Gold clusters and tetrasulfonatophenyl porphyrìn nanohybrids for sensing applications

Nanoscale materials are successfully employed as a versatile tool for applications in sensing, nanoelectronics, catalysis, imaging or nanomedicine. Au and Ag nanoparticles are widely used due to their interesting electronic and optical properties arising from the presence of surface plasmons, i.e. coherent oscillations of conduction band electrons at the nanoparticles surface, For instance, small molecules, ions or large chromophores adsorbed on the surface of metal nanoparticles can be detected at very low concentrations through Surface Enhanced Raman Spectroscopy (SERS), taking advantage of the enhanced electromagnetic field at the meta! surface caused by the excitation of the plasmon oscillation. In addition to these properties, metal clusters consisting of several to tens metal atoms and having sizes in the subnanometer range, display other fascinating electronic, optical and chernica! properties which derive from the confinement of the electrons into discrete energy levels due lo the spatial confinement being at themolecular scale. Several synthetic routes are reported in the literature, including gas phase methods for the synthesis of naked clusters, the use of protecting ligands such as phosphines or thiols to limit the clusters growth, and the employment of larger (bio)molecules like dendrirners or proteins, acting as macromolecular templates. In this work, the interaction between gold clusters composed of ten atoms (Au10) and the tetrasulphonatophenyl porphyrin (TPPS) has been investigated under acidic conditions through the combined use of various spectroscopic techniques. The synthesis of Au10 has been carried out according to a simple literature protocol in which Lhistidine acts both as reducing agent and protecting ligand of the resultant metallic clusters. The low pH has been chosen to allow the formation of porphyrin Jaggregates, which are known to form nanotubular structures in solution or on surfaces, and to lead to new organiclinorganic nanocomposites with potential applications as SERS-active substrates for the detection of chernically- and biologically-relevant species.