In most biological tissues, strong scattering of visible light limits the use of standard microscopy. New approaches and methods that focus on overcoming this particular problem have been developing recently. In this work, we have developed a novel technique for deep tissue imaging and sensing that utilizes optical WGM (whispering gallery mode) microresonators and their properties. The properties of spectrally narrow emission and high sensitivity to the environment make WGM microresonators superior to the standard fluorescent probes typically used in biological research. WGM microresonators placed under scattering materials or embedded inside them can be localized with high precision by decomposing the acquired spectral signal into contributions from individual microresonators. This is possible due to the fact that strong WGM resonances are mainly unaffected when propagating through a scattering medium. The spectral characteristics of each WGM microresonator are unique as long as they differ in size by only a very small margin. This can further be used to tag, identify and track individual cells. Additionally, sensing of different parameters such as refractive index, pH or temperature of the surroundings is also possible by analyzing the spectra. Simultaneous sensing and localization capabilities make the developed method a versatile tool in the field of deep tissue imaging.
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