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%\subsection{Radius measurement from anisotropy}  It is also possible to measure the hydrodynamic radius of a molecule by time-resolved Time-resolved  fluorescence anisotropy measurements, which measurements  canhelp  determine the molecule's rotational mobility which depends on the  molecular volume, mobility, rigidity, shape, volume  and the molecule's viscosity of the environment  surrounding environment.\cite{Lakowicz2006} the molecule.\cite{Lakowicz2006}  The sample solution is excited with a pulse of polarised light, and the fluorescence is collected in parallel and perpendicular polarisation directions as a function of time. The anisotropy $r(t)$ of a molecule undergoing rotational diffusion in the solution can be obtained from the measured intensities $I_\parallel$ and $I_\perp$ by \begin{equation}  r(t)= \frac{I(t)_\parallel-GI(t)_\perp}{I(t)_\parallel+2GI(t)_\perp} \label{eq:anisotropy}   \end{equation}  where $G$ is a correction factor that compensates for different transmission and detection efficiencies in the parallel and the perpendicular directions. directions.\cite{Suhling2014}  If the sample solution contains spherical molecules of homogeneous size, the anisotropy decay follows a single-exponential function \begin{equation}  r(t) = r_0\cdot e^{-\frac{t}{\phi}} \label{eq:1expfit}  \end{equation}  where $r_0$ is the initial anisotropy at $t=0$ and $\phi$ is the rotational correlation time. If the rotating unit is not spherical, a more complex multi-exponential  model is required. required.\cite{Phillips1984}  $\phi$ can thus be obtained by fitting eq~\ref{eq:1expfit} (or the more complex model) to the experimental anisotropy decay. $\phi$ is related to the volume $V$, and thus the effective radius, of the rotating molecule by the Stokes-Einstein-Debye equation \cite{VanHolde1998}  \begin{equation}