Toward Complete Resolution of DNA/Carbon Nanotube Hybrids by Aqueous Two-Phase Systems.

Sequence-dependent interactions between DNA and single-wall carbon nanotubes (SWCNTs) are shown to provide resolution for the atomic-structure-based sorting of DNA-wrapped SWCNTs. Previous studies have demonstrated that aqueous two-phase (ATP) systems are very effective for sorting DNA-wrapped SWCNTs (DNA-SWCNTs). However, most separations have been carried out with a polyethylene glycol (PEG)/polyacrylamide (PAM) ATP system, which shows severe interfacial trapping for many DNA-SWCNT dispersions, resulting in significant material loss and limiting multistage extraction. Here, we report a study of several new ATP systems for sorting DNA-SWCNTs. We have developed a convenient method to explore these systems without knowledge of the corresponding phase diagram. We further show that the molecular weight of the polymer strongly affects the partition behavior and separation results for DNA-SWCNTs in PEG/dextran (DX) ATP systems. This leads to the identification of the PEG1.5kDa/DX250kDa ATP system as an effective vehicle for the chirality separation of DNA-SWCNTs. Additionally, this ATP system exhibits greatly reduced interfacial trapping, enabling for the first time continuous multistep sorting of four species of SWCNTs from a single dispersion. Enhanced stability of DNA-SWCNTs in the PEG1.5kDa/DX250kDa ATP system also allows us to investigate pH dependent sorting of SWCNTs wrapped by C-rich sequences. Our observations suggest that hydrogen bonding may form between the DNA bases at lower pH, enabling a more ordered wrapping structure on the SWCNTs and improvement in sorting (11,0). Together, these findings reveal that the new ATP system is suitable for searching DNA sequences leading toward more complete resolution of DNA-SWCNTs. A new concept of "resolving sequences", evolved from the old notion of "recognition sequences", is proposed to describe a broader range of behaviors of DNA/SWCNT interactions and sorting.

Probing Trions at Chemically Tailored Trapping Defects.

Trions, charged excitons that are reminiscent of hydrogen and positronium ions, have been intensively studied for energy harvesting, light-emitting diodes, lasing, and quantum computing applications because of their inherent connection with electron spin and dark excitons. However, these quasi-particles are typically present as a minority species at room temperature making it difficult for quantitative experimental measurements. Here, we show that by chemically engineering the well depth of sp3 quantum defects through a series of alkyl functional groups covalently attached to semiconducting carbon nanotube hosts, trions can be efficiently generated and localized at the trapping chemical defects. The exciton-electron binding energy of the trapped trion approaches 119 meV, which more than doubles that of "free" trions in the same host material (54 meV) and other nanoscale systems (2-45 meV). Magnetoluminescence spectroscopy suggests the absence of dark states in the energetic vicinity of trapped trions. Unexpectedly, the trapped trions are approximately 7.3-fold brighter than the brightest previously reported and 16 times as bright as native nanotube excitons, with a photoluminescence lifetime that is more than 100 times larger than that of free trions. These intriguing observations are understood by an efficient conversion of dark excitons to bright trions at the defect sites. This work makes trions synthetically accessible and uncovers the rich photophysics of these tricarrier quasi-particles, which may find broad implications in bioimaging, chemical sensing, energy harvesting, and light emitting in the short-wave infrared.

Optically Triggered Melting of DNA on Individual Semiconducting Carbon Nanotubes.

Optical excitation of nanostructures is known to induce local heating, a phenomenon that has been intensely exploited for drug release, gene delivery, cancer thermotherapy, and energy harvesting. However, the effect is typically small requiring collective heating of a large concentration or aggregates of particles. Herein, we show that optical excitation of individual semiconducting single-walled carbon nanotubes triggers strongly localized heating adequate to melt non-covalently attached double-stranded oligonucleotides in solution. In contrast to conventional thermal dehybridization, this optically triggered DNA melting occurs at a solution temperature that is 22 °C lower than the DNA melting temperature. This unexpectedly large localized optical heating effect provides important new insights to design selective optical nanoheaters at the single particle level.

Capillary electrophoresis of covalently functionalized single-chirality carbon nanotubes.

We demonstrate the separation of chirality-enriched single-walled carbon nanotubes (SWCNTs) by degree of surface functionalization using high-performance CE. Controlled amounts of negatively charged and positively charged functional groups were attached to the sidewall of chirality-enriched SWCNTs through covalent functionalization using 4-carboxybenzenediazonium tetrafluoroborate or 4-diazo-N,N-diethylaniline tetrafluoroborate, respectively. Surfactant- and pH-dependent studies confirmed that under conditions that minimized ionic screening effects, separation of these functionalized SWCNTs was strongly dependent on the surface charge density introduced through covalent surface chemistry. For both heterogeneous mixtures and single-chirality-enriched samples, covalently functionalized SWCNTs showed substantially increased peak width in electropherogram spectra compared to nonfunctionalized SWCNTs, which can be attributed to a distribution of surface charges along the functionalized nanotubes. Successful separation of functionalized single-chirality SWCNTs by functional density was confirmed with UV-Vis-NIR absorption and Raman scattering spectroscopies of fraction collected samples. These results suggest a high degree of structural heterogeneity in covalently functionalized SWCNTs, even for chirality-enriched samples, and show the feasibility of applying CE for high-performance separation of nanomaterials based on differences in surface functional density.

Molecularly Tunable Fluorescent Quantum Defects.

We describe the chemical creation of molecularly tunable fluorescent quantum defects in semiconducting carbon nanotubes through covalently bonded surface functional groups that are themselves nonemitting. By variation of the surface functional groups, the same carbon nanotube crystal is chemically converted to create more than 30 distinct fluorescent nanostructures with unique near-infrared photoluminescence that is molecularly specific, systematically tunable, and significantly brighter than that of the parent semiconductor. This novel exciton-tailoring chemistry readily occurs in aqueous solution and creates functional defects on the sp(2) carbon lattice with highly predictable C-C bonding from virtually any iodine-containing hydrocarbon precursor. Our new ability to control nanostructure excitons through a single surface functional group opens up exciting possibilities for postsynthesis chemical engineering of carbon nanomaterials and suggests that the rational design and creation of a large variety of molecularly tunable quantum emitters-for applications ranging from in vivo bioimaging and chemical sensing to room-temperature single-photon sources-can now be anticipated.

Ammonium Laurate Surfactant for Cleaner Deposition of Carbon Nanotubes.

Experiments probing the properties of individual carbon nanotubes (CNTs) and those measuring bulk composites show vastly different results. One major issue limiting the results is that the procedures required to separate and test CNTs introduce contamination that changes the properties of the CNT. These contamination residues often come from the resist used in lithographic processing and the surfactant used to suspend and deposit the CNTs, commonly sodium dodecyl sulfate (SDS). Here we present ammonium laurate (AL), a surfactant that has previously not been used for this application, which differs from SDS only by substitution of ionic constituents but shows vastly cleaner depositions. In addition, we show that compared to SDS, AL-suspended CNTs have greater shelf stability and more selective dispersion. These results are verified using transmission electron microscopy, atomic force microscopy, ζ-potential measurements, and Raman and absorption optical spectroscopy. This surfactant is simple to prepare, and the nanotube solutions require minimal sonication and centrifugation in order to outperform SDS.

Pulmonary valve papillary fibroelastoma: a rare tumor and rare location.

Although rare, papillary fibroelastomas (PFEs) are considered the third most common cause of primary cardiac neoplasm. They are usually asymptomatic and are found during routine echocardiography. PFEs of the pulmonary valve are extremely rare. They are usually benign; however, because of their potential to embolize to the pulmonary circulation, it is often recommended that they be removed. This article reviews a case of an asymptomatic PFE of the pulmonary valve, incidentally found in a patient who presented with three-vessel coronary artery disease.

Brightening of carbon nanotube photoluminescence through the incorporation of sp3 defects.

Semiconducting carbon nanotubes promise a broad range of potential applications in optoelectronics and imaging, but their photon-conversion efficiency is relatively low. Quantum theory suggests that nanotube photoluminescence is intrinsically inefficient because of low-lying 'dark' exciton states. Here we demonstrate the significant brightening of nanotube photoluminescence (up to 28-fold) through the creation of an optically allowed defect state that resides below the predicted energy level of the dark excitons. Emission from this new state generates a photoluminescence peak that is red-shifted by as much as 254 meV from the nanotube's original excitonic transition. We also found that the attachment of electron-withdrawing substituents to carbon nanotubes systematically drives this defect state further down the energy ladder. Our experiments show that the material's photoluminescence quantum yield increases exponentially as a function of the shifted emission energy. This work lays the foundation for chemical control of defect quantum states in low-dimensional carbon materials.

Woman with chest pain and shortness of breath since age 18.

We present an interesting case of a 38-year-old woman who presented with a history of left-sided chest pain, dyspnoea and palpitations. The symptoms have been occurring since age 18, and were previously diagnosed as costochondritis. Because of the suspicious history, the patient underwent further cardiac investigations. The echocardiogram demonstrated an abnormal structure adjacent to the aortic valve. A diagnostic coronary angiogram revealed a large left circumflex artery fistulating into the right atrium. Surgery was performed to ligate the fistula. The patient recovered well and has been asymptomatic since.

Fever of unknown origin, with a twist.

The authors present a case of an 81-year-old man with fever of unknown origin. The case report is illustrated with the images which clarified the diagnosis in this challenging case. The cardiac MR images were of critical importance in arriving at a diagnosis of aortic root mycotic pseudoaneurysm with rupture into the right ventricle.

Acyclic cucurbit[n]uril molecular containers selectively solubilize single-walled carbon nanotubes in water.

Making single-walled carbon nanotubes (SWNTs) soluble in water is a challenging first step to use their remarkable electronic and optical properties in a variety of applications. We report that acyclic cucurbit[n]uril molecular containers 1 and 2 selectively solubilize small-diameter and low chiral angle SWNTs. The selectivity is tunable by increasing the concentration of the molecular containers or by adjusting the ionic strength of the solution. Even at a concentration 1000 times lower than typically required for surfactants, the molecular containers render SWNTs soluble in water. Molecular mechanics simulations suggest that these C-shaped acyclic molecules complex the SWNTs such that a large portion of nanotube sidewalls are exposed to the external environment. These "naked" nanotubes fluoresce upon patching the exposed surface with sodium dodecylbenzene sulfonate.

Kinetics of association and dissociation of HIV-1 reverse transcriptase subunits.

The biologically active form of HIV-1 reverse transcriptase (RT) is the p66/p51 heterodimer. The process of maturation of the heterodimer from precursor proteins is poorly understood. Previous studies indicated that association of p66 and p51 is very slow. Three techniques, a pre-steady-state activity assay, intrinsic tryptophan fluorescence, and a FRET assay, were used to monitor the dimerization kinetics of RT. Kinetic experiments were conducted with purified p66 and p51 proteins in aqueous buffer. All three techniques gave essentially the same results. The dissociation kinetics of p66/p51 were first-order with rate constants (k(diss)) of approximately 4 x 10(-6) s(-1) (t(1/2) = 48 h). The association kinetics of p66 and p51 were concentration-dependent with second-order rate constants (k(ass)) of approximately 1.7 M(-1) s(-1) for the simple bimolecular association reaction. The implications of slow dimerization of p66/p51 for the maturation process are discussed. A reaction-controlled model invoking conformational selection is proposed to explain the slow protein-protein association kinetics.

A sequence motif in the simian virus 40 (SV40) early core promoter affects alternative splicing of transcribed mRNA.

To identify new sequence elements in the promoter that affect splicing patterns of pre-mRNAs, we analyzed effects of different promoters on alternative splicing of model reporter genes. We compared the E1a alternative splicing pattern in transcripts expressed from the full-length cytomegalovirus, SV40 early, or a hybrid cytomegalovirus/SV40 early promoter and found that the hybrid promoter improved selection of the suboptimal E1a 5'SS-1. Expressing RNA from the hybrid promoter also enhanced selection of suboptimal splice sites in other alternatively spliced reporter genes, demonstrating the generality of this effect. Unlike previously defined promoter elements shown to affect alternative splicing, which were located in the enhancer/upstream activating sequences, the motif identified in this work is positioned within the core promoter; it is comprised of eight T-residues directly upstream of the SV40 early TATA box. This motif was previously implicated in DNA bending and negative regulation of transcription. Together, these results suggest that the identity of transcription complex assembled in the core promoter-dependent fashion can affect splice site selection during pre-mRNA splicing, perhaps by influencing the processivity of transcription elongation.

Detection of aspirin resistance by PFA-100: prevalence and aspirin compliance in patients with chronic stable angina.

Most acute coronary syndromes result from a platelet-rich occlusion of the coronary arteries. Antiplatelet drugs are of proven efficacy in preventing myocardial infarction, unstable angina, and stroke. However, not all patients on aspirin (ASA) benefit. We studied the phenomenon of aspirin resistance with a simple and reliable platelet function analyzer--the PFA-100. Studying 31 patients with unstable angina and 105 controls, we found aspirin resistance in 42% of patients, most of whom were shown to be compliant utilizing concomitant salicylate levels.