Carbon Nanotube Chemiresistors Research Articles

Peptide-Functionalized Carbon Nanotube Chemiresistors: The Effect of Nanotube Density on Gas Sensing.

Biorecognition element (BRE)-based carbon nanotube (CNT) chemiresistors have tremendous potential to serve as highly sensitive, selective, and power-efficient volatile organic compound (VOC) sensors. While many research groups have studied BRE-functionalized CNTs in material science and device development, little attention has been paid to optimizing CNT density to improve chemiresistor performance. To probe the effect of CNT density on VOC detection, we present the chemiresistor-based sensing results from two peptide-based CNT devices counting more than 60 different individual measurements. We find that a lower CNT density shows a significantly higher noise level and device-to-device variation while exhibiting mildly better sensitivity. Further investigation with SEM images suggests that moderately high CNT density with a stable connection of the nanotube network is desirable to achieve the best signal-to-noise ratio. Our results show an essential design guideline for tuning the nanotube density to provide sensitive and stable chemiresistors.

Potential of Carbon Nanotube Chemiresistor Array in Detecting Gas-Phase Mixtures of Toxic Chemical Compounds.

Toxic industrial chemicals (TICs), when accidentally released into the workplace or environment, often form a gaseous mixture that complicates detection and mitigation measures. However, most of the existing gas sensors are unsuitable for detecting such mixtures. In this study, we demonstrated the detection and identification of gaseous mixtures of TICs using a chemiresistor array of single-walled carbon nanotubes (SWCNTs). The array consists of three SWCNT chemiresistors coated with different molecular/ionic species, achieving a limit of detection (LOD) of 2.2 ppb for ammonia (NH3), 820 ppb for sulfur dioxide (SO2), and 2.4 ppm for ethylene oxide (EtO). By fitting the concentration-dependent sensor responses to an adsorption isotherm, we extracted parameters that characterize each analyte-coating combination, including the proportionality and equilibrium constants for adsorption. Principal component analysis confirmed that the sensor array detected and identified mixtures of two TIC gases: NH3/SO2, NH3/EtO, and SO2/EtO. Exposing the sensor array to three TIC mixtures with various EtO/SO2 ratios at a fixed NH3 concentration showed an excellent correlation between the sensor response and the mixture composition. Additionally, we proposed concentration ranges within which the sensor array can effectively detect the gaseous mixtures. Being highly sensitive and capable of analyzing both individual and mixed TICs, our gas sensor array has great potential for monitoring the safety and environmental effects of industrial chemical processes.

Evaluation of a Commercial Electronic Nose Based on Carbon Nanotube Chemiresistors.

Recently a hand-held, carbon-nanotube-based electronic nose became available on the market. Such an electronic nose could be interesting for applications in the food industry, health monitoring, environmental monitoring, and security services. However, not much is known about the performance of such an electronic nose. In a series of measurements, the instrument was exposed to low ppm vapor concentrations of four volatile organic compounds with different scent profiles and polarities. Detection limits, linearity of response, repeatability, reproducibility, and scent patterns were determined. The results indicate detection limits in the range of 0.1-0.5 ppm and a linear signal response in the range of 0.5-8.0 ppm. The repeatability of the scent patterns at compound concentrations of 2 ppm allowed the identification of the tested volatiles based on their scent pattern. However, the reproducibility was not sufficient, since different scent profiles were produced on different measurement days. In addition, it was noted that the response of the instrument diminished over time (over several months) possibly by sensor poisoning. The latter two aspects limit the use of the current instrument and make future improvements necessary.

An oligonucleotide-functionalized carbon nanotube chemiresistor for sensitive detection of mercury in saliva.

Divalent mercuric (Hg(2+)) ion and monomethyl mercury (CH3Hg(+)) are two forms of mercury that are known to be highly toxic to humans. In this work, we present a highly selective, sensitive and label-free chemiresistive biosensor for the detection of both, Hg(2+) and CH3Hg(+) ions using DNA-functionalized single-walled carbon nanotubes (SWNTs). The SWNTs were functionalized with the capture oligonucleotide, polyT, using a linker molecule. The polyT was hybridized with polyA to form a polyT:polyA duplex. Upon exposure to mercury ions, the polyT:polyA duplex dehybridizes and a T-Hg(2+)-T duplex is formed. This structure switch leads to the release of polyA from the SWNT surface and correspondingly a change in the resistance of the chemiresistive biosensor is observed, which is used to quantify the mercury ion concentration. The biosensor showed a wide dynamic range of 0.5 to 100 nM for the detection of CH3Hg(+) ions in buffer solution with a sensitivity of 28.34% per log (nM) of CH3Hg(+). Finally, real world application of the biosensor was demonstrated by the detection of Hg(2+) and CH3Hg(+) ions in simulated saliva samples spiked with a known concentration of mercury ions.

Epigallocatechin gallate decorated carbon nanotube chemiresistors for ultrasensitive glucose detection

The concentration of glucose in biological fluids is in the micromolar range, the detection of which requires devices with high sensitivity and low limit of detection (LOD). Here, we report the real-time electronic detection of glucose using an antioxidant found in green tea, namely, epigallocatechin gallate (EGCG), decorated on carbon nanotubes (CNTs) and tested in a chemiresistor configuration. The detection principle relies on the spontaneous reaction of EGCG with hydrogen peroxide, a reactive oxygen species released during glucose oxidation, which is translated electrically as a change in CNT conductance. Our results suggest that the response of EGCG decorated CNTs was far superior to that of the bare CNT based device. The sensor detected glucose ranging from 10 nM to 1 μM with LOD of ∼8.7 nM, which is much lower than the commercially available finger-pricking based glucose sensors. This could pave the way for developing simple resistivity-based sensors capable of glucose detection in biological fluids other than blood, such as sweat and saliva.

Environmental Monitoring of Low-ppb Ammonia Concentrations Based on Single-wall Carbon Nanotube Chemiresistor Gas Sensors: Detection Limits, Response Dynamics, and Moisture Effects

We present single-wall carbon nanotube (SWCNT) chemiresistor gas sensor (CGS) operating at room temperature, displaying an enhanced sensitivity to NH3. Ammonia concentrations in the full range of the average [NH3] in a urban environment have been measured, and a detection limit of 3 ppb is demonstrated, which is well below the sensitivities so far reported for non- functionalized SWCNTs operating at room temperature. Different materials were tested as substrates, including cheap plastic flexible substrates. In addition to a careful preparation of the SWCNT layers, the low-ppb limit is also attained by revealing and properly tracking a fast dynamics during the desorption process. On the basis of these results a model of the CGS response vs time is proposed. When functionalized with indium-tin oxide nanoparticles, a sensitivity increase is detected, along with a remarkable selectivity towards moisture.

Performance of graphene, carbon nanotube, and gold nanoparticle chemiresistor sensors for the detection of petroleum hydrocarbons in water

The performance of chemiresistor sensors made from thin film assemblies of single-wall carbon nanotubes, multiwall carbon nanotubes, reduced graphene oxide nanosheets (RGON), and gold nanoparticles (AuNP) was assessed with an immersible microelectrode array. Carbon nanotube and RGON chemiresistors were functionalized with octadecyl-1-amine and the AuNP chemiresistors were functionalized with 1-hexanethiol. The analytes examined were aqueous solutions of petroleum hydrocarbons: cyclohexane, naphthalene, benzene, toluene, ethylbenzene, and the three isomers of xylene (BTEX analytes). Titrations were performed to determine the detection limits of the different chemiresistors. The AuNP chemiresistor was the most sensitive to all the analytes with limits of detection between 0.2 and 0.6 ppm in water. In contrast, the multiwall carbon nanotube chemiresistor was the least sensitive to the analytes with limits of detection between 20 and 200 ppm. These sensitivities show that these nanomaterials have the potential, with further optimization, of being incorporated into devices that would respond to hydrocarbons in water at concentrations relevant to the regulations of the US Environmental Protection Agency. The stability of the sensors over 26 days was also assessed. Remarkably, there was a negligible change in the electrical resistance of the RGON sensors over this time. The nanotube sensors increased in resistance and the AuNP decreased in resistance over the same period. The drifting resistances did not affect the sensitivity of the nanomaterials, which remained constant with time.

Sensing properties of polyethylenimine coated carbon nanotubes in oxidized oil

Chemical detection is still a continuous challenge when it comes to designing single-walled carbon nanotube (SWCNT) sensors with high selectivity, especially in complex chemical environments. A perfect example of such an environment would be in thermally oxidized soybean oil. At elevated temperatures, oil oxidizes through a series of chemical reactions that results in the formation of monoacylglycerols, diacylglycerols, oxidized triacylglycerols, dimers, trimers, polymers, free fatty acids, ketones, aldehydes, alcohols, esters, and other minor products. In order to detect the rancidity of oxidized soybean oil, carbon nanotube chemiresistor sensors have been coated with polyethylenimine (PEI) to enhance the sensitivity and selectivity. PEI functionalized SWCNTs are known to have a high selectivity towards strong electron withdrawing molecules. The sensors were very responsive to different oil oxidation levels and furthermore, displayed a rapid recovery of more than 90% in ambient air without the need of heating or UV exposure.