Ion transport mediated pH-responsive MRI contrast in dual wrapped MSNs

Functionalised polymers act as environmentally-responsive coatings, capable of sterically gating water exchange between bulk and Gd(iii) centres within mesoporous silica nanoparticles (Gd-MSNs). Using poly(2-(dimethylamino)ethyl methacrylate) (pDMAEMA, pK _a = 6.4) this water barrier is pH actuated (MRI 'on' at pH < pK _a, MRI 'off' at pH > pK _a). Such pH-responsive polymer coated Gd-MSNs can subsequently be enveloped within vesicular structures, further limiting water access, providing an improved MRI 'off' state for the so-generated dual-wrapped structure (r ₁ = 2.4 ± 0.2 mM^-1 s^-1, pH 8.0, 1.41 T, 298 K) compared with the polymer alone. When ion carriers (here a chloride-transporting tripodal thiourea alongside a protonophore, carbonyl cyanide m-chlorophenyl hydrazone) are integrated, it is possible to couple this pH gating to specific symport ion transport such that, ultimately, ionic rcognition triggers a ∼500% magnitude enhancement in relaxivity (r ₁ = 14.5 ± 0.8 mM^-1 s^-1 at pH 5.0).