Temperature-Sensitive Europium (III) Probes and Their Use for

Anal. Chem. 2006, 78, 5094-5101

Temperature-Sensitive Europium(III) Probes and Their Use for Simultaneous Luminescent Sensing of Temperature and Oxygen Sergey M. Borisov and Otto S. Wolfbeis*

Institute of Analytical Chemistry, Chemo- and Biosensors, University of Regensburg, D-93040 Regensburg, Germany

Highly photostable and strongly luminescent europium(III) β-diketonate complexes are presented that can act as new probes for optical sensing of temperature. They can be excited with the light of a 405-nm LED and possess strong brightnesses. The decay times of the probes contained in a poly(vinyl methyl ketone) film and in poly(tert-butyl styrene) microparticles are highly temperaturedependent between 0 and 70 °C. The temperaturesensitive microparticles were dispersed, along with oxygensensitive microbeads consisting of a palladium porphyrin oxygen indicator in poly(styrene-co-acrylonitrile), in a thin layer of a hydrogel to give a dually sensing material which is excitable by a single light source. The two emissions can be separated by appropriate optical filters. The response to oxygen and temperature is described by 3D plots, and unbiased values can be obtained for temperature and oxygen, respectively, from the two luminescence signals if refined in an iteration step. The sensing scheme is intended for use in temperature-compensated sensing of oxygen, in contactless sensing of oxygen and temperature in (micro)biological and medical applications, in high-resolution oxygen profiling, and for simultaneous imaging of air pressure and temperature in wind tunnels. Temperature is a key parameter in numerous fields of science and technology. While not a chemical parameter by itself, its knowledge is of highest significance since temperature affects the response of all chemical sensors.1-3 Sensing temperature by itself is also desirable in marine research,4,5 in underground geochemical studies,6 in diagnosis of diseases,7 and in biotechnology.8,9 Luminescent temperature indicators have been employed lately * Corresponding author e-mail: [email protected]. (1) Wolfbeis, O. S. Anal. Chem. 2002, 74, 2663-2678; 2004, 76, 3269-3284; 2006, 78(12), in press; DOI 10.1021/ac060490z. (2) Demas, J. N.; DeGraff, B. A. Sens. Actuators, B 1993, 11, 35-41. (3) Ogurtsov, V. I.; Papkovsky, D. B. Sens. Actuators, B 2006, 113, 917-929. (4) Fernańdez-Valdivielso, C.; Egozkue, E.; Matıás, I. R.; Arregui, F. G.; Bariaín, C. Sens. Actuators, B 2003, 91, 231-240. (5) Zhao, Y.; Liao, Y. Sens. Actuators, B 2002, 86, 63-67. (6) Grosswig, S.; Hurtig, E.; Kuhn, K. Geophysics 1996, 61, 1065-1067. (7) Stefanadis, C.; Tsiamis, E.; Vaina, S.; Toutouzas, K.; Boudoulas, H.; Gialafos, J.; Toutouzas, P. Am. J. Cardiol. 2004, 93, 207-210. (8) Chuppa, S.; Tsai, Y.; Yoon, S.; Shackleford, S.; Rozales, C.; Bhat, R.; Tsay, G.; Matanguihan, C.; Konstantinov, K.; Naveh, D. Biotechnol. Bioeng. 1997, 55, 328-338. (9) Wunschel, D. S. J. Microbiol. Methods 2005, 62, 259-271.

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as components for pressure sensitive paints (PSP)10-13 in aircraft and car industries.14,15 Knowing temperature is particularly important in fluorescent sensing of oxygen since quenching by oxygen always is highly temperature dependent.10,16 Apart from IR thermoimaging17,18 (www.flirthermography. co.uk) luminescent temperature sensing has become popular for optical determination of temperature. Excitation by visible light is strongly preferred in luminescent sensing for the following reasons: (1) LEDs which are available for the visible part of the spectrum are usually more efficient and less costly than non-LED based UV light sources; (2) visible excitation causes less background fluorescence and thus less interferences; and (3) visible light sources are well compatible with inexpensive plastic (PMMA) optical fibers, while UV excitation requires the use of much more expensive glass fibers. Fluorescence intensity, rather than decay time, is measured in most of the optical temperature sensors reported so far.18-21 Unfortunately, intensity measurements can suffer from drifts of the optoelectronic system and variation in the optical properties of the sample including probe concentration, turbidity, intrinsic sample coloration, and refractive index.22 Determination of decay time is free from these drawbacks and therefore preferable. Luminescent probes that display a strong temperature dependence of their luminescence lifetime include ruthenium(II)-polypyridyl and europium(III)-β-diketonate complexes. Ruthenium(II)-tris(1,10-phenanthroline) () Ru-phen) has the highest temperature (10) Coyle, L. M.; Gouterman, M. Sens. Actuators, B 1999, 61, 92-99. (11) Zelelow, B.; Khalil, G.; Phelan, G.; Carlson, B.; Gouterman, M.; Callis, J. B.; Dalton, L. R. Sens. Actuators, B 2003, 96, 304-314. (12) Hradil, J.; Davis, C.; Mongey, K.; McDonagh, C.; MacCraith, B. D. Meas. Sci. Technol. 2002, 13, 1552-1557. (13) Koese, M. E.; Carrol, B. F.; Schanze, K. S. Langmuir 2005, 21, 91219129. (14) Demas, J. N.; DeGraff, B. A.; Coleman, P. B. Anal. Chem. 1999, 71, 793A800A. (15) Gouterman, M.; Callis, J.; Dalton, L.; Khalil, G.; Me´barki, Y.; Cooper, K. R.; Grenier, M. Meas. Sci. Technol. 2004, 15, 1986-1994. (16) Mongey, K. F.; Vos, J. G.; MacCraith, B. D.; McDonagh, C. M.; Coates, C.; McGarvey, J. J. J. Mater. Chem. 1997, 7, 1473-1479. (17) Hollandt, J.; Friedrich, R.; Gutschwager, B.; Taubert, D. R.; Hartmann, J. High Temp. - High Pressures 2004, 35/36, 379-415. (18) Ishii, J.; Shimizu, Y.; Shinzato, K.; Baba, T. Int. J. Thermophys. 2005, 26, 1861-1872. (19) Bai, F.; Melton, L. A. Appl. Spectrosc. 1997, 51, 1276-1280. (20) Uchiyama, S.; Matsumura, Y.; de Silva, A. P.; Iwai, K. Anal. Chem. 2003, 75, 5926-5935. (21) Iwai, K.; Matsumura, Y.; Uchiyama, S.; de Silva, A. P. J. Mater. Chem. 2005, 15, 2796-2800. (22) Lippitsch, M. E.; Draxler, S. Sens. Actuators, B 1993, 11, 97-101. 10.1021/ac060311d CCC: $33.50

© 2006 American Chemical Society Published on Web 05/26/2006

sensitivity23 among the polypyridyl complexes but suffers from moderate molar absorbance (445 ) 2.0 ‚ 104 L‚mol-1‚cm-1) and moderate emission quantum yield (0.019).24 Europium(III) complexes have a strongly temperature-dependent luminescence25,26 (that has led to the design of “thermographic” phosphors10) and rather narrow emission bands peaking at around 616 nm. The linelike emissions facilitate the separation of signals in the case of sensors where two signals are to be processed at the same time (so-called dual sensors). However, excitation in the UV region is required. Attempts have been made to extend the excitation wavelength into the visible. Khalil et al. synthesized a number of phenanthroline-based Eu(III) β-diketonate complexes11,25 with the absorption maximum extended up to 372 nm, but the absorptivity in the visible was rather small. Another approach is to use “antenna” chromophores which can be coordinated to tris(β-diketonate) Eu(III) complexes. Werts et al.28 observed complex formation between europium tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyloctane-3,5-dione () Eu(fod)3) and 4,4′-bis(N,N-dimethylamino)benzophenone which possessed visible absorption (414 ) 3.04‚104 M-1‚cm-1) and relatively high emission quantum yield (φ ) 0.2 in benzene). Unfortunately, the complex is stable only in apolar solvents such as benzene and toluene. Other europium(III) complexes that can be excited with visible light have relatively low emission quantum yields (φ < 0.06).29-31 Recently, Yang et al.32 reported on the highly efficient sensitization of europium luminescence by visible light using a dipyrazolyltriazine derivative coordinated to europium(III) tris(thenoyltrifluoroacetonate). The potential of such complexes as temperature indicators was, however, not recognized. We show here that the dipyrazolyltriazine tris(β-diketonate) europium(III) complexes are most viable temperature probes that combine high-temperature sensitivity with photostability and unusual brightness (defined as the product of exc, the molar absorbance at the wavelength of excitation, and φ, the luminescence quantum yield). We also show that such temperaturesensitive probes can be incorporated into microbeads in order to improve their stability and that such microbeads can be used along with beads dyed with metalloporphyrins to obtain composite materials suitable for simultaneous sensing of the two parameters. EXPERIMENTAL SECTION Materials. Europium(III) chloride hexahydrate, 2,4,6-trichlorotriazine, 3,5-dimethylpyrazole, 4-bromo-N,N-diethylaniline, 4,4,4(23) Liebsch, G.; Klimant, I.; Wolfbeis, O. S. Adv. Mater. 1999, 11, 1296-1299. (24) Alford, P. C.; Cook, M. J.; Lewis, A. P.; McAuliffe, S. G.; Skarda, V.; Thomson, A. J. J. Chem. Soc., Perkin Trans. 2 1985, 5, 705-709. (25) Berry, M. T.; May, P. S.; Xu, H. J. Phys. Chem. 1996, 100, 9216-9222. (26) Mitsuishi, M.; Kikuchi, S.; Miyashita, T.; Amao, Y. J. Mater. Chem. 2003, 13, 2875-2879. (27) Khalil, G. E.; Lau, K.; Phelan, G. D.; Carlson, B.; Gouterman, M.; Callis, J. B.; Dalton, L. D. Rev. Sci. Instrum. 2004, 75, 192-206. (28) Werts, M. H. V.; Duin, M. A.; Hofstraat, J. W.; Verhoeven, J. W. Chem. Commun. 1999, 799-800. (29) Dadabhoy, A.; Faulkner, S.; Sammes, P. G. J. Chem. Soc., Perkin Trans. 2 2000, 2359-2360. (30) Bretonniere, Y.; Cann, M. J.; Parker, D.; Slater, R. Org. Biomol. Chem. 2004, 2, 1624-1632. (31) Van Deun, R.; Nockemann, P.; Fias, P.; Van Hecke, K.; Van Meervelt, L.; Binnemans, K. Chem. Commun. 2005, 590-592. (32) Yang, C.; Fu, L. M.; Wang, Y.; Zhang, J. P.; Wong, W. T., Ai, X. C.; Qiao, Y. F.; Zou, B. S.; Gui, L. L. Angew. Chem., Int. Ed. 2004, 43, 5009-5013.

trifluoro-1-(2-naphthyl)butane-1,3-dione () nta), poly(vinyl methyl ketone) () PVMK, average MW 280 000), poly(4-tert-butyl styrene) () PTBS, average MW 50,000-100.000), and poly(styreneco-acrylonitrile) () PSAN; containing 30 wt % polyacrylonitrile, average MW 185 000) were obtained from Aldrich (www.sigmaaldrich.com). Tris (6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionato)europium () Eu(fod)3), europium(III) tris(thenoyltrifluoroacetonate) trihydrate () Eu(tta)3), and anhydrous tetrahydrofurane were bought from Fischer Scientific (www.fishersci. com). All other solvents were obtained from Fluka (www.sigmaaldrich.com). Palladium(II) 5,10,15,20-tetrakis(2,3,4,5,6-pentafluorophenyl)porphyrin (Pd-TFPP) was purchased from Porphyrin Systems (www.porphyrin-systems.de). Polyurethane hydrogel (type D4) was obtained from Cardiotech (www.cardiotech-inc.com), and polyester support (Mylar) was obtained from Goodfellow (www.goodfellow.com). All chemicals were used as received. Nitrogen and oxygen, both of 99.999% purity, were obtained from Linde (www.linde-gase.de). The ligand [4-(4,6dichloro-1,3,5-triazin-2-yl)-phenyl]diethylamine was prepared according to Golesworthy,33 and europium(III) tris(naphthoyltrifluoroacetonate) () Eu(nta)3) according to Charles.34 The dipyrazolyltriazine derivative L and the probes Eu(tta)3L, Eu(nta)3L, and Eu(fod)3L, respectively, were synthesized according to the procedure of Yang et al.32 Structures were also confirmed by elemental analyses. Eu(tta)3L: found C 45.55, H 3.30, N 8.87, S 8.05. calc. C 45.82, H 3.27, N 9.10, S 7.81. Eu(nta)3L: found C 56.36, H 4.07, N 9.02; calc. C 57.23, H 3.84, N 8.21. Eu(fod)3L: found C 44.11, H 4.01, N 7.92; calc. C 43.78, H 4.02, N 7.71. The preparation of the Pd-TFPP/PSAN microbeads was reported elsewhere.35 Preparation of the Eu(tta)3L/PVMK Temperature Sensing Layer. Three hundred milligrams of PVMK and 3 mg of Eu(tta)3L were dissolved in 2 g of dichloroethane. The “cocktail” was knife-coated onto a 100 µm thick polyester support and dried at ambient air to give a film in a thickness of ca. 10 µm. The same procedure was used for the preparation of sensor films composed of Eu(nta)3L/ PVMK and Eu(fod)3L/PVMK. Preparation of Eu(tta)3L/PTBS Temperature-Sensitive Microbeads. Five hundred milligrams of PTBS and 15 mg of Eu(tta)3L were dissolved in 5 g of toluene. The “cocktail” was spread onto a glass surface, and the solvent was evaporated at 60 °C. The material was mechanically ground, and the resulting particles were washed 8 times with ethanol. The ethanol suspension of the microbeads was spread onto a glass surface and dried at ambient air. To ensure a maximal size of the particles, they were passed through a 25-µm test sieve. Preparation of the Dually Sensing Material. Ten milligrams of Pd-TFPP/PSAN microbeads and 10 mg of Eu(tta)3L/PTBS microbeads were added to 1 g of a 5 wt % solution of hydrogel D4 in ethanol/water (9:1, v:v). The “cocktail” was stirred for 3 h, knifecoated onto a 100-µm polyester support, and dried at ambient air. Spectral Measurements. Absorption and emission spectra, respectively, were recorded on a Lambda 14 p Perkin-Elmer UVvis spectrophotometer (www.perkinelmer.com) and an Aminco (33) Golesworthy, R. C.; Shaw, R. A.; Smith, B. S. J. Chem. Soc. 1962, 15071508. (34) Charles, R. G.; Perrotto, A. J. Inorg. Nucl. Chem. 1964, 26, 373-376. (35) Borisov, S. M.; Vasylevska, G. S.; Krause, Ch.; Wolfbeis, O. S. Adv. Funct. Mater., accepted for publication.

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Figure 1. Chemical structures of the indicators used for sensing temperature (left panel) and oxygen (right).

AB 2 luminescence spectrometer (www.thermo.com). Relative luminescence quantum yields were determined according Demas and Crosby36 using 4-dicyanomethylene-2-methyl-6-(4-dimethylaminostyryl)-4H-pyrane (φ ) 0.57)37 as a standard (λexc 410 nm, slit width 1 nm; emission detected in steps of 0.4 nm at an emission slit width of 0.5 nm). The phase-modulation technique was used for measurement of decay time. A sensor film was placed in a homemade flowthrough cell, and its luminescence was excited with the light of a violet LED (λmax 405 nm; from www.roithner-laser.com). After passing a BG12 optical filter (Schott; www.schott.com), it was sinusoidally modulated at 300 Hz using a two-phase lock-in amplifier (SR830, Standford Research Inc.; www.thinksrs.com). A bifurcated fiber bundle was used to guide the excitation light to the sensor foil and to guide back the luminescence after passing the Chroma 615/10M or the Chroma 680/60M interference filters (AHF Analyzentechnik, www.ahf.de). Luminescence was detected with a photomultiplier tube (type H5701-02, from Hamamatsu, www.sales.hamamatsu.com). Temperature was controlled by a Lauda RC6 cryostat (Lauda, www.lauda.de). Gas calibration mixtures were obtained using a gas mixing device (MKS, Wilmington; www.mksinst.com). To humidify the test gases, the respective gas mixtures were bubbled through water until 100% relative humidity was achieved. Time-resolve emission was registered at an Aminco AB 2 luminescence spectrometer. Sensor films were placed at an angle of 45° relative to the exciting light beam and the photodetector. A delay time of 40 µs after each flash assisted in the elimination of reflected and stray light. This method is often referred to as gating. Experimental Data Fitting. 2D and 3D plots were fitted using Origin vs 6.1 (www.originlab.com) and TableCurve 3D vs 3.12 (www.systat.com) software, respectively. The equations were solved by Maple vs 8 (www.maplesoft.com) software. RESULTS AND DISCUSSION To sense both temperature and oxygen with a single sensor material, two indicators are required that can be incorporated into

a polymeric sensor matrix which in turn can be deposited as a planar foil or at the top of a fiber-optic waveguide, for example. While luminescent sensing oxygen alone is fairly established and a variety of quenchable probes are known, not any of them can be used in combination with temperature probes (which are much less established). We prefer Pd-TFPP as an oxygen indicator because it is photostable, has a good brightness (∼40 000 at 405nm excitation)38 in the unquenched state, can be excited with high efficiency at the Soret band (∼400 nm), and is strongly quenched by oxygen. Its chemical structure is shown in Figure 1. For example, the brightness of the widely used oxygen indicator ruthenium(II) tris(4,7-diphenyl-1,10-phenanthroline) is ∼10 500.22 If a luminescent temperature probe is used along with an oxygen probe, the two signals need to be separated so that they do not interfere mutually. The europium-derived probes described here appear to be particularly useful for three reasons: their luminescence is highly temperature-dependent; they can be excited at ∼ 400 nm (together with the oxygen probe); and their line type emission can be easily separated from the phosphorescence of Pd-TFPP. The Luminescence of the Temperature Indicators. The europium(III) complexes have the general formula EuX3L, where X is a β-diketonate ligand and L is the dipyrazolyltriazine derivative shown in Figure 1. The complexes containing naphthoyl-trifluoroacetone () Eu(nta)3L) and 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyloctane-3,5-dione () Eu(fod)3L) were prepared in complete analogy to the procedure of Yang et al.32 reported previously for the preparation of Eu(tta)3L. While all europium(III) tris(β-diketonate) complexes absorb exclusively in the UV region, the coordinated “antenna” chromophore (dipyrazolyltriazine derivative) allows visible light sensitization of Eu3+ luminescence upon excitation at the ligand charge-transfer band (Figure 2). The molar absorption coefficients for the charge-transfer band of Eu(tta)3L, Eu(nta)3L, and Eu(fod)3L in toluene are as high as 71 700 M-1‚cm-1 (λmax ) 402 nm), 61 300 M-1‚cm-1 (λmax ) 403 nm), and 62 300 M-1‚cm-1 (λmax ) 397 nm), respectively. Yang et al.32 report ) 55 000 M-1‚cm-1 for Eu(tta)3L.

(36) Demas, J. N.; Crosby, G. A. J. Phys Chem. 1971, 75, 991-1024. (37) Bondarev, S. L.; Knyukshto, V. N.; Stepuro, V. I.; Stupak, A. P.; Turbana, A. A. J. Appl. Spectrosc. 2004, 71, 194-201.

(38) Spellane, P. J.; Gouterman, M.; Antipas, A.; Kim, S.; Liu, Y. C. Inorg. Chem. 1980, 19, 386-391.

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Figure 2. Absorption and emission spectra of the europium(III) complexes: 1, 2, and 3, absorption spectra of Eu(tta)3L, Eu(nta)3L, and Eu(fod)3L in toluene (C ) 150 µM, 20 °C), respectively; 4, 5, 6, and 7, emission spectra of the Eu(tta)3L at 1, 25, 45, and 65 °C (λexc) 410 nm, C ) 3.5 µM).

The complexes exhibit an emission whose shape is typical for complexes of the Eu3+ ion (Figure 2). The luminescence intensity is highly temperature dependent and significantly decreases at higher temperatures. The emission quantum yields (λexc ) 410 nm, ∼3.5 µM solution in air-saturated toluene at 25 °C) were found to be 0.39, 0.52, and 0.42 for Eu(tta)3L, Eu(nta)3L, and Eu(fod)3L, respectively. Yang et al.32 report φ ) 0.52 for Eu(tta)3L at (a nonspecified) room temperature. The observed deviation is likely to be caused by temperature to which the luminescence of the complexes is very sensitive. In fact, luminescence quantum yields increase very strongly if solutions recooled. In air-saturated toluene at 1 °C they are as high as 0.67, 0.75, and 0.65 for Eu(tta)3L, Eu(nta)3L, and Eu(fod)3L, respectively. The emission quantum yields are thus much higher than those reported for other complexessuitableforvisiblelightsensitizationofEu3+ luminescence.28-31 The intense absorptions and high quantum yields result in very good brightnesses (Bs is defined as the product of quantum yield and the molar absorbance at the excitation wavelength). For Eu(tta)3L, Eu(nta)3L, and Eu(fod)3L, respectively, the Bs values are ∼28 000, 32 000, and 26 000 at 25 °C and ∼48 000, 46 000, and 41 000 at 1 °C. The long decay time of Eu(III) complexes is another attractive feature. Decay times were determined with phase modulation technique using the following equation

τ ) tanΦ/2πf

(1)

where Φ is the phase shift at the modulation frequency f, which was 300 Hz throughout this study. The time domain method was applied to obtain decay curves. Those for Eu(tta)3L (in air-saturated toluene solution) are shown in Figure 3a. As can be seen, the kinetics obeys a monoexponential decay law (with a corelation coefficient r2 of >0.999) both at 25 and 1 °C. Eu(nta)3L and Eu(fod)3L display a similar behavior. The decay times calculated with the data from the time domain experiment and the frequency domain experiments are virtually identical, the deviations not exceeding 2%. The decay times in air-saturated toluene at 25 °C were found to be 480, 430, and 560 µs for Eu(tta)3L, Eu(nta)3L, and Eu(fod)3L, respectively. For a temperature of 1 °C the data are 620, 560, and 800 µs, respectively. Evidently, temperature effects are less pronounced in the case of luminescence decay times compared to what is observed for quantum yields. Thus, it is likely that temperature significantly affects the efficiency of the energy transfer from the triplet state of the antenna chromophore to the 5D1 level of the Eu3+ ion. The luminescence of all the complexes investigated is weakly quenched by dissolved oxygen in that the emission quantum yields and decay times increased by ∼5% upon deoxygenation of air-saturated solutions. Materials for Temperature-Sensitive Sensor Layers. Most materials for optical sensing make use of an analyte-sensitive dye dissolved in a gas-permeable polymer. The polymer not only acts as a solvent for the dye but also allows the tuning of the quenchability of the indicator (and thus of the dynamic range of the sensor) and provides a certain permeation selectivity, thus eliminating cross-sensitivities to certain other species. On the other side, in the case of sensing temperature the use of a polymer with low gas permeability is preferred in order to avoid (or minimize) cross-sensitivity to oxygen. Polyacrylonitrile (PAN) has a very low gas permeability39 and therefore is usually the polymer of choice for optical temperature sensing.23 Unfortunately, the indicators are not stable in polar solvents such as DMF and ethanol where they almost completely dissociate into europium(III) tris(β-diketonate) and the free dipyrazolyltriazine ligand. Dilution of the solutions in such solvents as THF, dichloromethane, chloroform, and dichloroethane favors dissociation. Eu(tta)3L proved to be the most stable compound

Figure 3. (a) Time dependence of the emissions of Eu(tta)3L in air-saturated toluene solution (at λexc 410 nm). (1), at 1 °C; (b) at 25 °C. (b) Time dependence of the emissions of Eu(tta)3L and PdTFPP (λexc 410 nm) in polymer films at 25 °C: (3), Eu(tta)3L in poly(vinyl methyl ketone); (4), of Eu(tta)3L in poly(tert-butylstyrene), both at air saturation; (5), of Pd-TFPP in polymer PSAN in the absence of oxygen. Lines represent a fit via monoexponential decay model.


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Figure 4. Temperature dependence of the decay time of the europium(III) complexes dissolved in PVMK. Lines represent fitting via eq 2. The standard deviations do not exceed (0.04°.

with respect to dissociation. The complexes are, however, stable in concentrated solutions. Finally, the complexes showed the highest stability in apolar solvents such as toluene. The challenge is, therefore, to find a suitable combination of a polymer, preferably having low oxygen permeability, and a solvent which will not affect the stability of the probe. We find poly(vinyl methyl ketone) ()PVMK) to be a promising choice. Dichloroethane dissolves both this polymer and the temperature probes. Only minor dissociation of the complexes was observed at concentrations of about 0.6 mM. Their peaks of absorption undergo a slight longwave shift on immobilization in PVMK and are located at 411, 412, and 403 nm for Eu(tta)3L, Eu(nta)3L, and Eu(fod)3L, respectively. Longwave shifts of the absorption spectra on increasing the polarity of the solvent were observed earlier for some pushpull chromophores.40 The decay kinetics of the emission of the complexes dissolved in PVMK is purely monoexponential (Figure 3b, curve 3). The decay times obtained in both methods are identical (e.g. 440 and 450 µs, respectively, as obtained in the frequency and in the time domain for Eu(tta)3L/PVMK at 25 °C and air saturation). The temperature dependence of the decay time of the three europium(III) complexes dissolved in PVMK is presented in Figure 4. Obviously, the sensitivity is excellent in that the decay time drops more than 2-fold when temperature is increased from 1 °C to 66 °C. The materials based on Eu(tta)3L and Eu(nta)3L have negligible cross-sensitivity to oxygen. In fact, the decay time at air saturation is only ∼2% lower than that in the absence of oxygen. Eu(fod)3L in PVMK, however, exhibits higher cross-sensitivity to oxygen (the decay time is lower by ca. 5%). The response curves can be described (with a correlation coefficient r2 > 0.9995) by an Arrhenius-type equation10,23

(

(

τ ) k0 + k1 exp -

-1

)

∆E RT

(2)

where k0 is the temperature-independent decay rate for the excited-state deactivation, k1 is the pre-exponential factor, ∆E is the energy gap between emitting level and higher excited-state level, and R is the gas constant. The temperature dependence of the decay time (in the absence of oxygen) can be fitted with the (39) Polymer Handbook; Brandrup, J., Immergut, E. H., Grulke, E. A., Eds.; John Wiley & Sons Inc.: New York, 1999. (40) Boldrini, B.; Cavalli, E.; Painelli, A.; Terenziani, F. J. Phys. Chem. A 2002, 106, 6286-6294.

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following parameters: k0 ) 1880 s-1, k1 ) 2.55‚1010 s-1, ∆E ) 44.8 kJ‚mol-1 for Eu(tta)3L; k0 ) 2010 s-1, k1 ) 1.11‚1010 s-1, ∆E ) 41.6 kJ‚mol-1 for Eu(nta)3L; and k0 ) 1450 s-1, k1 ) 1.82‚109 s-1, ∆E ) 38.1 kJ‚mol-1 for Eu(fod)3L. The material was found to be highly photostable both in the absence of oxygen and under air. In fact, the phase shift for the Eu(tta)3L/PVMK system remained constant after 1 h of measurements when continuously excited with a 405-nm LED. Note: in a typical fiber-optic device used in practice (see, for example, www.presens.de), such an irradiation time would correspond to 18 000 single measurements. Under the same conditions only a minor decrease was detectable in luminescence intensity (-7%/h and -10%/h at 0 and 21.3 kPa of O2, respectively). Microbeads for Sensing Temperature. The materials presented above can be used for preparation of planar sensor foils or fiber-optic microsensors suitable for optical sensing of temperature. On a first glance, simultaneous sensing of two parameters (such as oxygen and temperature) simply requires two sensor “chemistries” to be mixed or to be spread layer by layer. These approaches are limited, however, for several reasons. These include (a) a strong spectral overlap of the luminescences of the indicators; (b) materials incompatibility of the sensor chemistries used for each single parameter (resulting in poor mechanical stability if not immiscibility); and (c) inner filter effects (in that one indicator absorbs distinctly more light than the second; this is particularly significant in the case of 2-layer sensors). The use of probes incorporated into nano- or microbeads offers one solution and the measurement of decay time rather than intensity the other. Beads of desired sensitivity and selectivity to a certain parameter (which is achieved by choosing the appropriate polymer) can be easily combined with beads sensitive to the other analyte. Generally, the desired quantities of both types of particles are mechanically dispersed in a matrix polymer, and the ratio of the two can be easily varied. Unfortunately, the europium(III) complexes could not be incorporated into PVMK in the form of microbeads but only as a film. Also, it was not possible to incorporate them into virtually oxygen impermeable PAN nanoparticles because the solvent dimethylformamide (which is needed to dissolve PAN) decomposes the probes. The complexes could be incorporated, however, in PTBS microparticles. PTBS, like the probes, is well soluble in toluene which does not adversely affect the stability of the indicators. The “cocktail” containing dissolved polymer and the temperature probe was spread onto a glass surface and dried at ambient air. Due to its high crystallinity, PTBS can be easily ground to give beads in a size of several µm. The particles were washed with ethanol to remove indicator molecules located on the surface. Even though ethanol and water decompose dissolved the europium complexes (as discussed before), those entrapped in the core of PTBS microbeads remain intact. The emission decay kinetics of the Eu(tta)3L dissolved in PTBS is again monoexponential (Figure 3b, curve 4). Figure 5 shows the temperature dependence of the luminescence decay time of the Eu(tta)3L complex in PTBS microbeads dispersed in a polyurethane hydrogel film that acts as the temperature-sensing layer. As expected, the cross-sensitivity to oxygen is significantly higher in PTBS than in PVMK due to its higher permeability for

Figure 5. Temperature dependence of luminescence decay time of Eu(tta)3L dissolved in PTBS microbeads that were dispersed in a hydrogel matrix.

Figure 6. Spectra of the materials and components used for dual sensing. 1 and 2, absorption of Eu(tta)3L/PTBS and Pd-TFPP/PSAN beads; 3-5, luminescence of the LED 405, Eu(tta)3L/PTBS, and PdTFPP/PSAN beads, respectively; 6-8, transmittance of optical filters BG 12, Chroma 615/10M, and Chroma 680/60M, respectively.

oxygen. Precise determination of temperature therefore is possible only if the oxygen concentration is kept constant or is known. The situation is quite similar to that of optical oxygen sensors and pressure-sensitive paints (PSPs) which are capable of precise oxygen determination only if temperature is known. The crosssensitivity of oxygen sensors to temperature results from an increase in the radiationless decay rate at higher temperatures, which is in fact the same process the optical temperature probes make use of. One limitation of PSPs results from the increasingly efficient quenching by oxygen at higher temperatures due to faster diffusion. This is a serious problem unless temperature is constant. It has been tackled by incorporating both a temperature-sensitive probe and an oxygen-sensitive probe in a single sensor layer and measuring fluorescence intensity10,11,13 or decay time.12,35 As expected, the photostability of Eu(tta)3L in PTBS in the presence of oxygen was lower than found for the Eu(tta)3L in PVMK. At 21.3 kPa of O2 (which is air pressure) an ∼30%/h decrease in luminescence intensity is observed, while in the absence of oxygen the rate was ∼10%/h. Similarly to the situation with PVMK, no drifts of phase shift were observed. In the next section it will be demonstrated that Eu3+-based temperature-sensitive microbeads (Eu(tta)3L/PTBS) can be used in a composite material suitable for simultaneous (dual) sensing of temperature and oxygen. Composite Material for Simultaneous Sensing of Oxygen and Temperature. The phosphorescence of the Pd(II) 5,10,15,20-tetrakis(2,3,4,5,6-pentafluorophenyl)porphyrin complex (PdTFPP) is strongly quenched by molecular oxygen, and this is the basis for its fluorometric sensing.41 The intense Soret band of the complex (λmax) 406 nm, ) 1.92‚105 M-1‚cm-1)38 perfectly overlaps the emission band of the 405-nm LED. Thus, efficient excitation is warranted. In fact, both indicators can be excited with the violet LED because the temperature-sensitive beads also strongly absorbed at around 405 nm (see Figure 6), The excitation light is filtered through a BG 12 glass filter whose spectral characteristics are also given in Figure 6. The emission the europium(III) complex (5D0f7F2 band) can be easily isolated using a Chroma 615/10M interference filter, while most of the phosphorescence of the palladium(II) porphyrin complex is filtered through a Chroma 680/60M interference filter. To adjust the response of the oxygen-sensitive probe to the range from 0 to 100% air saturation, the palladium complex was

incorporated into microparticles of a styrene/acrylonitrile (7:3) copolymer (referred to as PSAN). The presence of poly(acrylonitrile) reduces the efficiency of quenching by oxygen. Preparation and properties of such microbeads were described elsewhere.35 As can be seen from Figure 3b (curve 5) the decay kinetics in the absence of oxygen obeys a monoexponential law. However, a slight deviation is observed in the presence of oxygen. In this case decay times calculated using eq 1 in frequency domain measurements represent average values. The Eu(tta)3L/PTBS beads and the Pd-TFPP/PSAN beads were dispersed together in an ethanol/water (9:1, v:v) solution of the polyurethane hydrogel, and this “cocktail” was spread onto an optically transparent 100-µm poly(ethylene terephthalate) support foil to give, after solvent evaporation at room temperature, a sensor film in a thickness of about 14 µm. A cross section of the sensor layer is shown in Figure 7. The luminescence of the beads is excited from the bottom, and emission also is collected there. The cocktail may also be sprayed onto metal surfaces of airplane models as used in the aerospace industry. In this case excitation of the layer and collection of the luminescence is performed from top. Response Curves of the Composite Material. As shown, the decay times of the oxygen-sensitive beads and the temperature-sensitive beads mutually depend on oxygen partial pressure and temperature. Therefore, an iterative process has to be applied35 in order to compensate for the effect of temperature. It involves the following steps: (a) Stern-Volmer plots (τ0/τ vs pO2) are first established for different temperatures and by using a twosite quenching model;42,43 this results in Stern-Volmer constants

(41) Apostolidis, A.; Klimant, I.; Andrzejewski, D.; Wolfbeis, O. S. J. Comb. Chem. 2004, 6, 325-331.

(42) Carraway, E. R.; Demas, J. N.; DeGraff, B. A.; Bacon, J. R. Anal. Chem. 1991, 63, 337-342.

Figure 7. Cross section of the sensor layer for simultaneous optical sensing of oxygen and temperature using bead-immobilized fluorescent probes dispersed in a hydrogel matrix. Luminescence is excited from the bottom, and emission is also collected there. Oxygen can freely diffuse into the layer from a sample placed above the sensor.


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simple. A pair of such equations is arbitrarily chosen in order to demonstrate the applicability of the method. More complicated equations (which better describe the plots), of course, can also be used. The TableCurve 3D software describes the plots as Φ ) f(T, pO2), and the necessary transformations (that will solve the equations for T or pO2) were performed using the Maple software. For example, the following equation describes the behavior of the temperature-sensitive probe with a correlation coefficient r2 of 0.994

T)

x-bΦ1(aΦ1 + cΦ1xpO2 - 1) bΦ1

(5)

where a, b, and c, respectively, are numerical factors with values of 0.020434, 2.47025‚10-6, and 0.00037316. The response of the oxygen sensing system, in turn, can be described by the following empirical equation (r2 ) 0.9967) Figure 8. 3D plots of the response of the dual sensor to (a) temperature and (b) oxygen. Data points represent experimental data, and the surface is the result of a fit using TableCurve 3D software.

for each of the two quenching sites; (b) the temperature dependence of the two quenching constants is then analyzed via an Arrhenius type of equation; and (c) the parameters obtained in step (b) are then introduced into the equation for the two-site model to give the response function for oxygen

pO2 ) f1(τox, T)

(3)

where τox is the decay time of the oxygen-sensitive probe. Thus, if the decay time of the oxygen probe is measured and temperature is known, pO2 can be calculated. Temperature can be calculated unambiguously only if the probe is not affected by oxygen. While this is the case for the Eu(tta)3L/PVMK system, the situation is different in the case of the Eu(tta)3L/PTBS beads where oxygen acts as a fairly strong quencher. Therefore, an iterative approach is needed again to obtain a response function for temperature

T ) f2(τT, pO2)

(4)

where τT is the decay time of the temperature probe. Once τox and τT are known, eqs 3 and 4 can be used to obtain T and pO2. The situation may also be described by the 3D plots of response functions according to eqs 3 and 4, see Figure 8a,b. As can be seen, the phase shift Φ1 for the Eu(tta)3L/PTBS probe is highly temperature-dependent, while it shows only minor dependence on pO2. The situation is opposite in the case of Pd-TFPP/ PSAN beads (Φ2). The data have been analyzed empirically by the TableCurve 3D software which combines the capabilities of being a powerful surface fitter with those of finding almost perfect equations to describe 3D empirical data. In fact, the software offers a number of empirical equations, among which the following are relatively (43) Sacksteder, L.; Demas, J. N.; DeGraff, B. A.; Bacon, J. R. Anal. Chem. 1993, 65, 3480-3483.

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pO2 ) -eΦ2 +c + xe2Φ22 - 2ceΦ2 + c2 - 4fΦ22 - 4TdfΦ22 + 4afΦ2 + 4TbfΦ2 2fΦ2 (6) where the factors from a to f have values of 54.708, -0.560588, -0.05398, -0.0093588, 0.158319, and -0.0034438, respectively. Equations 5 and 6 can be further used in the iteration process when calculating temperature T and oxygen partial pressure pO2 once the phase shifts Φ1 and Φ2 are known. In the first step arbitrary temperature is introduced into eq 6 (alternatively, an arbitrary oxygen partial pressure is introduced into eq 5) and pO2 is calculated. Equation 4 is used then to calculate temperature, and the resulting value is used in eq 5. Up to 6 iterations are needed until both parameters become constant. Validation of the Method. To test the validity of the equations, gas mixtures of known pO2 were passed through the flowthrough cuvette containing the sensor layer. The test was performed at several temperatures. Table 1 shows data on the measured phase shifts Φ and on calculated temperatures and oxygen partial pressures. Six iterations were sufficient to calculate T and pO2, the parameters being constant after that. The calculated values are independent of the temperature, which is arbitrarily introduced for the first run (see n 1 and 2). As can be seen, the method is fairly precise. The error in the determination of temperature does not exceed 1.1 °C (with the exception of line 3, which was considered as an outlier) and 0.61 kPa for determination of oxygen. Performance of the Dually Sensing Material in Humidified Gas and in Water. In many biological, biotechnological, and medical applications oxygen and temperature sensing is performed in the aqueous phase. Therefore, the effect of humidity on the dually sensing material was investigated. The results are presented in Figure 9. As can be seen, humidity affects sensing properties of the material only slightly. The decay times of the temperature probe at 100% relative humidity is lower by 0.6-2% than for the case of dry gases, the effect being more pronounced at higher

Table 1. Measured Phase Shifts (Φ) in Degrees (°), Calculated Temperatures, and Calculated Oxygen Partial Pressures (pO2 in kPa) of Runs 1, 3, and 6 in the 6-Step Iteration Process and Real Temperatures (T, in °C) as Well as Oxygen Partial Pressures (pO2 in kPa) run 1

run 3

run 6

real values

error

n

Φ1,°

Φ2,°

T

pO2

T

pO2

T

pO2

T

pO2

∆T

∆pO2

1 2 3 4 5 6 7 8 9 10

45.30 45.30 45.68 43.16 41.78 40.83 39.65 36.11 35.24 35.24

20.81 20.81 29.14 16.30 18.50 30.87 34.15 12.00 11.88 31.52

5 50 50 50 50 50 50 50 50 50

13.45 5.25 2.53 8.29 6.55 2.17 1.61 15.38 15.77 2.05

11.77 13.17 16.07 22.42 30.59 37.20 41.52 48.22 51.29 54.90

11.84 11.53 5.03 18.00 10.52 2.963 2.02 16.64 14.92 1.769

12.09 12.09 15.84 21.26 30.44 37.18 41.51 48.19 51.31 54.90

11.76 11.76 5.04 18.84 10.56 2.96 2.02 16.67 14.90 1.77

11 11 14 21 31 37 41 48 51 54

12.16 12.16 5.07 18.23 10.13 3.04 2.03 17.22 15.20 2.03

1.09a 1.09a 1.84 0.26 0.56 0.18 0.51 0.19 0.31 0.90

0.40a 0.40a 0.03 0.61 0.43 0.08 0.01 0.55 0.30 0.26

a

A comparison of lines 1 and 2 shows that the same results are obtained independent of the temperature introduced in the first run.

Figure 9. Response of the dually sensing material to temperature and oxygen in dry and humidified gases and in aqueous medium: (a), response to temperature (in the absence of oxygen) and (b), response to oxygen (at 25 °C).

temperatures. The sensitivity of the material to oxygen is decreased slightly when the gas is humidified. The dually sensing material may also be used for sensing oxygen and temperature in aqueous media. Virtually no changes (in the case of oxygen sensing) or only minor changes (in the case of temperature sensing) were observed for the response in aqueous medium (compared to response in the humidified gases). This is demonstrated in Figure 9. Compared to the other dually sensing materials reported so far our material benefits from higher brightness. The materials which make use of Ru-phen as a temperature indicator13,35 require an additional light-reflective TiO2 layer to achieve sufficient brightnesses. Moreover, the broad emission band of the Ru-phen makes spectral separation of luminescences of both indicators more difficult. For example, for the material reported previously,35 which similarly to this work contained the Pd-TFPP/PSAN oxygen-sensitive microbeads but Ru-phen/PAN temperaturesensitive microparticles, excitation with two different LEDs was desirable to ensure unbiased luminescence signals. Due to line type emission of the Eu3+ ion spectral separation is very easy, as

was shown here. Finally, temperature sensitivity of the novel europium(III) probes is significantly higher than that of Ru-phen. It is worth mentioning that the europium(III) probes can also be used together with platinum(II) porphyrins (excited by a 405-nm LED, too). In this case the luminescences are easily separated by decay times (which differ by a factor of ∼10). The material is clearly advantageous to the dually sensing materials which made use of other europium(III) temperature-sensitive probes,10,11 because it allows visible (and not UV) excitation, apart from the increased brightness. In conclusion, we show that the novel europium(III) complexes introduced here are promising luminescent temperature indicators. They enable a lifetime-based determination of temperature. If immobilized in PVMK, their luminescence is virtually free of interference by oxygen, but if contained in PTBS microbeads the cross-sensitivity to oxygen becomes significant. A dual sensor was constructed using a palladium dye as a luminescent indicator for oxygen. Both indicators can be excited by a 405-nm LED, and their bright luminescence is separated using appropriate interference filters. Equations are presented that allow the two signals to be processed to give independent values for oxygen and temperature. The composite material enables simultaneous and contactless sensing of oxygen and temperature and may be used, for example, in wind tunnels, in high-resolution oxygen profiling, and in a variety of (micro)biological and medical applications. ACKNOWLEDGMENT This work was supported by the German Federal Ministry of Education and Research (BMBF) within project BP28 (“HYBOP”).

Received for review February 20, 2006. Accepted April 26, 2006. AC060311D


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Temperature-Sensitive Europium (III) Probes and Their Use for

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