Nonlinear photo-response in upconversion nanoparticles for in-depth super-resolution imaging

Upconversion nanoparticles (UCNP) is a lanthanide ion-doped nanocrystal that has a natural nonlinear photo-response from their upconverting energy transfer process. The nonlinearity can be further modified by changing the doping element and concentration. Here we present a strategy that applies UCNPs as near-infrared (NIR) nonlinear fluorescence probe for in-depth super-resolution imaging. We present a method that takes advantage of “non-diffractive” Bessel beam, further employs the photon-saturation of the NIR emission from UCNPs, so that enabling super-resolution mapping of single nanoparticles located 55 μm inside a spheroid, with a resolution of 98 nm, without adaptive optics compensation. We further apply the photon-conversion of UCNPs for a high efficient NIR nonlinear structured illumination microscopy (NIRNSIM) for a rapid in-depth super-resolution imaging. With 10 kW/cm2 continuous wave (CW) excitation, NIR-NSIM achieves a resolution of 130 nm, 1/7th of the excitation wavelength, and a frame rate of 1 fps, through 50 μm biological tissues.