Physical and Optical Properties of N,S-Codoped Carbon Quantum Dots Derived from Cigarette Butts for Mercury Ions Detection

Abstract

The accumulation of mercury metal ion (Hg2+) in polluted river water is a problem in the emergence of various diseases in humans, because it is accumulated in drinks and food eaten every day. Therefore, the development of an accurate, sensitive, efficient and convenient method is very important to detect the content of metal mercury ions in polluted water. In this study, Carbon Quantum Dots (CQDs) was made simultaneously doped with Nitrogen (N) and Sulphur (S) atoms, so called N,-S codoped CQDs for the application of mercury metal ion sensors in polluted water. CQDs were prepared from wasted cigarette butts, which contain cellulose acetate as a source of carbon and nicotine as N-atoms source. The preparation of carbon solutions precursors were made by two ways of treatments namely put wasted cigarette filters/butts with water for 6-12 months (precursor I) and atomic S doping was done by adding sulfuric acid (H2SO4) with a volume-percentage of 0, 10% and 20% in water. The second treatment was carried out through the washing of wasted cigarette filter with sulfuric acid solution with a volume percentage of 20% sulfuric acid in water (precursor II). CQDs were prepared by use of hydrothermal method at 180°C for 6 hours. The synthesized solution was neutralized with sodium carbonate (Na2CO3). The solution was then centrifuged at 10,000 rpm for 15 minutes and filtered with a 0.22 µm microsyringe filter. The result of solution is a transparent and turns into blue when irradiated by a 365 nm UV laser, which indicates that CQDs has been formed. The FTIR results of the CQDs solution without S doping showed the presence of groups containing N atoms, namely at wave numbers 1417 cm-1 (C–N), 1540 cm-1 (CON–H), 1650 cm-1 (C=N/C=O). This means that N-doped CQDs have been formed. The C–S functional group was identified at a wave number of 1361 cm-1 in the S atom-doped CQDs. The HR-TEM images showed that the CQDs had a size less than 10 nm, and the distance between graphene crystals plane is in the range of 0.22-0.28 nm. The absorption spectra of the CQDs are ranging from 200 nm to 350 nm. S atom doping does not affect the absorbance spectrum significantly. The CQDs without S-atom doping has a PL spectrum from 300 nm to 800 nm with an emission peak of 417 nm. While, the N,-S codoped CQDs have a PL emission peak at 404 nm. The addition of doping from 10% to 20% did not change the peak emission wavelength. The CQDs sizes, optical properties and FTIR spectra of N, S-codoped CQDs show no significant difference when were prepared from precursor I and precursor II. This shows that the preparation of N, S co-doped CQDs can be produced much faster by washing cigarette butts directly with sulfuric acid prior to hydrothermal process as compared to precursor I which needs 6-12 months.