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Konno, T.; Kirchhoff, J. R.; Heineman, W. R.; Deutsch, E. Inorg. .... Chemistry: Theory, Analysis, and Correlation, 2nd ed.; The C. V. Mosby Co. ... C...
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Anal. Chem. 1998, 70, 4326-4332

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 Beverly H. Swaile,† Elmo A. Blubaugh, Carl J. Seliskar,* and William R. Heineman*

Department of Chemistry, University of Cincinnati, P.O Box 210172, Cincinnati, Ohio 45221-0172 The interaction of [ReI(DMPE)3]+, where DMPE ) 1,2bis(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 gelmodified 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 gelmodified spectroscopic graphite was linear in the 10-710-5 M concentration range, with a detectable signal down into the 10-9 M range.

synthesized and studied is to find substitution-inert centers in which the chemical and biological properties of a compound can be readily changed by subtle ligand variations for the development of improved radiopharmaceuticals.2 For example, several species of cationic technetium complexes of bis(tertiary phosphine) ligands have been shown to be taken up by normal heart tissue and thus have potential utility in diagnostic nuclear medicine as myocardial imaging agents.2-5 [TcI(DMPE)3]+, where DMPE ) 1,2-bis(dimethylphosphino)ethane (Figure 1), is one such cation that has shown uptake in the myocardial tissue of animals6-8 and humans.8,9 The heart shows preferential uptake of monocations that are somewhat hydrophobic, and the bidentate DMPE ligands surrounding the metal center provide the hydrophobic character. The mechanism of the myocardial accumulation, however, is unclear.7 The overall charge of an imaging agent plays an important role in organ accumulation. A change in oxidation state of the metal ion that alters the overall charge of the complex can have a deleterious effect on organ imaging,5 and, consequently, in vivo redox reactions of an imaging agent are of concern. One possible way to monitor in vivo redox reactions of a radiopharmaceutical compound is with an electrochemical or an optical sensor.10,11 The in vivo concentrations of imaging agents are typically very low, which might preclude detection of these compounds at bare

† Present address: Department of Chemical Technology, University of Cincinnati, 2220 Victory Parkway, Cincinnati, OH 45206. * Address correspondence to these authors. Telephone: 513-556-9213 or 513556-9210. Fax: 513-556-9239. E-mail: [email protected] or william.heineman@ uc.edu.

(1) Clarke, M. J.; Podbielski, L. Coord. Chem. Rev. 1987, 78, 253-331. (2) Konno, T.; Kirchhoff, J. R.; Heineman, W. R.; Deutsch, E. Inorg. Chem. 1989, 28, 1174-1179. (3) Konno, T.; Heeg, M. J.; Stuckey, J. A.; Kirchoff. J. R.; Heineman, W. R.; Deutsch, E. Inorg. Chem. 1992, 31, 1173-1181. (4) Vanderheyden, J. Studies of Technetium and Rhenium Diphosphine Complexes, and Some Cationic Technetium Complexes of Schiff Base Ligands, as Myocardial Imaging Agents. Ph.D. Dissertation, University of Cincinnati, Cincinnati, OH, 1985. (5) Nowotnik, D. P.; Nunn, A. D. Drug News Perspect. 1992, 5, 174-183. (6) Kirchoff, J. R.; Heineman, W. R.; Deutsch. E. Inorg. Chem. 1987, 26, 31083113. (7) Deutsch, E.; Libson, K.; Vanderheyden, J.-L.; Kelissy, A. R.; Maxon, H. R. Nucl. Med. Biol. 1986, 13, 465-477. (8) Vanderheyden, J.-L.; Ketring, A. R.; Libson, K.; Heeg, M. J.; Roecker, L.; Motz, P.; Whittle, R.; Elder, R. C.; Deutsch, E. Inorg. Chem. 1984, 23, 31843191. (9) Vanderheyden, J.-L.; Heeg, M. J.; Deutsh, E. Inorg. Chem. 1985, 24, 16661673. (10) Seliskar, C. J.; Jeng, M. L.; Landis, D. A.; Swaile, B. H.; Blubaugh, E. A.; Heineman, W. R.; Deutsch, E. A. Proceedings Biosensors 92; Elsevier Science Publishers: Oxford, England, 1992. (11) Heineman, W. R.; Swaile, B. H.; Blubaugh, E. A.; Landis, D. A.; Seliskar, C. J.; Deutsch, E. Radiochim. Acta 1993, 63, 199-203.

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S0003-2700(98)00109-7 CCC: $15.00

Bis(tertiary phosphine) ligands have been shown to stabilize transition metals in a variety of oxidation states and coordination geometries.1 One of the reasons such compounds have been

© 1998 American Chemical Society Published on Web 09/18/1998

Figure 1. Structure of 1,2-bis(dimethylphosphino)ethane (DMPE) ligand.

electrode or optical fiber surfaces. A polymer-modified surface, however, would have an advantage over a bare surface if an appropriate polymer could be found that would enhance the electrochemical or optical signal of the radiopharmaceutical by preconcentrating it at the sensor surface, thus facilitating its detection at low concentration. A polymer film might also improve selectivity for the radiopharmaceutical by excluding interfering compounds present in biological tissue. Nafion, a perfluorosulfonated ionomer, has proved to be a good choice for preconcentration of cationic heart imaging agents because of its combination of cation-exchange and hydrophobic properties.12 Nafion is an un-cross-linked high-molecular-weight polymer with an ion-clustered morphology.13 The important characteristic of Nafion that makes it potentially useful for the detection of low concentrations of a cationic radiopharmaceutical is its ability to preconcentrate large organic cations and neutral species.14-17 Nafion’s affinity for hydrophobic cations has been demonstrated with compounds such as Ru(bpy)32+ 14,18-20 and Os(bpy)32+.19 We have examined the characteristics of [ReI(DMPE)3]+ at Nafion-modified glassy carbon electrodes and found a preconcentration factor of 1.1 × 106.12 This strong partitioning of [ReI(DMPE)3]+ into Nafion gave a 2-3 orders of magnitude improvement in detection limit by cyclic voltammetry. Detection limits of 2.5 × 10-9 M could be reached by differential pulse voltammetry at Nafion-modified glassy carbon, compared to 1.0 × 10-7 M at bare glassy carbon. One caveat is that Nafion-modified electrodes exhibit a very slow response due to the small diffusion coefficient for [ReI(DMPE)3]+ in Nafion. Another interesting but noncommercial form of Nafion is Nafion gel.21-24 Nafion gel is prepared by combining Nafion ion(12) Deng, Y,; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 1997, 69, 40454050. (13) Parthasarathy, A.; Martin, C. R.; Srinivasan, S. J. Electrochem. Soc. 1991, 138, 916-921. (14) Martin, C. R.; Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1982, 104, 48174824. (15) Steck, A.; Yeager, H. L. Anal. Chem. 1980, 52, 1215-1218. (16) Whiteley, L. D.; Martin, C. R. J. Phys. Chem. 1989, 93, 4650-4658. (17) Oyama, N.; Ohsaka, T.; Sato, K.; Yamamoto, H. Anal. Chem. 1983, 55, 1429-31. (18) Downey, T. M.; Neiman, T. A. Anal. Chem. 1992, 64, 261-268. (19) White, H. S.; Leddy, J.; Bard, A. J. J. Am. Chem. Soc. 1982, 104, 48114817. (20) Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1980, 102, 6641-6642. (21) Audebert, P.; Divisia-Blohorn, B.; Kern, J. M.; Aldebert, P.; Pineri, M. J. Chem. Soc., Chem. Commun. 1989, 939-941. (22) Andrieux, C. P.; Audebert, P.; Divisia-Blohorn, B.; Aldebert, P.; Michalak, F. J. Electroanal. Chem. 1990, 296, 117-128. (23) Andrieux, C. P.; Audebert, P.; Hapiot, P.; Divisia-Blohorn, B.; Aldebert, P. J. Electroanal. Chem. 1990, 296, 129-139.

exchange powder with solvents that act as plasticizing agents. Mixing Nafion powder with trialkyl phosphate solvents forms gel materials that behave as solids macroscopically and as liquids microscopically. In addition to the cationic exchange properties commonly associated with Nafion, the hydrophobicity of the resulting Nafion gel may be controlled by appropriate choice of plasticizing agent, such as tri-n-butyl phosphate (polar and hydrophobic). An important feature of Nafion gel with respect to sensor response time is the much faster diffusion that has been reported for electroactive compounds in Nafion gel films compared to Nafion films. Thus, Nafion gel is a good candidate for a polymer coating to be used in conjunction with optical or electrochemical sensors for monocationic, somewhat hydrophobic heart imaging agents. In this paper, we report a study of the partitioning/preconcentration of a monocationic complex into Nafion gel in the form of films that are free-standing or coated on electrodes. [ReI(DMPE)3]+, the nonradioactive analogue of [99mTcI(DMPE)3]+, is used as the model compound for a heart imaging agent for which an in vivo sensor has been developed.25 Spectroelectrochemical, spectroscopic, and voltammetric studies of [ReI(DMPE)3]+are reported. EXPERIMENTAL SECTION Reagents. [ReI(DMPE)3]CF3SO3 was prepared and purified as previously described.2 Sodium chloride and tri-n-butyl phosphate were used as received from Fisher Scientific. Nafion perfluorinated ion-exchange powder (in the form of a 5 wt % suspension in a mixture of low-molecular -weight aliphatic alcohols and water) and Nafion 117 perfluorinated membrane were purchased from Aldrich Chemicals. Water was purified using a Barnstead ORGANICpure/NANOpure water system. Burdick & Jackson brand high-purity acetonitrile UV was obtained from Baxter. Unless otherwise stated, all solutions were purged with argon gas for up to 1 h prior to use in electrochemical or spectroscopic experiments. Spectroelectrochemistry. Spectroelectrochemistry was performed with an optically transparent thin-layer electrode (OTTLE) consisting of a gold minigrid working electrode sandwiched between two quartz plates.26 A BAS Ag/AgCl (3 M NaCl) reference electrode and a platinum wire auxiliary electrode were used. An initial potential of -300 mV was applied to completely reduce the Re complex. A sequence of potentials stepped in 20 mV increments in the positive direction was then applied up to a potential of +400 mV, at which point the complex was completely oxidized. Potentials were applied with a BAS CV-27 voltammograph. Absorbance spectra were measured at each potential with a diode array spectrophotometer (Hewlett-Packard model 8452A) over the wavelength range of 190-820 nm. Preparation and Casting of Stiff Nafion Gel Film. Nafion gel was prepared by combining a ratio of Nafion perfluorinated ion-exchange powder suspension with tri-n-butyl phosphate with stirring. The usual ratio was 1 g of Nafion powder to 1.5 g of tri-n-butyl phosphate. The Nafion gel mixture was then either cast into free-standing sheets or coated as a film on electrode (24) Andrieux, C. P.; Divisia-Blohorn, B.; Aldebert, P.; Michalak, F. J. Electroanal. Chem. 1992, 322, 301-309. (25) Lee, M. T. B.; Seliskar, C. J.; Heineman, W. R.; McGoron, A. J. J. Am. Chem. Soc. 1997, 119, 6436-6435. (26) DeAngelis, T. P.; Heineman, W. R. J. Chem. Educ. 1976, 53, 594-597.

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surfaces. For casting into sheets, a known amount of gel/ethanol mixture was delivered into a specially designed Teflon mold with dimensions of 8 cm × 8 cm × 1 cm, and the mixture was allowed to stand covered in a hood to allow partial evaporation of the solvent. The mold was then placed in an oven at 60 °C for 12 h. The gel sheet was removed from the oven and allowed to cool. After cooling, the sheet could be removed from the mold and cut into disks with a diameter of 1.7 cm. Typically, the disks were about 0.5 mm thick, as measured by a caliper. Electrode Surface Modification with Nafion Gel. Teflonshrouded27 spectroscopic graphite electrodes (grade designation 366 BDFXI and 366 BS FXI, Poco Graphite, Decatur, TX) were polished on Fibermet polishing disks, first with 3-µm followed by 0.3-µm particulate mesh (Buehler, Lake Bluff, IL). A 5% (w/w) Nafion gel in ethanol solution was used in all experiments. Typically, a 5-µL aliquot of the 5% Nafion gel solution was applied to each polished electrode. This aliquot was allowed to dry overnight, giving a Nafion gel-modified spectroscopic graphite electrode. Each electrode was coated with approximately 150 mg of polymer. The uptake of [ReI(DMPE)3]+ at a Nafion gel-modified electrode was monitored by cyclic voltammetry. The Nafion gelmodified electrodes were placed in solutions of [ReI(DMPE)3]+ (1.0 × 10-4-1.0 × 10-7 M) for varying time periods (from less than 1 min to 24 h). Then each electrode was cycled in the potential range from -0.300 to +0.400 V vs Ag/AgCl. Optical Measurement of Partitioning of [ReI(DMPE)3]+ into Disks of Nafion and Nafion Gel. Both commercial Nafion membrane and the Nafion gel sheets were cut into disks of 1.7 cm in diameter using a cork borer. The Nafion gel disks were used directly without further cleaning, whereas the disks cut from commercial Nafion membrane were cleaned according to a procedure described by Parthasarathy et al.13 Once cut, the disks were weighed and placed in individual glass vials containing 15 mL of 0.150 M NaCl solution and allowed to soak for approximately 18 h. The saline concentration was chosen to mimic that found in biological systems.28 The disks were then removed from the saline solution, and each was placed in a vial containing a known volume of 1.0 × 10-4 M [ReI(DMPE)3]+ solution. The disks were allowed to soak in solution while being shaken by a gyrating shaker. After soaking for a given time, the polymer disks were removed from the vials and placed between two quartz slides in a modified cell holder. The absorbance spectrum of each disk was measured on the diode array spectrophotometer over the wavelength range of 190-820 nm. The absorbance spectra of the solutions in which the disks had soaked were also measured. It was assumed that the portion of each film interrogated by the light beam of the spectrophotometer was representative of the entire disk. The optical beam of the diode array has a diameter of 3/8 in. at the sample, which covered a significant fraction of the 1.7-cm-diameter disk. The thickness of each disk was uniform, as measured by the calipers. Irregularities in film thickness smaller than could be measured by the calipers would be averaged out by the relatively large optical beam cross section. (27) Hofbauer, M. Poly-N-vinylamide-Modified Electrodes as Potential Dopamine Sensors: Studies on the Polymer-Analyte Interactions. Diplomarbeit, Friedrich-Wilhelms University, Bonn, Germany, 1992. (28) Kaplan, L. A.; Pesce, A. J. Clinical Chemistry: Theory, Analysis, and Correlation, 2nd ed.; The C. V. Mosby Co.: St. Louis, MO, 1989; p 5.

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Voltammetry. Voltammetry experiments were done with a BAS 100A electrochemical analyzer. An electrochemical cell composed of three electrodes was used for all measurements. The working electrodes were Teflon-shrouded spectroscopic graphite (4.6-mm diameter) or gold (1.7-mm diameter, BAS), the reference electrode was Ag/AgCl, 3 M NaCl (BAS), and the auxiliary electrode was a large area platinum wire. RESULTS AND DISCUSSION The experimental plan involved first measuring the spectra of [ReI(DMPE)3]+ and its oxidation product [ReII(DMPE)3]2+ by thinlayer spectroelectrochemistry over a wavelength range of 190820 nm. Second, partitioning of [ReI(DMPE)3]+ into disks of freestanding Nafion and Nafion gel was measured spectroscopically in order to compare the preconcentrating ability of the two materials. We were especially interested in learning if the special properties of the Nafion gel21-24 are applicable to [ReI(DMPE)3]+. Last, partitioning of [ReI(DMPE)3]+ into thin films of Nafion gel on electrodes was examined by electrochemistry. Thin-Layer Spectroelectrochemistry of [ReI(DMPE)3]+. During the initial stages of experimentation, solutions of [ReI(DMPE)3]+, which are colorless, were observed to gradually turn pink. The cause was suspected to be slow oxidation of [ReI(DMPE)3]+ by oxygen dissolved in the solutions. Consequently, thin-layer spectroelectrochemical experiments were performed to obtain spectra of [ReI(DMPE)3]+ and the electrogenerated oxidized form [ReII(DMPE)3]2+ in order to confirm that this oxidation was occurring. The spectroelectrochemistry also provided information about the redox properties of the [ReI(DMPE)3]+/[ReII(DMPE)3]2+ couple. Cyclic voltammetry was used to define the potential range over which to perform the thin-layer spectroelectrochemistry experiment (vide infra). Absorbance spectra for spectroelectrochemistry on [ReI(DMPE)3]+ in an OTTLE with a gold minigrid working electrode are shown in Figure 2a. At -300 mV, two distinct absorbance peaks at 220 and 252 nm are obtained for the reduced form of the complex, [ReI(DMPE)3]+. These two peaks decrease in magnitude and shift into one peak at 234 nm as the potential is stepped to positive values to form [ReII(DMPE)3]2+ by oxidation at the minigrid. Also, a peak appears in the visible range at 526 nm (Figure 2b), which gives the characteristic pink color of [ReII(DMPE)3]2+. An isosbestic point exists in the visible region at about 320 nm, Figure 2b, that holds well until the most positive potentials are reached. We attribute the deviation from the isosbestic point to conversion of a small amount of [ReII(DMPE)3]2+ into another form. This conversion could be a slow substitution reaction of one of the DMPE ligands by Cl- from the supporting electrolyte or solvent, as has been observed in N,Ndimethylformamide,29 or possibly some oxidation to [ReIII(DMPE)3]3+ due to the close proximity to the next oxidation wave. A Nernst plot26 of the spectroelectrochemical data gives a straight line yielding an n value of 0.99 and an E°′ of +0.004 V. These data indicate a one-electron transfer as expected for the oxidation of the rhenium complex from the I to the II state. Partitioning of [ReI(DMPE)3]+ into Disks of Nafion Gel and Nafion Measured by Absorbance. Free-standing disks of (29) Kirchhoff, J. R.; Allen, M. R.; Cheesman, B. V.; Okamoto, K.; Heineman, W. R.; Deutsch, E. Inorg. Chim. Acta, in press.

Figure 3. Graph of preconcentration factors of 1.0 × 10-4 M [ReI(DMPE)3]+, 0.150 M NaCl into Nafion film and Nafion gel disks. Measurements were made at 252 nm. Each point is the average measurement for five disks.

Figure 2. (a) Thin-layer spectroelectrochemistry of 5.0 × 10-4 M [ReI(DMPE)3]CF3SO3, 0.150 M NaCl. Potentials were applied in 20mV increments over the range from -300 to +400 mV vs Ag/AgCl. (s) [ReI(DMPE)3]+ at -300 mV, (- - -) [ReII(DMPE)3]2+ at +400 mV, other spectra are for mixtures at intermediate potentials. (b) Enlargement of the range from 300 to 600 nm. (-‚‚-) [ReII(DMPE)3]2+ at +400 mV.

Nafion gel and Nafion were cast for the purpose of optically measuring the uptake of [ReI(DMPE)3]+ by these materials from aqueous solution. Optical clarity is an important property for materials to be used as preconcentrating media for absorbancebased optical fiber sensors. Most of the free-standing disks of both Nafion gel and Nafion exhibited some cloudiness, which seemed to vary with lot of Nafion used. The appearance of the disks may also be a function of casting conditions. However, the disks were sufficiently clear that partitioning of [ReI(DMPE)3]+ from aqueous solution into the disks could be measured by absorbance spectroscopy after background-subtracting the contributions from the disks. The spectra of the polymer disks alone were essentially featureless over the wavelength range of 220600 nm. Preconcentration factors (concentration of analyte in the film/concentration of analyte in solution) for the uptake of [ReI(DMPE)3]+ into the polymer disk were calculated from the absorbance spectra at 252 nm using Beer’s law. (We are using the term preconcentration factor rather than partition coefficient because the measurements were not being made at equilibrium. Also, we believe the concentration of [ReI(DMPE)3]+ within the films is inhomogeneous through the film thickness, being greatest at the region near the solution interface and smallest in the middle. This is a result of the slow diffusion within these relatively thick films.) Graphs of the average preconcentration factors for [ReI(DMPE)3]+ versus immersion time for five sets of Nafion gel and

Nafion disks are shown in Figure 3. After 4 h of contact with the solution, the maximum average preconcentration factors were 350 and 50 for Nafion gel and Nafion, respectively. Thus, over the same time period and under the same conditions, the Nafion gel exhibited a preconcentration factor enhancement of 7 times that of Nafion. Although it is clear that the equilibrium preconcentration factors (which would be indicated by a plateauing of the plots) had not been reached after 4 h, the experiments were discontinued at that time due to uncertainty about the filmcomplex composition raised after observing the appearance of absorbance peaks at 234 and 526 nm and noting a pink coloration of the disks. Both suggesed the presence of [ReII(DMPE)3]2+ in the polymer disks. Attempts to achieve equilibrium before compound degradation by working with thinner films and trying to more rigorously exclude oxygen were unsuccessful. The more rapid preconcentration of the radiopharmaceutical analogue by Nafion gel over the Nafion film is attributed to changes in the morphology of the polymer caused by the addition of the plasticizer.21-24 While both materials exhibit cationexchange capability, the addition of the tri-n-butyl phosphate plasticizer increases the hydrophobicity of the gel polymer, which enhances the uptake of a hydrophobic compound. Such behavior would be expected for [ReI(DMPE)3]+ because of its hydrophobicity. Since equilibrium partition coefficients were not achieved, we cannot say if the final uptake at equilibrium has been affected by the plasticizer. The plots in Figure 3 simply show that the compound partitions into the Nafion gel faster than into Nafion. Enhancement of diffusion coefficients in Nafion gel compared to Nafion is a property that has been observed for other compounds.21-24 By analogy, we assume the rate-limiting step in the overall partitioning process is the diffusion of the complex from the interface into the bulk of the film, and not the crossing of the solution/film interface. Thus, the ratio of the slopes of the two plots is a measure of the ratio of the diffusion coefficients of [ReI(DMPE)3]+ in Nafion gel:Nafion. This ratio is 8. Even larger differences in diffusion coefficients have been reported for ferrocene: 10-10 cm2/s in Nafion30 vs 10-6 cm2/s in Nafion gel.21,22 The difference in signal enhancement at Nafion gel and Nafion at short times is potentially important in the development of a practical sensor for [ReI(DMPE)3]+. More rapid partitioning into the polymer film will result in a faster responding sensor, which Analytical Chemistry, Vol. 70, No. 20, October 15, 1998

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Figure 4. Cyclic voltammogram of 1.0 × 10-4 M [ReI(DMPE)3]+, 0.150 M NaCl at bare gold. Scan rate was 100 mV/s.

is important for the application of in vivo measurement of heart imaging agents.25 This would apply to use of the polymer as a preconcentrating film for either an optical or an electrochemical sensor. Electrochemistry of [ReI(DMPE)3]+ at Bare and Nafion Gel-Coated Electrodes. Given the more rapid partitioning of [ReI(DMPE)3]+ into Nafion gel compared to Nafion, the gel form was further investigated as a thin film coated on an electrode surface. The main objective was to determine if the Nafion gel exhibited any deleterious effect on the electrochemistry of the [ReI(DMPE)3]+/[ReII(DMPE)3]2+ couple. Two types of electrodes were investigated: gold and spectroscopic graphite. Both electrode materials were of interest because of a long-term goal of making microelectrode sensors from either gold or carbon fibers. Bare Gold. A cyclic voltammogram of [ReI(DMPE)3]+ at a gold disk electrode (Figure 4) showed oxidation to [ReII(DMPE)3]2+ to occur with E°′ ) + 0.002 V vs Ag/AgCl, which is in good agreement with the spectroelectrochemical value (vide supra). The low concentration of complex, due to restricted solubility, results in a voltammogram that is not particularly well-defined. The anodic and cathodic peaks have a ∆Ep of 81 mV, with the ratio of peak currents, ipc/ipa, being 0.85 at 100 mV/s. Adsorption of the rhenium monocation, which is hydrophobic and solubility limited, is responsible for the anodic peak exhibiting higher current than the cathodic peak. This explanation is consistent with the behavior of the heights of ipc and ipa with varying scan rate over the range of 10-750 mV/s. ipc for the reduction of [ReII(DMPE)3]2+ electrogenerated in the reverse scan is linear versus ν1/2 but nonlinear with ν, which is consistent with a diffusionlimited reduction process with no detectable adsorption. However, analogous plots of ipa for oxidation of [ReI(DMPE)3]+ in the forward scan show linearity vs ν (and nonlinearity vs ν1/2) at high scan rates, which is consistent with oxidation of a surface-confined species under fast-scan conditions, whereas at low scan rates the plot versus ν loses linearity and the plot vs ν1/2 approaches linearity, which is consistent with oxidation of a diffusing species becoming dominant at slow scan rates. Nafion Gel-Modified Gold Electrode. The qualitative shape of the voltammogram at a Nafion gel-modified gold electrode in a solution of the same concentration of [ReI(DMPE)3]+ is similar to that at the bare electrode. The peak separation of 190 mV, however, was over twice that observed at a bare electrode. The 4330 Analytical Chemistry, Vol. 70, No. 20, October 15, 1998

Figure 5. Cyclic voltammogram of 1.0 × 10-4 M [ReI(DMPE)3]+, 0.150 M NaCl vs Ag/AgCl at bare spectroscopic graphite electrode. Scan rate was 100 mV/s.

E°′ for the couple was 0.130 V vs Ag/AgCl, which is a substantial positive shift compared to the bare electrode. An increase in peak separation as well as a positive shift in E°′ was also observed at Nafion-coated electrodes.12 A shift in this direction is consistent with the [ReI(DMPE)3]+ being stabilized by the gel, relative to the other form of the redox couple [ReII(DMPE)3]2+, which makes it more difficult to oxidize. This positive shift also suggests that the hydrophobic component of the interaction between complex and gel is more important than the ionic interaction. [ReII(DMPE)3]2+ would be expected to exhibit a stronger ionic interaction and a weaker hydrophobic interaction because of its greater charge density than [ReI(DMPE)3]+. The peak current ratio was 1.1 at 100 mV/s, which suggests that the adsorption of [ReI(DMPE)3]+ observed at bare gold has been eliminated by the presence of the gel at the gold interface. This is supported by the peak current behavior for scan rate studies. The current responses for both anodic and cathodic currents over the scan rate range of 10-500 mV/s adhere more closely to linearity when plotted vs ν1/2 than when plotted vs ν. Additionally, the peak currents were enhanced almost 3 times over that seen at the bare electrode due to the preconcentration of the complex into the Nafion gel. Bare Graphite Electrode. The cyclic voltammetry of [ReI(DMPE)3]+ was also evaluated at a bare polished graphite electrode (Figure 5). The formal redox potential was 0.105 V vs Ag/AgCl, which is shifted positive compared with that of bare gold. The peak current ratio (ipc/ipa ) 0.34 at 100 mV/s) is even lower than that at gold, suggesting that [ReI(DMPE)3]+ is adsorbed even more strongly at graphite than at gold. Plots of both ipc and ipa vs scan rate are relatively linear, while currents vs ν1/2 are nonlinear for oxidation and reduction over the scan rate range 10-500 mV/s. These results suggest that adsorption is a major factor in this electrode reaction. However, from the shape of the voltammogram, [ReI(DMPE)3]+ seems to adsorb more strongly, which is consistent with its more hydrophobic character. Nafion Gel-Modified Graphite Electrode. The addition of a Nafion gel coating to the graphite surface changes the cyclic voltammetry of [ReI(DMPE)3]+ (Figure 6) in a manner analogous to the changes observed at gold. A very clearly defined and symmetrical voltammogram was obtained at Nafion gel-coated graphite. Plots of anodic and cathodic peak currents over a scan rate of 10-500 mV/s are nonlinear vs ν but linear vs ν1/2, which is evidence of a diffusion-controlled process. The average ratio

Figure 6. Cyclic voltammogram of 1.0 × 10-4 M [ReI(DMPE)3]+, 0.150 M NaCl vs Ag/AgCl at Nafion gel-modified spectroscopic graphite electrode. Scan rate was 100 mV/s.

of ipa/ipc is changed to 1.0 at 100 mV/s. These changes are all consistent with a decrease in the adsorption phenomena observed at the bare electrode. The oxidation and reduction peak potentials, Epa and Epc, were approximately constant with scan rate and were +0.130 and +0.050 V vs Ag/AgCl, respectively, which corresponds to a ∆Ep of 80.0 mV and a formal reduction potential of +0.090 V vs Ag/AgCl. In the case of graphite, the presence of the film did not shift E°′ as much as with gold. The Nafion gel-modified electrodes exhibit a peak current signal enhancement of ∼4 times that observed for bare electrodes due to preconcentration of [ReI(DMPE)3]+ into the gel. Electrochemical Measurement of Partitioning into Nafion Gel Films. To measure the preconcentrating ability of Nafion gel, voltammetry was performed at bare and Nafion gel-coated spectroscopic graphite electrodes immersed in solutions of 1.0 × 10-4 M [ReI(DMPE)3]+. During these experiments, the potential was held constant at -0.300 V to maintain the rhenium complex in the monocationic form in the film during the uptake and to preclude uncontrolled oxidation, which had been a problem with the optical measurements on free-standing disks. Partitioning into the film was monitored with cyclic voltammograms taken periodically over the course of 18 h in order to observe the long-term behavior of the complex in the gel. This long study was also performed as an initial test of stability of the modified electrode, keeping in mind the application to in vivo measurements that might take substantial time. Cyclic voltammograms for bare and Nafion gel-coated electrodes during the first 1.5 h are shown in Figure 7a and b, respectively. It is evident from both sets of voltammograms that an increase in concentration of [ReI(DMPE)3]+ is occurring at both electrodes, although the increase is much greater at the Nafion gel-coated electrode. The increase at the bare graphite electrode is attributed to the adsorption mentioned above and perhaps some soaking into the somewhat porous electrode. Plots of peak current vs time from these voltammograms show that the compound increases in concentration at the electrode surface and then reaches equilibrium in less than 1 h. From voltammograms of five repetitions of this experiment, it is evident that the gel concentrates the compound for up to about 40 min, at which time the peaks begin to shift and change shape. The beginning of this behavior is evidenced in the last cyclic voltammogram in Figure 7b, which shows increased separation in peak potentials that might be attributed to either an increase in iR drop or a decrease in rate of electron transfer.

Figure 7. Repetitive cyclic voltammograms of 1.0 × 10-4 M [ReI(DMPE)3]+, 0.150 M NaCl at (a) bare spectroscopic graphite electrode with scans measured at 1, 3, 7, 18, 24, and 63 min, respectively from the inside to outside, and (b) Nafion gel-modified graphite electrode with scans measured at 1.5, 4.5, 10.5, 22.5, 46.5, and 96.0 min, respectively from the inside to outside, vs Ag/AgCl. Scan rate was 100 mV/s. (c) Graph of anodic peak current vs immersion time for bare and Nafion gel-modified graphite electrodes.

However, more drastic degradation occurs as the scanning continues, which is probably due to compound degradation within the gel film. Figure 7c is a graph of the anodic peak current for the Nafion gel-modified electrode vs uptake time. Ninety percent of this maximum uptake was reached in less than 1 h. The anodic peak currents obtained from the bare and the Nafion gel-modified electrode experiments were used to calculate current enhancement factors for [ReI(DMPE)3]+ into Nafion gel. A maximum enhancement value of 3 was achieved after 40 min of soaking. Polymer-modified electrodes have exhibited enhanced detection limits over bare electrodes as a result of favorable partitioning Analytical Chemistry, Vol. 70, No. 20, October 15, 1998

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respective concentration. These average peak currents for the electrodes at each concentration are plotted as a function of solution concentration in Figure 8. A linear relationship between peak current and concentration was obtained in the 10-5-10-7 M range, with plateauing at the higher concentrations due to saturation of the Nafion gel.31 Signal for the compound was observed to 1 × 10-9 M; however, the erratic behavior of the electrochemical signal at lower concentrations and the lack of linearity in relation to higher concentrations suggest that 1 × 10-7 M is a practical limit of quantitation. Similar behavior has been observed with [Re(DIARS)2Cl2]+ at low concentrations.32

Figure 8. Graph of average peak current obtained from differential pulse voltammograms plotted against concentration of [ReI(DMPE)3]+ in phosphate buffer, pH 7.4, at Nafion gel-modified spectroscopic graphite electrodes.

of the analyte into the polymer film.30 An analytical calibration plot was obtained for the Nafion gel-modified electrode with a series of [ReI(DMPE)3]+ solutions. Individual Nafion gel-modified spectroscopic graphite electrodes, which had been soaked in saline for 1 h prior to use, were immersed in analyte solutions for a period of 4 h. These electrodes were then removed and placed in a supporting electrolyte solution of 0.15 M NaCl, and differential pulse voltammograms were obtained immediately and at varying times thereafter, each electrode being used only once. There was a single peak within the potential window from -0.300 to +0.450 V vs Ag/AgCl, with a peak potential of +0.058 V vs Ag/AgCl. For each concentration studied, a series of five Nafion gel-coated electrodes were soaked in [ReI(DMPE)3]+ at each (30) Leddy, J.; Bard, A. J. J. Electroanal. Chem. 1985, 189, 203. (31) Whiteley, L. D.; Martin, C. R. Anal. Chem. 1987, 59, 1746-1751. (32) Ramos, B. L. Investigation of Polymer Modified Microelectrodes as Sensors for the Determination of Radiopharmaceutical Analogues. Ph.D. Dissertation, University of Cincinnati, Cincinnati, OH, 1994.

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CONCLUSIONS The radiopharmaceutical heart imaging agent analogue [ReI(DMPE)3]+ partitions into both Nafion and Nafion gel. As observed in the spectroscopic studies, partitioning occurs more rapidly into the Nafion gel than into to Nafion. Electrode surfaces modified with Nafion gel showed enhanced signal for the oxidation of [ReI(DMPE)3]+ by cyclic voltammetry over unmodified electrodes. In addition, the gel improved the definition of the voltammograms by providing for diffusion-controlled behavior vs the mixed adsorption/diffusion behavior obtained at unmodified surfaces. Thus, Nafion gel is a promising candidate for surface modification of electrodes and perhaps optrodes to be used as sensors for detection of cationic imaging agents. ACKNOWLEDGMENT This work was supported by the National Institutes of Health (Grant NIH-CA42179), The Department of Energy (Grant DOEFG02-86ER60487), and a North Atlantic Treaty Organization Grant (NATO Collaborative Research Grant). The authors also thank Dr. Thomas H. Ridgway for many helpful discussions. Received for review February 2, 1998. Accepted July 14, 1998. AC980109D