Assembling Vertical Nanogap Arrays with Nanoentities for Highly Sensitive Electrical Biosensing.

Nanogap biosensors have emerged as promising platforms for detecting and measuring biochemical substances at low concentrations. Although the nanogap biosensors provide high sensitivity, low limit of detection (LOD), and enhanced signal strength, it requires arduous fabrication processes and costly equipment to obtain micro/nanoelectrodes with extremely narrow gaps in a controlled manner. In this work, we report the novel design and fabrication processes of vertical nanogap structures that can electrically detect and quantify low-concentration biochemical substances. Approximately 40 nm gaps are facilely created by magnetically assembling antibody-coated nanowires onto a nanodisk patterned between a pair of microelectrodes. Analyte molecules tagged with conductive nanoparticles are captured and bound to nanowires and bridge over the nanogaps, which consequently causes an abrupt change in the electrical conductivity between the microelectrodes. Using biotin and streptavidin as model antibodies and analytes, we demonstrated that our nanogap biosensors can effectively measure the protein analytes with the LOD of ∼18 pM. The outcome of this research could inspire the design and fabrication of nanogap devices and nanobiosensors, and it would have a broad impact on the development of microfluidics, biochips, and lab-on-a-chip architectures.

An angle-compensating colorimetric strain sensor with wide working range and its fabrication method.

The visual response is one of the most intuitive principles of sensors. Therefore, emission and change of the colors are widely studied for development of chemical, thermal and mechanical sensors. And it is still a challenging issue to fabricate them with a simple working mechanism, high sensitivity, good reliability, and a cost-effective fabrication process. In this study, we propose a mechanical strain sensor, which has 2D photonic crystal structures in nanoscale on stretchable polydimethylsiloxane (PDMS) substrate. Due to the periodic nanostructures, the surface of the sensor produces structural colors. And when it is stretched, the periodicity of the nanostructures changes, which results in the shift of the colors. Multiple nanostructures with different periodicities are integrated on the sensor in order to extend the working range up to 150% with high sensitivity. In addition, reusable and robust molds, which are fabricated by self-assembly of nanoparticles, are used for multiple replications of sensor substrates. Thus, the fabrication process of this study is believed to be potential for possible industrial manufacturing. This study is expected to contribute to strain sensors in the future for the applications of health care, infrastructure monitoring, soft robotics, and wearable devices.

Fabrication of robust and reusable mold with nanostructures and its application to anti-counterfeiting surfaces based on structural colors.

In this study, we report a method to fabricate molds and flexible stamps with 2D photonic crystal structures. This includes self-assembly of polystyrene particles into monolayer, oxygen reactive ion etching, thin film (chromium (Cr)) deposition, and polydimethylsiloxane replication. By tuning the thickness of Cr layer, reusable master molds with nano bumps or nano concaves could be prepared selectively. We showed that the replicated flexible stamps out of these molds exhibited structural colors. Characteristics of the colors depended on viewing angle, brightness of background and light source. And the colors even faded out when the background is white or when the stamp was bent. By using this feature, possible strategies for anti-counterfeiting applications have been suggested in this study. Since the molds are reusable and the fabrication method is simple and cost-effective, this study is expected to contribute to nano devices for industries in future.

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.

Electrical, Structural, Optical, and Adhesive Characteristics of Aluminum-Doped Tin Oxide Thin Films for Transparent Flexible Thin-Film Transistor Applications.

The properties of Al-doped SnOx films deposited via reactive co-sputtering were examined in terms of their potential applications for the fabrication of transparent and flexible electronic devices. Al 2.2-atom %-doped SnOx thin-film transistors (TFTs) exhibit improved semiconductor characteristics compared to non-doped films, with a lower sub-threshold swing of ~0.68 Vdec-1, increased on/off current ratio of ~8 × 107, threshold voltage (Vth) near 0 V, and markedly reduced (by 81%) Vth instability in air, attributable to the decrease in oxygen vacancy defects induced by the strong oxidizing potential of Al. Al-doped SnOx films maintain amorphous crystallinity, an optical transmittance of ~97%, and an adhesive strength (to a plastic substrate) of over 0.7 kgf/mm; such films are thus promising semiconductor candidates for fabrication of transparent flexible TFTs.

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.

Man-made rotary nanomotors: a review of recent developments.

The development of rotary nanomotors is an essential step towards intelligent nanomachines and nanorobots. In this article, we review the concept, design, working mechanisms, and applications of state-of-the-art rotary nanomotors made from synthetic nanoentities. The rotary nanomotors are categorized according to the energy sources employed to drive the rotary motion, including biochemical, optical, magnetic, and electric fields. The unique advantages and limitations for each type of rotary nanomachines are discussed. The advances of rotary nanomotors is pivotal for realizing dream nanomachines for myriad applications including microfluidics, biodiagnosis, nano-surgery, and biosubstance delivery.

Ultra-durable rotary micromotors assembled from nanoentities by electric fields.

Recently, we reported an innovative type of micromotors consisting of nanowires as rotors and patterned Au/Ni/Cr nanodisks as bearings. The dimensions of micromotors were less than 1 µm, and could continuously rotate for 15 hours over 240 000 cycles. To understand the limitation of their lifetime, we systematically investigated the rotation dynamics by analytical modeling and determined the time-dependent torques and forces involved in the rotation. From the forces and torques, the extent of wear of micromotors was successfully derived, which agreed well with the experimental characterization. The results also proved that the frictional force linearly increases with the loading in such rotary nanodevices operating in suspension, consistent with the prediction of the non-adhesive multi-asperity friction theory. With these understandings, we enhanced the design of micromotors and achieved an operation lifetime of 80 hours and over 1.1 million total rotation cycles. This research, shedding new light on the frictional mechanism of recently reported nanowire micromotors with demonstration of the most durable rotary nanomechanical devices of similar dimensions to the best of our knowledge, can be inspiring for innovative design of future nanomechanical devices with ultra-long lifetime for practical applications.

Recent Progress on Man-Made Inorganic Nanomachines.

The successful development of nanoscale machinery, which can operate with high controllability, high precision, long lifetimes, and tunable driving powers, is pivotal for the realization of future intelligent nanorobots, nanofactories, and advanced biomedical devices. However, the development of nanomachines remains one of the most difficult research areas, largely due to the grand challenges in fabrication of devices with complex components and actuation with desired efficiency, precision, lifetime, and/or environmental friendliness. In this work, the cutting-edge efforts toward fabricating and actuating various types of nanomachines and their applications are reviewed, with a special focus on nanomotors made from inorganic nanoscale building blocks, which are introduced according to the employed actuation mechanism. The unique characteristics and obstacles for each type of nanomachine are discussed, and perspectives and challenges of this exciting field are presented.

Fabrication and robotization of ultrasensitive plasmonic nanosensors for molecule detection with Raman scattering.

In this work, we introduce the history and mechanisms of surface enhanced Raman scattering (SERS), discuss various techniques for fabrication of state-of-the-art SERS substrates, and review recent work on robotizing plasmonic nanoparticles, especially, the efforts we made on fabrication, characterization, and robotization of Raman nanosensors by design. Our nanosensors, consisting of tri-layer nanocapsule structures, are ultrasensitive, well reproducible, and can be robotized by either electric or magnetic tweezers. Three applications using such SERS nanosensors were demonstrated, including location predictable detection, single-cell bioanalysis, and tunable molecule release and monitoring. The integration of SERS and nanoelectromechanical system (NEMS) devices is innovative in both device concept and fabrication, and could potentially inspire a new device scheme for various bio-relevant applications.

Tunable release of multiplex biochemicals by plasmonically active rotary nanomotors.

It is highly desirable to precisely tune the molecule release rate from the surfaces of nanoparticles (NPs) that are relevant to cancer therapy and single-cell biology. An innovative mechanism is reported to actively tune the biochemical release rate by rotation of NPs. Plasmonic nanomotors were assembled from NPs and applied in multiplex biochemical release and detection. Both single and multiplex biosignals can be released in a tunable fashion by controlling the rotation speed of the nanomotors. The chemistry and release rate of individual chemicals can be revealed by Raman spectroscopy. The fundamental mechanism was modeled quantitatively and attributed to the fluidic boundary-layer reduction owing to the liquid convection. This work, which explored the synergistic attributes of surface enhanced Raman scattering and nanoelectromechanical systems, could inspire new sensors that are potentially interesting for various bio-applications.

Micromotors with step-motor characteristics by controlled magnetic interactions among assembled components.

In this study, we investigated the control of the rotation dynamics of an innovative type of rotary micromotors with desired performances by tuning the magnetic interactions among the assembled micro/nanoscale components. The micromotors are made of metallic nanowires as rotors, patterned magnetic nanodisks as bearings and actuated by external electric fields. The magnetic forces for anchoring the rotors on the bearings play an essential role in the rotation dynamics of the micromotors. By varying the moment, orientation, and dimension of the magnetic components, distinct rotation behaviors can be observed, including repeatable wobbling and rolling in addition to rotation. We understood the rotation behaviors by analytical modeling, designed and realized micromotors with step-motor characteristics. The outcome of this research could inspire the development of high-performance nanomachines assembled from synthetic nanoentities, relevant to nanorobotics, microfluidics, and biomedical research.

Location deterministic biosensing from quantum-dot-nanowire assemblies.

Semiconductor quantum dots (QDs) with high fluorescent brightness, stability, and tunable sizes, have received considerable interest for imaging, sensing, and delivery of biomolecules. In this research, we demonstrate location deterministic biochemical detection from arrays of QD-nanowire hybrid assemblies. QDs with diameters less than 10 nm are manipulated and precisely positioned on the tips of the assembled Gold (Au) nanowires. The manipulation mechanisms are quantitatively understood as the synergetic effects of dielectrophoretic (DEP) and alternating current electroosmosis (ACEO) due to AC electric fields. The QD-nanowire hybrid sensors operate uniquely by concentrating bioanalytes to QDs on the tips of nanowires before detection, offering much enhanced efficiency and sensitivity, in addition to the position-predictable rationality. This research could result in advances in QD-based biomedical detection and inspires an innovative approach for fabricating various QD-based nanodevices.

Ultrahigh-speed rotating nanoelectromechanical system devices assembled from nanoscale building blocks.

The development of rotary nanomotors is crucial for advancing nanoelectromechanical system technology. In this work, we report design, assembly and rotation of ordered arrays of nanomotors. The nanomotors are bottom-up assembled from nanoscale building blocks with nanowires as rotors, patterned nanomagnets as bearings and quadrupole microelectrodes as stators. Arrays of nanomotors rotate with controlled angle, speed (over 18,000 r.p.m.), and chirality by electric fields. Using analytical modelling, we reveal the fundamental nanoscale electrical, mechanical and magnetic interactions in the nanomotor system, which excellently agrees with experimental results and provides critical understanding for designing metallic nanoelectromechanical systems. The nanomotors can be continuously rotated for 15 h over 240,000 cycles. They are applied for controlled biochemical release and demonstrate releasing rate of biochemicals on nanoparticles that can be precisely tuned by mechanical rotations. The innovations reported in this research, from concept, design and actuation to application, are relevant to nanoelectromechanical system, nanomedicine, microfluidics and lab-on-a-chip architectures.

Suspended GaN nanowires as NO2 sensor for high temperature applications.

We propose a gas sensor operable over a wide temperature range and using suspended GaN nanowires functionalized with Pt-Pd. The sensor is batch-fabricated by directly integrating the GaN nanowires onto batch-processed silicon microelectrodes in parallel. The high thermal stability of the sensor originates from a large band gap of GaN nanowires that enables the detection of NO2 gas at an elevated temperature of up to 350 °C without a decrease in responsiveness. Exposed to NO2 at 100-1000 ppm at 350 °C, the sensor shows a linear increment in relative response with respect to the change in gas concentration. The sensor results in a two- to four-fold increase in responsiveness to NO2 at 100 ppm compared to NH3 at 100 ppm and CO2 at 1000 ppm. The nanowires suspended over a substrate provide increased surface area that could interact with gas molecules for enhanced responsiveness, and prevent any unnecessary interactions between the nanowires and the substrate.

Innovative mechanisms for precision assembly and actuation of arrays of nanowire oscillators.

Bottom-up assembling of Micro/Nano Electromechanical System (MEMS/NEMS) devices from nanoscale building blocks is highly desirable but extremely difficult to achieve. In this work, we report innovative mechanisms for precision assembly and actuation of arrays of nanowire NEMS devices that can synchronously oscillate between two designated positions for over 4000 cycles. The assembly and actuation mechanisms are based on unique magnetic interactions between nanoentities with perpendicular magnetic anisotropy (PMA) and electric-tweezer manipulation, our recent invention. Quantitative analysis of the dynamics of torques involved in the nano-oscillators reveals that the induced electrostatic torques due to the external electric fields between metallic NEMS components play a significant role in the mechanical actuation. These new findings are expected to inspire new in situ assembly and actuation strategies in the general field of NEMS devices such as nanomechanical switches for toggling on/off circuits and nanoresonators for biochemical sensors and radio frequency communication.

Ordered arrays of Raman nanosensors for ultrasensitive and location predictable biochemical detection.

Surface enhanced Raman scattering (SERS) is sensitive enough for single-molecule biochemical detection, but it is extremely difficult to obtain a large number of SERS hotspots for sensitive and reproducible detection. It is even more challenging to assemble the hotspots at designated positions for location predictable sensing. Here, we report an original strategy for the synthesis, manipulation, and assembling of plasmonic nanocapsule SERS sensors for high-sensitivity biochemical detection at predictable locations.

Lithography-Free Fabrication of Large Area Subwavelength Antireflection Structures Using Thermally Dewetted Pt/Pd Alloy Etch Mask.

We have demonstrated lithography-free, simple, and large area fabrication method for subwavelength antireflection structures (SAS) to achieve low reflectance of silicon (Si) surface. Thin film of Pt/Pd alloy on a Si substrate is melted and agglomerated into hemispheric nanodots by thermal dewetting process, and the array of the nanodots is used as etch mask for reactive ion etching (RIE) to form SAS on the Si surface. Two critical parameters, the temperature of thermal dewetting processes and the duration of RIE, have been experimentally studied to achieve very low reflectance from SAS. All the SAS have well-tapered shapes that the refractive index may be changed continuously and monotonously in the direction of incident light. In the wavelength range from 350 to 1800 nm, the measured reflectance of the fabricated SAS averages out to 5%. Especially in the wavelength range from 550 to 650 nm, which falls within visible light, the measured reflectance is under 0.01%.