A conceptual change in crystallisation mechanisms of oxide materials from solutions in closed systems.

Atomic and molecular level interactions in solutions dictate the structural and functional attributes of crystals. These features clearly dictate the properties of materials and their applicability in technologies. However, the microscopic phenomena of particle formation-nucleation and growth-in real systems are still not fully understood. Specifically, crystallisation occurring in closed systems are largely unproven. Combining coherent experimental data, we here demonstrate a fundamental nucleation-growth mechanism that occurs in a model zinc oxide system when particles are formed under continuous, rapid heating under closed reaction conditions. Defying all previous reports, we show that the nucleation commences only when the heating is terminated. A prenucleation clusters pathway is observed for nucleation, followed by crystallite assembly-growth. We show that the nucleation-growth processes result from temporal and dynamic activity of constituent ions and gaseous molecules in solution and by the irreversible expulsion of the dissolved gaseous molecules. We suggest that this nucleation process is generic to most closed systems that go through precipitation, and, therefore, important for the crystallisation of a variety of metal oxides, composites and minerals. We anticipate that the work may be a platform for future experimental and theoretical investigation promoting deeper understanding of the nucleation-growth phenomena of a variety of practical systems.

Antibacterial properties of F-doped ZnO visible light photocatalyst.

Nanocrystalline ZnO photocatalysts were prepared by a sol-gel method and modified with fluorine to improve their photocatalytic anti-bacterial activity in visible light. Pathogenic bacteria such as Escherichia coli (Gram-negative) and Staphylococcus aureus (Gram-positive) were employed to evaluate the antimicrobial properties of synthesized materials. The interaction with biological systems was assessed by analysis of the antibacterial properties of bacteria suspended in 2% (w/w) powder solutions. The F-doping was found to be effective against S. aureus (99.99% antibacterial activity) and E. coli (99.87% antibacterial activity) when irradiated with visible light. Production of reactive oxygen species is one of the major factors that negatively impact bacterial growth. In addition, the nanosize of the ZnO particles can also be toxic to microorganisms. The small size and high surface-to-volume ratio of the ZnO nanoparticles are believed to play a role in enhancing antimicrobial activity.

Photocatalytic Properties of g-C3N4-TiO2 Heterojunctions under UV and Visible Light Conditions.

Graphitic carbon nitride (g-C3N4) and titanium dioxide (TiO2) were chosen as a model system to investigate photocatalytic abilities of heterojunction system under UV and visible light conditions. The use of g-C3N4 has been shown to be effective in the reduction in recombination through the interaction between the two interfaces of TiO2 and g-C3N4. A simple method of preparing g-C3N4 through the pyrolysis of melamine was employed, which was then added to undoped TiO2 material to form the g-C3N4-TiO2 system. These materials were then fully characterized by X-ray diffraction (XRD), Brunauer Emmett Teller (BET), and various spectroscopic techniques including Raman, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), diffuse absorbance, and photoluminescence analysis. Photocatalysis studies were conducted using the model dye, rhodamine 6G utilizing visible and UV light irradiation. Raman spectroscopy confirmed that a composite of the materials was formed as opposed to a mixture of the two. Using XPS analysis, a shift in the nitrogen peak to that indicative of substitutional nitrogen was detected for all doped samples. This is then mirrored in the diffuse absorbance results, which show a clear decrease in band gap values for these samples, showing the effective band gap alteration achieved through this preparation process. When g-C3N4-TiO2 samples were analyzed under visible light irradiation, no significant improvement was observed compared that of pure TiO2. However, under UV light irradiation conditions, the photocatalytic ability of the doped samples exhibited an increased reactivity when compared to the undoped TiO2 (0.130 min-1), with 4% g-C3N4-TiO2 (0.187 min-1), showing a 43.9% increase in reactivity. Further doping to 8% g-C3N4-TiO2 lead to a decrease in reactivity against rhodamine 6G. BET analysis determined that the surface area of the 4% and 8% g-C3N4-TiO2 samples were very similar, with values of 29.4 and 28.5 m²/g, respectively, suggesting that the actual surface area is not a contributing factor. This could be due to an overloading of the system with covering of the active sites resulting in a lower reaction rate. XPS analysis showed that surface hydroxyl radicals and oxygen vacancies are not being formed throughout this preparation. Therefore, it can be suggested that the increased photocatalytic reaction rates are due to successful interfacial interactions with the g-C3N4-doped TiO2 systems.

Dual-action hygienic coatings: benefits of hydrophobicity and silver ion release for protection of environmental and clinical surfaces.

Coatings that demonstrate reduced attachment of crystalline precipitates and the medical device colonising Staphylococcus epidermidis were prepared by the immobilisation of silver doped perfluoropolyether-urethane siloxane thin films on glass substrates. The presence of stratified hydrophobic perfluoropolyether groups protects the coating surface from the attachment of crystalline hydrophilic species such as chlorides and phosphates, whilst silver ion release inhibited attachment of S. epidermidis and subsequent biofilm formation in vitro. The release of silver ions protects the perfluoro groups from the hydrophobic interactions of S. epidermidis cells, which can reduce the hydrophobicity of the protective coating. These coatings also exhibited significant antibacterial activity against planktonic Acinetobacter baumannii and S. epidermidis bacterial strains. Detailed elemental and chemical surface analysis obtained using X-ray photoelectron spectroscopy (XPS) provided useful information on the effect of bacterial incubation on key indicator hydrophobic and hydrophilic functional groups. XPS analysis indicated preferential adsorption of S. epidermidis cells at the hydrophobic sites along the polymeric chain. These dual-action hygienic coatings can be employed to protect against contamination environmental surfaces and bacterial colonisation on implanted medical devices.

Silver doped perfluoropolyether-urethane coatings: antibacterial activity and surface analysis.

The colonisation of clinical and industrial surfaces with pathogenic microorganisms has prompted increased research into the development of effective antibacterial and antifouling coatings. There is evidence that implanted biomedical surfaces coated with metallic silver can be inactivated by physiological fluids, thus reducing the bioactivity of the coating. In this work, we report the biofilm inhibition of Staphylococcus epidermidis using a room temperature processed silver doped perfluoropolyether-urethane coating. The release of silver ions from these fluoropolymers over a six-day period inhibited bacterial encrustation - as observed by scanning electron microscopy (SEM). X-ray photoelectron spectroscopy (XPS) analysis indicated differences in carbon, fluorine and sodium surface composition between silver doped and undoped fluoropolymers after exposure to nutrient rich media. These silver doped perfluoropolyether coatings also exhibited antibacterial activity against methicillin-resistant Staphylococcus aureus (MRSA), Escherichia coli, Pseudomonas aeruginosa and Acinetobacter baumannii; suggesting potential use in preventing transmission of pathogenic and opportunistic microbes on environmental surfaces in healthcare facilities. The broad-spectrum antibacterial activity of these silver release coatings may be exploited on biomaterials surfaces to combat the development of resistant Gram-negative Enterobacteriaceae that can occur during prophylactic treatment for urinary tract infections.

Prevention of Staphylococcus epidermidis biofilm formation using a low-temperature processed silver-doped phenyltriethoxysilane sol-gel coating.

Sol-gel coatings which elute bioactive silver ions are presented as a potential solution to the problem of biofilm formation on indwelling surfaces. There is evidence that high-temperature processing of such materials can lead to diffusion of silver away from the coating surface, reducing the amount of available silver. In this study, we report the biofilm inhibition of a Staphylococcus epidermidis biofilm using a low-temperature processed silver-doped phenyltriethoxysilane sol-gel coating. The incorporation of a silver salt into a sol-gel matrix resulted in an initial high release of silver in de-ionised water and physiological buffered saline (PBS), followed by a lower sustained release for at least 6 days-as determined by graphite furnace-atomic absorption spectroscopy (GF-AAS). The release of silver ions from the sol-gel coating reduced the adhesion and prevented formation of a S. epidermidis biofilm over a 10-day period. The presence of surface silver before and after 24 h immersion in PBS was confirmed by X-ray photoelectron spectroscopy (XPS). These silver-doped coatings also exhibited significant antibacterial activity against planktonic S. epidermidis. A simple test to visualise the antibacterial effect of silver release coatings on neighbouring bacterial cultures is also reported.