Thermophoresis of DNA determined by microfluidic fluorescence

Brownian Dynamics Simulation of Microscale Thermophoresis in Liquid

Microscale thermophoresis (MST) has garnered significant attention as a manipulation method for chemical species ranging from nanometers to micrometers in liquids. In particular, techniques for manipulating single nanometer-sized objects have been developed by driving MST through laser heating with near-infrared wavelengths focused down to submicron scales or via photothermal conversion of plasmonic nanoparticles. While MST simulations on a macroscopic scale can be addressed by solving the diffusion equation using the finite element method, alternative computational approaches are required to investigate thermophoretic behavior at the single-particle level. For this purpose, we have developed a numerical method for the thermophoresis of individual nanoparticles diffusing in a liquid by combining the finite element method for steady-state heat conduction with Brownian dynamics simulations. The scripts for the finite element method and Brownian dynamics calculations used in the present simulations are uploaded in the Supporting Information and freely available. The numerical results demonstrated satisfactory agreement with the experimental results of laser-induced thermophoresis performed on polystyrene nanoparticles with a diameter of 500 nm in water. This computational method is highly useful for controlling MST at the single-particle level, enabling the design of spatial temperature distributions and the evaluation of thermophoretic forces acting on individual nanoparticles.

Thermophoresis beyond Local Thermodynamic Equilibrium

We measure the thermophoresis of polysterene beads over a wide range of temperature gradients and find a pronounced nonlinear phoretic characteristic. The transition to the nonlinear behavior is marked by a drastic slowing down of thermophoretic motion and is characterized by a Péclet number of order unity as corroborated for different particle sizes and salt concentrations. The data follow a single master curve covering the entire nonlinear regime for all system parameters upon proper rescaling of the temperature gradients with the Péclet number. For low thermal gradients, the thermal drift velocity follows a theoretical linear model relying on the local-equilibrium assumption, while linear theoretical approaches based on hydrodynamic stresses, ignoring fluctuations, predict significantly slower thermophoretic motion for steeper thermal gradients. Our findings suggest that thermophoresis is fluctuation dominated for small gradients and crosses over to a drift-dominated regime for larger Péclet numbers in striking contrast to electrophoresis.

Manipulating nanoparticles based on a laser photothermal trap

A method of efficient directional optical manipulation of nanoparticles based on a laser photothermal trap is proposed, and the influence mechanism of external conditions on the photothermal trap is clarified. Through optical manipulation experiments and finite-element simulations, it is determined that the main cause of gold nanoparticle directional motion depends on the drag force. The laser power, boundary temperature, and thermal conductivity of the substrate at the bottom of the solution and liquid level essentially affect the intensity of the laser photothermal trap in the solution and then affect the directional movement and deposition speed of gold particles. The result shows the origin of the laser photothermal trap and the three-dimensional spatial velocity distribution of gold particles. It also clarifies the height boundary of photothermal effect onset, which clarifies the boundary between light force and photothermal effect. In addition, nanoplastics are manipulated successfully based on this theoretical study. In this study, the movement law of gold nanoparticles based on the photothermal effect is deeply analyzed through experiments and simulations, which is of significance to the theoretical study of the optical manipulation of nanoparticles using the photothermal effect.

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.

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.

Temperature profile characterization with fluorescence lifetime imaging microscopy in a thermophoretic chip

This study introduces a thermophoretic lab-on-a-chip device to measure the Soret coefficient. We use resistive heating of a microwire on the chip to induce a temperature gradient, which is measured by fluorescence lifetime imaging microscopy (FLIM). To verify the functionality of the device, we used dyed polystyrene particles with a diameter of 25 nm. A confocal microscope is utilized to monitor the concentration profile of colloidal particles in the temperature field. Based on the measured temperature and concentration differences, we calculate the corresponding Soret coefficient. The same particles have been recently investigated with thermal diffusion forced Rayleigh scattering (TDFRS) and we find that the obtained Soret coefficients agree with literature results. This chip offers a simple way to study the thermophoretic behavior of biological systems in multicomponent buffer solutions quantitatively, which are difficult to study with optical methods solely relying on the refractive index contrast.

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.

Anti-Stokes Thermometry in Nanoplasmonics

Whereas heating nanoparticles with light is straightforward, measuring the resulting nanoscale temperature increase is intricate and still a matter of active research in plasmonics, with envisioned applications in nanochemistry, biomedicine, and solar light harvesting, among others. Interestingly, this research line mostly belongs to the optics community today because light is not only used for heating but also often for probing temperature. In this Perspective, I present and discuss recent advances in the search for efficient and reliable thermometry techniques for nanoplasmonic systems by the nano-optics community. I focus on the recently proposed approach based on the spectral measurement of anti-Stokes emission from the plasmonic nanoparticles themselves.

Forced Crowding of Colloids by Thermophoresis and Convection in a Custom Liquid Clusius-Dickel Microdevice

We report a study demonstrating that simultaneous induction of a steady-state convection current and temperature gradient in a confined geometry can be an effective way to force crowding of dissolved particulates. To investigate this thermogravitationally driven concentration of particles in situ, we developed a microdevice capable of sustaining controlled transverse temperature gradients within a 5 cm long, 0.1 mm inner diameter capillary that allowed visualization of particle movement with standard optical microscopy. Experiments were conducted on two material systems representative of nanoscale small molecules and microscale particles. With the small molecules (aromatic dyes, 530-790 g/mol, 1-1.5 nm), thermophoretic and gravitational effects in the microdevice resulted in an asymmetrical 2× concentration change along the capillary height over 3 days. In contrast, the concentration change under similar conditions for 40-micron diameter latex colloids is 50-fold in 30 min. This dramatic difference in separation times is consistent with simulations and models of thermophoresis where the thermophoretic effect scales with particle size. Induced crowding of particulates leads to formation of accumulation and depletion zones at the bottom and top of the capillary, respectively. Both the concentration of dye molecules over time in the depletion zone and the spatial distribution of colloids over the entire capillary length were found to be good fits to simple first-order exponential decay functions. These results suggest potential applications of thermogravitational separation in developing new functional materials via thermophoretic and convective effects.

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.

Applications and challenges of thermoplasmonics

Over the past two decades, there has been a growing interest in the use of plasmonic nanoparticles as sources of heat remotely controlled by light, giving rise to the field of thermoplasmonics. The ability to release heat on the nanoscale has already impacted a broad range of research activities, from biomedicine to imaging and catalysis. Thermoplasmonics is now entering an important phase: some applications have engaged in an industrial stage, while others, originally full of promise, experience some difficulty in reaching their potential. Meanwhile, innovative fundamental areas of research are being developed. In this Review, we scrutinize the current research landscape in thermoplasmonics, with a specific focus on its applications and main challenges in many different fields of science, including nanomedicine, cell biology, photothermal and hot-electron chemistry, solar light harvesting, soft matter and nanofluidics.

Thermophoretic Micron-Scale Devices: Practical Approach and Review

In recent years, there has been increasing interest in the development of micron-scale devices utilizing thermal gradients to manipulate molecules and colloids, and to measure their thermophoretic properties quantitatively. Various devices have been realized, such as on-chip implements, micro-thermogravitational columns and other micron-scale thermophoretic cells. The advantage of the miniaturized devices lies in the reduced sample volume. Often, a direct observation of particles using various microscopic techniques is possible. On the other hand, the small dimensions lead to some technical problems, such as a precise temperature measurement on small length scale with high spatial resolution. In this review, we will focus on the "state of the art" thermophoretic micron-scale devices, covering various aspects such as generating temperature gradients, temperature measurement, and the analysis of the current micron-scale devices. We want to give researchers an orientation for their development of thermophoretic micron-scale devices for biological, chemical, analytical, and medical applications.

Analysis of biomolecular condensates and protein phase separation with microfluidic technology

An increasing body of evidence shows that membraneless organelles are key components in cellular organization. These observations open a variety of outstanding questions about the physico-chemical rules underlying their assembly, disassembly and functions. Some molecular determinants of biomolecular condensates are challenging to probe and understand in complex in vivo systems. Minimalistic in vitro reconstitution approaches can fill this gap, mimicking key biological features, while maintaining sufficient simplicity to enable the analysis of fundamental aspects of biomolecular condensates. In this context, microfluidic technologies are highly attractive tools for the analysis of biomolecular phase transitions. In addition to enabling high-throughput measurements on small sample volumes, microfluidic tools provide for exquisite control of self-assembly in both time and space, leading to accurate quantitative analysis of biomolecular phase transitions. Here, with a specific focus on droplet-based microfluidics, we describe the advantages of microfluidic technology for the analysis of several aspects of phase separation. These include phase diagrams, dynamics of assembly and disassembly, rheological and surface properties, exchange of materials with the surrounding environment and the coupling between compartmentalization and biochemical reactions. We illustrate these concepts with selected examples, ranging from simple solutions of individual proteins to more complex mixtures of proteins and RNA, which represent synthetic models of biological membraneless organelles. Finally, we discuss how this technology may impact the bottom-up fabrication of synthetic artificial cells and for the development of synthetic protein materials in biotechnology.

Plasmonic Manipulation of DNA using a Combination of Optical and Thermophoretic Forces: Separation of Different-Sized DNA from Mixture Solution

We demonstrate the size-dependent separation and permanent immobilization of DNA on plasmonic substrates by means of plasmonic optical tweezers. We found that a gold nanopyramidal dimer array enhanced the optical force exerted on the DNA, leading to permanent immobilization of the DNA on the plasmonic substrate. The immobilization was realized by a combination of the plasmon-enhanced optical force and the thermophoretic force induced by a photothermal effect of the plasmons. In this study, we applied this phenomenon to the separation and fixation of size-different DNA. During plasmon excitation, DNA strands of different sizes became permanently immobilized on the plasmonic substrate forming micro-rings of DNA. The diameter of the ring was larger for longer DNA (in base pairs). When we used plasmonic optical tweezers to trap DNA of two different lengths dissolved in solution (φx DNA (5.4 kbp) and λ-DNA (48.5 kbp), or φx DNA and T4 DNA (166 kbp)), the DNA were immobilized, creating a double micro-ring pattern. The DNA were optically separated and immobilized in the double ring, with the shorter sized DNA and the larger one forming the smaller and larger rings, respectively. This phenomenon can be quantitatively explained as being due to a combination of the plasmon-enhanced optical force and the thermophoretic force. Our plasmonic optical tweezers open up a new avenue for the separation and immobilization of DNA, foreshadowing the emergence of optical separation and fixation of biomolecules such as proteins and other ncuelic acids.

Thermophoresis: The Case of Streptavidin and Biotin

Thermophoretic behavior of a free protein changes upon ligand binding and gives access to information on the binding constants. The Soret effect has also been proven to be a promising tool to gain information on the hydration layer, as the temperature dependence of the thermodiffusion behavior is sensitive to solute–solvent interactions. In this work, we perform systematic thermophoretic measurements of the protein streptavidin (STV) and of the complex STV with biotin (B) using thermal diffusion forced Rayleigh scattering (TDFRS). Our experiments show that the temperature sensitivity of the Soret coefficient is reduced for the complex compared to the free protein. We discuss our data in comparison with recent quasi-elastic neutron scattering (QENS) measurements. As the QENS measurement has been performed in heavy water, we perform additional measurements in water/heavy water mixtures. Finally, we also elucidate the challenges arising from the quantiative thermophoretic study of complex multicomponent systems such as protein solutions.

Thermophoresis of biological and biocompatible compounds in aqueous solution

With rising popularity of microscale thermophoresis for the characterisation of protein-ligand binding reactions and possible applications in microfluidic devices, there is a growing interest in considering thermodiffusion in the context of life sciences. But although the understanding of thermodiffusion in non-polar mixtures has grown rapidly in recent years, predictions for associated mixtures like aqueous solutions remain challenging. This review aims to give an overview of the literature on thermodiffusion in aqueous systems, show the difficulties in theoretical description that arise from the non-ideal behaviour of water-mixtures, and highlight the relevance of thermodiffusion in a biological context. We find that the thermodiffusion in aqueous systems is dominated by contributions from heat of transfer, hydrogen bond interactions and charge effects. However, the separation of these effects is often difficult, especially in case of biological systems where a systematic exclusion of contributions may not be feasible.

Opto-Thermophoretic Tweezers and Assembly

Opto-thermophoretic manipulation is an emerging field, which exploits the thermophoretic migration of particles and colloidal species under a light-controlled temperature gradient field. The entropically favorable photon-phonon conversion and widely applicable heat-directed migration make it promising for low-power manipulation of variable particles in different fluidic environments. By exploiting an optothermal substrate, versatile opto-thermophoretic manipulation of colloidal particles and biological objects can be achieved via optical heating. In this paper, we summarize the working principles, concepts, and applications of the recently developed opto-thermophoretic techniques. Opto-thermophoretic trapping, tweezing, assembly, and printing of colloidal particles and biological objects are discussed thoroughly. With their low-power operation, simple optics, and diverse functionalities, opto-thermophoretic manipulation techniques will offer great opportunities in materials science, nanomanufacturing, life sciences, colloidal science, and nanomedicine.

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.

MicroScale Thermophoresis as a Tool to Study Protein-peptide Interactions in the Context of Large Eukaryotic Protein Complexes

Protein-peptide interactions are part of many physiological processes, for example, epigenetics where peptide regions of histone complexes are crucial for regulation of chromatin structure. Short peptides are often also used as alternatives to small molecule drugs to target protein complexes. Studying the interactions between proteins and peptides is thus an important task in systems biology, cell biology, biochemistry, and drug design. However, this task is often hampered by the drawbacks of classical biophysical methods for analysis of molecular interactions like surface plasmon resonance (SPR) or isothermal titration calorimetry (ITC), which require immobilization of the interaction partners or very high sample concentrations. MicroScale Thermophoresis (MST) is an innovative method that offers the possibility to determine the important parameters of a molecular interaction, such as dissociation constant, stoichiometry, and thermodynamics. Moreover, it does so in a rapid and precise manner, with free choice of buffers or biological liquids, no need for sample immobilization, and very low sample consumption. Here we describe two MST assays in detail, which analyze (i) the interactions between certain peptide stretches of the eukaryotic RNA polymerase II and a protein subunit of the eukaryotic transcription elongation complex and (ii) interactions between N-terminal histone tail peptides and epigenetic reader proteins. These experiments show that MST is able to characterize protein-peptide interactions that are triggered by only minor changes in the peptide, for example, only one phosphorylation at a specific serine residue.

Optical tweezing and binding at high irradiation powers on black-Si

Nowadays, optical tweezers have undergone explosive developments in accordance with a great progress of lasers. In the last decade, a breakthrough brought optical tweezers into the nano-world, overcoming the diffraction limit. This is called plasmonic optical tweezers (POT). POT are powerful tools used to manipulate nanomaterials. However, POT has several practical issues that need to be overcome. First, it is rather difficult to fabricate plasmonic nanogap structures regularly and rapidly at low cost. Second, in many cases, POT suffers from thermal effects (Marangoni convection and thermophoresis). Here, we propose an alternative approach using a nano-structured material that can enhance the optical force and be applied to optical tweezers. This material is metal-free black silicon (MFBS), the plasma etched nano-textured Si. We demonstrate that MFBS-based optical tweezers can efficiently manipulate small particles by trapping and binding. The advantages of MFBS-based optical tweezers are: (1) simple fabrication with high uniformity over wafer-sized areas, (2) free from thermal effects detrimental for trapping, (3) switchable trapping between one and two - dimensions, (4) tight trapping because of no detrimental thermal forces. This is the NON-PLASMONIC optical tweezers.

Thermophoretic migration of vesicles depends on mean temperature and head group chemistry

A number of colloidal systems, including polymers, proteins, micelles and hard spheres, have been studied in thermal gradients to observe and characterize their driven motion. Here we show experimentally the thermophoretic behaviour of unilamellar lipid vesicles, finding that mobility depends on the mean local temperature of the suspension and on the structure of the exposed polar lipid head groups. By tuning the temperature, vesicles can be directed towards hot or cold, forming a highly concentrated region. Binary mixtures of vesicles composed of different lipids can be segregated using thermophoresis, according to their head group. Our results demonstrate that thermophoresis enables robust and chemically specific directed motion of liposomes, which can be exploited in driven processes.

Thermal gradients are shown to provide a robust and chemically specific driving force to liposomes. Here the authors show controlled direction of migration of unilamellar lipid vesicles by varying the temperature in the suspension and the exposed polar lipid head groups.

Continuous Isotropic-Nematic Transition in Amyloid Fibril Suspensions Driven by Thermophoresis

The isotropic and nematic (I + N) coexistence for rod-like colloids is a signature of the first-order thermodynamics nature of this phase transition. However, in the case of amyloid fibrils, the biphasic region is too small to be experimentally detected, due to their extremely high aspect ratio. Herein, we study the thermophoretic behaviour of fluorescently labelled β-lactoglobulin amyloid fibrils by inducing a temperature gradient across a microfluidic channel. We discover that fibrils accumulate towards the hot side of the channel at the temperature range studied, thus presenting a negative Soret coefficient. By exploiting this thermophoretic behaviour, we show that it becomes possible to induce a continuous I-N transition with the I and N phases at the extremities of the channel, starting from an initially single N phase, by generating an appropriate concentration gradient along the width of the microchannel. Accordingly, we introduce a new methodology to control liquid crystal phase transitions in anisotropic colloidal suspensions. Because the induced order-order transitions are achieved under stationary conditions, this may have important implications in both applied colloidal science, such as in separation and fractionation of colloids, as well as in fundamental soft condensed matter, by widening the accessibility of target regions in the phase diagrams.

Mapping the Binding Site of an Aptamer on ATP Using MicroScale Thermophoresis

Characterization of molecular interactions in terms of basic binding parameters such as binding affinity, stoichiometry, and thermodynamics is an essential step in basic and applied science. MicroScale Thermophoresis (MST) is a sensitive biophysical method to obtain this important information. Relying on a physical effect called thermophoresis, which describes the movement of molecules through temperature gradients, this technology allows for the fast and precise determination of binding parameters in solution and allows the free choice of buffer conditions (from buffer to lysates/sera). MST uses the fact that an unbound molecule displays a different thermophoretic movement than a molecule that is in complex with a binding partner. The thermophoretic movement is altered in the moment of molecular interaction due to changes in size, charge, and hydration shell. By comparing the movement profiles of different molecular ratios of the two binding partners, quantitative information such as binding affinity (pM to mM) can be determined. Even challenging interactions between molecules of small sizes, such as aptamers and small compounds, can be studied by MST. Using the well-studied model interaction between the DH25.42 DNA aptamer and ATP, this manuscript provides a protocol to characterize aptamer-small molecule interactions. This study demonstrates that MST is highly sensitive and permits the mapping of the binding site of the 7.9 kDa DNA aptamer to the adenine of ATP.

MicroScale Thermophoresis: A Rapid and Precise Method to Quantify Protein-Nucleic Acid Interactions in Solution

Interactions between nucleic acids and proteins are driving gene expression programs and regulating the development of organisms. The binding affinities of transcription factors to their target sites are essential parameters to reveal their binding site occupancy and function in vivo. Microscale Thermophoresis (MST) is a rapid and precise method allowing for quantitative analysis of molecular interactions in solution on a microliter scale. The technique is based on the movement of molecules in temperature gradients, which is referred to as thermophoresis, and depends on molecule size, charge, and hydration shell. Since at least one of these parameters is typically affected upon binding of a ligand, the method can be used to analyze any kind of biomolecular interaction. This section provides a detailed protocol describing the analysis of DNA-protein interactions, using the transcription factor TTF-I as a model protein that recognizes a 10 bp long sequence motif.

Human neutrophil elastase inhibition studied by capillary electrophoresis with laser induced fluorescence detection and microscale thermophoresis

Capillary electrophoresis-laser induced fluorescence (CZE-LIF) and microscale thermophoresis (MST) were used for the first time to study the inhibition of human neutrophil elastase (HNE). We recently studied HNE kinetics (Km and Vmax) by developing an in-capillary CZE-LIF assay based on transverse diffusion of laminar flow profiles (TDLFP) for reactant mixing. In this work, the former assay was adapted to monitor HNE inhibition. Two natural well known HNE inhibitors from the triterpene family, ursolic acid and oleanolic acid, were tested to validate the developed assay. Since the solubility of pentacyclic triterpenes in aqueous media where the enzymatic reaction will take place is limited, the effect of DMSO and ethanol on HNE was studied using microscale thermophoresis (MST). An agglomeration of the enzyme was revealed when preparing the inhibitor in 5% (v/v) DMSO. This phenomenon did not occur in the presence of ethanol. Therefore, ethanol was used as inhibitor solvent, at a limited percentage of 20% (v/v). In these conditions and after optimization of the TDLFP approach, the repeatability (RSD on migration times and peak-areas inferior to 2.2%) of the CZE-LIF assay and the sensitivity (LOQ of few nM) were found to be satisfactory for conducting inhibition assays. IC50 values for ursolic and oleanolic acid were successfully determined. They were respectively equal to 5.62±0.10μM (r(2)=0.9807; n=3) and to 8.21±0.23μM (r(2)=0.9887; n=3). Excellent agreement was found between the results obtained by CE and those reported in literature which validates the developed method. Particularly, the CE-based assay is able to rank HNE inhibitors relative to each other. Furthermore, MST technique was used for evaluating HNE interaction with the ursolic acid. Up to 16 capillaries were automatically processed to obtain in one titration experiment the dissociation constant for the HNE-ursolic acid complex. Ki was found to be 2.72±0.66μM (n=3) which is in excellent agreement with the value determined by CE enzyme inhibition studies (Ki=2.81μM) confirming the reliability of the developed CE assay and the competitive inhibition mode of ursolic acid.

Active bioparticle manipulation in microfluidic systems

The motion of bioparticles in a microfluidic environment can be actively controlled using several tuneable mechanisms, including hydrodynamic, electrophoresis, dielectrophoresis, magnetophoresis, acoustophoresis, thermophoresis and optical forces.

Experimental study on thermophoresis of colloids in aqueous surfactant solutions

Thermophoresis refers to the motion of particles under a temperature gradient and it is one of the particle manipulation techniques. Regarding the thermophoresis of particles in liquid media, however, many open questions still remain, especially the role of the interfacial effect. This work reports on a systematic experimental investigation of surfactant effects, especially the induced interfacial effect, on the thermophoresis of colloids in aqueous solutions via a microfluidic approach. Two kinds of commonly used surfactants, sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB), are selected and the results show that from relatively large concentrations, the two surfactants can greatly enhance the thermophilic mobilities. Specifically, it is found that the colloid-water interfaces modified with more polar end groups can potentially lead to a stronger thermophilic tendency. Due to the complex effects of surfactants, further theoretical model development is needed to quantitatively describe the dependence of thermophoresis on the interface characteristics.

Ultrasensitive solution-processed broad-band photodetectors using CH₃NH₃PbI₃ perovskite hybrids and PbS quantum dots as light harvesters

Sensing from ultraviolet-visible to infrared is critical for both scientific and industrial applications. In this work, we demonstrate solution-processed ultrasensitive broad-band photodetectors (PDs) utilizing organolead halide perovskite materials (CH3NH3PbI3) and PbS quantum dots (QDs) as light harvesters. Through passivating the structural defects on the surface of PbS QDs with diminutive molecular-scaled CH3NH3PbI3, both trap states in the bandgap of PbS QDs for charge carrier recombination and the leakage currents occurring at the defect sites are significantly reduced. In addition, CH3NH3PbI3 itself is an excellent light harvester in photovoltaics, which contributes a great photoresponse in the ultraviolet-visible region. Consequently, operated at room temperature, the resultant PDs show a spectral response from 375 nm to 1100 nm, with high responsivities over 300 mA W(-1) and 130 mA W(-1), high detectivities exceeding 10(13) Jones (1 Jones = 1 cm Hz(1/2) W(-1)) and 5 × 10(12) Jones in the visible and near infrared regions, respectively. These device performance parameters are comparable to those from pristine inorganic counterparts. Thus, our results offer a facile and promising route for advancing the performance of broad-band PDs.

Effects of non-CpG site methylation on DNA thermal stability: a fluorescence study

Cytosine methylation is a widespread epigenetic regulation mechanism. In healthy mature cells, methylation occurs at CpG dinucleotides within promoters, where it primarily silences gene expression by modifying the binding affinity of transcription factors to the promoters. Conversely, a recent study showed that in stem cells and cancer cell precursors, methylation also occurs at non-CpG pairs and involves introns and even gene bodies. The epigenetic role of such methylations and the molecular mechanisms by which they induce gene regulation remain elusive. The topology of both physiological and aberrant non-CpG methylation patterns still has to be detailed and could be revealed by using the differential stability of the duplexes formed between site-specific oligonucleotide probes and the corresponding methylated regions of genomic DNA. Here, we present a systematic study of the thermal stability of a DNA oligonucleotide sequence as a function of the number and position of non-CpG methylation sites. The melting temperatures were determined by monitoring the fluorescence of donor-acceptor dual-labelled oligonucleotides at various temperatures. An empirical model that estimates the methylation-induced variations in the standard values of hybridization entropy and enthalpy was developed.

Studying small molecule-aptamer interactions using MicroScale Thermophoresis (MST)

Aptamers are potent and versatile binding molecules recognizing various classes of target molecules. Even challenging targets such as small molecules can be identified and bound by aptamers. Studying the interaction between aptamers and drugs, antibiotics or metabolites in detail is however difficult due to the lack of sophisticated analysis methods. Basic binding parameters of these small molecule-aptamer interactions such as binding affinity, stoichiometry and thermodynamics are elaborately to access using the state of the art technologies. The innovative MicroScale Thermophoresis (MST) is a novel, rapid and precise method to characterize these small molecule-aptamer interactions in solution at microliter scale. The technology is based on the movement of molecules through temperature gradients, a physical effect referred to as thermophoresis. The thermophoretic movement of a molecule depends - besides on its size - on charge and hydration shell. Upon the interaction of a small molecule and an aptamer, at least one of these parameters is altered, leading to a change in the movement behavior, which can be used to quantify molecular interactions independent of the size of the target molecule. The MST offers free choice of buffers, even measurements in complex bioliquids are possible. The dynamic affinity range covers the pM to mM range and is therefore perfectly suited to analyze small molecule-aptamer interactions. This section describes a protocol how quantitative binding parameters for aptamer-small molecule interactions can be obtained by MST. This is demonstrated by mapping down the binding site of the well-known ATP aptamer DH25.42 to a specific region at the adenine of the ATP molecule.

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.

Thermophoretic forces on DNA measured with a single-molecule spring balance

We stretch a single DNA molecule with thermophoretic forces and measure these forces with a spring balance: the DNA molecule itself. It is an entropic spring which we calibrate, using as a benchmark its Brownian motion in the nanochannel that contains and prestretches it. This direct measurement of the thermophoretic force in a static configuration finds forces up to 130 fN. This is eleven times stronger than the force experienced by the same molecule in the same thermal gradient in bulk, where the molecule shields itself. Our stronger forces stretch the middle of the molecule up to 80% of its contour length. We find the Soret coefficient per unit length of DNA at various ionic strengths. It agrees, with novel precision, with results obtained in bulk for DNA too short to shield itself and with the thermodynamic model of thermophoresis.

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.

Thermodiffusion in positively charged magnetic colloids: influence of the particle diameter

The Soret coefficient (ST) of positively charged magnetic colloids was measured as a function of the nanoparticles' diameter. The Z-scan technique and the generalization of the thermal lens model proved to be a reliable technique to measure ST. We show that ST is negative and increases with the particle's diameter, being best described by a functional dependence of the type ST∝d0. Potentiometric and conductometric experiments show that the particle's surface charge decreases as the temperature increases, changing the electrostatic interaction between the nanoparticles. The temperature gradient imposed in the ferrofluid by the Gaussian laser beam leads to the formation of the particle's concentration gradient. The origin of this phenomenon is discussed in terms of the decrease of the particle's surface charge in the hottest region of the sample and the thermoelectric field due to the inhomogeneous distribution of hydrogenous ions present in the colloidal suspension.

Bleaching-corrected fluorescence microspectroscopy with nanometer peak position resolution

Fluorescence microspectroscopy (FMS) with environmentally sensitive dyes provides information about local molecular surroundings at microscopic spatial resolution. Until recently, only probes exhibiting large spectral shifts due to local changes have been used. For filter-based experimental systems, where signal at different wavelengths is acquired sequentially, photostability has been required in addition. Herein, we systematically analyzed our spectral fitting models and bleaching correction algorithms which mitigate both limitations. We showed that careful analysis of data acquired by stochastic wavelength sampling enables nanometer spectral peak position resolution even for highly photosensitive fluorophores. To demonstrate how small spectral shifts and changes in bleaching rates can be exploited, we analyzed vesicles in different lipid phases. Our findings suggest that a wide range of dyes, commonly used in bulk spectrofluorimetry but largely avoided in microspectroscopy due to the above-mentioned restrictions, can be efficiently applied also in FMS.

Optically induced thermal gradients for protein characterization in nanolitre-scale samples in microfluidic devices

Proteins are the most vital biological functional units in every living cell. Measurement of protein stability is central to understanding their structure, function and role in diseases. While proteins are also sought as therapeutic agents, they can cause diseases by misfolding and aggregation in vivo. Here we demonstrate a novel method to measure protein stability and denaturation kinetics, on unprecedented timescales, through optically-induced heating of nanolitre samples in microfluidic capillaries. We obtain protein denaturation kinetics as a function of temperature, and accurate thermodynamic stability data, from a snapshot experiment on a single sample. We also report the first experimental characterization of optical heating in controlled microcapillary flow, verified by computational fluid dynamics modelling. Our results demonstrate that we now have the engineering science in hand to design integrated all-optical microfluidic chips for a diverse range of applications including in-vitro DNA amplification, healthcare diagnostics, and flow chemistry.

Effects of long DNA folding and small RNA stem-loop in thermophoresis

In thermophoresis, with the fluid at rest, suspensions move along a gradient of temperature. In an aqueous solution, a PEG polymer suspension is depleted from the hot region and builds a concentration gradient. In this gradient, DNA polymers of different sizes can be separated. In this work the effect of the polymer structure for genomic DNA and small RNA is studied. For genome-size DNA, individual single T4 DNA is visualized and tracked in a PEG solution under a temperature gradient built by infrared laser focusing. We find that T4 DNA follows steps of depletion, ring-like localization, and accumulation patterns as the PEG volume fraction is increased. Furthermore, a coil-globule transition for DNA is observed for a large enough PEG volume fraction. This drastically affects the localization position of T4 DNA. In a similar experiment, with small RNA such as ribozymes we find that the stem-loop folding of such polymers has important consequences. The RNA polymers having a long and rigid stem accumulate, whereas a polymer with stem length less than 4 base pairs shows depletion. Such measurements emphasize the crucial contribution of the double-stranded parts of RNA for thermal separation and selection under a temperature gradient. Because huge temperature gradients are present around hydrothermal vents in the deep ocean seafloor, this process might be relevant, at the origin of life, in an RNA world hypothesis. Ribozymes could be selected from a pool of random sequences depending on the length of their stems.

Thermal diffusion of nucleotides

We investigate the thermal diffusion behavior of aqueous solutions of nucleotides using an infrared thermal diffusion forced Rayleigh scattering (IR-TDFRS) setup. In this work we study 5 nucleotides: cyclic nucleotides adenosine and guanosine monophosphate, 5'-adenosine and 5'-cytidine monophosphate, and also adenosine diphosphate in water. The structures of nucleotides vary systematically, which results in different physical properties such as acidity, solubility, hydrophobicity, and melting point. We discuss the connection between the thermal diffusion behavior and the properties of the different nucleotides. Additionally, as in the case of the alkanes and monoscaccharides, we find a correlation between the thermal diffusion coefficient and the ratio of the thermal expansion coefficient and the kinematic viscosity.

Molecular interaction studies using microscale thermophoresis

Abstract The use of infrared laser sources for creation of localized temperature fields has opened new possibilities for basic research and drug discovery. A recently developed technology, Microscale Thermophoresis (MST), uses this temperature field to perform biomolecular interaction studies. Thermophoresis, the motion of molecules in temperature fields, is very sensitive to changes in size, charge, and solvation shell of a molecule and thus suited for bioanalytics. This review focuses on the theoretical background of MST and gives a detailed overview on various applications to demonstrate the broad applicability. Experiments range from the quantification of the affinity of low-molecular-weight binders using fluorescently labeled proteins, to interactions between macromolecules and multi-component complexes like receptor containing liposomes. Information regarding experiment and experimental setup is based on the Monolith NT.115 instrument (NanoTemper Technologies GmbH).

Laser-induced thermophoresis of individual particles in a viscous liquid

This paper presents a detailed investigation of the motion of individual micro-particles in a moderately-viscous liquid in direct response to a local, laser-induced temperature gradient. By measuring particle trajectories in 3D, and comparing them to a simulated temperature profile, it is confirmed that the thermally-induced particle motion is the direct result of thermophoresis. The elevated viscosity of the liquid provides for substantial differences in the behavior predicted by various models of thermophoresis, which in turn allows measured data to be most appropriately matched to a model proposed by Brenner. This model is then used to predict the effective force resulting from thermophoresis in an optical trap. Based on these results, we predict when thermophoresis will strongly inhibit the ability of radiation pressure to trap nano-scale particles. The model also predicts that the thermophoretic force scales linearly with the viscosity of the liquid, such that choice of liquid plays a key role in the relative strength of the thermophoretic and radiation forces.