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.

Illuminated (320 < < 370 nm), aqueous suspensions of transparent quantum-sized (Q-sized) ZnO semiconductor colloids in the presence of carboxylic acids and oxygen are shown to produce steady-state concentrations of H2O2 as high as 2 mM. Maximum H2O2 concentrations are observed only with added electron donors (i.e., hole scavengers). The order of efficiency of hole scavengers is as follows: formate > oxalate > acetate > citrate. Isotopic labeling experiments with 18C>2 are consistent with the hypothesis that hydrogen peroxide is produced directly by the reduction of adsorbed oxygen by conduction band electrons. Quantum yields for H2O2 production are near 30% at low photon fluxes. However, the quantum yield is shown to vary with the inverse square root of absorbed light intensity [ < ((/abs)"1)1/2], with the wavelength of excitation, and with the diameter of the Q-sized colloids. The initial rate of H2O2 production is 100-1000 times faster with Q-sized ZnO particles (Dp = 4-5 nm) than with bulksized ZnO particles (Dp = 0.1 µ ).

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.

Nanosized iron (< 100 nm in diameter) was synthesized in the laboratory and applied to the reduction of eight chlorinated ethanes (hexachloroethane (HCA), pentachloroethane (PCA), 1,1,2,2-tetrachloroethane (1,1,2,2-TeCA), 1,1,1,2-tetrachloroethane (1,1,1,2-TeCA), 1,1,2-trichloroethane (1,1,2-TCA), 1,1,1-trichloroethane (1,1,1-TCA), 1,2-dichloroethane (1,2-DCA), and 1,1-dichloroethane (1,1-DCA)) in batch reactors. Reduction of 1,1,1-TCA increased linearly with increasing iron loading between 0.01 and 0.05 g per 124 mL solution (0.08-0.4 g/L). Varying initial concentrations of PCA between 0.025 and 0.125 mM resulted in relatively constant pseudo-first-order rate constants, indicating PCA removal conforms to pseudo-first-order kinetics. The reduction of 1,1,2,2-TeCA decreased with increasing pH; however, dehydrohalogenation of 1,1,2,2-TeCA became important at high pH. All chlorinated ethanes except 1,2-DCA were transformed to less chlorinated ethanes or ethenes. The surface-area-normalized rate constants from first-order kinetics analysis ranged from < 4 x 10(-6) to 0.80 L m(-2) h(-1). In general, the reactivity increased with increasing chlorination. Among tri- and tetrasubstituted compounds, the reactivity was higher for compounds with chlorine atoms more localized on a single carbon (e.g., 1,1,1-TCA > 1,1,2-TCA). Reductive beta-elimination was the major pathway for the chlorinated ethanes possessing alpha,beta-pairs of chlorine atoms to form chlorinated ethenes, which subsequently reacted with nanosized iron. Reductive alpha-elimination and hydrogenolysis were concurrent pathways for compounds possessing chlorine substitution on one carbon only, forming less chlorinated ethanes.

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