Submillisecond Electric\nField Sensing with an Individual\nRare-Earth Doped Ferroelectric Nanocrystal

Understanding the dynamics of electrical signals within\nneuronal\nassemblies is crucial to unraveling complex brain functions. Despite\nrecent advances in employing optically active nanostructures in transmembrane\npotential sensing, there remains room for improvement in terms of\nresponse time and sensitivity. Here, we report the development of\nsuch a nanosensor capable of detecting electric fields with a submillisecond\nresponse time at the single-particle level. We achieve this by using\nferroelectric nanocrystals doped with rare-earth ions that produce\nupconversion (UC). When such a nanocrystal experiences a variation\nof surrounding electric potential, its surface charge density changes,\ninducing electric polarization modifications that vary, via a converse\npiezoelectric effect, the crystal field around the ions. The latter\nvariation is finally converted into UC spectral changes, enabling\noptical detection of the electric potential. To develop such a sensor,\nwe synthesized erbium and ytterbium-doped barium titanate crystals\nof ≈160 nm in size. We observed distinct changes in the UC\nspectrum when individual nanocrystals were subjected to an external\nfield via a conductive atomic force microscope tip, with a response\ntime of 100 μs. Furthermore, our sensor exhibits a remarkable\nsensitivity of 4.8 kV/cm/Hz, enabling time-resolved detection of a\nfast-changing electric field of amplitude comparable to that generated\nduring a neuron action potential.