Up/down-conversion photoluminescence from the rare-earth-doped nanocrystals have attracted a lot of interest because of their unique optical properties useful in various fields, including biophotonics and nanomedicine research [1-3]. Here, we use ferroelectric materials whose intrinsic polarization and crystal structure are responsive to external stimuli, e.g. electric field, mechanical stress, or temperature, as the matrix for welcoming the optically sensitive rare-earth elements [4-5]. As an illustration, we show here how we design a rare-earth-based ferroelectric sensor to detect the real-time local electric potential that, e.g. exists in biological neuronal networks for better understanding neuronal circuits functions in normal and pathological conditions. We prepare Yb3+/Er3+ co-doped BaTiO3 (BTO) nanocrystals via the hydrothermal method using a complex of BaCO3, CH3COOH, Yb(NO3)3, Er(CH3COO)3 and Ti(C3H7O)4 as precursors. The obtained Yb3+/Er3+ -doped BTO nanocrystals have a cubic shape with edge sizes of 150 nm and show up and down-conversion signals when optically excited with 980 nm and 488 nm laser wavelength, respectively. Under a bias voltage, an enhancement of 14 % in up-conversion emission intensity is observed at ~550nm line emission, which indicates that the single Yb3+/Er3+ -doped BTO nanocrystal has a good electric-field response and might be taken into consideration for the aforementioned biological applications. [1] J. Loo et al., Coordination Chemistry Reviews 400, 213042 (2019) [2] G. Chen et al., Chemical Reviews 114, 5161 (2014) [3] K. Shahzad et al., Materials Science and Engineering: C 119, 111444 (2021) [4] J. Hao et al., Angewandte Chemie International Edition 50, 6876 (2011) [5] C. Paillard et al., Advanced Materials 28, 5153 (2016)