a, Three components of an MRET sensor. A superparamagnetic nanoparticle quencher is connected to paramagnetic enhancers via interactive linker molecules. b, Nanometer-scale distance-dependent magnetic resonance tuning and resulting T1 contrast effect variation. The T1 contrast effect is negligibly low when d is shorter than the critical distance (dc), and it is enhanced when d is longer than dc. This distance-dependent T1 contrast variation can be exploited as a nanoscale ruler. c, Switching the T1 contrast effect using interactive linkers. The linker molecules can modulate d by changing their conformation upon reacting with targets. The possible modes of conformational change are folding/unfolding, cleavage, and binding. b adapted with permission from ref. 7, Macmillan Publishers.