Au Nanoparticle-Enhanced Surface Plasmon Resonance Sensing of Biocatalytic Transformations

N-(3-Aminopropyl)-N′-methyl-4,4′-bipyridinium is coupled to tiopronin-capped Au nanoparticles (diameter ca. 2 nm) to yield methyl(ami-nopropyl)viologen-functionalized Au nanoparticles (MPAV ²⁺-Au nanoparticles). In situ electrochemical surface plasmon resonance (SPR) measurements are used to follow the electrochemical deposition of the bipyridinium radical cation modified Au nanoparticles on an Au-coated glass surface and the reoxidation and dissolution of the bipyridinium radical cation film. The MPAV²⁺-functionalized Au nanoparticles are also employed for the amplified SPR detection of NAD⁺ and NADH cofactors. By SPR monitoring the partial biocatalyzed dissolution of the bipyridinium radical cation film in the presence of diaphorase (DP) NAD⁺ is detected in the concentration range of 1 × 10^-4 M to 2 × 10^-3 M. Similarly, the diaphorase-mediated formation of the bipyridinium radical cation film on the Au-coated glass surface by the reduction of the MPAV²⁺-functionalized Au nanoparticles by NADH is used for the amplified SPR detection of NADH in the concentration range of 1 × 10^-4 M to 1 × 10^-3 M.