Biomedical subjects

Joseph J Tulock

Publications and source records attributed to Joseph J Tulock.

4 recordsLinked to original sources

PubMed2005

Profiling pH gradients across nanocapillary array membranes connecting microfluidic channels.

Nanocapillary array membranes (NCAMs), comprised of thin (d approximately 5-10 microm) nuclear track-etched polycarbonate sheets containing approximately 10(8) cm(-2) nearly parallel nanometer-diameter capillaries, may act to gate fluid transport between microfluidic channels to effect, for example, sample collection. There is interest in H+-transport across these NCAMs because there is significant practical interest in being able to process analyte-containing samples under different pH conditions in adjacent layers of an integrated microfluidic circuit and because protons, with their inherently high mobility, present a challenge in separating microfluidic environments with different properties. To evaluate the capability of NCAMs to support pH gradients, the proton transport properties of NCAMs were studied using laser scanning confocal fluorescence microscopy (LSCFM). Spatiotemporal maps of [H+] in microfluidic channels adjacent to the NCAMs yield information regarding diffusive and electrokinetic transport of protons. The NCAMs studied here are characterized by a positive zeta potential, zeta > 0, so at small nanocapillary diameters, the overlap of electrical double layers associated with opposite walls of the nanocapillary establish an energy barrier for either diffusion or electrokinetic transport of cations through the nanometer-diameter capillaries due to the positive charge on the nanocapillary surface. Proton transfer through an NCAM into microchannels is reduced for pore diameters, d < or = 50 nm and ionic strengths I < or = 50 mM, while for large pore diameters or solution ionic strengths, the incomplete overlap of electric double layer allows more facile ionic transfer across the membranes. These results establish the operating conditions for the development of multilevel integrated nanofluidic/microfluidic architectures which can support multidimensional chemical analysis of mass-limited samples requiring sequential operations to be implemented at different pH values.

Electrochemistry↗

PubMed2005

Miniaturized lead sensor based on lead-specific DNAzyme in a nanocapillary interconnected microfluidic device.

A miniaturized lead sensor has been developed by combining a lead-specific DNAzyme with a microfabricated device containing a network of microfluidic channels that are fluidically coupled via a nanocapillary array interconnect. A DNAzyme construct, selective for cleavage in the presence of Pb2+ and derivatized with fluorophore (quencher) at the 5' (3') end of the substrate and enzyme strands, respectively, forms a molecular beacon that is used as the recognition element. The nanocapillary array membrane interconnect is used to manipulate fluid flows and deliver the small-volume sample to the beacon in a spatially confined detection window where the DNAzyme is interrogated using laser-induced fluorescence detection. A transformed log plot of the fluorescent signal exhibits a linear response (r2 = 0.982) over a Pb2+ concentration range of 0.1 - 100 microM, and a detection limit of 11 nM. The sensor has been applied to the determination of Pb2+ in an electroplating sludge reference material, the result agreeing with the certified value within 4.9%. Quantitative measurement of Pb2+ in this complex sample demonstrates the selectivity of this sensor scheme and points favorably to the application of such technologies to analysis of environmental samples. The unique combination of a DNAzyme with a microfluidic-nanofluidic hybrid device makes it possible to change the DNAzyme to select for other compounds of interest, and to incorporate multiple sensing systems within a single device for greater flexibility. This work represents the initial steps toward creation of a robust field sensor for lead in groundwater or drinking water.

Biosensing Techniques↗

PubMed2004

Microfluidic separation and gateable fraction collection for mass-limited samples.

Integrating multiple analytical processes into microfluidic devices is an important research area required for a variety of microchip-based analyses. A microfluidic system is described that achieves preparative separations by intelligent fraction collection of attomole quantities of sample. The device consists of a main microfluidic channel used to perform electrophoresis, which is interconnected at 90 degrees to two vertically displaced channels via a nanocapillary array membrane. The membrane interconnect contains nanometer-diameter pores that provide fluidic communication between the channels. Sample injection and analyte collection are controlled by application of an electrical bias between the microfluidic channels across the nanocapillary array. After the separation, the automated transfer of the FITC-labeled Arg, Gln, and Gly bands occurs; a fluorescence detector located at the separation/collection channel interconnect is used to generate a triggering signal that initiates suitable voltages to allow near-quantitative transfer of analyte from the separation channel to the second fluidic layer. The ability to achieve such sample manipulations from mass-limited samples enables a variety of postseparation processing events.

Algorithms↗

PubMed2003

Gateable nanofluidic interconnects for multilayered microfluidic separation systems.

The extension of microfluidic devices to include three-dimensional fluidic networks allows complex fluidic and chemical manipulations but requires innovative methods to interface fluidic layers. Externally controllable interconnects, employing nuclear track-etched polycarbonate membranes containing nanometer-diameter capillaries, are described that produce hybrid three-dimensional fluidic architectures. Controllable nanofluidic transfer is achieved by controlling applied bias, polarity, and density of the immobile nanopore surface charge and the impedance of the nanocapillary array relative to the microfluidic channels. Analyte transport between vertically separated microchannels has three stable transfer levels, corresponding to zero, reverse, and forward bias. The transfer can even depend on the properties of the analyte being transferred such as the molecular size, illustrating the flexible character of the analyte transfer. In a specific analysis implementation, nanochannel array gating is applied to capillary electrophoresis separations, allowing selected separated components to be isolated for further manipulation, thereby opening the way for preparative separations at attomole analyte mass levels.

Electrophoresis, Capillary↗