Ultrasmall Au nanoparticles coated with hexanethiol and anthraquinone/hexanethiol for enzyme-catalyzed oxygen reduction

• Quantum-sized gold nanoparticles modified with two thiols were synthesized.
• Enzyme bound to the gold nanoparticles is determined by SPR.
• The 3D architecture containing quantum-sized gold NPs and laccase leads to improved catalytic ORR.

Ultrasmall gold nanoparticles modified with hexanethiol and anthraquinone/hexanethiol monolayer obtained through a modified Brust method were anchored to gold electrodes by 1,4-benzenedimethanethiol and used to adsorb a redox enzyme—laccase. The electrical properties of the designed electrodes and the progress of laccase adsorption were monitored by impedance spectroscopy and surface plasmon resonance, respectively. The extent of enzyme adsorption determined by surface plasmon resonance spectroscopy was found to depend on the size of the gold nanoparticles. The optimal nanoparticles were those modified with an anthraquinone/hexanethiol monolayer with a core size of ca. 2 nm. The coverage by laccase was much larger than that achieved using monolayers of the respective compounds directly adsorbed onto the Au electrodes. The functionalized nanoparticle network on the gold electrode improved the connection of the enzyme molecules with the electrode substrate and lead to efficient enzyme catalyzed oxygen reduction at ca. +0.6 V vs. Ag/AgCl, the formal potential of laccase and close to the reversible oxygen reduction potential.

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