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126 Carbon-Based Nanocarriers for Drug Delivery
the rst step in this technique for synthesizing CNTsis to excite carbon atoms that
are in conjunction with metallic catalyst particles.
Generally, tubes are bored through silicon and subsequently loaded with iron
nanoparticles at the bottom. The substrates are later heated to break down a hydrocarbon. As soon as the carbon is in interaction with the metal atoms that have been
inserted into the holes, it starts to form nanotubes that take on the structure of the
tunnel. These characteristics allow CNTsto develop exceptionally long and perfectly
aligned forms in the tunnel’s angle. Asubstrate is prepared and processed in the
CVD methodat a temperature of around 700 °C by coating the particles ofmetal
catalysts. Iron, nickel, cobalt, or a mixture of these metalsare typical metal catalyst
particles used in CVD processing [2,19]. The purpose of utilizing metal nanoparticles in conjunction with catalyst support like Magnesium oxide (MgO)or Alumina
(Al2O3)is to increase the activesurface area for increasing by-products of the catalytic reaction of the metal particles and pure carbon. A carbon-containing gas,
like ethylene, acetylene, methane,and process gas, including ammonia, nitrogen,
or hydrogen,was utilized as the reactor’s fuel during the initial stage of nanotube
growth. The uidized bed reactors (FBRs)aremost frequentlyemployed inthe process of CVD [20,21]. The carbon-containing gas is fragmented across the catalyst
particle’s surface, and as a result, the carbon is now exposed around the edges of
the nanoparticle, wherein nanotubes could proliferate. However, the discussions over
the mechanism ofthis method arestill under investigation. According to studies, the
most widely recognized theories are thebase growth and thetip growth models [22].
The catalyst particles may remain at the bottom or within thenanotubesthroughout
theirdevelopment and expansion, relying on the attachment and adhesion of these
particles with thesubstrate [2]. The schematics of the CVD method for the fabrication of CNTs are presented in Figure5.2a [23].
This method leads to the formation of CNTs of requisite size and characteristicsif thepertinent factors are considered. Basically, the CVD processinvolves two
steps: 1. catalyst nucleation and deposition through chemical etching or thermal
annealing[24,25]; 2. nanotube growth onto the substrate at temperatures between
500 to 1000 °C [26]. Overall size and the length of the nanotubes depend mostly
on thereaction time; however, nanotubes of a maximumof 60mm in length can be
established through CVD [27].
5.3.2 laser aBlaTion
The laser ablation method demonstratesthesynthesis of CNTsthroughtargetingcarbonaceous feedstock gas using acontinuous laser pulse or extremely powerful laser
beam. The schematic of the laser ablation technique is illustrated in Figure5.2b [5].
It is made up of graphitic rods and a catalytic mixture of Co and Ni (50:50), which
is heated to around 1200 °C with argon gas owing through it. Graphite is used as a
source ofcarbon; however,this method is quite costly [28,29]. The employment of a
continuous laser has caused the graphite target to vaporize at 1200 °C in an oven. To
maintain the pressure at roughly 500 Torr, inertgases such asargon and heliumgas
are employed withinthe reactor chamber. This techniqueis quite similar to the arc
discharge approach. Due to the high cost of this process, it istypically utilized to
produce SWCNTs and is able to synthesize SWCNTswith higher yields [30].
(b)
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(a)
(c)
FIGURE 5.2 Schematic Representation for the Synthesis of Carbon Nanotubes (CNTs) by
(a) Chemical Vapor Deposition [23], (b) Laser Ablation, and (c) Arc Discharge Methods.
[Reprinted with permission from
Jha et al. (2020)] [5].
128 Carbon-Based Nanocarriers for Drug Delivery
Various laser ablation-assistedtechniques were employed for the synthesis CNTs,
such asthe free electron laser (FEL) technique and laser powder method. The FEL
method employsa pulse width of ~400 and a preheated argon gas jet to disperse carbon soot from the front of the carbon source [31].At the same time, the laser powder
approach employs a CO2 laser in an argon stream. In this method, a combination of
carbon source and catalyst powder is laser-ablated, whichsignicantly reduces the
losses inthermal conductivity. This approach yields around 5 gm/hr of CNTsusing a
Ni: Co (1:1) catalyst [32]. Moreover, with an increase in the laser power, the diameter
of CNTs decreases; therefore, the size of CNTs can be tailored through altering the
power provided by laser beams [2].
The properties CNTs synthesized via laser ablations techniques are strongly
affected bythe chemical and structural characteristics of the target material along
with the characteristics of the laser beam such aspower, wavelength,cw versus frequency, and energy uence. In addition, the spacing between the substrate and target, ow velocityand pressure of the inertgases, the chemical composition of the
chamber, and the ambient temperature also inuence the yield and characteristics
of CNTs. This process has the ability to produce SWCNTs with a high degree of
purity and quality. Although the fundamentals and mechanisms of the laser ablation
method are comparable to those of the arc discharge method,in this approach, the
required energy is delivered by a laser that strikes a pure graphite pellet containing
catalyst elements [2]. Since the metallic atoms display theirtendency toevaporate
out from the tip of the tube when it is closed, the major benets of this technology
include reasonably high yields and fairly lower metallic contaminants. The fundamental drawback of this method is that the nanotubes produced are not always consistently straight but occasionally branched.
Nevertheless, the laser ablation approach is not economically viable since it
involves high-grade graphite rods, demands strong laser power(in certain situations,
two laser beams are involved), and produces fewer nanotubes per day than the arc
discharge approach.
5.3.3 arc Discharge
Compared to previous approaches, the arc discharge process employs elevatedtemperatures (over 1700°C) for the synthesis of CNTs, resulting in CNTs with lower
structural aws. One of the most popular techniques involves arc discharge
forhigh-purity graphite electrodes, typically water-cooled and spaced 1 to 2mm
apart in a helium chamber at sub-atmospheric pressure[11]. The chamber includes
a cathode and anode composed of graphite, vaporized carbon molecules, and a tiny
proportion of metallic catalysts. The helium gas in the chamber can be replaced with
methane or hydrogen gas. The iron, cobalt, or nickel metallic particle are employed
as catalysts for synthesizing SWCNTs.The schematics of the arc discharge process
are illustrated in Figure5.2c [5].
The chamber is compressed, heated to about 4000 K, and direct current is
transmitted across the arch during the arcing process. The anode was consumed
throughout this process of arcing, and nearly half of the carbon accumulated over
the cathode electrode. The accumulated layer is known as a cylindrical hard deposit,
129Carbon Nanotubes for Drug Delivery System
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which continues to grow at the rate of ~1mm/min.The residual carbon, which forms
a hard-gray coating on edge, crystallizes to form “cathode soot” near the cathode
and “chamber soot” adjacent to the chamber walls. The SWCNTs or MWCNTsand
stacked polyhedral graphene nanosheets may be produced from the inner core,
chamber soot, and cathode soot thatare both soft and dark. The morphology and the
texture of the cathode deposit can be investigated via scanning electron microscopic
(SEM) analysis. Generally, two distinct textures are yielded by the cathode deposit.
The gray exterior shell is made up of curved and rigid graphene nano layers, while
the interior, softer, anddark core deposits are made up of bundle-like structures that
comprise irregularly distributed nanotubes [2].
There are two basic methods for the production and deposition of CNTs using
arc discharge: one uses various catalyst precursors, and the other does not. In overall, the formation ofMWCNTs maybe carried out in the absence ofcatalyst precursors; however, the formulation of SWCNTsuses various catalyst precursors.The
complex anode electrodes have mostly been employed for theexpansion in arc discharge,which can be formulated by the varied composite of metals such as iron,
cobalt, nickel, silver, palladium platinum, and graphite [12].According to previous reports, Ni-Y-graphite combinations may produce high yields (around 90%)
of SWCNTs with an average size of 1.4 nm [33], and this combination is presently
utilized all over the world to produce SWCNTs with a great yield. The ability and
possibility for producing a signicant number of nanotubes is the major benet of
the arc discharge method. In contrast, this method’s majordrawback is that it gives
very limited control over the nanotubes’ alignment, or chirality, which is crucial
fortheir classication, characteristics,and function. Furthermore, it is imperative to
purify the resulting products due to the metallic catalyst required for the reaction.
The developed SWCNTs thought this method exhibitsa 1.2 to 1.4 nm diameter. The
efciency of CNTs produced using the arc discharge method depended on factors
such as 1. selectivity ofthe inert gas, 2. inert gas pressure, and3.catalyst [34,35].
Similarly, MWCNTs of higher crystallinity and yield can be synthesized through
this arc discharge method. This approach may yield MWCNTs frompure graphite
arcs with an estimated inner diameter of 1–3 nm and an outside diameter of 10 nm
[36]. Since the catalyst is not employed in this process, there are no prerequisites
for a severe acidic purication process. Consequently, this method highlights the
fabrication ofMWCNTs with fewer defects. It has been demonstrated that the introduction of hydrogen gas into the formation zone allows for the best possible production of MWCNTs with few intrinsically entangled CNTs and high crystallinity [37].
MWCNTs can besynthesized using a variety of methods, including 1. liquid nitrogen
nanotube synthesis, 2. magnetic eld synthesis, 3. arc discharge by plasma rotation,
etc. [35].
5.3.4 Flame synThesis meThoD
In a regulated ame atmosphere, SWCNTs can be synthesizedfrom hydrocarbons
and thin metallic aerosol catalysts [38]. In comparison to other processes, ames are
signicantly less costly for producing nanotubes in bulk. Three essential elements
are required for the production of CNTs: a carbon precursor, metal catalyst particles,
130 Carbon-Based Nanocarriers for Drug Delivery
and a heating element. This process also nucleates and ultimately condenses the catalytic precursors, which are typically introduced via the ame process, into solid
metal spherical nanoparticles. Both the catalytic characteristics and the modication
of the ame parameters may inuence the structure of the nal product [39]. As a
result, several ame congurations, such as inverse diffusion, partly mixed, and premixed ames, would have been used to produce nanotubes and nanobers[40,41].
In the post-ame region of the premixed argon/oxygen/acetylene ame, performed at
around 50 Torr, SWCNTsmay have been identied by using the vapor of pentacarbonyl and iron as a source of a metal catalyst. In the space of around 30 ms, nanotubes have been seen to coalesce and assemble into clusters between 40 and 70nm
just above theburner [42].
5.4 PURIFICATION METHODS
Along with the large-scale synthesis, the purication of CNTs is a signicant issue.
The CNTs contain a number of contaminants whose concentrations vary depending
on the CNT production procedure. Depending on the approach utilized, quality and
quantity may vary. Carbonaceous compounds are the most prevalent impurities in
CNTs, although metals are the other sorts of impurities that are typically observed
[13,43].
Numerous contaminants can be found in the CNT soot as it is formed. Amorphous
carbon, metal catalyst, smaller fullerenes, and graphite (wrapped up) sheets are the
primary contaminants in the soot. The majority of the CNTs’ desirable qualities will
be hindered by these contaminants. It is essential to produce CNTs that are as pure
as feasible for basic research as well. The CNT specimens must also be as homogenous as reasonable to comprehend the measurements effectively. The oxidation and
acid-reuxing methods were used in typical industrial processes for the purication
of CNT, which affects the structural integrity of nanotubes. The CNTs’ insoluble
nature limits the use of liquid chromatography and makes purication challenges
rather severe. According to the kind of purication process, the purication stage of
CNTs eliminates amorphous carbon from CNTs, promotes or reduces mesopore or
micropore volume, breaks down the functional groups obstructing the pores’ entry,
or induces new functional groups. For instance, CNT purication processes, including heat or NH3 treatment, may be modied to improve mesopore volume and surface area in the event of bacterial adsorption. These methods essentially fall into two
categories: size- and structure-selective separations. The rst will isolate the CNTs
from contaminants, while the next will result in a relatively uniform distribution of
size or diameter of CNTs. Most of these processes are coupled with other techniques
to enhance purication and eliminate many contaminants at once [13].
It has been demonstrated that the CNTs produced by CVD are typically between
5 and 10% pure. Consequently, considerable purication is necessary before their
usage in biological applications [44]. The nanoparticulate systems contain several
residual metals, including Co, Ni, Mo, and Fe, in addition to specic organic contaminants and other impurities, including magnesium oxide, alumina, and silica. These
impurities and other carbonaceous contaminants present within the CNTs can be
removed through various purication techniques, which are discussed subsequently.
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5.4.1 air oXiDaTion
This method is the most effective for getting rid of amorphous carbon and metal
catalysts like Co, Fe, and Ni. Typically, the ideal airoxidation conditions have been
discovered as 40 minutes at 673 K [44]. Carbonaceous contaminants on the metal
surface may be effortlessly eliminated byutilizing oxidative processing. Since the
nanotubes are also oxidized during this process along with the impurities, it has
several disadvantages [45].Fortunately, CNT breakdown is not as severe as impurity
damage. These contaminants are more exposed or have higher defects. The fact that
such impurities are frequently bonded to the metal catalyst, which also serves as an
oxidizing catalyst, is another factor promoting impurity oxidation.
Overall, the variety of variables, including metal content, oxidation period, environment, oxidizing agent, and temperature, have a signicant impact on the effectiveness and yield of the process [13].
5.4.2 aciD TreaTmenT
The acid treatment is one of the simplest ways to reduce the concentration of impurities from the nanotubes. The higherconcentrations of metal particles and amorphous carbon (soot) can besuccessfully reduced by reuxing the sample in strong
acids such as HCl,H
reuxing agent [44]. The metal should initially undergo theoxidation or sonication
processto expose its surface. The metallic catalyst is subsequently solvated and subjected to acid treatment. In this process, the CNTs remain insuspended form. Only
the acid affects the metallic catalyst, particularly when applying an HNO3 treatment;
however, the CNTs and additional carbon atoms remain unaffected. The HNO3
reux and the moderate acid treatments through 4M HCl solution are essentially
the same, except thatthe metal must be completely subjected to the acid to solvate
it. According to the state-of-artliterature, the inuence of important factors, including type and concentration of acid, temperature, time, and pressure, arenot dened
thoroughly.As per their dependency, itmust be explored with adequate experimental
design to demonstrate thepossible interactions andeffects [13,46].
andHNO3,although HCl has been proven to be the best
2SO4,
5.4.3 UlTrasonicaTion
The foundation of this method is the segregation of particles brought on by ultrasonic
vibrations. In this method, diverse nanoparticle aggregations will be compelled to
vibrate and spread more widely. The choice ofsurfactants, solvents,orreagentsutilized is a crucial factor in separating the particles. The stability of the scattered tubes
in the system is affected by the solvent. If the CNTs are coupled to the metal particles, they will be more stable in weak solvents [47].
However, monodispersed nanoparticles are highly persistent in certain solvents,
like alcohols. The purity of the CNTs relies on the irradiation time once an acid is
applied. Only the metal particles getdissolved when the tubes are in the solution for
a short period; however, if the tubes are in the acid for a prolonged period of time, the
tubes will also undergo chemical degradation [13].
132 Carbon-Based Nanocarriers for Drug Delivery
5.4.4 micro-FilTraTion
The principle behind microltration is particle or size separation. In this method,
lters are used to capture CNTs and a limited number of carbon nanoparticles. The
lter allows the additional particles, such ascatalytic metal nanoparticles, carbon
nanoparticles, and fullerenes, to pass across. Soaking the freshly made CNTs in a
CS2 solution is one method of using microltration to separate fullerenes from CNTs.
Afterward, a lter traps the CS2 insoluble andallows the solubilized fullerenes from
the CS2 to trickle across [48–50].
Cross-ow ltration is a unique type of ltration technique used for the separation of CNTs. This technique uses a hollow beras the membrane. The solution
can pass through the membrane. The ltrate is continuously recycled through the
ber by pumping the ltrate through the ber’s bore under some head pressure. The
majority of the fast-owing solution that does not leak out on the sides of the ber
is then supplied directly to the reservoir. The membrane surface is swept by a rapid
hydrodynamic ow along the ber bore (cross ow), avoiding the formation of a lter
cake [51,52].
5.4.5 sUrFacTanT-BaseD annealing
Although the CNTs produced by the acid reux approach are generally pure, numerous contaminants may be trapped when the tubes convergeand may not be effectively
removed by a straightforward ltration procedure. Therefore, the surfactant-based
annealing method is used. In general, organic solvents like methanol or ethanol are
utilized within this technique when sodium dodecyl benzene sulphate (SDBS) is
employed. The ultra-ltration process is followed byannealing at a very high temperature (1273 K) for four hours since the nanotubes require more timeto settle. In
addition, annealing is an efcient method for improving CNT structure [44,53].
5.5 FUNCTIONALIZATION OF CARBON NANOTUBES
Perhaps one of the challenges associated with nanotubes for their application in the
biomedical eldis their tendency to aggregate due to weaker intermolecular interactions, which makes it difcult to disperse them into the suitable polymeric medium
and in various organic solvents. Such inadequacy of CNTs to dissolve in aqueous
media for biomedical and biological applications has been a signicant technical barrier [54]. To prevent the development of bundles owing to theiraggregation tendency,
CNTsare frequently functionalized tofurther enhance their diffusion in solvents and
other mediums. Functionalization refers to the addition of distinct functional groups
to the side chains or ends of CNTs [55]. This may be accomplished by making the
CNTs more hydrophilic withimproved solubility, utterly altering their biocompatibility prole by anchoring various functional groups,biomolecules, and other biocompatible nanomaterials to them via covalent or non-covalent functionalization
approaches.
Along with improving aqueous solubility, the functionalization of CNTs demonstrates their potential to minimize toxicity, improve biological compatibility, and
even offer the opportunity to load drug molecules, genes, or biomolecules for
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effective drug delivery systems (DDSs) [5]. Till now, numerous scientic studies
have presentedsurface functionalization strategies forCNTs with different molecules.As stated, two approaches have been frequently used for the functionalization
of CNTs, namely, covalent and non-covalent functionalization, which conjugates the
functionalization molecules through the chemical bonding and physical adsorption
phenomenon. The details related to these approaches are provided subsequently.
5.5.1 coValenT FUncTionaliZaTion
In the covalent functionalization method, the desirable functional groups can
bermly and irrevocably bound to the sidewalls or ends of the CNTs. Multiple functional entities,including the secondary dichloro-carbon groups, the uorine carboxylic group, and p-aminobenzoic acid, have been attached to the outer surface or to
the extremities of the nanotubes. The main advantages of chemical functionalization
are its ability to covalently bond with polymeric materials and its ability to diffuse
easily in a wide range of solvents. However, the development of defects in CNTs is
among the signicant disadvantages of this sort of functionalization [5,56].
There aretwo distinctmethods for thecovalent functionalization of CNTs, such
asthedirect functionalization ofside walls and indirect functionalization ofthe exterior of CNTswith oxygenated functional groups such as hydroxyl andcarboxylic
functionalgroups [57]. Regarding the two different types of CNTs, SWCNTs and
MWCNTs, SWCNTs were discovered to be signicantly convenient to functionalize since MWCNTs possess an outermost part with comparatively higher radii, and
the interior layers of graphene sheets are protected by the supercial layers, which
inevitablyprevent those from being paired with the exterior surface viathe functional groups. The CNTsarenot particularly reactive and typically require extreme
conditions for the reaction to occur. As a result, it has only been shown that the
surface chemistry of the CNTs may produce relatively modest chemical reactions.
Furthermore, it might be challenging to categorize functionalized SWCNTs, pinpoint the precise position of functionalized entities, and comprehend their anchoring
mechanism [58]. Although the covalent functionalization techniques are not entirely
distinctive, the nal compounds vary greatly depending on the properties of the conjugatedgroup or moieties.
The CNTs can be covalently functionalized through oxidation reactions, cycloaddition reactions, polymerization, and reaction with sulfoxides or acyl peroxides,
which are described as follows.
5.5.1.1 Oxidation
It is the most common method forthe covalent functionalization of CNTs, which
is frequently carried out by employing different oxidizing agents such as nitric
acid (HNO3) and developing hydrophilic functional entities like carboxylic or
hydroxylgroups within CNTs. These groups weretypically spotted at the terminal
along with the sidewall or surface of the CNTs [59]. After oxidation, theSWCNTs
3
adoptthe sp
carbon atom conguration, which allows theirsubsequent functional-
ization with diverse biomolecules like proteins or amino acids [60].
The solubility of CNT has certainly beenaugmented through oxidation; however,
this also causes an issue with CNT aggregation since it has been found that oxidized
134 Carbon-Based Nanocarriers for Drug Delivery
CNTs aggregate mostly in thevicinity of salts, which may be caused by the process
of charge screening. As a result, the CNTs are unstable in living organisms due to the
presence of various salts in the biologicaluid. The oxidized CNTs are employed to
address this agglomeration issue together with introducing a hydrophilic polymer to
its surface, such as polyethylene glycol (PEG), which renders nanotubes much soluble
and viable both inthein-vivo and in-vitro environment [61]. It has been demonstrated
that the oxidation of CNTs is a potential method of functionalization suitable for
small-scale manufacturing as well as the development of functionalized CNTs on a
large scale. Although oxidation using an acidic solution might shorten thenanotubes
by compromising specic properties, it is frequently required for specic biomedical
applicationslike oral drug delivery [62].
5.5.1.2 Reaction with Acyl Peroxides or Sulfoxides
As with oxidized CNTs, this reaction may also be utilized to functionalize CNTs on
the surface. It has the capacity to covalentlybindthevariety of functional groups
atthe edges or sidewalls of the CNTs, without causing defects or disrupting the structure ofCNTs[59]. As a result of the development of radicals with a carbon core,
acyl peroxide potentially contains terminal units for additional functionalization. For
instance, an amide could have resulted whenever the acyl chloride interacts with
the organic group comprising a carboxylic acid. Sulfoxides are also utilized for this
purpose in addition to acyl peroxides [5]. In addition to improving solvent dispersibility, these reacting functional groups linked to CNTs provide reactivesites for the
incorporation of monomers into polymeric structures. The major characteristics of
this free radical approach are its simplicity and sensible selection of radical-forming
compounds [1,63].
5.5.1.3 Cycloaddition Reaction
The cycloaddition reaction differs from the reaction discussed previously, asit occurs
near the sidewalls of CNTsrather than next to its defects or ends. This method is
also commonly employed for the covalent functionalization of CNTs. This reaction
can be split into three categories: 1. photoinduced cyclo addition, which is a photochemical reaction involving azides [64]; 2. Bingel reaction, which takes place when
a strong base is involved in the reaction and produces carbenes and which is also
known as the [2 + 1] cycloaddition process [65];and 3.1,3-dipolar cycloaddition
reaction, which has extensively beenemployed to functionalize CNTs in the current
scenarios[66].
5.5.1.4 Functionalization of CNTs with Polymers
Typically, thepolymer molecules are employed to improve the dispersion of CNTs
and to develop CNT-based compounds in order to investigate their novel properties. The main methods for modifying CNTs using polymer are covalent and noncovalent attachments [57]. However, the covalent attachment of polymeric molecules
through their grafting on the CNTs is the most popular andadaptable method.The
in-situ monomer polymerization, in which the monomer interacts with the elements
already present on the CNTs surface, has been used to attach the polymers to the
CNT’s surface in the presence of initiators [67]. In 2008, Ford and Qindeveloped
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a technique for synthesizing polymer/CNTs composites that allow polymers to be
covalently attached to CNTs [68]. The end-product composites can be easily dissolved in an aqueous medium and developstable colloidal dispersions without separating for longer time frames. CNTs that have been polymer functionalized can
also be disseminated into the parent polymer. CNTs have been functionalized, solubilized, and puried using this approach efciently and economically; however,
the stability of these dispersions is highly reliant on specic colloidal systems. Barrera etal. (2014)have suggested a three-step process in which functionalized CNTs
are rst employed to make polymer composites before being defunctionalized and
then reverting to their natural chemistry. In the rst step, functionalized CNTs are
dispersed in a solvent to obtain dispersion. In the second step, the obtaineddispersion is incorporated further into apolymer host matrix to produce a functionalized
CNTs-polymer composite. In the third step, the functionalized CNTs-polymer composite is modied with radiation, in which the alteration involves defunctionalizing
the functionalized CNTs using radiation chosen from the group composed of cosmic
radiation, heavy ions,protons, neutrons, and alpha particles [69].
The covalent functionalization of CNTs with polymers is primarily based on two
strategies: “grafting to” or “grafting from” approaches. The “grafting to” approach
can be established through coupling, nucleophilic addition, cycloaddition, and amidation reactions. At the same time, the “grafting from” approach was achieved by atom
transfer radical polymerization (ATRP), reversible addition–fragmentation chain
transfer (RAFT), and free radical polymerization reactions [70]. Zhang etal. (2017)
demonstrated the grafting to strategy for the functionalization of CNTs through a
nucleophilic addition reaction. They have presented the thiol-ene addition process
among low-density polyethylene (LDPE) with a vinyl terminal cap and MWCNTs
functionalized with trimethoxysilane at the moderate reaction environment. With
a high grafting degree of 18 weight percent, this approach produced an effective
reaction. It was reported that the end-grafted LDPE offered excellent compatibility
while promoting the homogeneous dispersion for the CNT-polymer matrix [71]. The
ATRP of polystyrene (PS) and poly(methylmethacrylate) (PMMA)from MWCNTs
was demonstrated by Baskaran et al. (2005). PMMA had a covalent anchorage of
70% by weight, whereas PS had a covalent anchorage of 18–34% by weight after
increasing the initiator amount. Therefore, it would seem that altering the initiator
content may be used to modify the polymer’s molecular weight [72].
Additionally, by derivatizing CNTs with a functional group that is an essential
component of the polymerization process, it is possible to develop composite materials
in which CNTs work chemically as a catalyst for polymer development. Although it
does not solve the issue of CNT dispersion, this technique ensures a great connection
between the matrix and CNTs since CNTs promote polymerization and the expansion
of polymer chains, making them more compatible with the host polymer [1].
5.5.2 non-coValenT FUncTionaliZaTion
On the surface of virgin CNTs, several tiny and big polymeric anticancer drugs
can be adsorbed through various non-covalent interactions. The hydrophobic and
π-πstacking interactions among thesemolecules and the surface of the CNTs are
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