Unraveling the Adsorption Behavior of Thymol on Carbon and Silica Nanospheres for Prolonged Antibacterial Activity: Experimental and DFT Studies

Functionalization of thymol (Thy) on nanocarriers is a key step in achieving prolonged antimicrobial activity. This requires nanomaterials with uniform particle diameters and suitable thymol sorption. Herein, hollow carbon (HC) and SiO2-carbon core–shell (SiO2@C) were investigated due to their diverse morphologies and ease of surface modification. HC (14 ± 1 nm size) and SiO2@C (10 ± 1.5 nm size) were synthesized by the Stöber method before thymol was loaded by incipient wetness impregnation. Nanoparticle physicochemical properties were characterized by advanced techniques, including X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS). Adsorption energies of thymol on the carbon and SiO2 surfaces were elucidated by density functional theory (DFT) simulations. Moreover, the in vitro thymol release profiles and antibacterial activity were evaluated. The experimental results indicated that the oxy-carbon surface species of HC led to longer thymol release profiles than the –OH group of SiO2@C. The DFT calculations revealed that the weaker physical interaction of thymol on HC was better for drug release than that on SiO2@C. Thus, a longer thymol release profile of HC with hollow structures showed better antibacterial performance against Gram-positive bacteria Staphylococcus aureus than that of SiO2@C with core–shell structures. This work confirms the important role of carbon morphology and specific functional groups in thymol release profiles for the further development of inhibition products.