observed on this type of ultrasmall nanoparticle. We hereby present second- and third-harmonic generation and multiphoton fluorescence of two thiolate-protected gold clusters: Au25(SCH2CH2Ph)18 and Au38(SCH2CH2Ph)24. At a fundamental wavelength of 800 nm, the Au38(SCH2CH2Ph)24 cluster is active. In contrast, Au25(SCH2CH2Ph)18 does not yield significant SHG signal. We ascribe this to the center of inversion in the Au25 cluster. Measurements on chiral Au25(capt)18 (capt: captopril) gave an SHG response, supporting this interpretation. We also observed third-harmonic generation at a fundamental wavelength of 1200 nm. At 800 and 1100 nm, the clusters decompose after short illumination time but are stable at illumination at 1200 nm. This may be exploited in combined deep tissue imaging and photothermal heating for theranostics applications.