Monitoring local temperature inside cells is crucial when interpreting biological activities as enhanced cellular metabolism leads to higher heat production and is commonly correlated with the presence of diseases such as cancer. In this study, we report on polymeric upconversion nanocapsules for potential use as local nanothermometers in cells by exploiting the temperature dependence of the triplet–triplet annihilation upconversion phenomenon. Nanocapsules synthesized by the miniemulsion solvent evaporation technique are composed of a polymer shell and a liquid core of rice bran oil, hosting triplet–triplet annihilation upconversion active dyes as sensitizer and emitter molecules. The sensitivity of the triplet–triplet annihilation upconversion to the local oxygen concentration was overcome by the oxygen reduction ability of the rice bran oil core. The triplet–triplet annihilation upconversion process could thus successfully be applied at different levels of oxygen presence including at ambient conditions. Using this method, the local temperature within a range of 22 to 40 °C could be determined when the upconversion nanocapsules were taken up by HeLa cells with good cellular viability. Thus, the higher cell temperatures where the cells show enhanced metabolic activity led to a significant increase in the delayed fluorescence spectrum of the upconversion nanocapsules. These findings are promising for further development of novel treatment and diagnostic tools in medicine.

Modified organogels are bicontinuous colloidal systems which form a three-dimensional network embedding well-solvated organic dyes sensitive to minor changes in its microenvironment. It was demonstrated that these natural wax/oils based organogels can be applied for minimally invasive temperature sensing in life-science objects. The desired temperature sensitivity of better than 100 mK, centered around the physiologically relevant temperature of 36°C, is warranted by using the process of triplet-triplet annihilation photon energy upconver-sion (TTA-UC) as a sensing mechanism. This all-optical sensing technique, based on ratiometric-type signal registration, ensures significant independence of the data obtained on excitation intensity instabilities, local molecular concentration fluctuations and field-of-view variations. The TTA-UC system is effectively protected for more than 1000 s against oxygeninduced damage, allowing stable performance of this temperature-sensing system even in ambient environments without lost of sensitivity and applying the same calibration curve.

Herein we demonstrate temperature-dependent sacrificial singlet oxygen scavenging properties of N-butyl-2-pyridone, ensuring efficient stimuli-responsive protection of densely populated excited triplet state ensembles against deactivation by molecular oxygen. As an acting external stimulus the temperature was chosen: it will be shown that at low temperature the concentration of singlet oxygen will be substantially lowered; in contrast, at elevated temperatures singlet oxygen will not be captured, and thus the optically excited densely populated triplet ensembles will be effectively depopulated. The singlet oxygen scavenging ability of N-butyl-2-pyridone demonstrates long-term protection of a triplet-triplet annihilation upconversion process against photooxidation.