Self-assisted optothermal trapping of gold nanorods under two-photon excitation

Abstract

We report a self-assisted optothermal trapping and patterning of gold nanorods (GNRs) on glass surfaces with a femtosecond laser. We show that GNRs are not only the trapping targets, but also can enhance the optothermal trapping of other particles. This trapping phenomenon is the net result of thermophoresis and a convective flow caused by localized heating. The heating is due to the conversion of absorbed photons into heat at GNR’s longitudinal surface plasmon resonance (LSPR) wavelength. First, we investigated the optothermal trapping of GNRs at their LSPR wavelength on the glass surface with as low as 0.5 mW laser power. The trapping range was observed to be larger than a typical field of view, e.g. 210 μm × 210 μm here. Second, by adjusting the distance between the laser focus and the glass surface, ring patterns of GNRs on the glass surface were obtained. These patterns could be controlled by the laser power and the numerical aperture of the microscope objective. Moreover, we examined the spectral emission of GNRs under different trapping conditions using the spectral phasor approach to reveal the temperature and association status of GNRs. Our study will help understanding manipulation of flows in solution and in biological systems that can be applied in future investigations of GNR-induced heating and flows.

Gold Nanorods: The Most Versatile Plasmonic Nanoparticles

“…On the other hand, the temperature gradient can cause nonuniform distributions in density and surface tension, which in turn generates natural convection and thermocapillary convection, respectively. By locally heating the solution or the substrate, Au NRs can move against the temperature gradient or along the thermal flow and assemble into highly concentrated aggregates at the heat source (Figure f). − …”

“…By locally heating the solution or the substrate, Au NRs can move against the temperature gradient or along the thermal flow and assemble into highly concentrated aggregates at the heat source (Figure 14f). 792 Although Au NRs do not have magnetism, conventional magnetic materials, such as nickel, Fe 3 O 4 , and LCs, can integrate with Au NRs through physical evaporation, chemical synthesis, and assembly to form functional nanostructures with plasmonic characteristics and magnetism. The magnetic field can then be applied to control the orientation and assembly of plasmonic−magnetic dual functional nanomaterials.…”

Heat-Mediated Optical Manipulation

“…Because the natural convective flow has a large working range in solutions and is universally applicable to manipulating various objects, it has been widely exploited for the larger-scale optothermal assembly of colloidal structures. 288,295,296 The heating source for the optothermal assembly can be (1) predesigned light-absorbing substrates, 79,297 (2) mobile lightabsorbing particles, 298 or (3) light-absorbing fluids. 72,299 As an example of substrate-based optical heating, Shoji et al fabricated Au nanopyramidal dimer arrays via angular-resolved nanosphere lithography to serve as a template for the optothermal assembly of 2D hexagonal arrays of PS particles.…”

“…296 In addition to the predesigned optothermal substrates, the natural convective flow can be generated by optical heating of the suspended nanoparticles or the solvents. 298 For instance, Dinh et al exploited the suspended Au nanorods as the heating source to induce a thermoplasmonic convective flow for 3D assembly of hydrogel microparticles (Figure 18a). 300 Similarly, Jin et al assembled Au nanoparticles on a glass substrate via selfgenerated thermoplasmonic convective flows without chemical ligands or substrate patterning (Figure 18b).…”

“…Because the natural convective flow has a large working range in solutions and is universally applicable to manipulating various objects, it has been widely exploited for the larger-scale optothermal assembly of colloidal structures. ,, The heating source for the optothermal assembly can be (1) predesigned light-absorbing substrates, , (2) mobile light-absorbing particles, or (3) light-absorbing fluids. , …”