Ultrahigh magnetic resonance contrast switching with water gated polymer-silica nanoparticles

Very high T₁ magnetic resonance imaging (MRI) switches can be obtained with pH-responsive polymer-coated paramagnetic mesoporous silica nanoparticles (MSNs), as the local environment traverses the pK_a of the polymer coat (Δr₁ ∼ 50 mM^-1 s^-1 at 1.5 T and Δr₁ ∼ 22 mM^-1 s^-1 at 3 T). We assign these characteristics to a strong peripheral hydration capping at the mesopores, impacting channel-confined water mobility such that outer sphere contributions to contrast are greatly enhanced.