Microbeads on microposts: an inverted architecture for bead microarrays

The rapid development of genomics and proteomics requires accelerated improvement of the microarrays density, multiplexing, readout capabilities and cost-effectiveness. The bead arrays are increasingly attractive because of their self-assembly-based fabrication, which alleviates many problems of top-down microfabrication. Here we present a simple, reliable, robust and modular technique for the fabrication of bead microarrays, which combines the directed assembling of beads in microstructures and PDMS-based replica molding. The beads are first self-assembled in pyramidal microwells fabricated by anisotropic etching of silicon substrates, then transferred on the apex of PDMS pyramids that replicate the silicon microstructures. The arrays are chemically and biochemically robust; they are spatially addressable and have the potential for being informationally addressable; and they appear to offer better readout capabilities than the classical microarrays.     

Biofabrication: using biological materials and biocatalysts to construct nanostructured assemblies

Emerging opportunities are placing greater demands on device fabrication: next-generation microelectronics will need minimum features of less than 100 nm, high-throughput drug screening will require facile methods to incorporate sensitive biological components into microelectromechanical systems (MEMS), and implantable devices will need to be built from biocompatible materials. Increasingly, these emerging demands are being addressed by combining traditional microfabrication methods with 'biofabrication': namely, the use of biologically derived materials and biocatalysts. Recent fabrication techniques are using biological construction materials as process aids or structural components, and enzymes are being considered for their potential to fabricate devices with high selectivity under mild conditions. If incompatibilities between biology and microfabrication can be eliminated, then biofabrication will be poised to emerge as the standard for nanoscale construction.     

Enriching libraries of high-aspect-ratio micro- or nanostructures by rapid, low-cost, benchtop nanofabrication

We provide a protocol for transforming the structure of an array of high-aspect-ratio (HAR) micro/nanostructures into various new geometries. Polymeric HAR arrays are replicated from a Bosch-etched silicon master pattern by soft lithography. By using various conditions, the original pattern is coated with metal, which acts as an electrode for the electrodeposition of conductive polymers, transforming the original structure into a wide range of user-defined new designs. These include scaled replicas with sub-100-nm-level control of feature sizes and complex 3D shapes such as tapered or bent columnar structures bearing hierarchical features. Gradients of patterns and shapes on a single substrate can also be produced. This benchtop fabrication protocol allows the production of customized libraries of arrays of closed-cell or isolated HAR micro/nanostructures at a very low cost within 1 week, when starting from a silicon master that otherwise would be very expensive and slow to produce using conventional fabrication techniques.     

Tungsten–Carbon Nanotube Composite Photonic Crystals as Thermally Stable Spectral‐Selective Absorbers and Emitters for Thermophotovoltaics

Thermophotovoltaics (TPVs) is a promising energy conversion technology which can harvest wide‐spectrum thermal radiation. However, the manufacturing complexity and thermal instability of the nanophotonic absorber and emitter, which are key components of TPV devices, significantly limit their scalability and practical deployment. Here, tungsten–carbon nanotube (W‐CNT) composite photonic crystals (PhCs) exhibiting outstanding spectral and angular selectivity of photon absorbance and thermal emission are presented. The W‐CNT PhCs are fabricated by nanoscale holographic interferometry‐based patterning of a thin‐film catalyst, modulated chemical vapor deposition synthesis of high‐density CNT forest nanostructures, and infiltration of the CNT forests with tungsten via atomic layer deposition. Owing to their highly stable structure and composition, the W‐CNT PhCs exhibit negligible degradation of optical properties after annealing for 168 hours at 1273 K, which exceeds all previously reported high‐temperature PhCs. Using the measured spectral properties of the W‐CNT PhCs, the system efficiency of a GaSb‐based solar TPV (STPV) that surpasses the Shockley–Queisser efficiency limit at modest operating temperatures and input powers is numerically predicted. These findings encourage further practical development of STPVs, and this scalable fabrication method for composite nanostructures could find other applications in electromagnetic metamaterials.     

Fabrication of Mechanically Tunable and Bioactive Metal Scaffolds for Biomedical Applications

Biometal systems have been widely used for biomedical applications, in particular, as load-bearing materials. However, major challenges are high stiffness and low bioactivity of metals. In this study, we have developed a new method towards fabricating a new type of bioactive and mechanically reliable porous metal scaffolds-densified porous Ti scaffolds. The method consists of two fabrication processes, 1) the fabrication of porous Ti scaffolds by dynamic freeze casting, and 2) coating and densification of the porous scaffolds. The dynamic freeze casting method to fabricate porous Ti scaffolds allowed the densification of porous scaffolds by minimizing the chemical contamination and structural defects. The densification process is distinctive for three reasons. First, the densification process is simple, because it requires a control of only one parameter (degree of densification). Second, it is effective, as it achieves mechanical enhancement and sustainable release of biomolecules from porous scaffolds. Third, it has broad applications, as it is also applicable to the fabrication of functionally graded porous scaffolds by spatially varied strain during densification.     

Macroporous Catalytic Carbon Nanotemplates for Sodium Metal Anodes

Because of its remarkably high theoretical capacity and favorable redox voltage (?2.71 V vs the standard hydrogen electrode), Na is a promising anode material for Na ion batteries. In this study, macroporous catalytic carbon nanotemplates (MC‐CNTs) based on nanoweb‐structured carbon nanofibers with various carbon microstructures are prepared from microbe‐derived cellulose via simple heating at 800 or 2400 °C. MC‐CNTs prepared at 800 °C have amorphous carbon structures with numerous topological defects, and exhibit a lower voltage overpotential of ≈8 mV in galvanostatic charge/discharge testing. In addition, MC‐CNT‐800s exhibit high Coulombic efficiencies of 99.4–99.9% during consecutive cycling at current densities ranging from 0.2 to 4 mA cm^?2. However, the carbon structures of MC‐CNTs prepared at 800 °C are gradually damaged by cycling. This results in significant capacity losses after about 200 cycles. In contrast, MC‐CNTs prepared at 2400 °C exhibit well‐developed graphitic structures, and maintain predominantly stable cycling behaviors over 1000 cycles with Coulombic efficiencies of ≈99.9%. This study demonstrates the superiority of catalytic carbon nanotemplates with well‐defined pore structures and graphitic microstructures for use in Na metal anodes.     

Freely Shapable and 3D Porous Carbon Nanotube Foam Using Rapid Solvent Evaporation Method for Flexible Thermoelectric Power Generators

A rapid solvent evaporation method based on the triple point of a processing solvent is presented to prepare carbon nanotube (CNT) foam with a porous structure for thermoelectric (TE) power generators. The rapid solvent evaporation process allows the preparation of CNT foam with various sizes and shapes. The obtained highly porous CNT foam with porosity exceeding 90% exhibits a low thermal conductivity of 0.17 W m^?1 K^?1 with increased phonon scattering, which is 100 times lower than that of a CNT film with a densely packed network. The aforementioned structural and thermal properties of the CNT foam are advantageous to develop a sufficient temperature gradient between the hot and cold parts to enhance TE output characteristics. To improve the electrical conductivity and Seebeck coefficient further, p‐ and n‐molecular dopants are easily introduced into the CNT foam, and the optimized condition is investigated based on the TE properties. Finally, optimized p‐ and n‐doped CNT foams are used to fabricate a vertical and flexible TE power generator with a combination of series and parallel mixed circuits. The maximum output power and output power per weight of the TE generator reach 1.5 µW and 82 µW g^?1, respectively, at a temperature difference of 13.9 K.     

Molecular dynamics study of core–shell structure from carbon nanotube and platinum nanowire

The interaction between the carbon nanotubes (CNTs) and platinum (Pt) nanowires (NWs) was investigated using forced field-based molecular dynamics (MD) simulations. Our results display that the Pt NW can induce the self-assembly of the CNTs to form a shell-core structure, this is because of the van der Waals interaction and the offset face-to-face π–π stacking interaction. The diameter of the CNT plays a major role in the formation of shell–core structure. Furthermore, the position of the CNT on the Pt NW also affects the formation of shell–core configuration, whereas the cross section of the NWs has a negligible effect on the fabrication process. Moreover, the interaction between multi-wall carbon nanotube and Pt nanowires was also discussed in detail, it is worth noting that the formation conformation of the CNT is also much more stable.     

Molding of deep polydimethylsiloxane microstructures for microfluidics and biological applications

Here we demonstrate the microfabrication of deep (> 25 microns) polymeric microstructures created by replica-molding polydimethylsiloxane (PDMS) from microfabricated Si substrates. The use of PDMS structures in microfluidics and biological applications is discussed. We investigated the feasibility of two methods for the microfabrication of the Si molds: deep plasma etch of silicon-on-insulator (SOI) wafers and photolithographic patterning of a spin-coated photoplastic layer. Although the SOI wafers can be patterned at higher resolution, we found that the inexpensive photoplastic yields similar replication fidelity. The latter is mostly limited by the mechanical stability of the replicated PDMS structures. As an example, we demonstrate the selective delivery of different cell suspensions to specific locations of a tissue culture substrate resulting in micropatterns of attached cells.     

Nanomoulding of Functional Materials,a Versatile Complementary Pattern Replication Method to Nanoimprinting

We describe a nanomoulding technique which allows low-cost nanoscale patterning of functional materials, materials stacks and full devices. Nanomoulding combined with layer transfer enables the replication of arbitrary surface patterns from a master structure onto the functional material. Nanomoulding can be performed on any nanoimprinting setup and can be applied to a wide range of materials and deposition processes. In particular we demonstrate the fabrication of patterned transparent zinc oxide electrodes for light trapping applications in solar cells.     

Study of Double-Side Ultrasonic Embossing for Fabrication of Microstructures on Thermoplastic Polymer Substrates

Double-side replication of polymer substrates is beneficial to the design and the fabrication of 3-demensional devices. The ultrasonic embossing method is a promising, high efficiency and low cost replication method for thermoplastic substrates. It is convenient to apply silicon molds in ultrasonic embossing, because microstructures can be easily fabricated on silicon wafers with etching techniques. To reduce the risk of damaging to silicon molds and to improve the replication uniformity on both sides of the polymer substrates, thermal assisted ultrasonic embossing method was proposed and tested. The processing parameters for the replication of polymethyl methacrylate (PMMA), including ultrasonic amplitude, ultrasonic force, ultrasonic time, and thermal assisted temperature were studied using orthogonal array experiments. The influences of the substrate thickness, pattern style and density were also investigated. The experiment results show that the principal parameters for the upper and lower surface replication are ultrasonic amplitude and thermal assisted temperature, respectively. As to the replication uniformity on both sides, the ultrasonic force has the maximal influence. Using the optimized parameters, the replication rate reached 97.5% on both sides of the PMMA substrate, and the cycle time was less than 50 s.     

Effect of the Si,Al and B doping on the sensing behaviour of carbon nanotubes toward ethylene oxide: a computational study

ABSTRACT

Exo– and endo–adsorption of ethylene oxide (EO) on pristine (9,0) (zigzag) carbon nanotube (CNT) and its doped forms with silicon (Si–CNT), aluminum (Al–CNT) and boron (B–CNT) were investigated using density functional theory (DFT) at M06–2X/6–311++G** level. The natural bond orbital (NBO) and the quantum theory of atoms in molecules (QTAIM) analyses were also performed by using the same level of theory. The effect of the doping on sensing behaviour of the CNT toward EO molecule was investigated through intermolecular interactions studies by calculation of total and partial density of states (DOS, PDOS). The enhanced sensitivity of doped–CNTs towards EO molecule associated with adsorption energies (E_ads) and the changes in geometric and electronic structures was examined and the global chemical reactivity parameters were calculated and comprehensively analysed. The thermodynamic property changes were calculated and compared. The results indicated that the EO adsorption on the pristine and doped CNTs was an exothermic spontaneous process. Moreover, based on the calculated E_g change (ΔE_g) and E_ads values, Al–CNT with superior sensitivity for sensing of EO molecule, indicates promising perspectives for its use in fabrication of new EO gas–sensing devices.     

Glucose sensors made of novel carbon nanotube-gold nanoparticle composites

Carbon nanotube and metal particle composites have been exploited to fabricate high performance electrochemical devices. However, the physical and chemical procedures to synthesize the composites are labor intensive and inefficient. Our study reveals an one-step wet chemistry method to accomplish fast and controllable production of gold nanoparticle (AuNP) and carbon naotube (CNT) composites. Such a process is sensitive to the surface charge. Especially, when functionalized with carboxyl groups, the CNTs carried negative charges and showed low level association with negatively charged AuNPs. Thermal treatment was employed to decompose the carboxyl groups and render each CNT a charge-free surface thereby achieving a high level AuNP-CNT association. The fabricated glucose sensors demonstrated dependence of their sensitivities to the amount of AuNPs on the CNTs. The enhancement of sensitivity can be attributed to an accelerated electron transfer rate from glucose oxidase Gox to the electrode. The Michaelis-Menten kinetics also indicated improved performance in the glucose sensor made of AuNP-CNTs. Therefore, our research revealed a novel approach to produce metallic nanoparticle and CNT composite for fabricating high performance electrochemical sensors.     

Interaction of nucleic acids with carbon nanotubes and dendrimers

Nucleic acid interaction with nanoscale objects like carbon nanotubes (CNTs) and dendrimers is of fundamental interest because of their potential application in CNT separation, gene therapy and antisense therapy. Combining nucleic acids with CNTs and dendrimers also opens the door towards controllable self-assembly to generate various supra-molecular and nano-structures with desired morphologies. The interaction between these nanoscale objects also serve as a model system for studying DNA compaction, which is a fundamental process in chromatin organization. By using fully atomistic simulations, here we report various aspects of the interactions and binding modes of DNA and small interfering RNA (siRNA) with CNTs, graphene and dendrimers. Our results give a microscopic picture and mechanism of the adsorption of single- and double-strand DNA (ssDNA and dsDNA) on CNT and graphene. The nucleic acid-CNT interaction is dominated by the dispersive van der Waals (vdW) interaction. In contrast, the complexation of DNA (both ssDNA and dsDNA) and siRNA with various generations of poly-amido-amine (PAMAM) dendrimers is governed by electrostatic interactions. Our results reveal that both the DNA and siRNA form stable complex with the PAMAM dendrimer at a physiological pH when the dendrimer is positively charged due to the protonation of the primary amines. The size and binding energy of the complex increase with increase in dendrimer generation. We also give a summary of the current status in these fields and discuss future prospects.     

Formation of non-base-pairing DNA microgels using directed phase transition of amphiphilic monomers

Programmability of DNA sequences enables the formation of synthetic DNA nanostructures and their macromolecular assemblies such as DNA hydrogels. The base pair-level interaction of DNA is a foundational and powerful mechanism to build DNA structures at the nanoscale; however, its temperature sensitivity and weak interaction force remain a barrier for the facile and scalable assembly of DNA structures toward higher-order structures. We conducted this study to provide an alternative, non-base-pairing approach to connect nanoscale DNA units to yield micrometer-sized gels based on the sequential phase transition of amphiphilic unit structures. Strong electrostatic interactions between DNA nanostructures and polyelectrolyte spermines led to the formation of giant phase-separated aggregates of monomer units. Gelation could be initiated by the addition of NaCl, which weakened the electrostatic DNA-spermine interaction while attractive interactions between cholesterols created stable networks by crosslinking DNA monomers. In contrast to the conventional DNA gelation techniques, our system used solid aggregates as a precursor for DNA microgels. Therefore, in situ gelation could be achieved by depositing aggregates on the desired substrate and subsequently initiating a phase transition. Our approach can expand the utility and functionality of DNA hydrogels by using more complex nucleic acid assemblies as unit structures and combining the technique with top-down microfabrication methods.     

Exploiting the Oxygen Inhibitory Effect on UV Curing in Microfabrication: A Modified Lithography Technique

Rapid prototyping (RP) of microfluidic channels in liquid photopolymers using standard lithography (SL) involves multiple deposition steps and curing by ultraviolet (UV) light for the construction of a microstructure layer. In this work, the conflicting effect of oxygen diffusion and UV curing of liquid polyurethane methacrylate (PUMA) is investigated in microfabrication and utilized to reduce the deposition steps and to obtain a monolithic product. The conventional fabrication process is altered to control for the best use of the oxygen presence in polymerization. A novel and modified lithography technique is introduced in which a single step of PUMA coating and two steps of UV exposure are used to create a microchannel. The first exposure is maskless and incorporates oxygen diffusion into PUMA for inhibition of the polymerization of a thin layer from the top surface while the UV rays penetrate the photopolymer. The second exposure is for transferring the patterns of the microfluidic channels from the contact photomask onto the uncured material. The UV curing of PUMA as the main substrate in the presence of oxygen is characterized analytically and experimentally. A few typical elastomeric microstructures are manufactured. It is demonstrated that the obtained heights of the fabricated structures in PUMA are associated with the oxygen concentration and the UV dose. The proposed technique is promising for the RP of molds and microfluidic channels in terms of shorter processing time, fewer fabrication steps and creation of microstructure layers with higher integrity.     

Optical Biochip with Multichannels for Detecting Biotin–Streptavidin Based on Localized Surface Plasmon Resonance

A rapid and accurate detection of molecular binding of antigen-antibody signaling in high throughput is of great importance for biosensing technology. We proposed a novel optical biochip with multichannels for the purpose of detection of biotin–streptavidin on the basis of localized surface plasmon resonance. The optical biochip was fabricated using photolithography to form the microarrays functioning with multichannels on glass substrate. There are different nanostructures in each microarray. Dry etching and nanosphere lithography techniques were applied to fabricate Ag nanostructures such as hemispheres, nanocylindricals, triangular, and rhombic nanostructures. We demonstrated that 100-nM target molecule (streptavidin) on these optical biochips can be easily detected by a UV-visible spectrometer. It indicated that period and shape of the nanostructures significantly affect the optical performance of the nanostructures with different shapes and geometrical parameters. Our experimental results demonstrated that the optical biochips with the multichannels can detect the target molecule using the microarrays structured with different shapes and periods simultaneously. Batch processing of immunoassay for different biomolecular through the different channels embedded on the same chip can be realized accordingly.     

Nanoscale Compositionally Graded Thin‐Film Electrolyte Membranes for Low‐Temperature Solid Oxide Fuel Cells

Controllable fabrication of compositionally graded Gd_0.1Ce_0.9O_2‐δ and Y_0.16Zr_0.84O_2‐δ electrolytes using co‐sputtering is demonstrated. Self‐supported membranes were lithographically fabricated to employ the new electrolytes into thin film solid oxide fuel cells. Devices integrating such electrolytes demonstrate performance of over 1175 mW cm^?2 and 665 mW cm^?2 at 520 °C using hydrogen and methane as fuel, respectively. The results present a general strategy to fabricate nanoscale functionally graded materials with selective interfacial functionality for energy conversion.     

N-Type Thermoelectric Performance of Functionalized Carbon Nanotube-Filled Polymer Composites

Carbon nanotubes (CNTs) were functionalized with polyethyleneimine (PEI) and made into composites with polyvinyl acetate (PVAc). CNTs were dispersed with different amounts of sodium dodecylbenzenesulfonate (SDBS) prior to the PEI functionalization. The resulting samples exhibit air-stable n-type characteristics with electrical conductivities as great as 1500 S/m and thermopowers as large as −100 µV/K. Electrical conductivity and thermopower were strongly affected by CNT dispersion, improving the properties with better dispersion with high concentrations of SDBS. This improvement is believed to be due to the increase in the number of tubes that are evenly coated with PEI in a better-dispersed sample. Increasing the amount of PEI relative to the other constituents positively affects thermopower but not conductivity. Air exposure reduces both thermopower and conductivity presumably due to oxygen doping (which makes CNTs p-type), but stable values were reached within seven days following sample fabrication.     

Biomolecular interaction analysis for carbon nanotubes and for biocompatibility prediction

The interactions between carbon nanotubes (CNTs) and biologics have been commonly studied by various microscopy and spectroscopy methods. We tried biomolecular interaction analysis to measure the kinetic interactions between proteins and CNTs. The analysis demonstrated that wheat germ agglutinin (WGA) and other proteins have high affinity toward carboxylated CNT (f-MWCNT) but essentially no binding to normal CNT (p-MWCNT). The binding of f-MWCNT–protein showed dose dependence, and the observed kinetic constants were in the range of 10⁻⁹ to 10⁻¹¹ M with very small off-rates (10⁻³ to 10⁻⁷ s⁻¹), indicating a relatively tight and stable f-MWCNT–protein complex formation. Interestingly in hemolysis assay, p-MWCNT showed good biocompatibility, f-MWCNT caused 30% hemolysis, but WGA-coated f-MWCNT did not show hemolysis. Furthermore, the f-MWCNT–WGA complex demonstrated enhanced cytotoxicity toward cancer cells, perhaps through the glycoproteins expressed on the cells' surface. Taken together, biomolecular interaction analysis is a precise method that might be useful in evaluating the binding affinity of biologics to CNTs and in predicting biological actions.