“…Many metal and metal oxide NPs (Au, Ag, Fe, CeO 2 , ZnO, CuO, SiO 2 and TiO 2 ) found to inhibit the microorganisms [10]. However, some metal NPs such as Au [11], hematite [12], Ag [13] and iron [14,15] enhanced the fermentative hydrogen production process. Earlier, the addition of Cu 2þ in an anaerobic digestion inhibited the formation of volatile fatty acids including acetate, butyrate and propionate [16].…”