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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5604_Библиотеки_им_академика_М_И_Перельмана.pdf
Carbon-Based Nanomaterials: An Overview 5
2 Carbon-Based Nanostructures
On the basis of size, shape and dimensional characteristics, CNMs can be divided
into zero-dimensional (0D) NMs (carbon quantum dots (CQDs), fullerenes [8]),
one-dimensional (1D) NMs (CNTs and carbon nanofibers [9]), two-dimensional
(2D) NMs (graphene, graphene oxide) and three-dimensional (3D) NMs (carbon
sponges [10]). To establish the active participation of CNMs in the recent research
scenario, Liu et al. [11] conducted a bibliographic investigation of CNM-related
keywords using CiteSpace. It is concluded that the keywords like carbon nanotubes,
nanomaterials, aqueous solution, graphene oxides and multiwalled carbon nanotubes
are regularly being used in research publications indicating that these particular
CNMs have been the focus of research based on carbonaceous nanomaterials (Fig. 2).
In this section, we aim at discussing regarding the various forms of CNMs in details
along with their physical and chemical characteristics.
2.1 Fullerene
Buckyballs or buckyball clusters (popularly known as endohedral fullerenes) which
include buckminsterfullerenes, C
ters with less than 300 atoms. Kroto’s immense interest in study of the quantum
, and fullerenes are comprised of carbon clus-
60
Fig. 2 Investigation of CNM-related keywords co-found using CiteSpace. Reproduced with
permission from Royal Society of Chemistry [11]
6 S.K.Swainetal.
mechanics of polyynes (–C≡C–C≡C) in an attempt to imitate the interstellar conditions leads to the birth of fullerenes. With the support of Curl and Smalley, Kroto
exposed a graphitic plate to high-frequency pulsed laser beams to form aggregates
with 60 atoms of carbon which were further identified by mass spectroscopy [12].
This led to t he wild race behind predicting the molecular structure of the clusters,
which can provides high-level stability to these C atom agglomerates. After few
attempts, Kroto came up with the idea of assembling the atoms in forms of geodesic
domes established by the American architect R. Buckminster Fuller, which consists
of hexagonal faces, arranged on a spherical surface by attachment with pentagons.
Hence, C
was predicted to be comprised of 20 hexagonal faces connected by
60
12 pentagons and was christened as “Buckminsterfullerene” [13] (Fig. 3a). Today,
various other highly stable forms of fullerenes have been successfully isolated, for
example, C
70,C20,C84,C78
, but still C60predominates as the most colloidally stable
and abundant form of fullerene.
Fullerenes are a class of highly functional water-soluble and non-toxic CNMs,
which offer functional points for chemical modifications by attachment of targeting
ligands in a 3D space, which can assist cellular targeting. This further permits
transformation of their allotropes to maximize the pharmacokinetic features, therapeutic applications along with other physical properties like hydrophilicity, dimensional attributes as well as stability and compatibility in biological atmosphere. C
60
fullerenes have the potential to capture the free radicals like methyl, hydroxyl and
superoxide anionic radicals, and hence preventing oxidation of cell membranes. The
act as potent-free radical scavengers in the development of fatigue and ischemia
Fig. 3 Structural attributes of a fullerenes, b carbon nanotubes and c graphene
Carbon-Based Nanomaterials: An Overview 7
processes in skeletal muscles [14]. Optimized dosages of C60can have positive therapeutic impacts in colorectal cancer, acute liver injury, obesity, hemiparkinsonism,
and acute cholangitis as well as used as an effective treatment against glyphosate
herbicide poisoning [15].
2.2 Carbon Nanotubes (CNTs)
CNTs are CNMs generated from cylindrically rolling up one or more graphene
sheets around the axis to form smooth tube-like structures with nano-dimensional
diameter and length ranging from few micrometres to centimetres. The existence of
such carbon-based tubular structures was evidenced for the first time in a Russian
article published in 1952. However, in 1978, Abrahamson and Wiles reported rolled
up layers of graphitic fibres having a hollow centre (Fig. 3b). However, finally in
1991, the research work by Iijima reported a complete structural investigation for the
first time and popularized the fibres as carbon nanotubes [16]. The carbon nanotubes
are yet another versatile class of CNMs with cylindrical-shaped long tube structures
comprised of neatly rolled up graphene sheets. They can be categorized into singlewalled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs)
and multiwalled carbon nanotubes (MWCNTs).
The SWCNTs consist of a one-atom thick single graphene layer with diameter
up to 3 nm. Meanwhile, the nano-dimensional attribute of a MWCNT can be more
than 100 nm. The multiple carbon layers of the MWCNTs provide extraordinary
mechanical strength to this special CNM as compared to SWCNTs. MWCNTs yield
robust materials suitable for a wide range of applications as composite materials.
However, SWCNTs are highly opted for fabrication of sensors due to their unusual
twisting behaviour.The CNTs tend to offerphenomenal Young’s modulus and tensile
properties just like steel and iron [17], which renders them applicable in biological,
electrical, mechanical and chemical fields [18]. These CNMs are utilized in energy
storage, electron field-emission systems, electrochemical and microelectronics applications due to their specificity and smooth morphology.Furthermore, the high absorption attributes of CNTs allow them to be used for hydrogen storage applications.
CNTs are usually synthesized via arc discharge, chemical vapour deposition (CVD),
laser ablation and hydrothermal methods to be used for different applications. Out
of these, CVD is the most sought after method as it can yield CNTs in various structural forms such as coiled, powdered and straight. Moreover, CVD allows easy CNT
growth regulation by careful manipulation of temperature condition.
2.3 Carbon Nanofibers (CNFs)
The first ever record regarding CNFs is a patent granted in name of Hughes and
Chambers in 1889, which reported fabrication of carbon filaments via gas pyrolysis
8 S.K.Swainetal.
followed by deposition of carbon and subsequent filament growth [19]. The beauty of
these marvellous structure was expressed much later after electron microscopy investigations were carried out by scientists Lukyanovich and Radushkevich in 1950s as
they published a paper reporting the hollow filamentous carbon structures possessing
diameter of 50 nm.
CNFs are 1D NMs with a far more complicated structural as compared to CNTs.
The directional assembling of the carbon layers is the main influencing factor for
2
its mechanical attributes. These CNMs are arranged as sp
-hybridized carbon atom
containing linear filaments in a discontinuous manner with very high aspect ratio
(100:1) [20]. Further detailed investigations revealed that not all carbon nanofibers
consist of graphitic layers adjusted along fibre axis [21]. Based on angles of graphitic
layers, CNFs are categorized into various shapes like platelet, ribbon (CNTs), tubular
and fishbone nanofibers (graphitic layers are arranged with an angle between the
perpendicular and principal axis). The unique structural orientations give rise to
remarkable properties, which renders CNFs applicable in diverse range of applications like polymer reinforcement, selective adsorption, catalysis, electronics and
hydrogen storage [21].
2.4 Graphene
Graphene is best described as a single layer of closely packed sp2-hybridized carbon
atoms arranged in hexagons [22]. The CNM was first observed by German scientist
Hanns-Peter Boehm and Ulrich Hofmann in the year 1962 and the term “graphene”
was first coined in relation to the graphite junction suffixed with “ene” attributing to
the existence of double bond interactions. However,after 40 years, recently graphene
was successfully isolated by the joint efforts of Geim and Novoselov, who reported
the detailed synthesis and structural properties of graphene in 2004 [23]. Novoselov
et al. substantially enhanced the yield and efficiency of the CNMs, in turn, accentuating its conductivity by decreasing the thickness to a single atomic layer. The
resultant ultrafine CNM exhibited outstanding electrical and thermal conductivities,
high aspect ratio, flexibility and mechanical strength. Their efforts were rewarded
with the Nobel Prize in physics in the year 2010.
The remarkable chemical, physical, optical, electrical and mechanical characteristics of graphene make it a promising material that has revolutionized the field of
nanotechnology. All the importance bagged by graphene is mainly due to the unique
atomic arrangements and 2D structure, which directly influences the properties it
exhibits. The structure of graphene is comprised of sp
present in a π-conjugated system oriented in monolayers in 2D sheets (Fig. 3c). The
C–C bond length is approximately 1.42 Å in the graphene structure with each atom
with a p-orbital present perpendicular to the molecular plane of the sheet, which
results in overlapping of orbitals to give valence and conduction bands leading
to planar conduction [24]. Through the passage of time, graphene has evolved to
form various derivatives like graphene oxide, reduced graphene oxide and graphene
2
-hybridized carbon atoms
Carbon-Based Nanomaterials: An Overview 9
Fig. 4 Structural attributes of a nanodiamonds, b nano-onions and c nanohorns
quantum dots for exhibition of enhanced behaviour towards several applications.
These chemically modified forms of graphene show great thermal and electrical
conductivities along with superior optical attributes [25]. Endowed with eminent
chemical and physical properties, graphene and its derivatives are extensivelyused in
tissue engineering, drug delivery, therapeutics, bioimaging, biosensing and genetics.
2.5 Nanodiamonds (NDs)
NDs are the most recently discovered allotropes of carbon with very largeaspect ratio,
usually generated by detonation and CVD process [26, 27]. NDs refer to a group of
CNMs possessing octahedrally arranged carbon atoms having an overall dimension of
about 5–50 nm. They are nanocrystalline structures with tetrahedral C atoms stacked
in 3D cubic trellis (Fig. 4a), giving them the diamond shape with an onion-like outer
graphitic shell coating. These 3D CNMs have seen attracted tremendous attention in
the field of theranostic applications because of their ease of functionalization, high
adsorption capability,thermal stability and biocompatibility [28, 29]. NDs are known
to generate photoluminosence (PL) emissions in near infrared (NIR) and red spectral
regions without the phenomenon of photoquenching due to the presence of colour
centres resulting in the ND matrix from nitrogen vacancies [30, 31]. Apart from
the optical properties, NDs show considerable magnetic characteristics arising from
their facet-dependent electrostatics [32]. This remarkable phenomenon of surface
electrostatics enables NDs as promising candidates for drug delivery. NDs can be
conveniently functionalized using biomolecules like deoxyribonucleic acid (DNA),
ribonucleic acid (RNA), lysozyme, proteins, cytochromes, antibodies and many more
to show synergistic effects towards theranostic applications [33, 34].
2.6 Nano-Onions (CNOs)
CNOs fall in the category of 0D CNMs that are characterized by their distinctive
structural features consisting of severallayers of closed concentric shells, resembling
10 S. K. Swain et al.
the structure of an onion (Fig. 4b). The approximate diameter of a conventional CNO
falls in the range of 1.4–50 nm with an inter-layer spacing of about 3.4 Å having
aC
fullerene core [35]. These CNMs are reportedly synthesized via DC arc
60/C80
discharge [36], underwater arc discharge [37], high vacuum thermal annealing [38],
and annealing under helium atmosphere [39] which yield CNOs of various shapes
and sizes. However, the CNOs achieved using the above techniques are found to
be hydrophobic in nature, which leads to higher level agglomeration in organic
and aqueous media. Covalent or non-covalent surface functionalization routes [40]
address the solubility issue. Especially, surface oxidation of the CNOs opens the
possibility of esterification reactions, which enhances their colloidal stability in water.
CNOs are widely utilized in the field of therapeutics.
2.7 Nanohorns (CNHs)
CNHs are considered as bridge connecting fullerenes and CNTs. The morpholog-
2
ical characteristics of CNHs predict that it is comprised of sp
-hybridized C atoms
arranged as five pentagons having a cone-shaped front tip, with a cone angle of 120
degrees and diameter of about 2 to 5 nm, and the sixth pentagonal structure forms
the CNT-type hexagonal wall along the axis [41] (Fig. 4c). In addition, the presence
of heptagons around the axis aids in the change of pentagon curvatures to form CNH
chemistry [42]. CNHs form aggregated structures to give rise to seeds, buds and
dahlia flower-like morphologies [43]. The dahlia form of CNH consists of a spherical structure with diameter falling in the range of 80–100 nm made up of almost
2000 tubular units, which serve as a promising candidate in nano-oncology [44]. The
fabrication route of CNHs is entirely based on vaporization of graphitic substrates
without any metal catalysts followed by quenching in inert atmosphere [45]. CNHs
are majorly synthesized via laser ablation, joule heating and arc discharge. The
nanohorns exhibit negligible toxicity, which is highly harnessed in different biomedical applications. Other major advantages of CNHs in comparison to other CNMs
are higher aspect ratio and porosity, which permits unhindered movement of the
encapsulated biomolecules inside the cavity of CNHs [46]. Moreover, the irregular
morphology allows controlled opening of cavity at stipulated sites, ensuring regulated
loading and release profiles of therapeutics.
2.8 Carbon Dots (CDs)
Nano-dimensional particles of carbon known as carbon dots or CDs also refer to a
recent discovery in the realm of CNMs but have drawn considerable focus towards
its remarkable optical properties, hydrophilicity as well as biocompatibility [47]. In
a recent investigation, surface-paralyzed CDs have shown reportedly strong photoluminescence with distinct properties in the visible range [48]. CDs with high PL
Carbon-Based Nanomaterials: An Overview 11
emission features can be broadly divided into two categories based on small size
and surface passivation [48]. Compared to primitive semiconductor quantum dots
and organic dyes, CDs exhibit excellent optical (fluorescence) properties like wide
range excitation and emission spectra as well as photostability [48]. This has led to
its increased popularity in the fields of drug delivery and bioimaging.
2.9 Nanoporous Activated Carbon
Activated charcoal or carbon is a form of carbonaceous material consisting of very
minute pores with minimized volumes. The main aim behind its synthesis is to
provide a large surface area as sites for chemical reactions and adsorption. Hence,
these are actively used as anti-fouling agents in water treatment process and in separation of gases, dyes and heavy metals fromwater. However,the efficiency of removal
is not that much impressive. This opens up a window for functionalizing activated
charcoal into nanoporous activated carbon to address the issue regarding removal
efficiency [49]. This nanoporous structure is mainly composed of carbon atoms
along with hydrogen and oxygen atoms in relatively lesser quantities. Based on the
synthesis technique, starting material and processing methods, some other inorganic,
sulphurous and phosphorus matter may also be traced on the surface. But oxygen and
oxygenated groups are primarily found on the surface of the nanoporous activated
carbon. The heavy demand of this form of activated carbon is directly dependent on
morphology of the pores, consisting of micropores or a combination of micro- and
mesopores, and the heteroatom present, like sulphur, nitrogen or oxygen [50].
3 Synthesis Techniques
Carbon-based nanomaterial synthesis utilizes two primary approaches:top-down and
bottom-up (Fig. 5). In the top-down method, larger bulk materials are broken down
into nanoscale dimensions using diverse methods like mechanical exfoliation, laser
ablation and ball milling. The extraction of layers from graphite characterizes the
top-down approach, while the bottom-up method entails assembling smaller building
blocks or molecular precursors to create nanoscale structures. Examples include
chemical vapour deposition (CVD) for graphene or the growth of carbon nanotubes
through chemical reactions. Each approach presents specific advantages and challenges, shaping the properties of the resulting nanomaterials. Researchers frequently
opt for these strategies based on their intended applications and the desired material characteristics. In the context of graphene, mechanical exfoliation is the process
of delaminating graphite layers using mechanical force. This method is crucial for
isolating single- or few-layer graphene sheets from the bulk structure of graphite,
enabling researchers to harness the exceptional electronic properties of graphene. In
12 S. K. Swain et al.
2004, Novoselovand collaborators achieveda ground-breaking milestone by demonstrating the production of graphene layers through mechanical exfoliation. Their
work highlighted the extraordinary electrical properties of graphene, displaying its
capability to exhibit the quantum Hall effect at room temperature. Films produced
via mechanical exfoliation, achieved by repeatedly peeling small mesas of highly
oriented pyrolytic graphite, demonstrated reliability and scalability up to sizes of
10 μm. Notably,this method accomplished the remarkable feat of preparing graphitic
sheets with thicknesses down to a few atomic layers, even isolating single-layer
graphene (SLG). Creating devices from these ultra-thin films has provided insights
into their electronic characteristics. Despite being atomically thin, these films maintain exceptionally high quality, facilitating 2D electronic transport that is ballistic at
submicrometre distances. Since then, this method has become a fundamental component in generating high-quality graphene samples, fostering widespread research
and applications across various fields, including electronics, materials science and
nanotechnology [7]. It is also feasible to produce graphene sheets through CVD
or laser ablation methods. These diverse approaches offer graphene and reduced
graphene oxide sheets of varying qualities, catering to the specific requirements
of respective applications. The production of moderate-quality graphene for structural applications at low costs is feasible, while high-quality graphene for electronic
devices, produced in smaller quantities, tends to be more expensive. Key methods
for mass production include thermal exfoliation and liquid phase of graphite, CVD
synthesis (considered cost-effective), and synthesis on silicon carbide [51].
In 1990, Huffman and Krätschmer pioneered through the evaporation of graphite
electrodes in a helium atmosphere fullerene should be produced [52, 53]. Subsequently, a reactor underwent modification through the establishment between two
graphite electrodes of an electric arc. The soot produced condenses on the reactor’s
Fig. 5 Top-down and bottom-up approaches for the synthesis of carbon-based nanomaterials
Carbon-Based Nanomaterials: An Overview 13
cold surface, and then undergoes collection and processing in boiling toluene, xylene
or other organic solvents. Upon the evaporation of the solvent, a dark condensate
emerges,containing approximately 10–15% of C60 and C70 fullerenes, accompanied
by minimal quantities of higher fullerenes. Based on synthesis parameters, the ratio
between C60 and C70 varies, typically with C60 being the dominant fraction. The
arc-discharge technique discussed belongs to the broad category of plasma methods,
widely preferred and frequently utilized when compared to alternative approaches
[54]. Nevertheless, the practical application of fullerenes faces limitations due to the
comparatively low productivity of current synthesis methods and elevated costs.
Various techniques have been developed for crafting CNT structures; gas-phase
processes are primarily cantered. Typically, three methods are employed for CNT
production: the CVD technique, the carbon arc discharge and laser ablation technique. The arc-discharge method utilizes elevatedtemperatures (surpassing 1,700 °C)
in the synthesis of CNTs, resulting in expanded CNTs with fewer structural defects
compared to alternative methods. The commonly employed methods involve arc
discharge between high-purity graphite electrodes, typically water-cooled electrodes
with diameters ranging from 6 to 12 mm. These electrodes are separated by 1 to 2 mm
in a chamber filled with helium (500 torr) under subatmospheric pressure (helium
substitution with hydrogen or methane is also feasible) [55]. Within the chamber,
there is a simultaneous presence of a graphite cathode and anode, accompanied by
evaporatedcarbon molecules and metal catalyst particles such as cobalt, nickel and/or
iron. The arcing process entails passing a direct current through the heated chamber
(around 4,000 K) and pressurized. Throughout this process, roughly 50 percent of
the evaporated carbon undergoes solidification on the cathode (negative electrode)
tip, resulting in the formation of a deposit at a rate of 1 mm/min, commonly termed
a “cylindrical hard deposit” or “cigar-like structure”. Meanwhile, the anode (positive electrode) undergoes consumption. The remaining carbon, forming a hard grey
shell, deposits on the periphery and condenses into “chamber soot” near the chamber
walls and “cathode soot” on the cathode. The inner core, consisting of dark and soft
cathode soot and chamber soot, produces either single-walled or multiwalled carbon
nanotubes and nested polyhedral graphene particles. The laser ablation technique
involves utilizing a high-power laser vaporization, specifically the yttrium aluminium
granet (YAG) type. In this process, a quartz tube holds a pure graphite block heated to
around 1,200 °C within a furnace, conducted in an Argon (Ar) atmosphere [56]. The
objective of utilizing a laser is to vaporize the graphite within the quartz. Similar to
the synthesis of single-walled carbon nanotubes through the arc-discharge method,
employing the laser technique requires the addition of metal particles as catalysts
to the graphite targets. This technique boasts notable advantages, including a relatively high yield and comparatively low metallic impurities. This is attributed to the
tendency of metallic atoms involved to evaporate from the end of the tube once it is
sealed. However, a notable drawback is that nanotubes produced through the laser
ablation method may not be uniformly straight and can display branching. Unfortunately, this technique is economically disadvantageous due to the requirement
for high-purity graphite rods, substantial laser powers (sometimes necessitating two
laser beams), and a lower daily synthesis quantity compared to the arc-discharge
14 S. K. Swain et al.
technique [57]. Furthermore, a standard method for carbon nanotube production is
CVD. This technique enables the growth of CNTs on various materials and involves
the chemical breakdown of a hydrocarbon on a substrate. The key advantage of CVD
lies in the high purity of the resulting material and the straightforward control of the
reaction course [58].
Several methods have been reported for producing carbon quantum dots (CQDs)
over the last decade. Hydrothermal synthesis, a solution reaction approach, involves
subjecting reaction materials to a high-temperature range in a Teflon-lined autoclave.
Pressure is generated in a Teflon-lined autoclave by securely sealing the valves. The
synthesis of CQDs from natural sources, such as betel leaves [59], Catharanthus
roseus (white flowering plant) leaves [60], Murraya koenigii leaves (curry leaves)
[61], hibiscus sabdariffa flowers (Roselle flower)[62], quince fruit (Cydonia oblonga)
powder [63], aloe vera leaf gel [64], etc., is also conducted through the hydrothermal
and carbonization method.
Sumio Iijima first documented the finding of spherical CNOs as a byproduct
in 1980 during an examination of synthesized carbon black in a vacuum using
a transmission electron microscope (TEM) [65]. Daniel Ugarte extensively revisited this process in 1992, thoroughly documenting an observed in situ formation
mechanism for the creation of spherical graphitic structures [66]. Chemical vapour
deposition has proven to be a viable method for obtaining CNOs. However, these
CNOs often contain contaminants in the form of catalyst particles encapsulated
within carbon cages [67, 68]. Nevertheless, it provides a straightforward approach to
obtaining CNOs encapsulated with catalyst particles, serving as efficient catalysts.
Sano et al. demonstrated the fabrication of CNOs through arc discharge between two
graphite rods immersed in deionized water, avoiding the utilization of metallic catalysts [69]. Additionally, Bian et al. successfully accomplished the electrochemical
synthesis of CNOs, including both solid (S-CNOs) and hollow (H-CNOs) varieties.
This was achieved from basic aromatic compounds in acetonitrile on a platinum
plate electrode using a multi-potential steps method [70]. Najafi et al. successfully
synthesized high-quality carbon nano-onions attached to graphene sheets through a
cost-effective and environmentally friendly bottom-up approach. They employed a
glucose/glycerol mixture easily accessible smart organic precursor and chose a onestep hydrothermal synthesis method [71]. Various synthetic techniques are available
for producing CNOs with diverse structures and properties. Therefore, selecting
the optimal synthetic technique for CNOs depends on the intended purpose of the
synthesis.
The synthesis of CNHs involves intricate processes aimed at creating horn-shaped
carbon nanostructures. Methods for CNH synthesis include CVD, joule heating, laser
ablation of pure graphite, and arc discharge of carbon rods. Li et al. demonstrated the
generation of single-walled carbon nanohorns (SWCNHs) with various morphologies through direct current arc discharge between pure graphite rods in different atmospheres, including CO, CO
in the air with carbon atoms, transforming it into CO, resulting in the formation
of O
2
of SWCNHs through a combination of CO and N
and air. The arc-discharge process involved the reaction
2
[72]. Pagura et al. conducted the
2