Mapping microscopic viscosity using molecular rotors
Marina K. Kuimova
Department of Chemistry, Imperial College London,
Exhibition Road, London, SW7 2AZ, UK
e-mail: m.kuimova@imperial.ac.uk

Viscosity is one of the main factors which influence diffusion in condensed media. In a cell viscosity can play a role in several diffusion mediated processes, such as drug delivery, signalling and mass transport. Previously, alterations in viscosity in cells and organs have been linked to malfunction; however, mapping viscosity on a single-cell scale remains a challenge.
We have imaged viscosity inside lipid mono- and bi-layers, in cells and in atmospheric aerosol particles using fluorescent probes, called molecular rotors [1]. In molecular rotors the speed of rotation about a sterically hindered bond is viscosity-dependent [1]. This approach enabled us to demonstrate that viscosity distribution in a cell is highly heterogeneous and that the local microviscosity in hydrophobic cell domains can be up to 100 times higher than that of water [2-5]. These conclusions have been confirmed by monitoring the decay and reaction rates of short-lived excited state of molecular oxygen, singlet oxygen, on a single cell level [3]. We also determine particle viscosity of complex, atmospherically relevant organic aerosols and monitor its temporal changes in real time, over time scales relevant for the atmosphere.


References
[1]    M. K. Kuimova, Phys Chem Chem Phys, 2012, 14, 12671
[2]    M. K. Kuimova, G. Yahioglu, J. A. Levitt, K. Suhling, J. Amer. Chem. Soc., 2008, 130, 6672
[3]    M. K. Kuimova, S. W. Botchway, A. W. Parker, M. Balaz, H. A. Collins, H. L. Anderson, K. Suhling, P. R. Ogilby, Nature Chem., 2009, 1, 69-73
[4]    N. A. Hosny, G. Mohamedi, P. Rademeyer, J. Owen, Y. Wu, M-X. Tang, R. J. Eckersley, E. Stride and M. K. Kuimova, Proc. Nat. Acad. Sci. USA, 2013, 110, 9225?9230
[5]    I. Lopez-Duarte, T. T. Vu, M. A. Izquierdo, J. A. Bull, M. K. Kuimova. Chem. Commun., 2014, DOI: 10.1039/C3CC47530A