Anal. Chem. 1989, 61,2643-2647
2643
Analytical Characteristics of ,8-CyclodextrinlSalt Mixtures in Room-Temperature Solid-Surface Luminescence Analysis M a r s h a D. Richmond a n d Robert J. Hurtubise* Chemistry Department, University of Wyoming, Laramie, Wyoming 82071 -3838
8-Cyclodextrin (@-CD)/sait mixtures were investigated as solid matrlces for obtaining room-temperature fluorescence (RTF) and room-temperature phosphorescence (RTP) from adsorbed compounds. A 30 % 8-CD mixture produced strong luminescence signals from adsorbed compounds wtthout the need for a heavy atom. Linear ranges, reproducibility, and lbntts of detectlon were obtained for p-amlnobenzoic acid and phenanthrene. The selectivity demonstrated for the &CD/ NaCl mixtures should prove useful in mixture analysis. Also, it was shown that a saturated solution of 8-CD was needed in the sample preparation step and also a matrix packing effect of the dried mixture was important to obtain strong luminescence signals. The spectra obtained with the 30% B-CD/NaCI mixture possessed more fine structure than those obtalned wtth filter paper. The RTF and RTP spectra obtained with the 30% @-CD mixture were practically the same in structural detail as those acquired at low temperature. The powdered &CD/NaCi mixtures were very easy to handle for low-temperature analysis, conventional luminescence equip ment could be used, and only small amounts of analyte were needed to ensure good spectra.
INTRODUCTION Room-temperature phosphorescence (RTP) analysis has developed into a very effective analytical technique for organic trace analysis. The utility of the approach has been discussed in reviews (1-3) as well as books (4,5). Although filter paper remains the most frequently used solid-surface in R T P analysis, other surfaces and solution media have also proved to yield good R T P signals from phosphors. Hurtubise and co-workers (6, 7) used sodium acetate to determine the luminescence parameters of p-aminobenzoic acid (PABA). Su and Winefordner (8) used a variety of inorganic substrates to obtain the R T P characteristics of polycyclic aromatic hydrocarbons (PAHs). Femia and Cline Love (9) have reported the use of micelle-stabilized solutions to obtain RTP. Donkerbroek et al. (10)employed sensitized phosphorescence for obtaining RTP signals. Typical applications for RTP may be found in biological analysis ( I I ) , drug analysis (12),and environmental analysis (13). Recently, the use of cyclodextrins was demonstrated in several areas of analytical chemistry. Cyclodextrins are glucose-containing oligosaccharides joined via a-1,4 linkages into a cone-shaped torus. The more commonly used cyclodextrins are those containing six, seven, and eight glucose units. These cyclodextrins are referred to as a-, @-, and y-cyclodextrins, respectively. Cyclodextrins have the unique ability to sequester molecules in their hydrophobic cavity (14). They have been used as organic modifiers in mobile phases and/or as part of the stationary phase in high-performance liquid chromatography (HPLC) (15, 16). Also, a-cyclodextrin was used as a spray reagent in thin-layer chromatography (TLC) (17). Blyshak et al. (28)employed cyclodextrins in aqueous solution extraction experiments. Scypinski and Cline Love (19) employed cyclodextrins in the presence of heavy atom
to induce R T P in solution. Filter paper impregnated with a-cyclodextrin showed an increase in the RTP signal intensity for adsorbed phosphors over untreated filter paper (20). Bello and Hurtubise (21) demonstrated that an 80% a-cyclodextrin/sodium chloride (a-CD/NaCl) mixture induced RTP from polycyclic aromatics of various size and funtionality. Bello and Hurtubise (22)have reported analytical figures of merit for the R T P and room-temperature fluorescence (RTF) of four model compounds with an 80% a-CD/NaCl mixture. They also investigated interactions responsible for observing R T P on a-CD/NaCl mixtures (23,24). Recently, it was demonstrated that a 1%a-CD/NaCl mixture could be used instead of an 80% a-CD/NaCl mixture with little loss in the analytical integrity of the signal (25). From these investigations, it was believed that P-cyclodextrin should also induce solid-surface RTP. @-Cyclodextrin (8-CD) has a somewhat larger cavity than a-CD, which enables P-CD to accommodate larger molecules than a-CD. The results in this paper illustrate the ability of P-CDINaCl mixtures to induce RTF and RTP and show the analytically utility of P-CDINaCl in solid surface luminescence analysis. EXPERIMENTAL SECTION Reagents. Benzo[a]pyrene, benzo[flquinoline, and benzo[elpyrene were all Aldrich Gold Label reagents and used as received. All other compounds were also purchased from Aldrich and recrystallized from distilled ethanol prior to use. Cyclohexane was Aldrich Gold Label. Ethylene dichloride (1,2-dichloroethane) was reagent grade and was purchased from Mallinkrodt (SpectrAR grade). The methanol and acetone were Photrex grade (J. T. Baker). The 2-propanol and water were HPLC grade and purchased from J. T. Baker. The P-cyclodextrin (Aldrich)was washed with distilled ethanol prior to use. The sodium chloride, sodium bromide, and sodium iodide were all reagent grade from J. T. Baker. All salts were washed with distilled ethanol prior to use. The nitrogen gas was passed through an Oxyclear tube (Alltech Associates, Inc.) to remove any oxygen. The filter paper used in acquiring room temperature spectra was developed in distilled ethanol twice to collect impurities at one end. Instrumentation. All room temperature fluorescence (RTF) and RTP intensity measurements were obtained by use of a Schoeffel SD 3000 spectrodensitometer with a SD 300 computer. The source was a 200-W Hg-Xe lamp (Canrad-Hanovia, Inc.), and the detector was a R928 PMT (Hamamatsu Corp.). RTF and RTP and low-temperature fluorescene and phosphorescence excitation and emission spectra were collected with a Fluorolog 2+2 spectrofluorometer (Spex Industries). The detector was a water-cooled R928 PMT and a 450-W Xe lamp was used to excite the fluorescence of the samples. The phosphorescence spectra were obtained by using the Spex 1934C phosphorimeter accessory with programmable pulsed source and selectable gated detector. The data were processed with a Spex Datamate computer. The sample holders were previously described (22). For low-temperature solid-surface spectra, the dry powder was placed in a quartz tube and slowly submerged in liquid nitrogen, and then the spectra were collected. Procedures. Determination of Solubility of p-CD i n Various Organic and Organic/Aqueous Solvents. Saturated solutions of p-CD were prepared and stirred for 1 h. The solution was then filtered and three 25.00-mL aliquots were transferred into weighed beakers, and the solvent was allowed to evaporate at room temperature. After the solvent had evaporated, the beakers were
0003-2700/89/0361-2643$01.50/00 1989 American Chemical Society
2644
ANALYTICAL CHEMISTRY, VOL.
61, NO. 23, DECEMBER 1, 1989
placed in an oven at 100 "C for 1 h for additional drying. After this, the beakers were placed in a desiccator and allowed to come to room temperature and weighed. The solubilities obtained were the average of six trials except for 5050 2-propanol/water which was the average of nine runs. The relative standard deviation for all solubility data was between 1.5 and 3.0%. Preparation of Cyclodextrin Mixture. The @-CD/NaClmixtures were prepared as follows. Appropriate amounts of 0-CD were weighed out and combined with the amount of ground sodium chloride to yield the desired percentage of @-CD/NaCl mixture. The mixture was then stirred in a ball mill for 30 min to ensure a homogeneous mixture of 0-CD and NaCl. Sample Preparation for Intensity Measurements. Sample preparation for intensity measurements was as previously described for a-CD/NaCl (22),except for this study a 30% @CD/NaCl mixture and a 5050 methanol (MeOH)/water solvent were used. Sample Preparation for Spectra Collection. Sample preparation for those samples used in the acquisition of excitation and emission spectra was carried out as follows. Into a small test tube, a 0.2-mL aliquot of 5050 MeOH/H20 was added along with a 5-pL aliquot of analyte solution and then 100 mg of 30% @CD/NaCl mixture. The contents of the test tube were then sonicated for 10 s. The slurry was placed in an oven at 110 "C for 40 min. The dried sample was then placed into the sample holder and spectra were obtained. The samples for RTP measurements were degassed with nitrogen for approximately 10 min before their spectra were collected.
RESULTS AND DISCUSSION Solvent Study. Previously it was believed that inclusion complex formation of cyclodextrins occurred exclusively in aqueous solution. However, Siege1 and Breslow (26)reported that inclusion complex formation did occur in dimethyl sulfoxide (DMSO) and dimethylformamide (DMF). In 1984, Harada and Takahashi (27)reported that p-CD was able to form inclusion complexes in a variety of organic solvents. Recently, Patonay et al. (28)proposed a ternary structure for y-cyclodextrin, pyrene, and 2-methyl-2-propanol (tert-butyl alcohol) in aqueous solutions. They reported that not only is size important for inclusion complex formation but hydrogen bonding with 2-methyl-2-propanol and the primary and secondary hydroxy groups of the cyclodextrin increased the effective hydrophobicity of the cyclodextrin cavity. Therefore, the solvent was found to play an active role in complex formation. Also, Patonay et d. (28) reported that other alcohols exhibited similar behavior but a less hydrophobic environment was found with these alcohols. Because the desirable characteristics of a solvent for solid-surface luminescence include a low boiling point and ease of evaporation, water is not a good solvent because of its high boiling point, and many organic compounds are insoluble in water. However, cyclodextrins are only sparingly soluble in organic solvents. Bello and Hurtubise (22) reported the effect of various solvents on the luminescent intensity of several compounds with an 80% cy-CD/NaCl mixture. They found that for the 80% a-CD/NaCl mixture, more intense signals were obtained by using methanol as a solvent over ethanol, 2-propanol, or other organic solvents such as cyclohexane, dichloroethane, and acetone. The larger solubility of a-CD in methanol compared to the other organic solvents was given as a factor for strong RTP signals. Table I gives the solubility results obtained in this work for @-CD. From Table I, it may be concluded that fi-CD is more soluble in alcohol/water solvents. For example, there is a significant increase in the solubility of @-CDwith the alcohol/water (5050) solvents compared to the solubility in the pure alcohol. In fact, for 2-propanol/water, the solubility of @-CD was very high, namely, 3.250 g/100 mL. Luminescence Characteristics of Compounds Adsorbed on P-CDINaCl. Bello and Hurtubise (22) reported the effects of water content in alcohol solvents on the lu-
Table I. Solubility of 8-Cyclodextrin in Various Organic and Aqueous/Organic Solvents solvent
solubility, g/100 mL
1,2-dichloroethane cyclohexane
0.000 0.000
acetone
0.024
2-propanol 2-propanol/watera ethanol ethanol/water" methanol methanol/water"
0.024
3.250 0.056 0.878
0.045 0.295
E\
5050 (v/v) 15.00
5...
2 co
12. 9 . 000 0
z z
m i
-
3'Mt
W
5w
1-1
0.00
\
1
0.00
1
20. 00
40.
00
En. Do
80.00
100.00
PERCENT ALCOHOL
Figure 1. RTP intensity of Cphenylphenol(lO0 ng) adsorbed on 30% P-CDINaCI as a function of water present in methanol (0)ethanol (0).
and 2-propanol (X). minescence properties of compounds adsorbed on 80% cyCD/NaCl mixture. They observed a decrease in luminescence signals for alcohol solvents which had greater than 10% water content. Also, it was demonstrated that little change in intensity was found in the region of &lo% water content. To determine if an increase in the /3-CD solubility would correspond to an increase in luminescence intensity from the adsorbed compound, RTF and R T P intensity measurements were collected by varying the amount of water present in methanol, ethanol, and 2-propanol. These solvents were used to adsorb the compounds on the P-CDINaCl mixture. For the compounds investigated, there was essentially no change in RTF intensity from 0 to 100% alcohol. For 4-phenylphenol, Figure 1 shows that the RTP intensity did not change much from 10% to 80% water. However, Figure 1indicates smaller intensities were observed for pure water and pure alcohol. A MeOH/H,O (5050) solvent was chosen for future RTF and RTP work with P-CDINaCl mixtures based on several criteria. This solvent composition was in the plateau region found in Figure 1,thereby allowing for good solubility of p-cyclodextrin without a large increase in drying time for the sample. Also, for most of the compounds studied, the methanol/water solvents gave the largest increase in signal over the other alcohol/water compositions investigated. Figure 2 is a representative example of R T F and RTP intensity changes with different ratios of millimoles of @-CD to millimoles of analyte in several D-CDINaCl mixtures. The following discussion pertains to RTP because it showed more dramatic changes in intensity than did RTF. There exists three regions of interest over the range of 0.01-80% p-CD in the @-CD/NaClmixtures (Figure 2). On the basis of the solubility of @-CDin MeOH/H20 (50:50),these regions may be defined in reference to the saturation point for p-CD in the solvent. The saturation point is indicated by the arrow in Figure 2. For the mixtures containing amounts of P-CD less than that required for a saturated solution in the sample preparation step, relatively low RTP intensities were observed.
ANALYTICAL CHEMISTRY, VOL. 61, NO. 23, DECEMBER 1, 1989
2645
1.65E-02
I
z
c
