Oxygen sensing based on lifetime of photoexcited triplet state of platinum porphyrin polystyrene film using time-resolved spectroscopy

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1 Journal of Porphyrins and Phthalocyanines J. Porphyrins Phthalocyanines 4, (2000) Oxygen sensing based on lifetime of photoexcited triplet state of platinum porphyrin polystyrene film using time-resolved spectroscopy YUTAKA AMAO 1 *, KEISUKE ASAI 1 and ICHIRO OKURA 2 1 Aerodynamic Division, National Aerospace Laboratory, Jindaiji-higashi, Chofu, Tokyo, Japan 2 Department of Bioengineering, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama, Japan Received 23 June 1999; Revised 18 August 1999 Accepted 26 August 1999 ABSTRACT: Optical oxygen-sensing systems based on the quenching of the photoexcited triplet state of platinum porphyrins platinum octaethylporphyrin (PtOEP) and platinum tetrakis(pentafluorophenyl)porphyrin (PtTFPP) in polystyrene (PS) using two different time-resolved spectroscopies (luminescence lifetime measurement and diffuse reflectance laser flash photolysis) have been developed. Using both spectroscopies, the same values of Stern Volmer constant K SV and quenching rate constant k q (K SV = k q t 0 ) are obtained. The decays of the luminescence and triplet triplet reflectance of the platinum porphyrins in PS consisted of two components (faster and slower lifetimes) in the absence and presence of oxygen. For both faster and slower components the lifetime decreases with increasing oxygen concentration. For both components a Stern Volmer plot of the platinum porphyrin PS films exhibits linearity. However, k q of the faster component is larger than that of the slower component (for PtOEP, three times larger; for PtTFPP, 40 times larger), indicating that two different oxygen-accessible sites exist in the platinum porphyrin PS films. The faster and slower components are related to oxygen-accessible sites on the surface and in the bulk of the platinum porphyrin films respectively. Concerning the fractional contributions of each lifetime component, the contribution of the faster component is greater than that of the slower component, indicating that the sensing site on the surface is important for optical sensing. The contribution of different oxygen-accessible sites in platinum porphyrin PS films for oxygen sensing is clarified by these techniques. Copyright # 2000 John Wiley & Sons, Ltd. KEYWORDS: optical oxygen sensing; time-resolved spectroscopy; platinum porphyrin; Stern Volmer constant; luminescence lifetime; laser flash photolysis INTRODUCTION Optical oxygen-sensing techniques based on the luminescence quenching of a probe by oxygen have attracted much interest [1]. Probes for this sensor are luminescent and oxygen-quenchable organic dyes such as polycyclic aromatic hydrocarbons [2 5], metalloporphyrins [6 12] and transition metal complexes [13 17]. Among these compounds, metalloporphyrins (platinum and palladium porphyrins) have been frequently used as oxygen-sensing dyes owing to their absorption in the visible region, high sensitivity to oxygen and large Stokes shifts. In particular, platinum octaethylporphyin (PtOEP) and platinum tetrakis(pentafluorophenyl)porphyrin (PtTFPP) are *Correspondence to: Y. Amao, Aerodynamic Division, National Aerospace Laboratory, Jindaiji-higashi, Chofu, Tokyo, Japan. Copyright # 2000 John Wiley & Sons, Ltd. typical compounds for optical oxygen-sensing dyes [18, 19]. For optical sensing, the luminescence intensity changes due to oxygen have been used. Although intensity measurements are simple, the intensity depends on the thickness of the sensing film, the light scattering from the background and the concentration of dye in the film. On the other hand, the lifetime of the photoexcited triplet state is concerned only with the light emission following a flash of excitation light and is not dependent on the intensity of the emitted light. The concept of using lifetime measurements of the photoexcited triplet state of dyes has proven especially attractive for optical oxygen sensing, and the method is more suitable than steady state luminescence-quenching methods. In general, optical oxygen-sensing systems based on luminescence (phosphorescence) lifetime have been widely used [20, 21]. As most organic compounds show no phosphorescence at room temperature, however, the number of oxygen-quenchable compounds is extre-

2 OXYGEN SENSING USING PLATINUM PORPHYRIN POLYSTYRENE FILM 293 mely limited. Triplet triplet absorption in a flash photolysis set-up allows us to determine the excited triplet lifetime of non-phosphorescent compounds. The photoexcited triplet state of organic compounds on a solid surface has been studied using diffuse reflectance laser flash photolysis (DRLFP) [22]. As previously reported, a system using the triplet triplet reflectance quenching of zinc porphyrin-doped films for measuring oxygen concentrations using timeresolved spectroscopy has been developed [23]. This method extends the number of indicators available for oxygen sensing. In this study we describe an optical oxygen-sensing system based on lifetime changes of the photoexcited triplet state of PtOEP and PtTFPP in polystyrene films by oxygen using two different time-resolved spectroscopies, namely luminescence lifetime measurement and diffuse reflectance laser flash photolysis. EXPERIMENTAL Materials PtOEP and PtTFPP (structures are shown in Fig. 1) were obtained from Porphyrin Products Inc. Polystyrene (PS, average MW , GPC grade) was purchased from Aldrich. Non-luminescent glass slides were obtained from Mitsubishi Chemical Co., Ltd. The other reagents were of the highest grade available. Spectroscopic Measurements Absorption spectra of the PtOEP PS and PtTFPP PS films were recorded using a Shimadzu Multispec-1500 spectrometer. Steady state luminescence spectra and excitation spectra of the films were measured using a Hitachi F-4010 fluorescence spectrometer with a 150 W xenon lamp as visible excitation light source. Excitation and emission bandpasses were 5.0 nm. Luminescence Lifetime Measurements Luminescence lifetime measurements were carried out using an Nd YAG laser (Spectra Physics; pulse width 10 ns) at room temperature. The excitation wavelength was the Q band of the platinum porphyrins. The light reflected by the oxygen-sensing films was transmitted by a quartz optical fibre to a monochromator and photomultiplier. The luminescence decays were stored in a storage oscilloscope (Sony-Tektronix TDS360). Diffuse Reflectance Laser Flash Photolysis Laser flash photolysis was carried out using an Nd YAG laser at room temperature. The excitation wavelength was the Q band of the platinum porphyrins. A xenon arc lamp as monitoring light beam was coupled into one end of an optical fibre (Dolan-Jenner Co.; core/ cladding diameter mm). The light reflected by the oxygen-sensing films was transmitted by the same fibre to a monochromator and photomultiplier. The transient Fig. 1. Chemical structures of platinum octaethylporphyrin (PtOEP) and platinum tetrakis(pentafluorophenyl)porphyrin (PtTFPP).

3 294 Y. AMAO ET AL. where R(t) is the time-resolved reflectance, t is the lifetime of the photoexcited triplet state and A n is the fractional contribution to each lifetime component. The calculated values of R c (t) are then compared with the experimental R(t) values and the goodness-of-fit is determined by minimization of the w 2 function 2 ˆ w n R t R c t 2 2 where w n is a statistical weighting factor that accounts Fig. 2. Absorption spectra of (a) PtOEP PS and (b) PtTFPP- PS films. spectra were stored in a storage oscilloscope (Sony- Tektronix TDS360). The experimental details were described previously [20, 21]. Analysis of Decay Data The decay curves were obtained by averaging 256 single shots. Decay curves were analysed without deconvolution by fitting with the equation R t ˆ A n exp t= n 1 Fig. 3. Typical luminescence decays of platinum porphyrin PS films: (a) PtOEP; (b) PtTFPP; (1) argon-saturated; (2) oxygen-saturated. Excitation wavelength: Q band of platinum porphyrins.

4 OXYGEN SENSING USING PLATINUM PORPHYRIN POLYSTYRENE FILM 295 Table 1. Luminescence lifetimes of platinum porphyrin films under deoxygenated (t 0 ) and oxygenated (t 100 ) conditions Compound t 0f (ms) t 0s (ms) t 100f (ms) t 100s (ms) w 2 PtOEP PtTFPP for the uncertainty in each R(t) value. When w n accurately describes the uncertainty in R(t), w 2 and the random distribution of residuals (R c (t) R(t)) are equal to one and zero respectively. Photostability of Sensing Films The stability of the sensing films against irradiation was measured by the change in absorption spectrum before and after film irradiation using a 100 mw Fig. 4. Luminescence lifetimes of platinum porphyrin PS films versus oxygen concentration (1) and Stern Volmer plot (2): (a) PtOEP; (b) PtTFPP.

5 296 Y. AMAO ET AL. Table 2. Stern Volmer constants K SV and quenching constants k q of platinum porphyrin films based on luminescence lifetime Compound K SV1 (% 1 ) k q1 (% 1 s 1 ) K SV2 (% 1 ) k q2 (% 1 s 1 ) PtOEP PtTFPP tungsten lamp. Light of wavelength less than 390 nm was removed using a cut-off filter (Toshiba L-39). The distance from the film to the lamp was 10 cm. Oxygen-sensing Systems The sample films were mounted at a 45 angle in a quartz cell to minimize light scattering from the sample and substrate. Different oxygen standards (in the range 0% 100%) in a gas stream were produced by controlling the flow rates of oxygen and argon gases entering a mixing chamber. The total pressure was maintained at 760 Torr. All experiments were carried out at room temperature. Preparation of Platinum Porphyrin PS Films The platinum porphyrin PS films were formed by casting a mixture of 30 wt% PS and platinum porphyrin in toluene onto non-luminescent glass slides. The concentration of platinum porphyrin in PS was approximately mol dm 3. The films were dried at room temperature and stored in the dark prior to use. The platinum porphyrin PS films have good photostability under laser irradiation. RESULTS AND DISCUSSION Absorption Spectra of Platinum Porphyrin PS Films Figure 2 shows the absorption spectra of the platinum porphyrin PS films. The shape of the spectrum in PS film is almost the same as in solution, indicating no interaction between platinum porphyrin and PS in the ground state. Fig. 5. Typical photoexcited triplet state decays of platinum porphyrin PS films monitored at 470 nm: (a) PtOEP; (b) PtTFPP; (1) argon-saturated; (2) oxygen-saturated. Excitation wavelength: Q band of platinum porphyrins. Oxygen Sensing Based on Luminescence Lifetime of Platinum Porphyrin PS Films Figure 3 shows typical luminescence decays of the platinum porphyrin films under argon (1) and oxygen (2) monitored at the emission peak of the platinum porphyrins. The decays consisted of two components (faster and slower lifetimes) in the absence and presence of oxygen. The faster luminescence decay of the platinum porphyrins under oxygen compared

6 OXYGEN SENSING USING PLATINUM PORPHYRIN POLYSTYRENE FILM 297 Fig. 6. Photoexcited triplet state lifetimes of platinum porphyrin PS films versus oxygen concentration (1) and Stern Volmer plot (2): (a) PtOEP; (b) PtTFPP. with that under argon indicates that quenching of the luminescence of the platinum porphyrins by oxygen occurred. Table 1 summarizes the luminescence lifetimes of the PtOEP PS and PtTFPP PS films. Figure 4 (1) shows the luminescence lifetimes of the platinum porphyrins versus oxygen concentration. The Table 3. Photoexcited triplet state lifetimes of platinum porphyrin films under deoxygenated (t t0 ) and oxygenated (t t100 ) conditions Compound t t0f (ms) t t0s (ms) t t100f (ms) t t100s (ms) w 2 PtOEP PtTFPP Subscripts f and s denote the faster and slower components of photoexcited triplet state decay respectively.

7 298 Y. AMAO ET AL. Table 4. Stern Volmer constants K SV and quenching constants k q of platinum porphyrin films based on photoexcited triplet state lifetime Compound K SV1 (% 1 ) k q1 (% 1 s 1 ) K SV2 (% 1 ) k q2 (% 1 s 1 ) PtOEP PtTFPP lifetimes of the platinum porphyrins were calculated by fitting with equation (1). For both faster and slower components the lifetime decreases with increasing oxygen concentration. Subscripts f and s denote the faster and slower components of luminescence decay respectively. Stern Volmer Relationship for Platinum Porphyrin PS Films Based on Luminescence Lifetime Figure 4(2) shows a Stern Volmer plot (t 0 /t 1 = K SV [O 2 ], where t 0 and t are the lifetimes in the absence and presence of oxygen respectively). For both components the Stern Volmer plot of the platinum porphyrin PS films exhibits linearity K SV values of the faster and slower lifetime components are almost the same, as shown in Table 2. However, K SV is strongly dependent on t 0 and the quenching rate constant k q (K SV = k q t 0 ). The quenching rate constant of the faster component is larger than that of the slower component, indicating that two different oxygenaccessible sites exist in the platinum porphyrin PS films. The faster and slower components are related to oxygen-accessible sites on the surface and in the bulk of the platinum porphyrin films respectively. Concerning the fractional contributions of each lifetime component, the contribution of the faster component is greater than that of the slower component, indicating that the sensing site on the surface is important for optical sensing. Oxygen Sensing Based on Triplet Lifetime of Platinum Porphyrin PS Films Figure 5 shows typical photoexcited triplet state decays of the platinum porphyrin films (monitored at 470 nm, the maximum of triplet triplet absorption of PtOEP and PtTFPP) under argon (1) and oxygen (2). In this case the decays also consisted of two components. Figure 6 (1) shows the photoexcited triplet state lifetimes of the platinum porphyrins versus oxygen concentration. The lifetimes of the platinum porphyrins were calculated by fitting with equation (1). Table 3 summarizes the photoexcited triplet state lifetimes of the PtOEP PS and PtTFPP PS films. K SV values of the faster and slower lifetime components are the almost same, as shown in Fig. 6 (2) and Table 4, and the quenching rate constant of the faster component is larger than that of the slower component. In general, the sensing properties are strongly affected by the thickness of the film. A thinner film needs less time for the oxygen inside it to reach equilibrium with the outside environment. In this system, however, K SV seems to be little affected by differences in film thickness. This result indicates that the system using triplet triplet absorption change is able to measure the oxygen concentration on the surface. CONCLUSION In this work, optical oxygen-sensing systems based on the quenching of platinum porphyrins by oxygen using two different time-resolved spectroscopies, namely luminescence lifetime measurement and triplet triplet reflectance using laser flash photolysis, have been developed. Using both spectroscopies, the same values of K SV and k q were obtained. The contribution of different oxygen-accessible sites in platinum porphyrin PS films for oxygen sensing is clarified by the above techniques. These techniques may be useful not only for improvement of the optical oxygen-sensing system but also for elucidation of the interaction mechanism between probe and oxygen. Acknowledgements This work was partially supported by a Grant-in-Aid for Scientific Research on Priority Areas Research from the Ministry of Education, Science, Sports, and Culture of Japan ( ).

8 OXYGEN SENSING USING PLATINUM PORPHYRIN POLYSTYRENE FILM 299 REFERENCES AND NOTES 1. Wolfbeis OS. Fiber Optic Chemical Sensors and Biosensors. CRC Press: Boca Raton, FL, Ishiji T, Kaneko M. Analyst 1995; 120: Sharma A, Wolfbeis OS. Appl. Spectrosc. 1988; 42: Lee ED, Werner TC, Seitz R. Anal. Chem. 1987; 59: Zilberstein J, Bromberg A, Berkovic G. J. Photochem. Photobiol. A: Chem. 1994; 77: Beswick RB, Pitt CW. Chem. Phys. Lett. 1988; 143: Twarowski AJ, Good L. J. Phys. Chem. 1987; 91: Lee S-K, Okura I. Spectrochim. Acta A 1998; 54: Lee S-K, Okura I. Analyst 1997; 122: Lee S-K, Okura I. Anal. Sci. 1997; 13: Papkovsky DB, Ponomarev GV, Trettnak W, O Leary P. Anal. Chem. 1995; 67: Lo L-W, Koch CJ, Wilson DF. Anal. Biochem. 1996; 236: Hartmann P, Leiner MJP, Lippitsch ME. Anal. Chem. 1995; 67: Xu W, McDonough III RC, Langsdorf B, Demas JN, DeGraff BA. Anal. Chem. 1994; 66: Carraway ER, Demas JN, DeGraff BA, Bacon JR. Anal. Chem. 1991; 63: Bambot S, Holavanahali R, Lakowicz JR, Carter GM, Rao G. Oxygen Transport to Tissue XVI, 1994; Bacon JR, Demas JN. Anal. Chem. 1987; 59: Lee S-K, Okura I. Anal. Commun. 1997; 34: Lee S-K, Okura I. Anal. Chim. Acta 1997; 342: Draxler S, Lippitsch ME. Anal. Chem. 1996; 68: Lippitsch ME, Pusterhofer J, Leiner MJP, Wolfbeis OS. Anal. Chim. Acta 1988; 205: Furuto T, Lee S-K, Asai K, Okura I. Chem. Lett. 1998; Wilkinson F, Kelly G. Diffuse reflectance laser flash photolysis. In Handbook of Organic Photochemistry, Vol. 1. Scaiano JC (ed.). CRC Press: Boca Raton, FL, 1989; 293.

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