Biomedical Engineering Reference
In-Depth Information
The literature contains a vast variety of attempts and concepts for biosensors and
the number of publications is still increasing. Because of the impact of various
technologies the improvements can be seen in all of these components with special
emphasis on their interfaces. For example, the communication between enzymes
and an electrode in an electrochemical sensor is of huge importance for its
performance. In an amperometric detection mode electrons are measured which
corresponds to the conversion of a substrate. To enhance the amount of electrons
traveling from the enzyme to the electrode, two different methods can be chosen.
One method is the possibility of using a sophisticated connecting layer in which the
enzyme can be embedded. By adding a redox-mediator to this layer there is the
possibility for an indirect electron transfer from the enzyme over the redox-
mediator to the electrode. In a recent example, Nagel et al. showed the synthesis and
application of a redox-polymer based on poly(N-isopropylacrylamide) (PNIPAM)
with incorporated ferrocene moieties for an indirect electron transfer using NAD-
dependent glucose dehydrogenase (NAD-GDH) or pyrroloquinoline quinone-
dependent GDH (PQQ-GDH) and glucose as the analyte ) [
15
]. The authors
detected a heterogeneous electron transfer rate of 80 s
-1
. This is twice as high
compared to a normal self-assembled monolayer of a ferrocenepentanoate. Hence,
by using hydrogels with incorporated mediators such as ferrocene a more effective
electron transfer may be achieved. The other method is to modify the recognition
element itself. To describe one example here, Demin and Hall modified a glucose
oxidase (GOx) [
10
]. By different methods such as NMR spectroscopy and in silico
calculations two considerations could be revealed: (i) oligosaccharide structures on
the surface of the GOx are responsible for a larger space between the enzyme and
the electrode; (ii) the path of the electron through the GOx could be shown hence
the hemisphere of the enzyme could be determined through which the electron can
pass to the electrode. From that, a genetically modified GOx was derived and
produced bearing no oligosaccharide structures and a certain surface modification
to facilitate direct immobilization. Hence, a better and direct electron transfer from
the enzyme to the electrode could be accomplished.
This is a nice example of how the modification of biological recognition
elements can lead to improved biosensor performance for applications such as
glucose detection. Nevertheless, there is a trend to overcome the limitations of
biological recognition elements such as stability problems under harsh conditions
or batch-variations and to replace them with artificial receptor molecules. To obtain
artificial receptors besides their chemical synthesis which is in most cases tedious
and time-consuming, two approaches have been established in the last few decades.
The first is the use of artificial DNA- or RNA-molecules called aptamers which may
act as an antibody-like recognition element. For their synthesis a process called
systematic evolution of ligands by exponential enrichment (SELEX), invented
simultaneously by Gold and Szostak [
9
,
25
], is used in which the tightest binding
DNA- (or RNA-) strands are selected via a selection process. Through a generic
approach aptamers against different molecules can be generated and used in
biosensor applications [
13
]. Since the binding event is not directly linked to a signal
generation
most
applications
using
aptamers
are
combined
with
an
optical
