Ultrathin two-dimensional MnO.sub.2 nanosheet as a stable coreactant of 3,3',5,5'-tetramethylbenzidine chromogenic substrate for visual and colorimetric detection of iron ion

The authors report on a chromogenic system based MnO.sub.2 nanosheet and the chomgenic substrate 3,3',5,5'-tetramethylbenzidine (TMB). The MnO.sub.2 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 H.sub.2O.sub.2. However, on addition of ferrous ion to the chromogenic system, the MnO.sub.2 nanosheet is decomposed via the redox reaction that occurs between Fe(II) and MnO.sub.2. 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.