Anal. Chem. 1997, 69, 4653-4659
Controlling the Response Characteristics of Luminescent Porphyrin Plastic Film Sensors for Oxygen Andrew Mills* and Anne Lepre
Department of Chemistry, University of WalessSwansea, Singleton Park, Swansea SA2 8PP, U.K.
Two porphyrins, platinum(II) octaethylporphyrin (PtOEP) and palladium(II) octaethylporphyrin (Pd-OEP), are incorporated into a wide variety of different encapsulating matricies and tested as oxygen sensors. The excited state lifetimes of the two porphyrins are quite different, 0.091 ms for Pt-OEP and 0.99 ms for Pd-OEP, and Pt-OEPbased oxygen sensors are found to be much less sensitive than Pd-OEP-based ones to quenching by oxygen. Two major response characteristics of an oxygen sensor are (i) its sensitivity toward oxygen and (ii) its response and recovery times when exposed to an alternating atmosphere of nitrogen and air. The response characteristics of a range of Pt-OEP, and Pd-OEP-based oxygen sensors were determined using cellulose acetate butyrate (CAB), poly(methyl methacrylate) (PMMA), and PMMA/CAB polymer blends as the encapsulating media. Pt-OEP and Pd-OEP oxygen sensors have better response characteristics (i.e., more sensitive and lower response and recovery times) when CAB is used as the encapsulating medium rather than PMMA. For both Pt-OEP- and PdOEP-based oxygen sensors, in either polymer, increasing the level of tributyl phosphate plasticizer improves the response characteristics of the final oxygen-sensitive film. Pt-OEP in different unplasticized PMMA/CAB blended films produced a range of oxygen sensors in which the response characteristics improved with increasing level of CAB present. Recent years have seen a growing interest in novel optical sensors for the detection of oxygen. The majority of optical sensors for oxygen employ a dye whose luminescence is quenched by oxygen. The choice of lumophore has been dominated by the ruthenium diimine complexes: tris(2,2′-bipyridyl)ruthenium(II) ([Ru(bpy)3]2+) and tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) ([Ru(dpp)3]2+). These fluorophores are very photostable and produce electronically excited states which have long lifetimes (τo ) 0.6 and 5.34 µs for [Ru(bpy)3]2+ and [Ru(dpp)3]2+, respectively) and high quantum yields (ΦL ) 0.042 and 0.30 for [Ru(bpy)3]2+ and [Ru(dpp)3]2+, respectively) and are readily quenched by oxygen (typically, kQ ) 3 × 109 dm3 mol-1 s-1).1,2 For most optical sensors for oxygen, the medium used to encapsulate the fluorophore is silicone rubber, a cross-linked polymer which has a high permeability toward oxygen (100 times (1) Carraway, E. R.; Demas, J. N.; Degraff, B. A.; Bacon, J. R. Anal. Chem. 1991, 63, 337-342. (2) Lin, C.-T.; Sutin, N. J. Phys. Chem. 1976, 80, 97-105. S0003-2700(97)00430-7 CCC: $14.00
© 1997 American Chemical Society
greater than that for any other organic polymer),3 high chemical and mechanical stability, and a hydrophobic nature. The latter property reduces problems associated with dye leaching and interference quenching by any ionic species present in the test medium. A very rough measure of the sensitivity of an oxygen sensor is the partial pressure of oxygen at which the initial (oxygen-free) luminescence exhibited by the sensor (Io) is reduced by 50%, i.e., pO2(S ) 1/2). Typical pO2(S ) 1/2) values of 377 and 29.8 Torr have been reported for [Ru(bpy)3]2+- and [Ru(dpp)3]2+-based oxygen sensors, respectively, encapsulated in silicone rubber.4 In most work on oxygen sensors, commercial silicone rubbers of proprietary composition have been used. This has limited somewhat our understanding of the effect of the nature of the polymer support on the sensitivity of the final oxygen sensor. The elegant work of Xu et al.5 has gone a long way to addressing this gap in our knowledge in a detailed examination of the variation in oxygen quenching of [Ru(dpp)32+(Cl-)2] encapsulated in mainly poly(dimethylsiloxane)-based polymers in which the amount of silica filler and amount and type of polar copolymer cross-linker were varied5 and the sensitivity, although not the response or recovery times, was studied as a function of the nature of the encapsulating medium. Recent work carried out by our group4 and others6 has established that the lumophoric, hydrophilic cations [Ru(bpy)3]2+ and [Ru(dpp)3]2+ are rendered hydrophobic and readily soluble in a range of different polymers, including silicone rubber (an elastomer)6 and poly(methyl methacrylate) (PMMA, a thermoplastic),4 by forming an ion-pair with a hydrophobic anion, such as tetraphenyl borate or dodecyl sulfate. We have demonstrated4 that a thermoplastic encapsulating medium, such as PMMA, offers a number of advantages over an elastomer, such as silicone rubber, for oxygen optical sensors. These advantages include the absence of a curing procedure, ease in varying polymer type and molecular weight, and dramatic alteration of oxygen sensitivity through the addition of a plasticizer. In an earlier paper,4 we were able to show that [Ru(dpp)32+(Ph4B-)2] encapsulated in PMMA, with tributyl phosphate as a plasticizer (133 parts per hundred resin, i.e., phr), gave rise to an oxygen sensor with a pO2(S ) 1/ ) value of 3.7 Torr, which is the most sensitive ruthenium-based 2 sensor reported to date. (3) Brandrup, J., Immergut, E. H., Eds. The Polymer Handbook, 3rd ed.; Wiley: New York, 1989; pp VI435-VI449. (4) Mills, A.; Thomas, M. Analyst 1997, 122, 63-68. (5) Xu, W.; McDonough, R. C.; Langsdorf, B.; Demas, J. N.; DeGraff, B. A. Anal. Chem. 1994, 66, 4133-4141. (6) Klimat, I.; Wolfbeis, O. S. Anal. Chem. 1995, 67, 3160-3166.
Analytical Chemistry, Vol. 69, No. 22, November 15, 1997 4653
Table 1. Previous Studies on Platinum and Palladium Porphyrin-Based Oxygen Sensors11-18 probea
τo/ ms
medium
(1/KSV)/ Torr
tV/s
water silicone rubber RTV 118 (GE) PS PMMA
0.535 3.57 7.2 27.1
65 15-20 15-20
7 8 8 8
