A novel and integrated membrane sensing platform for DNA detection is developed based on an anodic aluminum oxide (AAO) membrane. Platinum electrodes (∼50–100 nm thick) are coated directly on both sides of the alumina membrane to eliminate the solution resistance outside the nanopores. The electrochemical impedance technique is employed to monitor the impedance changes within the nanopores upon DNA binding. Pore resistance (Rp) linearly increases in response towards the increasing concentration of the target DNA in the range of 1 × 10−12 to 1 × 10−6 M. Moreover, the biosensor selectively differentiates the complementary sequence from single base mismatched (MM-1) strands and non-complementary strands. This study reveals a simple, selective and sensitive method to fabricate a label-free DNA biosensor.

A nanoporous alumina membrane-based ultrasensitive DNA biosensor is constructed using 5′-aminated DNA probes immobilized onto the alumina channel walls. Alumina nanoporous membrane-like structure is carved over platinum wire electrode of 76 µm diameter dimension by electrochemical anodization. The hybridization of complementary target DNA with probe DNA molecules attached inside the pores influences the pore size and ionic conductivity. The biosensor demonstrates linear range over 6 order of magnitude with ultrasensitive detection limit of 9.55×10−12 M for the quantification of ss-31 mer DNA sequence. Its applicability is challenged against real time cDNA PCR sample of dengue virus serotype1 derived from asymmetric PCR. Excellent specificity down to one nucleotide mismatch in target DNA sample of DENV3 is also demonstrated.

Porous alumina films with controllable pore sizes and having submicrometer film thicknesses were fabricated by the anodization of Al overlayers. The Al was deposited by sputtering onto either glass or onto silicon that had been coated with a layer of silicon nitride. Alumina membranes having thicknesses between 300 and 1000 nm were prepared analogously using a lithographic process to produce free-standing porous alumina films that were peripherally supported on a 500-µm-thick silicon substrate.Alumina structured with nanometer-sized pores has attracted significant recent attention as a template for the fabrication of nanostructures. 1-3 The formation of nanoporous alumina is generally performed by anodizing Al under a bias of 10-100 V in acidic media such as sulfuric acid, phosphoric acid, or oxalic acid. 4-8 This process produces a thin alumina barrier layer overlaid by a highly ordered hexagonal prismatic structure having a pore at the center of each hexagon. Variation in the electrochemical and etching conditions can provide control over the pore density and pore diameter, with available pore sizes between 10 and 200 nm. 7,9 Precise control of the channel growth process has been achieved using mechanical imprinting techniques to create nanosized indentations on the aluminum metal surface 10 or alternatively through the use of electron-beam 11 or focused-ion-beam lithographic methods. 12 The high structural regularity of the films produced by the electrochemical formation of porous alumina has been exploited as a template for the fabrication of nanowires of metals, 13,14 carbons, 15,16 polymers, 17 and semiconductors. 18,19 Porous alumina derived from thermally evaporated or sputtered aluminum films has also recently been used as a lithographic mask for the fabrication of microstructures on metals and semiconductors. 1,2 However, many potential applications of porous alumina require nanometer-scale film thicknesses that additionally provide access to a completely porous support structure with no remaining alumina barrier layer. Furneaux et al. described the use of progressive reduction in the anodizing voltage to create a perforation of the barrier layer and to achive separation of the alumina film from the aluminum, resulting in a porous micrometer-thick free-standing membrane. 20 Masuda et al. used the porous alumina as a template for making free-standing micrometer-thick metal films through a two-step replication process. 21 We describe herein an approach to making such free-standing nanoporous alumina membranes with controllable pore sizes and thicknesses in the nanometer regime, with the membranes peripherally supported on 500-µm-thick Si substrates to provide mechanical strength to the overall device design.Aluminum films were produced by magnetron sputtering (80 W; power density of 7.1 W cm -2 ) using a 99.999% Al target in an atmosphere of research-grade Ar at 5 × 10 -3 Torr. Substrates were either glass microscope slides (Corning) or (100) oriented, 13-23 Ω-cm resistivity, 500-550-µm-thick silicon wafe...

The use of nanosphere lithography to construct two-dimensional arrays of polystyrene (PS) particles coated with multilayered polyelectrolyte (PE) shells and truncated eggshell structures composed of PE thin layers is reported. The truncated eggshell PE structures were produced by extraction of the PS particle cores with toluene. The core-extraction process ruptures the apex of the PE coating and causes a slight expansion of the PE thin layers. Aniline hydrochloride was infiltrated into the PE shells and subsequently electropolymerized to yield an array of a composite containing polyaniline (PAni) and PE thin shells. Voltammetric, quartz crystal microbalance, and reflectance Fourier transform infrared spectroscopic measurements indicate that aniline monomers were confined within the thin PE shells and the electropolymerization occurred in the interior of the PE shell. The PE thickness governs the amount of infiltrated monomer and the ultimate loading of the PAni in the truncated eggshell structure. Surface-structure imaging by atomic force microscopy and scanning electron microscopy, carried out after each step of the fabrication process, shows the influence of the PE thickness on the organization and dimensions of the arrays. Thus, the PE thin shells composed of different layers can function as nanometer-sized vessels for the entrapment of charged species for further construction of composite materials and surface modifications. This approach affords a new avenue for the synthesis of new materials that combine the unique properties of conductive polymers and the controllability of template-directed surface reactions.

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