6.4.1　Particle size and its distribution

The particle size and its distribution of polymer nano-drug carriers influence their pharmacokinetics，in vivo distribution，transdermal efficiency，and residence time.The main methods to evaluate the particle size and its distribution of nanomaterials are dynamic light scattering（DLS）and scanning or transmission electron microscopy.

DLS is a technology that measures the fluctuations in the intensity of scattered light caused by the Brownian motion of particles.The average particle size can be calculated by the Stokes-Einstein equation.This technology provides repeatable overall sizes and distributions of NPs in a short time.Besides，this method measures the hydrodynamic diameters of NPs，which is not easy to be provided by scanning or transmission electron micrographs.However，there are also some limitations for the DLS technique.For example，since the intensities of scattered light is proportional to the sixth power of particle sizes，the presence of large-sized impurities such as dust in the NP solution will significantly affects the overall particle size.If the absorption of the nanomaterial coincides with the incident laser，the particle size cannot be measured.Meanwhile，for non-spherical NPs，DLS can only provide the equivalent spherical sizes.Besides，the stability of NPs，temperature，and other factors can also affect the final result.(https://www.daowen.com)

The size information of the NPs can be obtained from scanning or transmission electron microscopy as well.Generally，we need to calculate the average particle size and partide size distribution from a photograph of at least several hundreds of NPs.The advantage of these technologies is that they can provide the size for the solid part of nano-drug carrier（such as the core of a micelle），as well as the morphology of non-spherical or other structures（such as the hollow part of the vesicle），and avoid the influences of large sizes impurities.However，there are also some disadvantages.For example，these methods are timeconsuming，and they cannot provide the overall particle size information.Moreover，the morphology and size of NPs may change with the increase of concentration or during sampling.Besides，since the hydrophilic parts are generally invisible under the transmission electron microscope due to their low contrast，the particle size given by the transmission electron microscope is usually smaller than the hydrodynamic diameter given by DLS.Therefore，combining the results of DLS and electron microscopy can provide a more comprehensive characterization of the particle size and distribution information of nano-drug carriers.