Highly sensitive and label-free protein immunoassay-based biosensor comprising infrared metamaterial absorber inducing strong coupling.

A mid-infrared label-free immunoassay-based biosensor is an effective device to help identify and quantify biomolecules. This biosensor employs a surface-enhanced infrared absorption spectroscopy, which is a highly potent sensing technique for detecting minute quantities of analytes. In this study, a biosensor was constructed using a metamaterial absorber, which facilitated strong coupling effects. For maximum coupling effect, it is necessary to enhance the near-field intensity and the spatial and spectral overlap between the optical cavity resonance and the vibrational mode of the analyte. Due to significant peak splitting, conventional baseline correction methods fail to adequately analyze such a coupling system. Therefore, we employed a coupled harmonic oscillation model to analyze the spectral distortion resulting from the peak splitting induced by the strong coupling effect. The proposed biosensor with a thrombin-binding aptamer-based immunoassay could achieve a limit of detection of 267.4 pM, paving the way for more efficient protein detection in clinical practice.

Functional Asymmetry-Enabled Self-Adhesive Film via Phase Separation of Binary Polymer Mixtures for Soft Bio-Integrated Electronics.

Biocompatible adhesive films are important for many applications (e.g., wearable devices, implantable devices, and attachable sensors). In particular, achieving self-adhesion on one side of a film with biocompatible materials is a compelling goal in adhesion science. Herein, we report a simple and easy manufacturing process using water-soluble hyaluronic acid (HA) that allows adhesiveness on only one side using binary polymer mixtures based on a phase-separation strategy with an elastomer. HA influx allows for the entangled polymer chains of the elastomer to spontaneously deform, permitting tunable mechanical elasticity, conformability, and adhesion. The proposed adhesive film enables the transfer of nanopatterning and the attachment of various surfaces without the use of additional chemicals. In addition, the film can be used for measuring epidermal biopotential and for skin fixation of drug devices. Therefore, the developed facile asymmetric adhesion can block the interferences of other materials on the unnecessary adhesion side, providing considerable potential for the development of functional, multifunctional, and smart bioadhesives.

Multiple-Route Exciton Recombination Dynamics and Improved Stability of Perovskite Quantum Dots by Plasmonic Photonic Crystal.

We have studied the excited-state exciton recombination dynamics of perovskite quantum dots (QDs) through time-resolved photoluminescence (PL), PL blinking, PL intensity-dependent lifetime modulation, and long-term photostability tests. The various spectroscopic characterizations elucidate that the perovskite QDs have multiple intrinsic exciton recombination routes even in a single QD, i.e., exciton, biexciton, and positive/negative trions, which are dissimilarly contributed to ON and OFF state emissions. We also find that the enhanced radiative recombination from placing green QDs on a photonic Ag nanotip array induces notably improved long-term PL stability. We consider that the accelerated radiative recombination of QDs by strong coupling with the plasmonics of the photonic Ag nanotip array, while eliminating nonradiative pathways, is proven to be a critical factor for improved long-term stability.

Ultrasensitive Molecule Detection Based on Infrared Metamaterial Absorber with Vertical Nanogap.

Surface-enhanced infrared absorption (SEIRA) spectroscopy is a powerful methodology for sensing and identifying small quantities of analyte molecules via coupling between molecular vibrations and an enhanced near-field induced in engineered structures. A metamaterial absorber (MA) is proposed as an efficient SEIRA platform; however, its efficiency is limited because it requires the appropriate insulator thickness and has a limited accessible area for sensing. SEIRA spectroscopy is proposed using an MA with a 10 nm thick vertical nanogap, and a record-high reflection difference SEIRA signal of 36% is experimentally achieved using a 1-octadecanethiol monolayer target molecule. Theoretical and experimental comparative studies are conducted using MAs with three different vertical nanogaps. The MAs with a vertical nanogap are processed using nanoimprint lithography and isotropic dry etching, which allow cost-effective large-area patterning and mass production. The proposed structure may provide promising routes for ultrasensitive sensing and detection applications.

Enhancement of refractive index sensing for an infrared plasmonic metamaterial absorber with a nanogap.

An infrared plasmonic metamaterial absorber with a nanogap was numerically and experimentally investigated as a refractive index sensor. We experimentally demonstrated large enhancements of both sensitivity (approximately 1091 nm/refractive index unit) and figure of merit (FOM*; approximately 273) owing to the nanogap formation in the metamaterial absorber to achieve perfect absorption (99%). The refractive index sensing platform was fabricated by producible nanoimprint lithography and isotropic dry etching processes to have a large area and low cost while providing a practical solution for high-performance plasmonic biosensors.

Preparation of multilayer periodic nanopatterned WO3-based photoanode by reverse nanoimprinting for water splitting.

Photoelectrochemical (PEC) water splitting has been studied extensively as an environmentally friendly technology for hydrogen production using solar energy. WO3is considered a promising semiconducting material for photoanodes due to its high electron mobility, good hole diffusion length, and chemical stability. Periodic nanostructures of WO3have been investigated for enhancing the PEC performance of WO3-based photoanodes. In this study, facile fabrication of periodic nanostructures of WO3was achieved using reverse nanoimprint lithography, and the multilayer stacking of nanopatterned WO3film was also confirmed. The multilayer nanopatterned WO3films were used as photoanodes for PEC water splitting. The performance of the fabricated photoanode in PEC was 2 times higher than that of planar WO3film due to its higher light absorbance and lower charge transfer resistance.

Dual nanotransfer printing for complementary plasmonic biosensors.

One of the main challenges in the widespread utilization of localized plasmon resonance-based biosensors is the fabrication of large-area and low-cost plasmonic nanostructures. In this work, we fabricated large-area and low-cost complementary plasmonic biosensors such as nanohole and nanodisk arrays using dual nanotransfer printing (NTP) with a single metal deposition and a single reusable mold. The suspended nanohole arrays and the suspended nanodisk arrays were fabricated using the subsequent dry etching process. We confirmed a maximum enhancement in bulk sensitivity in experiments and simulations by controlling the vertical and lateral etching depths of the dielectric layer underneath the gold (Au) nanohole and nanodisk arrays. Furthermore, we show that the surface sensitivity evaluated by atomic layer deposition of aluminum oxide increased because appropriate vertical and lateral etching depths allow the target analyte to access the additional near-field formed at the bottom of the Au nanostructure. The dual NTP method provides a practical solution for the realization of large-area and low-cost label-free plasmonic biosensing systems, with a reduction in complexity and cost of the fabrication process of complementary plasmonic structures and metasurfaces.

Fano Metamaterials on Nanopedestals for Plasmon-Enhanced Infrared Spectroscopy.

We report a sensing platform for surface-enhanced infrared absorption (SEIRA) spectroscopy, based on Fano metamaterials (FMMs) on dielectric nanopedestals. FMMs consist of two parallel gold (Au) nanorod antennas, with a small horizontal coupler attached to one of the nanorod antenna. When placed on SiO2 dielectric nanopedestals, which exhibit strong field enhancements caused by the interference between subradiant and superradiant plasmonic resonances, they provide the highly enhanced E-field intensities formed near the Au nanoantenna, which can provide more enhanced molecular detection signals. Here, the sensing characteristics of FMMs on nanopedestals structure was confirmed by comparison with FMMs on an unetched SiO2 substrate as a control sample. The control FMMs and the FMMs on nanopedestals were carefully designed to excite Fano resonance near the target 1-octadecanethiol (ODT) fingerprint vibrations. The FMMs were fabricated by using nanoimprint lithography and the nanopedestal structures were formed by isotropic dry-etching. The experimental reflection spectra containing the enhanced absorption signals of the ODT monolayer molecules was analyzed using temporal coupled-mode theory. The FMMs on nanopedestals achieved over 7% of reflection difference signal, which was 1.7 times higher signal than the one from the control FMMs. Based on the FMMs on nanopedestal structures proposed in this study, it may be widely applied to future spectroscopy and sensor applications requiring ultrasensitive detection capability.

An ultraviolet and electric field activated photopolymer-ferroelectric nanoparticle composite for the performance enhancement of triboelectric nanogenerators.

For the development of high performance triboelectric generators (TENGs), it is required to have facile methods to adjust the triboelectric properties of the friction surfaces. In this work, we present the surface charge density modulation of the photopolymer-ferroelectric nanoparticle composite surface by applying ultraviolet (UV) and electric field. By using the photopolymer, the triboelectric surface property was modulated by exposure to UV. In addition, lithographic surface patterning can be easily adopted to enlarge the frictional surface area as well. Furthermore, the use of the PP allows a facile integration of ferroelectric nanoparticles (NPs) in the form of a nanocomposite structure, which can effectively increase the surface charge density by spontaneous dipole coupling of NPs embedded in the PP layer. As a result, approximately 4-fold higher output power has been achieved by applying this approach. The developed TENGs have also demonstrated superior mechanical durability, generating consistent outputs during 104 cyclic frictional contacts. The approach proposed here is a simple and reliable way to enhance the output performance of TENGs.

Microneedles integrated with a triboelectric nanogenerator: an electrically active drug delivery system.

In this study, a combined system of microneedles and a triboelectric nanogenerator (TENG) has been developed for drug delivery. A triboelectric device, which converts mechanical energy into alternating current (AC), was chosen to replace the electrophoresis (EP) effect. To directly generate triboelectricity from salmon deoxyribonucleic acid (SDNA)-based microneedles, a triboelectric series of SDNA film and chargeable polymers (polyimide and Teflon) was studied. The electrical output of the two charged polymers was compared to find a material that could be highly charged with SDNA. The electrical output was also compared as a function of the concentration of a drug embedded in the SDNA film, and the results confirmed that drug intercalation affected the carrier diffusion. The mechanical strength of the microneedles was assessed by histological analysis of their penetration into porcine cadaver skin. Furthermore, the output voltage of a system incorporating microneedles and TENG in cadaver skin, and in vitro drug release into gelatin were evaluated to examine potential application as an electrically active drug delivery system. The electrical output voltage of this system was ∼95 V. The mechanism of triboelectric perturbation to the skin has also been discussed. The system developed in this work is a new, facile approach toward effective drug delivery that replaces the existing EP method and expands the application of TENGs.

Transparent Displays Utilizing Nanopatterned Quantum Dot Films.

We report the realization of a transparent display using glass covered by a nanopatterned quantum dot (QD) film with good transmittance. The film was fabricated by nanoimprint lithography (NIL) and spin coating of colloidal QDs with specificexcitation maxima. The produced nanopatterned QD film was attached to transparent glass, enabling active image generation using a laser light source of a specific wavelength. Selective light emission was induced by strongly exciting the laser-exposed film surface, creating desired images, with color modulationenabled by controlling the QD layer (dozens of nanometers in size) via nanopatterning. The nanopatterned QD film used for image generation exhibits excellent transmittance (>80%), and can be used for transparent displays, with image realization in both bright and dark spaces. The fabricated displays have wide viewing anglesowing to their good light emission characteristics, and the fabrication through spin coating renders the fabrication process simple and applicable to large areas.

Flexible Plasmonic Color Filters Fabricated via Nanotransfer Printing with Nanoimprint-Based Planarization.

We investigated the preparation and performance of large-area transmission-type flexible plasmonic color filters (PCFs). These large-area PCFs were fabricated based on a nanotransfer printing (nTP) process that involves nanoimprint-based planarization. This process is a simple surface treatment for easy transfer of a metal to a flexible plastic substrate and formation of patterned aluminum nanodots and nanoholes on a substrate surface with poor roughness. Rabbit-ear structures can form during the nTP process, and this phenomenon was analyzed by numerical simulation. As defects were not detected in a 10 000-round bending test, the PCFs fabricated using this nTP process have excellent mechanical properties.

Three-dimensional plasmonic Ag/TiO2 nanocomposite architectures on flexible substrates for visible-light photocatalytic activity.

In this study, a periodic three-dimensional (3D) Ag/TiO2 nanocomposite architecture of nanowires was fabricated on a flexible substrate to enhance the plasmonic photocatalytic activity of the composite. Layer-by-layer nanofabrication based on nanoimprint lithography, vertical e-beam evaporation, nanotransfer, and nanowelding was applied in a new method to create different 3D Ag/TiO2 nanocomposite architectures. The fabricated samples were characterized by scanning electron microscopy, transmission electron microscopy, focused ion-beam imaging, X-ray photoelectron spectrometry, and UV-visible spectroscopy. The experiment indicated that the 3D nanocomposite architectures could effectively enhance photocatalytic activity in the degradation of methylene blue solution under visible light irradiation. We believe that our method is efficient and stable, which could be applied to various fields, including photocatalysis, solar energy conversion, and biotechnology.

Covalent bonding-assisted nanotransfer lithography for the fabrication of plasmonic nano-optical elements.

Many high-resolution patterning techniques have been developed to realize nano- and microscale applications of electric devices, sensors, and transistors. However, conventional patterning methods based on photo or e-beam lithography are not employed to fabricate optical elements of high aspect ratio and a sub-100 nm scale due to the limit of resolution, high costs and low throughput. In this study, covalent bonding-assisted nanotransfer lithography (CBNL) was proposed to fabricate various structures of high resolution and high aspect ratio at low cost by a robust and fast chemical reaction. The proposed process is based on the formation of covalent bonds between silicon of adhesive layers on a substrate and oxygen of the deposited material on the polymer stamp. The covalent bond is strong enough to detach multiple layers from the stamp for a large area without defects. The obtained nanostructures can be used for direct application or as a hard mask for etching. Two nano-optical applications were demonstrated in this study, i.e., a meta-surface and a wire-grid polarizer. A perfect absorption meta-surface was generated by transferring subwavelength hole arrays onto a substrate without any post-processing procedures. In addition, a wire-grid polarizer with high aspect ratio (1 : 3) and 50 nm line width was prepared by the nano-transfer of materials, which were used as a hard mask for etching. Therefore, CBNL provides a means of achieving large-area nano-optical elements with a simple roll-to-plate process at low cost.

Ferroelectric Zinc Oxide Nanowire Embedded Flexible Sensor for Motion and Temperature Sensing.

We report a simple method to realize multifunctional flexible motion sensor using ferroelectric lithium-doped ZnO-PDMS. The ferroelectric layer enables piezoelectric dynamic sensing and provides additional motion information to more precisely discriminate different motions. The PEDOT:PSS-functionalized AgNWs, working as electrode layers for the piezoelectric sensing layer, resistively detect a change of both movement or temperature. Thus, through the optimal integration of both elements, the sensing limit, accuracy, and functionality can be further expanded. The method introduced here is a simple and effective route to realize a high-performance flexible motion sensor with integrated multifunctionalities.

Infrared broadband metasurface absorber for reducing the thermal mass of a microbolometer.

We demonstrate an infrared broadband metasurface absorber that is suitable for increasing the response speed of a microbolometer by reducing its thermal mass. A large fraction of holes are made in a periodic pattern on a thin lossy metal layer characterised with a non-dispersive effective surface impedance. This can be used as a non-resonant metasurface that can be integrated with a Salisbury screen absorber to construct an absorbing membrane for a microbolometer that can significantly reduce the thermal mass while maintaining high infrared broadband absorption in the long wavelength infrared (LWIR) band. The non-dispersive effective surface impedance can be matched to the free space by optimising the surface resistance of the thin lossy metal layer depending on the size of the patterned holes by using a dc approximation method. In experiments a high broadband absorption was maintained even when the fill factor of the absorbing area was reduced to 28% (hole area: 72%), and it was theoretically maintained even when the fill factor of the absorbing area was reduced to 19% (hole area: 81%). Therefore, a metasurface with a non-dispersive effective surface impedance is a promising solution for reducing the thermal mass of infrared microbolometer pixels.

Scalable and enhanced triboelectric output power generation by surface functionalized nanoimprint patterns.

We report nanoimprint lithographic submicron surface patterning for scalable output power generation and performance enhancement in triboelectric nanogenerators (TENGs). Specifically, one contact surface of a TENG is nanoimprinted with polyurethane acrylate (PUA) lines in different pitches and the counter contact surface is coated with perfluoropolyether (PFPE). The results show that a TENG with 200 nm pitch PUA lines exhibits voltage and current up to ∼430 V and ∼55 µA cm(-2), generating about a sixfold higher output power than that with a flat PUA surface at an applied force of 0.3 MPa. In addition, scalable output power was obtained by adjusting line pitches. Further enhancement in output power was also demonstrated by chemically functionalizing the PUA line patterns with poly (diallyldimethylammonium chloride) (PDDA). The PDDA functionalization boosted voltage and current up to ∼500 V and ∼100 µA cm(-2), respectively, which corresponds to ∼50% power density enhancement. The approach introduced here is a simple, effective, scalable and reproducible way to fabricate TENGs.

A vanadium-doped ZnO nanosheets-polymer composite for flexible piezoelectric nanogenerators.

We report high performance flexible piezoelectric nanogenerators (PENGs) by employing vanadium (V)-doped ZnO nanosheets (NSs) and the polydimethylsiloxane (PDMS) composite structure. The V-doped ZnO NSs were synthesized to overcome the inherently low piezoelectric properties of intrinsic ZnO. Ferroelectric phase transition induced in the V-doped ZnO NSs contributed to significantly improve the performance of the PENGs after the poling process. Consequently, the PENGs exhibited high output voltage and current up to ∼32 V and ∼6.2 µA, respectively, under the applied strain, which are sufficient to directly turn on a number of light emitting diodes (LEDs). The composite approach for PENG fabrication is scalable, robust, and reproducible during periodic bending/releasing over extended cycles. The approach introduced here extends the performance limits of ZnO-based PENGs and demonstrates their potential as energy harvesting devices.

Triboelectric Hydrogen Gas Sensor with Pd Functionalized Surface.

Palladium (Pd)-based hydrogen (H2) gas sensors have been widely investigated thanks to its fast reaction and high sensitivity to hydrogen. Various sensing mechanisms have been adopted for H2 gas sensors; however, all the sensors must be powered through an external battery. We report here an H2 gas sensor that can detect H2 by measuring the output voltages generated during contact electrification between two friction surfaces. When the H2 sensor, composed of Pd-coated ITO (indium tin oxide) and PET (polyethylene Terephthalate) film, is exposed to H2, its output voltage is varied in proportion to H2 concentration because the work function (WF) of Pd-coated surface changes, altering triboelectric charging behavior. Specifically, the output voltage of the sensor is gradually increased as exposing H2 concentration increases. Reproducible and sensitive sensor response was observed up 1% H2 exposure. The approach introduced here can easily be adopted to development of triboelectric gas sensors detecting other gas species.

Controlled Patterning of Vertical Silicon Structures Using Polymer Lithography and Wet Chemical Etching.

In order to improve their performance for various applications, a facile method for the wafer-scale fabrication of micro/nano-patterned vertical silicon (Si) structures such as silicon nanowires (SiNWs), silicon nanorods (SiNRs), and porous silicon (p-Si) was developed. The method is based on the combination of lithography techniques (photolithography, thermal nano-imprint lithography, nanosphere lithography) and wet chemical etching (electro-chemical etching, metal-assisted chemical etching) processes. Micro-patterned p-Si with various pore diameters from 30 nm to 1.2 um were fabricated via electro-chemical etching. Micro/nano-patterned Si microstructures, nanorods, and nanowires were also successfully fabricated by changing the thickness of the metal layer of 5 nm or 20 nm in the metal-assisted chemical etching process. This study also investigated the effect of the etching time and patterning on the etched SiNWs length. This method provides advantages of simplicity, speed, large-scale production, easy size and shape manipulation, and low cost.