Facile Approach To Synthesize Nitrogen- and Oxygen-Rich Carbon Quantum Dots for pH Sensor, Fluorescent Indicator, and Invisible Ink Applications

Although few works on highly luminescent carbon quantum dots (CQD) with high photostability for sensing applications have already been reported in the literature, the study of photoluminescence of CQD in the presence of oxidation–reduction reaction and its use as fluorescent indicator for redox titration still remains in an infant stage. Here, we have synthesized green, eco-friendly, highly luminescent multicolored emissive CQDs from aspartic acid and urea using simple heat treatment at 200 °C for 2 h without using a solvent. The as-synthesized CQDs show dual emission peaks at shorter excitation (300 nm) wavelength, one due to quantum dots and the other due to defect states; however, at longer excitation (400 nm) wavelength, it shows only one peak due to defect states. At shorter excitation wavelength, the green emission peak is red-shifted as well as quantum yield increases from 38.81 to 57.68% with the increasing nitrogen content. These CQDs are very pH sensitive and it changes from blue to green color with the increasing pH from 1 to 13. We have used these highly photostable CQDs as a fluorescence indicator to study oxidation–reduction titration reaction and detect Fe2+, Cr3+ by fluorescence on–off–on technique. Although Fe2+, Cr3+ ions have no direct PL quenching effect, it is possible to detect due to different redox reactions occurring in solutions. We have estimated accurately the amounts of Fe2+ and Cr3+ present in the solutions using fluorescence titration results. Because of high photostability and high quantum yield these carbon dots are also useful as a unique invisible ink in paper and polymer film substrate in which written article changes color at different excitations for security reasons.