Langmuir 1996, 12, 3759-3762
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Articles Chemiluminescence in Microemulsions: Effect of Phase Composition Shlomo Cohen and Shlomo Magdassi* Casali Institute of Applied Chemistry, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel Received August 28, 1995. In Final Form: March 27, 1996X Chemiluminescence (CL) reactions were studied in a model system based on a phase diagram which was composed of Triton X-100, 2-butanol, toluene, and water. The CL reaction was achieved by using trichlorophenyl oxalate, H2O2, and perylene as the fluorophore. The intensity of light (I0) and t1/2 were found to depend on both reactant concentrations and phase composition. The CL reaction was conducted in W/O microemulsions, and an inverse relation between I0 and t1/2 was found. These two parameters could also indicate the transition from a W/O emulsion to a W/O microemulsion. The effect of medium polarity was evaluated by fluorescence, absorbance, and chemiluminescence spectra.
Introduction Chemiluminescence reactions have stirred up a great deal interest in the last few years, as they offer a unique and sensitive means for analyzing molecules at very low concentrations and are able to compete, in sensitivity, even with the classical radiometric methods. Chemiluminescence (CL) arises whenever a chemical reaction produces an electronically excited product that either emits direct light while returning to the ground state or transfers its energy to another molecule, a fluorophore, which emits the light.1 There are several groups of synthetic molecules which are used for CL, among them the various peroxyoxalate esters. This group of molecules was discovered by Chandross2 and is based on oxidation of an oxalate ester by hydrogen peroxide.3,4 The peroxyoxalate-type CL reaction reported by Rauhut5 is a very efficient nonenzymatic reaction, having quantum yields as high as 25%.6 However, since many analytical applications are required in aqueous solutions, the use of this group of materials is limited due to the insolubility of the component and the fast hydrolysis which the peroxalate esters undergo in protic solutions.7 It was therefore suggested to overcome these problems by using organized assembly systems such as micellar solutions,8,9 a combination of micellar solutions with polymers,10 and liposomes.11 Surprisingly, only very few reports were published on the use of microemulsions as a medium for X
Abstract published in Advance ACS Abstracts, July 1, 1996.
(1) Hinze, W. L.; Srinivasan, N.; Smith, I. K.; Igarashi, S.; Hoshino, H. Advances in Multidimensional Luminescence; JAI Press, Inc.: Greenwich, CT, 1991; Vol. 1, pp 149-206. (2) Chandross, E. A. Tetrahedron Lett. 1963, 761. (3) Rauhut, M. M.; Bollyky, L. J.; Roberts, B. G.; Loy, M.; Whitman, R. H.; Iannotta, A. V.; Semsel, A. M.; Klarke, R. A. J. Am. Chem. Soc. 1967, 89, 2S, Dec. 6. (4) Gundermann, K. D.; McCapa, F. Chemiluminescence in Organic Chemistry; Springer-Verlag: New York, 1987. (5) Rauhut, M. M. Acc. Chem. Res. 1969, 2, 80. (6) Akio, T.; Masako, M.; Hidetoshi, A. In Bio and Chem.: Instruments and Applications; Van-Dyke, Knox, Eds.; CRC-Press, Inc.: Boca Raton, FL, 1985; Vol. 1, Chapter 8. (7) Thompson, R. B.; Shaw-McBee, S. E. Langmuir 1988, 4, 106110. (8) Steijger, O. M.; Van-Mastbergere, H. M.; Holrhio, J. J. M. Anal. Chim. Acta 1989, 217, 229-237. (9) Grayeski, M. L. 32nd Annual Report on Research Assistance by the Petroleum Research Fund; American Chemical Society: Washington, DC, 1988; p 203.
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CL reactions.7,12,13 Thompson et al.7 have studied several microemulsion systems, which contained various fluorophores, and oxalate esters. They showed that CL can be obtained in microemulsions based on ionic and nonionic surfactants, that the emission was a pseudo-first-order process, and that the light intensity was dependent on microemulsion formulation. The purpose of the present research is to investigate the physicochemical parameters which affect the CL reaction in a model-dispersed system. Materials and Methods TCPO (trichlorophenyl oxalate) and perylene were obtained from Sigma, H2O2 30% w/w and 2-butanol were obtained from Frutarom, Israel, A.R. grade toluene was obtained from J.T. Baker, and Triton X-100 was obtained from Merck. The fluorescence and chemiluminescence spectra were measured by LS-5 fluorimeter (Perkin-Elmer), and absorption spectra was measured by a U-2000 spectrophotometer (Hitachi). Light emission intensity was measured by using a photoelectric cell (United Detector), linked to a recorder and digital voltmeter, placed in an apparatus built to contain a standard glass test tube, having a constant volume of the sample. Since some of the components, or the final mixtures, were very viscous, the mixing of the various samples was performed while the cuvette was outside the measuring photoelectric cell. Therefore, the first reading of light intensity could be performed only about 20-30 s after the addition of the last component. By using this method, a thorough mixing was achieved, and reproducibility problems which could result from difficulties related to the high viscosity, as indicated by Thompson et al.,7 were minimized. In order to allow comparison between initial light intensities of the various compositions, an apparent maximal light intensity, I0, was determined as the light intensity observed 45 s after all components of each composition were mixed. In most cases these apparent I0 values were also the maximal light intensities obtained, after which a decay was observed. Obviously, the true maximal light intensity was somewhat higher than the I0 value. (For comparison, data shown by Thompson (10) Baretz, B. H.; Tszaskos, W. J.; Elliot, L.; Douplais, D. L.; Hartala, R. R. Final Technical Report to the Office of Naval Research (Contribution No. N00014-82-C-0202); June 1994; p 39; Chem. Abstr. 1994, 102, 211989S. (11) Mandle, R. M.; Wong, Y. N. U.S. Pat. 4372745, 1983. (12) Thompson, R. B. U.S. Pat. 253,635, 1988. (13) Hubry, J. M. Fr. Pat. 87-3891-870-320.
© 1996 American Chemical Society
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a
Figure 1. Phase diagram for the toluene-water-Triton X-100-2-butanol system. T indicates the region in which a temperature-sensitive microemulsion is formed. a, b, and c indicate the compositions at which droplet size measurements were conducted. et al.7 indicate that the peak in light intensity was obtained about 1 min after the addition of H2O2.) By using the decay curve and the apparent maximal light intensity, a t1/2 parameter was determined, which is the time required to reach the light intensity of I0/2. These two parameters, I0 and t1/2 were expected to represent each intensity-time decay curve. Electrical conductivity measurements were performed by a CDM-83 conductivity meter (El-Hama) for samples in which the aqueous phase was 1 mM NaCl. Droplet size distributions of the microemulsions were evaluated by dynamic light scattering performed with a ZEM 5002 Malvern Zeta master. The measurements were conducted without further dilution of the sample. Each measurement was performed for three different samples at each composition which was studied. A phase diagram was constructed by mixing various compositions of water, toluene, Triton X-100, and 2-butanol, at room temperature. Formation of the microemulsion region was judged by the classical criteria of an optically clear, stable solution.
b
c
Results and Discussion The systems used in this study were composed of toluene (which is a good solvent for TCPO and perylene), water, a nonionic surfactant (Triton X-100), and a cosurfactant (2-butanol). The first step was to construct a phase diagram for this system, without the CL reactants. As shown in Figure 1, the phase diagram consists of two regions: a two-phase region and a one-phase region, which exists in a wide concentration range. As evaluated by electrical conductivity measurements (performed for systems which contained a 1 mM NaCl aqueous solution), this region is composed of O/W and W/O microemulsions. In addition, the region termed T indicates temperaturesensitive systems, in which slight heating causes a shift toward a turbid, two-phase system. It was also impossible to determine the type of continuous or dispersed phase in this region. It should be emphasized that the phase diagram was evaluated by the existence of turbidity, while changing the various compositions of the systems. The presence of microemulsion droplets was confirmed by dynamic light scattering measurements: The droplet diameters of compositions represented by points A, B, and C in the phase diagrams were 37 ( 1, 35 ( 2, and 34.8 ( 0.3 nm, respectively. The chemiluminescence reactions were performed at various phase compositions and various reactant concentrations. In general, in the W/O microemulsion region,
Figure 2. Effect of reagent concentration on initial light intensity, I0: (a) trichlorophenyl oxalate; (b) perylene; (c) H2O2. The composition of the microemulsion is 65% w/w toluene, 15% w/w aqueous phase, and 20% w/w surfactant-cosurfactant 4:1 mixture. Multiple points represent duplicate results obtained under the same experimental conditions.
an intense blue light was observed, while the apparent maximal intensity, I0, and t1/2 were dependent on both reactant concentrations and phase composition. The effect of reactant concentration was evaluated by measuring I0 and t1/2, for a constant composition of the W/O microemulsion (65% w/w toluene, 15% w/w H2O, 20% w/w 4:1 surfactant-cosurfactant), while changing the concentration of one reactant only for each set of experiments. As shown in Figure 2, increasing the concentration
Chemiluminescence in Microemulsions
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Figure 4. Relation between I0 and t1/2 for all compositions which were evaluated in the phase diagram.
Figure 3. Changes in I0 and t1/2 as a function of concentration of surfactant-cosurfactant (4:1 w/w). The toluene-water weight ratio was constant and equal to 1. The transition from emulsion to microemulsion is presented by a solid line.
of all three reactants leads to an increase in I0. Furthermore, light intensity, as a function of TCPO and H2O2 concentration, reaches a plateau value above 10 mg/mL and 20% w/w, respectively. Such a plateau was not observed for perylene even at its solubility limit in toluene. Since H2O2 is present at high concentration, the reason for this behavior is not clear (the fluorophore, perylene, is repeatedly excited and is not expected to be consumed). For comparison, Rauhut et al.3 found that for a one-phase system no simple dependence exists, while Thompson et al.7 observed a plateau value explained by self-quenching of the fluorophore at high concentrations. The role of the phase composition was evaluated by performing the CL reactions, at constant amounts of all three reactants in each phase (by controlling their concentrations). In all compositions the ratio of tolueneaqueous phase was equal to 1. Addition of surfactantcosurfactant mixtures at various concentrations led to initial formation of a W/O emulsion, followed by a W/O microemulsion, according to the phase diagram. As presented in Figure 3, a sharp decrease in t1/2 is observed between 20 and 30% w/w surfactant-cosurfactant mixture. Accordingly, at the same concentration range, a sharp increase in I0 is observed. From the phase diagram (Figure 1), the transition from a two-phase system (W/O emulsion) to a one-phase system (W/O microemulsion) occurs at 22.5% surfactant-cosurfactant concentration.
Since both I0 and t1/2 change significantly at the transition point from emulsion to microemulsion, it should be concluded that the changes do not result simply from the change in turbidity of the system but rather from a change in reaction kinetics. A logical possibility is that the dramatic increase of interfacial area which is obtained at the microemulsion (droplet diameter about 35 nm) leads to an increased probability of successful collisions between the reactants in the continuous phase and in the water droplets. It is interesting to note that, while keeping the amount of the various reactants constant, I0 increased and t1/2 decreased with the increase in surfactant-cosurfactant concentration. In addition, we found a general trend that I0 is inversly proportional to t1/2, for all points in the phase diagram, as presented in Figure 4. This observation is expected if the total amount of photons, which are emitted during the reaction, is constant. This is indeed the case, if the reactions are performed at constant reactant amounts and if the overall quantum yield does not change. However, it should be noted that the quantum yield was not measured in this study. This phenomena should be taken into consideration in application of CL reactions based on microemulsions, in which both the initial light intensity and the duration of light emission can be tuned by choosing the appropriate phase composition within the W/O microemulsion region. The CL emission in the peroxyoxalate system results from the fluorophore.5,7 Therefore, we measured the fluorescence spectra of perylene, in pure solvents and in O/W or W/O microemulsion. As shown in Figure 5, the fluorescence spectra are sensitive to the medium polarity: While perylene is dissolved in toluene, three peaks are observed; the lowest is at 416 nm. Increasing the polarity of the medium by using DMSO led to an increase of the intensity of this peak in relation to the peak at 446 nm. A similar trend is observed while the fluorescence is measured in W/O microemulsion or O/W microemulsion. The ratio of light intensity at 416 and 446 nm, I416/I446, is presented in Table 1 for these four systems. It is clear that increasing the polarity of the medium increases this ratio, in agreement with results obtained for pyrene as a probe for medium polarity.14 From this data it can be concluded that the continuous phase in the W/O micro(14) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039.
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Figure 5. Fluorescence spectra of perylene in (a) toluene, (b) DMSO, (c) W/O microemulsion (40% w/w toluene, 15% w/w H2O, 45% w/w Triton X-100-1-butanol), and (d) O/W microemulsion (5% w/w toluene, 47% w/w H2O, 47.5% w/w Triton X-100-2-butanol). Table 1. I416/I446 Ratio for Pure Solvents and Microemulsions Containing 10 mg/mL Perylene in the Final Systema medium
I416/I446
toluene DMSO W/O microemulsion O/W microemulsion
0.07 0.61 0.76 0.99
a W/O microemulsion composition: water 15% w/w, toluene 40% w/w, Triton X-100 36% w/w, 2-butanol 9% w/w. O/W microemulsion composition: water 47.5% w/w, toluene 5% w/w, 2-butanol 9.5% w/w, Triton X-100 38% w/w.
emulsion is even more polar than DMSO alone, probably due to the presence of the surfactant and cosurfactant. The highest I416/I446 is obtained for the O/W microemulsion, indicating that the perylene exists in a very polar media. This result is consistent with the observation that the O/W microemulsion was yellow, corresponding to the presence of perylene in the continuous phase. The CL spectra in the W/O microemulsion were also recorded and compared to the fluorescence and absorbance of perylene in W/O microemulsion (Figure 6). Although we expect that the CL spectrum will coincide with the fluorescence spectrum, the CL emission spectrum is very broad, and the high intensity peaks are shifted toward high wavelengths. This result can be explained by the
Figure 6. (a) Absorbance spectra and (b) fluorescence spectra (excitation at 370 nm) of perylene in W/O microemulsions (40% w/w toluene, 15% w/w H2O, 45% w/w Triton X-100-2-butanol) and (c) chemiluminescence spectra in the same microemulsion composition.
overlapping of the fluorescence spectrum and the absorption spectrum, which causes an inner-filter effect, as was also observed by Thompson et al.7 for a different CL system. It should be noted that the time dependence of the spectra was not measured, since the light emission was detected at a wide range of wavelengths by the photoelectric cell. In conclusion, we have shown that chemiluminescence can be achieved in a wide range of compositions of microemulsions, based on a nonionic surfactant, Triton X-100. The light intensity is inversely proportional to t1/2, while having constant amounts of reactants, and the transition from W/O emulsion to W/O microemulsion is also reflected by these two parameters. We expect that, while taking into consideration the changes in fluorescence spectra and the effect of the phase composition on I0 and t1/2, it will be possible to develop interesting analytical applications using such systems. It should be emphasized that, in most microemulsions studied here, the initial light intensities were very high, were visible even to the naked eye, and could be measured very simply by low-sensitivity instruments. Clearly, at these high concentrations only very few applications could be obtained. However, the high light intensities allow analytical applications at concentrations which are normally of analytical interest, by using more sensitive instruments such as a fluorimeter. LA950733A
