The possibility to easily and rapidly assess the presence of Gd 3+ ions in solution is of paramount importance in many domains like magnetic resonance imaging. In that context, the use of easy to implement colorimetric sensing probes based on gold nanoparticles (AuNPs) is of special interest. Herein, AuNPs functionalized with a commercial bis(p-sulfonatophenyl)phenyl phosphine ligand (BSPP) (AuNP@BSPP), bearing negatively charged sulfonate groups are used as a colorimetric sensing probe. The addition of Gd 3+ ions onto these NPs was studied through UV-visible absorbance measurements, Quartz Crystal Microbalance with Dissipation monitoring (QCM-D) and transmission electron microscopy and compared with citrate covered AuNPs. We evidenced interactions between the Gd 3+ ions and their water rich coordination sphere and sulfonate groups on the surface of AuNP@BSPP via electrostatic interactions and hydrogen bonding. These interactions induce the reversible aggregation of AuNP@BSPP in the presence of concentrations of Gd 3+ ions at a mM level. We took advantage of this phenomenon to develop a simple and fast bench colorimetric assay for the detection of free Gd 3+ ions, based on the determination of a flocculation parameter thanks to UV-visible measurements. Limits of detection and quantification were found equal to 0.74 mM and 4.76 mM of Gd 3+ ions, respectively, with a high sensitivity that competes with conventional methods used for lanthanide detection.