An optical absorption investigation of cross-linking of gold colloidal particles with a small dithiol molecule

The controlled aggregation of colloidal metal particles is a problem of technological importance in the generation of, for example, quantum wires. In this communication, we present results of an optical absorption spectroscopy investigation of the flocculation of gold colloidal particles capped with a small bifunctional molecule (1,3-propanedithiol). The self-assembly of the dithiol molecule on the gold surface leads to thiol surface functionalization of the colloidal particles, which may thereafter be used to chemically cross-link the gold clusters. Optical absorption measurements performed immediately after capping the gold colloidal particle surface with the dithiol molecules clearly show the growth of a long-wavelength component, indicating that the clusters aggregate in open string-like structures. Increasing the surface coverage of the dithiol molecules on the clusters leads to a greater flocculation rate, but prevents the formation of large gold clusters in direct contact, as indicated by X-ray diffraction measurements of the aggregates. This approach of controlled cross-linking of colloidal particles may have important applications in the generation of metal cluster networks.