Oligothiophene- and Oligopyrrole-Mediated Aggregation of Gold Nanoparticles

Gold nanocrystals in a toluene solution were aggregated by several thiophene and pyrrole oligomers as shown by surface plasmon (SP) spectroscopy and TEM analysis. The rate of aggregation can be controlled by varying the oligomer type and concentration. Two distinct regimes of aggregation kinetics are observed: (1) at low oligomer concentrations, aggregation is slow and dependent on the gold nanocrystal concentration with production of a SP band at ca. 700 nm and (2) at high oligomer concentrations, aggregation is very fast and produces a SP band at ca. 570 nm. The results suggest that, in general, at high oligomer concentrations, the linker molecules form more dense layers via perpendicular orientation toward the metal plane, whereas a parallel orientation is obtained at low concentrations. TEM analysis indicates in the high concentration regime that there is formation of spherical or dendritic superclusters depending on the aggregating oligomer. FTIR analysis of the terthiophene and dipyrrole gold aggregates has shown the presence of adsorbed ligand molecules with surface enhancements of some bands, giving features particular to parallel and perpendicular coordination, respectively.