This paper describes colloidal particles that are designed to induce hyper-intensity contrast (T-1 relaxation) in MRI. The contrast agents consist of discrete gadolinium complexes tethered to 10 rim diameter silver nanoparticles. The gadolinium complexes (1) [Gd(DTPA-bisamido cysteine)](2-) and (2) [Gd(cystine-NTA)(2)](3-), undergo chemisorption to particle Surfaces through thiol or disulfide groups, respectively, to form two new contrast agents. The resulting nanoparticulate constructs are characterized on the basis of their syntheses, composition, spectra and contrast enhancing power. The average r(1) relaxivities of the of the Surface bound complexes (obtained at 9.4 T and 25 degrees C) are 10.7 and 9.7 s(-1) mM(-1), respectively, as compared to 4.7 s(-1) mM(-1) for the clinical agent Magnevist(TM). Correspondingly, the respective whole particle relaxivities are 27927 and 13153 s(-1) mM(-1). (C) 2009 Elsevier Inc. All rights reserved.