Thermophoretic Tweezers for Low-Power and Versatile Manipulation of Biological Cells

Moving Meniscus-Assisted Template-Free Optothermofluidic Nanoparticle Patterning and Its Application in Optothermoconvective Particle Trapping

Moving meniscus-assisted vertical lifting is a commonly employed particle assembly technique to realize large-area particle patterning for the easy fabrication of colloidal photonic crystals and sensors. Though great success has been achieved for large-area patterning, inscribing desired patterns over the target substrate with precise control over the morphology remains a challenge. The target substrates need to be functionalized (physically or chemically) to realize desired patterns, which increases the complexity and limits their applicability to specific particle-liquid combinations. We demonstrate a new approach for the precise patterning of gold nanoparticles (Au NPs, diameter ∼60 nm) over solid substrates by the synergy of light-induced Marangoni flow and vertical lifting process (moving meniscus), without the requirement of photomasks or templates. The core idea relies on the particle accumulation due to light-induced Marangoni flow near the liquid meniscus in contact with a solid surface (due to plasmonic absorption of the particles) and the controlled lifting of the substrate. We present both the simulation and experimental results of the developed patterning technique. Various patterns such as continuous lines, intermittent lines with varying lengths, patterns with continuously varying widths, cross patterns, etc. are successfully inscribed. Dynamic control over the three-dimensional morphology of the deposited patterns is achieved by varying the lifting velocity, laser irradiation time, and lifting direction during the inscription process. Finally, we show the applicability of the developed plasmonically active surface for the large-area parallel manipulation of nonabsorbing microparticles based on optothermoconvective flow. The major advantage of the developed method compared to the existing light-controlled patterning techniques is its ability to inscribe patterns over large distances (up to several centimeters). We expect that the results presented in this paper will benefit different applications requiring precise particle patterning, such as optical elements, sensors, plasmonic substrates, microfluidic master templates, and electronic circuits.

A vibrating capillary for ultrasound rotation manipulation of zebrafish larvae

Multifunctional micromanipulation systems have garnered significant attention due to the growing interest in biological and medical research involving model organisms like zebrafish (Danio rerio). Here, we report a novel acoustofluidic rotational micromanipulation system that offers rapid trapping, high-speed rotation, multi-angle imaging, and 3D model reconstruction of zebrafish larvae. An ultrasound-activated oscillatory glass capillary is used to trap and rotate a zebrafish larva. Simulation and experimental results demonstrate that both the vibrating mode and geometric placement of the capillary contribute to the developed polarized vortices along the long axis of the capillary. Given its capacities for easy-to-operate, stable rotation, avoiding overheating, and high-throughput manipulation, our system poses the potential to accelerate zebrafish-directed biomedical research.

Solute-particle separation in microfluidics enhanced by symmetrical convection

The utilization of microfluidic technology for miniaturized and efficient particle sorting holds significant importance in fields such as biology, chemistry, and healthcare. Passive separation methods, achieved by modifying the geometric shapes of microchannels, enable gentle and straightforward enrichment and separation of particles. Building upon previous discussions regarding the effects of column arrays on fluid flow and particle separation within microchips, we introduced a column array structure into an H-shaped microfluidic chip. It was observed that this structure enhanced mass transfer between two fluids while simultaneously intercepting particles within one fluid, satisfying the requirements for particle interception. This enhancement was primarily achieved by transforming the originally single-mode diffusion-based mass transfer into dual-mode diffusion-convection mass transfer. By further optimizing the column array, it was possible to meet the basic requirements of mass transfer and particle interception with fewer microcolumns, thereby reducing device pressure drop and facilitating the realization of parallel and high-throughput microfluidic devices. These findings have enhanced the potential application of microfluidic systems in clinical and chemical engineering domains.

Bubble Printing of Layered Silicates: Surface Chemistry Effects and Picomolar Förster Resonance Energy Transfer Sensing

The assembly of nanoparticles on surfaces in defined patterns has long been achieved via template-assisted methods that involve long deposition and drying steps and the need for molds or masks to obtain the desired patterns. Control over deposition of materials on surfaces via laser-directed microbubbles is a nascent technique that holds promise for rapid fabrication of devices down to the micrometer scale. However, the influence of surface chemistry on the resulting assembly using such approaches has so far not been studied. Herein, the printing of layered silicate nanoclays using a laser-directed microbubble was established. Significant differences in the macroscale structure of the printed patterns were observed for hydrophilic, pristine layered silicates compared to hydrophobic, modified layered silicates, which provided the first example of how the surface chemistry of such nanoscale objects results in changes in assembly with this approach. Furthermore, the ability of layered silicates to adsorb molecules at the interface was retained, which allowed the fabrication of proof-of-concept sensors based on Förster resonance energy transfer (FRET) from quantum dots embedded in the assemblies to bound dye molecules. The detection limit for Rhodamine 800 sensing via FRET was found to be on the order of 10-12 M, suggesting signal enhancement due to favorable interactions between the dye and nanoclay. This work sets the stage for future advances in the control of hierarchical assembly of nanoparticles by modification of surface chemistry while also demonstrating a quick and versatile approach to achieve ultrasensitive molecular sensors.

Microfluidic Blood Separation: Key Technologies and Critical Figures of Merit

Blood is a complex sample comprised mostly of plasma, red blood cells (RBCs), and other cells whose concentrations correlate to physiological or pathological health conditions. There are also many blood-circulating biomarkers, such as circulating tumor cells (CTCs) and various pathogens, that can be used as measurands to diagnose certain diseases. Microfluidic devices are attractive analytical tools for separating blood components in point-of-care (POC) applications. These platforms have the potential advantage of, among other features, being compact and portable. These features can eventually be exploited in clinics and rapid tests performed in households and low-income scenarios. Microfluidic systems have the added benefit of only needing small volumes of blood drawn from patients (from nanoliters to milliliters) while integrating (within the devices) the steps required before detecting analytes. Hence, these systems will reduce the associated costs of purifying blood components of interest (e.g., specific groups of cells or blood biomarkers) for studying and quantifying collected blood fractions. The microfluidic blood separation field has grown since the 2000s, and important advances have been reported in the last few years. Nonetheless, real POC microfluidic blood separation platforms are still elusive. A widespread consensus on what key figures of merit should be reported to assess the quality and yield of these platforms has not been achieved. Knowing what parameters should be reported for microfluidic blood separations will help achieve that consensus and establish a clear road map to promote further commercialization of these devices and attain real POC applications. This review provides an overview of the separation techniques currently used to separate blood components for higher throughput separations (number of cells or particles per minute). We present a summary of the critical parameters that should be considered when designing such devices and the figures of merit that should be explicitly reported when presenting a device's separation capabilities. Ultimately, reporting the relevant figures of merit will benefit this growing community and help pave the road toward commercialization of these microfluidic systems.

Hypothermal opto-thermophoretic tweezers

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.

Optofluidic transport and assembly of nanoparticles using an all-dielectric quasi-BIC metasurface

Manipulating fluids by light at the micro/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, the counterpart of metals, has been 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 subwavelength scale 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 increases the flow velocity up to three times compared with the off-resonant situation by tuning the wavelength within several nanometers range. We also experimentally investigate the dynamics of particle aggregation with respect to laser wavelength and power. A physical model is presented and simulated to elucidate the phenomena and surfactants are added to the nanoparticle 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 hold enormous potential in high-sensitivity biosensing applications.

All-Optical Rapid Formation, Transport, and Sustenance of a Sessile Droplet in a Two-Dimensional Slit with Few-Micrometer Separation

We present a sessile droplet manipulation platform that enables the formation and transport of a droplet on a light-absorbing surface via local laser-beam irradiation. The mechanism relies on solutocapillary Marangoni flow arising from a concentration gradient in a binary mixture liquid. Because the mixture is strongly confined in a two-dimensional slit with a spacing of a few micrometers, the wetting film is stably sustained, enabling the rapid formation, deformation, and transport of a sessile droplet. In addition, to sustain the droplet in the absence of laser irradiation, we developed a method to bridge the droplet between the top and bottom walls of the slit. The bridge is stably sustained because of the hydrophilicity of the slit wall. Splitting and merging of the droplet bridges are also demonstrated.

Optofluidic Tweezers: Efficient and Versatile Micro/Nano-Manipulation Tools

Optical tweezers (OTs) can transfer light momentum to particles, achieving the precise manipulation of particles through optical forces. Due to the properties of non-contact and precise control, OTs have provided a gateway for exploring the mysteries behind nonlinear optics, soft-condensed-matter physics, molecular biology, and analytical chemistry. In recent years, OTs have been combined with microfluidic chips to overcome their limitations in, for instance, speed and efficiency, creating a technology known as "optofluidic tweezers." This paper describes static OTs briefly first. Next, we overview recent developments in optofluidic tweezers, summarizing advancements in capture, manipulation, sorting, and measurement based on different technologies. The focus is on various kinds of optofluidic tweezers, such as holographic optical tweezers, photonic-crystal optical tweezers, and waveguide optical tweezers. Moreover, there is a continuing trend of combining optofluidic tweezers with other techniques to achieve greater functionality, such as antigen-antibody interactions and Raman tweezers. We conclude by summarizing the main challenges and future directions in this research field.

Microfluidic systems for particle capture and release: A review

Microfluidic technology has emerged as a promising tool in various applications, including biosensing, disease diagnosis, and environmental monitoring. One of the notable features of microfluidic devices is their ability to selectively capture and release specific cells, biomolecules, bacteria, and particles. Compared to traditional bulk analysis instruments, microfluidic capture-and-release platforms offer several advantages, such as contactless operation, label-free detection, high accuracy, good sensitivity, and minimal reagent requirements. However, despite significant efforts dedicated to developing innovative capture mechanisms in the past, the release and recovery efficiency of trapped particles have often been overlooked. Many previous studies have focused primarily on particle capture techniques and their efficiency, disregarding the crucial role of successful particle release for subsequent analysis. In reality, the ability to effectively release trapped particles is particularly essential to ensure ongoing, high-throughput analysis. To address this gap, this review aims to highlight the importance of both capture and release mechanisms in microfluidic systems and assess their effectiveness. The methods are classified into two categories: those based on physical principles and those using biochemical approaches. Furthermore, the review offers a comprehensive summary of recent applications of microfluidic platforms specifically designed for particle capture and release. It outlines the designs and performance of these devices, highlighting their advantages and limitations in various target applications and purposes. Finally, the review concludes with discussions on the current challenges faced in the field and presents potential future directions.

Multimodal Optothermal Manipulations along Various Surfaces

Optical tweezers have provided tremendous opportunities for fundamental studies and applications in the life sciences, chemistry, and physics by offering contact-free manipulation of small objects. However, it requires sophisticated real-time imaging and feedback systems for conventional optical tweezers to achieve controlled motion of micro/nanoparticles along textured surfaces, which are required for such applications as high-resolution near-field characterizations of cell membranes with nanoparticles as probes. In addition, most optical tweezers systems are limited to single manipulation modes, restricting their broader applications. Herein, we develop an optothermal platform that enables the multimodal manipulation of micro/nanoparticles along various surfaces. Specifically, we achieve the manipulation of micro/nanoparticles through the synergy between the optical and thermal forces, which arise due to the temperature gradient self-generated by the particles absorbing the light. With a simple control of the laser beam, we achieve five switchable working modes [i.e., tweezing, rotating, rolling (toward), rolling (away), and shooting] for the versatile manipulation of both synthesized particles and biological cells along various substrates. More interestingly, we realize the manipulation of micro/nanoparticles on rough surfaces of live worms and their embryos for localized control of biological functions. By enabling the three-dimensional control of micro/nano-objects along various surfaces, including topologically uneven biological tissues, our multimodal optothermal platform will become a powerful tool in life sciences, nanotechnology, and colloidal sciences.

Photothermal Nanomaterials: A Powerful Light-to-Heat Converter

All forms of energy follow the law of conservation of energy, by which they can be neither created nor destroyed. Light-to-heat conversion as a traditional yet constantly evolving means of converting light into thermal energy has been of enduring appeal to researchers and the public. With the continuous development of advanced nanotechnologies, a variety of photothermal nanomaterials have been endowed with excellent light harvesting and photothermal conversion capabilities for exploring fascinating and prospective applications. Herein we review the latest progresses on photothermal nanomaterials, with a focus on their underlying mechanisms as powerful light-to-heat converters. We present an extensive catalogue of nanostructured photothermal materials, including metallic/semiconductor structures, carbon materials, organic polymers, and two-dimensional materials. The proper material selection and rational structural design for improving the photothermal performance are then discussed. We also provide a representative overview of the latest techniques for probing photothermally generated heat at the nanoscale. We finally review the recent significant developments of photothermal applications and give a brief outlook on the current challenges and future directions of photothermal nanomaterials.

Colloid thermophoresis in the dilute electrolyte concentration regime: from theory to experiment

Colloid thermophoresis in aqueous media is vital for numerous applications in nanoscience and life sciences. To date, a general description of colloid thermophoresis in DI water has not been determined. Here, we describe a theoretical model within the framework of the Fokker-Planck formalism and the flickering cluster concept to describe the hydration entropy effect on the thermophoretic behaviour of colloids suspended in DI water and compare this to new experimental results. We built an experimental platform to allow for rapid and robust temperature control and investigate the thermophoretic behaviour of silica microspheres with different sizes at various background temperatures for comparison. In this work, the ionic shielding effect is accounted for by using the well-known Duhr-Dhont's model, and the hydration layer effect is determined using the developed theoretical model. For the latter, our model reveals that the sign of the Soret coefficient is governed by the interplay between the binding energy and the chemical potential of water molecules, which were found to be in the same order of magnitude. We show that our analysis accurately describes the experimental behaviour of colloidal particles that opens a new avenue for developing versatile trapping and separation techniques for various colloidal particles in aqueous systems according to their size and background temperature.

Optical Manipulation Heats up: Present and Future of Optothermal Manipulation

Optothermal manipulation is a versatile technique that combines optical and thermal forces to control synthetic micro-/nanoparticles and biological entities. This emerging technique overcomes the limitations of traditional optical tweezers, including high laser power, photon and thermal damage to fragile objects, and the requirement of refractive-index contrast between target objects and the surrounding solvents. In this perspective, we discuss how the rich opto-thermo-fluidic multiphysics leads to a variety of working mechanisms and modes of optothermal manipulation in both liquid and solid media, underpinning a broad range of applications in biology, nanotechnology, and robotics. Moreover, we highlight current experimental and modeling challenges in the pursuit of optothermal manipulation and propose future directions and solutions to the challenges.

A review of sensors for classification and subtype discrimination of cancer: Insights into circulating tumor cells and tumor-derived extracellular vesicles

Liquid biopsy can reflect the state of tumors in vivo non-invasively, thus providing a strong basis for the early diagnosis, individualized treatment monitoring and prognosis of tumors. Circulating tumor cells (CTCs) and tumor-derived extracellular vesicles (tdEVs) contain information-rich components, such as nucleic acids and proteins, and they are essential markers for liquid biopsies. Their capture and analysis are of great importance for the study of disease occurrence and development and, consequently, have been the subject of many reviews. However, both CTCs and tdEVs carry the biological characteristics of their original tissue, and few reviews have focused on their function in the staging and classification of cancer. In this review, we focus on state-of-the-art sensors based on the simultaneous detection of multiple biomarkers within CTCs and tdEVs, with clinical applications centered on cancer classification and subtyping. We also provide a thorough discussion of the current challenges and prospects for novel sensors with the ultimate goal of cancer classification and staging. It is hoped that these most advanced technologies will bring new insights into the clinical practice of cancer screening and diagnosis.

Multiplexed Long-Range Electrohydrodynamic Transport and Nano-Optical Trapping with Cascaded Bowtie Photonic Crystal Nanobeams

Photonic crystal cavities with bowtie defects that combine ultrahigh Q and ultralow mode volume are theoretically studied for low-power nanoscale optical trapping. By harnessing the localized heating of the water layer near the bowtie region, combined with an applied alternating current electric field, this system provides long-range electrohydrodynamic transport of particles with average radial velocities of 30  μm/s towards the bowtie region on demand by switching the input wavelength. Once transported to a given bowtie region, synergistic interaction of optical gradient and attractive negative thermophoretic forces stably trap a 10 nm quantum dot in a potential well with a depth of 10  k_{B}T using a mW input power.

Optothermal rotation of micro-/nano-objects

Due to its contactless and fuel-free operation, optical rotation of micro-/nano-objects provides tremendous opportunities for cellular biology, three-dimensional (3D) imaging, and micro/nanorobotics. However, complex optics, extremely high operational power, and the applicability to limited objects restrict the broader use of optical rotation techniques. This Feature Article focuses on a rapidly emerging class of optical rotation techniques, termed optothermal rotation. Based on light-mediated thermal phenomena, optothermal rotation techniques overcome the bottlenecks of conventional optical rotation by enabling versatile rotary control of arbitrary objects with simpler optics using lower powers. We start with the fundamental thermal phenomena and concepts: thermophoresis, thermoelectricity, thermo-electrokinetics, thermo-osmosis, thermal convection, thermo-capillarity, and photophoresis. Then, we highlight various optothermal rotation techniques, categorizing them based on their rotation modes (i.e., in-plane and out-of-plane rotation) and the thermal phenomena involved. Next, we explore the potential applications of these optothermal manipulation techniques in areas such as single-cell mechanics, 3D bio-imaging, and micro/nanomotors. We conclude the Feature Article with our insights on the operating guidelines, existing challenges, and future directions of optothermal rotation.

Recent Progress on Optical Micro/Nanomanipulations: Structured Forces, Structured Particles, and Synergetic Applications

Optical manipulation has achieved great success in the fields of biology, micro/nano robotics and physical sciences in the past few decades. To date, the optical manipulation is still witnessing substantial progress powered by the growing accessibility of the complex light field, advanced nanofabrication and developed understandings of light-matter interactions. In this perspective, we highlight recent advancements of optical micro/nanomanipulations in cutting-edge applications, which can be fostered by structured optical forces enabled with diverse auxiliary multiphysical field/forces and structured particles. We conclude with our vision of ongoing and futuristic directions, including heat-avoided and heat-utilized manipulation, nonlinearity-mediated trapping and manipulation, metasurface/two-dimensional material based optical manipulation, as well as interface-based optical manipulation.

Investigating water/oil interfaces with opto-thermophoresis

Charging of interfaces between water and hydrophobic media is a mysterious feature whose nature and origin have been under debate. Here, we investigate the fundamentals of the interfacial behaviors of water by employing opto-thermophoretic tweezers to study temperature-gradient-induced perturbation of dipole arrangement at water/oil interfaces. With surfactant-free perfluoropentane-in-water emulsions as a model interface, additional polar organic solvents are introduced to systematically modify the structural aspects of the interface. Through our experimental measurements on the thermophoretic behaviors of oil droplets under a light-generated temperature gradient, in combination with theoretical analysis, we propose that water molecules and mobile negative charges are present at the water/oil interfaces with specific dipole arrangement to hydrate oil droplets, and that this arrangement is highly susceptible to the thermal perturbation due to the mobility of the negative charges. These findings suggest a potential of opto-thermophoresis in probing aqueous interfaces and could enrich understanding of the interfacial behaviors of water.

Universal optothermal micro/nanoscale rotors

Rotation of micro/nano-objects is important for micro/nanorobotics, three-dimensional imaging, and lab-on-a-chip systems. Optical rotation techniques are especially attractive because of their fuel-free and remote operation. However, current techniques require laser beams with designed intensity profile and polarization or objects with sophisticated shapes or optical birefringence. These requirements make it challenging to use simple optical setups for light-driven rotation of many highly symmetric or isotropic objects, including biological cells. Here, we report a universal approach to the out-of-plane rotation of various objects, including spherically symmetric and isotropic particles, using an arbitrary low-power laser beam. Moreover, the laser beam is positioned away from the objects to reduce optical damage from direct illumination. The rotation mechanism based on opto-thermoelectrical coupling is elucidated by rigorous experiments combined with multiscale simulations. With its general applicability and excellent biocompatibility, our universal light-driven rotation platform is instrumental for various scientific research and engineering applications.

Optothermal grid activation of microflow with magnetic nanoparticle thermophoresis for microfluidics

We report focused light-induced activation of intense magnetic microconvection mediated by suspended magnetic nanoparticles in microscale two-dimensional optothermal grids. Fully anisotropic control of microflow and mass transport fluxes is achieved by engaging the magnetic field along one or the other preferred directions. The effect is based on the recently described thermal diffusion-magnetomechanical coupling in synthetic magnetic nanofluids. We expect that the new phenomenon can be applied as an efficient all-optical mixing strategy in integrated microfluidic devices. This article is part of the theme issue 'Transport phenomena in complex systems (part 2)'.

Far-field position-tunable trapping of dielectric particles using a graphene-based plasmonic lens

In this report, a graphene-based plasmonic lens is designed for far-field position-tunable trapping of dielectric particles at a wavelength of 1550 nm, in which target particles can be floated at a variable z-position, using a variable gate voltage applied to the graphene ribbons. Preventing proximity of the trapped particle and the metallic lens structure, we can diminish general thermal issues in plasmonic tweezers, while realizing higher degrees of freedom in studying target characteristics of the particles by achieving position-tunable 3D trapping. These advantageous aspects are impossible in conventional plasmonic tweezers, because of the highly evanescent nature of the plasmonic field at the metal interface. The proposed structure is comprised of two concentric circular slit-sets (S₁, S₂), each capable of sending a directive beam, which can lead to a constructive interference, and forming a subwavelength focal spot in the far-field. Taking advantage of the epsilon-near-zero (ENZ) behavior of graphene, each of the radiating slit-sets can be switched ON/OFF, with a radiation switching ratio of about 49, by applying a small electric pulse of 80 meV to change the Fermi energy of the corresponding graphene ribbon from 0.535 eV to 0.615 eV. Hence, inverting the radiation state of the designed lens, from (S₁:ON, S₂:OFF) to (S₁:OFF, S₂:ON), we can change the z-position of the focal trapping site from 5000 nm to 9800 nm. This configuration can be proposed as a new generation of long-range, electrostatically tunable 3D plasmonic tweezing, without the need for any external bulky optomechanical equipment.

Heat-Mediated Optical Manipulation

Progress in optical manipulation has stimulated remarkable advances in a wide range of fields, including materials science, robotics, medical engineering, and nanotechnology. This Review focuses on an emerging class of optical manipulation techniques, termed heat-mediated optical manipulation. In comparison to conventional optical tweezers that rely on a tightly focused laser beam to trap objects, heat-mediated optical manipulation techniques exploit tailorable optothermo-matter interactions and rich mass transport dynamics to enable versatile control of matter of various compositions, shapes, and sizes. In addition to conventional tweezing, more distinct manipulation modes, including optothermal pulling, nudging, rotating, swimming, oscillating, and walking, have been demonstrated to enhance the functionalities using simple and low-power optics. We start with an introduction to basic physics involved in heat-mediated optical manipulation, highlighting major working mechanisms underpinning a variety of manipulation techniques. Next, we categorize the heat-mediated optical manipulation techniques based on different working mechanisms and discuss working modes, capabilities, and applications for each technique. We conclude this Review with our outlook on current challenges and future opportunities in this rapidly evolving field of heat-mediated optical manipulation.

Multiphysics microfluidics for cell manipulation and separation: a review

Multiphysics microfluidics, which combines multiple functional physical processes in a microfluidics platform, is an emerging research area that has attracted increasing interest for diverse biomedical applications. Multiphysics microfluidics is expected to overcome the limitations of individual physical phenomena through combining their advantages. Furthermore, multiphysics microfluidics is superior for cell manipulation due to its high precision, better sensitivity, real-time tunability, and multi-target sorting capabilities. These exciting features motivate us to review this state-of-the-art field and reassess the feasibility of coupling multiple physical processes. To confine the scope of this paper, we mainly focus on five common forces in microfluidics: inertial lift, elastic, dielectrophoresis (DEP), magnetophoresis (MP), and acoustic forces. This review first explains the working mechanisms of single physical phenomena. Next, we classify multiphysics techniques in terms of cascaded connections and physical coupling, and we elaborate on combinations of designs and working mechanisms in systems reported in the literature to date. Finally, we discuss the possibility of combining multiple physical processes and associated design schemes and propose several promising future directions.

Graphene-Based Opto-Thermoelectric Tweezers

Since the discovery of graphene, its excellent physical properties have greatly improved the performance of optoelectronic devices and brought important technological advances to optical research and its applications. Here, graphene is introduced to the field of optical-tweezer technology and demonstrate a new graphene-based opto-thermoelectric tweezer. This technology not only reduces the incident light energy required by two orders of magnitude (compared with traditional optical tweezers), it also brings new advantages such as a much broader working bandwidth and a larger working area compared to those of widely researched gold-film-based opto-thermoelectric tweezers. Compared with gold film, graphene exhibits higher thermal conductivity and higher uniformity and is easier to process. Thus, it is found that even monolayer graphene provides stable trapping for particles in a broad bandwidth and that performance is enhanced as the number of graphene layers increases. Furthermore, parallel trap multiple particles as desired shapes can be easily generated with structured graphene patterns. This work demonstrates the enormous application potential of graphene in optical-tweezer technology and will promote their application to the trapping or concentration of cells and biomolecules as well as to microfluidics and biosensors.

Microfluidic technologies for nanoparticle formation

Functional nanoparticles (NPs) hold immense promise in diverse fields due to their unique biological, chemical, and physical properties associated with size or morphology. Microfluidic technologies featuring precise fluid manipulation have become versatile toolkits for manufacturing NPs in a highly controlled manner with low batch-to-batch variability. In this review, we present the fundamentals of microfluidic fabrication strategies, including mixing-, droplet-, and multiple field-based microfluidic methods. We highlight the formation of functional NPs using these microfluidic reactors, with an emphasis on lipid NPs, polymer NPs, lipid-polymer hybrid NPs, supramolecular NPs, metal and metal-oxide NPs, metal-organic framework NPs, covalent organic framework NPs, quantum dots, perovskite nanocrystals, biomimetic NPs, etc. we discuss future directions in microfluidic fabrication for accelerated development of functional NPs, such as device parallelization for large-scale NP production, highly efficient optimization of NP formulations, and AI-guided design of multi-step microfluidic reactors.

A Microfluidic Dielectric Spectroscopy System for Characterization of Biological Cells in Physiological Media

Dielectric spectroscopy (DS) is a promising cell screening method that can be used for diagnostic and drug discovery purposes. The primary challenge of using DS in physiological buffers is the electrode polarization (EP) that overwhelms the impedance signal within a large frequency range. These effects further amplify with the miniaturization of the measurement electrodes. In this study, we present a microfluidic system and the associated equivalent circuit models for real-time measurements of cell membrane capacitance and cytoplasm resistance in physiological buffers with 10 s increments. The current device captures several hundreds of biological cells in individual microwells through gravitational settling and measures the system's impedance using microelectrodes covered with dendritic gold nanostructures. Using PC-3 cells (a highly metastatic prostate cancer cell line) suspended in cell growth media (CGM), we demonstrate stable measurements of cell membrane capacitance and cytoplasm resistance in the device for over 15 min. We also describe a consistent application of the equivalent circuit model, starting from the reference measurements used to determine the system parameters. The circuit model is tested using devices with varying dimensions, and the obtained cell parameters between different devices are nearly identical. Further analyses of the impedance data have shown that accurate cell membrane capacitance and cytoplasm resistance can be extracted using a limited number of measurements in the 5 MHz to 10 MHz range. This will potentially reduce the timescale required for real-time DS measurements below 1 s. Overall, the new microfluidic device can be used for the dielectric characterization of biological cells in physiological buffers for various cell screening applications.

Optoelectronic tweezers: a versatile toolbox for nano-/micro-manipulation

This review covers the fundamentals, recent progress and state-of-the-art applications of optoelectronic tweezers technology, and demonstrates that optoelectronic tweezers technology is a versatile and powerful toolbox for nano-/micro-manipulation.

Opto-thermophoretic trapping of micro and nanoparticles with a 2 µm Tm-doped fiber laser

We propose a method for opto-thermophoretic trapping with a 2 µm Tm-doped fiber laser. The infrared continuous-wave laser beam is directly and strongly absorbed by water solution, and some local temperature gradient is generated around a focus. The particles are migrated along the temperature gradient, and form a hexagonal close-packed structure at a bottom-glass solution interface. On the other hand, the particles are not trapped in heavy water which does not absorb 2 µm light. The fact indicates that the local temperature elevation is the origin of this phenomenon. We have investigated the dependence of the phenomenon on the material, particle size, and laser power. To the best of our knowledge, 2 µm is the longest wavelength used for the opto-thermophoretic trapping.

Nanopipette-Based Nanosensor for Label-Free Electrochemical Monitoring of Cell Membrane Rupture under H2O2 Treatment

H₂O₂ is an essential signaling molecule in living cells that can cause direct damage to lipids, proteins, and DNA, resulting in cell membrane rupture. However, current studies mostly focus on probe-based sensing of intracellular H₂O₂, and these methods usually require sophisticated probe synthesis and instruments. In particular, local H₂O₂ treatment induces cell membrane rupture, but the level of cell membrane destruction is unknown because the mechanical properties of the cell membrane are difficult to accurately determine. Therefore, highly sensitive and label-free methods are required to measure and reflect mechanical changes in the cell membrane. Here, using an ultrasmall quartz nanopipette with a tip diameter less than 90 nm as a nanosensor, label-free and noninvasive electrochemical single-cell measurement is achieved for real-time monitoring of cell membrane rupture under H₂O₂ treatment. By spatially controlling the nanopipette tip to precisely approach a specific location on the membrane of a single living cell, stable cyclic membrane oscillations are observed under a constant direct current voltage. Specifically, upon nanopipette advancement, the mechanical status of the cell membrane can be sensibly displayed by continuous current versus time traces. The electrical signals are collected and processed, ultimately revealing the mechanical properties of the cell membrane and the degree of cell apoptosis. This nanopipette-based nanosensor paves the way for developing a facile, label-free, and noninvasive strategy to assay the mechanical properties of the cell membrane during external stimulation at the single-cell level.

Integrated Assembly and Photopreservation of Topographical Micropatterns

Micromanipulation techniques that are capable of assembling nano/micromaterials into usable structures such as topographical micropatterns (TMPs) have proliferated rapidly in recent years, holding great promise in building artificial electronic and photonic microstructures. Here, a method is reported for forming TMPs based on optoelectronic tweezers in either "bottom-up" or "top-down" modes, combined with in situ photopolymerization to form permanent structures. This work demonstrates that the assembled/cured TMPs can be harvested and transferred to alternate substrates, and illustrates that how permanent conductive traces and capacitive circuits can be formed, paving the way toward applications in microelectronics. The integrated, optical assembly/preservation method described here is accessible, versatile, and applicable for a wide range of materials and structures, suggesting utility for myriad microassembly and microfabrication applications in the future.

Plasmonic Nanotweezers and Nanosensors for Point-of-Care Applications

The capabilities of manipulating and analyzing biological cells, bacteria, viruses, DNAs, and proteins at high resolution are significant in understanding biology and enabling early disease diagnosis. We discuss progress in developments and applications of plasmonic nanotweezers and nanosensors where the plasmon-enhanced light-matter interactions at the nanoscale improve the optical manipulation and analysis of biological objects. Selected examples are presented to illustrate their design and working principles. In the context of plasmofluidics, which merges plasmonics and fluidics, the integration of plasmonic nanotweezers and nanosensors with microfluidic systems for point-of-care (POC) applications is envisioned. We provide our perspectives on the challenges and opportunities in further developing and applying the plasmofluidic POC devices.

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. The combined action of far-field convection and a near-field trapping force (referred to as thermophoresis)-induced by highly localized plasmonic heating-enabled the 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.

Optothermally Assembled Nanostructures

Nanofabrication is one of the core techniques in rapidly evolving nanoscience and nanotechnology. Conventional top-down nanofabrication approaches such as photolithography and electron beam lithography can produce high-resolution nanostructures in a robust way. However, these methods usually involve multistep processing and sophisticated instruments and have difficulty in fabricating three-dimensional complex structures of multiple materials and reconfigurability. Recently, bottom-up techniques have emerged as promising alternatives to fabricating nanostructures via the assembly of individual building blocks. In comparison to top-down lithographical methods, bottom-up assembly features the on-demand construction of superstructures with controllable configurations at single-particle resolution. The size, shape, and composition of chemically synthesized building blocks can also be precisely tailored down to the atomic scale to fabricate multimaterial architectural structures of high flexibility. Many techniques have been reported to assemble individual nanoparticles into complex structures, such as self-assembly, DNA nanotechnology, patchy colloids, and optically controlled assembly. Among them, the optically controlled assembly has the advantages of remote control, site-specific manipulation of single components, applicability to a wide range of building blocks, and arbitrary configurations of the assembled structures. In this Account, we provide a concise review of our contributions to the optical assembly of architectural materials and structures using discrete nanoparticles as the building blocks. By exploiting entropically favorable optothermal conversion and controlling optothermal-matter interactions, we have developed optothermal assembly techniques to manipulate and assemble individual nanoparticles. Our techniques can be operated both in solution and on solid substrates. First, we discuss the opto-thermoelectric assembly (OTA) of colloidal particles into superstructures by coordinating thermophoresis and interparticle depletion bonding in the solution. Localized laser heating generates a temperature gradient field, where the thermal migration of ions creates a thermoelectric field to trap charged particles. The depletion of ion species at the gap between closely positioned particles under optical heating provides strong interparticle bonding to stabilize colloidal superstructures with precisely controlled configurations and interparticle distances. Second, we discuss bubble-pen lithography (BPL) for the rapid printing of nanoparticles using an optothermal microbubble. The long-range convection flow induced by the optothermal bubble drags the colloidal particles to the substrate with a high velocity. BPL represents a general method for printing all kinds of building blocks into desired patterns in a high-resolution and high-throughput way. Third, we present the optothermally-gated photon nudging (OPN) technique, which manipulates and assembles particles on a solid substrate. Our solid-phase optical control of particles synergizes the modulation of particle-substrate interactions by optothermal effects and photon nudging of the particles by optical scattering forces. Operated on the solid surfaces without liquid media, OPN can avoid the undesired Brownian motion of nanoparticles in solutions to manipulate individual particles with high accuracy. In addition, the assembled structures can be actively reassembled into new configurations for the fabrication of tunable functional devices. Next, we discuss applications of the optothermally assembled nanostructures in surface-enhanced Raman spectroscopy, color displays, biomolecule sensing, and fundamental research. Finally, we conclude this Account with our perspectives on the challenges, opportunities, and future directions in the development and application of optothermal assembly.

Opto-Thermophoretic Manipulation

The optical manipulation of tiny objects is significant to understand and to explore the unknown in the microworld, which has found many applications in materials science and life science. Physically speaking, these technologies arise from direct or indirect optomechanical coupling to convert incident optical energy to mechanical energy of target objects, while their efficiency and functionalities are determined by the coupling behavior. Traditional optical tweezers stem from direct light-to-matter momentum transfer, and the generation of an optical gradient force requires high optical power and rigorous optics. As a comparison, the opto-thermophoretic manipulation techniques proposed recently originate from high-efficiency opto-thermomechanical coupling and feature low optical power. Through rational design of the light-generated temperature gradient and exploring the mechanical response of diverse targets to the temperature gradient, a variety of opto-thermophoretic techniques were developed, which exhibit broad applicability to a wide range of target objects from colloid materials to biological cells to biomolecules. In this review, we will discuss the underlying mechanism of thermophoresis in different liquid environments, the cutting-edge technological innovation, and their applications in colloidal science and life science. We also provide a brief outlook on the existing challenges and anticipate their future development.

Damage-free light-induced assembly of intestinal bacteria with a bubble-mimetic substrate

Rapid evaluation of functions in densely assembled bacteria is a crucial issue in the efficient study of symbiotic mechanisms. If the interaction between many living microbes can be controlled and accelerated via remote assembly, a cultivation process requiring a few days can be ommitted, thus leading to a reduction in the time needed to analyze the bacterial functions. Here, we show the rapid, damage-free, and extremely dense light-induced assembly of microbes over a submillimeter area with the "bubble-mimetic substrate (BMS)". In particular, we successfully assembled 104-105 cells of lactic acid bacteria (Lactobacillus casei), achieving a survival rate higher than 95% within a few minutes without cultivation process. This type of light-induced assembly on substrates like BMS, with the maintenance of the inherent functions of various biological samples, can pave the way for the development of innovative methods for rapid and highly efficient analysis of functions in a variety of microbes.

Plasmonic tweezers: for nanoscale optical trapping and beyond

Optical tweezers and associated manipulation tools in the far field have had a major impact on scientific and engineering research by offering precise manipulation of small objects. More recently, the possibility of performing manipulation with surface plasmons has opened opportunities not feasible with conventional far-field optical methods. The use of surface plasmon techniques enables excitation of hotspots much smaller than the free-space wavelength; with this confinement, the plasmonic field facilitates trapping of various nanostructures and materials with higher precision. The successful manipulation of small particles has fostered numerous and expanding applications. In this paper, we review the principles of and developments in plasmonic tweezers techniques, including both nanostructure-assisted platforms and structureless systems. Construction methods and evaluation criteria of the techniques are presented, aiming to provide a guide for the design and optimization of the systems. The most common novel applications of plasmonic tweezers, namely, sorting and transport, sensing and imaging, and especially those in a biological context, are critically discussed. Finally, we consider the future of the development and new potential applications of this technique and discuss prospects for its impact on science.

Liquid Optothermoelectrics: Fundamentals and Applications

Liquid thermoelectricity describes the redistribution of ions in an electrolytic solution under the influence of temperature gradients, which leads to the formation of electric fields. The thermoelectric field is effective in driving the thermophoretic migration of charged colloidal particles for versatile manipulation. However, traditional macroscopic thermoelectric fields are not suitable for particle manipulations at high spatial resolution. Inspired by optical tweezers and relevant optical manipulation techniques, we employ laser interaction with light-absorbing nanostructures to achieve subtle heat management on the micro- and nanoscales. The resulting thermoelectric fields are exploited to develop new optical technologies, leading to a research field known as liquid optothermoelectrics. This Invited Feature Article highlights our recent works on advancing fundamentals, technologies, and applications of optothermoelectrics in colloidal solutions. The effects of light irradiation, substrates, electrolytes, and particles on the optothermoelectric manipulations of colloidal particles along with their theoretical limitations are discussed in detail. Our optothermoelectric technologies with the versatile capabilities of trapping, manipulating, and pulling colloidal particles at low optical power are finding applications in microswimmers and nanoscience. With its intricate interfacial processes and tremendous technological promise, optothermoelectrics in colloidal solutions will remain relevant for the foreseeable future.

The formation principle of micro-droplets induced by using optical tweezers

Utilizing droplets as micro-tools has become a valuable method in biology and chemistry. In previous work, we have demonstrated a novel droplet generation-manipulation method in a conventional optical tweezer system. Here, a further study of the droplet composition and its formation principle is performed. First, it is proved through Raman spectra that the principal component of the droplets is HPO₄ ^2- solution. Considering that the generated droplet size is at the μm level, we have adopted a variety of methods in experiments to reduce external interference. Second, using a confocal microscopic video camera, the growth process of the droplet is completely recorded in a common glass-based chamber. The finite element simulations help us to further understand that the droplet generation process using optical tweezers can be divided into two stages: "capture" caused by optical force field and "aggregation" induced by a photothermal phenomenon and thermal acceleration. Through these studies, the nature of the optical tweezer-generated droplets is revealed. As a general principle for the droplet generation, this method will provide inspiration and prospects in the fields of microfluidics and biophysics-chemistry.

Atomistic modeling and rational design of optothermal tweezers for targeted applications

Optical manipulation of micro/nanoscale objects is of importance in life sciences, colloidal science, and nanotechnology. Optothermal tweezers exhibit superior manipulation capability at low optical intensity. However, our implicit understanding of the working mechanism has limited the further applications and innovations of optothermal tweezers. Herein, we present an atomistic view of opto-thermo-electro-mechanic coupling in optothermal tweezers, which enables us to rationally design the tweezers for optimum performance in targeted applications. Specifically, we have revealed that the non-uniform temperature distribution induces water polarization and charge separation, which creates the thermoelectric field dominating the optothermal trapping. We further design experiments to systematically verify our atomistic simulations. Guided by our new model, we develop new types of optothermal tweezers of high performance using low-concentrated electrolytes. Moreover, we demonstrate the use of new tweezers in opto-thermophoretic separation of colloidal particles of the same size based on the difference in their surface charge, which has been challenging for conventional optical tweezers. With the atomistic understanding that enables the performance optimization and function expansion, optothermal tweezers will further their impacts.

Gold cauldrons as efficient candidates for plasmonic tweezers

In this report, gold cauldrons are proposed and proved as efficient candidates for plasmonic tweezers. Gold cauldrons benefit from high field localization in the vicinity of their apertures, leading to particle trapping by a reasonably low power source. The plasmonic trapping capability of a single gold cauldron and a cauldrons cluster are studied by investigating the plasmon-induced variations of the optical trap stiffness in a conventional optical tweezers configuration. This study shows that the localized plasmonic fields and the consequent plasmonic forces lead to enhanced trap stiffness in the vicinity of the cauldrons. This observation is pronounced for the cauldrons cluster, due to the additive plasmonic fields of the neighboring cauldrons. Strong direct plasmonic tweezing by the gold cauldrons cluster is also investigated and confirmed by our simulations and experimental results. In addition to the presented plasmonic trapping behavior, gold cauldrons benefit from a low cost and simple fabrication process with acceptable controllability over the structural average dimensions and plasmonic behavior, making them attractive for emerging lab-on-a-chip optophoresis applications.

Opto-Thermoelectric Tweezers: Principles and Applications

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.

Next-Generation Optical Nanotweezers for Dynamic Manipulation: From Surface to Bulk

Optical traps based on strongly confined electromagnetic fields at metal-dielectric interfaces are far more efficient than conventional optical tweezers. Specifically, these near-field nanotweezers allow the trapping of smaller particles at lower optical intensities, which can impact diverse research fields ranging from soft condensed matter physics to materials science and biology. A major thrust in the past decade has been focused on extending the capabilities of plasmonically enhanced nanotweezers beyond diffusion-limited trapping on surfaces such as to achieve dynamic control in the bulk of fluidic environments. Here, we review the recent efforts in optical nanotweezers, especially those involving hybrid forcing schemes, covering both surface and bulk-based techniques. We summarize the important capabilities demonstrated with this promising approach, with niche applications in reconfigurable nanopatterning and on-chip assembly as well as in sorting and separating colloidal nanoparticles.

Large-scale optothermal assembly of colloids mediated by a gold microplate

Light-activated colloidal assembly and swarming can act as model systems to explore non-equilibrium state of matter. In this context, creating new experimental platforms to facilitate and control two-dimensional assembly of colloidal crystals are of contemporary interest. In this paper, we present an experimental study of assembly of colloidal silica microparticles in the vicinity of a single-crystalline gold microplate evanescently excited by a 532 nm laser beam. The gold microplate acts as a source of heat and establishes a thermal gradient in the system. The created optothermal potential assembles colloids to form a two-dimensional poly-crystal, and we quantify the coordination number and hexagonal packing order of the assembly in such a driven system. Our experimental investigation shows that for a given particle size, the variation in assembly can be tuned as a function of excitation-polarization and surface to volume ratio of the gold microplates. Furthermore, we observe that the assembly is dependent on size of the particle and its material composition. Specifically, silica colloids assemble but polystyrene colloids do not, indicating an intricate behaviour of the forces under play. Our work highlights a promising direction in utilizing metallic microstructures that can be harnessed for optothermal colloidal crystal assembly and swarming studies. Our experimental system can be utilized to explore optically driven matter and photophoretic interactions in soft-matter including biological systems such as cells and micro organisms.

Two-state switchable plasmonic tweezers for dynamic manipulation of nano-objects

In this work, we present a plasmonic platform capable of trapping nano-objects in two different spatial configurations. The switch between the two trapping states, localized on the tip and on the outer wall of a vertical gold nanochannel, can be activated by varying the focusing position of the excitation laser along the main axis of the nanotube. We show that the switching of the trapping site is induced by changes in the distribution of the electromagnetic field and of the trapping force. The "inner" and "outer" trapping states are characterized by a static and a dynamic behavior respectively, and their stiffness is measured by analyzing the positions of the trapped specimens as a function of time. In addition, we demonstrate that the stiffness of the static state is high enough to trap particles with diameter as small as 40 nm. These results show a simple, controllable way to generate a switchable two-state trapping regime, which could be used as a model for the study of dynamic trapping or as a mechanism for the development of nanofluidic devices.

Opto-thermoelectric speckle tweezers

Opto-thermoelectric tweezers present a new paradigm for optical trapping and manipulation of particles using low-power and simple optics. New real-life applications of opto-thermoelectric tweezers in areas such as biophysics, microfluidics, and nanomanufacturing will require them to have large-scale and high-throughput manipulation capabilities in complex environments. Here, we present opto-thermoelectric speckle tweezers, which use speckle field consisting of many randomly distributed thermal hotspots that arise from an optical speckle pattern to trap multiple particles over large areas. By further integrating the speckle tweezers with a microfluidic system, we experimentally demonstrate their application for size-based nanoparticle filtration. With their low-power operation, simplicity, and versatility, opto-thermoelectric speckle tweezers will broaden the applications of optical manipulation techniques.