Comparison of 20 nm silver nanoparticles synthesized with and without a gold core: Structure, dissolution in cell culture media, and biological impact on macrophages

Abstract

Widespread use of silver nanoparticles raises questions of environmental and biological impact. Many synthesis approaches are used to produce pure silver and silver-shell gold-core particles optimized for specific applications. Since both nanoparticles and silver dissolved from the particles may impact the biological response, it is important to understand the physicochemical characteristics along with the biological impact of nanoparticles produced by different processes. The authors have examined the structure, dissolution, and impact of particle exposure to macrophage cells of two 20 nm silver particles synthesized in different ways, which have different internal structures. The structures were examined by electron microscopy and dissolution measured in Rosewell Park Memorial Institute media with 10% fetal bovine serum. Cytotoxicity and oxidative stress were used to measure biological impact on RAW 264.7 macrophage cells. The particles were polycrystalline, but 20 nm particles grown on gold seed particles had smaller crystallite size with many high-energy grain boundaries and defects, and an apparent higher solubility than 20 nm pure silver particles. Greater oxidative stress and cytotoxicity were observed for 20 nm particles containing the Au core than for 20 nm pure silver particles. A simple dissolution model described the time variation of particle size and dissolved silver for particle loadings larger than 9 μg/ml for the 24-h period characteristic of many in-vitro studies.

Fig. 1.

Representative HAADF STEM (columns 1 and 2) and HR-TEM (column 3) images of the (a) Ag^Au₂₀NPs, (b) Ag^pn₂₀NPs, and (c) Ag^pi₂₀NPs stock particles.

Fig. 2.

Dissolution profile showing amount of Ag in the supernatant and percent of particles dissolved for Ag^Au₂₀NPs, Ag^pn₂₀NPs, Ag^Au₁₁₀NPs, and Ag^pi₂₀NPs in FBS 10% volume + RPMI culture media.

Fig. 3.

HAADF STEM images of Ag^Au₂₀NPs and Ag^pn₂₀NPs before and after dissolution imaged as dispersed in cell culture media after 24 h. (a) Ag^Au₂₀NPs before and [(b) and (c)] after dissolution. Similarly for the Ag^pn₂₀NPs before (d) and [(e) and (f)] after dissolution.

Fig. 4.

Dissolution time plots for Ag^Au₂₀NPs [(a) and (c)], Ag^pi₂₀NPs [(b) and (d)] in FBS 10% volume + RPMI measured up to 24 h duration. [(a) and (b)] Plots showing ion concentration and [(c) and (d)] displaying data in terms of the percentage of the particles dissolved.

Fig. 5.

Experimental data and dissolution model fit to the dissolution data as a function of time for (a) Ag^Au₂₀NPs and (b) Ag^pi₂₀NPs. The vertical axis is plotted as [(M(0) − M(t))/M(0)] × 100 following Eq. (3). For each particle, the solid lines represent a single set of k₊ and K_sp values for that particle. Above solution concentrations of 9 μg/ml, the dissolution data can be approximately fit by single particle specific values. However, these values over predict dissolution that was observed for the lower concentrations of particles in solution, and the dashed lines are from fits to the 1 μg/ml data sets for each particle, with the values shown in Table II.

Fig. 6.

(a) Exposure to Ag^Au₂₀NPs (core) and Ag^PN₂₀NPs (pure) caused differential expression of HMOX, a marker for oxidative stress in RAW 264.7 cells. The insert shows HMOX expression blot with β actin as loading control. The graph is the average HMOX expression from two independent experiments, the error bars are range of the two experimental values. (b) Exposure to Ag^Au₂₀NPs (circle) and Ag^PN₂₀NPs (square) also showed differential cytotoxicity at high nanoparticle concentrations. In both tests, the Au core particles have a greater impact at relatively high particle exposures.

Scheme 1.

AgNP dispersion protocol followed for PNNL toxicology for in-vitro toxicity studies. Nanoparticles from the stock solution were directly diluted in concentrated FBS to minimize agglomeration of particles followed by the addition of RPMI 1640.

Scheme 2.

Steps and conditions followed for nanoparticles dissolution experiments. Additional details provided in the text.