CeO(2)–Co(3)O(4) nanocomposite with oxidase-like activity for colorimetric detection of ascorbic acid

A CeO(2)–Co(3)O(4) nanocomposite (NC) was prepared and characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction. The obtained CeO(2)–Co(3)O(4) 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 CeO(2)–Co(3)O(4) NC can be described as follows. TMB is adsorbed on the CeO(2)–Co(3)O(4) NC surface and provides lone-pair electrons to the CeO(2)–Co(3)O(4) NC, leading to an increase in electron density of the CeO(2)–Co(3)O(4) NC. An increased electron density can improve the electron transfer rate between TMB and the oxygen absorbed on its surface to generate O(2)˙(−) and ˙O(2), which further oxidize TMB.