Nanotechnology-Enabled Sensors

2.3 Physical Effects Employed for Signal Transduction 35
HYPSOCHROMI BATHOCHROMIC
Fig. 2.14 Hypsochromic and bathochromic frequency shifts occurring as a result
of the Doppler effect.
The observed Doppler shift in frequency is given by:
v
fobserved f source
v v ⎟
source
⎟
⎛ ⎞
= ⎜
(2.12)
⎝ + ⎠
where v is the speed of the wave in the medium, vsource is the speed of the
source with respect to the medium, and fsource frequency of the source
wave. If the wave source approaches the observer, then vsource is negative,
and conversely, if the wave is receding, then it takes on a positive value. A
familiar examples of the Doppler effect include the changing pitch of an
ambulance siren as it approaches and then drives past the observer.
Common examples of the Doppler effect in sensing include speed monitoring
devices and ultrasounds. Hypsochromic and bathochromic shifts are
used in measurement of large and distant bodies such as stars, galaxies and
gas clouds as their motion and spectrum can be studied with respect to the
observer. The Doppler effect also plays an important role in radar and sonar
detection systems.
The Doppler effect can play a significant role in the sensing and characterization
of nanomaterials. It is known that Doppler broadening (broadening
of spectral lines in UV-vis spectroscopy) is caused by the thermal
movement of small particles. 46 Doppler broadening generally places severe
constraints on precise spectroscopic measurements. However, the signatures
of the broadened spectrum (such as its bandwidth and shape) can be
utilized to extract information about the presence of atoms/molecules in
nanostructures, as well as providing information on their morphologies: by
decreasing the temperature, or by employing measurement methods such