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Surface Plasmon Resonance (SPR) is a powerful label free optical biosensing technology that relies on the measurement
of the refractive index or change of mass in close vicinity of the sensor surface. Therefore, there is an experimental
limitation in the molecular weight of the molecule that can be detected and consequently small molecules are
intrinsically more difficult to detect using SPR. One approach to overcoming this limitation is to first adsorb smaller
molecules onto the sensor surface, and to follow this by using their higher molecular weight antibodies counterparts
which ensure the specificity (and are easier to detect via SPR due to their higher weight). Although this has been
demonstrated with some success, it is not applicable in every case and some biomolecules such as enzyme are still
difficult to detect due to their specific reactivity (enzymatic reaction). In this paper, we present a powerful new method
that utilises specifically engineered spacers attached on one end to the sensor surface and on the other end to a
nanoparticle that behaves as a plasmonic label. These spacers are design to specifically react with the biomolecule to be
detected and release the (relatively large) plasmonic label, which in turn results in a measurable SPR shift (which is
much larger than the shift that would have been associated with the binding of the relatively small biomolecule). As a
proof of concept, this approach was used within a recently developed new form of SPR optical fibre sensor which relies
on the measurement of the re-emitted light by surface scattering of the plasmonic wave rather than transmission through
the fibre was used to detect an enzyme. Here trypsin (25kDa) was successfully sensed. This molecule is involved in both
intestinal and pancreatic diseases.
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