The bio-inspired copper(I) complex [Cu(TMG2tol)]OTf was used to investigate the mass transfer of a freely rising oxygen bubble in a thin-gap cell. Single rising oxygen bubbles of different diameters were generated with a so called Hele-Shaw cell set-up. The oxygen bubbles undergo an oxygenation reaction with the copper(I) complex solution forming an orange-colored copper(III) complex. As this species is not stable at ambient conditions, it immediately decays to a stable, green-colored copper(II) complex. Based on the colored signal of the final copper(II) complex, a high-speed grey level camera was calibrated using copper(II) complex solutions with varying concentrations. Finally gap-averaged concentration fields of the green coper(II) complex could be measured in the far-field wake of the single rising bubbles. Thereof mass flux densities, Sherwood numbers as well as Peclet numbers could be calculated.