Recently I participated in an online activity, related to the Bachelor of Science in Chemistry, organized by a university. The activity included an experiment with quantum nanodots that could be done at home, and, as you can imagine, I was beyond excited about this! In this blog about the experiment, I will first give some background information about quantum nanodots and Vitamin C, and I will then explain the whole experiment.
Quantum nanodots, what are they?
Quantum nanodots are semiconductive particles, with a size in the order of nanometers [1], that have many applications like bioimaging, catalysis, drug delivery, and optronic purposes like solar cells. [7] Because there are many different types of quantum nanodots, there are also many ways to produce them.
After the synthesis, the quantum nanodots can be used in many interesting ways. This is because of the electronic and optical properties. These properties are a bit different than those found in larger particles. Therefore, quantum nanodots have a special part in, for example, nanotechnology. [1]
Having optical properties means that quantum nanodots, among other things, can be illuminated by UV light. [6] UV light, which has a wavelength in the order of 10-7 meters (100-400 nm), has enough energy (around 10 electron Volts = 16,02176565 Joule) to get an electron in the quantum nanodots to a higher state of energy, an excited state. When the electron drops back to its original state, it releases the gained energy by the emission of light. This emission is called fluorescence (an optical effect) and can be used to do measurements. [5]
A specific type of quantum nanodots is carbon quantum (nano)dots. Positive properties from this type of quantum dots are the relatively simple synthesis, low toxicity (compared to inorganic quantum nanodots), and environmental friendliness, along with the fact that Carbon quantum dots have the same optical properties as 'regular' quantum nanodots. [6]
The synthesis of carbon quantum dots is divided into two categories, 'top-down' and 'bottom-up routes. ‘Top-down’ is a synthesis that works by breaking down larger carbon structures like carbon nanotubes or graphite with things like lasers or electrochemical techniques. ‘Bottom-up’, the route used in this experiment, is based on using small precursors (like with the quantum nanodots). An example of a precursor is citrate (citric acid). The precursor and a nanocomposite form carbon quantum dots through hydrothermal treatment or microwave synthetic routes. [7]
What makes carbon quantum dots even more interesting is, that in some cases, the fluorescence in these carbon quantum dots seems even stronger, compared to the regular quantum nanodots.
There are still a lot of uncertain explanations for this occurrence, among which the violation of Kasha's rule. Kasha's rule states that fluorescence always occurs in an appreciable yield from the lowest vibration level of the first excited state to the ground state. This means that fluorescence can only occur when an electron goes from the lowest excited state to the ground state, and not when an electron goes from a higher excited state to a lower, but still excited state. [8] Here the energy is lost in the form of heat via non-radiative vibrational relaxation and internal conversion. When Kasha's rule is 'broken', photoluminescence does occur between two excited states. This could explain why carbon quantum dots seem to emit more light than quantum nanodots, which is a positive property and makes Carbon quantum dots very suitable for measurements like the one discussed in this blog.
Vitamin C
Vitamin C is the more common name for ascorbic acid, a white or slightly yellow solid, and is one of the essential vitamins our body needs in order to perform certain processes.[2,4] Ascorbic acid is an organic molecule with C6H8O6 as a chemical formula. It has a molar mass of 176.124 grams per mole [3] and is soluble in water (330 grams per liter).
When it comes to chemical reactions, ascorbic acid will most likely participate in an acid-base reaction (as a weak acid). In this case, the ascorbic acid will give one positively charged hydrogen atom, a hydrogen ion, to a base molecule, and will become negatively charged as C6H7O6-. This anion called ascorbate [2] can form salts with several cations (positively charged) like calcium, potassium, and sodium. Next to this, ascorbic acid can also react with other organic acids as an alcohol-forming ester.
How the experiment works
The experiments consisted of two parts. One part was done to prepare the software of the shoebox spectrophotometer app by calibrating it. The other part was the actual measurement. Both steps will be explained in the following paragraphs.
List of needed things:
· Two syringes or pipettes (volumes: one milliliter and ten milliliters).
· One drop pipette.
· One heat-resistant mug.
· Four glasses.
· A cuvette.
· A UV light.
· Microwave.
· Filtration paper.
· A phone with a light sensor.
· A box that fits your phone and is covered with dark material on the inside.
Chemicals:
· Ascorbic acid (concentration: 0,1mg/mL or 0,00057 mol/L) (a volume of around two milliliter is enough)
· Urea (around five milliliters, 0,1 mg/mL)
· Juice of citrus fruits (can be whichever citrus fruit you have)
· 50 milliliters of water (doesn’t need to be distilled, tap water will do)
The calibration of the spectrophotometer app
Preparation of the calibration sample
To begin, one milliliter of the dissolved ascorbic acid (0,1 mg/mL) was taken with a syringe and put into a heat-resistant mug. One milliliter of urea was added to that. The mug was swirled, to make sure the two components became mixed together well. At this point, the solution was clear and colorless. Thereafter the mug was put in the microwave and heated on 600 or 700 Watts (depending on the settings of the used microwave). After every thirty seconds of heating, the mixture was taken out of the microwave to check the color. The heating would be done when the water in the solution was almost evaporated, causing the solution to become stickier. Instead of clear and colorless, the contents in the mug were orange/brown now, a sign that the desired Quantum Nanodots are made.
The mug was then put aside to cool down a bit. In this time, a clean glass was filled with water. When the mug was cool enough to touch, 10 milliliters of water were taken with the syringe and put in the mug. The mug was swirled again to dissolve all the Quantum Nanodots. The solution became clear and pale orange. To make sure that there wouldn't be any non-dissolved parts in the solution, everything was put through a piece of filter paper into a glass. The solution that came through the filter, was the desired product, a solution of Quantum Nanodots. [6]
Building and calibrating the measure installation
First, the software needed to be installed on a phone with a light sensor. The free app called ‘Shoebox Spectrophotometer’ is recommended for a phone with the Android operating system. This is downloadable in the Google Playstore, or with the following link: (Shoebox Spectrophotometer Android) For a phone with the IOS operating system, the app ‘Light Meter’ (Light Meter IOS) can be used, but this is free for only three days. Both of these apps use the light sensor on a phone to measure the light in the unit called lux.
To make this app work properly, the phone needs to be put in a box, that is covered with dark material. I could have used the box that contained the experiment materials. All that needed to be done with that box, was making some space on the side to slide the phone in and making a hole in the lid to be able to reach the cuvette (see picture below). Still, I decided to not use this box. This was because I figured that the setup that needed to be made, was almost the exact same as during the Spectrophotometer workshop at a conference I attended one and a half years ago. Therefore, I decided to use my wooden “spectrophotometer” box (see picture). I built that after the conference and this turned out to be much stronger. The fact that I already made this setup, caused the preparation to be a lot easier. I now only needed to put my phone in it and rearrange some objects. (see pictures)
The only thing left to do before the setup could be used was calibrating the app. This was done by filling the cuvette with the just made calibration standard by using a dropping pipette, turning the UV light on, closing the lid of the box, and clicking on the 'calibrate' button in the app. After that, the sample could be taken out of the cuvette again. This was all it took to make the setup ready to be used for the actual measurements.
The actual measurement
For the actual experiment, a new solution needed to be made. The goal of this experiment was to find how much vitamin C the juice of any citric fruit, in this case, an orange, contains. Therefore, a sample that existed out of this juice needed to be made. To do this, the earlier used heat-resistant mug was first cleaned and dried carefully. It was then used to mix one milliliter of orange juice and one milliliter of urea by swirling the mug. This solution maintained the slight orange / yellow color of the orange juice. The mug with the solution was placed in the microwave at 600 Watts for periods of thirty seconds. After roughly one and a half minutes the solution became brown, sticky, and bubbly (due to the cooking and evaporating water the orange juice contained). At this point, the mug had been taken out of the microwave and set aside to cool down a bit. After a short period of time, when the mug was touchable with bare hands, ten milliliters of water were added. The brown and sticky contents dissolved and became a slightly orange solution. After pouring this through a filter paper, this solution was now ready to be used to perform the measurements.
The measurements with this solution were performed in the exact same way as the sample to calibrate the app. Once the solution was put into the cuvette, the UV light was on and the lit was closed, the app showed a certain value of light received by the sensor. This value was given the unit called Lux. This value was saved in order to be able to make calculations with it later.
After I finished all these steps, I doubted a bit about the quality of the calibration I did. The values coming out of the experiment seemed to be a bit out of the expected range. Therefore, I decided to do the whole thing again, but to calibrate the app, I didn't use a self-made solution anymore. The package that was sent to me to do the experiment also contained a quantum nanodot solution that had already been made by the teachers at the University. This solution was given to us, so that the students that didn't have a microwave, could also participate. Even though I did have a microwave, I was still happy with this extra solution given to me, since that was basically an opportunity to do the experiment again, with the hope of better results. So, I did what I explained earlier. I calibrated the app with this solution, and after that, I tested my orange juice sample again. This time the results seemed to be more reliable, and after calculations, it turned out that it was actually the case.
The results
This little table was made after the measurements. The solution of know concentration gave a Lux of 86. The sample of which the concentration needed to be determined, marked with an X, gave a Lux of 78. These data were then put into a chart here below, including the Lux of 78. After some calculations, the unknown concentration turned out to be around 0,09 milligrams per milliliter. Sadly, this is a bit less than the expected 0,2 to 0,8 milligrams per milliliter. On the other hand, this could still be a good result, assuming that some ascorbic acid got destroyed while making the juice and while heating the solution to make the Carbon quantum dots.
In the end, this experiment turned out to be so much more than I expected! I learned a lot of new and awesome things and had a lot of fun. While writing this blog, I discovered that the 'quantum nanodots world' is big, and with big, I mean really big! There is so much information, and I believe that a lot is yet to come. In the future, I will for sure put my effort into this topic to learn more about it. This experiment was a great way to enjoy a small glimpse of this awesome quantum mechanical world.
References
1. Atkins, P., Jones, L., & Laverman, L. (2016). Chemical Principles (7de ed.). Macmillan Publishers.
2. Berg, J. M., Tymoczko, J. L., Gatto Jr., G. J., & Stryer, L. (2019). Biochemistry (9de ed.). Macmillan Publishers.
3. Bouwens, R. E. A., de Groot, P. A. M., Kranendonk, W., van Lune, J. P., Prop - van den Berg, C. M., van Riswick, J. A. M. H., & Westra, J. J. (2013). BINAS havo/vwo (6de editie). Noordhoff.
4. Clayden, J., Greeves, N., & Warren, S. (2012). Organic Chemistry (2nd ed.). Oxford University Press.
5. Driever, B. & Boswell-Bèta Utrecht. (2020). Boswell-Bèta Cursussyllabus Natuurkunde VWO (1.3 editie). Boswell-Bèta.
6. Sugiarti, S., & Darmawan, N. (2015). Synthesis of Fluorescence Carbon Nanoparticles from Ascorbic Acid. Indonesian Journal of Chemistry, 15(2), 141–145. https://doi.org/10.22146/ijc.21207
7. Wang, X., Feng, Y., Dong, P., & Huang, J. (2019). A Mini Review on Carbon Quantum Dots: Preparation, Properties, and Electrocatalytic Application. Frontiers in Chemistry, 7, 1–6. https://doi.org/10.3389/fchem.2019.00671
8. Wirz, J., & Klán, P. (2009). Photochemistry of Organic Compounds. Wiley. https://books.google.co.uk/books?id=8IstqQf6ZckC&pg=PA40&lpg=PA40&dq=kasha%27s+rule&source=bl&ots=ArmEU3BEC7&sig=RV1DG7mCXuWRSFcXM1Q6YZ8UY3w&hl=en&ei=sjvkS8zINYmEmgPXj5CCBg&sa=X&oi=book_result&ct=result&resnum=9&ved=0CDoQ6AEwCDgK#v=onepage&q=kasha’s%20rule&f=false
Oh it really interesting world cowantomy