Abstract
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.
Highlights
Gathering information on local temperature in cells promotes understanding their biological activities
The temperature probe should be small in comparison to the cell compartments, that is, it must be of nanoscale size to enable local heat monitoring, but not able to influence the real cell temperature
The temperature probe needs to be entirely biocompatible and it should not interfere with cell-metabolism processes during temperature recording
Summary
Gathering information on local temperature in cells promotes understanding their biological activities. We demonstrate the synthesis of polymeric upconversion nanocapsules and go on to use them for a temperature-dependent in vitro TTA-UC process in cells exposed to the ambient environment to study their applicability as nanothermometers. Polymeric Upconversion Nanocapsules Sensing Temperature in HeLa Cells (top) and Structures of the Sensitizer, meso-Tetraphenyl-tetrabenzo[2,3]porphyrin Palladium(II) (PdTBP, bottom left), and Emitter, 3,10Bis(3,3-dimethylbut-1-ynyl)perylene (BDMBP, bottom right). The absorbance (380−800 nm) of the samples was measured using a Tecan infinite plate reader in duplicate From this data, the absorbance values at 479 and 630 nm were used to estimate the concentration of encapsulated PdTBP and BDMBP dyes, respectively. For the cellular uptake study, HeLa cells resuspended in DMEM complete medium were seeded at a density of 8 × 104 cells per well in a 24-well plate for 24 h. The cell membrane was stained with CellMaskOrange (5 μg·mL−1, Life technologies, USA), excited with a DPSS laser (561 nm), and detected at 570−600 nm, pseudocolored in red
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