Khan, Ahmad; (2024) Towards novel antimicrobial formulations – an analysis of peracetic acid forming systems through nuclear magnetic resonance spectroscopy. Doctoral thesis (Eng.D), UCL (University College London).

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Abstract

Planktonic microbial species have the potential to colonise surfaces and form biofilms. The formation of biofilms is indicated as a major contributing factor in promoting disease and antimicrobial resistance. This thesis examines a strategy commonly used in denture cleansing and surface disinfecting formulations that aims to combat the microbial colonisation of surfaces through generating reactive oxygen species, as opposed to relying on traditional antimicrobial molecules. Haleon’s Polident formulation is an antimicrobial bleach-activated denture cleansing formulation that is indicated as having antimicrobial efficacy against planktonic and biofilm microbial species in 3 minutes and utilises peracetic acid as the main antimicrobial agent. The bleach activation system used in the Polident formulation – TAED and hydrogen peroxide – was probed by way of proton nuclear magnetic resonance spectroscopy (1H-NMR spectroscopy) to better understand the chemistry that allows for peracetic acid to be generated. Following this, four constituents of the Polident formulation – TAED, sodium percarbonate, potassium monopersulphate, and citric acid – were used as inspiration to create novel formulations. Through varying the molar equivalences of each of the reagents from 0 to 2 respectively, 27 novel formulations were created and studied through 1H-NMR spectroscopy. Such a route allowed for the determination of the benefits and negatives of each reagent’s inclusion in the formulation. It was found that the most determining factor for peracetic acid generation was the molar equivalence of the bleach activator – TAED. Further, it was found that both citric acid and potassium monopersulphate promote the formation of peracetic acid, however citric acid does so with a greater magnitude than potassium monopersulphate. It was also determined that doubling the molar equivalence of citric acid in the formulations led to a negligible gain in peracetic acid generation, and including potassium monopersulphate further negated the benefits of citric acid. Additional studies of selected formulations through 1H-NMR spectroscopy over eight hours revealed that increased molar equivalences of potassium monopersulphate led to the disruption of the TAED perhydrolysis mechanism and resulted in lower percentages of peracetic acid generation by the formulations. Following on from the 1H-NMR spectroscopy studies, the efficacy of 6 promising formulations was tested against 3 three prominent microbial species found in the oral cavity – S. mutans, E. faecalis, and C. albicans. All 6 formulations exhibited inhibitory and biocidal properties. Lastly, three essential oils – eugenol, eucalyptol, and t-cinnamaldehyde – indicated in literature as having antimicrobial efficacy against microorganisms commonly found in the oral cavity were examined as potential additives to the formulations, with the inhibition of microbial growth being observed against C. albicans and S. mutans. E. faecalis remained resistant to the individual essential oils, however microbial growth was inhibited through combining the essential oils. As such, the observations indicated that including essential oils as an additive to bleach activated formulations is feasible and shows promise, despite the high concentrations reported in this thesis.

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