Fig 1.
Structure of the charged SPIONs.
The amine and carboxyl functional groups are attached to the silica shell on the surface of the SPIONs.
Fig 2.
FTIR spectra of a) bare, b) positively and c) negatively charged SPIONs. In Fig 2a, the peaks at 1000 cm-1, 1100 cm-1 and 1660 cm-1 are indicated by black, blue and red arrows, respectively.
Fig 3.
Particle size distributions of the a) bare, b) positively and c) negatively charged SPIONs. The charged SPIONs, due to their silica shell, were slightly larger than the bare ones.
Table 1.
Zeta potential values of the SPIONs determined before and after incubation with the biofilm.
Fig 4.
TEM images of (a) bare, (b) positively charged and (c) negatively charged SPIONs.
Fig 5.
SEM images of (a,d) bare, (b,e) positively charged and (c,f) negatively charged SPIONs before and after incubation with the biofilms. Following incubation with the bacterial biofilms, the SPIONs appeared to be coated with soft matter.
Fig 6.
Positive TOF-SIMS spectra of (a) bare, (b) positively charged and (c) negatively charged SPIONs; negative TOF-SIMS spectra of (d) bare, (e) positively charged and (f) negatively charged SPIONs after incubation with the biofilm. The intensity of each peak was different for each sample. The intensity peak ratios changed from one sample to the others.
Fig 7.
Representative images of the bactericidal effect of (A) bare, (B) positively charged and (C) negatively charged SPIONs on the S. mutans biofilm for a SPION concentration of 2 μg mL-1. Images for exposures to 1 μg mL-1 and 3 μg mL-1 can be found in S2 and S3 Figs. For each image, green coloration indicates live cells while red coloration indicates dead cells.
Table 2.
The proportion of dead cells to total cells in the S. mutans biofilms following 3h of incubation with the bare, positively charged and negatively charged SPIONs.
Data were obtained after correction for the number of dead cells in the control images (stained bacteria without SPIONs), which was 10% ± 5%. N = 15.
Table 3.
CFU values (/mL) for S. mutans isolated from biofilms that were either exposed to bare, positively charged or negatively charged SPIONs or were incubated in the absence of SPIONs.