Defect-rich CeO_2-x nanosheets as efficient oxidase nanozymes for colorimetric Hg²⁺ detection and visible-light...

Defective, oxygen vacancy-rich CeO_2-x nanosheets were synthesised using a simple two-step process. The synthesised CeO_2-x nanosheets exhibit excellent oxidase-mimic activity compared with their CeO₂ nanosheet counterparts. Color change from colorless to red in aqueous dopamine solution and colorless to green coloration in aqueous 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid)diammonium salt (AzBTS) solution by CeO_2-x nanosheets confirms the creation of oxygen vacancies in the CeO₂ nanosheets. Again, Raman and XPS analysis of CeO_2-x nanosheets confirms a higher concentration of Ce³⁺ ions and lower oxidant content than CeO₂ nanosheets. The band gaps of CeO_2-x and CeO₂ nanosheets were found to be 2.82 eV and 2.97 eV, respectively. The remarkable oxidase mimic activity can strongly catalyze the oxidation of chromogenic substrates like 3,3',5,5'-tetramethylbenzidine (TMB) into a blue-colored cation radical without the need for an additional oxidizing agent. As mercury(ii) (Hg²⁺) ions are recognised as one of the most toxic metal ions in the world and exposure to Hg²⁺ ions, even in low concentration, can severely affect living organisms, here we detect Hg²⁺ ions in drinking water by a simple colorimetric detection method. Blue colored solution of oxidized TMB (ox-TMB) in the presence of glutathione can be converted to colorless TMB. Whereas, a blue-coloured solution of ox-TMB in the presence of both glutathione and Hg²⁺ ions can effectively be recovered. Based on this phenomenon, a simple and rapid colorimetric method for sensing Hg²⁺ ions with the naked eye was developed, showing a good linear response from 1 to 17.4 µM. The detection limit was estimated at 24.92 nM. Key operational parameters, including buffer composition, selectivity, pH, reaction time, and scavenger study, were established. The proposed method was successfully applied for Hg²⁺ detection in real water samples with good recovery results. In addition to the oxidase mimic Hg²⁺ ion sensing activity, a comparative photocatalytic efficiency of CeO₂ nanosheets and CeO_2-x nanosheets was also assessed by evaluating the degradation of methylene blue dye under visible light irradiation.