pH-Responsive Cobalt(II)-Coordinated Assembly Containing Quercetin for Antimicrobial Applications

The development of novel drug delivery systems (DDSs) with promising antibacterial properties is essential for facing the emergency of increasing resistance to antimicrobial agents. The antibacterial features of quercetin and its metal complexes have been broadly investigated. However, several drawbacks affect their activity and effectiveness. In this work, we propose a DDS based on a pH-responsive cobalt(II)-coordinated assembly containing quercetin and polyacrylic acid. This system is suggested to trigger the release of the model drug in a pH-dependent mode by exploiting the localized acidic environment at the bacterial infection sites under anaerobic conditions. The delivery system has been designed by accurately examining the species and the multiple equilibria occurring in solution among the assembly components. The formation of cobalt(II) complexes with quercetin in the absence or presence of the pH-responsive polyacrylic acid was investigated in buffered aqueous solution at pH 7.4 using spectrophotometric (UV-Vis) and calorimetric (ITC) techniques. The determined binding affinities and thermodynamic parameters that resulted are essential for the development of a DDS with improved binding and release capabilities. Furthermore, the affinity of the polymer–cobalt(II) complex toward the model antimicrobial flavonoid was explored at the solid–liquid interface by quartz crystal microbalance (QCM-D) experiments, which provided marked evidence for drug loading and release under pH control.