The antimicrobial efficacy and cytotoxic effects of 2D-MoS2 surfaces

Amin, Mohsin (2017) The antimicrobial efficacy and cytotoxic effects of 2D-MoS2 surfaces. Masters by Research thesis (MSc), Manchester Metropolitan University.

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Abstract

The requirement for antimicrobial surfaces to inhibit or control bacterial growth in the healthcare and environmental industries is increasing and the use of conventional antimicrobial surfaces are showing signs of microbial resistance. This study would investigate a potential antimicrobial and non-cytotoxic surface that would inhibit microbial retention and biofilm formation against two medically relevant pathogens, whilst having minimal effects on human cells. The roughness parameters of two dimensional molybdenum disulphide (2D-MoS2) surfaces produced using particles sizes of 90 nm, 2 μm and 6 μm at 5 %, 10 %, 15 % and 20 % concentrations were characterised using white light profilometery (micro-topography and surface features). Scanning electron microscopy was used to identify surface structure and determine if any differences were present in MoS2 distribution per particle size and concentration. Inductively coupled plasma emission spectroscopy (ICP-AES) was used to measure concentrations of molybdenum and sulphur ions that leached from the 2D-MoS2 surfaces. Microbiological assays were carried out in order to determine the retention and antimicrobial efficacy against planktonic and biofilm Staphylococcus aureus and Pseudomonas aeruginosa on the 2D-MoS2 surfaces. Optical profiling of the 2D-MoS2 surfaces showed that increases in particle sizes, increased the surface roughness. It was observed that an increase in 2D-MoS2 particle size of the surfaces resulted in an increase in heterogeneity of bacterial distribution. Results demonstrated an increasing trend of MoS2 leaching; this however was negligible due to results achieving the lower limits of ICP-AES detection. Zone of inhibition assays demonstrated no antimicrobial activity of the surfaces on the bacteria due to minimal leeching of the molybdenum in the surfaces. Crystal violet biofilm assays were successful in determining the antimicrobial effects of the surfaces against both S. aureus and P. aeruginosa. Growth curves demonstrated that, 2 μm was the most effective particle size of 2D-MoS2 at inhibiting both S. aureus and P. aeruginosa growth. Moreover, S. aureus showed the greatest susceptibility towards the 2D-MoS2 surfaces at all particle sizes. Retention assays demonstrated the surfaces influenced the retention of both bacteria and showed that P. aeruginosa, had a greater affinity towards binding to the 2D-MoS2 surface, resulting in increased retention. Scanning electron microscopy of the bacteria after contact with the 2D-MoS2 surfaces, alongside the analysis of the ICP-AES and zone of inhibition assays suggested that the surfaces mechanism of action was through contact kill, with S. aureus demonstrating different morphologies when in contact with the 2D-MoS2 and without. P. aeruginosa showed a hollowed bacterial cell structure after 0 h, with the bacteria being flattened or pressed. This effect had become more noticeable after 24 h with P. aeruginosa showing shrinkage at higher MoS2 concentrations. Cytotoxicity analysis against human kidney cells evaluated that after 24 h of incubation, 6 μm 2D-MoS2 demonstrated the least cytotoxicity, in contrast to 48 h where 90 nm 2D-MoS2 showed the least cytotoxicity. A potential application for these novel 2D-MoS2 surfaces would be as antimicrobial surfaces within the healthcare and environmental industries. Due to the antimicrobial efficacies of the surfaces alongside the low cytotoxic effects towards renal cells, catheter surface coatings may be a potential application for the 2D-MoS2 surfaces, due to existing complications with catheter acquired infections. This suggested that different particle sized surfaces of 2D-MoS2 should be used depending on the duration of contact with the host. This work suggests that 2 μm 2D-MoS2 was the most effective antimicrobial surface whereas, both 90 nm and 6 μm demonstrated the best biocompatibility, suggesting that a compromise of antimicrobial activity and human cell cytotoxicity would be required to achieve the most effective 2D-MoS2 surface.