Optothermal molecule trapping by opposing fluid flow with thermophoretic drift

Mts1 (S100A4) and Its Peptide Demonstrate Cytotoxic Activity in Complex with Tag7 (PGLYRP1) Peptide

Receptors of cytokines are major regulators of the immune response. In this work, we have discovered two new ligands that can activate the TNFR1 (tumor necrosis factor receptor 1) receptor. Earlier, we found that the peptide of the Tag (PGLYRP1) protein designated 17.1 can interact with the TNFR1 receptor. Here, we have found that the Mts1 (S100A4) protein interacts with this peptide with a high affinity (Kd = 1.28 × 10-8 M), and that this complex is cytotoxic to cancer cells that have the TNFR1 receptor on their surface. This complex induces both apoptosis and necroptosis in cancer cells with the involvement of mitochondria and lysosomes in cell death signal transduction. Moreover, we have succeeded in locating the Mts1 fragment that is responsible for protein-peptide interaction, which highly specifically interacts with the Tag7 protein (Kd = 2.96 nM). The isolated Mts1 peptide M7 also forms a complex with 17.1, and this peptide-peptide complex also induces the TNFR1 receptor-dependent cell death. Molecular docking and molecular dynamics experiments show the amino acids involved in peptide binding and that may be used for peptidomimetics' development. Thus, two new cytotoxic complexes were created that were able to induce the death of tumor cells via the TNFR1 receptor. These results may be used in therapy for both cancer and autoimmune diseases.

Determination of the critical aggregation concentration in water of Gum Arabic functionalized with curcumin oxidation products by micro-scale thermophoresis approach

Gum Arabic underwent enzymatic modification with curcumin oxidation products, prompting self-assembly in water at lower concentrations than native gum Arabic, which was fully soluble. The resulting particles displayed a narrow size distribution, suggestive of a micellization mechanism akin to Critical Micellization Concentration (CMC) in surfactants or Critical Aggregation Concentration (CAC) in polymers. Accurately determining CAC is vital for utilizing polymers in molecule encapsulation, but precise measurement is challenging, requiring multiple techniques. Initially, CAC was probed via turbidity measurements, dynamic light scattering (DLS), and isothermal calorimetric titration (ITC), yielding a range of 0.0015 to 0.01 %. Micro-scale thermophoresis (MST) was then employed for the first time to define CAC more precisely, facilitated by the intrinsic fluorescence of modified gum Arabic. Using MST, CAC was pinpointed at 0.001 % (w/v), a novel approach. Furthermore, MST revealed a low EC50 value of 0.007 % (w/t) for self-assembly, signifying uniformity among GAC sub-units and assembly stability upon dilution.

Opto-thermal manipulation with a 3 µm mid-infrared Er:ZBLAN fiber laser

Water has significantly high absorption around 3 µm wavelength region, originated by its fundamental OH vibrational modes. Here, we successfully demonstrate an opto-thermal manipulation of particles utilizing a 3 µm mid-infrared Er:ZBLAN fiber laser (adjustable from 2700 to 2826 nm) that can efficiently elevate the temperature at a laser focus with a low laser power. The 3 µm laser indeed accelerates the formation of the particle assembly by simply irradiating the laser into water. By altering the laser wavelengths, the assembling speed and size, instantaneous particle velocity, particle distribution, trapping stiffness and temperature elevation are evaluated systematically. We propose that the dynamics of particle assembly can be understood through thermo-osmotic slip flows, taking into account the effects of volume heating within the focal cone and point heating at the focus.

Short Peptides of Innate Immunity Protein Tag7 (PGLYRP1) Selectively Induce Inhibition or Activation of Tumor Cell Death via TNF Receptor

In this study, we have found two peptides of Tag7 (PGLYRP1) protein-17.1A (HRDVQRT) and 17.1B (RSNYVLKG), that have different affinities to the TNFR1 receptor and the Hsp70 protein. Peptide 17.1A is able to inhibit signal transduction through the TNFR1 receptor, and peptide 17.1B can activate this receptor in a complex with Hsp70. Thus, it is possible to modulate the activity of the TNFR1 receptor and further perform its specific inhibition or activation in the treatment of various autoimmune or oncological diseases.

Thermophoretic motion of a charged single colloidal particle

We calculate the thermophoretic drift of a charged single colloidal particle with hydrodynamically slipping surface immersed in an electrolyte solution in response to a small temperature gradient. Here we rely on a linearized hydrodynamic approach for the fluid flow and the motion of the electrolyte ions while keeping the full nonlinearity of the Poisson-Boltzmann equation of the unperturbed system to account for possible large surface charging. The partial differential equations are transformed into a coupled set of ordinary differential equations in linear response. Numerical solutions are elaborated for parameter regimes of small and large Debye shielding and different hydrodynamic boundary conditions encoded in a varying slip length. Our results are in good agreement with predictions from recent theoretical work and successfully describe experimental observations on thermophoresis of DNA. We also compare our numerical results with experimental data on polystyrene beads.

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.

A long-range order in a thermally driven system with temperature-dependent interactions

Aggregation of macro-molecules under an external force is far from being understood. An important driving situation is achieved by temperature difference. Inter-particle interactions in metallic nanoparticles with ligand capping are reported to be sensitive to temperature and the zeta potential of the particles being reduced in the cold region. Such particles form aggregates in the cold region of the system in the presence of temperature difference. Here we study the aggregation of particles in the presence of temperature difference with temperature-dependent interaction parameters using Brownian dynamics simulation. The particle interaction and particle diffusion are considered to be sensitive to the local temperature. We identify a long-range structural order in the cold region of the system using the Avrami equation for crystal growth kinetics. Our observations might be useful in designing ordered structures with macro-molecules under non-equilibrium steady-state conditions.

3D trapping of microbubbles by the Marangoni force

In this Letter, we show 3D steady-state trapping and manipulation of vapor bubbles in liquids employing a low-power continuous-wave laser using the Marangoni effect. Light absorption from photodeposited silver nanoparticles on the distal end of a multi-mode optical fiber is used to produce bubbles of different diameters. The thermal effects produced by either the nanoparticles on the fiber tip or the light bulk absorption modulate the surface tension of the bubble wall and creates both longitudinal and transversal forces just like optical forces, effectively creating a 3D potential well. Using numerical simulations, we obtain expressions for the temperature profiles and present analytical expressions for the Marangoni force. In addition, using an array of three fibers with photodeposited nanoparticles is used to demonstrate the transfer of bubbles from one fiber to another by sequentially switching on and off the lasers.

Microparticle manipulation using laser-induced thermophoresis and thermal convection flow

We demonstrate manipulation of microbeads with diameters from 1.5 to 10 µm and Jurkat cells within a thin fluidic device using the combined effect of thermophoresis and thermal convection. The heat flow is induced by localized absorption of laser light by a cluster of single walled carbon nanotubes, with no requirement for a treated substrate. Characterization of the system shows the speed of particle motion increases with optical power absorption and is also affected by particle size and corresponding particle suspension height within the fluid. Further analysis shows that the thermophoretic mobility (DT) is thermophobic in sign and increases linearly with particle diameter, reaching a value of 8 µm2 s-1 K-1 for a 10 µm polystyrene bead.

Optical trapping in the presence of laser-induced thermal effects

The inclusion of thermal effects in optical manipulation has been explored in diverse experiments, increasing the possibilities for applications in diverse areas. In this Letter, the results of combined optical and thermal manipulation in the vicinity of a highly absorbent hydrogenated amorphous silicon layer, which induces both the generation of convective currents and thermophoresis, are presented. In combination with the optical forces, thermal forces help reduce the optical power required to trap and manipulate micrometric polystyrene beads. Moreover, the inclusion of these effects allows the stacking and manipulation of multiple particles with a single optical trap along with the beam propagation, providing an extra tool for micromanipulation of a variety of samples.

Ancestrally Duplicated Conserved Noncoding Element Suggests Dual Regulatory Roles of HOTAIR in cis and trans

HOTAIR was proposed to regulate either HoxD cluster genes in trans or HoxC cluster genes in cis, a mechanism that remains unclear. We have identified a 32-nucleotide conserved noncoding element (CNE) as HOTAIR ancient sequence that likely originated at the root of vertebrate. The second round of whole-genome duplication resulted in one copy of the CNE within HOTAIR and another copy embedded in noncoding transcript of HOXD11. Paralogous CNEs underwent compensatory mutations, exhibit sequence complementarity with respect to transcripts directionality, and have high affinity in vitro. The HOTAIR CNE resembled a poised enhancer in stem cells and an active enhancer in HOTAIR-expressing cells. HOTAIR expression is positively correlated with HOXC11 in cis and negatively correlated with HOXD11 in trans. We propose a dual modality of HOTAIR regulation where transcription of HOTAIR and its embedded enhancer regulates HOXC11 in cis and sequence complementarity between paralogous CNEs suggests HOXD11 regulation in trans.

Thermophoretic tweezers for single nanoparticle manipulation

We present the trapping and manipulation of a single nano-object in an aqueous medium by optically induced temporally varying temperature gradients. By real-time object tracking and control of the position of the heating laser focus, we can precisely employ thermophoretic drift to oppose the random diffusive motion. As a result, a nano-object is confined in a micrometer-sized trap. Numerical modeling gives a quantitative prediction of the effect. Traps can be dynamically created and relocated, which we demonstrate by the controlled independent manipulation of two nanoparticles.

Growth of Colloidal Nanoplate Liquid Crystals Using Temperature Gradients

Controlling colloidal self-assemblies using external forces is essential to develop modern electro-optical and biomedical devices. Importantly, shape anisotropic colloids can provide optical properties such as birefringence. Here we demonstrate that external temperature gradients can be effective in controlling nematic liquid crystalline (LC) order in suspensions of plate-like colloids also known as nanoplates. Nanoplates, in an isotropic suspension, wherein their orientations are random, could be effectively moved using a temperature gradient environment causing a phase transition to LC nematic phase. Such controllably formed nematic phase featured large nematic monodomains and enabled topologically more stable structures that were evident from the absence of hedgehog-type defects which are typically found in nematics formed spontaneously via nucleation and growth mechanism in a sufficiently high concentration suspension of nanoplates. Due to their high surface area-to-volume ratio and excellent thermophoretic properties, nanoplates can prove to be ideal candidates for transport of biomolecules through temperature varying environments.

Fiber Optofluidic Technology Based on Optical Force and Photothermal Effects

Optofluidics is an exciting new area of study resulting from the fusion of microfluidics and photonics. It broadens the application and extends the functionality of microfluidics and has been extensively investigated in biocontrol, molecular diagnosis, material synthesis, and drug delivery. When light interacts with a microfluidic system, optical force and/or photothermal effects may occur due to the strong interaction between light and liquid. Such opto-physical effects can be used for optical manipulation and sensing due to their unique advantages over conventional microfluidics and photonics, including their simple fabrication process, flexible manipulation capability, compact configuration, and low cost. In this review, we summarize the latest progress in fiber optofluidic (FOF) technology based on optical force and photothermal effects in manipulation and sensing applications. Optical force can be used for optofluidic manipulation and sensing in two categories: stable single optical traps and stable combined optical traps. The photothermal effect can be applied to optofluidics based on two major structures: optical microfibers and optical fiber tips. The advantages and disadvantages of each FOF technology are also discussed.

Different measures for characterizing the motion of molecules along a temperature gradient

We study the motion of a Brownian particle in a medium with inhomogeneous temperature. In the overdamped regime of low Reynolds numbers, the probability distribution function (PDF) of the particle is obtained from the van Kampen diffusion equation [J. Phys. Chem. Solids 49, 673 (1988)JPCSAW0022-369710.1016/0022-3697(88)90199-0]. The thermophoretic behavior is commonly described by the Soret coefficient, a parameter that can be calculated from the steady-state PDF. Motivated by recent advances in experimental methods for observing and analyzing single nanoparticle trajectories, we here consider the time-dependent van Kampen equation from which the temporal evolution of the PDF of individual particles can be derived. We analytically calculate the PDF describing dynamics driven by a generalized thermophoretic force. Single-particle statistics are characterized by measures such as the mean displacement (drift) and the probability difference between moving along and against the temperature gradient (bias). We demonstrate that these quantities do not necessarily have the same sign as the Soret coefficient, which causes ambiguity in the distinction between thermophilic and thermophobic response (i.e., migration in and against the direction of the temperature gradient). The different factors determining the thermophoretic response and their influence on each measure are discussed.

Thermophoretic forces on a mesoscopic scale

We use a mesoscale-molecular simulation technique known as Multi-Particle Collision Dynamics (MPC) to study the forces acting on a stationary colloid inside a temperature gradient. We compare our measurements of the thermophoretic force to a theoretical prediction based on Onsager's reciprocal relations, assuming that the temperature gradient is constant across the colloid. We find a good agreement between our measurements and theoretical predictions over a wide range of system parameters, even when the condition of uniform gradients is not strictly fulfilled. Our measurements further suggest that the magnitude of the thermophoretic force depends on the hydrodynamic boundary condition at the colloidal surface, thus highlighting the hydrodynamic character of colloidal thermophoresis. We also investigate how fluid advection disturbs the interfacial layer around the colloid and introduce a dimensionless number to quantify the validity of local thermodynamic equilibrium.

Dynamics of a colloidal particle near a thermoosmotic wall under illumination

The effects of light on colloidal particles in solution are multiple, including transfer of photonic linear/angular momentum and heating of the particles or their surroundings. The temperature increase around colloidal particles due to light heating can drive a thermoosmotic flow along a nearby boundary wall, which significantly influences the motion of the particles. Here we perform mesoscopic dynamics simulations to study two typical scenarios, where thermoosmosis is critical. In the first scenario, we consider a light-absorbing colloidal particle heated by uniform illumination. Depending on the fluid-wall interactions, the thermoosmotic flow generated by the wall can exert a long-ranged hydrodynamic attraction or repulsion on the "hot" Brownian particle, which leads to a strong accumulation/depletion of the particle to/from the boundary. In the second scenario, we investigate the motion of a colloidal particle confined by an optical tweezer in a light-absorbing solution. In this case, thermoosmosis can induce a unidirectional rotation of the trapped particle, whose direction is determined by thermoosmotic properties of the wall. We show that colloidal particles near a thermoosmotic wall exhibit rich dynamics. Our results can be applied for the manipulation of colloidal particles in microfluidics.

A unified description of colloidal thermophoresis

We use the dynamic length and time scale separation in suspensions to formulate a general description of colloidal thermophoresis. Our approach allows an unambiguous definition of separate contributions to the colloidal flux and clarifies the physical mechanisms behind non-equilibrium motion of colloids. In particular, we derive an expression for the interfacial force density that drives single-particle thermophoresis in non-ideal fluids. The issuing relations for the transport coefficients explicitly show that interfacial thermophoresis has a hydrodynamic character that cannot be explained by a purely thermodynamic consideration. Our treatment generalises the results from other existing approaches, giving them a clear interpretation within the framework of non-equilibrium thermodynamics.

Assembly of a functional and responsive microstructure by heat bonding of DNA-grafted colloidal brick

A micromachine constructed to possess various chemical and mechanical functions is one of the ultimate targets of technology. Conventional lithographic processes can be used to form complicated structures. However, they are basically limited to rigid and static structures with poor surface properties. Here, we demonstrate a novel method for assembling responsive and functional microstructures from diverse particles modified with DNA strands. The DNA strands are designed to form hairpins at room temperature and denature when heated. Structures are assembled through the simultaneous manipulation and heating of particles with "hot" optical tweezers, which incorporates the particles one by one. The flexible connection formed by DNA strands allows the responsive deformation of the structures with local controllability of the structural flexibility. We assembled a microscopic robot arm actuated by an external magnet, a hinge structure with a locally controlled connection flexibility and a three-dimensional double helix structure. The method is simple and can also be applied to build complex biological tissues from cells.

Microfluidic Techniques for Analytes Concentration

Microfluidics has been undergoing fast development in the past two decades due to its promising applications in biotechnology, medicine, and chemistry. Towards these applications, enhancing concentration sensitivity and detection resolution are indispensable to meet the detection limits because of the dilute sample concentrations, ultra-small sample volumes and short detection lengths in microfluidic devices. A variety of microfluidic techniques for concentrating analytes have been developed. This article presents an overview of analyte concentration techniques in microfluidics. We focus on discussing the physical mechanism of each concentration technique with its representative advancements and applications. Finally, the article is concluded by highlighting and discussing advantages and disadvantages of the reviewed techniques.

Thermal gradient induced tweezers for the manipulation of particles and cells

Optical tweezers are a well-established tool for manipulating small objects. However, their integration with microfluidic devices often requires an objective lens. More importantly, trapping of non-transparent or optically sensitive targets is particularly challenging for optical tweezers. Here, for the first time, we present a photon-free trapping technique based on electro-thermally induced forces. We demonstrate that thermal-gradient-induced thermophoresis and thermal convection can lead to trapping of polystyrene spheres and live cells. While the subject of thermophoresis, particularly in the micro- and nano-scale, still remains to be fully explored, our experimental results have provided a reasonable explanation for the trapping effect. The so-called thermal tweezers, which can be readily fabricated by femtosecond laser writing, operate with low input power density and are highly versatile in terms of device configuration, thus rendering high potential for integration with microfluidic devices as well as lab-on-a-chip systems.

Thermo-Osmotic Flow in Thin Films

We report on the first microscale observation of the velocity field imposed by a nonuniform heat content along the solid-liquid boundary. We determine both radial and vertical velocity components of this thermo-osmotic flow field by tracking single tracer nanoparticles. The measured flow profiles are compared to an approximate analytical theory and to numerical calculations. From the measured slip velocity we deduce the thermo-osmotic coefficient for both bare glass and Pluronic F-127 covered surfaces. The value for Pluronic F-127 agrees well with Soret data for polyethylene glycol, whereas that for glass differs from literature values and indicates the complex boundary layer thermodynamics of glass-water interfaces.

Light-induced Soret effect and adsorption of nanocrystals in organic solvents

A light-induced Soret effect accompanied by photoinduced adsorption of pigment nanoparticles is described in organic solvents. We report an unexpected inversion of the nanoparticle flux which is directed along the temperarture gradient at short exposures to the light and switches against the gradient at longer exposures. This change of flux direction is due to light-induced adsorption of the nanocrystals onto the substrates of the cell.

Enhancement of binding kinetics on affinity substrates by laser point heating induced transport

Binding of analyte to an affinity substrate is significantly enhanced by laser point heating induced transport.

Trapping and manipulation of microparticles using laser-induced convection currents and photophoresis

In this work we demonstrate optical trapping and manipulation of microparticles suspended in water due to laser-induced convection currents. Convection currents are generated due to laser light absorption in an hydrogenated amorphous silicon (a:Si-H) thin film. The particles are dragged towards the beam's center by the convection currents (Stokes drag force) allowing trapping with powers as low as 0.8 mW. However, for powers >3 mW trapped particles form a ring around the beam due to two competing forces: Stokes drag and thermo-photophoretic forces. Additionally, we show that dynamic beam shaping can be used to trap and manipulate multiple particles by photophotophoresis without the need of lithographically created resistive heaters.

Single Molecules Trapped by Dynamic Inhomogeneous Temperature Fields

We demonstrate a single molecule trapping concept that modulates the actual driving force of Brownian motion--the temperature. By spatially and temporally varying the temperature at a plasmonic nanostructure, thermodiffusive drifts are induced that are used to trap single nano-objects. A feedback controlled switching of local temperature fields allows us to confine the motion of a single DNA molecule for minutes and tailoring complex effective trapping potentials. This new type of thermophoretic microbeaker even provides control over a well-defined number of single molecules and is scalable to large arrays of trapping structures.

Contribution to the benchmark for ternary mixtures: Transient analysis in microgravity conditions

We present a transient experimental analysis of the DCMIX1 project conducted onboard the International Space Station for a ternary tetrahydronaphtalene, isobutylbenzene, n-dodecane mixture. Raw images taken in microgravity environment using the SODI (Selectable Optical Diagnostic) apparatus which is equipped with two wavelength diagnostic were processed and the results were analyzed in this work. We measured the concentration profile of the mixture containing 80% THN, 10% IBB and 10% nC12 during the entire experiment using an advanced image processing technique and accordingly we determined the Soret coefficients using an advanced curve-fitting and post-processing technique. It must be noted that the experiment has been repeated five times to ensure the repeatability of the experiment.

Concentration-dependent thermophoretic accumulation for the detection of DNA using DNA-functionalized nanoparticles

Thermophoresis is a phenomenon about the migration of particles along a temperature gradient and is sensitive to the properties of particles and the surrounding medium. While a few studies have investigated its mechanisms and effects on particle motion in recent years, the applications of thermophoresis in biosensing has not been well explored. In this study, we demonstrate a thermophoresis-based method for detecting DNA. We use DNA-functionalized gold nanoparticles and fluorescent DNA probes to capture target DNA in free solution, and we demonstrate that the hybridization between the specially designed capture probes and the target DNA significantly changes the thermophoretic properties of the fluorescent probes. As a result, the target DNA can be specifically detected in serum-containing buffers based on the spatial distribution of the fluorescent probes in a laser-induced temperature gradient. The optical setup consists of only a laser and an epifluorescence microscope, and the detection does not rely on any micro- or nanofabricated devices. In addition, because the detection is based on the thermophoretic motion of molecules in free solution, no capture probes need to be immobilized on a fixed surface before detection, and no channels or pumps are needed for washing away unbound molecules. The thermophoresis-based biosensing method is found to be simple and effective for detecting DNA.

Modeling thermophoretic effects in solid-state nanopores

Local modulation of temperature has emerged as a new mechanism for regulation of molecular transport through nanopores. Predicting the effect of such modulations on nanopore transport requires simulation protocols capable of reproducing non-uniform temperature gradients observed in experiment. Conventional molecular dynamics (MD) method typically employs a single thermostat for maintaining a uniform distribution of temperature in the entire simulation domain, and, therefore, can not model local temperature variations. In this article, we describe a set of simulation protocols that enable modeling of nanopore systems featuring non-uniform distributions of temperature. First, we describe a method to impose a temperature gradient in all-atom MD simulations based on a boundary-driven non-equilibrium MD protocol. Then, we use this method to study the effect of temperature gradient on the distribution of ions in bulk solution (the thermophoretic effect). We show that DNA nucleotides exhibit differential response to the same temperature gradient. Next, we describe a method to directly compute the effective force of a thermal gradient on a prototypical biomolecule-a fragment of double-stranded DNA. Following that, we demonstrate an all-atom MD protocol for modeling thermophoretic effects in solid-state nanopores. We show that local heating of a nanopore volume can be used to regulate the nanopore ionic current. Finally, we show how continuum calculations can be coupled to a coarse-grained model of DNA to study the effect of local temperature modulation on electrophoretic motion of DNA through plasmonic nanopores. The computational methods described in this article are expected to find applications in rational design of temperature-responsive nanopore systems.

Measuring the Soret coefficient of nanoparticles in a dilute suspension

Thermophoresis is an efficient process for the manipulation of molecules and nanoparticles due to the strong force it generates on the nanoscale. Thermophoresis is characterized by the Soret coefficient. Conventionally, the Soret coefficient of nanosized species is obtained by fitting the concentration profile under a temperature gradient at the steady state to a continuous phase model. However, when the number density of the target is ultralow and the dispersed species cannot be treated as a continuous phase, the bulk concentration fluctuates spatially, preventing extraction of temperature-gradient induced concentration profile. The present work demonstrates a strategy to tackle this problem by superimposing snapshots of nanoparticle distribution. The resulting image is suitable for the extraction of the Soret coefficient through the conventional data fitting method. The strategy is first tested through a discrete phase model that illustrates the spatial fluctuation of the nanoparticle concentration in a dilute suspension in response to the temperature gradient. By superimposing snapshots of the stochastic distribution, a thermophoretic depletion profile with low standard error is constructed, indicative of the Soret coefficient. Next, confocal analysis of nanoparticle distribution in response to a temperature gradient is performed using polystyrene nanobeads down to 1e-5% (v/v). The experimental results also reveal that superimposing enhances the accuracy of extracted Soret coefficient. The critical particle number density in the superimposed image for predicting the Soret coefficient is hypothesized to depend on the spatial resolution of the image. This study also demonstrates that the discrete phase model is an effective tool to study particle migration under thermophoresis in the liquid phase.

Nonlinear thermokinetic phenomena due to the Seebeck effect

We propose a novel mechanism to produce nonlinear thermokinetic vortex flows around a circular cylinder with ideally high thermal conductivity in an electrolyte. That is, the nonlinear thermokinetic slip velocity, which is proportional to the square of the temperature gradient [∇(T)0(2)], is derived based on the electrolyte Seebeck effect, heat conduction equation, and Helmholtz–Smoluchowski formula. Different from conventional linear thermokinetic theory, our theory predicts that the inversion of the temperature gradient does not change the direction of the thermokinetic flows and thus a Janus particle using this phenomenon can move to the both hotter and colder regions in a temperature gradient field by changing the direction of its dielectric end. Our findings bridge the gap between the electro- and thermo-kinetic phenomena and provide an integrated physical viewpoint for the interface science.

Trapping of single nano-objects in dynamic temperature fields

In this article we explore the dynamics of a Brownian particle in a feedback-free dynamic thermophoretic trap. The trap contains a focused laser beam heating a circular gold structure locally and creating a repulsive thermal potential for a Brownian particle. In order to confine a particle the heating beam is steered along the circumference of the gold structure leading to a non-trivial motion of the particle. We theoretically find a stability condition by switching to a rotating frame, where the laser beam is at rest. Particle trajectories and stable points are calculated as a function of the laser rotation frequency and are experimentally confirmed. Additionally, the effect of Brownian motion is considered. The present study complements the dynamic thermophoretic trapping with a theoretical basis and will enhance the applicability in micro- and nanofluidic devices.

Computation of thermodynamic and transport properties to predict thermophoretic effects in an argon-krypton mixture

Thermophoresis is the movement of molecules caused by a temperature gradient. Here we report the results of a study of thermophoresis using non-equilibrium molecular dynamics simulations of a confined argon-krypton fluid subject to two different temperatures at thermostated walls. The resulting temperature profile between the walls is used along with the Soret coefficient to predict the concentration profile that develops across the channel. We obtain the Soret coefficient by calculating the mutual diffusion and thermal diffusion coefficients. We report an appropriate method for calculating the transport coefficients for binary systems, using the Green-Kubo integrals and radial distribution functions obtained from equilibrium molecular dynamics simulations of the bulk fluid. Our method has the unique advantage of separating the mutual diffusion and thermal diffusion coefficients, and calculating the sign and magnitude of their individual contributions to thermophoresis in binary mixtures.

Specific salt effects on thermophoresis of charged colloids

We study the Soret effect of charged polystyrene particles as a function of temperature and electrolyte composition. As a main result we find that the Soret coefficient is determined by charge effects, and that non-ionic contributions are small. In view of the well-known electric-double layer interactions, our thermal field-flow fractionation data lead us to the conclusion that the Soret effect originates to a large extent from diffusiophoresis in the salt gradient and from the electrolyte Seebeck effect, both of which show strong specific-ion effects. Moreover, we find that thermophoresis of polystyrene beads is fundamentally different from proteins and aqueous polymer solutions, which show a strong non-ionic contribution.

Graphene oxide-deposited microfiber: a new photothermal device for various microbubble generation

This study makes a claim of utilizing the photothermal effect of graphene oxide nanosheets (GONs) to effectively produce various microbubbles in an optical microfiber system at infrared optical communications band. A low power continuous-wave light at wavelength of 1527-1566 nm was launched into the microfiber to form GONs-deposition which acted as a linear heat source for creating various microbubbles. Both thermal convection flow and optical gradient force were responsible for the driving force to assemble GONs onto the microfiber. This simple optical fiber system can be used for assembling other micro/nanoscale particles and biomolecules, which has prospective applications in sensing, microfluidics, virus detection, and other biochip techniques.

Optically controlled thermophoretic trapping of single nano-objects

Brownian motion is driven by thermal fluctuations and becoming more efficient for decreasing size and elevated temperatures. Here, we show that despite the increased fluctuations local temperature fields can be used to localize and control single nano-objects in solution. By creating strong local temperature gradients in a liquid using optically heated gold nanostructures, we are able to trap single colloidal particles. The trapping is thermophoretic in nature, and thus no restoring body force is involved. The simplicity of the setup allows for an easy integration and scalability to large arrays of traps.

Stretching and controlled motion of single-stranded DNA in locally heated solid-state nanopores

Practical applications of solid-state nanopores for DNA detection and sequencing require the electrophoretic motion of DNA through the nanopores to be precisely controlled. Controlling the motion of single-stranded DNA presents a particular challenge, in part because of the multitude of conformations that a DNA strand can adopt in a nanopore. Through continuum, coarse-grained and atomistic modeling, we demonstrate that local heating of the nanopore volume can be used to alter the electrophoretic mobility and conformation of single-stranded DNA. In the nanopore systems considered, the temperature near the nanopore is modulated via a nanometer-size heater element that can be radiatively switched on and off. The local enhancement of temperature produces considerable stretching of the DNA fragment confined within the nanopore. Such stretching is reversible, so that the conformation of DNA can be toggled between compact (local heating is off) and extended (local heating is on) states. The effective thermophoretic force acting on single-stranded DNA in the vicinity of the nanopore is found to be sufficiently large (4-8 pN) to affect such changes in the DNA conformation. The local heating of the nanopore volume is observed to promote single-file translocation of DNA strands at transmembrane biases as low as 10 mV, which opens new avenues for using solid-state nanopores for detection and sequencing of DNA.

Escalation of polymerization in a thermal gradient

For the emergence of early life, the formation of biopolymers such as RNA is essential. However, the addition of nucleotide monomers to existing oligonucleotides requires millimolar concentrations. Even in such optimistic settings, no polymerization of RNA longer than about 20 bases could be demonstrated. How then could self-replicating ribozymes appear, for which recent experiments suggest a minimal length of 200 nt? Here, we demonstrate a mechanism to bridge this gap: the escalated polymerization of nucleotides by a spatially confined thermal gradient. The gradient accumulates monomers by thermophoresis and convection while retaining longer polymers exponentially better. Polymerization and accumulation become mutually self-enhancing and result in a hyperexponential escalation of polymer length. We describe this escalation theoretically under the conservative assumption of reversible polymerization. Taking into account the separately measured thermophoretic properties of RNA, we extrapolate the results for primordial RNA polymerization inside a temperature gradient in pores or fissures of rocks. With a dilute, nanomolar concentration of monomers the model predicts that a pore length of 5 cm and a temperature difference of 10 K suffice to polymerize 200-mers of RNA in micromolar concentrations. The probability to generate these long RNAs is raised by a factor of >10(600) compared with polymerization in a physical equilibrium. We experimentally validate the theory with the reversible polymerization of DNA blocks in a laser-driven thermal trap. The results confirm that a thermal gradient can significantly enlarge the available sequence space for the emergence of catalytically active polymers.

Optofluidics incorporating actively controlled micro- and nano-particles

The advent of optofluidic systems incorporating suspended particles has resulted in the emergence of novel applications. Such systems operate based on the fact that suspended particles can be manipulated using well-appointed active forces, and their motions, locations and local concentrations can be controlled. These forces can be exerted on both individual and clusters of particles. Having the capability to manipulate suspended particles gives users the ability for tuning the physical and, to some extent, the chemical properties of the suspension media, which addresses the needs of various advanced optofluidic systems. Additionally, the incorporation of particles results in the realization of novel optofluidic solutions used for creating optical components and sensing platforms. In this review, we present different types of active forces that are used for particle manipulations and the resulting optofluidic systems incorporating them. These systems include optical components, optofluidic detection and analysis platforms, plasmonics and Raman systems, thermal and energy related systems, and platforms specifically incorporating biological particles. We conclude the review with a discussion of future perspectives, which are expected to further advance this rapidly growing field.

Nanofluidic devices towards single DNA molecule sequence mapping

Nanofluidics enables the imaging of stretched single molecules with potential applications for single molecule sequence mapping. Lab-on-a-chip devices for single cell trapping and lyzing, genomic DNA extraction from single cells, and optical mapping of genomic length DNA has been demonstrated separately. Yet the pursuit for applying DNA optical mapping to solve real genomics challenges is still to come. We review lab-on-a-chip devices from literature that could be part of a complete system for the sequence mapping of single DNA molecules.

Reconfigurable optothermal microparticle trap in air-filled hollow-core photonic crystal fiber

We report a novel optothermal trapping mechanism that occurs in air-filled hollow-core photonic crystal fiber. In the confined environment of the core, the motion of a laser-guided particle is strongly influenced by the thermal-gradient-driven flow of air along the core surface. Known as "thermal creep flow," this can be induced either statically by local heating, or dynamically by the absorption (at a black mark placed on the fiber surface) of light scattered by the moving particle. The optothermal force on the particle, which can be accurately measured in hollow-core fiber by balancing it against the radiation forces, turns out to exceed the conventional thermophoretic force by 2 orders of magnitude. The system makes it possible to measure pN-scale forces accurately and to explore thermally driven flow in micron-scale structures.

Charging of heated colloidal particles using the electrolyte Seebeck effect

We propose a novel actuation mechanism for colloids, which is based on the Seebeck effect of the electrolyte solution: Laser heating of a nonionic particle accumulates in its vicinity a net charge Q, which is proportional to the excess temperature at the particle surface. The corresponding long-range thermoelectric field E is proportional to 1/r(2) provides a tool for controlled interactions with nearby beads or with additional molecular solutes. An external field E(ext) drags the thermocharged particle at a velocity that depends on its size and absorption properties; the latter point could be particularly relevant for separating carbon nanotubes according to their electronic band structure.

DNA transport and delivery in thermal gradients near optofluidic resonators

Heat generation and its impact on DNA transport in the vicinity of an optofluidic silicon photonic crystal resonator are studied theoretically and experimentally. The temperature rise is measured to be as high as 57 K for 10 mW of input power. The resulting optical trapping and biomolecular sensing properties of these devices are shown to be strongly affected by the combination of buoyancy driven flow and thermophoresis. Specifically, the region around the electromagnetic hot spot is depleted in biomolecules because of a high free energy barrier.

Accumulating microparticles and direct-writing micropatterns using a continuous-wave laser-induced vapor bubble

Through the enhanced photothermal effect, which was achieved using a silver film, a low power weakly focused continuous-wave laser (532 nm) was applied to create a vapor bubble. A convective flow was formed around the bubble. Microparticles dispersed in water were carried by the convective flow to the vapor bubble and accumulated on the silver film. By moving the laser spot, we easily manipulated the location of the bubble, allowing us to direct-write micropatterns on the silver film with accumulated particles. The reported simple controllable accumulation method can be applied to bimolecular detection, medical diagnosis, and other related biochip techniques.

Soret effect and photochemical reaction in liquids with laser-induced local heating

We report a theoretical model and experimental results for laser-induced local heating in liquids, and propose a method to detect and quantify the contributions of photochemical and Soret effects in several different situations. The time-dependent thermal and mass diffusion equations in the presence and absence of laser excitation are solved. The two effects can produce similar transients for the laser-on refractive index gradient, but very different laser-off behavior. The Soret effect, also called thermal diffusion, and photochemical reaction contributions in photochemically reacting aqueous Cr(VI)-diphenylcarbazide, Eosin Y, and Eosin Y-doped micellar solutions, are decoupled in this work. The extensive use of lasers in various optical techniques suggests that the results may have significance extending from physical-chemical to biological applications.