Au nanoparticle-catalyzed electron transfer from ammonia-borane to Ru(NH3)63+ for sensitive biosensing

Metal nanoparticle (NP)-catalyzed electron transfer (ET) from a reducing agent to a metal complex is useful for signal amplification in biosensors. For efficient ET, the metal complex must undergo rapid outer-sphere reactions, be highly water-soluble, and effectively penetrate bio/organic layers on metal NPs. Our study identifies Ru(NH₃)₆³⁺ as well-suited for this purpose. Among reducing agents, ammonia-borane (AB) enables rapid metal NP-catalyzed ET, with Au, Pt, and Pd NPs displaying similar catalytic activities. The pseudo second-order rate constant for 20-nm Au NP-catalyzed ET from AB to Ru(NH₃)₆³⁺ (1.4 × 10⁸ M⁻¹ s⁻¹) approaches the diffusion-controlled rate constant. Despite immunoglobulin G and bovine serum albumin passively adsorbed on Au NPs, catalytic activity remains largely unaffected. Applying Au NP-catalyzed ET to prostate-specific antigen detection in human serum achieves a low detection limit of 10 pg/mL. These findings highlight the potential of Ru(NH₃)₆³⁺ and AB in designing biosensors based on rapid catalytic reaction.