A Zn2+-responsive bimodal magnetic resonance imaging (MRI) and luminescence imaging probe GdL was synthesized. The relaxivity and luminescence properties were examined. In the presence of 0.5 equiv of Zn2+, the longitudinal relaxivity is increased from 3.8 mM(-1) s(-1) to 5.9 mM(-1) s(-1) at 23 MHz and 25 degrees C with 55% enhancement, whereas the fluorescence exhibits a 7-fold increase. The Zn2+ responsive imaging probe shows favorable selectivity and tolerance over a variety of biologically relevant anions and metal ions in physiological pH range for both relaxivity and luminescence. In vitro phantom images and confocal fluorescence images in living cells show that the bimodal Zn2+ probe can effectively enhance T-1-weighted imaging contrast and luminescence imaging effect through Zn2+ coordination with excellent cellmembrane permeability and biocompatibility. Spectral and electrospray ionization mass spectrometry (ESI-MS) studies indicate that two different Zn2+-bound species, (GdL)(2)Zn and GdLZn, are formed when 0.5 and 1 equiv of Zn2+ are bound to GdL complex, respectively. Crystal structural determination and dysprosium-induced O-17 NMR shift (DIS) experiment demonstrate that the increased molecular weight and the improved molecular rigidity upon complexation of Zn2+ with GdL is the primary factor for relaxivity enhancement. Significant enhancement of the luminescence is due to a heavy atom effect and much increased molecular rigidity upon Zn2+ binding to 8-sulfonamidoquinoline chromophore.