Phosphor thermometry at the cellular level

Luminescence from a thermographic phosphor (TGP) can be very sensitive to temperature, an attribute exploited in commercial and scientific applications at various levels. Depending on the formulation and the processing method, phosphor particle sizes may range from a few nanometers to dozens of microns across. The size, then, may be comparable to or even much smaller than a typical biological cell. In previous biomedical efforts we have explored using TGP´s as 1) flow tracers in simulations of positive pressure infusion for brain tumors, 2) their performance for nanoscale thermometry at physiological temperatures, and 3) as surrogates for green fluorescent protein-labelled cells in tests of cytometric neurocatheters. The present effort provides additional supporting quantitative information to aid deployment of phosphor thermometry for cellular level applications. One of the most temperature-sensitive phosphors is La₂O₂S:Eu. At normal body temperature, a one degree C change in temperature results in a greater than 5% change in decay time. In this work, it is demonstrated that temperature changes of at least 0.05 C can be resolved using this phosphor. Also demonstrated is detection of phosphor signal using small diameter multimode and even single-mode optical fiber. The latter fiber had a 5 micron diameter core and is therefore only slightly larger than biological cells. In conclusion, the results may serve as a guide to further investigations and application of phosphors to biomedical applications.