Upconversion-Luminescent Fiber Microchannel Sensors for Temperature Monitoring at High Spatial Resolution in the Brains of Freely Moving Animals.

Brain temperature is a critical factor affecting neural activity and function, whose fluctuations may result in acute life-threatening health complications and chronic neuropathology. To monitor brain temperature, luminescent nanothermometry (LN) based on upconversion nanoparticles (UCNPs) with low autofluorescence has received extensive attention for its advantages in high temperature sensitivity and high response speed. However, most of current the LNs are based on optical imaging, which fails in temperature monitoring in deep brain regions at high spatial resolution. Here, the fiber microchannel sensor (FMS) loaded with UCNPs (UCNP-FMS) is presented for temperature monitoring at high spatial resolution in the deep brains of freely moving animals. The UCNP-FMS is fabricated by incorporating UCNPs in microchannels of optical fibers, whose diameter is ∼50 µm processed by femtosecond laser micromachining for spatially resolved sensing. The UCNPs provide thermal-sensitive upconversion emissions at dual wavelengths for ratiometric temperature sensing, ensuring a detection accuracy of ± 0.3 °C at 37 °C. Superior performances of UCNP-FMS are demonstrated by real-time temperature monitoring in different brain regions of freely moving animals under various conditions such as taking food, undergoing anesthesia/wakefulness, and suffering external temperature changes. Moreover, this study shows the capability of UCNP-FMS in distributed temperature sensing in mammalian brains in vivo.