The Short Version
The available evidence does not substantiate the claim that published studies use seawater as a deliberate solvent or reaction medium for nanoparticle synthesis outside of algae/seaweed methods. The sole candidate study (Source 3) describes seawater as an operational electrolysis environment for a pre-synthesized catalyst, not as the synthesis medium itself. The broader evidence pool is dominated by algae-mediated approaches, and multiple sources note seawater's hostile ionic conditions for nanomaterial stability.