Superparamagnetic iron oxide nanoparticles have received great attention due to their applications as contrast agents for magnetic resonance imaging (MRI). This feature article briefly introduces the concepts of MRI and MRI contrast agents, and then mainly discusses the synthesis, surface modification, surface functionalization, colloidal stability and biocompatibility of iron oxide particles, followed by their MRI applications.

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Magnetic Fe(3)O(4) particle aggregates were prepared by cross-linking Fe(3)O(4) nanoparticles bearing surface carbonyl groups with poly-L-lysine. Upon further coupling with antiparaoxon methyl polyclonal antibody, the resultant particle aggregate-based probes were used in a lateral flow immunochromatographic assay (LFIA) of pesticide residue of paraoxon methyl. The results were compared with that achieved by using the mother Fe(3)O(4) nanoparticles. More quantitative results on the signal amplification effect endowed by the controlled aggregation of Fe(3)O(4) nanoparticles were extracted by relative optical density analysis. Under optimized conditions, a detection limit of 1.7 ng/mL for paraoxon methyl was achieved by using the particle aggregates, which is almost 40-fold lower than that based on the mother Fe(3)O(4) nanoparticles.

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Following on from our previous investigations, fluorescent core/shell CdTe@SiO 2 particles were prepared via the water-in-oil (W/O) reverse microemulsion method. It was found out that incubating the as-prepared aqueous CdTe quantum dots stabilized by thioglycolic acid in ammoniacal solution not only increased the fluorescence quantum yield of CdTe quantum dots but also gave rise to high retention of fluorescence throughout the silica coating. Under optimized conditions, the fluorescence quantum yield of CdTe quantum dots encapsulated in silica particles reached 47%. Both absorption and fluorescence spectroscopy were used in combination with X-ray photoelectron spectroscopy and electrophoresis to investigate the fluorescence enhancement effect occurring during incubation, the high retention of fluorescence quantum yield, as well as the formation of multicore/shell CdTe@SiO 2 particles as a result of the incubation process imposed on the as-prepared CdTe quantum dots.

Cellular uptake, endosomal/lysosomal escape, and the effective dissociation from the carrier are a series of hurdles for specific genes to be delivered both in vitro and in vivo. To construct siRNA delivery systems, poly(allylamine hydrochloride) (PAH) and siRNA were alternately assembled on the surface of 11.8 ± 0.9 nm Au nanoparticles (GNP), stabilized by denatured bovine serum albumin, by the ionic layer-by-layer (LbL) self-assembly method. By manipulating the outmost PAH layer, GNP-PAH vectors with different surface electric potentials were prepared. Then, the surface potential-dependent cytotoxicity of the resultant GNP-PAH particles was evaluated via sulforhodamine B (SRB) assay, while the surface potential-dependent cellular uptake efficiency was quantitatively analyzed by using the flow cytometry method based on carboxyfluorescein (FAM)-labeled siRNA. It was revealed that the GNP-PAH particles with surface potential of +25 mV exhibited the optimal cellular uptake efficiency and cytotoxicity for human breast cancer MCF-7 cells. Following these results, two more positively charged polyelectrolytes with different protonating abilities in comparison with PAH, i.e., polyethylenimine (PEI), and poly(diallyl dimethyl ammonium chloride) (PDDA), were chosen to fabricate similarly structured vectors. Confocal fluorescence microscopy studies indicated that siRNA delivered by GNP-PAH and GNP-PEI systems was better released than that delivered by the GNP-PDDA system. Further flow cytometric assays based on immunofluorescence staining of the epidermal growth factor receptor (EGFR) revealed that EGFR siRNA delivered by GNP-PAH and GNP-PEI exhibited similar down-regulation effects on EGFR expression in MCF-7 cells. The following dual fluorescence flow cytometry assays by co-staining phosphatidylserine and DNA suggested the EGFR siRNA delivered by GNP-PAH exhibited an improved silencing effect in comparison with that delivered by the commercial transfection reagent Lipofectamine 2000.

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Polyaniline/Fe(3)O(4) nanoparticle composite was prepared by polymerizing aniline in the presence of Fe(3)O(4) nanoparticles upon the use of H(2)O(2) as oxidant. The polymerization was monitored by ultraviolet-visible absorption spectroscopy. The microstructure of the resultant composite was characterized by transmission electron microscopy. The molecular structure of the resultant polyaniline in the composite was investigated by both Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy, while the magnetic property of the composite was characterized by vibrating sample magnetometer. Furthermore, the microwave absorption property of the resultant composite was measured in a frequency range of 2-18 GHz. Systematic investigations revealed that carboxylic acid in the buffer presented a determined role in the polymerization of aniline. To discover the role of carboxylic acid in the polymerization of aniline, more control experiments were designed and carried out by theoretical calculation in combination with electron spin resonance measurements. It was for the first time found out that carboxylic acid such as acetic acid and succinic acid can not only catalyze the polymerization of aniline but also facilitate the generation of hydroxyl radical via the decomposition of H(2)O(2).