Synthesis of Iron Oxide-Gold Core-Shell Nanoparticles

For the synthesis of the core-shell nanoparticles, iron oxide nanoparticles were firstly synthesized within the droplets of the reactor, as previously described^{24 (link),34 (link)}. Briefly, through the first two fused silica capillaries of the reactor, two aqueous solutions were injected at a flow rate of 10 µL/min. The first solution was 0.06 M of FeCl₃·6H₂O (Alfa Aesar, USA) and 0.03 M of FeCl₂∙4H₂O (Sigma-Aldrich, USA) dissolved in DI water, and the second solution was 4 M ammonia (Alfa Aesar, USA). As a continuous phase mineral oil (M5904, Sigma-Aldrich USA) with 0.075 vol % Triton X-100 (Samchun Chemical, Korea) and 1.75 vol % Abil EM 90 (Evonik Industrial, Germany) was used, the flowrate injected through the central Tygon tubing was 10 µL/min. For the synthesis of the gold shell around the iron cores, a gold precursor solution was injected into the existing droplets through the single capillaries at 100, 130, and 160 cm. The gold precursor solution consisted of 0.03 M HAuCl₄ (Sigma-Aldrich USA) dissolved in DI water. For all three injections, the same flowrate was used and flowrate was determined by a self-optimizing algorithm based on the transmission of the droplets and two initial guesses for the flowrate. To quench the reaction, core-shell nanoparticles were collected in a vial filled with water, once they left the capillary droplet reactor.