Highly efficient upconversion of Er3+ in Yb3+ codoped non-cytotoxic strontium lanthanum aluminate phosphor for low temperature sensors

Er3+ and Er3+/Yb3+ melilite-based SrLaAl3O7 (SLA) phosphors were synthesized by a facile Pechine method. The differences in emission intensities of I-4(13/2) -> I-4(15/2) transition in NIR region when excited with Ar+ and 980 nm lasers were explained in terms of energy transfer mechanisms. Temperature and power dependence of upconversion bands in the visible region centered at 528, 548 and 660 nm pertaining to H-2(11/2), S-4(3/2) and F-4(9/2) -> I-4(15/2) transitions were investigated. Fluorescence intensity ratio (FIR) technique was used to explore temperature sensing behaviour of the thermally coupled levels H-2(11/2)/S-4(3/2) of Er3+ ions in the phosphors within the temperature range 14-300 K and the results were extrapolated up to 600 K. Anomalous intensity trend observed in Er3+ doped SLA phosphor was discussed using energy level structure. Cytotoxicity of phosphors has been evaluated using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay in Bluegill sunfish cells (BF-2). The non-cytotoxic nature and high sensitivity of the present phosphors pay a way for their use in vitro studies and provide potential interest as a thermo graphic phosphor at the contact of biological products.