Transcript 328040_1_En_3_MOESM3_ESMx

Content:
• Fiber Optics
 Fundamentals of Fiber Optics
 Evanescent Waves
 Optical Fiber Biosensing
 Medical Applications of
Optical Fibers
• Optical Fiber Sensors Based on
Surface Plasmon Resonances
(SPRs)
 Surface Plasmon
Resonances and
Polaritons
 Fiber Optic SPR Sensors
 Examples of Utilizing
Optical Fibers as SPR
Biosensors
• Conclusions
Optical Fiber
• A potential area of nanomedicine involves clinical
applications of fiber optics.
• Optical fibers are useful for communication, endoscopy,
imaging, laser radiation delivery, and as biosensors.
• Fiber optics can help in the war against cancer by
closely monitoring various proteins or biomolecules that
are present in higher concentrations when cancer grows.
• A promising detection technique involves surface
plasmon resonances (SPRs).
• SPR-coupled optical fibers have the capability to detect
changes in the external refractive index, which will be a
focus of this lecture.
Fundamentals of Fiber Optics
• An optical fiber is a narrow, flexible cylindrical silica glass
rod core that is covered with a glass cladding.
• The fiber is then typically covered with a buffer and
protective jacket.
• Its main purpose is to act as a dielectric waveguide or
something that transmits light between its two ends.
• Since optical fibers can transmit light with low attenuation
or signal loss, information and images can be sent rapidly
through them.
• Optical fibers offer several advantages like low signal
degradation, high speed and capacity for information
transmission, and flexibility.
• Current applications include communication, sensing,
power transmission and imaging.
Fundamentals of Fiber Optics
(continued)
•
•
•
Two main principles that allow fiber optics to exist are Fermat’s
principle and Snell’s law of refraction.
Fermat’s principle states that light travels along a path of minimum
time. The optical path will remain straight if light travels in a
uniform medium.
Snell’s law of refraction governs the angle of incidence with which
an incoming ray encounters a change in the optical index. It is
described as
𝑛1 sin 𝜃1 = 𝑛2 sin 𝜃2 ,
where n1,2 are the respective indices of refraction (real component) of
two media, 𝜃1 is the angle of incidence from the normal to the boundary
of index change, and 𝜃2 is the angle of refraction of light in the second
medium.
Total Internal Reflection
•
•
•
Optical fibers guide light through total internal reflection, which
occurs when the refractive index of the core is higher than that of
the cladding.
In this case, when the ray of light comes a sufficiently high angle of
incidence, the light will be reflected back into the first medium from
the boundary of index change.
The angle at which this occurs is called a critical angle and is
defined as
𝜃𝑐𝑟 = arcsin
𝑛1
𝐶𝑙𝑎𝑑𝑑𝑖𝑛𝑔
𝜃𝑐𝑟
𝑛𝑐𝑜 > 𝑛𝑐𝑙
𝑛2
.
Evanescent Waves
• Optical fibers are useful in sensing applications which are
arranged through the light interaction with a fiber’s
surroundings.
• One way to achieve this is to use the evanescent field of
transmitted light.
• If the cladding of a particular fiber is removed, the
evanescent waves can penetrate into the surrounding
medium to the penetration depth, where the amplitude of
the evanescent light decreases to 1/e of the initial value E0.
• The equation for the field amplitude E(x) as a function of
distance x from the fiber core and penetration depth dp is
given as:
𝐸 𝑥 = 𝐸0 𝑒𝑥𝑝 −
𝑥
𝑑𝑝
.
Evanescent Waves (continued)
•
The penetration depth of the evanescent waves can be described by
the angle of incidence (θ1), wavelength of light (λ), and core (nco)
and cladding (ncl) indices of refraction as:

𝑑𝑝 =
.
2
2
2𝜋 𝑛𝑐𝑜
𝑠𝑖𝑛2 𝜃1 − 𝑛𝑐𝑙
•
As seen on the figure below, the light ray travels through the core;
however, there is a small portion of light, called the evanescent
waves, that decays outward from the core/cladding interface.
Fiber Optic Sensing
• Fiber optic sensing operates on the principle of modulation. One or
more of the optical properties of the transmitted light, like intensity,
phase, wavelength or polarization state, is modulated by a particular
parameter.
• The interaction with a medium causes a change in the optical
properties of light that can be detected by various means.
• Sources of light typically include lasers, lamps and light emitting
diodes.
• The light is guided by the optical fiber where it exits through a probe.
After it exits, it interacts with the parameter and is sent back to a
detector.
• Fiber optic sensors are advantageous because of their geometrical
versatility, materials and safety. Their flexibility and size allows them
to be inserted through needles and catheters. Optical fiber materials
are inert and biocompatible, and they can be reused.
Fiber Optic Biosensor
• A fiber optic biosensor (FOB) is a specific sensor that uses
optical properties in order to selectively detect biological
species like DNA and proteins.
• Biosensors are extremely useful for diagnostic and quantitative
purposes.
• Ideally, biosensors could detect analytes in low concentrations
and various environmental conditions like ones that are turbid.
• Detection techniques are being developed in vitro and in vivo.
• Since detection is highly selective, diseases and disorders could
be more quickly and accurately diagnosed.
• These new biosensing techniques will allow for real-time
analytical measurements and eliminate the lapse of traditional
sample preparations and measurements.
Fiber Optic Biosensor (continued)
• In general, most biosensors use enzymes.
• Enzymes are specialized proteins that catalyze and
facilitate reactions that take place in a living organism.
• Enzymes have high selectivity abilities for particular
analytes because they are developed to only catalyze
specific reactions.
• An enzyme is attached to the sensing core/probe of the
FOB. When the probe comes into contact with the analyte,
the enzyme catalyzes a designated reaction.
• When the reaction occurs, the optical properties of the
light change and can be detected.
• Some of these changes occur in absorbance/transmission,
fluorescence and luminescence.
Medical Applications of Optical Fibers
Endoscopy
• Currently, there are great uses for fiber optics in medicine.
• One of these uses is endoscopy, which involves looking inside the
human body by directly inserting into hollow cavities or organs.
• Due to the small size of the optical fibers, incisions tend to be small.
Additionally, the flexibility of the fibers allows the scope to twist and
turn through the body.
• An endoscope can be composed of multiple sets of optical fibers with
different functions. One set is used for lighting and to illuminate the
interior of the cavity or organ. Another set is used for imaging
purposes. Sometimes a third set is used as a laser to make minor
repairs.
• With the advancement of nanotechnology, endoscopes and optical
fiber sensors could be made even smaller and more powerful.
Medical Applications of Optical Fibers
Measuring Blood Flow
• Another example of the application of fiber optic sensing is
measuring blood flow.
• Light from a helium-neon laser is sent through an optical fiber
where it is guided toward the tissue or blood vessel.
• The light hits the blood vessels where some is absorbed and
some is scattered.
• Additionally, the motion of the blood cells causes the light to
undergo a Doppler shift.
• After the light is modulated, it is sent back to the detector
where the blood flow rate can be extracted by spectrum analysis
of the back-scattered signal.
• The Doppler shift depends on the flow rate and thus provides a
relationship to extract useful information.
Surface Plasmon Resonances
• A promising detection technique involves surface plasmon
resonances (SPRs), which occur on the surface of metal
nanoparticles or nanofilms.
• The metal contains a large group of negatively-charged free
electrons. Collectively, the density of the free electrons can be treated
as a plasma of particles.
• Additionally, the metal contains a lattice of positively-charged ions
that can be replaced by a constant positive background.
• Natural plasma oscillations occur when the free electrons oscillate
around the stationary protons at a particular frequency.
• The conversion of the electrostatic potential energy to the kinetic
energy of electrons causes these oscillations.
• A quantum of plasma oscillations is a plasmon.
• Surface plasmons occur along the direction of the metal-dielectric
interface.
Surface Plasmon Resonances and
Polaritons
• In order to excite surface plasmons, an external light radiation with
frequency in resonance with that of the plasmon oscillation
frequency must be applied.
• There are a few configurations of a metal-dielectric interface that
support SPRs, like a prism with one side coated with a gold nanofilm
and an optical fiber having a section covered by a micro- or nanosize thickness of metal film around the optical core.
• Adjusting the thickness of the nanofilm and frequency of the
incident light allows for optimization of the SPR.
• SPR is the collective oscillations of electrons stimulated by light in
resonance with the plasmon oscillation frequency.
• Evanescent waves from the core hit the metal nanofilm in the sensing
part of the fiber and cause surface plasmon polaritons to appear.
• The surface plasmon polaritons are electromagnetic waves that
oscillate along the direction of the metal and dielectric interface.
Fiber Optic SPR Sensors
• SPR fibers have the capability to detect changes in the
external refractive index.
• The optical fibers offer several advantages like small size,
flexibilit, and remote sensing. Remote sensing could
eventually occur within the human body itself.
• A fiber optic SPR sensor is basically an optical fiber with
one section of the cladding removed and replaced with the
metal nanofilm in order to promote SPRs.
• The nanofilm is then coated with the dielectric/sensing
layer.
• This sensing layer can be coated with molecules that will
bind to other target molecules.
Fiber Optic SPR Sensors (continued)
• When a testing medium containing the analyte passes over
the sensing core, molecular interactions occur between
the analyte and the molecules on the sensing layer.
• These interactions change the refractive index of the
medium.
• Incident light with resonance frequency enters the optical
fiber, strikes the metal-dielectric interface, and causes
SPRs to occur.
• A photodetector measures the decrease in absorbance due
to the change in the refractive index of the sensing layer.
• Each sensor depends on the specificity of its function of
detection of a particular analyte.
Examples of SPR Fiber Biosensors
Phenol Concentration
• Researchers from the Indiana Institute of Technology used
an optical fiber biosensor to detect phenolic compounds.
• One of their goals was to measure the response of SPRs
and relate this to the concentration of phenol.
• To construct the apparatus, they used an optical fiber with a
section of exposed cladding that was replaced with a 40 nm
silver film.
• The sensing layer was covered with immbolized tyrosinase
in polyacrylamide gel.
• Tyrosinase is an enzyme that catalyzes the breakdown of
phenols.
Examples of SPR Fiber Biosensors
Phenol Concentration (continued)
• A flow cell was constructed around the fiber in order to deliver the solutions
containing various concentrations of phenol.
• After the light passed through, it was analyzed by a spectrometer connected
to a computer.
• The resonance wavelength was varied in order to find the new optimal
wavelength for the SPRs. Each different concentration had a different
optimal wavelength.
• Different concentrations caused varying indices of refraction.
• With the data collected, the researchers were able to construct curves with
polynomial fits of the resonance wavelength versus concentration.
• These curves could be used as calibrations to detect and quantify unknown
concentrations of phenols.
• Phenols are typically biohazards, and this experiment offers a new and
effective detection method.
Examples of SPR Fiber Biosensors
(continued)
• There is much potential for fiber-SPR biosensors.
• In addition, nanoscale optical fibers have been created with unique
properties that give rise to great evanescent wave fields, and this
would allow for improved diagnostics and penetration depth.
• The next step involves making the SPR biosensors smaller and
effective inside the human body giving rise to lab-on-a-chip and array
technology, real-time monitoring, and superior diagnostic services.
• Instead of having to draw blood and send it to the lab, the analyte can
be analyzed within the body with a ‘middleman.’
• Nanotechnology can greatly assist in the process of miniaturizing
these sensors.
• Overall, SPR biosensors with the help of nanotechnology will be able
to operate inside the body and thus revolutionize diagnostics.
Conclusions
• Fiber optics has much potential in the field of
nanomedicine with its:
 superior signal transmission
 flexibility
 inertness
 remote ability.
• Fiber optics will assist in early detection and therapy of
cancer by closely monitoring various proteins or
biomolecules that are present in cancer.
• Fiber optic biosensors with the SPR technique will
become the next generation of diagnostic tools that can
occur within the human body with great specificity,
sensitivity and accuracy.