We report that ultra-small, monodisperse, water-dispersible magnetite (Fe₃O₄) nanoparticles can be synthesized by a facile one-pot approach using trisodium citrate as crystal grain growth inhibitor and stabilizer in polyol solution. The resultant Fe₃O₄ nanoparticles exhibit an excellent long-term colloidal stability in various buffer solutions without any modification. They are also superparamagnetic at room temperature and their magnetic property relies heavily on their size. Due to the low magnetization and good water-dispersibility, the 1.9 nm-sized Fe₃O₄ nanoparticles reveal a low r₂/r₁ ratio of 2.03 (r₁ = 1.415 mM⁻¹ s⁻¹, r₂ = 2.87 mM⁻¹ s⁻¹), demonstrating that they can be efficient T₁ contrast agents. On the other hand, because of the excellent magnetic responsivity, the 13.8 nm-sized Fe₃O₄ nanoparticles can be readily modified with nitrilotriacetic acid and used to separate the protein simply with the assistance of a magnet. In addition, these Fe₃O₄ nanoparticles may be useful in other fields, such as hyperthermia treatment of cancer and targeted drug delivery based on their size-dependent magnetic property and excellent stability.

A facile approach was established for fabricating photothermo-responsive nanogels for simultaneous optical temperature-sensing, cell imaging, and combined chemo-photothermal treatment. The strategy for preparing the hybrid Au@IPN-pNIPAAm nanogels involved two steps, consisting of growing an acrylamide monolayer on a single gold nanoparticle, followed by in situ polymerization and crosslinking of N-isopropylacrylamide and acrylamide. The hybrid Au@IPN-pNIPAAm nanogels exhibited not only biocompatibility at low concentration <200 μg ml⁻¹, but also good function on cellular imaging by using dark-field microscopy. Furthermore, the integration of the unique scattering properties of AuNP core and the reversible thermo-responsive volume phase transition of IPN-pNIPAAm shell could offer a thermo-/photo-triggered release of a model anticancer drug 5-fluorouracil from Au@IPN-pNIPAAm nanogels. Hence, the hybrid Au@IPN-pNIPAAm nanogels, may offer broad opportunities for combined diagnosis and therapy.