*3.4.2 Quartz crystal microbalance*

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 measurements are very precise (<1 part in 107 ) [193]. As illustrated in **Figure 5**, a 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* (Hz) in the area of the electrode (*A* cm<sup>−</sup><sup>2</sup> ) in terms of the mass increment, Δ*m* (g cm<sup>−</sup><sup>2</sup> ), loaded onto the crystal surface under a fundamental resonant frequency *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> ), the elastic modulus of the quartz crystal (2.947 × 1011 g cm−<sup>1</sup> s<sup>−</sup><sup>2</sup> ), and the surface area, respectively.

$$
\Delta F = \frac{2NF\_0^2}{\sqrt{\rho \mu}} \frac{\Delta m}{A} \tag{6}
$$

During the past few decades, the relationship between frequency shift and mass change, which was initially described by Sauerbrey, has been extensively applied for chemical sensing [182]. The Δ*F*, which is proportional to a mass adsorbed and/or sorbed on sensitive layers of distinct morphologies coated over the QCM electrode, is constantly monitored to identify and quantify the target analyte at the ng level (ng cm<sup>−</sup><sup>2</sup> ) [182].

The interaction between target molecules and sensitive coating layers (known as "guest-host interaction") plays an important role in the sensing mechanism. Such a guest-host interaction is considered as an adsorption process involving enrichment of guest species at the interface of a certain adsorbent, such as CP nanomaterials (**Figure 5**). In terms of high sensitivity and selectivity, the fabrication of CP nanomaterials is an important step toward the development of efficient advanced detection sensors. Since 1964, a QCM sensor had been implemented by King into a gas chromatography system for the detection of hydrocarbons [196]. Then after, QCM sensor device has been successfully applied as a sensitive tool to sense mass interfacial [197] and polymer film properties [198–201]. Accordingly, Δ*F* was investigated in terms of rigid mass changes, based on the Sauerbrey equation. The QCM technique has been used in the fields of gas sensing application including gas mixture analysis [202], discrimination of aromatic optical isomers [203], and VOC vapors detection [204, 205]. In the early trials, Gomes et al. [206] have used uncoated quartz crystals with gold electrodes to detect and quantify volatile amines, such as *iso*-propylamine, *n*-butylamine, *s*-butylamine, and *tert*-butylamine; however this method suffers from low sensitivity. Attention has been paid to the development of efficient QCM sensors which rely greatly on the utilization of CPs as sensitive coatings. Thin film-coated QCM sensors were pioneered by Ayad et al. [207–212]. For example, a QCM technique concomitant with sensitive layers of CPs, PANI in the form of ES, and EB prepared by the in situ chemical oxidative polymerization was explored to detect and quantify varieties of VOCs in air, such as chlorinated aliphatic hydrocarbons, like CCl4, CHCl3, CH2Cl2, and ClCH2-CH2Cl; aliphatic alcohols, like CH3OH, C2H5OH, C3H7OH, and C4H9OH; and aliphatic amines including CH3NH2, (CH3)NH, (CH3)3N, and (CH3CH2)3N [213–217]. The adsorption mechanism was discussed as a kind hydrogen bonding or a dipole/ dipole interaction formed between the imine and amine sites of PANI with chemical vapor. The difference in adsorption affinity was attributed to the differences in their chemical structure and strength of the electrostatic interactions. Interestingly, the PANI adsorption kinetics (Eq. (7)) [217] and diffusion of chemical vapor were carefully discussed, in terms of diffusion coefficient (*D*) using Fick's second equation (Eq. (8)), which has been reviewed by Crank [218].

$$\frac{\Delta F\_t}{\Delta F\_{\text{sys}}} = \mathbf{1} - \mathbf{e}^{-kt} \tag{7}$$

$$\frac{\Delta F\_t}{\Delta F\_{\text{ss}}} = 4 \sqrt{\frac{D}{\pi}} \frac{t^{1/2}}{L} \tag{8}$$

**139**

(g cm<sup>−</sup><sup>2</sup>

**Figure 5.**

*&Co. KGaA, Weinheim).*

tion of *t*

1/2/*L*, respectively.

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

frequency of the polymer film estimated from QCM. *L* (cm) is the film thickness and can be determined by the density of the polymer and mass change, and Δ*m*

*Schematic representation of the interaction of an analyte with PANI nanotube prepared with CSA coating on a QCM (modified and adapted with permission from Ref. [194]. Copyright 2014, Wiley-VCH Verlag GmbH* 

Li et al. constructed a sensor using water-soluble PANI and PANI-TiO2 nanocomposite-coated QCM for a selective detection of amine vapors [219, 220]. The nanocomposite exhibited a higher sensing affinity and good selectivity toward (CH3)3N and (CH3CH2)3N than other VOCs, such as C2H5OH, CH3-COO-CH2-CH3, CH2O, and CH3CHO. As the van der Waals absorption is the main interaction between PANI and amine, the sensor responses could be completely recovered after purging N2 at room temperature. Further, PANI/ES films doped with several dopants, such as HCl and DBSA, and 1,5-naphtalenedisulfonic acid (1,5-NDSA)) coated QCM sensor films have been fabricated to detect BTEX vapors [221]. The Δ*F* due to adsorption of VOCs is attributed to electrostatic interactions between vapor molecules and dopant in PANI/ES films. Interestingly, PANI-DBSA films were found to be highly sensitive and selective to *p*-xylene compared with toluene and benzene. Further, the adsorption behavior of poly(3-butoxythiophene) (P3BOT) mixed with stearic acid (SA) LB film-coated QCM was studied as a sensing material for series of chemical vapor analytes, such as chlorinated aliphatic hydrocarbons and some short-chain aliphatic alcohols [222]. On exposure to vapor analyte, the frequency of the QCM was changed, due to the dipole/dipole or hydrogen bonding interaction with P3BOT/SA film. Additionally, a control of sensitivity and selectivity of the sensor could be achieved through polymer functionalization with ether group, difference in molecular weight, and structure of the chemical vapors.

Gas sensing properties of the CPs have dramatically improved after incorporation of other nanomaterials such as CNTs, GO, metals, and other nanometal oxides. Very recently, Wang et al. fabricated a gas sensor by using PPY and PPY/TiO2 coating on QCM electrode for detecting different chemical vapors [223]. As a result, the sensor coated with PPY/TiO2 was found to exhibit a better sensing performance, longterm stability, and excellent reversibility, as well as acceptable selectivity toward NH3 in comparison to (CH3)3N, H2S, and C2H5OH. Based on QCM measurements, researchers could fabricate a PPY/TiO2 sensor for evaluating shelf-life quality changes of three typical foodstuffs (mango, egg, and fish) during 1-week storage.

) is from the Sauerbrey equation (Eq. (4)). The *k* and *D* can be calculated from the slope of linear graphs of ln(1 − Δ*Ft*/Δ*F∞*) against t and Δ*Ft*/Δ*F∞* as a func-

$$
\Delta F\_t = F\_{\text{polymer}} - F\_t \text{ and } \Delta F\_{\text{oo}} = F\_{\text{polymer}} - F\_{\text{oo}} \tag{9}
$$

where *k* is the pseudo-first-order rate constant for vapor uptake. Δ*Ft* and Δ*F∞* (Hz) are the frequency changes due to the adsorption uptake of the vapor into the polymer film at any time *t* and at the steady-state, respectively. *Fpolymer* is the

**Figure 5.**

*Gas Sensors*

(ng cm<sup>−</sup><sup>2</sup>

) [182].

(Eq. (8)), which has been reviewed by Crank [218].

\_ Δ*Ft* Δ*F*<sup>∞</sup>

\_ Δ*Ft* Δ*F*<sup>∞</sup> = 4 √ \_ \_ *D π t* 1/2 \_

where *k* is the pseudo-first-order rate constant for vapor uptake. Δ*Ft* and Δ*F∞* (Hz) are the frequency changes due to the adsorption uptake of the vapor into the polymer film at any time *t* and at the steady-state, respectively. *Fpolymer* is the

= 1 − *e*<sup>−</sup>*kt* (7)

Δ*Ft* = *Fpolymer*− *Ft and* Δ*F*∞ = *Fpolymer*− *F*∞ (9)

*<sup>L</sup>* (8)

<sup>Δ</sup>*F* = 2*<sup>N</sup> <sup>F</sup>*<sup>0</sup> \_

2 √ \_ \_ Δ*m*

During the past few decades, the relationship between frequency shift and mass change, which was initially described by Sauerbrey, has been extensively applied for chemical sensing [182]. The Δ*F*, which is proportional to a mass adsorbed and/or sorbed on sensitive layers of distinct morphologies coated over the QCM electrode, is constantly monitored to identify and quantify the target analyte at the ng level

The interaction between target molecules and sensitive coating layers (known as "guest-host interaction") plays an important role in the sensing mechanism. Such a guest-host interaction is considered as an adsorption process involving enrichment of guest species at the interface of a certain adsorbent, such as CP nanomaterials (**Figure 5**). In terms of high sensitivity and selectivity, the fabrication of CP nanomaterials is an important step toward the development of efficient advanced detection sensors. Since 1964, a QCM sensor had been implemented by King into a gas chromatography system for the detection of hydrocarbons [196]. Then after, QCM sensor device has been successfully applied as a sensitive tool to sense mass interfacial [197] and polymer film properties [198–201]. Accordingly, Δ*F* was investigated in terms of rigid mass changes, based on the Sauerbrey equation. The QCM technique has been used in the fields of gas sensing application including gas mixture analysis [202], discrimination of aromatic optical isomers [203], and VOC vapors detection [204, 205]. In the early trials, Gomes et al. [206] have used uncoated quartz crystals with gold electrodes to detect and quantify volatile amines, such as *iso*-propylamine, *n*-butylamine, *s*-butylamine, and *tert*-butylamine; however this method suffers from low sensitivity. Attention has been paid to the development of efficient QCM sensors which rely greatly on the utilization of CPs as sensitive coatings. Thin film-coated QCM sensors were pioneered by Ayad et al. [207–212]. For example, a QCM technique concomitant with sensitive layers of CPs, PANI in the form of ES, and EB prepared by the in situ chemical oxidative polymerization was explored to detect and quantify varieties of VOCs in air, such as chlorinated aliphatic hydrocarbons, like CCl4, CHCl3, CH2Cl2, and ClCH2-CH2Cl; aliphatic alcohols, like CH3OH, C2H5OH, C3H7OH, and C4H9OH; and aliphatic amines including CH3NH2, (CH3)NH, (CH3)3N, and (CH3CH2)3N [213–217]. The adsorption mechanism was discussed as a kind hydrogen bonding or a dipole/ dipole interaction formed between the imine and amine sites of PANI with chemical vapor. The difference in adsorption affinity was attributed to the differences in their chemical structure and strength of the electrostatic interactions. Interestingly, the PANI adsorption kinetics (Eq. (7)) [217] and diffusion of chemical vapor were carefully discussed, in terms of diffusion coefficient (*D*) using Fick's second equation

*<sup>A</sup>* (6)

**138**

*Schematic representation of the interaction of an analyte with PANI nanotube prepared with CSA coating on a QCM (modified and adapted with permission from Ref. [194]. Copyright 2014, Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim).*

frequency of the polymer film estimated from QCM. *L* (cm) is the film thickness and can be determined by the density of the polymer and mass change, and Δ*m* (g cm<sup>−</sup><sup>2</sup> ) is from the Sauerbrey equation (Eq. (4)). The *k* and *D* can be calculated from the slope of linear graphs of ln(1 − Δ*Ft*/Δ*F∞*) against t and Δ*Ft*/Δ*F∞* as a function of *t* 1/2/*L*, respectively.

Li et al. constructed a sensor using water-soluble PANI and PANI-TiO2 nanocomposite-coated QCM for a selective detection of amine vapors [219, 220]. The nanocomposite exhibited a higher sensing affinity and good selectivity toward (CH3)3N and (CH3CH2)3N than other VOCs, such as C2H5OH, CH3-COO-CH2-CH3, CH2O, and CH3CHO. As the van der Waals absorption is the main interaction between PANI and amine, the sensor responses could be completely recovered after purging N2 at room temperature. Further, PANI/ES films doped with several dopants, such as HCl and DBSA, and 1,5-naphtalenedisulfonic acid (1,5-NDSA)) coated QCM sensor films have been fabricated to detect BTEX vapors [221]. The Δ*F* due to adsorption of VOCs is attributed to electrostatic interactions between vapor molecules and dopant in PANI/ES films. Interestingly, PANI-DBSA films were found to be highly sensitive and selective to *p*-xylene compared with toluene and benzene. Further, the adsorption behavior of poly(3-butoxythiophene) (P3BOT) mixed with stearic acid (SA) LB film-coated QCM was studied as a sensing material for series of chemical vapor analytes, such as chlorinated aliphatic hydrocarbons and some short-chain aliphatic alcohols [222]. On exposure to vapor analyte, the frequency of the QCM was changed, due to the dipole/dipole or hydrogen bonding interaction with P3BOT/SA film. Additionally, a control of sensitivity and selectivity of the sensor could be achieved through polymer functionalization with ether group, difference in molecular weight, and structure of the chemical vapors.

Gas sensing properties of the CPs have dramatically improved after incorporation of other nanomaterials such as CNTs, GO, metals, and other nanometal oxides. Very recently, Wang et al. fabricated a gas sensor by using PPY and PPY/TiO2 coating on QCM electrode for detecting different chemical vapors [223]. As a result, the sensor coated with PPY/TiO2 was found to exhibit a better sensing performance, longterm stability, and excellent reversibility, as well as acceptable selectivity toward NH3 in comparison to (CH3)3N, H2S, and C2H5OH. Based on QCM measurements, researchers could fabricate a PPY/TiO2 sensor for evaluating shelf-life quality changes of three typical foodstuffs (mango, egg, and fish) during 1-week storage.

Novel low-humidity sensors were investigated by the in situ photopolymerization of PPY/Ag/TiO2 nanoparticle composite thin film coating on QCM [224]. Room temperature highly sensitive sensors with short response/recovery time for humidity based on GO/SnO2/PANI and PANI-GO coating on QCM were explored by Zhang et al. [225, 226]. The adsorption process of water molecules on QCM senor was carefully discussed using Langmuir adsorption isotherm model.
