Microparticle manipulation using laser-induced thermophoresis and thermal convection flow

We demonstrate manipulation of microbeads with diameters from 1.5 to 10 µm and Jurkat cells within a thin fluidic device using the combined effect of thermophoresis and thermal convection. The heat flow is induced by localized absorption of laser light by a cluster of single walled carbon nanotubes, with no requirement for a treated substrate. Characterization of the system shows the speed of particle motion increases with optical power absorption and is also affected by particle size and corresponding particle suspension height within the fluid. Further analysis shows that the thermophoretic mobility (DT) is thermophobic in sign and increases linearly with particle diameter, reaching a value of 8 µm2 s−1 K−1 for a 10 µm polystyrene bead

Optofluidic transport and particle trapping using an all-dielectric quasi-BIC metasurface

Manipulating fluids by light at the nanoscale has been a long-sought-after goal for lab-on-a-chip applications. Plasmonic heating has been demonstrated to control microfluidic dynamics due to the enhanced and confined light absorption from the intrinsic losses of metals. Dielectrics, counterpart of metals, is used to avoid undesired thermal effects due to its negligible light absorption. Here, we report an innovative optofluidic system that leverages a quasi-BIC driven all-dielectric metasurface to achieve nanoscale control of temperature and fluid motion. Our experiments show that suspended particles down to 200 nanometers can be rapidly aggregated to the center of the illuminated metasurface with a velocity of tens of micrometers per second, and up to millimeter-scale particle transport is demonstrated. The strong electromagnetic field enhancement of the quasi-BIC resonance can facilitate increasing the flow velocity up to 3-times compared with the off-resonant situation. We also experimentally investigate the dynamics of particle aggregation with respect to laser wavelength and power. A physical model is presented to elucidate the phenomena and surfactants are added to the particle colloid to validate the model. Our study demonstrates the application of the recently emerged all-dielectric thermonanophotonics in dealing with functional liquids and opens new frontiers in harnessing non-plasmonic nanophotonics to manipulate microfluidic dynamics. Moreover, the synergistic effects of optofluidics and high-Q all-dielectric nanostructures can hold enormous potential in high-sensitivity biosensing applications

Plasmonic Optical Tweezers for Particle Manipulation: Principles, Methods, and Applications

Inspired by the idea of combining conventional optical tweezers with plasmonic nanostructures, a technique named plasmonic optical tweezers (POT) has been widely explored from fundamental principles to applications. With the ability to break the diffraction barrier and enhance the localized electromagnetic field, POT techniques are especially effective for high spatial-resolution manipulation of nanoscale or even subnanoscale objects, from small bioparticles to atoms. In addition, POT can be easily integrated with other techniques such as lab-on-chip devices, which results in a very promising alternative technique for high-throughput single-bioparticle sensing or imaging. Despite its label-free, high-precision, and high-spatial-resolution nature, it also suffers from some limitations. One of the main obstacles is that the plasmonic nanostructures are located over the surfaces of a substrate, which makes the manipulation of bioparticles turn from a three-dimensional problem to a nearly two-dimensional problem. Meanwhile, the operation zone is limited to a predefined area. Therefore, the target objects must be delivered to the operation zone near the plasmonic structures. This review summarizes the state-of-the-art target delivery methods for the POT-based particle manipulating technique, along with its applications in single-bioparticle analysis/imaging, high-throughput bioparticle purifying, and single-atom manipulation. Future developmental perspectives of POT techniques are also discussed

Optothermal manipulation of colloidal particles and biological objects

Optical based manipulation techniques play an important role in bottom-up assembly of micro- and nano-structures, discovery of new materials, and biomedical diagnostics. Traditional optical tweezers have limitations for the requirement of rigorous optics and high optical power. Optothermal manipulation, which exploits light-heat conversion and particle migration under a light-directed temperature field, is an emerging strategy for achieving diverse manipulation functionalities in a low-power fashion. In this work, we have developed a series of optothermal manipulation techniques, including bubble-pen lithography, opto-thermophoretic tweezers, opto-thermoelectric tweezers, and opto-thermoeletric printing. In bubble-pen lithography, microbubbles generated at solid-liquid interfaces through laser heating of a plasmonic substrate are used to pattern diverse colloidal particles on the substrate. Through directing the laser beam to move the bubble, we create arbitrary single-particle patterns and particle assemblies with different resolutions and architectures. The key to optothermal tweezers is the ability to achieve negative Soret effect, or deliver colloidal particles from cold to hot regions in a temperature field. Two types of optothermal tweezers with different driving forces are explored for versatile manipulation of colloidal particles and biological objects. Opto-thermophoretic tweezers rely on an abnormal permittivity gradient built by layered solvent molecules at the particle-solvent interface, while opto-thermoelectric tweezers exploit a thermophoresis-induced thermoelectric field for low-power trapping of nanoparticles. Furthermore, we have demonstrated opto-thermoelectric printing of colloidal particles on substrates in salt solutions and hydrogel solutions. With the low-power operation, simple optics, and diverse functionalities, optothermal manipulation techniques will find a myriad of applications in colloidal science, materials science, nanotechnology, and life sciences, as well as in developing functional colloidal devices and biomedical devices.Materials Science and Engineerin

Plasmonic optical fiber for bacteria manipulation—characterization and visualization of accumulation behavior under plasmo-thermal trapping

In this article, we demonstrate a plasmo-thermal bacterial accumulation effect using a miniature plasmonic optical fiber. Combined action of far-field convection and a near-field trapping force (referred to as thermophoresis)—induced by highly localized plasmonic heating—enabled large-area accumulation of Escherichia coli. The estimated thermophoretic trapping force agreed with previous reports, and we applied speckle imaging analysis to map the in-plane bacterial velocities over large areas. This is the first time that spatial mapping of bacterial velocities has been achieved in this setting. Thus, this analysis technique provides opportunities to better understand this phenomenon and to drive it towards in vivo applications

Opto-Mechanical Manipulation Of Molecules And Chemical Reactions

We developed optical methods to manipulate molecules in a microfluidic environment. Optical tweezers can manipulate micro-spheres in solutions with the gradient force but are not practical for spheres smaller than 500 nm in diameter. Nanotweezers use the evanescent field out of waveguides, slot-waveguides, plasmonic resonances, and photonic crystal resonators. They were able to manipulate objects down to 40 nm. Proteins and many biomolecules are of sizes on the order of a few nanometers, a priori out of reach of these techniques. During my PhD, I developed nanophotonic and nano-optic systems aimed at applying electromagnetic potential wells to bias the motion of molecules against Brownian motion and eventually demonstrated that chemical reaction pathways could also be altered. I showed that photonic crystal resonators are a toolbox for nanoscale assembly enabling trapping, transport, and orientation of nano-objects. I also investigated the heat arising in optofluidic photonic crystals and found it to be higher than previously thought, up to 57 K for 10 mW of power input, which makes such devices incompatible with biological single molecule experiments. I then used electromagnetic fields shaped by waveguides-carbon nanotubes hybrids to trap immunoglobulin of mass down to 160 kDa. Last, I developed the optical manipulation of chemical reactions. I showed that electromagnetic gradient force can transport molecules across reaction barriers along a reaction coordinate demonstrating it experimentally by guiding the adsorption of immunoglobulin proteins onto carbon nanotubes. These techniques are part of a wider evolution that is changing the way we interact with molecules. Although originally dismissed for studying single molecules because of the diffraction limit, nano-optics and nanophotonics are becoming the center of this revolution

Thermophoresis or When Small Objects Meet Temperature Gradient: Numerous Applications

This mini review discusses the phenomenon of thermophoresis, also known as the thermophoretic effect. Thermophoretic effect arises from the combination of a temperature gradient and particles of very small dimensions, on the order of magnitude of the mean free path of the molecules of the surrounding gas. Despite being a little-known effect, it is critical to many physical and chemical processes and for characterising the properties of nanostructured materials that could be used in industry for sensing applications. A description and definition of otherwise very similar thermophoresis terms is provided, as well as a brief overview of the literature on this topic, with a focus on research in the twenty-first centur

Evanescent field trapping and propulsion of Janus particles along optical nanofibers

Small composite objects, known as Janus particles, drive sustained scientific interest primarily targeted at biomedical applications, where such objects act as micro- or nanoscale actuators, carriers, or imaging agents. A major practical challenge is to develop effective methods for the manipulation of Janus particles. The available long-range methods mostly rely on chemical reactions or thermal gradients, therefore having limited precision and strong dependency on the content and properties of the carrier fluid. To tackle these limitations, we propose the manipulation of Janus particles (here, silica microspheres half-coated with gold) by optical forces in the evanescent field of an optical nanofiber. We find that Janus particles exhibit strong transverse localization on the nanofiber and much faster propulsion compared to all-dielectric particles of the same size. These results establish the effectiveness of near-field geometries for optical manipulation of composite particles, where new waveguide-based or plasmonic solutions could be envisaged.journal articl

The synergistic role of light and heat in liquid-based nanoparticle manipulation

Light and heat are synergistic tools used in the manipulation of nanoparticles and biomolecules. When optical effects dominate over thermal effects, the motion of nanoparticles can be controlled by optical forces. Here, we study the motion of 100 nm gold particles within a 1D optical potential, created by interfering counterpropagating beams. Tracking of particle trajectories revealed a large and asymmetric reduction in the nanoparticle diffusion constant in the presence of the traps, in agreement with theoretical predictions. When thermal effects dominate, laser light can induce local temperature gradients. Here, this was achieved by absorption of near-infrared (NIR) laser light in a Chromium microdisc. This resulted in thermophoretic separation of sodium azide ions, causing a local electric field that was used to manipulate 26 nm polystyrene beads. The nanoparticles were observed to follow the NIR heating spot, enabling light-controlled nanoparticle swarming. The induced 3D temperature profiles were characterised by time-correlated single-photon counting microscopy, with a temperature-sensitive dye. Through analysis of the particle velocities, the thermoelectric field strength, as well as the previously unknown Soret coefficients of azide ions were quantified. Transmission of laser beams through nanoparticle suspensions can lead to strong nonlinear lensing and soliton-like propagation effects. Literature has attributed these to redistribution of particles by optical gradient forces, and the effect is commonly described as an effective Kerr nonlinearity. To test this hypothesis, beam propagation experiments through a suspension of 40 nm plasmonic gold nanoparticles were carried out, and were found to be in agreement with previously reported results. To verify the nature of the effect, a new time-resolved z-scan technique was developed to measure the timescale and magnitude of the refractive index change. Surprisingly, the data demonstrates that the timescales can only be explained by thermal-absorption, -diffusion, and thermo-optic effects. As a result, the nonlinear effects are non-local and z-scan measurements will underestimate their magnitude

Roadmap for Optical Tweezers 2023

Optical tweezers are tools made of light that enable contactless pushing, trapping, and manipulation of objects ranging from atoms to space light sails. Since the pioneering work by Arthur Ashkin in the 1970s, optical tweezers have evolved into sophisticated instruments and have been employed in a broad range of applications in life sciences, physics, and engineering. These include accurate force and torque measurement at the femtonewton level, microrheology of complex fluids, single micro- and nanoparticle spectroscopy, single-cell analysis, and statistical-physics experiments. This roadmap provides insights into current investigations involving optical forces and optical tweezers from their theoretical foundations to designs and setups. It also offers perspectives for applications to a wide range of research fields, from biophysics to space exploration