Nanoparticle Arrays on Surfaces Fabricated Using Anodic Alumina Films as Templates

High density nanoparticle arrays on surfaces have been created using a template‐assisted approach. Templates were produced by evaporating aluminum onto substrates and subsequently anodizing the aluminum to produce nanoporous alumina films. The resulting templates have a narrow distribution of pore sizes tunable from ～ 25 to ～ 70 nm. To demonstrate the flexibility of this approach for producing nanoparticle arrays on various substrates, templates have been fabricated on silicon oxide, silicon, and gold surfaces. In all cases, a final chemical etching step yielded pores that extended completely through the template to the underlying substrate. Because the templates remain in intimate contact with the substrate throughout processing, they may be used with either vacuum‐based or wet chemical deposition methods to direct the deposition of nanoparticles onto the underlying substrates. Here we have produced gold nanodot arrays using evaporation and gold nanorod arrays by electrodeposition. In each case, the diameter and height of the nanoparticles can be controlled using the confining dimensions of the templates, resulting in high density (～ 10¹⁰ cm^–2) arrays of nanoparticles over large areas (> 1 cm²).     

Designing Multicolor Micropatterns of Inverse Opals with Photonic Bandgap and Surface Plasmon Resonance

Structural coloration provides unique features over chemical coloration, such as nonfading, color tunability, and high color brightness, rendering it useful in various optical applications. To develop the structural colors, two different mechanisms of coloration–photonic bandgap (PBG) and surface plasmon resonance (SPR)–have been separately utilized. In this work, a new method is suggested to create structurally colored micropatterns by regioselectively employing SPR in a single film of inverse opal with PBG. The inverse opals are prepared by thermal embedding of opal into a negative photoresist and its subsequent removal. The inverse opals have a hexagonal array of open pores on the surface which serves as a template to make SPR‐active nanostructures through a directional deposition of gold, a perforated gold film and an array of curved gold disks are formed. With a shadow mask lithographically prepared, the gold is regioselectively deposited on the surface of the inverse opal, which results in two distinct regions of gold‐free inverse opal with PBG and gold nanostructure with SPR. As PBG and SPR develop their own structural colors respectively, the resultant micropatterns exhibit pronounced dual colors. More importantly, the micropatterns show the distinguished optical response for evaporation of volatile liquids that occupy the pores.     

Biologically Enabled Syntheses of Freestanding Metallic Structures Possessing Subwavelength Pore Arrays for Extraordinary (Surface Plasmon‐Mediated) Infrared Transmission

A scalable wet chemical process has been used to convert the intricate silica microshells (frustules) of diatoms into gold structures that retained the three‐dimensional (3‐D) frustule shapes and fine patterned features. Combined use of an amine‐enriching surface functionalization protocol and electroless deposition yielded thin (<100 nm) conformal nanocrystalline gold coatings that, upon selective silica dissolution, were converted into freestanding gold structures with frustule‐derived 3‐D morphologies. By selecting a diatom frustule template with a quasi‐regular hexagonal pore pattern (Coscinodiscus asteromphalus, CA), gold replica structures possessing such pore patterns were produced that exhibited infrared transmission maxima/reflection minima that were not observed for the starting silica diatom frustules or for flat nonporous gold films; that is, such extraordinary optical transmission (EOT) resulted from the combined effects of the quasi‐periodic hexagonal hole structure (inherited from the CA diatom frustules) and the gold chemistry. Calculated and measured IR transmission spectra obtained from planar gold films with quasi‐periodic hexagonal CA‐derived hole patterns, or with short‐range periodic hexagonal hole patterns, indicated that the enhanced IR transmission exhibited by the gold CA frustule replicas was enabled by the generation and transmission of surface plasmons. This scalable bio‐enabled process provides a new and attractive capability for fabricating self‐supporting, responsive, 3‐D metallic structures for use as dispersible/harvestable microparticles tailored for EOT‐based applications.     

Protein Rebinding to a Surface‐Confined Imprint

A novel strategy to prepare a selective ultrathin molecularly imprinted polymer (MIP) film directly on the gold‐based transducer surface for the peptide and protein detection in aqueous solution is demonstrated using a combination of epitope‐ and electrochemical surface imprinting approach. The synthetic peptide derived from the surface‐exposed C‐terminus of cytochrome c (Cyt c, residues 96–104) is selected as the template for the imprinting. It is labeled with a fluorescent dye in order to quantitatively evaluate all stages of the imprinting process in terms of changes in mean fluorescence intensity. The labeled peptide template is first chemisorbed on the gold surface as an oriented submonolayer through an additional C‐terminal cysteine. After electropolymerization, the template is stripped off electrochemically. To allow the imprinted sites to be confined to the surface, the film thickness is controlled to be comparable to the thickness of the peptide layer. This is achieved by the electropolymerization of scopoletin. Recognition capabilities of the films are characterized and the resulting MIP film is able to selectively capture the template peptide and the corresponding target protein. In case of the peptide recognition, the MIP film can discriminate even the single amino acid mismatched sequences of the target peptide.     

Fabrication of Conducting Polymer and Complementary Gold Microstructures Using Polymer Brushes as Templates

This article describes a strategy to fabricate a conducting polymer and complementary gold microstructures through selective electrodeposition and wet chemical etching on a chemically tethered polymer brush template that is prepared by surface‐initiated atomic transfer radical polymerization (ATRP) and subsequent photopatterning. The polymer brush acts as a sufficient insulating barrier and thus the polypyrrole (PPy) can be grown from the exposed area of the polymer brush template. Different polymer brushes provide different protection actions in selective etching, which is utilized to generate complementary (negative or positive) gold pattern on a single template in different manners.     

Novel Patterning of Gold Using Spin‐Coatable Gold Electron‐Beam Resist

Conventional lithography methods of gold patterning are based on deposition and lift‐off or deposition and etching. In this letter, we demonstrate a novel method of gold patterning using spin‐coatable gold electron‐beam resist which is functionalized gold nanocrystals with amine ligands. Amine‐stabilized gold electron beam resist exhibits good sensitivity, 3.0 mC/cm², compared to that of thiol‐stabilized gold electron beam resists. The proposed method reduces the number of processing steps and provides greater freedom in the patterning of complex nanostructures.     

Drug Delivery: Wavelength‐Selective Light‐Induced Release from Plasmon Resonant Liposomes (Adv. Funct. Mater. 6/2011)

Biodegradable, spectrally tunable plasmon resonant nanocapsules are created via the deposition of gold onto the surface of 100 nm diameter thermosensitive liposomes. These nanocapsules exhibit selective release of encapsulated contents upon illumination with light of a wavelength matching their distinct resonance bands. In this study, 760 and 1210 nm laser illumination elicits complete release from gold‐coated liposomes with a corresponding resonance, while causing minimal release from liposomes with an unmatching resonance. Spectrally selective release is accomplished through the use of multiple, low‐intensity laser pulses delivered over a period of minutes, ensuring that illumination affects the gold‐coated liposomes without heating the surrounding media. The use of pulsed illumination to achieve spectral selectivity is validated experimentally and through modeling of the heat equation. The result of this illumination scheme for selective release using multiple wavelengths of light is a biologically safe mechanism for realizing drug delivery, microfluidic, and sensor applications.     

Topographically Flat Substrates with Embedded Nanoplasmonic Devices for Biosensing

The ability to precisely control the topography, roughness, and chemical properties of metallic nanostructures is crucial for applications in plasmonics, nanofluidics, electronics, and biosensing. Here a simple method to produce embedded nanoplasmonic devices that can generate tunable plasmonic fields on ultraflat surfaces is demonstrated. Using a template‐stripping technique, isolated metallic nanodisks and wires are embedded in optical epoxy, which is capped with a thin silica overlayer using atomic layer deposition. The top silica surface is topographically flat and laterally homogeneous, providing a uniform, high‐quality biocompatible substrate, while the nanoplasmonic architecture hidden underneath creates a tunable plasmonic landscape for optical imaging and sensing. The localized surface plasmon resonance of gold nanodisks embedded underneath flat silica films is used for real‐time kinetic sensing of the formation of a supported lipid bilayer and subsequent receptor‐ligand binding. Gold nanodisks can also be embedded in elastomeric materials, which can be peeled off the substrate to create flexible plasmonic membranes that conform to non‐planar surfaces.     

Hollow Nanostars with Photothermal Gold Caps and Their Controlled Surface Functionalization for Complementary Therapies

Gold nanoparticles exhibiting absorption in the desirable near‐infrared region are attractive candidates for photothermal therapy (PTT). Furthermore, the construction of one nanoplatform employing gold nanoparticles for complementary therapy is still a great challenge. Here, well‐defined unique hollow silica nanostars with encapsulated gold caps (starlike Au@SiO₂) are readily synthesized using a sacrificial template method. Ethanolamine‐functionalized poly(glycidyl methacrylate) (denoted as BUCT‐PGEA) brushes are then grafted controllably from the surface of starlike Au@SiO₂ nanoparticles via surface‐initiated atom transfer radical polymerization to produce starlike Au@SiO₂‐PGEA. The photothermal effect of gold caps with a cross cavity can be utilized for PTT. The interior hollow feature of starlike Au@SiO₂ nanoparticles endows them with excellent drug loading capability for chemotherapy, while the polycationic BUCT‐PGEA brushes on the surface provide good transfection performances for gene therapy, which will overcome the penetration depth limitation of PTT for tumor therapy. Compared with ordinary spherical Au@SiO₂‐PGEA counterparts, the starlike Au@SiO₂‐PGEA hybrids with sharp horns favor endocytosis, which can contribute to enhanced antitumor effectiveness. The rational integration of photothermal gold caps, hollow nanostars, and polycations through the facile strategy might offer a promising avenue for complementary cancer therapy.     

Synthesis and Assembly of Gold Nanoparticles in Quasi‐Linear Lysine–Keggin‐Ion Colloidal Particles

The formation of fiber‐like colloidal particles of the amino acid lysine complexed with Keggin ions is demonstrated. The lysine–phosphotungstic acid (PTA) colloidal particles act as excellent templates for the synthesis and assembly of gold nanoparticles wherein the lysine‐PTA complex acts as a UV‐switchable reducing agent for gold ions. This novel bio‐organic–inorganic template shows excellent potential as a regulated nanoreactor for application in programmed nanoparticle synthesis and assembly in a single step.     

Self‐Assembled Monolayer Coatings on Nanostencils for the Reduction of Materials Adhesion

Nanostencils (shadow masks with submicrometer apertures in a thin silicon nitride membrane) are promising tools for the facile one‐step generation of nanopatterns of various materials by physical vapor deposition. Evaporation through a shadow mask is accompanied by gradual clogging of the apertures due to adhesion of evaporated material. In order to reduce this effect, nanostencils were coated with alkyl and perfluoroalkyl self‐assembled monolayers (SAMs). The formation and properties of SAMs on planar silicon nitride substrates were studied by contact angle goniometry, X‐ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The SAMs are stable under evaporation of gold at various angles. SAM‐coated nanostencils showed considerably less adhesion of gold compared to bare Si_xN_y stencils.     

Preparation and Organization of Nanoscale Polyelectrolyte‐Coated Gold Nanoparticles

The layer‐by‐layer (LbL) desposition of oppositely charged polyelectrolytes from adsorption solutions of different ionic strength onto ～7 nm diameter carboxylic acid‐derivatized gold nanoparticles has been studied. The polyelectrolyte‐modified nanoparticles were characterized by UV‐vis spectrophotometry, microelectrophoresis, analytical ultracentrifugation, and transmission electron microscopy. UV‐vis data showed that the peak plasmon absorption wavelength of the gold nanoparticles red‐shifted after each adsorption step, and microelectrophoresis experiments revealed a reversal in the surface charge of the nanoparticles following deposition of each layer. These data are consistent with the formation of polyelectrolyte layers on the nanoparticles. Analytical ultracentrifugation showed an increase in mean nanoparticle diameter on adsorption of the polyelectrolytes, confirming the formation of gold‐core/polyelectrolyte‐shell nanoparticles. Transmission electron microscopy studies showed no signs of aggregation of the polyelectrolyte‐coated nanoparticles. The adsorption of the polyelectrolyte‐coated gold nanoparticles onto oppositely charged planar supports has also been examined. UV‐vis spectrophotometry and atomic force microscopy showed increased amounts of nanoparticles were adsorbed with increasing ionic strength of the nanoparticle dispersions. This allows control of the nanoparticle surface loading by varying the salt content in the nanoparticle dispersions used for adsorption. The LbL strategy used in this work is expected to be applicable to other nanoparticles (e.g., semiconductors, phosphors), thus providing a facile means for their controlled surface modification through polyelectrolyte nanolayering. Such nanoparticles are envisaged to have applications in the biomedical and bioanalytical fields, and to be useful building blocks for the creation of advanced nanoparticle‐based films.     

Protein‐Enabled Layer‐by‐Layer Syntheses of Aligned,Porous‐Wall,High‐Aspect‐Ratio TiO2 Nanotube Arrays

An aqueous, protein‐enabled (biomimetic), layer‐by‐layer titania deposition process is developed, for the first time, to convert aligned‐nanochannel templates into high‐aspect‐ratio, aligned nanotube arrays with thin (34 nm) walls composed of co‐continuous networks of pores and titania nanocrystals (15 nm ave. size). Alumina templates with aligned open nanochannels are exposed in an alternating fashion to aqueous protamine‐bearing and titania precursor‐bearing (Ti(IV) bis‐ammonium‐lactato‐dihydroxide, TiBALDH) solutions. The ability of protamine to bind to alumina and titania, and to induce the formation of a Ti–O‐bearing coating upon exposure to the TiBALDH precursor, enables the layer‐by‐layer deposition of a conformal protamine/Ti–O‐bearing coating on the nanochannel surfaces within the porous alumina template. Subsequent protamine pyrolysis yields coatings composed of co‐continuous networks of pores and titania nanoparticles. Selective dissolution of the underlying alumina template through the porous coating then yields freestanding, aligned, porous‐wall titania nanotube arrays. The interconnected pores within the nanotube walls allow enhanced loading of functional molecules (such as a Ru‐based N719 dye), whereas the interconnected titania nanoparticles enable the high‐aspect‐ratio, aligned nanotube arrays to be used as electrodes (as demonstrated for dye‐sensitized solar cells with power conversion efficiencies of 5.2 ± 0.4%).     

介孔氧化铝组装金纳米颗粒阵列及其光学性质

采用真空蒸镀的方法在多孔氧化铝模板表面得到薄金膜,随后在真空管式炉中进行热处理,热处理中发生的热去湿过程使得金膜在多孔氧化铝表面形成有序的金纳米颗粒阵列。同时还研究了多孔氧化铝模板制备过程中二次氧化的作用,发现一次氧化对二次氧化进行具有一定指导作用;另外,研究了扩孔时间对模板孔径的影响,一定条件下,扩孔时间与孔径成正比例关系;最后研究了镀膜厚度对金纳米颗粒的影响,结果中可以看到,金膜的厚度直接影响金纳米颗粒阵列的形成。最后在分光光度计上的光学测量吸收光谱的结果中,出现了表面等离子体作用引起的很强的吸收峰。     

Preparation and characterisation of Langmuir–Blodgett films of 4‐(4‐(N‐octadecyl‐N‐methylamino) phenylazo)benzoic acid

Langmuir films of the title compound have been spread on an aqueous subphase at various values of pH at 10, 15 and 20°C. Increasing pH and increasing temperature favour stable films, but at the higher subphase pH and temperature values the pressure–area isotherms exhibit a transition to aggregated forms. Tilt angles between the alkyl chains and the normal to the subphase increase with increasing subphase pH and temperature. The area per molecule for films deposited at 15 °C decreased steadily with time at a fixed surface pressure, except at the highest pH, indicating poor stability. Langmuir–Blodgett films deposited at a surface pressure of 30 mN m⁻¹ and a subphase temperature of 15 °C were of Y‐type and showed transfer ratios above unity for the lower subphase pH values. UV/visible spectra of the LB films showed features characteristic of the formation of H aggregates for deposition at the higher subphase pH values. Over long periods of time the spectra for high pH showed evidence of increasing aggregation. Small‐angle X‐ray diffraction confirmed molecular tilts larger than those deduced in the floating monolayer. Reflection–absorption infrared spectroscopy of the LB films showed differences from the bulk also indicative of significant tilt, as did surface‐enhanced Raman spectroscopy. The LB films showed weak second‐harmonic generation from 1064 nm radiation consistent with a polar film structure parallel to the substrate. Copyright © 1999 John Wiley & Sons, Ltd.     

Ultrasmooth Gold Films via Pulsed Laser Deposition

A simple, one step technique for depositing ultrasmooth gold films using pulsed laser deposition is demonstrated by optimizing process para­meters. The smoothest film having a root‐mean‐square roughness of 0.17 nm (including the substrate roughness of 0.11 nm) for a 35 nm thick film on a silicon substrate are obtained by introducing a nitrogen flow in the chamber during deposition. We postulate that the reduction in surface roughness caused by nitrogen gas pressure in the chamber is due to the force of the gas flow acting against the flow of the plasma plume containing Au atoms. The gas acts as a filter that reduces the kinetic energy of the gold adatoms. This is the best result reported so far for a single step deposition of gold. It is a step towards low‐loss planar gold films for surface plasmon applications.     

Titania Nanotubes: Protein‐Enabled Layer‐by‐Layer Syntheses of Aligned,Porous‐Wall,High‐Aspect‐Ratio TiO2 Nanotube Arrays (Adv. Funct. Mater. 9/2011)

An aqueous, protein‐enabled (biomimetic), layer‐by‐layer titania deposition process is developed, for the first time, to convert aligned‐nanochannel templates into high‐aspect‐ratio, aligned nanotube arrays with thin (34 nm) walls composed of co‐continuous networks of pores and titania nanocrystals (15 nm ave. size). Alumina templates with aligned open nanochannels are exposed in an alternating fashion to aqueous protamine‐bearing and titania precursor‐bearing (Ti(IV) bis‐ammonium‐lactato‐dihydroxide, TiBALDH) solutions. The ability of protamine to bind to alumina and titania, and to induce the formation of a Ti–O‐bearing coating upon exposure to the TiBALDH precursor, enables the layer‐by‐layer deposition of a conformal protamine/Ti–O‐bearing coating on the nanochannel surfaces within the porous alumina template. Subsequent protamine pyrolysis yields coatings composed of co‐continuous networks of pores and titania nanoparticles. Selective dissolution of the underlying alumina template through the porous coating then yields freestanding, aligned, porous‐wall titania nanotube arrays. The interconnected pores within the nanotube walls allow enhanced loading of functional molecules (such as a Ru‐based N719 dye), whereas the interconnected titania nanoparticles enable the high‐aspect‐ratio, aligned nanotube arrays to be used as electrodes (as demonstrated for dye‐sensitized solar cells with power conversion efficiencies of 5.2 ± 0.4%).     

Multi‐Site Functionalization of Protein Scaffolds for Bimetallic Nanoparticle Templating

The use of biological scaffolds to template inorganic material offers a strategy to synthesize precise composite nanostructures of different sizes and shapes. Proteins are unique biological scaffolds that consist of multiple binding regions or epitope sites that site‐specifically associate with conserved amino acid sequences within protein‐binding partners. These binding regions can be exploited as synthesis sites for multiple inorganic species within the same protein scaffold, resulting in bimetallic inorganic nanostructures. This strategy is demonstrated with the scaffold protein clathrin, which self‐assembles into spherical cages. Specifically, tether peptides that noncovalently associate with distinct clathrin epitope sites, while initiating simultaneous synthesis of two inorganic species within the assembled clathrin protein cage, are designed. The flexibility and diversity of this unique biotemplating strategy is demonstrated by synthesizing two types of composite structures (silver–gold mixed bimetallic and silver–gold core–shell nanostructures) from a single clathrin template. This noncovalent, Template Engineering Through Epitope Recognition, or TEThER, strategy can be readily applied to any protein system with known epitope sites to template a variety of bimetallic structures without the need for chemical or genetic mutations.     

Engineering of Complex Macroporous Materials Through Controlled Electrodeposition in Colloidal Superstructures

Macroporous materials with a sophisticated architecture are obtained by electrochemical deposition of gold or polypyrrole in colloidal‐crystal templates. The Langmuir–Blodgett technique enables assembly of sub‐micrometric silica‐particle monolayers on conductive gold substrates, thus leading to colloidal superstructures with an unprecedented control of their features at the single‐bead‐layer level. This allows the integration of deliberate planar defects or the elaboration of well‐defined gradients in terms of sphere size. Controlled infiltration using electrochemical deposition preserves the architecture of the original templates and leads to inverse opals with well‐defined pore structures after the removal of the inorganic particles.     

A Three‐Dimensional Gold‐Decorated Nanoporous Copper Core–Shell Composite for Electrocatalysis and Nonenzymatic Biosensing

Bimetallic core–shell nanostructures have attracted increasing attention due to their low material costs along with enhanced chemico‐physical properties in comparison with their monometallic counterparts. Here, a novel gold‐decorated nanoporous copper (Au@NPC) core–shell composite fabricated by a facile in situ hydrometallurgy approach is reported. Thin gold shells with a controllable thickness are homogeneously deposited onto the internal surfaces of 3D nanoporous copper via a spontaneous displacement reaction while nanoporous copper is utilized as a reduction agent as well as 3D template and substrate. The resulting inexpensive core–shell nanostructure exhibits significant electrocatalytic activity for the oxidation of methanol and high non‐enzymatic sensitivity in detecting glucose.