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96 Carbon-Based Nanocarriers for Drug Delivery
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97Graphene-Based Nanocarriers as Drug Delivery System
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Drug Delivery with
4
Graphene Oxide-Based
Nanocarriers
4.1 AN OVERVIEW OF GO-BASED NANOCARRIERS
Carbon-based nanomaterials were extensively used in a variety of engineering elds
owing to their large surface area and affordable production cost. Graphite, a member
of the carbon allotropes, is a soft, elastic, and commonly accessible pure version of
carbon. Each carbon atom in graphite is covalently bonded to three adjacent carbon
atoms to produce the hexagonal layer. Graphene is designated as asingle layer of carbon atoms of graphite. Graphene has been widely employed for supercapacitors [1,2],
catalysis [3], hydrogen storage materials [2,4,5], battery electrodes [1], and biosensors
[2] ever since its discovery because of its remarkable physicochemical characteristics, including electrical, thermal, optical, and mechanical characteristics. Precisely,
the higher surface area and the optical properties of graphene nanomaterials have
attracted great interest in biomedical applications such as biosensing and drugs and
gene delivery [6,7]. However, the poor water solubility of graphene due to the π-π
staking limits its utilization for biomedical applications. Fortunately, the oxidative
and hydrophilic derivatives of hydrophobic graphene, such as graphene oxide (GO)
and reduced graphene oxide(rGO), are enriched in epoxy, carboxyl, and hydroxyl
functional entities, which provide them better stability and dispersibility in water [6].
Graphene, along with its derivative like graphene oxide, is currently being extensively researched, not only because of its fundamental physicochemical characteristics but also because of its agitating potential of applications in varied arenas of
biomedical engineering such as drug delivery, biosensing, cancer therapeutics, and
tissue engineering [8]. The modulation of defects in graphene-based nanomaterials is
directly relevant to a variety of applications. In the family of graphene-derived materials, GO and rGO are considered generic nanomaterials [9–11]. The oxidation of
graphene yields graphene oxide (GO), which has a variety of physicochemicalcharacteristics. It is a 2D single atomic thick honeycomb-like structure that is indeed
ahydrophilic derivative of graphene [12,13]. Other characteristics of GO, such as
controllable shape and biocompatibility, make it suitable for biomedical applications
[14–16]. It can be used for bioimaging and biosensing as it exhibits uorescence in
the visible and infrared regions of the EM spectrum and Raman signals in the D,
G, and 2D regions [14]. As illustrated in Figure4.1 [17], the oxygenatedfunctional
entities are located at the trailing edge and basal planes of the GO nanosheets. In
addition to the attributes listed previously, GO may be incorporated with polymers
and other additives such as polyacrylic acid (PAA), polyethylene glycol (PEG), folic
98 DOI: 10.1201/9781003358114-4
99Drug Delivery with Graphene Oxide-Based Nanocarriers
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acid, and chitosan to improve their biocompatibility, loading capacity, targetability
as well as structural properties.
Though the basic structure of rGO and GO are identical to that of graphene, they
also encompass oxygen-enrichedfunctional regions in lowerand higher quantities,
correspondingly [12,18]. The GO is made up of single-layer nanostructures that are
loaded with oxygen-richfunctional groups, offering itexcellent hydrophilicity. For
the preparation of GO, a conventional Hummers methodologyand its variants are
commonly implemented. The GO preparation method involves the use of strong
acids and oxidants that incorporate oxygenated functional entities into the GO.
Nonetheless, GO has been prepared using a variety of methodologies, the earliest
of which was described by Brodie (1859) [19] and was pursued by Staudenmaier
(1898) [20], Hummers (1958) [21], Tour (2010) [22], Sun (2013) [23] and Peng (2015)
[24]. The reduced degree of oxidation, challenging reaction environments, harmful
gases releases such as NO2, N2O4, or ClO2, the need for purication processes, and
the high production cost are some of the drawbacks of these methods. Nonetheless,
the Hummers approach and its modications have alleviated many of the constraints
to a certain degree. The specics are presented in the succeeding section. The better water solubility of GO is attributed to the existence of hydroxyl,carboxyl, and
epoxy functional groups; nonetheless, these are insufcient for biomedical applications without surface modications. Importantly, these oxygen-containing functional
units give a number of active sites for doping the elements or grafting additional
functional entitiesto improve the surface characteristics of GO while retaining its
fundamental features. [12,25]. In addition, the aqueous stability of GO is also a foremost issue for its application in drug delivery. GO has an aggregation tendency in
FIGURE 4.1 Chemical Structure of GO. [Replicated with permission from Song et al.
(2014)] [17].
100 Carbon-Based Nanocarriers for Drug Delivery
the physiological solutions with proteins and salt due to the nonspecic binding and
electrostatic interactions, which produces a hindrance to the development of biological probes [26]. The precise functionalization of GO boosts its solubility under
physiological circumstances and optimizes it for biological applications. In addition,
its unrivaled properties, such as higher surface area, have made GOone of the most
popular and extensively researched nanomaterials for environmental remediation
and drug delivery-relatedapplications [27–29]. Meanwhile, another graphene derivative, rGO, has been employed for diverse real-world applications, including drug
delivery, and can be simply produced by reducing the oxygenated function groups
of GO. The reduction mechanism can be accomplished using thermal and chemical
processes, mostly for stimuli-responsive functionalization [8,30]. Previously, GO and
rGO were extensively explored for energy, environmental, and drug delivery-related
applications, including cancer therapeutics, neurodegenerative diseases, gene delivery, and tissue engineering [27,31–33].
The present chapter offers substantial evidence associated with the synthesis of
pristine GO and rGO nanocarriers. It also debates their advantages, process features,
and inadequacies besides their chronological developments. This chapter elucidates
the signicance, physicochemical properties, and benets over the utilization of GO
in drug delivery-related applications. Various covalent and non-covalent functionalization approaches are also explored critically in considering the inadequacies of
GO in terms of aqueous stability and aggregation tendency, as well as biocompatibility for its application in the biomedical sector. Furthermore, the recent advances
in the structural, functional, and morphological modications of GO for enhancing
its therapeutic efcacy in cancer and other inrmities therapy are described in detail.
In addition, recommendations are made to minimize the shortcoming of GO-based
nanocarriers for their synthesis functionalization and to improve the adaptability of
GO in modern therapeutics. Ultimately, the outcomes of application-driven research
on GO-based nanocarriers related to cancer treatment, gene therapy, neurodegenerative diseases, and tissue engineering are reviewed to offer exhaustive knowledge
about its efciency toward drug delivery-related applications.
4.2 PREPARATION OF GRAPHENE OXIDE (GO)
Since GO is indeed a quasi-molecule, it developed in asynthetic product. Graphite,
the primary material, comprises many planes of hexagonal honeycomb-like structure. Exfoliation, on the other hand, can synthesize graphene asa single sheet of
graphite. GO is formed when pristine graphene sheets are oxidized. As a result,
GOis described as a graphene sheet or layer adorned by oxygenatedfunctionalities.
During the oxidation of graphite, the oxidizing agent such as KMnO4 reacts with the
carbon skeleton of graphite and further weakens the van der Waals force within the
stacked sheets of graphite while increasing the interlayer spacings, subsequently [34].
The extent and the degree of oxidation rmly affect an elemental composition
GO. The graphene sheets are completely exfoliated from graphite, hence depicting
the absence (0 at. wt. %) of oxygen (O). Meanwhile, in the case of GO, the highest O
content cannot exceed 50 at. wt.% as a result of sp
The atomic weight % of oxygen in the GO and hence its solubility is also dependent
2
hybridization of the carbon atom.
101Drug Delivery with Graphene Oxide-Based Nanocarriers
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on the temperature conditions. GO with higher solubility in physiological conditions
is generally recommended for biomedical applications [6,35]. GOwas found to have
an oxygen contentof about 10 to 50 % atomic weight % when restricted at temperatures of around 50 to 70 oC, with higher solubility and lower reduction at 35–50 oC.
Conversely, reduced graphene oxide (rGO) might be found to include up to 10%
atomic weight of oxygen (O), which can be accomplished by thermal or chemical
reduction processes [36]. The following section provides a comprehensive description of the processes for synthesizing GOand rGO.
4.2.1 synThesis oF prisTine go
Typically, two processes, dry media and wet medium, are used to synthesize
graphene oxide. During thedry media synthesis, graphene nanosheets are oxidized
in a vacuum chamber with atomic oxygen. This method of fabrication is expensive
since it necessitates the use of graphene sheets as a carbon source, an ultra-high vacuum condition, and molecular oxygen. [20,37]. On the other hand, the wet medium
technique is a less expensive option since it employs natural and easily available
synthetic graphite as a precursor to graphene/carbon and does not need exhaustive
experimentation.
The key chemical routes for the synthesis of GO are shown in Figure4.2 [27].
The rst method employs graphite for exfoliation to produce graphene sheets, which
are subsequently followed by the oxidation process. Furthermore, the oxidation process in the subsequent technique is driven by acoustic exfoliation in an aqueous phase
[38,39]. The last and third strategy is currently a well-known and commonly utilized
technology for producing GO. In this scenario, graphite is oxidized using powerful
oxidizing chemicals and exfoliated in an acidic media. The well-known Hummers,
Brodie, and Staudenmaier approaches have also applied this methodology.
FIGURE 4.2 Schematics for the Synthesis of GO.
102 Carbon-Based Nanocarriers for Drug Delivery
Although all three methods contribute to the synthesis of GO, the functional and
physicochemical characteristics of each method differ, including the degree of oxidation, composition, solubility, structure, reactive sites, and water solubility.
Over the past century, a variety of methods for synthesizing GO have been investigated. The three main ways are Brodie [19], Staudenmaier [20], and Hummers [21];
each of these approaches is a replacement for the earlier method. However, Sun etal.
(2013) [23] and Peng etal. (2015) [24] improved Hummers’s approach by utilizing
free water oxidation techniques to improve the quality and yield of GO, which is
currently the main method utilized for GO synthesis. Amodied version of each
of these essential GO synthesis methods was developed by Tour et al. (2010). The
details of these methods are as follows.
Brodie Method: The rst such description of a water-soluble GOwas made by
Brodie in 1859. His investigation, as was customary at thatperiod, was used
to determine the weight of graphite. Anumber of chemical reaction experiments were undertaken to highlight the physicochemical characteristics of
thisunique material. Here, fumic HNO3 was used to oxidize the graphite
after mixing it with KClO3. The oxidation that was taking place was monitored for any noticeable changes. The blend of GOthat was produced has an
elemental makeup of carbon (60%), hydrogen (2%), with oxygen(38%) [19].
Staudenmaier Method: Staudenmaier used sulfuric acid to further modify
Brodie’s process, which was employed in 1898 to oxidize graphite. Sulfuric
acid and a number of serial dilutions of KClO3 were added to the solution
well before thereaction to make the solution more acidic. However, much
as in Brodie’s reaction, explosive ClO2 gas was produced throughout the
reaction and contributed to the explosions since it decomposed quickly in
the air. Staudenmaier’s modications, on the other hand, were effective in
producing a highly oxidized variant of graphite [20].
Hummers Method: Hummers and Offeman (1958) had established an alter-
nate method for the oxidation of graphite to produce GO bytaking into
account the sluggish and risky reaction conditions of Staudenmaier’s
method. A water-soluble brownish-gray paste of GO was produced by
combining sodium nitrate (NaNO3), potassium permanganate (KMnO4),
and concentrated H2SO4 in different stoichiometric ratios and mixing with graphite. A yellowish-brown mixture was produced when the
reaction was subsequently suspended with water, and contaminants like
manganese were removed using hydrogen peroxide (H2O2). Eventual ly,
the combination underwent further ltration and water washing. Although
the degree of oxidation in GOattained by Hummers techniques is comparable to Staudenmaier, there is a signicant decrease in reaction time
under secured reaction circumstances. The time-consuming separating
procedure was the main aw in the Hummers approach [21]. The further
improvements to Hummers’s technique aremostly concentrated on overreaction time, quality, yield,and quantity of GOwith an improveddegree
of oxidation. As a modied Hummers method, several variants and optimization techniques are established over GO synthesis.
103Drug Delivery with Graphene Oxide-Based Nanocarriers
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To ur M et hod : By substituting sodium nitrate with phosphoric acid and intro-
ducing excess KMnO4, Marcano etal. (2010) established a modied version of the Hummers technique to increase the oxidation and reduce the
production of harmful gases like NO2, N2O4, or ClO2. It was asserted that
phosphoric acid offers graphite more integral basal planes. The GO that
was produced exhibits improved hydrophilicity, oxidation level, and reaction efciency [22]. Figure4.3 depicts a quick comparison of conventional
procedures with Hummers and its modied versions.
Due to the inertness of the inorganic carbon found in graphite, expanding graphite and
dispersing it in a solvent upon oxidation both necessitate strong protic or warm acids [36].
The Hummers method is augmented by the free water oxidation techniques,
which benet from the robust interaction among expanded graphite and oxidizing
reagents. The free water oxidation-based modied Hummers technique was recently
developed by Sun etal. (2013) and Peng etal. (2015). Theseapproachesovercome
the limitations of the conventional Hummers method, including producing poisonous
gases and using toxic chemicals.
Sun Me thod: Sun and Fugetsu (2013) recently unveiled the rst-ever environ-
mentally friendly strategy and more straightforward technology for synthesizing GO. Sulfuric acid was utilized as an acid media, while potassium
permanganate served as both an intercalator and an oxidizer. It was suggested that the volumetric proliferation of graphitic layers was caused by
the complexation of potassium permanganate, which increased the degree
of oxidation into the layers. They performed the oxidation reaction using
graphite: H2SO4 weight ratio of 1:20 and eliminated all other chemicals
from the GO reaction mechanism [23].
FIGURE 4.3 A Quick Comparison between Hummers’s Technique and Its Modied Versions.
[Replicated with permission from Marcano et al. (2010)] [22].
104 Carbon-Based Nanocarriers for Drug Delivery
Peng Me thod: Peng etal. (2015)have succeeded in synthesizing a substan-
tially water-soluble GOwith a higher degree of oxidation. They presented
a sustainable and scalable mannerGO synthesis technique that relied on
sulfuric acid and potassium ferrate (K2FeO4) as an oxidant. The proposed
approach was successful in preventing the reaction’s generation of hazardous gases and heavy metals. Here, sulfuric acid was used to make a suspension of graphite and potassium ferrate, which was then agitated at room
temperature for roughly an hour. Additionally, the combination underwent
centrifugation and water washing to produce a pure form of GO as the
end product [24]. The specics of the parameters and the features of the
techniques that have been proposed for the synthesis of GO are outlined in
Table4.1 [19–24,40].
4.2.2 synThesis oF reDUceD go (rgo)
Graphene, being a robust nanomaterial, presents several prospects for itsreal-world
applications like drug delivery, gene therapy, biosensors, and bioimaging due to its
structural and optical characteristics. Unfortunately, large-scale graphene production is a costly as well as time-consuming approach. Numerous initiatives have been
taken to eradicate the oxygen functional sites within GOin order to produce a substance with characteristics analogous to graphene [41].GO can be reduced via chemical, thermal,and electrochemical treatments. Nevertheless, the rGO produced by
each of the aforementioned methods exhibits distinct surface features, structure, and
additional optical and electrical attributes [39,41,42]. The important variables to be
considered for GO reduction are the following:
• The elemental composition of obtained rGO (C/O atomic ratio).
• Precision toward the elimination of oxygen group and reduction of oxygenated functional entities.
• Mitigation of surface defects of GO.
• Employing environmentally friendly reducing agents.
• Maintaining and enhancing the desirable chemical and physical attributes of
the parental GO.
Apart from oxygen functionalities, GO comprisesa persistent sp2 carbon structurethat
could be used for C-C bonding and is augmented by the chemical reduction of GO.
At elevated temperatures, the thermal reduction of GOis reliant upon the breakdown of oxygen-containing groups. Following the thermal reduction of GO, CO and
CO2 gases developed, resulting in nanosheet exfoliation. Other strategies for thermal reduction of GO include increased temperature annealing in an inert environment, microwave heating, and high energy light-assisted ash reduction of GO [43].
Perhaps the chemical reduction of GO is currently the simplest and most widely
used method for producing reduced graphene oxide (rGO). Chemicals like amino
acids,hydrazine, pyrrole, hydroxylamine, hydroquinone, hydrohalic acid, and metal
hydrides have been extensively used as reducing agents.
TABLE 4.1
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Specics of Process Conditions and Attributes of Various Methods for
GO Synthesis [19–24,40].
Method Reagents
Brodie KClO
, HNO360 oC, 3-4 h The very rst meth-
3
Staudenmaier KClO3, HNO3,
H2SO
4
Hummers H
Tour H
Sun H
Peng H
Panwar H
, NaNO3,
2SO4
KMnO
4
, NaNO3,
3PO4
KMnO
4
, KMnO4Room
2SO4
, K2FeO4Room
2SO4
, H3PO4,
2SO4
HNO3, KMnO
Reaction
temperature
and time Characteristics
odology for GO
synthesis
Evolution of toxic gas
ClO
Room
Temperature,
96 h
2
Successfully enhanced
the graphite oxidation
Evolution of toxic gas
ClO
2
35oC, 20 h Safe reaction
conditions
Reduced reaction time
35-40 oC, 12h Restraining the
discharge of toxic
gases like NO2, N2O4,
or ClO
2
A higher dose of
KMnO
4
Free water oxidation
Temperature,
2 h
method
Safe and efcient in
scalable applications
Free water oxidation
Temperature,
1 h
Heavy metals and
hazardous gases were
prevented
50oC, 3 h High yield - [40]
4
GO
thickness
(nm) Reference
- [19]
- [20]
- [21]
1.1 [22]
1.2 [23]
0.9 [24]
105Drug Delivery with Graphene Oxide-Based Nanocarriers
Shin etal. (2009) employed different amounts of sodium borohydride (NaBH4)
to chemically reduce GO. The rGOproduced upon reduction was further examined
using XRD; the 2ϴ was found at 23.98o, conrming a substantial reduction of GO
[44]. Stankovich etal. (2007) employed hydrazine hydrate as a reducing agent to
reduce a homogeneous dispersion of exfoliated GO. The resulting rGO exhibited a
BET surface area of 466m2/g, and elemental compositionrevealed a considerable
increase in the C/O ratio of rGO (10.2) upon GO reduction (2.7) [39]. Similarly, various reducing agents and thermal treatments were applied for the reduction of GO and
have been successfully used for drug delivery-related applications [45–47].
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