Pamela M. Norris

Toward a microchip-based solid-phase extraction method for isolation of nucleic acids

A silica‐based solid‐phase extraction system suitable for incorporation into a microchip platform (ν‐total analytical system; ν‐TAS) would find utility in a variety of genetic analysis protocols, including DNA sequencing. The extraction procedure utilized is based on adsorption of the DNA onto bare silica. The procedure involves three steps: (i) DNA adsorption in the presence of a chaotropic salt, (ii) removal of contaminants with an alcohol/water solution, and (iii) elution of the adsorbed DNA in a small volume of buffer suitable for polymerase chain reaction (PCR) amplification. Multiple approaches for incorporation of this protocol into a microchip were examined with regard to extraction efficiency, reproducibility, stability, and the potential to provide PCR‐amplifiable DNA. These included packing microchannels with silica beads only, generating a continuous silica network via sol‐gel chemistry, and combinations of these. The optimal approach was found to involve immobilizing silica beads packed into the channel using a sol‐gel network. This method allowed for successful extraction and elution of nanogram quantities of DNA in less than 25 min, with the DNA obtained in the elution buffer fraction. Evaluation of the eluted DNA indicated that it was of suitable quality for subsequent amplification by PCR.

Measurement of Thermal Boundary Conductance of a Series of Metal-Dielectric Interfaces by the Transient Thermoreflectance Technique

Measurement of the thermal boundary conductance (TBC) by use of a nondestructive optical technique, transient thermoreflectance (TTR), is presented. A simple thermal model for the TTR is presented with a discussion of its applicability and sensitivity. A specially prepared sample series of Cr, Al, Au, and Pt on four different substrates (Si, sapphire, GaN, and AlN) were tested at room temperature and the TTR signal fitted to the thermal model. The resulting TBC values vary by more than a factor of 3~0.71 310 8 ‐2.3310 8 W/m 2 K!. It is shown that the diffuse mismatch model (DMM) tended to overpredict the TBC of interfaces with materials having similar phonon spectra, while underpredicting the TBC for interfaces with dissimilar phonon spectra. The DMM only accounts for diffuse elastic scattering. Other scattering mechanisms are discussed which may explain the failure of the DMM at room temperature.@DOI: 10.1115/1.1857944#

Measurement of the electron-phonon coupling factor dependence on film thickness and grain size in Au, Cr, and Al.

Femtosecond thermoreflectance data for thin films and bulk quantities of Au, Cr, and Al are compared with the parabolic two-step thermal diffusion model for the purpose of determining the electron-phonon coupling factor. The thin films were evaporated and sputtered onto different substrates to produce films that vary structurally. The measurement of the electron-phonon coupling factor is shown to be sensitive to grain size and film thickness. The thin-film thermoreflectance data are compared with that of the corresponding bulk material and to a theoretical model relating the coupling rate to the grain-boundary scattering and size effects on the mean free path of the relevant energy carrier.

Influence of Inelastic Scattering at Metal-Dielectric Interfaces

Thermal boundary conductance is becoming increasingly important in microelectronic device design and thermal management. Although there has been much success in predicting and modeling thermal boundary conductance at low temperatures, the current models applied at temperatures more common in device operation are not adequate due to our current limited understanding of phonon transport channels. In this study, the scattering processes across Cr/Si, Al/Al 2 O 3 , Pt/Al 2 O 3 , and Pt/AIN interfaces were examined by transient thermoreflectance testing at high temperatures. At high temperatures, traditional models predict the thermal boundary conductance to be relatively constant in these systems due to assumptions about phonon elastic scattering. Experiments, however, show an increase in the conductance indicating inelastic phonon processes. Previous molecular dynamic simulations of simple interfaces indicate the presence of inelastic scattering, which increases interfacial transport linearly with temperature. The trends predicted computationally are similar to those found during experimental testing, exposing the role of multiple-phonon processes in thermal boundary conductance at high temperatures.

Influence of Interfacial Mixing on Thermal Boundary Conductance Across a Chromium/Silicon Interface

The thermal conductance at solid-solid interfaces is becoming increasingly important in thermal considerations dealing with devices on nanometer length scales. Specifically, interdiffusion or mixing around the interface, which is generally ignored, must be taken into account when the characteristic lengths of the devices are on the order of the thickness of this mixing region. To study the effect of this interfacial mixing on thermal conductance, a series of Cr films is grown on Si substrates subject to various deposition conditions to control the growth around the Cr/Si boundary. The Cr/Si interfaces are characterized with Auger electron spectroscopy. The thermal boundary conductance (h BD ) is measured with the transient thermoreflectance technique. Values of h BD are found to vary with both the thickness of the mixing region and the rate of compositional change in the mixing region. The effects of the varying mixing regions in each sample on h BD are discussed, and the results are compared to the diffuse mismatch model (DMM) and the virtual crystal DMM (VCDMM), which takes into account the effects of a two-phase region of finite thickness around the interface on h BD . An excellent agreement is shown between the measured h BD and that predicted by the VCDMM for a change in thickness of the two-phase region around the interface.

Femtosecond pump–probe nondestructive examination of materials (invited)

Ultrashort-pulsed lasers have been demonstrated as effective tools for the nondestructive examination (NDE) of energy transport properties in thin films. After the instantaneous heating of the surface of a 100 nm metal film, it will take ∼100 ps for the influence of the substrate to affect the surface temperature profile. Therefore, direct measurement of energy transport in a thin film sample requires a technique with picosecond temporal resolution. The pump–probe experimental technique is able to monitor the change in reflectance or transmittance of the sample surface as a function of time on a subpicosecond time scale. Changes in reflectance and transmittance can then be used to determine properties of the film. In the case of metals, the change in reflectance is related to changes in temperature and strain. The transient temperature profile at the surface is then used to determine the rate of coupling between the electron and phonon systems as well as the thermal conductivity of the material. In the case of semiconductors, the change in reflectance and transmittance is related to changes in the local electronic states and temperature. Transient thermotransmission experiments have been used extensively to observe electron-hole recombination phenomena and thermalization of hot electrons. Application of the transient thermoreflectance (TTR) and transient thermotransmittance (TTT) technique to the study of picosecond phenomena in metals and semiconductors will be discussed. The pump–probe experimental setup will be described, along with the details of the experimental apparatus in use at the University of Virginia. The thermal model applicable to ultrashort-pulsed laser heating of metals will be presented along with a discussion of the limitations of this model. Details of the data acquisition and interpretation of the experimental results will be given, including a discussion of the reflectance models used to relate the measured changes in reflectance to calculated changes in temperature. Finally, experimental results will be presented that demonstrate the use of the TTR technique for measuring the electron–phonon coupling factor and the thermal conductivity of thin metallic films. The use of the TTT technique to distinguish between different levels of doping and alloying in thin film samples of hydrogenated amorphous silicon will also be discussed briefly.

Effects of electron scattering at metal-nonmetal interfaces on electron-phonon equilibration in gold films

Electron scattering at interfaces between metals and dielectrics is a major concern in thermal boundary conductance studies. This aspect of energy transfer has been extensively studied and modeled on long time scales when the electrons and phonons are in equilibrium in the metal film. However, there are conflicting results concerning electron-interface scattering and energy transfer in the event of an electron-phonon nonequilibrium, specifically, how this mode of energy transfer affects the electron cooling during electron-phonon nonequilibration. Transient thermoreflectance (TTR) experiments utilizing ultrashort pulsed laser systems can resolve this electron-phonon nonequilibrium, and the thermophysical property relating rate of equilibration to electron-phonon scattering events G can be quantified. In this work, G in Au films of varying thicknesses are measured with the TTR technique. At large fluences (which result in high electron temperatures), the measured G is much larger than predicted from tradit...

Aerogels as biosensors: viral particle detection by bacteria immobilized on large pore aerogel

Abstract A proof-of-principle study is reported in which bacteria were immobilized within macroporous, supercritically dried silica sol–gel discs and signal induction was demonstrated by aerosolized virus particles. Escherischia coli (pET-gfp) bacteria-doped gels were used as an aerosol collector to detect bacteriophage. The bacteriophage (105 and 108 plaque forming units/ml) (pfu/ml) were aerosolized through the discs for 10 min, at a flow rate of 1.75 l/min and aerosol humidity of 70%. The discs were then incubated in bacterial growth media for 4 h and green fluorescent protein (GFP) expression monitored. The induction of GFP indicated that both bacteriophage and bacteria survived the stressful desiccating conditions of the aerosol challenge. Scanning confocal laser microscopic (SCLM) analysis demonstrated that the bacteriophage contacted viable bacteria and induced expression of the GFP in 35–95% of the bacterial cells. These findings indicate that virus particles can penetrate the structure of macroporous silica gels and trigger a detectable response in immobilized bacteria. The goal is to use microorganisms immobilized within these materials to facilitate the detection of chemicals and organisms within the environment.

THERMAL BOUNDARY RESISTANCE MEASUREMENTS USING A TRANSIENT THERMOREFLECTANCE TECHNIQUE

A transient thermoreflectance technique, using a 200-fs laser pulse, is demonstrated as a nondestructive method for measuring the thermal boundary resistance between a thin metallic film and dielectric substrate. Experimental results are presented for Au deposited on silicon and silicon dioxide substrates taken at room temperature and compared to a thermal model. The relevant thermal properties of the metal film and the substrate are known, leaving the thermal boundary resistance as the only free parameter in the least-squares fitting routine. It is shown that the sensitivity of this technique is related directly to the thermal diffusivity of the substrate. A comparison between the diffuse mismatch model, the phonon radiation limit, and the experimental results indicates that the phonon dispersion relations of the materials can be utilized to give a qualitative prediction of the thermal boundary resistance.A transient thermoreflectance technique, using a 200-fs laser pulse, is demonstrated as a nondestructive method for measuring the thermal boundary resistance between a thin metallic film and dielectric substrate. Experimental results are presented for Au deposited on silicon and silicon dioxide substrates taken at room temperature and compared to a thermal model. The relevant thermal properties of the metal film and the substrate are known, leaving the thermal boundary resistance as the only free parameter in the least-squares fitting routine. It is shown that the sensitivity of this technique is related directly to the thermal diffusivity of the substrate. A comparison between the diffuse mismatch model, the phonon radiation limit, and the experimental results indicates that the phonon dispersion relations of the materials can be utilized to give a qualitative prediction of the thermal boundary resistance.

Cells in Sol-Gels I: A Cytocompatible Route for the Production of Macroporous Silica Gels

A novel, high hydrolysis ratio sol-gel route for the biocompatible production of macroporous silica gels is presented. This route exploits the two step nature of the gelation reaction to remove undesired alcohol by-products from an acidic aqueous sol prior to gelation. These alcohol-free sols will gel when the pH is raised to the physiologic range in a two-step, acid/base catalyzed process. Furthermore, monolithic macroporous samples can be produced in a controlled manner by introducing water-soluble organic polymers into the sol.