Fig. 2: Proposed mechanism of iron oxide nanoparticle formation under photothermal conditions.

a–b TEM images of IONPs a, scale bars 50 nm, and SiO₂@AuBPs b, scale bars 200 nm, isolated post-reaction at different photothermal temperatures (160–200 °C) for 4 h. Nucleation of nanoparticles can be seen at 160 °C, and subsequent growth corresponding to an increase in photothermal temperatures. The yellow color of the outline corresponds to the temperature of 200 °C. c–d TEM images of intermediate IONPs c, scale bars 50 nm, and isolated SiO₂@AuBPs d, scale bars 100 nm, at different time intervals (45 min to 3 h) for reactions carried out at a photothermal temperature of 200 °C. A gradual increase in nanoparticle size can be seen as the reaction progressed, as indicated by the brown arrow above. Additionally, IONPs homogenously decorated AuBP encapsulation (IONP@AuBPs). e–f TEM images of non-uniform particle formation e, scale bar 50 nm, in the presence of OTMS-modified SiO₂@AuBPs (at 200 °C, 2 h) and isolated non-polar AuBPs demonstrating poor IONP attachment f, scale bar 100 nm. g Digital image of the hybrid IONP@AuBPs (black dotted box) exhibiting a magnetic response when approximating a magnet. Also, the characteristic color of the AuBPs plasmonic interaction can be seen. Scale bar 1 cm. h Zeta potential measurements exhibit weak electrostatic interaction between the negatively charged silica surface of AuBP and the iron precursor. Number of measurements (n = 5) with standard deviation (SD = 1) as error bar. Control represents absence of iron precursor. i FTIR analysis revealing the presence of capping ligands (oleic acid/oleylamine) over the AuBP silica surface after the reaction. j Scheme showing the proposed mechanism involved during the photothermal synthesis. Source data are provided as a Source Data file.