Anal. Chem. 2000, 72, 2943-2948
Electrogenerated Chemiluminescence of Tris(2,2′-bipyridyl)ruthenium(II) Ion-Exchanged in Nafion-Silica Composite Films Alexander N. Khramov and Maryanne M. Collinson*
Department of Chemistry, Kansas State University, 111 Willard Hall, Manhattan, Kansas 66506-3701
The voltammetry and electrogenerated chemiluminescence (ECL) of tris(2,2′-bipyridyl)ruthenium(II) (Ru(bpy)32+) ion-exchanged in Nafion and Nafion-silica composite materials have been investigated. The major goal of this work was to investigate and develop new materials and immobilization approaches for the fabrication of ECL-based sensors with improved reactivity and long-term stability. Nafion-silica composite materials with varying contents of Nafion (53-100 wt % relative to silica) were prepared via the two-step acid/base hydrolysis and condensation of tetramethoxysilane. The Nafion doped sols were spin cast on glassy carbon electrodes, dried, and then ion-exchanged with Ru(bpy)32+. The shapes of the cyclic voltammetric curves and the amount of Ru(bpy)32+ exchanged into the films strongly depends on the amount of Nafion incorporated into the hybrid sol. Nafionsilica films with a low content of Nafion ion-exchanged less Ru(bpy)32+ and exhibited tail-shaped voltammetry at 100 mV/s. The ECL of immobilized Ru(bpy)32+ in the presence of either tripropylamine or sodium oxalate in pH 5 acetate buffer was also strongly dependent on the amount of Nafion introduced into the composite with greater ECL observed for the Nafion-silica films relative to pure Nafion. Electrogenerated chemiluminescence (ECL) is the production of light from electrochemically generated reagents.1 Among numerous applications, ECL has been used extensively in chemical analysis as a sensitive and selective detection method for various analytes.1-4 One of the most highly studied ECL compounds in this regard has been tris(2,2′-bipyridyl)ruthenium(II) (Ru(bpy)32+).2-4 As a chemiluminescent reagent, Ru(bpy)32+ is well suited for chemical analysis due to its ability to undergo ECL reactions in aqueous solutions and its good chemical, electrochemical, and photochemical stability.2-4 During the past decade, considerable efforts have been directed toward the immobilization of Ru(bpy)32+ on an electrode * To whom correspondence should be addressed: (tel) (785) 532-1468; (fax) (785) 532-6665; (e-mail)
[email protected]. (1) Faulkner, L. R.; Bard, A. J. In Electroanalytical Chemistry, Bard, A. J., Ed.; Marcel Dekker: New York, 1977; Vol 10, pp 1-95. (2) Knight, A. W. Trends Anal. Chem. 1999, 18, 47-62 and references therein. (3) Lee, W.-Y. Mikrochim. Acta, 1997, 127, 19-39 and references therein. (4) Gerardi, R. D.; Barnett, N. W.; Lewis, S. W. Anal. Chim. Acta 1999, 378, 1-41 and references therein. 10.1021/ac9914519 CCC: $19.00 Published on Web 05/12/2000
© 2000 American Chemical Society
surface as a means to develop a cost-effective, regenerable chemical sensor.2-4 The direct immobilization of the chemiluminescent reagent on the electrode surface reduces the consumption of reagents and eliminates the need to incorporate an extra pump to deliver the reagent to the electrochemical cell.2-4 To date, several different approaches have been evaluated, including the immobilization of Ru(bpy)32+ in polymer layers on electrode surfaces or the direct attachment to an electrode as a monolayer via Langmuir-Blodgett or self-assembly techniques.5-15 The incorporation of Ru(bpy)32+ within the cation-exchange polymer Nafion, in particular, has received considerable attention.14,15 While these procedures have been shown to be promising, new materials and immobilization approaches are still needed to fabricate ECLbased sensors with improved reactivity and long-term stability. One promising approach for the immobilization of Ru(bpy)32+ at an electrode surface involves the use of Nafion-silica nanocomposites.16-21 These materials can be prepared by incorporating Nafion into a silica sol prepared by the acid- or basecatalyzed condensation of either tetramethoxysilane or tetraethoxysilane.16-21 Harmer and co-workers have previously shown that the acid groups in Nafion are more accessible in Nafion-silica composite gels, potentially offering greater activity.16 Heineman, Seliskar, and co-workers have also shown that the relatively slow diffusion of electroactive cations into Nafion can be improved via the use of Nafion-silica nanocomposites.17-21 The improved (5) Obeng, Y. S.; Bard, A. J. Langmuir 1991, 7, 195-201. (6) Miller, C. J.; McCord, P.; Bard, A. J. Langmuir 1991, 7, 2781-2787. (7) Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1980, 102, 6642-44. (8) Abruna, H. D.; Bard, A. J. J. Am. Chem. Soc. 1982, 104, 2641-2642. (9) Zhang, X.; Bard, A. J. J. Phys. Chem. 1988, 92, 5566. (10) Xu, X.-H.; Bard, A. J. Langmuir 1994, 10, 2409-2414. (11) Ghosh, P. K.; Bard, A. J. J. Electroanal. Chem. 1984, 169, 113 (12) Sykora, M. Meyer, T. J. Chem. Mater. 1999, 11, 1186. (13) (a) Zhao, C.-A.; Egashira, N.; Kurauchi, Y.; Ohga, K. Anal. Sci. 1998, 14, 439-441. (b) Zhao, C.-Z. Egashira, N.; Kurauchi, Y.; Ohga, K. Anal. Sci. 1997, 13, 333 (14) Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1981, 103, 5007-5013. (15) (a) Downey, T. M.; Nieman, T. A. Anal. Chem. 1992, 64, 261-268. (b) Lee, W. Y.; Nieman, T. A. Anal. Chem. 1995, 67, 1789-96. (16) Harmer, M. A.; Farneth, W. E.; Sun, Q. J. Am. Chem. Soc. 1996, 118, 77087715. (17) Slaterbeck, A. F.; Ridgway, T.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 1999, 71, 1196-1203. (18) Shi, Y.; Seliskar, C. J. Chem. Mater. 1997, 9, 821-829. (19) Hu, Z.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 1998, 70, 5230-5236. (20) Hu, Z.; Slaterbeck, A. F.; Seliskar, C. J.; Ridgway, T. H.; Heineman, W. R. Langmuir 1999, 15, 767-773. (21) Barroso-Fernandez, B.; Lee-Alvarez, M. T.; Seliskar, C. J.; Heineman, W. R. Anal. Chim. Acta 1998, 370, 221
Analytical Chemistry, Vol. 72, No. 13, July 1, 2000 2943
accessibility of these ion-exchange sites coupled with the faster uptake of reagents into these materials will be of benefit to ECLbased applications as the sensitivity of this method can be enhanced and detection limits lowered. In the present work, we describe the preparation of Nafionsilica composite films using a two-step acid-/base-catalyzed solgel process. The stability, electrochemical activity, and ECL of Ru(bpy)32+ immobilized within these composite films have been examined as a function of the amount of Nafion incorporated into the sol. These results show that the Nafion-silica composites have greater sensitivity and lower detection limits relative to pure Nafion films for the determination of tripropylamine or oxalate in solution. EXPERIMENTAL SECTION Reagents and Equipment. Tetramethoxysilane (TMOS, 99%), Nafion (5 wt % in aliphatic alcohols and water, EW ) 1100), tripropylamine (TPA, 98%), sodium oxalate (99.5%), and tris(2,2′bipyridyl)dichlororuthenium(II) (Ru(bpy)32+, 98%) were purchased from Aldrich and used as received. Hydrochloric acid, potassium nitrate, ammonium hydroxide, and potassium acetate were purchased from Fisher Scientific. Water was purified to type I using a Labconco Water Pro PS four-cartridge system. Electrochemical measurements were performed with a BAS CV-50W (Bioanalytical Systems, Inc.) voltammetric analyzer using a one-chamber, threeelectrode cell. The working electrode consisted of a glassy carbon electrode (A ) 0.2 cm2) prepared as previously described.22 The reference and auxiliary electrodes were a silver/silver chloride electrode (1 M KCl) and a platinum wire electrode, respectively. The same cell configuration was used in the ECL experiments. The one-chamber ECL cell was placed in a dark box ∼3.5 cm from the photocathode of a Hamamatsu 4632 photomultiplier tube. This tube was chosen because of its low dark counts (∼50-70 counts/s) at room temperature. A high-voltage power supply (Bertan Series 230) applied -800 V to the PMT. The PMT signal was amplified by a fast preamplifier (EGG Ortec VT 120A), and the data were collected with a multichannel scaler (i.e., photon counter, EGG Ortec T-914).23 The time per bin was 100 ms/point. Film thickness was measured with a Tencor Alpha Step 500 surface profiler. For the pH studies, the buffers (0.01 M) were prepared with either potassium phosphate (pH 6, 7, 8) or potassium acetate (pH 4, 5, 6). The ionic strength was kept constant via addition of 0.05 M potassium nitrate. Procedures. The composite Nafion-silica materials were prepared via the two-step acid-/base-catalyzed hydrolysis and condensation of TMOS in a manner similar to that previously described.16 The silica sol was prepared by mixing 0.5 mL of TMOS with 0.43 mL of deionized water and 0.1 mL of 0.1 M hydrochloric acid followed by stirring for 1 h. After 24 h, the hydrolyzed silica sol was added to a solution of Nafion and 0.1 M ammonium hydroxide (2:1, v/v). The volume of silica sol added to the Nafion solution was varied to change the mass ratio of Nafion to SiO2 from 53 to 88%, assuming complete conversion of TMOS to SiO2. The final mixture was vigorously stirred and allowed to sit at room temperature (20-22 °C) until just before gelation. The resultant viscous sol was cast on the surface of a glassy carbon electrode at ∼7000 rpm using an in-house-built (22) Collinson, M. M.; Rausch, C. G.; Voigt, A. Langmuir 1997, 13, 7245-7251. (23) Collinson, M. M.; Taussig, J.; Martin, S. M. Chem. Mater. 1999, 11, 25942599.
2944 Analytical Chemistry, Vol. 72, No. 13, July 1, 2000
Figure 1. Cyclic voltammograms of 0.01 mM Ru(bpy)32+ in 0.1 M acetate buffer, pH 5 at a bare glassy carbon electrode (dashed line) and at a Nafion-silica (83%) modified electrode (solid line). Scan rate, 100 mV/s. Inset: plot of anodic peak current versus immersion time in solution. (b) Nafion film; (O) Nafion-silica film (83%).
rotator. The films were uniformly and consistently blue in color. To decrease the gelation time, the samples with high contents of Nafion were often heated in a water bath at 50° C for 5-10 min after sitting for 1 h at room temperature. The 100% Nafion films were prepared by spin casting an aliquot of the Nafion/ammonium hydroxide solution on the glassy carbon electrode as described above. The thin films were typically dried for one to three days under room conditions (30-45% RH, ambient temperature). Prior to spin casting, the electrodes were polished with 0.05-µm alumina particles on a napless polishing cloth, sonicated in water for ∼10 min, and then dried. The film thickness was measured to be 120140 nm for the 56-88% Nafion-silica films using surface profilometry. The thickness of the pure Nafion films was difficult to accurately determine via profilometry but was estimated to be ∼100-200 nm. RESULTS AND DISCUSSION Voltammetric Characterization of Immobilized Ru(bpy)32+. The ion exchange and electroactivity of Ru(bpy)32+ in the twostep acid-/base-catalyzed Nafion-silica composite films were initially studied using cyclic voltammetry to provide information about reagent entrapment, activity, and stability. Ru(bpy)32+ can be readily incorporated into the Nafion-silica composite film via placing the electrode in a dilute Ru(bpy)32+ solution for a given period of time. Figure 1 shows the cyclic voltammograms (CVs) of 0.01 mM Ru(bpy)32+ in 0.1 M acetate buffer, pH 5, at a 83% Nafion-silica modified electrode (after 90 min in solution) and at a bare glassy carbon electrode. At the bare electrode, only a slight peak (shoulder) due to the oxidation of Ru(bpy)32+ in solution can be observed near 1.1 V. In contrast, a large distinguishable response can be seen at the modified electrode due to the preconcentration of Ru(bpy)32+ in the film as it sat in solution. The total peak current (Faradaic and non-Faradaic) was
10-fold higher at the modified electrode compared to that observed at the bare electrode. The anodic peak current acquired from the CVs obtained every 10 min after immersion in a 0.01 mM Ru(bpy)32+ solution is plotted as open circles in the Figure 1 inset. As can be seen, the ion exchange of Ru(bpy)32+ into the Nafion-silica composite film (83%) is relatively fast, with the maximum peak current occurring within 60-90 min in an unstirred 0.01 mM Ru(bpy)32+ in 0.1 M acetate buffer, pH 5. At higher concentrations, the uptake rate was considerably faster for this film, with the maximum current being achieved typically within 15 min in a 0.1 mM Ru(bpy)32+ solution. The ion exchange of Ru(bpy)32+ in a pure Nafion film under the same conditions is much slower, with it taking greater than 3 h to reach a similar maximum value in peak current (Figure 1, inset, closed circles). The ∼2-3-fold increase in the uptake rate of Ru(bpy)32+ into the composite films can be attributed to greater accessibility of the of the SO3- sites in the composite materials.16,19,20 Similar results have been observed in previous work for the ion exchange of Re(DMPE)3+ into Nafion-silica composites prepared from acid-catalyzed sols with high water content or lower Nafion content.19-21 Harmer and co-workers have also noted a faster uptake rate for 2-propanol into Nafion-silica composite gels relative to the pure polymer.16 The stability of Ru(bpy)32+ exchanged within the silica-Nafion composite films was very good. When the Ru(bpy)32+-immobilized composite electrode (83%) was placed in 0.1 M KNO3 or 0.1 M, pH 5 acetate buffer, the CV obtained was essentially indistinguishable from that obtained when the electrode was immersed in a 0.1 mM Ru(bpy)32+ solution. Little change in the shape or peak current was observed for immobilized Ru(bpy)32+ when the electrode was either stored dried or stored in 0.1 M KNO3 for up to 5-6 days. For both storage conditions, the electrodes were placed in 0.1 mM Ru(bpy)32+ solution for 5 min before the voltammograms were taken. After ∼5-6 days, the anodic peak current becomes smaller albeit the cathodic peak remains essentially the same. The decrease in peak current may be due to the partition of Ru(bpy)32+ in the more hydrophobic regions of Nafion as noted in prior work for pure Nafion films.15a In comparison, the electrochemical response for pure Nafion films typically diminished after 1 day of storage in buffer. The addition of the silica through the formation of a Nafion-silica composite film obviously improves the long-term stability of this immobilized redox system. The electrochemical stability of Ru(bpy)32+ immobilized in the Nafion-silica composite films was also good. When the electrode potential of a 83% Nafion-silica film modified electrode in acetate buffer was repetitively cycled at 100 mV/s for over 20 min, less than an 8% loss in Faradaic peak current was observed. In direct contrast, when Ru(bpy)32+ is immobilized in a silica film fabricated from the acid-catalyzed hydrolysis of TMOS, it leaches is out of the film in a relatively short period of time.22,24 Nafion exhibits very large ion-exchange selectivity coefficients (106-107) for hydrophobic ions25 such as Ru(bpy)32+ thus preventing the immobilized reagent from leaching into solution. The shape of the cyclic voltammetric curve is highly dependent on the amount of Nafion incorporated into the composite. Figure (24) Dvorak O.; DeArmond. M. K. J. Phys. Chem. 1993, 97, 2646-. (25) Szentirmay, M. N.; Martin, C. R. Anal. Chem. 1984, 56, 1898-1902.
Figure 2. Cyclic voltammograms of Ru(bpy)32+ ion-exchanged in a 53% and a 83% Nafion-silica composite film (solid line) and at a bare electrode (dashed line) in 0.01 M acetate buffer, pH 5, with 0.05 M KNO3. Scan rate, 100 mV/s. Inset: anodic peak current (one trial) versus scan rate (ν). The solid line represents the linear regression fit to the experimental data acquired at 5, 10, and 50 mV/s for the 53% composite and 10, 50, and 100 mV/s for the 83% composite.
2 shows the CVs acquired at 100 mV/s for Ru(bpy)32+ immobilized in a 53% and a 83% Nafion-silica composite film. When the amount of Nafion incorporated into the film is comparably low,
