The surface characteristics of colloidal particles have a significant effect on their behavior and stability in vivo. The surface charge on SLNs is usually due to the presence of ionic surfactants and/or surfactant-dispersion medium interactions, but can also be an intrinsic charge, particularly if the lipid used is an acid.
In any case, it is not possible to measure surface potentials directly, however, in particle interactions, the potential associated with close contact can be measured. This potential, the so-called zeta potential, is both correlated and measurable and thus serves as a characteristic parameter for charge selection on nanoparticles.
The spatial distribution of ions, traditionally named the EDL around a charged surface, determines its electrical state. The EDL is a physical model consisting of two layers: a fixed layer and a diffuse layer. The fixed layer is a firmly bound layer, while the diffuse layer is distributed in solution and in contact with the charged surface.
Fig.1 Schematic representation of electrical double layer. (Shah, 2015)
The zeta potential is usually measured by laser Doppler anemometry based on the principle of Doppler shift. Frequency shifts caused by light scattering are used to determine electrophoretic mobility (μ, particle velocity/strength of electric field). The zeta potential is usually derived from the electrophoretic mobility of the Helmholtz-Smoluchowski equation.
where ε is the permittivity and η is the viscosity of the dispersion medium. More complex models include the Henry equation and the Wiersema method, which are more suitable for nanoparticles but less frequently used. Therefore, zeta potential measurements for nanoparticle formulations should be qualitative rather than quantitative.
The zeta potential is affected by pH, ionic strength, and the types of ions in the dispersion medium. Zeta potential measurements on diluted samples are generally required to avoid multiple scattering effects.
Usually, lipid nanoparticles carry a negative charge developed by the surfactant system used in its stabilization. However, cationic lipid nanoparticles have also been prepared which find application in DNA and gene delivery. Zeta potential measurements have been undertaken to study the effect of electrolyte and pH on the stability of SLNs. The zeta potential of SLNs can be increased by adding surfactants and co-solvents, and decreased by sterilization and freeze-drying. In addition, the presence of cryoprotectants also affects the zeta potential of sentinel lymph nodes.
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