David R. Wheeler

Lithographically defined three-dimensional graphene structures.

A simple and facile method to fabricate 3D graphene architectures is presented. Pyrolyzed photoresist films (PPF) can easily be patterned into a variety of 2D and 3D structures. We demonstrate how prestructured PPF can be chemically converted into hollow, interconnected 3D multilayered graphene structures having pore sizes around 500 nm. Electrodes formed from these structures exhibit excellent electrochemical properties including high surface area and steady-state mass transport profiles due to a unique combination of 3D pore structure and the intrinsic advantages of electron transport in graphene, which makes this material a promising candidate for microbattery and sensing applications.

Detection of bioagents using a shear horizontal surface acoustic wave biosensor

Viruses are of high medical and biodefense concern and their detection at concentrations well below the threshold necessary to cause health hazards continues to be a challenge with respect to sensitivity, specificity, and selectivity. Ideally, assays for accurate and real time detection of viral agents would not necessitate any pre-processing of the analyte, which would make them applicable for example to bodily fluids (blood, sputum) and man-made as well as naturally occurring bodies of water (pools, rivers). We describe herein a robust biosensor that combines the sensitivity of surface acoustic waves (SAW) generated at a frequency of 325MHz with the specificity provided by antibodies for the detection of viral agents. A lithium tantalate-based SAW transducer with silicon dioxide waveguide sensor platform featuring three test and one reference delay lines was used to adsorb antibodies directed against either Coxsackie virus B4 or the category A bioagent Sin Nombre virus (SNV), a member of the genus Hantavirus, family Bunyaviridae, negative-stranded RNA viruses. Rapid detection (within seconds) of increasing concentrations of viral particles was linear over a range of order of magnitude for both viruses, although the sensor was approximately 5 x 10(5)-fold more sensitive for the detection of SNV. For both pathogens, the sensors selectivity for its target was not compromised by the presence of confounding Herpes Simplex virus type 1. The biosensor was able to detect SNV at doses lower than the load of virus typically found in a human patient suffering from hantavirus cardiopulmonary syndrome (HCPS). Further, in a proof-of-principle real world application, the SAW biosensor was capable to selectively detect SNV agents in complex solutions, such as naturally occurring bodies of water (river, sewage effluent) without analyte pre-processing. This is the first study that reports on the detection of viral agents using an antibody-based SAW biosensor that has the potential to be used as a hand-held and self-contained device for rapid viral detection in the field.

Maleimide-Activated Aryl Diazonium Salts for Electrode Surface Functionalization with Biological and Redox-Active Molecules

A versatile and simple method is introduced for formation of maleimide-functionalized surfaces using maleimide-activated aryl diazonium salts. We show for the first time electrodeposition of N-(4-diazophenyl)maleimide tetrafluoroborate on gold and carbon electrodes which was characterized via voltammetry, grazing angle FTIR, and ellipsometry. Electrodeposition conditions were used to control film thickness and yielded submonolayer-to-multilayer grafting. The resulting phenylmaleimide surfaces served as effective coupling agents for electrode functionalization with ferrocene and the redox-active protein cytochrome c. The utility of phenylmaleimide diazonium toward formation of a diazonium-activated conjugate, followed by direct electrodeposition of the diazonium-modified DNA onto the electrode surface, was also demonstrated. Effective electron transfer was obtained between immobilized molecules and the electrodes. This novel application of N-phenylmaleimide diazonium may facilitate the development of bioelectronic devices including biofuel cells, biosensors, and DNA and protein microarrays.

Multiplexed microneedle-based biosensor array for characterization of metabolic acidosis.

The development of a microneedle-based biosensor array for multiplexed in situ detection of exercise-induced metabolic acidosis, tumor microenvironment, and other variations in tissue chemistry is described. Simultaneous and selective amperometric detection of pH, glucose, and lactate over a range of physiologically relevant concentrations in complex media is demonstrated. Furthermore, materials modified with a cell-resistant (Lipidure(®)) coating were shown to inhibit macrophage adhesion; no signs of coating delamination were noted over a 48-h period.

Integrated carbon fiber electrodes within hollow polymer microneedles for transdermal electrochemical sensing.

In this study, carbon fiber electrodes were incorporated within a hollow microneedle array, which was fabricated using a digital micromirror device-based stereolithography instrument. Cell proliferation on the acrylate-based polymer used in microneedle fabrication was examined with human dermal fibroblasts and neonatal human epidermal keratinocytes. Studies involving full-thickness cadaveric porcine skin and trypan blue dye demonstrated that the hollow microneedles remained intact after puncturing the outermost layer of cadaveric porcine skin. The carbon fibers underwent chemical modification in order to enable detection of hydrogen peroxide and ascorbic acid; electrochemical measurements were demonstrated using integrated electrode-hollow microneedle devices.

Lithographically Defined Porous Carbon Electrodes

The special nature of the C C bond can lead to various polymorphic forms of carbon such as graphite, glassy-carbon, fullerenes (such as buckyballs), carbon nanotubes, and diamond. Electrodes made from carbon exhibit many useful properties including wide potential windows, low background capacitance, resistance to fouling, and catalytic activity for manyanalytes compared to solidmetal electrodes. In addition to the intrinsic material properties of carbon, functionalized films can be produced through chemical modification using a wide range of chemistries. Because of this flexibility and utility, fabrication of both macroand microporous carbon films, with their commensurate increase in surface area, continues to receive significant research interest. Some of the specific applications for porous carbon materials include fuel cells, electrochemical double layer capacitors, high surface area catalytic supports, water purification, and gas separation. Recently, it has been found that pyrolyzed photoresist films (PPFs) have the same unique properties of carbon electrodes with an advantage that they canbe lithographically defined. The goal of this work was to create lithographically defined porous pyrolyzedcarbonelectrodesandcharacterize thedepositionand electrochemical properties of metal nanoparticles on these electrodes. We report a robust fabrication method capable of producing large area ( 100s cm) submicrometer porous carbon films. In our approach, interferometric lithography (IL) is used topattern thickphotoresist films into 3Dperiodic lattices. These structures are then converted to carbon via pyrolysis under flowing forming gas. During pyrolysis, the non-carbon species in the resist polymer backbone are removed, while the bulk of the carbon remains. The patterned structures undergo significant shrinkage, but remarkably maintain their morphology and adhesion to the substrate. The degree of carbonization is a function of the pyrolysis temperature, which has a profound

Mass-Sensitive Microfabricated Chemical Preconcentrator

This paper describes a mass-sensitive microfabricated preconcentrator for use in chemical detection microsystems. The device combines mass sensing and preconcentration to create a smart preconcentrator (SPC) that determines when it has collected sufficient analyte for analysis by a downstream chemical microsystem. The SPC is constructed from a Lorentz-force-actuated pivot-plate resonator with an integrated heater. Subsequent to microfabrication, the SPC is coated with an adsorbent for collection of chemical analytes. The frequency of operation varies inversely with the mass of collected analyte. Such shifts can be measured by a back-EMF in the SPCs drive/transducer line. By using a calibrated vapor system, the limit of detection of the SPC was determined to be less than 50 ppb for dimethyl-methyl-phosphonate (DMMP) (actual limits of detection are omitted due to export control limitations). At 1 ppm of DMMP, 1-s collection was sufficient to trigger analysis in a downstream microsystem; other micropreconcentrators would require an arbitrary collection time, normally set at 1 min or longer. This paper describes the theory of operation, design, fabrication, coating, vapor system testing, and integration of the SPC into microanalytical systems. The theory of operation, which is applicable to other torsional oscillators, is used to predict a shear modulus of silicon (100) of G = 57.0 GPa plusmn2.2 GPa.

A Multifunctional Thin Film Au Electrode Surface Formed by Consecutive Electrochemical Reduction of Aryl Diazonium Salts

A multifunctional thin film surface capable of immobilizing two diverse molecules on a single gold electrode was prepared by consecutive electrodeposition of nitrophenyl and phenylboronic acid pinacol ester (PBA-PE) diazonium salts. Activation of the stacked film toward binding platinum nanoparticles (PtNPs) and yeast cells occurred via chemical deprotection of the pinacol ester followed by electroreduction of nitro to amino groups. FTIR spectral analysis was used to study and verify film composition at each stage of preparation. The affect of electrodeposition protocol over the thickness of the nitrophenyl and PBA-PE layers was explored and had a profound impact on the film properties. Thicker nitrophenyl films led to diminished PBA-PE diazonium reduction currents during assembly and decreased phenylboronic acid (PBA) layer thickness while allowing for higher PtNP loading and catalytic currents from PtNP-mediated peroxide reduction. Multilayer PBA films could be formed over the nitrophenyl film; however, only submonlayer PBA films permitted access to the underlying layer. The sequence of functional group activation toward binding was also shown to be significant, as perchlorate used to remove pinacol ester also converted aminophenyl groups accessible to the solution to nitrophenyl groups, preventing electrostatic PtNP binding. Finally, SEM images show PtNPs immobilized in close proximity (nanometers) to captured yeast cells on the PBA-aminophenyl-Au film. Such multibinding functionality films that maintain conductivity for subsequent electrochemical measurements hold promise for the development of electrochemical and/or optical platforms for fundamental cell studies, genomic and proteomic analysis, and biosensing.

Three dimensional nickel–graphene core–shell electrodes

The annealing of nickel-coated porous carbon structures results in a new three dimensional nanostructured graphene encapsulated nickel core–shell electrode. A highly interdependent and dynamic process is observed that results in the complete reversal of the spatial orientations of the two component system after the annealing process. We examine the mechanism of carbon diffusion and observe unexpected morphological changes of the nickel in response to carbon crystallization. The new nickel–graphene core–shell electrode demonstrates excellent electrochemical properties with promising applications in micro-batteries and biosensors.

Mass-fabricated one-dimensional silicon nanogaps for hybrid organic/nanoparticle arrays

Optical lithography based on microfabrication techniques was employed to fabricate one-dimensional nanogaps with micrometre edge lengths in silicon. These one-dimensional nanogaps served as a platform on which organic/nanoparticle films were assembled. Characterization of the gaps was performed with high-resolution TEM, SEM, and electrical measurements. Novel self-assembling attachment chemistry, based on the interaction of silicon with a diazonium salt, was used to iteratively build a multi-layer nanoparticle film across a 7 nm gap. By using nanoparticles capped with an easily displaced ligand, a variable conductive path was created across the 1D nanogap. Electrical measurements of the gap showed a dramatic change in the I(V) characteristics after assembly of the nanoparticle film.