Effect of bromide and chloride ions for the dissolution of colloidal gold

Gold organosol has been synthesized in toluene employing two-phase (water–toluene) extraction of AuCl4− followed by its reduction with sodium borohydride in presence of cationic surfactants with variable counter ions. The influence of the counterions of the phase transfer reagent and stabilizing ligand on the photochemical stability of the gold colloids in toluene has been investigated. The counterions of the surfactants, i.e. bromide or chloride ions at times generate the corresponding radicals under UV-irradiation, which oxidize the gold clusters. It is explored that photodiscoloration process is faster for bromide system as compared to chloride. The slower discoloration in presence of only chloride system has been explained in the light of electron affinity of chlorine and stability of chlorocomplex. The photodiscoloration is facilitated in chloroform. It has further been observed that HCl oxidizes the gold nanoparticles under ambient condition. Finally the thermal activation facilitates the oxidation process in presence of bromide ions.