Biomedical Engineering Reference
In-Depth Information
To provide electron transfer pathways between the enzyme active site and the
electrode one can use soluble or immobilized artifi cial mediators. This approach is
realized in the so-called second-generation biosensors [146]. The disadvantage of the
oxidase-based mediator systems results from the high oxidation ability of the mediator
required for successful competition with oxygen, the natural oxidant. To oxidize the
mediator a high electrode potential is required, which causes a problem with reduct-
ants (see above). In addition, being rather strong oxidants, the mediators exist in their
reduced states, and to bring into an active form it is necessary to oxidize them. Thus,
the signal of second-generation biosensors is due to mediator reoxidation, which obvi-
ously makes it impossible to access low detection limits. Moreover, some oxidases are
reported not to react with commonly used artifi cial mediators.
Application of transition metal hexacyanoferrates for development of biosensors
was fi rst announced by our group in 1994 [118]. The goal was to substitute platinum
as the most commonly used hydrogen peroxide transducer for Prussian blue-modifi ed
electrode. The enzyme glucose oxidase was immobilized on the top of the transducer
in the polymer (Nafi on) membrane. The resulting biosensor showed advantageous
characteristics of both sensitivity and selectivity in the presence of commonly tested
reductants, such as ascorbate and paracetamol.
Another approach for development of Prussian blue-based biosensors was published
in 1995 and involved enzyme immobilization by entrapment into Prussian blue fi lms
during its deposition [147]. However, as was mentioned, the best media for deposi-
tion of Prussian blue is 0.1 M HCl, which is not acceptable for enzymes, in particular
for glucose oxidase [148]. Moreover, the entrapment of the enzyme in metal hexacy-
anoferrates during their deposition does not provide enough enzyme activity of the
resulting fi lm, which results in a rather low sensitivity of the resulting biosensor [149]
compared to the sensitivity of the corresponding H 2 O 2 transducer [150].
13.5.2 Biosensors based on transition metal hexacyanoferrates
Except for Prussian blue activity in hydrogen peroxide, reduction has been shown for
a number of transition metal hexacyanoferrates. The latter were cobalt [151], nickel
[152], chromium [150], titanium [153], copper [154], manganese [33], and vanadium
[28] hexacyanoferrates. However, as was shown in review [117], catalytic activity of
the mentioned inorganic materials in H 2 O 2 reduction is either very low, or is provided
by impurities of Prussian blue in the material. Nevertheless, a number of biosensors
based on different transition metal hexacyanoferrates have been developed.
Metal hexacyanoferrates-based biosensors were developed for analysis of glucose
[11, 114, 118, 127, 147, 149, 152, 155-166], ethanol [11], D-alanine [147], oxalate
[167-169], cholesterol [170, 171], glutamate [114, 119], sucrose [172], and choline
[163]. Among the transducers used Prussian blue undoubtedly dominates especially
if one takes into account that instead of both chromium and cobalt hexacyanoferrates
the activity of the transducers in publications [149, 159, 167, 168] was most probably
provided by Prussian blue [117]. The sensitivity of cupric hexacyanoferrate is several
orders of magnitude lower compared to Prussian blue. However, chemically synthesized
Search WWH ::




Custom Search