Anal. Chem. 1992, 64, 1040-1844
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Characterization of Select Organic Analytes in Reverse Micelles Using Lanthanide Counterions as Acceptors Averrin G. Mwalupindi, Thilivhali T. NdouJ and Isiah M. Warner' Department of Chemistry, Emory University, Atlanta, Georgia 30322
A rlgntflcant enhancement of sendtlzed roomtemperature lumlnercence has been demonstratedudng synthedzed surfactants contalnlng the lanthanide counterlons Tb( 111) and Eu(II1). Varlou, organlc analytes have been used as senrHlzersto donate trlplet-state energy to the 4f energy kvel of the ianthanlde Ion. The enhancement facton, were obtalnud by use of lumlnercence measurements of lanthanide Ions in reverse mlcellar systems wlth organlc analytes solubilized In the aqueous core. The results are used, wlth other iumlnescence parameters, to correlate the Interaction between the donor and acceptor mokcuks. Proton (94)NMR spectroecopy has provided evldence that organlc analytes are solublllzed at the polar end of the reverse micelles.
INTRODUCTION Phosphorescence is rarely observed at room temperature unless measures are taken to provide a rigid molecular environment to reduce the quenching effect of dissolved oxygen and other impurities in solution. Phosphor-sensitized room-temperature phosphorescence is a technique developed to induce room-temperature luminescence from various molecules.' The technique involves the transfer of energy from a donor analyte to an acceptor molecule which can readily emit luminescence. The luminescence of the lanthanide compounds is usually very weak due to low absorption of the metal ion itself. However, the luminescence is dramatically enhanced when the metal ion is sensitized by use of an appropriate organic l i g a ~ ~ dThis . ~ J sensitization process consists of energy transfer from a triplet state of the organic ligand to some appropriate 4f energy level (resonance level) of the lanthanide ion. The lanthanide ion may then undergo a radiative transition, resulting in the characteristic and quantitative line emission of the ion. The luminescence intensity of the lanthanide ion is dependent on two important factors.4 First, to increase the probability of the transition from triplet to resonance level, the energy of the resonance level should be close to, but somewhat lower than, that of the triplet level. Second, the probability of nonradiative deactivation of the resonance level should be small compared to that of the radiative transition. On the basis of these factors, lanthanides which can be used as acceptors are Sm(III),Dy(III), Tb(III), and Eu(II1). The latter two ions are used in this work because of their longer decay times and, hence, intense luminescence intensities. In addition, the luminescence of Tb(II1) and Eu(II1) is not
* To whom correspondence
should be addressed. Present address: Gillette Research Institute, Gaithersburg, MD 20879. (1)Donkerbroek, J. J.; Elzas, J. J.; Groojier, C.; Frei, R. W.; Velthorst, N.Talanta 1981,28,717-721. (2)Weissman, S. I. J. Chem. Phys. 1942,10,214-217. (3)Crosby, G. A.; Whan, R. E.; Alire, R. M. J. Chem. Phys. 1961,34, 743-748. (4)Soini, E.; Lovgren, T. CRC C r i t . Reu. Anal. Chem. 1987,18,105154. +
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quenched by most organic molecules used as sensitizers in fluid solution.5 Recently, encapsulation of lanthanide ions into suitable ligands has yielded highly luminescent species which can be used as probes for a variety of applications. Examples of such ligands include diazapolyoxamacrobicyclics(cryptands),6 calixarenes? and crown ethers.8 The ligands provide protection against deactivating interactions between the luminescent lanthanide ion and solute quenchers. In these cases, the ligands contain strong absorbing chromophores which transfer the energy to the lanthanide ion. It is well established that the luminescence of the lanthanide ions is much more enhanced if the phosphor is incorporated into a micellar system.*l* The micelle organizes the donor and acceptor molecules into a very small volume, thus facilitating the energy-transfer process. In reverse micelles, the energy transfer becomes very efficient when both the donor and acceptor molecules are compartmentalized in the micellar core. The phosphors used in this work are counterions of the surfactant which can form reverse micelles when appropriate amounts are dissolved in cyclohexane and water. Significant enhancement of sensitized luminescence has already been reported using naphthalene acetic acid as the donor.12 The efficient energy transfer between the donor and acceptor was attributed to close solubilization of the molecules at the water/organic interface. In the present investigation, the nature of the interaction and the solubilization sites are simultaneously determined from sensitized luminescence measurements of Tb(II1) using select organic compounds as analytes and 1H NMR measurements of caffeine in the presence and absence of reverse micelles. In addition, the limits of detection of selected organic analytes in Tb(II1) reverse micelles are compared with those obtained using Tb(II1) salts which are dissolved in AOT reverse micelles.
EXPERIMENTAL SECTION Materials. The surfactant, AOT, waa obtained from Aldrich and purified as describedelsewhere.l2 Theophyllineand caffeine were purchased from Sigma Chemical Co. Acetylsalicylic acid, biphenyl, 2-naphthaleneaceticacid (NAA),terbium chloride,europium chloride,and deuterated chloroform were purchased from Aldrich Chemical Co., Milwaukee, WI. The 4,4-dichlorobiphenyl, 4-chlorobiphenyl, and 2-chlorobiphenyl compounds were (5)Filipescu, N.;Mushruah, G. W. J. Phys. Chem. 1968,72, 35163522. (6)Sabbatini, N.;Guardigli, M.; Mecati, A.; Balzani, V.; Ungaro, R.; Ghidini, E.; Casnati, A.; Pochini, A. J.Chem. SOC.,Chem. Commun. 1990, 878-879. (7)Pappalardo, S.;Bottino, F.; Giunta, L.; Pietraszkiewics, M.; Karpiuk, J. J . Inclusion Phenom. Mol. Recognit. Chem. 1991.10,387-392. (8)Tran, C. D.; Zhang, W. Anal. Chem. 1990,62, 835-840. (9)Almgren, M.; Greiser, F.; Thomas, J. K. J. Am. Chem. SOC.1979, 101,2023-2026. (10)Gelade, E.;DeSchryver, F. C. J.Am. Chem. SOC.1984,106,58715875. (11)Gelade, E.; Boens, N.; DeSchryver, F. C. J.Am. Chem. SOC.1982, 104,6288-6292. (12)Mwalupindi, A. G.; Blyshak, L. A.; Ndou, T. T.; Warner, I. M. Anal. Chem. 1991,63, 1328-1332. 0 1992 American Chemical Society
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