Adsorption of block copolymers in nanoporous alumina

We have studied the adsorption of end-attaching block copolymer chains inside the cylindrical pores of nanoporous alumina. Highly asymmetric PS-PEO block copolymers, with a small PEO anchoring block and a long PS dangling block, were allowed to adsorb onto porous alumina substrates with an average pore diameter of ∼200 nm from toluene solution. The adsorption process was monitored using FTIR spectroscopy, whereas depth profile analysis was performed by means of XPS and Ar⁺ ion sputtering. It is found that the PS-PEO adsorption kinetics in porous alumina are ∼4 orders of magnitude slower than the corresponding case of a flat alumina substrate. It appears that chains adsorbed near the pore entrance early on tend to form a barrier for chains entering the pore at later times, thereby slowing down the adsorption process significantly. This effect is much more pronounced for large chains whose dimensions are comparable with the pore diameter. The equilibrium adsorbance value is also affected by chain size due to the additional entropic penalty associated with chain confinement, the adsorbance falling substantially when the chain dimensions become comparable with the pore diameter. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1676–1682, 2010     

Paired cell for the preparation of AgI nanowires using nanoporous alumina membrane templates

This communication describes a relatively new and simple method for the preparation of AgI nanowires using nanoporous alumina membrane templates which can be easily extended to prepare nanowires of many other materials.     

Electrodeposition of tubular-rod structure gold nanowires using nanoporous anodic alumina oxide as template

One-dimensional tubular-rod structure gold nanowires have been prepared using electrodeposition method at constant current mode with confined nanochannels of porous anodic aluminum oxide template. The reduction mechanism of gold ions and formation process of tubular-rod structure gold nanowires are studied. Electron microscopy results show that the tubular-rod structure gold nanowires transform to solid nanorods when the electrodeposition time is long enough. The tubular-rod structure gold nanowires have an average diameter of 180 nm, which coincide with the diameter of the template used. X-ray diffraction results confirm that the tubular-rod structure gold nanowires are crystalline structure.     

Analysis of limiting factors of DNA band separation by a DNA sequencer using fluorescence detection.

The factors affecting the electrophoretic separation of DNA bands in DNA base sequencing using fluorescence detection are analyzed. All the factors contributing to DNA band spacing and band width are evaluated; DNA diffusion and thermal effects on gels are the main considerations. The dependence of the gel's electrical resistivity on gel temperature and the variation of temperature over gel thickness are associated with a broadening of DNA band width. As a result of the analyses the maximum separable base number is represented as a function of various electrophoretic variables. The best separations are possible with an electric field strength corresponding to gel thickness. The maximum separable base number increases as the gel thickness decreases. It also increases as the migration distance increases, but it becomes saturated and has an upper limit when the migration distance is long. This upper limit increases as gel thickness decreases. DNA fragments with 600 and 601 bases can be completely separated from each other under optimum conditions for a 0.2 mm thick gel plate. Furthermore, using the band spacing information, under the same conditions, 750 bases could be assigned separately.     

Highly sensitive and selective electrochemical detection of L-cysteine using nanoporous gold

Nanoporous gold (NPG) with uniform pore sizes and ligaments was prepared by using a simple dealloying method. NPG electrodes exhibit excellent electrocatalytic activity towards the oxidation of CySH and the mechanism for the electrochemical reaction of CySH on NPG has been discussed. Interestingly, if the operating potential is fixed at 0.65 V, a strong current is observed and interferences by tryptophan and tyrosine are avoided. The calibration plot is linear in the concentration range from 1 μM to 400 μM (R²?=?0.994), and the quantification limit is as low as 50 nM. The NPG-modified electrode has good reproducibility, high sensitivity and selectivity, can be used to sense CySH in aqueous solution.

Grafting of nanoporous alumina membranes and films with organic acids

In this study, direct surface grafting of nanoporous alumina membranes and glass‐supported alumina films was carried out with three different fluorinated organic acids: trifluoroacetic acid, perfluoropentanoic acid and 2,3,4,5,6‐pentafluorobenzoic acid. Elemental surface composition and chemical environment of alumina were investigated using X‐ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). Alumina surfaces grafted with fluoro‐organic acids exhibited increased hydrophobic properties compared to ungrafted surfaces when measured using goniometry and atomic force microscopy (AFM). This work describes the evidence for surface chemical modification of alumina using direct reaction with organic acids. An AFM study of the adsorption of the immunoglobulin G (IgG) molecules on the fluoro‐organic‐acid‐grafted surfaces is reported. The results show that an ordered arrangement of immunoglobulin G structures with in‐filling of pores could be achieved only on the more hydrophobic fluoro‐organic‐acid‐grafted alumina membranes. Copyright © 2007 John Wiley & Sons, Ltd.     

Direct detection of DNA using 3D surface enhanced Raman scattering hotspot matrix

Silver nanoparticles (AgNPs) are evaporatively self‐assembled into the 3D surface enhanced Raman scattering (SERS) hotspot matrix with the assistant of glycerol to improve the spectral reproducibility in direct DNA detection. AgNPs and DNA in the glycerol‐stabilized 3D SERS hotspot matrix are found to form flexible sandwich structures through electrostatic interaction where neighboring AgNPs create uniform and homogeneous localized surface plasmon resonance coupling environments for central DNA. Nearly two orders of magnitude extra SERS enhancement, more stable peak frequency and narrower peak full width at half maximum can therefore be obtained in DNA SERS spectra, which ensures highly stable and reproducible SERS signals in direct detection of both single strand DNA and double strand DNA utilizing the 3D SERS hotspot matrix. By normalizing the SERS spectra using phosphate backbone as internal standard, identification of single base variation in oligonucleotides, determination of DNA hybridization events and recognition of chemical modification on bases (hexanethiol‐capped at 5’ end) have been demonstrated experimentally. This proposed 3D SERS hotspot matrix opens a novel perspective in manipulating plasmonic nanoparticles to construct SERS platforms and would make the surface enhanced Raman spectroscopy a more practical and reliable tool in direct DNA detection.     

Direct selective detection of genomic DNA from coliform using a fiber optic biosensor

Fiber optic biosensors operated in a total internal reflection format were prepared based on covalent immobilization of 25mer lacZ single-stranded nucleic acid probe. Genomic DNA from Escherichia coli was extracted and then sheared by sonication to prepare fragments of approximately 300mer length. Other targets included a 25mer fully complementary lacZ sequence, 100mer polymerase chain reaction (PCR) products containing the lacZ sequence at various locations, and non-complementary DNA including genomic samples from salmon sperm. Non-selective adsorption of non-complementary oligonucleotides (ncDNA) was found to occur at a significantly faster rate than hybridization of complementary oligomers (cDNA) in all cases. The presence of ncDNA oligonucleotides did not inhibit selective interactions between immobilized DNA and cDNA in solution. The presence of high concentrations of non-complementary genomic DNA had little effect on extent or speed of hybridization of complementary oligonucleotides. Detection of genomic fragments containing the lacZ sequence was possible in as little as 20 s by observation of the steady-state fluorescence intensity increase or by time-dependent rate of fluorescence intensity changes.     

Formation of organometallic polymer nanorods using a nanoporous alumina template and the conversion to magnetic ceramic nanorods

Polyferrocenylsilane nanorods were prepared using a porous anodic aluminium oxide template followed by chemical etching; pyrolysis was used to obtain magnetic iron oxide-containing ceramic nanorods.     

A systematic evaluation of the role of crystalline order in nanoporous materials on DNA separation

The role of order within a porous separation matrix on the separation efficiency of DNA was studied systematically. DNA separation was based on a ratchet mechanism. Monodisperse colloidal suspensions of nanoparticles were used to fabricate highly ordered separation media with a hexagonal close-packed structure. Doping with a second particle size yielded structures with different degrees of disorder, depending upon the volume fraction of each particle size. Radial distribution functions and orientational order parameters were calculated from electron micrographs to characterize the scale of disorder. The peak separation distance, band broadening, and separation resolution of DNA molecules was quantified for each structure. DNA separation parameters using pulsed fields and the ratchet effect showed a strong dependence on order within the porous nanoparticle array. Ordered structures gave large separation distances, smaller band broadening and better resolution than highly disordered, nearly random, porous structures. The effect dominated these three parameters when compared to the effect of pore size. However, the effect of order on separation performance was not monotonic. A small, but statistically significant improvement was seen in structures with short range order compared to those with long range order.     

Electrochemical detection of hydrazine using a highly sensitive nanoporous gold electrode

A facile alloy–dealloy technique performed in aqueous media was employed to prepare a nanoporous gold (NPG) electrode that demonstrated extremely high sensitivity toward hydrazine oxidation. An Ag_∼60Au_∼40 alloy was electrodeposited at a constant potential on sequentially Cr- and Au-deposited indium tin oxide (Au/Cr/ITO) from a bath that contained sulfuric acid, thiourea, HAuCl₄·3H₂O, and AgNO₃. The dealloying step was performed in concentrated HNO₃, where Ag in the alloy was selectively oxidized to leave the NPG structure. The NPG electrode was employed to study the hydrazine oxidation in basic phosphate buffer solution (PBS), and the results were compared with those obtained using the gold nanoparticle (AuNP)-modified ITO (AuNP/ITO) electrode. The NPG electrode demonstrated an unusual surface-confined behavior, which probably resulted from the thin-layer characteristics of the nano-pores. Hydrazine was detected by hydrodynamic chronoamperometry (HCA) at +0.2 V (vs. Ag/AgCl). The steady-state oxidative current exhibited a linear dependence on the hydrazine concentration in the concentration range of 5.00 nM–2.05 mM, and the detection limit was 4.37 nM (σ = 3). This detection limit is the lower than the detection limits reported in the current literature concerning the electrochemical detection of hydrazine. The NPG electrode indeed demonstrates greater stability after hydrazine detection than the AuNP/ITO electrode.     

Nanoparticle based enhancement of electrochemical DNA hybridization signal using nanoporous electrodes

A novel nanoparticle-based enhanced methodology for the detection of ssDNA using nanoporous alumina filter membranes, containing pores of 200 nm in diameter, is reported. The blockage of the pores due to the hybridization is detected by measuring the decrease in the differential pulse voltammetric response of the [Fe(CN)(6)](4-/3-) redox indicator and using screen-printed carbon electrodes as transducing platform. Furthermore, 20 nm gold nanoparticle (AuNPs) tags are used in order to increase the sensitivity of the assay. The enhancement mechanism of DNA detection is due to an additional blocking effect induced by hybridization reaction by bringing AuNPs inside the pores. The developed methodology can be extended to other biosensing systems with interest not only for DNA but also for proteins and cells. The developed nanochannel/nanoparticle biosensing system would have enormous potential in future miniaturized designs adapted to mass production technologies such as screen-printing technology.     

Direct preparation of nanoporous carbon by nanocasting

Nanoporous carbon with narrow pore size distribution was prepared via a nanocasting technique by direct carbonization of cyclodextrin-silica organic-inorganic hybrid composite. The obtained carbon exhibited very high BET surface area and high pore volume.     

Direct fluorescence detection of point mutations in human genomic DNA using microbead-based ligase chain reaction

This report has described a convenient genotyping method capable of detecting point mutations directly in human genomic DNA based on the combination of ligase chain reaction (LCR) and microbead-enrichment technique. LCR primers, including a biotin-labeled common primer and two fluorescence-labeled allele-specific primers, are designed for two alleles of a mutated site. When genomic DNA carries the mutated site, the common primer and allele-specific primer are ligated to form exponential amplified biotin-labeled fluorescence ligation products. These ligated products are enriched by streptavidin-coated microbeads, and genotypes are identified conveniently according to the fluorescence color of microbeads using fluorescent microscopy. Due to amplification of LCR process and enrichment of microbeads, the detection limit of the proposed method is as low as 10⁻¹⁵ mol/L templates. The method provides a convenient and simple strategy to detect point mutation directly in human genome. We have confirmed the efficiency of this approach with the identification of β-globin gene point mutation, which results in the reduced production of globin in an inherited hemoglobin disorder thalassemia disease.     

Rapid detection of dengue virus in serum using magnetic separation and fluorescence detection

A magnetophoretic fluorescence sensor (MFS) has been developed to rapidly detect dengue virus in serum at a sensitivity that was approximately three orders of magnitude higher than conventional solid phase immunoassays. UV inactivated type 2 dengue virus was first reacted with a mixture of superparamagnetic and fluorescent microparticles functionalised with an anti-type 2 dengue virus monoclonal antibody in 10% fetal calf serum. The magnetic particles were separated from the serum based on their magnetophoretic mobility, and dengue virus was detected by the co-localization of magnetic and fluorescent particles at a specific point in the flow chamber. The MFS was capable of detecting dengue-2 virus at 10 PFU ml(-1) with a reaction time of 15 min. The MFS demonstrated a high specificity in the presence of yellow fever virus, a closely related flavivirus, which also did not produce any detectable increase in background signal. The improved performance of this technique appears to result from the rapid kinetics of the microparticle reaction, improved signal-to-noise ratio resulting from magnetophoretic separation, and rapid fluorescent particle detection. These results suggest that the MFS may be useful in early stage diagnosis of dengue infections, as well as other diseases.     

Lowering the detection limit of the acetylcholinesterase biosensor using a nanoporous carbon matrix

Nanostructured carbon matrix has been used for the immobilization and stabilization of the enzyme E.el. AChE. The use of this activated carbon matrix is shown to provide both, significant enzyme stabilization, as well as the means for lowering the detection limit of the biosensor. The enzyme is immobilized by adsorption into the nanostructured conductive carbon, which also acts as the working electrode. The proposed biosensor showed very good stability under continuous operation conditions (L₅₀ > 60 days), allowing its further use in inhibition mode. Using this biosensor, the monitoring of the organophosphorus pesticide dichlorvos at picomolar levels (1000 times lower than other systems reported so far) was achieved. The linear range of detection in flow injection system was six orders of magnitude (10⁻¹² to 10⁻⁶ M). It is suggested that the ability of activated carbon to selectively concentrate the pesticide, as well as the enzyme hyperactivity within the nanopores is the reason for the decrease in the detection limit of the biosensor.     

Ultrasensitive electrochemiluminescence detection of DNA based on nanoporous gold electrode and PdCu@carbon nanocrystal composites as labels

A sensitive electrochemiluminescence (ECL) DNA biosensor based on nanoporous gold (NPG) electrode and PdCu@carbon nanocrystals (CNCs) composites is developed. The CNCs were obtained simply by electrooxidation with abundant carboxyl groups at their surfaces. The NPG can be easily prepared by a selective dissolution of silver from silver-gold alloy in nitric acid, which has free-standing noble metal membranes with controllable three-dimensional (3D) porosity. The PdCu bimetallic nanocomposites with hierarchically hollow structures were fabricated through a simple replacement reaction using dealloyed nanoporous copper (NPC) as both a template and reducing agent. Structure characterization was obtained by means of transmission electron microscope (TEM) and scanning electron microscope (SEM) images. The PdCu@CNCs composites exhibit 6 times higher ECL intensity than the pure CNC-labeled reporter DNA. Taking advantage of dual-amplification effects of the developed probe, a limit of detection as low as 18 aM can be achieved and the assay exhibits excellent selectivity for single-mismatched DNA detection even in human serum. The proposed ECL based method should have wide applications in diagnosis of genetic diseases due to its simplicity, low cost, and high sensitivity at extremely low concentrations.     

Direct synthesis of nanoporous carbon nitride fibers using Al-based porous coordination polymers (Al-PCPs)

We report a new synthetic route for preparation of nanoporous carbon nitride fibers with graphitic carbon nitride polymers, by calcination of Al-based porous coordination polymers (Al-PCPs) with dicyandiamide (DCDA) under a nitrogen atmosphere.     

Novel fabrication process using nanoporous anodic aluminum oxidation and MEMS technologies for gas detection

A class of nanoporous TiO₂ gas sensors processed by novel anodic aluminum oxidation (AAO) of Al thin films and microelectromechnical systems (MEMS) techniques are presented. To enhance the sensitivity and reduce the sensing dimensions of a gas sensor, a nanoporous surface of the gas-sensitive material on the sensor is required. These sensors can be implemented on silicon or silicon dioxide substrate featuring a thin membrane of micro-hotplate structure featuring micro-heaters, thermometers and electrodes, and thus operate as chemoresistive devices. Combining the AAO method with dry-etch process, a homogeneous and nanoporous SiO₂ surface of the sensor can be effectively configured by modulating various hole diameters and depth, hence replacing conventional photolithography and electrochemical etch. The process integration including AAO, reactive ion etch (RIE) and microfabrication is mainly developed and a feasibility study of PVD TiO₂ thin film deposition upon the porous device is also provided. TiO₂ thin films deposited on the nanoporous surface are investigated and compared with non-porous TiO₂ films. It is encouraging that our fabrication process is able to provide relatively high surface area to enhance sensitivity of the sensor without additional doping steps. Our promising experimental results have revealed these miniature and cost-effective devices are not only compatible, but applicable to smart bio-chemical sensors of next generation.