327 research outputs found

    An Organocobalt–Carbon Nanotube Chemiresistive Carbon Monoxide Detector

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    A chemiresistive detector for carbon monoxide was created from single-walled carbon nanotubes (SWCNTs) by noncovalent modification with diiodo(η⁔:ηÂč-1-[2-(N,N-dimethylamino)ethyl]-2,3,4,5-tetramethylcyclopentadienyl)-cobalt(III) ([Cp[superscript ∧]CoI₂]), an organocobalt complex with an intramolecular amino ligand coordinated to the metal center that is displaced upon CO binding. The unbound amino group can subsequently be transduced chemiresistively by the SWCNT network. The resulting device was shown to have a ppm-level limit of detection and unprecedented selectivity for CO gas among CNT-based chemiresistors. This work, the first molecular-level mechanistic elucidation for a CNT-based chemiresistive detector for CO, demonstrates the efficacy of using an analyte’s reactivity to produce another chemical moiety that is readily transduced as a strategy for the rational design of chemiresistive CNT-based detectors.National Science Foundation (U.S.) (DMR-1410718)National Science Foundation (U.S.) (1122374

    Distinct Interfacial Fluorescence in Oil-in-Water Emulsions via Exciton Migration of Conjugated Polymers

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    Commercial dyes are extensively utilized to stain specific phases for the visualization applications in emulsions and bioimaging. In general, dyes emit only one specific fluorescence signal and thus, in order to stain various phases and/or interfaces, one needs to incorporate multiple dyes and carefully consider their compatibility to avoid undesirable interactions with each other and with the components in the system. Herein, surfactant‐type, perylene‐endcapped fluorescent conjugated polymers that exhibit two different emissions are reported, which are cyan in water and red at oil–water interfaces. The interfacially distinct red emission results from enhanced exciton migration from the higher‐bandgap polymer backbone to the lower‐bandgap perylene endgroup. The confocal microscopy images exhibit the localized red emission exclusively from the circumference of oil droplets. This exciton migration and dual fluorescence of the polymers in different physical environments can provide a new concept of visualization methods in many amphiphilic colloidal systems and bioimaging.National Science Foundation (U.S.) (National Science Foundation DMR‐1410718

    Retail Clerks International Protective Association, Local 872 (1936)

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    Improved methods for quickly identifying neutral organic compounds and differentiation of analytes with similar chemical structures are widely needed. We report a new approach to effectively “fingerprint” neutral organic molecules by using <sup>19</sup>F NMR and molecular containers. The encapsulation of analytes induces characteristic up- or downfield shifts of <sup>19</sup>F resonances that can be used as multidimensional parameters to fingerprint each analyte. The strategy can be achieved either with an array of fluorinated receptors or by incorporating multiple nonequivalent fluorine atoms in a single receptor. Spatial proximity of the analyte to the <sup>19</sup>F is important to induce the most pronounced NMR shifts and is crucial in the differentiation of analytes with similar structures. This new scheme allows for the precise and simultaneous identification of multiple analytes in a complex mixture

    High internal free volume compositions for low-k dielectric and other applications

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    The present invention provides materials, devices, and methods involving new heterocyclic, shape-persistent monomeric units with internal free volume. In some cases, materials the present invention may comprise monomers, oligomers, or polymers that incorporate a heterocyclic, shape-persistent iptycene. The present invention may provide materials having low dielectric constants and improved stability at high operating temperatures due to the electron-poor character of materials. In addition, compositions of the invention may be easily synthesized and readily modified to suit a particular application

    Detection of Per- and Polyfluoroalkyl Substances (PFAS) by Interrupted Energy Transfer

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    The ubiquitous presence of per‐ and polyfluoroalkyl substances (PFAS) in aqueous environments has aroused societal concern. Nonetheless, effective sensing technologies for continuous monitoring of PFAS within water distribution infrastructures currently do not exist. Herein, we describe a ratiometric sensing approach to selectively detect aqueous perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) at concentrations of ÎŒg ⋅ L−1. Our method relies on the excitonic transport in a highly fluorinated poly(p‐phenylene ethynylene) to amplify a ratiometric emission signal modulated by an embedded fluorinated squaraine dye. The electronic coupling between the polymer and dye occurs through overlap of π‐orbitals and is designed such that energy transfer is dominated by an electron‐exchange (Dexter) mechanism. Exposure to aqueous solutions of PFAS perturbs the orbital interactions between the squaraine dye and the polymer backbone, thereby diminishing the efficiency of the energy transfer and producing a “polymer‐ON/dye‐OFF” response. These polymer/dye combinations were evaluated in spin‐coated films and polymer nanoparticles and were able to selectively detect PFAS at concentrations of ca. 150 ppb and ca. 50 ppb, respectively. Both polymer films and nanoparticles are not affected by the type of water, and similar responses to PFAS were found in milliQ and well water

    Single-Walled Carbon Nanotube–Metalloporphyrin Chemiresistive Gas Sensor Arrays for Volatile Organic Compounds

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    National Science Foundation (U.S.) (DMR-1410718)National Science Foundation (U.S.) (1122374)Massachusetts Institute of Technology. Institute for Soldier NanotechnologiesUnited States. Defense Advanced Research Projects Agency (W911NF-14-1-0087

    Dinuclear pincer-palladium(II) complexes and their use as homogeneous or heterogeneous catalyst for the aldol reaction of methyl isocyanoacetate

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    Abstract Two new bimetallic Pd complexes have been synthesized and characterized and their catalytic activity checked for the aldol reaction of aldehydes with methyl isocyanoacetate. Each palladium atom is coordinated to an SCS-type ligand and the two pincer units are linked by a chiral spacer. The catalytic aldol reaction of methyl isocyanoacetate with aldehydes proceeds quickly but no significant diastereoselectivity and enantioselectivity is found. The comparison with a homologous mononuclear Pd complex shows no differences with the bimetallic compounds, concluding that there is no cooperativity between the metal centers. Two silica-supported catalysts prepared with a bimetallic compound show catalytic activity with very minor enantioselectivity

    Chemiresistive Sensor Arrays from Conductive 2D Metal–Organic Frameworks

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    Applications of porous metal–organic frameworks (MOFs) in electronic devices are rare, owing in large part to a lack of MOFs that display electrical conductivity. Here, we describe the use of conductive two-dimensional (2D) MOFs as a new class of materials for chemiresistive sensing of volatile organic compounds (VOCs). We demonstrate that a family of structurally analogous 2D MOFs can be used to construct a cross-reactive sensor array that allows for clear discrimination between different categories of VOCs. Experimental data show that multiple sensing mechanisms are operative with high degrees of orthogonality, establishing that the 2D MOFs used here are mechanistically unique and offer advantages relative to other known chemiresistor materials.Camille & Henry Dreyfus Foundation. Postdoctoral Program in Environmental ChemistrAlfred P. Sloan FoundationResearch Corporation for Science Advancement3M CompanyNational Science Foundation (U.S.). Graduate Research Fellowship Program (Grant 1122374)Massachusetts Institute of Technology. Institute for Soldier Nanotechnologie

    Redox Switchable Thianthrene Cavitands

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    A redox activated vase-to-kite conformational change is reported for a new resorcinarene-based cavitand appended with four quinoxaline-fused thianthrene units. In its neutral state, the thianthrene-containing cavitand was shown by 1H NMR to adopt a closed vase conformation. Upon oxidation the electrostatic repulsion among the thianthrene radical cations promotes a kite conformation in the thianthrene-containing cavitand. The addition of acid produced a shoulder feature below 300 nm in the cavitand’s UV-Vis spectrum that we have assigned to the vase-to-kite conformation change. UV-Vis spectroelectrochemical studies of the cavitand revealed a development of a similar shoulder peak consistent with the oxidation-induced vase-to-kite conformation change. To support that the shoulder peak is diagnostic for a vase-to-kite conformation change, a model molecule constituting a single quinoxaline wall of the cavitand was synthesized and studied. As expected UV-Vis spectroelectrochemical studies of the cavitand arm did not display a shoulder peak below 300 nm. The oxidation-induced vase-to-kite conformation is further confirmed by the distinctive upfield shift in 1H chemical shift of the methine signal. Key words: redox active, thianthrene, resorcinarene cavitands, electrochemical switching, conformation changeNational Science Foundation (U.S.). Center for Energy Efficient Electronics Science (Award ECCS0939514)Singapore. Agency for Science, Technology and Research (Graduate Scholarship

    Colorimetric Stimuli-Responsive Hydrogel Polymers for the Detection of Nerve Agent Surrogates

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    The threat of chemical warfare agents (CWAs) necessitates the development of functional materials that not only quickly detect the presence of CWAs but also actively protect against their toxicity. We have synthesized responsive units that exhibit colorimetric responses upon exposure to CWAs and incorporated them into a versatile detection platform based on copolymers prepared by ring-opening metathesis polymerization (ROMP). The theoretical detection limits for CWA simulants in solution for these polymers are as low as 1 ppm. By incorporating hydrogel-promoting units as pendant chains, we are able to obtain polymers that instantly respond to CWA vapors and are easy to regenerate to the deactivated state by simple treatment with ammonium hydroxide vapor. We further demonstrate a collapse of the polymer gels in response to trifluoroacetic acid (TFA), a strong acid that produces a more fully ionized state as a result of its more caustic nature.United States. Defense Threat Reduction Agency. Chemical and Biological Technologies Department (Grant BA12PHM123
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