Measuring Density and Viscosity of Samples as an Improvement of Determination of Hydrodynamic Diameter of Extracellular Particles by Interferometric Light Microscopy

We present technological advancement of the method for assessment of the hydrodynamic
diameter Dh of nanoparticles suspended in liquid, by interferometric light microscopy
(ILM). In ILM, Dh is determined by Einstein-Stokes equation which includes dynamic
viscosity η of the sample as an input. Hitherto, η was estimated by the respective
value for water at given temperature. It was of our interest to see how the results obtained
by direct measurement of η differ from this estimate. We measured the density of the
sample by oscillating U-tube Density Meter and used this value as an input to measure η by
a rolling and falling ball Micro viscometer. We determined Dh by interferometric light
microscope. An overestimation of Dh by using the “water” estimation was found to range
from 1% to 7%. Furthermore, a significant difference was observed between Dh obtained
by the respective data (t = 25.985; p < 0.001). The percentage difference between the two
methods of determining Dh showed a moderate and statistically significant negative correlation
with plasma density (Spearman coefficient ρ = -0.365, p < 0.001) and a very strong
and significant negative correlation with dynamic viscosity (ρ = -0.968, p < 0.001). ) The
inherently negative Spearman coefficients demonstrate that the overestimation error introduced
by the measurement using water dynamic viscosity increases significantly with
increase of the density and dynamic viscosity of the sample. The correction method bysed
on direct measurement of η offers a more accurate, scientifically sound measurement of
Dh of extracellular particles derived from different biological matrices (e.g., plants, microorganism
cultures, bodily fluids, etc.).