Biomedical Engineering Reference
In-Depth Information
species can be designed that function in a specific and predictable fashion for the
detection of analytes that behave in a manner similar to natural enzyme systems.
Looking ahead, these abiotic enzyme mimics should inspire the next generation of
functional synthetic metalloreceptors and may ultimately be rationally integrated
into highly sensitive biosensing technologies. The challenge will be to design
specific allosteric regulatory sites that are responsive to bioanalytes or surrogates
of interest.
4 Monolayer Receptors with Integrated Metal-Based Reporters
The fundamental limitation that has prohibited more widespread success of com-
mercial biosensors is the failure of most clinically relevant biomolecules to produce
an easily measured signal upon receptor binding that is specific over the many
potential interferants in a biological sample. Therefore, analytical approaches based
on analyte-receptor recognition (i.e., immunoassays) almost always require bur-
densome, multistep procedures that are often limited to laboratory settings. Thus, a
significant challenge remains to design functional receptor systems that can be
integrated with physicochemical transducers and report on specific biomolecular
interactions in complex matrices without intervention. Transition metal-modified
receptors hold exciting promise in this regard due to their inherent capacity for
participating in electron transfer reactions when integrated with signal transducing
electrodes.
Indeed, nanoscale charge transfer processes represent a rapidly advancing fron-
tier of fundamental science [
34
-
36
], with applications ranging from molecular
electronics [
37
-
40
] and information storage [
41
] in addition to chemical and
biological sensor fabrication [
42
]. Basic advances in our understanding of electron
transfer continue to fuel growth in electrochemical biosensor research due to the
tremendous commercial opportunity for fast, simple, scalable, and low-cost detec-
tion technologies that can be integrated with modern microelectronics. In particu-
lar, self-assembled monolayers (SAMs) of electroactive molecules adsorbed on
noble metal electrodes have been intensely investigated as model systems for
interfacial electron transfer events [
43
,
44
]. The majority of electroactive SAMs
studied to date comprise molecules with common design features, namely thiol-
terminated organic bridges anchored to gold electrodes through gold-thiolate
bonds with
-functionalized redox-active head groups. Since the pioneering work
of Chidsey and coworkers [
45
] in 1990, cyclic voltammetry (CV) studies of
ferrocene-terminated SAMs have been extensively reported in the literature and
the influence of bridge architecture, coadsorbed diluent molecules, and supporting
electrolytes on the redox behavior of these SAMs has been well established [
43
].
In voltammetry studies of redox-active SAMs, current is measured as a function
of potential between the SAM-modified working electrode and a reference elec-
trode. It is well known that the kinetics and thermodynamics of such interfacial
redox reactions are strongly affected by the nature of the medium in which they occur.
o
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