Biomedical Engineering Reference
In-Depth Information
18.3.2
Resonant Mirror
The resonant mirror (RM) sensor has been extensively used in the field of chemical and
biological sample analyses (25). The RM device consists of a high-index substrate, a thin
low-index spacer and a very thin monomode waveguiding layer (Figure 18.2). These
dielectric films are very durable, unlike the thin gold films used in SPR sensors, enabling
the devices to be cleaned and reused.
Light incident above the critical angle on the substrate-spacer interface is coupled into
the waveguiding layer via the evanescent field in the spacer layer when the propagation
constants in the substrate and the waveguide match. For monochromatic light, this occurs
over a very narrow range of angles, typically spanning less than one degree. Alternatively,
at a fixed input angle the coupling occurs over a narrow range of wavelengths. This device
has been termed the RM because it contains a resonant cavity (the waveguide) that acts as
an almost perfect reflector for light incident above the critical angle. However, if an opti-
cally absorbing species is present in the evanescent field above the waveguide layer, the
reflectivity of light overlapping the absorbance band will decrease. Similar modulations of
the reflected mode are also encountered on variations of the refractive index of the guid-
ing medium. Both these factors can be exploited to produce optical sensor devices. An RM
biosensor is commercially available as the IAsys immunosensor (affinity sensors).
Dmitreiv et al. (25,26) conducted a detailed kinetic-analysis study determining anti-
body-antigen (purified protein) interactions using IAsys sensor. Rasooly and Rasooly (27)
employed IAsys affinity sensor to detect staphylococcal enterotoxin A in various food
matrices including hotdog, potato salad, milk, and mushrooms at a concentration of
10-100 ng/g of product in less than 4 min. They used a sandwich-assay format to improve
sensitivity. Lathrop et al. (28) detected crude surface protein preparations from
L. monocy-
togenes
in IAsys at concentrations of 200 µg/ml in 20-30 min using a specific antibody. This
sensor gave several-fold higher signal for
L. monocytogenes
over other
Listeria
species. The
assay also gave positive signals with
L. innocua
, a nonpathogenic species, because of the
cross-reactivity of the antibody with this species (29,30). RM is most suitable for the detec-
tion of soluble toxins or proteins and was unable to detect whole bacterial cells because of
the limited depth (~300 nm) of the sensing layer (28).
Waveguiding layer
Evanescent field
TIR boundary
High-index substrate
Resonant
structure
Low-index spacer
∅
Reflected light
Prism block
FIGURE 18.2
Configuration of resonant mirror (RM) biosensor. (Adapted from IAsys Manual, Affinity Sensors,
Cambridgeshire, UK.)
