Geometrical confinement of gadolinium-based contrast agents in nanoporous particles enhances T₁ contrast

Magnetic resonance imaging contrast agents are currently designed by modifying their structural and physiochemical properties to improve relaxivity and to enhance image contrast. Here, we show a general method for increasing relaxivity by confining contrast agents inside the nanoporous structure of silicon particles. Magnevist, gadofullerenes and gadonanotubes were loaded inside the pores of quasi-hemispherical and discoidal particles. For all combinations of nanoconstructs, a boost in longitudinal proton relaxivity r ₁ was observed: Magnevist, r₁≈14 mM^-1 s ^-1/Gd³⁺ ion (∼8.15×10⁺⁷ mM ^-1s^-1/construct); gadofullerenes, r¹ ≈ 200 mM^-1s^-1/Gd³⁺ ion (∼7×10⁺⁹ mM^-1s^-1/construct); gadonanotubes, r¹ ≈ 150 mM^-1s^-1/Gd^3+ion(∼2×10 ^-1+9 mM^-1s^-1/construct). These relaxivity values are about 4 to 50 times larger than those of clinically available gadolinium-based agents (∼4 mM^-1s^-1/Gd ³⁺ ion). The enhancement in contrast is attributed to the geometrical confinement of the agents, which influences the paramagnetic behaviour of the Gd³⁺ ions. Thus, nanoscale confinement offers a new and general strategy for enhancing the contrast of gadolinium-based contrast agents.