3.2.1. FTIR Spectroscopy

The FTIR spectra depicted in Figure 1 have peaks at 3530 cm<sup>−</sup><sup>1</sup> for both nano and micro co-crystals, which is characteristic peak for water in the crystal structure [47,48]. The stretching band occurring at 1634 cm<sup>−</sup><sup>1</sup> is due to the carbonyl moiety (O=C–NR2) and is characteristic of 3TC. It partially overlaps with the band due to N–H bending at 1607 cm<sup>−</sup>1. The band at 1648 cm<sup>−</sup><sup>1</sup> is due to the stretching vibration of the imine group (R2-C=NR). Broad bands due to the stretching vibration of –NH2 and –OH functional groups are observed at 3300–3500 cm<sup>−</sup><sup>1</sup> and are indicative of 3TC in the co-crystal. Characteristic bands at 2170 cm<sup>−</sup><sup>1</sup> and 1652 cm<sup>−</sup><sup>1</sup> are due to –N3 and –N–H stretching vibrations and are indicative of AZT in the co-crystal.

**Figure 1.** FTIR absorption spectra of the micro (black) and the nano co-crystal (orange).
