Since the discovery of CNTs in 1991 by S. Iijima [4], a great deal of effort has been devoted to the fundamental understanding of their electrical, mechanical and chemical properties and of their use in a wide range of applications such as electronics and sensors. CNTs are divided into single walled carbon nanotubes (SWNT) and multiwalled carbon nanotubes (MWNT). A SWNT can be formed by rolling a graphene sheet (hexagonal structure) into a cylinder and a MWNT is composed of concentric graphene cylinders with an interlayer spacing of 0.34 nm. Most of CNTs are synthesized by arc discharge, laser ablation or chemical vapor deposition method.

CNT properties are highly dependent on their structure. SWNTs can be either metallic or semiconducting, a property which is determined by the atomic arrangement (chirality) and nanotube diameter. The roll-up vectors (n,m) of the cylinder describe the electrical properties of SWNTs [5,6]. Metallic SWNTs have roll-up vectors such that n – m = 3q (where q is any integer or zero) and semiconducting SWNTs have n – m ≠ 3q. Semiconducting SWNTs are p-type semiconductors with holes as the main charge carriers. The bandgap energies of semiconducting SWNTs are inversely related to their diameters. MWNTs show metallic electronic properties similar to metallic SWNTs. Interestingly, because of the one-dimensional structure, MWNTs and metallic SWNTs behave ballistic electronic transport properties without scattering over long lengths. These properties allow CNTs to carry high currents with negligible heating [7].

Semiconducting SWNTs play a central role in the operation of SWNT-based field-effect transistors (SWNT-FETs). The first SWNT-FET was demonstrated in 1998 by the groups of Dekker et al. [8] and Avouris et al. [9]. SWNT-FET devices are composed of individual SWNTs or random networks of SWNTs placed between a source (S) and a drain (D) electrode on a SiO₂/Si substrate. The Si layer can act as back gate, which is separated by an insulating layer of SiO₂. Since the work function of SWNTs is higher than that of most metals, the contact barrier between SWNTs and metals is usually a Schottky barrier (SB). The height of the SB in the SWNT-FET contact is determined by the work function of the electrode metal [10]. The conductance of SWNTs in devices can be modulated by applying a potential to the gate electrodes with a constant D-S bias voltage (V_DS).

Sensing gas or organic vapor molecules is central to environmental monitoring, control of chemical processes, health protection and agricultural applications. For example, NO₂, which can lead to respiratory symptoms in humans and dramatically affect agriculture, is produced from combustion or automotive emissions, and detection of NO₂ is important to monitoring environmental pollution. SWNT-FET devices are ideal for monitoring environmental gases and organic vapors. SWNTs are entirely composed of surface atoms, which make their electrical properties sensitive to their chemical environment. Since semiconducting SWNTs are p-type under ambient conditions, electron-donating molecules such as NH₃ that interact with SWNTs will result in charge-carrier (hole) recombination. This will cause a decrease in conductance and a shift in the transfer curve (I_DS – V_GS) to more negative voltages. However, electron-withdrawing molecules such as NO₂ and O₂ can increase hole concentration in the SWNT-FET and thus increase conductance, which leads to a shift of the I_DS – V_GS transfer curve to more positive voltages.

So far, SWNT-FET sensors have been shown to be sensitive to gases such as NH₃, NO₂, H₂, CH₄, CO, H₂S and some organic vapors such as ethanol and methanol. Firstly, the reported sensing devices are directly made from pristine CNTs. Subsequently, some gas sensor devices have been composed of functionalized CNTs, because gas sensors based on pristine CNTs have some shortcomings including low sensitivity to analytes due to low adsorption, low selectivity or long recovery time. To overcome the above shortcomings, many efforts have made to modify CNTs with different chemical materials to change their chemical nature and improve sensing performance.

In 2000, Dai et al. first demonstrated a CNT-based gas sensor [11]. The FET consisting of individual semiconducting SWNTs was made by growing SWNTs via CVD on SiO₂/Si substrates, then photolithographically patterning metal electrode on a single SWNT. The conductance of the SWNT-FET varied dramatically under various gate voltages when the device was exposed to NO₂ or NH₃ (shown in Figure 1). Upon exposure to NO₂, the conductance of the FET increased by about three orders of magnitude, and the transfer characteristics showed a shift of +4 V in gate voltage. Conversely, when the FET was exposed to NH₃ its conductance decreased by about three orders of magnitude and the transfer characteristics showed a shift of −4 V in gate voltage. The response time, defined as the time duration for signal stability from the introduction of sample, ranged from 2 to 10 s with 200 ppm of NO₂. The sensitivity, defined as the ratio between resistance after and before gas exposure, was ∼100 to 1,000. The used FET sensor recovers its original properties slowly at room temperature, and a typical recovery time is about 12 h. However, heating exposal sample in air at 200 °C led to recovery in ∼1 hour. As for sensing NH_3, the response time to 1% NH₃ was about 1 or 2 minutes, and sensitivity was from 10 to 100 ppm. Recently, Mattmann investigated CNT-FETs with aluminum oxide Al₂O₃ passivated contacts for NO₂ detection with very low limit concentration (80 ppb). They found that preventing metal contact exposure to the environment (especially oxygen) could benefit the NO₂ detection [12].

The sensing mechanism of NO₂ was explained as an attraction of electron charge from the SWNTs. The theoretical result shows that an electron charge of 0.1 transfers from the SWNT to an adsorbed NO₂ molecule. As for sensing NH₃, the sensing mechanism remains unclear, since calculation find no binding affinity between NH₃ molecules and SWNTs. The authors suggested that interaction between NH₃ and SWNTs was through binding of NH₃ to hydroxyl on a SiO₂ substrate, which could partially neutralize the negatively charged groups and lead to a positive electrostatic gate to the transistor. Recently, Marzari et al. systematically studied sensing mechanisms for CNT-based NH₃ sensors [13]. They found that the SB modulation at the CNT/metal contacts dominates the sensing performance at room temperature while at higher temperatures such as 150 °C or above, charge transfer process contributes to the sensing signal and NH₃ adsorption is to be facilitated by environmental oxygen.

To improve the sensitivity, SWNTs in the device were non-covalently functionalized with organic polymers made by drop-coating or dip-coating methods. Qi et al. demonstrated that non-covalently drop-coating of polyethyleneimine (PEI) and Nafion (a polymeric perfluorinated sulfonic acid ionomer) onto SWNT-FETs resulted in gas sensors with improved sensitivity and selectivity for NO₂ and NH₃ [14]. The PEI functionalization changed the SWNTs from p-type to n-type semiconductors, and sensors were able to detect less than 1 ppb NO₂ while being insensitive towards NH₃. In contrast to PEI-coated sensors, Nafion-coated SWNTs were insensitive to NO₂ while exhibiting a good sensitivity to NH₃. Star et al. also fabricated non-covalently functionalized SWNT FETs by simply submerging nanotube network FETs in an aqueous solution of PEI [15].

Since this first report on SWNT-FET based gas sensor, a great deal of effort in development of CNT based gas sensor has been made for various gases such as H₂, CH₄, CO, H₂S and some organic vapors. Since bare CNTs do not show obvious sensing for H₂, work has been done in functionalizing CNTs with H₂ or CH₄ sensitive materials. Dai et al. first reported the sensitivity of Pd decorated SWNTs to ppm levels of H₂ gas [16]. With the SWNT device coated with Pd nanoparticles by electron beam evaporation of Pd, the conductance of the Pd-modified device decreased upon exposure to H₂. This H₂ CNT sensor showed a response time of a few seconds and a recovery time of about 400 seconds. The sensing mechanism was explained as a dissociation of H₂ into atomic hydrogen on the Pd surface, which decrease the Pd work function and causes the electron donation to SWNTs.

SO₂ results from activities such as combustion and petroleum refining and can do harm to the environment since it can aid the formation of acid rain. SO₂ can be adsorbed onto bare CNTs with an effect similar to NO₂, but very few reports have been presented on CNT-based sensors for this gas. Suehiro et al. demonstrated that SWNT-FET shows a device behavior consistent with an electron donating species upon exposure to SO₂ [17]. It remains unclear why SO₂ can work as an electron donor in this procedure. H₂S is a dangerous gas because of its flammability and toxicity. Star et al. reported a metal decorated SWNT sensor array for detecting 50 ppm H₂S in air [15]. So far the reports of CNT-based sensors for the detection of SO₂ and H₂S are scarce, and further investigation into the interactions between CNTs and SO₂ and H₂S is needed.

O₂ is an important sensing target. Zettl et al.’s reports show that oxygen dramatically affects the physical properties of CNTs [18,19]. However, it is unclear how CNT-based devices respond to O₂ gas. Avouris et al. think that the response of CNT devices to O₂ is originated from interaction at the SWNT–metal-contact interface which results in a modification of the device SB [20]. It is apparent that considerable future effort will be needed for the development of a usable CNT-based O₂ sensor.

Organic chemical agents are toxic and very harmful to human health and the environment. These chemicals usually exist in the liquid form, but their vapors can be inhaled and absorbed through the skin. Fast and accurate detection of chemical agents is essential for protecting human health. Novak et al. demonstrated that a FET sensor based on a SWNT network can detect dimethyl methylphosphonate (DMMP), a stimulant for the nerve agent Sarin [21]. The sensors were reversible and capable of detecting DMMP at sub-ppb concentration levels (with V_GS = 0 V). A fast recovery (a few minutes) of the sensor was achieved by applying +3 V gate bias after exposure. The recovery was accelerated by the Coulombic interaction between the negative charge induced by the positive gate bias and DMMP which is a strong electron donor. Similar recovery using back gate bias was observed by Chang et al. [22] who found that a negative back gate voltage was required to regenerate SWNT-FET sensors exposed to ammonia and that a positive voltage was required to regenerate sensors exposed to NO₂. They suggested that this effect could be used to partly circumvent the low sensing specificity of pristine SWNT sensors and allow for a better identification of analytes. Similar FETs were tested with different alcohol vapors with good reversibility and reproducibility by Someya et al. [23]. The semiconducting SWNTs were synthesized by CVD and the electrode contacts were formed by evaporating Au with metal shadow masks. When exposed to several saturated vapors of methanol, ethanol, 1-propanol, 2-propanol and tert-butanol with V_GS = −20 V and V_DS = −100 mV, the response time was within 5–15 s.

Bao et al. fabricated thin-film transistor (TFT) sensors consisting of aligned, sorted nanotube networks [24]. This device allows achieving stable low-voltage operation under aqueous conditions. The authors developed a method to enrich semiconducting SWNTs and align them in a one-step solution deposition process by controlling substrate surface chemistry on silicon. These SWNT-TFTs were used to detect trace concentrations, down to 2 ppb, of dimethyl methylphosphonate (DMMP) and trinitrotoluene (TNT) in aqueous solutions. Along with reliable cycling underwater, the TFT sensors fabricated with aligned, sorted nanotube networks enriched with semiconductor SWNTs showed a higher sensitivity to analytes than those fabricated with random, unsorted networks with predominantly metallic charge transport.

Recently, several research groups are working on the functionalization of CNTs with different materials to enhance their sensing performance [25]. There are two main approaches for the surface functionalization of CNTs: covalent functionalization and non-covalent functionalization, depending on the types of linkages of the functional entities onto CNTs. Functionalized CNT sensors often offer a higher sensitivity and a better selectivity, compared with pristine CNT sensors. CNT/polymer nanocomposites as a non-covalent functionalization method without destruction of the physical properties of CNTs offer promising features as a sensing material. An et al. [26] fabricated SWNT and polypyrrole (PPy) nanocomposite based gas sensors. The sensor was formed by spin-coating nanocomposites onto prefabricated electrodes. The sensitivity of the nanocomposites was about ten times higher than that of polypyrrole. Abraham et al. developed a wireless gas sensor using a multi- walled carbon nanotube (MWNT) and PMMA composite film [27]. The sensor was fabricated by dip-coating the composite film on a pair of electrodes with interdigital fingers. The sensor showed a fast response and a large change of resistivity of an order of magnitude for sensing dichloromethane, acetone, and chloroform.

Challenges for the realization of commercially viable devices are numerous. They include: the development of a detailed fundamental understanding of the sensing mechanisms of these novel sensors and the utilization of this information for the rational design of nanostructured sensing materials; the development of analyte-specific, fast and stable sensors and sensor arrays together with appropriate numerical methods to analyze sensor array data; and the development of suitable high throughput nanomanufacturing techniques that enable mass production of high density sensor arrays.

Label-free biosensors with high miniaturization and integration have attracted intense research interest since they could potentially make advanced low cost molecular diagnostics routinely available [28–34]. Current sensing options mainly focus on optical detection, using fluorescent-labeled biomolecules with dyes [35–37] or quantum dots [38–40]. The artificial labeling of materials is time-consuming and cost-intensive [41,42], and introduction of labels can weaken the interaction between receptor and target [33]. Hence, the development of fully electronic and label-free sensing techniques is required [28–32]. The electrical conductance of a semiconducting SWNT is sensitive to its environment and varies significantly with surface adsorption of various chemicals and biomolecules [41–43]. This makes SWNT-FETs very promising candidates for label-free biosensing. The SWNT-FETs for biosensor applications are composed of SWNT networks or individual semi-conducting SWNTs. SWNT-FET based biosensors have been reported to detect various biological species such as DNA, proteins and cells.

Novoselov et al. demonstrated a graphene-based sensor with a detection resolution of an individual gas molecule, which is the first reported chemical sensor with the ultimate limit [98]. The graphene devices were prepared by micromechanical cleavage of graphite on an oxidized Si substrate, followed by fabrication of Ti/Au electrodes using electron beam lithography (Figure 7a). To assess the effect of gaseous chemicals on the graphene device, the response to NO₂, NH₃, H₂O and CO with a fixed concentration of 1 ppm was measured. In Figure 7b, region I corresponds to the device before exposure to chemicals. Obvious changes occurred within 1 minute of exposure (Figure 7b, region II) and then the response was followed by a saturation region (Figure 7b, region III). The device can be recovered to the undoped state by annealing in vacuum (region IV). The concentration (Δn) of chemically induced charge carriers as a function of gaseous concentration (C) was investigated by measuring the changes in longitudinal (ρ_xx) and Hall (ρ_xy) resistivity. The results showed that Δn depended linearly on the concentration of an examined chemical under the same exposure conditions, which is proposed to simplify the use of graphene-based sensors in practical applications.

To monitor the fundamental limit of graphene gas sensors, a series of techniques were exploited including high driving currents for suppression of the Johnson noise, annealed devices close to the neutrality point, and few-layer graphene devices with low contact resistance. Electrical signals from single-molecule were observed with the above operations (Figure 7c), revealing the adsorption and desorption of individual gas molecule. The chemically induced changes in ρ_xy occurred in a step-like manner when the device was exposed to diluted NO₂. Similar variations on the opposite direction were observed upon stopping NO₂ supply and starting to evacuate the sample chamber.

Since this first demonstration of a graphene gas sensor, significant research efforts have been made to this field. Recently, Chen et al. reported a graphene gas sensor by using partially reduced graphene oxide (GO) [99]. The graphene oxide transistor showed little response to chemical gases such as NO₂, and the device showed a typical p-type transistor behavior. However, the conductance of GO increased by annealing the device to partially reduce the graphene oxide. The partially reduced GO FET exhibited high sensitivity to exposures to NO₂ from ∼1.55 to 100 ppm.

Robinson et al. demonstrated reduced graphene oxide used for high performance chemical sensors [100]. The sensors are fabricated from a graphene oxide network produced by the Hummers’s method, with the graphene oxides reduced by hydrazine hydrate. The conductance of the graphite oxide device was highly sensitive to the exposure to acetone and some other toxic organics including hydrogen cyanide (HCN), 2, 4-dinitrotoluene (DNT) and dimethyl methylphosphonate (DMMP). The graphene oxide sensors achieved sensitivities at ppb levels for chemical toxics and explosives. The performance of the devices is tunable by varying the exposure time to hydrazine hydrate vapor and the level of reduction affects both the sensitivity and the 1/f noise.

Recently, several reports demonstrated that graphene-based FETs with electrolyte top gating can be efficiently used as chemical/biological sensors. Das et al. have discussed monitoring dopants by Raman Scattering in an electrochemically top-gated graphene-based FET [101]. They found that top gating graphene-based FETs was able to reach doping levels of up to 5 × 10¹³ cm⁻². Such high doping levels are possible because the Debye layer in the electrolyte gate provides much higher gate capacitance. Tao studied the transport properties of graphene FET in ionic solutions, they found that the transport characteristics changes systematically with the ionic concentration, which was qualitatively explained in terms of long-range scattering by charged impurities and ionictheory based on the PB Equation [102]

Modulation of the conductance channel in solution-gated FETs can be achieved by applying a gate potential across the electrolyte, which acts as a dielectric. Because of the ambipolar characteristics of graphene, both hydroxyl (OH⁻) and hydroxonium (H₃O⁺) ions are proposed to be able to modulate the channel conductance of graphene-based FETs. Loh et al. demonstrated solution-gated epitaxial graphene as a pH sensor [103]. The sensor was fabricated from few-layer graphene epitaxially grown on a SiC substrate. The electrochemical double layer at the graphene/electrolyte interface is observed to be very sensitive to pH and the conductance of the device responded to pH changes.

Rao et al. studied the noncovalent interaction of DNA nucleobases and nucleosides with graphene by isothermal titration calorimetry [104]. The results showed that the interaction energies of the nucleobases varies in the order guanine (G) > adenine (A) > cytosine (C) > thymine (T) in an aqueous solution. The same trend was found with the nucleosides. Interaction energies of A–T and G–C pairs are somewhere between those of the constituent bases. Theoretical calculations taking into account van der Waals interactions and solvation energies suggest that the trend should be G > A > T > C. Recently, Ohno et al. demonstrated an electrolyte-gated graphene-based FET for detecting protein adsorption [105]. The graphene-based FET, which has a non-functionalized single-layer graphene as the channel, exhibited a linear increase in conductance upon the electrolyte pH–thus indicating its potential use in a pH sensor. Further investigation revealed that the conductance of the graphene-based FET increased with the adsorbed protein (bovine serum albumin) at a level of several hundred picomolars, which implied that the graphene-based FET can be used for highly sensitive electrical biosensors. Mohanty and Berry developed a graphene-based single bacterium resolution biodevice and DNA FET [106]. They first investigated the interaction between chemical modified graphene and bioentities. Chemical modified graphene and their biohybrids were synthesized from graphene oxide (GO) or plasma-modified graphene amine (GA). The DNA with a terminal amine group was bonded covalently with carboxylic group of GO by immersing GO coated silica in the amine DNA and amide coupling reagent O-(7-azabenzotriazole-1-yl)-N,N,N,N′-tetramethyluronium hexafluorophosphate (HATU) solution. This tethering was verified by hybridizing the DNA with a fluorescent cDNA probe. The Gram-positive bacilli cells with highly negatively charged surface were immobilized on GA via electrostatic interactions, which are measured by means of fluorescence microscopy. The electronic conductances of the bacterium device and the DNA transistor were investigated: the results showed that the bacteria biodevice was highly sensitive with a single-bacterium attachment generating ∼1,400 charge carriers in a p-type FET and DNA tethered on graphene hybridizes with its complementary DNA strand to reversibly increase the hole density by 5.61 × 10¹² cm⁻². Very recently, Chen et al. demonstrated that CVD-grown large-sized graphene films consist-ingmonolayered and few layered graphene domains were used to fabricate liquid gated transistors for DNA sensor with a detection concentration as low as 0.01 nM, they found that adding AuNPs on the surface of graphene devices can extend the upper limit of DNA detectio due to the increase in loading of probe DNA molecules. [107] Leiber et al. demonstrated recording from eletrogenic cells using single layer graphene FET as well as simultaneous recording using Gra- and SiNW-FETs. Graphene-FET conductance signals recorded from spontaneously beating embryonic chicken cardiomyocytes which yielded well-defined extracellular signals [108].