Surface charge. Surface charge is the electric charge present at an interface. There are many different processes which can lead to a surface being charged, including ad-sorption of ions, protonation/deprotonation, and the application of an external electric field. Surface charge causes a particle to emit an electric field, which causes particle repulsions and attractions, and is responsible for many colloidal properties. The surface charge of particles can be measured with capillary electrophoresis (CE) which separates the particles based on their charge. Alternatively, the zeta potential can be determined as an indicator of the surface charge. The higher the zeta potential, the stronger the repulsion, the more stable the particle is. Zeta potential is not measurable directly but it can be calculated. Various instruments are available for determining the zeta potential of dispersed nanomaterials, i.e. DLS and NTA (Table 5). It has been suggested by Xxxx et al. (2010)1 that it is essential to determine the zeta potential as a function of pH, as this allows the determination of the point of zero charge where a dispersion of engineered nanomaterials exhibits the highest propensity to aggregate. The following relationship between zeta potential and stability are identified6: 6 Xxxx, X., Xxxxxxxxxx, X., Xxxxxx, X., Xxxxxx, X., Xxxxxxxx, X., Xxxxx, V., Xxxx, X., Xxxxxxxxx, A. 2010, "Weight of evidence approach for the relative ranking of nanomaterials", Nanotoxicology, 2011 Sep;5(3):445- 58. doi: 10.3109/17435390.2010.512986 . • Nanoparticles with zeta potential at pH 7 of > +30 mV or < -30 mV have a high water stability (i.e. no aggregation over time); • Nanoparticles with zeta potential at pH 7 of between -30 mV and + 30 mV have a low water stability (i.e. tendency to aggregate over time).
Surface charge. Surface charge influences the interaction of a particle with its surroundings (including agglomeration/aggregation behaviour, its stability and interaction with other with other materials), but also depends on the environment of the MNM (for example pH). Surface charge, as represented by zeta potential, influences the fate and transport of nanoparticles. Any surface charge on nanoparticles causes electrostatic repulsion between particles or charge that can counter the tendency to agglomerate. Zeta (ζ) potential is an abbreviation for the electrokinetic potential in colloidal systems and can be related to the stability of colloidal dispersions. The zeta potential indicates the degree of repulsion between adjacent, similarly-charged particles in dispersion. For molecules and particles that are small enough, a high zeta potential will confer stability, i.e. the solution or dispersion will resist agglomeration. When the potential is low, attraction exceeds repulsion and the dispersion will break and flocculate. In nanotoxicology, zeta potential (surface charge) plays a key role in determining (1) the degree of colloidal interaction, which is itself a function of the pH and ionic strength of the bulk solution, and (2) the bioavailability of a compound when considering mass transport through charged membranes as related to exposure (Sellers et al., 2015).
Surface charge. The surface charge determines whether MNMs can be dissolved in medium or whether they form aggregates; it can also influence their biocompatibility and ability to traverse biological barriers (XxXxxx 2005).
