PH-sensitive imaging agents

A magnetic resonance imaging agent comprising:

• (a) a superparamagnetic core;
• (b) a linker group;
• and
• (c) a pH sensitive macromolecule comprising a monomer unit comprising a dendron and having a pKa between 6 and 10;
• wherein,
• the linker group is affixed to the superparamagnetic core;
• the pH sensitive macromolecule is attached to the linker group;
• the dendron comprises a melamine, an imidazole, a polyimidazole, a vinyl imidazole, a nitrilotriacetic acid, a polyethylene glycol (PEG), a histidine, a lysine, an amino carboxylate, a histidine-lysine, or an amino sulfonate;
• the imaging agent exhibits a pH sensitive magnetic resonance response;
• and
• the imaging agent is biocompatible.