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Langmuir 1988,4, 106-110
Peroxyoxalate Chemiluminescence in Microemulsions Richard B. Thompson* and Susan E. Shaw McBee BiolMolecular Engineering Branch, Code 6190, Naval Research Laboratory, Washington, D.C. 20375-5000 Received December 15, 1986. In Final Form: July 16, 1987 We report here the first study of peroxyoxalate chemiluminescence in microemulsion media (Winsor IV phases). Peroxyoxalate chemiluminescence was observed in both ionic and nonionic microemulsions using different oxalate derivatives and fluorescers. The chemiluminescence emission was typically pseudo first order and sensitive to microemulsion formulation, temperature, and reactant concentration. Sodium salicylate exhibited little catalytic effect. The implications of this work for analytical chemistry are discussed. Introduction Chemiluminescence technology is of growing importance in fields as diverse as clinical chemistry, analytical chemistry, illumination, and aquatic safety. The most efficient chemiluminescent reaction not catalyzed by an enzyme is that of hydrogen peroxide with derivatives of oxalic acid, termed peroxyoxalate chemiluminescence. This reaction was first described by Chandrossl and studied in depth by Rauhut, Mohan, and their colleagues.2 An outline of the reaction appears as Figure 1: the oxalate bis-ester 1 is oxidized by hydrogen peroxide to form the (putative) transient dioxetanedione intermediate282, which then transfers its energy to a fluorescer, 3, which emits it as a photon. The details of the reaction remain unelucidated, although excitation of the fluorescer is probably via an electron-transfer p r o ~ e s s . ~The quantum yield of the reaction ranges up to 35%, and the emission wavelength may be widely varied by choosing a suitable f l u ~ r e s c e r . ~ Despite its efficiency and wide use for illumination, peroxyoxalate chemiluminescence has been little used for chemical analysis in comparison to luminol derivative^.^ This is primarily due to the insolubility and lability of the oxalate derivatives (and fluorescers) in water or protic solvents. Seitz and his colleagues described their problems finding compatible solvents in creating a peroxyoxalate chemiluminescence flow injection analysis system for quantitating aqueous hydrogen peroxide.sa Rauhut, Mohan, and their collaborators obtained little improvement by performing the reaction in cyclohexane in water emulsions but obtained much better results by including a detergent above its critical micelle concentration2dor polymer additives (B. H. Baretz, personal communication). The American Cyanamide group has also synthesized oxalate derivatives and fluorescers that are somewhat water (1) Chandross, E. Tetrahedron Lett. 1963,761. (2)(a) Rauhut, M. M. Acc. Chem. Res. 1969,2,80.(b) Mohan, A. G.; Rauhut, M. M. Nau. Res. Rev. 1984,36(4),17. (c) Mohan, A. G.; Narburgh, R. L. Chemiluminescent Systems: Development of High Light Capacity Formulations; AD755221; Defense Technical Information
Center, Cameron Station: Alexandria, VA, 22304-6145,1972.(d) Mohan, A. G.; et al. Aqueous Peroxyoxalate Chemiluminescence; AD121396; Defense Technical Information Center, Cameron Station: Alexandria, VA, 22304-6145,1982, (3)(a) Faulkner, L. R. In Methods in Enzymology; DeLuca, M. A,, Ed.; Academic: New York, 1978; Vol. 57, p 494. (b) Schuster, G. B.; Schmidt, S. P. In Aduances in Physical Organic Chemistry; Gold, V., Bethell, D., Eds.; Academic: New York, 1982;Vol. 18, p 187. (4)(a) Tseng, S. S.; Mohan, A. G.; Hainea, L. G.; Vizcarra, L. S.; Rauhut, M. M. J. Org. Chem. 1979,44,4113.(b) Rauhut, M. M.; Roberts, B. G.; Madding, D. R.; Bergmark, W.; Coleman, R. J.Org. Chem. 1975, 40, 330. (5) (a) Scott, G.; Seitz, W. R.; Ambrose, J. Anal. Chim. Acta 1980,115, 221. (b) Honda, K.;Sekino, J.; Imai, K. Anal. Chem. 1983,55,940. (c) Arakawa, H.; Maeda, M.; Tsuji, A. Clin. Chem. 1985,31,430.(d) Honda, K.;Miyaguchi, K.; Nishino, H.; Tanaka, H.; Yao, T.; Imai, K. Anal. Biochem. 1986,153,50.
0743-7463/88/2404-OlO6$01.50/0
Table I. Compositions of Microemulsions” volume fraction surf/cosurf surf/cosurf oil SDS/BuOH (35,w/w) 0.43 0.09 CTAB/BuOH (Ll,w/w) 0.61 0.06 Triton X-lOO/HexOH (4:1,w/w) 0.49 0.49 0.41 0.06 Brij 96/BuOH (2:1,w/w) 0.50 0.10 0.31 0.63
water 0.48 0.33 0.02 0.47 0.40 0.06
The volume fractions indicate the proportions of the surfactant/cosurfactant mixtures (surf/cosurf), oil (toluene for the cases reported here), and water (0.4 M aqueous NaCl or distilled water for the ionic or nonionic surfactants, respectively) included in the microemulsions. The assignments of oil in water (O/W) or W/O in Table I1 are based on the relative proportions of water and oil in the formulations given above.
soluble, but these compounds are not yet widely available.2di6 We felt that carrying out the reaction in a microemulsion might alleviate these and other problems seen in adapting the peroxyoxalate reaction to aqueous media. Microemulsions have been defined as “thermodynamically stable isotropic solutions of surfactant, oil, and water”.’ A great variety of microemulsions have been formulated, and their principles of formation are an active field of research.8 They have unique properties which are especially useful for carrying out reactions employing immiscible reagents such as peroxyoxalate chemiluminescence,especially in the presence of aqueous media. In particular, microemulsions are emulsions that may contain up to tens of percent oil in water (or vice versa) and yet be completely stable and optically clear. This is because the solubilized droplets have been shown by several means to be 2CTAB/ l-b~tanol,’~ Brij-96/1-butanol,’o or sodium oleate/l-b~tanol~~ were formulated (11)Cazabat, A. M.;Langevin, D.; Meunier, J.; Pouchelon, A. Adu. Colloid Interface Sci. 1982, 16, 175. (12)Balasubramanian, D.;Kumar, C. In Solution Behauior of Surfactants: Mittai, K.L., Fendler, E. J., Eds.; Plenum: New York, 1982. (13) Mackay, R. A,; Hermansky, C. J. Phys. Chem. 1981, 85, 739.
Figure 2. Kinetics of chemiluminescence emission from CPPO, H202, 1-Cl-BPEA, and sodium salicylate in SDS/BuOH/ toluene/0.4 M aqueous NaCl microemulsion. The microemulsion was mixed with fluorescer, oxalate ester, and catalyst, as described in Materials and Methods, and added ta a fluorescence cuvette. After background (just the PMT dark current in this case) is measured, H202is added at 1.5 min and the intensity monitored at 500 nm; the log-linear response suggests a pseudo-first-order process is rate limiting. The derived decay rate is 5.7 X 10” s-l.
based on cited recipes or phase diagrams. The compositionswere selected to be well within the boundaries of the microemulsion phase in the published phase diagram to ensure stability of the emulsion when the chemiluminescent reagents were added and are tabulated in Table I. Some of the literature formulations used oils other than toluene; we chose it because it seemed to solubilizethe reagents better than aliphatics such as tetradecane. Clausse and others have shown that microemulsion formation is much less sensitive to the molecular structure of the oil than that of the surfactant or cosurfactant. Formation of a microemulsion was judged by the classical criteria of an optically clear, stable solution; the microemulsions were not further characterized. Peroxyoxalate chemiluminescent reactions were typically carried out as follows: The fluorescer was dissolved in toluene to 1 mg/mL, and the aqueous phase was added to the surfactant/cosurfactant mixture and agitated to form a clear phase. The sodium salicylate catalyst2e(0.2 mg/mL total volume) and the oxalate derivative (1.3 mg/mL total volume) were then added and agitated in the above order. The sample was immediatelyplaced in the luminometer,and, after a background reading was acquired, 100 p L of 30% hydrogen peroxide was added with shaking to initiate the reaction. Reproducibilityof peak intensities and decay rates is limited by the difficulty of accurately pipetting and thoroughly mixing the viscous surfactant/cosurfactant mixtures. Initial intensities and decay rates were repeatable to *lo%, but decay rates were sometimes nonegponentialor poorly reproducible. The kinetics, intensity, and color of the emission were measured on a Spex Fluorolog 2 fluorimeter operated without a lamp; chemiluminescenceemission spectra were not corrected for the response of the Hamamatau R928P photomultiplier.
Results The kinetics of the chemiluminescent reaction of the oxalate CPPO with hydrogen peroxide in the presence of the fluorescer 1-C1-BPEA and sodium salicylate in a toluene/l-butanol/SDS/aqueous 0.4 M NaCl microemulsion is depicted in Figure 2. The bright-green chemiluminescent emission decays in a pseudo-first-order fashion as shown by the semilogarithmic plot. This behavior is similar to that displayed by the peroxyoxalate system in organic and other chemiluminescent systems. The peak intensities measured for several microemulsion systems, using different fluorescers and oxalate derivatives, are collected in Table 11. For comparison, we used toluene (14)Schulman, J. H.; McRoberts, T. S. Discuss Faraday SOC.1948, 165.
108 Langmuir, Vol. 4, No. I , 1988
surf SDS
Table 11. P e a k Intensities of Peroxyoxalate Chemiluminescence i n Microemulsions" cosurf fluor oxalate 1, comment BuOH 1-Cl-BPEA CPPO 1.00 O/W, as in Figure
1-CI-BPEA I
/*'
2
Brij-96 Brij-96 Triton Triton CTAB SDS SDS SDS SDS
BuOH BuOH HexOH HexOH BuOH BuOH BuOH BuOH BuOH SDS BuOH Triton HexOH
1-C1-BPEA 1-C1-BPEA 1-C1-BPEA 1-Cl-BPEA 1-Cl-BPEA Perylene 2-CDAA BTEN Rubrene 1-Cl-BPEA 1-Cl-BPEA
CPPO 0.22 CPPO 0.99 CPPO 0.33 CPPO 0.46 CPPO
