News
Understanding Alzheimer'sdiseased tissue The deposition of amyloid deposits in the brain and in the walls of cerebral blood vessels is a major pathological hallmark of Alzheimer's disease. Clinical diagnosis of the disease involves postmortem examination of brain tissue by using one of two histological dyes, Congo Red and thioflavin S, to identify the characteristic plaque. Aiming to further define the macromolecular structure and behavior of the primary component of the amyloid plaque called B-amyloid peptide S. Turrell and colleagues at the Universite des Sciences et Technologies de Lille (France) have investigated the reactive site and chemical mechanism of Congo Red and thioflavin S which bind to amyloid proteins
Collapsing bubbles influence voltammograms The expansion and violent collapse of microbubbles in a reaction solution exposed to power ultrasound (sonication) is called cavitation. The violent collapse of the bubbles produces an increase in temperature of up to several thousand Kelvin and an increase in pressure of several hundred atmospheres. Sonication significantly modifies voltammograms recorded at a stationary electrode, which results in sigmoidally shaped voltammograms with a limiting current that is the sum of steadystate and transient components. Chantal Degrand and colleagues at the Universite Blaise Pascal (France) have shown that the increase of the voltammetric current at a stationary electrode is caused by individual current pulses originating in
They performed optical microscopic and microspectrofluorometric analyses on several series of stained, diseased brain sections. Microspectrofluorometry showed mat the fluorescence maxima of Congo Red and thioflavin S bound to diseased tissue can be distinguished from those of the free dyes. They also found that both dyes compete for the same interacting site on diseased cerebral tissue and that the limiting concentration for staining efficiency of the two dyes is determined by the concentration at which aggregate formation begins to hinder the binding mechanism Staining efficiency reaches a maximum at about 1% dye concentration The researchers speculate that understanding the binding mode could lead to the design of chemical derivatives of the dyes that would be effective as agents for targeting amvloid denosits in the brain (Rinsfiprfrmrnfiv 1 QQ'i 7 S'SI-'Sfi)
For pharmaceutical analyses, ion-selective electrodes (ISEs) offer simplicity in design and operation, reasonable selectivity, fast response, the ability to operate in colored and turbid solutions, and the potential for interfacing with automated and computerized systems. In addition to their use for routine determination of pharmaceuticals, ISEs have also been used for detecting drugs of abuse. The key to constructing these types of electrodes is to incorporate the appropriate ion exchanger and solvent mediator into s, polyvinyl chloride) membrane matrix. Takashi Katsu and colleagues at Okayama Prefectural Police Headquarters (Japan) Okayama University Medical School (Japan) and Okayama University (Japan) investigated the improvement of an ISE for the detection of cocaine by the appropriate choice of an ion exchanger and solvent mediator
They used sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate as the ion exchanger and tetrakis(2-ethylhexyl) pyrojets, which arise from bubbles collapsing mellitate (TEHPY) as the solvent mediaduring cavitation in the near vicinity or at tor and found that TEHPY suppressed the surface of the electrode. They have de- response to the lipophilic quaternary amveloped a semiquantitative description of monium ions and strengthened the rethe integrated transient component of this sponse to cocaine. The electrode exhibphenomenon. ited a near-Nernstian response over the By investigating the one-electron reconcentration range 10"2-10~6 M, with a duction of methyl viologen in acetonitrile slope of 56 mV/decade and a detection at a 250-um platinum disc electrode ex- limit of 4 x 10~7 M. Interferences from posed to ultrasound, they estimated that other drugs, such as morphine and the transient cavitation involves bubbles deine were negligible When determining with radii of 160-200 um collapsing at cocaine in a mixture of cocaine and morthe surface of the electrode. As the bub- phine the researchers obtained an averbles collapse, jets of almost cylindrical age recovery of 99 9% with a relative shape develop and repeatedly strike the standard deviation of 0 56% (Anal Chim electrode's surface at a constant velocity Acta 1995 316 371-75) and submicrosecond duration. Voltammetric results from thefirstreduction step of methyl viologen are consistent with this theory and show that the increase in the current by sonication is caused primarily by the transient component (/ Electroanal Chem 1199 399 147-55)
Evolution, from left to right, of the shape of a collapsing bubble near a plane solid wall. (Adapted with permission of Elsevier Science.) 164 A
An ISE for cocaine
Analytical Chemistry News & Features, March 1, 1996
Comparison of the selectivity coefficients of the TEHPY electrode with those of the commonly used dioctyl phthalate (OOP) electrode. (Adapted with permission of Elsevier Science.)
