**4. Conducting polymers and CNT composites**

Conducting polymers have been widely used to enhance the sensing properties of CNTsbased sensors. CNTs unique characteristics combined with polymer's delocalized bonds, high permeability and low density have demonstrated that it is possible to detect many dif‐ ferent gases with high sensitivity, fast response and good reproducibility. Previous reports on polymer/CNTs-based sensor have been summarized and reviewed [7-9, 12]. However, some challenges to overcome are aggregation or agglomeration of CNTs, thermal stability and selectivity, among others. Polymer/CNTs composites used in resistors, SAW and QMB type of sensors are discussed.

CNTs were used to improve Polyaniline (PANi) poor thermal stability (Figure 4) [68]. The proposed solution to this was the uniform incorporation of CNTs in the polymer network. Considering that one of the problems of MWCNTs is their aggregation and agglomeration, they were oxyfluorinated under different conditions in order to obtain a better dispersion in aqueous solution and it was found that the better dispersion was obtained with the MWCNTs with the highest oxygen content. The oxyfluorinated MWCNTs were then mixed with aniline and ammonium persulfate (APS) and other chemicals, for in-situ polymeriza‐ tion. Changes in resistance as function of time were used to characterize and evaluate the resulting PANi/MWCNTs composite was for the detection of ammonia (NH3) in a concen‐ tration range of 1-50 ppm. PANi/MWCNTs composite with the highest oxygen content had a uniform composition, improved thermal stability and highest and faster response for 50 ppm of NH3. The composite was able to detect 1 ppm and showed excellent repeatability for cycling exposures to 50 ppm and it needed heat treatment to accelerate the NH3 desorption and thus the recovery of the sensor. A possible drawback is the selectivity to NH3 among gases that can extract protons (H+ ) from PANi.

**Metal oxide Nanotube Type Op T. oC Target (gas) DL Response Time Ref.**

MWNTs RT Ethanol 100ppm\* NS [60]

NOx 20ppm

H2 4%

NO2 1ppm

CO 2ppm 5min

LPG 21s

F-SWCNTs RT NH3 50ppm\* NS [62]

SnO2 SWNTs 200 NO2 2ppm NS [63]

WO3 MWNTs 350 H2 100ppm\* Ns [65]

Ethanol

RT NO2 100ppb

B-doped 1min

Conducting polymers have been widely used to enhance the sensing properties of CNTsbased sensors. CNTs unique characteristics combined with polymer's delocalized bonds, high permeability and low density have demonstrated that it is possible to detect many dif‐ ferent gases with high sensitivity, fast response and good reproducibility. Previous reports on polymer/CNTs-based sensor have been summarized and reviewed [7-9, 12]. However, some challenges to overcome are aggregation or agglomeration of CNTs, thermal stability and selectivity, among others. Polymer/CNTs composites used in resistors, SAW and QMB

CNTs were used to improve Polyaniline (PANi) poor thermal stability (Figure 4) [68]. The proposed solution to this was the uniform incorporation of CNTs in the polymer network.

**Table 1.** Summary of metal oxide NPs used to modify CNTs for gas sensor applications.

**4. Conducting polymers and CNT composites**

N-doped 1min [67]

NS [61]

3min (150C)

7min

4min (RT) [64]

[66]

ZnO SnO2 TiO2

ZnO

SnO2

\*Lowest tested concentration

type of sensors are discussed.

Co3O4 SWNTs RT, 250

346 Syntheses and Applications of Carbon Nanotubes and Their Composites

Pd-COOH SWCNTs

N-SWCNTs

SnO2 MWNTs RT, 150

SnO2 MWNTs 320

O-doped

**Figure 4.** Illustration of the steps to obtain oxyfluorinated CNTs modified with PANI. (From Yun et al. [68]. Copyright © 2012, with permission from Elsevier. )

Mangu *et al.* also worked in the preparation of PANi-MCNTs as well as Poly (3, 4-ethyl‐ enedioxythiophene) poly (styrenesulfonate) (PEDOT:PSS)-MWCNTs composites for the detection of 100 ppm of NO2 and NH3 [69]. This group studied the effect of dissolving the polymers in different solvents for the gas sensing properties. PANi was dissolved in dimethyl sulfoxide (DMSO), *N*, *N*-dimethyl formamide (DMF), ethylene glycol (EG) and 2-propanol. PEDOT: PSS was dissolved in DMSO, DMF and 0.1M sodium hydroxide (NaOH). Each polymer solution was spin-coated in plasma treated MWCNTs and evalu‐ ated as sensing material. All PANi-MWCNTs composited showed an increase in resist‐ ance for NH3 and a decrease in resistance for NO2, which is typical of p-type semiconducting composite films. All PANi-MWCNTs composites were selective to NO2. However, better sensitivities were achieved when PANi was dissolved in 2-propanol and DMSO for NH3 and for NO2, respectively. On the other hand, all PEDOT:PSS-MWCNTs composites were also excellent for the detection if both NO2 and NH3. PE‐ DOT: PSS-MWCNTs (prepared without any solvent) showed to be more sensitive to NH3 and PEDOT:PSS dissolved in NaOH to NO2.

Sayago, *et al.* have worked on the preparation of different composites using polymers with small percentages of CNTs as sensitive layers for surface acoustic wave (SAW) gas sensors [70-72]. Composites of polyisobutilene (PIB), polyepichlorohydrin (PECH) and polyetherur‐ ethane (PEUT) with 2% and 5% of MWCNTs were prepared and used to detect volatile or‐ ganic compounds (VOCs) at room temperature using SAW-sensor arrays. All the samples showed responses (frequency shifts) for octane and toluene (25-200 ppm), even though some samples were more sensitive than the others. For instance, samples with PIB/5%MWCNTs showed higher sensitivity for octane while PECH/2%MWCNTs and PEUT/2%MWCNTs were more sensitive to toluene. The difference in sensitivity is attributed to the difference in affinity between polymers and VOCs due to their respective polarities. The detection and re‐ covery times were fast and fully reversible, which means that the main interaction is physi‐ sorption. The role of the MWCNTs is unknown. In general, their presence in the composite showed to improve sensitivity and reduce the limits of detection (LOD) but did not affect selectivity, response and recovery times.

only showed improved the linearity and sensitivity but also lower resistance values. Lower resistance values might improve the performance at low RH range, a problem that PI resis‐

Yuana and coworkers prepared a sensor array based on polymer/MWCNTs composites for the selective detection of chloroform (CHCl3), tetrahydrofuran (THF) and methanol (MeOH) [76]. Ethyl cellulose (EC), poly [methyl vinyl ether-alt-maleic acid] (PMVEMA), hydroxypropyl methyl cellulose (HPMC), poly (alpha-methylstyrene) (PMS), poly (vinyl benzyl chloride) (PVBC) and poly (ethyleneadipate) (PEA) were the polymers used to prepare the polymer/MWCNTs composites and to provide uniqueness to each sensor in terms of their physical and chemical characteristics like molecular structure, polymer length, polarity and intermolecular forces. Changes in resistance as function of time were recorded for the sensor array when exposed to the different gases at different tem‐

three gases at the different temperatures when in presence of vapor molecules of chlor‐ ide, cyclic oxide and hydroxide groups. The decreasing order of sensitivity concurred with the order of decreasing conductivity: PEA/MWCNTs >EC/MWCNTs >PMVEMA/

SWCNTs modified with Poly- (D) glucosamine (Chitosan) (SWCNTs-CHIT) were as high performance hydrogen sensor [77]. Three types of sensors were prepared: SWCNTs de‐ posited on glass substrate (type 1), SWCNTs deposited over a glass substrate modified with CHIT-film (type 2), Chit-film deposited over SWCNTs deposited over a glass sub‐ strate. Each type of sensor showed a different changes in resistance when exposed to 4% H2 in air. Increase in resistance was observed for three types of sensors and good recov‐ ery but for type (3) sensor. Sensors modified with CHIT showed better sensitivity. More‐ over, the authors explained that the improved sensitivity was far higher than that reported for Pd-SWCNTs based sensors, which are commonly used for H2 detection. The enhanced performance of SWCNTs-CHIT can be explained by the strong interaction/ binding of the H2 molecules with/to the –OH and –NH3 groups contained in CHIT,

SWCNTs were modified with Polypyrrole (PPy) and 5, 10, 15, 20-tetraphenylporphyrine (TPP) to prepare composites for the detection of 1-butanol in nitrogen using a quartz microbalance (QMB) [78]. The QMB gold electrodes were coated with PPy/SWCNTs-COOH, PPy/SWCNTs-COOH/SWCNTs-TPP via electropolymerization. Frequency shifts of the quartz crystal resonator as function of time were measured for the sensors when exposed to 1-butanol in ppb concentration range. Even though both composites showed good and higher response magnitudes than QMB prepared with other composites that did not contain CNTs, PPy/SWCNTs-COOH/SWCNTs-TPP showed better performance than PPy/SWCNTs-COOH. The results demonstrate that the incorporation of CNTs en‐

Lu *et al*. reported a sensor array containing pristine SWCNTs, Rh-loaded SWCNTs, PEI/ SWCNTs and other CNTs with different coatings and loadings for the detection of hydrogen

C) and 50-60% R. H. The sensors showed to be selective to the

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349

tive-type sensors present because of their high resistance values.

MWCNTs >PVBC/MWCNTs >HPMC/MWCNTs >PMS/MWCNTs.

which also explains the poor recovery of type 3 sensor.

hanced the sensitivity towards the detection of 1-butanol.

peratures (30 40, 50, 60 o

Another SAW gas sensor was reported by Viespe *et al*. for the detection of methanol, etha‐ nol, toluene using different polyethyleneimine (PEI)-based nanocomposite as sensitive layer, including MWCNTs-PEI [73]. In general, frequency shifts were proportional to the gas con‐ centration and MWCNTs-PEI sensors showed better response time and higher sensitivity than the PEI-sensor. However, it did not show the best LOD when compared to the other PEI-based nanocomposite. The MWCNTs-PEI sensors showed higher sensitivity towards toluene and lower sensitivity to methanol when compared to ethanol.

Biopolymer/CNTs composites for chemical vapor sensors were produced by using two dif‐ ferent biopolymers, cellulose, the most naturally abundant one and poly (lactic acid) (PLA). Considering that previous studies have shown that a homogeneous distribution of MCNTs in the cellulose matrix can improve the polymer's mechanical and electrical characteristics, MWCNTs were functionalized with imidazolide groups and covalently attached it to cellu‐ lose chains [74]. The resulting material, a paper-like film, was then used as sensing element for the detection of methanol, ethanol, 1-propanol and 1-butanol at ppm levels. Responses were measured by changes in resistance and were found to be reversible and consistent for all the tested vapors. However, the sensor only showed linear responses as function of con‐ centration for 1-propanol in the range of 400-3600 ppm. The other composite, PLA/ MWCNTs was prepared by doping the biopolymer with2 and 5% of MWCNTs and anneal‐ ing, in order to understand the effect of MWCNTs in the crystallinity of the polymer and its performance in the detection of toluene, water, methanol and chloroform [75]. PLA/2%- MWCNTs showed highest responses for all gases, when compared to PLA/3%-MWCNTs. However, it was found that all samples were selective to chloroform. Moreover, annealing the samples showed a decrease in the responses that were significantly lower than the un‐ treated ones. Annealing did not affect the selectivity to chloroform but it considerably affect‐ ed its sensitivity.

Plasma-treated MWCNTs (p-MWCNTs) polyimide (PI) composite films (p-MWCNTs-PI) were developed by Yoo *et al.* in an effort to overcome some of the problems presented by PIbased resistive-type sensors, e. g. nonlinear sensitivity to relative humidity [33]. When tested as sensing material for 10-95% relative humidity (RH), p-MWCNTs-PI showed better sensi‐ tivity and linear response (resistance) as function of humidity, when compared to pristine-MWCNT-PI and pristine PI. The increase of p-MWCNTs content inp-MWCNTs-PI films not only showed improved the linearity and sensitivity but also lower resistance values. Lower resistance values might improve the performance at low RH range, a problem that PI resis‐ tive-type sensors present because of their high resistance values.

ethane (PEUT) with 2% and 5% of MWCNTs were prepared and used to detect volatile or‐ ganic compounds (VOCs) at room temperature using SAW-sensor arrays. All the samples showed responses (frequency shifts) for octane and toluene (25-200 ppm), even though some samples were more sensitive than the others. For instance, samples with PIB/5%MWCNTs showed higher sensitivity for octane while PECH/2%MWCNTs and PEUT/2%MWCNTs were more sensitive to toluene. The difference in sensitivity is attributed to the difference in affinity between polymers and VOCs due to their respective polarities. The detection and re‐ covery times were fast and fully reversible, which means that the main interaction is physi‐ sorption. The role of the MWCNTs is unknown. In general, their presence in the composite showed to improve sensitivity and reduce the limits of detection (LOD) but did not affect

Another SAW gas sensor was reported by Viespe *et al*. for the detection of methanol, etha‐ nol, toluene using different polyethyleneimine (PEI)-based nanocomposite as sensitive layer, including MWCNTs-PEI [73]. In general, frequency shifts were proportional to the gas con‐ centration and MWCNTs-PEI sensors showed better response time and higher sensitivity than the PEI-sensor. However, it did not show the best LOD when compared to the other PEI-based nanocomposite. The MWCNTs-PEI sensors showed higher sensitivity towards

Biopolymer/CNTs composites for chemical vapor sensors were produced by using two dif‐ ferent biopolymers, cellulose, the most naturally abundant one and poly (lactic acid) (PLA). Considering that previous studies have shown that a homogeneous distribution of MCNTs in the cellulose matrix can improve the polymer's mechanical and electrical characteristics, MWCNTs were functionalized with imidazolide groups and covalently attached it to cellu‐ lose chains [74]. The resulting material, a paper-like film, was then used as sensing element for the detection of methanol, ethanol, 1-propanol and 1-butanol at ppm levels. Responses were measured by changes in resistance and were found to be reversible and consistent for all the tested vapors. However, the sensor only showed linear responses as function of con‐ centration for 1-propanol in the range of 400-3600 ppm. The other composite, PLA/ MWCNTs was prepared by doping the biopolymer with2 and 5% of MWCNTs and anneal‐ ing, in order to understand the effect of MWCNTs in the crystallinity of the polymer and its performance in the detection of toluene, water, methanol and chloroform [75]. PLA/2%- MWCNTs showed highest responses for all gases, when compared to PLA/3%-MWCNTs. However, it was found that all samples were selective to chloroform. Moreover, annealing the samples showed a decrease in the responses that were significantly lower than the un‐ treated ones. Annealing did not affect the selectivity to chloroform but it considerably affect‐

Plasma-treated MWCNTs (p-MWCNTs) polyimide (PI) composite films (p-MWCNTs-PI) were developed by Yoo *et al.* in an effort to overcome some of the problems presented by PIbased resistive-type sensors, e. g. nonlinear sensitivity to relative humidity [33]. When tested as sensing material for 10-95% relative humidity (RH), p-MWCNTs-PI showed better sensi‐ tivity and linear response (resistance) as function of humidity, when compared to pristine-MWCNT-PI and pristine PI. The increase of p-MWCNTs content inp-MWCNTs-PI films not

toluene and lower sensitivity to methanol when compared to ethanol.

selectivity, response and recovery times.

348 Syntheses and Applications of Carbon Nanotubes and Their Composites

ed its sensitivity.

Yuana and coworkers prepared a sensor array based on polymer/MWCNTs composites for the selective detection of chloroform (CHCl3), tetrahydrofuran (THF) and methanol (MeOH) [76]. Ethyl cellulose (EC), poly [methyl vinyl ether-alt-maleic acid] (PMVEMA), hydroxypropyl methyl cellulose (HPMC), poly (alpha-methylstyrene) (PMS), poly (vinyl benzyl chloride) (PVBC) and poly (ethyleneadipate) (PEA) were the polymers used to prepare the polymer/MWCNTs composites and to provide uniqueness to each sensor in terms of their physical and chemical characteristics like molecular structure, polymer length, polarity and intermolecular forces. Changes in resistance as function of time were recorded for the sensor array when exposed to the different gases at different tem‐ peratures (30 40, 50, 60 o C) and 50-60% R. H. The sensors showed to be selective to the three gases at the different temperatures when in presence of vapor molecules of chlor‐ ide, cyclic oxide and hydroxide groups. The decreasing order of sensitivity concurred with the order of decreasing conductivity: PEA/MWCNTs >EC/MWCNTs >PMVEMA/ MWCNTs >PVBC/MWCNTs >HPMC/MWCNTs >PMS/MWCNTs.

SWCNTs modified with Poly- (D) glucosamine (Chitosan) (SWCNTs-CHIT) were as high performance hydrogen sensor [77]. Three types of sensors were prepared: SWCNTs de‐ posited on glass substrate (type 1), SWCNTs deposited over a glass substrate modified with CHIT-film (type 2), Chit-film deposited over SWCNTs deposited over a glass sub‐ strate. Each type of sensor showed a different changes in resistance when exposed to 4% H2 in air. Increase in resistance was observed for three types of sensors and good recov‐ ery but for type (3) sensor. Sensors modified with CHIT showed better sensitivity. More‐ over, the authors explained that the improved sensitivity was far higher than that reported for Pd-SWCNTs based sensors, which are commonly used for H2 detection. The enhanced performance of SWCNTs-CHIT can be explained by the strong interaction/ binding of the H2 molecules with/to the –OH and –NH3 groups contained in CHIT, which also explains the poor recovery of type 3 sensor.

SWCNTs were modified with Polypyrrole (PPy) and 5, 10, 15, 20-tetraphenylporphyrine (TPP) to prepare composites for the detection of 1-butanol in nitrogen using a quartz microbalance (QMB) [78]. The QMB gold electrodes were coated with PPy/SWCNTs-COOH, PPy/SWCNTs-COOH/SWCNTs-TPP via electropolymerization. Frequency shifts of the quartz crystal resonator as function of time were measured for the sensors when exposed to 1-butanol in ppb concentration range. Even though both composites showed good and higher response magnitudes than QMB prepared with other composites that did not contain CNTs, PPy/SWCNTs-COOH/SWCNTs-TPP showed better performance than PPy/SWCNTs-COOH. The results demonstrate that the incorporation of CNTs en‐ hanced the sensitivity towards the detection of 1-butanol.

Lu *et al*. reported a sensor array containing pristine SWCNTs, Rh-loaded SWCNTs, PEI/ SWCNTs and other CNTs with different coatings and loadings for the detection of hydrogen peroxide (H2O2) [79]. The measurements of changes in resistance as function of time were used to analyze the sensor array performance when exposed to H2O2. Pristine SWCNTs showed strong increases in resistance and fast responses for H2O2 and an estimated DL (by IUPAC definition) of 25 ppm. But when the sensor array was exposed to H2O and CH3OH in order to test its selectivity, pristine SWCNTs showed also good responses for both chemi‐ cals, which means that the discrimination capabilities towards H2O2 are limited. On the oth‐ er hand, the PEI/SWCNTs sensors were sensitive to H2O2, showing decreases in resistance for each exposure. The PEI/SWCNTs did not show significant changes in resistance when exposed to H2O and CH3OH, which makes it selective to H2O2 under the tested conditions.

PVBC PEA PPy

PPT

\* Lowest detected concentration

SWCNTs QMB 1-butanol 46ppb\* [78]

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351

PEI SWCNTs resistor H2O2 NS [79 80]

Electronic, physical and chemical properties of metallic nanoclusters are usually sensitive to the changes in environment [81]. CNTs decorated with metallic nanoparticles (NPs) have been widely used to achieve selectivity and improve the sensitivity, response time and DLs for a variety of gas detections. Layer by layer, electrodeposition, chemical deposition, elec‐ trochemical deposition and sputtering are the methods used to prepare the metallic NP-

SWCNTs films were modified with Pd NPs using sputtering method [82]. After apply differ‐ ent deposition times (40 – 160 s), it was found that 120 s was the optimum deposition time to

iair vs. time) showed differences in response and recovery between the first and following H2 sensing cycles. FTIR studies were used to support and explain those differences and mecha‐ nisms of detection. The first cycle showed an overall larger electrical current in the presence of H2and then it reached a new steady state. When the atmosphere was change to dry air, the current did not go to its original value but remained in the steady state, which is consid‐ ered as an irreversible response. The explanation to this is that is atomized by the Pd NPs and spilled to the surface of the MWCNTs, occurring the chemical and irreversible reaction of hydrogenation of the carbonyl groups of the MWCNTs at the first cycle. The second cycle and following ones started at the steady state where the first cycle finished and the electrical current showed a decrease in the presence of H2 and when the atmosphere was changed to dry air, the electrical current recovered back to where the cycle started. This reversible be‐

Pd/MWCNTs and Pt-Pd/MWCNTs composites were tested for the detection of H2 in a con‐ centration range of 20 ppm– 2% in N2 and 200 ppm – 2% in air [83]. Composites were pre‐ pared by growing CNTs yarns and then covered them with a layer of Pd NPs or sequentially deposited layers of Pd and Pt NPs, using a recently developed technique called self-fuelled electrodeposition (SFED). Exposure to 1% H2using N2 with 1% air as carrier gas. As with other Pd/CNTs-based sensors [82], an initial irreversible drop in resistance was ob‐

C. A typical response curve (igas/

**Table 2.** Summary of polymers used for the preparation of polymer/CNTs-based sensors.

**5. Metal nanoparticlesdecorated CNTs**

CNTs composites discussed in this section.

obtain enhanced sensor response for 1% H2 in dry air at 50 o

havior is explained as physisorption of H2 molecules onto Pd/SWCNTs.



\* Lowest detected concentration

peroxide (H2O2) [79]. The measurements of changes in resistance as function of time were used to analyze the sensor array performance when exposed to H2O2. Pristine SWCNTs showed strong increases in resistance and fast responses for H2O2 and an estimated DL (by IUPAC definition) of 25 ppm. But when the sensor array was exposed to H2O and CH3OH in order to test its selectivity, pristine SWCNTs showed also good responses for both chemi‐ cals, which means that the discrimination capabilities towards H2O2 are limited. On the oth‐ er hand, the PEI/SWCNTs sensors were sensitive to H2O2, showing decreases in resistance for each exposure. The PEI/SWCNTs did not show significant changes in resistance when exposed to H2O and CH3OH, which makes it selective to H2O2 under the tested conditions.

**Sensor**

PANi MWCNTs Resistor NH3 1ppm [68]

PI MWCNTs Resistor Humidity 10%\* [33]

PLA MWCNTs Resistor VOC NS [75] CHI SWCNTs Resistor H2 4%\* [77]

NH3

NO2

MWCNTs Resistor THF, CH3Cl2, MeOH NS [76]

**Configuration Target DL Ref.**

Toluene 1.7-12.2ppm

Methanol 650ppm\*

Ethanol 672ppm\* 1-propanol 635ppm\* 1-butanol 687ppm\*

Ethanol 176.5

Toluene 170.6

Methanol 184.2 [73]

Octane 9.2-12.7ppm [70 72]

NS

[74]

100ppm\* [69]

**Polymer CNT Type**

PECH, PEUT, PIB MWCNTs SAW

350 Syntheses and Applications of Carbon Nanotubes and Their Composites

Cellulose MWCNTs Resistor

PEDOT:PSS MWCNTs Resistor

PEI MWCNTs SAW

EC

PMVEMA HPMC PMS

**Table 2.** Summary of polymers used for the preparation of polymer/CNTs-based sensors.
