Sustained Regeneration of Hydrogen Peroxide at the Water–Gas Interface of Electrogenerated Microbubbles on an Electrode Surface

Microbubbles, inside-out microdroplets, act as extraordinary microreactors to facilitate thermodynamically unfavorable reactions in bulk solutions of water. We explored the formation of hydrogen peroxide (H2O2) and its sustained regeneration at the interface of water–gas microbubbles. For this purpose, the chemiluminescence of luminol was recorded by a digital camera to map the intensity of blue light emission over the time of about 20 min. The formation and regeneration of hydrogen peroxide were also monitored by fluorescence microscopic imaging of a hydrogen peroxide probe. The microscopic images consistently show a stable glow around the microbubbles over time during which the formed hydrogen peroxide diffuses into the bulk solution. This observation confirms that the concentration of H2O2 at the interface is 30 times higher than that in the water solution bulk after several minutes, which can be attributed to its regeneration at the water–gas interface. These findings increase our understanding of why the chemistries of gas microbubbles in water and water microdroplets surrounded by gas are so distinct from those of bulk-phase water.