Slowing Down Water: Enhanced and Cation‐responsive MRI Contrast

The confinement of paramagnetic centres to the internal channels of mesoporous nanoparticles brings with it a much greater contribution from outer sphere effects by virtue of tuneable side wall association and a resulting much stretched water diffusion coefficient. We specifically utilise the relative ease of side wall modifications to demonstrate that kosmotropic (water‐ordering) effects can be leveraged to greatly increase the viscosity of water in the vicinity of internalised paramagnetic complexes. This has the effect of supporting unprecedented levels of image contrast with q = 0 systems and much enhanced T 1 contrast for common q = 1 complexes, at clinically relevant field strengths. Physiologically relevant potassium‐responsive imaging was subsequently enabled by modulating the water organisation initially imparted by internalised crown ethers on specific cation binding.