Jon R. Kirchhoff

Internal standard method for the measurement of choline and acetylcholine by capillary electrophoresis with electrochemical detection

An internal standard method has been developed for the determination of the neurotransmitter acetylcholine and/or its metabolic precursor choline. This approach couples the high separation efficiency of capillary electrophoresis with the sensitivity and selectivity of electrochemical detection at an enzyme-modified electrode. Indirect electrochemical detection is accomplished at a 25 microm platinum electrode modified by cross-linking the enzymes choline oxidase and acetylcholinesterase with glutaraldehyde. Although in this simple form of electrode fabrication there is a gradual loss of response from the electrochemical detector with time, accurate quantitation is achieved by the addition of butyrylcholine, which is also a substrate for acetylcholinesterase, as an internal standard. A linear response is achieved between 0 and 125 microM with a limit of detection of 2 microM (25 fmol). The utility of this method was demonstrated by monitoring the kinetics of choline uptake in synaptosomal preparations.

Comparative X-ray crystallographic characterization of the mono- and bimetallic complexes of rhenium(I) tricarbonyl chloride with 2,3-bis(2-pyridyl)pyrazine

Abstract Comparative X-ray crystallographic characterization of [Re(dpp)(CO)3Cl] and [Re(CO)3Cl]2dpp, where dpp is 2,3-bis(2-pyridyl)pyrazine, has been conducted to examine the structural consequences resulting from sequential coordination of two [Re(CO)3Cl] molecular fragments to dpp. Addition of one metal center results in the coordinating portion of dpp adopting a relatively planar configuration. The dihedral angle between the pyrazine ring and the coordinated N(1)-pyridyl ring is 15.3(4)°, while the dihedral angle between the pyrazine ring and the uncoordinated N(2)-pyridyl ring is 45.8(3)°. The dpp ligand in the bimetallic complex is found to twist to accommodate the second metal center as manifested by a two-fold increase in the N(1)-pyridyl and pyrazine dihedral angle [29.7(4)°] and a decrease for the N(2)-pyridyl and pyrazine dihedral angle [31.6(3)°]. The direct structural consequence is an increase in the distortion of the central pyrazine moiety upon addition of each [Re(CO)3Cl] fragment. This observation confirms previously reported spectroscopic results that inferred the differences in electronic structure between [Re(dpp)(CO)3Cl] and [Re(CO)3Cl]2dpp were due to the dpp ligand adopting a nonplanar geometry in the bimetallic complex.

Application of scanning electrochemical microscopy and scanning electron microscopy for the characterization of carbon-spray modified electrodes

Different gold surfaces modified by carbon-spray have been investigated by scanning electron microscopy (SEM) and scanning electrochemical microscopy (SECM). A transformation of the SECM image to a distance-location profile is proposed which assists the correlation of both images. The structures found in the transformed SECM images of carbon-spray layers on gold substrates can be explained by the topographic features visible in the SEM pictures. Tempering the carbon spray results in an increased density of electrochemically reactive carbon particles which could be confirmed by cyclic voltammetric investigations. Gold minigrids modified with carbon spray expose some areas of especially large currents which could not be predicted from their SEM images. This effect may result from particles located at the edge of a wire intersection having relatively large active surfaces per particle. They contribute significantly to the total current of the minigrid.

Electrocatalytic microelectrode detectors for choline and acetylcholine following separation by capillary electrophoresis.

Two electrocatalytic enzyme modified microelectrode systems were employed as end-column amperometric detectors of choline (Ch) and acetylcholine (ACh) following separation by capillary electrophoresis (CE). Horseradish peroxidase cross-linked in an osmium based redox polymer hydrogel (HRP-Os) was physically adsorbed on Au microelectrodes followed by chemical cross-linking of the enzymes acetylcholinesterase (AChE) and choline oxidase (ChO). An alternative approach utilized the deposition of the transition metal catalyst, Prussian Blue (PB), on Pt microelectrodes as the electrocatalyst. Utilizing butyrylcholine (BuCh) as an internal standard, the HRP-Os/AChE-ChO and PB/AChE-ChO electrodes exhibited excellent linear responses from 2-2000 microM and 10-2000 microM, respectively, for both Ch and ACh. Detection limits of 0.1 microM or 38 amol were determined for the HRP-Os/AChE-ChO electrode. The limit of detection for ACh and Ch at the PB/AChE-ChO electrode was 5 microM or 9.5 fmol. The electrodes were operated at potentials of +0.10 and -0.10 V vs Ag/AgCl (3 M NaCl), respectively, and thus minimized the potential response from oxidizable interferences. In addition, both electrocatalytic electrodes showed good operational stability for more than 70 h. The enhanced detection capability of the HRP-Os/AChE-ChO and PB/AChE-ChO electrodes in combination with efficient CE separation of Ch and ACh provides a new sensitive and selective strategy for monitoring and quantifying these cholinergic biomarkers in biological fluids.

Spectroelectrochemistry and excited-state absorption spectroscopy of rhenium(I)α,α′-diimine complexes

Thin-layer electrochemistry and spectroelectrochemistry in 0.1 mol dm–3 NBu4PF6–dmf (dmf = dimethylformamide) have been used to characterize spectroscopically the sequentially reduced forms of [Re(bipy)(CO)3Cl], [Re(dpp)(CO)3Cl] and [{Re(CO)3Cl}2(dpp)][dpp = 2,3-bis(2-pyridyl)pyrazine, bipy = 2,2′-bipyridine]. Following the first reduction of each complex, the spectra exhibit similar absorption features in the near-UV region from 350 to 400 nm with a tailing absorption to ca. 500–600 nm. The excited-state absorption spectra of [Re(dpp)(CO)3Cl] and [{Re(CO)3Cl}2(dpp)] have also been obtained and exhibit similar features as in the spectroelectrochemical measurements. The spectra are consistent with reduction of the α,α′-dimine ligand and are dominated by π→π* transition of the ligand radical anion. Addition of a second electron to the monometallic complexes yields spectra that are characterized by a broad absorption band in the range 530–580 nm (Iµ= 10 900–12 100 dm3 mol–1 cm–1). In comparison, addition of a second electron to [{Re(CO)3Cl}2(dpp)] also yields a broad absorption at 550 nm (Iµ= 8800 dm3 mol–1 cm–1) and loss of the absorption at 357 nm, which is analogous to the spectrum for the doubly reduced [Re(dpp)(CO)3Cl] complex. Further reduction of the bimetallic complex results in identical spectral features as the doubly reduced complex with the main difference being an increase in absorbance over the entire UV/VIS region and a red shift of the lowest energy absorption to 576 nm. The spectroelectrochemical measurements for [{Re-(CO)3Cl}2(dpp)] suggest the second and third reductions result in reduction of the metal centres contrary to previous reports for electron localization in the bimetallic complex, which describe the second reduction as ligand based.

ELECTROCHEMISTRY AND SPECTROELECTROCHEMISTRY OF RE(1,2-BIS(DIMETHYLPHOSPHINO)ETHANE)3+

Abstract The electrochemical properties of [Re(dmpe) 3 ] + , where dmpe is 1,2-bis(dimethylphosphino)ethane, have been investigated in aqueous and non-aqueous solutions by cyclic and square wave voltammetry as well as by thin-layer electrochemical and spectroelectrochemical techniques. The redox behavior in N , N -dimethylformamide (DMF) is described in terms of three interrelated one-electron oxidations and the chemistry from decomposition of electrogenerated Re(III) species. The first and third redox processes are assigned to the oxidation of [Re(dmpe) 3 ] + to [Re(dmpe) 3 ] 2+ and [Re(dmpe) 3 ] 2+ to [Re(dmpe) 3 ] 3+ ; the corresponding E p values from square wave voltammetry are +0.336 and +1.120 V versus Ag/AgCl (3 M NaCl). The second redox process (+0.884 V) is described as the oxidation of an Re(II) species, [Re(dmpe) 2 (η 1 -dmpe)S] 2+ (S=a solvent molecule), which is proposed as an intermediate in the dissociation of a dmpe ligand. Complete dissociation of dmpe occurs from the Re(III) complexes to yield a species denoted as [Re(dmpe) 2 S 2 ] 3+ , which is reduced with a peak potential from square wave voltammetry at −0.288 V in DMF. Identification of this species as a bis(dmpe) complex is accomplished by comparison of its FAB mass spectrum, UV–Vis spectrum, HPLC retention time, and electrochemical properties to those of an independently synthesized complex, where S is replaced by Cl − .

Thiolato–technetium complexes. Part 6. Synthesis, characterization, electrochemical properties and crystal structure of [Tc(tdt)(dmpe)2]PF6[tdt = toluene-3,4-dithiolate, dmpe = 1,2-bis(dimethylphosphino)ethane]

The technetium(V) complex [Tc(OH)O(dmpe)2]2+ can be reduced by excess of toluene-3,4-dithiol (H2tdt) to yield the technetium(III) complex [Tc(tdt)(dmpe)2]PF6. The product has been characterized by elemental analysis, UV/VIS spectroscopy, fast atom bombardment mass spectrometry and X-ray crystallography. It crystallizes in the triclinic space group P with Z= 2, a= 9.1508(9), b= 12.794(2), c= 13.516(2)A, α= 93.94(1), β= 95.24(1) and γ= 108.791(9)°. The final R value was 0.037. The technetium co-ordination geometry is midway between octahedral and trigonal prismatic. Relevant structural parameters are Tc–S 2.318(6), S–C 1.738(4)A, S–Tc–S bite angle 84.49(4)° and Tc–P 2.402(7)A. The entire tdt ligand is planar. Electrochemical and spectroelectrochemical measurements have been made for the reversible TcIII–TcII and TcII–TcI couples at –0.600 and –1.217 V respectively vs. Ag–AgCl (3 mol dm–3 NaCl). A quasi-reversible TcIV–TcIII couple was observed at +0.680 V. The properties of [Tc(tdt)(dmpe)2]PF6 are compared to those observed previously for related cis- and trans-[Tc(SR)2(dmpe)2]+/0(R = alkyl or aryl) complexes.

Electropolymerized Pyrrole-Based Conductive Polymeric Ionic Liquids and their Application for Solid-Phase Microextraction

Pyrrole was covalently bonded to 1-methyl and 1-benzylimidazolium ionic liquids (ILs) via an N-substituted alkyl linkage to prepare electropolymerizable IL monomers with excellent thermal stability. The methylimidazolium IL, [pyrrole-C6MIm]+, was then electropolymerized on macro- and microelectrode materials to form conductive polymeric IL (CPIL)-modified surfaces. Electrochemical characterization of a 1.6 mm diameter Pt disk electrode modified with poly[pyrrole-C6MIm]+ demonstrated a selective uptake for an anionic redox probe while rejecting a cationic redox probe. Furthermore, electropolymerization of [pyrrole-C6MIm]+ doped with single-walled carbon nanotubes (SWNT) on 125 μm platinum wires produced 42 μm thick poly[pyrrole-C6MIm]+/SWNT films compared to 17 μm in the absence of SWNT and 5 μm for the previously reported poly[thiophene-C6MIm]+ coatings. The poly[pyrrole-C6MIm]+/SWNT films were prepared with reproducible thicknesses as well as thermal properties sufficient for high-temperature applications, such as solid-phase microextraction (SPME) with gas chromatographic analysis. The utilization of the CPIL sorbent materials in SPME experiments provided excellent extraction efficiencies and selectivity toward organic aromatic analytes. The CPIL sorbent coatings also yielded outstanding fiber-to-fiber reproducibility on the basis of extraction efficiencies and improved response for a range of analytes relative to commercial 100 μm poly(dimethylsiloxane) fibers when normalized for differences in film thickness. Poly[pyrrole-C6MIm]+ CPIL coatings doped with SWNT are therefore promising new sorbent materials for SPME analyses.

Evaluation of the inhibition of choline uptake in synaptosomes by capillary electrophoresis with electrochemical detection.

A direct method for evaluating choline uptake by the high‐affinity choline transport system in synaptosomes was developed using capillary electrophoresis (CE) with electrochemical (EC) detection. On‐column EC detection of choline and the internal standard, butyrylcholine, was accomplished with a 25 νm platinum electrode modified with the enzymes, choline oxidase and acetylcholinesterase. Choline uptake was evaluated as a function of choline concentration and a KM value of 1.7 νM was determined. The method was also used to evaluate a new class of redox affinity inhibitors of choline transport. In particular, the effectiveness of 3‐[(trimethylammonio)methyl]catechol (TMC) as an inhibitor of choline uptake was examined independently and relative to the inhibition of the well‐known inhibitor of choline transport, hemicholinium‐3. The IC50 and KI for TMC were determined to be 30 νM and 14 νM, respectively. The combination of the selectivity and sensitivity afforded by CEEC provides a relatively straightforward approach for monitoring choline transport in synaptosomes.

CVD of Alumina on Carbon and Silicon Carbide Microfiber Substrates for Microelectrode Development

Using a vacuum CVD technique, alumina films were deposited on 10 μm diameter carbon fibers, and 120 μm diameter silicon carbide fibers. This was accomplished by exposing resistively heated microfiber substrates to a sublimate of [Al(O- t Bu) 3 ] 2 , while the reaction chamber was maintained at 120 °C under a pressure of 0.15 torr. The coated substrates were then sealed into glass capillaries, using an epoxy sealant, to form microelectrode devices. The quality of the alumina films was found to be dependent on the substrate temperature, with optimum film quality being obtained at substrate temperatures between 750 °C and 800 °C. Film thickness was controlled by deposition time, and films from 1-100 μm were routinely produced. Films were characterized by microscopy and electrochemical techniques.