Biomedical Engineering Reference
In-Depth Information
Figure 12.3
One-eighth of the unit cell of KFe[Fe(CN)
6
] soluble Prussian blue. h e K
+
ions and the remaining interstitial or zeolitic water in the cubic sites have been omitted for
clarity from the scheme.
states of metals. h e cyanide ion can bind to metals in both terminal and
bridging M-CN-M´ modes; the bridges are commonly linear, and are pres-
ent in many polymeric metal types of cyanide [19], and in particular in PB
[20]. Depending on the specii c conditions of the preparation, several meth-
ods have typically been used to prepare these cyanide complexes. Addition of
[Fe(CN)
6
]
3-
to Fe
2+
aq
gives the deep blue complex Turnbull´s Blue (TB), while
if [Fe(CN)
6
]
4-
is added to aqueous Fe
3+
, the deep blue complex PB is produced
[14, 19]. Both PB and TB are hydrated salts of formula Fe
4
III
[Fe
II
(CN)
6
]
3
·xH
2
O
(x
14), and related to them is KFe[Fe(CN)
6
] - soluble PB [11].
As depicted in
Figure 12.3
, the zeolite-like structure possesses extended
lattices containing cubic arrangements of Fe
n+
centres linked by CN
-
bridges; each Fe
n+
(high- and low-spin) is an octahedral anionic building
block [Fe
n
(CN)
6
]
n-6
with a cubic unit cell of 10.2 Å along the Fe
III
-NC-Fe
II
-
CN-Fe
III
-sequence [13, 14].
h e selective dif usion of low molecular weight molecules (such as O
2
and H
2
O
2
) and some ions with small hydrated radius (such as Cs
+
, K
+
and
NH
4
+
) is due to its channel diameter of about 3.2 Å [11]. Consequently, the
degree of hydration, as well as the size of ion, are basic factors for the dif u-
sion of ions through the channels of the PB lattices [15].
∼
12.2.3
pH Stability and Deposition Method
h e chemical literature reports that to achieve a regular structure of
electro-deposited PB, two main factors have to be considered: the pH of
