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

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

*Application of Optical Fiber in Engineering*

the absorbance spectrum.

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.

sensors employing PANI is not as popular as the electrical one [42, 43].

a highly conductive filler as graphene and graphite [48].

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

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

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 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

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

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

casting, dip coating, spray, electrochemical deposition, or spin coating.

**50**

nanostructured thin film. These parameters affect gas sensing performance which will be discussed extensively here.
