Ghost Silica Nanoparticles of Host-Inherited Antibacterial Action

We fabricated surface-rough mesoporous silica nanoparticles (ghost SiO(2)NPs) by using composite mesoporous copper oxide nanoparticles (host CuONPs) as templates, which allowed us to mimic their surface morphology. The host CuONPs used here as templates, however, had a very high antibacterial effect, with or without functionalization. To evaluate the surface roughness effect on the ghost SiO(2)NPs antibacterial action, we functionalized them with (3-glycidyloxypropyl)trimethoxysilane (GLYMO) to permit additional covalent coupling of 4-hydroxyphenylboronic acid (4-HPBA). The diol groups on the bacterial membrane can form reversible covalent bonds with boronic acid (BA) groups on the ghost SiO(2)NPs surface and bind to the bacteria, resulting in a very strong amplification of their antibacterial activity, which does not depend on electrostatic adhesion. The BA-functionalized ghost SiO(2)NPs showed a very significant antibacterial effect as compared to smooth SiO(2)NPs of the same surface coating and particle size. We attribute this to the ghost SiO2 NPs mesoporous surface morphology, which mimics to a certain extent those of the original mesoporous CuONPs used as templates for their preparation. We envisage that the ghost SiO2 NPs effectively acquire some of the antibacterial properties from the host CuONPs, with the same functionality, despite being completely free of copper. The antibacterial effect of the functionalized ghost SiO2 NPs/GLYMO/4-HPBA on Rhodococcus rhodochrous (R. rhodochrous) and Escherichia coli (E. coli) is much higher than that of the nonfunctionalized ghost SiO(2)NPs or the ghost SiO(2)NPs/GLYMO. The results indicate that the combination of rough surface morphology and strong adhesion of the particle surface to the bacteria can make even benign material such as silica act as a strong antimicrobial agent. Additionally, our BA-functionalized nanoparticles (ghost SiO(2)NPs/GLYMO/4-HPBA) showed no detectable cytotoxic impact against human keratinocytes at particle concentrations, which are effective against bacteria.