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Carbon Nanotubes Based Sensor for Detectionof Traces of Gas Molecules- A Review

1Arvind Kumar, 2Jaspreet Kaur Rajput, 3Sukhvir Kaur1, 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 sensorsbased on technique and sensing materials used in therefabrication. Various sensors are designed making useof salient features of carbon nanotubes and its electrical,mechanical, and electromechanical properties. Effect of usingnano-composites on sensitivity and selectivity of gas sensorhave been studied.

Index Terms – Carbon Nanotubes (CNTs), Electrochemicalsensor (E-nose), sensors.

I. INTRODUCTIONSensors are the devices which convert any form of energyor any physical parameters into electricalsignals/measurable signals. Pressure sensor, gas sensor,humidity sensor, chemical sensors are some few examplesof sensors available in the market. Gas sensors are thedevices that are used to detect gas molecules in atmospherewhich may cause harm to our health.There are more than one hundred types of military andcivilian explosives and around twenty commonly useddrugs. A number of characteristics can be used for thedetection of gas molecules [1]:˗ Geometry (the presence a metallic detonator can bedetected using image shape analysis),˗ Material density (explosive material is denserthan most organic materials),˗ Elemental composition (e.g. vapor emissionanalysis can be used to detect them),˗ Vapor emissions (e.g. nitrogen or its compounds can bedetected in a vapor sample).

A. Detection methods

Vapor detection methods are used to measure traces ofcharacteristic volatile compounds that evaporate from theexplosive and other gas molecules [1]. However, theconcentration of explosive vapors inside the sensors ismany orders of magnitude lower than the pressure ofsaturated 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 forimproving sensitivity, selectivity and size. The mostpowerful pressure sensor contains microelectronicscircuits, which enable to install a digital pressuregauge just in sensor and software control starting ofdifferent electronic regulations in according to themeasured values. Special pressure sensors present sensorsfor explosive environment.

B. Electrochemical sensor (E-nose)

While some commercial products for detecting traceenergetic chemicals including explosives are nowavailable, the need of highly sensitive and fast detectionmethods is continuously increasing and the research intoalternative technologies continues at a pace. This is aresult of the fact that detecting trace chemicals in acomplex environment still remains a significanttechnological challenge because of the extremely lowconcentrations of the chemicals in solutions or low partialpressures in the air. For example, the partial pressure of2,4,6-trinitrotoluene (TNT) is a few parts per billion(ppb) at room temperature and in the subpart pertrillion range in the air above a buried mine. Nowadays,electronic nose has become a powerful tool to detect eventhe traces of explosive materials [2].To address this challenge, various detection methods withpotential to achieve low limit of detection (LOD) arebeing investigated for trace chemical detection, such aselectro-chemical sensors, biosensor, fluorescence andRaman-based opticalmethods, mass spectrometry (MS), ion mobilityspectrometry (IMS), and sensors based on nano andmicrofabrication technologies.Electrochemical sensors provide some selectivity butsuffer from limited sensitivity and require mobileelectrolytes, which may cause stability issues and delayedresponse time. In addition, electrodes can be easily fouled,and interfering problems may occur as some interferentsare electrochemically active. Nanomaterials, e.g., carbonnanotubes (CNTs) and nanowires, have been employed toconstruct electrochemical sensors for explosive detections[3]. CNTs applications include CNTs field-effecttransistors or resistors and CNTs modified glassy carbonelectrodes. More recently, CNTs have been employed inexplosive sensors by exploiting passive functions ratherthan transduction, such as enhanced Raman scattering andpreconcentration of target samples. LODs down to the ppblevel have been achieved for TNT detection by exploitingthe extremely large surface-to-volume ratio. Despite highsensitivity, significant challenges such as low selectivity,low signal-to-noise ratio (SNR), long recovery times,interference, and device fabrication difficulties still remainto be addressed [4-6].

C. CNTs as sensing material

Detection of trace explosives is still a challenging taskbecause of the extremely low vapor concentrations.Current explosive/gas sensors are mainly categorized intotwo modes of operation; chemical type operating by gasadsorption and physical type using ionization method.Chemical type conductivity-based explosive detectors arebulky, they require high operating temperatures and haveslow response time. Moreover most of them are capable of

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detecting only single type fumes of explosive materialsdue to their low selectivity. While on other handionization-based sensors have better selectivity andresponse time, but their huge size becomes its limitation.Both chemical and physical type gas detectors are usingsemiconductor materials as their sensing elements. Withthe discovery of nanomaterials, different types of sensingelements have been investigated to produce sensors whichare smaller in size, one of which is carbon nanotubes(CNTs). Development of high performance sensor isnow focused towards CNT- based sensors because oftheir inherent properties such as small size, large surfacearea and high electrical conductivity. Sensors designedusing CNTs are smaller in size, have lower powerconsumption, high sensitivity and better selectivitycompared to existing semiconductor based gas sensors.CNT- based gas sensors operate at room temperaturewhich will result in safer environment. Researchesinvestigates the structural and electrical characterizationof carbon nanotubes array assuitability and an effective sensing element in theionization-based sensor. The atomic structure of SWCNTcan be formed by wrapping a stripe of single atomiclayer of graphite sheet along a certain direction and thisdirection determines the diameter and chirality of thenanotubes. Experimental and theoretical studies havefound that these nano-meter sized CNTs have novelelectronic properties that is these can be metallic orsemiconducting, depending on their radius or chiralities.CNTs possess unique set of electrical and mechanicalproperties which make them useful in variety ofapplications [7]:• 100 times stronger than steel and 1/6th theweight of steel (Tensile strength value, 63GPa, exceeds that of any reported value forany type of material. Applications for very light-weight,high-strength cables and composites, where the carbonnanotubes 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 orinsulator based on their structure (patterns in whichgraphene sheets are folded). Carbon nanotubeslabeled as armchair show electrical properties similar tometals. When potential is applied to ends of CNTscurrent start flowing even they have conductivity betterthan copper [8].

II. CNTs AS A SENSORSeong jeen kim presented CNT based sensor for detectionof gas molecules, in this CNTs were used with silanehybrid thin film deposited by spray coating onsubstrate. It has shown great potential in sensitivity,operation at room temperature, small size. In this varioussilane binders are used to improve selectivity of thesensor, response property to different gas will bechanged if the binder in the solution is different. Herecomparison between MTMS and TEOS has beenobserved. With the improvement in technology to reducecost and improve sensitivity, metal oxides are being usedfor doping CNTs in order to improve selectivity [9].

Chatchawal wongchoosuk come with an idea ofelectronic nose based on carbon nanotubes doped withSnO2 used as a gas sensor. MOS based sensor generallyexhibit relatively poor selectivity, in this E- nose variousprocessing elements are used. Features extraction plays amain role for pre-processing steps, these help to enhanceperformance through more stable data representation. Thedoping of CNTs improves selectivity and sensitivity ofMOS SnO2 gassensor as CNT provide large surface area that increasegas reaction at the metal oxide.Sensor is fabricated using electron beam evaporation.Initially Cr/Al layer is deposited over the substrate, sensingfilm of thickness around 300 nm is deposited. NiCr layerwas evaporated over substrate backside used as heatingelement [11].

A. R. Kermany came forward with an idea of ionizationbased gas sensor. In this an array of CNTs are aligned in aparticular fashion for sensing gas molecules. This sensor isbased on fingerprinting the ionization characteristics ofdifferent gases. Every gas has unique breakdown voltagewhich is used for gas identification. An array of CNTs isworking as gas sensor; in this CNTs are used as anode andaluminum as cathode. Both plates are connected to voltagesource, this applied voltage provide energy for ionization[12-14].

Though they have high selectivity and good response timebut it still need improvement in its large size and high inputvoltage required for operation. To improve previouslydesigned sensors Hoel Guerin fabricated conductancebased gas sensor with the help of CNTs as sensingmaterial. Semiconducting CNTs exhibits change inresistance upon adsorption of gas molecules on CNTssurface. As to improve selective detection of analytesdifferent metals are used as electrode along with CNTseach distinct CNT-metal contact behaves differently withdistinct gas molecules. Now this difference in resistancevalue could be used as electronic signature to identify thegas.

Wenzhou Ruan emphasis on Low limit of detection (LOD)to detect traces of gas molecules deficiently, variousmethods is investigated to achieve LOD for trace chemicaldetection. Electrochemical sensor suffer from limitedselectivity to overcome this mobile electrolytes arerequired and they add up to stability issues and delay inresponse time. Microsensors are gaining importance intrace detection techniques. Microcantilevers andmicrocalorimeter are typically used techniques inmicrosensing. Microcalorimeter constitute three part amicrosuspended membrane, heater, thermistor and a CNTfilm on the surface of membrane. Single crystalline siliconis used for fabrication of heater and thermistor [16].Suspended membrane is heated to deflagration temperaturedue to high thermal conductivity CNT is also heated.When it is exposed to gas molecules they get adsorbed onCNT surface and go through endothermic or exothermicreactions. This change in temperature is measured bythermistor signifying detection of gas molecules. In this setup itutilizes high surface area and thermal conductivity of

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carbon nanotubes.In the above discussion various gas sensors havebeen studied for detection of gas molecules, every sensorcomes with its own pros and cons. Along with thereeffective output they do have some limitations which areneed to be improved.

III. CONCLUSIONElectrochemical sensors provide some selectivity butsuffer from limited sensitivity and require mobileelectrolytes [17], which may cause stability issues anddelayed response time. In addition, electrodes can beeasily fouled, and interfering problems may occur assome interferents are electrochemically active.Nanomaterials, e.g., carbon nanotubes (CNTs) andnanowires, have been employed to constructelectrochemical sensors for explosive detections. TypicalCNTs applications include CNTs field-effect transistorsor resistors and CNTs modified glassy carbon electrodes.More recently, CNTs have been employed in explosivesensors by exploiting passive functions rather thantransduction, such as enhanced Raman scattering andpreconcentration of target samples. LODs down to theppb level have been achieved for TNT detection byexploiting the extremely large surface to volume ratio.Despite high sensitivity, significant challenges such aslow selectivity, low signal-to-noise ratio (SNR), longrecovery times, interference, and device fabricationdifficulties still remain to be addressed [18].To further improve selectivity and sensitivity for sensinggas molecules, functionalization of CNT are required.Functionalization with particular metal oxide ornanocomposite can improve selectivity of the sensor.Use of hydrophobic compounds can protect false alarmfrom water vapours. Nano- materials provide largesurface area so they should be used as sensing materials.

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