Instrumentation & Measurement Magazine 24-5 - 50
Chemical and Biological
Applications Based on Plasmonic
Optical Fiber Sensors
Chiara Perri, Francesco Arcadio, Girolamo D'Agostino,
Nunzio Cennamo, Giovanni Porto, and Luigi Zeni
T
he use of optical fibers has evolved from an optical
transmission waveguide to a more intricated device
for bio-chemical sensing. The possibility of measuring
target substances with rapid and low-cost tools is still a
main challenge in different application fields, from environmental
monitoring to the medical-diagnostic sector. In recent
decades, plasmonic fiber-optic sensors have gained great
interest due to their excellent sensitivity and compactness, allowing
the possibility of remote sensing for different target
analytes that is real-time and label-free. Such sensors have constantly
evolved over time. In this work, a short overview on
plasmonic optical fiber sensors is presented, including illustrations
that focus on the applications and achievements so far.
Introduction to Plasmonic Sensors
Surface Plasmon Resonance (SPR) is one of the fastest growing
research fields which investigates the interaction at the
nanoscale between photons (light) and surface electrons of
a metallic layer [1]. In SPR sensors, the metallic element is in
contact with a Molecular Recognition Element (MRE), which
locally changes its refractive index (RI) when interacting with
the target analyte. This change affects the electromagnetic
wave propagating along the metal-dielectric interface in a
highly sensitive manner [2], [3].
By exploiting different types of MRE, it has been possible
to produce several applications up to now, linking scientists
from diverse backgrounds such as chemistry and physics
with medical sciences. Although the most common type of
plasmonic sensor is typically prism-based (Kretschmann
configuration) [4], this kind of sensor needs bulky setups that
do not allow field measurements and are challenging to the
development of small and low-cost Point-Of-Care (POC) devices
(medical diagnostic tests which can be performed at or
near the point of care of patients, for instance at bedside). By
replacing the prism with an optical fiber, in which light propagates
through the phenomenon of Total Internal Reflection
(TIR), all limitations due to the Kretschmann configuration
can be easily overcome. In recent decades, plasmonic sensors
based on optical fibers have constantly evolved, from
50
the first silica glass fibers-based to the more recent polymeric
ones [5]-[7].
In this work, we report on plasmonic optical fiber sensors
based on silica optical fibers, then on those based on plastic
optical fibers. After these sections on optical fiber sensor platforms,
the work recalls a simple and highly sensitive sensing
approach to realize optical chemical sensors: plastic optical fibers
(POFs) combined with molecularly imprinted polymers
(MIPs).
Surface Plasmon Resonance Sensors
Based on Silica Optical Fibers
Various designs and geometries have been proposed for the
realization of surface plasmon resonance sensors based on optical
fibers [8]. The simplest configuration is based on unclad
fibers, where a segment of the core is exposed by removing
the cladding and covered with a metallic layer or metallic
nanoparticles (for instance, gold). Unclad sensors have been
developed in recent years, from very large core fibers in the
range of 200 μm and 600 μm, instead of small single-mode core
fibers. P. Bhatia and B.D. Gupta realized a SPR fiber-optic sensor
for urea detection in biomedical applications [9]. In this
work, the authors prepared the probe using a silica fiber of 600
μm core diameter of 20 cm length, from which 1 cm length of
the cladding was removed from the middle. The unclad portion
of the fiber was first coated with a 40 nm layer of silver and
then an 8 nm layer of silicon to enhance the sensitivity, through
the evaporation technique. Finally, the urease enzyme was immobilized
on the silicon layer. The probe was characterized by
spectral interrogation and was able to measure solutions with
concentrations from 1 mM to 160 mM, which are close to physiological
blood concentrations.
In another work carried out by A.M. Shrivastav et al. [10],
an SPR fiber-optic sensor based on molecular imprinting was
fabricated for the detection of profenofos, a type of organophosphorus
pesticide (OPPs) used for controlling pests in
farming, which can cause injurious effects to the nervous system
of humans even at low concentrations. The sensing probe
was realized by coating the unclad core of a multimode fiber
IEEE Instrumentation & Measurement Magazine
1094-6969/21/$25.00©2021IEEE
August 2021
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