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 O₂ 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.