Fig. 1. Particle capture schemes of traditional optical tweezers, and G is gravity. (a) Unbalanced state of three-dimensional trapping by gradient force of light field generated by lens convergence beams; (b) equilibrium state of three-dimensional trapping; (c) SPR nanotweezers, the surface field intensity decays exponentially with the surface-particle distance, resulting in a trapping force to the gold nanostructure surface

Fig. 2. Schematic of refrigerative tweezers for nanoparticles manipulation^[42]

Fig. 3. Physical mechanisms of OTFT^[32]. (a) Diagram of thermophoresis; (b) diagram of thermoelectric effect

Fig. 4. Simulation results of natural convection^[58]. (a) Simulated convection around a gold nanodisk with a thickness of 40 nm and a diameter of 500 nm, and the heating of nanostructures from room temperature up to 80 ℃; (b) velocity fields for various chamber heights, induced by a gold disk of radius r=250 nm and T=80 ℃

Fig. 5. Distribution of PEG and DNA under non-uniform temperature field^[62]

Fig. 6. Marangoni convection section diagram (in the case of temperature field gradient), the tangential surface velocity uS of the fluid points to the cold zone near the liquid-gas interface, and then flows back to the higher end through the flow under the surface

Fig. 7. Schematic of OTFT based on gold nanoislands and gold films. Successive images of 500 nm PS particles trapping at (a) laser power of 1 mW and (b) laser power of 14.5 mW^[36]; (c) statistical analysis of the trapped PS number as a function of time for 500 nm particles,and the inset shows the schematic of the trapping potential well with a Gaussian profile, whose depth is described as U(r), γ is a constant, r is the coordinate, and a is the width of the potential well^[36]; (d) schematic of the experiment(thermal convection pushes the solution laterally into the hot zone and then axially out of the zone;thermophoresis drives the PS particle from cold zone to hot zone with a force larger than the axial convective drag force)^[76]

Fig. 8. Working principle of OTENT^[39]. (a) Surface charge modification of a metal nanoparticle by CTAC adsorption; (b) formation of CTAC micelles; (c) schematic of a Cl^-; (d) dispersion of a single metal particle and multiple ions in the solution without optical heating; (e) thermophoretic migration of the ions under optical heating; (f) steady ionic distribution under optical heating generates a thermoelectric field E_T for trapping the metal nanoparticle,and repulsive electric field E_r arises from the positive charges of the thermoplasmoic substrate and balances E_T; (g) simulated in-plane temperature gradient ∇T_r and direction of the corresponding trapping force; (h) simulated vertical-section temperature gradient ∇T_z and direction of the corresponding trapping force

Fig. 9. Illustration of the trapping mechanism and the sample geometry. A chrome ring (inner diameter is 10 μm, outer diameter is 12 μm, and height is 50 nm) is heated by a focused 808 nm laser rotating along the circumference of the ring at f=100 Hz^[83]

Fig. 10. Frame-by-frame demonstration of the trapped molecule's position under different AC frequencies^[77]: under 2 kHz AC frequency, the single molecule is trapped farther away from the pattern's edge; when the AC frequency is tuned higher, from 2 kHz to 5 kHz, the trapping position is shifted inward, closer to the pattern's edge; when the AC frequency increases to 10 kHz, the capture fails

Fig. 11. Mechanism of thermophoretic trapping of DNA^[35]. (a) Image of stained DNA before heating; (b) thermophoresis from central heating first repels DNA from the surrounding area; (c) thermophoresis and toroidal convection trap DNA in the center toward the lower chamber wall in a ring geometry, and the concentration enhancement is 13-fold; (d) the mechanism of thermophoretic trapping in the center is an interplay of lateral thermophoresis ①, ④ and axial thermophoresis ③ with convection ②

Fig. 12. Schematic of stretching single-molecule DNA via temperature gradient^[88]. (a) Stretching of two end tethered DNA by temperature gradient and force balance scheme; (b) DNA stretching for different laser powers (from left to right, no heating and 17.5, 19.5, and 21.5 mW of laser power; the scalar bar is 5 μm) and circular bright spots in the middle of the last three images are the laser converge positions

Fig. 13. DNA strand length screening by heat-driven fifilter^[89]. (a) Schematic of DNA capturing; (b) schematic of convection induced long strand DNA screening

Fig. 14. Design of ONSA for manipulating spherical (Staphylococcus aureus) and rod-shaped (Escherichia coli) bacteria^[98]. (a) Illustration of sorting and dynamics of bacteria in the ONSA ; (b) schematic of the dimension of the ONSA

Fig. 15. Schematic of fiber-based OTFT^[33,101]. (a) Experimental setup, the image was captured by CCD, which shows the SDF immerged in the particle suspensions; (b) propagating light along the taper-SDF-taper section and the negative photophoretic motions of particles radiated by the leaking light from SDF; microscope images of Escherichia coli after the laser on for (c) t_on=0 s (without incident optical power), (d) t_on=15 s (with an incident optical power of 125 mW), (e) t_on=47 s (with an incident optical power of 125 mW), (f) t_on=90 s (with an incident optical power of 125 mW),and the insert in Fig. 15(c) shows the image focused on the SDF; (g) experimental setup of optical fiber optothermal tweezers; (h) force components in the trap region; (i) side-view time-sequence images of trapping of Escherichia coli cells (the 0.65 mW laser was switched on at 0 s and switched off immediately before capturing of the last image; the excitation laser was 488 nm, and the laser sweeping speed was 3 s/frame; scale bar is 15 μm)

Fig. 16. Schematic of METH^[34]. (a) Setup of using a femtosecond laser to perform METH fabrication; (b) force analysis in the trapping scheme; (c) Escherichia coli images trapped at 24 mA

Fig. 17. Single cell manipulation and DNA amplification platform^[38]. (a) Optical guiding of a free-flowing living single cancer cell into a micro-well; (b) time-lapse images showing cell lysis during the first 60 s of laser illumination directed at the cell membrane; (c) procedures of DNA amplification in a particular micro-well; (d) fluorescence images of the 0 and 30 min RPA reaction; (e) procedures for DNA amplification of a targeted cancer cell; (f) fluorescence images showing amplified DNA of a single cell inside the micro-well at 0 and 30 min RPA reaction

Fig. 18. Schematic of the one-step thermophoretic AND gate operation (Tango) assay^[108]. (a) Tango assay combining AND gate operation on EV membranes, PEG-enhanced thermophoretic accumulation of EVs, two aptamer-based probes (PSMAL-Cy3 and Cy5-R-EpCAM) targeting PSMA and EpCAM, and one connector strand (connector) were designed; (b) Tango assay can directly detect tumor-derived EVs overexpression both PSMA and EpCAM within 15 min and a temperature gradient and a PEG concentration gradient were induced by local laser heating

Fig. 19. Schematic of OTF^[84]. (a) Optical setup of WSPRi part and optothermal excitation part (L1, L2: beam expanding lens group; L4, L5:collimating lens group; L3, L6: tube lens; Obj.1-2: objective lenses; MF:multimode optical fiber; BS:beam splitter); (b) side view of the entire system when the heating laser is switching on (the convective flow, which contains natural convective flow and thermo-osmotic flow, together with the thermophoresis, relocated and enriched the biomolecules from bulk solution to a ring shape region where the temperature is in relatively lower level); (c) top view of the ring shape biomolecule aggregation region in the background of near-field temperature map (R1: ring-shape biomolecule enrichment and binding region; R2: heating laser focusing center region; R3: background region with no laser heating); (d) zoom-in schematic of the biomolecule enrichment region when laser is switched on, the upward thermophoretic (UT) force induces inefficient binding of the biomolecules; (e) zoom-in schematic of the biomolecule enrichment region when laser is just switched off, the flipped thermophoretic (FT) force (downward) induces efficient binding of the biomolecules