Construction And Antibacterial Properties Of POM-Based Nanocomposite System

One of the principle advantages offered by hybrid materials is the possibility to construct modular composite assemblies derived from varied and responsive components,and whose properties are superior to those of the single components.In this context,polyoxometalates（POMs for short）are highly negatively charged clusters of ions consisting of early transition metals and oxygen atoms formed by self-assembly processes and characterized by a broad and versatile range of physicochemical properties.Due to their biological and biochemical effects,including antitumor,-viral and-bacterial properties,POMs and POM-based systems are considered as promising future metallodrugs.Despite the considerable potential of POMs as metallodrugs,applications of purely inorganic POMs in clinical application are prevented by both unstable in aqueous solutions at physiological pH and high toxicity.In the current study,one of the best ways to overcome the bottleneck is switching from inorganic POMs to POM-based organic-inorganic hybrids and nanocomposite structures.Such performances created new systems as drug,which could gain more promising therapeutic applications because the grafting of organic molecules onto the POM’s core or the incorporation of POMs into nanocomposites were clearly more effective in terms of increasing the POM’s stability and decreasing its inherent toxicity.Currently,bacteria pose a threat to global public health,especially due to the emergence and global rapid spread of bacterial resistance to currently available antibiotics underscore the urgent need for new alternative antibacterial agents.Thus this article focuses on antibacterial activity of POM-based hybrid nanocomposites and their therapeutic potential in the battle against bacteria,with emphasis on the synthesis and properties of biologically active POM-based hybrid and nanocomposites.By analysing the antibacterial activity and structure of POM-based hybrid and nanocomposites,putative mode of actions are provided,including potential targets for POM-bacterial cell interactions,and paving the way for POMs as next-generation powerful antibacterial drugs.The subject of the works was summarized as follows:1.Synthesis of POM-based organic-inorganic hybrid compoundsHerein,six new organically functionalized clusters,based on a Strandberg anionic core,were successfully obtained via a one-pot reaction using organic small molecule ligand,2/3/4-aminopyridine and imidazole/pyrazole,transition metal Cu ion,concentrated H₃PO₄ and Na₂MoO₄·2H₂O/（NH₄）₂Mo₂O₇·4H₂O in conventional aqueous conditions.Compounds 1–6 were well characterized in the solid state by X-ray single crystal diffraction,Fourier transform infrared（FT-IR）spectroscopy as well as powder X-ray diffraction（PXRD）analysis.The excellent stability was further confirmed by thermogravimetric analysis（TG）and UV spectroscopy（UV）.A nanometer bulk or rod-like structure with a smooth surface was observed by scanning electron microscopy（SEM）.And the elemental analysis and energy dispersive X-ray（EDX）mapping spectra were also performed,which were consistent with the X-ray single crystal diffraction analysis.2.Construction of POM-based nanocomposites systemIn this study,polydopamine（PDA）-coated magnetic microspheres with surface modification of POMs（designated as Fe₃O₄@PDA@POM）with a well-defined core-shell structure were designed and synthesized by employing layer-by-layer assembly technique in situ surface.In depth physicochemical characterisation of POMs surface modified Fe₃O₄ MPs established confidently that our synthesis strategy resulted in a stable POM external surface.The composition and properties of the as-prepared Fe₃O₄@PDA@POM have also been characterized and determined using powder X-ray diffraction（PXRD）patterns,FT-IR spectra,X-ray photoelectron spectroscopy（XPS）analysis,energy dispersive X-ray（EDX）spectroscopy and TG analysis.Coating thickness and morphology of the nanocomposites were determined ca the 15 nm size by scanning electron microscopy（SEM）and transmission electron microscopy（TEM）methods in combination with mapping scanning and the mode of high resolution（HRTEM）.The room-temperature magnetic hysteresis curves as well as the dispersion and magnetic separation experiment were conducted to confirm the strong magnetic responsiveness and rapid recovery.In addition,Zeta potentials were also determined to confirm the outstanding stability and high dispersion.3.Evaluation of antibacterial properties of POM-based nanocompositesQualitative and quantitative assessment of antibacterial potential of the obtained POM-based hybrid and nanocomposites were performed against Gram negative bacterium Escherichia coli（E.coli）and Gram positive bacterium Staphylococcus aureus（S.aureus）using colony counting methods.Moreover,the antibacterial activity test of POM-functionalised Fe₃O₄ MPs reveal that Fe₃O₄@PDA@POM showed significantly better antimicrobial performance over Fe₃O₄ MPs,wherein POM plays a significant role in improving antibacterial activity.Such a significant increase in the antibacterial activity of POM-functionalised Fe₃O₄ nanocomposite can be attributed to the synergistic effect of the presence of two antibacterial materials in a single system,while Fe₃O₄ also act as POM stabiliser and carrier inside the bacterial cells.The single system experiments revealed that pristine POM molecules had significantly lower antibacterial potential than Fe₃O₄@PDA@POM even at much higher concentrations than those present in Fe₃O₄@PDA@POM,wherein Cu²⁺in POM acts the most active part of its antibacterial activity.4.Study on the antibacterial model of the POM-based nanocompositesThe mode of interaction of the obtained POM-based hybrid and nanocomposites with Escherichia coli cells was further discussed.Overall,the antibacterial mechanism of action of the Fe₃O₄@PDA@POM nanocomposite might not be explained by a strict mechanism with a single substance but rather by multiple interaction effects of several components,and the combined effects of these disturbances ultimately results in bacterial cell death.It could be concluded that cell wall/membrane rupture,nucleic acid and protein leakage,interference in the respiratory chain dehydrogenase activity,reactive oxygen species accumulation and glutathione loss are all the factors responsible for the death of the bacterial cell.