Anal. Chem. 1901, 63,169-173
range have been demonstrated for several elements. The required computations are more complex than Beer's law but are well within the capabilities of a modern desktop microcomputer (15,16).
LITERATURE CITED (1) Epstein, M. S.;Winefordner, J. D. h o g . Anal. At. Spectrosc. 1984, 7 , 57-63. (2) . . Inale. J. D.;Crouch, S. R. SLh9ctrochemical Analysis; Prentice Hail: Englewood Cliffs, NJ, 1988. . (3) O'Haver. T. C.; Harnly, J. M: Zander, A. 1.Anal. Chem. 1978, 50, 1219-1221. . - . . . -_ . . (4) Moulton, G. P.; O'Haver, T. C.; Harnly, J. M. J. Anal. At. Spectrom. 1989, 4 , 673-676. (5) Moulton, G. P.; O'Haver, T. C.; Harnly, J. M. J. Anal. A t . Spectrom. 1990, 5 , 145-150. (6) Harnly, J. M. Anal. Chem. 1982, 5 4 , 1043-1048.
169
(7) Sturgeon, I?. E.; Chakrabarti, C. L.; hog. Anal. At. Spectrbsc. 1978, 1 , 165-177. (8) Fazakas, J. SpeCVOChifn. Acta 1983,388,455-458. (9) Fazakas, J.; Zugravescu, P. G. Spectrochim. Acta 1088, 438, 897-900.
(IO) Fazakas. J.; Hoenig, M. Talsnra 1988, 35,403-405. (11) Fazakas, J.; Zugravescu, P. G. Appl. Spectrosc. 1988. 42, 521-523. (12) Donega, H. M.; Burgess, T. E. Anal. Chem. 1970, 4 2 , 1521-1524. (13) Hassell, D. C.: Rettberg. T. M.; Fort, F. A,: Hoicombe, J. A. Anal. Chem. 1988, 6 0 , 2660-2683. (14) Piepmeier, E. Spectrochlm. Acta 1989, 448. 609-616. (15) O'Haver, T. C.; Kindervater, J. J. Anal. At. Specrrom. 1988, I, 89-91. (16) OHaver, T. C.; Chang, J . 4 . Spectrochim. Acta 1989, 448. 795-809.
RECEIVED for review August 23, 1990. Accepted October 4, 1990.
Luminescence Properties of Benzo[ f ]quinoline Adsorbed on ,8-CyclodextrinlSodium Chloride as a Function of Temperature Marsha D. Richmond' and Robert J. Hurtubise* Chemistry Department, University of Wyoming, Laramie, Wyoming 82071
Fluorescence and phosphorescence quantum yields, fluorescence and phosphorescence polarlratlon values, and phosphorescence llfetlmes were obtained for benro[t]qulnollne (B( t )0)adsorbed on @-cyclodextrln(B-CD)/NaCI matrlces as a functlon of temperature from 296 to 93 K. A varlety of fundamental lumlnescence parameters were calculated that were correlated with the Interactions of B(f)O with the BCD/NaCI matrlces. The phosphorescence quantum yield data revealed that B ( t ) Q on 30% B-CD/NaCI at 296 K almost gave the same quantum yleld value as at 93 K. I n addition, the fluorescence polarlratlon values for B( t ) O showed little change as a function temperature, whereas the phosphorescence polarlratlon values demonstrated a much more dramatic change with temperature. Very long rotational relaxation times were calculated for B(t)O adsorbed onto 30% 8-CD/NaCI, which Indicated that B( t )O was held very tightly on the 8-CD matrlx. Low temperature had very llttle effect on the luminescence quantum ylelds for B ( t ) O adsorbed on 30 % @-CD/NaCI, which showed that luminescence analysls could be carrled out at room temperature with little loss In analytlcal sensltlvlty.
INTRODUCTION Room-temperature phosphorescence (RTP) has been developed into a useful analytical technique for organic trace analysis (1,2). Both solution and solid-matrix RTP have been observed in mixed organized media ( 3 ) ,in biologically important systems ( 4 ) ,and from indicans (5). Recently, the use of cyclodextrins in R T P analysis was discussed (6-8). Cyclodextrins are cyclic oligosaccharides composed of a-1,4linkages having a structure similar to a truncated cone (9). There are three commercially available cyclodextrins. These are cy-, @-,and y-cyclodextrin. These cyclodextrins are comPresent address: Department of Chemistry, Iowa State Univ-
ersity, Ames, IA 50011.
prised of 6-8 glucose monomer units, respectively. Cyclodextrins, by their nature, possess a hydrophobic interior cavity. The ability of cyclodextrins to sequester small molecules into their cavitites has led to their use in R T P analysis (9). Vo-Dinh et al. ( 1 0 , I I ) and Ramasamy and Hurtubise (12) reported that filter paper previously treated with cyclodextrin yielded higher R T P signals over untreated filter paper. Bello and Hurtubise (13) reported that a variety of compounds yielded R T P with a a-cyclodextrin/sodium chloride (aCD/NaCl) solid matrix. Purdy and Hurtubise (14) demonstrated, relative to other solid matrices, that a 80% a-CD/ NaCl solid matrix gave the lowest R T P limit of detection, and emission bands of the compounds adsorbed onto 80% aCD/NaCl were better defined. Although a variety of solid matrices have been used in solid-matrix R T P analysis, less work has been done to study the interactions between phosphor and substrate responsible for the phosphorescence intensity. In addition, very little research has been done in investigating fluorophor-solidmatrix interactions. Ramasamy et al. (15)developed a method for determining luminescence quantum yields of compounds adsorbed on soild matrices. Also, Ramasamy and Hurtubise (16-18) discussed temperature effects on the luminescence properties of p-aminobenzoic acid on sodium acetate and benzo[flquinoline (BOQ) and 4-phenylphenol on filter paper. Bello and Hurtubise (19, 20) determined luminescence quantum yields and phosphorescence lifetimes at 296 and 93 K for p-aminobenzoic acid, 4-phenylphenol, B(f)Q, and phenanthrene on various a-CD/NaCl solid matrices. Also, Ramasamy and Hurtubise (16-18) and Bello and Hurtubise (19, 20) were able to calculate the rate constants for phosphorescence, the triplet formation efficiencies, and the rate constants for the radiationless transition from the excited triplet state to the ground state for compounds adsorbed onto filter paper and a-CD/NaCl solid matrices at various temperatures. Recently, we reported the use of @-cyclodextrin/sodium chloride (@-CD/NaCl)mixtures for room-temperature luminescence analysis (21). Several compounds of varying size,
0003-2700/91/0363-0169$02SO/O0 1991 American Chemical Society
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geometry, and functionality were found t o exhibit room-temperature fluorescence (RTF) and/or R T P with a 30% /3CD/NaCl solid matrix (21). We have also reported that results from solution interaction studies of ana1yte:P-CD inclusion formation in methano1:water (50:50) showed no simple correlations between solution interactions of model compounds and their observed luminescence intensities when adsorbed onto 30% $-CD/NaCl solid matrix (22). Additional work would be needed to clarify the interactions in the solution during the evaporation of the solvent in the sample preparation step. In this work, using the procedures developed by Ramasamy et al. (16-18), the luminescence quantum yields, phosphorescence lifetimes, rate constants, and the triplet formation efficiencies as a function of temperature were obtained for B(f)Q on two different P-CDINaCl solid matrices. In addition to these fundamental parameters, luminescence polarization data for B(flQ a t various temperatures were obtained by using a 30% P-CDINaCl solid matrix. Phosphorescence rotational relaxation times were calculated for B(f)Q on the P-CDINaCl solid matrix by employing Perrin’s equation. EXPERIMENTAL SECTION Reagents. The p-cyclodextrin was supplied by the American Maize-Products Corp. (Hammond, IN). The sodium chloride was Baker Analyzed and was purchased from J. T. Baker (Phillipsburg, NJ). The B(f)Q was gold-label quality (Aldrich Chemical Co., Milwaukee, WI) and used as received. Absolute ethanol was purified by distillation. The 1,6-dihydroxynaphthalene(Aldrich Chemical Co., Milwaukee, WI) was used as received. Sodium salicylate (Gold-Label,Aldrich, Milwaukee, WI) and anhydrous sodium acetate (Analyzed Reagent, J. T. Baker, Phillipsburg, NJ) were both used as received. Methanol and water were HPLC grade and supplied by Burdick and Jackson (Muskegon, MI). Instrumentation. Quantum Yields and Phosphorescence Lifetimes. The instrumental setup and temperature variation experiments for quantum yield and lifetime determinations were the same as described by Ramasamy and Hurtubise (26) Luminescence Polarization. Fluorescence and phosphorescence intensity values obtained for the calculation of polarization values were collected by using the polarization assembly for the Farrand MK-2 spectrofluorometer with the Farrand rotary chopper for the acquisition of phosphorescence data. Individual intensity values were averaged for a period of 2 min by using a computerized Bascom-Turner Model 4210 recorder. Sample alignment and temperature measurements were obtained by using the same procedures as for the phosphorescence lifetime and quantum yield measurements. Procedures. Quantum Yields and Lifetimes. Samples for quantum yield and lifetime studies were prepared as previously described with the following exceptions (16,191. The solvent used for the $-CD/NaCl samples was methano1:water (50:50). The analyte:P-CD/NaCl ratio was 300 ng:lO mg. After grinding, the sample was placed in a Delrin sample holder for all temperature measurements. Polarization Calculation. Samples for polarization measurements were prepared and aligned in the same manner as for the quantum yield samples. The analyte:P-CD/NaCl ratio was 600 ng:lO mg. There were four intensity measurements necessary for the calculation of polarization, and polarization was calculated by using the following equation: Ivv - G(IvH) = IVx
+ G(IvH)
where p is the polarization, Iw is the intensity of emitted radiation observed when both the excitation and emission polarizers are in the vertical (to the lab axis) position, and IVH is the intensity of emitted radiation when the excitation polarizer is in the vertical position and the emission polarizer is in the horizontal position. The term G ( G factor) is defined as Z H v / I H H . This factor is a correction factor for the polarization of emitted radiation due to the emission grating, the photomultiplier housing, the sample cell, and other optics through which the emitted radiation passes. This correction factor is wavelength dependent and must he determined
Table I. Luminescence Quantum Yields for B ( f ) Q with 30% &CD/NaCl and 1% 6-CD/NaCl Solid Matrices a t Various Temperatures 3070 b-CD/NaCl
T,K
o?
$Pb
256 273 233 193 153 53
0.55 0.54 0.60 0.62 0.62 0.54
0.20 f 0.01 0.20 f 0.04 0.24 f 0.07 0.26 f 0.02 0.22 f 0.03 0.23 f 0.03
170$-CD/NaCl
@?
@pe
0.55 d
0.046
0.62
0.17
“Pooled standard deviation on 3 0 7 ~P-CDINaCl was 0.07, and on 1% j3-CDjNaC1 it was 0.05. b 9 5 7 ~confidence level. cPooled standard deviation is 0.01. Values not determined. for each set of excitation and emission wavelengths used. For this study, an aqueous solution of 1,6-dihydroxynaphthalenewas used to determine the C factors as a function of wavelength. Corrections for polarization due to the P-CDINaCl solid matrix were made by subtracting intensity values for the blank from the sample intensity values using a procedure similar to that described by Chen (23). RESULTS AND DISCUSSION Luminescence Q u a n t u m Yields a n d Phosphorescence Lifetimes f o r B(f ) Q on P-CyclodextrinlSodium Chloride Solid Matrices at Various Temperatures. Both fluorescence and phosphorescence quantum yields were determined a t various temperatures from 296 to 93 K for B O Q adsorbed onto 30% P-CDINaCl solid matrix. They were also determined for B(f)Q on I ?& P-CDINaCl solid matrix a t 296 and 93 K. Recently, we reported that a saturated solution of $-cyclodextrin in the methano1:water (50:50) was needed in the sample preparation step for observing maximum luminescence intenstiy from the lumiphor adsorbed onto the $-CD/NaCl solid matrix (21). The 1% P-CDINaCl matrix was investigated because it represented a (3-CDINaCl mixture composition that gave a solution below the saturation point for /!I-cyclodextrin in the sample preparation step. Table I contains the luminescence quantum yield results as a function temperature for B(flQ on both P-CDINaCl solid matrices studied. Several interesting results appear in Table I. Of particular interest is the fact that not much change was observed for either the fluorescence or the phosphorescence quantum yields for B(f)Q on the 30% $-NaCl solid matrix in going from 296 to 93 K. The value of 0.20 for &, a t 296 K in Table I is the same as the value obtained at liquid nitrogen temperature for an ethanol solution of B(f)Q reported by Ramasamy et al. (15). Bello and Hurtubise (19) reported & value of 0.09 for BOQ with an 80% cu-CD/NaCl solid matrix a t 296 K. Also, with the 80% a-CD/NaCl solid matrix, they reported an increase in c#+,of 2.4 times a t 93 K for BWQ. Ramasamy and Hurtubise ( I 7) reported an increase in gpof 9.2 times for the protonated form of BWQ on filter paper over the same temperature range. The higher bP value for B O $ on the P-CDINaCl matrix shows that B(nQ is bonded much more tightly by the P-CDINaCl matrix compared to the a-CDINaCl and filter paper matrices. The effect of the amount of /S-cyclodextrin in the P-CD/ NaCl mixture on the luminescence quantum yields may be found in Table I by comparing the results obtained with the two different B-CD mixture compositions. Little change is noted in the fluorescence quantum yields based on the amount of $-CD used, and thus the fluorescence quantum yield is only slightly affected by the change in mixture composition. For example, there is only about a 1.1-fold increase in going from 296 to 93 K for the 1% J-CD/NaCl mixture, and on the basis of the experimental error for the fluorescence quantum yield determination. there was no increase in the fluorescence
ANALYTICAL CHEMISTRY, VOL. 63, NO. 2, JANUARY 15, 1991
Table 11. Phosphorescence Lifetime(s) for B(f)Q with 30% B-CDINaCl and 1% B-CDINaCl Solid Matrices at Various Temperaturesn
95%
T,K
30% D-CD/NaCl
296 273 233 193 153 93
1.71 f 0.013 2.18 2.82 2.97 3.45 3.83
f 0.011 0.008 f 0.018 f 0.015 f 0.045
Table 111. Triplet Formation Efficiency and Triplet-State Rate Constants for B(f)Q with 30% 8-CDINaCl and 1% B-CDINaCl Solid Matrices
1% &CD/NaCl 1.58
b
3.56
* 0.16 * 0.01
confidence level. Values not determined.
quantum yield for the 30% P-CDINaCl matrix. Comparison of the room-temperature 4, values in Table I for the two matrices shows that the 6, value from the 30% P-CD/NaCl solid matrix is 4.3 times the value obtained for the 1% pCD/NaCl matrix. These results imply that B(nQ is in a much more rigid and protective environment with 30% 0-CD/NaCl than with 1%P-CD/NaCl solid matrix at room temperature. At 93 K, the 6, value for the 30% P-CD/NaCl matrix is only 1.4 times larger than the 4, value obtained with the 1% 6CD/NaCl. This shows that a t the lower temperature B(nQ finds a more rigid environment with the 1% P-CDINaCl matrix than a t room temperature. However, with the 30% P-CDINaCl matrix, B O Q appears to experience very similar environmental conditions at room temperature and 93 K. For example, the ratio of the phosphorescence quantum yields at 93 and 296 K is 1.2. In fact, the phosphorescence quantum yield is almost constant from 296 to 93 K. We believe that this is the first report whereby the solid-matrix 4, value at room temperature is basically the same as that obtained a t low temperatures. The phosphorescence lifetimes (7,) of B(f)Q as a function of temperature and mixture composition are found in Table 11. The data in Table I1 indicate that longer lifetimes for B(nQ were found with the 30% P-CD/NaCl matrix compared to the 1%P-CDINaCl. An increase of 2.2 times was found for both the 30% P-CDINaCl and the 1%P-CDINaCl matrices in going from 296 to 93 K. In general, the 7, values for BWQ were larger on 30% p-CD/NaCl than on 80% a-CD/ NaCl(19) and the protonated form of B(f)Q on filter paper (18). The larger 7, values for B(f)Q on 30% P-CDINaCl indicated that quenching and/or vibrational deactivation processes were less with the 30% &CD/NaCl matrix compared to the 80% a-CD/NaCl matrix and filter paper. L u m i n e s c e n c e P a r a m e t e r s f o r B(f)Q w i t h t h e 8CD/NaCl Solid Matrices. Ramasamay and Hurtubise (17) employed a method for obtaining the activation energy, E,, and the preexponential factor, kl, for compounds on solid matrices. By constructing plots of In (T,-' - 7 , O - l ) vs 1/T, the resulting straight line would yield numerical values for E, and kl. The T,O term is the phosphorescence lifetime that shows no temperature dependence. This may be found graphically from lifetime vs temperature plots by an interative method (17). The lifetime of B(f)Q with the 30% P-CDINaCl does not exhibit a plateau region at very low temperatures (see Table 11). Therefore, ',7 was determined by an interative best fit technique originally described by Moriya (24) and used by Ramasamy and Hurtubise ( 17 ) for solid matrices. The E, and k , values determined for B(f)Q with the 30% p-CD/NaCl matrix were 416 cm-' and 1.8s-l, respectively. These values are less than those reported for protonated B O Q on filter paper (18). The smaller E, and k , values imply that B(f)Q is held tighter with the 30% @-CD/NaCl matrix than the protonated form of B(f)Q adsorbed onto filter paper (18). Ramasamy and Hurtubise (16-18) employed equations to calculate the rate constants for triplet-state to ground-state transitions for compounds adsorbed on solid matrices. By use
171
30 %
P-CD/NaCln T, K
6t
296 273 233 193 153 93
0.45 0.46 0.40 0.38 0.38 0.46
kp
km
1% @t
P-CD/NaClb kP
km
*
0.26 0.32 0.27 f 0.05 0.11 0.02 0.52 0.16 0.20 0.26 c 0.21 0.14 0.23 0.11 0.17 0.12 0.13 0.13 0.38 f 0.03 0.13 f 0.02 0.16 f 0.08
The pooled standard deviations for &, k,, and km were 0.07 and and 0.04 d,respectively. b95% confidence level. cValues not determined. 0.04
of these equations, the rate constant for phosphorescence ( k J , the rate constant for intersystem crossing from the triplet-state to ground-state transition (k,), and the triplet formation efficiency (4J were calculated for B(nQ for both the 30% and the 1% P-CDINaCl solid matrices. In order to calculate these parameters, two assumptions were made. First, it was assumed that there was no quenching due to oxygen and moisture. Experiments were conducted under dry nitrogen, and the samples were dried prior to the measurement step so oxygen and moisture quenching would be minimal. The second assumption was that no quenching due to impurities in the P-cyclodextrin occurred. The 0-CD was washed thoroughly with distilled ethanol prior to use, and because solid matrices were used diffusional quenching would be minimized. Table I11 contains the values calculated for k,, k,, and 4t for B(nQ with the two P-CD/NaCl solid matrices. As depicted in Table 111, k , for 30% P-CDINaCl does not stay constant over the entire temperature range studied. On the basis of the pooled standard deviation for k,, it appears that the k , value a t 93 K is somewhat lower than the k , values at the higher temperatures. Normally, K , does not show a temperature dependence. Comparison of the kp values obtained with the 30% P-CDINaCl matrix and the 1% P-CDINaCl matrix at room temperature suggests differences in solidmatrix properties for the two mixture compositions. The lower k , value obtained at room temperature for the 1% 0-CDINaCl indicates that a more rigid environment is provided for the phosphor on the 30% P-CDINaCl matrix than for the 1% P-CDINaCl matrix. The room-temperature k , value for B(f)Q on 80% a-CD/NaCl was reported as 0.18 (19),while a value of 0.11 was reported for the protonated form of B(nQ on filter paper at room temperature (18). The K , values listed in Table I11 for the two P-CDINaCl matrices show that the nonradiative-transition contribution is larger at room temperature than 93 K for both 0 - C D mixtures studied. However, the k , values obtained with the 30% P-CD/NaCl matrix showed that the 12, values are less for the 30% P-CDINaCl matrix than for the 1%P-CDINaCl matrix. The smaller k , values would favor the larger dP values obtained with 30% P-CDINaCl (Table I). The k , values in this work were lower at room temperature for B(f)Q compared to the corresponding k , values at room temperature for B(nQ on 80% a-CD/NaC1(19) and for the protonated form of B(f)Q adsorbed onto filter paper (18). For example, the ratio of k , values at room temperature for 80% a-CD/NaCl to 30% P-CDINaCl was 3.0, and for filter paper to 30% 0-CDINaCl the ratio was 2.0. The higher k , ratios indicate that B O Q is adsorbed more strongly onto the 30% P-CDINaCl mixture, which favors a higher 4, value on this solid matrix. The dt values shown in Table 111, within experimental error, stay constant for the 30% @-CD/NaClmatrix. The higher dt values for 30% P-CD/NaCl in Table I11 indicate a more rigid matrix
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 2, JANUARY 15, 1991
0.1EOO
= i
z
-O. Osoor
a
; -0.0550
