Biomedical Engineering Reference
In-Depth Information
frequency (9
10
6
Hz),
q
is the shear modulus of the quartz (2.947
10
11
g
cm
1
s
2
),
q
is the density of the quartz (2.648 g
cm
3
), and
A
is the electrode area (0.16 cm
2
).
2
F
0
2
m
A
(8.1)
F
(
q
q
)
1/2
By taking into account the characteristics of quartz resonators, the adsorbed mass can
be calculated from the frequency shift utilizing Eq. (8.2), which indicates that 1 Hz fre-
quency shift corresponds to a mass increase of about 0.9 ng.
m
A
F
1.83
10
8
(8.2)
The QCM data was also used to estimate the thickness,
d
(cm), of multilayer films by
means of Eq. (8.3), where
is the density of adsorbed materials (1.2 g cm
3
for polyelec-
trolytes and 1.3 g cm
3
for enzymes) [4] and
A
is the area of the QCM electrode. As
adsorption occurred on both sides of QCM electrode, total mass increase should be
divided by 2 [16].
m
2
A
d
(8.3)
When the assembly of multilayer structure was accompanied by alternate adsorption in
oppositely charged polyelectrolyte and enzyme solutions, for example at pH 7.0, PEI and
PEI-Fc are positively charged, and Alg-Na, DI, and G6PDH are negatively charged. The mul-
tilayer formation of the LBL method was investigated by QCM. A gold electrode modified
with 3-MPA self-assembled monolayer (SAM) was alternately adsorbed through sequential
immersion in PEI/Alg-Na, PEI-Fc/Alg-Na, PEI/DI, and PEI/G6PDH solution. The final
assembly of the multilayer film was referred to as Au/(PEI/Alg-Na)
2
(PEI-Fc/Alg-Na)
2
(PEI-
DI)
2
(PEI-G6PDH)
2
. After each adsorption step, the frequency of the Au electrode was meas-
ured and frequency shifts were recorded (Figure 8.2). A negative linear progression of
frequency shift was found to be dependent on adsorption cycles between four pairs of oppo-
sitely charged polyelectrolytes and enzymes, which indicated that multilayer assembly
could be successively carried out by the LBL method. From Eq. (8.3) and QCM data, the
5000
4000
3000
FIGURE 8.2
Frequency shift (-
F
) due to cycles of alter-
nate adsorption in PEI/Alg-Na (
2000
), PEI-
Fc/Alg-Na
(
),
PEI/DI
(
),
and
PEI/G6PDH (
) solutions. (From Zheng,
H., Okada, H., Nojima, S., Suye, S., Hori, T.
(2004) Layer-by-layer assembly of enzymes
and polymerized mediator on electrode sur-
face by electrostatic adsorption.
Sci. Technol.
Adv. Mater.
5:371-376.)
1000
0
0
5
10
15
Adsorption cycle
