Facile fabrication of nanoporous gold film electrodes

A novel one-step method has been developed for the fabrication of a three-dimensional (3D) nanoporous gold film (NPGF). The NPGF can be facilely made within 1 min from a pure gold substrate by applying a step potential just into the initial transition region of gold in an HCl medium. The pore formation and structural evolution have been revealed by scanning electron microscope, and the processes involve electrodissolution, disproportion, and deposition. The as-prepared 3D NPGF electrode has a large surface area and exhibits high catalytic activity in the electrooxidation of glucose. The NPGF electrode also shows excellent performance toward the electrooxidation of formic acid after being decorated with a tiny amount of Pt by electrodeposition.     

Simple fabrication of patterned gold nanoparticle arrays on functionalized block copolymer thin films

Patterned arrays of gold nanoparticles were fabricated using a simple dipping method that makes use of their specific interactions with nano-domains of carboxylic acid on a block copolymer template. Polystyrene-block-poly(tert-butyl acrylate) on the SU-8 photoresist pattern was selectively transformed to polystyrene-block-poly(acrylic acid). Au nanoparticles are selectively immobilized on the resulting carboxylic acid patterns to produce well-defined patterned Au nanoparticle arrays. This stable and robust template can be used to obtain any patterned nonaggregated metal or inorganic nanoparticle arrays.     

Densely integrated array of chemiresistor vapor sensors with electron-beam patterned monolayer-protected gold nanoparticle interface films

Use of electron-beam induced crosslinking to pattern films of monolayer-protected gold nanoparticles (MPNs) onto a chemiresistor (CR) sensor array is described. Each of the four CRs comprises a 100 μm(2) set of interdigital electrodes (IDEs) with 100 nm widths and spaces, separated from adjacent devices by 4 μm. Films of four MPNs, each with a different thiolate monolayer, were successively patterned on the IDEs. Vapor exposures yield rapid, reversible changes in CR resistances and differential vapor sensitivities comparable to those reported for larger CRs with unpatterned MPN films. The array response patterns facilitate vapor discrimination. This is the smallest MPN-coated CR array yet reported. The advantages of using such an array as the detector in microfabricated gas chromatographic analyzers are considered.     

Single-step fabrication of drug-encapsulated inorganic microspheres with complex form by sonication-induced nanoparticle assembly

Sonication of nanoparticle suspensions confined within aqueous droplets of drug molecules in toluene produces drug-loaded silica or titania porous microspheres with complex morphology and storage/release properties.     

Single-step fabrication of asymmetric dual-phase composite membranes for oxygen separation

Asymmetric dual-phase composite membranes for oxygen separation were conveniently fabricated by an acid leaching technique. A thin dense layer of Ce_0.85Sm_0.15O_1.925/Sm_0.6Sr_0.4FeO_3−δ was left by controlling the degree of acid leaching, and a porous substrate of Ce_0.85Sm_0.15O_1.925 with a fluorite structure was formed after dissolution of Sm_0.6Sr_0.4FeO_3−δ with a perovskite structure in HCl. Thus, a thin dense layer and a porous substrate can be fabricated in a single step in which traditional shrinkage mismatch and chemical reaction between thin dense layers and porous substrates can be avoided. The thickness of the dense layer can be controlled by varying the acid leaching time. Hence, dual-phase composite membranes with high oxygen flux can be obtained.     

Simultaneous detection of ultratrace lead and copper with gold nanoparticles patterned on carbon nanotube thin film

Highly sensitive detection of a Pb(2+)-Cu(2+) mixture using gold nanoparticles patterned on single-walled carbon nanotube (AuNP-SWCNT) film is reported. The gold nanoparticles were deposited electrochemically on carbon nanotube film using a cyclic voltammetry technique. The film showed a homogeneous size and density that could be easily controlled by the potential scanning cycle and gold precursor concentration. Square wave stripping voltammetry (SWSV) was applied to the simultaneous detection of Pb(2+) and Cu(2+) under optimized conditions. The AuNP-SWCNT electrode exhibited a high increase in sensitivity with a limit of detection of 0.546 ppb (R(2) = 0.984) and 0.613 ppb (R(2) = 0.991) for Pb(2+) and Cu(2+) ions, respectively, in a mixture of Pb(2+)-Cu(2+) solution (S/N = 3, n = 5), and a good linear response in the range from 3.31 ppb to 22.29 ppb. The electrode exhibited high reproducibility in repetitive measurements with a relative standard deviation as low as 4.2% and 2.6% for Pb(2+) and Cu(2+) ions, respectively. An interference study showed that Sb(3+), As(3+), Zn(2+), Ca(2+), and Na(+) ions did not have a significant effect. This study demonstrated an alternative approach to the rapid and reliable detection of heavy metals of environmental interest.     

Microcontact printing of monodiamond nanoparticles: an effective route to patterned diamond structure fabrication

By combining microcontact printing with a nanodiamond seeding technique, a precise micrometer-sized chemical vapor deposition (CVD) diamond pattern have been obtained. On the basis of the guidance of basic theoretical calculations, monodisperse detonation nanodiamonds (DNDs) were chosen as an "ink" material and oxidized poly(dimethylsiloxane) (PDMS) was selected to serve as a stamp because it features a higher interaction energy with the DNDs compared to that of the original PDMS. The adsorption kinetics shows an approximately exponential law with a maximum surface DND density of 3.4 × 10(10) cm(-2) after 20 min. To achieve a high transfer ratio of DNDs from the PDMS stamp to a silicon surface, a thin layer of poly(methyl methacrylate) (PMMA) was spin coated onto the substrates. A microwave plasma chemical vapor deposition system was used to synthesize the CVD diamond on the seeded substrate areas. Precise diamond patterns with a low expansion ratio (3.6%) were successfully prepared after 1.5 h of deposition. Further increases in the deposition time typically lead to a high expansion rate (～0.8 μm/h). The general pattern shape, however, did not show any significant change. Compared with conventional diamond pattern deposition methods, the technique described here offers the advantages of being simple, inexpensive, damage-free, and highly compatible, rendering it attractive for a broad variety of industrial applications.     

Combinatorial decoding: an approach for universal DNA array fabrication

A fiber optic microsphere-based oligonucleotide array is described that employs the sequence of the oligonucleotide probe attached to each microsphere as positional identifiers. Each microsphere serves as an immobilized array feature, functionalized with a unique single-stranded oligonucleotide sequence and randomly distributed into an array of microwells. To determine the sequences attached to individual microspheres, a series of fluorescently labeled combinatorial-pooled oligonucleotide target solutions was designed. Each combinatorial decoding solution is intended to identify the nucleotide at a particular position on every microsphere in the array. The combinatorial target solutions were synthesized by linking the four possible nucleotides at each position to four different fluorescent reporter dyes. As such, when the solutions were hybridized to the array, one of four possible fluorescent responses was generated for each position on a microsphere probe sequence. Adjusting the stringency of hybridization enabled single-base mismatch discrimination, and the signal with the highest intensity corresponded to the perfect nucleotide match. By consecutively exposing the array to a series of combinatorial decoding pool solutions, it was possible to simultaneously determine the sequence of every randomly positioned oligonucleotide-functionalized microsphere in the array. Once mapped, the microsphere array can be used for any typical genomic microarray experiment.     

High-density fabrication of normally closed microfluidic valves by patterned deactivation of oxidized polydimethylsiloxane

The use of polydimethylsiloxane (PDMS) in microfluidic devices is extensive in academic research. One of the most fundamental treatments is to expose PDMS to plasma oxidation in order to render its surface temporarily hydrophilic and capable of permanent bonding. Here, we show that changes in the surface chemistry induced by plasma oxidation can spatially be counteracted very cleanly and reliably in a scalable manner by subsequent microcontact printing of residual oligomers from a PDMS stamp. We characterize the surface modifications through contact angle, atomic force microscopy, X-ray photoelectron spectroscopy, and bond-strength measurements. We utilize this approach for negating the bonding of a flexible membrane layer within an elastomeric valve and demonstrate its effectiveness by integration of over one thousand normally closed elastomeric valves within a single substrate. In addition, we demonstrate that surface energy patterning can be used for "open microfluidic" applications that utilize spatial control of surface wetting.     

Facile fabrication of large area of aggregated gold nanorods film for efficient surface-enhanced Raman scattering

An effective and facile method for fabrication of large area of aggregated gold nanorods (AuNRs) film was proposed by self-assembly of AuNRs at a toluene/water interface for the first time. It was found that large area of aggregated AuNRs film could be formed at the interface of toluene and water due to the interfacial tension between the two phases. The obtained large area of aggregated AuNRs film exhibits strong surface-enhanced Raman scattering (SERS) activity with 4-aminothiophenol (4-ATP) and 2-aminothiophenol (2-ATP) as the probe molecules based on the strong electromagnetic coupling effect between the very adjacent AuNRs. Enhancement factors (EF) were used to estimate the SERS activity of the aggregated AuNRs film, which is obtained to be 1.7x10(5) for 7a vibration of 4-ATP. SERS intensity is compared with AuNRs deposited directly on glass, indicating high SERS activity and reproducibility of the aggregated AuNRs film. In addition, SERS activity has also been successfully demonstrated for dye molecule (Rhodamin 6G (R6G)) and biological small molecule (adenine) on the aggregated AuNRs film, showing great potential of the aggregated AuNRs film as a convenient and powerful SERS substrate for biological tags and biological molecular detection.     

Single-step fabrication and characterization of photonic crystal biosensors with polymer microfluidic channels

A method for simultaneously integrating label-free photonic crystal biosensor technology into microfluidic channels by a single-step replica molding process is presented. By fabricating both the sub-micron features of the photonic crystal sensor structure and the >10 microm features of a flow channel network in one step at room temperature on a plastic substrate, the sensors are automatically self-aligned with the flow channels, and patterns of arbitrary shape may be produced. By measuring changes in the resonant peak reflected wavelength from the photonic crystal structure induced by changes in dielectric permittivity within an evanescent field region near its surface, detection of bulk refractive index changes in the fluid channel or adsorption of biological material to the sensor surface is demonstrated. An imaging detection instrument is used to characterize the spatial distribution of the photonic crystal resonant wavelength, gathering thousands of independent sensor readings within a single fluid channel.     

Selective recruitment of membrane protein complexes onto gold substrates patterned by dip-pen nanolithography

Dip-pen nanolithography (DPN) is employed to develop a generic array platform for the selective recruitment of membrane protein complexes. An atomic force microscope tip inked with HS(CH2)16NH2 is used to generate amino-terminated domains on gold. These domains can be arranged into microscopic and submicroscopic patterns, and the untreated gold substrate is subsequently blocked with HS(CH2)2CONH(CH2CH2O)15CH3, a compound known to resist the unspecific binding of proteins and cells. The patterned gold substrate is exposed to an enriched membrane fraction from mutant Rhodobacter sphaeroides, which contains photosynthetic core complexes consisting of the reaction center and the light-harvesting complex LH1. The selective recruitment to the patterned domains, governed primarily by electrostatic interactions, is confirmed by contact mode atomic force microscopy.     

Progress in patterned paper sizing for fabrication of paper-based microfluidic sensors

In this paper, we report the progress in using paper sizing chemistry to fabricate patterned paper for chemical and biological sensing applications. Patterned paper sizing uses paper sizing agents to selectively hydrophobize certain area of a sheet. The hydrophilic-hydrophobic contrast of the pattern so created has an excellent ability to control capillary penetration of aqueous liquids in channels of the pattern. Incorporating this idea with digital ink jet printing technique, a new fabrication method of paper-based microfluidic devices is established. Ink jet printing can deliver biomolecules and chemicals with precision into the microfluidic patterns to form biological/chemical sensing sites within the patterns, forming the complete sensing devices. This study shows the potential of combining paper sizing chemistry and ink jet printing to produce paper-based sensors at low cost and at commercial volume.     

High-speed fabrication of patterned colloidal photonic structures in centrifugal microfluidic chips

In this paper, we report a rapid and facile method for fabricating colloidal photonic crystals inside microchannels of radially symmetric microfluidic chips which were made using soft-lithography. As the suspension of monodisperse silica or polystyrene latex spheres was driven to flow through the channels under the action of centrifugal force, the colloidal spheres were quickly assembled into face centered cubic arrangement which had a few photonic stop bands. The soft-microfluidic channels and cells confined the colloidal crystals into designed patterns. The optical reflectance was modulated by the refractive-index mismatch between the colloidal particles and the solvent in the interstices between the particles. Therefore, the present microfluidic chips with built-in colloidal photonic crystals can be used as in-situ optofluidic microsensors for high throughput screening or light filters in integrated adaptive optical devices.     

Electrostatic assembly of gold colloidal nanoparticles on organosilane monolayers patterned by microcontact electrochemical conversion

A new approach is introduced for electrostatically guided adsorption of colloidal nanoparticles onto a patterned self-assembled monolayer (SAM) with feature sizes ranging from nm to mm. Patterning of the adsorption templates is realized by electric-field-induced anodic oxidation of aminosilane SAM using an ink-free method. In this versatile method, both "positive" and "negative" type pattern transfers are possible. The chemically converted patterns are induced by localized electrical fields on the microcontacted areas, and the patterning resolution is insensitive to the diffusion of oxidizing agents because of the self-limiting oxidation kinetics, thereby enabling high-resolution, large-scale parallel patterning.     

Adsorption kinetics of an engineered gold binding Peptide by surface plasmon resonance spectroscopy and a quartz crystal microbalance

The adsorption kinetics of an engineered gold binding peptide on gold surface was studied by using both quartz crystal microbalance (QCM) and surface plasmon resonance (SPR) spectroscopy systems. The gold binding peptide was originally selected as a 14-amino acid sequence by cell surface display and then engineered to have a 3-repeat form (3R-GBP1) with improved binding characteristics. Both sets of adsorption data for 3R-GBP1 were fit to Langmuir models to extract kinetics and thermodynamics parameters. In SPR, the adsorption onto the surface shows a biexponential behavior and this is explained as the effect of bimodal surface topology of the polycrystalline gold substrate on 3R-GBP1 binding. Depending on the concentration of the peptide, a preferential adsorption on the surface takes place with different energy levels. The kinetic parameters (e.g., K(eq) approximately 10(7) M(-1)) and the binding energy (approximately -8.0 kcal/mol) are comparable to synthetic-based self-assembled monolayers. The results demonstrate the potential utilization of genetically engineered inorganic surface-specific peptides as molecular substrates due to their binding specificity, stability, and functionality in an aqueous-based environment.     

Self-assembly of vertically aligned gold nanorod arrays on patterned substrates

Nanorods standing at attention! A self-assembly technique based on convective and capillary forces was used for the direct fabrication of standing arrays of gold nanorods on lithographically predefined areas. The hexagonal close-packed structure of gold nanorods creates an ideal substrate for surface-enhanced Raman spectroscopy.     

Controlled wrinkling as a novel method for the fabrication of patterned surfaces

This article reviews recent applications of controlled wrinkling for creating structured and/or patterned interfaces, and its use in metrology. We discuss how wrinkles develop as a result of in-plane compression of thin sheets. As the wavelength of wrinkles is only dependent on elastic properties and thickness of the sheets, the phenomenon can be used in metrology for determination of elastic properties. The second aspect is its use for patterning and topographical structuring of surfaces. If mechanical properties and thickness are well controlled, wrinkle orientation and geometry can be tailored. Wavelengths between fractions of a micron and many micrometers are feasible. This process is based on a macroscopic deformation and upscaling to larger areas is possible which provides an attractive alternative to bottom-up or top-down approaches for surface patterning. We describe the formation of stable surface wrinkles in thin sheets of different materials having different surface chemistries, report on applications, and discuss the usefulness of wrinkles for building hierarchical structures.     

Spontaneous formation of cluster array of gold particles by convective self-assembly

Cluster arrays composed of metal nanoparticles are promising for application in sensing devices because of their interesting surface plasmon characteristics. Herein, we report the spontaneous formation of cluster arrays of gold colloids on flat substrates by vertical-deposition convective self-assembly. In this technique, under controlled temperature, a hydrophilic substrate is vertically immersed in a colloid suspension. Cluster arrays form when the particle concentration is extremely low (in the order of 10(-6)-10(-8) v/v). These arrays are arranged in a hierarchically ordered structure, where the particles form clusters that are deposited at a certain separation distance from each other, to form "dotted" lines that are in turn aligned with a constant spacing. The size of the cluster can be controlled by varying the particle concentration and temperature while an equal separation distance is maintained between the lines formed by the clusters. Our technique thus demonstrates a one-step, template-free fabrication method for cluster arrays. In addition, through the direct observation of the assembly process, the spacing between the dotted lines is found to result from the "stick-and-slip" behavior of the meniscus tip, which is entirely different from the formation processes observed for the striped patterns, which we reported previously at higher particle concentrations. The difference in the meniscus behavior possibly comes from the difference in colloidal morphology at the meniscus tip. These results demonstrate the self-regulating characteristics of the convective self-assembly process to produce colloidal patterns, whose structure depends on particle concentration and temperature.