Abstract
In ion-annihilation electrochemiluminescence (ECL), luminophore ions are generated by oxidation as well as reduction at electrodes surfaces, and subsequently recombine into an electronically excited state, which emits light. The intensity of the emitted light is often limited by the kinetic rate of recombination of the luminophore ion species. Recombination or annihilation rates are high ranging up to approximately 1010 M−1 s−1 and can be difficult to determine using scanning electrochemical microscopy or high-frequency oscillations of an electrode potential. Here, we propose determining annihilation kinetics by measuring the relative change of the emitted light intensity as a function of luminophore concentration. Using finite element simulations of annihilation ECL in a geometry of two closely spaced electrodes biased at constant potentials, we show that, with increasing concentrations, luminescence intensity crosses over from a quadratic dependence on concentration to a linear regime—depending on the rate of annihilation. Our numerical results are applicable to scanning electrochemical microscopy as well as nanofluidic electrochemical devices to determine fast ion-annihilation kinetics.
Highlights
Electrogenerated chemiluminescence [1] (ECL) is light generation by luminophore molecules which is triggered by electrochemical excitation
We predict that recombination kinetics of annihilation electrochemiluminescence can be determined by detecting the change in luminescence intensity when increasing the concentration of luminophore molecules in a setup of two closely spaced and constantly biased electrodes
Our numerical results show that this increase in intensity is expected to show a transition from a quadratic to a linear dependence on concentration
Summary
Electrogenerated chemiluminescence [1] (ECL) is light generation by luminophore molecules which is triggered by electrochemical excitation. A prototypical ECL reaction scheme for efficient light generation is the annihilation pathway In this pathway, luminophore molecules are oxidized and reduced with respect to their bulk oxidation state by applying anodic and cathodic potentials at two separate working electrodes, respectively, which are positioned at a close distance. Rates can be determined when using constantly biased doubleband microelectrodes and mapping the distribution of ECL emission in between both electrodes with a high spatial resolution [10] In such two-electrode direct current (DC) measurements, generally fast reaction rates or short lifetimes of intermediate reaction products can be investigated well for short distances between both electrodes. The intensity of emitted light depends on the reaction pathway as photons are emitted in the annihilation regime but not for 2-electron redox cycling (5)–(6) This fact has been exploited by the group of Bard to determine kann in rubrene and [Ru(bpy)3]2+ using an SECM [17]. For very short distances h, the intensity decreases as the 2-electron pathway starts to dominate. kann was determined by comparison of experimentally and numerically determined approach curves
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