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Hong Kong Chemistry Olympiad

for Secondary School (2014-2015)

Synthesis and Characterization of

Carbon Quantum Dots by

Combustion

Christian Alliance S C Chan Memorial College

Chow Chi Long (周智朗)

Lee Wan Ki (李韻琪)

Lee Tin Chun (李天進)

Shum Ching Yee (沈静儀)

Wu Hei Tung (鄔希彤)

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Abstract

In these few years, fluorescence quantum dots had attracted many attentions from the

scientists. Carbon quantum dots (CQDs), what we were amazed by, is one of the most

famous quantum dots as its beautiful fluorescence properties and low cytotoxicity. As we

desperately wanted to know how these little carbon particles throw away the dull

impression of carbon. We start our research.

In this project, we are investigating how to make CQDs and how to synthesize different

doped CQDs by burning different materials.

We carry out a general procedure which is burning papers soaked with different solutions

obtaining, N, S, P, to examine the amount of carbon quantum dots and try to create N

doped, S doped and P, doped carbon quantum dots.

Then, we choose foods with high percentage in N and S to try making up distinct doped

CQDs, like the first procedure but more related to our daily life.

Our experiments were be just carried out in the school laboratory.

We hoped that our tests can help obtaining carbon quantum dots in a more economical and

environmentally–friendly way.

Three of our CQDs, under the excitation of UV light, synthesized in our projects.

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Content

Introduction ........................................................................................................................ 4

Colours ........................................................................................................................... 4

Fluorescence ................................................................................................................... 7

Carbon Quantum Dots .................................................................................................... 8

Part A Synthesis of CQDs by combustion of paper soaking with different solutions ..... 10

Experimental procedures (Paper alone) ....................................................................... 12

Experimental procedures (Paper soaked with salt solutions) ....................................... 14

Results .......................................................................................................................... 16

Appearance of the resulting solution ........................................................................ 16

Fluorscence under UV light (365nm) ....................................................................... 17

Part B: Synthesis of CQDs by combustion of Domestic Materials (Foods). ................... 18

Food contains high percentage of Carbon – Rice and Sugar (Carbohydrates) ............ 19

Food contains high percentage of Nitrogen –Milk Powder and Soya bean (Protein) .. 20

Food contains high percentage of Sulphur – Beef , Garlic and Rotten eggs ................ 22

Experimental precedures .............................................................................................. 24

Results .......................................................................................................................... 25

Fluorscence under UV light (365nm) ....................................................................... 26

Fluorescence spectroscopy ....................................................................................... 26

Adding into KMnO4(aq) ........................................................................................... 27

Discussion ........................................................................................................................ 29

Conclusion ....................................................................................................................... 33

Limitation and future work .............................................................................................. 34

Acknowledgement ........................................................................................................... 35

References ........................................................................................................................ 36

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Introduction

Chemistry is always fascinated with colours. We can see colours changes in different

reactions. Different chemical substances such as organic pigments, transition ions and

nano-materials pose different colours. Colour has amazed human beings over centuries.

Before we get into the wonderful world of chemistry of colour, let us explore what

colour actually is.

Colours

Colour is the visual perceptual property corresponding in humans to the categories.

Different colour we can see is actually electromagnetic wave with different wavelength

that gets in our eyes.

There are many kind of electromagnetic wave in our world, such as UV light and

infra-red. Visible light has wavelength between about 700nm (Red) and 400nm (Violet).

A beam of white light we usually see is actually the combination of electromagnetic

wave with different wavelength. (Figure 1)

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Figure 1 A figure showing the wavelength of EM waves and their corresponding energy.

Colour categories are associated with objects or materials based on their physical

properties such as light absorption and reflection. For example, when we see a blue

object, it means that the object had reflected the blue light to your eyes and absorbed

light with other wavelength. An object can also transmit light or emit light themselves,

which also contribute to the colour. (Figure 2)

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Figure 2 A figure showing an object which absorb light other than blue, showing a blue colour in appearance.

According to the photoelectric effect proposed by Albert Einstein in 1905, other than

wave, light can be also particles called photons during energy transfer. When the energy

of photons was absorbed by an object, then the object will release the energy absorbed

by other different ways such as heat. It is the reason why we can see object with different

colours.

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Fluorescence

Fluorescence is the emission of light by a substance that has absorbed light or other

electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light

has a longer wavelength, and therefore lower energy, then the absorbed radiation the

most striking examples of fluorescence occur when the absorbed radiation is in the

ultraviolet region of spectrum, and thus invisible to human eyes, and the emitted light is

in the visible region. (Figure 3) Carbon Quantum Dots in our project show characteristic

fluorescence properties.3

Figure 3 A figure showing the energy transition during a fluorescence.

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Carbon Quantum Dots

Carbon quantum dots (CQDs) are generally small carbon nanoparticles (less than 10nm

in size) with various unique properties. Scientist have known more about it in the last

few years. CQDs shows fascinating fluorescence property and is easy to prepare in

secondary school laboratory so we have chosen CQDs in our research.

CQDs have many properties, such as good solubility, strong luminescence, and facile

modification. Also, it has low cytotoxicity and good biocompatibility.

CQDs can be synthesized by several synthetic methods. Up to date, chemical ablation,

electrochemical carbonization, laser ablation, microwaves irradiation and hydrothermal

treatments1 are common method to prepare CQDs in research laboratory. However, we

adopted a novel method in this project which is recently reported by Wei2.

Another important modification which makes CQDs versatile is doping. CQDs doping is

a widely used approach to tune the Photoluminescence properties of photo-luminescent

material. Various doping methods with different elements such as N and P have been

reported to tune the properties of CQDs.

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This project is divided into two parts:

Part A: Synthesis of CQDs by combustion of paper soaking with different solutions

Part B: Synthesis of CQDs by combustion of Domestic Materials (Foods).

The CQDs synthesized was characterized by fluorescence under UV light (wavelength:

365nm) and addition of KMnO4.

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Part A Synthesis of CQDs by combustion of paper soaking with

different solutions

Fluorescent CQDs can be prepared from a number of methods as nowadays many

researchers have been studying about that, for instant, laser ablation, arc-discharge,

electrochemical oxidation, acid dehydration, microwave heating etc. However, most of

them involve an expensive carbon source, complex reaction, time-consuming processes

and post-treatment processes, which limit the wide applications of CQDs.

However, a more convenient, simpler and low-cost way has been discovered recently.

Waste papers can be used as a primary source. Fluorescent CQDs are formed from the

ash after burning the papers. Our group adopted this synthetic method to obtain water

soluble fluorescent CQDs.

The mechanism for the formation of CQDs from this synthesis method involves the

carbonization of the major constituents of paper such as cellulose, hemicellulose and

lignin.1 The obtained CQDs have excellent water solubility, well distributed particle size,

acceptable fluorescent life time, fairly high photoluminescence with a quantum yield of

9.3%, high photo- stability and biocompatibility, which can be successfully applied in

cellular imaging.

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This synthetic method is low-cost and can be easily carried out in secondary school

laboratory. We simply use waste paper instead of expensive carbon source or strong acid.

In addition, it is a method of recycling of waste papers and large scale production of

CQDs. This is ’kill two birds with one stone’ strategy.

This synthesis method is simple and convenient, as the procedure and steps are very

straight forward. We have burnt waste papers to produce fluorescent CQDs from the

paper ash. The details of procedures will be discussed later.

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Experimental procedures (Paper alone)

1. 20 sheets of A4 papers was burnt and the ashes were collected. (Figure 4)

2. The ashes was added into 250mL deionized water and the solution was stirred

for 30 minutes.

3. The solution was then treat by ultrasonic bath for 5 minutes.

4. The resulting solution was placed in centrifuge and the centrifuge was operated

at 3000 rpm for 5 minutes.

5. The solution was filter by filter paper and filter funnel.

6. The solution is heated under Bunsen burner and the solution was concentrated

by evaporation. (Figure 5)

7. Concentrated CQDs solution was obtained. (Figure 6)

Figure 4

Figure 5

Figure 6

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After successfully obtain fluorescence CQDs solutions, we modify the procedure so as

to investigate if different CQDs could be obtain by immersing the paper with some

common salt solutions. It is commonly believe that CQDs can be doped with different

atoms and give out unique fluorescence. Different solutions were used and different

doping is expected.

1. Al (NO3)3 (aq): try to obtain Al doped CQDs

2. NaNO3 (aq): try to obtain N doped CQDs

3. Na2SO4 (aq): try to obtain S doped CQDs

4. Na3PO4 (aq): try to obtain P doped CQDs

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Experimental procedures (Paper soaked with salt solutions)

1. The papers immersed in different solutions. (Figure 7)

2. The papers were dried with oven for 60 minutes. (Figure 8)

3. The dried papers were burnt and the ashes were collected.

4. 250mL DI water was added and the solutions were stirred by magnetic stirrer

for 30 minutes

5. The resulting solution was placed in centrifuge and were operated at 3000 rpm

for 5 minutes.

6. The solution was filter by filter paper and filter funnel.

7. The solution is heated under Bunsen burner and the solution was concentrated

by evaporation. (Figure 5)

8. Concentrated CQDs solution was obtained.

9. The soluble impurities were removed by dialysis.

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Figure 7

Figure 8

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Results

After obtaining suspected CQDs solutions, we characterized the solution by 2 main

methods. Limited to instrumental analysis, we could only characterize the CQDs by

some simple chemical test.

1. To observe whether the solution has fluorescence property.

2. To observe whether the solution decolourized KMnO4(aq). 4

KMnO4(aq) was found to induced chemiluminiscence of CQDs, a tiny or catalytic

amount of CQDs could decolourize KMnO4(aq) solution. This could be used as a

chemical test to confirm the successful synthesis of CQDs.

Appearance of the resulting solution

Solutions N/A Al(NO3)3 Na2SO4 Na3PO4 NaNO3

Appearanc

e

We observed that the appearances of solutions of burning papers are similar. They are

mostly pale yellow or colourless.

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Fluorscence under UV light (365nm)

Solutions N/A Al(NO3)3 Na2SO4 Na3PO4 NaNO3

Fluorescence

under UV

We can observed that all of the above solution has fluorescence property, most of them

are green under UV. However, and it is still a little bit colour different between them

especially the NaNO3(aq) and the others. It is far brighter comparing to the others.

The result of the KMnO4(aq) test is collectively discussed in the below sections.

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Part B: Synthesis of CQDs by combustion of Domestic Materials

(Foods).

As what we mentioned above, through experiments, we discovered that CQDs can be

obtained by burning paper to ashes.

After doing a few experiments of burning paper, we started to think about burning some

other things that can be obtained easily in our daily life other than paper so as to make our

project more relevant to life. We decided to use food as our next target of experiment as

food is easily scorched in our daily life and it is one of the materials that can be obtained

extremely easily. We wonder whether a piece of seriously scorched food will produce

CQDs as what paper ashes did.

Also, food contains different kind of organic compounds, and so through burning food,

we may obtain doped CQDs.

This is how the project developed in to a novel one.

In this section, we will introduce the foods that are burnt to yield CQDs.

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Food contains high percentage of Carbon – Rice and Sugar (Carbohydrates)

Rice and white sugar are two of our most common foods in daily life. As a Chinese, we

usually take rice as our main course while we use sugar as our seasoning, so it is much

related to our daily life.

Both rice and white sugar contain very high percentage of carbohydrates. 80% of rice is

starch [(C6H10O5)n] while 99.98% of white sugar is sucrose (C12H22O11). Carbohydrates

have a high percentage of mass of carbon in its polymer structure. As a result, we expect

that we can obtain a large amount of CQDs by burning rice and white sugar. We expect

the fluorescence will be strong when we test it with ultra-violet light. The figure below

shown the black powder after combustion.

Figure 9 Sugar after burning.

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Food contains high percentage of Nitrogen –Milk Powder and Soya bean

(Protein)

Shunde Double Layer Milk Custard (雙皮奶)

Shunde Double Layer Milk Custard (雙皮奶) is a famous local Cantonese dessert which

is very nutritious. Lactose (C12H22011) made up the largest part of carbohydrates in the

dessert which can be easily absorbed by human beings. Beside, rich supply of proteins

(mainly consist of C, H, O, N and small amount of S) and a relevantly small amount of

lipids (made up with C, H, O) can be found in the food too.

In order to testify whether creating CQDs in carbon-containing food is possible. We

make double layer milk our option because it is one of the most famous local dessert in

Hong Kong which closely related to our daily life. Since double milk custard contains

nitrogen (N) and little amount of sulphur (S), we think there is a chance that we can

produce N-doped. Since we didn’t put high expectation into this experiment as by the

moment we have no clear evidence that CQDs can be produced in food and we do not

know the difference of carbon content between papers and milk custard, we predicted

we will be receiving very little CQDs. Then, we started our experiment with our general

procedures.

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Soya Bean Custard(豆腐花)

We are going to burn soya bean custard which is a traditional dessert of Chinese. Its

main ingredient is soya bean, which is a plant rich in protein and therefore nitrogen rich.

Soya bean is mainly formed by carbohydrates and proteins. Therefore it is possible to

produce CQDs which are highly possibly doped by Nitrogen. (Figure 10)

We expect that the solution with carbon quantum dots produced in this experiment will

show same colour as NaNO3 as it is expected that nitrogen is doped in the experiment.

Figure 10 A photo showing the Soya Bean Custard in burning.

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Food contains high percentage of Sulphur – Beef , Garlic and Rotten eggs

Beef

We have also chosen beef steak as a trail. It contains sulphur and nitrogen such that we

may obtain N/ S-doped CQDs.

As 3 pieces of beefsteaks can produce many ashes (totally about 24.87g) after burning,

we divided them into two separate set-ups.

N-Doped CQDs has enhanced PL emission when compared with normal. Therefore, we

expect the colour with absorbing UV light will be different if the CQDs is doped.

Figure 11 Beef when burning

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Garlics

S-doped carbon quantum dots have low cytotoxicity and excellent luminescence

properties, so we are trying to produce some by burning garlics which is high in sulphur.

It is expected that the CQDs will show the similar fluorescence colour, blue, as carbon

quantum dots produced by paper doped with sodium sulphate.

Rotten eggs

In this experiment,we are going to make CQDs by burning the rotten eggs. Eggs are rich

in sulphur and carbohydrates .Eggs are cheaper in prices compare to most of the foods.

We expected the CQDs will undergo excited(the fluorescence) like other general result

like burning paper.

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Experimental precedures

1. The foods were burnt and the ashes.

2. 250mL DI water was added and the solutions were stirred by magnetic stirrer

for 30 minutes.

3. The resulting solution was placed in centrifuge and the centrifuge was operated

at 3000 rpm for 5 minutes.

4. The solution was filter by filter paper and filter funnel.

5. The solution is heated under Bunsen burner and the solution was concentrated

by evaporation.

6. Concentrated CQDs solution was obtained.

7. The soluble impurities were removed by dialysis. (Figure 12)

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Figure 12 Solution undergoes dialysis. From the left, milk powder, soya bean custard, rice and sugar.

Results

Appearance of the solutions

Different from burning paper, we can observe that burning food may obtain darker

solutions except of eggs, which is yellow in colour while other was generally brown.

Appearance of CQDs from food

Steak Egg Rice Garlic Milk Soya bean Sugar

powder custard

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Fluorscence under UV light (365nm)

Food Steak Egg Rice Garlic Milk powder

Fluorescence

under UV

Soya bean

custard

Sugar

Fluorescence

under UV

Fluorescence spectroscopy

Figure 13 Fluorescence spectrum of CQDs sample Steak

0

100

200

300

400

500

600

700

800

350 450 550 650 750 850 950

Flu

ore

sce

nce

inte

nsi

ty

Excitation wavelength/ nm

Fluorescence spectrum of CQDs sample Steak

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Adding into KMnO4(aq)

Acidified KMnO4(aq) is well known as strong oxidizing agent. CQDs are electron rich

nanoparticles, which could be reacting with acidified KMnO4(aq). CQDs was known to

have chemiluminiscence properties under oxidizing agent such as acidified KMnO4(aq).

As a result, acidified KMnO4(aq) could serve as a chemical test for CQDs assuming

there is no other substances in the solution is reducing agent which should be a

reasonable assumption since the food was burnt and supposed to be oxidizing.

2 drops of CQDs solution was added to acidified KMnO4(aq) solution. If there is CQDs

presence, the solution will be decolourized very quickly in 5 minutes.

Here is the result of the test. (Figure 14)

Figure 14 CQDs solution added to KMnO4 and waited for 5 mins

KMnO4 Milk

powder

Soya

Bean

Custard

Eggs A Rice Garlic Beef

steak

Sugar Eggs B

Remarks: Eggs A is the solution outside the dialysis tubing while Eggs B is that inside.

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From the above diagram and photos, we can abserve that, as our expectation, Shunde

Double Milk Skin Custard, Soya Bean Custard, Garlic and Beefsteak contain pretty

large amount of CQDs in the solutions.

However, there is still something out of our expectation. First, burning rice and sugar,

which we think we can obtain more CQDs from than burning papers actually didn’t

work. From the testing above, we can observe that sugar didn’t show to contain any

CQDs while rice contains just a little.

For the case of egg, during the dialysis, there is some fluorescence solution form after

sometime. It may be due to leaking or the the size of CQDs are too small. But both the

fluorescence and KMnO4 test shows that it should be CQDs.

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Discussion

Part A

We characterized the CQDs solutions with a 365nm ultra-light lighter and potassium

permanganate solution (KMnO4).When we put the solutions under UV light, we

observed that the fluorescence are generally weak while some sample even did not show

any fluorescence. When we test these solutions with potassium permanganate solution,

the colour of the solution change from pink to pale pink at a low rate and some samples

have not colour change at all. These results suggest that CQDs can be made by burning

papers but the yield is very low. Also, a little difference in fluorescence colour under

UV light is observed among different solution. This suggest N, S or P doped CQDs

might be synthesized by this method. However, due to limitation of instrument, we

cannot confirm the real elemental composition of the CQDs.

In conclusion, CQDs can be made in burning papers though the amount is small and, by

soaking papers with different solutions, different types of doped CQDs can be produced.

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Part B

We characterized the final solutions with a 365nm ultra-light lighter and potassium

permanganate solution (KMnO4). When we put the solutions under UV-light, we

observed that except sugar, all solutions made up from burning food emit strong

fluorescence (either green or blue). This suggested that there is a larger amount of CQDs

made from burning of food rich in carbon. We also test the amounts of CQDs in the

solutions with potassium permanganate by observing the rate of colour change when

solution added. We observed that the in the table, colour change of solution made by

burning of carbon containing food from pink to colourless is generally faster than the

that of CQDs in part A. These results suggest that more CQDs can be found in burning

carbon containing food than in burning papers. This is out of our expectation and we are

amazed by the novel method we proposed.

The reason why the result is different from our expectations is as follow. We think that

carbohydrates is a poor CQDs precursor. As observed in burning paper, sugars and rice,

the yield of CQDs produced is low. They are all carbohydrates which are very similar in

chemical nature. This suggest all carbohydrates do not yield CQDs during burning.

While the egg and steak CQDs has exceptionally strong fluorescence under UV light.

Together with NaNO3(aq), this may suggest all of them are N – doped CQDs.

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The Fluorescence spectrum of steak was obtained. From the spectrum, we observed that

steak CQDs has very strong fluorescence at UV – violet range. This result support our

naked eye observation.

Based on the data stated above, we concluded that burning of carbon containing food

can produce a large amount of CQDs and even more than papers. Also, we successfully

proved that CQDs involved in daily life and can be founded during burning of food. In

conclusion, our experiment was very successful although the scorched smells released

during burning (especially garlic) challenge everyone’s temper in the laboratory.

Implication of this project

1) In incinerator

In many countries, there are incinerators. In this project, we discovered that CQDs can be

produced in the burning of food. We strongly believe that incinerators produced tones of

precious CQDs every day. If we can collect the CQDs from the incinerator, we may turn

the waste into something useful.

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2) Fluorescence in situ hybridization (FISH)

FISH is a technique developed to detect and locate the absence of specific DNA in

chromosomes in creatures by detecting fluorescence probe bound to chromosomes with

fluorescence microscopy. FISH is important especially in diagnosis. Parents with children

suffering developmental disability can have their DNA checked and decide whether shall

they have another baby. We believed with the discover of CQDs and cheap production of

CQDs, FISH can become easier and cheaper,more people will be benefit from the

discovery and the development of chemistry.

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Conclusion

The aim of our project is to study the synthesis and characteristics of fluorescent CQDs

by the method of burning papers or food. Different CQDs were produced successfully

from different precursor material. CQDs are characterized by two methods. First, the

resultant chemicals have strong fluorescence under UV-light. Second, it can decolourize

KMnO4.

Other than burning papers alone in order to produce CQDs, we had also try to burn

papers soaking with salt solution to study the effect of the different kinds of ions toward

the properties of CQDs. Although we cannot prove or ensure that doped-CQDs are

produced, we produced CQDs with different fluorescent colours under UV light. They

can give out different colour such as green or blue.

Apart from producing CQDs by papers, we believe it can also be done by food

containing carbon. We tried different food containing sulphur or nitrogen. The purpose

of it is also to study the effect of the different chemical elements toward the fluorescence

property of CQDs. It seems that the fluorescence is stronger than that of burning papers

because what we can observe is higher fluorescence colour intensity. Similarly, we

cannot confirm doping had been done; in fact the colours given out under UV light by

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different resultant CQDs are different. The CQDs from steaks and egg are highly

suspected to be N – doped CQDs due to its strong fluorescence under UV light.

In conclusion, water soluble CQDs was successfully produced by combustion of food.

The products show strong fluorescence under UV-light and decolourized KMnO4(aq).

Also, the method development in this project is found to be more efficient than that

reported before. 2

Limitation and future work

Due to limitation of instruments, further study of products is not possible. For example,

Transmission electron microscopy (TEM) can be used to measure the size of the CQDs.

Fluorescence spectroscopy can be used to characterize the fluorescence properties of

more samples of CQDs. X-ray photoelectron spectroscopy can be used to analyze the

elemental composition of the CQDs which further investigate whether the CQDs are

doped.

Also, the CQDs solutions that does not have strong fluorescence under UV may have

fluorescence in visible light range. A further study can be done to further investigate the

fluorescence properties of CQDs.

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Acknowledgement

Chemistry and Biology Panel of our school

Chemistry teacher – Mr. Ho Kwok Wai

Chemistry laboratory technician – Mr. Kwok Yu

Classmates for the support

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References

1. Y. Wang, A. Hu, J. Mater. Chem. C., 2014, 2, 6921-6939

2. J. Wei, J. Shen, X. Zhang, S. Guo, J. Pan, X. Hou, H. Zhang, L. Wang, B. Feng,

RSC Adv., 2013, 3, 13119–13122

3. Wikipedia:

a. Fluorescence: http://en.wikipedia.org/wiki/Fluorescence

b. Colour: http://en.wikipedia.org/wiki/Colour

c. Garlic: http://en.wikipedia.org/wiki/Garlic

d. Soybean: http://en.wikipedia.org/wiki/Soybean

e. Allicin: http://en.wikipedia.org/wiki/Allicin

f. Ajoene: http://en.wikipedia.org/wiki/Ajoene

4. L. Zhao, F. Di, D.Wang, L. H. Guo, Y. Yang, B. Wan, H. Zhang, Nanoscale, 2013,

5, 2655–2658

5. Livestrong.com ,“What food are high in nitrogen?”, By Jessica Bruso

http://www.livestrong.com/article/313163-what-foods-are-high-in-nitrogen/

6. New Senior Secondary Mastering Biology 1A, By H W Yung, K M Ho, Y K Ho, K

H Tam, L P Tong, pressed by Oxford University Press

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7. Bold sky Limitless Living, “12 Sulphur Rich Food For Your Skin”, posted by

Anwesha

http://www.boldsky.com/health/nutrition/2013/twelve-sulphur-foods-for-your-skin-0322

44.html

8. Health Alicious Ness, “Top 10 Food Highest in Phosphorus”

http://www.healthaliciousness.com/articles/high-phosphorus-foods.php

9. Jumeng Wei, Jianmin Shen, Xin Zhang, Shikuan Guo, Jiaqi Pan, Xiaogang Hou,

Haibin Zhang, Long Wang and Boxue Feng, RSC Advances, 2013, 3, 13119

10. Yofu Wang, Aiguo Hu, J. Mater. Chem. C. 2014. 2. 6921

11. Fluorescence In Situ Hybridization-FISH:

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