Single-nanometer iron oxide nanoparticles as tissue-permeable MRI contrast agents

Significance This work shows that iron oxide species as small as 1 nm in diameter can exhibit superparamagnetic properties and act as potent contrast agents for MRI. Because of their small size, the resulting particles effectively permeate dense biological tissue and undergo ultrasound-facilitated delivery from the vasculature to the brain. They also exhibit T 1 -weighted contrast-inducing properties similar to widely used paramagnetic MRI contrast agents but with far higher potency per molecule. Single-nanometer iron oxide agents, therefore, constitute a beneficial basis for the development of molecular imaging probes designed to detect targets in T 1 -weighted MRI. They might also function as alternatives to widely used gadolinium-based MRI agents in clinical contexts where gadolinium-related toxicity is a concern.