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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5932_Библиотеки_им_академика_М_И_Перельмана
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66 Carbon-Based Nanocarriers for Drug Delivery
FIGURE 3.2 Schematic Representation for the Synthesis of Graphene by (a) Liquid-Phase
Exfoliation (LPE) and (b) A Typical Experimental Setup of the Electrochemical Exfoliation
Process for the Synthesis of Graphene [31].
and the electrochemical exfoliation of graphite was tested at 10V for 10 min.This
technique successfully fabricated graphene nanosheetswith one to three layers and a
yield of roughly 60% [37]. According to the ndings of the aforementioned studies,
sulfuric acid is considered acompetent electrolyte for the electrochemical deposition
of graphene and exfoliation of graphite. The electrolyte process may be encouraged by
the sulfate ions’ size of 0.46 nm, which is comparable to the graphite interlayer spacing of 0.335 nm. Additionally, gases like SO2, O2, and H2 are released during the electrolysis of sulfate ions and co-intercalated water [38]. Graphite wasaked throughout
the complexation process due to the use of an acid electrolyte. However, in contrast to
what was anticipated, this technique produces multiple layers of graphene. Furthermore, Parvez etal. (2014) suggested a detailed mechanism for the electrochemical
exfoliation of graphite in an ammonium sulfate solution, as presented in Figure3.3.
In this process, the voltage output was adjusted to 10V, and graphite electrodes were
submerged in an electrolyte solution. During the electrolysis, water reduction at the
cathode results in the formation of hydroxyl (OH-) ions, which are effective nucleophiles. The corners and borders of the graphite grains are attacked by nucleophiles,
which furtherpromotes theoxidation processes and the physical adsorption of sulfate
ions in the graphite layer. The expansion and depolarization of the graphite layer are
driven by oxidation reactions. Throughout this reaction, water molecules co- intercalate
with SO
2-
ions. Gas molecules, namely, SO2 and O2, wereproduced as a result of SO
4
2-
4
ion reduction and oxidation ofwater molecules. The energy available to gas species
allows them to detach the graphite layer and develop multilayer graphene. The process
of exfoliating graphite is also inuenced by the concentration of electrolytesat the
applied voltage. To obtain 5 wt.% graphenes, the minimum electrolyte concentration
for the graphite electrolysis reaction was < 0.01M. It was suggested that the yield of
graphene-containing products might improveover 75 wt.%provided the concentration
was raised to a 0.01 to 1.0M limit. The overall mechanism of graphite exfoliation by
electrochemistry in inorganic salts is supported by this event [37,39].
Similarly, the hybrid method comprising sonication and an electrochemical oxidation process was also employed for the synthesis of graphene. Such combined
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FIGURE 3.3 Proposed Mechanism for the Electrochemical Exfoliation of Graphite in an
Ammonium Sulfate Solution. [Reprinted with permission from Parvez et al. (2014)] [39].
techniques can mitigate the requirement for rigorous operation stages, elevatedtemperatures, and excessive pressures in the synthesis method [31].
3.2.3 laser aBlaTion
A novel technique for synthesizing nanomaterials, particularly graphene, is laser
ablation. This approach offers a number of potential benets, such as environmental
friendliness, simple experimental modes, long-term stability of nanoparticles, the
absence of toxic synthesis reagents, and undesirable contaminants in nanoparticle
compositions [40,41]. Cappelli etal. (2015)produced graphene using laser ablation.
Their study was carried out utilizing a near-IR Nd: YAG laser (λ = 523 nm, frequency= 10 Hz, pulse width (τ)=7 ns, anddeposition period=15 min) on silicon
(Si) wafers with variablesurface temperatures (from ambient temperature to 900 °C)
[42]. The development process is then carried out under a range of congurations to
produce high-quality graphene [43].
Solid carbon sourceis essential inlaser ablation methodologies toenable the laser
source for carbon eradication and synthesize graphene [44,45]. Throughout the fabrication of graphene, a number of laser parameters need to be regulated [46]. The quality of the nal product may change depending on the way these parameters areset up
for the laser ablation device. The laser’s physical characteristics, such as its uence,
wavelength, frequency (repetition rate), and pulse duration, should be regulated as
the initial parameter. The next controllable variables arethe substrate’s temperature, distance, ambient pressure, and gas conditions. The product is also inuenced
by substrate choice. In a study employing the laser ablation technique, Koh etal.
(2012) examined the viability of different metals as a substrate for the production of
graphene. Nickel (Ni), cobalt (Co), copper (Cu),and iron (Fe)were the metals that
were examined. According to the ndings, graphene made from Ni, Co, Cu,and Fe
68 Carbon-Based Nanocarriers for Drug Delivery
frameworks does have a lattice constant of 0.357, 0.361, 0.352,0.251, and 0.287 nm,
correspondingly [45].
A high-grade bilayer graphene was produced utilizing a Ni/SiO2 substrate by
Hemani etal. (2013). This information was supported by Raman spectroscopy, which
demonstrated ndings that were 60% better than those obtained with other substrates
[44]. Pechlivani etal. (2017) evaluatedthe impact of pulses on diverse substrates. To
achieve the required wavelengths and pulse energies, ultra-short pulse lasers were
deployed. The outcomes of this investigation demonstrated that the development of
ultra-short pulse laser technology has the potential to promote micro-graphene as an
effective material in the manufacturing industry [47]. De Bonis etal. (2015)also used
ultra-short laser ablation, which resulted in exceptional graphene production [48].
Figure3.4 describes the general congurations of the laser ablation-based technique
for the synthesis of graphene, wherein direct laser contact with the carbon/graphite
solid causes it to lose some of its frameworks andyielding graphene [31].
3.2.4 chemical Vapor DeposiTion (cVD)
Chemical vapor deposition (CVD) is one of thebottom-up synthesis processes utilized on a larger scale to synthesize high-gradegraphene [49]. In this procedure, a
surface substrate and gas moleculesare combined inside a reactor vessel while the
reaction environment is controlled throughtemperature, gas ow rate, and pressure
[50]. Aconventional CVD instrument consists of a quartz reactor, a mass ow controller,thermocouples for temperature monitoring, a pump, gas distribution lines,
a power system, a vacuum system,and a computer for auto-control. For the CVD
process that produces graphene lms, a variety of substrates are employed, including
nickel (Ni), iron (Fe), andcopper (Cu). Typically, carbon sources include hydrocarbon gases likemethane (CH4) and acetylene (C2H2). The carbon source is stimulated
using two CVD techniques: thermal CVD and plasma-enhanced CVD (PECVD) [51].
To synthesize graphene, thermal CVD employs a vacuum tube, furnace, pressure
gauges, vacuum pump,and mass ow regulator to regulate the amount of hydrocarbon
FIGURE 3.4 A General Conguration of the Laser Ablation-Based Technique for the Synthesis of Graphene [31].
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and career gas. In PECVD, plasma prompts the gas source to break down before reacting
with the metal substrate to trigger the development of graphene laments [52]. Plasma has
been developed using many power sources, including radio frequency (RF), microwave,
and direct current (DC) [53]. The ability of graphene growth methodology to proceed at
low pressure and temperatures is a majorbenet of PECVD over thermal CVD [54]. High
temperature causes the decomposition of carbon sources into carbon and hydrogen atoms.
In this CVD technique, the undesirable compounds on the metallic surface of the
catalyst are eliminated and cleaned using H2and Argas as carrier gases. Graphene
has traditionally been grown via CVD on transitional metallic substrates like Cu and
Ni [55]. There are two primary phases in this growing process: 1. thermal decomposition of the gas reactant to produce carbon, and 2. exploitation of segmented carbon
on the surface of the metallic catalyst to produce the carbon framework of graphene
[56]. One method is to rst anneal nickel in an H2 environment at the required temperature of 900–1000 °C with grain size in order to produce graphene using polycrystalline nickel [5]. In this case, CH4 is employed as the carbon source, and the
substrate is subject to a combination of H2 and CH4. The carbon atom dissolves in
the Ni substrateduring the hydrocarbon’s breakdown andyields the solid solution.
Because Ni has the ability to dissolve at high temperatures, it may be solidied
and condensed in argon gas to yield a precipitate of Ni-C that can scratch graphene
(Figure 3.5) [5]. Ni is an excellent substrate toward the synthesis of graphene;
FIGURE 3.5 Growth Mechanism of Graphene on Cu/Ni Substrate Through Chemical
Vapor Deposition (CVD)(a) Graphene Growth by Dissolution-Precipitation Mechanism and
(b) Graphene Growth Direct by Deposition Mechanism. [Adapted with permission from
Mbayachi et al. (2021)] [5].
70 Carbon-Based Nanocarriers for Drug Delivery
however, the dimension and proportion of monolayer graphene might vary depending
on the purity of the Ni lm. The thickness and purity of graphene are inuenced by
the cooling rate, and the structure of Ni may also have an impact on the morphology
of obtained graphene[57].
To determine the relative signicance and relevance of the factors, Papon etal.
(2017)used a technique called “designs of experiments.” The interplay of a number of independent variables in the fabrication of graphene on a Cu substrate was
explained by this design. The outcome demonstrated a considerable effect of substrate temperature, duration, heating rate, and pre-annealing time on the nal purity
of the graphene produced. The time frame of the graphene development process and
the rates at which the temperature of the source of carbon is rising are the two major
factors that specically inuence the size of the graphene outcome [58]. It signies
that the researchers onlyneed to tweak these elements to affect the size substantially.
Additionally, Liu and Liu (2017) highlighted that strong control over the reaction
conditions could result in the production of large-area, high-grade graphene with
distinct structures and layers [59].
In general, the CVD technique continues to be one of the most effective ways
to produce signicant amounts of graphene. Comparing the CVD process to other
techniques like Scotch tape, thermal breakdown, reduction of GO, LPE, and even
otherbottom-up methods, the graphene obtained approximately 1.5 times better.
3.2.5 pyrolysis
The Greek pyro and lysis elements are where the word “pyrolysis” rst appeared.
Pyro denotes re, and lysis indicatesseparation. Few-layer graphene can besynthesized via a simplistic process of pyrolysisthat involves synthesizing carbon atoms on
a metal surface [60]. The thermal breakdown of silicon carbide (SiC)is one of the
frequently used methods of producing graphene.Si desorbs at elevatedtemperatures,
keeping C remaining to produce a few graphene layers. This method has signicantly
improved as a result of the ongoing mm-scale synthesisof graphene lms at a temperature of 750 °C on a thin nickel lm deposited on aSiC substrate [61]. The benet
of this technique is that graphene sheets are continuously produced all across the
whole SiC-coated surface. Unfortunately, large-scale production of graphene could
notbe possible using this process. At 1000 k, a similar system is used in the thermal
breakdown of ethylene. The ability to synthesize high-gradegraphene monolayers is
a benet of this synthesis technique [62].
3.2.6 arc Discharge
The arc discharge is indeed a comparatively economical and ecologically responsible technique for manufacturing graphene [63,64]. The arc discharge process may
synthesize graphene in the presence ofH2, He, or N2 [65]. To fabricate few-layered
graphene underneath a combination of He and carbon dioxide (CO
(2010) designed the arc discharge technique [66]. The outcome demonstrates that the
acquired graphene has fewer aws than graphene made using chemical processes. For
further applicability, the produced graphene may also be readily dispersed in organic
), Wu etal.
2
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solvents. Excellent graphene for constructing electrodes with different devices may
be produced under suitable He and CO2 environments within the arc discharge technique. The graphene made by this process also has the benet of being an excellent
option for an electrical charger utilized for conducting composites. Arepeatable and
sustainable aqueous arc discharge technique that yields excellent-grade dual and
triple-layer graphene was described by Kim etal. (2016) [67]. Meanwhile,it is necessary to design a purication technique since they discovered contaminants while
utilizing this approach.
Cheng etal. (2017)executed a studyon the exploration of the arc discharge technique to make graphene throughcombining a vacuum arc discharge with the CVD
process. Graphene is fabricated on a copper foil with the aid of a furnace operating
at an elevatedtemperature and a vacuum arc discharge. This fusion technique may
form a single sheet of graphene at high temperatures [68]. To fabricate graphene
nanosheets on a mass scale, Wu etal. (2016) revealed the mechanics of the arc discharge approach. In their study, an activated carbon (AC)was employed as the anode
and the cathode in the arc discharge process beneath themixed gases (H2 and N2).
Due to the concurrent reaction and evaporation caused by the alternating current
used throughout the process, sediments at the cathode are not formed. By doing this,
the temperature is raised, which is necessary to speed up the dispersion of carbon
atoms and clusters. As the diffusing rate increases, both thecarbon atoms and gas
molecules might interact with one another. Because H2gas hasan extremely rapid
cooling rate, graphene products may be easily developed. To achieve these circumstances and produce graphene with acceptable quality, they have blended H2gas with
an inert gas like N2, which possessesa lower thermal conductivity [69].
3.3 FUNCTIONALIZATION OF GRAPHENE
Despite the enormous potential for applications, it is imperative to perceive that the
graphene itself is hydrophobic and exhibits zero band gap, instability in aqueous
conditions, and inertness to reactions, which weaken its potential to compete in the
biomedical industry. This is among the factors contributing to the enormous rise
in research initiatives focused on the functionalization of graphene, encompassing
interactions between graphene (and its derivatives) and inorganic and organic molecules, chemical alteration of the large graphene surface, as well as a comprehensive description of numerous covalent and non-covalent interactions with graphene
[3,70,71]. Functionalization with oxygenated functional groups through the formation
of GO has multiplied the adaptability of graphene in drug delivery, cancer therapeutics, and other biomedical applications. Furthermore, the widening of the graphene
band gap induced by doping andintercalationmight aid in the development ofeffective nanoelectronics components. The graphene-based nanomaterials potentially provide a gateway to new domains of biotechnology unless they were biofunctionalized
with certainbiomolecules such as proteins, nucleic acids,enzymes, andpeptides [72].
In addition, due to its aptitude to quench a variety of chemical dyes, quantum dots
(QDs), and rapid DNA sequencing, graphene has recently been recognized as a viable element in the design of uorescence resonance energy transfer (FRET) biosensors [73]. Graphene can be functionalized via covalent and non-covalent interactions
72 Carbon-Based Nanocarriers for Drug Delivery
through numerous value-added functional entities, which further improves the surface characteristics of graphene; the details are provided subsequently.
3.3.1 coValenT FUncTionaliZaTion
The covalent functionalization of graphene has been the subject of several studies,
with the primary goals being to offer graphene with improved aqueous solubility,
ease of processing, reduced toxicity, and biocompatibility [3,12]. After molecules
are bonded covalently to graphene, the sp2 carbon atoms of the π-network undergo
rehybridization into an sp3 orientation. This results in a partial or complete breakdown of the π–πconjugation and impairments to the inherent chemical and physical
characteristics of graphene.
Graphene may be functionalized either at the basal surface or at the edges, albeit
it demands a variable amount of energy. As a result of their rehybridization to the
sp3 tetrahedral conguration, the dangling terminal bonds actually respond with
reduced energy barriers since it does not put excessive strain on the innermost carbon
elements. The covalent functionalization of graphene can occur in either two ways:
1. throughthe development of covalent cross-linking between reactive species like
dienophiles and free radicals and the C=C bonds of pure graphene; or 2. through
the development of covalent links between organic functionalities and the oxygen
groups of GO. The latter approach is most frequently used to attach solubilizing and
biologically active compounds to graphene. In fact, GO is a preferable contender for
biomedical application than pure graphene due to its oxygen-rich functional groups,
such as the carboxyl,epoxy, and hydroxylgroups, which offer improved dispersibility,
biocompatibility, and the potential for theirsubsequent surface modication [12,74].
Previously, numerous covalent functionalization strategies were adopted for the
surface modication of graphene through organic compounds. The functionalization of graphene with organic compounds has signicantly improved its dispersibility and, ultimately, colloidal stability as well as biocompatibility. The cycloaddition
reaction, free radical addition, and nucleophilic addition reactions drove organic
group functionalization.
3.3.1.1 Cycloaddition
The cycloaddition can be distinctively carried away through zwitterionic intermediate, the 1,3-dipolar cycloaddition of azomethine ylide, Diels–Alder cycloaddition, and
other organic intermediates such as nitrene, carbene, and aryne. The combination of
4-dimethylamino pyridine with an acetylene dicarboxylate produces a zwitterionic
intermediate, which reacts with an acetylene dicarboxylate to yield a ve-membered
ring. According tothe substituted functional groups, the functionalized nanoparticles of graphenecan be dispersed in organic solvents like DMF, CHCl3, or water [75].
Nitrenes are active compounds formed only after the photochemical or thermal ablation of an N2 molecule from the organic azides. They effortlessly combine with the graphene C=C double bonds to establish three-membered aziridine
rings that link the organic azide component to the surface ofgraphene nanosheets.
Graphene nanosheets are ultimately adorned with aromatic compounds [76],
polymers [77], or aliphatic chains that may then be supplemented with functional
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groups like carboxyl or hydroxyl groups depending upon the organic component [74,78]. These functional units may help to post-functionalize the obtained
graphene. For instance, carboxyl groups preferentially absorb gold nanoparticles
(NPs)distributed in a solution of carboxy-alkyl aziridine functionalized graphene,
enabling the gold NPstobe immobilized on the functionalized graphene. Graphene
that has undergone chemical modication is simple to dissolve in organic solvents.
Asignicant expansion in the sp3/sp2 ratio of graphene’s carbon atoms occurs concurrently with the development of aziridine rings, and this increase is seen as a rise
in the ID/IG ratioof functionalized graphene. When graphene combines with a surplus of the alkyl azides by a factor of 10,the ID/IG ratio is demonstrated to enhance
even more. This nding demonstrates a clear correlation across the reagent ratio
and the extent of graphene’s surface modication. In other terms, the proportion of
the reagents might inuence the extent of graphene functionalization [78]. Nitrene
addition might potentially be employed in polymer grafting on graphene nanosheets
[76]. Numerous azide groups must be present in the polymeric chains of these polymers. By adding nitrene, it is possible to covalently attach polyacetylene containing
alkyne azide groups on its side chain onto a surface ofgraphene. Because of the
chemical afnities of the obtained polymeric matrix, the functionalized polyacetylene exhibit improved dispersibility in conventional organic solvents. Previously,
asimilar method was employed to functionalize graphene with phenylalanine. The
reaction occurs once N-protected azido phenylalanine is combined with exfoliating
graphene nanosheets distributed in o- dichlorobenzene (Figure3.6) [74,79].
FIGURE 3.6 Formation of a Polyacetylene/Graphene Composite through an Aziridine Ring
Linker. [Reproduced with permission from V. Georgakilas (2014)] [74].
74 Carbon-Based Nanocarriers for Drug Delivery
Similar to nitrene, carbenes are extremely reactive organic precursors with low
electron density, which can target C-H bonds with sp3 carbon atoms in place of
hydrogen or C=C bonds in a [1 + 2] cycloaddition process. Due to the abundance of
C=C and C-H bonds at the edges and defective sites of graphene, the interactions
of graphene with carbenes results in the surface modication of graphene via both
possible mechanisms. Despite sufcient information from the early functionalization
of CNTs,and fullerenes employing carbene derivatives, the interaction ofgraphene
and carbene is still not fully utilized [74]. In the [1 + 2] reactions, dichlorocarbene
synthesized from chloroform that has been treated with sodium hydroxide (NaOH)is
introduced to graphene nanoplatelets to formulate three-memberedrings [3,74].
Ismaili et al. (2011) demonstrated the encapsulation of gold NPs on the surface ofgraphene through organic linkers, which resulted in a muchmore complex
graphene functionalization via carbene [80]. Carbene was synthesized by photochemically treating a 3-aryl-3(triuoromethyl)-diazirine derivative that had gold
NPs coupled at the side of the molecule by an Au-S interaction. Carbenes were
also formed through the breakdown of diazirines and the separation of nitrogen
atoms as N2, which may be accomplished via heating or incinerating diazirine compounds. The benet of not offering potential intramolecular recombination routes for
the associated carbenes, which results in by-products andlowers the productivity of
a carbene addition process, makes 3-Aryl-3(triuoromethyl)-diazirine compounds
a common choice for carbene synthesis. In this case, gold NPsencapsulated with
alkane thiol chains were partially functionalized by 3-aryl-3-(triuoromethyl)-diazirine molecules via a thiol-alkyloxy linker in which thiol is linked to gold and
oxygen onto the aryl ring [3].
As previously mentioned, arynes are another group of very reactive organic
intermediates that are synthesized from phenyl derivatives by removing two ortho
heteroatoms. As a result of their reactivity, arynes may easily undergo [2 + 2] or
[4 + 2] cycloadditions with dienes or C=C bonds. Zhong et al. (2010) employed
2-(trimethylsilyl) aryl triate as a precursorfor the synthesis of the aryne intermediate under the novel framework in which graphene nanosheets were functionalized by aryne cycloaddition. The dissolution rate of the functionalized graphene
was signicantly improved in an organic and polar solvent like DMF, o-DCB
(1,2-dichlorobenzene), ethanol, chloroform, and water whilesucceeding the interaction of the modied arene to graphene by a four-membered ring. Similarly, various
distinct functional groups cansupersede thearenes [81].
3.3.1.2 Free radical addition
Free radicals are extremely reactive organic mediators that engage sp2 carbon atoms
and establish covalent bonds. They are often synthesized from organic compounds
by meticulously removing an easily leaving component that dissociates a covalent
bonding. Acommon method for producing free radicals is to heat the diazonium salt
of an organic molecule. Meanwhile, the radicals are formed by eliminating an N2
molecule. The hybridization of the reacting carbon atoms shifts from sp2 to sp3 when
an organic radical is introduced to the surface ofgraphene. Such alteration disrupts
the aromatic structure, resulting in a signicant impact on the electrical characteristics of graphene [74]. According to Tour et al. (2010),the graphene conductivity
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decreases as a consequence of the time of the radical addition reaction, which maybe
regulated effectively [82].
In contrast, functionalizing graphene results in the introduction of a band gap that
can be specied, enabling graphene withremarkable semiconducting characteristics
[83]. This process has also been used to place aryl diazonium salts on graphene
surfaces, demonstrating itssignicantadaptability. These functional groups include
carboxy, chlorine, bromine, nitro,iodine,and cyano. Herein, the thermallyor chemically reduced graphene that has been made into nanostructures is the primary
component. Asurfactant is then employed to make the reduced graphene disperse.
The functionalized graphene compounds could be dissolved in polar aprotic solvents
[84]. Numerous types of graphene, including epitaxial graphene [85] and graphene
obtained through mechanical cleavage [86], have indeed been subjected to diazonium salt interactions.
To examine the antibacterial properties of the synthesized graphene compound,
chlorophenyl groups have also been introduced to graphene nanosheetsby the diazonium salt reactions. The extensive bactericidal potential of chlorine is a signicantcharacteristic of chlorophenyl-functionalized graphene [86]. Sun etal. (2010)
attempted to effectively functionalize theedges of graphene nanosheetswhile keeping the graphitic surface unaltered and furtheremployed the diazonium salt reaction
to accomplish this goal. It wasnot feasible to regulate the addition of free radicals to
the reacting graphene region upon full exfoliation of the monolayers since the edges,
and the remained surfaceare both equallyexposed to the radical species. This is not
the case for expanded graphite, in which the edges are fully exposed, whereas the
major graphene surface is shielded against large 4-bromophenyl radicals with a relatively tiny space between graphene nanosheets. As a result, the edges of the graphene
nanosheets were efciently functionalized with bromophenyl groups due tothe interaction between the 4-bromophenyl diazonium salt and thermally expanded graphite.
Following the process, the edge-functionalized graphene nanosheets dispersedreadily in DMF [87].
The incorporation of free radicals is also evident in polymer grafting on graphene
processes. Anumber of well-known free radical polymerization techniques, including
atom transfer radical polymerization (ATRP) and reversible addition- fragmentation
chain transfer (RAFT), have indeed been implemented to formulate polymer nanocomposites comprising polymericchains deposited on the surface of graphene [3,88].
Previously, a polystyrene-polyacrylamide copolymer was grafted onto the graphene
sheet via in-situ free radical polymerization of the monomers in the vicinity of distributed graphene nanosheets. Through adjusting the monomer ratio, the amphiphilic
characteristics of the graphene/copolymer composite may be modulated. Since the
acrylamide monomer is hydrophilic as well as the styrene monomer is organophilic,
they may both be dispersed in water and xylene, respectively [88].
3.3.2 non-coValenT FUncTionaliZaTion
Non-covalent functionalization is a potent technique frequentlyused for inducing
desirable characteristics in graphene nanosheetswhile preventing desired loss of its
parent properties. To establish non-covalent bonding, graphene delivers accessible
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