Rheological parameters (viscosity, creep compliance and elasticity) play an important role in cell function and viability. For this reason different strategies have been developed for their study. In this work, two new microrheometric techniques are presented. Both methods take advantage of the analysis of the polarized emission of an upconverting particle to determine its orientation inside the optical trap. Upconverting particles are optical materials that are able to convert infrared radiation into visible light. Their usefulness has been further boosted by the recent demonstration of their three-dimensional control and tracking by single beam infrared optical traps. In this work it is demonstrated that optical torques are responsible of the stable orientation of the upconverting particle inside the trap. Moreover, numerical calculations and experimental data allowed to use the rotation dynamics of the optically trapped upconverting particle for environmental sensing. In particular, the cytoplasm viscosity could be measured by using the rotation time and thermal fluctuations of an intracellular optically trapped upconverting particle, by means of the two previously mentioned microrheometric techniques.
Paloma Rodríguez-Sevilla, Yuhai Zhang, Nuno de Sousa, Manuel I. Marqués, Francisco Sanz-Rodríguez, Daniel Jaque, Xiaogang Liu, and Patricia Haro-González, "Microrheometric upconversion-based techniques for intracellular viscosity measurements," Proc. SPIE 10347, Optical Trapping and Optical Micromanipulation XIV, 103471S (Presented at SPIE Nanoscience + Engineering: August 09, 2017; Published: 25 August 2017); https://doi.org/10.1117/12.2275944.
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