Concept of Bioelectronic Nose - Bioelectronic Nose: Integration of Biotechnology and Nanotechnology - page 4

1.1.3 Bioelectronic Nose

In the late 1990s, a novel and more advanced concept of sensor devices was sug-
gested [ 44 ]. The challenge was to use ORs as a sensing material in order to mimic a
human or animal olfactory system. This new device is called a 'bioelectronic nose'.
The bioelectronic nose is based on OR proteins or cells expressing ORs on their
surface membrane. ORs are odorant-recognition elements, and are combined with
sensor devices that convert biological signals into electrical or optical signals.
Since ORs provide odorant-discriminating ability, the bioelectronic nose can
closely mimic a human or animal olfactory system. The concept of odorant analy-
sis using a bioelectronic nose fundamentally differs from the odor-discriminating
strategy of electronic noses based exclusively on pattern recognition using sensor
arrays. When the ORs are utilized as a primary sensing material, the sensors can
precisely distinguish a target molecule among a mixture of various compounds. In
addition, sensors based on ORs are more sensitive than electronic noses. The limit
of detection reaches the femto-molar range in liquid conditions and the ppt range
in gaseous conditions, which is similar to that of a human nose [ 45 , 46 ]. By virtue
of these excellent characteristics, the bioelectronic nose is now receiving great at-
tention from diverse fields such as disease diagnosis, food safety assessment, and
environmental monitoring.

1.2 Concept of Bioelectronic Nose

1.2.1 Biological Recognition Element

A bioelectronic nose consists of two main parts: an odorant-recognition element
and a signal transducer, as shown in Fig. 1.2 [ 47 ]. For the odorant-recognition,
cells expressing ORs in their surface membrane, OR proteins, and nanovesicles
have generally been used. In the human nose, approximately 390 different types of
functional ORs exist [ 48 ]. However, humans can discriminate thousands of types of
odors. This asymmetry originates from the excellent odorant-recognition ability of
ORs, which are capable of distinguishing between their specific ligands and partial
ligands, as well as irrelevant molecules [ 27 , 28 ]. A single odorant activates various
types of ORs, and one OR is activated by several odorants. Thus, numerous combi-
nations of activated ORs can be generated. These combinations are recognized as a
unique property of an odor in the brain [ 49 ].
A bioelectronic nose utilizes this odor-discriminating ability of ORs. Thus, they
can detect specific odors with great selectivity. For instance, the odor from decom-
posed seafood can be easily distinguished among other odors from various spoiled
foods when a bioelectronic nose is functionalized with receptors that can selectively
detect the odor of spoiled seafood [ 50 ]. However, problems still remain to be over-
come. ORs have a seven-transmembrane structure with a high hydrophobicity in