Hydrophobic coating- and surface active solvent-mediated self-assembly of charged gold and silver nanoparticles at water-air and water-oil interfaces

We report self-assembly of charge-stabilized gold and silver nanoparticles at water-air and water-oil interfaces, via manipulation of the interactions between the interfaces and the adsorbing nanoparticles. Nanoparticle adsorption from bulk colloids to an interface is an energy-favored, but finite sorption barrier-restrained (kinetics-controlled) process. Consequently, to successfully mediate self-assembly of nanoparticles, the finite sorption barrier should be decreased. That can be accomplished by manipulating its three controlling forces: the repulsive electrostatic force, the repulsive van der Waals force, and the attractive hydrophobic force between the interface and the adsorbing nanoparticles. It was found that hydrophobic coatings change nanoparticle hydrophobicity and greatly increase the attractive hydrophobic force. Surface active organic solvents ( methanol, ethanol, isopropanol, and acetone) decrease the attractive hydrophobic force to some extent. However, they decrease the repulsive electrostatic force to a larger extent, via a "charge dilution'' mechanism, due to their positive adsorption at the charged water-air and water-oil interfaces. Hydrophobic coatings and organic solvents consequently decrease the sorption barrier, facilitate nanoparticles overcoming the sorption barrier, and mediate the self-assembly of nanoparticles.Hunan Provincial Natural Science Foundation of China [06JJ3006]; Natural Science Foundation of China [20603008, 20703016, 20873037]; "985'' Foundation of the Ministry of Education of China

Correlating the Shape, Surface Plasmon Resonance, and Surface-Enhanced Raman Scattering of Gold Nanorods

We report the study correlating the shape, surface plasmon resonance (SPR), and surface-enhanced Raman scattering (SERS) of gold nanorods (NRs) in dilute colloids. A series of gold NRs with various aspect ratios was prepared via an improved seed-mediated technique. Increasing the aspect ratio finely tunes the position of the longitudinal plasmon mode of the NRs in a wide spectral range. This shape-dependent SPR behavior was simulated by Gans theory and the discrete dipole approximation method. The subtle influence of SPR on SERS was then demonstrated by gradually tuning the SPR wavelength across a fixed excitation line. SERS experiments and theoretically predicted electromagnetic enhancement by the three-dimensional finite-difference time domain method clearly demonstrate that overlapping SPR and the excitation line maximizes the SERS enhancement. This correlation thus enables a quick diagnosis of SERS intensity by looking at the position of the SPR band.Natural Science Foundation of China [20603008, 20703032]; 973 program [2009CB930703]; Hunan Provincial Natural Science Foundation of China [06JJ3006]; Natural Science Foundation of Fujian Province of China [E0710028]; State Key Laboratory for Physical Chemistry of Solid Surfaces ; "985" Foundation of Ministry of Education of Chin