A new processing methodology in presented for the direct synthesis of surface-functionalized nanoparticles through modification of a single-step continuous supercritical water process. The,processing methodology Utilizes inexpensive metal salt precursors that form nanoparticles upon mixing the metal salt solution with a supercritical water flow (24 MPa and 450 degrees C). Surface functionalization is achieved through introducing a supplementary flow of capping agent (citric acid in this exemple) to the stream of nascent (newly formed) nanoparticles, using a hovel reactor design. It was found that certain process attributes were key to effective functionalization Of the nascent nanopartide Stream, and that high grafting-densities of the capping agent were obtained in a relatively narrow process Window. We have also used the core design of the reactor to devise and test a scale-up methodology to produce large quantities of surface-functionalized nanoparticles. A method for scaling-up the reactor is described, using a newly developed pilot plant designed to process flow rates 20X.greater than the equivalent laboratory-scale process, which yields products at rates of ca. 1 kg/h (effectively semi-industrial-scale production): The method enables large-scale production without recourse to expensive or environmentally damaging reagents and uses water as the only process solvent, a significant advantage over many methods commonly used to produce surface-functionalized fianopartides. We report the synthesis and Characterization of citrate-functionalized Fe3O4 hanopartides as a model system. and present detailed characterization of the materials obtained at both processing scales.