Plasmon-driven selective deposition of au bipyramidal nanoparticles

We demonstrate the plasmon-selective and driven deposition of (bipyramidal) Au nanoparticles on transparent substrates (glass coverslips) utilizing total internal reflection (TIR) illumination. Near-IR laser light undergoing TIR at a glass-water interface causes colloidal Au bipyramids to irreversibly deposit onto the glass surface. We demonstrate that the deposition process has particle (i.e., shape) selectivity that is associated with resonant plasmon excitation. Specifically, the deposition is selective for the bipyramids over spheroidal particles that are also present in solution due to the former's surface plasmon resonance in the near-IR region. Our measurements, finite difference time domain simulations, and the results of an analytical model show that the optical (i.e., scattering and gradient) forces that act on the particles are large and cause the observed acceleration and directed motion of the bipyramids. These directional forces play a major role in the spatial pattern of particle deposition that is observed. In addition, the resonant photothermal heating of the Au bipyramids causes an irreversible loss in colloidal stability, thus allowing them to adhere to the surface. Structural (i.e., scanning electron microscopy) characterization of the deposited bipyramids reveals a slight reduction in aspect ratio relative to the ensemble, consistent with the proposed (heating) mechanism. To our knowledge this is the first demonstration of the plasmon-selective deposition of metal nanoparticles from a heterogeneous mixture.