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Anal. Chem. 1908, 6 0 , 2478-2482
(11) Bartlett, P. N.; Whltaker, R. G. J. Electroanal. Chem. 1987, 224. 37-48. (12) Lange, M. A.; Chambers, J. Q. Anal. Chlm. Acta 1985, 175, 89-97. (13) Claremnt, D. J.; Penton, C.; Pickup, J. C. J. Blomed. Eng. 1986, 8, 272-274. (14) Frew, J. E.; Green, M. J.; HiiI, H. A. 0. J . Chem. Techno/. Biotechnol. 1988, 36, 357-363. (15) Cass, A. E. G.; Davis, G.; Francis, G. D.: Hili, H. A. 0.; Aston, W. J.; Higgins, I. J.; Piotkin, E. V.; Scott, L. D. L.; Turner, A. P. F. Anal, Chem. 1984, 56, 667-671. (16) Lau, H. H.; Hart, H. J . Org. Chem. 1959, 24, 280-281. (17) VonStackeiburg, M.; Pilgram, M.; Toome, V. 2. Electrochem. 1953, 57, 342-350. (18) Cass, A. E. G.; Davis, G.; Green, M. J.; Hili, H. A. 0. J. Electroanal. Chem. 1985, 190, 117-127. (19) Nicholson, R. S.; Shain, I. Anal. Chem. 1984, 36, 706-723. (20) Weibel, M. K.; Bright, H. J. J . Biol. Chem. 1971, 246, 2734.
(21) Audebert, P.; Bidan, G.; Lapkowski. M. J. Chem. SOC.,Chem. Commun. 1988, 887-889. (22) Bidan, G.: Guglielmi, M. Synth. Met. 1986, 15, 49-58. (23) Diaz, A. F.; Castilio, H.: Kanazawa, K. K.; Logan, J. A,; Salmon, M.; Fajardo, 0. J. Nectroanal. C b m . 1982, 133, 233-239. (24) Laviron, E.; Rouiiier, L.; Degrand, C. J. Nectroanal. Chem. 1980, 112, 11-23. (25) Andrieux, C . P.; Saveant, J. M. J. Electroanal. Chem. 1978, 93, 163-168. (26) Davis, G. Biosensors 1985, 1 , 161-178. (27) Iannieiio, R. M.; Vacynych, A. M. Anal. Chem. 1981, 53, 2090-2095.
RECENEDfor review September 9,1987. Accepted August 10, 1988. The authors acknowledge the Science and Engineering Research Council for financial support.
Micellar Induced Simultaneous Enhancement of Fluorescence and Thermal Lensing Chieu D. Tran* and Timothy A. Van Fleet Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53233
Mlcelles have been found to slmultaneously enhance the fluorescence and thermal lens of solubilized substrates. From the studies of effects of varlous types of micelles on the fluorescence and thermal lens slgnals of the anthracene derivatives, It was found that the enhancement depends on the type of mlcelles and the structure of the substrate. Highest enhancement was achieved when there were strong interactions between them. For instance, the enhancement In the fluorescence and thermal lens of the 11-(9-anthroy1oxy)undecanoic acld, whose long hydrocarbon tall renders strong Interaction with micelles, specifically nonlonlc micelles, is about 5.5 and 3.7 times more than that for anthracene. An enhancement mechanlsm has been proposed In which the fluorescence Is enhanced because micelles Isolate and protect the analayte from quencher molecules as well as increase the viscosity of the medium. The thermal lens enhancement, on the other hand, Is due to the modlflcatlon of the thermooptlcal properties of water by micelles, namely, increase dn /dT and decrease thermal conductlvity.
Fluorescence and thermal lens are two of the most sensitive techniques in trace chemical analysis. The fluorescence technique is based on the measurement of emitted photons of an excited analyte while the thermal lens technique is based on the measurement of the heat generated by the nonradiative relaxation. The two techniques are thus complementary and have been used to determine fluorescent as well as nonfluorescent substances. Fluorescence can be enhanced by increasing the radiative processes of a molecule. In particular, isolation of the fluorescence molecule from quenching impurities and/or solvent molecules can be used to achieve this enhancement ( I , 2).
The thermal lens technique is based on the measurement of the temperature rise that is produced in an illuminated sample by nonradiative relaxation of the energy absorbed from a laser (3-7). Thus, its intensity can be enhanced by improving the nonradiative relaxation processes of the analyte and more significantly the thermooptical properties of the solvent. It
has been shown that higher sensitivity can be achieved by choosing solvents with a high temperature coefficient of the index of refraction, dnjdT, and low thermal conductivity, k, value. Generally, water is the worst medium for the thermal lens technique owing to its low d n / d T and high k value. Nonpolar solvents such as carbon tetrachloride and hydrocarbons are good thermooptical solvents because they have high d n j d T and low k values. At the same excitation laser intensity, thermal lens measurements in n-pentane and CCll are estimated to be 40 to 47 times more sensitive than those in water (7). I t is thus possible to enhance the fluorescence as well as thermal lens signals of an analyte by improving its radiative relaxation processes and performing the thermal lens measurement in a solvent that has high d n / d T and low k values (7). Surfactant organized assemblies such as micelles, reversed micelles, and microemulsions have been used extensively in recent years to enhance the fluorescence and to facilitate the measurement of phosphorescence at room temperature (8-18). These micellar effects are possible because of the unique characteristics of the surfactant-organized media such as their ability to compartmentalize the analyte. The radiative relaxation processes of the analyte are modified when it is solubilized in the micellar system because the micelles not only protect the analyte from quenching molecules but also modify the physical properties of the environment around it, i.e., viscosity, polarity, etc. It is possible to use micelles to simultaneously enhance the fluorescence as well as thermal lens of the analyte. The micelles enhance the fluorescence because they protect the analyte from quenching molecules. The thermal lens is enhanced because micelles modify the environment around the analyte from thermooptically poor water to thermooptically good hydrocarbon. In spite of its great potential, micelles have not been fully used to simultaneously enhance the sensitivity of these two techniques. All but one study has been reported but the reported work was based on the enhancement by reversed micelles (7). Such considerations prompted the present study, which aims to investigate the effect of aqueous micelles on the fluorescence and thermal lens of solubilized analyte. It will be demonstrated in this communication that micelles can simultaneously enhance the fluorescence as well as the thermal
0003-2700/88/0360-2478$01.50/0@ 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER 15, 1988
Chart I
\
2479
Table I. Thermooptical Properties of Solvents for Thermal Lens Measurementa /
solvent No
Anthracene ( A N )
2
CCl*
9-Nitroanthracene INAN)
n-pentane cyclohexane n-heptane n-hexane isooctane dioxane methanol water
9-Anthracene p r o p i o n i c a c i d (ANP)
HO
11.i9-Anthroyloxy)
104(dn/dT), K-1
1.03 1.13 1.24 1.26
-5.9
4.7
-5.5 -5.4
4.0 3.6 3.3
1.23 0.97 1.39 2.02 6.11
-5.0
-5.2 -4.9 -4.6 -4.7 -0.8
Elpb, MW-l
3.5 4.1 2.7
1.9 0.1
aData taken from Tran (7). *Enhancementper unit laser power in mW: X = 632.8 nm.
u n d e c a n o i c a c i d (ANb1
oH& OM 2-19-Anthroyloxy)
12, mW
cm-' K-'
p a l m i t i c a c i d (ANPA)
lens signal of an analyte. The enhancement was found to be dependent on the structure of the surfactant and the analyte. A mechanism for the enhancement will be proposed based on the results obtained from the systematic investigation of the effect of various types of micelles, i.e., cationic, anionic, nonionic, and zwitterionic, on anthracene-derivative substrates whose structures range from a nonsubstituent to the one having a long hydrocarbon tail.
EXPERIMENTAL SECTION Thermal lens signals were measured on a pump/probe thermal lens spectrometer using a Coherent Innova 100-10 argon ion laser operating at 351.1 nm as an excitation beam. Detailed information on this apparatus has been described previously (7). Absorption spectra were taken on a Perkin-Elmer 320 spectrophotometer. A Perkin-Elmer LS-5 spectrofluorometer interfaced with a Perkin-Elmer 3600 data station was used to measure fluorescence. The reported fluorescence intensity values were obtained by intergrating each fluorescence spectrum for the area under the curve. Anthracene (AN) and 9-nitroanthracene (NAN) were p u r c h e d from Aldrich. 9-Anthracenepropionic acid (ANP), 11-(9anthroy1oxy)undecanoicacid (ANU), and 2-(9-anthrolyoxy)palmitic acid (ANPA) (Chart I) were purchased from Molecular Probes (Junction City, OR) and used as received. Hexadecyltrimethylammonium bromide (CTAB), hexadecyltrimethylammonium chloride (CTAC),and sodium dodecyl sulfate (SDS) were products of Kodak. 3-(N-Dodecyl-N,N-dimethylammonium)propane-1-sulfonic acid (SB-12) was obtained from Serva Biochemical Co. (Westbury, NY). Tricosanethylene glycol mono-n-dodecyl ether (Cl2E, or Brij-35) was obtained from Pierce Chemical Co. (Rockford,IL) as ampuled aqueous solution of 10% (w/w). The homogeneous hexaethylene glycol mono-n-dodecyl ether (Cl2E6)was obtained from Nikko Chemical Co. (Tokyo, Japan) and used as received. Deionized water was distilled from all-glass distillation apparatus. M) of the antracene derivatives Stock solutions (1.25 X were prepared by dissolving AN, NAN, ANP, ANU, and ANPA in methanol. Micellar solutions were prepared by injecting 2-pL M (CTAB, CTAC, SDS, SB-12) aliquots into 10 mL of 1 X or 5% (w/w) (C12Es)or 3.3% (w/w) (Cl2EZ)surfactant solutions that had been filtered through a 0.2-wm Gelman membrane fiiter. The mixture was then mixed thoroughly with a vortex and sonicated by means of a Brasonic sonication bath. RESULTS AND DISCUSSION Thermal lens technique is based on the measurement of the heat generated in an illuminated sample by nonradiative relaxation of the energy absorbed from a laser. Its sensitivity
can, therefore, be enhanced by (a) increasing the yield for the nonradiative relaxation processes and (b) improving the thermooptical properties of the solvent. The latter possibility is deduced from the fact that when a modulated laser beam, which has a Gaussian intensity distribution is used to excite the sample, the heat generated is strongest at the center of the beam where the intensity is greatest. Consequently, a lenslike optical element is formed in the sample due to the temperature gradient between the center of the beam and the bulk sample. The effect of the thermal lens is generally measured as a relative change in the beam center intensity, L U ~ / in I ~the , far field. When a weak absorbing sample is located 31/2Zcbeyond the beam waist, this change is given by AIb,./Ibc
= 2.303EA
(1)
where 2, is the confocal distance and equals irw,2/X with w o the spot size of the beam at the beam waist and X the laser wavelength, A is the absorbance, and E is the enhancement in sensitivity over a conventional transmission measurement and is defined as
where d n / d T is the temperature coefficient of the index of refraction and k is the thermal conductivity of the solvent. It is clear from eq 2 that the sensitivity of the thermal lens technique also depends on the thermooptical properties of the solvent. Higher sensitivity per unit excitation laser power, i.e., high E / P value, can be achieved by choosing solvent with high d n / d T and low thermal conductivity values. The thermooptical and EIP values of some commonly employed solvents are summarized in Table I. Aqueous solutions are the worst medium for thermal lens technique owing to water's low d n / d T value and high k value. Generally, nonpolar solvents such as carbon tetrachloride and hydrocarbons are good thermooptical solvents because they have high d n / d T and low k values. At the same applied laser intensity, thermal lens measurements in n-pentane and CCll are calculated to be 40 and 47 times more sensitive than those in water (Table
I). The validity of this theory was investigated by studying the thermal lens signals of solute in different solvents having different thermooptical properties. Special attention was given in selecting the solute to ensure that it is soluble in a variety of solvents and the variation in its thermal lens signals is not due to the change in the photophysical processes but rather due only to the change in the thermooptical properties of the solvent. 9-Nitroanthracene was selected for this study because it seems to satisfy the requirements: it is soluble in polar solvents and at concentrations below M and it is soluble in nonpolar solvents as well. Furthermore, the photophysical
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER 15, 1988
Table 11. Relative Thermeal Lens Signal Intensity of 9-Nitroanthracene in Different Solvents"
(RI)exptlb
solvent
cc1,
n-heptane isooctane
dioxane methanol water
47 40 36 35 33 41 27 17 1
aConcentration of 9-nitroanthracene used was 2.5 X low8M. (RILPtl = (Sample - Sbhk)/Swater where Ssample and Swater are thermal lens signals of equal concentration of 9-nitroanthracene in a solvent and in water, respectively, and in the blank signal. (RI)dd = (E/P)aolv/(E/P)wabr where the values for thermal lens enhancement per unit laser power in solvent, (E/P)ao,v, and in water, are obtained from Table I. properties of this molecule are the same at room temperature in different solvents because it does not fluoresce or phosphoresce in any solvents a t room temperature. The lack of radiative relaxation of this molecule has been attributed to the quenching effect of the nitro group substituent (19,20). The variation on the thermal lens signal of this molecule can, therefore, be ascribed to the change in the properties of solvents. The relative thermal lens signal intensities of 9nitroanthracene in different solvents are listed in Table 11. For comparison, these values are listed as the relative intensity, (RI)exptlto that in water, which is defined as (R1)exptl
=
(Ssample
thermal lensing
(RU~c~c
38 f 4 40 f 3 32 f 2 26 f 4 30 f 3 30 f 5 21 f 3 26 i= 4 1.0 f 0.5
n-pentane cyclohexane n-hexane
Table 111. Relative Fluorescence and Thermal Lens Signal Intensity of Anthracene Derivatives in Different Media
substrate
(&TL)
fluorescence (4F)
nitroanthracene water (NAN)
water
(AN)
CTAB CTAC
anthracenepropionic acid (ANP)
1.0 f 0.1 1.6 f 0.1 1.00 f 0.03 SDS 1.7 f 0.1 SB-12 1.3 f 0.1 Brij-35 1.5 f 0.1 CIZE6 1.8 f 0.1
1.00 f 0.01 0.95 f 0.02 1.85 f 0.05 1.91 f 0.02 2.21 f 0.04 2.10 f 0.04 2.21 f 0.01
2.0 f 0.1 2.6 f 0.1 2.85 f 0.06 3.6 f 0.1 3.5 f 0.1 3.6 f 0.1 4.0 f 0.1
water CTAB CTAC
1.00 f 0.01 1.53 f 0.01 1.63 f 0.03 2.04 f 0.03 2.04 f 0.04 2.41 f 0.02 2.52 f 0.03
2.0 f 0.1 2.9 f 0.1 3.4 f 0.1 3.6 f 0.1 3.1 f 0.1 3.5 f 0.1 4.2 f 0.1
1.00 f 0.01 2.14 f 0.03 2.23 f 0.02 1.62 f 0.02 3.13 f 0.02 2.99 f 0.03 3.18 f 0.03
2.0 f 0.1 3.76 f 0.05 3.8 f 0.1 5.8 f 0.2 4.5 f 0.1 6.4 f 0.1 6.9 f 0.2
1.0 f 0.1 1.4 f 0.1 1.8 f 0.1 SDS 1.6 f 0.1 SB-12 1.1 f 0.1 Brij-35 1.1 f 0.1 C&6 1.7 f 0.1
anthracenewater 1.0 f 0.1 palmitic acid CTAB 1.62 f 0.04 (ANPA)
CTAC
1.6 f 0.1 4.2 f 0.2 SB-12 1.4 f 0.1 Brij-35 3.4 f 0.1 C1&j 3.7 f 0.2
SDS
anthraceneundecanoic acid
~ T + L @F
1.0 f 0.1 2.0 f 0.1 2.2 f 0.1 2.5 f 0.1 1.7 f 0.1 2.1 f 0.1 2.3 f 0.1
1.0 f 0.1 CTAB 2.0 f 0.1 CTAC 2.2 f 0.1 SDS 2.5 f 0.1 SB-12 1.7 f 0.1 Brij-35 2.1 f 0.07 ClzE6 2.3 f 0.1
anthracene
- Sblank)/Swater
where Sample and Swater are the thermal lens signals of equal concentrations of 9-nitroanthracene in the solvent and in water, respectively, and &lank is the blank signal. It is noteworthy to add that the measurements of these values were performed by using a digital oscilloscope and a personal computer for data acquisition rather than the lock-in amplifier. Reasons for this selection include the relatively narrow dynamic range of the lock-in amplifier; i.e., it became saturated for the same concentration of 9-NAN when the solvent was changed from water to a nonpolar one (21). Table I1 also lists the calculated relative intensity values, (RI)dcd,which were defined as (RUdd = (E/P)wlv/(E/P)wter where the values for thermal lens enhancement per unit laser power in solvent, (E/P)solv, and in water, (E/P)ws&r, were obtained from Table I. As expected from the theory, the thermal lens signal intensity of 9-NAN increased as the thermooptical properties of the solvent improves. Furthermore, it is pleasing to see that the observed enhancement values are in good agreement with the calculated values. For example, carbon tetrachloride was found to enhance the thermal lens signal by 38 times, which is in relatively good agreement with the calculated value of 47. Good agreement was also found for other solvents: 40, 32, 30, 30, 26, 26, and 21 time enhancements were found for n-pentane, cyclohexane, n-heptane, isooctane, n-hexane, methanol, and dioxane, respectively. These values are in good agreement with the calculated values of 40, 36,33, 41,35, 17, and 27, respectively. Additional information on the environmental effect on thermal lens signal intensity was obtained from the study of micellar effect on 9-NAN. The thermal lens signals of 9-NAN in different micellar systems, Le., cationic (CTAB and CTAC), anionic (SDS), nonionic (ClZEz3or Brij-35 and C12Es)),and zwitterionic (SB-12) micelles, were measured and the results are shown in Table 111. Compared to that in water, the thermal lens signal intensity of 9-NAN increased by 2.0 times when it was solubilized in CTAR micelles. Enhancements
solvent
water 1.0 f 0.1 CTAB 3.8 f 0.2 CTAC 3.5 f 0.2 SDS 4.2 f 0.1 SB-12 1.0 f 0.1 Brij-35 2.0 f 0.1 ClSEG 2.4 f 0.1
1.00 f 0.01 2.0 f 0.1 6.31 f 0.03 10.1 f 0.2 6.39 f 0.02 9.9 f 0.2 2.19 f 0.01 6.4 f 0.1 8.37 f 0.02 9.4 f 0.1 10.30 f 0.03 12.3 f 0.1 12.22 f 0.04 14.6 f 0.1
were also obtained when the probe molecule was in CTAC, SB-12, Brij-35, or Clz& micelles: 2.2 times in CTAC, 1.7 times in SB-12,2.1 times in Brij-35, and 2.3 times in ClzEs micelles. Larger enhancement was found, however, when the molecule is in SDS anionic micelles (2.5X). The observations can be explained in terms of solubilization and relative location of 9-NA in micelles. The NOz group of the 9-NAN is probably partly negatively charged and is attracted to the cationic headgroups of the CTAB and CTAC micelles through electrostatic interactions. Consequently, the anthracene moiety probably located very close to the micellar headgroups. Conversely, the 9-NAN probably situated in a relatively more hydrophobic region of the SDS micelles because of the electrostatic repulsion between between the NOz group and the micellar anionic headgroups. The larger enhancement observed in SDS as compared to these in other micellar systems is because as aforementioned, the thermal lens signal intensity is proportional to the hydrophobicity of the environment around the solute. The lack of effect by the bromide and chloride counterions on the thermal lens in CTAB and CTAC lends further credence to the explanation. Detail information on the enhancement mechanism was investigated by employing various anthracene derivatives whose structures range from nonsubstituent (anthracene, AN) to ones having different chain lengths (anthracenepropionic acid, ANP; 11-(9-anthroyloxy)undecanoicacid, ANU; and 2-(9-anthroyloxy)palmiticacid, ANPA) (Chart I) in different micellar systems. Compared to 9-NAN, the effects of micelles on these probes are relatively more complex because these
ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER 15, 1988
probe molecules have relatively higher fluorescence quantum yield than 9-NAN. The micelles will affect not only the thermal lens but also the fluorescence as well. Accordingly, the fluorescence and thermal lens signal of these probes in micellar systems were measured and the results are shown in Table 111. Similar to the relative thermal lens values, $TL, the fluorescence quantum yield values, $F, listed in Table I11 are the relative values compared to those in water and are defined as
4~ = &le/ 4water and dwater are the areas under the fluorescence
where $sample spectra of equal concentration of the substrate in micellar system and in water, respectively. Compared to that in water, the relative fluorescence quantum yield of anthracene enhanced substantially when the compound was solubilized in micellar solutions. The enhancements range from 1.85times in CTAC to 1.91 times in SDS and 2.10 times in Brij-35 with the maximum enhancement of 2.21 times in SB-12 and C12E6. The lesser degrees of enhancement observed in CTAC as compared to other micellar systems can be explained in terms of interaction between the ?r electrons of the anthracene and the positive ammonium headgroups of the micelles. As a consequence, the micellar effect is less than those in other micellar systems. Furthermore, because of this interaction, the anthracene is prone to the quenching effect by the counterions as can be seen by the fluorescence quenching by the bromide ions in CTAB: compared to that in water, the fluorescence of AN decreased by 49% in CTAB micellar solution. As mentioned earlier, micelles affect the thermal lens signal of substrate in two ways, directly and indirectly. The thermooptical properties of the environment around the substrate is improved when it is solubilized in micelles and this solubilization process leads to the direct enhancement of the thermal lens signal. Thermal lens signal is also varied when the photophysical properties of a substrate are modified by micelles. The contribution of these two effects to the thermal lens signals makes it rather difficult to use the & values alone to get insight into the effect of micelles on the thermooptical properties of the environment. Fortunately, detailed information can be obtained by discussing in terms of the sum of the fluorescence quantum yield and thermal lens signal together, i.e., (dF+ $m). The use of this sum renders feasibility of distinguishing the two effects. That is because, if the variation on the $m is only due to the change in $F, then the sum of ($TL $F) should be the same as that in water, i.e., ($TL &) = 2, as there are no changes in the thermooptical properties of the medium. Therefore, any ($TL + $F) value larger than 2 is a clear indication of micellar effect on the environment of the substrate. This effect is expected to be directly proportional to the magnitude of the ($TL $F) value. A larger (4% + $F) value is expected when the micellar effect is large, i.e., when the substrate is well solubilized in the micelles. As expected, large micellar effects were found for anthracene. The (dTL + $F) values were found to be 2.6, 2.85, 3.6, 3.5, 3.6, and 4.0 in CTAB, CTAC, SDS, SB-12, Brij-35, and C12E6,respectively. It is interesting to note that there is a pronounced difference in the micellar effect of CTAB and CTAC. Changing the counterion of the micelles from bromide to chloride leads to 9% enhancement in the (& + &) value. This observation can be explained in terms of the relative size of the micelles. CTAB micelles, having aggregation number of 61, are relatively smaller than CTAC, which has an aggregation number of 78 (22). The micellar effect is, therefore, more pronounced in the larger CTAC micelles. For the similar reason, the ( 4 + ~$F) ~value in the nonionic C12E6 micelles
+
+
+
2481
is 11% higher than that in Brij-35 micelles. These two nonionic surfactants belong to the same poly(oxyethy1ene glycol) monoether type of compounds having a general formula CH3(CH2)i-10(CH2CH20)jH or CiEP The Brij-35 is tricosanethylene glycol mono-n-dodecyl ether or C12E23 while the C12& is hexaethylene glycol mono-n-dodecyl ether. They have the same hydrophobic moiety (Clz) but different polyoxyethylene hydrophilic parts. As a consequence, the aggregation number of the ClzE6 micelles is 179, which is relatively larger than the value of 40 for the Brij-35 (22, 23). Compared to CTAB and CTAC micelles, the difference in the aggregation numbers of Brij-35 and C12E6 is relatively larger. In addition, the AN molecule is probably located deeper inside the hydrocarbon core of the nonionic micelles than the cationic micelles in which, as explained earlier, it may locate very close to the micellar surface. The difference in the micellar effect is, therefore, larger for the Brij-35 and C&6 (11%) than for the CTAB and CTAC. This observation clearly demonstrates that the thermal lens technique is very sensitive to the variation of the medium. The micellar effect on the thermooptical properties is lowest in CTAB and CTAC as compared to other micellar systems. This is in agreement with the fluorescence results and can be explained in terms of interaction between the micellar cationic headgroups and ?r electrons of the anthracene. Because of this interaction, the AN molecule located very close to the micellar surface, thereby benefiting very little from the hydrophobic nature of the micellar core. Larger micellar effects were observed with SDS, Brij-35, and C & &with the latter having the maximum (dTL+ $F) value of 4.0. This may be due to the strong interaction between the nonpolar AN and the hydrophobic moiety of the large nonionic C&6 micelles. This interaction enables the AN to locate well inside in the micelles, thereby achieving maximum micellar effect. The results obtained so far seem to indicate that a larger thermal lens signal is accomplished when the substrate is solubilized well into the core of micelles. To further confirm this deduction, we have chosen three additional molecules, i.e., 9-anthracenepropionic acid, ANP, 2-(9-anthroyloxy)palmitic acid, ANPA, and 11-(9-anthroyloxy)undecanoic acid, ANU, with the anticipation that the subtle differences in the molecular structure of these probes would provide more insight into the mechanism of micellar enhanced thermal lens signal. Similar to AN, the fluorescence of ANP was enhanced substantially when the molecule was incorporated into micellar solutions. The $F values were found to increase from 1 in water to 1.53, 1.63, 2.04, 2.04, 2.41, and 2.52 in CTAC, CTAB, SDS, SB-12, Brij-35, and C12E6, respectively. Compared to AN, several noted differences were found for ANP. These include the smaller quenching effect by bromide counterions ($F in CTAB and CTAC were 1.53 and 1.63 as compared to 1.0 in pure water), the C12E6 and Brij-35 micelles being the medium that gave the highest enhancement, and, for the same micellar solution, the fluorescence of ANP being more enhanced than that of AN, e.g., $F for ANP and AN in C12E6 were 2.52 and 2.21, respectively. A variety of reasons may account for these differences but the most likely one is the difference in the locations of ANP and AN. The propionic acid substituent in the ANP is expected to facilitate the incorporating of the probe into micelles. Because of this incorporation, the ANP is relatively far from the micellar surfaces. Therefore, quenching by bromide counterions was not as efficient as for the AN. The hydrocarbon chain of the ANP promotes the stronger interaction with the hydrophobic moiety of the micelles and thus leads to the further penetrating of the ANP into the nonionic micelles. As a consequence, the ANP was well protected, and hence, highest fluorescence
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CHEMISTRY,
VOL. 60, NO. 22, NOVEMBER 15, 1988
enhancement was achieved in ClzEs micellar solution. The same explanation can also be used to explain the differences in the degree of thermal lens enhancement as reflected by the (+TL + +F) values. The (+TL + +F) value for ANP in C12E6 was found to be 4.2, which is the highest compared to other micellar solutions (values of 2.9, 3.4, 3.6, 3.1, and 3.5 were found for ANP in CTAB, CTAC, SDS, SB-12, and Brij-35, respectively). Additional information on the effect of the chain length on the fluorescence and thermal lens enhancement can be obtained from results of ANPA and ANU. ANPA has a structure similar to ANP, i.e., an anthracene and a carboxylic group, but the two groups are farther apart in the ANPA than in ANP as there are three atoms between these two groups in the former but only two in the latter (Chart I). In addition, the ANPA also has a long hydrocarbon tail attached to it. Due to these differences, it is expected that the ANPA may locate further inside the micelles as compared to ANP. The fluorescence and thermal lens results in Table I11 show that this is indeed the case. In all cases, the observed fluorescence and thermal lens enhancements for ANPA are relatively higher than those for ANP. For example, the relative fluorescence quantum yield of ANPA in water, CTAB, CTAC, SDS, SB-12 Brij-35, and C12E6 were 1, 2.14, 2.23, 1.62, 3.13, 2.99, and 3.18, respectively. Except for SDS these values are about 24-53% higher than those for ANP. The thermal lens enhancement (in terms of +TL + +F) is relatively higher for ANPA than ANP. It is pleasing to see that similar to ANP, maximum enhancement for ANPA was provided by CI2E6 micelles. The presence of the long hydrocarbon tail in ANPA probably enables it to be solubilized well into the nonionic micelles, which as a consequence give maximum thermal lens enhancement. The effect of the long hydrocarbon tail and the relative distance between the anthracene moiety and the carboxylic polar group can be better evaluated when the data on ANPA and ANU are compared. Both of these molecules have a long hydrocarbon tail but the anthracene and the carboxylic acid groups in the ANU are relatively farther apart than in ANPA. It is, therefore, anticipated that the fluorescence and thermal lens enhancement for ANU are relatively higher than those for ANPA. In fact, as seen in Table 111,the strongest micellar effect is seen for ANU. The enhanced fluorescence and thermal lens are highest for ANU as compared to other probe molecules. The relative fluorescence quantum yield in water, CTAB, CTAC, SDS, SB-12, Brij-35, and ClzEs are 1.00, 6.31, 6.39, 2.19, 8.37, 10.30, and 12.22 respectively. These values are about 35-284% higher than those for ANPA. The thermal lens enhancement (in terms of 4 T L + +F) was also improved by the same extend. Again it is pleasing to observe the maximum thermal lens enhancement in CIZEBmicelles. Collectively, these results provide clear evidence for the simultaneous enhancement of the fluorescence and thermal lens signals of various substances by micellar systems. The fluorescence is enhanced because the micelles isolate and protect the analyte from quencher molecules as well as increase the viscosity of the medium (8). The thermal lens enhancement is due to modification of the thermooptical properties of water by micelles, i.e., increased d n l d T and decreased thermal conductivity k values. Micellar effects such as changing the structure of water and providing a nonpolar
medium for the analyte are probably the main mechanism for the modification of the thermooptical properties. Water is a poor thermooptical solvent because it has strong hydrogen bonding network. As a consequence, the variations in its density with temperature and its d n l d T values are very small. Adding a substance that is capable of breaking these hydrogen bonds will lead to a greater variation in the density with temperature and, hence, larger dn/dT. Surfactant molecules are excellent additives for such tasks, as it is known to break hydrogen bonds of water (8). Accordingly, the d n l d T value for the aqueous micellar solution should be higher than that of the pure water. Furthermore, the hydrocarbon region of the micelles provides a good thermooptical nonpolar environment for analyte. This effect is reflected by the observation of maximum enhancement in the thermal lens signal for ANU, which is expected to be incorporated well into the hydrophobic region of the micelles. It has been demonstrated that the sensitivity of two fundamentally different techniques, fluorescence and thermal lens, can be enhanced simultaneously by micelles. This enhancement facilitates the use of these techniques as a complementary means for the detection and identification of fluorescent as well as nonfluorescent trace chemical species. Experiments are now in progress to expand the application of this technique in the area of general trace chemical analysis. Registry No. AN, 120-12-7;NAN, 602-60-8; ANP, 41034-83-7; ANU, 73038-57-0;ANPA, 67708-96-7; CTAB, 57-09-0; CTAC, 112-02-7;SDS, 151-21-3;SB-12, 14933-08-5; Brij-35, 9002-92-0; CiZEs, 3055-96-7.
LITERATURE CITED
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RECEIVED for review May 23,1988. Accepted August 2,1988. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and to the Marquette University Committee on Research for financial support of this research.
