Functionalized Silver Nanocapsules with Improved Antibacterial Activity Using Silica Shells Modified with Quaternary Ammonium Polyethyleneimine as a Bacterial Cell-Targeting Agent

Silver nanoparticles (AgNPs) have remarkable and broad-spectrum antibacterial activities against Gram-positive (G+) and Gram-negative bacteria (G−). However, the negative surface potential of AgNPs limits their antibacterial activities due to the electrostatic repulsion with the negatively charged bacterial cell membrane. To address the limitation, AgNPs were loaded in the mesoporous silica nanoparticles by preparing silver core–mesoporous silica shell nanocapsules (Ag@MSNs), and then, a cationic antibacterial polymer, quaternary ammonium polyethyleneimine (QPEI), was used to modify Ag@MSNs for improving their surface potential and antibacterial activities. The results showed that the obtained Ag@MSN-QPEI exhibited a high positive surface potential (+39.6 mV) and a strong electrostatic attraction with Pseudomonas syringae pv. lachrymans cells in coculture, resulting in an excellent bacterial cell-targeting effect. At the same concentration, Ag@MSN-QPEI exhibited less silver content (reducing the silver content of Ag@MSNs by 19%), higher antibacterial activities, and longer effective duration against Clavibacter michiganensis subsp. michiganensis (G+) and P. syringae pv. lachrymans (G−) than Ag@MSNs and QPEI alone. The excellent bacterial cell-targeting effect and synergistic antibacterial action combined with QPEI accounted for the significantly enhanced antibacterial activities of Ag@MSN-QPEI. Therefore, using a cationic antibacterial polymer to confer the bacterial cell-targeting effect and synergistic antibacterial action would be extended to other antimicrobial materials.