Preparation, microscopy, and flow cytometry with excitation into surface plasmon resonance bands of gold or silver nanoparticles on aminodextran-coated polystyrene beads

Spherical polystyrene latex beads of about 2.0 mum diameter were coated with islands of silver or gold metal, about 5-200 nm in diameter, by reduction of aqueous silver or gold ions in the presence of sugar-coated polystyrene latex beads. The metal islands are held on the bead surface by a polymeric sugar derivative, aminodextran, covalently bound to the polystyrene aldehyde/sulfate bead. Images of the gold or silver nanoparticle-coated polystyrene beads, obtained with an optical microscope, show that gold, silver, and uncoated polystyrene beads can be distinguished by their different colors, red-purple, black, and colorless, respectively. Also, scanning electron micrographs of coated versus uncoated polystyrene beads show a distinct granular bead surface when metal nanostructures are present versus a smooth bead surface when they are absent. Nanoparticles of gold greater than about 50 nm in diameter on polystyrene beads showed enhanced 90 degrees or side light scatter (resonant Rayleigh scattering) with excitation at 633 nm but no enhancement (same light scatter intensity as uncoated polystyrene beads) at excitation wavelengths of 544, 488, and 458 nm. On the other hand, nanoparticles of silver greater than about 50 nm in diameter on polystyrene beads showed enhanced 90 degrees or side light scatter with excitation wavelengths of 458, 488, 544, and 633 nm but no enhancement with 351-365 nm excitation. Amplification of elastic scatter from both silver and gold colloids was maximally achieved with structures of 50-200 nm diameter, as shown in forward versus side scatter histograms obtained by flow cytometry. Side scatter enhancements of 2- to 10-fold were observed for gold- or silver-coated polystyrene beads over uncoated polystyrene beads of the same diameter. The origin of this wavelength-dependent, light scattering amplification that was observed by flow cytometry has been identified with excitation into surface plasmon resonance bands of gold and silver nanoparticles on polystyrene latex beads.