Niobium oxide exists in several crystal phases and valence states, yielding a variety of properties. Phase transitions between polymorphs significantly alter the properties of functional niobium oxide nanostructures, which are applied in many different renewable energy applications. This study uses environmental transmission electron microscopy to investigate the oxidation of NbO nanowires to T-Nb2O5. Small oxygen partial pressures (<1 mbar) suffice to initiate the oxidation which initially causes the destruction of the lattice ordering prior to the crystallization of the new phase. Different oxygen partial pressures and temperatures are used to manipulate the reaction kinetics. The crystallization is slow at room temperature and confined to the nanowire surface. Elevated temperatures accelerate the crystallization, resulting in the transformation of the complete nanowire. The transformation proceeds via dislocations, yielding the woven-like structure of the transformed nanowire. Despite the dramatic changes of the atomic composition of the material, an orientation relationship between the different phases is preserved. The phase transformation of NbO to T-Nb2O5 observed in situ complies with ex situ phase diagrams. This illustrates the importance of environmental transmission electron microscopy for the investigation of oxidation and reduction reactions of metal oxide nanostructures to identify the impact of shape and intrinsic defects.

Anisotropic colloidal hybrid nanoparticles exhibit superior optical and physical properties compared to their counterparts with regular architectures. We herein developed a controlled, stepwise strategy to build novel, anisotropic, branched, gold nanoarchitectures (Au-tripods) with predetermined composition and morphology for bioimaging. The resultant Au-tripods with size less than 20 nm showed great promise as contrast agents for in vivo photoacoustic imaging (PAI). We further identified Au-tripods with two possible configurations as high-absorbance nanomaterials from various gold multipods using a numerical simulation analysis. The PAI signals were linearly correlated with their concentrations after subcutaneous injection. The in vivo biodistribution of Au-tripods favorable for molecular imaging was confirmed using small animal positron emission tomography (PET). Intravenous administration of cyclic Arg-Gly-Asp-d-Phe-Cys (RGDfC) peptide conjugated Au-tripods (RGD-Au-tripods) to U87MG tumor-bearing mice showed PAI contrasts in tumors almost 3-fold higher than for the blocking group. PAI results correlated well with the corresponding PET images. Quantitative biodistribution data revealed that 7.9% ID/g of RGD-Au-tripods had accumulated in the U87MG tumor after 24 h post-injection. A pilot mouse toxicology study confirmed that no evidence of significant acute or systemic toxicity was observed in histopathological examination. Our study suggests that Au-tripods can be reliably synthesized through stringently controlled chemical synthesis and could serve as a new generation of platform with high selectivity and sensitivity for multimodality molecular imaging.