Intrinsic peroxidase-like activity of mesoporous nickel oxide for selective cysteine sensing.

Mesoporous nickel oxide nanoflowers (NiO NFs) can be easily synthesized by a two-step synthetic procedure based on modified hydrothermal (MHT) treatment of nickel acetate and ethanol amine in water followed by thermal decomposition at 350 °C for 4 h. After thermal treatment, the porosity is increased by 18% with retention of parental nickel hydroxide size. In this study, for the first time, a new catalytic application of NiO NFs has been revealed in terms of peroxidase-like activity where colorless 3,3',5,5' tetramethylbenzidine (TMB) is oxidized to blue color product in the presence of HO at room temperature. Comparative study confirms that mesoporous NiO NFs exhibit superior catalytic activity to the parent analogues, i.e. Ni(OH) or bulk NiO. This intrinsic peroxidase-like activity from an easily synthesized inorganic nanomaterial provides an alternative to horseradish peroxidase (HRP) enzyme. The lower Michaelis constant (K) value indicates that the catalyst NiO NFs bind efficiently to the test substrate, i.e. TMB. Interestingly, the NiO NFs-catalyzed TMB oxidation, i.e. blue color formation, has been found to be selectively and successively inhibited by a variable amount of cysteine among a set of 21 congeners. Thus our adopted simple, low-cost and novel colorimetric assay stands to be a highly efficient approach for selective detection of cysteine with a limit of detection (LOD) value of ∼1.1 μM using a simple UV-vis spectrophotometer. The proposed method also exhibits outstanding selectivity and accuracy for N-acetyl cysteine (an analogue of cysteine) estimation in real pharmaceutical samples.