Activity and reusability of immobilized gold nanoparticles for the catalysis of both oxidation and reduction reactions

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Colloidal gold nanoparticles (AuNP) are well-known to present a catalytic activity. However, they are characterized by certain limitations, as a rather poor stability and lack of both handling and reusability. Their immobilization on surfaces could help to overcome these drawbacks. This work evaluated the impact of the immobilization of AuNP towards their catalytic activity on reduction and oxidation reactions. Colloidal AuNP stabilized with citrate ions were synthesized, characterized, and immobilized by the dip-coating method, the obtained immobilized AuNP (iAuNP) were characterized. The catalytic activity of AuNP and iAuNP was evaluated and compared to each other through the p-nitrophenol reduction reaction. The kinetic of the catalytic activity of iAuNP was slower than colloidal AuNP. The catalytic activity appeared to be impacted by the less available surface of iAuNP. The reduction efficiency of iAuNP was also evaluated with the DPPH•/DPPH2 reduction reaction. The capacity of iAuNP to catalyze the reduction of DPPH• in DPPH2 was shown even after 130 days and 20 reuses of the nanostructured surface on the contrary to non-reusable colloidal AuNP. The oxidation efficiency of iAuNP was measured by the ability to catalyze the auto-oxidation of curcumin (HPLC-UV/vis). For all these experiments, the addition of 4-mercapto-phenol on the surface of iAuNP passivated the surface, decreasing the catalytic activity of the materials. In conclusion, iAuNP are redox catalysts with both anti- and pro-oxidant effects and high reusability over a very long period.