3.2.3. FTIR Spectroscopy

The FTIR spectra depicted in Figure 6 show peaks at 3530 cm<sup>−</sup><sup>1</sup> (\*), which are characteristic of the hydrogen-bonded water in the crystal for both coated and uncoated nano co-crystals, [54,55]. The stretching band at 1635 cm<sup>−</sup><sup>1</sup> (@) is due to the carbonyl moiety (O=C–NR2) and is characteristic for 3TC and AZT. It partially overlaps with the N–H (bending) band at 1607 cm<sup>−</sup><sup>1</sup> (→). The stretching vibration of the imine group (R2-C=NR) is observed at 1648 cm<sup>−</sup><sup>1</sup> (#). Characteristic bands for AZT are observed at 2170 cm<sup>−</sup><sup>1</sup> (‡) and 1652 cm<sup>−</sup>1, due to –N3 and –N–H stretching vibrations. A comparison of the wavenumbers for the co-crystal and the OPT-NCC are reported and plotted in Table S2 and Figure S2 respectively and the spectra for the NCC, AZT and 3TC are depicted in Figure S3 in the Supplementary Information.

**Figure 6.** FTIR absorption spectra of the uncoated NCC (black) and the OPT-NCC (orange).
