While the sites of action of liposome-cell interactions are largely determined by size, knowing the zeta potential of a liposome preparation
can help predict the fate of the liposomes in vivo. The zeta potential of
a particle is the overall charge that the particle acquires in a particular
medium. In gene therapy for instance, zeta potential measurements can
be used to optimize the ratio for particular liposomes with various DNA
plasmids in order to minimize aggregation (Figure 4).
The well-established technique of dynamic light scattering (DLS) is
widely used in the characterization of liposomes and, since zeta potential
is also such a vitally important parameter, analytical systems, such as the
Zetasizer Nano from Malvern Instruments, that offer the measurement
of both, are finding widespread application. While DLS is used to measure particle size, the technique of laser doppler microelectrophoresis is
used for zeta potential measurement.
Once again, light scattering resulting from the Brownian motion of the
particles is at the heart of the DLS technique. DLS measures the time-de-pendent fluctuations in the intensity of the scattered light and determines
the diffusion coefficients of the particles. Again using the Stokes-Einstein
equation, this information is converted into a particle size distribution.
Using laser doppler microelectrophoresis to measure zeta potential,
an electric field is applied to a solution of molecules or a dispersion of
particles, which then move with a velocity related to their zeta potential.
Measurement of this velocity enables the calculation of electrophoretic
mobility and from this the zeta potential and zeta potential distribution.
The physical characterization of liposomes is of great importance in
understanding their suitability for a range of applications with rapid,
repeatable characterization, an important consideration as part of the
development and quality control processes. The techniques described
above provide complementary information about the size, concentration
and zeta potential of liposome preparations.
Senior Application Scientist
and Mike Kaszuba,
Technical Support Manager
Figure 4: Cationic liposomes (positively charged) are complexed with DNA
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