Influence of carbon nano-dots in water on sonoluminescence

Sonoluminescence (SL) occurs when acoustically induced oscillating bubbles in a liquid collapse. The SL from pure water normally generates ultraviolet to blue emission which is related to hydroxyl plasma formed in and around the bubbles. It is known that carbon nano-dots (CNDs) can serve as free radical captors, where the C-bonds can couple strongly with free radicals and form C-based functional groups. In this work, a SL experiment is conducted via placing CND aqueous solution (CNDAS) in the focal area of the SL apparatus. Unexpectedly and dramatically, it is found that the color of SL now turns orange, which is so bright that it can be seen even by the naked eye. By examining the CNDAS before and after the SL experiment, it is observed that the influence of CNDs on optical absorption, photoluminescence and SL is mainly achieved via coupling between the C-bonds in the CNDs and the free hydroxyl radicals generated during the processes of acoustically driven cavitation and SL. The interesting and important findings from this work demonstrate that the CNDs in water can modify significantly the SL effect. Thus, CNDs can provide a new test medium for studying and revealing the microscopic mechanism of the SL phenomenon.