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Fluorescent Microresonator Biochemical Sensors: Applications, Limitations and Perspectives

Optical microresonators have emerged over the last decades as the state of the art for biochemical sensing applications with several approaches having been demonstrated for single molecule, ion and nanoparticle detection. This has been achieved by taking advantage of the circulating light inside the microresonators, also known as whispering gallery modes (WGMs), which enable probing of the environment surrounding the resonator after each round trip. Despite the tremendous sensing performances, such approaches have not yet made their way to any commercial applications and are still largely secluded to research labs due to their inherent complexity and operating cost, requiring a phase-matched fiber taper or prism to couple the probing light into the resonator in addition to highly sophisticated equipment. Fluorescent microresonators, containing a gain medium such as organic dyes, quantum dots or inorganic nanocrystals, on the other hand, might alleviate the aforementioned limitations. Upon remote illumination with the appropriate light source, a WGM modulated fluorescence spectrum is generated, which can also be remotely collected. Besides their simplicity of operation, fluorescent microresonators are inherently easier to fabricate with multiple examples of dye doped polymer microspheres or fluorescent coated microcapillaries existing which can be prepared in batch, not requiring any of the advanced microfabrication facilities required for toroids or individually fabricated microspheres. Here, the emerging sensing applications for fluorescent microresonators will be discussed, from standard assays to real life protein detection and quantification in human serum. The limitations of these resonators, and especially their limited quality factors and detection limits will also be discussed as well as the avenues for improving their sensing performance and the addition of specific functionalities that exploit the unique nature of active resonators. Finally, we will explore the alternative sensing applications such as in-vivo sensing, only permitted by the remote excitation/collection scheme of these devices, and we will present a different approach for sensing exploiting the distinctive light emitting capability of these devices.