Improving the sensitivity of magnetic resonance imaging (MRI), a powerful non‐invasive medical imaging technique, requires the development of novel contrast agents with a higher efficiency than gadolinium chelates such as DTPA:Gd (DTPA: diethylenetriaminepentaacetic acid) that are currently used for clinical diagnosis. To achieve this objective, the strategy that we have explored involves the use of gold nanoparticles as carriers for gadolinium chelates. These nanoparticles are obtained by reducing a gold salt in the presence of a dithiolated derivative of DTPA. Characterization of these particles by transmission electron microscopy (TEM), X‐ray diffraction (XRD), thermogravimetric analysis (TGA), colorimetric titration, and X‐ray photoelectron spectroscopy (XPS) reveals the presence of a multilayered shell containing about 150 ligands on 2–2.5 nm sized particles. These particles exhibit a high relaxivity (r1 = 585 mM–1 s–1 as compared to 3.0 mM–1 s–1 for DTPA:Gd), rendering them very attractive as contrast agents for MRI.

It is shown that chromophores can be efficiently grafted on nanometer sized gold particles dispersible in an aqueous solution. The grafting is carried out in two steps: first, a primary grafting of thiols consisting both of stabilisers (2-mercaptoethane sulfonate and thiomaleic acid) and of linker molecules (4-mercaptophenol) and secondly the grafting of lissamine rhodamine B on these previously attached linkers. The efficiency of the grafting has been evaluated by dosing the remaining dye molecules in solution after forcing the precipitation of particles by salt addition. It is found that all the linkers participate in the efficient grafting of lissamine rhodamine B.