Kinetics of Dimethyl Methylphosphonate Adsorption and Decomposition on Zirconium Hydroxide Using Variable Temperature In Situ Attenuated Total Reflection Infrared Spectroscopy.

The decomposition mechanisms of dimethyl methylphosphonate (DMMP), a widely used simulant for organophosphorus chemical warfare agents (CWAs), are relatively well understood from previous studies. However, there still lacks a quantitative description of DMMP decomposition kinetics under ambient conditions that is relevant for sequestration applications. We investigated adsorption and decomposition kinetics of DMMP on amorphous zirconium hydroxide (ZH) using variable-temperature in situ attenuated total reflection (ATR) infrared spectroscopy. We demonstrate that quantifying DMMP decomposition kinetics using conventional methods, where the integrated absorbance of P-O vibrational modes is monitored, can be inaccurate because these spectra are also convoluted with C-O vibrational modes from transient surface methoxy species that are not proportional to DMMP decomposition due to methanol desorption. Here, we propose to use the ρ(PCH3) modes as an alternative way to track DMMP adsorption and decomposition reactions. On the basis of density functional theory (DFT) simulations and comparisons to relatively unreactive monoclinic zirconia (m-ZrO2), we assign the deconvoluted components of the ρ(PCH3) region and use it to monitor decomposition products over time at various temperatures. Because the PCH3 group is present in many toxic organophosphorus compounds, tracking the PCH3 bands in time-dependent IR spectra is useful for measuring surface kinetics of CWAs and their simulants on various decontamination materials.

Silicon nanowire arrays for the preconcentration and separation of trace explosives vapors.

Silicon nanowire (SiNW) arrays are demonstrated as a suitable platform for the preconcentration of trace nitroaromatic compounds and subsequent desorption via Joule heating of the array. Arrays are fabricated from Si wafers containing an epitaxially grown layer of low conductivity intrinsic Si sandwiched between layers of high conductivity p-type Si. Passage of current through the nanowires results in nanowire temperatures in excess of 200 °C during heating of the arrays as verified by using the temperature-dependent shift of the Si Raman band at ˜520 cm-1. Analyte vapor preconcentration and partial separation is achieved on the array at analyte concentrations nearly two orders-of-magnitude below saturated vapor concentrations at room temperature. The effects of desorption carrier gas flow rate and temperature on the ability to preconcentrate and resolve the analytes of interest are determined. 2,6-dinitrotoluene (2,6-DNT) and 2,4-dinitrotoluene (2,4-DNT) were detected at nominal vapor concentrations of 800 pptv with a 1 min sample time (1.1 ng nominal mass load) and trinitrotoluene (TNT) was detected at a nominal vapor concentration of 65 pptv with a 10 min sample time (1.1 ng nominal mass load).

Environmental Effects on Zirconium Hydroxide Nanoparticles and Chemical Warfare Agent Decomposition: Implications of Atmospheric Water and Carbon Dioxide.

Zirconium hydroxide (Zr(OH)4) has excellent sorption properties and wide-ranging reactivity toward numerous types of chemical warfare agents (CWAs) and toxic industrial chemicals. Under pristine laboratory conditions, the effectiveness of Zr(OH)4 has been attributed to a combination of diverse surface hydroxyl species and defects; however, atmospheric components (e.g., CO2, H2O, etc.) and trace contaminants can form adsorbates with potentially detrimental impact to the chemical reactivity of Zr(OH)4. Here, we report the hydrolysis of a CWA simulant, dimethyl methylphosphonate (DMMP) on Zr(OH)4 determined by gas chromatography-mass spectrometry and in situ attenuated total reflectance Fourier transform infrared spectroscopy under ambient conditions. DMMP dosing on Zr(OH)4 formed methyl methylphosphonate and methoxy degradation products on free bridging and terminal hydroxyl sites of Zr(OH)4 under all evaluated environmental conditions. CO2 dosing on Zr(OH)4 formed adsorbed (bi)carbonates and interfacial carbonate complexes with relative stability dependent on CO2 and H2O partial pressures. High concentrations of CO2 reduced DMMP decomposition kinetics by occupying Zr(OH)4 active sites with carbonaceous adsorbates. Elevated humidity promoted hydrolysis of adsorbed DMMP on Zr(OH)4 to produce methanol and regenerated free hydroxyl species. Hydrolysis of DMMP by Zr(OH)4 occurred under all conditions evaluated, demonstrating promise for chemical decontamination under diverse, real-world conditions.

Quantification of Efficient Plasmonic Hot-Electron Injection in Gold Nanoparticle-TiO2 Films.

Excitation of localized surface plasmons in metal nanostructures generates hot electrons that can be transferred to an adjacent semiconductor, greatly enhancing the potential light-harvesting capabilities of photovoltaic and photocatalytic devices. Typically, the external quantum efficiency of these hot-electron devices is too low for practical applications (<1%), and the physics underlying this low yield remains unclear. Here, we use transient absorption spectroscopy to quantify the efficiency of the initial electron transfer in model systems composed of gold nanoparticles (NPs) fully embedded in TiO2 or Al2O3 films. In independent experiments, we measure free carrier absorption and electron-phonon decay in the model systems and determine that the electron-injection efficiency from the Au NPs to the TiO2 ranges from about 25% to 45%. While much higher than some previous estimates, the measured injection efficiency is within an upper-bound estimate based on a simple approximation for the Au hot-electron energy distribution. These results have important implications for understanding the achievable injection efficiencies of hot-electron plasmonic devices and show that the injection efficiency can be high for Au NPs fully embedded within a semiconductor with dimensions less than the Au electron mean free path.

Surface plasmon polariton-induced hot carrier generation for photocatalysis.

Non-radiative plasmon decay in noble metals generates highly energetic carriers under visible light irradiation, which opens new prospects in the fields of photocatalysis, photovoltaics, and photodetection. While localized surface plasmon-induced hot carrier generation occurs in diverse metal nanostructures, inhomogeneities typical of many metal-semiconductor plasmonic nanostructures hinder predictable control of photocarrier generation and therefore reproducible carrier-mediated photochemistry. Here, we generate traveling surface plasmon polaritons (SPPs) at the interface between a noble metal/titanium dioxide (TiO2) heterostructure film and aqueous solution, enabling simultaneous optical and electrochemical interrogation of plasmon-mediated chemistry in a system whose resonance may be continuously tuned via the incident optical excitation angle. To the best of our knowledge, this is the first experimental demonstration of SPP-induced hot carrier generation for photocatalysis. We found electrochemical photovoltage and photocurrent responses as SPP-induced hot carriers drive both solution-based oxidation of methanol and the anodic half-reaction of photoelectrochemical water-splitting in sodium hydroxide solution. A strong excitation angle dependence and linear power dependence in the electrochemical photocurrent confirm that the photoelectrochemical reactions are SPP-driven. SPP-generated hot carrier chemistry was recorded on gold and silver and with two different excitation wavelengths, demonstrating potential for mapping resonant charge transfer processes with this technique. These results will provide the design criteria for a metal-semiconductor hybrid system with enhanced hot carrier generation and transport, which is important for the understanding and application of plasmon-induced photocatalysis.

Fabrication of metallic nanodisc hexagonal arrays using nanosphere lithography and two-step lift-off.

Nanosphere lithography (NSL) has been widely used as an inexpensive method to create periodic arrays of metallic nanoparticles or nanodiscs on substrates. However, most nanodisc arrays derived from a NSL template are restricted to hexagonally-ordered triangular arrays because the metal layer is deposited onto the interstices between the nanospheres. Metallic nanodisc arrays with the same arrangement as the original nanosphere array have been rarely reported. Here, we demonstrate a facile, low-cost method to fabricate large-area hexagonal arrays of metallic nanodiscs using an NSL template combined with a two-step lift-off process. We employ a bi-layer of two dissimilar metals to create a re-entrant sidewall profile to undercut the sacrificial layer and facilitate the final lift-off of the metallic nanodiscs. The quality of the nanodisc pattern and the array periodicity is determined using statistical image analysis and compared to the original nanosphere array in terms of size distribution, surface smoothness, and array pitch. This nanodisc array is used as an etch mask to create a vertically-aligned Si nanowire array. This combined approach is a scalable and inexpensive fabrication method for creating relatively large-area, ordered arrays of various nanostructures.

The importance of correcting for variable probe-sample interactions in AFM-IR spectroscopy: AFM-IR of dried bacteria on a polyurethane film.

AFM-IR is a combined atomic force microscopy-infrared spectroscopy method that shows promise for nanoscale chemical characterization of biological-materials interactions. In an effort to apply this method to quantitatively probe mechanisms of microbiologically induced polyurethane degradation, we have investigated monolayer clusters of ∼200 nm thick Pseudomonas protegens Pf-5 bacteria (Pf) on a 300 nm thick polyether-polyurethane (PU) film. Here, the impact of the different biological and polymer mechanical properties on the thermomechanical AFM-IR detection mechanism was first assessed without the additional complication of polymer degradation. AFM-IR spectra of Pf and PU were compared with FTIR and showed good agreement. Local AFM-IR spectra of Pf on PU (Pf-PU) exhibited bands from both constituents, showing that AFM-IR is sensitive to chemical composition both at and below the surface. One distinct difference in local AFM-IR spectra on Pf-PU was an anomalous ∼4× increase in IR peak intensities for the probe in contact with Pf versus PU. This was attributed to differences in probe-sample interactions. In particular, significantly higher cantilever damping was observed for probe contact with PU, with a ∼10× smaller Q factor. AFM-IR chemical mapping at single wavelengths was also affected. We demonstrate ratioing of mapping data for chemical analysis as a simple method to cancel the extreme effects of the variable probe-sample interactions.

Influence of inhomogeneous porosity on silicon nanowire Raman enhancement and leaky mode modulated photoluminescence.

Metal-assisted chemical etching (MACE) offers an inexpensive, massively parallel fabrication process for producing silicon nanowires (SiNWs). These nanowires can possess a degree of porosity depending on etch conditions. Because the porosity is often spatially inhomogeneous, there is a need to better understand its nature if applications exploiting the porosity are to be pursued. Here, the resolution afforded by micro-Raman and micro-photoluminescence (PL) is used to elucidate the effects of porosity heterogeneity on the optical properties of individual SiNWs produced in large arrays with MACE, while also determining the spatial character of the heterogeneity. For highly porous SiNWs, there is a dramatic reduction in Raman signal and an increase in PL near the SiNW tips. PL spectra collected along the SiNW length exhibit peaks due to leaky mode resonances. Analysis of the PL resonance peaks, Raman spectrum line shape, SEM images, and EDS spectra indicate that the SiNWs possess both radial and axial heterogeneity wherein, from base to SiNW tip, the SiNWs comprise a shell of increasingly thick porous Si surrounding a tapering core of bulk Si. This work describes how structural porosity variation shapes SiNW optical properties, which will influence the design of new SiNW-based photonic devices and chemical/biological sensors.

The impact of culture medium on the development and physiology of biofilms of Pseudomonas fluorescens formed on polyurethane paint.

Microbial biofilms cause the deterioration of polymeric coatings such as polyurethanes (PUs). In many cases, microbes have been shown to use the PU as a nutrient source. The interaction between biofilms and nutritive substrata is complex, since both the medium and the substratum can provide nutrients that affect biofilm formation and biodeterioration. Historically, studies of PU biodeterioration have monitored the planktonic cells in the medium surrounding the material, not the biofilm. This study monitored planktonic and biofilm cell counts, and biofilm morphology, in long-term growth experiments conducted with Pseudomonas fluorescens under different nutrient conditions. Nutrients affected planktonic and biofilm cell numbers differently, and neither was representative of the system as a whole. Microscopic examination of the biofilm revealed the presence of intracellular storage granules in biofilms grown in M9 but not yeast extract salts medium. These granules are indicative of nutrient limitation and/or entry into stationary phase, which may impact the biodegradative capability of the biofilm.

Quantitative LSPR imaging for biosensing with single nanostructure resolution.

Localized surface plasmon resonance (LSPR) imaging has the potential to map complex spatio-temporal variations in analyte concentration, such as those produced by protein secretions from live cells. A fundamental roadblock to the realization of such applications is the challenge of calibrating a nanoscale sensor for quantitative analysis. Here, we introduce a new, to our knowledge, LSPR imaging and analysis technique that enables the calibration of hundreds of individual gold nanostructures in parallel. The calibration allowed us to map the fractional occupancy of surface-bound receptors at individual nanostructures with nanomolar sensitivity and a temporal resolution of 225 ms. As a demonstration of the technique's applicability to molecular and cell biology, the calibrated array was used for the quantitative LSPR imaging of anti-c-myc antibodies harvested from a cultured 9E10 hybridoma cell line without the need for further purification or processing.

Biotemplating rod-like viruses for the synthesis of copper nanorods and nanowires.

BACKGROUND: In the past decade spherical and rod-like viruses have been used for the design and synthesis of new kind of nanomaterials with unique chemical positioning, shape, and dimensions in the nanosize regime. Wild type and genetic engineered viruses have served as excellent templates and scaffolds for the synthesis of hybrid materials with unique properties imparted by the incorporation of biological and organic moieties and inorganic nanoparticles. Although great advances have been accomplished, still there is a broad interest in developing reaction conditions suitable for biological templates while not limiting the material property of the product. RESULTS: We demonstrate the controlled synthesis of copper nanorods and nanowires by electroless deposition of Cu on three types of Pd-activated rod-like viruses. Our aqueous solution-based method is scalable and versatile for biotemplating, resulting in Cu-nanorods 24-46 nm in diameter as measured by transmission electron microscopy. Cu2+ was chemically reduced onto Pd activated tobacco mosaic virus, fd and M13 bacteriophages to produce a complete and uniform Cu coverage. The Cu coating was a combination of Cu0 and Cu2O as determined by X- ray photoelectron spectroscopy analysis. A capping agent, synthesized in house, was used to disperse Cu-nanorods in aqueous and organic solvents. Likewise, reactions were developed to produce Cu-nanowires by metallization of polyaniline-coated tobacco mosaic virus. CONCLUSIONS: Synthesis conditions described in the current work are scalable and amenable for biological templates. The synthesized structures preserve the dimensions and shape of the rod-like viruses utilized during the study. The current work opens the possibility of generating a variety of nanorods and nanowires of different lengths ranging from 300 nm to micron sizes. Such biological-based materials may find ample use in nanoelectronics, sensing, and cancer therapy.

Periodically porous top electrodes on vertical nanowire arrays for highly sensitive gas detection.

Nanowires of various materials and configurations have been shown to be highly effective in the detection of chemical and biological species. In this paper, we report a novel, nanosphere-enabled approach to fabricating highly sensitive gas sensors based on ordered arrays of vertically aligned silicon nanowires topped with a periodically porous top electrode. The vertical array configuration helps to greatly increase the sensitivity of the sensor while the pores in the top electrode layer significantly improve sensing response times by allowing analyte gases to pass through freely. Herein, we show highly sensitive detection to both nitrogen dioxide (NO(2)) and ammonia (NH(3)) in humidified air. NO(2) detection down to 10 parts per billion (ppb) is demonstrated and an order-of-magnitude improvement in sensor response time is shown in the detection of NH(3).

Vapor detection performance of vertically aligned, ordered arrays of silicon nanowires with a porous electrode.

Vertically aligned, ordered arrays of silicon nanowires capped with a porous top electrode are used to detect gas phase ammonia and nitrogen dioxide in humidified air. The sensors had very fast response times and large signal-to-noise ratios. Calibration curves were created using both an initial slope method and a fixed-time point method. The initial-slope method had a power law dependence that correlates well with concentration, demonstrating a viable alternative for eventual quantitative vapor detection and enabling shorter sampling and regeneration times.

Controllable redox reaction of chemically purified DNA-single walled carbon nanotube hybrids with hydrogen peroxide.

We report for the first time the controllable redox reaction of chemically purified ssDNA-HiPco SWNT hybrids with hydrogen peroxide. Compared with the suspensions before separation, the purified SWNT suspensions become inert with hydrogen peroxide which may serve as a platform for further chemical manipulation. In the presence of thiocyanate ions, the reaction of SWNTs with hydrogen peroxide is initiated and accelerated at the earlier reaction stage, accompanied with the near-infrared spectral suppression. At the later stage, the suppressed spectral intensity is recovered overtime. The thiocyanate ions may work as a mediator being able to control the reaction rate as well as the tunable properties of the reaction. The tunable redox reaction of SWNTs and H2O2 mediated by thiocyanate ions may offer a new sensing scheme for continuously monitoring H2O2 concentrations.

Phenylphosphonic acid functionalization of indium tin oxide: surface chemistry and work functions.

The work function of indium tin oxide (ITO) substrates was modified with phosphonic acid molecular films. The ITO surfaces were treated prior to functionalization with a base cleaning procedure. The film growth and coverage were quantified by contact angle goniometry and XPS. Film orientation was determined by reflection/absorption infrared spectroscopy using ITO-on-Cr substrates. The absolute work functions of nitrophenyl- and cyanophenyl-phosphonic acid films in ITO were determined by Kelvin probe measurement to be 5.60 and 5.77 eV, respectively.

Optical pH response of DNA wrapped HiPco carbon nanotubes.

Solubilization of single-walled carbon nanotubes in aqueous solution by surface functionalization is of great interest for biosensor applications and separation of individual nanotubes. Here we have observed that HiPco nanotubes can be stably dispersed into double-stranded DNA aqueous solutions. Interestingly, the first optical interband transitions of the DNA wrapped semiconducting HiPco nanotubes possess a unique pH dependence, a phenomenon observed in SDS-encased and carboxylic group functionalized single-walled carbon nanotubes. The unique optical pH dependence of surface modified semiconducting nanotubes may have promising applications in optical biosensors.

Optically sensing additional sonication effects on dispersed HiPco nanotubes in aerated water.

Ultrasonication is a necessary process to make single-walled carbon nanotubes (SWNTs) soluble in aqueous solution with surfactants such as sodium dodecyl sulfate (SDS). However, an understanding of the sonication effects on the electronic and optical properties of SWNTs in aqueous solution is still lacking. Here, we have observed that sonication-induced pH changes suppress the optical transitions of the first interband transition pair (S11) in the density of states of semiconducting SWNTs while other possible intermediates induced by sonication contribute less significantly to the observed spectral changes without the involvement of sonication-induced pH decrease. The suppressed S11 peaks can be restored by adding basic solution, suggesting that the lattice structure of SWNTs is undisrupted by the sonication used here. The absorbance of S11 peaks shows a nearly linear relationship with sonication-induced pH changes in the narrow pH range of 5.2 and 6.1. The results indicate that SDS-encased SWNTs may be used as an indicator for sonolysis-related applications.

Aggregation effects on the Raman spectroscopy of dielectrophoretically deposited single-walled carbon nanotubes.

The effect of aggregation on surfactant-suspended individual single-walled carbon nanotube (SWNT) Raman spectroscopy has been explored in the context of dielectrophoretic separation. The Raman spectra of individual surfactant-suspended HiPco SWNTs deposited on a substrate and the same suspension deposited via dielectrophoresis were compared as a function of iterative aggregation states. The evolution of the samples' radial breathing modes and tangential modes at multiple excitation wavelengths (514, 633, and 785 nm) illustrates a direct correlation between changes in the Raman spectra and a broadening and downshifting of resonance transition energies. Dielectrophoresis samples exhibited Raman changes similar to control samples, indicating characterization of electronic separation is compromised by aggregation effects.

Recoverable solution reaction of HiPco carbon nanotubes with hydrogen peroxide.

There is increasing interest in developing single-walled carbon nanotubes (SWNTs)-based optical biosensors for remote or in vitro and in vivo sensing because the near-IR optical properties of SWNTs are very sensitive to surrounding environmental changes. Many enzyme-catalyzed reactions yield hydrogen peroxide (H(2)O(2)) as a product. To our knowledge, there is no report on the interaction of H(2)O(2) with SWNTs from the optical sensing point of view. Here, we study the reaction of H(2)O(2) with an aqueous suspension of water-soluble (ws) HiPco SWNTs encased in the surfactant sodium dodecyl sulfate (SDS). The SWNTs are optically sensitive to hydrogen peroxide in pH 6.0 buffer solutions through suppression of the near-IR absorption band intensity. Interestingly, the suppressed spectral intensity of the nanotubes recovers by increasing the pH, by decomposing the H(2)O(2) into H(2)O and O(2) with the enzyme catalase, and by dialytically removing H(2)O(2). Preliminary studies on the mechanisms suggest that H(2)O(2) withdraws electrons from the SWNT valence band by charge transfer, which suppresses the nanotube spectral intensity. The findings suggest possible enzyme-assisted molecular recognition applications by selective optical detection of biological species whose enzyme-catalyzed products include hydrogen peroxide.

Solubilization of single-wall carbon nanotubes by supramolecular encapsulation of helical amylose.

We have developed a simple, efficient process for solubilization of single-wall carbon nanotubes (SWNTs) with amylose in aqueous DMSO. This process requires two important conditions, presonication of SWNTs and subsequent amylose treatment in an optimum mixture of DMSO/H2O. The former step separates SWNT bundles, and the latter step provides a maximum cooperative interaction of SWNTs with amylose, leading to the immediate and complete solubilization. The best solvent condition for this is around 10-20% DMSO, in which amylose assumes a random conformation or an interrupted helix. This indicates that the amylose helix is not the prerequisite for encapsulation of SWNTs. The SEM and AFM images of the encapsulated SWNTs manifest loosely twisted ribbons wrapping around SWNTs, which are locally intertwined as a multiple twist, but no clumps of the host amylose are seen on SWNT capsules.