Temperature-Induced Energy Transfer in Dye-Conjugated Upconverting Nanoparticles: A New Candidate for Nanothermometry

Lanthanide-doped upconverting nanoparticles (UCNPs) are highly promising candidates for bioimaging and for cellular nanothermometry as a novel diagnostic tool. Aiming for the diagnosis of diseases at very early stages in order to optimize therapy and recovery of the patient, it must be taken into account that thermal singularities are often one of the first indicators of a disease. It is therefore our goal to develop a nanothermometer based on UCNPs that is suitable to detect the temperature at a subcellular level in the physiological range. Thus, upconverting NaGdF4:Er-3+,Yb3+ nanoparticles that convert near-infrared (NIR) into visible (VIS) light are synthesized by thermal decomposition. Appropriate surface modification with a thermoresponsive polymer pNIPAM (poly(N-isopropylacrylamide)) guarantees dispersibility in aqueous media required for biomedical applications. In a further step, the combination of the obtained UCNPs with an organic dye (FluoProbe532A) provides potential donor-acceptor-pairs allowing for energy transfer processes, whereas the light emitted by the Er3+ ions (donors) is absorbed by the organic dye (acceptor). It has been demonstrated that the dye-conjugated UCNPs undergo a temperature-dependent energy transfer process inducing a temperature-dependent increase in the thermal sensitivity when compared to unlabeled UCNPs. This result indicates the great potential of the presented nanoprobes for applications in nanothermometry.