Light-stimulated micromotor swarms in an electric field with accurate spatial, temporal, and mode control.

Swarming, a phenomenon widely present in nature, is a hallmark of nonequilibrium living systems that harness external energy into collective locomotion. The creation and study of manmade swarms may provide insights into their biological counterparts and shed light to the rules of life. Here, we propose an innovative mechanism for rationally creating multimodal swarms with unprecedented spatial, temporal, and mode control. The research is realized in a system made of optoelectric semiconductor nanorods that can rapidly morph into three distinct modes, i.e., network formation, collectively enhanced rotation, and droplet-like clustering, pattern, and switch in-between under light stimulation in an electric field. Theoretical analysis and semiquantitative modeling well explain the observation by understanding the competition between two countering effects: the electrostatic assembly for orderliness and electrospinning-induced disassembly for disorderliness. This work could inspire the rational creation of new classes of reconfigurable swarms for both fundamental research and emerging applications.

Precise electrokinetic position and three-dimensional orientation control of a nanowire bioprobe in solution.

Owing to Brownian-motion effects, the precise manipulation of individual micro- and nanoparticles in solution is challenging. Therefore, scanning-probe-based techniques, such as atomic force microscopy, attach particles to cantilevers to enable their use as nanoprobes. Here we demonstrate a versatile electrokinetic trap that simultaneously controls the two-dimensional position with a precision of 20 nm and 0.5° in the three-dimensional orientation of an untethered nanowire, as small as 300 nm in length, under an optical microscope. The method permits the active transport of nanowires with a speed-dependent accuracy reaching 90 nm at 2.7 µm s-1. It also allows for their synchronous three-dimensional alignment and rotation during translocation along complex trajectories. We use the electrokinetic trap to accurately move a nanoprobe and stably position it on the surface of a single bacterial cell for sensing secreted metabolites for extended periods. The precision-controlled manipulation underpins developing nanorobotic tools for assembly, micromanipulation and biological measurements with subcellular resolution.

Portable Bulk-Water Disinfection by Live Capture of Bacteria with Divergently Branched Porous Graphite in Electric Fields.

Easy access to clean water is essential to functioning and development of modern society. However, it remains arduous to develop energy-efficient, facile, and portable water treatment systems for point-of-use (POU) applications, which is particularly imperative for the safety and resilience of society during extreme weather and critical situations. Here, we propose and validate a meritorious working scheme for water disinfection via directly capturing and removing pathogen cells from bulk water using strategically designed three-dimensional (3D) porous dendritic graphite foams (PDGFs) in a high-frequency AC field. The prototype, integrated in a 3D-printed portable water-purification module, can reproducibly remove 99.997% E. coli bacteria in bulk water at a few voltages with among the lowest energy consumption at 435.5 J·L-1. The PDGFs, costing $1.47 per piece, can robustly operate at least 20 times for more than 8 h in total without functional degradation. Furthermore, we successfully unravel the involved disinfection mechanism with one-dimensional Brownian dynamics simulation. The system is practically applied that brings natural water in Waller Creek at UT Austin to the safe drinking level. This research, including the working mechanism based on dendritically porous graphite and the design scheme, could inspire a future device paradigm for POU water treatment.

Acceleration of Biomolecule Enrichment and Detection with Rotationally Motorized Opto-Plasmonic Microsensors and the Working Mechanism.

Vigorous research efforts have advanced the state-of-the-art nanosensors with ultrahigh sensitivity for bioanalysis. However, a dilemmatic challenge remains: it is extremely difficult to obtain nanosensors that are both sensitive and high-speed for the detection of low-concentration molecules in aqueous samples. Herein, we report how the controlled mechanical rotation (or rotary motorization) of designed opto-plasmonic microsensors can substantially and robustly accelerate the enrichment and detection speed of deoxyribonucleic acid (DNA) with retained high sensitivity. At least 4-fold augmentation of the capture speed of DNA molecules is obtained from a microsensor rotating at 1200 rpm. Theoretical analysis and modeling shed light on the underlying working mechanism, governed by the molecule-motor-flow interaction as well as its application range and limitation. This work provides a device scheme that alleviates the dilemmatic challenge in biomolecule sensing and offers the understanding of the complex interactions of molecules and moving microobjects in suspension. The results may assist the rational design of efficient microrobotic systems for the capture, translocation, sensing, and release of biocargoes.

Scalable Fabrication of Molybdenum Disulfide Nanostructures and their Assembly.

Molybdenum disulfide (MoS2 ) is a multifunctional material that can be used for various applications. In the single-crystalline form, MoS2 shows superior electronic properties. It is also an exceptionally useful nanomaterial in its polycrystalline form with applications in catalysis, energy storage, water treatment, and gas sensing. Here, the scalable fabrication of longitudinal MoS2 nanostructures, i.e., nanoribbons, and their oxide hybrids with tunable dimensions in a rational and well-reproducible fashion, is reported. The nanoribbons, obtained at different reaction stages, that is, MoO3 , MoS2 /MoO2 hybrid, and MoS2 , are fully characterized. The growth method presented herein has a high yield and is particularly robust. The MoS2 nanoribbons can readily be removed from its substrate and dispersed in solution. It is shown that functionalized MoS2 nanoribbons can be manipulated in solution and assembled in controlled patterns and directly on microelectrodes with UV-click-chemistry. Owing to the high chemical purity and polycrystalline nature, the MoS2 nanostructures demonstrate rapid optoelectronic response to wavelengths from 450 to 750 nm, and successfully remove mercury contaminants from water. The scalable fabrication and manipulation followed by light-directed assembly of MoS2 nanoribbons, and their unique properties, will be inspiring for device fabrication and applications of the transition metal dichalcogenides.

Light programmable micro/nanomotors with optically tunable in-phase electric polarization.

To develop active nanomaterials that can instantly respond to external stimuli with designed mechanical motions is an important step towards the realization of nanorobots. Herein, we present our finding of a versatile working mechanism that allows instantaneous change of alignment direction and speed of semiconductor nanowires in an external electric field with simple visible-light exposure. The light induced alignment switch can be cycled over hundreds of times and programmed to express words in Morse code. With theoretical analysis and simulation, the working principle can be attributed to the optically tuned real-part (in-phase) electrical polarization of a semiconductor nanowire in aqueous suspension. The manipulation principle is exploited to create a new type of microscale stepper motor that can readily switch between in-phase and out-phase modes, and agilely operate independent of neighboring motors with patterned light. This work could inspire the development of new types of micro/nanomachines with individual and reconfigurable maneuverability for many applications.

Visible light-gated reconfigurable rotary actuation of electric nanomotors.

Highly efficient and widely applicable working mechanisms that allow nanomaterials and devices to respond to external stimuli with controlled mechanical motions could make far-reaching impact to reconfigurable, adaptive, and robotic nanodevices. We report an innovative mechanism that allows multifold reconfiguration of mechanical rotation of semiconductor nanoentities in electric (E) fields by visible light stimulation. When illuminated by light in the visible-to-infrared regime, the rotation speed of semiconductor Si nanowires in E-fields can instantly increase, decrease, and even reverse the orientation, depending on the intensity of the applied light and the AC E-field frequency. This multifold rotational reconfiguration is highly efficient, instant, and facile. Switching between different modes can be simply controlled by the light intensity at an AC frequency. We carry out experiments, theoretical analysis, and simulations to understand the underlying principle, which can be attributed to the optically tunable polarization of Si nanowires in an aqueous suspension and an external E-field. Finally, leveraging this newly discovered effect, we successfully differentiate semiconductor and metallic nanoentities in a noncontact and nondestructive manner. This research could inspire a new class of reconfigurable nanoelectromechanical and nanorobotic devices for optical sensing, communication, molecule release, detection, nanoparticle separation, and microfluidic automation.

Electrokinetic Manipulation Integrated Plasmonic-Photonic Hybrid Raman Nanosensors with Dually Enhanced Sensitivity.

To detect biochemicals with ultrahigh sensitivity, efficiency, reproducibility, and specificity has been the holy grail in the development of nanosensors. In this work, we report an innovative type of photonic-plasmonic hybrid Raman nanosensor integrated with electrokinetic manipulation by rational design, which offers dual mechanisms that enhance the sensitivity for molecule detection directly in solution. For the first time, we integrate large arrays of synthesized plasmonic nanocapsules with densely surface distributed silver (Ag) nanoparticles (NPs) on lithographically patterned photonic crystal slabs via electric-field assembling. With the interdigital microelectrodes, the applied electric fields not only assemble the hybrid plasmonic nanocapsules on photonic crystal slabs, but also generate electrokinetic flows that focus analyte molecules to the Ag hot spots on the nanocapsules for surface-enhanced Raman scattering (SERS) detection. The synergistic effects of plasmonic-photonic resonance and the electrokinetic molecular focusing can promote the SERS enhancement factor (EF) robustly to ∼2 × 109. Various molecules including SERS probing molecules, nucleobases, and unsafe food additives can be detected directly from suspension. The innovative mechanism, design, and fabrication reported in this work can inspire a new paradigm for achieving high-performance Raman nanosensors, which is pivotal for lab-on-chip disease diagnosis and environmental protection.

Biobased High-Performance Rotary Micromotors for Individually Reconfigurable Micromachine Arrays and Microfluidic Applications.

In this work, we report an innovative type of rotary biomicromachines by using diatom frustules as integrated active components, including the assembling, operation, and performance characterization. We further investigate and demonstrate unique applications of the biomicromachines in achieving individually reconfigurable micromachine arrays and microfluidic mixing. Diatom frustules are porous cell walls of diatoms made of silica. We assembled rotary micromachines consisting of diatom frustules serving as rotors and patterned magnets serving as bearings in electric fields. Ordered arrays of micromotors can be integrated and rotated with controlled orientation and a speed up to ∼3000 rpm, one of the highest rotational speeds in biomaterial-based rotary micromachines. Moreover, by exploiting the distinct electromechanical properties of diatom frustules and metallic nanowires, we realized the first reconfigurable rotary micro/nanomachine arrays with controllability in individual motors. Finally, the diatom micromachines are successfully integrated in microfluidic channels and operated as mixers. This work demonstrated the high-performance rotary micromachines by using bioinspired diatom frustules and their applications, which are essential for low-cost bio-microelectromechanical system/nanoelectromechanical system (bio-MEMS/NEMS) devices and relevant to microfluidics.

Gel mesh as "brake" to slow down DNA translocation through solid-state nanopores.

Agarose gel is introduced onto the cis side of silicon nitride nanopores by a simple and low-cost method to slow down the speed of DNA translocation. DNA translocation speed is slowed by roughly an order of magnitude without losing signal to noise ratio for different DNA lengths and applied voltages in gel-meshed nanopores. The existence of the gel moves the center-of-mass position of the DNA conformation further from the nanopore center, contributing to the observed slowing of translocation speed. A reduced velocity fluctuation is also noted, which is beneficial for further applications of gel-meshed nanopores. The reptation model is considered in simulation and agrees well with the experimental results.

Interaction prolonged DNA translocation through solid-state nanopores.

An interesting smooth blocked nanopore and corresponding "current ladder" phenomenon was observed in DNA translocation experiments through solid-state nanopores. The ionic current shows several drop steps (current levels in the current ladder) with an identical drop interval, which corresponds to an individual unfolded DNA translocation event. This indicates that multiple anchored DNA molecules have one end inside the nanopore to cause such a current ladder. On each current level, normal DNA translocation events were detected. The event duration time increases as the level number increases, which means DNA translocates more slowly when more DNA molecules are inside the nanopore due to DNA-DNA interactions. The Langevin dynamic model was used to explain the experimental observations. This finding strongly suggests that DNA-DNA interactions greatly impact the translocation dynamics when DNA passes through the nanopores.