P. 1

Hong Kong Chemistry Olympiad for

Secondary School (2014-2015)

Synthesis and Characterization of

Carbon Quantum Dots by

Electrochemical Carbonization

Christian Alliance S C Chan Memorial College

Law Hoi Yiu (羅凱謠)

Leung Hiu Man (梁曉雯)

Ng Wai Ching (吳緯靜)

Tsui Kwan Lok (徐君樂)

Wong Shu Ting (黃舒婷)

P. 2

Abstract

Carbon quantum dots (CQDs) are versatile nanomaterials that are capable of

carrying out photoluminiscence (PL). They are easy to produce, quite

variative, highly biocompatible, and have a bright future in various aspects

including bioimaging and chemical sensors. In this research, the method of

synthesis, properties of CQDs and the factors affecting their properties are

introduced and extensively investigated.

In this research, CQDs are synthesized by means of electrochemical

carbonization. It was followed by purification with methods such as column

chromatography, salting out and centrifugation. The samples are then

examined concerning their PL properties under UV and visible lights, and

how they can be altered. The chemiluminiscence and catalytic properties are

also probed.

It was discovered that the CQDs fabricated have the fluorescence under

excitation by widest range of EM waves, when 6V of current and copper

plates are used in electrolysis, with ethanol as the main carbon source. The

PL response under excitation by UV and coloured lights successfully

occurred. The concentration of the CQDs solution also affects its PL emission:

the more concentrated the solution is, the more obvious the fluorescence is.

Chemiluminiscence (CL) induced by a strong oxidizing agent KMnO4

was also attempted. Although the CL emission cannot be observed under

naked eyes or digital camera, it was accidentally found out that the CQDs

may have catalysed the redox reaction between the permanganate ion and

water.

P. 3

These tests are convenient, economic, and can be carried out even in

secondary school laboratories with bench chemicals. We hope that this

research can enlighten the future of this omnipotent nanomaterial, and

provide a great insight in its potential applications.

P. 4

Table of Contents Abstract ........................................................................................................................................... 2 Table of Contents ............................................................................................................................ 4 1. Introduction ................................................................................................................................. 5

1.1 Introduction of carbon quantum dots ................................................................................ 5 1.2 Properties .......................................................................................................................... 5 1.3 Application ........................................................................................................................ 6 1.4 Wavelength of light ........................................................................................................... 7

2. Methods ....................................................................................................................................... 8 2.1 Electrochemical carbonization .......................................................................................... 8 2.2 Electrolysis ........................................................................................................................ 8 2.3 Chromatography................................................................................................................ 9 2.4 Column Chromatography ................................................................................................ 10 2.5 Thin layer chromatography (TLC) .................................................................................. 11 2.6 Centrifuge........................................................................................................................ 11 2.6 Characterization .............................................................................................................. 12

3. Procedures ................................................................................................................................. 14 3.1 Synthesis ......................................................................................................................... 14 3.2 Analysis ........................................................................................................................... 17

4. Results ....................................................................................................................................... 19 4.1 Electrolysis (graphite electrode group) ........................................................................... 19 4.3 Chemiluminescence properties ....................................................................................... 20 4.4 Catalytic property ............................................................................................................ 21 4.5 Effect of concentration on the photoluminescence property of CQDs samples ............. 21 4.6 Fluorescence spectroscopy .............................................................................................. 22

5. Discussions................................................................................................................................ 23 5.1 Colours of solutions ........................................................................................................ 24 5.2 Effect of voltage on the synthesis of CQDs .................................................................... 26 5.3 Effect of concentration on the fluorescence of CQDs .................................................... 27 5.4 The trend of emitted lights in photoluminiscence ........................................................... 28 5.5 Chemiluminescence of CQDs ......................................................................................... 30 5.6 Catalytic property of CQDs ............................................................................................ 31

6. Limitations ................................................................................................................................ 32 7. Possible Errors .......................................................................................................................... 33 8. Conclusion ................................................................................................................................ 34 9. References ................................................................................................................................. 35 10. Acknowledgement ................................................................................................................... 37 11. Appendix I Wavelength of the colour filters ........................................................................... 38 12. Appendix II Fluorescence result of CQDs .............................................................................. 39

P. 5

1. Introduction

1.1 Introduction of carbon quantum dots Carbon quantum dots (CQDs) are small carbon nanoparticles with sizes below 10 nm. CQDs are

versatile and can be synthesized by many means such as chemical ablation and electrochemical

oxidation.1 Different method of synthesizing of CQDs carry different features. CQDs of different

size can undergo different modification, such as doping with different element, enabling it to

carry different propertities.

1.2 Properties One of the most fascinating features of CQDs is fluorescence. This property allows CQDs to

absorb light at UV – visible range which has higher energy, and then emit light which has lower

energy.

Figure 1 Figure showing fluorescence properties of inorganic quantum dots (CdS)

Phosphorescence and chemiluminescence are opticial properties besides fluorescence. On the

other hand, it also has various merits including low biotoxicity and high biocompatibility (as the

main components of CQDs are carbon compounds small in size).

P. 6

1.3 Application With various unique properties, CQDs have found more and more wide use since the past few

years.1 With the biological properties of CQDs, such as low toxicity and good biocompatibility,

CQDs show great potential applications in biomedicine. As CQDs are fluorescent nanomaterials,

bioimaging is also one of the applications of CQDs. Apart from this, biosensor is one of CQDs’

application because of its solubility in water. The figure 2 showed CQD-involved bioimaging in

mouse.

Figure 2 Figure showing bioimaging using CQDs

In addition, the chemiluminescence and electrochemiluminescence properties of CQDs enable

them with wide potentials in optronics, catalysis and sensors. 1

Figure 3 Figure showing some examples of application of CQDs

P. 7

1.4 Wavelength of light

Our eyes are sensitive to light which lies in a wavelength range of 400 - 700 nm and a colour

range of violet through red. The human eye is not eable to see radiation with wavelengths outside

this range. The visible lights are (from shortest to longest wavelength): violet, blue, green, yellow,

orange, and red. Ultraviolet radiation has a shorter wavelength than the visible violet light.

Infrared radiation has a longer wavelength than visible red light. The white light is a mixture of

the light with different colour of the visible spectrum. Black is a total absence of light.

Figure 4 Figure showing the wavelength of light and the corresponding energy

By some mechanisms introduced in the following sections, CQDs can emit EM waves (such as

visible lights) upon excitation by high-energy EM waves.

With these in mind, the following objectives are proposed:

-To synthesis different size of CQDs by electrochemical oxidation of alcohol

-To study the properties of CQDs

-To study the factors affecting the properties of CQDs

P. 8

2. Methods

2.1 Electrochemical carbonization Electrochemical carbonization was used as the synthesis method because it is a powerful method

to prepare CQDs using alcohols as precursors. However, there are only a few reports about

electrochemically carbonizing small molecules to CQDs. 2

It arouses our interest on studying the

properties of CQDs produced in such a method.

Also, this method is advantageous because of its low cost and easy manipulation, which makes it

become available to produce in common secondary school laboratory.

2.2 Electrolysis Graphite rods were first adopted as both anode and cathode, and NaOH solution as electrolyte

and alkali media. It was hypothesized that the CQDs are produce from the graphite and the

solvent ethanol.

Also, different applied voltage will affect the diameter of CQDs.

However, during experiment, we discovered using graphite as electrode would produce great

amount of dirt which is hardly separated from CQDs. As a result, copper plates were used

instead of graphite rods and different currents were used to produce CQDs of various diameters.

P. 9

2.3 Chromatography Chromatography is used to separate mixtures of substances into their components. It includes

column chromatography, thin layer chromatography (TLC), paper chromatography etc. All forms

of chromatography work on the same principle.

They all have a stationary phase (solvent contained with dissolved solute) and a mobile phase (a

liquid or a gas). The mobile phase flows through the stationary phase and carries the components

of the mixture with it. Different components with different polarities travel at different rates. The

components with lower polarities will be retained on the stationary phase first whereas the

components with higher polarities will retain afterwards. As a consequence, components with

different polarities can be separated by this method.

In this experiment, column chromatography and thin layer chromatography were attempted.

Silica gel was used as the stationary phrase.

P. 10

2.4 Column Chromatography Column chromatography was used as a type of purification method to separate different factions

of CQDs from the solution after electrolysis. Column chromatography is used as preparative skill

in order to separate product from impuries.

Hexane-ethyl acetate mixture (in 1:8 proportion) was used as the mobile phase. Due to the lack

of column, burette was used instead of column. The solution with different component can be

collected above the burette. CQDs of be obtained from a mixture of compounds in a crude

mixture.

Figure 5 Figure showing column chromatography, obtained from

http://www.chemguide.co.uk/analysis/chromatography/column.html

P. 11

2.5 Thin layer chromatography (TLC) In TLC method, ethyl acetate : n-hexane (1:1) was used as the mobile phase

In contrast to, Column Chromatography, it was used to analyze the constituents of the CQD

solutions by their different polarities. It was used as an analysis method. Data can be collected

by this method quickly and researchers instantly have a brief idea on the reaction mixture for

further investigation.

Figure 6 Figure showing thin layer chromatography, obtained from

http://www.chemguide.co.uk/analysis/chromatography/thinlayer.html

2.6 Centrifuge The centrifuge involves principle of sedimentation, where the centripetal acceleration is used to

separate substances of greater and lesser density. By the same concept lighter objects will tend to

move to the top of the tube; in the rotating picture, move to the center. In a solution, particles

whose density is higher than that of the solvent sink (sediment), and particles that are lighter than

it float to the top. The CQDs contained in solvent can be easily separated from the sediment of

MgSO4 and other unwanted materials.

P. 12

2.6 Characterization 2.6.1 Fluorescence property

CQDs typically show optical absorption in the UV region with a tail extending to the visible

range. After the absorption, different frequencies of lights are emitted at the same time, which

was the phenomenon called fluorescence. The detailed principles of CQDs absorption and

fluorescence properties will be shown on the discussion part below.

In this experiment, UV light or monochromatic lights produced by a colour filter were used as

the light resource for exciting the CQDs. Also fluorescence spectroscopy is used to objectively

characterize the sample.

Figure 7 Figure showing the energy transition involved in fluorescence

Source: http://elchem.kaist.ac.kr/vt/chem-ed/spec/molec/mol-fluo.htm

P. 13

2.6.2 Chemiluminescence property

Chemiluminescence properties of CQDs were mainly discovered when the CQDs coexisted with

some oxidants, such as potassium permanganate (KMnO4) and cerium(IV) ion. Since

chemiluminescence is a process of releasing light energy from a chemical reaction, it is believed

that CQDs gives chemiluminescence property by oxidation/reduction reaction. Moreover, the

increasing temperature had a positive effect on the chemiluminescence in CQDs because of the

thermal equilibrium of electron distribution.

In this experiment, acidified KMnO4(aq) will be used as the oxidant. The solution is warmed to

increase the chemiluminescence emission.

P. 14

3. Procedures

3.1 Synthesis

3.1.1 Preparation of solution

1. 0.68g (~6 granules) of NaOH was dissolved in about 1mL of deionized water. (Figure 8)

2. The solution was stirred until all NaOH dissolved.

3. The saturated NaOH solution was poured into 75ml of ethanol.

Figure 8 Preparation of solutions

P. 15

3.1.2 Electrolysis

1. The solution was electrolyzed under room condition for 48 hours. Graphite rods were used

as electrode. (Figure 9)

2. Step 1 was repeated with copper plate as electrode. (for another set of samples)

3. Steps 1 and 2 were repeated with different current densities.

Figure 9 Electrolysis by graphite rods

P. 16

3.1.3 Purification by centrifuge or filtration method

1. The solution was treated with MgSO4 to remove the salts and water.

2. The solution was placed in centrifuge, the solid and solution was then separated.

3. The solution was then filtered by filter paper and funnel.

4. The resulting solution was heated with water bath until all the solvent evaporate.

5. The CQDs yellow semi solid was obtained.

3.1.4 Purification by Column Chromatography(CC) Method

1. MgSO4 was added into the solution to remove the salts and water.

2. The bottom of the burette was sealed by cotton

3. Hexane-ethyl acetate mixture (in 1:8 proportion) was added into the silica gel to put it into

mobile state and packed into the burette.

4. The solution is poured into the burette, with hexane-ethyl acetate mixture as the developing

agent.

5. The solution is be collected by test tubes according to the different colour bands. The CQDs

products can be obtained under the burette.

Figure 10 Silica-gel column chromatography

P. 17

3.2 Analysis

3.2.1 Analysis by Thin Layer Chromatography(TLC) method

1. CQD products were dissolved with small amount of solvent (ethyl ethanoate :n-hexane 1:1).

2. Silica gel TLC plates were cut into 2.5cmX4cm.

3. A horizontal line was drawn 0.5cm above the bottom by pencil.

4. The dissolved CQDs product was spotted by 10µL micro-capillary tube. Pipette filler was

used to dry the TLC plate.

5. TLC plate was immersed in to the solvent. Components in the CQD solution with different

solubility were separated by solvent. The result can be easily observed under UV

Figure 11 A developed slica-gel TLC plate viewed under UV light

P. 18

3.2.2 Analysis of Fluorescence property

1. Different wavelength of light were projected to different solution by a torch covered with

colour filters. Emission of coloured lights was observed.

2. Step 1 was repeated with different concentration of CQDs solution.

3.2.3 Analysis of chemiluminescence property

1. A few drops of acidified KMnO4 (aq) was added into the warmed solution

2. Camera with exposure mode was used to film the solution in a black box.

3. Step 1and 2 was repeated with acidified K2Cr2O7 (aq) instead of acidified KMnO4 (aq) .

3.2.4 Analysis of catalytic property

1. 2 drops of CQDs solution was added into a test tube of acidified KMnO4(aq).

2. The solution was shaken and waited until it decolourized completely.

3. Time is measured by stopwatch.

4. Step 1 and 2 was repeated with 0, 4, 6, 8, 10 drops of CQDs solution and the same amount

of KMnO4 (aq).

Figure 12 Experimental set-up to investigate the mentioned catalytic property

P. 19

4. Results

4.1 Electrolysis (graphite electrode group) The chemicals used were NaOH, deionized water and absolute ethanol.

During the reaction, it was found that a lot of heat was given out (possibly due to electrical

resistance, electrical energy was dissipated as heat energy.)

Due to lack of solvent and time, using column chromatography to separate a dirty mixture was

abandoned. We did literature research and found that using Platinum as electrode will result in

much cleaner solution. However, Platinum is too expensive, we tried another inert metal copper

instead. Although in our knowledge, copper metal will lose electron at anode, it is worth to try

since alcohol was used as solvent. The result was encouraging.

Figure 15 Solution under UV light, weak, green fluorescence was observed

Figure 14 Table showing the result of CQDs obtain by column chromatography

X: there was no fluorescent CQDs was observed

/ : The experiments were not performed.

√ : fluorescent CQDs was observed

Excitation by

Figure 13 Table showing the fluorescence property of CQDs of different batch of products

P. 20

4.2 Colour light anaylsis (copper group)

Colour light analysis before and after dilution by exactly known amount of ethanol:

Figure 16 Table showing the fluorescence property of CQDs of different batch of products using copper as electrode

It is observed that the fluorescence properties were observed when the CQDs are synthesized in

different condition. It is observed that the higher the voltage, the fluorescence range will shift to

the red side. For example, CQDs produced by 1.5V, has maximum fluorescence range near UV

and violet while that produced by 6V has maximum fluorescence range near green light.

After dilution, some samples that did not exhibit photoluminescence (PL) property or absorbed

all the lights upon excitation by coloured lights showed a response. It can be deduced that the PL

properties of the samples can be masked by their own colours.

4.3 Chemiluminescence properties

No observable changes were found by naked eye and also camera (in exposure mode).

Figure 17 Diagram showing after oxidizing agent KMnO4 was added into the CQDs.

P. 21

4.4 Catalytic property During experiment of the CQDs solutions' chemiluminescence property, it was discovered

accidentially that it also possesses catalytic property. This will be explained and discussed in

further sections.

When KMnO4 (aq) was added to the CQDs solutions, it decolourizes.

The graph below shows the decolourization process:

0

1

2

3

4

5

6

0 2 4 6 8 10 12

Rat

e /

min

-1

No. of drops of CQDs solution added

Figure 18 Graph showing the relationship between the concentration of CQDs and rate of reaction

Figure 19 The process of decolourization with time.

P. 22

4.5 Effect of concentration on the photoluminescence property of CQDs

samples

Figure 20 Table showing the relationship of conc. with fluorescence

X: no colour change, light passe through the solution directly

The more concentrated the solution, the deeper the colour of light it emitted.

4.6 Fluorescence spectroscopy

The 6V copper sample was found to have widest range of fluorescence under visible light range,

this property was further investigated by fluorescence spectroscopy. The result is as follow.

Figure 21 Fluorescence spectrum of the CQDs sample – 6V coppper

0

2

4

6

8

10

12

14

350 450 550 650 750 850 950

Flu

ore

sce

nce

inte

nsi

ty

Excitation wavelength/ nm

Fluorescence spectrum of CQDs sample 6V - copper

P. 23

5. Discussions In our synthesis process, the alcohol (ethanol) in the reaction mixture serves as the main carbon

source for the formation of CQDs, while the copper strip/previously-used graphite rods serve as

simple electrodes. Alkaline medium is also necessary for the formation of CQDs, therefore

sodium hydroxide and a small amount of water is also added. The sodium hydroxide solution

also acts as an electrolyte.

After the designated period of time described in the procedures part, the electric current was

stopped.

In the graphite electrode group, we attempted column chromatography (CC) in purifying the

products obtained. The CQDs were hypothesized to be moderately polar, thus silica-gel CC with

hexane-ethyl acetate mixture (in 1:8 proportion) as developing agent was used. Although

solutions (with different colour) that exhibits fluorescence properties can be extracted, this

method is relatively slow and costly.

Also, the graphite method is not very productive. In further examination, it was discovered that

the solutions somehow contained a visible amount of solid particles. It was suscepted that in the

grahpite electrolysis process, a considerable amount graphite particles was produced instead of

electrochemical oxidation alcohol to produce CQDs. Thus compared to the copper electrode

group, higher voltage was needed in this method. Sometimes, due to the high voltage applied,

cooling measures such as placement in a water trough and also re-addition of water (as all the

water are decomposed by electrolysis, current dropped to zero) becomes necessary. These are the

reasons why graphite electrodes were abandoned in further experiments.

In the copper electrode group, to remove the remaining water and sodium hydroxide present,

some MgSO4 powder were added to remove the residual water. This is known as the “salt-out”

process, as some unwanted water-soluble salts in the solution (in this case, NaOH), will

crystallize and precipitate out after all water was removed by MgSO4.

After the salting out process, two methods were attempted to remove the MgSO4 added: filtration

P. 24

and centrifuge. While both methods can effectively remove the residual MgSO4, centrifuge was

way more efficient and rapid, which is very ideal given an ongoing large-scale CQD production.

5.1 Colours of solutions

It was observed that the solutions obtained after purification possesses various hues: some

yellowish, some orange or even brown. The unidentified organic compounds originated from

alcohol molecules due to electrochemical oxidation reactions, instead of the compared trace

amounts of CQDs, contributed to these hues.

In the synthesis, utmost efforts are paid in order to ensure that the voltage applied was as

accurate as possible. However, errors still arise, as slight differences of experimental conditions,

approximately 0.1~0.5V in voltage, still occurred. Apart from the as-said different colours, it

may also lead to slight differences in the diameter and amount of CQDs fabricated.

After synthesis, the obtained solutions were examined using both UV light (wavelength 365nm)

and coloured lights. In these examinations, a vast array of the solutions exhibits fluorescence

behaviours (i.e. illuminations) as illustrated by pictures in the results section.

The following diagram shows the possibilities of monochromatic light going through a sample.

The sample may have absorption, transmission or fluorescence towards the light. (Figure 22) We

do not have UV – Vis fluorescence spectroscopy to quantitatively investigate the fluorescence

property of CQDs so we use monochromatic light produced by white LED passing through a

colour filter of school colorimeter ins If the light is absorbed, no light will be observed or the

light will become dimmer. If the light is transmitted, the colour will have no change. If the

sample has fluorescence property, the colour of light would change.tead. The change of colour

after passing through a CQDs solution indicated the fluorescence properties of CQDs in visible

range. The fluorescence photo is attached in appendix II.

P. 25

Figure 22 The sample may have a) absorption b) transmission or c) fluorescence towards a monochromatic light

source.

It was hypothesized that the CQD solutions after working-up process consists of CQDs of

different diameters, therefore having different energy levels. So, upon excitation by UV light,

different frequencies of lights are emitted at the same time to give the greenish/bluish white light.

However, in some solutions, these behaviours are not observed. It may be due to the very small

amount, or even absence of CQDs in the resultant solution, which was related to the amount of

current applied during electrolysis.

P. 26

5.2 Effect of voltage on the synthesis of CQDs

In the experiments using copper electrode, CQDs were synthesized using 1.5V, 3V, 4.5V, 6V,

12V and 18V. When 12V and 18V are used, we couldn’t see any change when the solutions are

put under different colour light. It was observed that copper oxides easily form on the copper

electrodes during electrolysis. It was believed that the copper oxides formed reduced the

electrical conductivity of copper electrodes, thus lowering the current and negatively affected the

synthesis of CQDs, so the presence of CQDs cannot be detected.

When voltage in the range 1.5V-6V is used, CQDs are present in the solutions. In the solutions of

1.5V and 4.5V, the CQDs only absorb blue light to give green light. A change of green to

yellowish orange can also be seen in the solution of 3V. However, the change in colour in these

solutions are insignificant. When the voltage used is too low, the production of CQDs is little in

quantity, which makes the changes difficult to be seen.

After several trials, we discovered that the CQDs solution synthesized under 6V changes blue

light to green, dark green to red/orange, and light green to yellow. Compared with CQDs

solutions synthesized under other voltage, it shows the greatest variations of colour change. This

means it has a wide fluorescence range in visible light region.

P. 27

5.3 Effect of concentration on the fluorescence of CQDs

We used the product synthesized under 6V to carry out the test for concentration effect of CQDs.

We diluted the solution one time, concentrated it once and then twice to test for its

photoluminescence properties respectively.

When the CQDs solution is diluted once, the green light emitted after absorbing the blue light is

lighter than the original solution. The difference between blue and green is small. When the

CQDs solution is concentrated once, the green light emitted is darker. When the CQDs solution

is concentrated twice, the difference between the blue and green light is very obvious. Moreover,

the CQDs solution absorbs green light to emit yellow light and absorbs purple light without

giving out any visible light.

We found that a more diluted CQDs solution shows a smaller colour change in light emission

while a more concentrated CQDs solution shows a greater colour change between the light

absorbed and the light emitted. As the concentration of CQDs solution decreases, the amount of

CQDs in the solution decreases, which makes their fluorescence less obvious, and vice versa.

P. 28

5.4 The trend of emitted lights in photoluminiscence

Under examination by coloured lights, most solutions produced a recgonizable

photoluminiscence (PL) response. An interesting trend is also discovered.

Figure 23 Figure showing the wavelength of light and the corresponding energy

The above diagram manifested the electromagnetic spectrum, and also the visible spectrum. It

was discovered that when blue lights were applied, green lights were given off; when green

lights were applied, pale green/ yellowish orange/ red lights were given off. The high energy

light was applied while the low energy light was emitted. This is known as the redshift

phenomenon. This trend was universally observed in numerous samples different in colour,

indicating that this change is not under influence of the solution's own colour.

P. 29

Figure 24 Figure showing the energy transition involved in fluorescence, obtained from:

http://en.wikipedia.org/wiki/File:Stimulated_Emission.svg

During photoluminiscence, the energy from incident photons are absorbed by atom in ground

state, which led to its elevation to a higher energy level (posessing a higher amount of potential

energy). After a short period of time, e.g. a few nanoseconds [6]

the excited atom dropped back to

a lower energy level, releasing a photon; the energy (in eV) posessed by the photon is the

potential energy difference between the two levels. During this process, some energy was lost as

the vibrational or rotational energy so that the energy difference between the excited state and the

ground states was reduced.

According to the formula, as the energy (E) of the photon decreases, its

wavelength (λ) will also increase, implying a shift of wavelength to the right side of the visible

spectrum, producing the redshift phenomenon.

Higher energy

Lower energy

P. 30

5.5 Chemiluminescence of CQDs

We tried to test for the properties of chemiluminescence of CQDs. Chemiluminescence is

releasing the chemical energy in the reactants in the form of light energy by oxidation/reduction

reaction. It is believed that when CQDs coexisted with some oxidants, such as potassium

permanganate (KMnO4) and cerium (IV) ions, there will be energy release in the form of CL

emission. However, no CL emission could be observed by naked eyes in our experiment.

However, the emission of light is only 10-14

watt4, which can only be detected by means of a very

sensitive photomultiplier. Although CQDs do exhibit CL properties, the light given out is too

weak to be observed by naked eyes or digital camera.

P. 31

5.6 Catalytic property of CQDs

During the research in the chemiluminiscence property of CQDs, it was discovered accidentally

that when acidified potassium permanganate solution was added to the CQDs, it decolourised.

The decolourization is not due to the reaction of ethanol with KMnO4, as the CQDs samples,

originally in ethanol, was evaporated to dryness and redissolved in water.

To understand whether the reaction took place in stoichiometric amounts, the experiment stated

in the Procedures section was carried out. It was discovered that whether 2,4, or even 10 drops of

KMnO4 were added to the same volume of water-based CQDs solution, the decolourization took

place in the same way, but at a different rate. Our findings was affirmed, when some KMnO4

solution was added again to the already decolourized solution, the decolourization happened

again, impling that the CQDs were not destroyed. Therefore, the reaction is in a catalytic amount

rather than a stiochiometric amount.

From the observation, it was believed that a redox reaction took place: permanganate ion, a

powerful O.A., was reduced to colourless Mn (II) ions; while hydroxide ions (from ionization of

water), the most abundant R.A in the reaction mixture, was oxidised to oxygen. We strongly

believe that the CQDs have redox catalytical property and it is reasonable as CQDs has

numerous electrons and surface functional group.

This may have implication in water fuel cells, in which water was oxidised to oxygen and

reduced to hydrogen, and electricity is produced. Originally, the fuel cell reactions are catalysed

by traditional catalysts such as platinum and some transitional metal oxides, which are quite

expensive and may be noxious to the environment. Now, CQDs, which are easy to fabricate and

have low toxicity, can be used in fuel cells.

P. 32

6. Limitations In the project, lack of equipment was the major limitation.

Column for column chromatography was lacked, so burette was used instead. As the diameter

of the burette was too small, the column chromatography process was slow.

The wavelength of photoluminescence could not be known by naked eyes. The colour recorded

by camera is not accurate and subject to software and setting of the camera. A fluorometer

would be able to objectively collect fluorescence data. Due to lack of resouces, we could only

run one sample.

An electron microscope was needed to observe the carbon quantum dots and determine their

diameters since they were too small to be observed with light microscopes.

A photomultipler was required as the intensity of light emission from chemiluminescence of

carbon quantum dot was 10-14

watt 4, which could not be observed by naked eyes.

A thick layer of copper oxide was formed on the copper electrode during electrolysis. The

problem can be solved by using platinum electrode instead of copper electrode. However

platinum was not available due to limitation of resources.

P. 33

7. Possible Errors Sodium hydroxide granules were used instead of standardized sodium hydroxide solution. Due to

the hygroscopic nature of this alkali, the amount of sodium hydroxide used was not exactly the

same as that of the experiment required.

There was an error in voltage during electrolysis. As stated in the discussions section, attempts

were made in calibrating the voltages to the most accurate ones, but slight errors were still

present. For example, a voltage of 6V was preferred, but it turns out that the de facto voltages

applied were not the same.

The exact volume of solvent used to dilute (or concentrate) the CQDs solution was not

accurately known, thus an error was present.

Since the luminescence was observed with naked eyes, the colour of luminescence was hard to

be determined. For example, it was difficult to classify some vague luminescence as green or

blue.

P. 34

8. Conclusion Different CQDs were successfully synthesized by means of electrochemical carbonization. Most

CQDs solutions showed a PL response. There was an obvious trend of emitted lights, known as

the redshift phenomenon. Also, the catalytic property of CQDs was discovered circumstantially.

The decolourization always occurred when a powerful O.A. was added, which indicated that a

redox reaction took place. Unfortunately, the CL emission was unable to be tested but it was

believed that CQDs do exhibit CL properties according to the sources.

Besides, the current density during electrolysis and effect of concentration affected the

fluorescence property of CQDs. Because of its great variations of colour change, 6V CQDs was

found to have a widest range of visble light fluorescence property. Moreover, the fluorescence of

CQDs showed that the concentration of CQDs solution was directly proportional to its amount of

CQDs obtained.

Finally, as CQDs can be easy –made and have various applications in biomedicine, we hope that

CQDs can be widely used in the future.

P. 35

9. References [1] Carbon Quantum Dots: Synthesis, Properties and Applications Youfu Wang, Aiguo Hu, J

Mater. Chem. C. 2014, 2, 6921-6939.

[2] Water-Souble Fluorescent Carbon Quantum Dots and Photocatalyst Design Haitao Li,

Xiaodie He et al., Angew. Chem. Int. Ed. 2010, 49, 4430-4434.

[3] Chemiluminescence of carbon dots under strong alkaline solutions: a novel insight into

carbon dot optical properties† Lixia Zhao, Fan Di, Dabin Wang, Liang-Hong Guo, Yu Yang, Bin

Wan and Hui Zhang, Nanoscale, 2013, 5, 2655

[4] Classical oxidant induced chemiluminescence of fluorescent carbon dotsw Zhen Lin, Wei

Xue, Hui Chen and Jin-Ming Lin* Chem. Commun., 2012, 48, 1051–1053

[5] Wikipedia,. 'Quantum Dot'. N.p., 2015. Web. 2 May 2015.

Retrieved from http://en.wikipedia.org/wiki/Quantum_dot

[6] Wikipedia,. 'Luminescence'. N.p., 2015. Web. 2 May 2015. Retrieved from

http://en.wikipedia.org/wiki/Luminescence

[7] National Institute of Opening School., 'Biochemistry lesson 28'. N.p., 2015. Web. 3 May

2015. Retrieved from http://www.nios.ac.in/media/documents/dmlt/Biochemistry/Lesson-28.pdf

[8] Clark, Jim. 'Column Chromatography'. Chemguide.co.uk. N.p., 2015. Web. 3 May 2015.

Retrieved from http://www.chemguide.co.uk/analysis/chromatography/column.html

[9] Clark, Jim. 'Thin Layer Chromatography'. Chemguide.co.uk. N.p., 2015. Web. 3 May 2015.

Retrieved from http://www.chemguide.co.uk/analysis/chromatography/thinlayer.html

[10] Tissue, Brian. 'Molecular Fluorescence Spectroscopy'. Science Hypermedia. N.p., 2015.

Web. 3 May 2015. Retrieved from http://elchem.kaist.ac.kr/vt/chem-ed/spec/molec/mol-fluo.htm

P. 36

[11] Wikipedia,. 'Heterogenous water oxidation'. N.p., 2015. Web. 2 May 2015. Retrieved from

http://en.wikipedia.org/wiki/Heterogeneous_water_oxidation

P. 37

10. Acknowledgement We would like to thank the Department of Chemistry of our school for providing laboratory,

chemicals, materials and equipments. Thanks must also be given to the Department of Biology

for providing a centrifuge. Also, the academic reference is greatly acknowledged for their

inspirations to our project.

We would like to show sincere gratitude towards our chemistry teacher, Mr. Ho Kwok Wai, for

his unceasing support and guidance. We would also like to thank the laboratory technician Mr.

Kwok Yu for his kind help. Appreciation should also be given to our school janitors for

maintaining a clean laboratory for our experiments.

Thanks are given to Hong Kong Baptist Univerisity, Prof. Leung Cham Fai research group for

providing fluorescence spectroscopy experiment.

Last but not the least, we are truely grateful to our school, Christian Alliance S C Chan Memorial

College, for the funding and the high regard towards our research.

P. 38

11. Appendix I Wavelength of the colour filters

Colour Wavelength (nm) Filter with a

white LED

Purple 440

Blue 470

Dark green 490

Green 520

Light green 550

Yellow 580

Orange 590

Red 680

P. 39

12. Appendix II Fluorescence result of CQDs

Absorbed Blue to Green Dark green to

light green

Light green to

yellow

Orange to red

Before

After