The green synthesis of a reduced graphene oxide (rGO) silica (SiO2) nanocomposite using Nigella sativa seed extract includes mixing a quantity of carbon source in an acid solution while stirring to obtain a solution; adding a first oxidant gradually into the solution to oxidize the soot and obtain a first suspension; stirring the first suspension while maintaining a temperature of the suspension to about 35° C.; adding plant seeds extract to the first suspension while raising the temperature of the suspension to about 60° C.; adding a second oxidant to the suspension to form the reduced graphene oxide nanoparticles; isolating the reduced graphene oxide nanoparticles by centrifugation; suspending the reduced graphene oxide nanoparticles in water; adding a solution comprising tetraethyl orthosilicate (TEOS), concentrated aqueous ammonia solution and a plant seeds extract under ultrasonication; and increasing the temperature to about 90° C. to form reduced graphene oxide-silicon dioxide nanocomposite suspension.