Gold‐Cobalt Nanoparticle Alloys Exhibiting Tunable Compositions, Near‐Infrared Emission, and High T2 Relaxivity

We demonstrate the synthesis of discrete, composition‐tunable gold‐cobalt nanoparticle alloys (% Co = 0–100%; diameter = 2–3 nm), in contrast with bulk behavior, which shows immiscibility of Au and Co at room temperature across all composition space. These particles are characterized by transmission electron microscopy and 1 H NMR techniques, as well as inductively coupled plasma mass spectrometry, X‐ray photoelectron spectroscopy, and photoluminescence spectroscopy. In particular, 1 H NMR methods allow the simultaneous evaluation of composition‐tunable magnetic properties as well as molecular characterization of the colloid, including ligand environment and hydrodynamic diameter. These experiments also demonstrate a route to optimize bimodal imaging modalities, where we identify Au x Co y NP compositions that exhibit both bright NIR emission (2884 m −1 cm −1 ) as well as some of the highest per‐particle T 2 relaxivities (12200 m m NP −1 s −1 ) reported to date for this particle size range.