Biomedical Engineering Reference
In-Depth Information
Currently, there is significant interest in coupling the electron transfer properties of
surface-confined ferrocenes (and other redox probes) with the molecular recognition
capabilities of designed receptors to prepare highly sensitive and selective integrated
electrochemical sensors for a broad range of analytes.
Ferrocene scaffolds offer a number of advantages in the construction of
integrated SAM-based electrochemical sensor platforms. These include (1) tunable
[
46
] and fully reversible oxidation/reduction processes within a convenient electro-
chemical window for maintaining SAM integrity, (2) stability in aqueous media, and
(3) commercial availability in many forms offering synthetic versatility and facile
routes toward functionalization with anchoring organosulfur groups and/or recogni-
tion motifs such as DNA, peptides, and receptor ligands. In this section, recent
examples of biosensing monolayers that incorporate ferrocene-receptor conjugates
will be highlighted. In particular, emphasis will be placed on receptor architectures
that enable reagentless electrochemical detection of different classes of analytes. For
a more comprehensive treatment of ferrocene bioconjugates, the reader is directed to
the recent reviews by Metzler-Nolte et al. [
47
] and Martic et al. [
48
].
4.1 Ferrocene-DNA Conjugates
A potentially general solution to the problem of signal detection in biosensors is
based on the ability to program binding-induced “folding” responses of electrode-
bound DNA probes. In particular, E-DNA sensors that utilize oligonucleotide
receptors bound at one terminus to an electrode with the other covalently labeled
with a ferrocene (or alternate) redox probe have become mainstay architectures for
reagentless DNA detection [
49
]. In these devices, target DNA induced change in
receptor strand conformation (i.e., folding, unfolding) and/or dynamics influences
the electron transfer characteristics between the redox probe and the electrode
leading to a detectable electrochemical signal.
The first reported E-DNA sensor system was based on a self-complementary
ferrocene-functionalized DNA probe that formed a stable stem-loop structure [
50
].
A schematic illustration of the sensor is shown in Fig.
7
. The sensor architecture
closely resembles a surface-immobilized DNA hairpin molecular beacon [
51
]. In
the absence of target single-stranded DNA, the stem-loop probe holds the ferrocene
reporter in sufficiently close proximity to the electrode to enable efficient charge
transfer during voltammetric scanning resulting in a large, quantifiable faradaic
current response. Upon target DNA hybridization, the stem-loop structure is
compromised and the ferrocene probe is displaced from frequent neighboring
interactions with the electrode, significantly decreasing the current observed during
scanning, thus providing a “signal-off” measurement for target DNA in the sample
as low as 10 pM. Because the ferrocene moiety simply imparts redox activity to the
DNA recognition sequence, it was envisaged that the platform could be translated
to alternate electrochemical probes.
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