**2. Unmodified carbon nanotubes**

Pristine CNTs are known for their high stability because of their strong sp2 carbon-carbon bonds and thus insensitive when used as sensing material for certain gases. However, the de‐ tection of NO, NO2 and NH3 has been previously reported. In order to improve their sensitivi‐ ty and recovery time, different approaches like dispersion techniques to debundling the CNTs ropes, humidity assisted detection, application of an electric field, continuous use of ultravio‐ let (UV) light and even separation of semi-conductive types from conductive have been report‐ ed. The detection of NO, NO2 and NH3, as well as other gases like formaldehyde and dimethyl methylphosphonate (DMMP) with pristine SWCNTs are discussed in this section.

A MWCNTs based sensor was used for the detection of 50 ppm of nitrogen monoxide (NO) [36]. With the purpose to increase the sensitivity, an electric field was applied between two copper plates as electrodes, one of them containing MWCNTs-silicon wafer. It was found that when a positive potential was applied to the copper plate and the negative potential ap‐ plied to the copper plate containing the MWCNTs-sensor, NO, being an electron acceptor, moves to the electron enriched zone, which is the one containing the MWCNTs-sensor and thus enhancement in the sensitivity is observed. The stronger the applied electric field, the better the sensitivity. However, applying a negative electric field was applied to the copper plate and a positive potential was applied to the copper plate containing the MWCNTs-sen‐ sor, the NO molecules moved away from the MWCNTs sensor and thus a decrease in the sensitivity is observed. The more negative the applied electric field, the lower the sensitivity of the sensor. Recovery of the sensors was achieved by applying reverse potential from the one used to perform the gas sensing experiments.

tion/functionalization capability has been exploited throughout the last years, especially for the development of devices with enhanced selectivity and sensitivity for the room tempera‐ ture detection of a wide variety of gases. Numerous articles and reviews focused on differ‐ ent aspect of CNTs-based sensors and summarizing their progress and potential have been published throughout the years. Some of them are listed in references [3-34]. A most recent review [35] addressing the technological and commercial aspects of CNTs sensors presents evidence of the continuous active research in the area and that they have real potential to

This chapter presents a summary of selected original research articles that have been pub‐ lished between 2010 and present in which the main subject are modified/functionalized SWCNTs, DWCNTs and MWCNTs and their use as gas sensing material. The majority of the references included in this chapter content are based on experimental results. However, theoretical studies based on computational science are also included because of their impor‐ tance in the study of CNT-based sensors. The use of different methods of calculations and simulation has been useful to design new structures and materials and to study, evaluate and predict the interactions and adsorption energies between those materials and gaseous molecules. First, we present current research activities on pristine CNTs-based sensors and the different approaches used to improve their sensitivity and selectivity without modifying the CNTs structure, followed by the review of CNTs modified with conducting polymers, metallic nanoparticles (NPs), nanostructured oxides and sidewall modification, doping and others. Different modification/functionalization techniques like chemical deposition, plasma, sputtering and electrodeposition are discussed. Gas sensors based on changes of electrical conductivity caused by adsorption of gas molecules (resistors) are the most common sensor type discussed in this review. Other sensing platforms like surface acoustic wave (SAW) and

Pristine CNTs are known for their high stability because of their strong sp2 carbon-carbon bonds and thus insensitive when used as sensing material for certain gases. However, the de‐ tection of NO, NO2 and NH3 has been previously reported. In order to improve their sensitivi‐ ty and recovery time, different approaches like dispersion techniques to debundling the CNTs ropes, humidity assisted detection, application of an electric field, continuous use of ultravio‐ let (UV) light and even separation of semi-conductive types from conductive have been report‐ ed. The detection of NO, NO2 and NH3, as well as other gases like formaldehyde and dimethyl

A MWCNTs based sensor was used for the detection of 50 ppm of nitrogen monoxide (NO) [36]. With the purpose to increase the sensitivity, an electric field was applied between two copper plates as electrodes, one of them containing MWCNTs-silicon wafer. It was found that when a positive potential was applied to the copper plate and the negative potential ap‐ plied to the copper plate containing the MWCNTs-sensor, NO, being an electron acceptor,

methylphosphonate (DMMP) with pristine SWCNTs are discussed in this section.

complement or substitute current technologies.

338 Syntheses and Applications of Carbon Nanotubes and Their Composites

quartz microbalance (QMB)are also included.

**2. Unmodified carbon nanotubes**

Cava, et al. proposed the use of a homogeneous film of MWCNTs prepared by the self-as‐ sembly technique and use it as an active layer for an oxygen gas sensor with increased sensi‐ tivity [37]. When the sensors were exposed to 10% O2 in Nitrogen at 160 o C, the electrical resistance decreased and showed a better oxygen sensitivity when compared to sensors pre‐ pared under the same condition but using the drop-cast method. The reason for this is that the self-assembly technique provides a better distribution of the nanotubes and thus pro‐ moting a better gas adsorption between nanotubes (inter-tube contact).

The high van der walls attraction between CNTs causes them to remain in bundles or ag‐ glomerated. This can represent a problem for their application as gas sensors because it re‐ sults in less adsorption/interaction (binding) sites, which translates in less sensitivity. Considering this, different dispersion techniques were used by Ndiaye and coworkers for the preparation of CNTs based sensors for NO2 detection [38]. SWCNTs were dispersed in a surfactant, sodium dodecylbenzene sulfonate (NaDDBs) and an organic solvent, chloroform (CHCl3), drop-casted in IDEs and tested for the detection of 50, 100, 120, 200 ppb of NO2 at 80 o C. Sensors prepared with SWCNTs dispersed in NaDDBs showed better sensitivity than those with SWCNTs dispersed in chloroform. The explanation to this is that the surfactant was more effective in debundle the SWCNTs than the organic solvent. It was stated that even though the surfactant was not completely removed after several rinsing steps and heat‐ ing treatment at 150o C, it does not have significant effect on the electronic behavior of the sensor. Both surfactant-dispersed and organic solvent-dispersed samples showed a decrease in resistance with increasing temperature, which demonstrate the semi-conductive behavior of the SWCNTs and thus no effect of the solvent.

A SWCNT-based gas sensor selective for NO2 and SO2 at room temperature and ambient pressure was developed by Yao *et al.* [39]. High sensitivity and selectivity for 2 ppm of each gas was achieved by controlling the humidity levels. For instance, at low humidity levels, the sensors showed to be selective for NO2and insensitive to SO2. At high humidity levels (92%), both gases were detected. However, NO2 showed a decrease in resistance and SO2 showed an increase in resistance.

Continuous *in situ* UV illumination on SWCNTs during gas sensing experiments was used to enhance the sensor's overall performance in the detection of NO, NO2 and NH3 (Figure 1) [40]. Changes in conductance (∆G/G0) as function of time were recorded and used to prepare calibration curves in order to determine sensors sensitivity. It was found that the continuous exposure of the sensors to UV light under inert atmosphere (N2, Ar) regenerating the sur‐ face, therefore, enhances their sensitivity for the detection of NO and NO2. Linear responses were achieved at low concentrations and up to 50 ppm. Detection limits (DL), derived from the noise of the baseline and the slope obtained from the calibration curve, were found to be as low as 590 parts per quadrillion (ppq) and 1.51ppt for NO and NO2, respectively. For NH3 it was found not only that the *in situ* UV illumination reverses the direction of the changes in conductance, but it was also confirmed that it helps to improve the DL from 5.67 ppm to 27.8 ppt when tested under identical conditions. The achieved DL outperformed by several orders of magnitude the sensitivity of other CNTs-based NO, NO2and NH3 sensors that have been previously reported. This is attributed to the UV light inducing surface regenera‐ tion and actively removal of all gases adsorbed on SWCNTs surface. It is worth noticing that this *in situ* cleaning with continuous UV-light exposure without device degradation was just achieved under inert atmospheres.

decrease in resistance and increase in resistance as exposed to NO2 and NH3, respectively. Full recovery was achieved by applying heat after NH3 exposure and vacuum after NO2 ex‐ posure. However, semi-conducting SWCNTs films were more sensitive for NH3 than to NO2

Latest Advances in Modified/Functionalized Carbon Nanotube-Based Gas Sensors

http://dx.doi.org/10.5772/52173

341

Horrillo *et al.* used SWCNTs films for the room temperature detection of Chemical Warfare Agents (CWA) [42]. Changes in resistance were measured as samples were exposed to simu‐ lants of CWA at different ppm levels, DL of 0.01 ppm, 0.1 ppm and 50 ppm were achieved for DMMP, dipropylene glycol methyl ether (DPGME) and dimethylacetamide (DMA), re‐ spectively. The most remarkable advantage is that the sensors perform better and at more

CNTS modified with different functional groups have been used for the development of sensors for detection of volatile organic compounds (VOCs) in the environment as well as in exhaled breath. For the detection of VOCs in air, Wang *et al.* worked in the preparation of a sensor array based on MWCNTs covalently modified with different functional groups like propargyl, allyl, alkyltriazole, thiochain, thioacid, hexafluoroisopropanol (HFIP) [43] and Shrisat, *et al.* reported another one based on SWCNTs modified with different porphyrins (organic macrocyclic compounds) likeoctaethyl porphyrin (OEP), ruthenium OEP (RuOEP), iron OEP (FeOEP), tetraphenylporphyrin (TPP), among others [44]. Penza *et al*. also worked in the modification of MWCNTs with TPP for the room temperature detection of VOCs [45]. In this case, the TTP contained two different metals, Zn (CNT:ZnTPP) and Mn (CNT:MnTPP). Sensors were exposed to ethanol, acetone, ethylacetate, toluene and Triethyl‐ amine at ppm levels and all showed increase in resistance when exposed to the different gases. CNT: MnTPP showed the highest sensitivity towards all gases with respect to un‐ modified CNTs but for triethylamine and CNT: ZnTPP was more sensitive to ethylacetate. Two different CNT-based sensor arrays have been reported for the detection and pattern recognition of VOCs present in exhaled breath samples for medical diagnosis, Tisch *et al*. presented a sensor array containing different nanomaterials including organically function‐ alized random networks of SWCNTs for the detection of VOCs related to Parkinson disease and that were present in exhaled breath collected from rats [46]. Ionescu and coworkers re‐ ported a sensor array based on bilayers of SWCNTS and polycyclic aromatic hydrocarbons (PAH) for the detection of multiple sclerosis in exhaled human breath [47]. In general, the incorporation of organic functional groups provided not only enhanced sensitivity but also provided better selectivity for each gas when compared to pristine CNTs. The use of statisti‐ cal techniques like principal component analysis (PCA), discriminant factor analysis (DFA) and linear discriminant analysis (LDA) was possible to determine the discrimination capa‐

SWCNTs were functionalized with tetrafluorohydroquinone (TFQ) at the room temperature for detection of dimethyl methylphosponate (DMMP) at parts per trillion (ppt) levels (Fig‐

sensitive at room temperature, when tested at different temperatures.

at different concentrations at ppm level.

**3. Surface modified carbon nanotubes**

bility of the sensors toward each VOC.

**Figure 1.** SWCNTs-sensor responses to (A) NO (10 – 200ppt) under *in situ* UV illumination, (B) NO2 (40 – 1000 ppt) under *in situ* UV illumination, (C) NH3 (5 – 500 ppm) without *in situ* UV illumination, and (D) NH3 (200 – 4000 ppt) under *in situ* UV illumination. (From Chen *et al.* [40]. Copyright © 2012, with permission from Nature Publishing Group. )

Battie and coworkers used sorted semi-conducting SWCNTs as sensing film for the detec‐ tion of NO2 and NH3 [41]. The density gradient ultracentrifugation (DGU) technique was used to separate semi-conducting from as produced SWCNTs. Films of as produced and sorted semi-conducting SWCNTs were exposed to NO2 and NH3 in air. Both films showed decrease in resistance and increase in resistance as exposed to NO2 and NH3, respectively. Full recovery was achieved by applying heat after NH3 exposure and vacuum after NO2 ex‐ posure. However, semi-conducting SWCNTs films were more sensitive for NH3 than to NO2 at different concentrations at ppm level.

Horrillo *et al.* used SWCNTs films for the room temperature detection of Chemical Warfare Agents (CWA) [42]. Changes in resistance were measured as samples were exposed to simu‐ lants of CWA at different ppm levels, DL of 0.01 ppm, 0.1 ppm and 50 ppm were achieved for DMMP, dipropylene glycol methyl ether (DPGME) and dimethylacetamide (DMA), re‐ spectively. The most remarkable advantage is that the sensors perform better and at more sensitive at room temperature, when tested at different temperatures.
