title: Optical Torques on Upconverting Particles for Intracellular Microrheometry.
abstract: Precise knowledge and control over the orientation of individual upconverting particles is extremely important for full exploiting their capabilities as multifunctional bioprobes for interdisciplinary applications. In this work, we report on how time-resolved, single particle polarized spectroscopy can be used to determine the orientation dynamics of a single upconverting particle when entering into an optical trap. Experimental results have unequivocally evidenced the existence of a unique stable configuration. Numerical simulations and simple numerical calculations have demonstrated that the dipole magnetic interactions between the upconverting particle and trapping radiation are the main mechanisms responsible of the optical torques that drive the upconverting particle to its stable orientation. Finally, how a proper analysis of the rotation dynamics of a single upconverting particle within an optical trap can provide valuable information about the properties of the medium in which it is suspended is demonstrated. A proof of concept is given in which the laser driven intracellular rotation of upconverting particles is used to successfully determine the intracellular dynamic viscosity by a passive and an active method.
authors.affiliation: Fluorescence Imaging Group, Departamento de Física de Materiales, Universidad Autónoma de Madrid , 28049 Madrid, Spain.
authors.forename: Paloma
authors.initials: P
authors.lastname: Rodríguez-Sevilla
authors.affiliation: Department of Chemistry, National University of Singapore , Science Drive 3, Singapore 117543, Singapore.
authors.forename: Yuhai
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: Departamento de Física de la Materia Condensada, Condensed Matter Physics Center (IFIMAC), and Nicolás Cabrera Institute, Universidad Autónoma de Madrid , 28049 Madrid, Spain.|Donostia International Physics Center (DIPC) , Donostia-San Sebastián 20018, Spain.
authors.forename: Nuno
authors.initials: N
authors.lastname: de Sousa
authors.affiliation: Departamento de Física de Materiales, Condensed Matter Physics Center (IFIMAC), and Nicolás Cabrera Institute, Universidad Autónoma de Madrid , 28049 Madrid, Spain.
authors.forename: Manuel I
authors.initials: MI
authors.lastname: Marqués
authors.affiliation: Fluorescence Imaging Group, Departamento de Física de Materiales, Universidad Autónoma de Madrid , 28049 Madrid, Spain.|Instituto Ramón y Cajal de Investigaciones Sanitarias, Hospital Ramón y Cajal , Madrid 28034, Spain.
authors.forename: Francisco
authors.initials: F
authors.lastname: Sanz-Rodríguez
authors.affiliation: Fluorescence Imaging Group, Departamento de Física de Materiales, Universidad Autónoma de Madrid , 28049 Madrid, Spain.|Instituto Ramón y Cajal de Investigaciones Sanitarias, Hospital Ramón y Cajal , Madrid 28034, Spain.
authors.forename: Daniel
authors.initials: D
authors.lastname: Jaque
authors.affiliation: Department of Chemistry, National University of Singapore , Science Drive 3, Singapore 117543, Singapore.
authors.forename: Xiaogang
authors.initials: X
authors.lastname: Liu
authors.affiliation: Fluorescence Imaging Group, Departamento de Física de Materiales, Universidad Autónoma de Madrid , 28049 Madrid, Spain.
authors.forename: Patricia
authors.initials: P
authors.lastname: Haro-González
keywords: Optical trapping; finite element method; microrheometry; optical torque; upconverting particle; viscosity