The ability to non-invasively observe changes in Ca2+ concentration is important for neuroscience. We have therefore developed a series of gadolinium chelate complexes based on DO3A (Scheme 1), which is hypothesized to change relaxivity in magnetic resonance experiments dynamically with Ca2+ concentration. Different lengths of the phosphonate side chains are expected to lead to different binding constants of the phosphonate - gadolinium bonds. The latter property can be exploited for fine-tuning the sensitivity of the agent to calcium ion concentration.