Long-range optofluidic control with plasmon heating

Leveraging dielectrophoresis in inertial flow for versatile manipulation of micro and nanoparticles

The manipulation of micro and nanoparticles has extensive applications in biomedical research, clinical diagnostics, environmental monitoring, drug discovery, and the mining industry. Dielectrophoresis (DEP) utilises nonuniform electric fields to manipulate particles, offering a label-free, high-precision, and non-invasive method for both natural and synthetic particles. DEP manipulation has been well studied in the Stokes flow region with ultra-low Reynolds numbers (Re ≪ 1), where viscous effects dominate. However, its application in the inertial flow regime remains largely unexplored. This study aims to bridge the gap by coupling DEP and inertial flow for the manipulation of particles across micro and nano scales. First, we theoretically analysed the physical coupling of DEP and inertial lift forces along the vertical direction in microchannels, utilising symmetrical interdigitated electrode (IDE) arrays patterned on the top and bottom channel surfaces. We then experimentally investigated how the vertical coupling of DEP and inertial lift forces affects particle lateral focusing properties. The effects of DEP along the vertical direction were leveraged and amplified by the inertial effects along the lateral direction. Finally, we applied DEP in the inertial flow regime for size-based and dielectric property-based separation of particles and cells, as well as nanoparticle focusing and filtration. We believe that leveraging DEP in inertial flow will advance the field by providing a versatile and powerful method for the manipulation of micro and nanoparticles.

Synergizing microfluidics and plasmonics: advances, applications, and future directions

In the past decade, interest in nanoplasmonic structures has experienced significant growth, owing to rapid advancements in materials science and the evolution of novel nanofabrication techniques. The activities in the area are not only leading to remarkable progress in specific applications in photonics, but also permeating to and synergizing with other fields. This review delves into the symbiosis between nanoplasmonics and microfluidics, elucidating fundamental principles on nanophotonics centered on surface plasmon-polaritons, and key achievements arising from the intricate interplay between light and fluids at small scales. This review underscores the unparalleled capabilities of subwavelength plasmonic structures to manipulate light beyond the diffraction limit, concurrently serving as fluidic entities or synergistically combining with micro- and nanofluidic structures. Noteworthy phenomena, techniques and applications arising from this synergy are explored, including the manipulation of fluids at nanoscopic dimensions, the trapping of individual nanoscopic entities like molecules or nanoparticles, and the harnessing of light within a fluidic environment. Additionally, it discusses light-driven fabrication methodologies for microfluidic platforms and, contrariwise, the use of microfluidics in the fabrication of plasmonic nanostructures. Pondering future prospects, this review offers insights into potential future developments, particularly focusing on the integration of two-dimensional materials endowed with exceptional optical, structural and electrical properties, such as goldene and borophene, which enable higher carrier densities and higher plasmonic frequencies. Such advancements could catalyze innovations in diverse applications, including energy harvesting, advanced photothermal cancer therapies, and catalytic processes for hydrogen generation and CO2 conversion.

Artificial thermal flow control on thermoelectric device by tuning electrode absorptivity

Thermoelectric conversion is a direct conversion of thermal energy to electricity, triggered by the Seebeck effect. Typically, the configuration of a thermoelectric device and the absorptivity of both electrodes exhibit symmetrical optical characteristics between the hot and cold ends, and these factors usually are not expected to affect the direction of the thermal gradient. Here, we first demonstrate the ability to reverse the direction of thermal flow across a thermoelectric element by adjusting the absorptivity of electrodes at both ends in an environment with uniform thermal radiation. For example, when the metamaterial or fullerene electrodes were attached to one end a p-type thermoelectric element, they generated output voltages of 19.0 and -4.0 V, respectively, in an environment with uniform thermal radiation at 364 K. Using this insight, we demonstrated power generation on a π -shaped thermoelectric device consisting only of p-type thermoelectric legs by designing the absorptivity of the electrode at each end. Our findings will provide valuable insights as a device guideline for conventional thermoelectric devices.

Recent Advancements in Nanophotonics for Optofluidics

Optofluidics is dedicated to achieving integrated control of particle and fluid motion, particularly on the micrometer scale, by utilizing light to direct fluid flow and particle motion. The field has seen significant growth recently, driven by the concerted efforts of researchers across various scientific disciplines, notably for its successful applications in biomedical science. In this review, we explore a range of optofluidic architectures developed over the past decade, with a primary focus on mechanisms for precise control of micro and nanoscale biological objects and their applications in sensing. Regarding nanoparticle manipulation, we delve into mechanisms based on optical nanotweezers using nanolocalized light fields and light-based hybrid effects with dramatically improved performance and capabilities. In the context of sensing, we emphasize those works that used optofluidics to aggregate molecules or particles to promote sensing and detection. Additionally, we highlight emerging research directions, encompassing both fundamental principles and practical applications in the field.

Metasurface absorber enhanced thermoelectric conversion

Metasurfaces are artificial thin materials that achieve optical thickness through thin geometrical structure. This feature of metasurfaces results in unprecedented benefits for enhancing the performance of optoelectronic devices. In this study, we report that this metasurface feature is also essential to drive photo-thermoelectric conversion, which requires the accumulation of thermal energy and effective heat conduction. For example, a metasurface-attached thermoelectric device placed in an environment with uniform thermal radiation generates an output voltage by gathering the thermal energies existing in the environment and creating an additional thermal gradient across the thermoelectric element. In contrast, when a 100-μm-thick-carbon-black-coated electrode was used instead of the metasurface, the device showed lower thermoelectric performance than that of the metasurface-attached device although carbon black exhibits higher infrared absorption than the metasurface. These results indicate that metasurface characteristics of optical thickness and thin geometrical structure for achieving the high thermal conductance are essential in enhancing the performance of photo-thermoelectric devices in terms of the effective collection of thermal energies and conduction of local heating.

Thermometries for Single Nanoparticles Heated with Light

The development of efficient nanoscale photon absorbers, such as plasmonic or high-index dielectric nanostructures, allows the remotely controlled release of heat on the nanoscale using light. These photothermal nanomaterials have found applications in various research and technological fields, ranging from materials science to biology. However, measuring the nanoscale thermal fields remains an open challenge, hindering full comprehension and control of nanoscale photothermal phenomena. Here, we review and discuss existent thermometries suitable for single nanoparticles heated under illumination. These methods are classified in four categories according to the region where they assess temperature: (1) the average temperature within a diffraction-limited volume, (2) the average temperature at the immediate vicinity of the nanoparticle surface, (3) the temperature of the nanoparticle itself, and (4) a map of the temperature around the nanoparticle with nanoscale spatial resolution. In the latter, because it is the most challenging and informative type of method, we also envisage new combinations of technologies that could be helpful in retrieving nanoscale temperature maps. Finally, we analyze and provide examples of strategies to validate the results obtained using different thermometry methods.

Non-steady state thermometry with optical diffraction tomography

Label-free thermometry is a pivotal tool for many disciplines. However, most current approaches are only suitable for planar heat sources in steady state, thereby restricting the range of systems that can be reliably studied. Here, we introduce pump probe-based optical diffraction tomography (ODT) as a method to map temperature precisely and accurately in three dimensions (3D) at the single-particle level. To do so, we first systematically characterize the thermal landscape in a model system consisting of gold nanorods in a microchamber and then benchmark the results against simulations and quantitative phase imaging thermometry. We then apply ODT thermometry to resolve thermal landscapes inaccessible to other label-free approaches in the form of nonplanar heat sources embedded in complex environments and freely diffusing gold nanorods in a microchamber. Last, we foresee that our approach will find many applications where routine thermal characterization of heterogeneous nanoparticles samples in 3D or in non-steady state is required.

Manipulation of Convection Using Infrared Light Emitted from Human Hands

Control of convection plays an important role in heat transfer regulation, bio/chemical sensing, phase separation, etc. Current convection controlling systems generally depend on engineered energy sources to drive and manipulate the convection, which brings additional energy consumption into the system. Here the use of human hand as a natural and sustainable infrared (IR) radiation source for the manipulation of liquid convection is demonstrated. The fluid can sense the change of the relative position or the shape of the hand with the formation of different convection patterns. Besides the generation of static complex patterns, dynamic manipulation of convections can also be realized via moving of hand or finger. The use of such sustainable convections to control the movement of a floating "boat" is further achieved. The use of human hands as the natural energy sources provides a promising approach for the manipulation of liquid convection without the need of extra external energy, which may be further utilized for low-cost and intelligent bio/chemical sensing and separation.

Light-to-Heat Conversion of Optically Trapped Hot Brownian Particles

Anisotropic hybrid nanostructures stand out as promising therapeutic agents in photothermal conversion-based treatments. Accordingly, understanding local heat generation mediated by light-to-heat conversion of absorbing multicomponent nanoparticles at the single-particle level has forthwith become a subject of broad and current interest. Nonetheless, evaluating reliable temperature profiles around a single trapped nanoparticle is challenging from all of the experimental, computational, and fundamental viewpoints. Committed to filling this gap, the heat generation of an anisotropic hybrid nanostructure is explored by means of two different experimental approaches from which the local temperature is measured in a direct or indirect way, all in the context of hot Brownian motion theory. The results were compared with analytical results supported by the numerical computation of the wavelength-dependent absorption efficiencies in the discrete dipole approximation for scattering calculations, which has been extended to inhomogeneous nanostructures. Overall, we provide a consistent and comprehensive view of the heat generation in optical traps of highly absorbing particles from the viewpoint of the hot Brownian motion theory.

Quantitative phase imaging for characterization of single cell growth dynamics

Quantitative phase imaging (QPI) has emerged as an indispensable tool in the field of biomedicine, offering the ability to obtain quantitative maps of phase changes due to optical path length delays without the need for contrast agents. These maps provide valuable information about cellular morphology and dynamics, unperturbed by the introduction of exogenous substances. In this review, a summary of recent studies that have focused on elucidating the growth dynamics of individual cells using QPI is presented. Specifically, investigations into cellular changes occurring during mitosis, the differentiation of cellular organelles, the assessment of distinct cell death processes (i.e., apoptosis, necrosis, and oncosis) and the precise measurement of live cell temperature are explored. Furthermore, the captivating applications of QPI in theragnostics, where its potential for transformative impact is prominently showcased, are highlighted. Finally, the challenges that need to be overcome for its wider adoption and successful integration into biomedical research are outlined.

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.

Electrothermoplasmonic flow in gold nanoparticles suspensions: Nonlinear dependence of flow velocity on aggregate concentration

Efficient mixing and pumping of liquids at the microscale is a technology that is still to be optimized. The combination of an AC electric field with a small temperature gradient leads to a strong electrothermal flow that can be used for multiple purposes. Combining simulations and experiments, an analysis of the performance of electrothermal flow is provided when the temperature gradient is generated by illuminating plasmonic nanoparticles in suspension with a near-resonance laser. Fluid flow is measured by tracking the velocity of fluorescent tracer microparticles in suspension as a function of the electric field, laser power, and concentration of plasmonic particles. Among other results, a non-linear relationship is found between the velocity of the fluid and particle concentration, which is justified in terms of multiple scattering-absorption events, involving aggregates of nanoparticles, that lead to enhanced absorption when the concentration is raised. Simulations provide a description of the phenomenon that is compatible with experiments and constitute a way to understand and estimate the absorption and scattering cross-sections of both dispersed particles and/or aggregates. A comparison of experiments and simulations suggests that there is some aggregation of the gold nanoparticles by forming clusters of about 2-7 particles, but no information about their structure can be obtained without further theoretical and experimental developments. This nonlinear behavior could be useful to get very high ETP velocities by inducing some controlled aggregation of the particles.

A Review on Self-Assembly of Colloidal Nanoparticles into Clusters, Patterns, and Films: Emerging Synthesis Techniques and Applications

The colloidal synthesis of functional nanoparticles has gained tremendous scientific attention in the last decades. In parallel to these advancements, another rapidly growing area is the self-assembly or self-organization of these colloidal nanoparticles. First, the organization of nanoparticles into ordered structures is important for obtaining functional interfaces that extend or even amplify the intrinsic properties of the constituting nanoparticles at a larger scale. The synthesis of large-scale interfaces using complex or intricately designed nanostructures as building blocks, requires highly controllable self-assembly techniques down to the nanoscale. In certain cases, for example, when dealing with plasmonic nanoparticles, the assembly of the nanoparticles further enhances their properties by coupling phenomena. In other cases, the process of self-assembly itself is useful in the final application such as in sensing and drug delivery, amongst others. In view of the growing importance of this field, this review provides a comprehensive overview of the recent developments in the field of nanoparticle self-assembly and their applications. For clarity, the self-assembled nanostructures are classified into two broad categories: finite clusters/patterns, and infinite films. Different state-of-the-art techniques to obtain these nanostructures are discussed in detail, before discussing the applications where the self-assembly significantly enhances the performance of the process.

The role of temperature-induced effects generated by plasmonic nanostructures on particle delivery and manipulation: a review

Plasmonic optical tweezers that stem from the need to trap and manipulate ever smaller particles using non-invasive optical forces, have made significant contributions to precise particle motion control at the nanoscale. In addition to the optical forces, other effects have been explored for particle manipulation. For instance, the plasmonic heat delivery mechanism generates micro- and nanoscale optothermal hydrodynamic effects, such as natural fluid convection, Marangoni fluid convection and thermophoretic effects that influence the motion of a wide range of particles from dielectric to biomolecules. In this review, a discussion of optothermal effects generated by heated plasmonic nanostructures is presented with a specific focus on applications to optical trapping and particle manipulation. It provides a discussion on the existing challenges of optothermal mechanisms generated by plasmonic optical tweezers and comments on their future opportunities in life sciences.

A versatile interferometric technique for probing the thermophysical properties of complex fluids

Laser-induced thermocapillary deformation of liquid surfaces has emerged as a promising tool to precisely characterize the thermophysical properties of pure fluids. However, challenges arise for nanofluid (NF) and soft bio-fluid systems where the direct interaction of the laser generates an intriguing interplay between heating, momentum, and scattering forces which can even damage soft biofluids. Here, we report a versatile, pump-probe-based, rapid, and non-contact interferometric technique that resolves interface dynamics of complex fluids with the precision of ~1 nm in thick-film and 150 pm in thin-film regimes below the thermal limit without the use of lock-in or modulated beams. We characterize the thermophysical properties of complex NF in three exclusively different types of configurations. First, when the NF is heated from the bottom through an opaque substrate, we demonstrate that our methodology permits the measurement of thermophysical properties (viscosity, surface tension, and diffusivity) of complex NF and biofluids. Second, in a top illumination configuration, we show a precise characterization of NF by quantitively isolating the competing forces, taking advantage of the different time scales of these forces. Third, we show the measurement of NF confined in a metal cavity, in which the transient thermoelastic deformation of the metal surface provides the properties of the NF as well as thermo-mechanical properties of the metal. Our results reveal how the dissipative nature of the heatwave allows us to investigate thick-film dynamics in the thin-film regime, thereby suggesting a general approach for precision measurements of complex NFs, biofluids, and optofluidic devices.

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.

Hydrodynamic manipulation of nano-objects by optically induced thermo-osmotic flows

Manipulation of nano-objects at the microscale is of great technological importance for constructing new functional materials, manipulating tiny amounts of fluids, reconfiguring sensor systems, or detecting tiny concentrations of analytes in medical screening. Here, we show that hydrodynamic boundary flows enable the trapping and manipulation of nano-objects near surfaces. We trigger thermo-osmotic flows by modulating the van der Waals and double layer interactions at a gold-liquid interface with optically generated local temperature fields. The hydrodynamic flows, attractive van der Waals and repulsive double layer forces acting on the suspended nanoparticles enable precise nanoparticle positioning and guidance. A rapid multiplexing of flow fields permits the parallel manipulation of many nano-objects and the generation of complex flow fields. Our findings have direct implications for the field of plasmonic nanotweezers and other thermo-plasmonic trapping systems, paving the way for nanoscopic manipulation with boundary flows.

Laser-induced convection shifts size distributions in nanoparticle tracking analysis

This work discusses the effects of increasing laser power on the size data derived from NTA for particles of known size and scatterers in solutions of flufenamic acid in ethanol. We find that whilst a higher laser power reveals more particles as expected, their residence time changes due to laser-induced convection. This reduced residence time decreases the number of tracks available for individual particle size determination, shifting the size distribution to smaller values. This problem is overcome by using a shutter to inhibit the development of convection currents, increasing particle residence time and reducing the error on the size distribution. The detailed understanding of laser-induced convection permits more robust size characterisation of mesoscopic organic clusters, which play a key role in two-step nucleation theory.

Switching between laser-induced thermophoresis and thermal convection of liquid suspension in a microgap with variable dimension

Phoretic motion of particles along a temperature gradient formed in a fluid, known as thermophoresis, often takes place under the influence of bulk motion caused by thermal convection. In this paper, using a laser heating method, the significance of two competing effects, that is, thermophoresis and thermal convection, for the particle transport in a liquid phase confined in a microgap is investigated experimentally by changing the gap size as a control parameter. It is found that there is a threshold of the gap size, above which the particles tend to accumulate around the heated spot, forming a ring-like particle distribution. On the contrary, if the gap size is below the threshold, the particles are depleted from the heated spot. Switching between these accumulation and depletion modes is expected to develop novel manipulation techniques.