Dynamic mucus secretion in ventral surfaces of toe pads of the tree frog (Dryophytes japonica).

The tree frog is a prominent amphibian among terrestrial vertebrates known for its ability to adhere to various surfaces through the capillary forces of water in the microchannels between micropillars on its disc-shaped toe pads, a phenomenon known as wet adhesion. However, the secretion pattern of mucus on the attachment surface of living tree frog toe pads and the distribution of active mucus pores (AMPs) have not yet been fully elucidated. In this study, we utilized synchrotron X-ray micro-computed tomography and interference reflection microscopy to obtain the spatial distribution of the entire population of ventral mucus glands on the toe pads of living tree frogs and the real-time mucus secretion patterns from the ventral mucus pores on the contact surface under different environmental conditions. We observed that the number and secretion frequency of AMPs on the toe pad are regulated according to environmental conditions. Such dynamic mucus secretion on the tree frog's toe pad could contribute to the understanding of capillary force regulation for wet adhesion and the development of adhesive surfaces by mimicking the mucus-secreting toe pad.

Reusable Wrinkled Nanoporous Silver Film Fabricated by Plasma Treatment for Surface-Enhanced Raman Scattering Applications.

A nanoporous silver film (npAgF), a promising structure for surface-enhanced Raman spectroscopy (SERS), can be fabricated by using successive O2 and Ar plasma treatments on a planar silver film. The common dealloying method for producing an npAgF involves annealing at high temperatures to produce an alloy film, as well as harsh etching using corrosive chemicals. By contrast, the plasma-based method can be applied directly to various functional substrates to produce more sophisticated npAgF structures. Herein, we report a facile fabrication method for a wrinkled npAgF (w-npAgF) for SERS applications using a thermally contractible polystyrene substrate. The w-npAgF had 3D wrinkles of the nanoporous structure and showed approximately 8 times higher SERS enhancement than did the flat npAgF. Moreover, the w-npAgF could be reused for multiple SERS measurements of different molecules by mild O2 and Ar plasma treatments after each use, in which the O2 plasma effectively removed the adsorbed organic molecules and the Ar plasma reduced silver oxide to pristine silver for subsequent SERS measurements. The wrinkled nanoporous structure was maintained after multiple mild plasma treatments for reuse. The simplicity of plasma-based fabrication and high sensitivity of w-npAgFs are promising features for the green production of low-cost and reusable 3D SERS substrates.

Regulation of Interfacial Anchoring Orientation of Anisotropic Nanodumbbells.

Nanoparticles exhibiting geometrical and chemical anisotropies hold promise for environmentally responsive materials with tunable mechanical properties. However, a comprehensive understanding of their interfacial behaviors remains elusive. In this paper, we control the interfacial anchoring orientation of polystyrene nanodumbbells by adjusting interparticle forces. The film nanostructure is characterized by the orientation angle analysis of individual dumbbells from cross-sectional EM data: dumbbells undergo orientation transitions from a distinctive horizontal bilayer to an isotropic anchoring when electrostatic repulsion is suppressed by either an ionic strength increase or surface amine-modification. This anchoring orientation influences the film's mechanical properties and foam stability, as investigated by a 2D isotherm and dark/bright-field microscopy measurements. Our findings highlight the potential for precise control of supra-colloidal structures by modulating particle alignment, paving the way for smart delivery systems.

Early on-site detection of strawberry anthracnose using portable Raman spectroscopy.

We developed a method for the early on-site detection of strawberry anthracnose using a portable Raman system with multivariate statistical analysis algorithms. By using molecular markers based on Raman spectra, the proposed method can detect anthracnose in strawberry stems 3 days after exposure to Colletotrichum gloeosporioides. A fiber-optic probe was applied for the portable Raman system, and the acquisition time was 10 s. We found that the molecular markers were closely related to the following subjects: i) an increase in amide III and fatty acids of C. gloeosporioides invading strawberry stems (Raman bands at 1180-1310 cm-1) and ii) a decrease in metabolites in strawberry plants, such as phenolic compounds and terpenoids (Raman bands at 760, 800, and 1523 cm-1). We also found that the increased fluorescence background caused by various chromophores within the invading C. gloeosporioides could serve as a marker. A two-dimensional cluster plot obtained by principal component analysis (PCA) showed that the three groups (control, fungal infection, and pathogen) were distinguishable. The linear discriminant analysis (LDA)-based prediction algorithm could identify C. gloeosporioides infection with a posterior probability of over 40%, even when no symptoms were visible on the inoculated strawberry plants.

Reusable, Ultrasensitive, Patterned Conjugated Polyelectrolyte-Surfactant Complex Film with a Wide Detection Range for Copper Ion Detection.

Conjugated polyelectrolytes (CPEs) are emerging as promising materials in the sensor field because they enable high-sensitivity detection of various substances in aqueous media. However, most CPE-based sensors have serious problems in real-world application because the sensor system is operated only when the CPE is dissolved in aqueous media. Here, the fabrication and performance of a water-swellable (WS) CPE-based sensor driven in the solid state are demonstrated. The WS CPE films are prepared by immersing a water-soluble CPE film in cationic surfactants of different alkyl chain lengths in a chloroform solution. The prepared film exhibits rapid, limited water swellability despite the absence of chemical crosslinking. The water swellability of the film enables the highly sensitive and selective detection of Cu2+ in water. The fluorescence quenching constant and the detection limit of the film are 7.24 × 106 L mol-1 and 4.38 nM (0.278 ppb), respectively. Moreover, the film is reusable via a facile treatment. Furthermore, various fluorescent patterns introduced by different surfactants are successfully fabricated by a simple stamping method. By integrating the patterns, Cu2+ detection in a wide concentration range (nM-mM) can be achieved.

Near-UV light emitting diode with on-chip photocatalysts for purification applications.

A new design for light-emitting diodes (LEDs) with on-chip photocatalysts is presented for purification applications. An array of disk-shaped TiO2, with a diameter of several hundred nanometers, combined with SiO2 pedestals was fabricated directly on the surface of an InGaN-based near-ultraviolet (UV) LED using a dry etching process. The high refractive-index contrast at the boundary and the circular shape can effectively confine the near-UV light generated from the LED through multiple internal reflections inside the TiO2 nanodisks. Such a feature results in the enhancement of light absorption by the photocatalytic TiO2. The degradation of the organic dye malachite green was monitored as a model photocatalytic reaction. The proposed structure of LEDs with TiO2/SiO2 nanodisk/pedestal array exhibited a photocatalytic activity that was three times higher than the activity of LEDs with a TiO2 planar layer. The integration of photocatalytic materials with near-UV LEDs in a single system is promising for various purification applications, such as sterilization and disinfection.

Photocatalytic Degradation of Rhodamine B Dye by TiO2 and Gold Nanoparticles Supported on a Floating Porous Polydimethylsiloxane Sponge under Ultraviolet and Visible Light Irradiation.

A combination of plasmonic nanoparticles (NPs) with semiconductor photocatalysts, called plasmonic photocatalysts, can be a good candidate for highly efficient photocatalysts using broadband solar light because it can greatly enhance overall photocatalytic efficiency by extending the working wavelength range of light from ultraviolet (UV) to visible. In particular, fixation of plasmonic photocatalysts on a floating porous substrate can have additional advantages for their recycling after water treatment. Here, we report on a floating porous plasmonic photocatalyst based on a polydimethylsiloxane (PDMS)-TiO2-gold (Au) composite sponge, in which TiO2 and Au NPs are simultaneously immobilized on the surface of interconnected pores in the PDMS sponge. This can be easily fabricated by a simple sugar-template method with TiO2 NPs and in situ reduction of Au NPs by the PDMS without extra chemicals. Its ability to decompose the organic pollutant rhodamine B in water was tested under UV and visible light, respectively. The results showed highly enhanced photocatalytic activity under both UV and visible light compared to the PDMS-TiO2 sponge and the PDMS-Au sponge. Furthermore, its recyclability was also demonstrated for multiple cycles. The simplicity of fabrication and high photocatalytic performance of our PDMS-TiO2-Au sponge can be promising in environmental applications to treat water pollution.

Hollow Porous Gold Nanoshells with Controlled Nanojunctions for Highly Tunable Plasmon Resonances and Intense Field Enhancements for Surface-Enhanced Raman Scattering.

Plasmonic metal nanostructures with nanogaps have attracted great interest owing to their controllable optical properties and intense electromagnetic fields that can be useful for a variety of applications, but precise and reliable control of nanogaps in three-dimensional nanostructures remains a great challenge. Here, we report the control of nanojunctions of hollow porous gold nanoshell (HPAuNS) structures by a facile oxygen plasma-etching process and the influence of changes in nanocrevices of the interparticle junction on the optical and sensing characteristics of HPAuNSs. We demonstrate a high tunability of the localized surface plasmon resonance (LSPR) peaks and surface-enhanced Raman scattering (SERS) detection of rhodamine 6G (R6G) using HPAuNS structures with different nanojunctions by varying the degree of gold sintering. As the neck region of the nanojunction is further sintered, the main LSPR peak shifts from 785 to 1350 nm with broadening because the charge transfer plasmon mode becomes more dominant than the dipolar plasmon mode, resulting from the increase of conductance at the interparticle junctions. In addition, it is demonstrated that an increase in the sharpness of the nanojunction neck can enhance the SERS enhancement factor of the HPAuNS by up to 4.8-fold. This enhancement can be ascribed to the more intense local electromagnetic fields at the sharper nanocrevices of interparticle junctions. The delicate change of nanojunction structures in HPAuNSs can significantly affect their optical spectrum and electromagnetic field intensity, which are critical for their practical use in a SERS-based analytical sensor as well as multiple-wavelength compatible applications.

An optical fiber-based LSPR aptasensor for simple and rapid in-situ detection of ochratoxin A.

Label-free biosensing methods that rely on the use of localized surface plasmon resonance (LSPR) have attracted great attention as a result of their simplicity, high sensitivity, and relatively low cost. However, in-situ analysis of real samples using these techniques has remained challenging because colloidal nanoparticles (NPs) can be unstable at certain levels of pH and salt concentration. Even in the case of a chip-type LSPR sensor that can resolve the instability problem by employing NPs immobilized on the substrate, loading of a sample to sensor chip with exact volume control can be difficult for unskilled users. Herein, we report an optical-fiber-based LSPR aptasensor that can avoid these problems and serve as a portable and simple system for sensitive detection of a small mycotoxin, ochratoxin A (OTA), in real samples. The optical fiber coated with aptamer-modified gold nanorods (GNRs) is simply dipped into a solution containing OTA and subjected to LSPR analysis. Quantitative analysis of OTA is performed by measuring the spectral red shift of the LSPR peak of GNRs. Under optimized conditions, the LSPR peak shift displays a linear response (R2 = 0.9887) to OTA in the concentration range from 10pM to 100nM, with a limit of detection of 12.0pM (3S). The developed sensor shows a high selectivity for OTA over other mycotoxins such as zearalenone (ZEN) and ochratoxin B (OTB), and shows an accurate detection capability for OTA in real grape juice samples.

Two-step adsorption kinetics of malachite green on anionic polystyrene microspheres in aqueous solution probed by second harmonic generation.

Adsorption of organic cations of malachite green (MG) on an anionic microsphere surface was monitored in real time using a surface-specific second-harmonic generation (SHG) technique. This adsorption was found to consist of two processes of different timescales for all MG concentrations, which has not been considered in previous SHG experiments. The two-step adsorption can be well explained by our suggested model, which consists of an initial fast electrostatic alignment of weakly bound MG followed by slow non-electrostatic adsorption of MG on the surface of a microsphere. This study shows the importance of kinetics measurements in providing full understanding of adsorption of charged molecules on colloid surfaces.

Titanium oxide nanotube arrays for high light extraction efficiency of GaN-based vertical light-emitting diodes.

TiO2 nanotube (NT) arrays were fabricated on the surface of n-GaN through a liquid-phase conversion process using ZnO nanorods (NRs) as a template for high-efficiency InGaN/GaN multiple quantum well (MQW) vertical light-emitting diodes (VLEDs). The optical output power of the VLEDs with TiO2 NTs was remarkably enhanced by 23% and 189% at an injection current of 350 mA compared to those of VLEDs with ZnO NRs and planar VLEDs, respectively. The large enhancement in optical output is attributed to a synergistic effect of efficient light injection from the n-GaN layer of the VLED to TiO2 NTs because of the well-matched refractive indices and superior light extraction into air at the end of the TiO2 NTs. Light propagation along various configurations of TiO2 NTs on the VLEDs was investigated using finite-difference time domain simulations and the results indicated that the wall thickness of the TiO2 NTs should be maintained close to 20 nm for superior light extraction from the VLEDs.

Low internal pressure in femtoliter water capillary bridges reduces evaporation rates.

Capillary bridges are usually formed by a small liquid volume in a confined space between two solid surfaces. They can have a lower internal pressure than the surrounding pressure for volumes of the order of femtoliters. Femtoliter capillary bridges with relatively rapid evaporation rates are difficult to explore experimentally. To understand in detail the evaporation of femtoliter capillary bridges, we present a feasible experimental method to directly visualize how water bridges evaporate between a microsphere and a flat substrate in still air using transmission X-ray microscopy. Precise measurements of evaporation rates for water bridges show that lower water pressure than surrounding pressure can significantly decrease evaporation through the suppression of vapor diffusion. This finding provides insight into the evaporation of ultrasmall capillary bridges.

Light-Emitting Diodes with Hierarchical and Multifunctional Surface Structures for High Light Extraction and an Antifouling Effect.

Bioinspired hierarchical structures on the surface of vertical light-emitting diodes (VLEDs) are demonstrated by combining a self-assembled dip-coating process and nanopatterning transfer method using thermal release tape. This versatile surface structure can efficiently reduce the total internal reflection and add functions, such as superhydrophobicity and high oleophobicity, to achieve an antifouling effect for VLEDs.

Morphological and SERS Properties of Silver Nanorod Array Films Fabricated by Oblique Thermal Evaporation at Various Substrate Temperatures.

Aligned silver nanorod (AgNR) array films were fabricated by oblique thermal evaporation. The substrate temperature during evaporation was varied from 10 to 100 °C using a home-built water cooling system. Deposition angle and substrate temperature were found to be the most important parameters for the morphology of fabricated films. Especially, it was found that there exists a critical temperature at ~90 °C for the formation of the AgNR array. The highest enhancement factor of the surface-enhanced Raman scattering (SERS), observed in the Ag films coated with benzenethiol monolayer, was ~6 × 10(7). Hot spots, excited in narrow gaps between nanorods, were attributed to the huge enhancement factor by our finite-difference time-domain (FDTD) simulation reflecting the real morphology.

Fluorescence turn-on response of a conjugated polyelectrolyte with intramolecular stack structure to biomacromolecules.

An anionic conjugated polyelectrolyte based on polydiphenylacetylene showed a significant fluorescence turn-on response to positively-charged proteins through a conformational relaxation of its intramolecular stack structure.

In-situ observation of the inside-to-outside molecular transport of a liposome.

The first in-situ and real-time observation of the molecular transport from inside to outside of a liposome was shown by using the second harmonic generation (SHG) technique. The transport of an organic cationic molecule across the liposome bilayer could be switched on and off using the structural change of the lipid bilayer caused by temperature change. This approach can be helpful for the understanding and control of the molecular transport in the liposome vehicle.

Topographic control of lipid-raft reconstitution in model membranes.

Liquid-ordered (L(O)) domains reconstituted in model membranes have provided a useful platform for in vitro studies of the lipid-raft model, in which signalling membrane molecules are thought to be compartmentalized in sphingolipid- and cholesterol-rich domains. These in vitro studies, however, have relied on an uncontrolled phase-separation process that gives a random distribution of L(O) domains. Obviously, a precise control of the size and spatial distribution of the L(O) domains would enable a more systematic large-scale in vitro study of the lipid-raft model. The prerequisite for such capability would be the generation of a well-defined energy landscape for reconstituting the L(O) domain without disrupting the two-dimensional (2D) fluidity of the model membrane. Here we report controlling the reconstitution of the L(O) domains in a spatially selective manner by predefining a landscape of energy barriers using topographic surface modifications. We show that the selective reconstitution spontaneously arises from the 2D brownian motion of nanoscale L(O) domains and signalling molecules captured in these nanodomains, which in turn produce a prescribed, concentrated downstream biochemical process. Our approach opens up the possibility of engineering model biological membranes by taking advantage of the intrinsic 2D fluidity. Moreover, our results indicate that the topographic configuration of cellular membranes could be an important machinery for controlling the lipid raft in vivo.