Anal. Chem. 1996, 68, 2615-2620
Effects of Polymer Matrices on Calibration Functions of Luminescent Oxygen Sensors Based on Porphyrin Ketone Complexes Paul Hartmann*
AVL List GmbH, Biomedical Research and Development, Kleiststrasse 48, A-8020 Graz, Austria Wolfgang Trettnak
Joanneum Research, Institute for Chemical and Optical Sensors, Steyrergasse 17, A-8010 Graz, Austria
The design of luminescent oxygen sensors is guided by optimizing sensitivity and/or the form of the calibration function. Both qualities are governed by the molecular processes of luminescence quenching. To evaluate the influence of matrix effects, we prepared membranes based on oxygen-sensitive phosphorescent complexes of porphyrin ketones dissolved in plasticizer-free poly(vinyl chloride) (PVC) and polystyrene (PS). In a PVC matrix, both platinum(II) and palladium(II) octaethylporphyrin ketones exhibited perfectly linear Stern-Volmer intensity plots and almost single-exponential excited state decays. In a PS matrix, the sensitivity of palladium(II) octaethylporphyrin ketone was among the highest reported to date. Yet, slightly nonlinear Stern-Volmer plots and nonexponential decays illustrate the significance of matrix effects of PS. Addition of plasticizers to PVC-based sensors allowed tuning of the oxygen sensitivity in a wide range, while the Stern-Volmer plots became pronouncedly nonlinear. For the plasticizer bis(2-ethylhexyl) adipate, the decay profile was single-exponential in the absence but nonexponential in the presence of oxygen, which is attributed to a distribution of quenching rate constants. Luminescent oxygen sensors are of special interest in environmental monitoring, industrial process control,1 and medical and biological applications2 (e.g., blood gas analysis3 and respiratory monitoring). Each task requires sensors with optimized properties in the concentration range of interest. For polymer-based sensors, many key factors of their performance are subject to matrix effects arising from a microheterogeneity of the local environment of the dye molecules in the sensing layer.4 In particular, nonlinear Stern-Volmer plots and nonexponential excited state decays have been observed for sensors based on polycyclic aromatic hydrocarbons5 or ruthenium complexes.6-9 (1) Fiber optic chemical sensors and biosensors; Wolfbeis, O. S., Ed.; CRC Press: Boca Raton, FL, 1991; Vols. 1 and 2, Chapters 1 and 10. (2) Trettnak, W. Fluorescence Spectroscopy; Wolfbeis, O. S., Ed.; Springer: Berlin, 1993; p 79-89. (3) Leiner, M. J. P. Sens. Actuators B 1995, 29, 169-173. (4) Carraway, E. R.; Demas, J. N.; DeGraff, B. A. Anal. Chem. 1991, 63, 332336. (5) Kroneis, H.; Marsoner, H. J. Sens. Actuators B 1983, 4, 587-592. (6) Wolfbeis, O. S.; Leiner, M. J. P.; Posch, H. E. Mikrochim. Acta 1986, 3, 359-366. (7) Bacon, J. R.; Demas, J. N. Anal. Chem. 1987, 59, 2780-2785. S0003-2700(96)00008-X CCC: $12.00
© 1996 American Chemical Society
Application of long-lived phosphorescent dyes may offer significant advantages in terms of sensitivity to oxygen and matrix effects. Previous studies indicate10-14 that sensors based on phosphorescent porphyrin complexes are highly sensitive even in thermoplastic polymers, which are good solvents for these dyes, and may lead to improved calibration functions. In practice, recalibration of the sensors is necessary prior to measurements. The number of calibration points required depends on the form of the calibration function and thus on matrix effects. Sensors with negligible matrix effects are expected to show a single-exponential decay and a hyperbolic decrease of the emission intensity or decay time with oxygen concentration, so a linear Stern-Volmer equation15 (eq 1) can be applied, where I0
I0/I ) τ0/τ ) 1 + kqτ0PO2
(1)
and I are the luminescence intensities in the absence and in the presence of the quencher, respectively, τ0 and τ are the time constants of the related decay profiles, kq is the bimolecular quenching rate constant, and PO2 is the oxygen partial pressure. The product of kq and τ0 is the Stern-Volmer quenching constant, KSV. This function requires two calibration points (e.g., two different calibration media) if both I0 (or τ0) and KSV are unknown. This reduces to one calibration point for decay time-based sensors, provided that τ0 is constant. If the quenching constant KSV is sufficiently reproducible and stable, another calibration point can be eliminated, offering the prospect of calibration-free sensors. Description of the quenching response of many polymer-based oxygen sensors requires nonlinear Stern-Volmer equations with more than two variable parameters.4,7,16 Thus, additional points are required for sensor calibration. The decay profiles of such sensors can frequently be described by sums of exponentials. (8) Sacksteder, L. A.; Demas, J. N.; DeGraff, B. A. Anal. Chem. 1993, 65, 3480. (9) Hartmann, P.; Leiner, M. J. P.; Lippitsch, M. E. Anal. Chem. 1995, 67, 88-93. (10) Vanderkooi, J. M.; Wilson, D. F. Adv. Exp. Med. Biol. 1986, 200, 189-193. (11) Khalil, G.-E.; Gouterman, M. P.; Green, E. U.S. Patent 5,043,286, 1991. (12) Gewehr, P. M.; Delpy, D. T. Med. Biol. Eng. Comput. 1993, 31, 11-21. (13) Papkovsky, D. B. Sens. Actuators B 1995, 29, 213-218. (14) Papkovsky, D. B.; Ponomarev, G. V.; Trettnak, W.; O’Leary, P. Anal. Chem. 1995, 67, 4112-4117. (15) Stern, O.; Volmer, M. Phys. Z. 1919, 20, 183-188. (16) Hartmann, P.; Leiner, M. J. P.; Lippitsch, M. E. Sens. Actuators B 1995, 29, 251-257.
Analytical Chemistry, Vol. 68, No. 15, August 1, 1996 2615
Nonlinear Stern-Volmer equations and a sum of exponential decays of oxygen sensors have to be considered as phenomenological approaches to complex photophysical processes,17 which rather give rise to distributions of the photophysical parameters due to ground state heterogeneity of the local environment.18-20 Hence, the macroscopically observed properties (e.g., sensitivity and calibration function) of luminescent chemical sensors depend on the molecular processes on a microscopic scale. The quenching behavior is governed by the decay profile of the dye (τ0) and by the diffusion properties of oxygen in the polymer matrix (kq). Based on these key factors, we investigated the quenching behavior of platinum(II) and palladium(II) porphyrin ketones13,14 immobilized in poly(vinyl chloride) (PVC) and polystyrene (PS). Typical phosphorescent molecules such as metalloporphyrins usually exhibit certain disadvantages, like poor photostability. Metal complexes of porphyrin ketones offer the advantages of improved chemical and photochemical stability, compatibility with LED light sources, and acceptable quantum yields of 1-10%. Excited state decay times (at room temperature) of τ0 ) 61µs for a Pt porphyrin ketone, or even τ0 ) 480 µs for a Pd porphyrin ketone, in PS matrices have been reported.14 In this work, the investigations focus on (1) promising combinations of dyes (τ0) and plasticizer-free polymers (kq) and (2) improvement of oxygen permeation properties (with prospects of sensitivity and response time tuning) by addition of plasticizers to the polymer (i.e., variation of kq). Based on the results of luminescence intensity and decay time quenching experiments, we demonstrate the dependencies of calibration functions on the underlying molecular processes. EXPERIMENTAL SECTION Poly(vinyl chloride) Membranes. A 1 mg sample of platinum(II) octaethylporphyrin ketone (PtOEPK, Joanneum Research, Graz, Austria) or palladium(II) octaethylporphyrin ketone (PdOEPK, Joanneum Research) was dissolved in a solution of 1 g of PVC (high molecular weight; Sigma, Steinheim, Germany, or Fluka, Buchs, Switzerland) in 10 mL of tetrahydrofuran (THF; Merck, Darmstadt, Germany). Polymer/plasticizer solutions were obtained by mixing solutions of 1 g of PVC and various amounts of bis(2-ethylhexyl) sebacate (DOS; Fluka) or alternatively, bis(2ethylhexyl) adipate (DOA; Fluka) in 10 mL of THF. Membranes were cast on a transparent polyester support (Mylar; DuPont, Bad Homburg, Germany) having a wet-layer thickness of 20 or 50 µm and left to dry at room temperature at least for 24 h. Polystyrene Membranes. A 1 mg sample of PtOEPK or PdOEPK was dissolved in 1 mL of a solution of 5% (w/w) PS (average MW ) 280 000; Tg ) 100 °C; Sigma) in toluene.14 The dye/polymer solution was cast onto Mylar having a wet-layer thickness of 20 µm and left to dry at room temperature at least for 24 h. Luminescence Intensity Measurements. Luminescence intensity data were obtained with a SPEX Fluorolog II fluorometer. The excitation and emission wavelengths for the intensity quenching experiments were λex ) 592 nm and λem ) 758 nm for PtOEPK and λex ) 602 nm and λem ) 790 nm for PdOEPK. (17) James, D. R.; Ware, W. R. Chem. Phys. Lett. 1985, 120, 455-459. (18) James, D. R.; Liu, Y. S.; de Mayo, P.; Ware, W. R. Chem. Phys. Lett. 1985, 120, 460-465. (19) Siemiarczuk, A.; Wagner, B. D.; Ware, W. R. J. Phys. Chem. 1990, 94, 16611666. (20) Brochon, J.-C.; Livesey, A. K.; Pouget, J.; Valeur, B. Chem. Phys. Lett. 1990, 174, 517-522.
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The following nonlinear Stern-Volmer equation was fitted to the data:21
[
]
I0 f0 ) + (1 - f0) I 1 + KSVPO2
-1
(2)
A more widely used approach is the multicomponent model,22,23
I0 ) I
[∑ i
foi
]
-1
1 + KSViPO2
(3)
where f0 are fractional contributions. Dry gas mixtures were provided by a UTAH Medical Products PGM-3 gas mixing device with an accuracy of 0.25% absolute. Nitrogen served as the inert gas component. Further experimental details are described elsewhere.9 Luminescence Decay Time Measurements. Decay time measurements were performed with a PRA LN102/103 dye laser (Rhodamine 6G in ethanol; Lambda Physik, Go¨ttingen, Germany), providing pulse widths of t ) 300 ps, and a fast photomultiplier (Hamamatsu R2949), coupled to a Tektronix TDS 350 sampling oscilloscope. Emission was monitored through cutoff filters (Schott OG 665 and OG 695). The decay curves were obtained by averaging 2560 single shots. The signal-to-noise ratio (S/N) of the initial decay amplitude was approximately S/N ) 60, unless stated otherwise. Decay curves were analyzed without deconvolution by fitting a sum of a minimum number of exponentials by a least-squares algorithm (Microcal Origin). From the respective parameters, the preexponential weighted mean decay time, τm, can be calculated:4 n
τm )
∑ j)1
n
Bjτj/
∑B
j
(4)
j)1
where Bj are the amplitudes and τj the time constants of the multiexponential model. Unless stated otherwise, all measurements were performed at T ) 25 °C. RESULTS (1) Plasticizer-Free Membranes. As a first approach to oxygen sensors with favorable quenching sensitivities and calibration functions, we investigated promising combinations of metal porphyrin ketones and plasticizer-free polymers. Table 1 summarizes the parameters used to describe the quenching behavior and the decay profiles of the realized sensors. PtOEPK in Plasticizer-Free PVC. The Stern-Volmer intensity plot of PtOEPK in plasticizer-free PVC was perfectly linear (Figure 1a). Application of a two-component intensity quenching model (eq 3) did not improve the quality of the fit. The excited state decay profile was single-exponential in the absence of oxygen, while the fits to the quenched decay profiles required slight corrections of a second component. However, the relative amplitudes of the second exponential component used in the fits were almost constant for all applied oxygen partial (21) Trettnak, W.; Leiner, M. J. P.; Wolfbeis, O. S. Analyst 1988, 113, 15191523. (22) Lehrer, S. S. Biochemistry 1971, 10, 3254-3263. (23) Eftink, M. R.; Ghiron, C. A. Biochemistry 1976, 15, 672-680.
Table 1. Intensity Quenching Parameters (Eq 3) and Decay Parameters for PtOEPK in Plasticizer-Free PVC and PS decay parameters intensity
parametersa
dye
polymer
f01
KSV1 (10-3 Torr-1)
PtOEPK PtOEPKb PdOEPK PdOEPK
PVC PS PVC PS
1 0.70 ( 0.01 1 0.91 ( 0.01
1.46 ( 0.01 31.0 ( 0.3 11.2 ( 0.1 1690 ( 30
no. of exponentials KSV2 (10-3 Torr-1) 10.2 ( 0.2 270 ( 30
τ0 (µs)
N2c
O2d
64 ( 1 58 ( 2e 442 ( 8 459 ( 5
1 2 (1)f (1)f
(2)g 2 2
a Corrected for background emission (PdOEPK only). b at T ) 30 °C. c Number of exponentials required for the unquenched decays. d Number of exponentials required for the quenched decays. e τm, calculated from eq 4. f S/N ) 12. g Second exponential comprised less than 10% of the emission intensity.
a
b
Figure 1. Stern-Volmer intensity (0) and weighted mean decay time plots ( ) for (a) PtOEPK in pure PVC and (b) PtOEPK in PS (at * T ) 30 °C) versus applied oxygen partial pressure. Solid lines are best fits of a two-component model (eq 3) to the intensity quenching data.
pressures and did not exceed 10% (therefore, in Table 1, the double-exponential decay is written in parentheses). Agreement of the Stern-Volmer intensity and mean decay time plots was satisfactory. PtOEPK in PS. Compared to the PVC-based optode, the PS sensor14 shows a higher sensitivity to oxygen. The Stern-Volmer intensity plot is given in Figure 1b. It appears to be linear, but a fit of a linear Stern-Volmer model (eq 1) gave nonrandom residuals, while a fit of the two-component model (eq 3, Table 1) revealed a significant downward curvature with respect to the x-axis.
The excited state decay of PtOEPK in PS was nonexponential both in the absence and in the presence of the quencher: The qualities of single-exponential fits were unsatisfactory, while double-exponential fits gave random residuals for the given experimental accuracy. In contrast to the results for the decays of the dye’s phosphorescence in a pure PVC matrix, the weight of the second exponential component increased strongly with increasing oxygen concentration. Nonetheless, Figure 1b reveals good agreement of the Stern-Volmer plots of mean decay time and steady-state intensity. PdOEPK in Plasticizer-Free PVC. The oxygen sensitivity of PdOEPK in plasticizer-free PVC was much higher than that of the corresponding PtOEPK/PVC sensor (due to a higher decay time of the Pd(II) complex) but lower than that of PtOEPK in polystyrene. The Stern-Volmer intensity plot of this sensor was slightly nonlinear (Figure 2a) for the following reason: Comparison of the emission properties of the dye and a blank PVC layer cast on Mylar suggested the presence of a significant background emission of 0.7% of I0 at λem ) 790 nm. This quantity was subtracted from the measured total emission intensities at various oxygen concentrations. As a result, the corrected Stern-Volmer intensity plot became perfectly linear (Table 1). PdOEPK in PS. The oxygen sensitivity of PdOEPK in PS was the highest of all investigated samples. As for the same complex in pure PVC, the Stern-Volmer intensity plot was slightly nonlinear, but background subtraction (0.2% of I0 at λem ) 790 nm) did not lead to a linearization of the plot (Figure 2b). The excited state decay in the absence of the quencher was singleexponential (for S/N ) 12) but became markedly nonexponential in the presence of 7% oxygen in the analytical gas. (2) Plasticized PVC Membranes. Addition of plasticizers improves the permeability of polymers to oxygen. We chose PtOEPK in a PVC matrix to investigate the effects of added plasticizers on the quenching behavior of the sensor. Figure 3 shows the Stern-Volmer intensity and decay time plots of PtOEPK in PVC with 30% and 60% (w/w) DOA. In contrast to plasticizer-free PVC membranes of this dye, the plots were downward curved. Table 2 comprises the parameters fitted to the decay data at various oxygen partial pressures. The SternVolmer plots of the preexponential weighted mean decay times roughly agreed with the respective intensity plots (Figure 3). We performed a systematic investigation of the intensity quenching behavior of plasticized PVC sensor membranes in a wide concentration range of a second plasticizer (DOS). Intensity data were analyzed with the help of a simple yet phenomenological nonlinear Stern-Volmer equation (eq 2). The obtained model Analytical Chemistry, Vol. 68, No. 15, August 1, 1996
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a
Table 2. Decay Parameters for PtOEPK in PVC/DOA for Various Oxygen Partial Pressures PO2 (Torr)
B1 (%)
0 145.6 728
100.0 38.5 37.4
30% DOA in PVC 64.1 ( 0.1a 12.5 ( 0.5 3.9 ( 0.1 3.1 ( 0.2 0.83 ( 0.03
64.1 7.2 1.7
0 145.6 728
100.0 30.9 31.0
60% DOA in PVC 60.7 ( 0.2 6.4 ( 0.7 2.12 ( 0.08 3(1 0.58 ( 0.04
60.7 3.4 1.3
a
τ1 (µs)
τ2 (µs)
τm (µs)
Errors are given as standard deviations from the fit.
b
Figure 4. Fitted parameters (1 - f0) (*) and KSV (9) of a nonlinear Stern-Volmer model equation (eq 2) applied to intensity quenching data of PtOEPK in PVC/DOS with various amounts of plasticizer. Figure 2. Stern-Volmer intensity plots of original (9 symbols) and background-corrected (0) data for (a) PdOEPK in pure PVC and (b) PdOEPK in PS versus applied oxygen partial pressure.
Figure 3. Stern-Volmer intensity and preexponential weighted mean decay time plots for PtOEPK in plasticized PVC/DOA versus applied oxygen partial pressure. (+) I0/I for 60% (w/w) DOA in PVC; (×) respective data for τm0/τm; (O) I0/I for 30% (w/w) DOA in PVC; and (4) respective data for τm0/τm.
parameters are given in Figure 4 for membranes with varying plasticizer concentration. While the quenching sensitivity (KSV) increased with increasing [DOS] in PVC (strongly for [DOS] > 20% (w/w)), the expression (1 - f0) increased only up to [DOS] ≈ 15%-20% (w/w) before it decreased strongly. In a pure DOS 2618 Analytical Chemistry, Vol. 68, No. 15, August 1, 1996
solvent, the dye again shows linear Stern-Volmer plots, with a quenching constant of KSV ) 0.39 Torr-1 (in pure DOA, the corresponding value was KSV ) 0.49 Torr-1). DISCUSSION Oxygen sensors based on Pt and Pd porphyrin ketones are suitable for measuring a wide range of oxygen partial pressures. This has also been demonstrated for some metalloporphyrins in previous work.10-12 However, frequently the molecular processes responsible for the observed quenching properties have not been regarded. Up to this point, favorable linear Stern-Volmer plots sufficiently free of experimental errors have not yet been described, and the excited state lifetime measurements did not allow more detailed mechanistic interpretations. The latter point is especially regrettable, since lifetime data provide insight into the effects of matrix-indicator interactions, which exert their influence on the shapes of Stern-Volmer plots and decay profiles. (1) Plasticizer-Free PVC Membranes. In a pure PVC matrix, both dyes employed show perfectly linear Stern-Volmer plots (PdOEPK only after background correction). This excludes the significance of any effects that may serve to explain nonlinear Stern-Volmer plots (e.g., decay time distributions or dual sorption mechanisms,24 which are frequently discussed when the applied partial pressure of the gas is used instead of its bulk concentration as the independent variable16,25-27). (24) Vieth, W. R.; Tam, P. M.; Michaels, A. S. J. Colloid Interface Sci. 1966, 22, 360-370.
Due to the long unquenched decay times of the metal porphyrin ketones and the structure of PVC, matrix heterogeneity is supposed to average out on the time scale of the decay. The observed downward curvature of the Stern-Volmer intensity plot of the PdOEPK/PVC sensor is a result of unquenchable background emission from the Mylar support and, thus, a matter of low quantum yield of the dye (Φ ) 0.01 in micellar sulfite solution14). The sensitivities of the PVC-based sensors are convenient for applications both in the physiological PO2 range and in environmental monitoring. Higher sensitivities have been obtained with a PS matrix due to an increased permeability to oxygen: (2) PS Membranes. The observed sensitivity of the PdOEPK/ PS sensor was among the highest reported in the literature for luminescent oxygen sensors. However, since the Stern-Volmer plots of both PS-based sensors remain slightly downward curved, a linear model is insufficient for calibration. It has been suggested25-27 that dual-sorption effects might contribute to the frequently observed curvature of the SternVolmer intensity plots. However, typical values of Langmuir affinity constants for noncondensing gases in PS24 indicate that this effect should have negligible influence. Further, a plain Langmuir adsorption model predicts single-exponential decays for all oxygen concentrations, which was not the case for the PSbased sensors. Instead, the observed nonlinear Stern-Volmer plots and the fits of double-exponential decays for the PS-based sensors are in agreement with simulation results of James et al.17,18 for distributions of decay times. First, due to the limited accuracy of experimental data, such distributions can be fitted by a doubleexponential model, as long as their widths exceed 15% of the standard deviations. Second, the distributions become broader upon quenching, and so the weight of the shorter decay component of a double-exponential fit increases. An interference from the emission of precipitated dye molecules to the decay can be excluded for the experimental data in this work. The mean decay time of PtOEPK powder was τ ) 0.32 µs and was insensitive to oxygen. Based on these considerations, decay time distributions are most probably the reason for the observed photophysical features of metal porphyrin ketones in PS. In comparison to PVC, PS exhibits a higher glass transition temperature (Tg ) 100 °C vs 80 °C for PVC),28 which is the result of reduced chain mobility and stronger steric effects. Therefore, the PVC structure appears to play a special role in that it does not provide measurable matrix effects, while the rigid structure of PS is supposed to give rise to significant decay time distributions of incorporated dyes. A secondary effect of oxygen might influence the observed relaxation dynamics of guest molecules in PS in a way similar to rigidity effects: oxygen is known to be able to form weak ground state complexes with aromatic groups.29 The charge transfer states of molecular oxygen styrene complexes alter the polarity of the polymer host. If such sites are adjacent to the dye molecule, this may, in turn, influence the decay law of the dye. If present, (25) Carraway, E. R.; Demas, J. N.; DeGraff, B. A. Langmuir 1991, 7, 29912998. (26) Li, X.-M.; Ruan, F.-C.; Wong, K.-Y. Analyst 1993, 118, 289-292. (27) Demas, J. N.; DeGraff, B. A.; Xu, W. Anal. Chem. 1995, 67, 1377-1380. (28) Cowie, J. M. G. Chemie und Physik der Polymeren; Verlag Chemie: Weinheim, 1976. (29) Ogilby, P. R.; Kristiansen, M.; Clough, R. L. Macromolecules 1990, 23, 26982704.
the strength of this effect would increase with increasing oxygen concentrations in the polymer. This mechanism should be considered for chemically homogeneous sensors showing increasingly nonexponential decay profiles with increasing oxygen concentration. (3) Plasticized PVC Membranes. Although effects similar to the PS-based sensors were observed, the quenching properties of plasticized PVC membranes require different models. First, the decay profile in the absence of the quencher was single-exponential, even at S/N ) 60. This is reasonable, since the rigidity of the matrix is weakened by the plasticizer, and PtOEPK in a deoxygenated plasticizer (DOA) environment has a single-exponential decay time of τ0 ) 57 ( 1 µs, close to that for PtOEPK in PVC (see Table 1). Second, on adding oxygen, the decays became clearly nonexponential. We propose that this effect originates in the diffusion properties of oxygen in the layer: oxygen diffusion in plasticized PVC is supported by an increase in free volume. A size distribution of this void space may cause a distribution of quenching rate constants, which, in turn, leads to a distribution of decay times in presence of oxygen, while the unquenched decay profiles remain single exponential. At elevated plasticizer concentrations, the polymer becomes overplasticized, and spare plasticizer molecules tend to form occlusions.30 Within these domains, the quenching rate constant kq is very high. Thus, the diffusion length of oxygen within the lifetime of the excited state of the dye is likely to exceed the dimensions of single domains, which gives rise to additional distributions of quenching rate constants. This effect is expected to vanish for plasticizer concentrations approaching 100%. The intensity quenching behavior for membranes with different plasticizer concentrations supports this picture. The course of the deviations from a linear quenching response (1 - f0) and of the quenching sensitivity (KSV, Figure 4) indicates the onset of a structural transition (e.g., formation of occlusions) near [DOS] ) 20% (w/w) in the polymer. An improved model of the luminescence quenching of dyes in plasticized PVC membranes should consider dye molecules located both in the local vicinity of PVC chains and in plasticizer occlusions as well as dye aggregates at phase boundaries. CONCLUSIONS Matrix effects on the quenching properties of luminescent dyes in solid solutions strongly depend on the polymer used. The simple calibration functions obtained with PVC-based sensors suggest that a favorable polymer structure is a key to successful sensor design. The interactions of oxygen with the polymers also need to be better understood, since oxygen potentially influences the polarity of the polymer host and, in turn, the matrix effects. The obtained results with plasticizer-free membranes point to a convenient way to tailor sensor properties in a wide range from physiological applications to environmental or even trace analysis. This could be achieved, for example, by simply combining polymers such as PVC or PS with dyes of various decay times. Oxygen sensors based on plasticizer-free membranes can be produced easily, at large scale and low cost. Plasticized PVC-based luminescent sensors attract interest, since their sensitivities and response times can be tuned by controlled addition of plasticizers. This is of importance, e.g., for (30) Simon, M. A.; Kusy, R. P. Polymer 1993, 34, 5106-5115.
Analytical Chemistry, Vol. 68, No. 15, August 1, 1996
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respiratory monitoring. However, it has to be taken into account that the favorable calibration function of plasticizer-free PVC sensors cannot be retained.
Dolezal (Joanneum Research); and stimulating discussions with Dr. M. J. P. Leiner (AVL) and Dr. K. Biebernik (Joanneum Research).
ACKNOWLEDGMENT The authors acknowledge synthesis of the porphyrin ketones by Prof. G. Ponomarev, Institute of Biomedical Chemistry, Moscow; support of the dye laser by Prof. H. F. Kauffmann, University of Vienna; assistance in sensor preparation by Mrs. C.
Received for review January 3, 1996. Accepted May 7, 1996.X
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AC960008K X
Abstract published in Advance ACS Abstracts, June 15, 1996.
