*3.1.2 Preparation using two-step method approach*

Two step approach is designed in a manner that the surface of MO-NPs is sufficiently wet so to mitigate the particle impingement [39]. This is done by either using mechanical mixing [40] or sonication or the combination of both [41, 42] as shown in **Figure 1**.

During the mechanical mixing, the large flocs are broken down and the individual particles are kept separated from each other. During the sonication stage, the ultrasound waves, by stretching the molecular spacing of fluid, creates cavities within, which cause subsequently the chemical bonds to break [44]. However, due to the intrinsic properties of MO-NPs, a poor dispersion is often yielded which is mitigated by either extending the mixing time, adding few drops of acidifying agents [45] or even bubbling gas during the preparation [43].
