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Sensors and Actuators A 205 (2014) 5862
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical
journal homepage: www.elsevier .com/ locate /sna
Conducting polymer coated optical microfiber sensor
for alcohol detection
Yeong Siang Chiam a, Kok Sing Lim b, Sulaiman Wadi Harun b,Seng
Neon Gan a, Sook Wai Phang a,
a Department of Chemistry, Faculty of Science, University
ofMalaya, 50603 Kuala Lumpur, Malaysiab Photonics Research Centre,
Department of Physics, University ofMalaya, 50603 Kuala Lumpur,
Malaysia
a r t i c l e i n f o
Article history:
Received 12 April 2013
Received in revised form 21 October 2013
Accepted 24 October 2013
Available online 1 November 2013
Keywords:
Polyaniline
Microfiber sensor
Alcohol
Wavelength shift
a b s t r a c t
In this study, an optical microfiber sensor coated with
conducting polymer, polyaniline (PAni) is investi-
gated as a detector for alcohols. PAni was synthesized by
chemical oxidation method and dioctyl sodium
sulfosuccinate (AOT) was used as the dopant. PAni was then
coated onto the microfiber by drop coating
method. The proposed sensor exhibits wavelength shift in the
output spectrum in response to various
types of alcohols at different concentrations. The output
response can be associated with the solution
refractive index and number ofalkyl substituent ofalcohol.
Furthermore, this sensor also shows excellent
repeatability in the detection. In conclusion, a conducting
polymer based microfiber optical sensor for
alcohol detection with simple, low cost and effective set-up was
successfully achieved in this study.
2013 Published by Elsevier B.V.
1. Introduction
Fiber optic sensors have attracted a great deal of interest
dueto their excellent performance in numerous sensing
applications
include chemistry, biomedicines and environmental
engineering
[14].
Generally, fiber optics sensors can be divided into two cat-
egories, namely extrinsic and intrinsic sensors. With
extrinsic
sensors, light is emitted from the fiber and is blocked or
reflected
back into thefiber-optic systemsuch as optodes. While
forintrinsic
fiber, light is transported at internal sensing system [5]. In
this type
of sensors, modifications are made at the structure of optical
fiber
such as fiber cladding removal and tapered fibers.
The operating principle of most electrical based chemical
sen-
sors is based on electrical conductivity of the chemical
analytes
which is a function of the amount of ions in solution. The
mea-
sured electrical conductivity is then translated into the degree
of
ion concentration.However, this technique is susceptible to
electri-
cal interference, corrosionto the sensor[6].
Therefore,opticalbased
sensors arepreferable dueto their many promisingproperties
such
as non-invasive nature, immunity to electromagnetic
interference,
highsensitivity, compact size,low cost,possibilityof realtime
mon-
itoring and remote sensing [7,8]. Besides, these sensors can
also be
used for refractive index measurement [9,10] in which only a
small
volume of sample is required. The principle of the
measurement
Corresponding author. Tel.: +60 3 79674122; fax: +60 3
79674193.
E-mail address: [email protected](S.W. Phang).
is based on the between the guided light in the single mode
fiber
with surrounding medium. The resonant wavelength varies with
different refractive index of the surrounding medium.
Therefore,this sensor can also be employed for chemical sensing and
liquid
concentration measurement. Subsequently, optical measurement
can be highly appealing. However, the drawback of this system
is
its strong dependence on temperature which can be eliminated
through incorporating a temperature control into the system
or
adopting temperature discriminative configuration [11].
Optical microfiber devices are well known as low-loss silica
waveguides in the forms of loops [12], knots [13], or coils
[14,15],
and havebeen demonstrated for different applications include
add-
drop filters [16], lasing systems [1719], nonlinear optical
systems
[20], and sensing devices [21,22]. Due to the small diameter
of
microfiber, the evanescent field in the surrounding of
microfiber
is large and thus the microfiber devices are sensitive to
changes in
of the surrounding medium, particularly the refractive index
(RI)
of the medium. Ultraviolet (UV) light detection using
microfiber
devicehas been demonstratedby coating theknotareaof
thedevice
with photoresponsive liquid crystal, which refractive index can
be
influenced by UV light irradiated on the device. As a result,
the out-
put spectrum of the device shifts when exposed to the UV
light
[23].
Applying the voltage on the rod tunes the spectral signal
res-
onating from the microfiberring resonator. The operating
principle
of optical microfiber sensors is followed the path whereby
light
measurement is taken based on the resonant coupling of the
light
guided by the single mode fiber. Response to external
physical,
chemical, biological, or other influences will have effects on
the
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Y.S.Chiam et al. / Sensors and Actuators A205 (2014) 5862 59
light guided within the fiber optic sensor [6]. Therefore,
fiber-optic
sensor with cladding modification is very attractive due to its
large
dynamicrange, highsensitivity, andsuperiorintegration
withother
fiber systems.
In 1960, Heeger et al. discovered that conducting polymer is
a
new generation of polymer which displays the combined
mechani-
calpropertiesand processibility ofpolymer as well as
theelectronic
and optical properties of metals or semi-conductors.
Conducting
polymer, while being organic polymer, has the unusual
property
of possessing high electrical conductivity and exhibits a
range
of properties from semiconductor to near-metallic behavior.
Fur-
thermore, they also display the optical properties of metals
or
semi-conductors [24]. Some well known examples of conducting
polymers are polyaniline (PAni), polypyrrole (PPy) and
polythio-
phine (PTh), which are being used as detection medium in
fiber
optic sensor.
Conducting polymer is used in optical sensors due to its
many
advantages such as easy fabrication, excellent ambient
stability,
rapid and reversible adsorption and desorption kinetics, has
low
poisoning effect and low power consumption [25,26]. Besides,
conducting material such as PAni is independent on detecting
envi-
ronment such as surrounding temperature and air humidity
while
traditional sensing material, metal oxide such as SnO2 and
Fe2O3are highly dependent on detecting environment. This make
the
conducting polymer favorable use as sensing material
compared
with traditional material [26]. Furthermore, conducting
polymer
sensor is light, cheap and interacts more favorably with
organic
compounds compared with traditional material [27]. On the
others
hand, PAni can shows multiple color change depending on its
oxi-
dation states. These macroscopic color changes can be used as
an
indicator in optical sensing system [28].
In this study, an optical microfiber alcohol sensor
fabricated
and experimentally investigated. This device is manufactured
by
depositing PAni on a microfiber resonator, particularly at
the
microfiber knot resonator the sensing area of the sensor
which
is then used for the detection of alcohols of different
concentra-
tions. This proposed sensor is a simple, robust and low cost
device
that can quantitate a measurable attribute and converts it into
asignal which can be read by an observer or by an instrument.
Dif-
ferent types of alcohol such as methanol, ethanol, 1-propanol
and
2-proapanol are examined in thisstudy. Besides, alcohols
withvari-
ousconcentrations arealso being studied. From ourknowledge,
we
arethe first toreport on this novel lowcostand
effectiveconducting
polymer-based optical sensor for alcohol detection.
2. Experimental
2.1. Chemicals
All the chemicals such as aniline (Ani), dioctyl sodium sul-
fosuccinate (AOT) and ammonium persulphate (APS) used were
procured from SigmaAldrich (USA) while hydrochloric acid
(HCl)
used was from RCI Labscan (Thailand). Besides, methanol
(CH3OH),
ethanol (C2H5OH),1-propanol and2-propanol (C3H7OH)usedwere
purchased from SYSTERM (Malaysia). All the chemicals used
were
analytical grade and were used without purification.
Deionized
water was collected from Millipore water purification system.
Pre-
clean microscope glass slide with width and length of 75mm
and
25mm, respectively, were used as substrate for the
microfiber
structure during measurement.
2.2. Synthesis of polyaniline (PAni)
In the optical set-up, the sensing layer of coated fiber is
con-
sisting of chemically oxidized PAni using Ani as monomer, AOT
as
dopantand APSas oxidant. Aniwas slowlyaddedintothe AOTsolu-
tion and stirred for 2 h. It was followed byslowaddition of APS
into
the above solution. The polymerization reaction was carried out
at
low temperature for 24h. The resultant sample was washed
with
distilled water to remove the unreacted AOT, APS and
monomer.
Extraction and dilution were carried out to obtain the desired
PAni
concentration.
2.3. Chemical characterizations
2.3.1. Fourier transforms infrared (FTIR) spectroscopy
FTIR spectroscopy of PAni was recorded using FT-IR spec-
trometer (Perkin Elmer RX 1 model) from wavelength range of
4004000nm to confirm the chemical structure of PAni.
2.3.2. UV-vis spectroscopy
Ultravioletvisible (UVvis) spectroscopy of PAni was recorded
using UVVis spectrophotometer (UV-1650 PC model) in the
wave-
length range of 300900 nm to determine the conducting
behavior
of PAni.
2.4. Preparation of tapered fiber coated with PAni
In this study, single mode glass fiber (SMF) was used. The
optic
fiber device composes of a microfiber knot resonator (MKR) in
a
Sagnac loop reflector. Using the flame brushing method,
optical
microfiber with length of 56cm is fabricated from an SMF.
After
that, the microfiber knot resonator was made and fixed at a
diame-
ter of 0.52.0 mm. Then, the microfiber structure was embedded
in
Teflon in order to maintain the physical structure and
resonance
condition [29]. PAni solution was then deposited onto the
MKR
region using drop coating method. The device was left to dry
for
30min.
2.5. Application of PAni based tapered fiber in alcohol
detection
Fig.1a showsthe optical set-up for microfibersensor
whichlaser
with wavelength of 1550 nm was used as a light source for
the
microfiber device. This microfiber device consists of a
microfiber
knot resonator which is coated with PAni. In the experiment,
this
device is connected to a 3-port circulator so that the output
wave
from the microfiber device can be circulated out of the system.
The
incident wave,Einenter into port 1 and exits through port 2
before
it enters the microfiber device. Then, the waves pass through
a
coupler which splits the wave into two. The two waves
counter-
propagate in the Sagnac loop and enter the MKR (as shown in
Fig. 1b) through its two arms. Finally, the waves recombined at
the
coupler and produces the output waves, Eout and it is routed
out
from the device through port 3. The output wave is analyzed
andrecorded using an optical spectrum analyser (OSA) [29]. To
elimi-
nate the temperature influence on the microfiber, the
experiment
is conducted in air-conditioned laboratory in which the
temper-
ature is regulated at 26C. The microfiber device is placed on
a
large aluminum optical table. This stabilizes the temperature
of
the microfiber device and the wavelength fluctuation is less
than
0.02nm/h.
This microfiber device which coated with PAni is used to
study
alcohols at different concentrations such as 1%, 5%, 10%, 20%
and
50%. The alcohols which were investigated in this study
include
methanol, ethanol, 1-propanol and 2-propanol. Refractive
indexes
for each alcohol with different concentration are recorded.
The
optical property of PAni toward different types of alcohols
was
analyzed by OSA.
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60 Y.S.Chiam et al. / Sensors and Actuators A205 (2014) 5862
Fig. 1. (a)Optical set-upfor microfiber sensor. (b)Microscope
image of microfiberknot resonator (MKR). Inset shows thedrawing of
theMKR.
3. Results and discussion
3.1. Chemical characterizations
3.1.1. Fourier transform infra-red (FTIR) characterizationFig.2
shows theFTIR spectrum of theresultedPAni.The peaks at
1614cm1 and 1460cm1 are assigned to quinoid and benzenoid
rings vibration in the polymer chain, respectively. The
absorption
band at1614cm1 alsosupportsthe presence of Anirepeatingunits
in the PAni chain being connected via a head-to-tail monomer
cou-
pling during polymerization. Furthermore, the peaks at 1291
cm1
and 1197cm1 indicating the C N stretching in bipolaron poly-
mer is highly doped and exist in the conducting form. The
peaks
at 36003000 cm1 and 30002800 cm1 correspond t o the N H
and C H stretchingvibrations ofPAni,respectively.TheFTIR
spec-
trum reveals the PAni exist in the emeraldine salt form
[30,31].
Fig. 2. FTIR spectrumof PAni.
Fig. 3. UV spectrum of PAni.
3.1.2. Ultravioletvisible (UVvis) characterization
Refer to the UV spectrum on Fig. 3, the peak observed at 353
nm
is correspond to * transition of benzenoid rings. The shoul-
der peak appears at 434 nm is due to the localized polaron
bands
of protonated PAni while the strong absorption peak appears
at
755 nm indicates the conducting emeraldine salt phase of
dopedPAni [30]. FTIR spectrum in Fig. 2 and UVvis spectrum in Fig.
3
confirm the polymerization of PANi. Since the microfiber
structure
is in thescaleof micrometer, an optical microscope equipped with
a
CCDcamera is used to observe andcapturethe image of PAni
coated
microfiber device. The above figure shows that the PAni has
been
deposited onto the MKR. The electrical conductivity
measurement
has been carried out and the measured conductivity of the
PAni
film is 1102 S/cm2.
3.2. Application of PAni coated microfiber in alcohols
detection
Fig. 4 shows the outputspectra of PAni coated microfiber
device
before and after its immersion in 1% ofn-propyl alcohol. Based
on
Fig. 4, there is a wavelength shift for PAni coated microfiber
from
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Y.S.Chiam et al. / Sensors and Actuators A205 (2014) 5862 61
Fig.4. Wavelength shiftbefore andafterimmersion in
1%ofn-propylalcohol during
optical measurement.
Fig. 5. Electrostatic interaction beween PAni andn-propyl
alcohol.
Fig.6. Wavelength shift during optical measurementfor
differenttypes of alcohols.
1540.42nm (before immersion) to 1539.87nm (after immersion).
This wavelength shift is due to electrostatic interaction
between
the partial positive amine group ( NH+) from PAni backbone
and
the partial negative charge of hydroxyl group ( OH) from
alco-
hol solution (Fig. 5). Due to this electrostatic interaction,
PAni
backbone is restricted from twisting by the hydroxyl group
with
different alkyl substituent which cause solvachromic effect
and
thus decrease the dihedral angle between adjacent aromatic
ringsof the polymer. This theory is proposed by Ginder et al.
through
theoretical consideration [32,33].
The transformation from the planar to the tetrahedral struc-
ture will significantlytwist the torsion angle between the
aromatic
groups of PAni due to the steric hindrance of the aromatic
groups.
The increase in the band gap significantly reduce the
absolute
coefficient of solvent and cause the solvachromic shift. Thus,
the
excitation peak of PAni after immersion in alcohol solution
shift to
the lower wavelength or hysochromoc shift (blue shift) and
appear
at higher energy compare to the excitation peak of the PAni
before
immersion in alcohol solution [34].
From wavelength shift obtained in Fig. 6, the increase of
carbon
atoms in the alkyl group of alcohol molecules (methanol,
ethanol,
1-propanol and 2-propanol) will increase the effect of
steric
Fig. 7. Output responseof thesensor to therefractive indices of
differentalcohols.
Fig. 8. Recycle ability of PAni coated microfiber toward 5%
ethanol.
hindrance and cause red shift to be more pronounced in the
optical
measurement. With increasing of carbon atoms in the alkyl
group,
steric hindrance effect will weaken the electrostatic
interaction
between the NH+ groups from PAni backbone with the hydroxyl
group of alcohol. From methanol to propanol, this effect
signif-
icantly increases the dihedral angle between adjacent
aromaticrings of PAni. Hence, the increase in band gap
significantly reduce
theabsolutecoefficient of solvent andcausethe
solvachromicshift.
Thus, increasing size of alkyl group foralcohol molecules will
cause
an increase in bathochromic shift (red shift) in optical
measure-
ments [35].
Fig. 7 shows that the refractive index is proportional to the
con-
centration of alcohols. With increasing concentration,
wavelength
showsa bathochromic shift(red shift) during optical
measurement.
The increase in alcohol concentrations (from 1% to 30%)
signif-
icantly increases the refractive index of the solution and
effective
refractive index at the microfiber. The change in effective
refrac-
tive index of the microfiber causes wavelength shift in the
output
spectrum of the microfiber sensor. The relationship between
wave-
length shift and refractive index change in the solution can
bewell explained with a linear expression, =kn where is
the wavelength shift,n is the refractive index change and k is
the
sensitivity coefficient.
From Fig. 7, it shows that refractive index increases with
the
increment of alcohol concentration. Besides, optical
measurement
shows increase in the wavelength shift with increasing in
alcohol
concentration (Refer Fig. 6). Thus, the wavelength shift is
directly
proportional to the refractive index of each alcohol.
According to Plancks constant formula, E=hc/, as the wave-
length increase, the energy required for electron transition
between PAni and alcohol will be less which results to a red
shift
(bathochromic shift) in the absorbance spectrum.
PAni coated microfiber shows good recyclability in alcohol
detection as shown in Fig. 8. According to Fig. 8, there is a
constant
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