Sensor Used in E-nose - The Electronic Nose Artificial Olfaction Technology - page 148

Direct sensor and indirect sensor [ 14 ]

1. Direct-detection sensors The biological significance is directly measured in real
time, and they are typically noncatalytic elements such as cell receptors or
antibodies. The most common direct-detection biosensor systems use surface
Plasmon resonance (SPR), resonant mirror, or quartz resonator transducers. All
of these systems use detection methods that directly sense the presence of the
analyte itself.
2. Indirect-detection sensors These rely on a primary recognition reaction that
binds the analyte to a substrate followed by a secondary recognition reaction
that binds a labeled molecule to the joined analyte to allow detection. Indirect-
detection systems use fluorescent labels, catalytic elements such as enzymes
(e.g., alkaline phosphates), microorganisms, and plant or mammalian tissue to
enable or enhance detection. The most common indirect-detection biosensors
use optical fluorescence or electrochemical transducers, including light-
addressable potentiometric sensors (LAPS) that combine electro-optics and
electrochemistry. Indirect-detection biosensors often show higher sensitivity
than direct-detection sensors because of the signal amplification possible with
the classification step.

Direct biosensor technologies enable real-time detection, while indirect bio-
sensors, which are typically rely on a sandwich assay, are basically slower;
therefore, direct biosensors are preferable when speed is a major factor. On the
other hand, indirect biosensors are likely to be more sensitive, simpler, and less
expensive than many direct biosensors. For example, fluorescent-based biosensors
are low priced to configure in a multichannel design, whereas the direct biosensors
typically are more costly.
Classification based on the detection method
Biosensors are also frequently classified by the type of transducer employed
(electrochemical, optical, piezoelectric, Gravimetric, and thermal) as shown in
Fig. 6.5 .

Fig. 6.5 Classification of biosensor (detection method)