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Journal of The Electrochemical
Society
OPEN ACCESS
Review—Recent Advances in Carbon Nanomaterials as Electrochemical
Biosensors
To cite this article: Ravinder Kour et al 2020 J. Electrochem. Soc. 167 037555
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Journal of The Electrochemical Society, 2020 167 037555
Review—Recent Advances in Carbon Nanomaterials as
Electrochemical Biosensors
Ravinder Kour,1 Sandeep Arya,2,z Sheng-Joue Young,3,* Vinay Gupta,4 Pankaj Bandhoria,5
and Ajit Khosla6,*,z
1
Department of Physics, Government Degree College for Women, Kathua, Jammu and Kashmir 184102, India
2
Department of Physics, University of Jammu, Jammu, Jammu and Kashmir 180006, India
3
Department of Electronic Engineering, National Formosa University, Yunlin 63201, Taiwan
4
Department of Mechanical Engineering, Khalifa University of Science and Technology, Masdar campus, Abu Dhabi 54224,
UAE
5
Department of Physics, Government Gandhi Memorial Science College, Jammu, Jammu and Kashmir 180001, India
6
Department of Mechanical System Science, Graduate School of Science and Engineering, Yamagata University, Yamagata,
992-8510, Japan
In the last three decades, a lot of scientific research has been carried out in the field of Carbon nanomaterials all over the world due
to their significant electronic, optical, mechanical, chemical and thermal properties. The zero, one, two and three dimensional
Carbon nanomaterials (i.e. fullerenes, Carbon nanotubes, Graphene, Carbon quantum dots, Carbon Nanohorns, Nanodiamonds,
Carbon Nanofibres and Carbon black) have exhibited such inherent features that can be easily exploited in the development of
advanced technology for sensing applications. The employment of nanomaterials within sensors has paved new way and
opportunities for the detection of analytes or target molecules. Carbon nanomaterials based electrochemical biosensors have
reported biocompatibility, better sensitivity, better selectivity and lower limits of detection to detect a wide range of chemical to
biological molecules. In this paper, a comprehensive review has been made to cover recent developments in the field of Carbon
based nanomaterials as electrochemical biosensors. The characteristic features of a variety of nanomaterials like fullerenes, Carbon
nanotubes, Graphene, Carbon quantum dots, Carbon Nanohorns, Carbon Nanodiamonds, Carbon Nanofibres, Carbon black etc.
have been discussed along with their synthesis methods. The recent application of all these nanomaterials as electrochemical
biosensors for the detection of various biomolecules have been highlighted; the future prospects and possibilities in this field have
been outlined.
© 2020 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited. This is an open access
article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/
by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/
1945-7111/ab6bc4]
Manuscript submitted September 10, 2019; revised manuscript received January 13, 2020. Published January 31, 2020. This paper
is part of the JES Focus Issue on Sensor Reviews.
The Carbon nanomaterials have laid down their historical Carbon Based Nanomaterials
footprints in the field of scientific research with the first investiga-
The Carbon atoms, possessing a valency of four, have the ability
tions on fullerenes and related compounds during mid-eighties.
to form single, double and triple covalent bonds among themselves
Since then there has been a remarkable increase in the vast scientific
or with other elements. Not only this, they have got the ability to
research all over the world to innovate technology to study and
form long chains of atoms, thus exhibiting the phenomenon of
develop this wonderful class of materials for a wide range of
polymerization. The Carbon atoms do posses such an electronic
applications. Several new classes of nanomaterials have been
structure and atomic size that makes them capable to exhibit distinct
investigated and reported since then with inherent intrinsic features
physical structures with distinct physical properties in spite of the
for their viable application in the development of sensing and bio
same chemical composition. The Carbon atoms can undergo sp, sp2,
sensing devices. The Carbon has been used for the electrochemical
sp3 hybridizations with a narrow band gap between their 2s and 2p
electrodes due to its unique electrochemical properties i.e. a large
electronic shells.1–3 The diamond with sp3 hybridization and
potential window, low cost, a very small background current. The
graphite with sp2 hybridization are the two widely known allotropic
biocompatibility of Carbon nanomaterials has revolutionized the
forms of Carbon. The geometrical structure of the particles in
field of electrochemical detection of various analytes or targets.
nanomaterials is the basic criterion for their classification. The
The electrochemical analysis has been used for the qualitative and
particles can have shapes of tubes, horns, spheres or ellipsoids. The
quantitative determination of amount of electro active analytes.
tube or horn shaped particles are called as Carbon nanotubes (CNTs)
These methods have been highly accurate, reliable and cheap. The
or Carbon nanohorns (CNHs) respectively; the spherical or ellip-
various characterization techniques have been employed for getting
soidal nanoparticles are present in fullerenes.4–6 The CNMs find vast
the electrochemical response like cyclic voltammetry (CV),
technical applications in micro and nanoelectronics, gas storage,
Differential Pulse Voltammetry (DPV), Square Wave Voltammetry
production of conductive plastics, composites, paints, textiles,
(SWV) or Pulsed Amperometry (PA).
batteries with enhanced life times, biosensors, etc. attributed to their
In this paper, a comprehensive study has been made to highlight
low toxic nature and large scale production for use.7,8 The fullerene,
the characteristic features of a variety of nanomaterials along with
Carbon nanotubes (CNTs), Graphene, Carbon Nanodiamonds
the methods employed for their synthesis. The recent developments
(CNDs) and Carbon dots (CDs) are the most significant allotropic
in the field of Carbon based nanomaterials for their use as
modifications of the nanocarbon.9 The 0D nanodiamonds, 1D
electrochemical biosensors have been thoroughly discussed along
nanotubes, 2D Graphene nanosheets can act as a prototype for the
with the future prospects and possibilities in this field.
nano composites.
Fullerenes.—The allotropic modification of Carbon, known as
fullerene, was discovered in 1985 by H. W. Kroto, R. F. Curl, and
R. F. Smalley.10 It was the first nanomaterial to be successfully
*Electrochemical Society Member.
isolated. The characteristic feature of fullerenes is the formation of a
z
E-mail: snp09arya@gmail.com; khosla@yz.yamagata-u.ac.jp number of atomic Cn clusters (n > 20) of carbon atoms on a
Journal of The Electrochemical Society, 2020 167 037555
spherical surface. The carbon atoms form covalent bonds with each SWCNTs depend upon the roll-up vectors (n, m). If the roll-up vectors
other in the sp2 hybridization in fullerenes. They are most commonly n-m = 3q where q is any integer/zero, the SWCNTs are metallic. If
present on the surface of the sphere at the vertices of pentagons and n-m = 3q, the SWCNTs are semi conductive in nature.35,36 If n = m,
hexagons. C60 is the fullerene that has been extensively studied and the nanotubes are known as armchair. If m = 0, they are known as
investigated. It has highly symmetric spherical molecules consisting zigzag, otherwise they are known as chiral.
of 60 carbon atoms, present at the vertices of 20 hexagons and 12 Also, the SWCNTs can exhibit electrical conductivity or semi
pentagons or 60 Carbon atoms comprising of 12-five member rings conductive properties that depend upon the diameter of the
and 20-six member rings.11 The diameter of fullerene is 0.7 nm.12 tubes.37–41 The armchair SWCNTs have electrical conductivity
Fullerenes have been used in the medical field such as in cancer more than that of copper whereas zigzag and chiral SWCNTs do
therapies, MRI and for gynecological malignancies.13–17 display semi conductive properties for their use in sensor
fabrication.40,42 The MWCNTs are composed of multiple Carbon
Synthesis.—Fullerenes are basically 0D form of Graphitic carbon layers with inconsistent chirality and can display extraordinary
and can be visualized as an irregular sheet of Graphene being curled mechanical attributes instead of exceptional electrical characteris-
up into a sphere by adding pentagons at its structure. They come in tics. These nanomaterials do posses such characteristic feature that
various forms and sizes ranging from 30 to 300 Carbon atoms. They makes them potential candidates for use in technological fields. The
can be synthesized by employing methods such as electric arc- CNTs have been used as an electrode in electrochemical reactions
discharge, electron beam ablation and sputtering.18,19 Fullerenes are due to their significant electron transfer capabilities.43 They can be
also present in the soot of combustion flames20–22 and can also be used in electrochemical sensors as they do have the ability to make
synthesized by using Graphitic electrodes.23,24 Fullerenes were electron transfer possible in chemical reactions at the electrode
firstly synthesized by evaporating Graphite electrodes in a Helium interface.44–47 The CNTs find immense applications in the field of
atmosphere.25,26 However, the practical use of fullerenes is limited nano-electro-mechanical systems.48–50 Table I shows the values of
due to their high synthesis cost and low yields of the methods significant physical, electronic and mechanical characteristic fea-
currently available for their production. tures of CNTs.
Carbon nanotubes.—One of the allotropic modifications of Synthesis.—The CNTs have been synthesized by using Carbon
carbon, known as Carbon nanotubes (CNTs) were discovered in arc discharge, Chemical Vapour Deposition (CVD) and laser
1991 by the Japanese scientist S. Ijima.27 In CNTs, each carbon ablation methods.52–54 The Carbon arc discharge with a suitable
atom with 3 electrons forms trigonally coordinated s bonds to three catalyst was firstly used to synthesize SWCNTs or MWCNTs with a
carbon atoms by using sp2 hybridization.27–29 CNT is basically one high yield and better control over the size of the synthesized
layer of Graphene rolled in the form of a hollow tube seamlessly. nanotubes.27,55,56 The CVD method has resulted in the production
The rolled Graphene sheets stacked in cylindrical/tubular structures of CNTs with smaller diameters and lower yield but finer quality.57
with a diameter of several nanometers is the characteristic feature of The Laser ablation method gives a lower yield and much smaller
carbon nanotubes. The CNTs can have variable length, diameter, the diameter but much finer quality.58,59 The metallic and semi
number of layers and chirality vectors (symmetry of the nulled conductive CNTs can be synthesized through selective
Graphite sheet). Based on their structures, CNTs can be divided into functionalization,60 selective destruction by electrical heating61 or
two basic groups: single walled Carbon nanotubes (SWCNTs) and separation by density gradient ultra centrifugation.62 The CVD has
multi-walled Carbon nanotubes (MWCNTs).30,31 The SWCNTs been used to produce high quality SWCNTs and MWCNTs in
have a diameter around 1–3 nm and a length of few micrometers vertically aligned array by using transition metal nanoparticle
whereas MWCNTs have a diameter of 5–25 nm and a length around catalysts.63,64 They have been synthesized on a very large scale by
10 μm. However, recently the synthesis of CNTs with a length of using arc discharge and CVD methods (Co-Mo Catalysts). The CVD
550 nm has been investigated and reported.32 The CNTs have method needs simple equipment and mild temperature and pressure
excellent physical properties like rigidity, strength and elasticity as conditions and is more suitable for the large scale production of
compared to other fibrous materials. They do posses high value of CNTs than the other two methods.65
aspect ratio (length to diameter ratio) than other materials. The high The metallic and quasi crystalline substrates have been used to
aspect ratios of CNTs may vary from 102 to 107. The larger aspect synthesize vertically aligned arrays of CNTs.66,67 The synthesis of
ratio comes out for SWCNTs than MWCNTs as a consequence of CNTs has been reported by pyrolysing metal carbonyls in the
their smaller diameter. Not only this, they do posses high thermal presence of other hydrocarbons.68,69 The transition metals present in
and electrical conductivities in comparison to other conductive Graphite electrodes have produced CNTs with more product output
materials. The strength of CNTs is 10–100 times larger than the and reproducibility.70 The transition metal catalysts along with CVD
strong steel at a fraction of steel weight.33 method have been researched to get good quality CNTs in vertically
The one layer of Graphene in CNTs can be rolled in different aligned arrays.71,72 The CVD synthesis employs the use of catalysts
ways. Based on the rolling of Graphene sheets, the CNTs are in substrates on which nanotubes grow. The metallic nanoparticles
classified as zigzag, armchair, chiral, depending on the number of are employed as catalysts and their size depends on the diameter of
unit vectors in the crystal lattice of Graphene along two directions in the nanotubes to be synthesized (0.5–5 nm for SWCNTs, 8 to 10 nm
honey comb structure. The chirality has a significant effect on the for MWCNTs). The nanoparticles Ni, Co, Fe have been used as nano
properties of CNTs. The electrical properties of SWCNTs are a catalysts for the synthesis of CNTs. The CVD reactors use inert gas
function of their chirality or hexagon orientation with respect to the methane for SWCNT production and ethylene for MWCNTs. In case
tube axis. The chirality decides whether a particular CNT is metallic of SWCNTs, the substrate is heated up to 850 °C–1000 °C and
or semiconducting in nature.34 The electrochemical properties of 550 °C–700 °C for MWCNTs synthesis. The thermal decomposition
of hydrocarbons produces Carbon which is dissolved in the metal
Table I. Significant physical, electronic and mechanical character- nano catalyst. When a certain concentration of Carbon is attained, its
istic features of CNTs.51 semi-fullerene cap is formed that acts as a basic unit for the growth
of the nanotube. The continuous flow of Carbon from the hydro-
Specific surface area 200–900 m2J−1 carbon source to the catalyst particle is maintained. Finally, the
Specific gravity 0.8–2 g−1cm−2 CNTs are obtained after purification process and removal of
Electrical conductivity 2 × 10−2−0.25 Scm−1 catalysts from the tips and surface of nanotubes. The research is
Thermal conductivity 6600 Wm−1K−1 going on for the last step so that high quality of the synthesized
Elastic Modulus >1 TPA material may be obtained.73,74 After the production of CNTs there is
Tensile strength >100 GPa a need to purify the material to remove the amorphous carbon
Journal of The Electrochemical Society, 2020 167 037555
materials. Although, arc discharge and laser ablation methods Graphene is a semiconductor material with zero band gaps,
produce SWCNTs in a high quantity but they suffer from drawbacks ambipolar electric field with charge carrier mobility more than
also as there is a need to evaporate C-atom from solid state source at 15000 to 20000 cm2 Vs−1 at room temperature. It possesses
a very high temperature (>3000 °C) and the nanotubes bundle excellent mechanical, physical, chemical and thermal properties
together during the formation which limits their applications.75–77 and is transparent to light up to 97.7%. That’s why it is a potential
The length of CNT depends on the time taken for their growth. The candidate for the application in highly sensitive electrochemical
diameter of synthesized SWCNTs varies from 0.7 to 3 nm78 and 10 sensors.86–89 The mobility of electrons in the layers of Graphene is
to 200 nm for MWCNTs.79 The different types of drugs can be one hundred times more than that in Silicon.90 That’s why it is
effectively loaded on the internal and external surfaces of CNTs due predicted that one day it will replace Silicon in the electronic
to their large surface area.80,81 industry. Graphene finds immense application in sensors due to its
large specific surface area and high charge carrier mobility.91,92
Graphene.—It is a 2D allotropic form of Carbon comprising of a Table II shows the values of significant physical, electronic and
single layer of Carbon atoms. The Carbon atoms exhibit a hexagonal mechanical characteristic features of Graphene.
crystal lattice joined to each other by s and p bonds in sp2
hybridization with an interatomic distance of 0.142 nm of Carbon Synthesis.—The Graphene was firstly isolated in 2004 at the
hexagons. The Graphene was first explored, searched by a Canadian University of Manchester by Novaselov and Geim by isolating
theoretical physicist, P. R. Wallace in 1947 whereas the samples individual Graphite layers by applying the peeling off method with a
were later investigated by a Dutch-British physicist A. Geim and a scotch tape.82 This method produces a high quality Graphene devoid
Russian-British physicist K. Novoselov.82–84 Although, the theore- of any defect. However, the small size of the sample restricts its use
tical investigations on the Graphene have been conducted exten- for lab research only and not for commercial applications. The
sively, the real material has been synthesized only recently. The peeling of Graphite in solvents like N-methyl pyrrolidone94 and
research on the characteristic features of Graphene is still going on. surfactant sodium dodecyl benzene sulphonate solution95 has been
It do possess extremely high mechanical rigidity and a high thermal reported. Due to the small fabrication cost and less number of
stability. The electrical properties of this carbon allotrope basically processing steps, this technique has been widely used for the large
distinguish from the properties of 3D materials. Graphene is a scale synthesis of Graphene. The layered Graphene has been
building block of other allotropes of carbon as it can be wrapped up, fabricated by employing CVD peeling off from Graphite,82 the
rolled up cylindrically or stacked up to get 0D fullerenes, 1D carbon epitaxial route,96 solvothermal production,97 liquid phase
nanotubes and 3D Graphite respectively.85 Thus it depicts the exfoliation,94 microwave assisted exfoliation98 and other oxidation
structural element of some other Carbon allotropes such as full- techniques.99,100 The Graphene can also be synthesized from the
erenes, CNTs and Graphite. The Graphene rolled into 0D bucky- reduction of Graphene oxide but the synthesized material has large
balls, 1D nanotube and stacked up into 3D graphite is shown in the number of defects101 whereas the Graphene synthesized from
Fig. 1.85 Graphite consist of a small density of defects.102
Figure 1. Graphene rolled into 0D buckyballs, 1D nanotube and stacked up into 3D Graphite, Reprinted with permission from Ref. 85 (Copyright (2007)
Springer Nature).
Journal of The Electrochemical Society, 2020 167 037555
Table II. Significant physical, electronic and mechanical character- are chemically stable, their photo luminescent behavior can be used
istic features of Graphene. for the several in-vivo and in-vitro applications.127 The CNDs are
the potential fluorescent probes for use as biomarkers and in bio
In plane modulus 1̃ TPa 87 labeling studies.128
Strength 130
̃ GPa 87
Specific surface area 2630 m2 g−1 93 Synthesis.—The CNDs are synthesized artificially by the detona-
Thermal conductivity 5000̃ W mK−1 93 tion of explosive to produce these nanoparticles.129 As diamond
Electron mobility at room temperature 105 ̃ cm 2
Vs−1 93 exhibits fluorescence due to the presence of a complex defect (N-V),
containing nitrogen (N) and a vacancy (V), the fluorescent CNDs
can be synthesized by doping of N vacancies by means of electron
irradiation and annealing in the free space.130 The scientific
The CVD technique has been applied for the synthesis of investigations have reported the fluorescent CNDs consisting of
Graphene modified electrodes and devices to be used in electro- roughly 400 Carbon atoms and Silicon vacancies that have the
chemical sensors. The Graphene has also been fabricated by the use potential for use in sensing applications.131
of transition metal substrates like Ni,103,104 Po,105 Pt,106 Cu107 on a The CNDs can be functionalized by the covalent or non-covalent
very large scale. As a consequence of a very low stability of Carbon method to provide extra stability to them.132 The covalent modifica-
in Copper, the CVD growth of Graphene over Copper results in a tions of CNDs have produced stable complexes of drugs, whereas
highly crystalline Graphene layers.108 The Epitaxial Graphene can with non-covalent methods, the drugs can be easily attached to the
be synthesized by graphitization of doped SiC single crystal wafers CNDs but with decreased stabilities. Since the covalent modifica-
at high temperature as well as of undoped crystals of SiC.109,110 tions of CNDs involve complex processes133,134 due to which the
Graphene Oxide (GO) has been produced by the chemical non-covalent method is widely used to build CND based drug
oxidation of Graphite at a very low production cost. The delivery systems. The colloidal behavior of the nanodiamonds can
Hummers method has been employed for the growth of GO as it be enhanced by transforming the surface of CNDs.135 A number of
takes very small time for the growth and does not dissolve harmful functional bio molecules and drugs have been attached on the
chemicals.99 The synthesis of Graphene oxide (GO) has also been surface of CNDs by non-covalent methods.136 The thermal induction
reported by making use of potassium permanganate and concen- and plasma treatment methods have been employed to attach
trated Sulphuric acid as oxidation agent and for peeling off chlorine, ketonic and carboxylic groups on their surface.137–139
Graphite.99 The acidic treatment accounts for the hydrophilic The surface modified CND films were used to absorb small
character of GO. The GO sheets can be dispersed well in water.11 molecules like alkyl alcohol, Sulphonic acids, thiols and complex
The GO can be reduced back to Graphene by using chemical structures like DNA and enzymes.140–144 Using alkyl chains, fluorine
reduction methods such as by the direct addition of reducing agents and Si, the covalent attachment on the surface of CNDs is
like hydrazine111 or by thermal reduction at high temperatures.112 feasible.145–147 It has been reported that the ketonic, carboxylic
The solution growth of Graphene has been reported to produce and amino groups can modify the surface of CNDs.148–153 These
GO in which Graphite is oxidized due to which an aqueous colloidal groups are further changed by a chemical modification in order to
form of GO flakes is produced. As a result, the basal plane functionalize the CND particle. A functionalized CND particle with
of the Graphene is functionalized with hydrophilic functional alkyl group can be distributed in the organic solvents uniformly
groups.113–115 GO has a high density of oxygen functional groups whereas the non-modified CNDs can be dispersed in water but not in
(carboxyl, hydroxyl, carbonyl, and epoxy) at its basal plane and its organic solvents. A functionalized CND particle with a Silane
edges due to which it forms a colloidal solution in water and polar coupling reagent can transform a glass substrate.154 The functiona-
solvents and is a novel Graphene material. lization of CNDs results in the fluorescent behavior without N-V
defects. The protein and biotin- supported CNDs were investigated
Reduced graphene oxide (rGO).—The electrochemical reduction in order to increase their affinity towards the biological molecules.
method has also been applied to obtain reduced Graphene oxide These modified CNDs have the potential to be used for drug delivery
(rGO).116 Various reduction methods have been employed to reduce systems.155 The fluorescent CNDs can be used for designing
Go partially to form reduced Graphene oxide (rGO) by using laser nanosensors due to the presence of N-V centers.156 The CNDs can
radiation,117 annealing118 and chemical methods.119 However, the be very easily functionalized with biomolecules after undergoing
harsh use of chemicals for oxidation degrades the properties of purification by the ozone gas.157,158
Graphene by damaging the basal plane of the Graphene. That’s why
peeling off of Graphene from Graphite is done under suitable Carbon nanohorns.—Carbon Nanohorns (CNHs) are one of the
solvents and surfactants.94,95 The Graphene has a tendency to allotrope of Carbon consisting of closet cages of Carbon atoms with
aggregate into Graphite in some solvents. Thus it is difficult and a diameter of 2–5 nm and length 40–50 nm.159 They are more
challenging to prepare pure and uniformly dispersed single layer beneficial to use than CNTs as they can be synthesized at a larger
Graphene in the solvents. The mechanical peeling of the Graphite is scale at room temperature without any use of metal catalysts. They
done to obtain pure 2D Graphene by making use of the adhesive can be synthesized by using arc discharge of Carbon rods,160 laser
tapes4 that has lesser density of defects. The structures of Graphene ablation of pure Graphite161 and Joule heating. The CNHs do posses
based nanomaterials are shown in the Fig. 2.120 high surface area and good porosity which can be exploited for their
potential application in the field of biosensing.161,162
Carbon nanodiamonds.—Another allotrope of carbon, Carbon
Nanodiamonds (CNDs) is the nanoparticles with the crystal structure Carbon dots.—The Carbon Dots (CDs) are zero-dimensional
of Diamond, and exhibit excellent properties of diamond.121,122 The CNMs consisting of Carbon atoms with a size below 10nm. These
CNDs consist of a crystalline Diamond core which is surrounded by materials do possess significant electronic and optical properties as
a anion like amorphous Graphite shell.123 They do possess very exhibited by Quantum Dots.163 They do possess low toxicity,
small size, large surface area and large adsorption capacity for the stability and biocompatibility for their application as electrochemical
attachment of chemical to biological molecules.124,125 They exhibit biosensors.164–166 The CDs have been synthesized by using laser
exceptional hardness, thermal conductivity, refractive index, coeffi- ablation method applied to the Carbon atoms.167 The various
cient of friction, insulation characteristics and have very low processes like pyrolysis,168 hydrothermal synthesis,169 electroche-
toxicity.126 mical methods170 and microwave synthesis171 have been used to
The CNDs display fluorescence due to the presence of a complex synthesize CDs. They can also be prepared by using the soot of the
defect N-V, containing nitrogen (N) and a vacancy (V). Since CNDs candle flame.172
Journal of The Electrochemical Society, 2020 167 037555
Figure 2. Structures of Graphene based nanomaterials (a) Carbon atoms in pure Graphene with sp2 hybridization (b) Graphene Oxide (GO) (c) Reduced
Graphene Oxide (rGO) (d) Graphene Carbon Quantum Dot (GCQD), Reprinted from120 (CC BY-NC 4.0).
CDs can be classified into Carbon Quantum Dots (CQDs) and is further divided into three types based on the way in which the
Graphene Quantum Dots (GQDs). The CQDs and GQDs have a catalyst is employed i.e. Substrate method, Spray method and Gas
diameter range from 1 to 10 nm. The GQDs consist of Graphene phase flow catalytic method.
layers of size less than 10 nm. They can be synthesized by using During PECVD method, the high energy electrons present in the
thermal plasma jet technique with low fabrication cost. They can be plasma collide with the gas molecules. As a result, they transfer their
an alternative to the nanodiamonds.173–176 kinetic energy to them, thereby, causing excitation, ionization and
decomposition which results in production of CNFs.184–186
Carbon nanofibres.—Carbon Nanofibres (CNFs) are cylindrical During electro spinning process, the polymers like silk, DNA,
wire shaped nanostructures in which graphene sheets are piled in collagen and polyester have been used to obtain CNFs. The
different arrangements such as ribbon-like, platelet or herringbone. polymeric solution is firstly subjected to a potential of very high
The length of CNFs varies in order of micrometers and can be up to volts for getting charged, then to a spinning port where it is moved at
10 μm whereas their diameters vary from 10 to 500 nm. Their a very fast rate. As a result, the nanofibres get deposited at the
mechanical strength and electric properties are just like that of collecting plate in the form of a mat. The fiber mat undergoes
CNTS.177 As a consequence of stacking of graphene sheets with oxidation and is carbonized in nitrogen atmosphere to produce
different shapes in different arrangements, CNFs have more edge CNFs.187–190 The CVD produces CNFs with impurities which
sites on their outer walls in comparison to CNTs. The presence of require a further complicated purification process whereas electro
edge sites makes it feasible to transfer electrons with electro active spinning produces CNFS through a very easy process with high
species in solution and the detector substrate.178,179 The CNFs do purity.191,192 The Fig. 3 shows the SEM images of alignment of
possess attributes like good electrical conductivity, large surface CNFs grown on a Silicon substrate in plasma growth process in the
area, biocompatibility and easy fabrication process that are vital for presence of electric field (a) & (b) and CNFs exposed density (c).193
electrochemical sensing applications. Moreover, CNFs can be easily
functionalized to suit a particular detection mechanism. Carbon black.—The Carbon black (CB) is a nanomaterial
prepared from the combustion of petroleum products. They are the
Synthesis.—The CNFs can be prepared by employing arc nanoparticles spherical in shape and are strongly bonded to each
discharge180 and laser ablation181 methods. The thermal chemical other to form aggregates. The size of Carbon black particles varies
vapour deposition (CVD),Plasma enhanced chemical vapour deposi- from 3.0 to 100nM. The significant physical, electronic and
tion(PECVD) and electro spinning have also been employed for the mechanical characteristic features of Carbon black are given in
preparation of CNFs.182 During thermal CVD method, a compound Table III. The conductivity of Carbon black can be enhanced by
containing hydrogen and carbon is thermally decomposed by heating up to 7000 °C because more number of electrons in sp2
employing a metal catalyst at a constant temperature.183 This method hybridization state with delocalized pi-bonds is available for the
Figure 3. SEM images of CNF array. (a) & (b) Alignment of CNFs grown on a Silicon substrate in plasma growth process (c) CNFs exposed density, reprinted
with permission from.193
Journal of The Electrochemical Society, 2020 167 037555
Table III. Significant physical, electronic and mechanical charac- and crystalline properties for a particular application. The CNMs
teristic features of Carbon black. based surfaces can be easily tailored by means of functionalization
by a large number of covalent and non covalent methods which
Surface area 15–1000 m2g−1 195,196 enhances their electrochemical sensing capabilities. These materials
Electrical conductivity 1.0–2 × 103 Sm−1 197 are also highly biocompatible. The CNT sensors do posses the
Thermal conductivity 0.2–0.3 Wm−1K−1 198 ability to transport electrons faster, highly sensitive and are
Young’s Modulus 1–50 MNm−2 199 capable of detection even at very low limits. The Graphene has
Tensile strength 20–50 MPa 200 also been effectively used for the electrochemical sensing due to
their significant electron transport features as described in the
Table no I and II. The CNT or Graphene based electrochemical
sensors have higher sensitivity, higher selectivity, fast electron
conduction of current.194 Due to their large surface area; a large transfer rate and low limits of detection. The doping can significantly
number of oxygenated groups are formed at the edges of the Carbon influence the electronic, mechanical and conducting properties of
black nanoparticles. It is the presence of sp2 hybridized Carbon atom CNTs. Not only this, the distinct forms of CNMs have a varying
edge planes and oxygenated groups over the Carbon black nanoma- density of states. The density of states of CNMs based electrode
terials that make them capable to attach biomolecules on their determines the electron transfer capabilities with the target mole-
surface to act as electrochemical biosensors. They can be used for cules. For a faster electron transfer process, the energy of electrons
the detection of analytes for the biosensing applications. in the electrode should be equal to that in the redox reaction. A
higher density of state enhances the possibility of existence of
Synthesis.—The Carbon black nanoparticles can be prepared by electrons with enough high energy needed for their transfer to the
employing furnance, channel and acetylene processes.201,202 The redox system.207 The density of states for CNMs varies with their
preparation process is very easy and has low cost. The properties of structure and can be adjusted by making changes in their atomic
Carbon black can be easily tailored by introducing other materials bonding structures. It also depends upon the tube diameter in case of
such as polymers203 or metallic nanoparticles into them for better CNTS. The density of states can be increased by peeling off CNTs in
electrochemical sensing applications.204 a controlled manner.208
The controlled oxidation of MWCNTS can enhance their
Biosensors electrochemical performance by modifying their electronic structure.
Biosensors have been extensively used for the detection of The inorganic particles can be strongly chemically coupled to the
biological molecules, pathogens and other disease causing agents CNMs to change the electronic structure of each individual
in the healthcare field. The biosensors are basically chemical sensors component to provide synergistic electro catalytic activities to the
which make use of the recognition properties of the biomolecules in resulting hybrid systems.209
its sensitive layer. The CNMs have been extensively used for The planar geometry of Graphene and tubular geometry of
providing immobilization aid to the recognition molecules in the nanotubes makes it possible to expose the surface atoms for forming
biosensors. A typical biosensor consists of three parts (i) a chemical bonds with a large number of molecules of the target
recognition molecule that can be an enzyme, protein, antibody or material for the biosensing applications. Every atom on the surface
DNA etc. (ii) a transducer element which records the interaction as a of Graphene is exposed due to its high exceptional surface area.
signal between the analyte or target and the recognition molecule Thus, the high molecular functionalization is feasible in Graphene in
(iii) a signal processor. An electrochemical biosensor is attached comparison to other carbon nanomaterials.
with sensitive biological molecules on the surface of solid electrodes The CNMs do possess a high surface to volume ratio, electrical
by employing recognition properties of biomolecules in order to hold conductivity and mechanical strength that makes them potential for
the target molecules on the surface of electrode. As a result of this use in electrochemical biosensors.210–212 The large surface area
process, a reaction signal is converted into an electrical signal like electrical, thermal conductivity and strength of CNTs make them
voltage, current, impedance etc. which can be easily detected. Most suitable for use in electrochemical biosensors.51 The basal planes of
of the biosensors that have been developed are electrochemical in Graphene play a major role in electrochemical process than the edge
nature. The electrochemical biosensors are potentiometric, ampero- planes.213,214 The Carbon nanostructures have the outstanding photo
metric or conductometric depending upon the signal generated from thermal response. The photo thermal technique has been used to
the electrochemical process which can be a resistance, current or reduce/eliminate the size of tumors.215,216
voltage signal respectively. The electrochemical biosensors have
been used to study the qualitative as well as the quantitative aspects Carbon Nanotubes as Electrochemical Biosensors
of the detected molecule. The electrochemical biosensors are highly The CNTs have been widely explored for their use in the
sensitive to ensure detection, highly selective to avoid the inter- electrochemical sensing of biomolecules for various biomedical
ference of other species, small in size, easy to use and cost applications as shown in the Fig. 4.217–220 The properties of CNTs
effective.205,206 can be customized to suit their potential as biosensors. The
antibodies and enzymes can customize the features of CNTs in the
Why CNMs for Electrochemical Biosensors electrochemical biosensors. The characteristics of CNTs as biosen-
The CNMs have been extensively used for the electrochemical sors can also be tailored by the peptides and nucleic acids as they do
biosensors due to their large surface area due to which many have the inherent capability to be acquainted with bio-elements or
detection events can occur simultaneously on their surface and biomolecules. The method of analysis i.e. invivo or invitro deter-
also, the attachment of the biomolecules is possible very easily. mines the design of a biosensor. The biomolecules are attached on
These materials have such electronic, optical, physical and mechan- the surface of CNTs in order to prepare the surface for a particular
ical properties which make them potential candidates for use in detection process. The various biomolecules such as enzymes,
biosensors. Their charge storage and electron transfer properties can proteins or nucleic acids have been extensively used in the CNT
be engineered for the electrochemical applications. These materials biosensors for this purpose.
have low cost, wide potential range over which the CNMs electrode
can operate, high electro catalytic activities for a large number of Covalent or non-covalent functionalization of CNTS.—The
redox-active chemical and biological systems. The electrochemical physical and chemical properties of nanoparticles can be engineered
performance of the biosensors can be optimized by modifying the by subjecting them to functionalization, by attaching some mole-
structure of these molecules to engineer their electronic, chemical cules on their surface.222 The CNTs are not soluble in aqueous
Journal of The Electrochemical Society, 2020 167 037555
Figure 4. CNTS as biosensors for different applications, Reprinted with permission from221 (Copyright 2014, Elsevier).
solutions but when they undergo oxidation in a mixture of acids, the studied and investigated. The SWCNTs modified with diazonium
carboxylic groups attach to the surface and side walls of the have displayed the highest electron transfer in cellobiose hydro-
nanotubes making them soluble in aqueous solutions. Thus, functio- genase from phanerochaete Sordida with small values of lactose
nalization has proved to be a boon to the CNTs for modifying their oxidation potential.231
physical and chemical properties.222,223 The functionalization of However, the covalent functionalization of CNTs has an influ-
CNTs with chemicals or suspension in a surfactant containing ence on its intrinsic properties as the change in CNT surface by
solution results in decrease in their bundle formation.224 covalent attachment can cause hybridization to change from sp2 to
The most commonly used tailoring technique for CNTs in order sp3. As a consequence of it, the mechanical strength and electrical
to enhance their electrochemical sensing performance (sensitivity properties could be hampered due to the decrease in conjugation
and selectivity) are covalent or non-covalent functionalization. The abilities of the CNTs.232,233
biomolecules are attached to the surface of CNTs by covalent or The non-covalent functionalization of CNTs has been significant
non-covalent functionalization. In covalent functionalization, the for attaching the biomolecules on CNTs as it does not affect the
various chemical functional groups like carboxylic and amine groups intrinsic properties of CNTs. As a consequence of that the mechanical
are attached to the surface and side walls of CNTs by certain and electrical properties are not affected.232,233 The CNTs are non-
chemical processes. These functional groups on the CNTs react with covalent functionalized as a result of pi-pi electrostatic interactions
the functional groups present in the bimolecular structure resulting in between the CNTs and the biomolecules.233,234 The adsorption of
the formation of a covalent or a non-covalent bond. The CNT aromatic molecules and benzene derivatives on the surface of
functionalized with poly (amidoamine) dendrimer through covalent SWCNTs has been achieved due to the establishment of pi-pi
functionalization has been used for the attachment of glucose interactions between the CNTs and the benzene derivatives.235 The
oxidase and Horseradish peroxidase.225 The use of covalent func- aromatic compounds have been employed for attaching the biomole-
tionalization for the attachment of biomolecules on CNTs for cules on the surface of CNTs by non-covalent functionalization such as
sensing glucose,225,226 H2O2,227 aflatoxin,228 carcinoembroyonic ferrocane,236–238 anthracene,239–241 pyrene242–244 etc for the develop-
antigen detection229 have been reported. The amine functionalized ment of electrochemical biosensors. The non-covalent functionalized
CNTs interact with the amino groups on biomolecules such as CNTs by aromatic compounds have been used for the bioelectroca-
enzymes, proteins and nucleic acids. The glutaraldehyde, active ester talysis of oxygen,239,242,245–248 glucose biosensors,236,237,249 H2O2
or epoxy has been used to attach the amine containing biomolecules detection,250 ethanol biosensor,251 and trichloroacetic acid
to the CNTs.226,229 biosensor.252 The polymers have been employed for the non-covalent
An electrochemical immunosensor has been developed for the functionalization of CNTs for use in biosensors. The polymers like
detection of carcinoembryonic antigen in saliva and serum in which polypyrroles,253,254 glycolipids,255,256 polyethyleneimine257–260 have
monoclonal anti-carcinoembryonic antigen antibodies are attached been used for the non-covalent functionalization of CNTs. The
on polyethylene amine treated MWCNTs side walls by using commonly used tailoring techniques for CNTs have been functiona-
covalent functionalization with the help of glutaraldehyde.229 lization with conducting polymers,261–263 mixing with surfactants or
The vertically aligned SWCNTs are attached on a GCE with a polyelectrolyte,264–266 using metal oxides or nanoparticles,267–270
covalent bonding230 or on a gold surface by diazonium has been adding enzymes56,266,271–275 and doping with heteroatom.55,276
Journal of The Electrochemical Society, 2020 167 037555
Detection of dopamine.—The first use of CNT electrochemical of detection of approx. 100 nM and 1 μM has been reported for the
biosensor was reported in 1996 where it was used for the detection functionalized electrode and as-fabricated electrodes respectively.
of dopamine.220 The CNT biosensor exhibited better detection in The functionalized SWCNT electrode exhibited better selectivity to
comparison to other Carbon based sensors due to the fact that there dopamine in the presence of ascorbic acid and uric acid than the as-
was the presence of pores due to the packaging of CNTs as a result fabricated electrodes.280
of which its surface area was increased for the attachment of
dopamine.277 The formation of oxygen containing groups at the CNT based glucose biosensors.—The CVD synthesized CNT
surface of nanotube during oxidation process has also a significant fibers has been used as a sensing electrode to detect the presence of
effect on the detection process with CNTs. The multi-walled carbon glucose. The CNT based biosensors have exhibited a very fast
nanotubes have been mixed with bromoform to obtain a paste which amperometric response to the presence of glucose.281–283 The
is packed into a glass tube forming a CNT Carbon paste sensor. The glucose based CNTs have been extensively investigated and
CV and DPV characterization techniques have been employed to reported.284
study the oxidation of dopamine by the traditional Carbon sensor and A glucose biosensor has been fabricated by using phase separa-
CNT based carbon paste sensor.220 tion method by employing MWCNT-grafted chitosan (CS)-nanowire
The dopamine has also been detected in the presence of ascorbic (NW) to which glucose oxidase is attached to obtain the biosensor.
acid by employing MWCNTs functionalized with Sodium dedecyl The electrochemical detection of glucose was done by employing
sulphate on a Ta substrate. The DPV was employed to detect the cyclic voltammetry and amperometry. The fabricated biosensor
dopamine in the presence of ascorbic acid. However, the detection exhibited a high sensitivity of 5.03 μA/mM in a concentration range
levels were in the micrometer range i.e. limit of detection is 3.75 μM of 1–100 mM and a low response time to the detection of glucose.
for dopamine in the presence of ascorbic acid with range from The MWCNTs-CS-NW facilitates the conduction of electrons
0.02mM to 0.2 mM which restricts the applicability of the sensor for between glucose oxidase and target molecules.285 In another study,
the practical use.264 The modification of a MWCNTs electrode with Glucose oxidase was attached on CNT nanoelectrode ensembles by
RuO2 has an influence on the enhancement of limit of detection covalent bonding with the formation of amide linkages between their
using DPV than the MWCNTs. A much higher concentration of amine residues and the carboxylic acid groups present on the tips of
dopamine, ascorbic acid and uric acid in the μM to mM range have CNTs. The release of H2O2 from the enzymatic reaction of glucose
been tested using this investigation and the electrode is highly oxidase upon the glucose and oxygen on CNT nanoelectrodes causes
selective to the presence of dopamine, ascorbic acid and uric acid.267 the detection of glucose.286
The Over Oxidized polypyrrole (OPPY)-MWCNT-modified In another study, a biosensor employing Ni-nanoparticles dis-
Glassy Carbon Electrode (GCE) has reported an enhanced sensitivity persed in vertically aligned CNTs grown on Si/SiO2 substrate has
to the detection of dopamine. The electrochemical deposition has been reported in which the Ni-nanoparticles have deposited uni-
been employed to tailor MWCNTs with cadmium oxide (CdO) formly inside and on the top of the CNT forest. The fabricated
nanoparticles of size approx. 50 nm diameters. The resulting CdO/ biosensor exhibited a sensitivity of 1433μAmM1cm2 with a limit of
MWCNT sensor does not exhibit any signal response to uric acid, detection 2 μM over a linear range between 5 μM to 7 μM.287 In
ascorbic acid and dopamine up to 100 μM but it has a better another study, CuO nanoparticles deposited on the side walls and
selectivity for H2O2.269 The enzyme laccase has also been used to tips of vertically well-aligned MWCNTs array by a two step electro
detect the presence of dopamine in the presence of ascorbic acid and deposition method has been investigated. The CuO-modified
3, 4-dihydroxyphenyl acetic acid (DOPAC). In this technique, the MWCNT electrochemical biosensor exhibited a limit of detection
limit of detection of 0.4 μM has been reported with high of 800nM and a very high sensitivity of 2190 μAmM−1cm−2 with a
selectivity.271 The Tyrosinase and Nafion has been employed for linear range response up to 3.0mM glucose concentration with rapid
the detection of dopamine by using MWCNTs but the limit of response and good stability.288 The Pt nanoparticles loaded CNTs
detection was reported to be very low of 0.5 μM.266 The doping of composites have been prepared under hydrothermal conditions by
CNTs with certain elements can customize their electronic proper- polymerization reaction of glucose and reduction deposition of a
ties, conductivity and mechanical strength.276 The doping of CNTs platinum source in the pores of anodic alumina membranes. The
with Boron for the detection of dopamine is significant due to the fabricated electrode has been used as a amperometric sensor for the
presence of more functionalized groups at the defect sites of CNTs low potential detection of H2O2. The as-prepared Pt-CNT glucose
and presence of extra edge plane sites.55 The limit of detection of 1.4 biosensor displayed a limit of detection of 0.055mM with a linear
nM has been reported in Boron doped CNTs which has been range of 0.16–11.5 nM glucose concentration with a very high
enhanced by incorporating Boron in CNTs. The SWCNTs functio- sensitivity and selectivity.289 Another CuO-MWCNTs fabricated
nalized by over-oxidized polypyrrole has been electro-copolymer- biosensor exhibited a high sensitivity of 2596 μAmM−1cm−2 to the
ized on the surface of the electrode.262,263 The films of over-oxidized detection of glucose with a limit of detection of 0.22 μM in a linear
polypyrrole allowed the cations of dopamine to pass through them range over a concentration up to 1.2 mM.290
but repelled ascorbic acid and serotonin thus exhibiting better
selectivity.261 The use of untreated DWCNT has been reported to CNT based enzymatic biosensors.—The CNT based electroche-
be the best choice for the development of amperometric sensors. The mical biosensors have been fabricated by using dehydrogenase
CV, XPS and BET analysis have shown that the smaller length enzymes such as alcohol-dehydrogenase,291 phosphatase,292 D-
DWCNTs are more effected by the acid functionalization which fructose dehydrogenase and glucose dehydrogenase. A biosensor
decreases the electro active area of the respective modified electrode employing alcohol-dehydrogenase enzyme along with MWCNT and
due to which the amperometric sensitivity in the detection of poly (vinyl alcohol) has reported a response time of about 8 s for the
dopamine and catechol on the functionalized DWCNTs modified ethanol detection. The electro oxidation of NADH which is
electrode is decreased.278 produced during the enzymatic activity produces a current that is
The MWCNTs, SWCNTs, GO, rGO and CQDs have been taken into account for generating the response by the biosensor.291
employed for the electrode modification in the detection of dopa- Other electrochemical biosensors have been fabricated by using
mine. However, due to the presence of one type of nanostructures, acetyl cholinesterase, alkalinephosphatase, organophosphorus hy-
these materials almost provide same sensitivity and linear range of drolase and urease enzymes. The voltammetric biosensors have been
detection for dopamine. The linear range and lower limit of detection fabricated by attaching urease and acetyl cholinesterase enzymes on
can be improved by employing the hybrid or composite materials of the surface of the SWCNT modified electrode by means of sol-gel
Carbon nanostructures.279 The electrochemical functionalization material for the detection of urea and acetylthiocholine
plays a very important role in increasing limit of detection and respectively.291 In another study, alkaline phosphatase enzyme is
sensitivity in dopamine sensor with the SWCNT electrode. The limit attached on the surface of CNTs in the presence of streptavidin by
Journal of The Electrochemical Society, 2020 167 037555
using layer by layer method.293 The lactate level monitoring is anti-PSA antibodies have been reported to detect PSA.311 The
significant for the use in biotechnology, food processing and in immunosensors have been reported for the detection of osteopontin
sports medicine. The amperometric detection of the level of lactate that causes prostate cancer. They have been fabricated on the glass
has been done by using CNT and mineral oil paste having lactate substrate which is coated with SWCNTs with attached osteopontin
oxidase.292 molecules on their surface.312 Another biosensor, A CNT (EDC/
The electronic structures of SWCNTs have an influence on the NHS functionalized) transistor and an antibody which is a hybrid to
detection in CNTs enzymatic biosensor. The [FeFe]-hydrogenase the genetically engineered antibody have been used for the detection
enzyme from clostridium acetobutylicum when attached to metallic of osteopontin.313 The DNA strands functionalized MWCNTs and
SWCNTs have reported better electro catalytic activity.273 This is SWCNTs have been reported for the detection of PSA in the blood
because of the better coordination between clostridium acetobuty- samples with more sensitivity and selectivity.314 The electroche-
licum redox-active sites and the electron surface.273 The glutamate mical biosensors used for the detection of prostate cancer biomarker
hydrogenase was attached to the top of CNTs by covalent bonding to using nanoparticles have been briefly reviewed and reported.315
fabricate a glutamate biosensor that has been able to display the limit
of detection up to 10 nM. The CNT electrodes are better candidates Detection of blood cholesterol level.—The amount of blood
for the fabrication of enzymatic biosensors with a high sensitivity cholesterol level has been detected by the amperometric electro-
and high selectivity.274 chemical biosensors.316–318 The amperometric biosensors employing
An electrochemical biosensor for the determination of tyrosine sol gel chitosin/silica and MWCNTs organic-inorganic nanohybrid
has been reported. The sensor was fabricated by dissolving poly- composite material has been reported for the detection of cholesterol
sulfone (PSF) in dichloromethane and depositing it on a GCE. The in the blood.318 The cholesterol oxidase enzymes are used for the
nitric acid functionalized MWCNTs are drop coated on the PSF oxidation of cholesterol to 4-cholesten-3-one producing hydrogen
layer. The Tyrosinase enzyme (TyOx) is deposited on MWCNT/ peroxide. The electrochemical detection of level of H2O2 produced
PSF/GCE after cross linking with glutaraldehyde. The characteriza- during the enzymatic activity gives the level of cholesterol in the
tion of the biosensor has been done by using the CV and electric blood. An amperometric cholesterol biosensor has been fabricated
impedance spectroscopy. The biosensor exhibited a very low limit of through layer by layer deposition of (poly (diallyl dimethyl
detection 0.3 nM and a very high sensitivity 1.988 μAμM−1cm−2 to ammonium) chloride) and cholesterol oxidase enzyme on a
detect the tyrosine.294 The doping of CNTs by nitrogen has an MWCNT electrode modified by the gold nanoparticles. A protective
influence on the electrochemical performance for the detection of coating of non conducting (poly (o-phenylenediamine) film has been
H2O2 and has been used to make H2O2 based enzymatic biosensor at produced over it by means of the electrochemical methods. A limit
a lower potential.295,296 of detection of 0.2 mM to the detection of cholesterol has been
The Hydrogen peroxide biosensors have been fabricated by using reported by the fabricated biosensor.316
enzyme horseradish peroxidase(HRP) in which the enzyme is
attached on the surface of MWCNTs with the aid of mediator Detection of cancer cells.—A poly dopamine coated CNT
methylene blue and by crossing linkage between HRP and BSA functionalized by the folic acid has been used for the detection of
composite film.297,298 A hydrogen peroxide detector has been HeLa and HL60 cancer cells.319 The biosensors have been reported
reported by employing CNTs modified with Pt nanoparticles to detect O2 released from HeLa cells which have been fabricated
fabricated by means of chemical reduction method on the surface from the hollow Carbon cubic (HCC) and porous Carbon cubic
of a waxed graphite electrode.299 The glucose oxidase was attached (PCC) nanomaterials. The etching of zeolitic imidazolate framework
on the side walls of SWCNTs in order to detect the presence of K-8(ZIF-8) with tannic acid followed by calcinations process
glucose. The glucose oxidase attached SWCNTs exhibited an produces HCC nanomaterials whereas PCC nanomaterials were
enhancement in conductance upon adding glucose and thus acting obtained by direct pyrolysis of ZIF-8. The HCC and PCC are
as a biosensor for the enzymatic activities.56 attached on the surface of Screen printed Carbon electrode (SPCE).
The HCC based sensor have exhibited a limit of detection 207 nM
Detection of proteins.—The cellular prion protein has been whereas the PCC electrode have reported a limit of detection
detected by making use of a fluorescent-label aptamer which involves 140 nM to detect the O2 released from HeLa cells. The selectivity
the quenching of the fluorescence by non-covalent modification of the of the biosensors has been examined by studying the amperometric
MWCNTs with the aptamer. But the quenching is restored in the response in the presence of interfering agents like 4-acetaimido-
presence of cellular prion protein. The biosensor exhibited a limit of at phenol, uric acid, ascorbic acid, D-glucose and dopamine. The
least 4.1 nM. The sensor demonstrated outstanding selectivity for the biosensors displayed very small current response to them, thereby,
cellular prion protein in the presence of amino acid and other exhibiting excellent selectivity.320 The synthesis of Hollow Carbon
proteins.300 The SWCNTs optical biosensor has been reported for Cubic (HCC) and Porous Carbon Cubic (PCC) nanomaterial and
the detection of protein-protein interaction with limit of detection 10 various steps involved in the detection of superoxide anions in HeLa
pM.301 The insulin has been detected by SWCNTs upon functiona- cell are shown in Fig. 5.320
lization with insulin-binding aptamer.302 The CNT electrochemical The surface ITO electrode coated with an assembly of CNT
biosensors for the detection of nucleic acids have been investigated multilayer and antibodies to the epithelial cell adhesion molecules
vastly and have been brought in notice.303–308 have been used to detect the cancer cells of liver.321 The Au-Ag
alloy coated MWCNTs have been used for the detection of volatile
Detection of prostate specific antigen.—A highly sensitive biomarkers of the gastric cancer cells MGC-803.322 The femtomolar
electrochemical biosensor based on CNT-bioconjugates and level gastric cancer biomarker miRNA-106a has also been detected
SWCNT forest platform has been reported for the detection of and reported.323
prostate specific antigen (PSA) detection by using horseradish The carcinoembryonic antigen has been detected by the CNT
peroxidase (HRP)/secondary antibody ratio. The device exhibited a based electrochemical immunosensor employing gold nanoparticles-
limit of detection of 4 pgmL−1 in 10 μl calf serum without any encapsulated dendrimers as a sensor surface with the attachment of
dilution.309 With the replacement of SWCNTs with gold nanopar- two enzymes Glucose oxidase and horseradish peroxidase. The
ticles customized MWCNTs, the limit of detection of PSA changed electron transfer is facilitated by the encapsulation of gold nano-
to 0.40 pgmL−1,thereby, significantly enhancing the sensitivity and particles in the interior structure of the device. The immunosensor
selectivity of detection.310 The CNT based biosensors have been has exhibited a wide concentration range from 10 pgmL−1 to
coated with the antibodies for the detection of the biomolecules/ 50 ngmL−1 and the limit of detection has been reported to be lower
disease causing agents. The CNTS-arrayed electrodes coated with as compared to the ELISA test.324You can also read
