Microsphere nanoscopy for imaging of actin proteins

The use of contact microlenses first started being used as a simple method which allows resolving nanometer sized objects using visible light. The resolution beyond the classical diffraction limit has been reported for nanoplasmonic structures. However the mechanism is not fully understood whether it is from photonic nanojets, plasmon-polaritons, localized surface plasmon resonances, coherent modal excitation in metallic objects, or optical resonances in the microspherical lens itself is still debated in the literature. In this work, we applied contact high-index microspheres for fluorescence imaging of actin protein filaments. The microspheres were embedded in elastopolymer slabs. The fact that the filaments have nanometer-scale width, much smaller than the diffraction limit, significantly simplified the resolution analysis. Using microscope objective with a limited numerical aperture (NA=0.6), we achieve a resolution of ~λ/1.5 for proteins through the microsphere and a resolution of ~λ/0.5 without a microsphere.