Instrumentation & Measurement Magazine 24-5 - 51

with a 40 nm silver layer and a film of MIP specific for profenofos.
The realized sensor detected profenofos in concentrations
ranging from 10−4
μg/L.
-10−1
of 2.5 × 10−6
Plasmonic sensors can be realized also using side-polished
(or D-shaped) fibers, which have a planar segment exposing
the core used as the sensing region [11], tapered fibers or Ushaped
fibers to enhance the optical performances.
Tapered fibers are formed by heating and softly stretching
along the propagation axis. This method makes optical fibers
thinner, typically a few millimeters or centimeters over a certain
length. The fiber core is also thinner by the same factor as
the overall fiber, and the evanescent wave from the core ultimately
hits the outer surface and is exposed to the medium
around it. The SPR sensor results as a metallic layer is placed
over the tapered region of the fiber [12].
Similarly, with the help of a heat source, U-shaped plasmonic
fiber-optic sensors are realized by bending. The
obtained U-bent region is de-cladded, and a metallic film or
nanoparticles are deposited on the surface of the probe. A
U-shaped fiber optic probe coated with glucose-capped silver
nanoparticles (Ag NPs) for the detection of mercury ions
in aqueous solution was presented by Shukla et al. [13]. The
authors prepared the probe by cutting 1 cm of the length of
the cladding in the middle of the fiber and bending the optical
fiber using a butane flame. After a washing step of the
U-bent region based on an acid-alkali procedure, the probe
was dipped in glucose-capped Ag NPs solution. The glucosecapped
Ag NPs-coated optical fiber probe was then dipped in
mercury solutions of different concentrations, reporting a limit
of detection for mercury of 2 ppb [13].
Plasmonic Sensors Based on Plastic
Optical Fibers
Due to their excellent flexibility, simple handling, wide numerical
aperture, large diameter, and the fact that plastic is
capable of withstanding smaller bend radii than glass, POFs
are particularly advantageous over silica fibers. Thanks to
such properties, which increased their popularity and competitiveness
in telecommunications, they are preferred in the
development of optical sensors as well, since they have easier
manufacturing and handling processes [14].
Recently, Boruah et al. developed a portable optical Ushaped
fiber sensor for heavy metals detection in aqueous
medium [15]. The authors manufactured the sensing probe
from a 10 cm length plastic optical fiber (core diameter 600
μm), from which 2 cm cladding was removed from the middle
using a surgical blade. The unclad portion was inserted in
a glass capillary tube of diameter 1 cm and heated to develop
U-shaped probe of 1 cm radius. Next, the sensing probe was
coated with oxalic acid-functionalized gold nanoparticles (Au
NPs) for the selective binding of Pb2+ ions. Sensing operations
were performed using a setup consisting of a white LED as
light source and an optical detector. The use of a LED instead
of a spectrometer allowed the sensing setup to be compact and
eased the handling process. The sensing region of the prepared
August 2021
μg/L, with a Limit Of Detection (LOD)
probe was then placed in contact with liquid samples of Pb2+
and the obtained detection limit was 2.1 ppb, which is below
the WHO guidelines value of 10 ppb [15].
Plasmonic sensors can be implemented in plastic optical
fibers using different geometries, such as D-shaped fibers as
well. In 2011 Cennamo et al. designed a SPR sensor configuration
based on a D-shaped POF [16], aiming at producing a very
highly sensitive, robust, low-cost, and reliable sensor. The authors
realized the SPR sensor as a 10 mm long D-shaped POF
that can be monitored and exploits only two components: a
white light source and a spectrometer. First, a POF was embedded
in a resin block with a specific trench. Then, the D-shaped
was obtained by removing the cladding of a 980 μm core plastic
optical fiber along half the circumference by using two
different polishing papers (5 μm and 1 μm grit). A buffer of Microposit
S1813 photoresist was spun on the exposed core, and
finally a thin gold film was deposited by sputtering technique
[16]. Since then, several applications were developed with this
optical platform in the environmental, industrial and medical
sectors by coupling the D-shaped POF sensor with biological
receptors (antibodies, aptamers, etc.) or synthetic receptors
such as MIPs [17]-[20].
Other SPR sensors implemented in D-shaped POF have
been realized. Recently, Junxia Sun et al. realized a sensitive
and selective SPR sensor that employs gold-supported graphene
composite film/D-shaped fiber to detect dopamine,
an important neurotransmitter in the human body [21]. To
obtain the D-shaped sensor, the authors first exposed part of
the core in the middle of a 20 cm long POF to form a 15 mm
long D-shape area. Then, the D-shape region was polished
with sandpaper, cleaned and modified with a hybrid gold/
graphene layer. In particular, the sensitive layer was produced
using a one-step procedure where the gold film was
used as a supportive layer for graphene transferring, so
that the sensitive layer could be fixed directly in the D-POF.
Finally, a dopamine binding aptamer (DBA) was immobilized
on the Au film/graphene D-POF sensor. The proposed
sensor demonstrated good sensitivity and selectivity by detecting
dopamine in the range of concentration from 10−10
M [21].
M
to 10−6
Up to now, several POF-SPR sensors have been realized
using biological receptors (antibodies, enzymes, aptamers,
etc.) and chemical receptors (nanomaterials, Molecularly Imprinted
Polymers, etc.). Chemical receptors present some
advantages over the biological ones, thanks to their low cost
and resistance in wide ranges of pH (from acids to alkaline values)
and temperature.
Low-Cost Optical-Chemical Sensors:
Plasmonic POFs Combined with MIPs
Among receptors, MIPs are gaining great interest in low-cost
sensor development. MIPs are completely synthetic materials
with molecular recognition sites. The synthetic process
implies a co-polymerization between functional monomers
and a cross-linking reagent with a template molecule (target
analyte). The template molecule is coordinated in specific
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51
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