Effect of -OH on the thermal enhancement properties of NIR-II lanthanide-doped nanoparticles in water

Thermo-enhanced nanothermometry has attracted considerable attention owing to its high thermal sensitivity; however, most studies are focused on upconversion nanoparticles (NPs) in the form of powder. In the field of bioapplication, it is imperative to investigate the performance of NPs dispersed in aqueous solutions to expedite their in vivo utilization. Particularly, the influence of -OH on the thermal properties of these NPs remains unclear. Herein, we investigated the thermal properties of alpha-NaYbF4:2%Er,2%Ce@NaYF4 NPs with different shell thicknesses emitting in the second near-infrared (NIR-II) region. Both powder and aqueous solution-dispersed NPs exhibit thermal enhancements due to a reduction in the -OH content and a blue shift of -OH vibration at elevated temperatures, respectively. The observed blue shift of -OH vibration is attributed to the conversion of 4-coordinated hydrogen-bound water into 2-coordinated hydrogen-bound water. By separately introducing LiCl and D2O to an aqueous solution, we demonstrated how the blue shift and -OH content affect the thermal enhancement factor in aqueous solutions. These findings underscore the crucial role of -OH groups in nanothermometry for bioapplications while also providing insights for designing highly thermally sensitive nanothermometers. The -OH group plays a key role in the thermal enhancement effect of nanoparticles (NPs). By separately introducing LiCl and D2O into aqueous solutions, we demonstrated how the blue shift and -OH content affect the thermal enhancement factor of NPs.