Nanobubble Reactivity: Evaluating Hydroxyl Radical Generation (or Lack Thereof) under Ambient Conditions.

Nanobubble (NB) generation of reactive oxygen species (ROS), especially hydroxyl radical (OH), has been controversial. In this work, we extensively characterize NBs in solution, with a focus on ROS generation (as OH), through a number of methods including degradation of OH-specific target compounds, electron paramagnetic resonance (EPR), and a fluorescence-based indicator. Generated NBs exhibit consistent physical characteristics (size, surface potential, and concentration) when compared with previous studies. For conditions described, which are considered as high O NB concentrations, no degradation of benzoic acid (BA), a well-studied OH scavenger, was observed in the presence of NBs (over 24 h) and no EPR signal for OH was detected. While a positive fluorescence response was measured when using a fluorescence probe for OH, aminophenyl fluorescein (APF), we provide an alternate explanation for the result. Gas/liquid interfacial characterization indicates that the surface of a NB is proton-rich and capable of inducing acid-catalyzed hydrolysis of APF, which results in a false (positive) fluorescence response. Given these negative results, we conclude that NB-induced OH generation is minimal, if at all, for conditions evaluated.