Carbon nanotubes sensor properties a review

Carbon nanotubes Sensor properties A Review Author’s Accepted Manuscript Carbon nanotubes Sensor properties A Review Irina V Zaporotskova, Natalia P Boroznina, Yuri N Parkhomenko, Lev V Kozhitov PII S[.]

Author’s Accepted Manuscript

Carbon nanotubes: Sensor properties A Review

Irina V Zaporotskova, Natalia P Boroznina, Yuri

N Parkhomenko, Lev V Kozhitov

PII: S2452-1779(17)30017-8

DOI: http://dx.doi.org/10.1016/j.moem.2017.02.002

Reference: MOEM50

To appear in: Modern Electronic Materials

Cite this article as: Irina V Zaporotskova, Natalia P Boroznina, Yuri N Parkhomenko and Lev V Kozhitov, Carbon nanotubes: Sensor properties A

http://dx.doi.org/10.1016/j.moem.2017.02.002

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Carbon nanotubes: Sensor properties A Review

Irina V Zaporotskova1,*, Natalia P Boroznina1, Yuri N Parkhomenko2, Lev V Kozhitov2

and their ions included in salts and alkali

Keywords: carbon nanotubes, sensor properties, sensors on the basis of carbon nanotubes,

boundary modified nanotubes, carboxyl group, amino group

Introduction

The current stage of research into the nanotubular forms of materials is characterized by a great interest to their synthesis methods, improvement of these synthesis methods, study of the properties and attempts of industrial applications of these nanomaterials Systems of this type attract interest thanks to a combination of multiple properties that cannot be achieved in conventional single crystal and polycrystalline structures Nanomaterials are defined as materials the sizes of which in at least one dimension are in the 1—100 nm range [1—3] Their shapes may be zero-dimensional (0D) and one-dimensional (1D) nanostructures 0D nanostructures include, for example, quantum dots [1] Quantum dots were used as a structural material for multiple applications including memory modules, quantum lasers and optical sensors The discovery of carbon nanotubes (1D nanostructures) is one of the most important achievements of the advanced science This existence form of carbon is intermediate between graphite and fullerenes However, many of nanotube properties are drastically different from those of the abovementioned forms of carbon Therefore nanotubes (or nanotubulenes) should be considered

as a new material with unique physicochemical properties showing good promise for a wide range of applications [4—8]

Carbon nanotubes (CNT) can find applications in a great number of areas such as additives to polymers and catalysts, in autoelectron emission for cathode rays of lighting components, flat displays, gas discharge tubes in telecommunication networks, absorption and screening of electromagnetic waves, energy conversion, lithium battery anodes, hydrogen storage, composite materials (fillers or coatings), nanoprobes, sensors, supercapacitors etc [9, 10] The great variety of the new unconventional mechanical, electrical and magnetic properties

of nanotubes can become the starting point for a breakthrough in nanoelectronics

As a nanotube is a surface structure, its whole weight is concentrated in its surface layers This feature is the origin of the uniquely large unit surface of tubulenes which in turn predetermines their electrochemical and adsorption properties [11] The high sensitivity of the electronic properties of nanotubes to molecules adsorbed on their surface and the unparalleled unit surface providing for this high sensitivity make CNT a promising starting material for the development of superminiaturized chemical and biological sensors [12, 13] The operation principle of these sensors is based on changes in the V—I curves of nanotubes as a result of adsorption of specific molecules on their surface The use of CNT in sensor devices is one of their most promising applications in electronics These sensors should have high sensitivity and selectivity, as well as fast response and recovery

Below we provide a review of recent works that dealt with the development of CNT based sensors and study of their working mechanisms, and generalize available theoretical and experimental literary data on the alkaline metal sensitivity of carbon tubulenes boundary modified by functional groups

Structural features of carbon nanotubes

Carbon nanotubes were discovered and described by S Injima, Japan, in 1991 One of the amazing phenomena associated with the nanotubes is the dependence of their properties on their shape Nanotubes are elongated cylindrical structures with diameters of 1 to several dozens of nanometers and lengths of up to several microns consisting of one or several hexagonal graphite planes rolled in tubes Their surface consists of regular hexagonal carbon cycles (hexagons) [4—10] Depending on nanotube synthesis conditions, one- or multilayered tubulenes with open or closed terminations may form

The structure of tubulenes is typically described in terms of infinite cylindrical surfaces accommodating carbon atoms interconnected into a single network with hexagonal cells, i.e the

sp2-network The mutual orientation of the hexagonal network and the longitudinal axis of a nanotube determines an important structural property of the tubulene, i.e its chirality The

chirality of a nanotube is described by two integers (n and m) that locate the hexagon of the

network which will match after nanotube rolling with the hexagon that is in the origin of coordinates The chirality of a nanotube can also be uniquely specified by the angle Θ (Θ is the orientation angle or the chiral angle) formed by the nanotube rolling direction and the direction

of the common edge of two adjacent hexagons There are multiple nanotube rolling options, but

of special interest are those which do not distort the structure of the hexagonal network These

optional rolling directions are those at the angles Θ = 0 and 30 arc deg corresponding to the (n, 0) and (n, n) chiralities, respectively The orientation (or rolling) angle determines the electrical

properties of CNT They can exhibit either metallic or semiconductor conductivity types However, most nanotubes are semiconductors with a 0.1 to 0.2 eV band gap Controlling their band structure one can obtain a variety of electronic devices [10]

It is a common practice to subdivide the CNT in two types, i.e the achiral and chiral ones The chiral tubulenes have a screw symmetry, while the achiral ones have a cylindrical symmetry and are further divided in two types In one of the achiral CNT types, two edges of

each hexagon are parallel to the cylinder axis These are the so-called zig-zag nanotubes (Fig 1

a) In the other type of the achiral CNT two edges of each hexagon are perpendicular to the cylinder axis, these being the so-called arm-chair nanotubes (Fig 1 b)

Generally, the CNT can be described by specifying the chiral vector C h:

as well as the tube diameter dt, the chiral angle Θ and the basic translation vector T (Fig 2)

The vector C h connects the two crystallographically equivalent states O and A on a dimensional (2D) graphene plane in which carbon atoms are located Figure 2 shows the chiral

two-angle Θ of a zig-zag type nanotube (Θ = 0) and the unit vectors a1 of а 2 of the hexagonal lattice The angle Θ = 30 arc deg corresponds to an arm-chair tubulene The pairs of the symbols (n, m)

specify different methods of graphene surface rolling to a nanotube The differences in the chiral

angle Θ and the tube diameter dt cause differences in the properties of the CNT In the (n, m)

notation system used for exactly specifying the chiral vector C h, the notation (n, m) in Eq (1) refers to chiral symmetry tubulenes, (n, 0) refers to zig-zag tubulenes and (n, n) refers to arm- chair tubulenes The higher the value of n the greater the diameter of the tube

In the terms of the (n, m) indexes, the diameter of a tubulene can be written as follows:

– 3

,

c c

a m mnn



where ac–c is the difference between the nearest carbon atoms (0.1421 nm for graphite) and Сh is

the length of the chiral vector C h The chiral angle Θ is specified by the following expression:

To study the properties of the CNT as one-dimensional (1D) systems one should specify

the lattice vector Т oriented along the tubulene axis orthogonally to the chiral vector C h (Fig 2)

The vector Т of a chiral tubulene can be written as follows:

whereas the following statement is true for dk:

where d is the greatest common divisor of (n, m)

Gas sensors based on carbon nanotubes

As a nanotube is a surface structure, its whole weight is concentrated in the surface of its layers This feature is the origin of the uniquely large unit surface of tubulenes which in turn predetermines their electrochemical and adsorption properties The extremely high adsorption capacity of the CNT and the excellent sensitivity of the CNT properties to atoms and molecules adsorbed on their surface [8] provide the possibility of designing sensors on the basis of nanotubes [12—14] Currently, several types of gas sensors (detectors) on the basis of the CNT are discussed in luterature:

– sorption gas sensors;

– ionization gas sensors;

– capacitance gas sensors;

– resonance frequency shift gas sensors We will consider these types of sensors in detail

Sorption gas sensors

Sorption gas sensors are the largest group of gas sensors [13] Their main operation principle is adsorption during which an adsorbed gas molecule transfers an electron to or takes it from a nanotube This changes the electrical properties of the CNT, and this change can be detected There are gas sensors based on pure CNT including mono- and multilayered ones, as well as those based on CNT modified by functional groups, metals, polymers or metal oxides

It is well known that monolayer CNT are sensitive to gases, e.g NO2, NH3 and some volatile organic compounds due to a change in the conductivity of the nanotubes as a result of gas molecule adsorption on their surface [15, 16] A sensor was designed [16] for detecting gases and organic vapors at room temperature the detection limit of which was as low as 44 ppb for

d if n – m is not a multiple of 3d

3d if n = m is a multiple of 3d

dk =

NO2 and 262 ppb for nitrotoluene The recovery time of that sensor was ~10 h due to the high bond energy between the CNT and some gases Then [17] this recovery time was reduced to 10 min by exposing to UV radiation which facilitated the desorption of gas molecules

The same gases were detected with another sensor [18] based on a field-effect transistor

in which the conducting channel was one semiconducting monolayer CNT The response time of the device was within a few seconds, and the response defined as the ratio between the resistivity before and after gas exposure was approx 100—1000 ppm for NO2 Three models were proposed for explaining the action of that sensor:

– charge transfer between a nanotube and a molecule adsorbed on its surface;

– molecular strobing of nonpolar molecules e.g NO2 which shift the conduction threshold of the CNT;

– change of the Schottky barrier between the nanotube and the electrodes [19, 20]

In transistor based sensors the energy barrier of CNT adsorption for dimethylmethylphosphonate [21], NH3 [22] or NOx [23] can be reduced by applying positive bias

to the gate This causes electron tunneling through a narrow barrier

To reduce the sensor recovery time after gas detection by a sorption mechanism, attempts were made to accelerate gas desorption by heating sensor detectors The operation of a monolayer CNT based sensor for NH3 detection was analyzed [24] Gas exposure leads to electron transfer from NH3 to the tube resulting in the formation of a spatial charge region on the surface of the semiconductor CNT and hence an increase in its electrical resistivity The device reached saturation at a concentration of ~40 ppm The sensor recovered completely to the initial state at 80 °C Fabrication of sensor detectors by template printing followed by annealing in air

at different temperatures for 2 h was reported [25] Those sensors were used for NH3 detection After 10 min gas exposure at room temperature the sensor resistivity increased by 8% compared

to the initial level The conduction type of the CNT changed from semiconducting at moderate temperatures (< 350 °С) to metallic at high temperatures (> 350 °C)

The possibility of fabricating multilayered CNT (MCNT) based sensors was discussed

[26, 27] The resistivity of the sensor proved to change due to the p conductivity type in

semiconducting MCNT and the formation of Schottky barriers between nanotubes having metallic conductivity type and those having semiconductor conductivity type during gas adsorption An electrochemical gas sensor was designed on the basis of modified multilayered CNT films for Cl2 detection [28] The sensor’s surface which was the cathode was exposed to

chlorine gas, and the resulting galvanic effect was measured The nanotubes acted as the microelectrode The recovery time of that sensor was ~150 s Another sensor on the basis of ultrathin CNT films [29] was used for NO2 and NH3 detection at room temperature The authors proposed a method of synthesizing ~5 nm thick films with a high density of nanotubes ensuring high sensitivity and reproducibility of the sensor, i.e 1 ppm for NO2 and 7 ppm for NH3 Gas desorption was accelerated by UV exposure

Gas sensors on the basis of oriented CNT were described [30] The resistivity of the CNT films declined after NO2 exposure and increased after NH3, ethanol and C6H6 exposure A nanotube film can be described as a network of highly efficient resistors consisting of the resistances of every single CNT and the resistivity of the sites and tunnels between adjacent nanotubes A vertical transport type detector was suggested [31] on the basis of regular CNT arrays for an NH3 gas sensing The detector had high sensitivity and response time (less than 1 min) and good recovery at atmospheric pressure and room temperature It provided NH3detection in the 0.1—6 % range

CNT modification by functional groups, metal nanoparticles, oxides and polymers changes the electronic properties of the nanotubes and increases their selectivity and response to specific gases Noteworthy, the interaction of target molecules with different functional groups

or additives varies significantly CNT are often modified by adding the carboxyl group —СООН This group creates reactive sections at the terminations and the side walls of the CNT where active interaction with various compounds occurs For example, it was shown [32] that sensors synthesized from carboxylated monolayer CNT were sensitive to CO with a 1 ppm detection limit whereas pure monolayer CNT did not respond to this gas The NO2 gas sensitivity

of monolayer CNT functionalized by the amino group —NH2 was studied [33] The amino group acts as a charge transfer agent of the semiconducting CNT that increases the number of electrons transferred from the nanotube to the NO2 molecule

There are also gas sensors on the basis of CNT functionalized by polymers that show good performance at room temperature [34, 35] They can be used as conductometric, potentiometric, amperometric and volt-amperometric converters for the detection of a wide range

of gases it was shown [36] that field effect transistors based on monolayer CNT modified by polyethyleneimine can be used as gas sensors with improved response and selectivity for NO2,

CO, CO2, CH4, H2 and O2 These sensors were able to detect less than 1 ppm NO2 within a response time of 1—2 min It was demonstrated [37] that functionalized monolayer CNT with attached poly(sulfonic acid m-aminobenzene) have higher sensitivity to NH3 and NO2 compared with carboxylated nanotubes These systems exhibited sensitivity to 5 ppm NH3 CNT

modification by polymers also improves their sensitivity to organic compound vapors A compact wireless gas sensor was designed [38] on the basis of monolayer CNT + polymethylmetacrylate (PMMA) The sensor exhibited a fast response (2—5 s) and an increase

in resistivity by 100 orders of magnitude after exposure to dichloromethane, chloroform and acetone vapors The sensor recovered to the initial state immediately after gas removal The sensor’s action mechanism was accounted for by polymer response to the adsorption of organic vapors by PMMA and charge transfer from polar organic molecules adsorbed on the surface of the nanotubes The working principle of an integrated system on the basis of monolayer CNT and polymer cellulose was described [39] A cellulose layer was applied to the surface of the conducting CNT which was used as a gas sensor for the detection of benzene, toluene and xylene vapors

There are gas sensors based on CNT modified by metallic nanoparticles [40] The working principle of a sensor on the basis of monolayer CNT with palladium (Pd) nanoparticles for hydrogen detection at room temperature was described [41] The response time of the sensor was 5—10 s, the recovery time being ~400 s Adsorbed H2 molecules are known to dissociate at room temperature into hydrogen atoms that are dissolved in Pd and reduce the metal work function As a result the carrier concentration in the nanotubes decreases and their conductivity drops The process is reversible for dissolved atomic hydrogen can react with atmospheric oxygen to form OH This causes the formation of water which eventually leaves the Pd-CNT system and the initial conductivity of the sensor is restored Two methods of monolayer CNT functionalizing by palladium for the fabrication of hydrogen detectors was described [42] Nanotubes can be either chemically functionalized by Pd or coated with sputtered metal films In another work [43] an H2 nanosensor functionalizing method was developed that implied electrodeposition of Pd particles on monolayer CNT The sensor exhibited a good room temperature response The detection limit was 100 ppm, the recovery time being 20 min

Other metals can also be used for the design of CNT based gas sensors Sensors on the basis of multilayered nanotubes functionalized by Pt or Pd were fabricated [44, 45] They showed good H2 sensitivity and recovery at room temperature The response and recovery times were 10 min for CNT functionalized by Pd and 15 min for CNT functionalized by Pt Another hydrogen detector was designed on the basis of monolayer CNT decorated by gold particles [46] The effect of point heterocontacts between CNT and gold microwires on the detection of NH3and NO2 with fast response and recovery was demonstrated [47] The working mechanism of the probe was based on the formation of a thin conducting channel between Au and a nanotube and a change in the resistivity of the tubulene Gas detectors on the basis of monolayer nanotubes

modified by Au, Pt, Pd and Rh were reported [48] The difference in the catalytic activity of the metal nanoparticles determines the selectivity of the sensors for Н2, CH4, CO, H2S, NH3 and

NO2 The working principle of a high-efficiency gas sensor based on MCNT–Pt composite material sensitive to toluene C7H8 was described [49] The sensor responses at a concentration of

1 ppm and 150 °C were measured The efficiency of the sensor was noticeably higher than that

of earlier described sensors [50]

There were also reports on the fabrication of gas sensors on the basis of CNT and nanostructured metal oxides [50—56] Sensors modified by SnO2 or TiO2 were sensitive to NO2,

CO, NH3 and ethanol vapors at low working temperatures Nanotubes in metal oxide matrices produced the main conducting channels which efficiently changed the conductivity of the composite material during gas adsorption The recovery time of the sensors depended on the energy of the bond between gas molecules and the CNT surface A sensor on the basis of MCNT coated by SnO2 was described [57] that exhibited a good response to oil gas and ethanol vapors and recovered within a few seconds at 335 °C The sensor’s response grew with gas concentration The high sensitivity and low resistivity of that system was accounted for by the specific features of its electron transfer mechanism Electrons move through SnO2 grains in MCNT that have a low resistivity Furthermore, the sensor’s gas response could increase due to

the formation of the p—n junction between the nanotubes and the SnO2 nanoparticles [58] Acetone and NH3 can be detected with TiO2 + MCNT composition sensors fabricated using the sol gel method [59] Sensors on the basis of SnO2–TiO2 oxide mixture and MCNT embedded into thin SnO2–TiO2 films were described [60] The response and recovery times of those sensors were less than 10 s at working temperatures of 210—400 °C The improved sensor characteristics and the lower working temperatures can be attributed to an enhancement of the

p—n junction influence in addition to the grain boundary effects

An interesting working principle of a CNT based sensor device was demonstrated by a scientists team of the Research Center at the Toulouse University, France [61] They found a significant dependence of microwave radiation transmission pattern in a material containing two-layered nanotubes on the concentration of impurities in atmosphere [61] Specimens of two-layered nanotubes ~2 nm in diameter and ~10 m in length that had high purity and high reproducibility of the electric, magnetic and optical parameters were introduced in a powdered form into the cavity of a silicon waveguide mounted on a thin dielectric membrane The membrane material had a dielectric constant close to unity and a high microwave radiation transmission coefficient in the 1—110 GHz range To study the sensor characteristics the authors exposed the device to nitrogen at a 5 atm pressure for 16 h Experimental data on the microwave

radiation transmission coefficient and the wave phase shift in the abovementioned frequency range demonstrated substantial changes in these parameters as a result of gas sorption The recovery time to the initial device parameters was several hours at room temperature However, this time decreased by many times if the device was heated

Many researchers dealt with CNT based gas sensors containing various surface defects For example, CNT based sensors doped with boron and nitrogen were described [62] These sensors were used for detecting low NO2, CO, C2H4 and H2O concentrations at room temperature and at 150 °C It was found that nitrogen doped CNT were more sensitive to nitrogen dioxide and carbon oxide while boron doped tubes exhibited better sensitivity to ethylene All the nanotubes were highly sensitive to humidity variations Another study [63] dealt with sensors on the basis of monolayer CNT containing vacancy surface defects formed as a result of high temperature exposure (300—800 °C) Measurements of the sensitivity of those sensors to NO2,

NH3 and H2 showed higher sensitivity of defect containing sensors compared to defect free ones

at room temperature The authors hypothesized [63] that part of gas molecules are adsorbed on nanotube surfaces while others penetrate into openings produced on nanotube walls as a result of high temperature exposure (Fig 3)

Thus, sorption sensors on the basis of CNT exhibit high sensitivity but are not free from a number of disadvantages:

– inability to identify gases with low adsorption energies;

– lack of selectivity;

– high nanotube sensitivity to variations of ambient conditions (humidity, temperature and gas flowrate);

– long exposure time (from decades of seconds to several minutes);

– long sensitive element recovery time (from several minutes to several hours);

– possible irreversible changes of CNT conductivity due to chemisorption

Ionization gas sensors

The problem of detecting gas molecules with low adsorption energies was resolved by using ionization gas sensors The working principle of these sensors is based on the determination of gas ionization parameters during accelerated ion collision with gas molecules Due to the absence of adsorption and chemical interaction between the sensitive element and the

tested gas one can identify gases with low adsorption energies However, ionization type gas sensors do not find general application due to the following disadvantages:

– unacceptable weight and dimensions;

– high operation voltages ((102—103

V) and hence high power consumption

The use of CNT as one of the sensor electrodes is a key to partial solution of these problems The design of these sensors includes an anode in the form of an array of vertically arrange CNT, an aluminum cathode and a 150 m thick glass insulation layer inserted between the anode and the cathode If voltage is applied between the anode and the cathode the nanotubes induce high electric field at their terminations due to their high aspect ratio [64, 65] These conditions are favorable for the formation of self-sustained electrode discharge at lower voltages required Results for NH3, CO2, N2, O2, He and Ar gas detection with these sensors were reported [66] It was found that with an increase in gas concentration the breakdown voltage of the sensors changed but slightly while the discharge current increased linearly for each gas This

is accounted for by the influence of the volume concentration of gas molecules on the discharge current and the dependence of the breakdown voltage primarily on electric field magnitude and the bond energy of gas molecules

Thus, the low power consumption and breakdown voltage, high selectivity and process compatibility with standard microelectronics technologies as well as compact dimensions of planar ionization sensors on the basis of CNT show good promise for their general use However, their wide application is hindered by the necessity of using high sensitivity signal processing devices and degradation of the CNT sensitive element due to coronary discharges

Capacitance gas sensors

One more type of sensors in which CNT arrays are used as sensitive elements are capacitance gas sensors A capacitance sensor was described [67] the sensitive element of which was an array of misoriented nanotubes grown on a SiO2 layer The first plate of the sensor was a CNT array, the other plate being silicon If external voltage is supplied between the two plates, high magnitude electric field is generated at the CNT terminations causing polarization of adsorbed molecules and an increase in the capacity High sensitivity of that sensor to vapors of benzene, hexane, heptanes, toluene, isopropyl alcohol, ethanol, chlorobenzene, methyl alcohol, acetone and dinitrotoluene was demonstrated [68] The main drawback of capacitance gas sensors are irreversible CNT changes caused by gas chemisorption necessitating sensitive

element regeneration or replacement Moreover, sensors of this type cannot perform well at high humidity and therefore their application areas are further restricted

Resonance frequency shift gas sensors

A change in the electrical properties of the CNT during their interaction with gases was used as a basis for the development of resonance frequency shift gas sensors [69, 70] The sensitive elements of these sensors can be disk resonators with nanotubes grown on the outer surfaces of the disks When the CNT on the resonator are exposed to gases the dielectric permeability of the disk with the nanotubes changes resulting in a shift of its resonance frequency Because of the difference in the frequency shifts caused by different gases these sensors exhibit good sensitivity and selectivity This allows detecting a large number of gases at low concentrations including NH3, CO, N2, He, O2 and Ar A drawback of this type of sensors is the necessity of using additional equipment for analyzing the dielectric permeability and resonance frequency

Electrochemical and biological sensors on the basis of CNT

A special group of sensors are electrochemical and biological sensors (biosensors) that contain CNT Their typical working principle is based on oxidation and reduction reactions occurring during the interaction with biomolecules Electrochemical sensors with CNT have found general application in biomedical research [71]

Electrochemical sensors and biosensors were studied [72] the electrodes of which were CNT modified by redox polymers acting as catalysts of the electron transfer reaction between biomolecules and the basis of the electrode, i.e nanotubes [73] This combination of CNT with polymer improves the electrical conductivity and mechanical strength of the hybrid material Redox polymers can be selected from different groups of polymers capable of reversible oxidation and reduction reactions, e.g azine group polymers (phenazines, phenothiazines, phenoxazines etc.) [74—78] These biosensors allow detecting glucose, ethanol, hydrogen peroxide, nitride, sorbitol, uric and ascorbic acids, dopamine etc An amperometric device for glucose detection was described [79] Glucose oxidase was introduced into a composite material and attached to the terminations of the sensor CNT by creating amide bonds between the N–acetylglucosamine residues and the carboxyl groups of the modified nanotubes Glucose was oxidized by oxygen under the catalytic effect of glucose oxidase, the reaction product being

gluconolactone and hydrogen peroxide The concentration of the product hydrogen peroxide is proportional to the initial glucose concentration Therefore the sensor signal caused by hydrogen peroxide in the sample was used for characterizing glucose concentration If the composite material contained 10 wt.% glucose oxidase the device signal was linearly proportional to glucose concentration in the 0—5.4 g/l range, the glucose detection limit being approx 0.11 g/l [80]

Biosensors on the basis of CNT arrays are also suitable for the analysis of deoxyribonucleic or ribonucleic acids (DNA or RNA) For this application the sensor nanotubes are modified by oligonucleotides e.g guanine The abovementioned valuable property originates from the ability of oligonucleotides to readily bind with respective complementary DNA or RNA nucleotides The signal characterizing the content of DNA or RNA in the sample is measured using the complex [Ru(bpy)3]2+ compound which detects guanine oxidation A decrease in the density of CNT on the sensor surface causes an increase in the sensor sensitivity [81] The study showed that guanine oxidation produced a far higher signal in that sensor compared to a graphite electrode signal The detection limit for a 21-term oligonucleotide was 2 g/l, the DNA detection limit being 170 g/l [82]

Boundary modified CNT as active components of sensor devices

Carboxylated CNT

Sensors can also be based on boundary modified CNT This can be for example an atomic force microscope the probe of which has a chemically modified nanotube with a specially selected functional group Experiments were reported [83] in which CNT were obtained with one

of the boundaries being modified by an attached carboxyl group In the experiments the authors used a multilayered nanotube attached to the golden pyramid of the microscope’s silicon cantilever The nanotube termination was truncated in an oxygen containing atmosphere by applying voltage between the tube and mica surface with a niobium layer sputtered on it A carboxyl group (—СООН, Fig 4) formed at the open nanotube termination It was reported [84, 85] that carboxylated CNT are sensitive to ethanol vapors and NO, СО and NO2 gases If necessary the carboxyl group can be substituted for other functional groups using methods applied in organic chemistry The probe with the modifying group interacts with specimen surfaces having different chemical compositions in different manners Thus the probe of an atomic force microscope fitted with a nanotube and a specially selected chemical group becomes chemically sensitive It is logical to assume that the use of modified CNT as sensors may not be

restricted to gas detection Other chemical elements, e.g metals, can also be sensed It is also possible to differentiate between metal atoms and their ions contained in salts and alkali

The mechanism of —СООН functional group attachment to a monolayer semi-infinite carbon tubulene was studied [86] and the activity of this modified system to several metals was

investigated Zig-zag (6, 0) type tubulenes were simulated within the molecular cluster model

using a semiempirical calculation method (MNDO) [87, 88] and the DFT calculation method [89] One of the cluster boundaries was terminated by pseudoatoms for which hydrogen atoms were selected, and a carboxyl group was attached to the carbon atom at the other CNT termination (Fig 5) Specific features were identified in the spatial orientation of the carboxyl group relative to the nanotube boundaries, as well as its geometry and inner charge distribution

The mechanism of —СООН functional group attachment to a selected carbon atom at an open nanotube boundary was simulated by stepwise approximation of the carboxyl group position with a 0.01 nm step along a perpendicular line drawn towards the tube boundary and oriented to a C atom [90] As a result the formation of a chemical bond between the nanotube and —СООН was observed testifying to the possibility of monolayer CNT functionalizing by carboxyl groups for providing highly sensitive chemically active probes on their basis

Then the authors studied the interaction mechanism between potassium, sodium and lithium atoms with terminal oxygen and hydrogen atoms of the carboxyl group The process was simulated by stepwise approximation of the selected metal atoms to the O or H atom of the functional group Potential energy surface profiles were plotted for the nanotube + СООН—metal atom system (Fig 6) Each profile had a minimum corresponding to the formation of bonds at specific distances Table 1 summarizes the calculation results characterizing the main parameters of К, Li and Na atom attachment to the terminal atoms of the carboxyl group that modifies an open CNT boundary As the interaction distances corresponding to the minima in the energy profiles are quite large one can assume that the interaction between the functional group atoms and the selected metal atoms is the weak Van-der-Waals one This is an important result confirming the possibility of multiple reusing of these probes without destruction which could otherwise be caused by chemical interaction with the selected alkaline metal atoms

Table 1 Main Parameters of К, Li and Na Attachment to the Terminal O and H Atoms of

Carboxyl Group that Modifies CNT (6, 0)

rint is the interaction distance between the metal atom or the O (or H) atom of the

functional group and Еint is the energy of the respective interaction

The authors further studied the scanning of an arbitrary surface containing sodium, potassium or lithium atoms to be initialized and determined the sensitivity of CNT with terminal functional groups to selected chemical elements The process was simulated by stepwise approximation of the selected metal (ion) atoms to the functional group that was parallel to the modified nanotube boundary (Fig 7) Analysis of the interaction energy profiles plotted based on the calculations (Fig 8) showed that the modified tubulene became chemically sensitive to the selected metals The energy profiles had typical minima indicating the formation of stable interaction between elements and the CNT + COOH system The binding energies are summarized in Table 2 The results substantiate the possibility of using modified CNT as sensors for some elements and radicals Their presence can be experimentally detected by controlling the change in the potential in the probing system based on a nanotube with a functional group

Table 2 Main parameters of interaction of carboxylated CNT (6, 0) with metal atoms and

ions as determined by surface scanning

of probes with clearly specified chemical characteristics

Carbon Nanotubes Boundary Modified by Amino Group

As noted above, a carboxyl group can be substituted for other functional groups using methods applied in organic chemistry, e.g for the quite abundant and well studied amino group

NH2 The reactivity of the amino group originates from the presence of an unshared pair of electrons The interaction of monolayer CNT functionalized by–NH2 group with NO2 gas was studied [91] It was shown that the amino group acts as a charge transfer agent in the semiconducting CNT and hence the number of electrons transferred from the nanotubes to the

NO2 molecule increases

There is a report [92] on an investigation of amino group attachment to an open boundary

of semiconducting monolayer CNT forming a chemically active probe for sensor devices and the interaction between simulated boundary modified systems with atoms and ions of metals The authors simulated the attachment of an amino group to an open boundary of a semi-infinite CNT

(6, 0) Analysis of the potential energy surface profiles plotted for the nanotube + NН2 system revealed the formation of a chemical bond between CNT and the functional group

Analysis of the charge distribution in the system showed that the carbon atom of the nanotube to which the amino group is attached acquires the charge qС = +0.2 The negative charge acquired by the nitrogen atom of the functional group suggests that the attachment of —

NН2 to the tubulene boundary causes a transfer of electron density from the carbon atom of the nanotube to the nitrogen atom of the amino group This activates the sensor working mechanism according to which the resultant system acting as a sensor has a different concentration of charge carriers that triggers conductivity in the nanosystem

The authors studied the interaction mechanism between potassium, sodium and lithium atoms and monolayer CNT functionalized by amino group The process was simulated by stepwise approximation of the selected metal atoms to the H atom of the functional group The potential energy surface profiles plotted for the nanotube + NH2 —metal atom system (Fig 9) have minima corresponding to the presence of interaction at specific distances Table 3 summarizes the calculation results characterizing the main parameters of К, Li and Na atom attachment to the boundary modified CNT system The presence of the weak Van-der-Waals interaction indicates the possibility of multiple reusing of these probes Moreover, a probing system based on nanotubes modified by functional groups may undergo a charge in the height of the Schottky barrier between the nanotube + NН2 system and the sensor device electrodes as a result of the interaction with metal atoms, and this change will be detected during sensor operation The interaction parameters obtained using various theoretical methods (MNDO and DFT) proved to be in a good agreement confirming the correctness of the results Analysis of the charge state of the system showed that the electron density is transferred from the metal atoms to the probe system This leads to an increase in the concentration of charge carriers and changes the electrical properties of the system

Table 3 Main parameters of К, Li and Na attachment to CNT (6, 6) modified by amino

group

Interatomic Bond

rint, nm Еint, eV

Table 4 Main parameters of interaction of CNT (6, 0) modified by amino group with sodium, potassium and lithium atoms and ions as determined by arbitrary surface