Characterization Techniques of Living Cells Encapsulated with Nanomaterials - Cell Surface Engineering: Fabrication of Functional Nanoshells - page 95

media. The nature of this electrical double determines the stability of the
colloidal particles in the liquid media. The higher the zeta potential the
higher the stability of the particles in the suspension. The zeta potential can
be measured from the mobility of particles by monitoring the frequency or
phase shift of a laser beam impinging onto colloidal suspension as particles
move around using Smoluchowski theory. 30 The solvent viscosity used to
prepare the suspension and dielectric permittivity should be known for the
calculation of zeta potential.
The change in surface potential of B. subtilis spores was monitored as their
surface was modified with a cationic and anionic polyelectrolyte. 31 The na-
tive B. substilis spores had a negative surface potential (

d n 8 y 4 n g | 0

17 mV) in water
at pH 6.8. When the spore surface was coated with poly(dimethyldiallyl
ammonium chloride) (PDDA, a positively charged polyelectrolyte, the zeta
potential shifted to about 20 mV. When these PDDA-coated spores were
coated with sodium poly(styrene sulfonate) (PSS a negatively charged poly-
electrolyte), the zeta potential shifted to about 65 mV. The periodic shift
between negative and positive layers continues as the number of polyelec-
trolyte added onto the top of each other.
A quartz crystal microbalance (QCM) is an instrument that is composed of
an electrode sensitive to small mass changes. The working principle of a
QCM is explained with the piezoelectric effect. Certain materials such as
quartz crystal can generate potential upon application of a physical strain on
them. In a similar way, an applied voltage can cause physical strain on the
material. The alternative potential application results with the oscillation of
the quartz with a certain frequency. When a material adheres to the quartz
electrode surface, it causes a shift in the frequency, which can be monitored.
Note that similar to other transducers used in sensing, a receptor phase
or molecular structure should be placed for selective capturing target mol-
ecule or molecular structure since QCM responds unselectively to all mass
changes.
The use of QCM was first demonstrated by Sauerbrey. 32 The earlier ap-
plications of QCMs were to monitor the thickness of the material deposited
onto the quartz surface in air. However, it was later realized that the QCMs
can be used in liquid media. 33 In the following years, several reports dem-
onstrated the applicability of QCMs in biological sensing applications and
even in nanomedicine more recently. 34-36 In living-cell modification appli-
cations, positively or negatively charged polymeric materials are simul-
taneously deposited onto the cell surface. When integration of a
nanomaterial into the cell surface is desired, it is sandwiched in between the
polymeric layers. The thickness of the layer during a layer-by-layer de-
position process can be monitored with a QCM. There are several reports
employing QCM for the estimation of film thickness on a cell surface. 37-39
Note that the material is deposited onto the QCM surface not to the cell
surface and it is used to estimate the material thickness onto the cell surface.
However, mimicking the cell surface may help to obtain more realistic
results.

B

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