Biomedical Engineering Reference
In-Depth Information
photobleaching, and triplet states. In practice, the prepared nanoparticles are polydis-
persed not only in size but also in brightness. Thus, a multicomponent fitting procedure
is commonly performed. Such fitting algorithms are provided by most commercial
fCS systems.
Conceptually, the fCS approach is similar to a more common DlS technique
for measuring the size of the nanoparticles; however, fCS is much more sensitive.
When analyzing the particles of sizes less than 10 nm using DlS, the uncertainty
of a signal becomes large, and the interference from other particles in solution
makes it difficult to obtain accurate results [95]. Besides, DlS requires relatively
high sample concentrations (μM-mM) and large volumes (ml) compared to fSC
(concentration pM-nM, volume < fl (femtoliter)). The drawback of fCS is that the
method requires highly photostable fluorophores; otherwise, the system considers
bleached fluorophores as having “left the confocal volume” resulting in a lower
size measurement.
Using fCS, Balaji
et al
. measured the hydrodynamic radius of gold nanorods that
have weak fluorescence but are resistant to photobleaching [96]. fCS has been
utilized to reveal the mechanism of nanorod growth [97] and the size of photolumi-
nescent carbon dots [98]. fCS has also been applied to measure the size and shape of
reverse micelles loaded with Cy3, which have revealed prolate ellipsoid-shaped
particles [99].
6.8 miscellaNeous methods
6.8.1 electrophoresis
Electrophoretic techniques separate charged molecules and nanoparticles under an
applied uniform electric field where they migrate toward the electrode with opposite
polarity. The rate of this migration (mobility
ν
) is linearly proportional to the applied
electric field
E
and electrophoretic mobility
μ
e
:
ν=µ
e
E
where
E
is an applied electric field and
μ
e
is the electrophoretic mobility proportional
to the particle's charge and inversely proportional to particle's size and shape.
Quite commonly found in biological and biochemical research, electrophoretic
techniques have only recently (in the last decade) been adopted by nanoparticle
researchers. In gel electrophoresis, for example, particles with different sizes, charges,
and even shapes can be distinguished due to distinct bands resulting from different
migration mobilities. an illustration of this method for several optical nanoparticles
is shown in figure 6.25. Hence, a great number of publications selected gel
electrophoresis for routine characterization of gold nanoparticles [100], quantum
dots [101], polymeric nanocapsules [40], and viral nanoparticles [102].
one common application of gel electrophoresis is to control nanoparticle syn-
thesis in order to evaluate the conjugation efficiency of a targeting moiety. Method
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