Biomedical Engineering Reference
In-Depth Information
Except for their low cost and simplicity, sensors allow continuous monitoring of key
analytes, which is important in certain cases of clinical, industrial, and environmental
analysis. Besides that, modern clinical diagnostics requires analysis of blood or tissue
liquids directly in the target organ, because a conventional procedure, which includes
cutting a piece of tissue and delivering it to an analytical instrument, in some cases
becomes non-informational due to decomposition of certain key metabolites. Obviously,
such clinical analysis can be realized only with chemical or biological sensors.
To provide its successful operation in real samples one has to address suffi cient
selectivity of chemical or biological sensors. That's why the use of common electro-
catalysts like platinum, which requires membrane technologies to shield selectively
electrode surface, is not convenient. On the contrary, the use of inert electrode supports
modifi ed with certain monomers or polymers may result in highly selective and even
more sensitive chemical sensors. The use of a biological recognition element, which
distinguishes biological sensors, already provides high selectivity. However, to pro-
vide biosensor operation a successful coupling of biological and electrode reactions is
required, and some of the selective chemical sensors can serve as suitable transducers.
Among the variety of materials used for electrode modifi cation the electroactive
organic and inorganic polymers seem to be the most prominant ones. In this chapter
the electroactive polycrystals of transition metals, hexacyanoferrates, will be discussed
for the development of chemical and biological sensors.
13.2 PROPERTIES OF TRANSITION METAL
HEXACYANOFERRATES
Prussian blue, or ferric hexacyanoferrate, is defi nitely one of the most ancient coor-
dination materials. The earliest announcements were from the very beginning of
the eighteenth century [1, 2]. However, a quite recent investigation by Neff [3], that
Prussian blue forms electroactive layers after electrochemical or chemical deposition
onto the electrode surface, has opened a new area in fundamental investigation of this
unique inorganic polycrystal.
13.2.1 Structure of transition metal hexacyanoferrates
The fact that Prussian blue is indeed ferric ferrocyanide (Fe
4
III
[Fe
II
(CN)
6
]
3
) with
iron(III) atom coordinated to nitrogen and iron(II) atom coordinated to carbon has
been established by spectroscopic investigations [4]. Prussian blue can be synthesized
chemically by the mixing of ferric (ferrous) and hexacyanoferrate ions with different
oxidation state of iron atoms: either Fe
3
[Fe
III
(CN)
6
]
3
.
After mixing, an immediate formation of the dark blue colloid is observed. However,
the mixed solutions of ferric (ferrous) and hexacyanoferrate ions with the same oxida-
tion state of iron atoms are apparently stable.
The crystalline structure of Prussian blue was fi rst discussed by Keggin and Miles on
the basis of powder diffraction patterns [5] and then has been determined more precisely
[Fe
II
(CN)
6
]
4
or Fe
2
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