Hypothermal opto-thermophoretic tweezers

Abstract

Optical tweezers have profound importance across fields ranging from manufacturing to biotechnology. However, the requirement of refractive index contrast and high laser power results in potential photon and thermal damage to the trapped objects, such as nanoparticles and biological cells. Optothermal tweezers have been developed to trap particles and biological cells via opto-thermophoresis with much lower laser powers. However, the intense laser heating and stringent requirement of the solution environment prevent their use for general biological applications. Here, we propose hypothermal opto-thermophoretic tweezers (HOTTs) to achieve low-power trapping of diverse colloids and biological cells in their native fluids. HOTTs exploit an environmental cooling strategy to simultaneously enhance the thermophoretic trapping force at sub-ambient temperatures and suppress the thermal damage to target objects. We further apply HOTTs to demonstrate the three-dimensional manipulation of functional plasmonic vesicles for controlled cargo delivery. With their noninvasiveness and versatile capabilities, HOTTs present a promising tool for fundamental studies and practical applications in materials science and biotechnology.

Fig. 1. Working principle of HOTTs.

(a) At ambient temperature, thermophoretic force (${\mathit{F}}_{\mathit{th}}$) repels the particle away from the laser in most conditions. White arrows indicate ${\mathit{F}}_{\mathit{th}}$ decomposed along and perpendicular to the substrate (b) In HOTTs, ${\mathit{F}}_{\mathit{th}}$ becomes attractive to trap particles at a sub-ambient temperature. c Schematic and timelapse optical images showing the repelling of a 1 μm PS particle in DI water by the laser beam at an ambient temperature of 27 °C. d The same particle was trapped at the laser beam at a sub-ambient temperature of 4 °C. The green crosshair indicates the laser beam center. Laser wavelength: 532 nm, laser power: 40 μW, beam radius: 850 nm, scale bars: 2 μm.

Fig. 2. Performance evaluation of trapping microparticles using HOTTs.

(a) Particle trajectories of 1 μm PS particles (low colloid concentration) in 1 mM NaCl_0.2OH_0.8 solution at varying environmental temperature. b Trapping stiffness dependence on sample temperature at single-particle concentration indicates the enhancement of trapping efficiency at lower temperatures for varying laser powers (i–iii), sizes (ii, iv), materials (ii, v), and solutions (i–iii, iv-v). Data is presented as mean values ± standard error of mean. c, d Optical images of repulsion at 27 °C (yellow panels) and trapping at 4 °C (blue panels) of 1 μm PS particle in DI water at a high concentration of 28.6 mg/mL (c) and low concentration (d) of 0.29 mg/mL. Laser power is 50 μW. e Trapping probability of 1 μm PS particles as a function of sample temperature and colloidal concentration. The highest concentration and volume fraction (Φ) of 1 μm PS particles at 100% relative concentration is 28.6 mg/mL and 0.3, respectively. Laser power is 50 μW. Scale bars: (a) 30 nm (c, d) 5 μm.

Fig. 3. Trapping erythrocytes in different tonicities using HOTTs.

(a) Schematic of erythrocytes in isotonic PBS. b Timelapse optical images showing the trapping and thermal rupture of erythrocytes at ambient temperature. Laser power: 0.67 mW. c Timelapse images of erythrocyte trapping at 4 °C at the same laser power. No cell rupture is observed. d Schematic of erythrocytes in hypotonic PBS. e Timelapse images of hypotonic erythrocytes repelled at ambient temperature (yellow panels) and trapped at 4 °C (blue panels). Laser power: 0.44 mW. f Schematic of erythrocytes in hypertonic PBS. g Timelapse images of hypertonic erythrocytes repelled at ambient temperature (yellow panels) and trapped at 4 °C (blue panels). Laser power: 0.34 mW. Scale bars: 5 μm.

Fig. 4. 3D manipulation of plasmonic vesicles using HOTTs.

(a) Schematic of a plasmonic vesicle under 660 nm excitation. b Force analysis of the plasmonic vesicle at ambient temperature and sub-ambient temperature. c Schematic of vesicle rupture and cargo release due to 532 nm laser beam excitation (532 nm). Schematic and optical images showing (d) the levitation of a plasmonic vesicle, (e) in-plane manipulation of a plasmonic vesicle, and (f) manipulation of the vesicle and subsequent rupture for controlled cargo release. The power of the 660 nm laser for vesicle manipulation is 0.67 mW with a beam radius of 814 nm. The power of the 532 nm laser for vesicle rupture is 0.1 mW with a beam radius of 810 nm. Scale bars: 5 μm.