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submicrometer-sized pipets. For simple ion-transfer reactions, such as the Electrochemical techniques for probing transfer of acetylcholine mass and charge transport in various me(ACh+) from water to 1,2dia and studying the kinetics of multistep dichloroethane (DCE), the reactions have traditionally relied on the generator pipet contains oxidation or reduction of the analyte of inthe cation, which is transterest at one electrode (the "generator") ferred to the outer DCE and the detection of the product by a secsolution when the pipet is ond electrode (the "collector"). Michael V. biased at a sufficiently posi- Ionic reactions studied with a dual-pipet device. Mirkin and co-workers at Queens College- tive potential. The collector CUNY describe a new generation/collecpipet is negatively biased so that a signifiHowever, collection efficiency drops when tion approach for monitoring homogecant fraction of the ejected cations are a species in the outer organic solution acts transferred back to the neous ionic reactions that do not involve phase. as a ligand and forms a complex with the oxidation/reduction steps. The collection efficiency depends on ejected cation. The technique is based on a device con- the potential of the collector electrode (E) In some cases, the complexed cation is sisting of two water-filled, micrometer- or and is optimized to about 40% at Ec=0.1 V. transferred to the collector along with the free cation; however, in other cases, only the free cation is transferred. Ion binding to high molecular weight DNA is one exT N T in land mines amide substituents, which exhibit elecample in which only the free cation is transtron-withdrawing behavior. Conse2,4,6-Trinitrotoluene (TNT) and 2,4-diniferred to the aqueous phase. On the other trotoluene (DNT) are the primary constit- quently, the polymer exhibits smaller hand, in the facilitated transfer of K+ from fluorescence response to oxidative uents of the estimated 120 million unexwater to DCE by dibenzo-18-crown-6 quenchers than polymer 1. ploded land mines throughout the world. (DB18C6), both the complexed product Methods for the direct Structural (cavity[K+DB18C6] and the free K+ are transdetection of TNT and size) effects are studied ferred back to the aqueous phase. The DNT, such as neutron by comparing polymer two simultaneous charge-transfer proactivation, electron 1 with polymer 3, which readily identified because their capture detection, and has similar electronic transfer waives occur at different potenion mobility spectromeproperties to polymer 1 tials The dual-pipet device allows direct try, offer fewer false but a different thin-film observation of the transfer of IC+ from Weipositives than metal morphology because of ter to DCE which has not previously detectors for locating an extension of the penbeen reported The technique offers a buried land mines. tiptycene groups. The method for monitoring numerous other However, a simple, average cavity size of reactions occurring at high positive or low-cost, real-time TNT polymer 3 is smaller negative potentials (J Am Chem Soc chemosensor is needed than that of polymer 1. 1998 120 12700-01) to complement these Larger cavities enhance methods. Timothy M. the diffusion of analytes Swager and Jye-Shane Chemical structures of phenylene- through the films thus As the protein unfolds ethynylene polymers 1-4. Yang of the Massachufavoring TNT binding Previous investigations of electrospray ionsetts Institute of Technology investiPolymer 4 is the most electron-rich of the ization (ESI) MS in the positive ion mode gated the use offluorescentpolymer four polymers; however it shows the least have indicated that the charge state distribufilms as TNT/DNT chernosensors. The fluorescence quenching toward dNT and tion can be used to characterize the tertiary electronic and structural properties of other electron-deficient analvtes because structure (but not the secondary structure) four pentiptycene-derived phenyleneitsplanarstructure lacks porosity of proteins in solution. Some studies in the ethynylene polymers studied and Overall, films of polymers 1-3 are negative ion mode have also shown that untheir effects onfluorescencequenching highly selective toward nitro-aromatic folded protein states lead to higher charge with a variety of analytes including nitro- compounds; however, selectivity depends states than the more compact native state. aromatic compounds and quinones are on film thickness. The thinner the film, D. J. Douglas and Lars Konermann of reported the larger its response to nitro-aromatic the University of British Columbia (Canacompounds and the smaller its response Electron-rich polymer 1 responds da) investigated whether the positive and to quinones. The authors explain the difwithin seconds to TNT/DNT vapors. negative ion modes are equivalent methods The authors attribute the rapid response ferences in terms of polymer-analyte inof monitoring protein conformational teractions, which are believed to be electo electrostatic interactions between the states, particularly during protein unfoldpolymer and the electron-deficient TNT/ trostatic. (J. Am. Chem. Soc. 1998, 120, ing. They studied different types of unfold11864-73) DNT molecules. Polymer 2 contains ing in three proteins the unfolding of lysozyme by the reduction of its disulfide
Dual-pipet generation/collection
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Analytical Chemistry News & Features, February 1, 1999
