Carolyn M. Matzke

Motor-protein “roundabouts”: Microtubules moving on kinesin-coated tracks through engineered networks

Nanotechnology promises to enhance the functionality and sensitivity of miniaturized analytical systems. For example, nanoscale transport systems, which are driven by molecular motors, permit the controlled movement of select cargo along predetermined paths. Such shuttle systems may enhance the detection efficiency of an analytical system or facilitate the controlled assembly of sophisticated nanostructures if transport can be coordinated through complex track networks. This study determines the feasibility of complex track networks using kinesin motor proteins to actively transport microtubule shuttles along micropatterned surfaces. In particular, we describe the performance of three basic structural motifs: (1) crossing junctions, (2) directional sorters, and (3) concentrators. We also designed track networks that successfully sort and collect microtubule shuttles, pointing the way towards lab-on-a-chip devices powered by active transport instead of pressure-driven or electroosmotic flow.

Spatially-resolved analysis of nanoparticle nucleation and growth in a microfluidic reactor

Microfluidic systems provide a unique platform for investigation of fundamental reaction processes, which is critical to understanding how to control nanostructure synthesis on a production scale. We have examined the synthesis of cysteine-capped CdS quantum dot nanocrystals (CdS-Cys) between two interdiffusing reagent streams in a continuous-flow microfluidic reactor. Using spatially resolved photoluminescence imaging and spectroscopy of the microreactor, we have acquired kinetic and mechanistic data on the CdS-Cys nanoparticle nucleation and growth, and observed a binary shift in the particle emission spectrum from a higher (2.9 eV) to lower (2.5 eV) energy emission peak within the first second of residence time. Several reactor models have been tested against the spatially and spectrally resolved signals, which suggest that homogeneous reaction and particle nucleation are diffusion-limited and occur only at the boundary between the two laminar streams, while a slower activation process occurs on a longer (seconds) time scale. The results provide direct insight into the rapid processes that occur during crystallization in microfluidic mixing channels, and demonstrate the potential of using controlled microfluidic environments with spatially resolved monitoring to conduct fundamental studies of nanocrystal nucleation and growth.

Microfabricated silicon gas chromatographic micro-channels: fabrication and performance

Using both wet and plasma etching, we have fabricated micro- channels in silicon substrates suitable for use as gas chromatography (GC) columns. Micro-channel dimensions range from 10 to 80 micrometer wide, 200 to 400 micrometer deep, and 10 cm to 100 cm long. Micro-channels 100 cm long take up as little as 1 cm2 on the substrate when fabricated with a high aspect ratio silicon etch (HARSE) process. Channels are sealed by anodically bonding Pyrex lids to the Si substrates. We have studied micro-channel flow characteristics to establish model parameters for system optimization. We have also coated these micro-channels with stationary phases and demonstrated GC separations. We believe separation performance can be improved by increasing stationary phase coating uniformity through micro-channel surface treatment prior to stationary phase deposition. To this end, we have developed microfabrication techniques to etch through silicon wafers using the HARSE process. Etching completely through the Si substrate facilitates the treatment and characterization of the micro-channel sidewalls, which dominate the GC physico- chemical interaction. With this approach, we separately treat the Pyrex lid surfaces that form the top and bottom surfaces of the GC flow channel.

Integrated Chemical Analysis Systems for Gas Phase CW Agent Detection

A miniature, integrated chemical laboratory (μChemLab) is being developed that utilizes microfabrication to provide faster response, smaller size, and an ability to utilize multiple analysis channels for enhanced versatility and chemical discrimination. Improved sensitivity and selectivity are achieved by using a cascaded approach where each channel includes a sample collector/concentrator, a gas chromatographic (GC) separator, and a chemically selective surface acoustic wave (SAW) array detector. Prototypes of all three components have been developed and demonstrated individually and current work is focused on integrating these into a complete analysis system.

Fabrication of high performance microlenses for an integrated capillary channel electrochromatograph with fluorescence detection

We describe the microfabrication of an extremely compact optical system as a key element in an integrated capillary channel electrochromatograph with fluorescence detection. The optical system consists of a vertical cavity surface-emitting laser (VCSEL), two high performance microlenses and a commercial photodetector. The microlenses are multilevel diffractive optics patterned by electron beam lithography and etched by reactive ion etching in fused silica. The design uses substrate-mode propagation within the fused silica substrate. Two generations of optical subsystems are described. The first generation design has a 6 mm optical length and is integrated directly onto the capillary channel-containing substrate. The second generation design separates the optical system onto its own substrate module and the optical path length is further compressed to 3.5 mm. The first generation design has been tested using direct fluorescence detection with a 750 nm VCSEL pumping a 10{sup {minus}4}M solution of CY-7 dye. The observed signal-to-noise ratio of better than 100:1 demonstrates that the background signal from scattered pump light is low despite the compact size of the optical system and is adequate for system sensitivity requirements.

Hand-Held Miniature Chemical Analysis System (μChemlab) for Detection of Trace Concentrations of Gas Phase Analytes

A miniature, integrated chemical laboratory (μChemLab) is being developed that utilizes microfabrication to provide faster response, smaller size, lower power operation, and an ability to utilize multiple analysis channels for enhanced versatility and chemical discrimination. Improved sensitivity and selectivity are achieved with three cascaded components: (1) a sample collector/concentrator, (2) a gas chromatographic (GC) separator, and (3) a chemically selective surface acoustic wave (SAW) array detector. Prototypes of all three components have been developed and demonstrated both individually and when integrated on a novel electrical and fluidic printed circuit board. A hand-held autonomous system containing two analysis channels and all supporting electronics and user interfaces is currently being assembled and tested.

Microfabricated gas phase chemical analysis systems

A portable, autonomous, hand-held chemical laboratory (/spl mu/ChemLab/sup TM/) is being developed for trace detection (ppb) of chemical warfare (CW) agents and explosives in real-world environments containing high concentrations of interfering compounds. Microfabrication is utilized to provide miniature, low-power components that are characterized by rapid, sensitive and selective response. Sensitivity and selectivity are enhanced using two parallel analysis channels, each containing the sequential connection of a front-end sample collector/concentrator, a gas chromatographic (GC) separator, and a surface acoustic wave (SAW) detector. Component design and fabrication and system performance are described.

Ballistic electron and photocurrent transport in Au-molecular layer-GaAs diodes

We present a study on hot electron transport through Au∕molecule∕n-GaAs(001) diodes via ballistic electron emission microcopy (BEEM). The molecules in the structure form a monolayer of either octanedithiol [HS–(CH2)8–SH] or hexadecanethiol [HS–(CH2)15–CH3]. For the dithiol case, the presence of the molecular interlayer leads to undetectable BEEM transmission. Whereas a small photoinduced collector current is detected at random locations at a forward (reverse) scanning tunneling microscopy (STM) tip voltage of −1.43±0.01V (+1.50±0.02V). In comparison, with monothiol diodes, or diodes where the molecules are sandwiched between two Au films (Au∕molecule∕Au∕GaAs), the BEEM transmission remains a significant fraction of the reference diode signal (30%–80%) with a slight increase in the ballistic transport threshold voltage (−1.0to−1.1V) from that of the reference Au∕GaAs diodes (−0.89V). Auger depth profiling and cross-sectional transmission electron microscopy show that Au-molecule intermixing occurs in Au/he...

Expanding the Capabilities and Applications of Gas Phase Miniature Chemical Analysis Systems (µChemLab

Sandia National Laboratories has developed both gas and liquid phase chemical analysis systems. An autonomous hand-held system has been fabricated and demonstrated for sensitive and selective detection of gas phase chemical warfare agents. Recent efforts have focused on maturing this technology and extending its application to other analytical needs. We have now fabricated an on-chip packed gas chromatography column and demonstrated its ability to separate gases such as methane, ethane, ethylene, and acetylene. We have also demonstrated the use of a thermally isolated membrane to rapidly modify and pyrolize fatty acids to form volatile fatty acid methyl esters (FAMEs). This new tool is useful in analyzing biological samples. Finally, we have used rapid temperature ramping of our on-chip open tubular columns to enable separation of low volatility analytes such as explosives and FAMEs. These new capabilities significantly extend the applicability of Sandia’s µChemLab™ technology.

Integrated Micro-Optical Fluorescence Detection System for Microfluidic Electrochromatography

We describe the design and microfabrication of an extremely compact optical system as a key element in an integrated capillary-channel electrochromatograph with laser induced fluorescence detection. The optical design uses substrate-mode propagation within the fused silica substrate. The optical system includes a vertical cavity surface-emitting laser (VCSEL) array, two high performance microlenses and a commercial photodetector. The microlenses are multilevel diffractive optics patterned by electron beam lithography and etched by reactive ion etching in fused silica. Two generations of optical subsystems are described. The first generation design is integrated directly onto the capillary channel-containing substrate with a 6 mm separation between the VCSEL and photodetector. The second generation design separates the optical system onto its own module and the source to detector length is further compressed to 3.5 mm. The systems are designed for indirect fluorescence detection using infrared dyes. The first generation design has been tested with a 750 nm VCSEL exciting a 10-4 M solution of CY-7 dye. The observed signal-to-noise ratio of better than 100:1 demonstrates that the background signal from scattered pump light is low despite the compact size of the optical system and meets the system sensitivity requirements.