Exploitation of Diatom Frustules for Nanotechnology: Tethering Active Biomolecules

Diatoms are single‐celled microalgae with rigid walls (frustules) composed of amorphous silica. The intricate 3D microstructure of diatoms results in a high surface area formed by myriad pores and channels. The combination of the silica chemistry of the frustule coupled with the high surface area makes it particularly suitable for applications such as microscale total analysis systems. Here it is demonstrated that the diatom frustule can be chemically modified for the attachment of antibodies, and that the attached antibodies retain biological activity. These modified structures have potential applications in antibody arrays and may have use in techniques such as immunoprecipitation. These silica structures are produced in diatoms using only light and minimal nutrients and, therefore, generate an exceptionally cheap and renewable material.     

Photoluminescence Detection of Biomolecules by Antibody‐Functionalized Diatom Biosilica

Diatoms are single‐celled algae that make microscale silica shells called “frustules”, which possess intricate nanoscale features imbedded within periodic two‐dimensional pore arrays. In this study, antibody‐functionalized diatom biosilica frustules serve as a microscale biosensor platform for selective and label‐free photoluminescence (PL)‐based detection of immunocomplex formation. The model antibody rabbit immunoglobulin G (IgG) is covalently attached to the frustule biosilica of the disk‐shaped, 10‐µm diatom Cyclotella sp. by silanol amination and crosslinking steps to a surface site density of 3948 ± 499 IgG molecules µm^?2. Functionalization of the diatom biosilica with the nucleophilic IgG antibody amplifies the intrinsic blue PL of diatom biosilica by a factor of six. Furthermore, immunocomplex formation with the complimentary antigen anti‐rabbit IgG further increases the peak PL intensity by at least a factor of three, whereas a non‐complimentary antigen (goat anti‐human IgG) does not. The nucleophilic immunocomplex increases the PL intensity by donating electrons to non‐radiative defect sites on the photoluminescent diatom biosilica, thereby decreasing non‐radiative electron decay and increasing radiative emission. This unique enhancement in PL emission is correlated to the antigen (goat anti‐rabbit IgG) concentration, where immunocomplex binding follows a Langmuir isotherm with binding constant of 2.8 ± 0.7 × 10^?7 M .     

Rapid Fabrication of Micro‐ and Nanoscale Patterns by Replica Molding from Diatom Biosilica

Diatoms are single‐celled micro‐algae that possess an exoskeleton (called frustule) comprised of diverse and highly ordered 3D porous silica structures and that hold considerable promise for biological or biomimetic fabrication of nanostructured materials and devices. We have used, for the first time, a soft lithographic approach of replica molding to replicate porous diatom structures into polymers. Two centric diatom species, Coscinodiscus sp., Thalassiosira eccentrica cultured in our laboratory were used as masters for replication. In the first step, replica molding onto soft and elastic polymer using poly(dimethylsiloxane) PDMS produced a negative replica of the diatom frustule. These PDMS replicas were then used as a mold to fabricate the positive polymer replicas of diatoms using a mercaptol ester type UV curable polymer (NOA 60). Fabricated polymer replicas were characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM). In all cases, diatom micro‐ and nanoscale porous structures were successfully transferred with high precision into polymer replicas. Such an accomplishment effectively demonstrates the potential for using diatoms as blueprints for rapid and simple fabrication of polymer nanostructures. The prepared replicas were used as diffraction gratings and as nanowells to hold polymeric nanoparticles effectively demonstrating the functional properties of these biomimetic structures.     

Highly Ordered Mesoporous Silicon Carbide Ceramics with Large Surface Areas and High Stability

Highly ordered mesoporous silicon carbide ceramics have been successfully synthesized with yields higher than 75 % via a one‐step nanocasting process using commercial polycarbosilane (PCS) as a precursor and mesoporous silica as hard templates. Mesoporous SiC nanowires in two‐dimensional (2D) hexagonal arrays (p6m) can be easily replicated from a mesoporous silica SBA‐15 template. Small‐angle X‐ray diffraction (XRD) patterns and transmission electron microscopy (TEM) images show that the SiC nanowires have long‐range regularity over large areas because of the interwire pillar connections. A three‐dimensional (3D) bicontinuous cubic mesoporous SiC structure (Ia3d) can be fabricated using mesoporous silica KIT‐6 as the mother template. The structure shows higher thermal stability than the 2D hexagonal mesoporous SiC, mostly because of the 3D network connections. The major constituent of the products is SiC, with 12 % excess carbon and 14 % oxygen measured by elemental analysis. The obtained mesoporous SiC ceramics are amorphous below 1200 °C and are mainly composed of randomly oriented β‐SiC crystallites after treatment at 1400 °C. N₂‐sorption isotherms reveal that these ordered mesoporous SiC ceramics have high Brunauer–Emmett–Teller (BET) specific surface areas (up to 720 m² g^–1), large pore volumes (～ 0.8 cm³ g^–1), and narrow pore‐size distributions (mean values of 2.0–3.7 nm), even upon calcination at temperatures as high as 1400 °C. The rough surface and high order of the nanowire arrays result from the strong interconnections of the SiC products and are the main reasons for such high surface areas. XRD, N₂‐sorption, and TEM measurements show that the mesoporous SiC ceramics have ultrahigh stability even after re‐treatment at 1400 °C under a N₂ atmosphere. Compared with 2D hexagonal SiC nanowire arrays, 3D cubic mesoporous SiC shows superior thermal stability, as well as higher surface areas (590 m² g^–1) and larger pore volumes (～ 0.71 cm³ g^–1).     

In Situ Observation of Strain Development and Porosity Evolution in Nanoporous Gold Foils

The formation of nanoporous gold by open circuit dealloying of 100 nm AgAu foils in nitric acid is investigated in situ and in real time by combining synchrotron small angle X‐ray scattering (SAXS) and X‐ray diffraction (XRD). The time dependence of the dealloying is followed as a function of acid concentration. For all concentrations, several characteristic dealloying stages are observed. Firstly, there is a fast initial dissolution stage with an increase in surface area due to pore and mound formation; this leads to strain in the nanoporous gold that results from an increase in capillary pressure. After dissolution is complete, there is rapid coarsening of the quasi‐periodic, pore–ligament morphology. During this later stage, we deduce strong strain anisotropies that can be explained by preferred crystallographic orientation of ligaments. This rapid coarsening stage is followed by a slow coarsening stage where the SAXS patterns, and hence the quasi‐periodic morphology, is self‐similar in time. There is a strong correlation between the morphology evolution and strain development, which can be explained by capillary forces.     

Hybrid Photonic Nanostructures by In Vivo Incorporation of an Organic Fluorophore into Diatom Algae

Biotechnological processes harnessing living organisms' metabolism are low‐cost routes to nanostructured materials for applications in photonics, electronics, and nanomedicine. In the pursuit of photonic biohybrids, diatoms microalgae are attractive given the properties of the porous micro‐to‐nanoscale structures of the biosilica shells (frustules) they produce. The investigations have focused on in vivo incorporation of tailored molecular fluorophores into the frustules of Thalassiosira weissflogii diatoms, using a procedure that paves the way for easy biotechnological production of photonic nanostructures. The procedure ensures uniform staining of shells in the treated culture and permits the resulting biohybrid photonic nanostructures to be isolated with no damage to the dye and periodic biosilica network. Significantly, this approach ensures that light emission from the dye embedded in the isolated biohybrid silica is modulated by the silica's nanostructure, whereas no modulation of photoluminescence is observed upon grafting the fluorophore onto frustules by an in vitro approach based on surface chemistry. These results pave the way to the possibility of easy production of photonic nanostructures with tunable properties by simple feeding the diatoms algae with tailored photoactive molecules.     

In Situ Fabrication of Three‐Dimensional Graphene Films on Gold Substrates with Controllable Pore Structures for High‐Performance Electrochemical Sensing

In this work, novel three‐dimensional graphene films (3D GFs) with controllable pore structures are directly fabricated on gold substrates through the hydrothermal reduction. An interfacial technique of the self‐assembled monolayer is successfully introduced to address the binding issue between the graphene film and substrate. Adscititious silica spheres, serving as new connection centers, effectively regulate the dimensions of framework in graphene films, and secondary pore structures are produced once removing the spheres. Based on hierarchically porous 3D GFs with large surface area, excellent binding strength, high conductivity, and distinct interfacial micro‐environments, selected examples of electrochemical aptasensors are constructed for the assay of adenosine triphosphate (ATP) and thrombin (Tob) respectively. Sensitive ATP and Tob aptasensors, with high selectivity, excellent stability, and promising potential in real serum sample analysis, are established on 3D GFs with different structures. The results demonstrate that the surface area, as well as interfacial micro‐environments, plays a critical role in the molecular recognition. The developed reliable and scalable protocol is envisaged to become a general path for in situ fabrication of more graphene films and the as‐synthesized 3D GFs would open up a wide horizon for potential applications in electronic and energy‐related systems.     

金属复合结构的反常透射增强效应的数值分析

利用全矢量的三维时域有限差分法,分析了一种金属银复合结构的反常透射增强效应.该结构是在一层打有六角排列的圆孔阵列的金属银层的上方放置一层六角排列的银圆环阵列构成.与普通的单层银打孔阵列相比,该结构明显具有更高的透射峰值和更窄的透射带宽.系统分析了结构的场强分布、坡印廷矢量能流分布图、透射峰的频率色散关系,结果表明该复合结构激发的更强的独特的表面等离激元模式主导了整个透射过程.     

3D Hexagonal (R‐3m) Mesostructured Nanocrystalline Titania Thin Films: Synthesis and Characterization

A straightforward and reproducible synthesis of crack‐free large‐area thin films of 3D hexagonal (R‐3m) mesostructured nanocrystalline titania (meso‐nc‐TiO₂) using a Pluronic triblock copolymer (P123)/1‐butanol templating system is described. The characterization of the films is achieved using a combination of electron microscopy (high‐resolution scanning electron microscopy and scanning transmission electron microscopy), grazing‐incidence small‐angle X‐ray scattering, in situ high‐temperature X‐ray diffraction, and variable‐angle spectroscopic ellipsometry. The mesostructure of the obtained films is found to be based upon a 3D periodic array of large elliptically shaped cages with diameters around 20 nm interconnected by windows of about 5 nm in size. The mesopores of the film calcined at 300 °C are very highly ordered, and the titania framework of the film has a crystallinity of 40 % being composed of 5.8 nm sized anatase crystallites. The film displays high thermal stability in that the collapse of the pore architecture is incomplete even at 600 °C. The accessible surface area of 3D hexagonal meso‐nc‐TiO₂ estimated by the absorption of methylene blue is nearly twice as large as that of 2D hexagonal meso‐nc‐TiO₂ at the same annealing temperature.     

Evaporation‐Induced Coating and Self‐Assembly of Ordered Mesoporous Carbon‐Silica Composite Monoliths with Macroporous Architecture on Polyurethane Foams

A facile approach of solvent‐evaporation‐induced coating and self‐assembly is demonstrated for the mass preparation of ordered mesoporous carbon‐silica composite monoliths by using a polyether polyol‐based polyurethane (PU) foam as a sacrificial scaffold. The preparation is carried out using resol as a carbon precursor, tetraethyl orthosilicate (TEOS) as a silica source and Pluronic F127 triblock copolymer as a template. The PU foam with its macrostructure provides a large, 3D, interconnecting interface for evaporation‐induced coating of the phenolic resin‐silica block‐copolymer composites and self‐assembly of the mesostructure, and endows the composite monoliths with a diversity of macroporous architectures. Small‐angle X‐ray scattering, X‐ray diffraction and transmission electron microscopy results indicate that the obtained composite monoliths have an ordered mesostructure with 2D hexagonal symmetry (p6m) and good thermal stability. By simply changing the mass ratio of the resol to TEOS over a wide range (10–90%), a series of ordered, mesoporous composite foams with different compositions can be obtained. The composite monoliths with hierarchical macro/mesopores exhibit large pore volumes (0.3–0.8 cm³ g^?1), uniform pore sizes (4.2–9.0 nm), and surface areas (230–610 m² g^?1). A formation process for the hierarchical porous composite monoliths on the struts of the PU foam through the evaporation‐induced coating and self‐assembly method is described in detail. This simple strategy performed on commercial PU foam is a good candidate for mass production of interface‐assembly materials.     

Diatom Frustule‐Inspired Metamaterial Absorbers: The Effect of Hierarchical Pattern Arrays

Diatoms are photosynthetic algae that exist ubiquitously throughout the planet in water environments. Over the preceding decades, the diatom exoskeletons, termed frustules, featuring abundant micro‐ and nanopores, have served as the source material and inspiration for myriad research efforts. In this work, it is demonstrated that frustule‐inspired hierarchical nanostructure designs may be utilized in the fabrication of metamaterial absorbers, thereby realizing a broadband infrared (IR) absorber with excellent performance in terms of absorption. In an effort to investigate the origin of this absorption characteristic, numerical models are developed to study these structures, revealing that the hierarchical organization of the constituent nanoparticulate metamaterial unit cells introduce an additional resonance mode to the device, broadening the absorption spectrum. It is further demonstrated that the resonant peaks shift linearly as a function of inter‐unit‐cell spacing in the metamaterial, which is attributed to the induced collective dipole mode by the nanoparticles. Ultimately, the work herein represents an innovative perspective in terms of the design and fabrication of IR absorbers inspired by naturally occurring biomaterials, offering the potential to lead to advances in metamaterial absorber technology.     

Linewidth‐Optimized Extraordinary Optical Transmission in Water with Template‐Stripped Metallic Nanohole Arrays

The experimental observation of unusually sharp plasmon resonance peaks in periodic Ag nanohole arrays made using template stripping is reported. The extraordinary optical transmission (EOT) peak associated with the surface plasmon polaritons at the smooth Ag‐water interface shows a well‐defined Fano‐type profile with a linewidth below 10 nm at a wavelength of around 700 nm. Notably, this sharp and intense radiant peak (Q factor of 71) is obtained at visible frequencies in water and at normally incident illumination. This is accomplished by obtaining high‐quality Ag surfaces with a roughness below 1 nm, which reduces the imaginary component of the Ag dielectric function that is associated with material damping, as well as shrinking the nanohole radius to decrease radiative damping of plasmons. The localized spectral response of the radiant plasmon peak is characterized using the nanohole array in water in a layer‐by‐layer fashion via sequential atomic layer deposition of Al₂O₃. Because the ultrasharp EOT peak is obtained with excellent uniformity over a centimeter‐sized area from the metallic nanohole array in water, these template‐stripped nanohole arrays will benefit many practical applications based on EOT.     

基于开口谐振环的多频太赫兹透射特性研究

通过在双层金属开圆孔阵列并嵌入对称开口谐振环结构,设计了一种新型金属-介质-金属透射增强结构.用电磁仿真软件对该结构的透射特性进行模拟,研究了单元结构的几何参数对透射峰的影响.结果表明:增加开口谐振环可以有效增加透射峰数量,实现了0.2～1.1 THz内的多频超强透射.根据透射峰处金属表面的场分布,透射峰与磁谐振、局域型表面等离子体、传播型表面等离子体、法布里-珀罗谐振及他们之间的杂化耦合有关.结果对深入研究超强透射特性及透射机理具有一定的指导意义,也为太赫兹微波器件设计和性能分析提供了重要参考.     

Fabrication of periodic square arrays by angle-resolved nanosphere lithography

We investigate the fabrication of periodic square arrays of solid gold islands by angle-resolved nanosphere lithography (ARNSL) in conjunction with thermal evaporation and etching. By varying θ (the tilt angle between the direction of gold deposition beam and the substrate surface normal) and ? (the substrate rotation angle about the beam axis), adjacent islands on a deposited hexagonal gold array will have a constant and periodic difference in height. Upon etching, this height bias will result in the shorter structures being removed to produce an array with a different symmetry from the original hexagonal symmetry of the parent mask. By depositing at three directions of ? = 0°, 120° and −120° with a constant θ = 20°, experimental results show that deposited two-dimensional gold periodic arrays will have a measurable difference in height between adjacent islands. Etching of the resulting patterns produced periodic near-square arrays with triangular nanostructures. Thus the combination of ARNSL and etching can allow selective periodic nanostructures to be removed, increasing the diversity of array symmetries available through nanosphere lithography.     

Mesostructured Arrays of Nanometer‐spaced Gold Nanoparticles for Ultrahigh Number Density of SERS Hot Spots

A novel one‐trough synthesis via an air‐water interface is demonstrated to provide hexagonally packed arrays of densely spaced metallic nanoparticles (NPs). In the synthesis, a mesostructured polyoxometalate (POM)‐silicatropic template (PSS) is first self‐assembled at the air‐water interface; upon UV irradiation, anion exchange cycles enable the free‐floating PSS film to continuously uptake gold precursors from the solution subphase for diffusion‐controlled and POM‐site‐directed photoreduction inside the silica channels. NPs ≈ 2 nm can hence be homogeneously formed inside the silica‐surfactant channels until saturation. As revealed via X‐ray diffraction, small‐angle X‐ray scattering (SAXS), grazing incidence SAXS, and transmission electron microscopy, the Au NPs directed by the PSS template are arrayed into a 2D hexagonal lattice with inter‐channel spacing of 3.2 nm and a mean along‐channel NP spacing of 2.8 nm. This corresponds to an ultra‐high number density (≈10¹⁹ NPs cm^?3) of narrowly spaced Au NPs in the Au‐NP@PSS composite, leading to 3D densely deployed hot‐spots along and across the mesostructured POM‐silica channels for surface‐enhanced Raman scattering (SERS). Consequently, the Au‐NP@PSS composite exhibits prominent SERS with 4‐mercaptobenzoic acid (4‐MBA) adsorbed onto Au NPs. The best 4‐MBA detection limit is 5 nm , with corresponding SERS enhancement factors above 10⁸.     

Synthesis of Self‐Supported Ordered Mesoporous Cobalt and Chromium Nitrides

Herein, we demonstrate an ammonia nitridation approach to synthesize self‐supported ordered mesoporous metal nitrides (CoN and CrN) from mesostructured metal oxide replicas (Co₃O₄ and Cr₂O₃), which were nanocastly prepared by using mesoporous silica SBA‐15 as a hard template. Two synthetic routes are adopted. One route is the direct nitridation of mesoporous metal oxide nanowire replicas templated from SBA‐15 to metal nitrides. By this method, highly ordered mesoporous cobalt nitrides (CoN) can be obtained by the transformation of Co₃O₄ nanowire replica under ammonia atmosphere from 275 to 350 °C, without a distinct lose of the mesostructural regularity. Treating the samples above 375 °C leads to the formation of metallic cobalt and the collapse of the mesostructure due to large volume shrinkage. The other route is to transform mesostructured metal oxides/silica composites to nitrides/silica composites at 750–1000 °C under ammonia. Ordered mesoporous CrN nanowire arrays can be obtained after the silica template removal by NaOH erosion. A slowly temperature‐program‐decrease process can reduce the influence of silica nitridation and improve the purity of final CrN product. Small‐angle XRD patterns and TEM images showed the 2‐D ordered hexagonal structure of the obtained mesoporous CoN and CrN nanowires. Wide‐angle XRD patterns, HRTEM images, and SAED patterns revealed the formation of crystallized metal nitrides. Nitrogen sorption analyses showed that the obtained materials possessed high surface areas (70–90 m² g^?1) and large pore volumes (about 0.2 cm³ g^?1).     

Learning from Diatoms: Nature's Tools for the Production of Nanostructured Silica

Diatoms are eukaryotic, unicellular algae that are ubiquitously present in almost any water habitat on earth. Diatoms dominate phytoplankton populations and algal blooms in the oceans. They are responsible for about 25 % of the world's net primary production. Apart from this ecological significance, diatoms are mainly known for the intricate geometries and spectacular patterns of their silica‐based cell walls. These patterns are species specific. They are precisely reproduced in each generation documenting a genetic control of this biomineralization process. Biogenesis of the diatom cell wall is considered to be a paradigm for the controlled production of nanostructured silica. Biochemical studies demonstrated that diatom biosilica is a composite material containing zwitterionic proteins (silaffins) and long‐chain polyamines in addition to silica. Functional studies indicate a crucial role of these organic components in guiding silica precipitation as well as in the formation of species‐specific nanopatterns. These activities can be explained by molecular self‐assembly and phase‐separation processes. Moreover, diatom cell walls also exhibit very exciting properties from the physical point of view: they are extremely stable and they may act as photonic crystals.     

Design of Catalytically Active Cylindrical and Macroporous Gold Microelectrodes

Porous gold structures with a well‐defined pore size and thickness are obtained through electrochemical deposition of gold in a colloidal crystal template synthesized by the Langmuir–Blodgett (LB) technique. Cylindrical gold wires were used as substrates for the LB deposition of successive monolayers of silica particles of various sizes, and the electrodeposition of gold through this inorganic template led to a homogeneous, porous metal structure. These materials were characterized through typical electrochemical experiments and showed high surface‐to‐volume ratios with promising features for their further use in miniaturized electrocatalytic devices.     

Quasi‐One Dimensional in‐Plane Conductivity in Filamentary Films of PEDOT:PSS

The mechanism and magnitude of the in‐plane conductivity of poly(3,4‐ethy‐lenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) thin films is determined using temperature dependent conductivity measurements for various PEDOT:PSS weight ratios with and without a high boiling solvent (HBS). Without the HBS the in‐plane conductivity of PEDOT:PSS is lower and for all studied weight ratios well described by the relation $ \sigma = \sigma _0 {\rm exp}[- \left({{{{T_0}}\over{T}}} \right)^{0.5}] $ with T₀ a characteristic temperature. The exponent 0.5 indicates quasi‐one dimensional (quasi‐1D) variable range hopping (VRH). The conductivity prefactor σ₀ varies over three orders of magnitudes and follows a power law σ₀∝c^3.5_PEDOT with c_PEDOT the weight fraction of PEDOT in PEDOT:PSS. The field dependent conductivity is consistent with quasi‐1D VRH. Combined, these observations suggest that conductance takes place via a percolating network of quasi‐1D filaments. Using transmission electron microscopy (TEM) filamentary structures are observed in vitrified dispersions and dried films. For PEDOT:PSS films with HBS, the conductivity also exhibits quasi‐1D VRH behavior when the temperature is less than 200 K. The low characteristic temperature T₀ indicates that HBS‐treated films are close to the critical regime between a metal and an insulator. In this case, the conductivity prefactor scales linearly with c_PEDOT, indicating the conduction is no longer limited by a percolation of filaments. The lack of observable changes in TEM upon processing with the HBS suggests that the changes in conductivity are due to a smaller spread in the conductivities of individual filaments, or a higher probability for neighboring filaments to be connected rather than being caused by major morphological modification of the material.     

Fully Integrated Microscale Quasi‐2D Crystalline Molecular Field‐Effect Transistors

Monolithic integration of microscale organic field‐effect transistors (micro‐OFETs) is the only and inevitable path toward low‐cost large‐area electronics and displays. However, to date, such an ultimate technology has not yet evolved due to challenges in positioning and patterning highly crystalline microscale molecular layers as well as in developing micrometer scale integration schemes. In this work, by mastering the local growth of molecular semiconductors on pre‐defined terraces, single‐crystal quasi‐2D molecular layers tens of square micrometers in size are created in dense periodic arrays on a Si substrate. Nondestructive photolithographic processes are developed to pattern micro‐OFETs with mobilities up to 34.6 cm² V^?1 s^?1. This work demonstrates the feasibility to integrate arrays of short‐channel micro‐OFETs into electronic circuitry by highly parallel and size scalable fabrication technologies.