In situ formation of fluorescent polydopamine catalyzed by peroxidase-mimicking FeCo-LDH for pyrophosphate ion and pyrophosphatase activity detection

As pyrophosphate ion (PPi) and pyrophosphatase (PPase) play crucial roles in the pathological process of arthritis, determination of PPi and PPase in biological fluids turns to be of great importance for clinical diagnosis and therapy of arthritic diseases. In this work, we proposed a new fluorescent assay for PPi and PPase activity detection based on the competitive coordination chemistry of Fe3+ between PPi and an in situ formed fluorescent polydopamine (PDA). FeCo layered double hydroxide (FeCo-LDH) was explored as a peroxidase mimic to facilitate the in situ formation of fluorescent PDA from dopamine mediated by low-concentration H2O2 within 30 min; The formed fluorescent PDA could be significantly quenched by Fe3+ through forming a PDA-Fe3+ complex structure; When PPi existed, it coordinated Fe3+ competitively against PDA and inhibited the fluorescence quenching of PDA by Fe3+; When PPi was hydrolyzed under the catalysis of PPase, the Fe3+ ion could quench the fluorescence of the formed PDA again. With these principles, our fluorescent assay was able to detect PPi and PPase activity specifically, providing detection limits down to 54 μM and 0.13 U/L, respectively. Furthermore, accurate determination of PPi and PPase activity in spiked human serum was also demonstrated using the developed assay.