| The size transition from bulk silver metals to nanoparticles and eventually to single atoms passes through the relatively unexplored, yet intriguing nanocluster size regime. These materials, while retaining many of the discrete energy levels of atoms, possess multiple free electrons, enabling strong, size-dependent optical transitions, including fluorescence, in the visible spectrum. Stabilization of the clusters in aqueous solutions requires the use of encapsulating agents to both coordinate the silver ions and protect the clusters, after reduction and initial agglomeration, from further uncontrolled aggregation. Single-stranded oligonucleotides and amine dendrimers, have served this function well, and the resulting properties have been previously reported. Here we expand our investigation of these fascinating materials by exploring their notable nonlinear optical properties, including the strong two-photon absorption of oligonucleotide-encapsulated clusters and the two-photon absorption and hyper-Rayleigh scattering of dendrimer-encapsulated clusters. Radiative lifetimes of several tens of picoseconds observed for the dendrimer-encapsulated clusters have proven useful in time-gated biological imaging. The silver cluster can be discriminated despite high fluorescent background by selectively imaging at times immediately following a given laser pulse. In addition to the endogenous effects of the cluster, its small size of only a few atoms renders it highly susceptible to surface and environmental effects, which manifests, for example, in the observed photoinduced charge transfer between the silver cluster and oligonucleotide. Known for its ability to stabilize surplus charges, the oligonucleotide can readily accept donated electrons from silver, resulting in a long-lived charge transfer state. This state has been shown to be highly advantageous in imaging applications, as control of this state enables better control over the time-averaged emission rate of the molecule. The mechanism of charge transfer, and the possible means by which this state can be controlled will be also be investigated in this work. |
