Technical Notes Anal. Chem. 1994, 66. 2788-2790
Molecular Fluorescence Thermometry Kimberley F. Schrum, Angela M. Williams, Stacey A. Haerther, and Dor Ben-Amotz* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-1393 Shifts in the wavelength of the fluorescence maximum and intensity in the blue tail of the fluorescence of BTBP [N,N'bis(2,5-di-terM>utylphenyl)-3,4,9,10-perylenedicarboximide] are shown to correlate with the temperature of the fluid in which it is dissolved. These are used to measure temperature optically witha precision of better than ±2 °C in both associated (methanol) and nonpolar (mineral oil) fluids at concentrations as low as 10"7
M.
of great importance in many and research commercial, industrial, applications. In this Temperature is a variable
study, we report a new fluorescence-based molecular thermometer that may find unique applications in monitoring microscopic (e.g., solid-state circuit or biological cell) or hostile (e.g., high-voltage) environments, where conventional thermometry is impractical or impossible. Here we demonstrate the technique in liquid methanol and mineral oil over a 15-70
°C temperature range. A number of previously reported optical thermometers have been based on nonlinear as well as linear Raman spectroscopy,1'2 and have been used in many different environments including internal combustion and jet engines, flames, plasmas, C VD processors, and industrial furnaces.3 These are all based on the spectroscopic determination of the Boltzmann distribution of molecular rotational and/or vibrational populations.2’4 Limitations of these techniques include the inherent weakness of Raman signals, thus requiring the use of an intense radiation source such as laser, and monochromators or holographic Bragg gratings to filter out the large Rayleigh scattering (elastic light scattering) signal. Furthermore, Raman spectra are often obscured by fluorescence from the sample, which cannot readily be eliminated. Alternative optical approaches to the measurement of temperature based on visible or ultraviolet absorption and/or fluorescence have been developed for specific applications but have not been widely applied. For example, excimer-forming mixtures of organic compounds have been used to measure temperature in fuel sprays.5 Alternatively, the temperature (1) Attal-Tretout, B.; Bouchardy, P.; Magre, P.; Pealat, M.; Taran, J. P. Appl. Phys. B. 1990, 51, 17-24. (2) Kip, B. J.; Meier, R. J. Appl. Spectrosc. 1990, 44, 707-711. (3) Rosaco, G. J.; Hurst, W. S. In Temperature-. It’s Measurement and Control in Science and Industry-, Schooley, J. F., Ed.; American Institute of Physics: New York, 1992; Vol. 6, Part 2, pp 655-660. (4) Liepertz, A.; Seeger, T.; Spiegel, H.; Magens, E. In Temperature: It’s Measurement and Control in Science and Industry-, Schooley, J. F., Ed.; American Institute of Physics: New York, 1992; Vol. 6, Part 2, pp 661-666. (5) Murray, A. M.; Melton, L. A. Appl. Opt. 1985, 24, 2783-2787.
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dependence of the fluorescence spectrum of rare-earth ion phosphors has been used in surface thermometry6 6and to look at temperature fluctuations in surfaces of turbine engines.7 Additionally, the red tail of the electronic absorption spectra of a polyatomic probe molecule has been used to measure dynamic heat transfer in chemical reactions.8 All of these techniques, however, require higher chromophore concentrations than the molecular fluorescence thermometer described
in this work. Our technique relies on the measurement of changes in the fluorescence maximum and/or the intensity of the blue hotband tail of the fluorescence of an organic probe chromophore. Two mechanisms are likely to contribute to changes in the fluorescence spectrum; one is thermal excitation of vibronic states in the electronically excited state, termed fluorescence hot-band emission, and the other is solvent index of refractioninduced shifts in the fluorescence maximum. The particular fluorescent probe molecule used in these studies, BTBP,9 is chosen for its large absorption cross-section, high fluorescence quantum yield, reasonable solubility in a both polar and nonpolar solvents, and negligible fluorescence Stokes shift as well as its previous application as a molecular viscosity probe.10
EXPERIMENTAL SECTION Apparatus. Light from a 50-W tungsten lamp was collected
(f /1), collimated, and focused (f/3) into the sample cell (Figure An excitation wavelength of 480 ± 10 nm was selected using a three-cavity band pass interference filter (Corion Corp.). The temperature of the sample was regulated using a cylindrical water-jacketed quartz cell (Hellma Cells Inc.) and a thermostated water circulator (Lauda RM-6). Fluorescence from the sample was collected (with f/4) at a 90° angle using a camera lens (Olympus 50 mm, f/1.8), a grating monochromator (ISA H-20 with 1 -mm entrance and exit slits), and a photomultiplier tube (RCA 1P28). The monochromator was scanned using a stepper motor and interfaced using Labview II (National Instruments) software on a Macintosh II computer. The photoelectric pulses from the phototube, representing the intensity of the fluorescence, were amplified (EG&G Ortec VT120), discriminated (Mechtronics 512), 1).
(6) Goss, L. P.; Smith, A. A.; Post, M. E. Rev. Scl. Instrum. 1989,60,3702-3706. (7) Noel, B. W.; Borella, H. M.; Franks, L. A.; Marshall, B. R.; Allison, S. W.; Cates, M. R.; Stange, W. A. J. Propul. Power 1986, 2, 565-568. (8) Seilmeier, A.; Scherer, P. O. J.; Kaiser, W. Chem. Phys. Lett. 1984, 105, 140-146. (9) Ben-Amotz, D.; Drake, J. M. J. Chem. Phys. 1988, 89, 1019-1029. (10) Wiliams, A. M.; Ben-Amotz, D. Anal. Chem. 1992, 64, 700-703.
0003-2700/94/0366-2788$04.50/0 1994 American Chemical Society
©
Wavelength (nm) Photon Counter
Figure 2. Fluorescence spectra of 10^M solution of BTBP In methanol. The solid line spectrum was taken at 20.9 °C while the dotted spectrum was taken at 56.5 °C.
Flgura 1. Diagram of Instrumental setup.
and then counted (National Instruments and Labview software).
NB-MIO-16 board
Reagents. The probe molecule Ar,Ar'-bis(2,5-di-ferfbutylphenyl)-3,4,9,10-perylenedicarboximide (BTBP; 8305480-2; Aldrich, 97% purity), was used without further purification, as were the two solvents; methanol (Burdick & Jackson, 99.9%) and light mineral oil (Fischer). Procedure. Solution concentrations (between 10^ and 10-8 M) were determined from the absorbance of the samples at 520 nm (t ~ 9 X 104 L/mobcm).11 Solutions were placed into the fluorescence cell and allowed to equilibrate (15-20 min or until the temperature was stable for 1 min). Temperatures were measured using a thermocouple, which had been calibrated against a mercury thermometer. Fluorescence counts were accumulated for 0.5 s at 0.5-nm wavelength increments. The wavelength ranges of the spectra were selected so as to include both the fluorescence maximum and a baseline on the blue side; 490-545 nm for methanol solutions and 490-530 nm for the mineral oil solutions. Six scans were averaged at each temperature to obtain a spectrum. Each set of data consists of five spectra collected at different
temperatures between 15 and 70 °C.
RESULTS Representative examples of the fluorescence spectra obtained at two different temperatures are shown in Figure 2. As the temperature increases, the peak shifts to shorter wavelengths, and the intensity in the blue tail of the spectrum increases. Such spectra have been analyzed in three different ways in order to obtain quantitative temperature correlations as well as to distinguish the peak shift and the fluorescence hot-band contributions to the spectra. One relatively simple analysis algorithm is based on the determination of the ratio of the intensity of the fluorescence in the blue tail to that at a reference window around the fluorescence peak, with temperature-independent integration windows. In particular, the reference intensity is measured by integrating the counts over a 6-nm-wide wavelength window (11) Rademacher, A.; Marklc, S.; Langhals, H. Chem. Ber. 1982, 115, 29272934.
(12) Castncr, E. W.; Maroncelli. M.; Fleming, G. R. J. Chem. Phys. 1987, 86, 1090-1097.
centered on the fluorescence maximum at a reference temperature (the lowest temperature of interest). This same reference window is used when integrating the spectra obtained at all other temperatures. The intensity of the blue tail is integrated over a 15-nm window located 10-25 nm to the blue of the reference peak. The resulting ratios of the tail intensity (/tail) to that of the reference intensity (/peak) are plotted as a function of temperature in methanol and light mineral oil. Linear fits of the data [/toii//peak = 0-36 + 0.00627" (°C) in methanol and /t*ii//peak = 0.13 + 0.00247"(°C) in mineral oil] yield standard deviations of the ratios from the line of 0.002 in 10-6 M methanol and 0.003 in 10~7 M light mineral oil, respectively, which correspond to a temperature resolution of ± 0.35 and ± 1.32 °C.
An alternative method of analysis of the data is obtained by correlating the shift in the fluorescence maximum (Xmax) with temperature. This may be done quite accurately by performing Gaussian fits to the fluorescence spectra over a 20-nm window about the maximum. Figure 3a,b shows the peak (Xmax) positions thus obtained, plotted as a function of temperature in methanol and mineral oil, respectively. Linear fits to these data [Xmax = 537.5 + 0.0437" (°C) in methanol and Xmax= 523.7 + 0.020T(°C) in mineral oil] yield standard deviations from the fit line of 0.041 a nd 0.018 nm for methanol and light mineral oil solutions, respectively, corresponding to temperature resolutions of ± 0.95 and ± 0.91 °C. In an effort to extract the pure fluorescence hot-band contribution to the spectra, the intensity of the blue tail may be measured relative to that at the fluorescence maximum. In other words, the tail and peak integration windows are shifted with temperature so as to track the fluorescence maximum. The spectra are then reanalyzed to obtain the ratio of the integrated intensity of the tail (/ui]) to the integrated intensity of the peak (/p«,k) for each spectrum. These results are plotted as a function of temperature, as shown in Figure 3c,d. Linear fits to these data [/taii//peak = 0-54 + 0.00237" (°C) in methanol and /Uii//peak = 0.16 + 0.00137 (°C) in mineral oil] yield standard deviations from the fit line for methanol and mineral oil solutions of 0.0020 and 0.0026, respectively, which correspond to temperature resolutions of ± 0.87 and ± 1.99 °C, respectively. Analytical Chemistry, Vol. 66,
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Temperature (°C) Figure 3. (a and b) Peak position (Xmu) versus temperature for 10~* M BTBP/methanol and 10*7 M BTBP/light mineral oil solutions, respectively, (c and d) Ratio of the integrated tall intensity (/tan) to the integrated peak Intensity (/pnk) versus temperature for the abovementioned solutions. The error bars represent a single standard deviation of the mean of six measurements (as described In the Experimental Section).
DISCUSSION The measured changes in the fluorescence spectrum of BTBP appear to result from the combined influence of a shift in the fluorescence maximum and a change in the intensity in the blue tail. The former effect, which probably derives from the temperature dependence of the index of refraction of the solvent, yields slightly better temperature correlations (Figure 3a,b) while the latter effect (Figure 3c,d), which we believe reflects the changing thermal population of excited vibronic states, may be of more general interest as a universal temperature probe (as discussed below). The blue shift in the fluorescence maximum undoubtedly reflects the thermal expansion of the solvent and, thus, the decrease in the index of refraction of the fluid with increasing temperature. Both the absorption and fluorescence spectra of BTBP undergo a shift of about the same magnitude and sign (0.5 A/°C for methanol). This behavior is typical of that of other chromophores in nonaqueous solvents. The blue shift in the spectra corresponds to a shift toward the vaporphase spectrum of the chromophore as the index of refraction of the solvent decreases. For this reason, any other variable which produces a change in density (e.g., pressure) is also expected to produce a shift in the fluorescence (and absorption)
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maximum. Thus, shifts in a fluorescence maxima may not correlate simply with temperature when other variables that couple to the density of the system are also changing. The vibronic hot-band population in the excited electronic state, on the other hand, may yield a more universal probe of temperature, even when pressure and other variables are changing, as long as a near-equilibrium Boltzmann population distribution is maintained in the excited-state vibronic manifold. Thus, changes in the intensity of fluorescence in the blue tail of the spectrum, when referenced to the intensity at the peak (as in Figure 3c,d), are expected to yield a more universal temperature probe. On the other hand, there are also reasons to expect that not all chromophore/solvent systems will offer as simple a fluorescence hot-band probe as BTBP. For example, chromophores which undergo significant fluorescence Stokes shifts may display a more complicated temperature dependence of the blue tail fluorescence. In particular, in such systems the blue tail intensity is expected to depend on the time scale over which the fluorescence Stokes shift occurs.12 This may be related to the viscosity (or longitudinal relaxation time) of the solvent and, thus, only indirectly to temperature (as well as pressure, etc.). In conclusion, our results suggest that the fluorescence of BTBP may be useful as an optical thermometer, as long as some caution is exercised. Under conditions in which temperature is the only variable that affects the density of the system, the shift in the fluorescence maximum may be used as a temperature probe with about 1-2 °C temperature resolution. When simultaneous changes in temperature, pressure, and/or other variables are taking place, then the somewhat less sensitive but probably more reliable, vibronic hot-band effect may be employed. Further studies (e.g., under variable pressure and temperature) are required in order to verify this expectation as well as to investigate the degree to which the above results may be generalized to chromophores other than BTBP. Current applications under investigation include immobilizing BTBP in a polymer matrix for use as an optrode coupled to the end of an optical fiber, as well as using BTBP-doped lubricants to measure temperature profiles at ball bearing contacts.
ACKNOWLEDGMENT Support for this work from the Office of Naval Research (N00014-92-1559) is gratefully acknowledged. Received for review January 31, 1994. Accepted April 20, 1994.® •Abstract published in Advance ACS Abstracts, June 15, 1994.
