Hyperpolarized Xe NMR signal advancement by metal-organic framework entrapment in aqueous solution

Significance

Hyperpolarized ¹²⁹Xe NMR/MRI is a useful method for diagnosis of diseases of the respiratory system. However, the sensitive detection of specific compounds in blood remains a challenge because of the weak ¹²⁹Xe signal in aqueous solution. We developed a way, Hyper-SAME, to promote the ¹²⁹Xe signal in aqueous solution. The ¹²⁹Xe signal intensity is four times beyond that of free ¹²⁹Xe in water and 200 times better than the benchmark molecular cage, cryptophane-A, in its saturated aqueous solution. Additionally, the hyperpolarized ¹²⁹Xe signal can be amplified further by combining Hyper-SAME with hyperpolarized ¹²⁹Xe chemical exchange saturation transfer.

Abstract

We report hyperpolarized Xe signal advancement by metal-organic framework (MOF) entrapment (Hyper-SAME) in aqueous solution. The ¹²⁹Xe NMR signal is drastically promoted by entrapping the Xe into the pores of MOFs. The chemical shift of entrapped ¹²⁹Xe is clearly distinguishable from that of free ¹²⁹Xe in water, due to the surface and pore environment of MOFs. The influences from the crystal size of MOFs and their concentration in water are studied. A zinc imidazole MOF, zeolitic imidazole framework-8 (ZIF-8), with particle size of 110 nm at a concentration of 100 mg/mL, was used to give an NMR signal with intensity four times that of free ¹²⁹Xe in water. Additionally, Hyper-SAME is compatible with hyperpolarized ¹²⁹Xe chemical exchange saturation transfer. The ¹²⁹Xe NMR signal can be amplified further by combining the two techniques. More importantly, Hyper-SAME provides a way to make detection of hyperpolarized ¹²⁹Xe in aqueous solution convenient and broadens the application area of MOFs.