synthesis, property, characterization and application.

1. ADAMA SCIENCE AND TECHNOLOGY UNIVERSITY
DEPARTMENT OF APPLIED CHEMISTRY
MATERIALS CHEMISTRY PhD PROGRAM
SEMINAR – I PRESENTATION ON
CARBON QUANTUM DOTS
SYNTHESIS, PROPERTIES AND APPLICATION
ENDALE KEBEDE FEYIE
MONDAY JULY 1, 2019

2. 2
Fig. One-pot synthesis and purification route for CDs with distinct fluorescence characteristics. (B) Eight CD
samples under 365 nm UV light. (C) Corresponding PL emission spectra of the eight samples, with maxima
at 440, 458, 517, 553, 566, 580, 594, and 625 nm.

3. CONTENT
S
Introduction
Structure
Properties
Synthesis
Characterization
Application
Summary and outlook 3

4. 1. INTRODUCTION
What are carbon quantum dots?
 Novel class of carbon nanomaterials with prominent fluorescence and are composed
of discrete, quasi-spherical carbogenic material with sizes less than 10 nm. (H. Yu et al.
2016)
 Are known as carbon dots(CDs), carbon nanodots(CNDs), carbon quantum dots
(CQDs).
Discovery
• Xu et al., 2004; synthesis of SWCNTs from graphite
Composition and structure
• Their core is mostly carbon
• Contain 5-50 wt.% oxygen and other elements (H, N, S, P, …)
• Their morphology is mostly quasi-spherical, and the structure can be graphitic or
amorphous (K. Hola et al. 2014, L. Bao et al. 2011) 4

5. WHAT IS UNIQUE ABOUT CARBON QUANTUM
DOTS?
• exhibit some distinctive properties due to occurrence of
the quantum confinement and edge effect.
• tunable and stable photoluminescence (PL)
• Upconverted photoluminescence (UPCL)
• Biocompatible, inert, non toxic
• Tunable surface functionalization
• high resistance to photobleaching, photoblinking
• marked electron donating and accepting capabilities
• Excellent water solubility
• low cost and ease of synthesis
 Can replace semiconductor quantum dots and dyes in
different areas of applications ranging from sensing to
optoelectronics. (X. Wang et al. 2009, W. Kwon et al. 2014)
5

6. WHY CARBON QUANTUM
DOTS?
• As a group of newly emerging fluorescent nanomaterials, CQDs have shown
tremendous potential as versatile nanomaterials for a wide range of applications,
including (R. Q. Ye et al. 2013)
• chemical sensing, biosensing,
• bioimaging,
• drug delivery,
• photodynamic therapy, photothermal therapy
• Photocatalysis, electrocatalysis
• Optoelectronics
• Solar cells
• Light emitting devices
• Capacitors, …. 6

7. 2. STRUCTURE
• CQDs are commonly described in terms of a carbogenic
core with surface shell. (X. Li et al. 2015)
• Core structure: Mostly carbon
• Could be amorphous or crystalline/sp2 or sp3 carbon
• Graphitic, graphene like, amorphous
• surface shell
• envelops the core
• Results from surface functionalization and passivation
• hosts a variety of functional groups
• Hydroxyl, Carboxyl, Carboxylate, Amine, and Amide
• complex molecules: polyethylene glycol (PEG), or
polyethyleneimine
7

8. STRUCTURE:
DEFECTS
• Core and surface structures of CDs are quite
synthesis-dependent
• Defects are created during synthesis through
surface functionalization and doping
• Density of defects is related to synthesis method
and precursors used
• may serve as capture centres for excitons, thus
giving rise to surface-state-related PL
• Can be revealed by characterization techniques
such as:
• X- ray diffraction (XRD)
• Raman spectroscopy
• X-ray photo electron spectroscopy (XPS)
• Fourier transform infrared spectroscopy ((FTIR)
• TEM/HRTEM
8

9. DEFECTS IN CQDs
• XRD: peak at 26 corresponding to the (002)
planes of graphite, two new peaks emerged at
22.59 (labelled *) and 18.20, which refer to the
amorphous carbon and (103) planes (belonging
to hexagonal carbon) in the XRD pattern
• Raman spectra: The intensity ratio of the D and
G band (ID/IG) is a measurement of the disorder
extent, as well as the ratio of sp3/sp2 carbons.
• XPS: the XPS spectra of C1s, from which we can
see that from graphite to C-dots, the peak
intensity (located at about 288 eV) has a
noticeable increase, indicating more oxidation
groups (C–O, C=O) appeared in C-dots, which is
consistent with the Raman results.
Figure (A) XRD patterns, (B) Raman spectra λex =
633 nm), (C) C1s XPS spectra and (D) FTIR spectra of
graphite and CQDs produced electrochemically from
graphite. (H. Ming et al. 2012)
9

10. 3.
SYNTHESIS
10

11. 3.1. TOP-DOWN
METHODS
• Involve the disruption of bulk carbon
precursors or nanomaterials such as:
• graphite (bulk),
• carbon nanotubes,
• activated carbon.
• The methods include:
• arc-discharge,
• laser ablation,
• electrochemical oxidation,
• chemical oxidation
• Require additional passivating and
functionalizing their surface with various
polar/apolar moieties.
(A. Sciortino et al. 1018)
11
Advantages
• abundant raw materials
• produce oxygen containing functional
groups at the edge,
• facilitating their solubility and
functionalization.
Limitation
• low yield,
• large density of defects,
• Little fluorescent or have low
fluorescence quantum yield
• control of size and shape

12. 3.1.1. LASER ABLATION
 Involves ablating a piece of bulk carbon material using a laser beam
 Only after an acidic treatment of the surface and the following
surface passivation by organic molecules, these carbon nanoparticles
become bright luminescent
 Sun et al.: graphite target in a flow of argon gas carrying water
vapor at 900 °C and 75 kPa.
Gonçalves et al.: carbon targets immersed in deionized water
Hu et al.: graphite flakes dispersed in PEG solution
Reyes et al. graphite target in acetone
Size of CQDs can be controlled by:
Wave length of the laser
Laser pulse with (pulse duration)
12
Y.-P. Sun et al., 2006
S.-L. Hu et al. 2009
D. Reyes et al. 2016

13. 3.1.2. ELECTROCHEMICAL OXIDATION
the most common method to synthesize CQDs with
the advantages of high purity, low cost, high yield,
easy manipulation of size and good reproducibility
Carried out in an electrochemical cell using the
precursors graphite/carbon nano tubes as electrodes
Size control can be achieved by manipulating applied
potential, current density, nature of electrolyte/pH,
temperature
J. G. Zhou et al.: MWCNTs used as electrode
H. Li et al.: graphite electrode with NaOH/ethanol as
the electrolyte
Shinde et al.: MWCNTs as electrode and propylene
carbonate/LiClO4 electrolyte
13
Fig. The schematic diagram of
electrochemical fabrication of C-dots.

14. 3.1.3. CHEMICAL OXIDATION
• an effective and convenient approach for large scale production and requires no complicated devices.
• provides a simple approach to synthesize CQDs using cheap, abundant precursors: carbon fibre, coal,
carbon soot
• The precursor heated in the mixture of strong acids (sulfuric acid and nitric acid) and requires
additional passivation step
• The oxygen-containing groups such as C=O, C–O, O–H are introduced to the surface of CQDs during
the oxidation.
• The strength/concentration of the acid and duration of treatment determines size, PL quantum yield
14
Qiao et al. 2010
S. Hu et al. 2013

15. 3.2. BOTTOM-UP APPROACHES
• fabricate CQDs from molecular precursors such as citric acid,
sucrose and glucose through
• microwave synthesis,
• thermal decomposition,
• Hydrothermal/solvothermal treatment,
• template-based routes.
• Involves carbonization of the molecular precursors carried out
at relatively low temperatures.
• Besides “pure” carbon-core CQDs, mixing the carbon sources
with other molecular precursors, as urea and thiourea can be
used as a method to dope the structure of CQDs with nitrogen,
sulphur, or other heteroatoms.
• They are particularly simple and surface passivation can be
usually achieved in “one pot” without the need of post-synthesis
chemical processing. 15
Advantages
• fewer defects
• controllable size and
morphology;
Disadvantage
• poor solubility,
• small dot size
• aggregation issue

16. 3.2.1. HYDROTHERMAL/SOLVOTHERMAL SYNTHESIS
• are economical, eco-friendly, easy to handle, and route to synthesize
CDs from diverse carbon-based precursors.
• In a typical procedure,
• the precursors are dissolved in a suitable solvent and heated to high
temperature (100−200 °C) in the absence of air in a Teflon-lined
autoclave.
• The small organic moieties join together to form carbogenic cores
and then grow into CDs ranging from 2 to 10 nm in size.
• The PL can be modulated by varying the experimental conditions, i.e.,
varying the molar mass of precursors, the nature of the solvent, heating
time, and temperature.
• Zboril et al. prepared CDs from citric acid and urea in N,N-
dimethylformamide
• Mehta et al. prepared CQDs from sugar cane juice in ethanol
• Bourlinos et al. synthesized CQDs from ammonium citrate in water
• Although not the very latest and has been used in practice for many
years, it is still a very efficient, facile, versatile, and cost-effective
approach for the synthesis of CDs.
16
Fig a) Preparation of MCBF-CQDs from blue to
red by solvothermal treatment of CA and DAN. b)
Photographs of MCBF-CQDs under daylight(left)
and fluorescence images (right) under UV light
(excited at 365 nm) . Yuan F. et al.

17. 3.2.2. MICROWAVE-ASSISTED HEATING METHOD
• is an augmentation to the solvothermal/ hydrothermal technique where microwave is utilize
instead of heat.
• can effectively shorten the reaction time and provide simultaneous, homogeneous heating,
which leads to uniform size distribution of quantum dots
• is facile, less energy/time consuming, and easily scalable for the preparation of highly
fluorescent CQDs
• Zhu et al.: sugar and polyethylene glycol (PEG) in distilled water
• Tang et al.: glucose + water
17

18. 3.2.3. THERMAL DECOMPOSITION (PYROLYSIS)
• involves heating of the precursor organic molecules at high temperature for a predefined
time followed by dissolution in water and separation
• offers advantages of easy operation, solvent-free approach, wide precursor tolerance, short
reaction time, low cost and scalable production
• Ma et al. synthesized N-doped GQDs with graphene-like structures by the direct carbonization of
ethylene diamine tetra acetic acid (EDTA) heated in a sand bath at 260–280 ℃
• Martindale et al. prepared CQDs with a high quantum yield of 45% by the straightforward
pyrolysis of citric acid at 180 ℃ for 40 h .
18

19. 3.2.4. TEMPLATE BASED METHOD – CONFINED PYROLYSIS
• Discrete CQDs with tunable and uniform sizes can be prepared via
confined pyrolysis of an organic precursor in nanoreactors. The
synthesis involved three steps:
• absorbing the organic precursor into porous nanoreactors via
capillary force,
• pyrolysis of the organic precursor confined in the nanoreactors into
carbonaceous matter,
• release of the as-synthesized CQDs by removing the nanoreactors.
• J. Zong et al. synthesized hydrophilic CQDs with mesoporous
silica nanospheres as nanoreactors by impregnation of a citric
acid precursor
• Polymeric core–shell nanoparticles are also effective nanoreactors with
thermally cross-linkable core and thermally removable shell
• pyrolysis of PAN@PMMA core–shell nanoparticles
Schematic illustration of the
preparation of CQDs via confined
pyrolysis of an organic precursor in
nanoreactors.
19
Y. Wang et al. 2013
X. Guo et al. 2012

20. 3.3. TAILORING THE PROPERTIES OF CQDs
• CQDs prepared by most of the methods are generally not fluorescent or
fluorescence quantum yields are low, limiting their application.
• surfaces of CQDs is sensitive to contaminants in their environment,
• In order to alleviate these problems, surface passivation, functionalization
and doping of CQDs is performed to stabilize fluorescence and improve
the fluorescence quantum yields.
• Surface passivation is usually attained by the formation of a thin
insulating layer, usually by the attachment of polymeric materials, such as
oligomeric PEG
• Functionalization of CQDs
• oxidative treatment using strong acids
• Attaching various organic molecules
• doping with heteroatoms, nitrogen in particular, has shown great
potential to significantly enhance the quantum yield of CQDs
• Doping is carried out during synthesis by using appropriate precursors 20
H. P. Liu et al. 2007
Y. Q. Dong et al 2010
X. Zhai et al. 2012
S. Zhu et al. 2013

21. 4. PHOTOPHYSICAL
PROPERTIES
4.1. LIGHT ABSORPTION
• typically show optical absorption in the UV region with a tail extending to the visible
range
• Most of the C-dots, have an absorption band around 260–320 nm.
• The peaks are usually imputed to π-π* transitions of conjugated C=C system and n-
π* transitions of C=O, C-N, or C−S groups.
• The absorption band could be modulated via various surface
passivation/functionalization techniques
• absorbance of C-dots was found to increase to longer wavelength after surface
passivation with 4,7,10-trioxa-1,13-tridecanediamine (TTDDA) or organosilane (350–
550 nm; 340–410 nm, 360 nm center, respectively).
(Y.-P. Sun et al. 2006; S.-L. Hu et al. 2009) 21

22. 4.2. PHOTOLUMINESCENCE (PL)
• Origin of PL: not clearly known
• bandgap transitions corresponding to conjugated π-domains - quantum size effect
• Surface states - defects in the graphene structures
• Most CDs exhibit excitation-dependent photoluminescence, with strong emission in the blue-
wavelength region that decays rapidly in the red-wavelength region
• For CDs with well-defined crystalline cores, photoluminescence often depends strongly on the size
• The intensity and wavelength of PL depends on various factors:
• Synthesis methods, synthesis parameters, nature of precursors
• CD surface: degree of oxidation, functional group, doping
• CD core crystallinity/amorphous nature
Quantum yield:
• CDs possessed rather low QYs
• QY depends on the synthesis route and the surface chemistry
22

23. PHOTOLUMINESCENCE (PL)
23
Fig Wavelength and size
dependence of fluorescence of
CQDs. (H. Ding et al. 2016)

24. 4.3. UP-CONVERTED PHOTOLUMINESCENCE (UCPL)
• For UCPL emission the emission wavelength is shorter than the excitation wavelength
• the mechanism of this unique character is not fully understood
• multi-photon excitation mechanism (Cao et al.)
• relaxation of electrons from 𝜋∗
to 𝜎 (Shen et al. )
A) UCPL spectra of the CQDs dispersed
in water at excitation wavelengths
progressively increasing from 700
nm.
B) B) UCPL properties of CQDs
dispersed in water at excitation
wavelengths from 805 nm to 1035
nm
(M. Li et al. 2012; Q. Feng et al. 2013)
24

25. 5. CHARACTERIZATION
• Varied analytical methods are routinely
applied to characterize CQDs and their
physical properties:
• shape and size
• crystalline organization of the carbon
atoms,
• type and abundance of functional units
displayed upon the CQDs’ surface.
• optical properties (light absorption and
luminescence)
25
The commonly used characterization methods
include:
• Fourier transform infrared spectroscopy
(FTIR),
• Nuclear magnetic resonance (NMR),
• Transmission electron microscope
(TEM)/high resolution TEM(HRTEM)
• X-ray photoelectron spectroscopy (XPS),
• Raman spectroscopy,
• X-ray diffraction (XRD)
• Uv –Vis absorption spectroscopy
• Fluorimetry

26. TRANSMISSION ELECTRON MICROSCOPE (TEM)/HRTEM
• A primary technique for visualization of Carbon-Dots, providing important information
upon particle morphology, size distribution, and crystalline organization.
• High-resolution TEM (HRTEM) experiments have been applied to confirm the periodicity of
the graphitic core, reflecting its crystalline nature.
X-ray diffraction (XRD)
• evaluation of the crystalline nature of Carbon-Dots.
• information upon the unit cell dimensions and crystal spacing within the crystalline
carbon cores.
Raman spectroscopy
• Disorder in the structure/degree of defect
13C – NMR
• type of carbon (sp2/sp3)
26

27. X-RAY PHOTOELECTRON SPECTROSCOPY (XPS)
• provides information upon specific atomic units present upon CQDs’ surface.
27
• An example of an XPS analysis of a carbon dot sample surface containing C=C, C-OH, C-N, C=N and
C=O groups is provided in Figure.
• The spectral analysis reveals the distinct nitrogen-, oxygen-, and carbon-bonded units displayed upon
the CQDs’ surface.
Fig XPS spectra of a CQD

28. FOURIER TRANSFORM INFRARED (FTIR)
• FTIR usually complements XPS, illuminating distinct
functional units through recording of typical vibration
bands
• FTIR can be used to characterize the modified CQDs
in order to determine whether they were effectively
passivated.
Fig FTIR spectra contrast of Carbon Fiber and CQD
prepared by chemical oxidation of the CF
• Characteristic absorption peaks at 3307 cm-1
and 1724 cm-1 suggested the presence of
carboxyl groups on their surface;
• absorption peak at 1579 cm-1 demonstrated the
existence of a double bond;
• absorption peak at 1097 cm-1 implied the
existence of ether linkage.
28

29. 6. APPLICATIONS
• Because of their unique blend of properties, CQDs
are very promising for many applications
• Their bright emission, combined with the marked
electron-donor capability can be exploited in
optoelectronic devices.
• The sensitivity of the PL emission to ions and
other molecules in solution can be exploited to
create nanosensors
• Their non-toxicity and biocompatibility is a key
advantage to perform in vivo and in vitro
bioimaging experiments and drug delivery
• Their light absorption properties have been
exploited as means to enhance photocatalysis
29

30. 6.1. SENSING
• could serve as sensors for a broad range of analytes, such as
ions, small molecules, macromolecules, cells and bacteria.
• Based on change in intensity, wavelength, anisotropy, or lifetime
of fluorescence
• Nanosensors based on CQDs were developed by two different
strategies:
• the nanosensors simply consisted of “pure” CDs, as-synthesized or
passivated through specific target groups;
• the functional sensing material was a nano-composite fabricating by
coupling CQDs with other nano- or micro-materials
• Applicable in sensing of:
• Ions
• Small molecules
• Macromolecules
• Cells, bacteria and viruses 30
Fig. Schematic illustration of the
heavy metal ions detection
mechanism via CQD fluorescence
quenching in
a) absence and
b) b) presence of Hg2+ ions.

31. 6.2. BIOIMAGING
• CQDs possess great potential for fluorescent bioimaging due to their superior fluorescent properties,
possibility of multimodal bioimaging of cells and tissues, biocompatibility and low toxicity
• Sun et al. used PEGylated CQDs for in vivo optical imaging of different organs including bladder, kidney
and liver of a mice
31

32. 6.3. NANOMEDICINE
Photodynamic Therapy
• Photodynamic therapy is a clinical treatment mainly
for superficial tumours.
• It involves the localisation and accumulation of
CQDs in the tumour tissue, following which they are
irradiated with a specific wavelength, triggering the
formation of singlet oxygen species that result in cell
death.
Drug and Gene delivery
• CQDs could function as nano-carriers for tracking
and delivery of drugs or genes
• Besides serving as drug carriers and fluorescent
tracers, CQDs were found to be able to control drug
release.
Figure. A schematic illustration for the gene delivery
and real-time monitoring of cellular trafficking
utilizing CD-PEI/Au-PEI/pDNA assembled
nanohybrids
32

33. 6.4. PHOTOCATALYSIS/ELECTROCATALYSIS
• A photocatalyst is a substance which, upon photo-excitation,
becomes capable of speeding up a chemical process.
• Their light absorption and electron transfer properties, and the
ease of coupling to other materials such as TiO2 , Fe2O3 , ZnO
are particularly beneficial for these applications
• In general, CDs are employed in two different ways
• as a photo-sensitizers: improves light harvesting capability of
semiconductors
• as acceptor of charge carriers from the photoexcited semiconductor
Examples:
• Green synthesis of organic compounds:
• oxidation of organic cpds with H2O2 in visible light
• Degradation of dyes using TiO2
• Splitting of water using light and TiO2 as a catalyst
• Reduction of oxygen in fuel cells
33
Fig. photocatalytic mechanism for the
CQDs/TiO2 nanotube composite under
visible light irradiation
Fig Oxidation process of benzyl alcohol to
benzaldehyde in the presence of CQDs
under NIR light irradiation

34. 34
Fig. Mechanisms of photocatalysis enhanced by carbon-dots coupled to a semiconductor
material.
a. Carbon-Dots (small green spheres) act as light absorbers, transferring the photoexcited electrons to
the semiconductor;
b. Carbon-Dots serve as “electron sinks” thereby extending the lifetimes of the electron-hole pairs.
Xie et al.

35. 6.6. OPTOELECTRONICS
Solar cells
• sensitizers in dye-sensitized solar cells or organic solar
cells to improve the photoelectric conversion efficiency
Light-emitting devices (LED)
• CDs are used in the construction of light emitting diodes
(LED), CDs can be used in two different ways:
• as fluorescent downconverters – phosphor applications
• as the active layer in an electroluminescent device
• Advanced Information Encryption
• electric double-layer capacitors (EDLCs)
• photodetectors
35
Illustrative
diagrams and
results on the
application of
CQDs in
LEDs.
Diagrams
on the
different
structures
of solar
cells based
on CQDs.

36. 7. CONCLUSION AND OUTLOOK
In this article,
• recent developments in the field of CQDs, concentrating on their synthetic approaches, surface
modification methods, various optical properties and their applications in bioimaging, photocatalysis,
sensing and medicine and optoelectronics have been discussed.
• The synthesis of CQDs is usually quite easy and only requires cheap and abundant materials. Furthermore,
simple chemical experiments are needed for surface modification of CQDs that could be performed in a
standard elementary level chemistry laboratory.
• Compared to QDs, due to nontoxic behaviour of CQDs, they stand to have an enormous influence on
environmental and biotechnological applications.
• Furthermore, because of excellent light absorbing ability of CQDs as well as their unique photo-induced
electron transfer capability, they are considered as an excellent candidate for photocatalytic applications.
• High QY, high photo and chemical-stability, beside non-blinking behaviour of CQDs encourage researchers
to develop highly sensitive biosensors in different environments.
• It seems clear that the future of CQDs remains promising.
36

37. CHALLENGES
• Although there are many important advantages and potential applications of CQDs,
further research to enhance the properties of the material is required in order to meet
the application requirements.
• Therefore, the studies of CQDs should continue to address issues such as
• Low product yield
• Low quantum yield
• Control of size and shape
• Mechanism of photoluminescence
• Narrow spectral coverage
37