Synthesis of inorganic optical thermometers based on Cr3+ within the pores of mesoporous silica nanoparticles

Recently, contactless optical thermometers have attracted a great research interest, finding application in a wide variety of technological fields, moving from biological intracellular sensing to microelectronics, catalysis and microfluidic. Specifically, ratiometric optical thermometers exploit the ratio between the emission intensities of a metal center, typically a lanthanide or a transition metal, in a crystalline host. The aim of this research was to characterize the innovative Ga2O3:Cr3+ system in nanometric dimensions through a synthesis within the pores of mesoporous silica nanoparticles. The effects of nanoparticles’ size, heat treatment temperature and percentage of chromium on the phase stabilization (different Ga2O3 polymorphs) and the absorption/emission bands were investigated. The crystal field variation was then studied at the progressive substitution of gallium by aluminum. The results demonstrated that a ratiometric optical thermometer based on chromium doped gallium oxide in nanometric dimensions lower than 25 nm was not possible due to the weak crystal field experienced by Cr3+ in the cubic Ga2O3 polymorph. However, the unprecedent investigation of Cr3+ in the cubic Ga2O3 phase suggested the potential of this system as phosphor in the field of near-infrared LEDs (NIR-LEDs). When gallium is partially substituted with aluminum (5% Al), the presence of chromium emission peaks allowing the realization of the ratiometric thermometer was instead observed.