A comparative study of oregano (Origanum vulgare L.) essential oil-based polycaprolactone nanocapsules/microspheres: Preparation, physicochemical characterization, and storage stability

Essential oils have been used in clinical therapies and healing practices for several chronic diseases, but it is still challenging their extent use due to their poor stability. Nanotechnology delivery systems have been proposed as an alternative to improve their stability. Oregano (Origanwn vulgare L.) is one of the most important medicinal plant highly reputed as an efficient remedy for infectious diseases. This study proposes the use of polycaprolactone, a biodegradable polymer, as a wall material for the encapsulation of Origantun vulgare L. essential oil, a carvacrol-rich source, using the nanoprecipitation and double emulsion methods. Nanocapsules formulations showed different values of mean particle size ranged between 175.17 +/- 0.31 nm and 220.93 +/- 2.73 nm, negative zeta potential values ranged between -18.7 +/- 0.50 mV up to - 42.53 +/- 1.01 mV, and encapsulation efficiencies ranging from 50.36 +/- 1.86% and 85.89 +/- 2.40%. The optimized nano-formulation presented a nano-scale size of 181.6 +/- 2.17 nm, a monomodal size distribution with a polydispersity index of 0.133 +/- 0.01, a pH value of 4.92 and a zeta potential value of - 40.9 +/- 0.93 mV. The encapsulation efficiency of 85.89 +/- 2.40% was achieved. In addition, microspheres produced by double emulsion, demonstrated porous microspheres morphology in the size of 1759.00 +/- 162.6 nm, pH value of 5.98, a zeta potential value of - 15.7 +/- 1.56 mV and encapsulation efficiency of 47.52 +/- 0.52%. Dynamic light scattering and scanning electron microscopy were performed to verify the size and the colloidal morphology. Thermal properties were assessed by thermogravimetric analysis and differential scanning calorimetry and the measurements revealed the thermal stability of the particles and showed that the crystalline state was less ordered in nanocapsules. Attenuated Total Reflectance Fourier Transform Infrared results indicated a slight shift in the C=O band of the polymer from 1723 cm(-1) induced by the change in its crystallinity degree due to the successful incorporation of oregano within its shell. Moreover, the long-term stability study was carried out for a period of 60 days at 4 +/- 2 degrees C, 25 +/- 2 degrees C and 40 +/- 2 degrees C. Results showed that nanocapsules were physically stable with high carvacrol retention at different temperatures. However when microspheres were thermally treated at 25 degrees C and 40 degrees C, an increase in particle size was observed. Besides, carvacrol retention decreased to 64.56% and 61.84% at 25 degrees C and 40 degrees C, respectively. From gathered results, the good physical stability of nanocapsules in long-term storage with high carvacrol retention stability were observed mainly at 4 degrees C.