Research on non-cyanide electroless gold plating reactions for sustainable use

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Aiming to establish non-cyanide electroless gold plating reactions for sustainable use, I have investigated redox reactions in the plating solution containing sulfite (SO_3^<2->) and thiosulfate as ligands for Au(I) ion, thiourea (TU) as a reducing agent, and hydroquinone (HQ). The following results were obtained1) In the course of the plating reaction, HQ is oxidized to benzoquinone (Q) by thiyl radical which is a product of the reaction between Au(I) complex and TU, followed by the formation of hydroquinone sulfonic acid (HQSA) through the reaction between S0_3^<2-> and Q.HQSA is further sulfonated according to a similar reaction mechanism to that of HQ2) It was found that TU is able to accelerate the reduction of Au(I) complexes. This acceleration is specifically observed on the gold electrode surface, not on glassy carbon (GC) electrodes3) The possibility of the electrochemical desulfonation of HQSA was assessed in aqueous buffers (pH 5, 8) and organic solvents (DMSO, DMF) using Au, GC, Pt, and Au amalgam electrodes. The desulfonation reaction did not occur within the potential windows of these solvent-electrode systems4) The supply of Au(I) complex as [Au(SO_3^<2->)_2]^<3-> during the plating was recommended, since the increase in the concentration of SO_3^<2-> in the plating solution, which is caused by the supply of [Au(SO_3^<2->)_2]^<3->, does not significantly affect the reduction of Au(I) complex5) I proposed a new approach toward the establishment of the non-cyanide electroless gold plating reaction for sustainable use, in which dimethyl disulfonyl hydroquinone is used instead of HQ