Resolving the Bubble Puzzle: Hydrogen Peroxide Formation Precedes Hydroxyl Radicals in Microbubbles and Is Governed by Solid–Water Interfaces

Abstract

Recent reports attribute sustained chemiluminescence (CL) and electrochemiluminescence (ECL) from electrogenerated microbubbles on steel or copper electrodes in aqueous luminol solutions to the spontaneous formation of HO• radicals at the gas–water interface. In response, our experiments with a broad set of electrodes, namely, stainless steel, copper, aluminum, and platinum, reveal that while microbubbles can be electrogenerated on all electrodes, CL/ECL is exhibited by steel and Cu only and not by Al and Pt. These observations establish that the gas–water interface of microbubbles is not the site for HO• radical generation (else CL/ECL would be recorded in all cases). Parallel quantification of H2O2 in these experiments reveals the following electrode dependence: Al > Cu > Steel ≫ Pt (undetectable, limit of detection 50 nM). This reveals that depending on the electrode, H2O2 forms first, and in some cases, HO• radicals are observed (i.e., where CL/ECL is seen). Experiments with 1H NMR and EPR demonstrate that (i) H2O2 formation occurs only when O2 is present; and (ii) while steel and copper generate HO• radicals through one-electron reduction of H2O2, Al does not, and Pt preferentially promotes disproportionation of H2O2 to H2O and O2. In fact, H2O2 and HO• radicals are observed at specific metal–water interfaces even without microbubbles, confirming that the solid surface is the reactive site. This work provides an electrode-based predictability of whether electrogenerated microbubbles yield CL/ECL in luminol solutions and also calls into question the broader notion of spontaneous formation of HO• radicals at gas–water interfaces.