A New Application to Purifying Drinking Water Using Carbon Quantum Dots
Matthew Dougherty, Stevenson University
Abstract: Toxicity in drinking water is a problem that most people are familiar with. Well-known occurrences in Flint Michigan, Florida, and in countries around the globe, drinking water is not always safe to drink and sometimes hard to come by. My research conducted over the course of this 2020-2021 academic school year took a look at a potential approach to recognize these problems and pinpoint/detect when metal ions are present in water. From this research, further experiments can be conducted regarding the actual process of purifying the water and finding the most optimal solutions for detecting toxicity in water.
One of the biggest problems people are faced with in our world is something that most people who are reading this article have probably never experienced before. However, it is an extremely dangerous issue in many places around the world. What might be the most well-known occurrences in the United States of this issue was fairly recently. In Flint, Michigan, a city-wide issue was discovered in 2014. Toxic chemicals and poisonous lead were discovered to be in the water system and this issue persisted until 2019 when a solution was finally introduced. Since then water pipes have been replaced and thoroughly tested to ensure that the water will soon be safe again.  While we will not be researching a way to purify the water directly, we will be showcasing an innovative, low-toxicity, and cost-efficient approach to recognize toxicity flowing through tap water around the world. This method which can be applied, is also more accurate than the traditional water testing kits that can be purchased at a typical hardware store.
This method will be achieved through the use of Goo-Gone: a mass-produced, readily-available substance containing the main ingredient of PPG(Propylene Glycol)-3 Methyl Ether. The goal will be to detect metal ions in the water. This topic was introduced through a prior experiment which was found to be effective at synthesizing CQDs. The replicated experiment, which was conducted on the basis of a previous experiment which used grass to synthesize the dots, used orange juice to synthesize Carbon Quantum Dots (CQDs) which were then used to bind to metal ions successfully .
For this experiment to be conducted properly, it was important to choose a substance which contains Carbon. A pressure-cooking method was used in the previous experiment to synthesize the dots. CQDs are a class of carbon nanoparticles that have recently emerged as a low toxicity, chemically stable, and cost effective approach to chemical sensing, biosensing, bioimaging, nanomedicine, photocatalysis and even in detecting cancer. However, our purpose for using CQDs is due to their capability of reacting with and detecting metal ions in water to help purify drinking water. However, the goal was to find an original source that has never been tested before. PPG-3 Methyl Ether is the original source out of which we decided to synthesize CQDs. PPG-3 Methyl Ether is the main ingredient in the common cleaning and adhesive remover Goo Gone. The reason for pursuing this substance was simple; Goo Gone is mass produced, easily accessible, and relatively cheap. A small quart-sized bottle was acquired for only 4$ at a local hardware store. Moreover, the substance is a liquid which allows us to follow a similar procedure and approach as used before. Additionally, PPG is chemically made up of Carbon and Hydrogen molecules so it appeared as a strong solution with which to synthesize the CQDs.
Materials & Methods
In order to achieve results and discover if the CQDs are effective at detecting metal ions, a simple hydrothermal synthesis route based on a water system was used to achieve the CQDs. The PPG-3 will be sealed in an enclosed teflon autoclave and then pressure cooked at 180 C for 3 hours. Assuming the dots are effectively synthesized, the intensity of their fluorescence will be measured with a fluorometer. As found in our replicated experiment, if the CQDs do in fact bind to the ions, their fluorescence will decrease. The reason for the decrease in fluorescence is because the metals reflect light as well, however it is both not fluorescent and not nearly as strong of a reading of fluorescence from the CQDs. Therefore when the dots bind to the metal they will lose a considerable level of fluorescence.
The orange juice-synthesized CQDs were only effective at detecting Cu2+ ions, one of the four metal ions used in the experiment. The four metal ions used were Cu 2+, Ni 2+, Co 2+, and Zn 2+. Ultimately, six solutions in total were measured by the fluorometer to acquire all data. When replicating the first experiment, our initial pressure-cooked sample was significantly more concentrated than the optimal solution that would be used for the intensity readings. In an attempt to avoid this issue for the PPG-3 experiment, two initial samples were pressure cooked. The first sample consisted of 100% Goo Gone
and the second sample consisted of 50% Goo Gone and 50% water. Upon removing the samples from the oven, it was discovered that the uncooked solution of Goo Gone directly from the bottle had already contained CQDs. However, the use of two samples turned out to be effective as when the samples were removed from the oven, the 100% cooked solution had actually significantly lost fluorescence in comparison to the 100% uncooked sample.
From here, our initial goal shifted from simply trying to synthesize CQDs from the solution, but rather to attempting to increase the intensity of fluorescence and seeing if an optimal fluorescence could be achieved through the method. As mentioned before, the 100% cooked sample had lowered in fluorescence. However, the 50% cooked solution revealed stronger CQDs than the original sample. The 50% mix of water and Goo Gone separated when in the cooker, but what came out was a two-layered solution. The top layer consisted of Goo Gone and displayed no fluorescence, however, the bottom layer was different. The bottom layer was not clear water, it was a yellow-brown layer of Goo Gone-infused water which fluorescence was very strong compared to the uncooked Goo Gone. Next, two additional samples were produced from the 50% solution in an attempt to optimize the solution to find its peak fluorescence. The 50% solution was diluted by 50% again and by 90% to create a 25% concentration sample and a 5% solution, respectively. The final solution consisted of water added to the 100% cooked solution after the Goo Gone was first pressure cooked. Samples are shown in Figure 1 below.
The aqueous metal solutions were then created to be used in order to test the binding potential of the CQDs that were produced. The final metal solutions each consisted of 100 uL of CQDs from the optimal concentration of CQDs, 800 uL of buffer solution, and 100 uL of aqueous metal solution. Two samples were produced for each metal in addition to two control samples to ultimately create 12 samples in total to be measured with the fluorometer.
After measuring the values for each concentration of solution, it was found that the 50% cooked solution experienced the highest fluorescence of all tested samples at approximately 30,000 units based on an average of two samples as seen in Figure 2.
In order to numerically compare the intensity of fluorescence for each acquired sample, a ratio was produced. As seen in the graph, the green bar which represents the 50% cooked solution was nearly at a 3 times stronger intensity than the original solution which is represented at the red bar on the graph (see Figure 3).
The optimal solution of 50% was then used in the metal additive solutions and readings were taken again in order to determine if the metal ions would bind to the CQDs. As mentioned before, two of each metal solution along with two controls were measured and the average of the two results was found (see Figure 4).
Again, a ratio was found to compare the different samples. The sample containing water added to the 100% cooked sample was not included as there was no fluorescence shown. The teal bar represents the control of the CQDs only, and as shown, none of the metal samples being observed resulted in a decrease in intensity.
From the results we can conclude that because none of the samples experienced a decrease in intensity, that the synthesized CQDs did not bond to any of the metal ions included in this experiment. Ultimately, PPG-3 Methyl Ether is an effective, efficient chemical out of which CQDs can be synthesized. However, those synthesized dots are not effective at detecting the four metal ions included in this experiment. Further experiments can be conducted to find CQDs that are more effective for detecting metal ions as it has been proven to be effective in prior experiments.
 Denchak, M. (2018). Flint water crisis: Everything you need to know. NRDC. https://www.nrdc.org/stories/flint-water-crisis-everything-you-need-know
 Li, Z., Zhang, Y., Niu, Q., et al. (2017). A fluorescence probe based on the nitrogen-doped carbon dots prepared from orange juice for detecting Hg2+ in water. Luminescence, 187, pp 274-80. https://eds.a.ebscohost.com/eds/detail/detail?vid=5&sid=85352135-3405-453b-9023-8e6e53955e05%40sdc-v-sessmgr01&bdata=JnNpdGU9ZWRzLWxpdmUmc2NvcGU9c2l0ZQ%3d%3d#AN=S0022231316313035&db=edselp
 Liang, Q., Ma, W., Shi, Y., et al. (2013). Easy synthesis of highly fluorescent carbon quantum dots from gelatin and their luminescent properties and applications. Carbon, 60, pp 421-28. https://www.scien cedirect.com/science/article/pii/S0008622313003515
 Lui, S., Tian, J., Wang, L., et al. (2012). Hydrothermal treatment of grass: a low-cost, green route to nitrogen-doped, carbon-rich, photoluminescent polymer nanodots as an effective fluorescent sensing platform for label-free detection of Cu(II) ions. Advanced Materials. 24(15), pp 2037-41. DOI 10.1002/adma.201200164
 Ying Lim, S., Shen, W., Gao, Z. (2015). Carbon quantum dots and their applications. Chemical Society Reviews, 44(1), pp 362-81. https://pubs.rsc.org/en/ content/articlehtm l/2014/cs/c4cs00269e
 Behara, B., Maiti, T., Mohapatra, S. (2012). Simple one-step synthesis of highly luminescent carbon dots from orange juice: Application as excellent bio-imaging agents. Chemical Communications, 48, pp 8835–37. https://pubs.rsc.org/en/content/articlelanding/2012/cc /c2cc33 796g/unauth#!divAbstract
Experiment conducted and study completed by Matthew Dougherty and Lauryn Howell. Article written and prepared by Matthew Dougherty.