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

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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 1H NMR techniques, as well as inductively coupled plasma mass spectrometry, X‐ray photoelectron spectroscopy, and photoluminescence spectroscopy. In particular, 1H 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 AuxCoyNP compositions that exhibit both bright NIR emission (2884 m −1cm−1) as well as some of the highest per‐particle T 2 relaxivities (12200 mm NP −1s−1) reported to date for this particle size range.