Reversible Photoinduced Formation and Manipulation of a Two-Dimensional Closely Packed Assembly of Polystyrene Nanospheres on a Metallic Nanostructure

Abstract

Nanostructure-enhanced optical trapping of polymer beads was investigated by means of fluorescence microspectroscopy. It was found that trapping behavior was quite sensitive to the particle size as well as excitation light intensity. We present a 2D closely packed assembly of polystyrene nanospheres on a gold nanostructure that is triggered by gap-mode localized surface plasmon (LSP) excitation. We discuss the trapping mechanism from the viewpoints of not only the radiation force but also of the thermal force (thermophoresis and thermal convection) induced by near-infrared laser irradiation. Thermophoresis worked as a repulsive force whose direction was opposed to that of the radiation force. On the other hand, thermal convection acted in favor of trapping: It supplied nanospheres toward the LSP excitation area. By suppressing the repulsive force, the assembled trapped nanospheres took the form of hexagonal shapes on a gold nanostructure. By optimizing irradiation parameters, we achieved 2D manipulation of nanospheres on a substrate. Our method has advantages over the conventional optical tweezers technique because of its weak light intensity, and could be a promising method of creating and manipulating a 2D colloidal crystal on a plasmonic substrate. ■ INTRODUCTIONLocalized surface plasmons (LSP) have been investigated because they demonstrate highly sensitive spectroscopies 1−5 and the enhancement of photochemical reactions. 6−10 These applications are enabled by the enhancement effect of an incident resonant electromagnetic field (EMF) at the surfaces of noble metallic nanostructures. 11−13 In particular, the application of LSPs at the nanogaps between adjacent noble metallic nanostructures (gap-mode LSP) has recently attracted much attention for achieving the effective optical trapping (OT) of nanoparticles at the nanogaps; this is known as LSPbased OT (LSP-OT). Since Grigorenko et al. first experimentally demonstrated the LSP-OT of polystyrene microspheres in 2008, 14 various researchers have been exploring the phenomenon to reveal the features and mechanisms of the LSP-OT of polymer beads, metallic nanoparticles, and bacteria. 15−24 We also demonstrated the LSP-OT of semiconductor nanocrystals (quantum dots) and polystyrene nanospheres and by means of confocal fluorescence microspectroscopy. 22−24 Such LSP-OT has a great advantage with respect to incident light intensity: the laser intensity can be much decreased (to the order of kW/cm 2 ) and still achieve stable trapping, as compared to conventional optical tweezers (∼MW/cm 2 ). 25−29 Thus, LSP-OT could enable a new technique for manipulating not only nanoparticles but also smaller molecules such as polymer chains, 24 proteins, and DNA.According to recent research, however, the process of LSP-OT should not be so simple. Simultaneously with LSP excitation, other favorable or unfavorable physical processes take place competing with the enhanced radiation force (RF), which is the driving force for OT. We currently consider that photothermal effects (local t...

Insight into plasmonic hot-electron transfer and plasmon molecular drive: new dimensions in energy conversion and nanofabrication

“…Figure 13, depicts the plasmonic NS-enhanced optical force-based fabrication of a 2D close-packed assembly of polystyrene (PS) nanospheres. 66 As expected for optical trapping, the 2D colloidal crystal formation is reversible and is only formed during laser illumination. With the aid of a plasmonic structure (Figure 13), the laser intensity for stable trapping decreased to~10 3 W cm − 2 , which compares favorably with that of conventional optical tweezers of~10 7 -10 8 W cm − 2 .…”

“…66 Other platforms such as Au nano-islands and random arrays of particles have been employed to gather/assemble particles. For instance, a close-packed Au NP film on a glass coverslip immersed in a solution containing PS beads or DNA was irradiated with a laser with a wavelength of 633 nm (o 25 mW) on a microscope stage.…”

“…The interpretation contrasts that of the Tsuboi group. 66 Using Au nanoisland substrates, another study demonstrated the optical trapping and assembly of PS microspheres and living cells into highly organized patterns with low laser power density (~10 4 W cm − 2 at 785 nm). 68 The observed trapping is attributed to the net contribution from a near-field optical trapping force and a long-range thermophoretic force that overcomes the axial convective drag force.…”

“…In this case, the thermophoretic force was assumed to have dragged the PS particles from cold to hot, the opposite direction to that suggested by other studies. 58,66 To realize plasmon-assisted optical trapping, hot spots that exhibit enormous enhancements are a prerequisite. 55,56 For Au nanoisland films, reproducible formation of such hot spots may largely depend on technical ability.…”

Potential energy surfaces and reaction pathways for light-mediated self-organization of metal nanoparticle clusters

“…Simultaneous control of multiple NPs is possible via holographic beam shaping using a spatial light modulator (SLM) 16 . Alternatively, (an array of) optical traps can be generated by localized surface plasmons in metallic nanostructures 17 or guided-resonance modes of photoniccrystal slabs 18 , leading to templated self-assembly of NPs.…”