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Structural coloration is promising as an alternative to chemical coloration because it has characteristics of their high color brightness, no fading, and low toxicity. Here, we report a pragmatic micromolding technique to create functional photonic microcylinders which are useful as structural color pigments. Photocurable dispersions of silica particles with interparticle repulsion are molded to spontaneously form regular arrays in confined volumes, which are instantly stabilized by photopolymerization. The resulting photonic microcylinders, released from the mold, exhibit pronounced structural colors from the entire visible range. In addition, multiple compartments can be integrated into single microcylinders through volatile-solvent-mediated sequential molding. As each compartment can be independently rendered to be structurally colored, transparent, or magneto-responsive, the multicompartment microcylinders show advanced functionalities, such as color-brightness tunability and switchable color properti...

Droplet-guiding superhydrophobic SERS substrates are created by a combinatorial lithographic technique. Photolithography defines the pattern of a micropillar array with a radial density gradient, whereas colloidal lithography features a nanotip array on the top surface of each micropillar. The nanotip array renders the surface superhydrophobic, and the pattern of micropillars endows the radial gradient of the contact angle, enabling the spontaneous droplet migration toward the center of the pattern. Water droplets containing target molecules are guided to the center, and the molecules dissolved in the droplets are concentrated at the surface of the central micropillar during droplet evaporation. Therefore, the molecules can be analyzed at the predefined position by Raman spectra without scanning the entire substrate. At the same time, the SERS-active nanotip array provides high sensitivity of Raman measurement.

Enhancing light–matter interactions is essential to improving nanophotonic and optoelectronic device performance. In the present work, we developed a new design for 3D plasmonic nanostructures with enhanced near-field interactions among the plasmonic nanomaterials. The 3D plasmonic nanostructures consisted of multilayered bottom Ag/polydimethylsiloxane (PDMS) nanostructures, an alumina middle layer, and top Ag nanoparticles (NPs). High areal density PDMS nanoprotrusions were self-organized by a simple maskless plasma etching process. The conformal deposition of alumina using atomic layer deposition and Ag deposition produced 3D plasmonic nanostructures. These structures induced multiple near-field interactions between the ultrahigh-areal-density (1400 μm−2) top Ag NPs and the underlying Ag nanostructures, and among the top Ag NPs themselves. The high density of hot spots across the 3D space yielded highly efficient and widely tunable plasmonic responses across the entire visible range. The SERS signal enhancement measured at the 3D plasmonic nanostructures was 3.9 times the signal measured at the 2D multilayered structures and 48.0 times the signal measured at a Ag NP layer deposited onto a Si substrate. Finally, the 3D plasmonic nanostructures exhibited excellent uniformity with a variation of 6.8%, based on a microscale Raman mapping analysis. The excellent Raman signal uniformity can be attributed to the ultrahigh areal density of the Ag NPs and the uniform thickness of the alumina spacing layer.

Herein, a simple interference lithographic technique to design omniphobic surface-enhanced Raman scattering active surfaces composed of stacked-disk nanotower arrays is reported. To create the nanotower arrays in a highly reproducible fashion, the authors use a single laser exposure onto the photoresist-on-dielectric mirror using a phase mask. The interference by the phase shift produces a nanotower array with a submicrometer periodicity, whereas the standing wave effect by reflection from the mirror defines the stacked-disk structure at each nanotower at sub-100 nm periodicity. The stacked-disk nanotowers with silver outer-coating provide a strong electric field localization at the vertically integrated nanogaps on the side wall. At the same time, the series of reentrant geometry on the side walls can efficiently pin the liquid–air interface for a variety of liquids, including water and toluene, thereby providing omniphobic surface property. The molecules dissolved in either water or toluene at low concentration can be enriched in the drop on the surface in a course of evaporation, which are finally deposited onto a tiny spot. Raman spectra of the molecules are further amplified by the strong electric field at the nanogaps on the side walls of the nanotowers. Therefore, the surfaces provide high sensitivity of Raman analysis for both polar and nonpolar molecules.

Highly roughened Au-decorated 3D ZnO nano-structures were prepared using a combination of prism holographic lithography and atomic layer deposition techniques. Prism holographic lithography is a simple and rapid method for fabricating ordered 3D nanostructures using the optical interference effects of multiple beams derived from a specially designed prism. Highly ordered reproducible surface-enhanced Raman scattering (SERS) substrates are needed for the reliable calibration of target analyte concentrations. A high density of Au nanoparticles separated by nanoscale gaps was generated on the Au-coated ZnO inverse structures. The nanogaps may function as strong hot spots for highly sensitive SERS-based chemical/biological sensors. The optimized SERS intensity from the prepared Au-coated 3D ZnO inverse structures was 20 times the intensity obtained from an Au-coated flat glass control substrate. The surfaces could be reused after the photocatalytic degradation and removal of adsorbates in the presence of ZnO. The Au-coated 3D ZnO structures described here offer an alternative to traditional single-use SERS substrates.

We report a microfluidic strategy for creating semipermeable microgels containing metal nanoparticles to directly detect small molecules included in the solution of large adhesive proteins using surface-enhanced Raman scattering. With a capillary microfluidic device, gold nanoparticle-laden microgels are prepared to have uniform size. The microgels allow diffusion of smaller molecules than mesh size of their gel network while excluding larger molecules. This enables the selective infusion of small analytes onto the surface of gold nanoparticles from the solutions of adhesive proteins, thereby providing high Raman intensity by metal-surface enhancement; otherwise, proteins adsorb the surface, which significantly reduces the intensity. Therefore, this microgel platform enables the direct detection of analytes from biological fluids and obviates complicated pre- or post-treatment of samples. In addition, the microgels are able to be injected into target volume such as vessels or living organisms, which are t...

We report a novel method to create 2D hierarchical nanopatterns with high structural complexity using phase-shift lithography. With phase masks with large lattice periodicities relative to the wavelength of the light source, several different diffraction beams are generated from a single grating, which then interfere to form a highly complex 3D intensity profile at the Fresnel region. We transfer the horizontal slice of the intensity profile into the thin film of negative photoresist, making 2D nanostructures. Because the diffraction order determines a length scale of intensity variation in a horizontal surface, interference of several different diffraction orders leads to the formation of complex and hierarchical nanopatterns, which are difficult to create with conventional phase-shift lithography. In addition, as the 2D profile is modulated along the light propagation direction, a variety of complex nanopatterns can be fabricated from a single phase mask by adjusting the distance between the diffraction grating and photoresist film. A full 3D intensity profile formed by interference is calculated using the finite-difference time domain (FDTD) method, which enables us to anticipate the shape and morphology of the resulting 2D nanostructures.

3D hybrid plasmonic nanomaterials are composed of 3D-stacked Ag nanowires and nanoparticles separated by a nanoscale-thick alumina interlayer. The 3D hybrid plasmonic nanostructures exhibit strong plasmonic coupling between the ultrahigh populations of plasmonic nanomaterials, overcoming the physical limitation of inefficient plasmonic coupling of the Ag nanowire stacks.

Inverse opals have been used for structural coloration and photonic applications owing to their photonic bandgap properties. When the photonic structures contain planar defects, they provide defect modes, which are useful for lasing, sensing, and waveguiding. However, it remains a challenge to insert a planar defect into inverse opals in a reproducible manner. Here, we report a new method for producing planar-defect-inserted inverse opals using sequential capillary wetting of colloidal crystals and creating micropatterns through photolithography. Three cycles of deposition and thermal embedding of colloidal crystals into the underlying film of negative photoresist were performed. In the three cycles, opal, particle monolayer, and opal were sequentially employed, which yielded the monolayer-templated planar defect sandwiched by two inverse opals after particle removal. The planar defect provided a passband whose wavelength can be controlled by adjusting the diameter of particles for the defect layer. Moreover, the defect-inserted inverse opals can be micropatterned by photolithography as the negative photoresist is used as a matrix. The resulting micropatterns deliver a unique spectral code featured by a combination of stop band and defect mode and a graphical code dictated by photolithography, being useful for anticounterfeiting applications.

Highly uniform 3D ZnO hollow shell structures were prepared by combining prism holographic lithography (PHL) and atomic layer deposition (ALD). As a dense ZnO film was obtained by using the ALD process, no volume shrinkage occurred during the subsequent calcination to remove the sacrificial polymer template. No volume shrinkage during heat treatment is crucial for achieving excellent optical properties and mechanical stability of inverse photonic crystals (PCs).

This work introduces a robust means for excellent position registry of microparticles via a forced assembly technique on flexible or stretchable substrates. It is based on the dry powder rubbing process which allows assembly of a microparticle monolayer in a short time without requiring any solvent or thermal treatment. Elastic physical templates are used as substrates for the forced assembly in this study. Since the elastic templates can reduce the stress accumulation between the closely packed particles, they can minimize the defect formation in the particle assembly in large areas. The method can be used with powders comprising irregularly shaped particles with a relatively large size distribution that cannot be periodically ordered by conventional self-assembly. Furthermore, a non-closely packed particle array can be fabricated readily in large area, which is highly desirable for practical uses of the particle monolayers. The particle monolayers formed on the elastomer templates can be transferred to surfaces coated with thermoplastic block copolymers. Once transferred, the particle monolayers are flexible and stretchable over their entire surface. This work uses the particle monolayers on a large-area flexible substrate as photomasks to produce various photoresist patterns.

Plasmonic nanostructures strongly localize electric fields on their surfaces via the collective oscillations of conducting electrons under stimulation by incident light at a certain wavelength. Molecules adsorbed onto the surfaces of plasmonic structures experience a strongly enhanced electric field due to the localized surface plasmon resonance (LSPR), which amplifies the Raman scattering signal obtained from these adsorbed molecules. This phenomenon is referred to as surface-enhanced Raman scattering (SERS). Because Raman spectra serve as molecular fingerprints, SERS has been intensively studied for its ability to facilely detect molecules and provide a chemical analysis of a solution. Further enhancements in the Raman intensity and therefore higher sensitivity in SERS-based molecular analysis have been achieved by designing plasmonic nanostructures with a controlled size, shape, composition, and arrangement. This review paper focuses on the current state of the art in the fabrication of SERS-active substrates and their use as chemical and biosensors. Starting with a brief description of the basic principles underlying LSPR and SERS, we discuss three distinct nanofabrication methods, including the bottom-up assembly of nanoparticles, top-down nanolithography, and lithography-free random nanoarray formation. Finally, typical applications of SERS-based sensors are discussed, along with their perspectives and challenges.

In this paper, the authors review the optical principles underlying the fabrication of 3D periodic nanostructures prepared using holographic lithography (HL) as well as their applications toward chemical sensors and energy storage devices using 3D functional nanomaterials. HL is potentially useful for the simple and rapid fabrication of defect‐free large‐area periodic nanostructures with various structural geometries. The use of optical elements, such as well‐designed prisms and diffraction gratings, improves the reproducibility of the manufacturing process by simplifying complicated optical setups. 3D functional nanostructures, including semiconductor and metallic nanomaterials, are useful in highly sensitive photonic crystals and plasmonic sensors, and high‐performance 3D electrodes for use in energy production and storage devices.

Semipermeable microcapsules are appealing for controlled release of drugs, study of cell-to-cell communication, and isolation of enzymes or artificial catalysts. Here, we report a microfluidic strategy for creating monodisperse microcapsules with size-selective permeability using polymerization-induced phase separation. Monodisperse water-in-oil-in-water (W/O/W) double-emulsion drops, whose ultrathin middle layer is composed of photocurable resin and inert oil, are generated in a capillary microfluidic device, and irradiated by UV light. Upon UV illumination, the monomers are photopolymerized, which leads to phase separation between the polymerized resin and the oil within the ultrathin shell. Subsequent dissolution of the oil leaves behind regular pores in the polymerized membrane that interconnect the interior and exterior of the microcapsules, thereby providing size-selective permeability. The degree of phase separation can be further tuned by adjusting the fraction of oil in the shell or the affinity of the oil to the monomers, thereby enabling the control of the cutoff value of permeation. High mechanical stability and chemical resistance of the microcapsules, as well as controllable permeability and high encapsulation efficiency, will provide new opportunity in a wide range of applications.

Surface-enhanced Raman scattering (SERS) is a promising technique for molecular analysis as the molecular fingerprints (Raman spectra) are amplified to detectable levels compared with common spectroscopy. Metal nanostructures localize electromagnetic field on their surfaces, which can lead to dramatic increase of Raman intensity of molecules adsorbed. However, the metal surfaces are prone to contamination, thereby requiring pretreatment of samples to remove adhesive molecules. To avoid the pretreatment and potentially achieve point-of-care (POC) analysis, we have developed SERS-active microgels using the droplet-microfluidic system. As the microgels are composed of water-swollen network with consistent mesh size, they selectively allow diffusion of molecules smaller than the mesh, thereby excluding large adhesives. To render the microgels highly SERS-active, we destabilize silver nanocubes to form agglomerates, which are embedded in the matrix of microgels. The nanogaps in the agglomerates provide high sensitivity in Raman measurement and size-selective permeability of the microgel matrix obviates the pretreatment of samples. To validate the functions, we demonstrate the direct detection of Aspirin dissolved in whole blood without any pretreatment.

Isotropic microparticles prepared from a suspension that undergoes polymerization have long been used for a variety of applications. Bulk emulsification procedures produce polydisperse emulsion droplets that are transformed into spherical microparticles through chemical or physical consolidation. Recent advances in droplet microfluidics have enabled the production of monodisperse emulsions that yield highly uniform microparticles, albeit only on a drop-by-drop basis. In addition, microfluidic devices have provided a variety of means for particle functionalization through shaping, compartmentalizing, and microstructuring. These functionalized particles have significant potential for practical applications as a new class of colloidal materials. This feature article describes the current state of the art in the microfluidic-based synthesis of monodisperse functional microparticles. The three main sections of this feature article discuss the formation of isotropic microparticles, engineered microparticles, and hybrid microparticles. The complexities of the shape, compartment, and microstructure of these microparticles increase systematically from the isotropic to the hybrid types. Each section discusses the key idea underlying the design of the particles, their functionalities, and their applications. Finally, we outline the current limitations and future perspectives on microfluidic techniques used to produce microparticles.

Anisotropic nanostructures with precise orientations or sharp corners display unique properties that may be useful in a variety of applications; however, precise control over the anisotropy of geometric features, using a simple and reproducible large-area fabrication technique, remains a challenge. Here, we report the fabrication of highly uniform polymeric and metallic nanostructure arrays prepared using prism holographic lithography (HL) in such a way that the isotropy that can be readily and continuously tuned. The prism position on the sample stage was laterally translated to vary the relative intensities of the four split beams, thereby tuning the isotropy of the resulting polymer nanostructures through the following shapes: circular nanoholes, elliptical nanoholes, and zigzag-shaped nanoarrays. Corresponding large-area, defect-free anisotropic metallic nanostructures could then be fabricated using an HL-featured porous polymer structure as a milling mask. Removal of the polymer mask left zigzag-shaped metallic nanostructure arrays in which nanogaps separated adjacent sharp edges. These structures displayed two distinct optical properties, depending on the direction along which the excitation beam was polarized (longitudinal and transverse modes) incident on the array. Furthermore, bidirectional anisotropic wetting was observed on the anisotropic polymer nanowall array surface.

Three-dimensional hierarchical architectures are fabricated using a simple, cost-effective, durable colloidal phase mask containing a colloidal monolayer embedded in a flexible polydimethylsiloxane (PDMS) membrane. These structures give rise to a photonic bandgap that can be tuned over a wide spectral range from the visible to the near-infrared regions.

We present a simple, easy method for fabricating high-quality titania inverted replicas of 3D holographically featured structures. A combination of single-prism holographic lithography and sol-gel chemistry was used to prepare 3D titania inverse structures with flat and completely open surfaces without the use of additional postprocessing steps, such as reactive ion etching, ion-beam milling, and/or polishing steps. A hydrophobic, stable liquid titania precursor facilitated the complete infiltration of the precursor into the hydrophobic 3D SU-8 polymer template, which produced very uniform high-quality titania inverse structures. Although the degree of film shrinkage during the calcination process was large (∼34%), the optical strength of the 3D titania inverse photonic crystals doubled because of the high-refractive-index contrast. Compared to titania inverse opal structures, the filling fraction (∼27%) of titania materials has been doubled. This is the first work to fabricate titania inverse photonic crystals with a high filling fraction by utilizing prism holographic lithography and the sol-gel chemistry reaction of a stable titania precursor. The X-ray diffraction patterns indicated the presence of a crystalline anatase or rutile phase depending on the calcination temperature.

Large-area, highly ordered, Ag-nanostructured arrays with various geometrical features were prepared for use as surface-enhanced Raman scattering (SERS)-active substrates by the self-assembly of inorganic particles on an SU-8 surface, followed by particle embedding and Ag vapor deposition. By adjusting the embedding time of the inorganic particles, the size of the Ag nanogap between the geometrically separated hole arrays and bowl-shaped arrays could be controlled in the range of 60 nm to 190 nm. More importantly, the SU-8 surface was covered with hexagonally ordered nanopillars, which were formed as a result of isotropic dry etching of the interstices, leading to triangular-shaped Ag plates on nanopillar arrays after Ag vapor deposition. The size and sharpness of the triangular Ag nanoplates and nanoscale roughness of the bottom surface were adjusted by controlling the etching time. The potential of the various Ag nanostructures for use as practical SERS substrates was verified by the detection of a low concentration of benzenethiol. Finite-difference time-domain (FDTD) methodology was used to demonstrate the SERS-activities of these highly controllable substrates by calculating the electric field intensity distribution on the metallic nanostructures. These substrates, with high sensitivity and simple shape-controllability, provide a practical SERS-based sensing platform.