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.
Thermoresponsive
phase separation mechanisms of aqueous poly(N-isopropylacrylamide)
(PNIPAM) solutions were investigated
using an optical tweezer combined with a Raman microspectroscope.
A near-infrared laser beam (λ = 1064 nm) was focused into the
solution to produce and trap a single polymer microdroplet under an
optical microscope. The laser beam played two important roles: The
first role is to locally heat the solution to induce phase separation
in which numerous polymer microdroplets are generated around the focus,
while the second one is to collect these microdroplets. Eventually,
a single polymer droplet was stably produced and trapped at the focus.
Our method enabled us to perform two types of microanalysis for the
droplet. Analysis I is real-time monitoring the growth of the polymer
droplets by which we can determine the growth rate of droplets. Analysis
II is Raman microspectroscopy to reveal chemical components of the
droplets. By means of these two analyses, we revealed important phase
separation mechanisms in terms of stereoregularity (isotacticity)
dependence. From analysis I, we show that droplet growth is governed
by the Ostwald ripening mechanism and the growth is accelerated by
increasing the isotacticity. From analysis II, we show that the gelation
is promoted in the droplet (physical gel formation) with increasing
isotacticity. Our technique should be a versatile tool to explore
liquid–liquid phase separation mechanisms for various binary
solution systems.
The ability to modulate, tune, and control fluorescence colour has attracted much attention in photonics-related research fields. Thus far, it has been impossible to achieve fluorescence colour control (FCC) for material with a fixed structure, size, surrounding medium, and concentration. Here, we propose a novel approach to FCC using optical tweezers. We demonstrate an optical trapping technique using nanotextured Si (black-Si) that can efficiently trap polymer chains. By increasing the laser intensity, the local concentration of perylene-labelled water-soluble polymer chains increased inside the trapping potential. Accordingly, the excimer fluorescence of perylene increased while the monomer fluorescence decreased, evidenced by a fluorescence colour change from blue to orange. Using nanostructure-assisted optical tweezing, we demonstrate control of the relative intensity ratio of fluorescence of the two fluorophores, thus showing remote and reversible FCC of the polymer assembly.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.