Microwave-enhanced antibacterial activity of polydopamine-silver hybrid nanoparticles

Abstract

The ever-increasing risks posed by antibiotic-resistant bacteria have stimulated considerable interest in the development of novel antimicrobial strategies, including the use of nanomaterials that can be activated on demand and result in irreversible damage to pathogens. Microwave electric field-assisted bactericidal effects on representative Gram-negative and Gram-positive bacterial strains were achieved in the presence of hybrid polydopamine-silver nanoparticles (PDA-Ag NPs) under low-power microwave irradiation using a resonant cavity (1.3 W, 2.45 GHz). A 3-log reduction in the viability of bacterial populations was observed within 30 minutes which was attributed to the attachment of PDA-Ag NPs and associated membrane disruption in conjunction with the production of intra-bacterial reactive oxygen species (ROS). A synergistic effect between PDA and Ag has been demonstrated whereby PDA acts both as an Ag NP carrier and a microwave enhancer. These properties together with the remarkable adhesivity of PDA are opening a route to design of antibacterial adhesives and surface coatings for prevention of biofilm formation.

Fig. 1. Synthesis and characterisation of PDA–Ag. (a) An overview of the synthesis of PDA–Ag, (b) UV-Vis spectroscopy of PDA, PDA–Ag and Ag NP, (c) hydrodynamic diameter information obtained by DLS of PDA, PDA–Ag and Ag NP, (d) SEM and (e) TEM images of PDA–Ag.

Fig. 2. Zone of inhibition studies for PDA–Ag. (a) Overview of the experimental procedure for the zone of inhibition studies. (b) The zone of inhibition sizes for 0.2 mg mL⁻¹ PDA–Ag, 15 μg mL⁻¹ Ag NP and 0.2 mg mL⁻¹ PDA on the same plate. This indicates 1 of 5 plates used to calculate the average inhibition zone. Discs are 6.5 mm in diameter.

Fig. 3. Bacterial cell viability and corresponding ROS production. Schematic diagram summarising (a) the experimental design, (b) cell viability of B. subtilis, (c) cell viability of E. coli, (d) bacterial ROS production for B. subtilis, (e) bacterial ROS production for E. coli before and after 30 min exposure to microwaves. The experiments were conducted with 3 biological replicates.

Fig. 4. TEM images of PDA–Ag TEM images of E. coli and B. subtilis before and after exposure to microwaves in the presence of 0.1 mg mL⁻¹ PDA–Ag, 0.2 mg mL⁻¹ PDA–Ag and corresponding amount of Ag NP (7.5 μg mL⁻¹).

Fig. 5. The detection of Ag⁺ released from PDA–Ag. TMB assay used for detection of released Ag⁺, showing Ag⁺ release over 60 minutes for PDA–Ag and Ag NP, with or without exposure to microwaves.

Fig. 6. The production of reactive oxygen species from PDA–Ag. Relative absorbance of ROS indicators showing the amount of (a) singlet oxygen, (b) hydroxyl, (c) hydrogen peroxide and (d) superoxide produced in presence of 0.1 mg mL⁻¹ PDA–Ag NPs in absence (black) and presence (grey) of microwaves. 1.3 W, 2.45 GHz microwaves were used over 1 h.