**2. Modified single mode optical fiber (SMF) sensors**

Researchers showed intensive focus on modified optical fiber platforms as sensing tools since they are more sensitive compared to the conventional fibers. The high sensitivity in the modified fiber is a consequence of the evanescent wave or some portion of the optical power propagates outside of the core layer [3].

**47**

itself is very expensive.

**2.2 Tapered optical fiber sensors**

*Modified Single Mode Optical Fiber Ammonia Sensors Deploying PANI Thin Films*

Evanescence wave-based intensity sensors require fiber modification to expand

Etched optical fiber is widely used for evanescent based sensors. The common process is to etch the cladding part of the fiber. One way to remove the cladding is to immerse the optical fiber using a strong acid in a chemical bath. Chemical etching produces shorter tapers with larger cone angles, resulting in higher transmission efficiency [12]. Depending on the composition of the glass, different acidic solutions are used. The etching rate depends on the dopant concentration in the structure of the optical fiber and concentration of the chemical solution. Etching parameters such as solvent type, acid concentration, etching time, and temperature are critical factors for the resultant optical and geometrical characteristics of the optical fiber [11]. Even though the modified optical fiber sensors enhance the evanescent wave to interact with the surrounding, each technique has its drawbacks. Side-polished fiber is made through polishing a fiber that is implanted on a block, such as a quartz block. The weakness of this technique is that the long time consumed in the fabrication procedures and it is difficult to produce a long sensing region. Hence, it difficult to develop a high sensitivity sensor [13]. The D-shape fiber is an optical fiber with removing half of the cladding layer. It has an advantage of long evanescent field interaction length. The removed-clad, namely etched fiber, sensor offers a simple and inexpensive fabrication method, especially compared with mechanical pulling and D fibers. On the other hand, chemical etching process is not easily controllable. The fabrication of tapered optical fibers is more reproducible and the controllable with the advance in tapering machine technology. However, the tapering machine

In tapered optical fiber, the fiber diameter is reduced at a specific area called taper waist through heating and pulling the fiber ends in opposite directions as

the penetration depth of the evanescence wave to increase its interaction with the surroundings. As a result, different optical fiber modification techniques are deployed including side polishing [4], D-fibers [5], chemical etching [6, 7] or tapering [8, 9] as shown the **Figure 1** [10] to improve the evanescence field for sensing

*Modified optical fiber sensors (a) side-polished, (b) etched and (c) tapered fibers [10, 11].*

*DOI: http://dx.doi.org/10.5772/intechopen.94001*

applications [11].

**Figure 1.**

**2.1 Etched optical fiber sensor**

*Modified Single Mode Optical Fiber Ammonia Sensors Deploying PANI Thin Films DOI: http://dx.doi.org/10.5772/intechopen.94001*

**Figure 1.** *Modified optical fiber sensors (a) side-polished, (b) etched and (c) tapered fibers [10, 11].*

Evanescence wave-based intensity sensors require fiber modification to expand the penetration depth of the evanescence wave to increase its interaction with the surroundings. As a result, different optical fiber modification techniques are deployed including side polishing [4], D-fibers [5], chemical etching [6, 7] or tapering [8, 9] as shown the **Figure 1** [10] to improve the evanescence field for sensing applications [11].

#### **2.1 Etched optical fiber sensor**

*Application of Optical Fiber in Engineering*

achieve this research project are as follows:

sensors in some previous studies.

length ranges.

Researchers showed intensive focus on modified optical fiber platforms as sensing

tools since they are more sensitive compared to the conventional fibers. Cladding modified SMF sensors with high sensitivity integrated with nanostructured thin films against ammonia can be deployed to avoid crises resulted from gas leakage such as ammonia [2]. These sensors have been gained popularity as practical tool to detect chemicals with low concentrations such as gases. By utilizing these configurations, it

The aim of this chapter is to design and demonstrate an etched-tapered SMF optical fiber gas sensor for remote monitoring application. The gas under testing is ammonia due to its high severity and deployment in the industry. The objectives to

• To present different modified optical fiber transducing platforms and nano-

• To design, fabricate and characterize modified SMF transducing platforms,

• To synthesize, deposit, characterize and evaluate gas sensing characteristics of the PANI nanofiber as a sensing layer onto developed modified SMF transducing platforms towards different concentrations of NH3 gas within C-band wave-

• To compare performance criteria of developed SMF sensors with the reported

The next section presents a description of the modified optical fiber platforms as sensing tools since they are more sensitive compared to the conventional fibers. The etched, tapered, etched-tapered platforms as modified optical fiber platforms will be elaborated. After that, the Polyaniline nanofiber employed as a sensing layer is introduced in details. The properties of PANI thin films will be discussed by highlighting its attraction and factors that influence the sensing performance. Later, a detailed review on previous works that use PANI as a sensing layer for ammonia sensors will be presented. Moreover, PANI nanostructured thin film preparation and deposition onto the SMF transducing platforms will be highlighted. Several micro-characterizations of the fabricated nanostructured thin films were carried out to investigate sensing layer morphology and thickness of the nanostructured thin film. These parameters affect gas sensing performance. The sensing performance of the modified SMF sensors including etched, tapered and etched-tapered sensors coated with sprayed PANI nanofibers will be investigated and analyzed when exposed to ammonia with different concentrations. The investigation performed in the range of C-band wavelengths at room temperature. Based on author's knowledge, the investigation of the SMF sensors coated with PANI in C-band wavelengths ranges is not explored yet. Finally, chapter conclusions will be summarizing the performance properties behind the deployment of modified SMF

is expected to fabricate sensors with high sensitivity and fast response.

materials used as sensing layers particularly, polyaniline.

that are etched, tapered and etched-tapered SMF platforms.

sensing platforms Integrated with PANI nanofibers at room temperature.

Researchers showed intensive focus on modified optical fiber platforms as sensing tools since they are more sensitive compared to the conventional fibers. The high sensitivity in the modified fiber is a consequence of the evanescent wave or some portion of the optical power propagates outside of the core layer [3].

**2. Modified single mode optical fiber (SMF) sensors**

**46**

Etched optical fiber is widely used for evanescent based sensors. The common process is to etch the cladding part of the fiber. One way to remove the cladding is to immerse the optical fiber using a strong acid in a chemical bath. Chemical etching produces shorter tapers with larger cone angles, resulting in higher transmission efficiency [12]. Depending on the composition of the glass, different acidic solutions are used. The etching rate depends on the dopant concentration in the structure of the optical fiber and concentration of the chemical solution. Etching parameters such as solvent type, acid concentration, etching time, and temperature are critical factors for the resultant optical and geometrical characteristics of the optical fiber [11].

Even though the modified optical fiber sensors enhance the evanescent wave to interact with the surrounding, each technique has its drawbacks. Side-polished fiber is made through polishing a fiber that is implanted on a block, such as a quartz block. The weakness of this technique is that the long time consumed in the fabrication procedures and it is difficult to produce a long sensing region. Hence, it difficult to develop a high sensitivity sensor [13]. The D-shape fiber is an optical fiber with removing half of the cladding layer. It has an advantage of long evanescent field interaction length. The removed-clad, namely etched fiber, sensor offers a simple and inexpensive fabrication method, especially compared with mechanical pulling and D fibers. On the other hand, chemical etching process is not easily controllable. The fabrication of tapered optical fibers is more reproducible and the controllable with the advance in tapering machine technology. However, the tapering machine itself is very expensive.

#### **2.2 Tapered optical fiber sensors**

In tapered optical fiber, the fiber diameter is reduced at a specific area called taper waist through heating and pulling the fiber ends in opposite directions as

**Figure 2.** *Demonstration of tapered optical fiber [11].*

shown in **Figure 2** [11, 14]. Based on the figure, Dt, Wl and Ut represents the down taper, waist length and up taper regions which are the tapered fiber profile [11].

Several techniques have been developed to fabricate the tapered optical fibers such as etching [15] and flame heating. The flame heating technique has been verified to be the most flexible technique that results in robust physical properties. Later, the flame was replaced with microheater that is more controllable. Recently, computer-controlled machines that are able to fabricate tapers with desired dimensions are available in the market. Conventional SMF or multimode optical fiber (MMF) with standard diameter of 125 μm can be tapered down to 5 μm using the machine [16, 17].

Tapering process improves the sensitivity of the optical fiber sensors by easing the access to the evanescent field, which enables strong interaction between the light and the analyte. To prepare a qualified tapered fiber based devices, the tapered fibers used should be fabricated with high adiabaticity, uniform microfiber diameter and suitable microfiber diameter with large evanescent wave. Fundamentally, strong evanescent wave is obtainable with thinner tapering fiber diameters. Accordingly, the tapered fibers are made with small diameter in the range of 0.8–3 μm for most devices uses tapered fibers.

The strong evanescent wave on the taper waist surface make it more sensitive to its surrounding. The optical fiber sensors fabricated either by etching or tapering processes can be coated with suitable nanostructured sensing layer in order to improve the sensitivity. When sensing layer reacts towards the target analyte, its optical properties may change. Hence, the amount of evanescent wave absorbed by the sensing layer is changed according to the analyte concentrations [11].

#### **2.3 Etched-tapered optical fiber sensors**

The Etched-tapered optical fiber platform comprises of the etching and tapering processes abovementioned. Firstly, hydrofluoric acid is used to remove some of the cladding of the SMF as illustrated previously. The etched area is then tapered using different tapering methods based on the recommended configurations. Dealing with etched fibers to perform the tapering process is a critical and more challenging as compared to tapering the standard fiber [11]. The removal of the cladding layer of the SMF increases its fragility and increase difficulty of fiber handling. This is may be overcome by utilizing a customized holder in which the etched fiber is positioned while it is tapered.

Optical fiber sensors are deployed for detection of different hazardous gases including ammonia (NH3). Ammonia is widely used gas in different applications such as chemical industries, agricultures and medicines [18, 19] . Natural NH3 level present in the atmosphere is in low ppb (1–5 ppb) levels. NH3 can be characterized by its colorless, pungent smell, and explosive, toxic at a high-concentration NH3 atmosphere [20, 21]. Generally, upon exposure to around 50 ppm NH3 gas in air

**49**

*Modified Single Mode Optical Fiber Ammonia Sensors Deploying PANI Thin Films*

may cause acute poisoning or life-threatening situations such as permanent blindness, lung disease, respiratory disease, skin disease. The Occupational Safety and Health Administration (OSHA) has set a limit of 25 ppm in the workplace throughout an 8 hours shift and a short-term limit (15 min) of 35 ppm [11, 22]. Additionally, NH3 sensors remain the potential candidates employed in agriculture, chemical industries, pharmaceutical, hydrogen fuels, defense and food processing industries to monitor the NH3 leak in controlled atmospheres. Hence, the development of highly-sensitive and reliable NH3 sensors to continuously detect leakages NH3 is a

More worrying, the gas is flammable at 50°C at very high concentration (150,000 ppm) [11, 23]. In chemical leakage alarm for NH3 detection, the detection limit ≥1000 ppm with operating temperature range up to 500°C and required response time may be in the range of minutes. The concentrations can be very high at NH3 plants and can even be explosive [24]. Recently, on Aug 2016, Petronas Chemicals Group stated that two workers were killed and three injured by an ammonia leak at a Malaysian chemical plant [25]. Hence, the development of highly-sensitive and reliable sensors to continuously detect leakages of NH3 is a key issue for the safety of

In general, the performance of the nanomaterial based sensors, particularly sensitivity, is controlled by three factors, material characteristics, transducer function and variability for sensor development [30]. The material characteristics indicate its surface ability to detect a specified chemical. The transducer function refers to the ability of converting the response of interaction between the analyte and nanomaterial surface into readable signal [31]. For the purpose of effectively upgrading the performance of the sensor, the surface properties might be enhanced by depositing nanostructured thin film as a sensing layer. In nanometer dimensions, the majority of the particles (atoms) are surface or near surface of the sensing platform. Therefore, the effective number of existing sites to interact with analyte molecules is high [11]. The deployment of nanomaterial sensing layers reduces the size of the detecting parts and transducer as well as reduced cost and response time. This results in scaling down of the detecting devices and simplicity. Moreover, nanomaterial sensing layers provides high surface to volume ratio leads to better detection limits [32]. The sensors incorporating the nanostructured sensing layer including conducting polymers such as polyaniline (PANI) has showed ability to

Conducting polymers have become popular since early 1980s [33] due to their low cost, ease of synthesis and processing with ability to sense in room temperature [34]. Polyaniline (PANI) is of the important conducting polymers exploited extensively and studied as sensing materials. The light weight, high conductivity, mechanical flexibility and low-cost leads to the use of PANI in many applications. PANI exist in several oxidation states with different colors. Generally, the fundamental form of polyaniline known as emeraldine. Emeraldine forms can either be in emeraldine base (EB) or protonated emeraldine salt (ES) forms. Reducing emeraldine base generates the leucoemeraldine base (LEB) or pernigraniline base (PEB) in oxidized forms. [35]. The acid/base doping response makes PANI attractive for acid/base chemical vapor sensors, super capacitors, as well as biosensors. Potential aspects of PANI make it promising for sensing applications since it is presents different oxidation states each with different

*DOI: http://dx.doi.org/10.5772/intechopen.94001*

key issue for the safety of environments [19].

integrate with different transducing platforms [11, 32].

**3. Polyaniline nanostructures**

color, changes and conformations [11].

environments [19, 26–29].

#### *Modified Single Mode Optical Fiber Ammonia Sensors Deploying PANI Thin Films DOI: http://dx.doi.org/10.5772/intechopen.94001*

may cause acute poisoning or life-threatening situations such as permanent blindness, lung disease, respiratory disease, skin disease. The Occupational Safety and Health Administration (OSHA) has set a limit of 25 ppm in the workplace throughout an 8 hours shift and a short-term limit (15 min) of 35 ppm [11, 22]. Additionally, NH3 sensors remain the potential candidates employed in agriculture, chemical industries, pharmaceutical, hydrogen fuels, defense and food processing industries to monitor the NH3 leak in controlled atmospheres. Hence, the development of highly-sensitive and reliable NH3 sensors to continuously detect leakages NH3 is a key issue for the safety of environments [19].

More worrying, the gas is flammable at 50°C at very high concentration (150,000 ppm) [11, 23]. In chemical leakage alarm for NH3 detection, the detection limit ≥1000 ppm with operating temperature range up to 500°C and required response time may be in the range of minutes. The concentrations can be very high at NH3 plants and can even be explosive [24]. Recently, on Aug 2016, Petronas Chemicals Group stated that two workers were killed and three injured by an ammonia leak at a Malaysian chemical plant [25]. Hence, the development of highly-sensitive and reliable sensors to continuously detect leakages of NH3 is a key issue for the safety of environments [19, 26–29].

In general, the performance of the nanomaterial based sensors, particularly sensitivity, is controlled by three factors, material characteristics, transducer function and variability for sensor development [30]. The material characteristics indicate its surface ability to detect a specified chemical. The transducer function refers to the ability of converting the response of interaction between the analyte and nanomaterial surface into readable signal [31]. For the purpose of effectively upgrading the performance of the sensor, the surface properties might be enhanced by depositing nanostructured thin film as a sensing layer. In nanometer dimensions, the majority of the particles (atoms) are surface or near surface of the sensing platform. Therefore, the effective number of existing sites to interact with analyte molecules is high [11]. The deployment of nanomaterial sensing layers reduces the size of the detecting parts and transducer as well as reduced cost and response time. This results in scaling down of the detecting devices and simplicity. Moreover, nanomaterial sensing layers provides high surface to volume ratio leads to better detection limits [32]. The sensors incorporating the nanostructured sensing layer including conducting polymers such as polyaniline (PANI) has showed ability to integrate with different transducing platforms [11, 32].

## **3. Polyaniline nanostructures**

Conducting polymers have become popular since early 1980s [33] due to their low cost, ease of synthesis and processing with ability to sense in room temperature [34]. Polyaniline (PANI) is of the important conducting polymers exploited extensively and studied as sensing materials. The light weight, high conductivity, mechanical flexibility and low-cost leads to the use of PANI in many applications.

PANI exist in several oxidation states with different colors. Generally, the fundamental form of polyaniline known as emeraldine. Emeraldine forms can either be in emeraldine base (EB) or protonated emeraldine salt (ES) forms. Reducing emeraldine base generates the leucoemeraldine base (LEB) or pernigraniline base (PEB) in oxidized forms. [35]. The acid/base doping response makes PANI attractive for acid/base chemical vapor sensors, super capacitors, as well as biosensors. Potential aspects of PANI make it promising for sensing applications since it is presents different oxidation states each with different color, changes and conformations [11].

*Application of Optical Fiber in Engineering*

*Demonstration of tapered optical fiber [11].*

machine [16, 17].

**Figure 2.**

devices uses tapered fibers.

positioned while it is tapered.

shown in **Figure 2** [11, 14]. Based on the figure, Dt, Wl and Ut represents the down taper, waist length and up taper regions which are the tapered fiber profile [11]. Several techniques have been developed to fabricate the tapered optical fibers such as etching [15] and flame heating. The flame heating technique has been verified to be the most flexible technique that results in robust physical properties. Later, the flame was replaced with microheater that is more controllable. Recently, computer-controlled machines that are able to fabricate tapers with desired dimensions are available in the market. Conventional SMF or multimode optical fiber (MMF) with standard diameter of 125 μm can be tapered down to 5 μm using the

Tapering process improves the sensitivity of the optical fiber sensors by easing the access to the evanescent field, which enables strong interaction between the light and the analyte. To prepare a qualified tapered fiber based devices, the tapered fibers used should be fabricated with high adiabaticity, uniform microfiber diameter and suitable microfiber diameter with large evanescent wave. Fundamentally, strong evanescent wave is obtainable with thinner tapering fiber diameters. Accordingly, the tapered fibers are made with small diameter in the range of 0.8–3 μm for most

The strong evanescent wave on the taper waist surface make it more sensitive to its surrounding. The optical fiber sensors fabricated either by etching or tapering processes can be coated with suitable nanostructured sensing layer in order to improve the sensitivity. When sensing layer reacts towards the target analyte, its optical properties may change. Hence, the amount of evanescent wave absorbed by the sensing

The Etched-tapered optical fiber platform comprises of the etching and tapering processes abovementioned. Firstly, hydrofluoric acid is used to remove some of the cladding of the SMF as illustrated previously. The etched area is then tapered using different tapering methods based on the recommended configurations. Dealing with etched fibers to perform the tapering process is a critical and more challenging as compared to tapering the standard fiber [11]. The removal of the cladding layer of the SMF increases its fragility and increase difficulty of fiber handling. This is may be overcome by utilizing a customized holder in which the etched fiber is

Optical fiber sensors are deployed for detection of different hazardous gases including ammonia (NH3). Ammonia is widely used gas in different applications such as chemical industries, agricultures and medicines [18, 19] . Natural NH3 level present in the atmosphere is in low ppb (1–5 ppb) levels. NH3 can be characterized by its colorless, pungent smell, and explosive, toxic at a high-concentration NH3 atmosphere [20, 21]. Generally, upon exposure to around 50 ppm NH3 gas in air

layer is changed according to the analyte concentrations [11].

**2.3 Etched-tapered optical fiber sensors**

**48**

PANI-ES is the only conducting form of PANI with approximately 15 S cm−1 conductivity. Meanwhile, other forms are normally insulating with conductivity below 10–5 S cm-1 [36, 37]. The PANI EB and ES form can be identified through their colors, where EB is blue and ES is green [36]. PANI-ES can be obtained through doping process, either by oxidation of leucoemeraldine base or by protonation of the PANI-EB [36]. The protonation is carried out by processing the PANI-EB with a strong acid such as HCl that induces the protonation of the imine sites.

PANI is attractive to be used as a sensing layer because it can rapidly switch between the EB and ES forms as it is exposed to certain analytes. This reversible process is also known as doping (ES) or dedoping (EB). This reversible pHswitching property not only changes its electrical conductivity, but also its optical property. The change in optical properties can be observed through the change in the absorbance spectrum.

PANI has been proposed for sensing NH3 since there were variations in the electrical conductivity and optical absorption on exposure to NH3. The properties change with the condition of oxidation and protonation of the polymer. At the point when exposing PANI-ES (the acid form) to NH3, it will be deprotonated and transferred into a non-conducting PANI-EB [11, 21]. While there are many reported studies on the PANI based electrical sensors [38–41], the optical fiber based NH3 sensors employing PANI is not as popular as the electrical one [42, 43].

Sensors that use PANI in nanostructure forms such as nanofibers or nanorods have shown a significantly better performance in terms of response time and sensitivity compared to the ones that use conventional PANI films [44]. This is as a result of increased surface area, high porosity, and small structure diameter which enhances the diffusion of the analyte molecules into the nanostructures [44]. PANI nanofibers can be obtained through various methods such as template synthesis, phase separation and electrospinning [45]. Several approaches have been adopted to enhance the PANI sensing performance (sensitivity and selectivity) [46]. This includes polymer molecular structures modification, using different dopants, and integrating the conducting PANI with different types of inorganic materials such as graphene-like materials [47]. The conductivity of PANI can be also enhanced using a highly conductive filler as graphene and graphite [48].

### **4. Review of ammonia sensors based on polyaniline**

Limited SMF based NH3 sensors employing PANI nanocomposite have been proposed so far. The developed optical sensors utilized a few types of substrates including glass substrate, waveguide, and modified optical fibers. Different optical measurement techniques such as absorption, transmittance, reflectance, resonance wavelength shift and fluorescence are used in the development of NH3 optical sensors coated with PANI. The development of NH3 optical sensors coated with PANI can be carried out using different deposition methods. This includes in-situ deposition, drop casting, dip coating, spray, electrochemical deposition, or spin coating.

The influence of synthesis methods, deposition methods, dopant types on PANI morphology and NH3 sensing properties was studied in [49]. Glass substrate was used and absorbance measurement was done at wavelength of 632 nm. They experimented with three synthesis methods (interfacial, rapid mixing and dropwise mixing), two deposition methods (in-situ and drop-coating) with three types of dopants (HCl, CSA, and I2). The results demonstrated that in-situ deposited PANI formed a cauliflower-like nanoparticles structure with a thickness of approximately 400 nm and diameter of 300 nm. While in the case of drop-casted PANI, a PANI nanofibers was formed with measured diameters of approximately

**51**

*Modified Single Mode Optical Fiber Ammonia Sensors Deploying PANI Thin Films*

60–90 nm and 350 nm length. The in-situ deposited PANI nanostructure showed a shorter response time with higher sensitivity compared to drop-casted PANI, mainly because of more uniform coating. Drop-casting method suffered from the problem of non-uniform coating due to agglomeration of the PANI at certain areas on the glass surface. In-situ deposition of PANI-HCl was used for other experiments to investigate the best synthesis method. Rapid mixing method with oxidant-to-monomer mole ratio of equal to 1 was found to give the best result. This was contributed from the highest porosity and highest surface area of PANI nanogranules with size of 200 nm – 300 nm. PANI-CSA was found to give the highest sensitivity and the most stable NH3 sensor as compared to PANI-HCl and I2-doped PANI. The UV–Vis and FTIR measurements also confirmed that the sensing mechanism is based on the deprotonation process. From this work, it is learnt that to achieve high sensitivity, the highest surface area nanostructure is

In [50], a super Fiber Bragg grating (FBG) NH3 sensor was developed based on optical reflectance measurement method. The FBG sensor was fabricated by removing some part of its cladding using chemical etching with 14 μm reduced diameter. The PANI sensing layer was deposited on the etched area with 300 nm thickness. The FBG showed a blue shift in Bragg's wavelength towards the shorter wavelengths as exposed to higher NH3 concentration with 0.073 pm/ppm sensitivity.

Surface plasmon resonance-based NH3 plastic optical fiber sensor coated with PANI as sensing layer was reported in [51]. The sensor was fabricated by uncladding 1 cm length of a 600 μm fiber diameter. The unclad area was coated with different thickness of indium tin oxide (ITO) and PANI on top with the use of thermal evaporation technique. Then, the ITO coated fiber was dipped into ammonium hydroxide (as adhesive), followed by PANI solution. They found that the resonance wavelength increases as the NH3 concentration increases and the sensor with ITO

Fiber sensors based on evanescent wave absorption were proposed using bent optical fiber [52] and removed-clad fiber [53]. In [52], NH3 sensors were proposed using bent optical fibers with PANI and Fe (III) porphyrin-doped PMMA have detection limit in the range of ppm. The cladding of silica MMF was removed using chemical etching and replaced with thin PANI layer (less than 1 μm) in [53] for NH3 sensing. It was observed that the absorbance spectra increase over certain wavelength (between 500 to 800 nm) as the sensors were exposed to NH3. However, there is no detailed explanation on the synthesis methods and the type of PANI used in this work. Even though this work is quite dated (2003), but it gave a useful indication that thin PANI layer is a good candidate as an absorbance-based NH3 sensor.

**5. Optical fiber modification, nanomaterials deposition and** 

In this section, the development and characterizations of the modified SMF sensing platforms including etched, tapered and etched-tapered platforms will be elaborated. The etching process based on the use of chemical to remove some of the cladding layer. These platforms were characterized in term of output optical power. In the second section, PANI nanostructured thin film preparation and deposition onto the SMF transducing platforms will be highlighted. Finally, the PANI nanofibers fabrication and deposition onto the SMF transducing platforms will be explained. Several micro-characterizations of the fabricated nanostructured thin films were carried out to investigate sensing layer morphology and thickness of the

*DOI: http://dx.doi.org/10.5772/intechopen.94001*

desirable together with high doping level.

layer of 60 nm gave the best response.

**characterizations**

The value is low so that it is hardly to be measured.

#### *Modified Single Mode Optical Fiber Ammonia Sensors Deploying PANI Thin Films DOI: http://dx.doi.org/10.5772/intechopen.94001*

60–90 nm and 350 nm length. The in-situ deposited PANI nanostructure showed a shorter response time with higher sensitivity compared to drop-casted PANI, mainly because of more uniform coating. Drop-casting method suffered from the problem of non-uniform coating due to agglomeration of the PANI at certain areas on the glass surface. In-situ deposition of PANI-HCl was used for other experiments to investigate the best synthesis method. Rapid mixing method with oxidant-to-monomer mole ratio of equal to 1 was found to give the best result. This was contributed from the highest porosity and highest surface area of PANI nanogranules with size of 200 nm – 300 nm. PANI-CSA was found to give the highest sensitivity and the most stable NH3 sensor as compared to PANI-HCl and I2-doped PANI. The UV–Vis and FTIR measurements also confirmed that the sensing mechanism is based on the deprotonation process. From this work, it is learnt that to achieve high sensitivity, the highest surface area nanostructure is desirable together with high doping level.

In [50], a super Fiber Bragg grating (FBG) NH3 sensor was developed based on optical reflectance measurement method. The FBG sensor was fabricated by removing some part of its cladding using chemical etching with 14 μm reduced diameter. The PANI sensing layer was deposited on the etched area with 300 nm thickness. The FBG showed a blue shift in Bragg's wavelength towards the shorter wavelengths as exposed to higher NH3 concentration with 0.073 pm/ppm sensitivity. The value is low so that it is hardly to be measured.

Surface plasmon resonance-based NH3 plastic optical fiber sensor coated with PANI as sensing layer was reported in [51]. The sensor was fabricated by uncladding 1 cm length of a 600 μm fiber diameter. The unclad area was coated with different thickness of indium tin oxide (ITO) and PANI on top with the use of thermal evaporation technique. Then, the ITO coated fiber was dipped into ammonium hydroxide (as adhesive), followed by PANI solution. They found that the resonance wavelength increases as the NH3 concentration increases and the sensor with ITO layer of 60 nm gave the best response.

Fiber sensors based on evanescent wave absorption were proposed using bent optical fiber [52] and removed-clad fiber [53]. In [52], NH3 sensors were proposed using bent optical fibers with PANI and Fe (III) porphyrin-doped PMMA have detection limit in the range of ppm. The cladding of silica MMF was removed using chemical etching and replaced with thin PANI layer (less than 1 μm) in [53] for NH3 sensing. It was observed that the absorbance spectra increase over certain wavelength (between 500 to 800 nm) as the sensors were exposed to NH3. However, there is no detailed explanation on the synthesis methods and the type of PANI used in this work. Even though this work is quite dated (2003), but it gave a useful indication that thin PANI layer is a good candidate as an absorbance-based NH3 sensor.
