Timothy Cantrell

A novel enzymatic microreactor with Aspergillus oryzae β-galactosidase immobilized on silicon dioxide nanosprings.

The use of silicon dioxide (SiO2) nanosprings as supports for immobilized enzymes in a continuous microreactor is described. A nanospring mat (2.2 cm2 × 60 μm thick) was functionalized with γ‐aminopropyltriethoxysilane, then treated with N‐succinimidyl‐3‐(2‐pyridyldithio)‐propionate (SPDP) and dithiothreitol (DTT) to produce surface thiol (SH) groups. SPDP‐modified β‐galactosidase from Aspergillus oryzae was immobilized on the thiolated nanosprings by reversible disulfide linkages. The enzyme‐coated nanospring mat was placed into a 175‐μm high microchannel, with the mat partially occluding the channel. The kinetics and steady‐state conversion of hydrolysis of o‐nitrophenyl β‐D‐galactosylpyranoside at various substrate flow rates and concentrations were measured. Substantial flow was observed through the nanosprings, for which the Darcy permeability κ ≈ 3 × 10−6 cm2. A simple, one‐parameter numerical model coupling Navier‐Stokes and Darcy flow with a pseudo‐first‐order reaction was used to fit the experimental data. Simulated reactor performance was sensitive to changes in κ and the height of the nanospring mat. Permeabilities lower than 10−8 cm2 practically eliminated convective flow through the nanosprings, and substantially decreased conversion. Increasing the height of the mat increased conversion in simulations, but requires more enzymes and could cause sealing issues if grown above channel walls. Preliminary results indicate that in situ regeneration by reduction with DTT and incubation with SPDP‐modified β‐galactosidase is possible. Nanosprings provide high solvent‐accessible surface area with good permeability and mechanical stability, can be patterned into existing microdevices, and are amenable to immobilization of biomolecules. Nanosprings offer a novel and useful support for enzymatic microreactors, biosensors, and lab‐on‐chip devices.

Differential cytotoxicity exhibited by silica nanowires and nanoparticles

Silica nanowires are one-dimensional nanomaterials that are being developed for use in biological systems. Unfortunately, little is known regarding the cytotoxic potential of this type of nanomaterial. Here, using two different human epithelial cell lines we have examined the cytotoxicity of silica nanowires over a broad concentration range. The results indicate that silica nanowires are nontoxic at concentrations below 190 µg/ml but exhibit considerable cytotoxicity at higher concentrations. Examination of the mechanisms responsible for nanowire-induced cytotoxicity indicates that apoptotic pathways are not activated. Instead, cytotoxicity appears to be primarily due to increased necrosis in cells exposed to high concentrations of nanowires. In contrast to what was seen with silica nanowires, analysis of silica nanoparticles revealed very little cytotoxicity even at the highest concentrations tested. These results indicate that structural differences between silica nanomaterials can have dramatic effects on interaction of these nanomaterials with cells.

ZnO coated nanospring-based chemiresistors

Chemiresistors were constructed using 3-D silica nanospring mats coated with a contiguous film of ZnO nanocrystals. Chemiresistors with an average ZnO nanocrystal radius  20 nm, were found to exhibit a relative change in conductance of a factor of 50 upon exposure to a gas flow of 20% O2 and 80% N2 with ∼500 ppm of toluene and an operational temperature of 400 °C. Samples with an average ZnO nanocrystal radius of 15 nm were found to be the most responsive with a relative conductance change of a factor of 1000. The addition of metal nanoparticles (average radius equal to 2.4 nm) onto the surface of the ZnO nanocrystals (average radius equal to 15 nm) produced a relative change in conductance of a factor of 1500. For the optimum conditions (T = 400 °C, grain size ∼15 nm) well-defined spikes in conductance to explosive vapors (TNT, TATP) were obtained for 0.1 ms exposure time at ppb levels.

Thermal and optical activation mechanisms of nanospring-based chemiresistors.

Chemiresistors (conductometric sensor) were fabricated on the basis of novel nanomaterials—silica nanosprings ALD coated with ZnO. The effects of high temperature and UV illumination on the electronic and gas sensing properties of chemiresistors are reported. For the thermally activated chemiresistors, a discrimination mechanism was developed and an integrated sensor-array for simultaneous real-time resistance scans was built. The integrated sensor response was tested using linear discriminant analysis (LDA). The distinguished electronic signatures of various chemical vapors were obtained at ppm level. It was found that the recovery rate at high temperature drastically increases upon UV illumination. The feasibility study of the activation method by UV illumination at room temperature was conducted.

Characterization of a vertically aligned silica nanospring-based sensor by alternating current impedance spectroscopy

In this study, the initial phase of development of a vertically aligned (silica) nanospring (VANS)-based sensor utilizing alternating current impedance spectroscopy is presented. The sensor is a capacitor consisting of two glass substrates coated with indium tin oxide, where the VANS are grown on one substrate, following a top-down approach, serving as the dielectric spacer layer. The sensitivity of the VANS sensors was evaluated using deionized water (of an effective 10?3 mM monovalent ion concentration) and saline-phosphate (SP) solutions of pH 7.3 with concentrations 0.1, 1, 10 and 100 mM. Similar tests were performed with sensors without VANS or blank sensors. The modeling of the VANS impedance spectra required an equivalent circuit consisting of eight elements compared to four elements for the blank sensor. VANS sensors exhibited greater sensitivity to changes in the SP concentration relative to the blank sensors. The enhanced sensitivity is attributed to the addition of an ionic diffusion barrier at the VANS?solution interface and to ionic diffusion within the VANS.

Tris(1H-inden-1-yl)phosphane

In the structure of the title compound, C27H21P, at 89 (2) K the P atom is bound to the allylic carbon of the indenyl unit with P—C distances of ca 1.88 A.