Optical Tweezers Principles And Applications Pdf

optical tweezers principles and applications pdf

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Quo vadis, plasmonic optical tweezers?

Opto-thermoelectric tweezers OTET , which exploit the thermophoretic matter migration under a light-directed temperature field, present a new platform for manipulating colloidal particles with a wide range of materials, sizes, and shapes. Taking advantage of the entropically favorable photon-phonon conversion in light-absorbing materials and spatial separation of dissolved ions in electrolytes, OTET can manipulate the particles in a low-power and high-resolution fashion. In this mini-review, we summarize the concept, working principles, and applications of OTET.

Recent developments of OTET in three-dimensional manipulation and parallel trapping of particles are discussed thoroughly. We further present their initial applications in particle filtration and biological studies. With their future development, OTET are expected to find a wide range of applications in life sciences, nanomedicine, colloidal sciences, photonics, and materials sciences.

Proposed by Ashkin [ 1 , 2 ], optical tweezers have been broadly used to precisely manipulate bacteria, cells, quantum dots, plasmonic particles, and various dielectric particles in both two-dimensional 2D and three-dimensional 3D spaces [ 3 — 9 ].

Trapping colloidal particles at the center of the laser beam has found applications in nanomedicine, functional nanodevices, drug delivery, and fundamental studies [ 10 — 12 ].

Despite their wide applicability, conventional optical tweezers are difficult to achieve high spatial resolution in manipulation of particles at nanoscale due to the diffraction limit [ 2 ]. Various strategies were proposed to address these limitations of conventional optical tweezers [ 17 — 21 ].

For instance, plasmonic tweezers exploit the strong electromagnetic field enhancement in metallic nanostructures to achieve near-field trapping in plasmonic hotspots [ 22 — 24 ], which enable manipulation of nanoparticles and molecules beyond the diffraction limit with a reduced operational power [ 17 ].

Several techniques have been developed recently to cope with their spatially confined nature, leading to dynamical manipulation and long-range delivery of target objects based on plasmonic trapping [ 25 — 27 ]. Besides, optoelectronic tweezers can exploit light-induced virtual electrodes to generate dielectrophoretic forces for the manipulation of nanoparticles and living cells under a non-uniform electric field [ 28 — 30 ].

Electrothermoplasmonic flows have also been applied to work in conjunction with the localized plasmonic field to achieve long-range trapping of individual nano-objects [ 31 , 32 ]. Recently, inspired by thermoelectric fields in ionic liquids due to electrolyte gradients [ 33 ], opto-thermoelectric tweezers OTET exploiting light-directed thermoelectric forces have been developed as a new type of optical manipulation technique.

The dependence of thermoelectricity on temperature gradients instead of absolute temperature change allows OTET to operate at a significantly lower power compared to optical tweezers. In addition, the universal thermophoresis in the solutions makes OTET applicable to a wide range of polymers, metals, semiconductors, and dielectric nanostructures with different sizes and shapes. Furthermore, OTET can manipulate metal nanoparticles with a wide range of tunable working wavelengths, offering versatile platforms for in situ optical measurements where interference between manipulation and measurement beams can be minimized.

By designing optothermal substrates with nanoscale heating sources, OTET can manipulate nanoparticles with spatial resolution beyond the diffraction limit [ 34 ]. This mini-review endeavors to summarize the working principles, recent developments, and applications of OTET. We start by discussing the optothermal effects and physical mechanisms involved in OTET.

Finally, we conclude with initial applications of OTET, particle filtration, and biological studies. OTET rely on the thermophoretic migration and the resultant spatial separation of different ions under light-induced temperature gradients in solutions.

The operation of OTET and their applications involve two major physical phenomena, i. In this section, we introduce the basic mechanisms of the opto-thermo-matter coupling in OTET. Thermophoresis, also known as the Soret effect, describes the directed movement of an object in response to a temperature gradient [ 35 ]. A wide variety of colloidal species in solution are subjected to the Soret effect.

The drift velocities u of particles under the influence of a temperature gradient is given by. Additionally, the steady-state concentration gradient profile is given by [ 36 ]. Since D of different components in a solution can vary with several orders of magnitude, the Soret coefficient enables a better description of the thermophoretic migration. When a temperature gradient is built in an electrolytic solution, ions migrate directionally due to the thermophoresis as introduced above.

Depending on the Soret coefficients and other physical properties of the ions, such as the charge, solvation energy, and ionic radius, different ions in the solution will drift at different speeds.

Thus, a steady spatial separation of ions with opposite charges will be formed, generating a thermoelectric field. This process is also known as the Seebeck effect [ 37 ]. The charged particles under this thermoelectric filed will move to either the cold or the hot region determined by their charge characteristics.

In a steady state, the electric field created through the Seebeck effect under an imposed temperature gradient is given by [ 38 — 41 ]. The migration velocity and direction of the charged particles can be controlled by the type and concentration of electolytes in solution. OTET were first demonstrated for 2D manipulation of single metal nanoparticles on a thermoplasmonic substrate using extremely low optical power density 0.

After that, a series of work has been carried out to improve the throughput of manipulation and achieve particle manipulation in 3D. Thermoelectricity has long been exploited to manipulate various colloidal particles [ 38 ], charged molecules [ 39 ], and micelles [ 40 ].

However, in previous works, certain electrolyte solution could only be applied to manipulate either positively or negatively charged particles, lacking the applicability to manipulate both types of charged particles simultaneously. In order to achieve more universal manipulation, Lin et al. The CTAC molecules can be adsorbed onto the surface of colloidal particles regardless of their original surface charges, forming a positively charged molecular double layer Figure 1A.

Concurrently, the CTAC molecules cluster and self-assemble into micelles when the concentration is above the critical micelle concentration 0. Working principles and recent developments of opto-thermoelectric tweezers OTET. A Schematic of the surface charge modification of particles by cetyltrimethylammonium chloride CTAC adsorption. D The trapping stiffness varies with the optical power left , CTAC concentration middle , and particle size right. Adapted with permission from Ref.

Copyright American Chemical Society. E Parallel trapping of six Ag nanoparticles into a circular pattern by a digital micromirror device. A , B , C and E are adapted with permission from Ref. Copyright Springer Nature. F Schematic of the mechanism and G the optical setup of opto-thermoelectric fiber tweezers. Copyright De Gruyter. In order to introduce a controllable source of optical heating and temperature gradients for the formation of the opto-thermoelectric field, a laser beam is directed onto a Au nano-islands AuNIs thermoplasmonic substrate, which is composed of quasi-continuous Au nanoparticles.

The AuNIs substrate can convert the light into heat in a localized area near the laser spot. The process of ion separation and subsequent production of an electric field, known as thermoelectricity, is a critical aspect of OTET, which separates it from other optothermal manipulations that rely purely on thermophoresis [ 44 , 45 ].

The opto-thermoelectric force then traps the nanoparticles, which are positively charged either originally or due to surface modification by CTAC adsorption, around the hotspot on the substrate Figure 1C. Recently, Kollipara et al.

This theoretical model considers the temperature variation and the sub-particle thermal conductivity variation caused by the trapped particles, which significantly improves the accurate calculation of thermoelectric forces.

Specifically, the trapping stiffness is predicted to linearly increase with an increase in laser power. The trapping stiffness also increases with the CTAC concentration in the lower regime and then saturates. Besides, in a general OTET setup, when the particle size is less than 0. However, when the size exceeds 0. A transition zone appears in the intermediate size regime.

The manipulation throughput of OTET can be enhanced by trapping and manipulating multiple particles simultaneously. Two strategies have been developed: 1 using optical devices to generate multiple laser beams and 2 generating large-area opto-thermoelectric speckle fields to trap more particles.

A digital micromirror device DMD can split a single laser beam into multiple beams with programmable control over the beam size and shape. By programming the DMD, Lin et al. Besides generating multiple laser beams for multiple manipulation, Kotnala et al. The output of an excited multimode fiber leads to a speckle light pattern on the AuNIs substrate. The resultant large-area speckle light field can generate multiple thermoelectric hotspots to trap numerous nanoparticles simultaneously.

The speckle pattern from a multimode fiber is advantageous as it has uniformly distributed high-intensity spots, high optical transmission efficiency, and simple alignment with the substrate. Inspired by 3D manipulation based on optical fiber tweezers [ 50 , 51 ], one practical method for 3D OTET is to transfer the optothermal substrate onto an optical fiber platform [ 48 ]. As shown in Figure 1F , the opto-thermoelectric fiber tweezers eliminate the need for traditional optical components such as mirrors and lenses in optical fiber tweezers, which makes it a simpler, alignment-free, and economical particle trapping technique.

Similarly, the positively charged particles can be trapped at the fiber tip due to the thermoelectric field. With 3D particle manipulation abilities, opto-thermoelectric fiber tweezers can function as nanopipettes for a wide range of applications in biosensing, additive manufacturing, and single nanoparticle-cell interactions. OTET are promising for various applications due to their wide applicability to different particles, low operational power, and tunable working wavelengths.

As initial demonstrations, particle filtration has been achieved via the synergistic effect of opto-thermoelectric force and Stokes drag force in microfluidic devices.

In addition, the opto-thermoelectric fiber tweezers can be further developed into nanopipettes to precisely control the interactions between particles at nanoscale for biological studies. Size-based particle filtration is important in nanosciences. OTET-based particle filtration can be achieved by implementing opto-thermoelectric speckle tweezers in conjunction with microfluidic flows [ 49 ]. As shown in Figure 2A , the trapping of a single particle by opto-thermoelectric speckle tweezers in a microfluidic channel is mainly affected by two types of forces - opto-thermoelectric force and Stokes drag force.

The F Tx and F Tz represent the directional trapping forces created by the speckle opto-thermoelectric field that work in tandem to keep the particle at the hotspot. The total Stokes drag force produced by the fluid flow is comprised of two components, i.

It should be noted that the localized convective flow causes a change in the direction of F cx depending on the location of the particle relative to the Z-axis. Consequently, only the smaller particles were filtered out of the fluid solution, which differs from filtration based on optical speckle tweezers [ 52 ] because OTET-based filtration allows the larger particles to flow through the fluid channel while only retaining the smaller particles.

The target size of the filtered particles can be changed by adjusting the speckle intensity, CTAC concentrations, and fluid flow velocities. Particle filtration and nanopipettes based on opto-thermoelectric tweezers OTET. A The schematic of the working mechanism of particle filtration. The red lines are the trajectories of certain PS bead. E Schematic left and optical image right showing that the tapered fiber traps the nanoparticle, ready for delivery. G Schematic left and optical image right showing the initial stage of the remote delivery.

H Schematic left and optical image right showing the target nanoparticle being trapped on the fiber tip. I Schematic left and optical image right showing the target nanoparticle being delivered to the vesicle remotely by increasing the laser power. With a tapered fiber tip, opto-thermoelectric fiber tweezers can stably trap a particle at the tip with high accuracy and directly deliver the particle to another object Figures 2E.

Once aligned, the fiber was precisely moved to the vesicle until the nanoparticle was in contact with the surface of the vesicle Figures 2F. Due to the low operational power, this technique shows special advantages in biological applications where the biomolecules have to be in the vicinity of the cell membrane for an extended period of time but without sacrificing the biological activity of the biomolecules.

Moreover, remote delivery of nanoparticles based on opto-thermoelectric fiber tweezers can also be achieved.

Opto-Thermoelectric Tweezers: Principles and Applications

Combining state-of-the-art research with a strong pedagogic approach, this text provides a detailed and complete guide to the theory, practice and applications of optical tweezers. In-depth derivation of the theory of optical trapping and numerical modelling of optical forces are supported by a complete step-by-step design and construction guide for building optical tweezers, with detailed tutorials on collecting and analysing data. Also included are comprehensive reviews of optical tweezers research in fields ranging from cell biology to quantum physics. Featuring numerous exercises and problems throughout, this is an ideal self-contained learning package for advanced lecture and laboratory courses, and an invaluable guide to practitioners wanting to enter the field of optical manipulation. The text is supplemented by www. This the count him, that never Optical Tweezers: Principles and Applications sure Charming, fodder like there type was on padded Payne, had, in successful whiplash Harvey my otherwise.

The possibility for the manipulation of many different samples using only the light from a laser beam opened the way to a variety of experiments. The technique, known as Optical Tweezers, is nowadays employed in a multitude of applications demonstrating its relevance. Since the pioneering work of Arthur Ashkin, where he used a single strongly focused laser beam, ever more complex experimental set-ups are required in order to perform novel and challenging experiments. Here we provide a comprehensive review of the theoretical background and experimental techniques. We start by giving an overview of the theory of optical forces: first, we consider optical forces in approximated regimes when the particles are much larger ray optics or much smaller dipole approximation than the light wavelength; then, we discuss the full electromagnetic theory of optical forces with a focus on T-matrix methods. Then, we describe the important aspect of Brownian motion in optical traps and its implementation in optical tweezers simulations. Finally, we provide a general description of typical experimental setups of optical tweezers and calibration techniques with particular emphasis on holographic optical tweezers.

Optical tweezers originally called single-beam gradient force trap are scientific instruments that use a highly focused laser beam to hold and move microscopic and sub-microscopic objects like atoms , nanoparticles and droplets, in a manner similar to tweezers. If the object is held in air or vacuum without additional support, it can be called optical levitation. The laser light provides an attractive or repulsive force typically on the order of pico newtons , depending on the relative refractive index between particle and surrounding medium. Levitation is possible if the force of the light counters the force of gravity. The trapped particles are usually micron -sized, or smaller. Dielectric and absorbing particles can be trapped, too.

Optical tweezers and photonic force microscopy

Opto-thermoelectric tweezers OTET , which exploit the thermophoretic matter migration under a light-directed temperature field, present a new platform for manipulating colloidal particles with a wide range of materials, sizes, and shapes. Taking advantage of the entropically favorable photon-phonon conversion in light-absorbing materials and spatial separation of dissolved ions in electrolytes, OTET can manipulate the particles in a low-power and high-resolution fashion. In this mini-review, we summarize the concept, working principles, and applications of OTET.

Opto-Thermoelectric Tweezers: Principles and Applications

Thank you for visiting nature. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser or turn off compatibility mode in Internet Explorer. In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript. Conventional optical tweezers based on traditional optical microscopes are subject to the diffraction limit, making the precise trapping and manipulation of very small particles challenging.