Dr. Mario Fischer Profile

Dr. Mario Fischer

at Niels Bohr Institute

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Proceedings Article | 29 August 2008 Paper

Validation of FDT calibration method in complex media

Mario Fischer, Andrew Richardson, S. Nader Reihani, Lene Oddershede, Kirstine Berg-Sørensen

Proceedings Volume 7038, 703822 (2008) https://doi.org/10.1117/12.796274

KEYWORDS: Particles, Calibration, Error analysis, Optical tweezers, Fourier transforms, Optical calibration, Motion measurement, Time metrology, Remote sensing, In vivo imaging

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Optical tweezers constitute an obvious choice as the experimental technique for manipulation and trapping of organelles in living cells. For quantitative determination of the forces exerted in such in vivo systems, however, tools for reliable calibration of the optical tweezers are required. This is complicated by the fact that the viscoelastic properties of the cytoplasm are a priori unknown. We elaborate on a previously reported theoretical calibration procedure and verify its authenticity experimentally. With this approach, we may at the same time determine the trapping characteristics of the optical tweezers and the viscoelastic properties of the cytoplasm. The method employs the fluctuation-dissipation theorem (FDT) which is assumed valid for the situations considered. This allows for extracting the requested properties from two types of measurements that we denote as passive and active. In the passive part, the Brownian motion of a particle inside the trap is observed. In the active part, the system is slightly perturbed and the response of the trapped particle is tracked. Gently oscillating the stage on which the sample is mounted allows the delay between the position of the stage and the response of the trapped bead, using a quadrant photodiode, to be quantified. No assumptions about the particle radius or geometry or about the frequency-dependent friction coefficient are needed. The paper contains the theoretical background of the method in terms of convenient formulations of the fluctuation-dissipation theorem and application of the method in two types of experiments. Further we discuss experimental concerns which are i) the choice of driving characteristics in the active part of the calibration procedure and ii) statistical errors.

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