Transmission Electron microscopy (TEM) images were acquired on a Jeol 2100 HR operating at 200 kV. Samples were prepared by depositing a drop of diluted NP suspension on 300 mesh TEM grid were dried under vacuum for 2 h. Inductively coupled plasma-mass spectrometer (ICP-MS) was used to estimate the amount of iron for calculating mol% and turn over frequency (TOF), Agilent Technologies 7700 series.