16 R&DMagazine April 2014 www.rdmag.com
Characterizing Liposomes for
Researchers examine the application and usefulness of nanoparticle tracking analysis and
light scattering techniques in the characterization of liposomes used as drug delivery vehicles.
When considering potential drug delivery vehicles, liposomes are an important option and have already been approved for use with a number of therapeutic formulations. Liposomes
are comprised of phospholipids and may be
single- or multi-layered, can be produced in
different sizes and have a hydrophilic interior
and hydrophobic shell. They are biodegradable and essentially non-toxic and, importantly, are capable of encapsulating both hydrophilic and hydrophobic materials. In addition,
the surface of the liposome can be modified in
order to enable targeting of drugs at specific
biological sites, promote longevity of the liposome in vivo or engineer a diagnostic tool.
As with all such developments, it’s essential
to ensure that the physical properties of the
liposomes are suitable for their application.
How will they behave once they are in the body,
and are they stable enough to ensure the drug is
delivered to the required site? Are the particles
sized correctly for clinical applications, or will
they be lost from the bloodstream?
Knowing the size, concentration and zeta
potential of a liposome preparation can help
predict its fate in vivo, while the association
of charged liposomes with oppositely charged
molecules can be monitored by measuring the
zeta potential of the resulting complex. These
factors can have a marked effect on the effi-
ciency of drug delivery, and their analysis can
assist formulators when considering a specific
liposome as a suitable vehicle. Reliable ana-
lytical systems that provide robust data make
an important contribution to the formulation
process. Nanoparticle tracking analysis and
dynamic light scattering tech-
niques are two approaches that
provide essential and comple-
Nanoparticle tracking analysis
(NTA) uses laser light scattering
to examine nanoparticles in
solution (Figure 1). It enables
the visualization of individual
particles and tracking of their
Brownian motion so that size
distributions, based on individual particles, are built up in a
matter of seconds.
Light scattered by the parti-
cles in solution is captured using
a scientific digital camera and the motion of
each particle is tracked from frame to frame by
the instrument software (Figure 2).
This rate of particle movement is related to
a sphere equivalent hydrodynamic radius as
calculated through the Stokes-Einstein equation
(Figure 3). The technique calculates particle size
on a particle-by-particle basis and, because video
clips form the basis of the analysis, it’s possible to
accurately characterize real-time events.
Since NTA technology allows the visual-
ization of nanoparticles simultaneously but
separately, it’s possible to obtain additional
information. One such possibility is measure-
ment of the relative light scattering intensity
of a particle. The resulting data can be plotted
against the independently obtained mea-
surement of particle size, allowing particles
of different refractive index (RI) or material
composition to be resolved in even greater
detail. This unique ability potentially allows
the user to probe whether nanoscale drug
delivery structures such as liposomes vary in
their contents: Empty liposomes may have a
lower RI (light scattering power) than those
loaded with a higher RI material. Such differ-
ences could allow their discrimination even if
they were of very similar size.
This single particle detection system also allows
the measurement of particle concentration.
Figure 1: NTA measurement set up. Images: Malvern Instruments
Figure 2: Particles are seen as light points mov-
ing under Brownian motion.