in ac research
In AC Research contains brief introductions to the research articles appearing in the March 1 issue. A free updated table of contents is available on the Web (http://pubs.acs.org/ac).
Concentration jumps, prime rate constant. Determining rate constants in the 0.1- to 100-s–1 range for the binding of Mg2+ to 8-hydroxyquinoline and other biochemical systems is now more accurate with an improved concentration jump experiment. E. Grell and co-workers at the MaxPlanck-Institut für Biophysik, the Technische Universität Darmstadt (both in Germany), the Université Louis Pasteur, Bio-Logic–Science Instrument S.A., and the Commissariat à l’énergie atomique (all in France) use step-motor-driven mixing and achieve large 60- to 300-fold dilutions for precise kinetic studies. (“Concentration Jump Experiments for the Precise Determination of Rate Constants of Reverse Reactions in the Millisecond Time Range”; 10.1021/ac0007229; p 857) Interferon immunosensor. W. P. van Bennekom and colleagues at Utrecht University (The Netherlands) describe an electrochemical immunosensor for direct detection of the 15.5-kDa protein interferon-␥ at attomolar concentrations. The immunosensor can be regenerated by a sequence of potential pulses that remove the self-assembled monolayer, the MD-2, and the interferon. The response of the immunosensor is reproducible within 10%. (“Development of an Electrochemical Immunosensor for Direct Detection of Interferon-␥ at the Attomolar Level”; 10.1021/ac001051h; p 901) Detecting oxidative stress. An optical method for indirectly detecting oxidative stress based on the accumulation of an iron-regulatory protein, IRP1, is described by F. Lisdat and colleagues at the University of Potsdam, the Research Institute of Molecular Pharmacology, and the German Institute of Nutrition (all in Germany). The method takes advantage of the binding between IRP1 and an iron-responsive element, which has been immobilized on a surface plasmon resonance chip. Reproducible detection of 20- to 200-nM IRP1 is reported. (“An Optical Method for the Detection of Oxidative Stress Using Protein–RNA Interaction”; 10.1021/ac000786j; p 957) ICATS MALDI QqTOF. Timothy J. Griffin and colleagues at the University of Washington, MDA Sciex, and the University of Manitoba (both in Canada) quantitatively analyze complex protein mixtures with cysteinyl isotope-coded
affinity tags (ICATS) of chemically identical d(0) and d(8) reagents, which act as internal standards for each other. After enzymatic digestion and separation by multidimensional chromatography, MALDI-QqTOF analyzes the sample quantitatively by an initial MS scan followed by tandem MS to determine the sequence of expressed peptides for identification. (“Quantitative Proteomic Analysis Using a MALDI Quadrupole Time-of-Flight Mass Spectrometer”; 10.1021/ac001169y; p 978)
NIR sorts combinatorial/primordial soup. Chieu D. Tran and Troy Alexander of Marquette University present a near-IR spectrometry/partial least squares method for distinguishing amino acid residues in combinatorially produced dipeptides and tripeptides. This noninvasive technique also distinguishes between similar peptides with the same amino acid residues when they differ only in sequence. (“Near-Infrared Spectrometric Determination of Di- and Tripeptides Synthesized by a Combinatorial Solid-Phase Method”; 10.1021/ac0010274; p 1062)
Electron hopwatch. Marcin Majda and co-workers at the University of California–Berkeley develop a two-dimensional, electrochemical time-of-flight method to determine lateral mobility of amphiphiles and electron hopping in Langmuir monolayers. Diffusion constants are measured without knowing the concentration of the diffusing species, a new advantage in the study of biomembranes. (“2D Electrochemical Time of Flight and Its Application in the Measurements of the Kinetics of Lateral Electron Hopping in Monolayer Films at the Air/Water Interface”; 10.1021/ac001015i; p 870) Valuable tips for STM. Yoshio Umezawa and co-workers at the University of Tokyo and the Science University of Tokyo (both in Japan) modify scanning tunneling microscopy (STM) gold tips with 4-mercaptopyridine, which interacts with zinc(II) 5,15–bis(4-octadecyloxyphenyl)porphyrin (PorZn) sample centers. In the enhanced imaging that results, Por-Zn can be differentiated from the zinc-free Por-2H and Por-Ni. (“Scanning Tunneling Microscopy with Chemically Modified Tips: Discrimination of Porphyrin Centers Based M A R C H 1 , 2 0 0 1 / A N A LY T I C A L C H E M I S T R Y
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on Metal Coordination and Hydrogen Bond Interactions”; 10.1021/ac001056e; p 878)
Checking for simulated nerve agents. Nathan S. Lewis and Alan R. Hopkins at the California Institute of Technology evaluate arrays of conducting polymer composite vapor detectors for detecting simulated nerve agents. They obtain detection limits that are lower than the concentration of real nerve agents needed to induce severe effects. The arrays could easily resolve the signatures of the simulants from each other and other test analytes. (“Detection and Classification Characteristics of Arrays of Carbon Black/Organic Polymer Composite Chemiresistive Vapor Detectors for the Nerve Agent Simulants Dimethylmethylphosphonate and Diisopropylmethylphosphonate”; 10.1021/ac0008439; p 884) Pyrolysis leaves electrodes flat. Richard L. McCreery and Srikanth Ranganathan of Ohio State University develop a process to pyrolyze a commercially available photoresist after spin-coating it onto a silicon wafer, creating an extremely flat (~0.5-nm rms) surface. Improvements over conventional glassy carbon include low capacitance; weak adsorption properties; and eliminating the polishing step, which permits mass fabrication into unusual shapes. (“Electroanalytical Performance of Carbon Films with Near-Atomic Flatness”; 10.1021/ac0007534; p 893) Diamond electrodes. Greg M. Swain and colleagues at Michigan State University, Silesian Technical University (Poland), and the Naval Research Laboratory report on an electrically conductive, optically transparent, diamond thinfilm electrode. The electrode has a short wavelength cutoff of ~225 nm, which is the indirect band gap of the material, and transmits light out to at least 1000 nm. In theory, the electrode has an optical window from 225 nm well out into the far-IR. (“Diamond Optically Transparent Electrodes: Demonstration of Concept with Ferri/Ferrocyanide and Methyl Viologen”; 10.1021/ac001257i; p 908) Carbon nanotubes draw attention. Nanqiang Li and co-workers at Peking University (Peoples’s Republic of China) functionalize a film of single-wall carbon nanotubes on a glassy carbon electrode, with carboxylic acid groups resulting in more stable cyclic voltammetric behavior and a larger surface area and charge current. Good electrocatalytic behavior is reported toward the oxidation of dopamine, epinephrine, and ascorbic acid. (“Investigation of the Electrochemical and Electrocatalytic Behavior of Single-Wall Carbon Nanotube Film on a Glassy Carbon Electrode”; 10.1021/ ac000967l; p 915) Charged monolayer-protected clusters. Royce W. Murray and Deon T. Miles at the University of North Carolina prepare monolayer-protected gold clusters (MPCs) with mixed monolayers of alkanethiolates and alkanethiolates terminally -functionalized with phenothiazine. The mixed 120 A
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MPCs can contain as many as 10 phenothiazines/MPC; these electron donors are electroactive in rapid, successive 1-electron reactions. (“Redox and Double-Layer Charging of Phenothiazine Functionalized Monolayer-Protected Clusters; 10.1021/ac0012647; p 921)
Positionable microcell for SECM. John E. Baur and Thomas W. Spaine at Illinois State University construct positionable voltammetric cells with tip diameters of 400,000 theoretical plates/m are achieved. (“Sol–Gel Open Tubular ODS Columns with Reversed Electroosmotic Flow for Capillary Electrochromatography”; 10.1021/ac000817a; p 987) SPME-MS checks dairy air. Christophe Pérès and coworkers at the INRA de Theix (France) couple solid-phase microextraction (SPME) with MS for a mass spectra fingerprint of Camembert-type cheeses at different stages of ripeness. Sampling the volatile compounds by the static headspace method minimizes modifications of the matrix and reduces analytical artifacts. (“Solid-Phase Microextraction-Mass Spec-
trometry: A New Approach to the Rapid Characterization of Cheeses”; 10.1021/ac001146j; p 1030)
Transfer system brings better FRET. Fluorescence resonance energy-transfer (FRET) peptide probes in aqueous solutions exhibit quenched fluorophore emissions because donor and acceptor moieties are in close contact. Tetsuo Nagano and co-workers at the University of Tokyo (Japan) add -cyclodextrin, which separates the fluorophores by forming a complex with coumarin, increasing fluorescence intensity and its value as a tool in ratio imaging and highthroughput screening of combinatorial libraries. (“Intramolecular Fluorescence Resonance Energy Transfer System with Coumarin Donor Included in –Cyclodextrin”; 10.1021/ ac001016a; p 939) Partnering optical fibers. Peter Geissinger and colleagues at the University of Wisconsin–Milwaukee solve a spatial resolution problem for fluorescent chemosensors. With a second fiber providing evanescent excitation of the fluorophores and acting as an optical delay line, they obtain minimum spacing between adjacent sensor regions, which is well below the fluorescence lifetime limit. (“Readout Scheme Providing High Spatial Resolution for Distributed Fluorescent Sensors on Optical Fibers”; 10.1021/ac0011437; p 1007) Optical measurement of amines. Because the detection of neutral analytes is a challenge for chemical sensors, Gerhard Mohr and colleagues at ETH Technopark (Switzerland) and the University of Regensburg (Germany) have developed an optical sensor for the job. Neutral analytes interact with a fluorescent indicator dye, which changes the dye’s luminescence intensity. Phosphorescent beads serve as internal controls to ensure that the variations are not due to fluctuations in the light source or background. (“Fluoro Reactands and Dual Luminophore Referencing: A Technique To Optically Measure Amines”; 10.1021/ac000945z; p 1053) Novel PIV micro-markers. Novel fluorescent liposome markers for particle-image velocimetry (PIV) are synthesized by Anup K. Singh and colleagues at Sandia National Laboratories. These markers are an advance over latex particles because they are designed with a fluorescent dye encapsulated in the aqueous core and a fluorophore-labeled lipid contained in the bilayer that maximizes fluorescence. In addition, the markers have a net negative charge that imparts hydrophilicity and minimizes aggregation and interaction with negatively charged glass microchannels, and they can be easily removed by breaking them into monomers with organic solvents or into micelles by detergents. (“Fluorescent Liposome Flow Markers for Microscale Particle-Image Velocimetry”; 10.1021/ac001159x; p 1057)
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