Radical Generation and Bactericidal Activity of Nanobubbles Produced by Ultrasonic Irradiation of Carbonated Water

Our previous study showed that nanobubbles (NBs) encapsulating CO gas have bactericidal activity due to reactive oxygen species (ROS) (Yamaguchi et al., 2020). Here, we report that bulk NBs encapsulating CO can be efficiently generated by ultrasonically irradiating carbonated water using a piezoelectric transducer with a frequency of 1.7 MHz. The generated NBs were less than 100 nm in size and had a lifetime of 500 h. Furthermore, generation of ROS in the NB suspension was investigated using electron spin resonance spectroscopy and fluorescence spectrometry. The main ROS was found to be the hydroxyl radical, which is consistent with our previous observations. The bactericidal activity lasted for at least one week. Furthermore, a mist generated by atomizing the NB suspension with ultrasonic waves was confirmed to have the same bactericidal activity as the suspension itself. We believe that the strong, persistent bactericidal activity and radical generation phenomenon are unique to NBs produced by ultrasonic irradiation of carbonated water. We propose that entrapped CO molecules strongly interact with water at the NB interface to weaken the interface, and high-pressure CO gas erupts from this weakened interface to generate ROS with bactericidal activity.