Critical considerations in determining the surface charge of small extracellular vesicles

Small extracellular vesicles (EVs) have emerged as a focal point of EV research due to their significant role in a wide range of physiological and pathological processes within living systems. However, uncertainties about the nature of these vesicles have added considerable complexity to the already difficult task of developing EV-based diagnostics and therapeutics. Whereas small EVs have been shown to be negatively charged, their surface charge has not yet been properly quantified. This gap in knowledge has made it challenging to fully understand the nature of these particles and the way they interact with one another, and with other biological structures like cells. Most published studies have evaluated EV charge by focusing on zeta potential calculated using classical theoretical approaches. However, these approaches tend to underestimate zeta potential at the nanoscale. Moreover, zeta potential alone cannot provide a complete picture of the electrical properties of small EVs since it ignores the effect of ions that bind tightly to the surface of these particles. The absence of validated methods to accurately estimate the actual surface charge (electrical valence) and determine the zeta potential of EVs is a significant knowledge gap, as it limits the development of effective label-free methods for EV isolation and detection.

In this study, for the first time, University of Calgary researchers show how the electrical charge of small EVs can be more accurately determined by accounting for the impact of tightly bound ions. This was accomplished by measuring the electrophoretic mobility of EVs, and then analytically correlating the measured values to their charge in the form of zeta potential and electrical valence. In contrast to the currently used theoretical expressions, the employed analytical method in this study enabled a more accurate estimation of EV surface charge, which will facilitate the development of EV-based diagnostic and therapeutic applications.

Distribution of electric potential in an electrolyte surrounding
a charged particle including small EVs

The electric potential (ψ) decays away from the charged surface as a function of distance due to the formation of the electric double layer (EDL) which is composed of two parallel layers of ions surrounding a charged object: Stern layer and diffuse layer. The thickness of the EDL is usually given at a distance termed as Debye length (λ), where the electric potential decreases in magnitude by e⁻¹ of its maximum value (ψ₀). The Stern plane is the interface separating the ions tightly associated with the particle surface (ions within the Stern layer) from the ions within the diffuse layer. The slip plane is the interface between the mobile phase of the surrounding electrolyte and the static phase of the electrolyte remaining attached to the particle, where the relative velocity of electrolyte to the particle is zero.