Iron oxide nanoparticles have long been studied as a T-2 contrast agent in MRI due to their superparamagnetic behavior. T-1-based positive contrast, being much more favorable for clinical application due to brighter and more accurate signaling is, however, still limited to gadolinium- or manganese-based imaging tools. Though being the only available commercial positive-contrast agents, they lack an efficient argument when it comes to biological toxicity and their circulatory half-life in blood. The need arises to design a biocompatible contrast agent with a scope for easy surface functionalization for long circulation in blood and/or targeted imaging. We hereby propose an extremely fast microwave synthesis for fluorescein-labeled extremely-small iron oxide nanoparticles (fdIONP), in a single step, as a viable tool for cell labeling and T-1-MRI. We demonstrate the capabilities of such an approach through high-quality magnetic resonance angiographic images of mice.This study was supported by a grant from Ministerio de Economia y Competitividad (MAT2013-47303-P) and European Union (Cardionext, ITN-FP7-608027).S
