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Temperature fields provide a noninvasive approach for manipulating individual macromolecules in solution. Utilizing thermophoresis and other secondary effects resulting from the inhomogeneous distribution of crowding agents, one may gain valuable insights into the interactions of molecular mixtures. In this report, we examine the steady-state concentration distribution and dynamics of DNA molecules in a poly(ethylene glycol) (PEG)/water solution when exposed to localized temperature gradients generated by optical heating of a thin chrome layer at a liquid–solid boundary. This allowed us to experimentally investigate the interplay between DNA thermophoresis and PEG-induced entropic depletion effects. Our quantitative analysis demonstrates that the depletion effects dominate over DNA thermophoresis, causing the DNA polymers to migrate toward the heat source. Additionally, we explore the transient stretching of individual DNA molecules in thermally induced PEG gradients and estimate the contributing forces.

A cornerstone of the directed motion of microscopic self-propelling particles is an asymmetric particle structure defining a polarity axis along which these tiny machines move. This structural asymmetry ties the orientational Brownian motion to the microswimmers directional motion, limiting their persistence and making the long time motion effectively diffusive. Here, we demonstrate a completely symmetric thermoplasmonic microswimmer, which is propelled by laser-induced self-thermophoresis. The propulsion direction is imprinted externally to the particle by the heating laser position. The orientational Brownian motion, thus, becomes irrelevant for the propulsion, allowing enhanced control over the particles dynamics with almost arbitrary steering capability. We characterize the particle motion in experiments and simulations and also theoretically. The analysis reveals additional noise appearing in these systems, which is conjectured to be relevant for biological systems. Our experimental results show that even very small particles can be precisely controlled, enabling more advanced applications of these micromachines.

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.

Thermophoresis is a common mechanism that can drive autonomous motion of Janus particles under the right environment. Despite recent efforts to investigate the mechanism underlying the self-propulsion of thermophoretic particles, the interaction of particles with the substrate underneath the particle has remained unclear. In this work, we explore the impact of poly(N-isopropylacrylamide) (PNIPAM)-functionalized substrate with various chain lengths on the active motion of a single polystyrene particle half-coated with gold (Au–PS). We show how the modification of the substrate with polymer brushes enhances the particle velocity, where brush chain length plays a significant role as well. The results demonstrate the intrinsic dependence of particle velocity on the flow boundary condition and the thermo-osmotic slip at the interface.

A method for analyzing single molecule tracking data is presented, which allows the measurement of flow profiles in shear flows of ultrathin liquid films. The results show that the velocity profile detected by single molecule tracking is in general different from the analytical solution of the Navier−Stokes equation due to the diffusion of the probe molecules. However, the theory presented allows the extraction of even slip boundary phenomena, which for the first time provides access to boundary conditions on a molecular scale. A verification of the theory and its capabilities is presented by means of tracking single polystyrene particles in a 500 nm thin water film confined in a surface forces apparatus.

The study of the aggregation of soluble proteins into highly ordered, insoluble amyloid fibrils is fundamental for the understanding of neurodegenerative disorders. Here, we present a method for the observation of single amyloid fibrils that allows the investigation of fibril growth, secondary nucleation or fibril breakup that is typically hidden in the average ensemble. Our approach of thermophoretic trapping and rotational diffusion measurements is demonstrated for single Aβ40, Aβ42and pyroglutamyl-modified amyloid-β variant (pGlu3-Aβ3–40) amyloid fibrils.

Optical trapping of metal nanoparticles with conventional ‘tweezers’ can be challenging due to absorption and heating. Now, an opto-thermoelectric trapping approach that exploits heating has been demonstrated.

We describe the transmission characteristics for the interaction of an arbitrary beam with (possibly multilayered) spherical particles of arbitrary size and electric permeability. Within the generalized Lorenz–Mie theory, expressions that generalize the total cross sections to their fractional counterparts are presented, which allow for an analytic quantification of transmission signals, both on-axis and off-axis. For Gaussian (Davis) beams, the relative angular domain of collection as compared to the beam’s angle of divergence determines sensitively the shape and magnitude of the interference signal. Depending on the particle’s position within the beam, the transmission signatures related to a pure energy redistribution as well as to accompanying absorption are discussed for Rayleigh particles in terms of their complex-valued polarizability. Implications for positioning, temperature control, spectroscopy, and optimized extinction measurements are discussed.

Force-free trapping and steering of single photophoretically self-propelled Janus-type particles using a feedback mechanism is experimentally demonstrated. Realtime information on particle position and orientation is used to switch the self-propulsion mechanism of the particle optically. The orientational Brownian motion of the particle thereby provides the reorientation mechanism for the microswimmer. The particle size dependence of the photophoretic propulsion velocity reveals that photon nudging provides an increased position accuracy for decreasing particle radius. The explored steering mechanism is suitable for navigation in complex biological environments and in-depth studies of collective swimming effects.

We present a simple scheme for the manipulation of light intensity by light mediated by a dissipative process. The implementation employs the heat released by an optically excited plasmonic metal nanoparticle to control the size of an isotropic bubble in a nematic liquid crystal film. The nematic film is designed as a zero-order half-wave plate that rotates an incident probe light polarization by π/2 and is blocked by an analyzing polarizer behind the structure. The growing isotropic bubble disturbs the half-wave plate and causes the probe to be transmitted through the modulator structure. Our results demonstrate that dissipative processes may be advantageously used to control light by light.

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.

We explore the intuitive lensing picture of laser-heated nanoparticles occurring in single particle photothermal (PT) microscopy. The effective focal length of the thermal lens (TL) is derived from a ray-optics treatment and used to transform the probing focused Gaussian beam with ABCD Gaussian matrix optics. The relative PT signal is obtained from the relative beam-waist change far from the TL. The analytical expression is semiquantitative, capable of describing the entire phenomenology of single particle PT microscopy, and shows that the signal is the product of the point-spread functions of the involved lasers times a linear function of the axial coordinate. The presented particularly simple and intuitive Gaussian beam lensing picture compares favorably to the experimental results for 60 nm gold nanoparticles and provides the prescription for optimum setup calibration.

Combining quantitative photothermal microscopy and light scattering microscopy as well as accurate MIE scattering calculations on single gold nanoparticles, we reveal that the mechanism of photothermal single-molecule/particle detection is quantitatively explained by a nanolensing effect. The lensing action is the result of the long-range character of the refractive index profile. It splits the focal detection volume into two regions. Our results lay the foundation for future developments and quantitative applications of single-molecule absorption microscopy.

Steady-state photoluminescence (PL) quenching of colloidal CdSe/ZnS and CdSe quantum dots (QDs) induced by functionalized porphyrin molecules was investigated for various QD sizes. The majority of the observed strong quenching of the QD photoluminescence can be assigned to neither Förster resonant energy transfer (FRET) nor photoinduced charge transfer between the QDs and the chromophore. Using the remaining small FRET efficiency to monitor the formation of QD/chromophore nanoassemblies, the major contribution to PL quenching was found to be proportional to the calculated quantum-confined exciton wave function at the QD surface. The quenching depends on the QD size and shell and is stronger for smaller quantum dots. Upon comparison of experimentally determined quenching rates and calculations of the exciton wave function, it was concluded that the attachment of only one chromophore induces a pertubation of the wave function that is accompanied by a strong increase of the radiationless decay rate.

Solvent-dependent ultraviolet–visible (UV–vis) absorption and Stokes shifts including strong hydrogen-bond-donating (HBD) solvents such as 2,2,2-trifluoroethanol and 1,1,1,3,3,3-hexafluoro-2-propanol of two coumarine dyes (Co 151 and Co 153) were analyzed with multiple-square analyses of linear solvation energy relationships and the Kamlet–Taft solvent parameter set to α (HBD capacity), β (hydrogen-bond-accepting capacity), and π* (dipolarity/polarizability). The UV–vis absorption and emission spectra of Co 151 and Co 153 were measured when adsorbed on various polysaccharides such as different cellulose batches, carboxymethylcelluloses with different degrees of substitution, and chitine. As a result of this evaluation, Co 153 is recommended as an alternative UV–vis probe for evaluating the dipolarity/polarizability of cellulose and cellulose derivates. Multiple adsorption of Co 153 on Linters cellulose took place indicating a wide-surface polarity distribution, which makes the determination of a rigid polarity parameter questionable. Thus, fluorescence measurements of adsorbed Co 153 are suitable to detect inhomogenities on a surface but not for the determination of empirical polarity parameters. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1210–1218, 2003

The diffusion of individual rhodamine 6G molecules in ethylene glycol close to a glass interface has been studied. Diffusion coefficients are analyzed by photon burst analysis. It is shown that the diffusion of dye molecules becomes slower near the interface as compared to the bulk value. We attribute this to anomalous diffusion of molecules next to the interface, due to attachment and detachment of molecules caused by molecule surface interaction. The analysis of the measurements is tested by computer simulations of fluorescence bursts.

Thermophoretic Swimmers Ring‐shaped DNA origami with defined diameters are designed and synthesized. Site‐specific functionalization with capture strands allows directed assembly of nanoparticles to form tailored sphere dimers for application as self‐thermophoretic nano swimmers. Electron microscopy confirms predefined size and structural integrity of the dimers. More details can be found in article number 1800775by Andreas Heerwig, Michael Mertig, and co‐workers.

This work reports on the construction of mechanically stable DNA origami ring structures that can act as thermophoretic active elements of self‐thermophoretic nanoswimmers or as coupling elements either to other thermophoretic active particles or to cargo elements. The DNA origami structures are designed according to given geometric parameters and physical requirements, synthesized, as well as characterized by agarose gel electrophoresis and transmission electron microscopy. The tailored binding of gold nanoparticles is proven by STEM. The authors present design and synthesis of a DNA origami ring, which can further be functionalized or decorated either with gold or polystyrene nanoparticles paving the way to well controlled and massively parallel self‐assembly of self‐thermophoretic nanoswimmers. The DNA origami construct is characterized in terms of gel electrophoresis and TEM imaging. The accessibility of the origami ring edges by particles and their controlled binding there is verified by STEM imaging.

We present a novel concept for the synthesis of a self‐thermophoretic nanoswimmer, a construct consisting of a gold nanoparticle as a heating element and an artificial DNA structure as a thermophoretic active part. For the latter, DNA origami technique was applied to design and synthesize rod‐like bundle structures as well as a more complex 2‐leg construct. This novel concept is based on the versatile and easy‐to‐apply DNA origami toolbox to realize Janus particle‐like structures with various geometries. We synthesized a variety of different nanoswimmers and characterized them in terms of structure, yield, and thermal stability. Schematic representation of the construction principle of a self‐thermophoretic nanoswimmer consisting of a DNA origami structure and a gold nanoparticle. Inset: TEM image of such a nanoswimmer structure.