Anal. Chem. 2004, 76, 1768-1772
Effect of Nonionic Fluorosurfactant on the Electrogenerated Chemiluminescence of the Tris(2,2′-bipyridine)ruthenium(II)/Tri-n-propylamine System: Lower Oxidation Potential and Higher Emission Intensity Feng Li and Yanbing Zu*
Department of Chemistry, The University of Hong Kong, Hong Kong, P.R. China
Fluorosurfactants are commercially available, and their applications in electrochemical systems have been the interest of many studies. Here, we describe a novel effect of a nonionic fluorosurfactant (Zonyl FSN) on the electrogenerated chemiluminescence (ECL) of the tris(2,2′bipyridine)ruthenium(II)/tri-n-propylamine (TPrA) system at gold and platinum electrodes. Compared with its hydrocarbon analogue (Triton X-100), the adsorbed fluorosurfactant species not only rendered the electrode surfaces more hydrophobic but also significantly retarded the growth of the electrode oxide layers. As a result, more facile direct oxidation of TPrA was achieved, which led to the appearance of a low oxidation potential ECL signal (below 1.0 V vs SCE). At the gold electrode, the ECL peak appeared at 0.82 V, ∼400 mV more negative than usual; while its intensity was ∼50 times higher. The generation of the intense ECL signal at low oxidation potential may lead to the development of more efficient ECL analysis. The electrogenerated chemiluminescence (ECL) of Ru(bpy)32+ (bpy ) 2,2′-bipyridine) with aliphatic amines has been studied for more than one decade.1-9 Due to its high ECL efficiency, the Ru(bpy)32+/tri-n-propylamine (TPrA) system is usually adopted in analytical ECL applications, such as immunoassay and DNA analysis, with Ru(bpy)32+ or its derivatives as ECL labels.7 Several reaction mechanisms have been proposed for the ECL process. The formation of the excited state, Ru(bpy)32+*, is generally believed to be based on the electron transfer between Ru(bpy)33+ and a strongly reducing species, TPrA•, generated upon the (1) Noffsinger, J. B.; Danielson, N. D. Anal. Chem. 1987, 59, 865. (2) Leland, J. K.; Powell, M. J. J. Electrochem. Soc. 1990, 137, 3127. (3) Zu, Y.; Bard, A. J. Anal. Chem. 2000, 72, 3223. (4) Kanoufi, F.; Zu, Y.; Bard, A. J. J. Phys. Chem. B 2001, 105, 210. (5) Zu, Y.; Bard, A. J. Anal. Chem. 2001, 73, 3960. (6) Gross, E. M.; Pastore, P.; Wightman, R. M. J. Phys. Chem. B 2001, 105, 8732. (7) Bard, A. J.; Debad, J. D.; Leland, J. K.; Sigal, G. B.; Wilbur, J. L.; Wohlstadter, J. N. Chemiluminescence, Electrogenerated. In Encyclopedia of Analytical Chemistry: Applications, Theory and Instrumentation; Meyers, R. A., Ed.; John Wiley & Sons: New York, 2000; Vol. 11, p 9842, and references therein. (8) Miao, W.; Choi, J.-P.; Bard, A. J. J. Am. Chem. Soc. 2002, 124, 14478. (9) (a) Factor, B.; Muegge, B.; Workman, S.; Bolton, E.; Bos, J.; Richter, M. M. Anal. Chem. 2001, 73, 4621. (b) Cole, C.; Muegge, B. D.; Richter, M. M. Anal. Chem. 2003, 75, 601.
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oxidation of TPrA.1-4 Recently, a new route involving the reaction of Ru(bpy)3+ with TPrA cation radical (TPrA•+) was found to play an important role in producing the excited state.8 Some other important features of this ECL system, such as the role of direct coreactant oxidation,3,8 the dependence of emission intensity on the electrode material,3 and the surfactant effects,5,9 have also been investigated. Now, it is clear that the light emission of the Ru(bpy)32+/TPrA system depends significantly on the direct oxidation rate of the coreactant, especially when the Ru(bpy)32+ concentration is low ( 0.01 wt %, a fairly stable ECL intensity was obtained with 6-7-fold enhancement (see Figure 3 inset). In the presence of dilute FSN ([FSN] < 0.005 wt %), a similar enhancement effect on both TPrA oxidation current and the ECL intensity was observed, as shown in Figure 4. The intensity of the ECL peak at ∼1.25 V reached its maximum at [FSN] ∼ 0.005 wt %, and then an obvious drop occurred (see Figure 4 inset). When [FSN] > 0.005 wt %, a larger TPrA oxidation current was observed in the potential region between 0.8 and 1.1 V. Simultaneously, a new intense ECL peak appeared below 1.0 V. The intensity of this low oxidation potential ECL peak increased with increasing [FSN], and the maximum value was achieved at [FSN] ∼ 0.05 wt %. The peak potential of the ECL signal was ∼0.82 V and its maximum intensity was ∼50 times larger than that obtained at ∼1.25 V in the surfactant-free solution. Further increase of FSN quantity led to a drop of the ECL intensity, while TPrA oxidation current did not decrease (see Figure S2 in the Supporting Information), the reason for which is unclear. Platinum Electrode. A platinum electrode usually produces a weaker ECL signal compared with a gold electrode, which has been attributed to the slower oxidation rate of TPrA.3 The effect of Triton X-100 on the ECL at the Pt electrode was very similar to that at the gold electrode, as shown in Figure 5. TPrA oxidation current as well as the ECL intensity increased in the presence of Triton X-100. The addition of FSN also enhanced the intensity of the ECL peak at ∼1.1 V. When [FSN] > 0.01 wt %, a low oxidation potential ECL peak appeared at ∼0.85 V; however, its intensity was much lower than that observed at the gold electrode.
Figure 4. Cyclic voltammograms and corresponding ECL signals for 1 µM Ru(bpy)32+, 0.1 M TPrA, and 0.15 M PBS (pH 7.6) at a 2-mm-diameter gold electrode in the absence (solid line) and presence of 0.005 (dashed line) or 0.05 wt % FSN (dotted line). Scan rate, 0.1 V/s. Inset: ECL peak intensity as a function of [FSN] (ECL1 at ∼0.82 V, ECL2 at ∼1.25 V).
Figure 5. Cyclic voltammograms and corresponding ECL signals for 1 µM Ru(bpy)32+, 0.1 M TPrA, and 0.15 M PBS (pH 7.6) at a 2-mm-diameter platinum electrode in the absence (solid line) and presence of 0.1 wt % Triton X-100 (dashed line) or 0.5 wt % FSN (dotted line). Scan rate, 0.1 V/s. Inset: ECL peak intensity as a function of [Triton X-100] or [FSN] (FSN ECL1 at ∼0.85 V, FSN ECL2 at ∼1.1 V.)
The standard oxidation potential of TPrA in aqueous solution has been determined to lie between 0.8 and 0.9 V versus SCE.4 Previous studies demonstrated that the oxidation of the TPrA molecule at Au or Pt electrodes is inhibited by the electrode surface oxide layer3 and is also significantly affected by the hydrophobicity of the electrode surface.5,9 As discussed above, the adsorption of Triton X-100 species did not influence the oxide layer growth at the Au electrode and only slightly affected the same process at the Pt electrode. The incomplete FSN adsorption layer (when [FSN] < 0.005 wt %) also exhibited similar behavior. In other words, in the presence of Triton X-100 or dilute FSN, both the Au and Pt electrodes were covered with oxide layers in the potential region where TPrA oxidation occurred and the ECL signals were produced. However, the electrode surfaces were rendered more hydrophobic by the adsorbed surfactant molecules.
In these cases, the enhancements of both TPrA oxidation current and the ECL intensity should be attributed to the alteration of electrode surface hydrophobicity. In the presence of concentrated FSN in solution, a compact FSN adsorption layer formed at the electrode/electrolyte interface and the growth of the electrode oxide layer was significantly retarded. At the Au and Pt electrodes, evident oxidation of the electrode surface occurred only when the potential was positively scanned beyond ∼0.85 and ∼0.7 V, respectively. Therefore, the compact FSN adsorption layer not only rendered the electrode surface more hydrophobic but also effectively suppressed the influence of the electrode oxide layer on TPrA oxidation (see Figure 2). This led to the more remarkable enhancement of TPrA oxidation current below 1.0 V. The FSN effect was much weaker at the Pt electrode due to the earlier rearrangement/desorption of the adsorbed surfactant species. Following the removal of the compact FSN adsorption layer, the electrode surface was oxidized suddenly, resulting in a rapid drop of TPrA oxidation rate. As shown in Figures 4 and 5, a low oxidation potential ECL peak appeared at ∼0.8-0.9 V when concentrated FSN existed. The appearance of the large ECL peak at 0.82 V at the gold electrode was striking. The mechanism for the ECL signal of the Ru(bpy)32+/TPrA system in the potential region more negative than 1.0 V has been studied recently.8 It was proposed that this ECL process may involve the intermediacy of TPrA cation radicals in the generation of Ru(bpy)32+*:
TPrA - e f TPrA•+ f TPrA• + H+
(1)
Ru(bpy)32+ + TPrA• f Ru(bpy)3+ + products
(2)
Ru(bpy)3+ + TPrA•+ f Ru(bpy)32+* + TPrA
(3)
Ru(bpy)32+*f Ru(bpy)32+ + hv
(4)
where TPrA•+ ) (CH3CH2CH2)3N•+ and TPrA• ) (CH3CH2CH2)2NC•HCH2CH3. In this route, only TPrA is oxidized on the electrode while the oxidation of Ru(bpy)32+ is not required. The low oxidation potential ECL signal is evident at a clean glassy carbon electrode due to the fast TPrA oxidation rate.3,8 However, since the reproducibility of a glassy carbon surface is generally bad in an aqueous solution,17 it is difficult to adopt the ECL signal below 1.0 V in analytical applications. On the contrary, a gold electrode surface is easy to clean electrochemically in situ, and this work demonstrated that the adsorption of FSN species could make the gold electrode suitable to generate the low oxidation potential ECL with high efficiency. Therefore, it is expected that a more efficient analytical method based on the ECL at lower oxidation potential but with higher emission intensity may be developed. A detailed characterization of the low oxidation potential ECL signal is now underway. CONCLUSIONS The adsorption behavior of a nonionic fluorosurfactant, FSN, at gold and platinum electrodes was examined and compared with its hydrocarbon analogue, Triton X-100. Although the head-on (17) McCreery, R. L. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1992; Vol. 17.
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adsorption mode is favorable for both surfactant species, their influences on the cyclic voltammograms of the noble metal electrodes are quite different. A compact FSN adsorption layer significantly retarded the growth of the electrode oxide layer, while Triton X-100 exhibited much weaker impact. This suggests that the assembly of the more hydrophobic fluorinated chains adjacent to the electrode may preclude the access of H2O to the electrode surface much more effectively compared with the hydrocarbon chains. The ECL of the Ru(bpy)32+/TPrA system is influenced by electrode surface hydrophobicity as well as the surface oxide layer. Triton X-100 and dilute FSN could render the electrode surface more hydrophobic and facilitate the direct oxidation of TPrA, leading to the enhancement of ECL intensity. In these cases, the ECL peak potential remained unchanged and no evident ECL signal was observed below 1.0 V. In the presence of concentrated FSN, however, the compact fluorosurfactant adsorption layer formed at the electrode/electrolyte interface would not only alter the hydrophobicity of the electrode surface but also inhibit the growth of electrode surface oxide layer. At the gold electrode, TPrA oxidation current was significantly enhanced between 0.8 and 1.1 V, and a new ECL peak at ∼0.82 V was obtained. The
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maximum intensity of the low oxidation potential ECL peak was ∼50 times larger than that of the ECL peak at 1.25 V in the surfactant-free solution. A low oxidation potential is usually preferred to avoid undesirable anodic reactions when using Ru(bpy)32+ or its derivatives as ECL labels in biosystems. The intense low oxidation potential ECL signal generated at the gold electrode in the presence of the nonionic fluorosurfactant may lead to the development of more efficient ECL analysis. ACKNOWLEDGMENT We thank Professor Allen J. Bard and Professor Chuansin Cha for helpful discussions. This work was supported by a grant from the University of Hong Kong. SUPPORTING INFORMATION AVAILABLE Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.
Received for review October 6, 2003. Accepted January 2, 2004. AC035181C
