Thanks to their versatile magnetic and luminescence features, lanthanide complexes have gained a central position in biomedical imaging as magnetic resonance imaging (MRI) contrast agents and optical imaging probes. In addition, appropriate chemical design allows modification of the magnetic relaxation properties of Gd-III complexes and the optical properties of visible- or near-infrared (NIR)-emitting lanthanide chelates upon interaction with various biomarkers, which makes them ideal candidates for the creation of responsive agents. In this Forum Article, we demonstrate such design principles as well as the difficulties encountered in the context of neurotransmitter (NT) detection. Lanthanide(III) complexes of a macrocyclic ligand incorporating a benzophenone chromophore and a monoazacrown ether (LnL(3)) have been synthesized as responsive probes to monitor amino acid NTs either in MRI (Ln = Gd) or in NIR optical detection (Ln = Nd or Yb). The parameters characterizing the water exchange and rotational dynamics of the gadolinium(III) complex were assessed by O-17 NMR and H-1 NMRD. In the presence of zwitterionic NTs, the inner-sphere water molecule is replaced by the carboxylate function of the NTs in the gadolinium(III) complex, leading to a decrease of the longitudinal relaxivity from 6.7 to 22.5 mM(-1) s(-1) (300 MHz and 37 degrees C). The apparent affinity constants range from K-a = 35 for gamma-aminobutyric acid (GABA) to 80 M-1 for glycine and glutamate, and there is no selectivity with respect to hydrogen carbonate (K-a = 232; pH 7.4). The gadolinium(III) complex interacts with human serum albumin (HSA), resulting in a 60% increase in the relaxivity (20 MHz, 37 degrees C) in the absence of NTs. The HSA-bound complex, however, was revealed to be less responsive to NTs because of displacement of the Gd-III-bound water by HSA, which was confirmed by the hydration number calculated from luminescence lifetimes of the HSA-bound europium(III) complex. The creation of an imaging agent suitable for NIR detection of NTs for an enhanced sensitivity in biological systems using the benzophenone (BP) moiety as the sensitizer of lanthanide luminescence was also attempted. Upon excitation at 300 nm of the BP chromophore in aqueous solutions of NdL3 and YbL3, characteristic NIR emissions of Nd-III and Yb-III were observed because of F-4(3/2) (R) I-4(J) (J = 9/2-13/2) and F-2(5/2) (R) F-2(7/2) transitions, respectively, indicating that this chromophore is a suitable antenna. Despite these promising results, luminescence titrations of Nd-III and Yb-III complexes with NTs were not conclusive because of chemical conversion of the ligand triggered by light, preventing quantitative analysis. The observed photochemical reaction of the ligand is strongly dependent on the nature of the lanthanide chelated; it is considerably slowed down in the presence of Nd-III and Eu-III.