Evolving Trends in Transition Metal-Modified Receptor Design and Function - Designing Receptors for the Next Generation of Biosensors - page 250

Fig. 9 (a) Structure of the ferrocene-pepstatin conjugate and schematic representation of the
diluted self-assembled peptide film on a gold surface. The initial cyclic voltammogram of
ferrocene probe SAM is in red .(b) The binding of HIV-1 PR to the ferrocene-pepstatin probe
causes a significant shift in the formal potential and a decrease in the current intensity as shown in
blue . Reprinted with permission from [ 61 ]. Copyright 2007 Royal Chemical Society
Kraatz and coworkers first reported integrated sensors with unsymmetric ferro-
cene architectures for the electrochemical detection of papain [ 60 ] and HIV type 1
protease (HIV-1 PR) [ 61 ] using conjugates of peptide enzyme inhibitors. Figure 9
shows the sensor for HIV-1 PR. In this example, the ferrocene probe is conjugated
with a cystamine-terminated oligoproline anchoring group and the known aspartic
protease inhibitor pepstatin. Mixed SAMs of the probe and hexanethiol diluent
were prepared and the initial faradaic response characterized by CV. Upon expo-
sure to target HIV-1 PR, the formal potential of the surface-bound ferrocene probe
shifted to a higher potential suggesting protein-probe interaction. Increasing
concentrations of target (from 40 to 100 nM) resulted in a linear increase in redox
potential accompanied by progressively higher current loss. The analytical response
was attributed to HIV-1 PR encapsulation of the surface-bound ferrocene probe
resulting in ferrocenium destabilization and partial shielding of counteranion access
during oxidation. Recent extension of this label-free biosensing platform to the
detection of a panel of HIV proteins [ 62 ] using different peptide receptors suggests
that the approach may be general.
Ultimately, the integrated electrochemical biosensors discussed above demon-
strate that redox probe-receptor conjugates provide a means of signaling specific
biomolecular recognition events. Our laboratory is pursuing a number of electroac-
tive SAM building blocks [ 63 ] amenable to biofunctionalization to further explore
similar modes of analyte-induced electronic signal transduction. The challenge of
developing label-free, low cost, and multiplexed biosensor platforms provides
significant inspiration and we anticipate significant advances in this area in the
coming years.