By Anne Jing
Background and Introduction
Carbon quantum dots are quantum dots derived from any carbon source. This report exclusively uses gelatin as the carbon source. Through experimentation, a repeatable procedure is developed for the synthesis of quantum dots using gelatin through the process of hydrothermal synthesis. The carbon source was dissolved in water and then acidified. The mixture was then poured into the hydrothermal synthesis reactor with minimal oxygen—allowing for incomplete combustion. The reactor was then placed in an oven at a temperature set accordingly to the desired colour of the quantum dot and incubated for a duration of time. After the reaction was done, the compound was then diluted to a specific ratio of solution to water.
Many tests were also performed to gather various information about the solutions. First, the wavelengths emitted were measured using a fluorescent spectroscopy analyzer. Two samples synthesized at different temperatures displayed different wavelengths on the graph. This proves that the size, and therefore colour, was able to be manipulated by changing the temperature. An electron microscope was also used to measure the diameter of the quantum dots. Through the images, it is apparent that the diameter of the dots changed within various solutions synthesized at different temperatures.
The effectiveness of carbon quantum dots is determined by comparing qualities to those of CdSe quantum dots. Some include, but not limited to, are the shelf life of the substance, intensity of light, percent yield, cost, and the rate of reproduction.
Quantum dots have many applications ranging from bio imaging to making TV screens (2,3). Carbon quantum dots not only have the potential to substitute CdSe quantum dots but can be more efficient as the production of these dots are financially reduced and the toxicity is much lower than CdSe quantum dots.
Quantum dots are nanocrystals of a semiconducting material that emit a specific wavelength under an UV-A light (known as a black light).5 These wavelengths are physically observed as different colours along the colour spectrum. Their quantum sized dimensions allow for usual properties with the most evident being luminous and fluorescent under ultraviolet light (1). The colour produced is directly proportional to the size of the particle. Smaller particles emit shorter wavelengths and thus, produce a colour near the blue end of the spectrum. Inversely, as the radius of the particle expands to grow in size, it results in a longer wavelength and thus, radiates a colour near the red end of the spectrum.1 Through hydrothermal synthesis— the process of which crystals can be synthesized under high temperatures and pressures— the colour emitted can be adjusted by manipulating the size of the particle with the addition or absence of heat.
Microscopically, this property is observed as the quantum confinement effect—the phenomenon that gives quantum dots the unique properties (1). As particles start to become Nano-sized, considered to have zero dimension, the quantum confinement effect starts to take over (6). As particles of semiconductors decrease in size, the electrons are squeezed and confined into a smaller and non-preferred orbital; thus, giving unusual properties to the atoms. Chemists use the analogy of “particle in a box” to explain this phenomenon (1). As the particle size decreases, the radius of the semiconductor approaches the exciton Bohr radius—the average distance between the electron in the conduction band and the hole that was once filled by said electron (1). As illustrated by Pauli’s exclusion principle, there is a quantization of energy levels. In a bulk semiconductor, there is an infinite number of states at which the electron can be at.1 This is because as the size of the “box” increases, there are numerous positions at which the electron’s wave can be (1). As the “box” gets smaller and smaller, getting closer to the electron, the number of possible states decreases, causing an increase in the band gap—the gap that must be passed for an electron to move from the valence band to the conduction band (1). As the quantum confinement approaches to existence, the energy of the electron becomes more discontinued—meaning that there are distinct states at which the electron can and cannot be (1). The quantum confinement effect is only applicable when the size of the particle is too small to be compared to de Broglie’s wave equation:
Carbon Quantum DotsCarbon quantum dots (CQD) are quantum dots synthesized from carbon, as opposed to a metal alloy compound such as cadmium selenide (CdSe) (1). The synthesis of CQD uses the same principles as traditional quantum dots. CQD do not compose of toxic compounds like CdSe and therefore is applicable to applications such as bio-imaging. Carbon is also an easily accessible material as it is an organic compound which will make mass production financially friendly.
The purpose of this experiment is to create a reproducible, controllable procedure for the synthesis of quantum dots using gelatin to replace the toxic CdSe quantum dots.
It is hypothesized that, the bonds between the carbons are able to be broken and the quantum confinement effect will allow for the synthesis of CQD through heat and acid. It is also hypothesized that colour can be manipulated by changing temperature during synthesis.
MATERIALS AND METHODS
Three substances were used to synthesize CQD: Knox gelatin (found in local grocery stores), distilled water (found in local grocery stores), and 12 M Hydrochloric acid (HCl(aq)) (found on www.sigmaaldrich.com). Many equipment were used in the procedure. The hydrothermal synthesis reactor consisted of a 1 ¼ in. X 4 in. galvanized steel pipe nipple, two 1 ¼ in. pipe caps, and Gas Teflon tape—these materials can be found in a local hardware store. Some common lab equipment used consists of: beakers (one must be 200mL), beaker tongs, test tubes, test tube rack, rubber stoppers, glass stirring rod, hot plate, electronic scale, channel locks/pipe wrenches, vials with lids, UV light, vice grip, blast shield, small oven, fume hood, heat resisting gloves, squeeze bottle, graduated pipette, transfer pipette, graduated cylinder, paraffin paper, filter paper, funnel, 200 nm syringe filter and tweezers. These items can be found online at www.sigmaaldrich.com. The equipment used to analyze the results were a spectrometer (from Assumption College School), spectrofluorometer (from the University of Waterloo), a FEI Titan 80-300 HB Electron microscope, and carbon grids (from the Canadian Centre for Electron Microscopy).
Four trials were conducted in this experiment. The purpose of the first trial was to conclude the right ratio between the reactants and the right dilutions to allow for optimal intensity. 0.8 mL of HCl(aq) was added to 100 mL of boiled, distilled water. Next, 42.21 g of gelatin was added into the solution. The mixture was then sealed in a reactor and incubated at 453 K (180°C) for eight hours. The ratio was diluted to one part gelatin mixture to thirty-nine parts distilled water. The solution was filtered through a qualitative filter paper and a 200 nm syringe filter to ensure purity. This was labeled V1, D. The purpose of the second trial was to replicate the same results ensuring the procedure developed from the previous trial was repeatable. However, the reactor was not sealed property so the solution had oxidized and solidified, producing a black solid. 1 g of the black solid was then re-dissolved in 39 mL of water—the same ratio as trial 1. Due to the error, a third trial was performed to ensure reproducibility. This was labeled V1T2, D. The purpose of the third lab was to test the controllability of the CQD and determine the colour range that is able to be produced. The temperature was raised to 505 K (232°C) and solution was diluted to 1:79. The rest of the procedure remained constant. By raising the temperature, it is hypothesized that the colour would change, specifically towards the red end of the spectrum. This was labeled V1T3, E. A fourth trial was completed to replicate the initial trial to ensure reproducibility. The initial procedure was used. This was labeled V1T4, D.
Trial 1 concluded a procedure to synthesize CQD. The CQD glowed a distinctive blue colour. It was then compared to trial 3 which glowed a distinctive green colour (Figure 1). Trial 4 was conducted to ensure the reproducibility of the solution. Figure 2 compares trial to trial 4. Trial 2 was originally intended as a test to replicate trial 1, however, an error occurred, producing a new outcome. It is apparent that by re-dissolving the solidified gelatin, it produced a green mixture. Figure 3 displays the comparison between V1, D and V1T2, E. It is noticed that V1T2, E glowed a slightly brighter green than V1T2, D.
An optical spectrometer was used to detect the quantum dot’s wavelength and intensity. Four different dilutions of the V1, D sample were analyzed (Figure 4).
A fluorescent spectroscopy analyzer was used to measure the wavelength of each sample. Figure 5 compares the wavelengths emitted from V1, D (blue) and V1T3, E (green). A shift in the graph’s peaks towards longer wavelengths is apparent. It is observed that V1T3, E has a lower intensity, however, it is determined that intensity is independent from temperature but rather depended on the concentration of the solution.
The wavelength emitted from sample V1T2, D was measured (Figure 6). The graphs showed two peaks. This suggests that there are two different wavelengths emitted from the sample and thus, containing two different sized QD within the sample. The higher peak suggests that there are more CQD with the longer wavelength therefore, producing a green colour.
An electron microscope was also used to determine the diameter of the dots. Figure 7 shows an image of the CQD of the V1, D sample under an electron microscope. It is evident these dots show relatively uniform diameter (of around 10 nm - 11nm) and shape.
A sample of V1T3, E was also measured by an electron microscope (Figure 8). These dots also show uniform diameter (of around 25 nm) and shape. A change in diameter was observed—more than double in size. This proves that the size, and therefore wavelength, of CQD are able to be manipulated by a change in temperature. V1T3, E was conducted 505 K. It is evident that there is an increase in diameter (as proven by images from the electron microscope) and an increase in wavelength (as proven by the spectroscopy graphs).
The diameter of the dots within V1T2, D was also measured using an electron microscope. Figure 9 shows two different sized dots within the sample. It was also noticed that some dots are stacked on top of each other. It is observed that there are more quantum dots with diameters of approx. 35 nm than those of approx. 20 nm. These images, along with the fluorescent spectroscopy graph of V1T2, D support the presence of a greater amount of larger sized CQD within the V1T2, D sample.
The hypothesis of creating a reproducible and controllable was supported. A repeatable and alterable process of synthesizing CQD was developed through the process of hydrothermal synthesis. Two colours were able to be achieved by increasing the temperature during synthesis. The apparatus used to synthesize CQD limited the range of possible colours produced as a higher temperature than 505 K was unachievable. When compared to CdSe quantum dots, it is concluded that the process of hydrothermal synthesis has poor control over the size of the CQD as CdSe produced a wider range of colours. The price to synthesize 25mL of CQD is approx. $7, compared to the price to synthesize 25mL of CdSe quantum dots of $607, is 1.15% of the cost to make CdSe quantum dots. CQD displays no evidence of serious health hazards—unlike the carcinogenetic CdSe quantum dots. CdSe quantum dots have a self-life of approx. 3 years. The self-life of CQD is inconclusive and is effective to present (April 16, 2018). However, it is also concluded that the synthesizing process is eight times longer than synthesizing CdSe quantum dots and is more difficult to achieve specific colours. If time and specific colour do not limit the practical use of quantum dots, it can be concluded that CQD hydrothermally synthesized from gelatin is an effective substitute for CdSe quantum dots.
I express my gratitude towards my chemistry teacher, Mr. William Dunlop and Mr. David Page, Department Head of Science at Assumption College. I would also like to thank the University of Waterloo, Dr. Laura Ingram and Dr. Kristina Lekin in the Department of Chemistry. In addition, I would like to thank Dr. Andreas Korinek of the Canadian Centre for Electron Microscopy.
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About the Author
Hi! My name is Anne Jing and I’m a grade twelve student at Assumption College. I have a passion for science and innovation. In my grade eleventh year of high school, decided to apply my love for chemistry in a real-world problem. My hobbies include cooking, stargazing, and playing the piano. I also love sports; I play volleyball, tennis, badminton, and I like to snowboard and swim.