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Search Results: 1 - 10 of 227350 matches for " R. Bruce Weisman "
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Single-Walled Carbon Nanotube Sample Purification
Saunab Ghosh,Sergei M. Bachilo,R. Bruce Weisman
Physics , 2013,
Abstract: Removal of impurities and aggregates from as-grown single walled-carbon nanotube (SWCNT) materials remains an important challenge in nanotube research and applications. We report a simple but effective purification method that uses permanent magnets to remove nanotube aggregates, as well as residual metallic catalyst, from surfactant-coated aqueous SWCNT suspensions. Samples have been characterized by optical absorption, fluorescence and Raman spectroscopies; AFM and near-IR fluorescence microscopies; and TGA. We found that magnetic purification gives samples with reduced absorption backgrounds and high fluorescence efficiencies comparable to those prepared by ultracentrifugation. Nanotubes in magnetically processed HiPco samples are at least 80% individualized. We propose that SWCNT aggregates are preferentially removed because they contain disproportionate amounts of ferromagnetic catalyst particles. As compared to ultracentrifugation, magnetic processing offers major advantages in cost, simplicity, energy consumption, and scalability.
Structure-Dependent Fluorescence Efficiencies of Individual Single-Walled Carbon Nanotubes
Dmitri A. Tsyboulski,John-David R. Rocha,Sergei M. Bachilo,Laurent Cognet,R. Bruce Weisman
Physics , 2007, DOI: 10.1021/nl071561s
Abstract: Single-nanotube photometry was used to measure the product of absorption cross-section and fluorescence quantum yield for 12 (n,m) structural species of semiconducting SWNTs in aqueous SDBS suspension. These products ranged from 1.7 to 4.5 x 10(-19) cm2/C atom, generally increasing with optical band gap as described by the energy gap law. The findings suggest fluorescent quantum yields of ~8% for the brightest, (10,2) species and introduce the empirical calibration factors needed to deduce quantitative (n,m) distributions from bulk fluorimetric intensities.
Carbon Nanotubes: Solution for the Therapeutic Delivery of?siRNA?
D. Lynn Kirkpatrick,Michelle Weiss,Anton Naumov,Geoffrey Bartholomeusz,R. Bruce Weisman,Olga Gliko
Materials , 2012, DOI: 10.3390/ma5020278
Abstract: Carbon nanotubes have many unique physical and chemical properties that are being widely explored for potential applications in biomedicine especially as transporters of drugs, proteins, DNA and RNA into cells. Specifically, single-walled carbon nanotubes (SWCNT) have been shown to deliver siRNA to tumors in vivo. The low toxicity, the excellent membrane penetration ability, the protection afforded against blood breakdown of the siRNA payload and the good biological activity seen in vivo suggests that SWCNT may become universal transfection vehicles for siRNA and other RNAs for therapeutic applications. This paper will introduce a short review of a number of therapeutic applications for carbon nanotubes and provide recent data suggesting SWCNT are an excellent option for the delivery of siRNA clinically.
Absorption spectroscopy of individual single-walled carbon nanotubes
Stéphane Berciaud,Laurent Cognet,Philippe Poulin,R. Bruce Weisman,Brahim Lounis
Physics , 2007, DOI: 10.1021/nl062933k
Abstract: Current methods for producing single-walled carbon nanotubes (SWNTs) lead to heterogeneous samples containing mixtures of metallic and semiconducting species with a variety of lengths and defects. Optical detection at the single nanotube level should thus offer the possibility to examine these heterogeneities provided that both SWNT species are equally well detected. Here, we used photothermal heterodyne detection to record absorption images and spectra of individual SWNTs. Because this photothermal method relies only on light absorption, it readily detects metallic nanotubes as well as the emissive semiconducting species. The first and second optical transitions in individual semicontucting nanotubes have been probed. Comparison between the emission and absorption spectra of the lowest-lying optical transition reveal mainly small Stokes shifts. Side bands in the near-infrared absorption spectra are observed and assigned to exciton-phonon bound states. No such sidebands are detected around the lowest transition of metallic nanotubes.
Stepwise Quenching of Exciton Fluorescence in Carbon Nanotubes by Single Molecule Reactions
Laurent Cognet,Dmitri A. Tsyboulski,John-David R. Rocha,Condell D. Doyle,James M. Tour,R. Bruce Weisman
Physics , 2007, DOI: 10.1126/science.1141316
Abstract: Single-molecule chemical reactions with individual single-walled carbon nanotubes were observed through near-infrared photoluminescence microscopy. The emission intensity within distinct submicrometer segments of single nanotubes changes in discrete steps after exposure to acid, base, or diazonium reactants. The steps are uncorrelated in space and time, and reflect the quenching of mobile excitons at localized sites of reversible or irreversible chemical attack. Analysis of step amplitudes reveals an exciton diffusional range of about 90 nanometers, independent of nanotube structure. Each exciton visits approximately 104 atomic sites during its lifetime, providing highly efficient sensing of local chemical and physical perturbations.
Biexciton, single carrier, and trion generation dynamics in single-walled carbon nanotubes
Bertrand Yuma,Stéphane Berciaud,Jean Besbas,Jonah Shaver,Silvia M. Santos,Saunab Gosh,R. Bruce Weisman,Laurent Cognet,Mathieu Gallart,Marc Ziegler,Bernd H?nerlage,Brahim Lounis,Pierre Gilliot
Physics , 2013, DOI: 10.1103/PhysRevB.87.205412
Abstract: We present a study of free carrier photo-generation and multi-carrier bound states, such as biexcitons and trions (ionized excitons), in semiconducting single-walled carbon nanotubes. Pump-and-probe measurements performed with fs pulses reveal the effects of strong Coulomb interactions between carriers on their dynamics. Biexciton formation by optical transition from exciton population results in an induced absorption line (binding energy 130 meV). Exciton-exciton annihilation process is shown to evolve at high densities towards an Auger process that can expel carriers from nanotubes. The remaining carriers give rise to an induced absorption due to trion formation (binding energy 190 meV). These features show the dynamics of exciton and free carriers populations.
Emittance increase caused by core depletion in collisions
R. Bruce
Physics , 2009,
Abstract: A new effect is presented, which changes the emittance during colliding-beam operation in circular colliders. If the initial transverse distribution is Gaussian, the collision probability is much higher for particles in the core of the beam than in the tails. When small-amplitude particles are removed, the remaining ones therefore have a larger transverse emittance. This effect, called core depletion, may cause a decrease in luminosity. An approximate analytic model is developed to study the effect and benchmarked against a multiparticle tracking simulation. Finally, the time evolution of the intensity and emittances of a Pb bunch in the Large Hadron Collider (LHC) at CERN is calculated, taking into account also other processes than collisions. The results show that integrated luminosity drops by 3--4% if core depletion is taken into account. It is also found that core depletion causes the transverse emittance to be larger when more experiments are active. This observation could be checked against experimental data once the LHC is operational.
Identification and Prediction of Diabetic Sensorimotor Polyneuropathy Using Individual and Simple Combinations of Nerve Conduction Study Parameters
Alanna Weisman, Vera Bril, Mylan Ngo, Leif E. Lovblom, Elise M. Halpern, Andrej Orszag, Bruce A. Perkins
PLOS ONE , 2013, DOI: 10.1371/journal.pone.0058783
Abstract: Objective Evaluation of diabetic sensorimotor polyneuropathy (DSP) is hindered by the need for complex nerve conduction study (NCS) protocols and lack of predictive biomarkers. We aimed to determine the performance of single and simple combinations of NCS parameters for identification and future prediction of DSP. Materials and Methods 406 participants (61 with type 1 diabetes and 345 with type 2 diabetes) with a broad spectrum of neuropathy, from none to severe, underwent NCS to determine presence or absence of DSP for cross-sectional (concurrent validity) analysis. The 109 participants without baseline DSP were re-evaluated for its future onset (predictive validity). Performance of NCS parameters was compared by area under the receiver operating characteristic curve (AROC). Results At baseline there were 246 (60%) Prevalent Cases. After 3.9 years mean follow-up, 25 (23%) of the 109 Prevalent Controls that were followed became Incident DSP Cases. Threshold values for peroneal conduction velocity and sural amplitude potential best identified Prevalent Cases (AROC 0.90 and 0.83, sensitivity 80 and 83%, specificity 89 and 72%, respectively). Baseline tibial F-wave latency, peroneal conduction velocity and the sum of three lower limb nerve conduction velocities (sural, peroneal, and tibial) best predicted 4-year incidence (AROC 0.79, 0.79, and 0.85; sensitivity 79, 70, and 81%; specificity 63, 74 and 77%, respectively). Discussion Individual NCS parameters or their simple combinations are valid measures for identification and future prediction of DSP. Further research into the predictive roles of tibial F-wave latencies, peroneal conduction velocity, and sum of conduction velocities as markers of incipient nerve injury is needed to risk-stratify individuals for clinical and research protocols.
The Pain Frequency-Severity-Duration Scale as a Measure of Pain: Preliminary Validation in a Pediatric Chronic Pain Sample
Katherine S. Salamon,W. Hobart Davies,Melissa R. Fuentes,Steven J. Weisman,Keri R. Hainsworth
Pain Research and Treatment , 2014, DOI: 10.1155/2014/653592
Abstract: Typically, pain is measured by intensity and sensory characteristics. Although intensity is one of the most common dimensions of pain assessment, it has been suggested that measuring pain intensity in isolation is only capturing part of the pain experience and may not lead to an accurate measurement of how pain impacts a child’s daily functioning. The current study aimed to develop a measure that would capture pain intensity along with frequency and duration in a clinical sample of youth diagnosed with chronic pain. The pain-frequency-severity-duration (PFSD) scale was developed and data were collected from a multidisciplinary pain clinic at a large, midwestern children’s hospital. Validated measures of functional limitations and health related quality of life were also collected. Significant correlations were found between the PFSD composite score, functional limitations, and health related quality of life. Future research should continue to evaluate this questionnaire utilizing other validated pain measures and other areas potentially impacted by chronic pain and with more diverse samples. This initial finding suggests that the PFSD is a convenient self-reported measure and is strongly related to health related quality of life and functional disability. 1. Introduction Idiopathic chronic pain is persistent pain, lasting longer than three months, that does not serve as a somatic warning sign of tissue damage or injury [1]. It is estimated that one in five children in the United States is affected by chronic pain [1]. Typically, pain is measured by its intensity and sensory characteristics (e.g., location and pattern, including frequency and duration) [2]. Specifically, pain frequency, severity/intensity, and duration are often assessed simultaneously at medical appointments and during hospital visits. Self-reported pain intensity continues to be the most widely used measurement of a child’s pain [3]. Although intensity is the most common dimension of pain assessment, von Baeyer has suggested that measuring pain intensity alone is only capturing part of the clinical picture [4, 5]. Given that pain is typically thought of as a subjective experience, having an accurate measurement of pain intensity does not guarantee an accurate measurement of how that pain impacts a child’s daily functioning. For example, two youths may report similar intensity of pain on a 10 point scale; however, one youth may limit social and physical involvement while the other continues to engage in daily activities with minimal impairment. Therefore, it is important to have a pain
Surgimiento, formación y persistencia de organizaciones sociales para la pesca ribere?a de la península de Baja California
Ramírez Sánchez, Saudiel;McCay, Bonnie J.;Johnson, Teresa R.;Weisman, Wendy;
Región y sociedad , 2011,
Abstract: in this preliminary study we use a comparative approach to explore the conditions under which small-scale fishing organizations (sfos) emerged and have persisted in the baja california peninsula.there are more than 300 sfos operating in the peninsula, including fishing cooperatives (222), sociedades de producción rural [rural production companies] (73), sociedades de solidaridad social [social solidarity companies] (5) and fishing unions (1). fishers' rationale, institutional incentives, and external agents are important factors in the creation of sfos, and their survival seems to be affected by organizational incentives, investment in physical and human capital, and social solidarity as a guiding principle of their activities. our assessment is a first step towards understanding the conditions under which sfos have participated and could participate in improving the management and conservation of fisheries in the region.
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