Int. Journal of Electrical & Electronics Engg.
Vol. 2, Spl. Issue 1 (2015)
e-ISSN: 1694-2310 | p-ISSN: 1694-2426
Carbon Nanotubes Based Sensor for Detection of Traces of Gas Molecules- A Review 1
Arvind Kumar, 2Jaspreet Kaur Rajput, 3Sukhvir Kaur 1, Department of ECE,UIET Panjab University, Chandigarh 160025 2, Dr. B.R. Ambedkar National Institute of Technology, Jalandhar 144011 Abstract- In this review paper, we discuss various gas sensors based on technique and sensing materials used in there fabrication. Various sensors are designed making use of salient features of carbon nanotubes and its electrical, mechanical, and electromechanical properties. Effect of using nano-composites on sensitivity and selectivity of gas sensor have been studied.
While some commercial products for detecting trace energetic chemicals including explosives are now available, the need of highly sensitive and fast detection methods is continuously increasing and the research into alternative technologies continues at a pace. This is a result of the fact that detecting trace chemicals in a complex environment still remains a significant technological challenge because of the extremely low concentrations of the chemicals in solutions or low partial pressures in the air. For example, the partial pressure of 2,4,6-trinitrotoluene (TNT) is a few parts per billion (ppb) at room temperature and in the subpart per trillion range in the air above a buried mine. Nowadays, electronic nose has become a powerful tool to detect even the traces of explosive materials [2]. To address this challenge, various detection methods with potential to achieve low limit of detection (LOD) are being investigated for trace chemical detection, such as electro-chemical sensors, biosensor, fluorescence and Raman-based optical methods, mass spectrometry (MS), ion mobility spectrometry (IMS), and sensors based on nano and microfabrication technologies. Electrochemical sensors provide some selectivity but suffer from limited sensitivity and require mobile electrolytes, which may cause stability issues and delayed response time. In addition, electrodes can be easily fouled, and interfering problems may occur as some interferents are electrochemically active. Nanomaterials, e.g., carbon nanotubes (CNTs) and nanowires, have been employed to construct electrochemical sensors for explosive detections [3]. CNTs applications include CNTs field-effect transistors or resistors and CNTs modified glassy carbon electrodes. More recently, CNTs have been employed in explosive sensors by exploiting passive functions rather than transduction, such as enhanced Raman scattering and preconcentration of target samples. LODs down to the ppb level have been achieved for TNT detection by exploiting the extremely large surface-to-volume ratio. Despite high sensitivity, significant challenges such as low selectivity, low signal-to-noise ratio (SNR), long recovery times, interference, and device fabrication difficulties still remain to be addressed [4-6].
Index Terms – Carbon Nanotubes (CNTs), Electrochemical sensor (E-nose), sensors.
I. INTRODUCTION Sensors are the devices which convert any form of energy or any physical parameters into electrical signals/measurable signals. Pressure sensor, gas sensor, humidity sensor, chemical sensors are some few examples of sensors available in the market. Gas sensors are the devices that are used to detect gas molecules in atmosphere which may cause harm to our health. There are more than one hundred types of military and civilian explosives and around twenty commonly used drugs. A number of characteristics can be used for the detection of gas molecules [1]: ˗ Geometry (the presence a metallic detonator can be detected using image shape analysis), ˗ Material density (explosive material is denser than most organic materials), ˗ Elemental composition (e.g. vapor emission analysis can be used to detect them), ˗ Vapor emissions (e.g. nitrogen or its compounds can be detected in a vapor sample). A. Detection methods Vapor detection methods are used to measure traces of characteristic volatile compounds that evaporate from the explosive and other gas molecules [1]. However, the concentration of explosive vapors inside the sensors is many orders of magnitude lower than the pressure of saturated vapor on the explosives surface. Vapors and traces are commonly detected by means of: - Electronic/chemical sensors, - Optical sensors, - Biosensors.. A new type of pressure sensor propose approaches for improving sensitivity, selectivity and size. The most powerful pressure sensor contains microelectronics circuits, which enable to install a digital pressure gauge just in sensor and software control starting of different electronic regulations in according to the measured values. Special pressure sensors present sensors for explosive environment. B. Electrochemical sensor (E-nose) 133
C. CNTs as sensing material
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Detection of trace explosives is still a challenging task because of the extremely low vapor concentrations. Current explosive/gas sensors are mainly categorized into two modes of operation; chemical type operating by gas adsorption and physical type using ionization method. Chemical type conductivity-based explosive detectors are bulky, they require high operating temperatures and have slow response time. Moreover most of them are capable of
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Int. Journal of Electrical & Electronics Engg.
Vol. 2, Spl. Issue 1 (2015)
detecting only single type fumes of explosive materials due to their low selectivity. While on other hand ionization-based sensors have better selectivity and response time, but their huge size becomes its limitation. Both chemical and physical type gas detectors are using semiconductor materials as their sensing elements. With the discovery of nanomaterials, different types of sensing elements have been investigated to produce sensors which are smaller in size, one of which is carbon nanotubes (CNTs). Development of high performance sensor is now focused towards CNT- based sensors because of their inherent properties such as small size, large surface area and high electrical conductivity. Sensors designed using CNTs are smaller in size, have lower power consumption, high sensitivity and better selectivity compared to existing semiconductor based gas sensors. CNT- based gas sensors operate at room temperature which will result in safer environment. Researches investigates the structural and electrical characterization of carbon nanotubes array as suitability and an effective sensing element in the ionization-based sensor. The atomic structure of SWCNT can be formed by wrapping a stripe of single atomic layer of graphite sheet along a certain direction and this direction determines the diameter and chirality of the nanotubes. Experimental and theoretical studies have found that these nano-meter sized CNTs have novel electronic properties that is these can be metallic or semiconducting, depending on their radius or chiralities. CNTs possess unique set of electrical and mechanical properties which make them useful in variety of applications [7]: • 100 times stronger than steel and 1/6th the weight of steel (Tensile strength value, 63 GPa, exceeds that of any reported value for any type of material. Applications for very light-weight, high-strength cables and composites, where the carbon nanotubes are the load-carrying element.) • Electrical conductivity as high as copper, thermal conductivity as high as diamond. • Average diameter of 1.2 – 1.4 nm (10000 times smaller than a human hair). CNTs are categorized as conductor, semiconductor or insulator based on their structure (patterns in which graphene sheets are folded). Carbon nanotubes labeled as armchair show electrical properties similar to metals. When potential is applied to ends of CNTs current start flowing even they have conductivity better than copper [8]. II. CNTs AS A SENSOR Seong jeen kim presented CNT based sensor for detection of gas molecules, in this CNTs were used with silane hybrid thin film deposited by spray coating on substrate. It has shown great potential in sensitivity, operation at room temperature, small size. In this various silane binders are used to improve selectivity of the sensor, response property to different gas will be changed if the binder in the solution is different. Here comparison between MTMS and TEOS has been observed. With the improvement in technology to reduce cost and improve sensitivity, metal oxides are being used for doping CNTs in order to improve selectivity [9]. NITTTR, Chandigarh
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e-ISSN: 1694-2310 | p-ISSN: 1694-2426
Chatchawal wongchoosuk come with an idea of electronic nose based on carbon nanotubes doped with SnO2 used as a gas sensor. MOS based sensor generally exhibit relatively poor selectivity, in this E- nose various processing elements are used. Features extraction plays a main role for pre-processing steps, these help to enhance performance through more stable data representation. The doping of CNTs improves selectivity and sensitivity of MOS SnO2 gas sensor as CNT provide large surface area that increase gas reaction at the metal oxide. Sensor is fabricated using electron beam evaporation. Initially Cr/Al layer is deposited over the substrate, sensing film of thickness around 300 nm is deposited. NiCr layer was evaporated over substrate backside used as heating element [11]. A. R. Kermany came forward with an idea of ionization based gas sensor. In this an array of CNTs are aligned in a particular fashion for sensing gas molecules. This sensor is based on fingerprinting the ionization characteristics of different gases. Every gas has unique breakdown voltage which is used for gas identification. An array of CNTs is working as gas sensor; in this CNTs are used as anode and aluminum as cathode. Both plates are connected to voltage source, this applied voltage provide energy for ionization [12-14]. Though they have high selectivity and good response time but it still need improvement in its large size and high input voltage required for operation. To improve previously designed sensors Hoel Guerin fabricated conductance based gas sensor with the help of CNTs as sensing material. Semiconducting CNTs exhibits change in resistance upon adsorption of gas molecules on CNTs surface. As to improve selective detection of analytes different metals are used as electrode along with CNTs each distinct CNT-metal contact behaves differently with distinct gas molecules. Now this difference in resistance value could be used as electronic signature to identify the gas. Wenzhou Ruan emphasis on Low limit of detection (LOD) to detect traces of gas molecules deficiently, various methods is investigated to achieve LOD for trace chemical detection. Electrochemical sensor suffer from limited selectivity to overcome this mobile electrolytes are required and they add up to stability issues and delay in response time. Microsensors are gaining importance in trace detection techniques. Microcantilevers and microcalorimeter are typically used techniques in microsensing. Microcalorimeter constitute three part a microsuspended membrane, heater, thermistor and a CNT film on the surface of membrane. Single crystalline silicon is used for fabrication of heater and thermistor [16]. Suspended membrane is heated to deflagration temperature due to high thermal conductivity CNT is also heated. When it is exposed to gas molecules they get adsorbed on CNT surface and go through endothermic or exothermic reactions. This change in temperature is measured by thermistor signifying detection of gas molecules. In this set up it utilizes high surface area and thermal conductivity of 134
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Vol. 2, Spl. Issue 1 (2015)
e-ISSN: 1694-2310 | p-ISSN: 1694-2426
alcoholic beverages “, trends anal. Chem., 24, pp. 57-66, 2005. [3] H. Yu and J. Wang, “discrimination of long jing green tea by electronic nose”, sens. Actuators B,122, pp. 134-140, 2007. [4]. D. S.Moore, “Instrumentation for trace detection of high explosives,” Rev. Sci. Instrum., vol. 75, no. 8, pp. 2499–2512, Aug. 2004. [5] J. Wang, “Electrochemical sensing of explosives,” Electroanalysis, vol. 19, no. 4, pp. 415– 423, Feb. 2007. [6] S. V. Patel, T. E. Mlsna, B. Fruhberger, E. Klaassen, S. Cemalovic, and D. R. Baselt, “Chemicapacitive microsensors for volatile organic compound detection,” Sens. Actuators B, Chem., vol. 96, no. 3, pp. 541– 553, Dec. 2003. [7]. Valentin N. Popov, “Carbon nanotubes: properties and application”, Materials Science and Engineering R 43 (2004) 61–102, 2004. [8]. Phaedon Avouris, Zhihong Chen, “carbon based electronics”, Nature Nanotechnology 2, 605 - 615 (2007). [9]. S. jeen kim, “The Effect on the Gas Selectivity of CNT-Based Gas Sensors by Binder in SWNT/Silane Sol Solution”, IEEE SENSORS JOURNAL, VOL. 10, NO. 1, JANUARY 2010. [10]. C. Wongchoosuk, A. Wisitsoraat, “Mobile Electronic Nose Based on Carbon Nanotube- SnO2 Gas Sensors: Feature Extraction Techniques and Its Application”, 978-1-4244-3388-9, 2009 IEEE. [11]. A. Wisitsoraat, A. Tuantranont, C. Thanachayanont, V. Patthanasettakul, and P. Singjai, “Electron beam evaporated carbon nanotube dispersed SnO2 thin film gas sensor”, J. Electroceram., 17, pp. 45–49, 2006. [12]. A. R. Kermany, N. M. Mohamed, “Ionization- Based Gas Sensor using Aligned MWCNTs Array”, ICSE Proc. 2010, Melaka, Malaysia 2010 IEEE. [13]. A. Pham, “Carbon Nanotubes Resonator Sensors for Remote Sensing Systems,” in IEEE Topical Conference on Wireless Communication Technology 2003. [14]. J. Huang, J. Wang, C. Gu, K. Yu, F. Meng, and J. Liu, “A novel highly sensitive gas ionization sensor for ammonia detection,” Sensors and Actuators, A 150, pp. 218–223 2009. [15]. J. Wu, h. Liu, Y. Wang, D. Xu, and Y. Zhang, “A MEMS-Based Ionization Gas Sensor Using Carbon Nanotubes and dielectric Barrier,” Proceeding of the 3rd IEEE int. Conf. on Nano/ Micro Engineered and Molecular Systems 2008. [16]. Wenzhou Ruan, Zheyao Wang, “A Microcalorimeter Integrated With Carbon Nanotube Interface Layers for Fast Detection of Trace Energetic Chemicals”, JOURNAL OF Microelectromechanical Systems, vol. 22, no. 1, February 2013. [17]. J. Wang and S. Thongngamdee, “On-line electrochemical monitoring of (TNT) 2,4,6- trinitrotoluene in natural waters,” Anal. Chim. Acta, vol. 485, no. 2, pp. 139–144, Jun. 2003. [18]. M. Sergio and M. Arben, “Nanomaterials based electrochemical sensing applications for safety and security,” Electroanalysis, vol. 24, no. 3, pp. 459– 469, Mar. 2012.
carbon nanotubes. In the above discussion various gas sensors have been studied for detection of gas molecules, every sensor comes with its own pros and cons. Along with there effective output they do have some limitations which are need to be improved. III. CONCLUSION Electrochemical sensors provide some selectivity but suffer from limited sensitivity and require mobile electrolytes [17], which may cause stability issues and delayed response time. In addition, electrodes can be easily fouled, and interfering problems may occur as some interferents are electrochemically active. Nanomaterials, e.g., carbon nanotubes (CNTs) and nanowires, have been employed to construct electrochemical sensors for explosive detections. Typical CNTs applications include CNTs field-effect transistors or resistors and CNTs modified glassy carbon electrodes. More recently, CNTs have been employed in explosive sensors by exploiting passive functions rather than transduction, such as enhanced Raman scattering and preconcentration of target samples. LODs down to the ppb level have been achieved for TNT detection by exploiting the extremely large surface to volume ratio. Despite high sensitivity, significant challenges such as low selectivity, low signal-to-noise ratio (SNR), long recovery times, interference, and device fabrication difficulties still remain to be addressed [18]. To further improve selectivity and sensitivity for sensing gas molecules, functionalization of CNT are required. Functionalization with particular metal oxide or nanocomposite can improve selectivity of the sensor. Use of hydrophobic compounds can protect false alarm from water vapours. Nano- materials provide large surface area so they should be used as sensing materials. REFERENCES [1]. Z. Bielecki, J. Janucki, “Sensors and Systems for the detection of explosive devices – an overview”, Metrol. Meas. Syst., Vol. 19 (2012), No. 1, pp. 3-28, 2012. [2]. M.P. Marti, R.Boque, “electronic nose in the quality control of
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