Fig. 1.

Illustrations of Hyper-SAME and ¹²⁹Xe NMR spectra for ZIF-8 in aqueous solution. (A) Illustration of ¹²⁹Xe NMR signal enhanced by ZIF-8 in aqueous media. (B) NMR spectra of hyperpolarized ¹²⁹Xe in ultrapure water, saturated CrypA, and ZIF-8 (5.0 mg/mL) in aqueous solution. For all spectra, numbers of scan (NS) = 16, LB = 10 Hz. (C) Entrapment of hyperpolarized ¹²⁹Xe in ZIF-8 crystals. The aperture of the six-membered ring is built by six 2-methylimidazole and six zinc ions. The distance between two opposite zinc atoms is 11 Å. The diameter of the largest sphere which will pass through the window of the six-membered ring is 3.8 Å. However, the flexibility of the structure allows for an effective diameter from 3.8 Å to 4.3 Å, which is suitable for the entrance of Xe. The four-membered ring (3.2 Å) is too small for Xe atom entry and escape. Entrapped Xe is in orange and the dissolved Xe in red. (D) NMR spectra of hyperpolarized ¹²⁹Xe in ZIF-8 nanocrystals (110 nm) at different concentrations (2.5, 5.0, 10, 15, 20, and 25 mg/mL) in aqueous solution, NS = 16, LB = 10 Hz. For these ¹²⁹Xe NMR spectra, a zg sequence was applied for acquisition (rectangular pulse, the pulse length p1 = 31.8 μs).