MnO2 nanostructures of different morphologies from amino acids–MnO4− reactions in aqueous solutions

In this paper, we describe a simple method for the synthesis of MnO2 nanostructures (quantum dots) by permanganate oxidations of methionine and cysteine. Upon addition of permanganate to a solution of reductant (methionine and/or cysteine), yellow-brown color species appears within the time of mixing which was stable for several weeks. The UV–vis spectroscopy and transmission electron microscopy (TEM) techniques were used to characterize the nature of color formed. TEM images show that the MnO2 sol consists of aggregated spherical nonoparticles (size in the range ca ≤ 7 nm) of various forms. In addition, we observed the different morphologies of the MnO2 nanostructures (MnO2 nanoparticles, nanosheet, and fiber-like aggregated nanostructures) under different experimental conditions. The effect of various parameters, such as [H+], [MnO4−], and [reductants] was also investigated.