Upconversion luminescent nanoparticles in physical sensing and in monitoring physical processes in biological samples

The most frequently used strategy for sensing is based on the emission variation of a photoactive system and it can be classified as chemical or physical depending on its response to either a chemical or a physical stimulus. There are a large number of luminescent nanomaterials that respond to chemical species, but comparatively, with the exception of temperature sensing, to date there are few examples showing the capacity of photoactive nanosystems responsive to physical stimuli, such as magnetic and electric fields, high radiation energy, tension/pressure, viscosity, etc, and very few of them comprise upconversion nanoparticles (UCNPs). These nanomaterials consist of an inorganic matrix doped with rare earth (RE) ions and can be advantageously used in sensing due to their unique optical features: near-infrared excitation, anti-Stokes emission, multiplexed emissions that span over a broad range (i.e. from the UV to NIR region depending on the nature of the RE ions), narrow emission bands, long emission lifetimes, high photostability, and non-photoblinking, among others. In this review we focus on the advances in the application of the luminescence of UCNPs for sensing environmental physical changes and for tracking physical processes in living systems.