Dual lanthanide Complexes-Loaded ratiometric nanothermometer for elucidating intracellular variations of temperature and calcium transport

Herein, we report a ratiometric nanothermometer to study the correlation of heat/temperature and intracellular calcium transport. Firstly, poly(methyl methacrylate)-based copolymeric nanoparticles (NPs) from nanoprecipitation encapsulates two lanthanide complexes (Eu(DNPD)3Phen and Sm(DBM)3Phen) to enhance fluorescence of lanthanide ions (Ln3+). NPs were thereafter coated with an amphiphilic block copolymer pluronic F-127 to prevent nonspecific interaction between NPs and proteins, which ensures internalization of the nanothermometer into living cells. The back energy transfer (BET) from the excited state of Ln3+ to the triplet energy levels of ligand endows the temperature dependence on ratiometric fluorescence of Eu3+ and Sm3+. This nanothermometer has excellent anti-interference ability, ensuring accurate cell temperature measurement within a range of 29.0–40.0℃. It exhibits a maximum relative thermal sensitivity of 1.9 % ℃-1 at 29.0℃ and a minimum temperature resolution of 0.1℃ at 39.0℃ in live HeLa cells. The nanothermometer was applied for studying temperature or heat induced intracellular Ca2+ transport. It illustrated that free cytoplasmic Ca2+ concentration ([Ca2+]i) increased only when the cell temperature reached a certain threshold under photothermal stimulation, and the rise of temperature corresponds to the increase of [Ca2+]i. Furthermore, Ca2+ burst induced Ca2+ transport from cytoplasm to endoplasmic reticulum, which promoted ATP hydrolysis along with the rise of intracellular temperature. 200 s stimulation with FCCP enhanced ca. 1.7℃ of intracellular temperature along with 2.2 folds increment of [Ca2+]i, as FCCP induces Ca2+ efflux from mitochondria by increasing proton concentration therein. These results indicate that a close relationship between intracellular temperature and Ca2+ transport, which is of great significance for further understanding the role of temperature in signaling pathways and neurotransmitter release events.