The diffraction pattern of DIO-NPs and SIO-NPs (Figure ) shows the peaks that correspond to an fcc maghemite structure (ICSD card no. 01-083-0112) characterized by diffraction planes (220), (311), (400), (422), (511), and (440). No additional diffraction peaks of any impurity were detected, demonstrating the high purity of the synthesized samples. The average particle size was deduced from the full width at half maximum of six lines using Scherrer's relation [28
X-ray diffraction pattern of synthesized DIO-NPs and SIO-NPs.
is the averaged length of coherence domains (which is of perfectly ordered crystalline domains) taken in the direction normal to the lattice plane that corresponds to the diffraction line taken into account, β
is the line broadening due to the small crystallite size, λ
is the wavelength of X-rays (1.5406 Å), θ
is the Bragg angle, B
is the linewidth, and b
is the instrument line broadening. For linewidths measured as the full width at half maximum peak intensity, K
is 0.89 [30
]. The experimental linewidth was deduced assuming Gaussian profiles for experimental and instrumental broadening in accord with [31
]. The line broadening is essentially due to the size effect. The average size, deduced from the full width at half maximum, has a value of 5.8 (±0.5) nm for DIO-NPs and 7.3 (±0.5) nm for SIO-NPs. They are consistent with the mean sizes deduced from high-resolution (HR)-TEM observations (Figure ). From the d
value of the peaks, the estimated lattice parameter is 0.835 nm for both samples (DIO-NPs and SIO-NPs), which is consistent with the literature value [32
Large-area TEM and HR-TEM images and size distributions of DIO-NPs and SIO-NPs. Large-area TEM images of synthesized (A) DIO-NPs and (B) SIO-NPs. HR-TEM images of (C) DIO-NPs and (D) SIO-NPs. Size distribution of (E) DIO-NPs and (F) SIO-NPs.
The DIO-NPs and SIO-NPs synthesized by the coprecipitation method is shown in Figure A,B. The synthesized DIO-NPs and SIO-NPs showed well-shaped spherical nanostructure morphology. Grain size distribution was determined by measuring the mean diameter, D, of approximately 500 particles on the micrographs. These monodisperse nanoparticles have an average grain size of 6 nm (DIO-NPs) and 8 nm (SIO-NPs). The grain size distribution was shown in Figure E,F. In Figure C,D, we show a HR-TEM picture. The clear lattice fringe in the HR-TEM image demonstrates the well crystalline nature of resultant nanoparticles. The interplanar distances (for DIO-NPs) of 2.51, 2.08, and 1.60 Å were attributed to the (311), (400), and (511) planes of maghemite, respectively. In the SIO-NP sample, the interplanar distances of 2.51 and 2.08 Å were consistent with the (311) and (400) planes of maghemite. Therefore, both X-ray diffraction patterns and HR-TEM give feature characteristics of the maghemite structure.
Figure shows magnetization as a function of the applied magnetic field when the magnetization is normalized by the weight of the maghemite nanoparticles. No significant difference was seen among samples both in their superparamagnetic and frozen states. The saturation magnetization (MS
) at 5 K was approximately 22 emu/g for DIO-NPs and 26 emu/g for SIO-NPs. A decreasing size of particles leads to a decrease of MS
due to increased dispersion in the internal exchange [33
]. The surface spin disorder, arising from reduced coordination and broken exchange bonds between surface spins, is expected to give reduced magnetization for samples with smaller diameter.
Hysteresis loops measured at 5 K for DIO-NP and SIO-NP samples.
The antibacterial activity of the samples (DIO-NPs and SIO-NPs) was observed using common bacterial pathogens, E. coli, P. aeruginosa (Gram-negative), E. faecalis (Gram-positive), and a species of fungus (C. krusei). The antibacterial effect of DIO-NPs on the Gram-negative bacteria E. coli ATCC 25922 was less visible than that on the Gram-positive bacteria E. faecalis ATCC 29212 and C. krusei 963 (a species of fungus) for all concentrations. DIO-NPs showed highly significant toxicity to all three bacterial species (Figure ). The Gram-negative bacteria P. aeruginosa 1397 is not inhibited in the presence of DIO-NPs.
Antibacterial activity of DIO-NPs using E. coli, E. faecalis, C. krusei, and P. aeruginosa.
Concerning the effect of DIO-NPs on the microbial growth of the tested strains, we could observe that different concentrations of the tested compound either inhibited or stimulated the growth of E. faecalis
ATCC 29212, E. coli
ATCC 25922, and C. krusei
963 strains in the suspension. At concentrations lower than 2.5 mg/mL, the DIO-NPs inhibit the growth of the E. coli
ATCC 25922 strain. The growth of E. faecalis
ATCC 29212 was inhibited at low concentrations of DIO-NPs (from 0.01 to 1.25 mg/mL). The C. krusei
963 strain is inhibited at concentrations between 0.01 and 0.625 mg/mL. The antibacterial activity of DIO-NPS on the Gram-negative bacteria E. coli
ATCC 2912 was higher than that on the Gram-positive bacteria E. faecalis
ATCC29212. This is in accord to the previous result using PEGylated ZnO nanoparticles, which exhibited a much stronger antibacterial effect on Gram-negative bacteria [34
]. On the other hand, in our study, the SIO-NPs proved to stimulate the growth of microbial cells, as demonstrated by the absorbance measurements at 620 nm of the obtained cultures (Figure ).
Antibacterial activity of SIO-NPs using E. coli, E. faecalis, C. krusei, and P. aeruginosa.
The intensity of the stimulatory effect on the microbial growth proved to be proportional with the concentration of SIO-NPs, as proved by the linear trend lines. In exchange, all tested concentrations of DIO-NPs and SIO-NPs slightly stimulated the growth of P. aeruginosa
In general, the stimulatory effect on the microbial growth was higher for sucrose than for dextran. These results may be due to the fact that natural sucrose has a vital role as a transport carbohydrate and sometimes also as a storage carbohydrate. Wu and Birch [35
] showed that some microbes convert sucrose with remarkable yields into the structural isomers isomaltulose and trehalulose, possibly to sequester the sugar in a form that confers an advantage against competing species. On the other hand, the use of sucrose isomers is currently limited by the expense of microbial or enzymatic conversion from more abundant plant-derived sucrose [36
]. The last studies demonstrated that sucrose content of pulp from the roots decreased when the number of bacteria increased under anaerobic storage. A difference in the antimicrobial activity of DIO-NPs and SIO-NPs may come from active oxygen species generated by the powder in solution. Indeed, every bacterium responds unevenly to oxidative stress due to differences in the permeability of cell membranes [37
]. Some microbial strains succumb to damage to cell walls by O2−
and others, while others show greater sensitivity to H2
, as is the case for E. coli
]. Shi et al. [39
] showed that the increase of antibacterial activity is directly related to the increase of active oxygen generated on the surface of particles of oxide nanoparticle, reducing the size of the particle. Moreover, the nanoparticle of oxides in solution enhances the possibility of interaction between the particle and the bacterial cell due to its surface charge and surface energy [38
Finally, it is important to emphasize that the antimicrobial activity of the DIO-NPs was more potent than that of the SIO-NPs. NP bacterial interactions are influenced by interfacial forces, especially electrostatic, that can control the interaction between NPs and the bacterial surface [41
]. The results showed that antibacterial DIO-NPs were performed by attaching dextran to iron oxide using a coprecipitation method. Importantly, the antibacterial activities are due to the surfactant from the surface of the iron oxide nanoparticles. The hydroxyl groups present in dextran offer many sites for derivatization, and these functionalized glycoconjugates represent a largely unexplored class of biocompatible and environmentally safe compounds. Therefore, it could be concluded that dextran-coated iron oxide nanoparticles could be released through aqueous carbohydrate solutions owing to the stable dispersion at molecular level and the slow diffusion from the stabilizing medium [42
]. The mechanism of iron oxide NP antibacterial activity and the properties related to toxicity are still not clearly understood.