Step by step optimization of luminescence thermometry in MgTiO3:Cr3+, Nd3+@SiO2 nanoparticles towards bioapplications

The increasing popularity of luminescent nanothermometry in recent years can be attributed to its application potential in biomedicine. In response to this need, we describe a biocompatible bimodal luminescent ther-mometer that operates in ratiometric and luminescence lifetime modes based on particles of MgTiO3:Cr3+, Nd3+@SiO2. The introduction of Cr3+ and Nd3+ dopants enabled the luminescence of Ti3+ ions to be observed, and the difference in the thermal quenching rates of Cr3+ (4T2 -> 4A2), Ti3+ (2T2 -> 2E) and Nd3+ (4F3/2 -> 4I11/2) ions enabled the ratiometric thermometers. The highest sensitivity reaching SR = 1.00%K-1 was obtained for MgTiO3:0.1 % Cr3+, 0.1 % Nd3+ at 203 K. The shortening of the lifetime of the 4T2 level of Cr3+ ions associated with its thermal depopulation allows to develop a lifetime-based thermometer with a relative sensitivity reaching 0.85-1.18%K-1 in the physiological temperature range. The deposition of a SiO2 shell on a MgTiO3:Cr3+,Nd3+ did not introduce significant changes in the shape of the emission spectrum and slightly elongates the lifetime by reducing the probability of surface-related nonradiative processes. More importantly, the thermometric performance of this luminescence thermometer was preserved. The low cytotoxicity of the obtained materials underlines their potential in bioapplications of the described luminescent thermometers.