Biology Reference
In-Depth Information
length
κ
−
1
, where [46]
2
n
0
z
2
e
2
εε
0
kT
κ
=
(8.10)
in which
n
0
and
z
are the concentration and charge of ions in the
electrolyte,
e
is the charge on an electron,
ε
is the permittivity of the
medium,
ε
0
is the permittivity of free space, and
k
is Boltzmann's
constant. The overall interaction between the latex particles is then
the sum of their attractive and repulsive forces, and is described
quantitativelybyDVLOtheory[58,59].Theimportantexperimental
parameter here is the electrolyte concentration, since this does not
effect van der Waals forces but when increased causes the diffuse
layer to shrink (e.g., from
∼
300
˚
Ato3A going from 1
×
10
−
4
Mto
1 M for a 1:1 electrolyte at 25
◦
C [46]). Thus, increasing electrolyte
concentration can cause coagulation. (This should also be noted
for solution phase nanoparticles since the physical principles are
exactly the same.)
8.5.4
Layer-by-Layer Deposition: Theory
In 1966, Iler demonstrated that films of alternating positively
charged alumina fibrils and negatively charged silica colloids could
be built up on hydrophilic glass [60]. In the early 1990s, Decher and
coworkers extended this procedure to the deposition from solution
of oppositely charged polyelectrolytes [61-63]. The technique,
knownas“layer-by-layer”(l-b-l)depositionhassincebecomewidely
applied. The method is relatively simple, and as shown in Fig. 8.8,
consists of (1) derivatizing a substrate with a stable surface charge
excess, (2) immersing the substrate in a solution of an oppositely
charged polyelectrolyte (PE), (3) immersing in water to remove
weakly bound PE, and (4) immersing in a solution of a second PE,
oppositely charged to the first. Steps (2) to (4) can be repeated as
manytimesasnecessarytogivetherequiredthickness.Thereasons
for the popularity of the method are that, in addition to simplicity,
it allows us to control the resulting film thickness down to the level
of a few Angstroms, films of more than 1000 PE layers are possible,
thefilmsarephysicallystableandarepermeabletosolutionspecies,
enabling a film-confined catalyst to react with substrate. The l-b-l
