*3.3.2 Fiber-optic devices*

Fiber-optic sensors are a class of optical sensors that use optical fibers to detect chemical analytes. Light is generated by a light source and is sent through an optical fiber, then reflects the absorption property of the CP surface when it returns through the optical fiber, and finally is captured by a photo detector [170]. Sensors based on fiber optics used the light guiding properties of the optical fibers to carry the light into and from the CP active layer [171]. However, this type of optical sensor has some drawback concerning the complication of associated electronics and software, cost-effectiveness, concentration limitation, short lifetime due to photobleaching, and limited ability to transmit light through optical fiber over long distances [167, 172]. A fiber-optic device based on PANI was used to detect HCl, NH3, hydrazine (H4N2), and dimethyl methylphosphonate (DMMP, a nerve agent, sarin stimulant) [173]. Muthusamy and coworkers developed gas sensors

based on PPY and PPY/Prussian blue (PPY-PB) nanocomposite coating on fiber optic to monitor NH3, acetone, and ethanol gases at room temperature [174]. The PPY-PB nanocomposite-based fiber-optic sensor exhibited an enhanced sensitivity for ethanol than pure PPY nanoparticles, and spectral intensity increases linearly with increasing the concentrations of gas. Very recently, Mohammed et al. fabricated an etched-tapered single-mode fiber (SMF) coated with a high surface area PANI/graphite nanofiber (GNF) nanocomposite as optical sensor for NH3 gas at room temperature in the visible wavelength range [175]. The sensor exhibited a good response time, sensitivity, and reproducibility for NH3, compared with pure PANI-coated SMF. Furthermore, an optical microfiber sensor was designed by drop coating of PANI doped with dioctyl sodium sulfosuccinate onto a microfiber resonator as a sensor for alcohols [176]. The sensor output spectrum showed red shift in wavelength upon response to various alcohols at different concentrations, due to the increase in dihedral angle and average band gap (lower energy) of PANI fiber. In a recent study by Kim and coworkers reported the application of fiber-optic reflectance sensors (FORS) coated with PPY film for sensing VOCs up to 1 ppm under atmospheric conditions [177]. The variation in the reflected light intensity was caused by the formation of polaron-bipolaron and film swelling when interacted with VOCs.

## *3.3.3 Surface plasmon resonance (SPR)*

Surface plasmon resonance is another class of optical sensors which referred to excitation of surface plasmon-based optical sensor for chemical sensing utilizing light. SPR optical sensor is a thin film refractometer sensing device which measures the changes in refractive index that occurred at the surface of a plasmon-supported metal film. On excitation by the monochromatic light, a change in the refractive index of a dielectric material gives rise to a change in propagation constant of the surface plasmon (prism coupled, i.e., attenuated total reflectance (ATR), waveguide coupled, and grating coupled) [178, 179]. The propagation constant of a radiation alters the characteristics of light wave coupled to the surface plasmon, e.g., coupling angle, coupling wavelength, and intensity phase [180]. After exposing to analytes, the minimum in the reflectance curve can be shifted, indicating the presence of analyte. The sensitivity of this type of sensors is high, but the detecting procedures are complicated. A SPR device was explored by Agbor and coworkers using PANI thin films to detect NO2 and H2S gases, resulting in an increase in reflectivity and resonance angle [180].

#### **3.4 Mass-sensitive device sensors**

Mass-sensitive devices transform the mass change at a specially modified surface into a change of a property of the piezoelectric material. Surface acoustic wave (SAW) and the quartz crystal microbalance (QCM) techniques are the main categories of piezoelectric gas sensing devices [181]. SAW and QCM are the simplest piezoelectric devices with a selective coating deposited on the surface to serve as an adsorptive surface capable of measuring an extremely small mass change at room temperatures [182]. Interestingly, SAW and QCM sensors are very promising and are widely accepted as smart transducers for their miniaturized design, possibility of wireless integration, high thermal stability, inertness, and room temperature operation. In addition, they can be easily combined with a variety of recognition sensitive layers for sensing applications ranging from small gas molecules to large biomolecules or even whole cell structures.

**137**

(g cm<sup>−</sup><sup>2</sup>

respectively.

*Gas Sensors Based on Conducting Polymers DOI: http://dx.doi.org/10.5772/intechopen.89888*

Surface acoustic wave resonators represent one of the most prominent acoustic devices for their exceptionally high frequency from several hundred MHz to GHz, which can record remarkably diminutive frequency shifts resulting from exceptionally small mass loadings making them potentially suitable in mass sensing applications [183]. Clearly, SAW resonator is a sensitive layer coated on the gap between a transmitter (an input) and a receptor (an output) interdigital transducers (IDTs) coating on the top of the piezoelectric crystal to design a SAW gas sensor [184]. An input radio-frequency voltage is applied across the transmitter IDTs, inducing deformations in the piezoelectric crystal that give rise to an acoustic wave, traversing the gap between two IDTs. When it reaches the receptor IDTs, the mechanical energy was converted back to radio-frequency voltage. The adsorption/desorption of gas on the CP film on the gap modulates the wave propagation characters, and a frequency shift can be recorded between the input and output voltages. In the sense of applications, a SAW sensor based on fibrous PANI nanocomposites layers prepared by chemical oxidative polymerization of aniline in the presence of finely divided metal oxides deposited on ZnO/64° YX LiNbO3 SAW transducer to detect H2, NO2, NO, CO2 and CO gases [185–188]. The designed SAW sensor exhibited improved sensitivity and repeatability of the gas molecules in ppm level at room temperature. Attractive studies utilized PPY nanocomposite-based SAW gas sensors deposited onto 128° YX-LiNbO3 substrate via Langmuir-Blodgett (LB) method due to high sensitivity and low cost. The LB PPY nanocomposite-based SAW exhibited excellent selectivity toward low NH3 concentration of 20 ppm, with respect to other interfering gases, such as CO, CH4, H2, and O2, at room ambient temperature [189–191]. By merging with electrical conductivity gas response, the sensing mechanism for gas detection has been investigated as elastic loading. Very recently, SAW integrated with PPY and PPY/TiO2 films has been utilized for NO2 and H2S detection at room temperature by the self-assembly method [192]. Upon exposure to NO2, the SAW sensors coated with PPY film showed a negative frequency shift (Δ*F*) tendency, in contrast to PPY/TiO2 that exhibited a faster sensor response and a higher sensitivity.

Besides, the selectivity was greatly improved by addition of TiO2 to PPY.

The advantage of conceptual simplicity, relative ease of modification, chemical inertness of the substrates, ruggedness, low cost, and ready availability of piezoelectric transducers have encouraged the development of QCM technique in various sensor applications. In addition, the sensitivity of piezoelectric transducers is based on the mass per unit area, suggesting miniaturization without losses in their sensitivity. The associated electronics are fairly simple, and frequency

QCM sensor consists of a quartz disk coated with metal electrodes on both sides (usually Pt or Au). When a voltage is applied to the quartz crystal plate, it can oscillate at a specific frequency, and the relation between frequency change (Δ*F*) of the oscillating crystal and the mass change (∆*m*) on the quartz surface was described by Sauerbrey empirical derivation (Eq. (6)) [195]. The change of Δ*F*

*F*0 can be estimated from Eq. (1), where *N*, *F*0, *ρ*, *μ*, and *A* are the harmonic overtone, the fundamental resonance frequency, the crystal density (2.649 g cm<sup>−</sup><sup>3</sup>

), loaded onto the crystal surface under a fundamental resonant frequency

) [193]. As illustrated in **Figure 5**, a

), the

), and the surface area,

) in terms of the mass increment, Δ*m*

s<sup>−</sup><sup>2</sup>

*3.4.2 Quartz crystal microbalance*

measurements are very precise (<1 part in 107

(Hz) in the area of the electrode (*A* cm<sup>−</sup><sup>2</sup>

elastic modulus of the quartz crystal (2.947 × 1011 g cm−<sup>1</sup>

*3.4.1 Surface acoustic wave*
