Biosensor technology in the treatment of cardiovascular disease - Biomaterials and Devices for the Circulatory System - page 284

11.2.2 Optical biosensors
Apart from electrochemical measurement, optical detection is the other
main technique used in biosensors, based on either fl uorescence or chemi-
luminescence methods. In fl uorescent detection, the amount of light emitted
by fl uorescent markers at specifi c wavelengths is altered by the binding of
the substance being measured, which includes cells, protein and DNA. Such
detection methods are frequently used in DNA microarrays. Fluorescent
probes are available for enzymes, proteins, DNA hybridisation, growth
factors, drugs, peptides, lipids and hormones. 10 The immobilisation of the
fl uorescent-labelled reagent on the end of the optic fi bre (which carries the
excitation light and collects the measured signal) is often desirable. This is
known as an intrinsic sensor; in an extrinsic sensor the optic fi bre is placed
into a solution containing the sample plus fl uorescent probe. The most
popular method of immobilising fl uorescent probes is a host matrix known
as sol-gel, which is made from organic salts. Recent reviews of fi bre-optic
biosensors are provided by Monk & Walt 11 and Leung et al. 12
Biosensors based on the technique of surface plasmon resonance (SPR)
are also common, for the immunosensing of various biochemical ana-
lytes. 13,14 SPR has the advantage over other optical techniques of not requir-
ing a fl uorophore label. Detection takes place by the measurement of
changes in properties of refl ected light from a thin metal fi lm, as shown in
Fig. 11.3. The presence of analyte molecules attached to the antibodies
immobilised on the metal fi lm causes a change in the surface plasmon wave
(SPW) that is proportional to the amount of analyte detected. The

Analyte of
interest

￿ ￿ ￿ ￿ ￿

Immobilised
antibodies

Silver/gold layer

Incident
light

Reflected light
to detector

Prism for
coupling light
to surface

11.3 Basic principle of surface plasmon resonance. Incident light,
prism-coupled to a thin metal fi lm, excites surface plasmons to
produce a surface plasmon wave (SPW). As analytes are captured by
immobilised antibodies, the SPW changes, causing variations in the
wavelength or incident angle of the refl ected light that are
proportional to the amount of analyte adsorbed.