Anal. Chem. 1992, 64,700-703
700
The data shown in Figure 4 were reproducible to within 10Y0. Since the magnitude of the stripping peak currents is largely controlled by the concentration of the solution-phase analyte and not by that of the electroactive zeolite ion for a given loading level, problems with the reproducibility of the response of the zeolite electrodes as often reported in the literature26are somewhat minimized. At typical concentrations used in these experiments (Le. between 1 and 50 ppm) and with a 10% exchanged AgY electrode, the response remained stable to within 10% for several days. Variations in the response and long-term stability of the zeolite electrodes as a function of the concentration of electroactive ion, the nature of the solvent, and the analyte type and concentration are now under scrutiny. Since the mechanism operating for both ion and water detection is the same, it is likely that the detection limits in both cases will be similar. The projected detection limit (sub-ppm) is considerably superior to those reported recently for water sensors.27
CONCLUSION The most important features of this paper can be summarized as follows: Zeolite electrodes can be used as amperometric solution-phasesensors for alkali-metal cations and water with sensitivities for solution-phase cations in the sub-ppm regime. The zeolite-modified electrodes can be used to amperometrically detect most cationic species including, for example; alkali metals, alkaline earths, organics, and heavy metal ions." ACKNOWLEDGMENT We gratefully acknowledge the Natural Science and Engineering Research Council (NSERC) and the Institute for Chemical Science and Technology (ICST) for funding this research program. REFERENCES (1) Chemlcal Sensors and MlwOnstrumentat&n; Murray, R. W., Dessy, R. E., Janata, J., and S&, W. R., Eds.; ACS Symposium Series NO. 403; American Chemlcal Soclety: Washlngton, DC. 1989. (2) Fundamentals and Appllcatlons of Chemlcal Sensors; Schuetzle. D., Hammerle, R., Eds.; ACS Symposium Series No. 309; American
Chemlcal Society: Washington, DC, 1986. PfOCeedings of the Symposium on Chemlcal Sensors; Turner, D. R., Ed.; The Electrochemical Society Inc.: Pennington, NJ, 1987; Vol. 87-9. Deakin, M. R.; Byrd, H. Anal. Chem. 1989. 61, 290-295. Murray, C. 0.;Nowak, R. J.: Roilson, D. R. J . Electrmnal. Chem. Interfac&l Electrochem. 1984, 164, 205. Rolison, D. R.; Nowak, R. J.; Welsh, T. A.; Murray, C. G. Talanta 1991, 38, 27-35. Zdffe Mlecu&r Sleves; Breck, D. W., Ed.; R. E. Kriger Publishing: Malabar, FL, 1984. Csicery, S. M. Zeolites 1984, 4 , 202-213 and references cited therein. Meisei, S. L.; McCulbugh. J. P.; Lechthaier, C. H.; Weisz, P.B. Chemtech 1978, 86-89. Maxwell, I. E. J . Inclus&n Rmnom. 1988, 4 , 1-29. Baker, M. D.; Senaratne, C. United Kingdom Patent Application. Filed Feb 13. 1991. No. 9103053.6. Ozin. 0.A.: Baker, M. D.; W b e r , J.; Shihua, W. J . Am. Chem. Soc. 1985, 707, 1995-2000. Baker, M. D.;Zhang, J. J . Phys. Chem. 1990, 94, 8703-8706. Shaw, B. R.; Creasy, K. E.; Lanczycki, C. L.; Sergeant, J. A. J . E k trochem. SOC. 1988, 735, 669. Murr, E. L.; Kerkeni, M.; Sellami, A.; Ben Tarrit, Y. J . Ektrmnal. Chem. Interfacial Electrochem. 1988, 246, 461. Sherry, H. S. The Ion-EXchenge RqeH&s of Zeolites; IonIxchange Voi. 2; McGraw Hill: New York, 1962; pp 89-134. Narayana, N.; Kevan. L. J . Chem. Phys. 1982, 76, 3999. Ken, G.T. zeoines 1983, 3, 295-297. Barrer, R. M.;James, S. D. J . Phys. Chem. 1880, 6 4 , 421. Amos, L. J.; Duggal, A.; Mlrsky, E. J.; Ragonee, P.; Bocarsly, A. 6.; Fitrgerald-Bocarsly, P. A. Anal. Chem. 1988, 60, 245-249. Thomsen, K. N.; Baldwin, R. P. Anal. Chem. 1989, 67, 2594-2598. Ion Chromatography; Tarter, J. G., Ed.; Chromatographic Science Series Vol. 37; M. Dekker: New York, 1987. Small. H. Ion Chromat0gr;ePhy; Plenum Press: New York, 1989. Daegupta, P. K. J . Chromatogr. Scl. 1989, 2 7 , 422-448. Rockiin, R. D.; Rey, M. A.; Stllllan, J. R.; Campbell, D. L. J . Chromatogr. Sci. 1989, 2 7 , 474-479. Rolison, D. R. Chem. Rev. 1990, 90, 867-878 and references cited therein. Huang, H.; Dasgupta, P. K. Anal. Chem. 1990, 6 2 , 1935-1942.
M. D. Baker* Chandana Senaratne Guelph-Waterloo Centre for Graduate Work in Chemistry Department of Chemistry and Biochemistry University of Guelph Guelph, Ontario N1G 2W1, Canada RECEIVED for review May 22, 1991. Accepted December 20, 1991.
TECHNICAL NOTES Molecular-Optical Viscometer Based on Fluorescence Depolarization Angela M. Williams and Dor Ben-Amotz* Department of Chemistry, Purdue Uniuersity, West Lafayette, Indiana 47907 INTRODUCTION Conventional viscometers such as capillary flow, falling-ball, and rotating-disk viscometers1v2measure the mechanical drag on a macroscopic object in the fluid of interest. These techniques require relatively large sample volumes and are difficult to implement for in situ viscosity determination as, for example, under operating-engine or on-line manufacturing processes. In this study we report the development of a new microscopicviscometer design which can be used to remotely measure the viscosity of small fluid samples. This molecular-optical technique can be viewed as an extension of reant microscopic viscometer designs utilizing small mechanical probes and optical detection methods. For example, a ferrofluid viscometer, based on optical measurement of the birefringence decay of a solution containing a colloidal
suspension of iron clusters (4000 nm3)has recently been a p plied to the study of the dependence of viscosity on temperature in gly~erol.~ Although this technique requires only a few cubic millimeters of sample volume, it is restricted by the requirement for a pulsed external magnetic field to align the colloidal iron particles. Other microscopic techniques based on molecular fluorescence measurements have been developed and used primarily in micelle and biological membrane fluidity ~ t u d i e s .These ~ are based on rotational diffusion and empirical relations between excimer lormation, fluorescence quenching, and intramolecular rotational relaxation on microscopic viscosity of a fluid. Although such molecular-opticalmethods only require small samples, they may be complicated by specific molecular effects, such as hydrogen bonding and solvent cage structure,
0003-2700/92/0364-0700$03.00/00 1992 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 64, NO. 6, MARCH 15, 1992
Filter.
Polarizer /
n
methanol (Fisher, 99.9%), 1-octanol (Aldrich, 99.6%). Zerofrequency shear viscosities of alkanes and alcohols were obtained from Landolt-Bornsteinviscosity tables: and mixture viscosities were measured with a Gilmon 2302 falling-ball viscometer calibrated with viscosity standards from Brinkman at room temperature. The estimated error of the falling-ball viscosity measurements is *4%.
FLUORESCENCE DEPOLARIZATION Fluorescence depolarization measurements are performed using vertically polarized light to excite an oriented population of probe molecules. Rotational diffusion of the probe molecules, which depends strongly on viscosity, leads to depolarization of the probe molecule’s fluorescence. The fluorescence depolarization is characterized by the emission anisotropy p=
GzW
- IVH
GzVV -k zrVH
n
1-.......... ......................... ....
Photon Counter
701
(1)
where Zw and Zw refer to the intensity of the vertically and horizontally polarized fluorescence, respectively, relative to the vertical excitation polarization. The instnunental constant is G = ZHH/ZHV where ZHH and ZHV represent the horizontal and vertical fluorescence intensity with a horizontal polarized excitation. This constant serves to correct the anisotropy signal for nonuniform sensitivity of the detection system to polarization. The emission anisotropy is related directly to the rotational diffusion time, Trot, of the probe molecule Trot
- 7 f l ~ P0 r F ~
(2)
where 7flu,ris the fluorescence lifetime and ro is the emission anisotropy of the probe molecule at zero time. Although ro = 0.4 is predicted for molecules with collinear absorption and emission dipoles? somewhat lower values are often observed, presumably as a result of vibronic and/or solvent cage fluctuation effects. For BTBP, picosecond fluorescence depolarization studies have established that ro is equal to 0.37 f 0.03: In addition, the fluorescence time, 7flu0r,may vary with solvent as a result of solvent-mediated intersystem crossing or internal conversion mechanisms. For BTBP, however, rnuor = 3.72 f 0.16 ns has been found in degassed n-alkane and n-alcohol solvents.6 A linear relationship between shear viscosity over temperature, s/T, and rotational diffusion time, rmt,is predicted by simple hydrodynamic diffusion model^.^ This linear relationship has been quantitatively verified for BTBP dissolved in low-viscosity alkane and alcohol solvents a t various temperatures? Experimental rotational diffusion results can thus be approximately represented by a straight line 7r0t = C ( v / T ) (3) where the constant C is essentially independent of temperature and the nature of the solvent. Our results (presented in the following section)yield a beat value of C = 81.1 (ns*K)/cP over the entire viscosity range studied, while a somewhat higher value of C = 103 (ns.K)/cP is found to better represent the lowest viscosity data. The rotational time can be eliminated from the above expressions in order to express viscosity directly in terms of 7flu0r,ro, C, G, and the fluorescence signal intensities, Z w and Zw.
702
ANALYTICAL CHEMISTRY, VOL. 64, NO. 6, MARCH 15, 1992 o Ethanol/Glycerol e Dodecane/MlnerolO11
O r
T
' ' ' 5 ' ' ' ' 10 I ' ' ' ' 15 I ' ' ' ' 20 I ' ' '
25 -I
ViscosHy ( 11 ) , CP
Correlation of rotatlonal diffusion times of BTBP measured by fluorescence depolarization and shear vlscositles of low viscosity fluids. The solid line Is the best fit through the orlgln with a slope of 0.348 nslcP (C = 103 (ns.K)/cPin eq 3). The sample temperatures are 24 f 1 'C. Flgurr 2.
0 Ethanol/Glycerol e DodecanelMineral 0 1 1
Viscosity ( 11 ) ,cP
Correlation of rotatlonal ditfudon times and measured shear Viscosities at hlgh Viscosity. The solM line is the best fit to all the data with a slope of 0.273 nslcP (C = 81.1 ns.K)/cP In eq 3). For comparlson the dotted line represents the best fit to the low vlscosity data (from Figure 2). The sample temperatures are 24 f 1 'C. This equation represents the direct connection between the experimentally measured fluorescence polarization ratio, Zvv/ZvH, and the shear viscosity of the i :id. RESULTS AND DISCUSSION Figure 2 displays the correlation between the rotational diffusion time of BTBP and the shear viscosities of n-alkanes, n-alcohols, ethanol/glycerol mixtures, and paraffin oil/dodecane mixtures, either tabulateds or measured by falling-ball viscometry. The linear relationship between the steady-state rotational time and viscosity in all four of these systems is clearly evident. The relatively small difference between the strongly associated ethanol/glycerol mixture and the nonpolar dodecane/mineral oil mixture is remarkable in view of the extreme difference in molecular interaction in these two complex mixed-liquid systems. Figure 3 represents the results obtained in the mixed fluids over a higher viscosity range. Notice that the best fit slope to the high-viscosity data (solid line) is somewhat smaller than that fit to the low-viscosity data (dashed line). This presumably reflects the onset of saturation at high viscosity, as tracer molecules cease to rotate on the time scale of their Flgurr 3.
Calculated relationship between vlscosity and the fluorescence polarization ratio, G*(Iw/IvH),obtalned uslng eq 4 with r,, = 0.37, 7- = 3.72 ns, and C = 81.1 (ns.K)/cP). This plot can thus be used to directly relate measured polarization ratios of BTBP to solvent vlscosity. Flgurr 4.
fluorescencedecay. In viscometric applications the important point is that there is a good, although slightly nonlinear, correlation between viscosity and rotational time, even in solvents of very different molecular structure and composition. The propagation of errors in these measurements can be most clearly represented using a plot of viscosity as a function of G(Zw/ZvH) according to eq 4. The solid curve in Figure 4 represents the predictions of eq 4 using r0 = 0.37, T~~~~ = 3.72 ns, and C = 81.1 (ns.K)/cP obtained from a fit of all the experimental data to eq 3. The slope of the line in Figure 4 directly reflects the sensitivity of the device to errors in Zw/ZvH. Thus a 1% error in Zw/Zw will produce a 20% viscosity error in the vicinity of 1 CPbut only a 2% error in viscosity at around 20 cP. Of the other parameters in eq 4 only T~~~~ is expected to vary significantly from sample to sample. Previous measurements reveal 9% standard deviation in T~~~~ for BTBP in degassed n-alkane and n-alcohol solvents. In addition, we have measured the fluorescence time of BTBP in an ethanol/glycerol mixture (55% by weight ethanol) before and after degassing with bubbling argon and found a 25% increase in T ~ , , after degassing (from 3.61 to 4.53 ns). Thus although oxygen quenching may significantly affect rfiuor,in a series of solvents with similar oxygen concentration fluorescence quenching is estimated to contribute less than 10% uncertainty to the correlation between viscosity and rmtor ZW/Zw. Implementation of this device in laboratory measurements of fluid viscosities can be performed using components similar to those shown in Figure 1. Several refinements and simplifications of this device can be readily envisioned. Simultaneous collection of the Zw and ZVH signals can be achieved by splitting the emission and detecting it using two polarized detectors. For this purpose, amplified photodiodes with band-pass filters could replace the monochromator and photomultiplier tube used in this work. In addition, although a mechanical rotation of the input polarizer would still be required for determination of the instrument factor, G, the actual viscosity measurements would not involve any mechanical moving parts. Furthermore, polarization preserving optical fibers could, in principle, be used to apply this molecular-optical viscometer in remotely probing the viscosity of fluids under in situ engineering conditions. The practicality of such fiber-optic detection schemes is currently under investigation.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 6, MARCH 15, 1992
Various approaches to extending the device to higher viscosity can be inferred by inspection of eq 4. The polarization ratio is fundamentally restricted to values in the range 1 < G(Zw/Zw) < l / r o (-2.5). Thus in order to access a higher viscosity range, a decrease in C or an increase in rfluor is required. A decrease in C could be realized by employing a smaller tracer chromophore. This approach is complicated by the fact that small molecules typically have a nonlinear viscosity dependence whose form is sensitive to molecular solvent/solute interaction^.^^^ A more promising tactic is to extend the lifetime of the tracer. This could be done using longer lived phosphorescent tracers, as demonstrated by Edigar and Co-workers.8,1Oin order to extend the viscosity range of the device by more than a factor of lo3. Finally, extension of the device up to viscosities approaching the glass transition, 7 1013P,could be achieved by permanent photobleaching of the tracer dye using a polarized high-power laser pulse. This would effectively make the lifetime of the tracer infiiite and so could be used to probe rotational reequilibration of the unbleached dye population over an arbitrarily long time and thus up to an arbitrarily high viscosity. ACKNOWLEDGMENT This work was supported in part by a grant from the Exxon
-
703
Education Foundation. We would also like to thank Sylvia James and Jinsong Li for their assistance in the performance of some of the viscosity and fluorescence measurements reported here. Registry No. BTBP,83054-80-2; ethanol, 64-17-5; glycerol, 56-81-5; dodecane, 112-40-3.
REFERENCES (1) Touloukian. Y. S.; Saxena, S. C.; I-lestertnans. P. Thermophyslcel Ropertles of Matter; IF1 Plenum: New York, 1975;Vol. 11, pp 45a-
46a. (2) Dinsdale. A.; Moore, F.; Vbw/fy and Its Measuement; Relnho# Publlshing Corp.: New York, 1962. (3) Bacrl, J. C.;Dumas, J.; Gorse, D.; Perzynski, R.; Sailn, D. J . Pnp. Lett. 1985, 46. L-1199-L-1205. (4) aieser, F.; Drummond, C. J. J . Pnp. Chem. 1988. 92. 5580-5593. (5) Ben-Amotz, D.; Scott, T. W. J . Chem. phvs. 1987,87, 3739-3748. (6) Ben-Amotz, D.; Drake, J. M. J . Chem. Pnys. 1988,89, 1019-1029. (7) Perrin, P. F. J . phvs. Radium lQ34, 5 , 497. (8) Tao, T. 8/0po&mers 1989. 8, 609-632. (9) Landott-Bomsteln. z8hlewette u#d Functkmen; Springer: Berlin, 1969 Vol. 11, Part 5a. (lo) Hyde, P. D.; Evert, T. E.; Edigar, M. D. J . Chem. Pnys. 1990, 93, 2274.
RECEIVED for review September 26,1991. Accepted December 6, 1991.
