Frontiers of Microwaves and Optoelectronics; Anamaya Publishers, New Delhi, 2008;

ISBN 978-81-89927-19-6 (468-474)

Modified Cladding Optical Fiber

Chemical Sensor Using Polyaniline
Doped with Acrylic Acid

K.P. Kakde, H.J. Kharat, P.A Savale, K. Datta,

P. Ghosh and M.D. Shirsat
Optoelectronics and Sensor Research Laboratory, Department of Physics
Dr. Babasaheb Ambedkar Marathwada University, Aurangabad - 431 004, India

Abstract: We have developed a fiber-optic chemical sensor by replacing a

certain portion of the original cladding by a chemically sensitive material,
specifically, polyaniline. The sensor is based on the change of optical power or
optical intensity modulation induced within modified multimode optical fibers.
The sensor design is based on modified cladding technique; the conducting
polymer film of the polyaniline doped with (Acrylic acid) AA, sensitive to
ammonia gas with optimized synthesis parameters was, coated on a small section
of the uncladed fiber. Coating technique i.e. in-situ chemical polymerization
was used. The best dopant, processing technique and substrate nature were
selected and investigated for better sensitivity to the ammonia. The sensing
element length, source intensity and source wavelength, shows a dramatic
influence on the sensor response.

Keywords: Fiber-optic chemical sensor modified cladding, Conducting polymer,

Sensor technology.

Introduction
In recent years, a lot of attention has been given to the use of conducting polymers
in chemical sensors, as sensing layers for gases detection, because of merits such
that easy fabrication, low power consumption, and low poisoning effect [1–8].
Conducting polymers are a new class of sensing materials, which can be prepared
by a simple oxidative polymerization method. They exhibit reversible pH-induced
spectroscopic and gas-induced conductivity changes. They also provide a suitable
structure for immobilization of ligands, enzymes and antibodies. Therefore, their
use in the development of novel chemical and biological sensors has received
considerable attention [9-12]. The sensitive parameters in these sensors are changes
Modified Cladding Optical Fiber Chemical Sensor Using Polyaniline 469
in the work function, the conductivity or optical absorption coefficient of the
polymer. Considerable effort has directed towards the development of chemical
sensors by the change in optical properties [13-16]. Till now, metal oxides such
as SnO2 and Fe2O3 are mostly used as sensing materials. The principal
disadvantages of such materials include high dependence on the detecting
environments. However, the optical method shows independence from
environmental interference.
In this paper, the low cost optical fiber chemical sensor, i.e. ammonia sensor
is developed. The plastic (PMMA) optical fiber has been used for the sensing
application. The fiber -optic sensing element is prepared by replacing the
original cladding material with a chemical sensitive material, polyaniline, on a
certain portion of an optical fiber. The ammonia sensitive film of the conducting
polymer (PANI) doped with Acrylic acid (AA) as a novel material, has
been optimized. The parameters like conductivity, adhesitivity to substrate,
porosity, uniformity and sensitivity to ammonia gas of the film have been
optimized on the PMMA substrate. The gas sensing properties of the synthesized
PANI film in terms of change in resistance of the film has been carried out by
indigenously developed computer controlled gas sensing system. Then the
film with optimized parameters was deposited on the optical fiber. The optical
properties of the sensor have been studied by indigenously developed gas
sensing chamber and fiber optic bench, when the sensor is exposed to the
ammonia vapor.

Experimental

Synthesis of Polyaniline Film by Chemical Polymerization

PANI-AA was synthesized using in-situ polymerization of aniline monomer by
using ammonium peroxydisulfate (APS) as an oxidant in the presence of AA as
a dopant. The polymerization was carried out at 10°C ± 0.5 in a temperature
controlled water bath for 20 hour. In this process, 0.50 M of AA aqueous solution
and 0.25 M of aniline were added into 10 ml of distilled water, and (then) the
solution was stirred by an electromagnetic stirrer for about half hour. Afterwards
the solution was cooled down to 10°C and 10 ml of APS aqueous solution (0.25
M) was added drop wise to the solution containing AA and aniline monomer with
continuous stirring. The PMMA substrate was submerged in the reaction mixture
of aniline and APS and as a result PANI film was deposited on PMMA substrate.
Then the resulting film was removed from the solution, washed with distilled
water and dried.

Preparation of Optical Fiber Sensing Element

Preparation of the sensing element i.e. the modified cladding region involves
three steps, (a) stripping off the jacket (b) removal of the passive cladding, and
(c) application of active cladding. A plastic multimode fiber with core/cladding/
jacket dimension of 960/40/250 ∝m was used in this work. A meter length of
optical fiber is used and a small section (1cm-4cm) of the jacket was stripped off
470 KAKDE ET AL

L F F D
C

Fig. 1. Schematic diagram of the experimental set up. L: Light emitting diode,
F: optical fiber, C: airtight chamber, V: ammonia vapor, D: photodetector
and S: signal processor.

the center of the optical fiber. The sensor is prepared by removing the cladding
of a small portion of the fiber by polishing with the abrasive paper and application
of the acetone and water on the fiber and polishing with the tissue [17].

Coating of Polyaniline Film on Optical Fiber

The in-situ deposition of the chemically active polyaniline on the fiber modified
section is achieved by suspending the uncladed region of the optical fiber in the
reaction container, consisting of monomer, oxidant and the dopant acid. The
plastic optical fiber with core/cladding/jacket
dimension of 960/40/250 ∝m was used in this
work. The fiber with removed cladding (1-4
cm) is suspended in the reaction container,
containing the aniline, ammonium
peroxidisulphate and the acrylic acid with
the optimized reaction and the process
parameters discussed above. The resulting
coated fiber was removed from the solution, Fig. 2. SEM picture of optical fiber
washed with distilled water and dried. The coated with polyaniline film.
SEM picture of the optical fiber sensor coated with PANI film by in situ chemical
deposition method is shown in Fig. 2.

Determination of Sensing Properties of Optical Fiber Sensor

An experimental set-up used for the characterization of optical fiber sensor is
shown in Fig. 1. The light source was focused onto the one end of the modified
optical fiber sensor. At the other end, a photo detector was positioned to receive
the optical signal, and convert the same to an equivalent electrical signal. The
change in output power is measured when the sensor is exposed to different
concentrations of ammonia vapors (20-200 ppm) at room temperature.
The influence of the sensing length on the sensor response was investigated.
The sensor with different sensing length (1-4 cm) was used and the response
of the sensor was investigated. The effect of the source wavelength was studied,
in which sources with different wavelength 450 nm; 550 nm and 650 nm were
used to test the influence of the wavelength on the sensitivity of the sensor. The
Modified Cladding Optical Fiber Chemical Sensor Using Polyaniline 471
effect of power variations of the source on the sensor response was investigated
by varying the power of the source (1-3.5 ∝w).

Result and Discussion

Sensing Behavior of Synthesized PANI Films

Sensing behavior of the synthesized PANI films was studied using indigenously
developed computer controlled gas sensing chamber. The synthesized PANI
films were exposed to ammonia gas for 7 minutes. The recovery time was
measured by exposing the film to the air for 7 minutes. The change in resistance
of the film was recorded at an interval of 15 second. We have tested synthesized
PANI-AA films for 20, 100 and 250 ppm of ammonia. The relationship between
change in resistivity of the synthesized PANI film with time when exposed to
20 ppm, 100 ppm and 250 ppm concentration of ammonia gas is shown in
Fig. 3. It was observed that the resistance of the polyaniline film increases when
exposed to ammonia; it reaches a maximum value and becomes constant.
The resistance decreases steadily to a minimum value, when the ammonia gas
was removed; however, a drift from its original value was observed. The
response time for the film was found to be 180 s and the recovery time
is found to be 300 s.
30
c Air
25 Air
20 b

15 a
10
5
NH3 NH3
00 300 600 900 1200 1500 1800
Time (s)
Fig. 3. Response of the synthesized PANI film to ammonia gas: (a) 20 ppm,
(b) 100 ppm and (c) 250 ppm.

Effect of the Ammonia Vapor on the Output Intensity of the Sensor

Figure 4 shows the response curve of the sensor when exposed to ammonia. For
the purpose of investigation of reproducibility and response characteristics of the
sensor three measurements was continuously carried out. The 2 cm sensor was
exposed to 20 ppm of ammonia vapor and change in output power of the sensor
was observed. All of the measurements are almost same. This is the one of the
important characteristics of the sensor. In addition to this the sensor has recovery
time below 10 min. The sensor has fast recovery time and good repeatability.
Figure 5 shows the response of the sensor for different concentration of ammonia
vapor. The length of sensor was 2 cm. It shows linear response for 50 ppm to
200 ppm of ammonia concentration.
472 KAKDE ET AL

0.1
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
0 10 20 30 40 50 60
Time (min)
Fig. 4. The response curve of the sensing system to 20 ppm of ammonia.

0.25
0.23
0.21
0.19
0.17
0.15
0.13
0.11
0.09
0.07
0.05
0 50 100 150 200
Concentration (ppm)
Fig. 5. The response of the optical fiber sensor to different ammonia concentration.

Effect of the Sensing Length of the Sensor

The different sensing elements i.e. 1 cm to 4 cm were prepared and coated with
polyaniline film doped with acrylic acid with the optimized parameters.. We
observed best response for 2 cm sensing element, when it was exposed to 50
ppm of ammonia vapor. The decrease in output power (intensity) with increasing
sensing length is attributed to the increase in the number of leaky modes. The
increase in the sensor response is due to the increase in sensor length from 1
cm to 2 cm, but for 3 cm and 4 cm sensor response is less i.e. the change in power
(intensity) is less, which may be due to the more sensing length which incorporates
more leaky modes and hence less light can interact with the film and therefore
the sensor response is less.

Influence of the Wavelength of Light on Fiber Optic Sensors

The light sources with wavelength 450 nm, 550 nm and 650 nm were used to
study the influence of the light wavelength on the sensitivity of the sensor.
The 2 cm sensor coated with sensing film was used and the sensor was
exposed to 200 ppm of ammonia vapor. We observed excellent response i.e.
change in power (intensity) for 650 nm wavelength as compared to 550 nm
Modified Cladding Optical Fiber Chemical Sensor Using Polyaniline 473
and 450 nm of wavelength. Thus the sensor response is highly dependent
on the source wavelength.

Effect of the Source Power

The influence of light intensity of source on sensor response (when it is exposed
ammonia vapor) has also been investigated using Optical Fiber test bench, Ruby
Optosystems, Pune, India. The sensor response was obtained for 1 ∝w, 2 ∝w,
3 ∝w, 3.5 ∝w. The change in output power was maximum for 3.5 ∝w source
power. This may be due to fact the more source power has the more evanescent
power available at the sensor, which incorporates more interaction with the film.

Conclusion
We have designed and developed an optical fiber based chemical sensor for
ammonia gas sensing. The sensor is based on modified cladding approach. i.e.
ammonia sensitive layer was deposited on the core of the sensor. A simple
approach was used to design the sensor. An optimization of the ammonia sensitive
layer was carried out on the PMMA substrate first, which has advantages to carry
out different characterizations easily. A sensitive, simple and low-cost fiber-optic
sensor was successfully designed and developed. The sensing properties of the
optical fiber sensor for ammonia vapors at room temperature have been studied.
We observed linear response for 20-200 ppm of ammonia. It also exhibits good
reversibility and repeatability. These experimental results have demonstrated that a
low cost plastic optical fiber sensor for ammonia gas can be designed and
developed.

Acknowledgement
Authors are thankful to the University Grants Commission, New Delhi, India for
the financial assistance and one of the authors K.P. Kakde expresses his gratitude
for the award of Teacher fellowship under FIP scheme of UGC, New Delhi, India

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