Instrumentation & Measurement Magazine 24-9 - 70

The Plasmonic Optical Fiber as the
Instrument: The Rising Trend of InSitu
Biomedical Measurement
Xile Han, Tuan Guo, and Gaozhi Xiao
O
ne of the key elements for early diagnosis of acute
and severe diseases is the development of ultrasensitive
detection methods, which are required to
analyze the pathological state of the human body from trace
amounts of substances in blood or urine. Surface plasmon resonance
(SPR) optical fiber biosensors, particularly those based
on tilted fiber Bragg gratings (TFBG), have emerged in recent
years as a novel solution for in-situ biomedical detection.
TFBGs can sensitively detect the physical and chemical interactions
between biomolecules in real-time by sensing minute
changes in refractive index, even in vivo. The TFBG-based SPR
method can achieve rapid and accurate analysis of biological
samples by demodulating the wavelength, intensity, phase
and polarization state of the optical spectrum. In addition,
TFBG-based SPR fiber optic sensors are capable of simultaneously
detecting the absolute or relative values of multiple
parameters. This effectively eliminates the interference from
the ambient environment and ensures the stability and reliability
of the sensor.
SPR Optical Fiber Sensors
Surface plasmon resonance sensors are employed in a very
large proportion of applications in biomedical sensing. Compared
with traditional biosensors, SPR biosensors exhibit the
advantages of having extremely high interfacial refractive index
sensitivity and fast response time and are label-free. They
have become the dominant detection means for biochemistry
research. Since the surface plasmon polaritons (SPPs) generated
by the interaction between a metal film on the surface of
the SPR sensor and the surrounding dielectric are very sensitive
to biomolecules attached to the metal surface, SPR sensors
can accurately provide quantitative results of the molecular interaction
process such as adhesion, binding and dissociation.
The most common approach to exciting SPPs in thin metal
films is the Kretschmann-Raether prism-coupled configuration
shown in Fig. 1a [1], [2]. In this configuration, light is
injected through a prism towards a plane face covered by a
thin metal layer. The incident angle at the glass-metal interface
is chosen to be greater than the critical angle so that light
is totally internally reflected. Therefore, the evanescent waves
associated with total internal reflection will propagate in the
metal layer and surrounding dielectric layer, and their penetration
depth does not exceed the wavelength of the working
light wave. In addition, one of the necessary conditions for
the excitation of SPP wave is that the polarization of light be
perpendicular to the metal surface. In this way, microscopic
physical and chemical reactions, such as the action of biomolecules
on the surface of the metal film, will change the effective
refractive index of the local SPR, which is manifested as a shift
in the SPR spectrum. This method can achieve high sensitivity
refractive index measurement in the order of 10−6
~10−8
RIU.
As an alternative to prisms, optical fibers with specific
structural designs can also stimulate the SPR effect. A photo
of a gold-coated optical fiber is shown in Fig. 1b [1]. In sensors
based on modified optical fiber, the core-guided light can
be directly in contact with the surrounding medium, and the
consequent SPR response is obtained from the transmission
spectrum. Advantages of optical fiber configurations over
prism-coupled SPR sensors are their ease of getting light to
and from the sensing area with ultra-low loss and the ability
to use widely available fiber-coupled sources. Of particular interest
for biological samples, optical fiber SPR sensing devices
working in the near-infrared band can obtain a larger penetration
depth in biological tissues than visible wavelengths. In
addition, the compact optical fiber SPR sensor can be used for
in-situ detection of trace objects in physically confined measurement
environments.
In order to excite SPR on the optical fiber surface, the fiber
structure needs to be modified to couple the light from the fiber
core into the cladding, and subsequently into the SPP-generating
metallic film. The most straightforward configurations
are realized by partly or entirely removing the cladding by
Dr. Guo received the 2018 IMS Technical Award for outstanding contributions
to the advancement of energy and health monitoring technologies.
70
IEEE Instrumentation & Measurement Magazine
1094-6969/21/$25.00©2021IEEE
December 2021

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