Biomedical Engineering Reference
In-Depth Information
communication between the bound analyte and the electrode transducer, causing
a substantial attenuation to the readout signal associated with the recognition events.
Au NPs possess several attractive characteristics in serving as building blocks
for molecularly imprinted sensing platforms on electrodes: (1) The nanometric
dimensions of the particles and the well-developed, yet simple, surface chemistry
associated with the functionalization of Au surfaces, imply that the particles may
be chemically modified and aggregated by different means to yield a high surface
area, porous, Au NPs assemblies. Using the appropriate synthetic cross-linking
conditions, and introducing a target analyte-molecule into the reaction chamber,
imprinted sites corresponding to the molecular contours of the target analyte
can, then, be generated within the cross-linked Au NPs composite. The high
porosity associated with the imprinted Au NPs composite is expected to facilitate
the diffusion of the analyte within the layer, and, thus, the association of the analyte
with the recognition sites becomes rapid, leading to fast response times of the
sensing system. (2) The chemical modification of the exterior capping layer of
the Au NPs by functional molecules, such as electropolymerizable and/or ligand
units, yields a simple and controllable route for an electrochemical synthesis of Au
NPs arrays, acting as receptors for specific target analytes. Also, the versatility of
the available methods for the size- and shape- controlled synthesis of the Au NPs is
expected to affect the resulting sensing performance by the imprinted matrices and
to play a major role in the optimization of the sensors. (3) Several intrinsic features
of Au NPs, such as the occurrence of a localized plasmon upon their excitation by
light, provide unique optical enhancement phenomena in close proximity to the
interface of the NPs, and can be, thus, used for the transduction and amplification of
recognition events. Specifically, when functionalized Au NPs are electropo-
lymerized on a Au-coated surface, the coupling between the localized surface
plasmon of the metal NPs and the surface plasmon wave associated with the thin
metal film leads to a shift in the surface plasmon resonance (SPR) energy.
The effects of this coupling were theoretically addressed [
48
], and the changes in
the SPR curves, resulting from the variations in the dielectric properties at the
vicinity of the Au surface, were discussed.
2 Molecularly Imprinted Bis-Aniline-Cross-linked Au NPs
Matrices for Sensing
Recently, we developed a generic method to electropolymerize assemblies of
bis-aniline-cross-linked Au NPs on Au electrodes (Fig.
1a
). The procedure involves
the synthesis of Au NPs (ca. 4 nm diameter) modified with a protective capping layer
consisting of electropolymerizable thioaniline groups (1), and mercaptoethane sul-
fonic acid (2) stabilizing units. The subsequent electropolymerization of the
functionalized Au NPs is carried out by the application of a fixed number of potential
cycles, between
0.35 and 0.8 V vs. a Ag quasi reference electrode (Ag QRE), on a
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