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Anal. Chem. 1995, 65, 1491-1492
TECHNICAL NOTES
Fiber-optic Temperature Sensor Based on the Temperature Dependence of the pK, of Tris Amy E. Straub and W. Rudolf Seitz' Department of Chemistry, University of New Hampshire, Durham, New Hampshire 03824
INTRODUCTION The development of fiber optic chemical sensors is an active area of research.15 Many projected applications, e.g., invasive blood gas sensing and bioreactor monitoring, require simultaneous measurement of temperature. We point out here how this can be accomplished by exploiting the highly temperature dependent pKa of tris(hydroxymethy1)aminomethane (Tris). The sensing element is a small volume of Tris buffer containinga pH indicator dye. As the temperature varies, the pK, of Tris changes, altering the ratio of acid to base forms of the indicator. This ratio is monitored remotely through fiber optics. The primary advantage is that the measured parameter can be similar or identical to one of the chemical parameters being monitored. In many applications an indicator dye used to measure pH can also be used to follow the variation in the pKa of Tris. This means that the instrumentation for fiber optic chemical sensing can also be used to monitor temperature. No additional apparatus is required. A second advantage is that the measured parameter can be an intensity ratio, which is insensitive to instrumental drift.
THEORY The expression for pH in a Tris buffedindicator solution is pH = pKa,, + log [Trisl/[TrisH+l = PK,,~,+ log [Inl/[HInl (1)
where the subscripts t and in designate Tris and indicator, respectively. Solving for the log of the ratio of acid to base indicator forms yields log [In]/[HIn] = pKa,, - PK,,~,+ log [Trisl/[TrisH+l
(2)
In the sensor, the concentrations of Tris and TrisH+ are essentially constant because they are in large excess relative to the indicator. Therefore, the log [In]/[HIn] term varies with the pK, difference. Tris has a pKa of 8.214 at 20 "C with a temperature coefficient of -0.027/°C.6 Phenol red, the indicator used in this study, has a pK, of 7.9 at 20 "C with a temperature coefficient of -0.006/°C.7 This means that the theoretical
* Author to whom reprint requests should be addressed.
(1) Angel, S. M. Spectroscopy 1987,2 (41, 38-48. (2) Wolfbeis, 0.S.In Molecular LuminescenceSpectmscopy:Methods and Applications; Schulman, S . G.,Ed.; Wiley Interscience: New York, 1988; Part 11, pp 129-281. (3) Krull, U.M.; Brown, R. S. In Laser Remote Chemical Analysis; Measures, R. M., Ed.; Wiley-Interscience: New York, 1988; pp 505-532. (4) Seitz, W. R. CRC Crit.Reu. Anal. Chem. 1988, 19,135-178. (5)Wolfbeis, 0.S.Fiber Optic ChemicalSensors andBiosensors;CRC Press: Boca Raton, FL, 1991; Vola. I and 11. (6) Bates, R.; Hetzer, H. J . Phys. Chem. 1961, 65,667-671. (7)Kolthoff, I. M. R e d . Trau. Chim. 1921, 40, 775-786. 0003-2700/93/0365- 149 1$04.00/0
net temperature coefficient for the system, Le., the pKnt pK,,i, term, is 4).021/"C.
EXPERIMENTAL SECTION Reagents. Tris(hydroxymethyl)aminomethane,acrylamide, potassium persulfate, sodium hydroxide, hydrochloric acid, and standard pH buffers were purchased from Fisher. The phenol red was from Matheson, Coleman and Bell. NJV-Methylenebis(acrylamide) was obtained from Sigma. Apparatus. The temperature sensor was constructed from aluminum. The design is shown in Figure 1. The reflector is a hollow cap that contains the buffer/indicator system. Indicator absorbance is measured through two optical fibers held in a single SMA connector which has been drilled out to accept both fibers. The reflector cap is connected to the optical fiber connector via a union. A lock nut allows the distance between the reflector and the optical fibers to be controlled. Separate 200-pm core diameter plastic clad fused-silica fibers from Ensign Bickford conduct light from the source to the fiber and back to a detector. The reflector cap contains a rubber washer to make a tight seal with the union. Alljoints are wrappedwith Teflontape to prevent leaking. Measurements using the fiber optic arrangement were made using the source,excitation monochromator,and detector module of an SLM 8OOO spectrofluorometer. Fiber optics were connected to the spectrofluorometer through aluminum housings that fit over the lens holders in the sample compartment. SMA connectors coupled the fibers to the housings. The molar absorptivities of phenol red in the acid and base forms were measured with a Shimadzu 200 UV spectrophotometer. The variation in Tris/indicator absorbance with temperature is solutionwas measuredwith a Cary 219 spectrophotometer with a thermostated sample chamber. A Thermomix 1420 circulating water bath was used to control temperature for the fiber optic experiments. Procedures. Cross-linkedpoly(acry1amide) with immobilized phenol red was prepared by a procedure adapted from the literature.8 Acrylamide(3.600 g),NJV-methylenebis(acrylamide) (0.400 g), and phenol red (0.0080 g) were dissolved in 50.0 mL of water. Potassium persulfate (0.500 g) was added to initiate polymerization. The resulting orange solution was poured into the reflector cap and allowed to gel in place. The gel was then soaked in 50 mL of pH 8.00 Tris buffer until the gel turned red. Tris buffer was prepared by mixing 50.0 mL of 0.10 M Tris with 26.2 mL of 0.10 M HC1and diluting to 100.0 mL with water. This solution has a pH of 8.0 at 25 "C. Absorption measurements in solution were taken after the thermocouple in the Cary 219 indicated that the temperature had stabilized. Measurements with the fiber optic sensor were obtained 10 min after the thermometer in the water bath showed that the temperature had reached a constant value. The reference solution for the solution measurement was Tris buffer with no added indicator. With the fiber optic sensor, it is not possibleto measure a reference intensity directly. Instead, the reflected intensity at 700 nm, a wavelength where neither form of the indicator absorbs, served as the reference. To (8) Hicks, G.;Updike, S.Anal. Chem. 1966, 38, 726-730. Q 1993 American Chemlcal Society
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ANALYTICAL CHEMISTRY. VOL. 65. NO. 10, MAY 15, 1993
To Dmeclion System
Source
intervals hetween 15 and 50 OC. The change in intercept indicates that the PK.,~- p K a term in eq 2 changes by 0.86. When phenol red is immobilized on polyacrylamide, its pK, shifts to 7.57.9 This accounts for part of the change in intercept. The remaining change in intercept must be due to Tris. If the uncharged base form of Tris is stabilized, the hydrophobic polyacrylamide matrix would cause the pK, of Tris to increase. The slopes of the cuvette and sensor curves are similar. Because the polyacrylamide matrix may affect the pK. temperature dependence, we do not expect the two values to necessarily be identical. We cannot explain the relatively small discrepancy between the theoretical and experimental slopes. It could reflect uncertainty in the literature temperature coefficient for phenol red since this value was not determined with high precision. Because our sensor has a relatively large thermal mass, it took several minutes to reach a constant value after the temperature changed. The response time will be a function of thermal mass and can he readily reduced by miniaturizing the sensor. We did not experimentally determine the lifetime of our sensor. We anticipate that the sensor lifetime will be limited by photodegradation of the indicator, which will depend on several factors including duty cycle, illumination intensity, and total indicator amount in the sensor. As long as the measured parameter is the ratio of acid to base absorbance, photodegradationwill not cause a systematicerror. However, as the indicator concentration decreases, the precision of the absorbance measurements will necessarily decrease as well.
GSMA Optical Fibers
Connector Lock Nul
i l l
I
Union
I
indicator
I
kgiector I
i
npuro 1. Dlagam of sensing apparatus.
determinetheabaorbanceoftheindicator,itisnecessmytoknow the relationship between the intensities at 700 nm and the measurement wavelength, i.e., the values of k , and kl in the following expressions: A559
= log kII7dI559
(3)
A431 = 1%
kJ7dI431 (4) The subscripts, 550 and 431, refer to the wavelengths (in nanometers) where the base and acid forms of phenol red have their maximum absorbances. The values of k , and kp were found to be 1.07 and 0.637 in an experiment performed using polyacrylamide gel without phenol red in the sensor. These values were then used to compute absorbances at the two wavelengths.
RESULTS AND DISCUSSION In the cuvette study, log [Inl/[HInl varied linearly with temperature between 13 and 45 "C. For seven points measured atapproximatelyequalintervalsbetweentwothese temperatures, the slope was -0.0191/°C with a standard deviation of 0.003,and the intercept at 0 O C was 0.26 with a standard deviation of 0.009. The correlation coefficient was 0.999. In the sensor experiment, the slope was -0).0180/"Cwith a standard deviation of 0.007,the intercept was -0.60at 0 "C with a standard deviation of 0.024, and the correlation coefficient was 0.991. Nine data points were taken at 5-deg (9) Peterson, J. I.; Goldstein, S. R.; Anal. Chem. 1980,52,864-869.
Fitzgerald, R.V.;Buckhold, D.K.
CONCLUSIONS The results confirm the feasibility of a fiher optic temperature sensor based on the temperature dependence of the pK. of Tris. We anticipate that this kind of sensor will be usefulinconte~wherechemiealparametersandtemperature are simultaneously monitored with fiber optic sensors. The indicator used to monitor the change in pK. in temperature can be selected to he most suitable for the particular system. We chose phenol red for this study because it is widely used as an indicator for physiological samples. However, a fluorescentindicator would be preferredif theother chemical parameters are being measured by fluorescence. Altematively, a longer wavelength pH indicator could he selected for compatibilitywith instruments that use lighbemitting diodes as sources. Note, however, that indicator must have a pK, close to that of Tris. While the temperature-sensing concept caninprinciple beapplied withotherhuffers, theyhavemuch smaller temperature coefficients and, therefore, will be much less sensitive.
RECEIVEDfor review November 10, 1992. Accepted January 29, 1993.
