The determination of electrophoretic mobility and zeta potential is crucial to understanding the behavior and stability of colloids and suspensions over time. Traditional light scattering methods that are based upon the shifted frequency spectrum are relatively insensitive and can fail when confronted with high viscosity oils, non-polar liquids, high salt concentrations or particles that are close to their isoelectric point.
The first company to develop an instrument that can easily cope under these conditions was Brookhaven Instruments Corporation. Brookhaven’s ZetaPALS system uses Phase Analysis Light Scattering (PALS) to take measurements that are 1000 times more sensitive than traditional methods without needing extremely high voltages.
Its capabilities make it highly suited to a wide variety of applications. For example, David Lynn, a postdoctoral fellow in the Chemical Engineering Department at the Massachusetts Institute of Technology, is working on the delivery and controlled release of pharmaceuticals. “We characterize a wide variety of particles, ranging from polymer/DNA complexes to micelles and polymer-based microparticles and nanoparticles,” he explained. “It was important to us that the instrument we chose could determine zeta potential without compromising the accuracy of particle sizing. The ZetaPALS is unique in being suitable for a wide range of experimental conditions such as organic solvents or the ‘high salt‘ typical of some biological buffers.”
Other researchers have seen similar advantages. Mark Davis, professor at the Department of Chemical Engineering, California Institute of Technology, explained: “We routinely take measurements in high ionic strength conditions of up to 150 mM salt and need an instrument that can determine the size and zeta potential of the new particles that we are synthesizing as non-viral gene carriers. These particles are composites of nucleic acid and polymers and the measurements are very important to our understanding of how the characteristics of these particles relate to gene delivery into cells.” Themis Matsoukas, professor at the Department of Chemical Engineering, Pennsylvania State University said: “We work with titania nanoparticles at a pH of between 0.5 and 2. These particles are formed as aggregates and we use acid to help to disperse them. However, the conductivity of the solution at this pH can be up to 2500 mS/m and it is important for us to be able to take measurements under these conditions.”
Finally, the instrument has also proved itself extremely useful in real time studies according to Professor Stefan Highsmith of the Department of Biochemistry, University of the Pacific. “We study the electrical properties of contractile proteins like actin, myosin, tubulin and members of the kinesin family. When mixed with ATP, these proteins move around and change their shape, which is reflected in their hydrodynamic size, and we want to find out what conformational changes could be responsible for this. In combination with a BI-APD avalanche photodiode detector, the ZetaPALS system gives us a ten-fold increase in sensitivity and allows us to collect data more quickly. By fitting a BI-SM50 small volume cell we can manipulate tiny quantities of samples to follow reactions in real-time, something we have been unable to do before.”