Recent developments in optical imaging techniques, particularly multi-photon excitation microscopy that allows studies of biological interactions at a deep cellular level, have motivated intensive research in developing multi-photon absorption fluorophores. Biological tissues are optically transparent in the near-infrared region. Therefore, fluorophores that can absorb light in the near-infrared (NIR) region by multi-photon absorption are particularly useful in bio-imaging. For instance, photoluminescence from ligand-protected gold nanoclusters has drawn extensive research interest in the past decade due to their bright, non-blinking, stable emission and tunable from the blue to the NIR emission. In this work, using the control of single metal doping on silver nanoclusters (Ag25 protected by thiolate SR=2,4-dimethylbenzenethiol (DMBT) ligand), we aim to explore the effects of metal doping on (photo)stability and nonlinear optical response of liganded nanoclusters. We study two-photon excited photoluminescence and second harmonic response upon excitation in NIR (780-950 nm) range. A particular emphasis is paid to the effect of metal doping on the second-order nonlinear optical scattering properties (first hyperpolarizability, β(2ω)) of Ag25 nanoclusters. In addition, β(2ω) values are one order higher than the one reported for Au25 nanoclusters and represent the largest values ever reported for ligand-protected nanoclusters. Such enhanced hyperpolarizability leads to strong second harmonic response and renders them attractive targets in bioimaging.
