Evaluation of electrocatalysis of hourglass-shaped polyoxometallates with different transition metals towards hydrogen peroxide transformed from superoxide radicals in living cell mitochondria

In this work, we study the electrocatalytic detection of H2O2 at electrodes correspondingly modified by four hourglass-shaped polyoxometallates (POMs) with Cu, Co, Mn and Cs as their individual central metal atoms, and four representative Wells-Dawson POMs with the same metal atoms. In this way, we have related their analytical performance to the open structures of hourglass-shaped POMs and the cage-like structures of Wells-Dawson POMs. Among them, we found the three-dimensional hourglass phosphomolybdate (POM 1) with plentiful exposed Cu active sites exhibits a dynamic range of 0.1-10 mu M, a sensitivity of 11.6 mu A mu M-1 cm-2, and a limit of detection as low as 44 nM for H2O2. It also offers high selectivity, fast response time (2 s), a reproducibility of-1.69% and retains-97.9% of the initial current after 15 days, verifying its good stability. Accordingly, the POM 1-modified electrode was applied to monitoring H2O2 converted from superoxide anion radical (O2 center dot- ) generated by mitochondria isolated and purified from human liver cancer cells (HepG2) without pharmaceutical stimulation. Meanwhile, by exploiting four specific inhibitors (rotenone, 2-thenoyltrifluoroacetone, antimycin A and paclitaxel) known to bind with targeted sites in the corresponding complex within mitochondria to produce a high H2O2 concentration, our results indicated that O2 center dot- was mainly produced by mitochondrial complex I (ubiquinone oxidoreductase) via electron leakage in the mitochondrial electron transport chain of liver cancer. The electrochemical detection strategy in our work has provided a direct insight into the electron leakage of mitochondrial complexes caused by targeted oxidative stress diseases.