Measure the Zeta Potential of Individual Particles with TRPS

In a mixture, it is common to assume that all of the particles have the same zeta potential but this is often not the case. When particles have polydisperse size and surface charge distributions, the most widely used technique (Laser doppler velocimetry, PALS) to measure the zeta potential becomes quite unreliable. The solution is to individually measure the electrophoretic mobility of a large enough number of single particles, which TRPS does very well. The electrophoretic mobility is converted to a zeta potential via their linear relationship. This results in zeta potential data that is unbiased by the size distribution and can be provided as a size vs charge plot, a concentration vs charge plot, or a 3d concentration vs charge and size. That level of information is essential for sophisticated applications like nanomedicine development and nanomedicine QA. It can be used to further identify surface biomolecular properties or biomolecular activity.

Phase analysis light scattering (PALS), an ensemble technique based on laser doppler velocimetry, is the best known technology for the determination of zeta potential of nanoparticle suspensions. Although readily available, ensemble techniques (e.g. PALS), as opposed to single particle measurement techniques, such as TRPS, can only measure and calculate the average particle mobility and hence detailed single particle information is lost, in particular when measuring polydisperse samples. TRPS is the only available technology that provides simultaneous in-suspension information about particle size and zeta potential on a particle-by-particle basis, guaranteeing accurate analysis of multi-modal and polydisperse samples (see figure). Whilst a mixed sample of bare and carboxylated polystyrene spheres with equivalent sizes (~400 nm) but different zeta potentials was perfectly resolved with TRPS, the two particle populations could not be distinguished with PALS.

Particle dispersions and formulations are stabilized by electrostatic repulsion, steric hindrance, or a combination of these two forces. Particles will eventually aggregate in the absence of sufficient stabilisation. Researchers use zeta potential as an indicator of electrostatic stabilisation of particles. Zeta potential is a modelled quantity derived by measuring electrophoretic mobility of particles in suspension. Electrophoretic mobility is critically dependent on particle and solution properties (ionic strength, ionic composition, and viscosity). Thus, it’s important to perform zeta analysis for nanoparticles or nano-formulations in physiological buffer(s) designed for biological applications.

High-resolution single particle zeta analysis will provide an advanced understanding of particle behaviour in different pH and salt conditions and aid in monitoring particle corona evolution over time for biokinetic studies. Moreover, determination of accurate charge on each particle could provide potential insights into nano-bio interactions in varying physiological buffers, critical for determining particle stability and uptake; impacting the overall delivered particle dosage.

Tunable Resistive Pulse Sensing (TRPS) alone provides the means to distinguish particles through fine scale mapping of zeta potential of individual particles. The surface charge of membranous vesicles arise from the combined net charge of extrinsic moieties on the vesicle surface. Using TRPS, subtle changes in surface charge can be resolved as the membrane composition is altered.  

TRPS can also be used to monitor changes in zeta potential through particle-ligand interaction in real time. This is demonstrated below in the binding of biotinylated single stranded DNA (35 bases) to the surface of a streptavidin coated particle. The diffusion dependent binding reaction goes to completion within approximately 170 sec after the addition of DNA at 30 sec, the binding of the DNA coincides with a reduction in the zeta potential. The resolution limit was 10% of the oligo coverage. For example, the resolution limit was 44 oligos per particle for particles with a DNA/particle ratio of 400. The control reaction which uses streptavidin that has been enzymatically inactivated by proteinase K shows no DNA interaction. With longer oligos the sensitivity is increased. TRPS provides researchers with the tool for advanced charge studies of particle-particle interactions, zeta based vesicle characterisation, receptor-ligand interactions that may result in altered charge of the carrier particle, charge reporting antibody-antigen reactions and aptamer based detection technologies.