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 tec hnological importance in the generation of, for example, quantum wires. In this communication, we present results of an optical absorption spectroscop y investigation of the flocculation of gold colloidal particles capped with a small bifunctional molecule (1,3-propanedithiol). The self-assembly of t he dithiol molecule on the gold surface leads to thiol surface functionaliz ation of the colloidal particles, which may thereafter be used to chemicall y cross-link the gold clusters. Optical absorption measurements performed i mmediately after capping the gold colloidal particle surface with the dithi ol molecules clearly show the growth of a long-wavelength component, indica ting that the clusters aggregate in open string-like structures. Increasing the surface coverage of the dithiol molecules on the clusters leads to a g reater flocculation rate, but prevents the formation of large gold clusters in direct contact, as indicated by X-ray diffraction measurements of the a ggregates. This approach of controlled cross-linking of colloidal particles may have important applications in the generation of metal cluster network s.