Introduction to Biosensor Technology - Sensor Technology in Neuroscience - page 32

monitored. A resonant transfer of electromagnetic energy into the surface
plasmon wave is observed through a minimum in the plot of incident angle
versus reflected intensity (Figure 1.24(b)). The key feature experimentally is
how well a specific configuration is capable of sensing a change in the intensity
of the reflected light, often termed RIU. Most commercial instruments have a
limit of detection of around 10 6 to 10 7 RIU.
A simplified schematic of a typical SPR experiment is shown in Figure 1.25.
This type of instrument designed for work in the biosensor arena is generally
capable of analyzing several channels (gold surfaces) using microfluidic
sample introduction in a real-time, label-free fashion through flow-injection
technology. In certain cases, for the purpose of biomolecule attachment to the
sensor 'chip' and for enhancement of sensitivity, a dextran layer some 100 nm
thick is imposed at the device-liquid interface. The system generates a response
plot, which is often referred to in the field as a 'sensorgram', presumably in light
of chromatograms and the like! An example of SPR-based plots is depicted in
Figure 1.26. With respect to applications, the SPR technique has been
employed in a wide variety of cases such as the adsorption of proteins, cells and
nucleic acids on gold and modified metal surfaces, and the detection of
biomolecular interactions such as found in immunochemistry and nucleic acid
duplex formation. 117-122 The method has proven to be particularly helpful for
epitope determination in the former area.
Other instrumental arrangements for observing SPR have appeared in the
literature. One example among several is the waveguide coupling of photons
with plasmons. In one example of this arrangement, plasmons on a surface of a
thin metal film are excited by a Gaussian-like leaky mode of an effectively single
mode photonic crystal waveguide. 123 This allows ecient phase matching with
plasmons at any wavelength of choice while retaining highly sensitive response
to changes. Another important advance in terms of an additional form of
plasmon resonance detection is SPR imaging, sometimes referred to as SPRi,
which was introduced in the 1980s. 124,125 At that time it was shown that

d n 4 t 3 n g | 1

d n 3 .

Figure 1.25 Typical SPR experiment involving flow injection on target into flow cell.