Mohammed started his PhD project in Paunov Research Group at the Department of Chemistry of the University of Hull in April 2011. He graduated with PhD in 2016.
PhD supervisors: Prof. Vesselin N. Paunov (Co-PI) and Prof Gillian Greenway (PI)
Research project 1:
Nanotoxicity of polyelectrolyte-functionalized titania nanoparticles: Role of the particle size, surface charge and concentration
(Dr M. Al-Awady, Prof G.M. Greenway and Prof V.N. Paunov)
We studied the role of the various factors controlling the nanotoxicity of titania nanoparticles (TiO2NPs). We studied the nanotoxicity of TiO2NPs of various hydrodynamic diameters and crystallite sizes on C. Reinhardtii (microalgae) and S. cerevisiae (yeast) upon illumination with UV/visible light. The cell viability was assessed for a range of nanoparticle concentrations and incubation times. Bare TiO2NPs affect the microalgae viability at much lower particle concentrations than for yeast. We also found an increased nanotoxicity upon illumination with visible light which indicates that they may also interfere with the microalgae photosynthetic system leading to decreased chlorophyll content upon exposure to TiO2NPs. The results indicate that the larger the hydrodynamic diameter of the TiO2NPs the lower is their nanotoxicity, with anatase TiO2NPs generally being more cytotoxic than rutile TiO2NPs .
Fig. 1. Schematics of the effect of polyelectrolyte-coated titania nanoparticles on microalgae cells.
We also prepared a range of polyelectrolyte-coated TiO2NPs using the layer by-layer method and studied their nanotoxicity on yeast and microalgae. The toxicity of the coated TiO2NPs alternates with their surface charge. TiO2NPs coated with cationic polyelectrolyte as an outer layer exhibit much higher nanotoxicity than the ones with an outer layer of anionic polyelectrolyte. TEM images of sectioned microalgae and yeast cells exposed to different polyelectrolyte-coated TiO2NPs confirmed the formation of a significant build-up of nanoparticles on the cell surface for bare- and cationic polyelectrolyte-coated TiO2NPs. The effect is coming from the increased adhesion of cationic nanoparticles to the cell walls. Significantly, coating the TiO2NPs with an anionic polyelectrolyte as an outer layer led to a reduced adhesion and much lower nanotoxicity due to electrostatic repulsion with the cell walls (Fig. 2). This suggest a new way of making the TiO2NPs potentially safer for use in different formulations by pre-coating them with anionic polyelectrolytes.
Fig. 2. The effect of anatase TiO2NPs coated with different number of layers of anionic (PSS) and cationic (PAH) polyelectrolytes on the viability of C. Reinhardtii microalgae at different particle concentrations (0, 100 and 500 µg mL-1). The cells were incubated with the TiO2NPs for at 0 h, 3 h and 6 h exposure times in dark conditions (A-D) and in UV light (E-H), respectively. The cytotoxic effect on the microalgae cells was assessed for: (A,E) bare TiO2NPs; (B, F) TiO2NPs/PSS; (C,G) TiO2NPs/PSS/PAH and (D, H) TiO2NPs/PSS/PAH/PSS at different nanoparticle concentrations and exposure times .
1. Al-Awady, M.J., Greenway, G.M., Paunov, V.N., Nanotoxicity of polyelectrolyte-functionalized titania nanoparticles towards microalgae and yeast: role of the particle concentration, size and surface charge, RSC Advances, 5 (2015), 37044-37059.
Research project 2:
Enhanced antimicrobial effect of berberine in nanogel carriers with cationic surface functionality
(Dr M. Al-Awady, A. Fauchet, Prof G.M. Greenway and Prof V.N. Paunov)
We report a strong enhancement in the antimicrobial action of berberine encapsulated into polyacrylic acid-based nanogels followed by further surface functionalisation with a cationic polyelectrolyte (PDAC). Due to the highly developed surface area, the nanogel carrier amplifies the contact of berberine with microbial cells and increases its antimicrobial efficiency. We show that such cationic nanogel carriers of berberine can adhere directly to the cell membranes and maintain a very high concentration of berberine directly on the cell surface. We demonstrated that the antimicrobial action of the PDAC-coated nanogel loaded with berberine on E. coli and C. reinhardtii is much higher than that of the equivalent solution of free berberine due to the electrostatic adhesion between the positively charged nanogel particles and the cell membranes .
Fig. 1. TOP: Schematic diagram of the encapsulation of berberine (from berberine hydrochloride) into polyacrylic nanogels by using swelling/deswelling cycles of the nanogel in basic (pH 8) and acidic (pH 5) medium, respectively. The berberine-loaded nanogel was surface-functionalised with a cationic polyelectrolyte (PDAC). BOTTOM: SEM image of E. coli cells after incubation with a solution of 0.0088 wt% PDAC-coated 0.006 wt% BLC suspension (Carbopol nanogel loaded with berberine).
Our results also showed a marked increase in their antimicrobial action at shorter incubation times compared to the non-coated nanogel particles loaded with berberine under the same conditions. We attribute this boost in the antimicrobial effect of these cationic nanocarriers to their accumulation on the cell membranes which sustains a high concentration of released berberine causing cell death within much shorter incubation times (Fig. 2). This study can provide a blueprint for boosting the action of other cationic antimicrobial agents by encapsulating them into nanogel carriers functionalised with a cationic surface layer. This nanotechnology-based approach could lead to the development of more effective wound dressings, disinfecting agents, antimicrobial surfaces, and antiseptic and antialgal/antibiofouling formulations. .
Fig. 2. The viability of C. reinhardtii microalgae cells upon incubation with 0.0045 wt% PDAC, 0.0045 wt% PDAC-coated Carbopol, 0.0375 wt% free berberine, 0.00375 wt% BLC, and 0.0045 wt% PDAC-coated 0.00375 wt% BLC suspension for different incubation times (5 min, 1 h and 1.5 h). After incubation, the cells were washed from the respective formulation and their viability was determined using the FDA live/dead cell assay. .
1. Al-Awady, M.J., Fauchet, A., Greenway , G.M., Paunov, V.N. Enhanced antimicrobial effect of berberine in nanogel carriers with cationic surface functionality, J. Mater. Chem. B, 5 (2017) 7885 – 7897.