Yuanfang Gao

A micro sensing probe for detecting individual biological cells

While detecting individual biological cells is essential in cytometry for counting cells and analyzing the cellular contents, miniaturized cell-detecting devices are desirable not only for on-site biomedical analysis but also for in situ monitoring of cell dynamics. To study the electrical detection mechanism, a micro sensing probe has been developed on silicon wafers using microfabrication technology. The device consists of an array of through-holes in diameter of 20, 60, and 100 /spl mu/m, with gating electrodes around the holes for cell sensing. An interfacing system including demodulation and signal processing circuitry is built to convert impedance change into electrical pulses. The pulse signals were acquired by a Lab View program for automatic pulse detection and analysis. The sensing mechanism has been tested on microbeads with high-frequency AC excitation and preliminary results indicate that the pulse amplitude distributions correlate with the size of beads. Further refinement on the sensor design and testing of the device for cell detection and analysis are expected.

A Novel On-Chip Diagnostic Method to Measure Burn Rates of Energetic Materials

ABSTRACT A novel on-chip diagnostic method has been developed to measure burn rates of energetic materials patterned on a 1 inch × 3 inch glass chip. The method is based on time-varying resistance (TVR) of a sputter-coated thin platinum (Pt) film, in which resistance of the film changes because of the propagation of ignition of the nanoenergetic material over it. The corresponding voltage differential is captured by a high-speed data acquisition system (1.25 × 106 samples/s). We have measured burn rates as high as 504 m/s for thermites of copper oxide (CuO)/aluminum (Al) and 155 m/s for bismuth oxide (Bi2O3)/Al nanoparticles using this method. We have provided an explanation for the change of resistance upon ignition, based on the microstructural characterization and energy dispersive spectroscopy.

Electrochemical Properties of Carbon Nanoparticles Entrapped in Silica Matrix

Carbon-based electrode materials have been widely used for many years for electrochemical charge storage, energy generation, and catalysis. We have developed an electrode material with high specific capacitance by entrapping graphite nanoparticles into a sol-gel network. Films from the resulting colloidal suspensions were highly porous due to the removal of the entrapped organic solvents from sol-gel matrix giving rise to high Brunauer-Emmett-Teller (BET) specific surface areas (654 m(2)/g) and a high capacitance density ( approximately 37 F/g). An exponential increase of capacitance was observed with decreasing scan rates in cyclic voltammetry studies on these films suggesting the presence of pores ranging from micro (< 2 nm) to mesopores. BET surface analysis and scanning electron microscope images of these films also confirmed the presence of the micropores as well as mesopores. A steep drop in the double layer capacitance with polar electrolytes was observed when the films were rendered hydrophilic upon exposure to a mild oxygen plasma. We propose a model whereby the microporous hydrophobic sol-gel matrix perturbs the hydration of ions which moves ions closer to the graphite nanoparticles and consequently increase the capacitance of the film.

Crystallization of amorphous silicon by self-propagation of nanoengineered thermites

Crystallization of amorphous silicon (a-Si) thin film occurred by the self-propagation of copper oxide/aluminum thermite nanocomposites. Amorphous Si films were prepared on glass at a temperature of 250°C by plasma enhanced chemical vapor deposition. The platinum heater was patterned on the edge of the substrate and the CuO∕Al nanoengineered thermite was spin coated on the substrate that connects the heater and the a-Si film. A voltage source was used to ignite the thermites followed by a piranha solution (4:1 of H2SO4:H2O2) etch for the removal of residual products of thermite reaction. Raman spectroscopy was used to confirm the crystallization of a-Si.

Optimization of Design and Fabrication Processes for Realization of a PDMS-SOG-Silicon DNA Amplification Chip

A novel on-chip inexpensive platform to perform DNA amplification has been fabricated by optimizing the design and microfabrication processes using polydimethyl siloxane (PDMS) and silicon. The silicon base contains a set of microfabricated platinum heater structures on the bottom with a 140-nm-thick spin-on-glass (SOG) layer on the top and a 3 mul replica molded PDMS chamber with feed channels and inlet-outlet ports bonded to this film. The plasma exposed SOG surface is found to undergo recovery of hydrophobicity with time as indicated by an increase in advancing contact angle and bonds very well to another plasma exposed PDMS piece. The bonding protocol developed can be used for a diverse range of substrates, which may form a basis for integration of fluidic assays with microelectronics. The hydrophobic recovery of the microchamber and channels also eliminate the need for various surface passivation techniques for polymerase chain reaction (PCR) chips. A thermal cycler with flexible PCR cycle control is designed and implemented by using a sensing thermocouple. The amplification has been tested using picogram-level template DNA concentration. We have further been able to show negligible nonspecific binding of the template DNA to the hydrophobic interiors of our device by fluorescence measurements and have been able to successfully demonstrate the possibility of multiple usage of this chip without cross-contamination from the previous run

On-Chip Initiation and Burn Rate Measurements of Thermite Energetic Reactions

Burn rates of various nano-energetic composites were measured by two techniques; on-chip method and conventional optical method. A comparison is presented to confirm the validity of on-chip method. On-chip initiators were prepared using platinum heater films and nanoenergetic composites. Thin film Pt heaters were fabricated with different dimensions and ignition delay was studied using a nano-energetic composite of CuO nano-rods and Al-nano-particles. The ignition delay as a function of electrical power is presented for the same energetic composite. Heater with smaller surface area is found to be more efficient, which may be due to the lower heat losses.

Optimization of Fabrication Process for a PDMS-SOG-Silicon Based PCR Micro Chip through System Identification Techniques

A polymerase chain reaction (PCR) micro-chip with integrated thin film heaters and temperature detectors has been realized on a silicon-SOG-PDMS (poly-di(methyl) siloxane) platform. Accurate temperature sensing and control is important for a PCR reaction. This precludes the placement of the temperature sensor anywhere else but within the PCR chamber which can, in certain microchip designs complicate the fabrication methodology. This paper presents the design and optimal placement of a thin film resistance based temperature detector (RTD) for sensing of temperature response on the bottom of the chip (heater side) and predicting the temperature response on the top of the chip (PCR chamber side). Thermal modeling of the system has been performed using a parametric black-box approach based on the input-output data. From the steady state response of the system, pseudo random binary sequences (PRBS) have been generated and used to excite it. Second and fourth order ARX (auto regressive with exogenous inputs) models have been derived for optimal control and their performances have been compared. Reduction of fabrication complexity in regards to optimal placement of temperature sensor has been proposed