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complex colloids; fluorescent probes; Janus particles; microfluidics; microlenses; total internal reflection
Abstract:
Materials capable of dynamically and reversibly altering their emission are relevant for numerous optical applications. Here, the anisotropic morphology-directed light emission from fluorescent Janus emulsion droplets, an intrinsically chemo-responsive material platform, is investigated. Informed by experimental observations of morphology-dependent optical confinement of internally emitted light within the higher refractive index phases, ray-tracing is used to predict and fine-tune the droplets’ optical properties and their ability to concentrate light. Theoretical prediction and closely matching experimental results show that the collection of incident light and the confinement of emitted light in the internal droplet phase due to total internal reflection both contribute to the droplets’ anisotropic light emission profile. A novel ratiometric dual-angle fluorescence detection approach that exploits the gravitational alignment of the droplets is implemented to quantify the morphology-dependent large-scale chemically-induced modulation of the anisotropic emission of droplet layers. Relevant emulsion design parameters are systematically examined to enhance the signal-to-noise ratio, and a second emitter is co-compartmentalized inside the droplets to amplify the anisotropic light confinement via an absorption–emission cascade. Preferential excitation of dyes in proximity to the internal droplet interface enhances the collected light intensity, demonstrating that dye-loaded Janus emulsion droplets function as stimuli-responsive, tunable, fluorescent optical elements.
