Biomedical Engineering Reference
In-Depth Information
after proper pretreatment. The LBL alternating assembly of oppositely charged polyelec-
trolytes provides a method to construct nanoscale films with various kinds of substances in
a preset sequence. A wide range of materials, for example polyelectrolytes [9,10], dyes
[11,12], ceramics [13,14], and others, have been used in the fabrication of multilayer struc-
tures by the LBL method, which makes it possible to incorporate substances with desired
chemical or physical properties. Because the macromolecules are adsorbed from aqueous
solution in the LBL procedure, it opens up the possibility to immobilize biomolecules such
as proteins [4,15,16] and DNA [17,18] in their active state.
The method of LBL adsorption can be easily manipulated. A concrete example of
LBL adsorption of enzymes and polymerized mediator on a carbon electrode is shown
as follows [19]. A negative or positive charge was first introduced onto a base material
such as a carbon (PG) or gold electrode. The introduction of a carboxy group to a
carbon electrode is achieved by electrochemical oxidation in nitric acid solution contain-
ing 2.5% K
2
Cr
2
O
7
. A potential of 2.2 V vs. Ag/AgCl was applied to a bare electrode dipped
into the above solution for 10 s. In the assembly of multilayer structures by the LBL
method, PEI (polyethyleneimine) was used as polycation, and glucose-6-phosphate dehy-
drogenase (G6PDH), diaphorase (DI), and sodium alginate (Alg-Na) as polyanions. The
modified electrode was first immersed in solution (5.0 mg mL
1
) for 20 min, and then thor-
oughly rinsed with water. It was then immersed in Alg-Na solution (5.0 mg mL
1
) for
another 20 min and rinsed again to construct one layer of mediator. The multilayer medi-
ator film was constructed by repeating the above procedure, and the electrode was
referred to as PG/PEI-Fc
n1
. The multilayers of DI and G6PDH were immobilized
by repeated alternate adsorption in PEI solution (1.0 mg mL
1
) and enzyme solution
(1.0 mg mL
1
) under 4°C.
8.3
Properties of Layer-by-Layer Film
Data about the structure of these multilayer assembly constructed using LBL methods
have been evaluated by various kinds of instrumental techniques, such as quartz crystal
microbalance (QCM), ultraviolet adsorption, atomic force microscopy (AFM), and
scanning electron microscopy [4,17,20-25]. It has been found that a linear mass increase
occurred with the layer number, and many factors such as salt concentration [22], adsorp-
tion time, dry conditions [21], pH [26], and charge density [27] affected the formation
of multilayer films. It has also been demonstrated that an ultrathin organized multilayer
structure could be formed utilizing the LBL procedure. The LBL procedure was
also found to allow the adsorption amount and thickness of layers to be controlled,
which made it possible to build up desired multilayer architectures with multiple
components [25].
8.3.1
Characterization of Multicomponent Films by Quartz Crystal Microbalance
Mass-controlled LBL sequential adsorption technique was analyzed using QCM to moni-
tor the immobilization procedure [6,16,22,24,28-30]. It has been proven that LBL method
is a simple and elegant method which facilitates nanoscale preparation of ultrathin films
with defined composition and uniform thickness.
The Sauerbrey equation describes the relationship between frequency shift and area
mass density for one cycle of adsorption (Eq. 8.1), where
F
0
is the fundamental resonant
