The optical trapping of polymer beads of different diameters d of 100, 200, 500, 1000, and 3000 nm on planar (F-Si) and nanostructured (BSi) crystalline silicon was investigated at laser wavelengths λ = 808 and 1064 nm, respectively. We demonstrate that BSi surfaces can enhance the optical gradient force required to grip nanoparticles (100 nm) in contrast to F-Si surfaces, significantly changing the trapping behavior. Thus, different modes of optical tweezing were characterized by modification of Si surface topography (nanostructuring), wavelength λ, laser irradiation intensity I, and irradiation area. Specifically, we present four separate modes of optical tweezing using planar and nanostructured Si for (i) a single particle trapped by tightly and loosely focused irradiation on F-Si, (ii) a large number of 3D assembled beads trapped by tightly focused irradiation on BSi, (iii) a small number of particles trapped on F-Si, and (iv) a large number of particles trapped on BSi, resulting in a 2D self-ordered assembly. The mechanisms of the optical manipulation of particles in the range 100–1000 nm were discussed with implications for the trapping of bacteria or viruses by using nanostructured semiconductor assisted optical tweezing (NASSCA/OT) and their response to high mechanical shear forces at locally changed temperatures on nanostructured surfaces.

Nano-objects, such as quantum dots (QDs), are essential units for the construction of functional materials and devices in current technologies. The establishment of a versatile scheme to sort desired components from a crude product is crucial for bringing out the full potential of the original materials. However, it is still challenging to separate QDs with the same composition on the basis of size and to sort QDs with the same size but different optical properties. Here, we demonstrate such sorting for the first time by combining plasmonic optical trapping with thin-layer chromatography (TLC), which is a widely used tool. LED photoexcitation of the localized surface plasmon resonance of Au nanoparticles immobilized on a TLC plate affected the distance QDs traveled depending on the wavelength and intensity of irradiated light, which led to clear separation according to the size and/or optical properties of the QDs. Since optical property-based separation cannot be achieved by conventional chromatography, in which the interactions between stationary phases of chromatographs and QDs are simply based on differences in the size or surface functionality of the QDs, the present strategy will be a key solution for the establishment of a versatile scheme for sorting nano-objects.

Optical trapping of inorganic nanocrystals at oil/water interfaces was investigated under a loose focus condition. The target nanocrystals were octahedral gold nanoparticles (OGPs, 70 nm in size), rice-shaped (10 nm × 20 nm) ZnS-AgInS 2 (ZAIS) nanoparticles, and octahedral titania (TiO 2 ) nanoparticles (OTPs, 100 nm). While we were unable to trap these nanoparticles in homogeneous liquids (n-hexane or water), they were successfully trapped at an oil/water interface. We also investigated trapping for a binary system at an interface, where OGPs and ZAIS nanoparticles were dispersed in water (lower layer) and n-hexane (upper layer), respectively. We observed the formation of a complex (hybrid) of an OGP and ZAIS particle trapped at the oil/water interface. Interestingly, the complex particle kept rotating during trapping even under irradiation of linearly polarized light. This phenomenon suggested the spontaneous breakdown of rotational symmetry in the optically coupled nanoparticles. This interfacial trapping technique was applied to a photocatalytic reaction. We observed the generation of Au nanoparticles from OTPs trapped at an interface under UV light irradiation. Liquid/liquid interfaces can strongly assist optical trapping and induce characteristic chemical behavior.

Recently, we have developed an optical tweezers technique based on nanostructured Si (Black-Si). The spike-like nanostructure enhances the electric field of the incident light, thereby strengthening the grip made by the tweezers. In the present study, we applied the tweezers to poly(N-isopropylacrylamide) (PNIPAM) in aqueous solutions. This polymer has frequently been used as a target in the field of optical manipulation chemistry. Using the technique, we succeeded in forming a PNIPAM microdroplet on the surface of the Black-Si. The formation of PNIPAM droplets is important in polymer chemistry, analytical chemistry, and droplet chemistry. Importantly, herein, we demonstrate that the trapping behavior is very sensitive to the wavelength of the trapping laser light (1064 or 808 nm). Using 808 nm laser light, the polymer can be trapped with a 100-fold-greater efficiency compared to that using 1064 nm laser light. Moreover, with 808 nm laser light, the chemical structure of the droplet can be modulated. By changing the laser intensity at 808 nm, we succeeded in varying the internal polarity of the droplet, while at 1064 nm the polarity is fixed. The production of the PNIPAM microdroplet is purely due to the enhanced optical force of the 808 nm laser light. The selection of laser wavelength is very important in the Black-Si-based optical tweezers.

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