Ultrathin two-dimensional MnO2 nanosheet as a stable coreactant of 3,3′,5,5′-tetramethylbenzidine chromogenic substrate for visual and colorimetric detection of iron(II) ion

The authors report on a chromogenic system based MnO2 nanosheet and the chomgenic substrate 3,3', 5,5'-tetramethylbenzidine (TMB). The MnO2 nanosheet can oxidize TMB in acidic environment to form a yellow solution with an absorption peak at 450 nm. The process does not require the presence of an enzyme or H2O2. However, on addition of ferrous ion to the chromogenic system, the MnO2 nanosheet is decomposed via the redox reaction that occurs between Fe(II) and MnO2. As a result, the intensity of the absorption at 450 nm is reduced. This finding is exploited in a photometric method for determination of Fe(II) that shows high selectivity and a 0.3 mu M detection limit (based on the 3 sigma/slope criterion). Fe(II) can also be detected visually in concentrations down to 100 mu M. The method was applied to the determination of Fe(II) in spiked water samples and gave satisfactory recoveries.