Author’s Accepted Manuscript

Carbon nanotubes: Sensor properties. A Review

Irina V. Zaporotskova, Natalia P. Boroznina, YuriN. Parkhomenko, Lev V. Kozhitov

PII: S2452-1779(17)30017-8DOI: http://dx.doi.org/10.1016/j.moem.2017.02.002Reference: 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. AR e v i e w , Modern Electronic Materials,http://dx.doi.org/10.1016/j.moem.2017.02.002

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1

Carbon nanotubes: Sensor properties. A Review

Irina V. Zaporotskova1,*

, Natalia P. Boroznina1, Yuri N. Parkhomenko

2, Lev V. Kozhitov

2

1Volgograd State University, 100 Universitetskii Prospekt, Volgograd 400062, Russia

2National University of Science and Technology MISiS, 4 Leninskiy Prospekt, Moscow 119049,

Russia

irinazaporotskova@gmail.com

n.z.1103@mail.ru

parkh@rambler.ru

kozitov@misis.ru

*Corresponding author.

Abstract

Recent publications dealing with dealing with the fabrication of gas and electrochemical

biosensors based on carbon nanotubes have been reviewed. Experimental and theoretical data on

the working principles of nanotubes have been presented. The main regularities of the structure,

energy parameters and sensor properties of modified semiconducting systems on the basis of

cabon nanotubes have been studied by analyzing the mechanisms of nanotubule interaction with

functional groups (including carboxyl and amino groups), metallic nanoparticles and polymers

leading to the formation of chemically active sensors. The possibility of using boundary

modified nanotubes for the identification of metals has been discussed. Simulation results have

been reported for the interaction of nanotubes boundary modified by —СООН and —NH2

groups with atoms and ions of potassium, sodium and lithium. The simulation has been carried

out using the molecular cluster model and the MNDO and DFT calculation methods. Sensors

fabricated using this technology will find wide application for the detection of metallic atoms

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.

2

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.

3

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

4

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 Ch:

Ch = na1 + ma2, (1)

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

The vector Ch connects the two crystallographically equivalent states O and A on a two-

dimensional (2D) graphene plane in which carbon atoms are located. Figure 2 shows the chiral

angle Θ of a zig-zag type nanotube (Θ = 0) and the unit vectors a1 of а2 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 Ch, 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:

dt = Ch/π =

1/22 2

–3,

c ca 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 Ch. The chiral angle Θ is specified by the following expression:

1 3tan .

2

m

m n

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 Ch (Fig. 2).

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

5

1 2

k

2 2.

m n a n m a

d

T

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 =

6

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

7

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 NH3

detection 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

8

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 NH3

and 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

9

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

10

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

11

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—10

3 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

12

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

13

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

14

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.

15

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).

Interatomic

Bond rint, nm

Еint, eV

MNDO DFT

Na—О 0.22 -4.23 -3.21

Na—Н 0.18 -3.03 -1.77

K—О 0.25 -4.00 -4.3

K—Н 0.18 -2.41 -1.04

Li—O 0.20 -5.45 -4.39

Li—H 0.19 -5.90 -4.62

Notations. Hereinafter:

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.

16

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

ions as determined by surface scanning

Atom/Ion rs-int, nm Е s-int, eV

Na 0.3 -0.64

Na+ 0.26 -1.73

K 0.25 -1.77

K+ 0.28 -1.76

Li 0.3 -0.93

Li+ 0.3 -1.63

Notations. Hereinafter: rs-int and Е s-int are the distance and energy of sensor interaction,

respectively.

These theoretical studies provided an explanation of the mechanism whereby the

boundary of a monolayer CNT is modified by a functional carboxyl group resulting in the

formation of a sensor that was later fabricated in the course of experiments and proved to be

sensitive to some gases [83—85]. The sensor responded to the presence of ultrafine quantities of

materials, and this shows good promise for its applications in chemistry, biology, medicine etc.

The use of chemically modified nanotubes in atomic force microscopy is a way to the fabrication

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

17

(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

MNDO DFT

Na 0.16 -1.90 -2.43

К 0.16 -3.60 -3.22

18

Li 0.18 -1.17 -1.0

Na+ 0.12 -2.78 -3.21

K+ 0.20 -5.54 -4.30

Li+ 0.15 -2.15 -3.39

The authors studied the sensor properties of the probe fabricated on the basis of CNT

modified by amino group for sodium, potassium and lithium atoms and ions by simulating the

scanning of a surface containing selected atoms (or ions). Analysis of the results showed that

tubulenes with functional amino groups become sensitive to the selected elements. The

interaction energies are summarized in Table 4.

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

scanning simulation.

Atom/Ion rs-int, nm Еs-int, eV

MNDO DFT

K 0.20 -5.47 -5.21

Li 0.20 -2.25 -2.00

Na 0.19 -3.12 -3.48

Na+ 0.12 -2.05 -2.23

K+ 0.14 -5.54 -5.15

Li+ 0.15 -2.15 -2.36

Thus, the experimental and theoretical results confirm the possibility of using CNT

modified by amino and carboxyl groups as sensors for specific elements and radicals. Their

presence can be experimentally detected by controlling the change in the potential of the probing

system based on a nanotube with a functional group. The resultant sensor element will be highly

selective: as can be seen from Tables 3 and 4, the interaction energies of the sensor system with

19

different elements are different. Therefore the system response to the presence of atoms or their

ion will also be different.

Summary

Experimental and theoretical studies showed that CNT are an extremely promising

material for further use in the field. Further development of nanotube technologies will provide

new physical objects the properties of which will be of great scientific and practical interest.

Thanks to their unique structure and properties the CNT can be used as active elements of

sensors for the detection of numerous materials including gases, organic compounds etc. CNT

modification by functional groups, metal nanoparticles, polymers and metal oxides greatly

increase the selectivity of the detectors fabricated on their basis. Their high electric catalytic

activity and fast electron transfer combined with high stability of nanotube compounds with

redox polymers provide for CNT application as electrochemical biosensors. Current

investigations focus on the search for new modifying additives that will improve the parameters

of CNT based sensors. This review aims at stressing not only the unique physicochemical

properties of sensor components but also possible synergistic effects that may occur as a result of

CNT modification by chemically active groups and particles. Sensors fabricated on their basis

will have high selectivity and response to the presence of ultrafine quantities of materials, e.g.

metals included in salts and alkali, and this shows good promise for their use in chemistry,

biology, medicine etc.

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Figure 1. Idealized models of (а) zig-zag and (b) arm-chair monolayer nanotubes.

30

Figure 2. Main parameters of nanotube lattice:

OA = Сh = na1 + ma2 is the chiral vector specified by the unit vectors а1 and а2; Θ is the chiral

angle, ОВ = Т is the lattice unit cell vector and is the translation vector.

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Figure 3. (a) Model of sensor on the basis of monolayer CNT modified by defects and (b) SEM

image of CNT on sensor.

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Figure 4. Nanotube with functional chemical group as probe of cantilever of scanning atomic

force microscope. Shown is probe movement for measurement of interaction energy between

functional group and specimen surface.

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Figure 5. Model of semi-infinite CNT (6, 0) with edge functional carboxyl group.

34

Figure 6. Energy profiles of interaction between CNT modified by carboxyl group —СООН and

Na, K and Li depending on distance between (a) metal atoms and hydrogen atom of the group

and (b) metal atoms and oxygen atom of the group.

35

Figure 7. Simulation of scanning of an arbitrary surface area containing Na atom (shown as

purple ball). Dashed line shows sodium atom migration relative to nanotube with functional

carboxyl group. Green balls in the figure show carbon atoms, red balls are oxygen atoms and

white ones are hydrogen atoms.

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Figure 8. Profiles of energy interaction between (a) atoms or (b) ions of metal (K, Li, Na) and

CNT + СООН system obtained by simulation of scanning. r = 0 is the point under the hydrogen

atom of the carboxyl group.

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Figure 9. Profiles of energy interaction of CNT (6, 6) modified by amino group with (a) Na, K

and Li metal atoms and (b) Li+, Na

+ and K

+ ions calculated using MNDO method.