Chemical Sensors for Radiopharmaceuticals

Oxidation-state speciation of

The analytical utility of chemically modified microelectrodes for oxidation-state speciation of redox couples by cyclic voltammetry has been explored. [Re(I)(DMPE)3]+/[Re(II)(DMPE)3]2+, where DMPE = 1,2-bis(dimethylphosphino)ethane, was studied at carbon-fiber microelectrodes of approximately 5 microm in radius coated with Nafion-entrapped solgel-derived silica (Nafion-silica) composite. The results are compared with cyclic voltammetry of [Fe(CN)6]3-/[Fe(CN)6]4- at bare carbon-fiber microelectrodes. At both microelectrodes, the cathodic and anodic limiting currents are linearly proportional to the concentrations of the reducible and oxidizable species of a redox couple, respectively. The shape of the cyclic voltammogram and the magnitude of the steady-state limiting current are not affected by the potential at which the scan starts. Speciation of both forms of a redox couple could be achieved voltammetrically at the microelectrodes. However, a considerably slower scan rate was required to achieve steady state at the modified electrode because of the smaller diffusion coefficients of [Re(I)(DMPE)3]+ and [Re(II)(DMPE)3]2+ in the Nafion-silica composite. The detection limit at the modified electrode was considerably lower (5 x 10(-9) M for [Re(I)(DMPE)3]+) than at the bare electrode (6 x 10(-5) M for [Fe(CN)6]3- and [Fe(CN)6]4-) because of the substantial preconcentration of [Re(I)(DMPE)3]+ by the Nafion-silica composite.

Spectroscopic and electrochemical evaluation of a perfluorosulfonated ionomer and its gel as preconcentrating media for [ReI(DMPE)3]+, where DMPE = 1,2-bis(dimethylphosphino)ethane

The interaction of [ReI(DMPE)3]+, where DMPE = 1,2-bis(dimethylphosphino)ethane, a nonradioactive analogue of a heart imaging agent, with Nafion gel, which is Nafion plasticized with tri-n-butyl phosphate, has been evaluated spectroscopically and electrochemically. Thin-layer spectroelectrochemistry on the rhenium compound yields a linear Nernst plot with an n value of 0.99 and E degree' of 0.049 V vs Ag/AgCl. The electrochemistry is consistent with a reversible one-electron transfer between the mono- and dicationic forms of the complex. The UV-visible spectrum of electrogenerated [ReII(DMPE)3]2+ is identical to that obtained by air oxidation of [ReI(DMPE)3]+. Thin, free-standing films of Nafion gel and Nafion that were sufficiently clear to record visible spectra were cast. Spectroscopic measurement of the partitioning of [ReI-(DMPE)3]+ from aqueous solution into these films shows a more rapid uptake of the complex by the Nafion gel. Preconcentration factors into Nafion gel and Nafion were 350 and 50, respectively, after 4 h of soaking. Cyclic voltammetry of 1.0 x 10(-4)-1.0 x 10(-7) M (ReI(DMPE)3]+ in 0.15 M supporting electrolyte aqueous solution at bare gold and spectroscopic graphite electrodes suggests that the complex adsorbs to these electrodes. By comparison, the well-defined cyclic voltammograms at Nafion gel-modified electrodes exhibit diffusion-controlled behavior. The formal reduction potential at Nafion gel-modified electrodes is shifted positively compared to bare electrodes. A current enhancement of approximately 4 was observed at Nafion gel-modified spectroscopic graphite over a bare electrode. A calibration plot of peak current for differential pulse voltammetry vs concentration at Nafion gel-modified spectroscopic graphite was linear in the 10(-7)-10(-5) M concentration range, with a detectable signal down into the 10(-9) M range.

Electrochemical behavior of [ReI(DMPE)3]+, where DMPE = 1,2-bis(dimethylphosphino)ethane, at perfluorosulfonated ionomer-modified electrodes

The perfluorosulfonated ionomer Nafion shows potential utility as a polymer film to enhance the electrochemical detection of [ReI(DMPE)3]+, where DMPE = 1,2-bis-(dimethylphosphino)ethane. [ReI(DMPE)3]+, a nonradioactive radiopharmaceutical analog for heart imaging, partitions strongly into Nafion films on glassy carbon. Well-defined, chemically reversible cyclic voltammograms are obtained for the [ReI(DMPE)3]+/[ReII(DMPE)3]2+ couple with Eo' shifted positively by 60 mV relative to its value on bare glassy carbon. [ReI(DMPE)3]+ partitions into Nafion more strongly than the oxidized form, [ReII-(DMPE)3]2+. The detection limit for [ReI(DMPE)3]+ by cyclic voltammetry was improved by 2-3 orders of magnitude by the Nafion film. Differential pulse voltammetry for oxidation of [ReI(DMPE)3]+ at the Nafion-modified electrode has a detection limit of 2.5 x 10(-9) M compared to 1.0 x 10(-7) M at the bare electrode. A preconcentration factor of 1 x 10(6) for partitioning of [ReI(DMPE)3]+ from 0.05 M NaCl into Nafion on a glassy carbon electrode was measured.