## 7 General Synthesis of Nanomaterials  
There are two major ways of synthesizing nanomaterials: the top-down and bottom-up approaches. Top-down methods are physical means, which are expensive and consume a lot of energy and time, whereas bottom-up approaches are wet-chemical or biological means and are preferred by most researchers. Top-down methods are physical means and include the mechanical/ball milling, photolithography, sputtering, chemical etching, etc., while bottom-up approaches involve physical, chemical and biological means and include sonochemical, microwave, photochemical, vapor deposition, sol-gel, chemical and electrochemical deposition, atomic and molecular condensation, spray, laser pyrolysis, etc. [141].  
The biological method is one of the emerging green methods of synthesis because it uses eco-friendly materials and aqueous solvents, saves time and energy, and is cost effective, sustainable and non-toxic. The ultimate objectives of using biological means of synthesis, phytonanotechnology or green nanotechnology is to reduce the environmental and human risks associated with other methods of engineering nanomaterials. They provide nanomaterials that serve the essential needs of the present century. Biological means of synthesis consist of the microorganism-mediated method and plant-assisted green nanotechnology. More focus and interest have been give to the use of the plant method, as all plant parts ranging, including the roots, banks, leaf, stem, fruits, sap, and even fruit wastes, are applied in fabricating nanomaterials for various uses. Plants are considered as natural reservoirs for various components such as flavonoids, phenolics, terpenoids, carbohydrates, proteins, saponins, and acids that have potentials for reducing, stabilizing and capping metal or metal oxide nanoparticles as well as functionalizing carbon-based nanomaterials. Since these biocomponents in plants have functional groups that can act as organic ligands, they serve as electron donors and effectively reduce the bulk metal or metal oxides to their nanoparticulate forms. This method provides nanoparticles that have interesting characteristics including small sizes, eco-friendly nature, biocompatibility, low toxicity, simple reaction procedures and enhanced surface morphologies with unlimited applications.  
Figure 2: Schematic diagram of influence of micronutrients on crops Adapted from [137, 138, 139, 140].  
In the application of the biological method of synthesis, the plant-assisted method in particular is seriously gaining attention because it is cost effective, easily manipulated, of high purity, sustainable, parametric efficient and greens. It has thus far outweighed other options. For instance, in a typical synthesis of SeNPs, sodium selenite, selenous acid or selenium oxide can be used as the precursor compound and plants substrates from _Vitis vinifera_ fruits, _Bougainvillea spectabilis_ wild flower, etc. have been used [142; 143]. The synthesis involves different concentrations of the precursor compound and the plant extract. The solution is mixed and stir-heated at a temperature the plant biocomponents can withstand--not too high as to render them inactive. After a period, there could be observable color changes, indicative of nanoparticles formation and the solution is centrifuged, washed and dried to get the nanoparticles. [PERSON] reported different positive influences of silver nanoparticles synthesized by different plant extracts on improving crop production [144]. From the results, there was interesting development on the root and shoot and the fruit yield, among others. Although there are many reports on plant-mediated synthesis of nanoparticles, some gaps still exist on their application in crop improvement.