Atomically Precise Nanoclusters as Co-Catalysts for Light-Activated Microswimmer Motility.

Microswimmers are self-propelled particles that navigate fluid environments, offering significant potential for applications in environmental pollutant decomposition, biosensing, and targeted drug delivery. Their performance relies on engineered catalytic surfaces. Gold nanoclusters (AuNCs), with atomically precise structures, tunable optical properties, and high surface area-to-volume ratio, provide a new optimal catalyst for enhancing microswimmer propulsion.

Controlling the Speed of Light-Activated Colloids with a Constant, Uniform Magnetic Field.

It is demonstrated how the strength of activation for photocatalytic, self-propelled colloids can be enhanced with a constant, uniform magnetic field. When exposed to ultraviolet light and hydrogen peroxide, the titanium dioxide-based colloids become actively propelled. Due to the iron oxide core, a uniform field oriented perpendicular to the surface where motion takes place causes the asymmetrically shaped particles to rotate, which consequently leads to an increase in activity.

Material-dependent performance of fuel-free, light-activated, self-propelling colloids.

Self-propelling, light-activated colloidal particles can be actuated in water alone. Here we study the effect of adding different amounts of a gold/palladium alloy to titanium dioxide-based, active colloids. We observe a correlation between alloy-thickness and the average speed of the particles, and we discover an intermediate thickness leads to the highest activity for this system.

Shape- and Material-Dependent Self-Propulsion of Photocatalytic Active Colloids, Interfacial Effects, and Dynamic Interparticle Interactions.

Active colloids powered by self-generated, local chemical concentration gradients exhibit dynamics that are a function of the particles' morphology and material properties. These characteristics also govern how the active colloids interact with surfaces, including other particles and nearby walls. Thus, by targeted design, the dynamic behavior, on average, can be engineered, despite a lack of "external" control such as an applied magnetic field.

Shape-Dependent, Chiro-optical Response of UV-Active, Nanohelix Metamaterials.

We harness a synergy between morphology and the electromagnetic response of semiconducting material to engineer the chiro-optical properties of metamaterials that are active at ultraviolet (UV) wavelengths. Chiral metamaterials have recently ushered in new research directions in fundamental light-matter interactions, while simultaneously opening a range of promising photonics-based applications from polarization control to improved biosensing methods. Despite these recent advances, to date, very little attention has been focused upon engineered large UV-chiro-optical activity, where naturally occurring molecular optical activity bands are most typically encountered.

Self-Phoretic Microswimmers Propel at Speeds Dependent upon an Adjacent Surface's Physicochemical Properties.

Self-phoretic colloids are emerging as critical components of programmable nano- and microscale active matter and may usher in a new area of complex, small-scale machinery. To date, most studies have focused upon active particles confined by gravity to a plane located just above a solid/liquid interface. Despite this ubiquity, little attention has been directed at how the physicochemical qualities of this interface might affect motion.

Light-Activated, Multi-Semiconductor Hybrid Microswimmers.

Using a dynamic fabrication process, hybrid, photoactivated microswimmers made from two different semiconductors, titanium dioxide (TiO ) and cuprous oxide (Cu O) are developed, where each material occupies a distinct portion of the multiconstituent particles. Structured light-activated microswimmers made from only TiO or Cu O are observed to be driven in hydrogen peroxide and water most vigorously under UV or blue light, respectively, whereas hybrid structures made from both of these materials exhibit wavelength-dependent modes of motion due to the disparate responses of each photocatalyst. It is also found that the hybrid particles are activated in water alone, a behavior which is not observed in those made from a single semiconductor, and thus, the system may open up a new class of fuel-free photoactive colloids that take advantage of semiconductor heterojunctions.

Shape-Dependent Motion of Structured Photoactive Microswimmers.

We investigate the dynamics of structured photoactive microswimmers and show that morphology sensitively determines the swimming behavior. Particular to this study, a major portion of the light-activated particles' underlying structure is built from a photocatalytic material, made possible by dynamic physical vapor deposition (DPVD). We find that swimmers of this type exhibit unique shape-dependent autonomous swimming that is distinct from what is seen in systems with similar structural morphology but not fabricated directly from the catalyst.

Engineering Contactless Particle-Particle Interactions in Active Microswimmers.

Artificial self-propelled colloidal particles have recently served as effective building blocks for investigating many dynamic behaviors exhibited by nonequilibrium systems. However, most studies have relied upon excluded volume interactions between the active particles. Experimental systems in which the mobile entities interact over long distances in a well-defined and controllable manner are valuable so that new modes of multiparticle dynamics can be studied systematically in the laboratory.

Dynamic stabilization of Janus sphere trans-dimers.

We experimentally investigated the self-assembly of chemically active colloidal Janus spheres into dimers. The trans-dimer conformation, in which the two active sites are oriented roughly in opposite directions and the particles are osculated at their equators, becomes dominant as the hydrogen peroxide fuel concentration increases. Our observations suggest high spinning frequency combined with little translational motion is at least partially responsible for the stabilization of the trans-dimer as activity increases.

Spiral diffusion of rotating self-propellers with stochastic perturbation.

Translationally diffusive behavior arising from the combination of orientational diffusion and powered motion at microscopic scales is a known phenomenon, but the peculiarities of the evolution of expected position conditioned on initial position and orientation have been neglected. A theory is given of the spiral motion of the mean trajectory depending upon propulsion speed, angular velocity, orientational diffusion, and rate of random chirality reversal. We demonstrate the experimental accessibility of this effect using both tadpole-like and Janus sphere dimer rotating motors.

Active Nanorheology with Plasmonics.

Nanoplasmonic systems are valued for their strong optical response and their small size. Most plasmonic sensors and systems to date have been rigid and passive. However, rendering these structures dynamic opens new possibilities for applications.

Rotational Analysis of Spherical, Optically Anisotropic Janus Particles by Dynamic Microscopy.

We analyze the rotational dynamics of spherical colloidal Janus particles made from silica (SiO2) with a hemispherical gold/palladium (Au/Pd) cap. Since the refractive index difference between the surrounding fluid and a two-faced, optically anisotropic Janus microsphere is a function of the particle's orientation, it is possible to observe its rotational dynamics with bright-field optical microscopy. We investigate rotational diffusion and constant rotation of single Janus microspheres which are partially tethered to a solid surface so they are free to rotate but show little or no translational motion.

Surface roughness-induced speed increase for active Janus micromotors.

We demonstrate a simple physical fabrication method to control surface roughness of Janus micromotors and fabricate self-propelled active Janus microparticles with rough catalytic platinum surfaces that show a four-fold increase in their propulsion speed compared to conventional Janus particles coated with a smooth Pt layer.

Nanopropellers and their actuation in complex viscoelastic media.

Tissue and biological fluids are complex viscoelastic media with a nanoporous macromolecular structure. Here, we demonstrate that helical nanopropellers can be controllably steered through such a biological gel. The screw-propellers have a filament diameter of about 70 nm and are smaller than previously reported nanopropellers as well as any swimming microorganism.

Shape control in wafer-based aperiodic 3D nanostructures.

Controlled local fabrication of three-dimensional (3D) nanostructures is important to explore and enhance the function of single nanodevices, but is experimentally challenging. We present a scheme based on e-beam lithography (EBL) written seeds, and glancing angle deposition (GLAD) grown structures to create nanoscale objects with defined shapes but in aperiodic arrangements. By using a continuous sacrificial corral surrounding the features of interest we grow isolated 3D nanostructures that have complex cross-sections and sidewall morphology that are surrounded by zones of clean substrate.

Nanohelices by shadow growth.

The helix has remarkable qualities and is prevalent in many fields including mathematics, physics, chemistry, and biology. This shape, which is chiral by nature, is ubiquitous in biology with perhaps the most famous example being DNA. Other naturally occurring helices are common at the nanoscale in the form of protein secondary structures and in various macromolecules.

Self-propelling nanomotors in the presence of strong Brownian forces.

Motility in living systems is due to an array of complex molecular nanomotors that are essential for the function and survival of cells. These protein nanomotors operate not only despite of but also because of stochastic forces. Artificial means of realizing motility rely on local concentration or temperature gradients that are established across a particle, resulting in slip velocities at the particle surface and thus motion of the particle relative to the fluid.

Chiral colloidal molecules and observation of the propeller effect.

Chiral molecules play an important role in biological and chemical processes, but physical effects due to their symmetry-breaking are generally weak. Several physical chiral separation schemes which could potentially be useful, including the propeller effect, have therefore not yet been demonstrated at the molecular scale. However, it has been proposed that complex nonspherical colloidal particles could act as "colloidal molecules" in mesoscopic model systems to permit the visualization of molecular phenomena that are otherwise difficult to observe.

Hybrid nanocolloids with programmed three-dimensional shape and material composition.

Tuning the optical, electromagnetic and mechanical properties of a material requires simultaneous control over its composition and shape. This is particularly challenging for complex structures at the nanoscale because surface-energy minimization generally causes small structures to be highly symmetric. Here we combine low-temperature shadow deposition with nanoscale patterning to realize nanocolloids with anisotropic three-dimensional shapes, feature sizes down to 20 nm and a wide choice of materials.

Self-organized multiconstituent catalytic nanomotors.

Self-organized catalytic nanomotors consisting of more than one individual component are presented. Tadpole-like catalytic nanomotors fabricated by dynamic shadowing growth (DSG) self-organize randomly to form two-nanomotor clusters (approximately 1-3% yield) that spin as opposed to circular motion exhibited by the individual structures. By introducing magnetic materials to another system, self-assembled "helicopter" nanomotors consisting of a V-shaped nanomotor and a microbead are formed with approximately 25% yield, showing a significantly higher yield than the control (0%).