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°‘ 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. Preconcentraion factors into Nafion gel and Nafion were 350 and 50, respectively, after 4 h of soaking. Cyclic voltammetry of 1.0 × 10-4−1.0 × 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 ∼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.