Chemistry Reference
In-Depth Information
2.3 The Origins and Working Principles of Holographic
Sensors
In late 1970s, those preparing art holograms realised that so called
”
effects could be obtained by using several exposures of one scene with the
632.8 nm beam from a HeNe laser with each exposure only illuminating different
sections of that scene. Before each exposure, the moisture level or pH of the gelatin
emulsion was changed, so that each exposure had the emulsion with a different
degree of swelling. This resulted in objects having different colours when the
“
pseudo-colour
cally,
the thickness of the emulsion can be varied through pre-swelling/shrinking the
emulsion before laser exposure. Since gelatin
finished hologram was replayed under a white light source [
66
69
]. Speci
-
is thickness is greatly affected by
its moisture content, moisture control techniques were utilised to create pseudo-
colour holograms [
68
,
70
]. In the 1980s, emulsion pre-treatment was optimised to
obtain a range of output wavelengths from a
lm
'
fixed exposure wavelength [
71
,
72
]. In
the 1990s, the tuning technique of holograms led to the realisation that re
ection
holograms could be used as sensors to quantify humidity [
73
] and chemical sub-
stances [
74
fl
76
]. Figure
2.3
shows the timeline in the development of holographic
sensors. Any physical or chemical stimulant that changes the lattice spacing (d)or
the effective index of refraction (n) of the
-
film cause observable changes in the
wavelength (
le (colour distribution), or the intensity (brightness) of
the hologram. The intensity output by the hologram depends on the modulation
depth of the refractive index (dark and bright fringes), and the number of planes
present in the polymer matrix. Swelling in the polymer matrix increases the distance
between NP spacings and produces a red Bragg peak shift, whereas shrinkage in the
matrix shifts the peak to shorter wavelengths. The diffraction grating acts as an
optical transducer, whose properties are determined by the physical changes in the
polymer matrix. For example, when the polymer matrix is functionalised with a
receptor comonomer, which has the ability to draw or expel water from the system
upon binding, the degree of swelling indirectly represents the concentration of the
target analyte. The shift in the Bragg peak can be monitored using a spectropho-
tometer, and the sensor can be calibrated based on the inputted physical or chemical
change. Hence, the same sensor can be optically or visually interpreted to quantify
λ
peak
) or its pro
Fig. 2.3 Historical development of holographic sensors
