A detailed study of the photophysics and photochemistry of polymer-immobilized luminescent transition-metal complex oxygen sensors is presented. Emphasis is on understanding the underlying origin of the nonlinear Stern-Volmer quenching response. Microheterogeneity is important in both photophysical and photochemical behavior, and the nonlinear quenching responses in RTV 118 silicone rubber can be adequately described by a two-site model, although detailed lifetime measurements suggest a more complex Underlying system. Counterion studies with quenching counterions are shown to be useful probes of the structure of the complex in the polymer. While oxygen enhances photochemical instability, singlet oxygen is not directly implicated in sensor decomposition. I n the photochemistry there is at least one reactive and one much less reactive site, although the photochemistry and quenching measurements probably sample different populations of sites. The existence of reactive sites suggests that stability can be enhanced by a preliminary photolysis to eliminate the more reactive sites.

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Determining quenching mechanisms for luminescent species adsorbed or bound to a variety of heterogeneous systems (e.g., silicas, organic, inorganic, and biopolymers) is quite difficult In the absence of detailed information on system heterogeneity. A method for assessing the relative contributions of static and dynamic quenching In heterogeneous systems is presented. While the method does not provide direct information on the details of system heterogeneity, it requires no a priori information on the nature of the heterogeneity. This approach is based on a comparison of intensity quenching data with lifetime quenching data using a preexponential weighted lifetime, rM. rM Is calculated by fitting the observed decay curves to a sum of a relatively small number (2-4) of exponentials. For time-correlated single-photon counting the parameters obtained from a statistically acceptable fit can be used to accurately estimate rM, even though the computed model may bear no resemblance to the true decay kinetics. Simulations confirm that the method works for a wide range of heterogeneous systems. The technique Is applied to oxygen quenching of a luminescent metal complex on a silica surface.

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Oxygen quenching of [Ru(Ph2phen)3]Cl2 (Ph2phen = 4,7-diphenyl-1,10-phenanthroline) has been studied in a diverse series of polymers, most with a common poly-(dimethylsiloxane) (PDMS) component. Systematic variations in the polymer properties have been made in order to delineate the structural features important for satisfactory use of supports for oxygen sensors. Most measurements were made using homo- or copolymers containing a PDMS region, although some measurements were made on small ring siloxane polymers. In particular, quenching behavior was examined as a function of polymer structure as well as the type of and amount of polar copolymer cross-linkers. Cross-linkers were added to enhance the solubility of oxygen probes in an otherwise nonpolar polymer. In addition, hydrophobic silica was added to alter quenching properties. Domain models are used to explain the variations in oxygen quenching properties as a function of additives and cross-linkers. These considerations have led to the most sensitive ruthenium-based sensor reported to date. The relative affinity of the different domains for the complex and the efficacy of the domains for oxygen quenching control the overall behavior of the sensing response. Guidelines for design of suitable polymer supports for oxygen sensors are proposed.

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Optical oxygen sensors are being used in applications ranging from blood gas analysis to pressure measurements in wind tunnels.

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