(35d) Plasmonics Enhanced Fluorescent and Catalytic Properties of Shape-Controlled Ag-Cu and Ag-Pd Bimetallic Nanoparticles

The ability of noble metal nanostructures to manipulate light at the nanoscale is called plasmonics and has promising applications in biodiagnostics, such as metal enhanced fluorescence (MEF), and photocatalytic applications. MEF involves the enhancement of fluorescent activity of fluorophores in the presence of vicinal plasmonic metal nanoparticles, whereas the catalytic activity of nanoparticles can be increased due to photothermal effects of plasmonic metal nanoparticles. Ag, with its narrow and intense surface plasmon resonance (SPR) peak, demonstrates the best plasmonic properties among all metals. However, alloying nanoparticles is an attractive option because important optical properties, like the wavelength corresponding to the SPR peak and the scattering efficiency that dictate the enhancement/quenching of fluorophore molecules, can easily be controlled. Hence, we studied Ag-Cu bimetallic nanoparticles for fluorescence enhancements. Ag-Pd nanoparticles were studied as well, because Pd is an important metal for hydrogenation reactions and Ag-Pd would demonstrate the effect of plasmonic enhancement on nanocatalysis. The plasmonic properties can be manipulated by controlling particle size, aspect ratio, shape, particle-to-particle distance and surrounding dielectric medium. Therefore, using Finite Difference Time Domain (FDTD) calculations, we calculated the extinction spectra and the electric field distribution of various core-shell bimetallic nanoparticles optimizing the size, shape and shell thickness. Based on the calculations, the Ag-Cu nanocubes with a Ag core of 25nm and Cu shell thickness of 2-4 nm seem to be the best candidate for maximum MEF. The modified polyol synthesis method was used to synthesize these core-shell Ag-Cu and Ag-Pd nanocubes with a Ag core of 25 nm and a Cu or Pd shell thickness of 2-4 nm. To the best of our knowledge, this is the first instance of synthesis of such core-shell bimetallic nanocubes.  The enhancement of fluorescence emission of the Cy3 dye in the vicinity of Ag-Cu nanocubes was demonstrated using spectrophotometric measurements. The plasmonic enhanced catalytic activity of the Ag-Pd nanocubes under optical excitation was demonstrated for the ethylene hydrogenation reaction.