Malic acid-coated iridium nanoparticle-induced cascade enzymatic reactions for norepinephrine detection

In this work, malic acid (MA) stabilized iridium nanoparticles (MA-IrNPs) were prepared by one-pot synthesis using MA and IrCl3 as the precursors and NaBH4 as the reducing agent. The as prepared MA-IrNPs have exhibited multi-enzyme activities, especially an excellent laccase-like activity and improved environment adaptability. For the substrate 2,4-dichlorophenol (2,4-DP), a low Km value of 0.117 mM and a high Vmax value of 0.002 mM s−1 were obtained. Under the catalysis of MA-IrNPs, the colourless norepinephrine (NE) can be selectively oxidized by dissolved oxygen to form an orange-brown product and the reaction can be substantially accelerated in the presence of H2O2 due to the synchronic effect of catalase-like and laccase-like activity of MA-IrNPs. Based on this chromogenic reaction, a sensitive colourimetric method was developed for the determination of NE in urine samples. A limit of detection as low as 0.05 μM and a wide linear range of 0.1–120 μM were achieved, showing a promising application potential of MA-IrNPs in disease diagnosis.