yin Xiao

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.

Highly ordered multilayered 3D graphene decorated with metal nanoparticles

Highly ordered multi-layered three-dimensional (3D) graphene structures decorated with Pd, Pt and Au metal nanoparticles are prepared and characterized. The ability to control the morphology, distribution and size of the metal nanoparticles on the 3D graphene support upon changing the electro- and electroless-deposition conditions is demonstrated. Tailor-made Pt nanostructures, with nanospike and nanoparticle shapes, are prepared using electroless deposition techniques. Au nanoflowers and nanoparticle structures and Pd nanocubes are obtained following electrodeposition onto the 3D graphene support. The deposition patterns and trends are characterized. The greatly enhanced electrocatalytic activity of the metal-NP–graphene surfaces has been illustrated in connection to voltammetric measurements of ORR and hydrogen peroxide at 3D-graphene coated with Pt and Pd nanoparticles, respectively. Such metal nanoparticles decorated multi-layer 3D graphene allows for high mass transport access and catalytic activity for a diverse range of applications, including sensor and fuel-cell technologies.

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.

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.

Microneedle-based transdermal sensor for on-chip potentiometric determination of K(+).

The determination of electrolytes is invaluable for point of care diagnostic applications. An ion selective transdermal microneedle sensor is demonstrated for potassium by integrating a hollow microneedle with a microfluidic chip to extract fluid through a channel towards a downstream solid-state ion-selective-electrode (ISE). 3D porous carbon and 3D porous graphene electrodes, made via interference lithography, are compared as solid-state transducers for ISEs and evaluated for electrochemical performance, stability, and selectivity. The porous carbon K(+) ISEs show better performance than the porous graphene K(+) ISEs, capable of measuring potassium across normal physiological concentrations in the presence of interfering ions with greater stability. This new microfluidic/microneedle platform shows promise for medical applications.

Highly ordered tailored three-dimensional hierarchical nano/microporous gold–carbon architectures

The preparation and characterization of three-dimensional hierarchical architectures, consisting of monolithic nanoporous gold or silver films formed on highly ordered 3D microporous carbon supports, are described. The formation of these nano/microporous structures involves the electrodeposition or sputtering of metal alloys onto the lithographically patterned multi-layered microporous carbon, followed by preferential chemical dealloying of the less noble component. The resulting hierarchical structure displays a highly developed 3D interconnected network of micropores with a nanoporous metal coating. Tailoring the nanoporosity of the metal films and the diameter of the large micropores has been accomplished by systematically changing the alloy compositions via control of the deposition potential, plating solution and coarsening time. SEM imaging illustrates the formation of unique biomimetic nanocoral- or nanocauliflower-like self-supporting structures, depending on the specific preparation conditions. The new 3D hierarchical nano/microporous architectures allow for enhanced mass transport and catalytic activity compared to common nanoporous films prepared on planar substrates. The functionality of this new carbon–gold hierarchical structure is illustrated for the greatly enhanced performance of enzymatic biofuel cells where a substantially higher power output is observed compared to the bare microporous carbon substrate.

Quantum-Size-Controlled Photoelectrochemical Fabrication of Epitaxial InGaN Quantum Dots

We demonstrate a new route to the precision fabrication of epitaxial semiconductor nanostructures in the sub-10 nm size regime: quantum-size-controlled photoelectrochemical (QSC-PEC) etching. We show that quantum dots (QDs) can be QSC-PEC-etched from epitaxial InGaN thin films using narrowband laser photoexcitation, and that the QD sizes (and hence bandgaps and photoluminescence wavelengths) are determined by the photoexcitation wavelength. Low-temperature photoluminescence from ensembles of such QDs have peak wavelengths that can be tunably blue shifted by 35 nm (from 440 to 405 nm) and have line widths that narrow by 3 times (from 19 to 6 nm).

Lithographically defined 3D nanoporous nonenzymatic glucose sensors.

Nonenzymatic glucose oxidation is demonstrated on highly faceted palladium nanowflower-modified porous carbon electrodes fabricated by interference lithography. Varying electrodeposition parameters were used to control the final shape and morphology of the deposited nanoparticles on the 3D porous carbon which showed a 12 times increase in the electrochemically active surface area over analogous planar electrodes. Extremely fast amperometric glucose responses (achieving 95% of the steady state limiting current in less than 5s) with a linear range from 1 to 10mM and a detection limit of 10 μM were demonstrated. The unusual surface properties of the pyrolyzed photoresist films produced strongly adhered palladium crystal structures that were stable for hundreds of cycles towards glucose oxidation without noticeable current decay.

Surface charge dependent nanoparticle disruption and deposition of lipid bilayer assemblies.

Electrostatic interaction plays a leading role in nanoparticle interactions with membrane architectures and can lead to effects such as nanoparticle binding and membrane disruption. In this work, the effects of nanoparticles (NPs) interacting with mixed lipid systems were investigated, indicating an ability to tune both NP binding to membranes and membrane disruption. Lipid membrane assemblies (LBAs) were created using a combination of charged, neutral, and gel-phase lipids. Depending on the lipid composition, nanostructured networks could be observed using in situ atomic force microscopy representing an asymmetrical distribution of lipids that rendered varying effects on NP interaction and membrane disruption that were domain-specific. LBA charge could be localized to fluidic domains that were selectively disrupted when interacting with negatively charged Au nanoparticles or quantum dots. Disruption was observed to be related to the charge density of the membrane, with a maximum amount of disruption occurring at ∼40% positively charged lipid membrane concentration. Conversely, particle deposition was determined to begin at charged lipid concentrations greater than 40% and increased with charge density. The results demonstrate that the modulation of NP and membrane charge distribution can play a pivitol role in determining NP-induced membrane disruption and NP surface assembly.