Formation, properties and stability of poly(hexamethylene biguanide) stabilised silver nanoparticles from the perspective of novel antimicrobial agents

Abstract

Rising antimicrobial resistance has increased pressure to develop novel antimicrobial agents. Poly(hexamethylene biguanide) stabilised silver nanoparticles have been suggested as a potential candidate exhibiting strong biocidal properties. This study researches the formation, properties, and stability over time of the PHMB-AgNPs complex. Silver nanoparticles were synthesised from silver nitrate using a chemical reduction procedure with sodium borohydride in the presence of a stabiliser, PHMB. The obtained complex was characterised by a wide range of complementary methods relying on different analytical principles such as scattering, spectroscopy and microscopy. This allowed for a determination of the physical properties of the complex and the monitoring of changes over time. The preliminary antimicrobial efficacy was assessed using a well diffusion technique and flow cytometer. AgNPs stabilised with PHMB (concentration from 20 μg/mL to 200 μg/mL) were successfully synthesised and characterised. A noticeable shift in the peak position from typically observed 390 nm for Bare-AgNPs to 420 nm evidenced by UV-Vis and the change in the direction of electrophoretic mobility, confirmed that AgNPs were indeed stabilised with PHMB. The size of spherical silver nanoparticle cores was calculated from Small-Angle X-ray Scattering patterns to be around 13 nm for Bare-AgNPs and 4 nm for PHMB-AgNPs. However, the two-population distribution observed with Dynamic Light Scattering exhibited significant number fluctuation indicating flocculation caused by the polymer. Limits of the PHMB concentration available for complexing were observed at around 60 μg/mL, beyond which polymer presence was recorded in the filtrate residue following membrane filtration. The antimicrobial activity indicated that silver nanoparticles did not exhibit any activity against E. Coli or S. Aureus, in contrast to the PHMB stabiliser and the PHMB-AgNPs. Long term stability has shown that a minimum PHMB concentration at 60 μg/mL must be achieved to prevent the flocculation of AgNPs over time.