1
A Surface Plasmon Resonance Bio-Sensor based on
Dual Core D-Shaped Photonic Crystal Fibre
Embedded with Silver Nanowires for Multi-Sensing
Suoda Chu, K. Nakkeeran, Senior Member, IEEE, Abdosllam M. Abobaker, Member, IEEE, Sumeet S. Aphale,
Senior Member, IEEE, S. Sivabalan, P. Ramesh Babu and K. Senthilnathan
Abstract—In this paper, for sensing and monitoring the bio-
chemical analyte dissolved in liquid, antigen-antibody interaction
or protein-DNA/RNA binding process, we design a surface plas-
mon resonance refractive index based biosensor by using a dual
core D-shaped six-fold photonic crystal fibre which is embedded
with silver nanowires for multi-detection. We numerically analyze
both the dispersion relations and the loss spectra for various
analytes by finite element method. This optical fibre bio-sensor
monitors the changes of the refractive index for different analytes
by measuring the spectral shifts of the fibre loss peaks at
their resonance wavelengths. With the wavelength interrogation
method, we find that the proposed biosensor with two sensing
channels exhibits a maximum refractive index sensitivity of 3400
nm/RIU and a resolution of 2.94 × 10
−5
RIU for a large
sensing range from 1.35 to 1.50, which covers most known
analytes of proteins, viruses or DNA/RNA. By utilizing 200
nm silver nanowires in the sensing channels, the sensitivity
can be enhanced up to 4000 nm/RIU. Due to its special two-
channel design for multi-sensing, it is possible to distinguish/study
the binding possibility/capability of unknown analyte with two
different target proteins simultaneously. Further, by introducing
another critical channel, the confinement loss for either channel
I or channel II can be greatly enhanced for high accurate result
and more reliable sensing. Moreover, we numerically prove that
the diameter of nano silver wires has great influences on the
sensing peaks and sensitivity of the proposed biosensor.
Index Terms—D-shaped, Nano-sized metallic wire, Photonic
Crystal Fibre, Refractive Index Sensor, Surface Plasmon Reso-
nance, Sensitivity.
I. INTRODUCTION
S
URFACE plasmon resonance (SPR) based fibre sensors
have shown remarkable developments in numerous fields
for label-free sensing application since they are highly robust
and versatile optical tools. With the several outstanding ad-
vantages, in particular, high sensitivity, fast response, low-cost,
capability of removable detection and real-time sensing, it now
gains a considerable attention in a variety of research disci-
plines that range from genomics, proteomics, drug discovery,
Suoda Chu, K. Nakkeeran and Sumeet S. Aphale are with School of
Engineering, Fraser Noble Building, University of Aberdeen, Aberdeen
AB24 3UE, UK (email: r05sc15@abdn.ac.uk; K.Nakkeeran@abdn.ac.uk;
S.Aphale@abdn.ac.uk).
Abdosllam M. Abobaker is with Department of Communications En-
gineering, College of Electronic Technology, Bani Walid, Libya (email:
almahjub11@gmail.com).
S. Sivabalan is with School of Electrical Engineering, VIT University,
Vellore-632 014, Tamil Nadu, India (email: ssivabalan@vit.ac.in).
P. Ramesh Babu and K. Senthilnathan are with Department of Physics,
School of Advanced Sciences, VIT University, Vellore-632 014, Tamil Nadu,
India (email: prameshbabu@vit.ac.in; senthee@gmail.com)
food safety control, environment monitoring and development
to medical diagnostics [1].
SPR is essentially an electromagnetic mode coupling phe-
nomenon due to the collective resonant oscillation of free
electrons in plasmonic metal, which is stimulated by p-
polarized incident light at the interface of metal and dielectric
materials [2]. For a SPR based fibre sensor system, when
the frequencies of incident light through the fibre and free
electrons in the metal coated on the fibre are matched, the
surface plasmons can be excited. This resonance condition is
known as the SPR. Under this condition, a small portion of
energy from the fundamental mode will be transferred to the
surface plasmon mode, which in turn results in a dip in the
fibre loss spectrum. Further, there would be a wavelength shift
of the loss spectrum as and when the refractive index of the
analyte is changed due to the surrounding environment [3]. It is
also called propagating SPR as the enhanced surface plasmons
will propagate along the thin metal film in the same direction
of light propagation in the optical fibre.
Photonic crystal fibre (PCF), also called holey fibre or
micro-structured fibre is a new class of special optical fibre.
The cladding of such fibre consists of an arrangement of tiny
and closely spaced air holes which go through the whole length
of fibre [4]. It is possible to control the fibre optical character-
istics by varying the diameter of the air holes and/or distance
between the air holes and/or changing the arrangement of
the air holes. Thus, the combination of PCF and SPR sensor
technologies help in enhancing the sensitivity of the sensor
by manipulating the structural parameters. In 2011, Zhou et
al. proposed a PCF sensor based on silver nanocubes with
a sensitivity of 3774.2 nm/RIU [5]. In 2015, Otupuri et al.
presented a novel PCF multi-channel biosensor with Gold and
Ta
2
TO
5
by measuring two fundamental modes [6]. Later, in
2017, Lu et al. demonstrated a multilayer-coated SPR sensor
for dual refractive index range measurements with a capillary
structure [7]. But PCF-SPR sensors in multi-sensing still suffer
from low sensitivity or low contrast in the loss spectra of two
analytes samples.
In this paper, we propose a six-fold dual core D-shaped
PCF biosensor based on SPR phenomenon embedded with
silver nanowires for multi-sensing and carry out the sensitivity
analysis for a broad range of analytes of refractive index from
1.35 to 1.50. Moreover, according to the loss spectra results
in comparison with gold nanowires based biosensor, we find
that silver nanowire is more suitable for multi-sensing due to
2
low internal impact between the two resonance peaks from
the coupling dynamics of the two channels for multi-sensing.
Most importantly, the novel use of the critical channel with the
additional silver nanowire increases the loss coefficient of the
target analyte between two samples which can greatly reduce
the measuring error for accurate and practical multi-sensing.
The highest sensitivity can be obtained by applying 200 nm
silver nanowires is 4000 nm/RIU. Therefore, the proposed
biosensor has a steady and stable sensitivity characteristics and
meet the requirements for multi-sensing with high efficiency
that can be utilized as a biomedical tool for test-measurement
of the molecular levels of biological samples or clinical
identification of antigen/antibody for their binding reaction,
etc.
II. GEOMETRIC STRUCTURE AND THEORETICAL
MODELLING
The cross sectional structure of the proposed SPR biosensor
is shown in Fig. 1. The side polished depth of the dual core
D-shaped PCF is set as d=7.35 µm. Two half polished air
holes are designed to fill in with nanowires and immobilized
antibodies as sensing channels. Entire PCF-SPR sensor com-
posed of four layers of air holes which are arranged in a fixed
distance of lattice pitch, Λ=2 µm. The diameter of air holes
is set as 1.2 µm and the diameter of silver nanowire is kept
as 300 nm. Those silver nanowires are embedded into the half
polished air hole at the top surface, along the whole length
of fibre sensor which is exactly between analyte and silica
cladding for generating the SPR. Two different antibodies
were immobilized around the silver nanowires within two
sensing channels and the phosphate buffered saline (PBS)
buffer solution with antigen flows through the biosensor’s D-
shaped surface. A three dimensional (3D) schematic of the
proposed sensor is presented in Fig. 2 with silver nanowires,
immobilized antibody and antigen in PBS buffer solution.
Fig. 1. Cross-section of the proposed six-fold dual core PCF-SPR biosensor.
In this structure, the cladding material is made of pure silica
and the wavelength dependence of the refractive index of silica
can be calculated from Sellmeier equation [8]:
Fig. 2. 3D diagram of the proposed six-fold dual core PCF-SPR biosensor.
n(λ) =
s
1 +
B
1
2
λ
2
− C
1
+
B
2
2
λ
2
− C
2
+
B
3
2
λ
2
− C
3
(1)
where B
1
=0.691663, B
2
=0.407943, B
3
=0.897479,
C
1
=0.004679, C
2
=0.013512, and C
3
=97.934003. Here,
λ represents the incident light wavelength in vacuum.
Following Drude model can be used to describe the dielectric
constant of silver or gold [9]:
ε(λ) = 1 −
λ
2
λ
c
λ
2
p
(λ
c
+ iλ)
(2)
where λ
p
is the plasma wavelength and λ
c
is the collision
wavelength of the metal. For silver, λ
p
is 0.14541 µm and λ
c
is
17.614 µm [10]. In the case of gold, λ
p
is 0.16826 µm and λ
c
is 8.9342 µm [11]. As the binding process between the analyte
and the protein/antibody surface results in an increase in the
refractive index, the refractive indices in two sensing channels
are considered as the channel I is fixed as 1.35 and channel
II varies from 1.36 to 1.50 for comparison. The refractive
index range is referenced from the research work of Voros
[12]. The refractive index of the running PBS buffer solution
with antigen/virus/DNA is considered as 1.34 [13]. For the
numerical study, we use a finite element method (FEM) with
a perfectly matched layer (PML) as the boundary condition.
The proposed dual core PCF biosensor can be fabricated by
the state-of-art technique of stack-and-draw process [14] and
side polishing method [15]. The stacked preform arrangement
contains two solid fused silica rods for dual core and four
layers of hollow silica capillaries for air holes in the fibre
fabrication process as shown in Fig. 3. The well developed
and controllable side polishing method is available to polish
the desired depth for the D-shaped cross-section as required.
Such D-shaped PCF with similar structure of half-open air-
hole channels was successfully fabricated by Kim et al., [16].
On the other hand, the metallic nanowire placement can be
realized by filling nanowire colloids into the air-hole channels
by capillary force and air pressure [17].
III. ANALYSIS OF SENSING PERFORMANCE
For the proposed dual core PCF-SPR based biosensor with
silver nanowires, due to the SPR phenomenon, it is well
known that the energy of plasmon mode would arise as a
consequence of the light energy transfer from the fundamental
mode. Figures 4 illustrate both the distribution of electric field
of the fundamental mode and the surface plasmon mode for a
3
Fig. 3. Stacked preforms of the proposed dual-core PCF and side polishing
method for the D-shaped surface with half-open air-hole channels.
range of wavelengths from 600 nm to 900 nm. Here, the arrows
indicate the direction of the electric field. It can be clearly seen
from Figs. 4(a), 4(b) and 4(I) that a part of energy of core-
guided fundamental mode from core area is transferred into
plasmon mode around the nano-sized silver wire in channel I
for n
a
=1.35. Hence, the strongest surface plasmon resonance
is excited as this energy transfer reaches to a maximum due to
the phase matching between core-guided mode and plasmon
mode occurs at this particular wavelength (697 nm). This
phenomenon takes place as both the core-guided fundamental
and the surface plasmon modes have the same real part of their
propagation constants at this resonance wavelength. Hence, the
intersection point of the dispersion relations for core-guided
fundamental mode and plasmon mode can be used to locate
the resonance wavelength. Results in Figs. 4(b), 4(c) and 4(II),
depict another resonance for n
a
=1.38 when the wavelength
approaches to 780 nm in channel II. As the monoclonal
antibody binds only to one ligand site of a particular antigen
due to the specific binding principle [18], the two channels will
have no influence with each other as long as their refractive
indices of binding layer are different. This helps the biosensor
to achieve highly accurate and stable sensing results. Finally,
results presented in Figs. 4(c) and 4(d), show that the wave-
length of the incident light is not creating any considerable
resonance, as the energy can be barely transferred to the
surface plasmon mode. Owing to a shift in the confinement
loss peak of the resonance wavelength for analytes of different
refractive indices, the minute change of surrounding refractive
index can be detected. Therefore, we can make use of those
confinement loss peaks for multi-sensing purpose. Moreover,
because of the association and dissociation in the process of
antigen-antibody binding reaction, this biosensor can also be
used to monitor the reaction duration by detecting their SPR
refractive index changes based on the confinement loss data
over time scale. According to [19], the confinement loss α
loss
is defined by:
α
loss
=
40π
λ ln 10
= (n
eff
) , (3)
where λ is the wavelength of incident light in vacuum and
= (n
eff
) represents the imaginary part of the effective refractive
index of the core-guided fundamental mode.
The dispersion relation of the core-guided fundamental
mode and surface plasmonic mode along with the confinement
loss is presented in Fig. 5 when the refractive indices of
Fig. 4. Light energy flow distributions in the cross-section of dual core D-
shaped PCF-SPR sensor for different wavelengths for the analyte refractive
index of n
a
= 1.35 and n
a
= 1.38. (a) is the fundamental mode at 600 nm.
(b) and (I) are the fundamental mode and surface plasmonic mode at first
resonance wavelength of 697 nm for n
a
=1.35 of channel I. (c) and (II) are the
fundamental mode and plasmonic mode at the second resonance wavelength
of 780 nm for n
a
=1.38 of channel II. (d) is the fundamental mode at 900
nm. The arrows indicate the direction of the electric field.
0 . 6 0 0 . 6 5 0 . 7 0 0 . 7 5 0 . 8 0 0 . 8 5 0 . 9 0
1 . 4 4
1 . 4 6
1 . 4 8
1 . 5 0
R e f r a c t i v e I n d e x
W a v e l e n g t h [μ m ]
C o r e M o d e
P l a s m o n i c M o d e ( C h a n n e l 1 )
P l a s m o n i c M o d e ( C h a n n e l 2 )
C o n f i n e m e n t L o s s
0 . 0 0
0 . 0 2
0 . 0 4
0 . 0 6
0 . 0 8
0 . 1 0
0 . 1 2
R e s o n a n c e P e a k 2
L o s s [ d B / c m ]
R e s o n a n c e P e a k 1
Fig. 5. The dispersion relation of the fundamental mode (black line), two
surface plasmon modes for n
a
=1.35 (red line, channel I) and n
a
=1.38 (green
line, channel II) at 697 nm and 780 nm, respectively and confinement loss
(blue line) for wavelength range from 600 nm to 900 nm.
antigen-antibody bindings are 1.35 and 1.38. It is obvious that
the confinement loss approaches maximum at the intersection
point of core mode and plasmonic mode due to the maxi-
mum energy leakage of fundamental mode at the resonance
wavelength. As discussed before, when the real part of the
effective refractive index of the fundamental mode coincide
with the two SPR modes (red and green lines in Fig. 5), the
phase matching conditions for two coupling dynamics between
the two sensing channels take place at 697 nm and 780 nm,
respectively.
For a refractive index based biosensor, the SPR wavelength
is extremely sensitive to the surrounding refractive index
changes. The variations of loss spectra against wavelength are
presented in Fig. 6 for various analytes of refractive index
varying from 1.35 to 1.45. The detailed sensitivity data is
presented in Table. I for a full set of analyte of refractive index
from 1.35 to 1.50. From the simulation results, it is obvious
that as the refractive index of the analyte increases, the loss
4
Fig. 6. Confinement loss spectra for n
a
of channel I fixed at 1.35 while
channel II is set as a range of n
a
from 1.36 to 1.45 for comparison. The
enlarged region shows the details of the peaks of n
a
=1.35 in channel I and
the peak of n
a
=1.36 in channel II.
peak is getting shifted towards the longer wavelength with
increase in their respective loss amplitudes. The enlarged graph
shows that for the detection of n
a
=1.35 and 1.36, due to the
sub-peak influence [20], the two loss curves become broader
and blunt, making it difficult to detect. Hence, according to the
loss spectra, the detectable analytes refractive index threshold
for multi-sensing is limited to a resolution of 0.01. Also, from
the simulation results, the maximum sensitivity is computed to
be 3400 nm/RIU using the wavelength interrogation method
[21]. For a 0.1 nm peak-wavelength resolution instrument,
the refractive index resolution of corresponding sensor is
2.94 × 10
−5
RIU. How much sharp/blunt (narrow/broad) the
confinement loss characteristics is indicated by the full-width
at half-maximum (FWHM) bandwidth of the the curve and for
n
a
=1.36, the loss curve is too blunt and broad to be calculated
(mentioned as N/A in Table. I). The average figure of merit
(FOM) value for the proposed biosensor is calculated to be
74.5 RIU
−1
.
By swapping the silver nanowires immobilized with anti-
bodys, which corresponds to the change of refractive indices
of the binding layers to 1.38 for channel I and 1.35 for
channel II, we find that the loss spectra remain the same as
shown in Fig. 5. This proves that the resonant characteristics
of this biosensor with such nanowire configuration are not
related to the channel position but it is determined by the
refractive indices of the binding layers. On the other hand,
this inference brings the limitation on the application of the
proposed biosensor to distinguish the target binding analytes
TABLE I
SENSITIVITY DATA OF THE D-SHAPED DUAL CORE PCF-SPR
BIOSENSOR
(BINDING LAYER RANGES FROM 1.35 TO 1.50 [SILVER NANOWIRE])
Analyte Resonant Peak FWHM Sensitivity
Refractive Wavelength Loss Bandwidth
Index (nm) (dB/cm) (nm) (nm/RIU)
1.35 697 0.030 37.2 –
1.36 722 0.056 N/A 2500
1.37 751 0.068 38.5 2900
1.38 780 0.092 37.5 3000
1.39 810 0.126 38.3 3100
1.40 841 0.174 39.4 3100
1.41 872 0.240 40.4 3200
1.42 904 0.329 41.7 3200
1.43 936 0.447 42.9 3200
1.44 969 0.605 44.2 3300
1.45 1002 0.811 45.5 3300
1.46 1036 1.080 46.5 3400
1.47 1069 1.429 47.6 3300
1.48 1103 1.877 48.6 3400
1.49 1137 2.448 49.7 3400
1.50 1170 3.171 49.1 3400
Fig. 7. Cross-section of the D-shaped dual core PCF-SPR biosensor with
critical channel of silver nanowires.
between the two channels simultaneously. Moreover, as listed
in Table. I, the confinement losses are less than 0.1 dB/cm
for a range of analyte refractive indices from 1.35 to 1.38.
Those confinement loss values will cause lower power loss
that cannot be detected appropriately and accurately using an
optical spectrum analyzer (OSA). In order to overcome those
limitations, a critical channel with an addition silver nanowire
is introduced in the central half-polished air hole of D-shaped
surface, as illustrated in Fig. 7. Three different scenarios are
considered for comparison of this modified biosensor: (1)
critical channel with n
a
=1.35, (2) no critical channel and
(3) critical channel with n
a
=1.38. The results reported in
Fig. 8 show that with a single nanowire (either n
a
=1.35 or
1.38) present in the critical channel, the confinement loss
gets significant enhancement than the case without any critical
channel. For RI values of n
a
=1.35 and 1.38, the confinement
losses can be increased from 0.03 dB/cm to 0.157 dB/cm and
0.092 dB/cm to 0.445 dB/cm, respectively. Typically, PCF-
SPR biosensors require only a short length (L) of optical
fibre (usually L = 10 mm to 25 mm) [22]. Hence, for a 10
mm long PCF-SPR biosensor, the minimum transmission loss
could be 0.157 dB for a RI of 1.35 and 0.445 dB for a RI of
5
0 . 6 0 . 7 0 . 8 0 . 9
0 . 0
0 . 1
0 . 2
0 . 3
0 . 4
0 . 5
0 . 6
C h a n n e l 2
n
1
= 1 . 3 5 a n d n
2
= 1 . 3 8
( w i t h c r i t i c a l c h a n n e l f o r 1 . 3 5 )
n
1
= 1 . 3 5 a n d n
2
= 1 . 3 8
( n o c r i t i c a l c h a n n e l )
n
1
= 1 . 3 5 a n d n
2
= 1 . 3 8
( w i t h c r i t i c a l c h a n n e l f o r 1 . 3 8 )
W a v e le n g th ( μ m )
L o s s [ d B / c m ]
C h a n n e l 1
Fig. 8. The loss spectra as a function of wavelength when the refractive
index of binding layers are set as 1.35 and 1.38 for multi-sensing enhanced
by critical channel with an additional silver nanowire.
Fig. 9. Cross-section of the D-shaped dual core PCF-SPR biosensor with two
loss enhanced channels of silver nanowires.
1.38, which is 3.55% and 9.74% variation in the transmitted
intensities, respectively. According to [22], the power loss of
the enhanced biosensor can meet the requirement of at least
1% variation in the transmitted intensity with distinguishable
loss peak for proper sensing. Also, due to the low confinement
loss characteristics of the proposed PCF-SPR biosensor it
might be required to use a lengthier PCF than the typical value
to achieve the minimum required power loss.
Another way to achieve more distinguishable loss peak
(confinement loss enhancement) for better sensing is to use
a PCF-SPR biosensor design structure as shown in Fig. 9.
This D-shaped dual core PCF-SPR biosensor with two chan-
nels of silver nanowires configuration exhibits a significant
confinement loss enhancement as shown in Fig. 10. For a
RI value of n
a
=1.38, the confinement loss can reach up to
4.728 dB/cm, which is 50 times more than the first proposed
D-shaped configuration (Fig. 1), and 10 times more than the
configuration with critical channel (Fig. 7). With the enhanced
confinement loss characteristic (Fig. 10) of the two-channels
PCF-SPR biosensor with nanowires (Fig. 9), it is possible to
keep the length of the proposed biosensors within the typical
value of the sensing region. However, further improvement
of the two-channel nanowires configuration by adding another
Fig. 10. The comparison of loss spectra as a function of wavelength when the
refractive index of binding layers are set as 1.35 and 1.38 for multi-sensing
with (solid lines) and without (dashed lines) loss enhanced channels.
0 . 8 0 . 9 1 . 0 1 . 1 1 . 2 1 . 3 1 . 4
0
2
4
6
8
1 0
1 2
1 4
S u b P e a k
( C h a n n e l I I )
S P R P e a k
( C h a n n e l I I )
n
1
= 1 . 4 5 a n d n
2
= 1 . 5 0 ( l o s s e n h a n c e d w i t h o u t c r i t i c a l c h a n n e l )
n
1
= 1 . 4 5 a n d n
2
= 1 . 5 0 ( l o s s e n h a n c e d w i t h c r i t i c a l c h a n n e l )
W a v e le n g t h (μ m )
L o s s [ d B / c m ]
S P R P e a k
( C h a n n e l I )
Fig. 11. The loss spectra as a function of wavelength when the refractive
indices of binding layers are set as 1.45/1.50 in the confinement loss enhanced
configuration with and without the critical channel (nanowire diameter =200
nm).
critical channel with nanowire for multi-sensing to distinguish
the binding reactions in two different channels will not be pos-
sible. It is because two adjacent nanowires in the configuration
would cause two surface plasmons for higher refractive index
analyte, especially when the diameter size of the nanowires
are very small. The unwanted sub-peak of SPR illustrated in
Fig. 11 might affect the accuracy and stability of the PCF-SPR
biosensor [20].
Gold (noble metal with higher chemical stability) is the most
commonly used material in practice for SPR sensor designs
![TABLE II THE RESONANT WAVELENGTH OF THE D-SHAPED DUAL CORE PCF-SPR BIOSENSOR (BINDING LAYER RANGES FROM 1.35 TO 1.50 [SILVER NANOWIRE])](/figures/table-ii-the-resonant-wavelength-of-the-d-shaped-dual-core-3hmbi6rf.webp)
