Nanoparticles such as extracellular vesicles (EVs), liposomes, and lipid nanoparticles (LNPs) hold great potential for advancing medicine. To ensure their efficacy and safety in medical applications, thorough characterisation of these nanoparticles is essential. One characterisation measurement you may be overlooking – or measuring inaccurately – is zeta potential.
Understanding zeta potential and its role in colloidal system stability
Zeta potential provides valuable insights into nanoparticle stability, as it reflects the surface charge of particles and their interaction with the surrounding medium. More specifically, zeta potential is a measure of the electrical potential at the slipping plane of a particle in a colloidal system (Figure 1).
A colloidal system occurs when a substance is dispersed throughout a continuous “dispersion” medium.2 Stability is crucial when it comes to the application of colloidal systems, as attraction between colloidal particles can induce particle coagulation and flocculation. One way to prevent this particle aggregation is to incorporate high-resolution measurements of zeta potential into your development.
When a colloidal system is stable, all particles are evenly distributed through the dispersion medium. However, electrostatic and steric effects can lead to fluctuations in this system, leading to aggregation and ultimately, phase separation where the dispersed particles coagulate and settle out of the dispersion medium. For therapeutic purposes, it is important to maintain a stable colloidal system where the particles are evenly dispersed. Measuring zeta potential can help predict the stability of a colloidal system.
Zeta potential can be measured on a uniquely single-particle basis using tunable resistive pulse sensing (TRPS). Using the Exoid, our high-resolution TRPS instrument, you can simultaneously collect size and zeta potential measurements in a particle-by-particle manner. It is important to note that not all analytical methods used to measure zeta potential offer this single-particle resolution and subsequent precision.
The role of zeta potential in nanoparticle stability and implications for therapeutics
Obtaining the zeta potential of a particle provides insights into surface charge and stability. Stable nanoparticles maintain their size, shape, and dispersion over time, while unstable particles tend to aggregate and precipitate. Zeta potential plays a pivotal role in determining this stability. Large zeta potential values, whether positive or negative, suggest strong electrostatic repulsion between particles, leading to a stable dispersion. Conversely, low zeta potential values indicate weak repulsion, increasing the likelihood of aggregation.
In the context of therapeutic development, determining the zeta potential of nanoparticles is a crucial aspect of optimisation and meeting regulatory compliance.
Optimising extracellular vesicles for therapeutic efficacy
Extracellular vesicles (EVs) are promising candidates for both non-invasive diagnostics and targeted therapeutics.3 As has been shown for other particle types, zeta potential measurements are important for understanding EV stability and their therapeutic potential. By measuring zeta potential of EVs prepared under different conditions, researchers can optimise EV preparations for enhanced stability and functionality, such as improved cellular interactions, thereby making them more effective in clinical applications.
Studies have shown that EVs with optimised zeta potential are less likely to aggregate, thereby enhancing their delivery efficiency and interaction with target cells.4 The insights gained from zeta potential measurements helps in the journey of pursuing EVs as drug delivery vectors which can encapsulate and deliver therapeutic agents to target cells. Here, having strong zeta potential contributes towards a stable shelf-life. A stable formulation will have higher bioavailability than one that is aggregating, allowing the therapeutic to deliver its cargo effectively.5,6 Additionally, zeta potential measurements can be used as a quality control metric during the production and purification of EVs. This ensures that EVs are of consistent quality, with minimal batch-to-batch variation. Therefore, measuring zeta potential is an essential step in optimising and regulating EV formulations for nanomedicine.
Ensuring vaccine stability and shelf life
When it comes to vaccine stability and shelf life, determining zeta potential can provide valuable insights. A large absolute zeta potential indicates that the particles will strongly repel one another, leading to a lower risk of aggregation and maintaining the vaccine's efficacy over time. Vaccine formulations can also be optimised with the help of zeta potential, enabling them to remain stable under various conditions. This is crucial for transport and storage, as it can take months before a vaccine is used.
A closer look at lipid nanoparticles (LNPs) can demonstrate the value of measuring zeta potential for stability purposes. LNPs are particles that contain a lipid core and a surfactant outer layer. They are often used as drug vectors and came into the spotlight when they were used as mRNA-based vaccines against SARS-CoV-2. However, challenges surrounding their transport and storage stability remain an active area of investigation. In one in-house study, we measured the zeta potential of empty LNPs and mRNA-loaded LNPs using the Exoid, our high-resolution instrument for particle characterisation. The empty LNPs had a zeta potential (mode ± SD) of –10 ± 1.1 mV, while mRNA-loaded LNPs had a zeta potential of –17 ± 1.1 mV. This could be due to the loading of the negatively charged mRNA, but what is clear is that the mRNA-loaded LNPs had a stronger zeta potential than the empty LNPs. This finding highlights the capability of the Exoid and its ability to measure zeta potential to a high degree of resolution, allowing meaningful sample comparisons.
Zeta potential and considerations for gene therapy
In addition to aiding vaccine and EV-based R&D, zeta potential has a role to play in the design of gene delivery vectors, helping ensure they encapsulate and protect genetic material from degradation effectively. Stable nanoparticles with higher zeta potential will enable more efficient delivery to target cells, which is beneficial for enhancing gene therapy outcomes.
Particle charge also influences how gene therapy nanoparticles interact with the membrane of target cells; for example, positively charged nanoparticles can facilitate binding and uptake by negatively charged cell membranes, improving transfection efficiency.7 Finding the right balance is crucial though, as excessive positive charge can lead to cytotoxicity.
Thorough zeta potential characterisation can help minimise non-specific interactions with blood components and off-target cells, reducing side effects and improving the specificity of gene delivery. By optimising zeta potential, nanoparticles can be engineered to target specific cell types, enhancing the precision of gene therapy.
Zeta potential as an invaluable tool in the field of nanomedicine
Measuring zeta potential is essential for ensuring the stability, efficacy, and safety of biological nanoparticles in both therapeutics and diagnostics. By understanding and optimising zeta potential, researchers can improve nanoparticle performance, meet regulatory requirements, and advance the development of innovative medical treatments. The Exoid provides a reliable and accurate platform for zeta potential measurement, making it an invaluable tool for researchers in the field of nanomedicine.
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