Wavelength-Sensitive Optical Tweezers Using Black-Si Nanospikes for Controlling the Internal Polarity of a Polymer Droplet

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.