title: Synthesis and Characterisation of Nanocomposite Coatings for Antibacterial/Antifouling Applications
Creators: name: Michailidis, M
Publisher: name: University of Liverpool Repository
Year: 2018

description: Biofouling can occur everywhere water is present and is a major problem for a wide variety of materials and devices such as biomedical devices and implants, as well as industrial and marine equipment. After the ban of the efficient but environmentally harmful antifouling paints based on tributyltin, the creation of new, effective and low-toxic antifouling paints is a major challenge. The aim of this thesis is to develop novel modified nanoparticles (NPs) as functional fillers for low toxic and environmentally friendly antibacterial/antifouling coatings for mobile and stationary applications in maritime, hospitals, industries, etc. Application of mesoporous silica nanoparticles (MSNs) as antifouling/antibacterial carriers attracted a few attention so far, specifically with a dual synergetic effect. In the present work, MSNs modified with quaternary ammonium salts (QASs) and loaded with the active agent DCOIT were synthesized as functional fillers for antifouling/antibacterial coatings. From the family of the MSNs, MCM-48 was selected as a carrier because of its cubic pore structure, high surface area, and high specific pore volume. Chapter 3 details the synthesis and characterisation of spherical MCM-48 MSNs with two different average sizes; 400 nm and 120 nm. As revealed from the X-ray diffraction and nitrogen isotherms, both types of MSNs showed highly ordered cubic mesostructure, high BET surface area, high pore volume and narrow pore size distribution. From the morphological studies using scanning electron microscopy and transmission electron microscopy, the MCM-48 with average size of 400 nm exhibited spherical morphology and some aggregates or fused together particles were present, while the MCM-48 with average size of 120 nm showed spherical morphology and all the NPs were monodispersed. Chapter 4 documents the surface modification of MCM-48 NPs with two different types of QASs; dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride and dimethyltetradecyl[3-(triethoxysilyl)propyl]ammonium chloride. The QAS modified MCM-48 revealed strong covalent bonds between the QAS and the surface of the NPs. The surface functionalization was confirmed by diffuse reflectance infrared Fourier transform spectroscopy, thermogravimetric analysis, elemental analysis, and ζ-potential measurements. Additional loading of the QAS modified MCM-48 with a commercially available biocide (Parmetol S15) resulted in a synergetic dual antibacterial/antifouling effect. Either loaded or unloaded QAS-modified MSNs exhibited high antibacterial performance confirming their dual activity. In the case of the modified NPs with average size of 400 nm, the loaded QAS-modified MCM-48 (dual functionalised) killed all exposed bacteria after 3 h of incubation and presented 100% reduction at the antibacterial tests against Gram-negative and Gram-positive bacteria. Furthermore, the QAS-modified MCM-48 (single functionalised) presented 77−89% reduction against the exposed Gram-negative bacteria and 78−94% reduction against the exposed Gram-positive bacteria. Furthermore, spherical MCM-48 NPs with smaller average size of 120 nm were surface modified with the two types of QASs and tested against Gram-negative and Gram-positive bacteria in order to investigate if there is size-dependency at the antibacterial properties of silica-QAS core-shell NPs. The smaller sized modified Marios Michailidis – April 2018 8 | P a g e NPs showed increased antibacterial properties (83-94% reduction against Gram-negative bacteria and 87-97% reduction against Gram-positive bacteria) compared to their counterparts with average size of 400 nm implying that smaller size of modified NPs could provide better antibacterial properties. In chapter 5, the modified NPs were homogeneously added in coating formulations. The nanocontainer-doped paints showed good distribution of the functionalised nanocontainers in the coating matrix, increased hydrophobicity and similar roughness values with the pristine coating formulation. In the antibacterial tests, the formulated paints revealed excellent antibacterial properties where all the bacteria were dead after 16 hours of exposure. Additionally, the nanocontainer-doped paints presented high anti-macrofouling properties against mussels. The paints containing the dual functionalised NPs presented 100% inhibition of mussels’ attachment after 72 hours exposure of mussels in culture plates coated with the nanocontainer-doped paints. Furthermore, all of the nanocontainer-treated paints illustrated low toxicity against the Red Sea mussels Brachidontes pharaonis and the brine shrimps Artemia salina. Finally, PVC panels were coated with the formulated paints (containing either 2 or 5 wt % modified nanoparticles) and immersed in Red Sea (Eilat, Israel) for field test. After six months of exposure, the paints containing the NPs with dual effect showed significantly lower biofouling coverage (below 10%) compared to the biofouling coverage of pristine paint (50%) indicating their high antifouling properties in real sea water conditions. The results of this work show that (i) the dual functionalised MCM-48 can be used as functional fillers for coating formulations providing excellent antibacterial/antifouling properties to the coatings, (ii) the dual functionalised MCM-48 can be used as environmental-friendly low-toxic alternatives for tin-based paints and (iii) the covalently attached QASs on the surface of the nanoparticles remain active even after complete release of the biocide, which considerably increases their functional lifetime in the coating formulations.