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158 Sonali Loya and Swati Chandravanshi
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are employed in different tumour cells. Various types of cancer treatment have some advantages and also some disadvantages, affecting the quality of life of the patient. There
are many chemotherapy drugs that affect not only the cancer cells but also the healthy
cells, due to which there are some side effects such as vomiting and hair loss in patients
[8]. For the new era of cancer research, CDs could be a promising candidate for the diagnosis and drug delivery [9].
The properties of CDs are governed by quantum confinement and surface state
[13], which can also be changed by using different precursors or methods [15, 16]. CD
can be made in such a way that they can exhibit different functional groups like
amine, carbonyl, hydroxyl, carboxyl, and ether. Due to this versatile design capability,
we can obtain CDs of different size and surface functional groups. This helps us to
modulate the physical and chemical properties of CDs. These features of CDs make
them the promising materials in cancer treatment by using them for bioimaging to
drug delivery and also good agents for photodynamic therapy (PDT) [10] and photothermal therapy (PTT) [11].
8.2 Preparation of CDs
There are various methods to synthesize nanoparticles. All these methods are divided
into two basic approaches: top-down approach and bottom-up approach. Top-down
approach involves the exfoliation and cutting down of macroscopic carbon structures
to the carbon particle in a nanoscale range. Chemical exfoliation, laser ablation, and
ultrasonic-assisted treatment are some examples of this method. Bottom-up approach
involves the building up of a nanomaterial from bottom by combining atom to atom,
molecule to molecule, and cluster to cluster. Solvothermal method, pyrolysis, and
chemical vapour deposition (CVD) are few examples of this approach.
8.2.1 Top-down approach
8.2.1.1 Chemical exfoliation
Chemical exfoliation is a very si mple method to pro duce high-quality CDs in large
scale without using any complex technique. In this method, various precursors like
carbon nanotubes, graphene oxide (GO), and carbon fibre are cleaved to a desired
nanostructure size by using strong acids or oxidizing agents. From the past years,
many researchers have prepared different types of CDs by this method which has
been described as follows:
In 2007, Mao and co-workers prepared multicolour fluorescent CNPs from the
combustion soot of candles by means of oxidation using strong acids. This was then
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purified using polyacrylamide gel electrophoresis. The CNPs thus formed were small,
having size less than 2 nm and were soluble in H
O [17].
2
In 2011, Peng et al. synthesized GQDs by c hemical oxida tion and bre aking of
micro-meter-sized carbon fibres using H
and HNO3as in Figure 8.1a [18].
2SO4
In 2016, Zhao et al. prepared GQDs by chemical exfoliation of petroleum asphal-
tene by using mild oxidizing agents, that is, the mixture of concentrated HNO
. They were then neutralized by aqueous NH3. GQDs comprise two layers of gra-
H
2SO4
3
and
phene nanosheets, and their surface is composed of mainly O- and N-based functional
groups [19]. They showed strong FL properties.
In 2019, Gunjal et al. prepared CDs by chemical oxidation of wast e tea residue
using 0.1 M HNO
particles. It was then neutralized with Na
[20]. The solution is then cooled and centrifuged to remove large
3
. After a process of dialysis, a clear yel-
2CO3
low suspension of CDs was obtained.
In 2014, Sun et al. prepar ed fluorinated GQDs by chemical exfoliation of fluorinated graphene oxide with concentrated HNO
and concentrated H2SO4[21]. The solu-
3
tion was refluxed in the presence of microwave radia tion for 6 h. After cooling the
solution mild ultrasonication was performed. It was then treated with Sodium Carbonate. The solution was filtered. Here they obtained yellow solution, which after dialysis produces blue fluorescent GQD-F.
In 2017–2018, Soni et al. prepared N-, S-co-doped carbon quantum dots (CQDs) by
chemical oxidative cleavage of palm shell powder using triflic acid as shown in
Figure 8.1b [22]. The CQDs thus formed showed strong photoluminescence and good
dispersibility. It was also shown that their size could be changed by changing the
length of amino acid chains.
Kailasa and co-workers synthesized three (blue, green, and yellow) fluorescent
colour CDs by acidic (H
) oxidation of tomato [23]. CDs thus produced showed
2SO4
good water dispersibility and high quantum yield (QY).
In 2019, Desai et al. prepared Cucumis melo CDs by the acidic oxidation of muskmelon (C. melo) fruit [24]. The acids used were H
2SO4
and H3PO4.
In 2017, Nair et al. synthesized high-quality GQDs by the oxidative cleavage of GO
by using potassium permanganate in 30 min [25]. The QY of GQDs was up to 23.8%,
and also the product yield was high around 75–81%.
In 2015, Zhu et al. prepared GQDs by the oxidative cleavage of GO using hydroxyl
radicals. This was obtained by the decomposition of H
tungsten oxide nanowire (W
). This method does not produce any by-product [26].
18O49
in the presence of a catalyst
2O2
8.2.1.2 Laser ablation method
In laser ablation method, a part of the material is evaporated from the surface by
means of laser irradiation. The surface of the target material absorbs high-powered
laser beam incident on it which makes the temperature of the absorbing material to
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Figure 8.1: (a) Oxidative cutting of carbon fibre into GQDs [18] (Copyright 2012, American Chemical
Society) and (b) chemical exfoliation process of palm shell powder [22] (Copyright 2018, Elsevier).
increase rapidly. This makes the material of the surface to vaporize into laser plume.
Sometimes, the vaporized material condensates into cluster of particles, which are
then either deposited on the substrate or collected through the filter system. Various
attempts were made to synthesize CDs from laser ablation method, which is described
as follows:
In 2006, Sun et al. prepared CDs by laser ablation of a mixture of graphite powder
and cement in the presence of water vapour with argon as a carrier gas. A Q-switched
Nd-YAG laser was used for ablation, as shown in Figure 8.2a [27].
In 2016, Kang et al. prepared GQDs from multi-walled carbon nanotube by using
pulsed laser ablation (PLA) technique, as shown in Figure 8.2b [29].
In 2019, Ren et al. prepared N-doped micropore CQDs from sustainable and waste
Platanus biomass using PLA technique as shown in Figure 8.2c [28]. The QY was 32.4%.
In 2019–2020, Cui et al. prepared CQDs from low-cost carbon cloth by using dualbeam PLA system. In this process, a single laser beam was used, which was divided into
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two laser beams in order to reduce the laser ablation time, as shown in Figure 8.2d [30].
The QY of CQDs was 35.4%.
Figure 8.2: (a) Representation of carbon dots with PEG1500N species attached to the surface [27]
(Copyright 2006, American Chemical Society); (b) figure showing the exfoliation of multi-walled carbon
nanotube to GQDs (Scientific reports, Open access); (c) synthesis of N-doped micropore carbon quantum
dots from waste Platanus biomass; (MDPI open access); and (d) mechanism of DMSO-CQD domains
passivated by DMSO molecules [30] (Copyright 2020, Elsevier).
8.2.1.3 Ultrasonic–assisted treatment
Here, alternate high-pressure and low-pressure waves are created, resulting in the
formation and disruption of small bubbles in solution. From the cavitation of small
bubbles, a strong hydrodynamic shear force is generated which cuts the macroscopic
carbon materials into nanoscale CDs.
In 2011–2012, Zhuo et al. synthesized GQDs from graphene by ultrasonic method.
First, the graphene was oxidized using concentrated H
and concentrated HNO3,
2SO4
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which was then treated ultrasonically with an ultrasonic instrument. Th en the concentrated H
and HNO3were removed. After re-dispersion in H2Ofiltrationand
2SO4
dialysis, they obtained GQDs. Since then, bulk carbon materials such as GO, carbon
nanofibres, MWCNTs, and graphite were investigated as starting materials for the synthesis of GQDs using ultrasound in aqueous solution or organic solvents [31].
In 2014, Song et al. prepared high-quality GQDs from graphene intercalation compounds (GICs) with controlled oxidation [32]. First of all, potassium–sodium tar trate
was grinded and mixed with graphite which then reacted to the autoclave vessel for
24 h at 250 °C. The GICs thus formed are exfoliated in water under ultrasonic-assisted
method, resulting in the formation of GQDs.
Figure 8.3: (a) Formation of polymer-functionalized CQDs by ultrasonic-assisted treatment [33] (Copyright
2018, Elsevier). (b) Large-scale synthesis of G-dots from large food waste [34] (Copyright 2014, American
Chemical Society).
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In 2018, Huan g et al. synthesized high-quality methoxy polyethylene glycol
(PEG)-functionalized fluorescent CNPs using cigarette ash through a one-pot ultrasonic irradiation treatment using thiol group-terminated PEG as precursor as shown
in Figure 8.3a [33].
In 2013–2014, Park et al. prepared CNDs from waste food materials by simple ultrasonic irradiation treatment, as shown in Figure 8.3b [34].
8.2.2 Bottom-up approach
8.2.2.1 Microwave synthesis
This method involves the formation of nanoparticles by microwave irradiation of solution. Microwave irradiation has good penetration effect, which homogeneous ly
heats up the reaction solution. This results in uniform nucleation and rapid crystal
growth. This method has various advantages like cost-effectiveness, provide uniform
heat, and short time of reaction.
In 2012, Li et al. prepared stabilizer-free greenish yellow luminescent GQDs from
graphene oxide nanosheets under acid conditions through microwave-assisted treatment [35].
In 2017, Yao et al. synthesized fluorescent CQDs by the use of waste crab shell
using microwave-assisted approach as shown in Figure 8.4a [36]. They used transition
metal ions such as Gd
In 2016, Kumawat and co-workers prepared GQDs through microwav e-assisted
green sy nthesis route using Mangifera indica (mango) lea ves as a carbon source as
shown in Figure 8.4b [37]. The size of GQDs from 2 to 8 nm shows bright red luminescence. The obtained GQDs showed excellent biocompatibility and photostability, making
them suitable for magnetic resonance (MR) imaging and thermal sensing of live cells.
In 2014– 2015, Pires et al. synthesized CQDs using microwave-assisted technique
from an aqueous solution of raw cashew gum [38]. No passivation reagent was used.
In 2019, Ren et al. synthesized nitrogen-doped GQDs using sodium citrate and triethanol amine as raw materials through mi crowave-assisted approach [39]. No other
harsh chemical was used. The QY was 8%.
In 2019, Ricardo et al. prepared CDs from the aqueous solution of citric acid and
urea placed in a glass beaker through microwave-assisted treatment [40].
3+
,Mn2+, and Eu3+to incorporate into carbon matrix.
8.2.2.2 Hydrothermal method
In hydrothermal method, the materials are dissolved in water under high temperature and high pressure. The dissolved substance is then crystallized to obtain the desired end products. The process is usually performed in a steel pressure vessel also
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Figure 8.4: (a) Preparation of MFCQDs by a microwave-assisted hydrothermal method [36] (Copyright
2017, American Chemical Society). (b) Preparation of soluble CDs from mango leaves and their various
applications [37] (Copyright 2017, American Chemical Society).
known as autoclave, which may or may not be coated with protective Teflon coatings.
This process offers an advantage of low-cost, non-toxic, and simple approach.
In 2009, Pan et al. synthesized ultrafine GQDs from preoxidized graphene sheets
(GSs) by using hydrothermal method. The cutting of GSs involves the complete breaking of mixed epoxy chains present on the surface, which consists of less epoxy groups
and more carbonyl groups into CQDs [41].
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In 2016–2017, Z hao et al. prepared GQDs from GO using hydrothermal method
with assistance of KO
, as shown in Figure 8.5a [42]. The QY was 8.9%.
2
In 2018, Halder et al. synthesized GQDs from pre-synthesized GO as a precursor
and low amount of H
as an oxidant by simple hydrothermal method (Figure 8.5b)
2O2
[43]. The synthesized GQDs were uniformly small-sized of approximately 5 nm.
In 2013–2014, Mehta et al. synthesized water-dispersible fluorescent CDs from Sac-
charum officinarum juice (Figure 8.5c) [44].
In 2012, Lu et al. prepared fluorescent CNPs from wastes of pomelo peel as a carbon source through hydrothermal method, as shown in Figure 8.5d [45]. The prepared
CNPs were water soluble and have QY of 6.9%.
In 2013–2014, Liu et al. synthesized CQDs using bamboo leaves via green hydrothermal method. Branched polyethylenimine (BPEI)-capped CQDs were prep ared by
coating CQDs with BPEI by electrostatic adsorption, as shown in Figure 8.5d [46]. The
average size of CQDs prepared was 3.6 nm and the QY was 7.1%.
In 2017–2018, Essner et al. prepared a series of CDs from citric acid using hydrothermal and microwave r outes followed by dialysis or ultrafiltration purification
steps. They showed that impurities which are produced during CD synthesis should
be removed to get good results [47].
8.2.2.3 Chemical vapour deposition
CVD method is a well-known approach used to fabricate CQDs. In this method, the
carbon source is taken in gaseous phase, and the source of energy such as plasma or
resistively heated coil is used to transfer energy to a gaseous carbon molecule. In general, hydrocarbons such as methane and CO are made to flow through the quartz
tube placed in an oven at high temperature around 720 °C. At such high temperatures,
the hydrocarbons are broken down to produce pure carbon molecules, which then
diffuse towards the substrate that is heated and coated with a catalyst. Here, the carbon molecules bind with the substrate. There undergo some reactions producing
CNPs of desired size. The size of the final product could be determined by modulating
some parameters like source of carbon, flow rate, growth time, and temperature of
the substrate.
Fan et al. prepared CQDs by the CVD method. They used methane gas as a carbon
source. The copper foil was rinsed with HCl and alcohol to remove the oxidized surface. It
was then heated to 1,000 °C in the presence of H
(10 mL min−1)andargon(200mLmin−1)
2
for 40 min. Then, hydrogen was turned off and argon was kept for another 10 min to
−1
remove residual hydrogen. Methane (2 mL min
) was introduced in the reaction tube for
3 s. The synthesized CQDs had size in the range of 5–15 nm [48].
In 2015, Huang et al. prepared GQDs fabricated on silicon wafer by CVD [49]. This
method was simple, cost-effective, eco-friendly, and absence of chemical functional
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Figure 8.5: (a) Hydrothermal cutting procedure for GO with the assistance of KO2[42] (Copyright 2017,
Elsevier); (b) synthesis of GQDs from GO and the picture with and without 360 nm wavelength UV
excitation [43] (Copyright 2018, American Chemical Society); (c) synthesis of carbon dots from Saccharum
officinarum juice by hydrothermal method [44]; (Copyright 2014, Elsevier); (d) formation of CDs from
pomelo peel [45] (Copyright 2012, American Chemical Society); and (e) synthesis and application of CQDs
(Copyright 2014, Elsevier).
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groups. The prepared GQDs were single crystalline and highly pure with an average
thickness of 1.2 nm and an average diameter of 7.5 nm.
The N-GQDs were also prepared by using chitosan as a carbon source (Figure 8.6)
[50]. The synthesized N-GQDs had an average diameter of 12 nm and thickness of 3 nm.
Figure 8.6: Synthesis of N-GQDs by CVD [50] (Copyright 2018, American Chemical Society).
8.2.2.4 Pyrolysis
Pyrolysis is a ver y influential technique to for m fluorescent CDs using macroscopic
carbon structures as a starting material. This involves four main steps, that is, heating, dehydration, degradation, and carbonization, which act as important factors for
converting the organic carbon containing substance into CQDs under high temperature. The carbon precursors are broken down into CNPs by using highly concentrated
alkali or acid.
In 2011, Zhou et al. synthesized water-soluble fluorescent CDs from the lowtemperature carbonization and simple filtration of watermelon peel as a carbon source
as shown in Figure 8.7a [51]. The process involves two steps. First, the carbonization of
watermelon peel at 220 °C for 2 h in ambient air condition is followed by the ultrasonic
treatment for 30 min, filtration, and centrifugation.
In 2019, Praneerad et al. prepared CDs by the pyrolysis of durian peel waste [52].
The formed CDs had a QY of 11% and an average size of about 10 nm.
In 2013, Sun et al. synthesized sulphur- and nitrogen-co-doped CDs (S–N-C-dots) by
using sulphuric acid, carbonization, and etching of hair fibre, as shown in Figure 8.7b [53].
In 2013, Wee et al. prepared CDs through one-pot carbonization of bovine serum
albumin (BSA) protein. About 1 mL of BSA was mixed with 3 mL of concentrated H
2SO4
.
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