Publications

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Bioweapons and emerging infectious diseases pose growing threats to our national security. Both natural disease outbreak and outbreaks due to a bioterrorist attack are a challenge to detect, taking days after the outbreak to identify since most outbreaks are only recognized through reportable diseases by health departments and reports of unusual diseases by clinicians. In recent decades, arthropod-borne viruses (arboviruses) have emerged as some of the most significant threats to human health. They emerge, often unexpectedly, from cryptic transmission foci causing localized outbreaks that can rapidly spread to multiple continents due to increased human travel and trade. Currently, diagnosis of acute infections requires amplification of viral nucleic acids, which can be costly, highly specific, technically challenging and time consuming. No diagnostic devices suitable for use at the bedside or in an outbreak setting currently exist. The original goals of this project were to 1) develop two highly sensitive and specific diagnostic assays for detecting RNA from a wide range of arboviruses; one based on an electrochemical approach and the other a fluorescent based assay and 2) develop prototype microfluidic diagnostic platforms for preclinical and field testing that utilize the assays developed in goal 1. We generated and characterized suitable primers for West Nile Virus RNA detection. Both optical and electrochemical transduction technologies were developed for DNA-RNA hybridization detection and were implemented in microfluidic diagnostic sensing platforms that were developed in this project.

Chemical functionalization is required to adapt graphenes properties to many applications. However, most covalent functionalization schemes are spontaneous or defect driven and are not suitable for applications requiring directed assembly of molecules on graphene substrates. In this work, we demonstrated electrochemically driven covalent bonding of phenyl iodoniums onto epitaxial graphene. The amount of chemisorption was demonstrated by varying the duration of the electrochemical driving potential. Chemical, electronic, and defect states of phenyl-modified graphene were studied by photoemission spectroscopy, spatially resolved Raman spectroscopy, and water contact angle measurement. Covalent attachment rehybridized some of the delocalized graphene sp2 orbitals to localized sp3 states. Control over the relative spontaneity (reaction rate) of covalent graphene functionalization is an important first step to the practical realization of directed molecular assembly on graphene. More than 10 publications, conference presentations, and program highlights were produced (some invited), and follow-on funding was obtained to continue this work.

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Field-structured chemiresistors (FSCRs) are polymer based sensors that exhibit a resistance change when exposed to an analyte of interest. The amount of resistance change depends on the polymer-analyte affinity. The affinity can be manipulated by modifying the polymer within the FSCRs. In this paper, we investigate the ability of chemically modified FSCRs to sense hydrogen peroxide vapor. Five chemical species were chosen based on their hydrophobicity or reactivity with hydrogen peroxide. Of the five investigated, FSCRs modified with allyl methyl sulfide exhibited a significant response to hydrogen peroxide vapor. Additionally, these same FSCRs were evaluated against a common interferrant in hydrogen peroxide detection, water vapor. For the conditions investigated, the FSCRs modified with allyl methyl sulfide were able to successfully distinguish between water vapor and hydrogen peroxide vapor. A portion of the results presented here will be submitted to the Sensors and Actuators journal.

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The advancement of materials technology towards the development of novel 3D nanostructures for energy applications has been a long-standing challenge. The purpose of this project was to explore photolithographically defineable pyrolyzed photoresist carbon films for possible energy applications. The key attributes that we explored were as follows: (1) Photo-interferometric fabrication methods to produce highly porous (meso, micro, and nano) 3-D electrode structures, and (2) conducting polymer and nanoparticle-modification strategies on these structures to provide enhanced catalytic capabilities and increase conductivity. The resulting electrodes were then explored for specific applications towards possible use in battery and energy platforms.

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There is national interest in the development of sophisticated materials that can automatically detect and respond to chemical and biological threats without the need for human intervention. In living systems, cell membranes perform such functions on a routine basis, detecting threats, communicating with the cell, and triggering automatic responses such as the opening and closing of ion channels. The purpose of this project was to learn how to replicate simple threat detection and response functions within artificial membrane systems. The original goals toward developing 'smart skin' assemblies included: (1) synthesizing functionalized nanoparticles to produce electrochemically responsive systems within a lipid bilayer host matrices, (2) calculating the energetics of nanoparticle-lipid interactions and pore formation, and (3) determining the mechanism of insertion of nanoparticles in lipid bilayers via imaging and electrochemistry. There are a few reports of the use of programmable materials to open and close pores in rigid hosts such as mesoporous materials using either heat or light activation. However, none of these materials can regulate themselves in response to the detection of threats. The strategies we investigated in this project involve learning how to use programmable nanomaterials to automatically eliminate open channels within a lipid bilayer host when 'threats' are detected. We generated and characterized functionalized nanoparticles that can be used to create synthetic pores through the membrane and investigated methods of eliminating the pores either through electrochemistry, change in pH, etc. We also focused on characterizing the behavior of functionalized gold NPs in different lipid membranes and lipid vesicles and coupled these results to modeling efforts designed to gain an understanding of the interaction of nanoparticles within lipid assemblies.

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Formation of complex nanomaterials would ideally involve single-pot reaction conditions with one reactive site per nanoparticle, resulting in a high yield of incrementally modified or oriented structures. Many studies in nanoparticle functionalization have sought to generate highly uniform nanoparticles with tailorable surface chemistry necessary to produce such conjugates, with limited success. In order to overcome these limitations, we have modified commercially available nanoparticles with multiple potential reaction sites for conjugation with single ssDNAs, proteins, and small unilamellar vesicles. These approaches combined heterobifunctional and biochemical template chemistries with single molecule optical methods for improved control of nanomaterial functionalization. Several interesting analytical results have been achieved by leveraging techniques unique to SNL, and provide multiple paths for future improvements for multiplex nanoparticle synthesis and characterization. Hyperspectral imaging has proven especially useful for assaying substrate immobilized fluorescent particles. In dynamic environments, temporal correlation spectroscopies have been employed for tracking changes in diffusion/hydrodynamic radii, particle size distributions, and identifying mobile versus immobile sample fractions at unbounded dilution. Finally, Raman fingerprinting of biological conjugates has been enabled by resonant signal enhancement provided by intimate interactions with nanoparticles and composite nanoshells.

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Conductive polymers have become an extremely useful class of materials for many optical applications. We have developed an electrochemical growth method for depositing highly conductive ({approx}100 S/cm) polypyrrole. Additionally, we have adapted advanced fabrication methods for use with the polypyrrole resulting in gratings with submicron features. This conductive polymer micro-wire grid provides an optical polarizer with unique properties. When the polymer is exposed to ionizing radiation, its conductivity is affected and the polarization properties of the device, specifically the extinction ratio, change in a corresponding manner. This change in polarization properties can be determined by optically interrogating the device, possibly from a remote location. The result is a passive radiation-sensitive sensor with very low optical visibility. The ability to interrogate the device from a safe standoff distance provides a device useful in potentially dangerous environments. Also, the passive nature of the device make it applicable in applications where external power is not available. We will review the polymer deposition, fabrication methods and device design and modeling. The characterization of the polymer's sensitivity to ionizing radiation and optical testing of infrared polarizers before and after irradiation will also be presented. These experimental results will highlight the usefulness of the conductive infrared polarizer to many security and monitoring applications.

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Polymer electronic devices and materials have vast potential for future microsystems and could have many advantages over conventional inorganic semiconductor based systems, including ease of manufacturing, cost, weight, flexibility, and the ability to integrate a wide variety of functions on a single platform. Starting materials and substrates are relatively inexpensive and amenable to mass manufacturing methods. This project attempted to plant the seeds for a new core competency in polymer electronics at Sandia National Laboratories. As part of this effort a wide variety of polymer components and devices, ranging from simple resistors to infrared sensitive devices, were fabricated and characterized. Ink jet printing capabilities were established. In addition to promising results on prototype devices the project highlighted the directions where future investments must be made to establish a viable polymer electronics competency.

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The gas-phase {mu}ChemLab{trademark} developed by Sandia can detect volatile organics and semi-volatiles organics via gas phase sampling . The goal of this three year Laboratory Directed Research and Development (LDRD) project was to adapt the components and concepts used by the {mu}ChemLab{trademark} system towards the analysis of water-borne chemicals of current concern. In essence, interfacing the gas-phase {mu}ChemLab{trademark} with water to bring the significant prior investment of Sandia and the advantages of microfabrication and portable analysis to a whole new world of important analytes. These include both chemical weapons agents and their hydrolysis products and disinfection by-products such as Trihalomethanes (THMs) and haloacetic acids (HAAs). THMs and HAAs are currently regulated by EPA due to health issues, yet water utilities do not have rapid on-site methods of detection that would allow them to adjust their processes quickly; protecting consumers, meeting water quality standards, and obeying regulations more easily and with greater confidence. This report documents the results, unique hardware and devices, and methods designed during the project toward the goal stated above. It also presents and discusses the portable field system to measure THMs developed in the course of this project.

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Molecular electronic based chemical vapor sensors were assembled using noble metal nanoparticles and short conjugated phenylene ethynylene (PE) based molecules. Sacrificial capping ligands on the nanoparticles were replaced by tighter binding PE ligands. The films were assembled between pairs of electrodes by iteratively exposing the substrates to solutions of the nanoparticles and PE crosslinking bridging ligands. Some of the conjugated bridging molecules contained an electron deficient phenol to provide a simple platform for developing sensor applications. The phenol is calculated to have a significant change in its HOMO/LUMO gap in the presence of specific analytes. Judicious combination of nanoparticle size and ligand structure provides a film in which the organic bridging ligands dramatically affect film conductance. Specifically, π-conjugated ligands lower resistance more in films with smaller nanoparticles. Thus the sensing mechanism of these films is not based on the typical swelling mechanism but rather on the modulation of the molecular electronic structure of the conducting PE bridging ligands.

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Nanoparticles have received much attention and have been the subject of many reviews. Nanoparticles have also been used to form super molecular structures for molecular electronic, and sensor applications. However, many limitations exist when using nanoparticles, including the ability to manipulate the particles post synthesis. Current methods to prepare nanoparticles employ functionalities like thiols, amines, phosphines, isocyanides, or a citrate as the metal capping agent. While these capping agents prevent agglomeration or precipitation of the particles, most are difficult to displace or impede packing in nanoparticle films due to coulombic repulsion. It is in this vein that we undertook the synthesis of nanoparticles that have a weakly bound capping agent that is strong enough to prevent agglomeration and in the case of the platinum particles allow for purification, but yet, easily displaced by other strongly binding ligands. The nanoparticles where synthesized according to the Brust method except stearonitrile was used instead of an aliphatic thiol. Both platinum and gold were examined in this manner. A representative procedure for the synthesis of platinum nanoparticles involved the phase transfer of chloroplatinic acid (0.37 g, 0.90 mmol) dissolved in water (30 mL) to a solution of tetraoctylammonium bromide (2.2 g, 4.0 mmol) in toluene (80 mL). After the chloroplatinic acid was transferred into the organic phase the aqueous phase was removed. Stearonitrile (0.23 g, 0.87 mmol) was added and sodium borohydride (0.38 g, 49 mmol) in water (25 mL) was added. The solution turned black almost immediately and after 15 min the organic phase was separated and passed through a 0.45 {micro}m Teflon filter. The resulting solution was concentrated and twice precipitated into ethanol ({approx}200 mL) to yield 0.11 g of black platinum nanoparticles. TGA experiments showed that the Pt particles contained 35% by mass stearonitrile. TEM images showed an average particle size of 1.3 {+-} 0.3 nm. A representative procedure for the synthesis of gold nanoparticles involved the transfer of hydrogen tetrachloroaurate (0.18 g, 0.53 mmol) dissolved in water (15 mL) to a solution of tetraoctylammonium bromide (1.1 g, 2.0 mmol) in toluene (40 mL). After the gold salt transferred into the organic phase the aqueous phase was removed. Stearonitrile (0.23 g, 0.87 mmol) was added and sodium borohydride (0.19 g, 5.0 mmol) in water (13 mL) was added. The solution turned dark red almost immediately, and after 15 min the organic phase was separated and passed through a 0.45 {micro}m Teflon filter. The resulting solution was used without purification via precipitation because attempts at precipitation with ethanol resulted in agglomeration. TEM images showed an average particle size of 5.3 {+-} 1.3 nm. The nanoparticles synthesized were also characterized using atomic force microscopy in tapping mode. The AFM images agree with the TEM images and show a relatively monodispersed collection of nanoparticles. Platinum nanoparticles were synthesized without stearonitrile to show that the particles were in fact capped with the stearonitrile and not the tetraoctylammonium bromide. In the absence of stearonitrile the nanoparticles would not redissolve in hexane or toluene after precipitation. While it is possible the tetraoctylammonium bromide helps prevent agglomeration by solvation into the capping stearonitrile ligand layer on the particles recovery of a quantitative amount of the starting tetraoctylammonium bromide was difficult and we cannot rule out that some small amount of tetraoctylammonium bromide serves in a synergistic capacity to help solubilize the isolated platinum particles. Several exchange reactions were carried out using the isolated Pt nanoparticles. The stearonitrile cap was exchanged for hexadecylmercaptan, octanethiol, and benzeneethylthiol. In a typical exchange reaction, Pt nanoparticles (10 mg) were suspended in hexane (10 mL) and the exchange ligand was added (50 {micro}L). The solutions were allowed to stir overnight and precipitated twice using ethanol. TGA experiments confirmed ligand exchange. We have also shown that these particles may be assembled in a layer by layer (LBL) fashion to build up three dimensional assemblies. As an example of this LBL assembly a substrate consisting of gold electrodes separated by 8 {micro}m on a quartz wafer was first functionalized by immersing in a solution of 1,8-octanedithiol (50 {micro}L) in hexane (10 mL) for 15 min, rinsed with hexane (10 mL), ethanol (10 mL), and dried under a stream of nitrogen. The scaffold was then placed in a toluene solution containing Au nanoparticles capped with stearonitrile (10 mg/mL) for 15 minutes. The scaffold was then rinsed with hexane (10 mL), ethanol (10 mL), and dried under a stream of nitrogen. The substrate was then immersed iteratively between the 1,8-octanedithiol and the Au nanoparticle solution 4 more times.

The synthesis and characterization of soluble and processable high molecular weight polysilsesquioxanes with carboxylate functionalities was discussed. It was found that the tert-butyl functionality in these polymers was eliminated to give carboxylic acids functionalized polysilsesquioxane or methyltin carboxylatosilsesquioxane gels. The analysis showed that the polysilsesquioxane binds and removes tin through gelation.

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A variety of organic and hybrid organic-inorganic polymer systems were prepared and evaluated for their bulk response to optical, thermal and chemical environmental changes. These included modeling studies of polyene-bridged metal porphyrin systems, metal-mediated oligomerization of phosphaalkynes as heteroatomic analogues to polyacetylene monomers, investigations of chemically amplified degradation of acid- and base-sensitive polymers and thermally responsive thermoplastic thermosets based on Diels-Alder cycloaddition chemistry. The latter class of materials was utilized to initiate work to develop a new technique for rapidly building a library of systems with varying depolymerization temperatures.

Four of the better developed resist schemes that are outgrowths of DUV (248 and 193 nm) resist development are considered as candidates for EUV. They are as follows: trilayer, a thin imaging layer on top of a refractor masking/pattern transfer layer on top of a planarizing and processing layer (PPL); solution developed, organometallic bilayer where the imaging and masking layer have been combined into one material on top of a PPL; and finally silylated resists. They are examined in a very general form without regard to the specifics of chemistry of the variations within each group, but rather to what is common to each group and how that affects their effectiveness as candidates for a near term EUV resist. In particular they are examined with respect to sensitivity, potential resolution, optical density, etching selectivity during pattern transfer, and any issues associated with pattern fidelity such as swelling.

We recently described a near-surface imaging scheme that employs disilanes and a bilayer resist scheme which together dramatically improve silicon contrast. A relatively thin 0.25 to 0.1 {mu}m imaging layer of a chemically amplified photo-crosslinking resist (Shipley XP-8844 or XP-9472) is spin coated on top of a thicker (0.25-0.5 {mu}m) layer of hard-baked resist (such as Shipley MP-1807). This bilayer scheme improves silicon contrast and provides additional advantages such as providing a planarizing layer and a processing layer.

Recent results in the use of disilanes as silylating reagents for near-surface imaging with deep-UV (248 nm) and EUV (13.5 nm) lithography are reported. A relatively thin imaging layer of a photo-cross-linking resist is spun over a thicker layer of hard-baked resist that functions as a planarizing layer and antireflective coating. Photoinduced acid generation and subsequent heating crosslinks and renders exposed areas impermeable to an aminodisilane that reacts with the unexposed regions. Subsequent silylation and reactive ion etching afford a positive-tone image. The use of disilanes introduces a higher concentration of silicon into the polymer than is possible with silicon reagents that incorporate only one silicon atom per reactive site. The higher silicon content in the silylated polymer increases etching selectivity between exposed and unexposed regions and thereby increases the contrast. Additional improvements that help to minimize flow during silylation are also discussed, including the addition of bifunctional disilanes. We have resolved high aspect ratio, very high quality 0.20 {mu}m line and space patterns at 248 nm with a stepper having a numerical aperture (NA)= 0.53, and have resolved {<=} 0.15 {mu}m line and spaces at 13.5 nm.