3.2.2. Raman Spectroscopy

The Raman spectra illustrated in Figure 2 show characteristic peaks for both 3TC and AZT. In the region < 1500 cm<sup>−</sup><sup>1</sup> the Raman spectrum for 3TC exhibits several unique bands at approximately 1290 cm<sup>−</sup>1, 1250 cm<sup>−</sup><sup>1</sup> and 790 cm<sup>−</sup><sup>1</sup> [49]. A carbonyl stretching mode is observed at 1650 cm<sup>−</sup><sup>1</sup> in addition to C=N stretching at 1530 cm<sup>−</sup>1, both of which are confirmed in previous reports [49,50]. The most intense bands for AZT at 1650 cm<sup>−</sup><sup>1</sup> are due to the symmetric stretching vibration of the C=C bond of the pyrimidine ring. Another marker band for AZT is that corresponding to the breathing vibration of the pyrimidine ring located at 760 and at 790 cm<sup>−</sup>1. The peak at 1480 cm<sup>−</sup><sup>1</sup> is due to the carbonyl C=O stretching vibration of the pyrimidine ring. The band for the N≡N stretching vibration of the azide group present in the Raman spectrum at 2100 cm<sup>−</sup><sup>1</sup> is characteristic of AZT. The signals observed are in close agreemen<sup>t</sup> with previously reported data [51].

**Figure 2.** A stacked plot of the Raman spectra for the micro (black) and nano co-crystals (orange).
