6.4.2　Zeta（ζ）potential

The charge properties of nano-drug carriers are related to the stability of materials and the interactions between the NPs and biological interfaces such as cell phagocytosis，distribution in vivo，and adsorption of protein.Therefore，it is meaningful to measure the charge properties of polymer nano-drug carriers.The charge properties of nano-drug carriers can be measured by zeta（ζ）potential.Theζpotential of a nano-drug carrier can be described by the Stern electric double layer theory（Figure 6-7）.The Stern electric double layer theory indicated that the surface of charged NPs in solution could be divided into a Stern layer and a diffuse layer.The Stern layer refers to a layer of ions with opposite charges tightly adsorbed on the surface of NPs.The diffuse layer consists of two layers—a fluid layer containing ions that are close to the Stern layer and can stably move with the Stern layer，and a dispersion layer on the outer side.The interface between the two layers is the slipping plane，where the potential is called ζ potential.Most ofζpotential meters are based on electrophoretic light scattering technology.NPs with different charge properties in the solution move at different rates under the electric field，resulting in a Doppler shift in the frequency of the scattered light，and then theζpotential can be calculated.Theζpotential of NPs varies with the surroundings of the solution，such as pH and ionic strength.Therefore these conditions should be constant during measurement.(https://www.daowen.com)