Biomedical Engineering Reference
In-Depth Information
trinitrotoluene (TNT), dinitrotoluene (DNT), and mononitrotoluene (MNT) was
demonstrated [
49
]. Whereas this methodology was proved to be efficient, facilitating
a detection limit of 200 pM for TNT with an impressive selectivity between the
structurally related nitoaromatic explosives, a major drawback of the electrochemical
detection scheme is in the necessity for the target analyte to be redox active (the redox
activity must also be outside the potential region associated with the broad quasi-
reversible electrical response of the bis-aniline bridges). To overcome these obstacles,
other readout techniques, such as surface plasmon resonance, SPR, were further
implemented to follow the sensing capabilities of the bis-aniline-cross-linked Au
NPs matrices.
SPR is a common method to probe refractive index changes occurring on thin
metal films as a result of recognition events or chemical transformations [
64
,
65
].
Due to its high sensitivity, SPR spectroscopy has been widely used to develop
optical sensors and biosensors. For example, protein-protein interactions [
66
,
67
]
and the formation of antigen-antibody complexes [
68
,
69
] were probed using SPR.
The method is, however, limited to large molecules, such as proteins, or low-
molecular-weight substrates which change their color upon various chemical
transformations leading to substantial refractive index changes (that are translated
to shifts in the SPR spectrum). Nonetheless, for systems with low coverage of the
surface with the analytes, or when only small refractive index changes are induced
by the binding of low-molecular-weight analytes, amplification of the SPR signal is
required. This limitation can be resolved by the conjugation of labels that intensify
the refractive index changes. For example, the conjugation of latex particles [
70
],
liposomes [
71
], or secondary proteins [
72
] was used to amplify small refractive
index changes, therefore leading to observable SPR shifts upon the recognition
processes.
Metal NPs, such as Au or AgNPs, exhibiting a localized plasmon, were extensively
used to amplify SPR signals originating from recognition events. The coupling of the
localized plasmon of the NPs with the surface plasmon wave was found to affect the
SPR energy and, thus, to enhance the SPR shifts [
48
]. For example, the formation of
immuno-complexes [
20
], DNA hybridization [
18
], and biocatalytic processes [
73
]
were followed by the amplification of the SPR signals using the Au NPs as labels.
The following sections will describe the SPR sensing of a variety of compounds on
the respective molecularly imprinted bis-aniline-cross-linked Au NPs matrices. The
different target analytes are associated with the matrices through different physical
and chemical interactions that will be addressed in detail.
2.1 Molecular Imprinting of Recognition Sites in Au NPs
Composites Through Donor-Acceptor Interactions
The electropolymerization of thioaniline-functionalized Au NPs onto a thioaniline-
modified Au surface yields a thin film containing layers of Au NPs. The redox state of
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