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Nanomaterials 2020, 10, 2255 8 of 11 one year of storage may be due to the decreasing charge repulsion with the changes of the ion strength in the medium. There were no observable changes in size and shapes of PVP and PEG-coated AgNPs, but a slight aggregation of PVP and PEG-coated AgNPs were observed after one year of storage. The relative stability of PEG and PVP-coated AgNPs in comparison to Citrate AgNPs may be because PEG and PVP are sterically stabilized AgNPs, which are strongly bound to the core and potentially permeable to solutes and solvents. BPEI AgNPs showed only slightly decreased size and aggregation after one year of storage. BPEI AgNPs are electrosterically stabilized and have abundant amine motifs that can form stable complexes with Ag atoms, which can explain their relative stability. 4.2.2. Hydrodynamic Size and Zeta Potential of AgNPs by DLS Analysis The DLS analysis did not show significant hydrodynamic size changes for any AgNPs in media after one year of storage. The findings of the TEM images analysis discussed above contradict these results. Because DLS provides information on the hydrodynamic radii of particles in solution, an increase in AgNPs’ size from DLS over TEM measurements is expected. It can indicate a successful capping of AgNPs with coatings [32]. Our results are not consistent with the results of others, maybe due to the analytical deficiencies in the DLS methodology we used. However, the results for the hydrodynamic size indicated that AgNPs in the medium were still stable after one year of storage. Citrate AgNPs and PVP AgNPs showed increased zeta potential but PEG AgNPs and BPEI AgNPs showed decreased zeta potential after one year of storage. The increased zeta potential of Citrate and PVP-coated AgNPs demonstrated a more stable dispersion of AgNPs than in the beginning. This is due to the degree of electrostatic repulsion between adjacent, similarly charged particles in a dispersion and can magnify the zeta potential of particles. A high zeta potential confirms stability for small molecules and particles. PEG AgNPs showed significantly decreased zeta potential, but the low molecular weight PEG can neutralize the surface charge and stabilize AgNPs through steric hindrance. 4.2.3. Dissolution of AgNPs The surface coating was found to be important for the dissolution of AgNPs based on the analysis by ICP-MS and ultracentrifugation. The positively charged BPEI AgNPs showed the highest dissolution (371.1 μg/L) but the negatively charged Citrate AgNPs, PEG AgNPs, and PVP AgNPs showed lower dissolution of 3.12 μg/L, 67.36 μg/L, and 130.60 μg/L, respectively (Table 1). NP surfaces can be covered imperfectly because the capping molecules can be very big for BPEI AgNPs. In the exposed areas, the AgNPs can be easily dissolved and oxidized. Another contribution to their faster dissolution is related to the ionic strength of positively charged BPEI. The AgNPs’ dispersions can be higher than in the case of the neutral or negatively stabilized particles. The capping molecules in the case of the negatively charged AgNPs are very small, resulting in a thin surface layer and their faster dissolution. This may be the reason why we also detected high ionic Ag in PVP AgNPs and not in Citrate AgNPs during storage. This is inconsistent with other studies, which showed that the dissolution of AgNPs decreased with increasing pH [33]. Therefore, the dissolution of AgNPs during storage under optimal conditions is relevant to the surface coatings. 4.3. Aging Impact on the Toxicity of AgNPs’ to Hepa1c1c7 Cells We found that one-year storage significantly changes the physicochemical properties of AgNPs. The 24-h toxicity of AgNPs aged for one year to Hepa1c1c7 cells is higher than the toxicity of pristine AgNPs (Figure 4). The observed effect concentration of the initial AgNPs (citrate AgNPs, PVP AgNPs, and BPEI AgNPs) to Hepa1c1c7 cells was 5 μg/L, which was five times higher than the 1 μg/L of the aged AgNPs, including citrate AgNPs, PVP AgNPs, and BPEI AgNPs. The observed effect concentration of the aged PEG AgNPs to Hepa1c1c7 cells was increased 50 times (Figure 4). Aged BPEI AgNPs also showed much higher toxicity than the pristine BPEI AgNPs (Figure 4C). The increased toxicity of BPEIPDF Image | Silver Nanoparticles Undergoing Long-Term Aging
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