Multicore iron oxide nanoparticles, also known as colloidal nanocrystal clusters, are magnetic materials with diverse applications in biomedicine and photonics. Here, we examine how both of their characteristic dimensional features, the primary particle and sub-micron colloid diameters, influence their magnetic properties and performance in two different applications. The characterization of these basic size-dependent properties is enabled by a synthetic strategy that provides independent control over both the primary nanocrystal and cluster dimensions. Over a wide range of conditions, electron microscopy and X-ray diffraction reveal that the oriented attachment of smaller nanocrystals results in their crystallographic alignment throughout the entire superstructure. We apply a sulfonated polymer with high charge density to prevent cluster aggregation and conjugate molecular dyes to particle surfaces so as to visualize their collection using handheld magnets. These libraries of colloidal clusters, indexed both by primary nanocrystal dimension (d p ) and overall cluster diameter (D c ), form magnetic photonic crystals with relatively weak size-dependent properties. In contrast, their performance as MRI T 2 contrast agents is highly sensitive to cluster diameter, not primary particle size, and is optimized for materials of 50 nm diameter (r 2 = 364 mM −1 s −1 ). These results exemplify the relevance of dimensional control in developing applications for these versatile materials.

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Here we report a solution-phase strategy for depositing ultrathin graphene-like carbon onto iron oxide nanocrystals (NCs) for corrosion resistance in magnetic and electrocatalytic applications. Thermal decomposition of iron carboxylates is a well-known method for generating uniform, size-tunable iron oxide NCs. When this reaction is completed at elevated temperatures and for longer times, the nanomaterials become unreactive to further growth and the magnetic nanomaterial survives treatment with concentrated nitric acid. X-ray photoelectron and Raman spectroscopies reveal that these materials contain graphene-like carbon. Metal carboxylates can decompose and yield carbon monoxide (CO), which we detect via gas chromatography−mass spectrometry. We speculate that when this CO is generated near a growing iron oxide surface, it disproportionates to yield carbon dioxide and carbon. Our approach is notable given that a low-temperature, solution-phase route for forming carbon materials such as graphene from the bottom up has remained elusive.

Nanoparticle ceria is a remarkable antioxidant due to its large cerium (III) content and the possibility of recovering cerium (III) from cerium (IV) after its use. Here we increase the...