Adsorption of ciprofloxacin on carbon nanotubes: Insights from molecular dynamics simulations

The adsorption of the neutral and zwitterionic forms of ciprofloxacin (CFX) on a single-walled carbon nanotube (SWCNT) has been studied by molecular dynamics (MD) simulations in gas phase and water. In both phases CFX remain adsorbed on the SWCNT during all simulation time. The interaction energies (E(i)nt) are strongly negative and suggest a preferential adsorption on the internal wall of the SWCNT. The CFX molecules adsorb mostly oriented parallel to the surface interacting via pi - pi interactions and, in vacuum, they tend to aggregate by forming intermolecular H-bonds. In vacuum, other arrangements ("sandwich-like") are detected when at least 4 CFX are present The analysis of CFX-CFX Fl-bonds in water shows that they are more dispersed on the SWCNT. Nevertheless, when 8 CFX are present in the confined space inside the SWCNT some aggregation is observed. Free energy calculations show that adsorption is a spontaneous process and is more favorable on the inner wall of the nanotube for both forms of CFX. This implies at neutral pH zCFX would be preferentially absorbed on the inner part of the SWCNT. The E(i)nt for the minimum energy configuration of the CFX-SWCNT adduct is significantly more negative than the obtained Delta G(a)ds. This suggests a strong contribution of desolvation upon adsorption, as evidenced by the net decrease of the number of water molecules hydrating the adsorbed CFX with respect to the free state in bulk water. Desolvation results to be more pronounced inside the SWCNT. These results for the adsorption of zCFX are in good agreement with experimental data which suggest a thermodynamically favored process with a positive entropy term. (C) 2019 Elsevier B.V. All rights reserved.