Bulk nanobubbles (gas-filled cavities typically below 200 nm that persist in water for hours to days) have been claimed both to spontaneously generate reactive oxygen species (ROS) — hydroxyl radicals, hydrogen peroxide, superoxide — with no dynamic stimulus or external energy input, and to generate no radicals at all. The capacity is invoked for nanobubble water treatment, disinfection, and agriculture, yet two peer-reviewed 2026 studies reach opposite conclusions: molecular simulations and experiments indicating no spontaneous radical generation, versus concurrent evidence for ROS formation associated with bulk nanobubbles in pure water. The problem is to determine whether bulk nanobubbles spontaneously generate ROS without external energy input and, if so, at what yield, under which conditions, and by what interfacial mechanism, and in either case to reconcile the contradictory reports.

Background

Bulk nanobubbles are gas-filled cavities with diameters typically below 200 nm that persist in water for hours to days — far longer than classical dissolution theory predicts — and their stability and reactivity remain active research subjects. Reactive oxygen species (ROS) — most importantly the hydroxyl radical ⋅\cdotOH and hydrogen peroxide H2_2O2_2, but also superoxide O2⋅−_2^{\cdot -} and singlet oxygen — are strong oxidants central to advanced oxidation processes (AOPs), water-treatment technologies that degrade organic pollutants by generating such oxidants. In classical acoustic cavitation (sonochemistry), a violently collapsing bubble concentrates energy into a localized 'hot spot' reaching thousands of kelvin, and water vapor homolyzes into ⋅\cdotOH and H radicals; this mechanism ordinarily requires strong dynamic driving such as ultrasound. The radical question for quiescent bubbles arose when Takahashi, Chiba and Li (2007) reported electron spin resonance (ESR, equivalently EPR) evidence that collapsing microbubbles generate free radicals 'in the absence of a dynamic stimulus'. ESR spin trapping infers short-lived radicals from the spectrum of a stable adduct formed with a trap molecule such as DMPO (5,5-dimethyl-1-pyrroline N-oxide, the standard hydroxyl-radical spin trap), and fluorescence probes such as aminophenyl fluorescein (APF) report ⋅\cdotOH through increased fluorescence. The subsequent literature split. On the positive side, Liu et al. (2016) identified ⋅\cdotOH in nanobubble water using APF and scavenger chemistry and connected it to seed-germination effects; Takahashi et al. (2021) detected ESR spin-adduct signals in microbubble-treated water months after dispersion, attributed them to bulk nanobubbles stabilized by iron-ion shells, and found radical generation re-triggered when acid destroys those shells; Yamaguchi et al. (2021) and Mokudai et al. (2024) reported bactericidal ROS, mainly ⋅\cdotOH, specific to CO2_2 nanobubbles; and Liu et al. (2025) attributed catalyst-free CO2_2 reduction to spontaneously generated hydrogen radicals in a nanobubble-water system. On the negative side, numerical simulations by Yasui et al. (2016, 2018) indicated that a dissolving air nanobubble produces no ⋅\cdotOH, and proposed that H2_2O2_2 formed during hydrodynamic-cavitation bubble generation could mimic positive detections; and Chae et al. (2023) found, for oxygen nanobubbles under ambient conditions, no degradation of the ⋅\cdotOH-specific scavenger benzoic acid over 24 h and no EPR signal, while reinterpreting the widely used APF positive response as acid-catalyzed probe hydrolysis at the proton-rich nanobubble interface — a concrete false-positive mechanism. In a middle position, Huang et al. (2025) showed that micro-nanobubbles redirect the aqueous Fe(II)/O2_2 electron-transfer pathway toward efficient ⋅\cdotOH production — nanobubbles assisting externally supplied redox chemistry rather than oxidizing on their own. Wang et al. (2026) combined experiments with atomistic and ab initio molecular dynamics (MD/AIMD), in which interatomic forces are computed from first-principles electronic-structure theory rather than empirical potentials: during free nanobubble collapse the local pressure transiently exceeds 24,124 atm, yet the interfacial solvent temperature stays below 162 °C, so the classical hot-spot mechanism appears inapplicable to radical generation under the modeled nanoscale conditions; the interfacial electric field lowers the vertical ionization energy of adsorbed hydroxide ions — the energy required to remove an electron at the adsorbed geometry, before the remaining system relaxes — by about 6 kcal/mol, but this is thermodynamically insufficient to detach electrons without exogenous energy; and experimentally, nanobubbles alone barely oxidize Rhodamine B, a model organic-pollutant dye used to assay oxidative degradation (below 8% degradation), while strongly enhancing UV-driven degradation (53.7%), leading the authors to describe bulk nanobubbles as field-assisted energy-reducing catalysts rather than intrinsic oxidants. Yet in the same period Kawada et al. (2026) reported evidence for ROS formation associated with bulk nanobubbles in pure water — their preprint version is titled 'Spontaneous Generation of Hydrogen Peroxide and Radicals from Bulk Nanobubbles in Pure Water'. Whether bulk nanobubbles spontaneously generate ROS is therefore concurrently contested in the peer-reviewed literature rather than settled.

Problem Statement

Determine whether bulk nanobubbles in water spontaneously generate reactive oxygen species — that is, generate ⋅\cdotOH, H2_2O2_2, O2⋅−_2^{\cdot -}, or other reactive species without any dynamic stimulus or external energy input — and, if so, at what yield, under which conditions, and by what interfacial mechanism. The question is directly contested by the concurrent 2026 reports: Wang et al. (2026) conclude that bulk nanobubbles do not spontaneously generate radicals but function as field-assisted energy-reducing catalysts, whereas Kawada et al. (2026) report persistent oxidative probe responses — H2_2O2_2-equivalent, ESR spin-trap, and APF signals — for O2_2- and N2_2-derived bulk nanobubbles in pure water after the generating ultrasound is stopped, offering their proposed mechanism only as a tentative working hypothesis. A resolving answer must provide a verdict on whether spontaneous generation occurs, report the yield, conditions, and interfacial mechanism if it does, and explain the origin of the claimed positive detections and the null results in the cited lineage, taking into account the known artifact mechanisms and the contested characterization of the detected entities — whether the light-scattering signals arise from genuine gas-filled nanobubbles or from particulate contaminants (Fattahi et al., ref. 16).

References

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