Gold nanostructures on chemically reinforced PDMS microwell arrays

This paper describes a facile strategy for fabricating arrays of two- and three-dimensional gold nanostructures using PDMS-infiltrated polystyrene (PS) colloidal crystals. PDMS molding of colloidal crystal, gold vapor deposition, and subsequent calcination of PS produced gold thin layers over hexagonal PDMS microwell arrays with hemispherical air-voids of approximately 140 nm on glass substrates. Vapor deposition of perfluoroalkylsilane thin layers improved the thermal stability of the colloidal template over 100 °C, providing a route to preparation of hollow architectures with gold thin layers supported by PDMS nanostructures. Surface modification of the PDMS using poly(allylamine hydrochloride) induced two-dimensional colloidal crystals of PS and PMMA spheres through electrostatic interactions. Particle aggregation of 13 nm gold nanoparticles in the PDMS microwells demonstrated a surface plasmon resonance band red-shifted to 810 nm, in comparison with that on the flat surface at 720 nm.     

Influence of the substrate,metal overlayer and lattice neighbors on the focusing properties of colloidal microspheres

Calculations of the field distribution in colloidal SiO₂ microspheres are presented. Two cases are considered: small particles on a Si substrate irradiated by the 266 nm light, and larger ones, covered with a gold film and irradiated at 800 nm. Substrate, neighboring spheres and sputtered metal overlayer all significantly modify the field pattern and magnitude. Reflected light is focused inside the spheres, which may lead to their damage. The results can be useful in the analysis of microspheres-assisted nano-patterning.     

Thiolated polyethylene oxide as a non-fouling element for nano-patterned bio-devices

This work describes the synthesis of a thiolated polyethylene oxide that self-assembles on gold to create a non-fouling surface. Thiolated polyethylene oxide was synthesised by reacting 16-mercaptohexadecanoic acid with polyethylene glycol mono methyl ether. The coverage of the thiolated polyethylene oxide on gold was studied by cyclic voltammetry, and the modified surfaces were characterised by X-ray photoelectron spectroscopy and ellipsometry. Protein resistance was assessed using quartz crystal microbalance. Results showed a non-fouling character produced by the thiolated polyethylene oxide. The synthesised product was used as the passivation layer on nano-patterned surfaces consisting of arrayed nano-spots, fabricated by plasma based colloidal lithography. The specific adsorption of anti-bovine serum albumin in the mercaptohexadecanoic acid spots was verified by atomic force microscopy.     

Large-scale protein/antibody patterning with limiting unspecific adsorption

A simple synthetic route based on nanosphere lithography has been developed in order to design a large-scale nanoarray for specific control of protein anchoring. This technique based on two-dimensional (2D) colloidal crystals composed of polystyrene spheres allows the easy and inexpensive fabrication of large arrays (up to several centimeters) by reducing the cost. A silicon wafer coated with a thin adhesion layer of chromium (15 nm) and a layer of gold (50 nm) is used as a substrate. PS spheres are deposited on the gold surface using the floating-transferring technique. The PS spheres were then functionalized with PEG-biotin and the defects by self-assembly monolayer (SAM) PEG to prevent unspecific adsorption. Using epifluorescence microscopy, we show that after immersion of sample on target protein (avidin and anti-avidin) solution, the latter are specifically located on polystyrene spheres. Thus, these results are meaningful for exploration of devices based on a large-scale nanoarray of PS spheres and can be used for detection of target proteins or simply to pattern a surface with specific proteins.     

Long range ordering in self-assembled Ni arrays on patterned Si

We have succeeded in aligning self-assembled structures by using a lithographically defined stripe. The 140 nm wide by 100 nm high SiO₂ strip is shown to guide the assembly of 500 nm latex spheres so that spheres are aligned along the strip and are in registration on either side of the strip. This method can be used to increase long-range ordering in magnetic storage systems without compromising the density. Inverse sphere Ni arrays were made by electrodeposition through the latex template. We also show that the hexagonal symmetry of the resulting inverse sphere Ni arrays can be simulated using the approach presented below.     

Visible transmission through metal-coated colloidal crystals

Large-scale periodically structured metal films with enhanced optical transmission in visible frequencies were fabricated by depositing silver onto colloidal crystals. The obtained transmission properties are similar to those observed through periodical hole arrays in planar metal films. We have experimentally observed two enhanced transmission pass bands in visible frequencies in these metal films due to the excitation of surface plasmon polaritons. The peak positions of the pass bands depend on the size of the colloidal spheres. The transmission spectra highly depend on the incident angle for p-polarized light but are weak for s-polarized light. Our fabrication method provides a promising approach for the fabrication of large-scale low-cost plasmonic crystals with submicrometer periodicity.     

Fabrication of Colloidal Photonic Crystals with Heterostructure by Spin-Coating Method

Colloidal photonic crystal heterostructures, composed of two opaline photonic crystal films of silica spheres with different diameters, are fabricated by a two-step spin-coating method. Scanning electron microscopy （SEM） and UV-vis spectrophotometer are used to characterize the heterostructures. The SEM images show good ordering of the two-layer colloidal crystals constituting the heterostructures. The transmission spectra measured from the （111） plane in the heterostructure show that the composite colloidal photonic crystals have double photonic stop bands. Furthermore, when the sizes of the silica spheres used for fabricating the composite photonic crystal are slightly different, the transmission spectrum shows that the composite photonic crystals have more extended bandgap than that of the individual photonic crystals due to partial overlapping of its two photonic stop bands.     

Synthesis, characterization and antibacterial action of hollow titania spheres

Hollow titania spheres were synthesized using the cationic polystyrene lattices which were prepared by polymerization in suspension of styrene using 2,2′-azobis (2-methylpropionamidine) dihydrochloride (AMPA) as an initiator. These cationic colloidal particles were dispersed in absolute ethanol in the presence of poly(vinylpyrrolidone) (PVP), solution of sodium chloride (NaCl) and mixed with ethanolic solutions of titanium tetraisopropoxide (TTIP). Subsequently, hollow spheres of titania compounds were obtained by calcinations of the so-coated polystyrene lattices at elevated temperature in air. The hollow titania spheres were characterized with scanning electron microscopy (SEM), FT-infrared spectroscopy (FT-IR), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and differential thermal analysis (DTA). Moreover, antibacterial action of illuminated hollow titania spheres on pure culture of Escherichia coli (E. coli) was studied. A decrease of E. coli concentration was observed after illumination of bacteria in the presence of hollow titania spheres.     

Self-assembly of length-tunable gold nanoparticle chains in organic solvents

The one-dimensional coagulation of gold colloidal particles dispersed in organic solvent was investigated with transmission electron microscopy. The results indicate that the length of the nanoparticle chains can be modulated by changing the concentration of the solutions. It was also demonstrated that the wetting of the substrate surface hardly influenced the morphology of the nanoparticle chains, which revealed that the particle chains had been formed in the solution before deposition on the substrates. A general theoretical interpretation is provided to explain the linear coagulation of gold colloidal particles, on the basis of the asymmetrical distribution of the charges absorbed on the surface of the gold colloidal particles, as well as the action of the solvent molecules. Received: 8 April 2002 / Accepted: 1 July 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +86-025/361-9983, E-mail: jhliao@seu.edu.cn     

Fabrication of Two-Dimensional Arrays of Micron-Sized Gold Rings Based on Preferential Nucleation at Reentrant Sites

A templating method for fabricating two-dimensional （2D） arrays of micron-sized goM rings is reported. The microstructures are formed by electroless plating in a through-porous polymer membrane on a silicon substrate obtained from a closed-packed silica colloidal crystal. Our results show that the sizes of gold rings can be altered by varying electroless plating conditions for the porous polystyrene membranes. Moreover, we explain the growth mechanism of gold rings using the classical crystal growth theory that is preferential nucleation at reentrant sites.     

Confinement-induced colloidal attractions in equilibrium

The Poisson-Boltzmann theory for colloidal electrostatic interactions predicts that charged colloidal spheres dispersed in water should repel each other, even when confined by charged surfaces. Direct measurements on highly charged polystyrene spheres, however, reveal strong, long-ranged confinement-induced attractions that have yet to be explained. We demonstrate that anomalous attractions also characterize the equilibrium pair potential for more weakly charged colloidal silica spheres sedimented into a monolayer above a glass surface. This observation substantially expands the range of conditions for which mean-field theory incorrectly predicts the sign of macroions' interactions, and provides new insights into how confinement induces long-ranged like-charge attractions.     

Experimental determination of the band structure of photonic crystals of colloidal silica spheres

A photonic band structure of colloidal crystals of silica spheres is analytically determined by a band model with three fitting parameters: the sphere size, the effective refractive index, and the band-gap. Optical properties of the crystals annealed at various temperatures were characterized by a procedure similar to X-ray diffraction technique, and the width of photonic band-gap measured from the transmission spectra experimentally servers as an additional check on the validation of the model. The photonic band structures defined by the band-gap, the refractive index, and the Brillouin zone are obviously superior to the use of the Bragg's expression involving simple zone folding.     

Enhanced photoelectrochemical performance of ZnO photoanode with scattering hollow cavities

Abstract ZnO nanoparticles with average diameter of 12 nm were used to fabricate ZnO photoanodes by electrohydrodynamic (EHD) technique for dye-sensitized solar cells (DSSCs). To enhance the light scattering and conversion efficiency, the ZnO film with scattering hollow cavities (HCs) was realized by calcining polystyrene spheres (PSs) in the film. The films had strong light scattering ability and the overall light to electricity conversion efficiency (η) was improved and reached 5.5% under illumination of simulated solar light (AM-1.5, 100 mW/cm²).     

A sensor for adenosine triphosphate fabricated by laser-induced forward transfer of luciferase onto a poly(dimethylsiloxane) microchip

Laser-induced forward transfer (LIFT) of the enzyme luciferase was explored as a potential technique to be used in the fabrication of a microchip adenosine triphosphate (ATP) sensor. Poly(dimethylsiloxane) (PDMS) was selected as the substrate for deposition of the luciferase. In comparison with other solid substrates, such as glass and polystyrene, it was found that the flexibility of PDMS made it a superior substrate for the immobilization of micro-spots of luciferase. LIFT of luciferase onto a PDMS substrate using a 355 nm laser was successfully carried out, while the bioactivity of the enzyme was maintained. Yellow luminescence ascribed to luciferase was observed from a transferred spot on the PDMS chip from the enzymatic reaction between luciferin and ATP. A microchip ATP sensor was also fabricated by attaching a small photodiode to the PDMS chip. On the basis of the fabricated microchip, the Michaelis-Menten relation between the luminescence intensity from the spot, and the ATP concentration was confirmed. The potential for fabricating biosensors using a combination of the LIFT technique with a PDMS substrate was shown to be very good.     

A Mechanism for the Self-Organization of Colloidal Gold Nanoparticles

We report a two-stage mechanism of formation of the two-dimensional surperlattices in colloidal gold nanoparticles. The first stage is the formation and growth of holes. When the film is thinner than the “pancake“ thickness and because the solvent volatility holes nucleate and grow, the dry solid surface is exposed. The second stage corresponds to the reorganization of the colloids in the impacted areas between several holes, in which the particles gathering along the rims order due to the van der Waals-type attraction between colloids and the confinement by the length of dodecanethiol. Then at a compromised distance and perfect position giving the minimum free energy, ordering occurs in small domains along the boundary of the hole. When the ordered domains become closer because of the hole growth, they rotate at certain angles and the further ordering appears.     

Kinetic Monte Carlo simulation of spatially ordered growth of quantum dots on patterned substrate

We report the growth of well-ordered InAs QD chains by molecular beam epitaxy system. In order to analyze and extend the results of our experiment, a detailed kinetic Monte Carlo simulation is developed to investigate the effects of different growth conditions to the selective growth of InAs quantum dots (QDs). We find that growth temperature plays a more important role than growth rate in the spatial ordering of the QDs. We also investigate the effect of periodic stress on the shape of QDs in simulation. The simulation results are in good qualitative agreement with our experiment.     

Two-dimensional nanopatterning by PDMS relief structures of polymeric colloidal crystals

A new constructive method of fabricating a nanoparticle self-assembly on the patterned surface of a poly(dimethylsiloxane) (PDMS) relief nanostructure was demonstrated. Patterned PDMS templates with close-packed microwells were fabricated by molding against a self-assembled monolayer of polystyrene spheres. Alkanethiol-functionalized gold nanoparticles with an average particle size of 2.5 nm were selectively deposited onto a hydrophobic self-assembled monolayer printed on the substrate by the micro-contact printing (μCP) of the prepared PDMS microwell, in which the patterned gold nanoparticles consisted of close-packed hexagons with an average diameter of 370 nm. In addition, two-dimensional colloidal crystals derived from PMMA microspheres with a diameter of 380 nm and a negative surface charge were successfully formed on the hemispherical microwells by electrostatic force using positively charged PAH-coated PDMS as a template to produce multidimensional nanostructures.     

Self-organization of quasi-equilibrium steady-state condensation in accumulative ion-plasma devices

We consider both theoretically and experimentally self-organization process of quasi-equilibrium steady-state condensation of sputtered substance in accumulative ion-plasma devices. It has been shown that the self-organization effect is caused by self-consistent variations of the condensate temperature and the supersaturation of depositing atoms. Two possible types of self-organization process have been found out on the basis of the phase-plane method. The aluminium condensation experimental data confirming the self-organization nature of quasi-equilibrium steady-state condensation are discussed.     

Linear and nonlinear optical studies in photonic crystal alloys

We report on linear transmittance and reflectance as well as on third-harmonic generation in photonic crystal alloys formed by various compositions of polystyrene and poly (methyl methacrylate) colloidal spheres of the same size. These photonic crystal alloys are structurally ordered but contain refractive-index disorder and thus provide a random variation of scattering potential. The stopgap shows a monotonic shift in wavelength as a function of composition that can be fitted by assuming an effective dielectric constant for the colloidal spheres. In each alloy a dramatic enhancement of third-harmonic generation is observed, always on the short-wavelength side of the stopgap.