Concept of Bioelectronic Nose - Bioelectronic Nose: Integration of Biotechnology and Nanotechnology - page 7

developed in 2012 [ 80 ]. Jin et al. demonstrated that the odorants can be selectively
and sensitively detected through the olfactory signaling pathway in nanovesicles.
Nanovesicle-based sensors have been applied in diverse fields, such as in the as-
sessment of food quality and the diagnosis of diseases [ 81 , 82 ].

1.2.2 Signal Transducer

The biological signals generated from ORs are mainly divided into three types: con-
formational changes in olfactory receptors, dissociation of the α-subunit of G pro-
teins from activated ORs, and ion influx caused by the signal transduction in cells.
These biological signals should be converted into signals that can be measured.
Devices for this role are called signal transducers or secondary transducers [ 47 ].

1.2.2.1 Quartz Crystal Microbalance

Quartz crystal microbalances (QCMs) have been used for the development of
receptor-based bioelectronic noses [ 62 - 66 ]. The functionalized surface of quartz
crystals is coated with olfactory receptors. The whole mass of OR increases by the
binding between ORs and odorants. QCM can specifically detect this change. The
adsorption of specific molecules onto the surface of quartz crystals coated with ORs
results in the reduction of the resonance frequency of the quartz crystal. Odorants
can be detected using this principle.
Research on QCM-based bioelectronic noses using ORs isolated from bullfrogs
was reported in 1999 [ 66 ]. The specific recognition of odorants could be monitored
in real-time using the QCM device. It was demonstrated that the ORs overexpressed
in E. coli and in mammalian cells could also be utilized for the functionalization
of QCM chips [ 62 , 65 ]. These studies represent that the piezoelectric method can
be used for the development of highly sensitive and selective bioelectronic noses.
However, QCM devices have an issue that must be overcome. The oscillation fre-
quency of quartz crystals can be easily affected by various external factors, such
as electromagnetic fields and pressure. Hence, non-specific noise signals are fre-
quently generated [ 83 , 84 ].

1.2.2.2 Surface Plasmon Resonance

Surface plasmon resonance (SPR) has been broadly used to measure the association
and dissociation of analytes on a surface. The SPR is an optical phenomenon that
occurs when p-polarized light hits a prism covered with a metal surface [ 85 ]. At a
specific incident angle, the intensity of the reflected light is reduced due to reso-
nance energy transfer. This resonance angle is affected by the adsorption of specific
molecules onto the metal surface. When the surface of SPR is coated with ORs, the
binding of odorants to ORs influences the resonance angle.