CeO₂-Co₃O₄ nanocomposite with oxidase-like activity for colorimetric detection of ascorbic acid

A CeO₂-Co₃O₄ nanocomposite (NC) was prepared and characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction. The obtained CeO₂-Co₃O₄ NC displayed biomimicking oxidase-like activity, which can catalytically oxidize the 3, 3', 5, 5'-tetramethylbenzidine (TMB) substrate from colorless to the blue oxidized TMB (ox-TMB) product with a characteristic absorption peak at 652 nm. When ascorbic acid (AA) was present, ox-TMB would be reduced, resulting in a lighter blue and lower absorbance. On the basis of these facts, a simple colorimetric method for detection of AA was established with a linear relationship ranging from 1.0 to 500 μM and a detection limit of 0.25 μM. When this method was used to detect AA in human serum and commercially available vitamin C tablet samples, a good recovery of 92.0% to 109.0% was obtained. Besides, the catalytic oxidation mechanism was investigated, and the possible catalytic mechanism of CeO₂-Co₃O₄ NC can be described as follows. TMB is adsorbed on the CeO₂-Co₃O₄ NC surface and provides lone-pair electrons to the CeO₂-Co₃O₄ NC, leading to an increase in electron density of the CeO₂-Co₃O₄ NC. An increased electron density can improve the electron transfer rate between TMB and the oxygen absorbed on its surface to generate O₂˙^- and ˙O₂, which further oxidize TMB.