ICE Issue 9

18 Scientific Article The Israel Chemist and Chemical Engineer Issue 9 · January 2023 · Tevet 5783 over phospholipid coated Sm:SrF2 (i.e., PL-Sm:SrF2, and thus referred to as non-glycosylated nanofluorides), in realtime, in the same inflamed tissue by presenting their spatial distribution as a multicolor 19F-MRI map (Figure 3d-i). In addition to the ability to use them as imaging agents for 19F-MRI, in general, and the potential to use them for multicolor MRI applications in particular (Figure 3), we developed a liquid-state NMR approach with which to study the formation pathways of nanofluorides with a conventional NMR setup, without the need to disturb the reaction conditions. Synthesizing nanofluorides under in situ NMR conditions, we were able to probe their sub-nm growth over the entire course of their formation, highlighting their controllable growth mechanisms (coalescence vs. classical simple-growth), which resulted in different morphological and functional features [18, 19]. Examining the correlation between the crystallographic features of the nanofluorides and their relaxation properties, we have developed an approach to shorten the T1 relaxation times of the fluoride content in nanofluorides by 10-fold only by introducing a defect in their crystals. Such an approach for nanocrystallinedefects relaxation enhancement (NDRE) demonstrates that, while avoiding the use of paramagnetic elements and without introducing the PRE-effect to shorten T1 values, we were able to extensively enhance the longitudinal relaxation rates of small-sized fluoride NCs to improve 19F-MRI performance [20]. To overcome this pitfall, nanofluorides were doped through their fabrication with the paramagnetic dopant Sm3+, which induced a significant paramagnetic relaxation enhancement (PRE) effect. Specifical ly, the T1 of nanofluorides was shortened more than 200-fold, from a T1 of ca. 15 s for nondoped CaF2 to an ultrashort T1 of 70 ms for Sm:CaF2 resulting in an eight-fold enhancement in their 19F-MRI SNR [17]. Then, with the introduction of paramagnetic nanofluorides (with Sm:CaF2 as a putative example), the ability to classify different types of synthetic nanofluorides and present them in a “multicolor” fashion was also examined. In this regard, the large range of chemical shifts of nanofluorides, which spans from BaF2 (ca. -10 ppm) to MgF2 (ca. -200 ppm) [16], provided a platform for the development of a series of fluoride-based NCs with different 19F-NMR chemical shifts. To demonstrate this ability, paramagnetic nanofluorides of the Sm:SrF2 type were synthesized to have a size and shape similar to Sm:CaF2 (Figure 3a). Dispersed in water, well-resolved, high-resolution 19F NMR peaks that differed from one another by more than ∼20 ppm (Figure 3b) were obtained with the expected characteristic resonances for SrF2 (-88 ppm) and CaF2 (-109 ppm). Such relatively large difference in their chemical shifts allowed spatial mapping of the two types of nanofluorides (Sm:CaF2 vs. Sm:SrF2) in the same imaging frame, without overlapping signals and without affecting the two detectable 19F-MRI signals (Figure 3c). Specifically, we demonstrated the immune specificity of lactose-phospholipid coated Sm:CaF2 (i.e., LPL-Sm:CaF2, namely paramagnetic glyconanofluorides) Figure 3. Multicolor 19F-MRI with nanofluorides. (a) TEM images (scale bar: 50 nm) of Sm:SrF 2 and Sm:CaF2 nanofluorides and (b) their 19F-NMR spectra when dispersed in water. (c) Multicolor 19F-MRI of a phantom containing SrF 2, CaF2, or a mixture of these. (d) Schematic representation of nonglycosylated PL-Sm:SrF2 and glycosylated LPL-Sm:CaF2 NCs injected as a mixture into the footpad of an inflamed mouse. (e) 1H-MRI of the inflamed mouse; white arrow indicates the inflamed lymph node, and yellow arrow represents the injection site. (f) In vivo 19F-NMR spectrum (total injected PL-Sm:SrF2 and LPL-Sm:CaF2). (g) 19F-MRI acquired with the center of the frequency offset set at either −88 ppm (left, yellow) or −109 ppm (right, light blue). (h) Representative 1H/19F MRI showing the higher accumulation of LPL-Sm:CaF 2 NCs in the LN. (i) Dot graph presenting the 19F-MRI signal of either PL-Sm:SrF 2 or LPL-Sm:CaF2 in the lymph node ROI (n = 4, Student’s t test, * represents a p value <0.05). Scale bar: 50 nm. Modified with permission from ref. 17