Biomedical Engineering Reference
In-Depth Information
is rather hard to distinguish between the alkali metal ions and ammonium ion. Indeed,
the same metal hexacyanoferrates were used in some cases as potassium, cesium or
rubidium sensors, and in other cases as ammonium sensors (Table 13.1).
Particular cases are potassium selective potentiometric sensors based on cobalt [41]
and nickel [38, 42] hexacyanoferrates. As mentioned, these hexacyanoferrates possess
quite satisfactory redox activity with sodium as counter-cation [18]. According to the
two possible mechanisms of such redox activity (either sodium ions penetrate the lattice
or charge compensation occurs due to entrapment of anions) there is no thermodynamic
background for selectivity of these sensors. In these cases electroactive fi lms seem to
operate as “smart materials” similar to conductive polymers in electronic noses.
Except for sensor applications, the intercalation of alkali metal ions in metal hex-
acyanoferrates was used for adsorption and separation of cesium ions from different
aqueous solutions with Prussian blue [43, 44] and cupric hexacyanoferrate [45, 46].
13.3.2 Amperometric sensors for electroactive compounds
Whereas detection of electroinactive ions was principally worked out at the end of last
century, the use of transition metal hexacyanoferrates as sensors for various electroactive
compounds still attracts particular interest of scientists. Although the cross-selectivity of
such compounds must be low, a number of them have been successfully used for analy-
sis of real objects.
Electrocatalysis in oxidation has apparently fi rst been shown for ascorbic acid oxi-
dation by Prussian blue [60] and later by nickel hexacyanoferrate [61]. More valua-
ble for analytical applications was the discovery in the early 1990s of the oxidation
of sulfi te [62] and thiosulfate [18, 63] at nickel [62, 63] and also ferric, indium, and
cobalt [18] hexacyanoferrates. More recently electrocatalytic activity in thiosulfate
oxidation was shown also for zinc [23] hexacyanoferrate. Prussian blue-modifi ed elec-
trodes allowed sulfi te determination in wine products [64], which is important for the
wine industry.
A particular interest for clinical applications was a possibility for detection of
dopamine by its oxidation on nickel [19], cobalt [65], and osmium [66] hexacyanofer-
ates. Except for oxidation of dopamine, cobalt and osmium hexacyanoferrates were
active in oxidation of epinephrine and norepinephrine. For clinical analysis it is also
important to carry out the detection of morphine on cobalt [67] and ferric [68] hexa-
cyanoferrates, as well as the detection of oxidizable amino acids (cystein, methionine)
by manganous [69] and ruthenium [70] hexacyanoferrate-modifi ed electrodes. In gen-
eral, oxidation of thiols was fi rst shown for Prussian blue [71] and nickel hexacyano-
ferrate [72]. This approach has been used for the detection of thiols in rat striatum
microdialysate [73]. Alternatively, the detection of thiocholine with Prussian blue was
employed for pesticide determination in acetylcholine-esterase test [74].
Nitric oxide (NO) and nitrite were found to be oxidized by Prussian blue and indium
hexacyanoferrate-modifi ed electrodes [75-77]. For pharmaceutical application oxida-
tion of isoprenaline [78] and vitamin B-6 [79] at cupric hexacyanoferrate-modifi ed
electrodes was shown.
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