Synthesis and anti-bacterial/fungal activities of amphiphilic polysiloxanes primary ammonium salts

Amphiphilic cationic polymers are currently used as antimicrobial agents to disrupt microbial membranes with long-termed stability and low bio-toxicity. However, there are few studies on the relationship between molecular structure and antimicrobial properties of amphiphilic polymers containing primary amine salts against fungi. In this work, a series of polysiloxanes graft primary ammonium salts (PDMS-g-AH) with tunable molecular weights (3 kDa, 5 kDa, 8 kDa, and 10 kDa) and grafting ratios (50% and 75%) were synthesized via thiol-ene "click reaction", and the effect of their structural parameters on the broad-spectrum inhibition of bacteria (Escherichia coli and Staphylococcus albus) and pathogenic fungi (Candida albicans), especially on two phytopathogenic fungi (Fusarium oxysporum f. sp. cubense race 4 (Foc4) and Rhizoctonia solani (R. solani)) were systematically assessed. The aggregation of polymers in water was characterized by the critical micelle concentration (CMC) values, zeta potentials, and particle sizes and distributions. All PDMS-g-AH had positive zeta-potential values and exhibited high micellar stability in water. At the same time, the aggregation characteristics of polymers are affected by the structure and molecular weight of polymers. Small aggregates and high zeta-potentials interacted easily with the microbial cell membranes, which affected the antimicrobial activity together. These results indicated that the block type of PDMS-g-AH, i.e., PDMS-b-AH, showed better antimicrobial activities than the random type (PDMS-co-AH), and the optimal antimicrobial activities were obtained for moderate polymer chain length (about 5 kDa). This work reveals the relationship between the chemical structure of PDMS-g-AH, their aggregation behavior in water as well as interaction with biological membranes during the sterilization process, which provides the potential applications as fungicidal agents for effectively controlling plant diseases, such as rice sheath blight and banana Fusarium wilt.