ICE Issue 9

17 Scientific Article The Israel Chemist and Chemical Engineer Issue 9 · January 2023 · Tevet 5783 as a quantitative “hot-spot” map over anatomical 1H-MRI. In this regard, perfluorocarbons (PFCs), fluorine-rich materials, have been successfully used in a wide range of 19F-MRI applications [12, 13], including clinical setups [14]. Relying on the relatively large range of their chemical shift appearances in the 19F-NMR spectrum (a few tens of ppm), PFCs have been proposed for multiplexed in vivo 19F-MRI [7, 9, 10]. Nevertheless, their introduction as emulsions of a typical 100– 200 nm size (i.e., PFC nanoemulsions) and because they are organic formulations, PFC nanoemulsions are not applicable for studies for which ultrasmall (<10 nm) nanoformulations are desired, and they lack the well-established and diverse chemistries of inorganic nanocrystals (NCs). Moreover, they do not cover the whole range of 19F-NMR chemical shifts, which span over almost 200 ppm when using inorganic fluorides (as shown by solid-state NMR). An inorganic, small-size alternative to PFC nanoemulsions may, therefore, be fluoride-based NCs (MxFy, M = metal ion, F = F-), which had not been studied in solutions with highresolution 19F-NMR and were not used in 19F-MRI until very recently. This is because in NC-based formulations, the restricted mobility of the elements within the crystal frequently results in NMR line-broadening, and highresolution NMR signals from the core of the NCs’ nuclei cannot be obtained using liquid-state NMR experiments. Overcoming such l imitations and, thus, successful ly performing liquid-state 19F-NMR experiments of M xFy in solution, we offered a novel kind of 19F-nanotracers for 19FMR imaging, which are based, for the first time, on inorganic fluoride NCs, namely nanofluorides [15]. These nanofluorides combine the advantages of inorganic NCs (e.g., small and controllable sizes, dense fluoride content, monodispersity, colloidal stability, surface modifiability, designed as nonspherical materials, etc.) with the merits of 19F-MRI. In addition, the large chemical shifts of nanofluorides, which can span over almost 200 ppm, provide a platform for the development of a series of fluoride-based NCs with different 19F-NMR chemical shifts, which can serve as artificial “multicolor” tracers for multiplexed MRI. Demonstrating that high-resolution 19F-NMR spectra can be achieved by sufficient averaging of homonuclear dipolar interactions of 19F-nuclear spins within small-size fluoridecontaining NCs (i.e., nanofluorides), we showed that CaF2 NCs can be used as nano-sized molecular tracers for 19F-MRI [15]. First, small, water-dispersed CaF2 NCs were synthesized and found to be highly crystalline and monodispersed (Figure 2a) with preserved monodispersity in water, as determined by dynamic light scattering (DLS, Figure 2b). The XRD pattern of the synthetic CaF2 NCs (Figure 2c) features a typical cubicphase, fluorite-type, fcc structure, where all fluorides are expected to be magnetically equivalent, as reflected by the first coordination sphere scheme (inset, Figure 2c). Indeed, a singlet peak was observed in the high-resolution 19F-NMR spectrum of water-dispersed CaF2 NCs at -109 ppm (Figure 2d), similar to the frequency obtained for CaF2 powder with solid-state NMR [16]. Then, the potential of using the proposed CaF2 NCs as imaging tracers for in vivo 19F-MRI was evaluated in a mouse model of inflammation. To this end, polyethylene-glycol (PEG)-coated CaF2 NCs were injected into a group of inflamed mice. A clear 19F-MRI signal was observed at the region of the popliteal lymph node of NCinjected mice in the same leg as the injection site (Figure 2e) one hour post-injection. Although their potential to be used in vivo was evident, the T1 relaxation times of nanofluorides are relatively long (>10 sec) [15], which limits signal averaging and, thus, the signal-tonoise ratio (SNR) in 19F-MR images at a given imaging time. Figure 2. 19F-NMR and 19F-MRI of water-soluble CaF 2 NCs. (a) TEM images. (b) DLS histogram. (c) XRD pattern with schematic of the Ca2+ first coordination sphere (red spheres represent 19F-atoms). (d) High-resolution 19F-NMR in water. (e) In vivo imaging of PEGylated CaF2 showing anatomical 1H-MRI of a representative mouse (left) and matched 19F-MRI (middle) shown as a pseudo-color map overlaid on 1H-MRI (right). Modified from reference 15 with permission. e