This work reports a detailed investigation about the physicochemical properties of superparamagnetic γ-Fe₂O₃ nanomaterial synthesized by the co-precipitation method and coated with two silica shells, and its application as support for the immobilization of oxovanadium(IV) acetylacetonate ([VO(acac)₂]). The influence of the silica coatings on the surface composition and physicochemical interactions of the core-shell nanocomposites is discussed based on the combination of several techniques: electron microscopy techniques (SEM and TEM with EDS), DLS, powder XRD, XPS, FTIR and magnetic characterization. The identity of the iron oxide, γ-Fe₂O₃, was confirmed by XPS, FTIR and by the Rietveld refinement of the PXRD pattern. The results obtained by electron microscopy techniques, XRD and magnetization indicated that the γ-Fe₂O₃ nanoparticles are superparamagnetic and present an average size of ∼6.5 nm. The first silica coating leads to a core-shell nanomaterial with an average particle size of 21 nm and upon the second coating, the average size increases to 240 nm. Magnetic measurements revealed that the silica-coated nanomaterials maintain the superparamagnetic state at room temperature, although with an expected reduction of the magnetization saturation due to the increase of the silica shell thickness. Furthermore, a numerical fit of the temperature dependence of magnetization was performed to determine the core size distribution and the effect of the silica coatings on the dipolar magnetic interactions. [VO(acac)₂] was covalently immobilized on the surface of the silica-coated magnetic nanoparticles functionalized with amine groups, as confirmed by chemical analysis and XPS. In a proof-of-principle experiment, we demonstrated the catalytic performance of the novel magnetic hybrid nanomaterial in the epoxidation of geraniol, which presented high selectivity towards the 2,3-epoxygeraniol product and easy recovery by magnetic separation.

A novel synthetic methodology for star shaped gold-coated magnetic nanoparticles is reported. The coating is performed in two steps: formation of gold nuclei at the surface of magnetite nanoparticles followed by growth of the gold nuclei into a complete star shaped shell. The star-shaped gold-coated magnetic nanoparticles thus obtained preserve the magnetic properties of the precursor magnetite nanoparticles, e.g. they can be easily separated with a magnet. In addition, the gold coating provides interesting optical properties while simultaneously allowing for biofunctionalization that may be advantageous for biological applications, such as (bio)detection via surface-enhanced Raman spectroscopy (SERS). As a proof-of-concept, a capping agent terminated with a nickel(II)-nitrilotriacetate group showing high affinity for histidine was used to modify the surface of the nanoparticles. The resulting star-shaped nanoparticles were used to selectively capture histidine-tagged maltose-binding protein from a crude cell extract. Finally, the performance of star shaped gold-coated magnetic nanoparticles as SERS platforms was demonstrated through the detection of Raman active dye (Astra Blue).

In this study, we report the synthesis of gold-coated iron oxide nanoparticles capped with polyvinylpyrrolidone (Fe@Au NPs). The as-synthesized nanoparticles (NPs) exhibited good stability in aqueous media and excellent features as contrast agents (CA) for both magnetic resonance imaging (MRI) and X-ray computed tomography (CT). Additionally, due to the presence of the local surface plasmon resonances of gold, the NPs showed exploitable “light-to-heat” conversion ability in the near-infrared (NIR) region, a key attribute for effective photothermal therapies (PTT). In vitro experiments revealed biocompatibility as well as excellent efficiency in killing glioblastoma cells via PTT. The in vivo nontoxicity of the NPs was demonstrated using zebrafish embryos as an intermediate step between cells and rodent models. To warrant that an effective therapeutic dose was achieved inside the tumor, both intratumoral and intravenous routes were screened in rodent models by MRI and CT. The pharmacokinetics and biodistribution confirmed the multimodal imaging CA capabilities of the Fe@AuNPs and revealed constraints of the intravenous route for tumor targeting, dictating intratumoral administration for therapeutic applications. Finally, Fe@Au NPs were successfully used for an in vivo proof of concept of imaging-guided focused PTT against glioblastoma multiforme in a mouse model.

A photocatalytic approach was used to synthesize triangular nanoplates in aqueous solution. The synthesis is based on the reduction of a gold salt using a tin(IV) porphyrin as photocatalyst, cetyltrimethylammonium bromide (CTAB) as a stabilizing agent, and triethanolamine (TEA) as the final electron donor. The average edge length of the triangular nanoplates can be easily changed in the range 45–250 nm by varying the concentration of photocatalyst, and fine-tuning of the average edge length is achieved by varying the concentration of CTAB. Study of the mechanism of formation of the nanoplates by UV-vis and by transmission electron microscopy (TEM) shows that there is a first stage where formation of 5 nm seeds takes place, further growth is probably by fusion and by direct reduction of gold onto the preformed nanoparticles. The nanoparticles formed during the photocatalytic reduction of the gold precursor show an irregular shape that evolves to regular triangular nanoplates after ripening in solution for 24 h.

Magnetic force microscopy (MFM) is a very powerful technique, which can potentially be used to detect and localize the magnetic fields arising from nanoscopic magnetic domains, such as magnetic nanoparticles. However, in order to achieve this, we must be able to use MFM to discriminate between magnetic forces arising from the magnetic nanoparticles and nonmagnetic forces from other particles and sample features. Unfortunately, MFM can show a significant response even for nonmagnetic nanoparticles, giving rise to potentially misleading results. The literature to date lacks evidence for MFM detection of magnetic nanoparticles with nonmagnetic nanoparticles as a control. In this work, we studied magnetite particles of two sizes and with a silica shell, and compared them to nonmagnetic metallic and silica nanoparticles. We found that even on conducting, grounded substrates, significant electrostatic interaction between atomic force microscopy probes and nanoparticles can be detected, causing nonmagnetic signals that might be mistaken for a true MFM response. Nevertheless, we show that MFM can be used to discriminate between magnetic and nonmagnetic nanoparticles by using an electromagnetic shielding technique or by analysis of the phase shift data. On the basis of our experimental evidence we propose a methodology that enables MFM to be reliably used to study unknown samples containing magnetic nanoparticles, and correctly interpret the data obtained.