Superatom Clusters of Bi-Icosahedral Gold: An Optical and Electrochemical Study

Gold nanostructures have been the focus of research over the past two decades because of their potential as excellent two-photon absorbers, which make them good sensors and imaging agents. This is due, in part, to their size-specific properties. Of particular interest are a class of gold nanostructures called superatoms, or magic numbered clusters, that exhibit quantum confinement effects. These structures are also referred to as nanomolecules (AuNCs). They have small enough optical band gaps to absorb in the near infrared I (near IR) region (700-900 nm) which is beneficial for biological applications. One of the most investigated superatom clusters is Au₂₅L₁₈(L = alkyl or aryl thiols) in the icosahedron geometry. Its properties have been characterized both experimentally and theoretically. Au₂₅L₁₈<b>, </b>where L = hexanethiol exhibits luminescence efficiencies of ∼2.5 × 10^-4 in the nearIR region with cross-sections of 2700 GM at 1290 nm which can be useful for two-photon imaging with IR light. Doping the AuNC metal core with foreign atoms is considered a powerful method for transforming Au₂₅L₁₈into stable clusters with different electronic structures resulting in increased luminescence. In addition, this can induce magnetism which is important for magnetic imaging, and computing applications. The electronic states of these clusters can further be manipulated by adding ligands containing a chromophore or possessing redox capability. While these effects are documented, the mechanisms involved remain controversial and little work has been done with respect to its bi-counterpart. The results of this study will offer more insight to better designed AuNCs with bi-icosahedral geometrical structures that are doped (Pt, Ag, Cd, Hg) for use as novel nonlinear optical materials (NLO). The electrochemical band gap and brightness factors will also be presented.