Analysis of Ag nanoparticle-cell interactions affecting

A set of analysis methods (imaging flow cytometry, ICP-MS, dark field micro-scopy) was used to study NP-cell interactions. Flow cytometry relies on increased light scattering ability of cells upon their association with NPs. Flow cytometry side scatter (SSC) signal has been shown to describe cell-NP interactions^{77, 78}. Imaging flow cytometry enables high-throughput semi-quantitative analysis of the level of cellular interaction with NPs and cell viability when bright field, dark field and fluorescence images are collected simultaneously. No significant increase in flow cytometry SSC signal or obvious binding in dark field microscopy was seen with most PEG- (near-neutral surface charge) and citrate- (negative surface charge) coated Ag particles. Only cells exposed to the largest tested concentration (25.0 μg/mL) of 70 nm PEG- and citrate-coated particles showed slight increase in SSC signal.

However, notable increase in SSC signal was revealed after 24 h exposure to bPEI-coated (positive surface charge) particles at a concentration as low as 0.1 μg/mL (Figure 6, Table 3). bPEI coated particles were also seen to interact tightly with cells under enhanced dark field microscope which allows visuali-zation at higher magnification (Paper III Figure 3). In both, imaging flow cyto-metry and dark field microscopy, in the case of larger particles the binding of particles to cells was more obvious. Cell counting and ICP-MS analysis was used to obtain quantitative results for cell-bound Ag. The analysis revealed that although larger particles were more visible in flow cytometry and dark field microscopy, the amount of Ag bound to each cell was similar for all bPEI-coated particles. ICP-MS analysis further clarified that compared to bPEI-coated particles, the cellular binding of citrate-coated and PEG-coated particles was 4-fold and 25-4-fold lower, respectively (Table 3).

Cellular localization of Ag particles was analysed using selective chemical etching⁸¹ which allowed to separate cell surface associated and intracellular Ag.

The removal of cell surface associated particles was observable from changes in the flow cytometry SSC signal (Figure 7). It was seen that at IC50 concentration the total amount of Ag NPs bound to cells was higher for bPEI-coated particles, but intracellular concentration of Ag was remarkably similar (Table 3). There-fore, we suggest that the cytotoxicity of different Ag NPs is mostly influenced by their internalization capability and is not directly influenced by cell surface associated Ag NPs. Dissolution of internalized Ag particles has been indicated^{30, 137} and therefore we indirectly evaluated the dissolution of internalized Ag NPs by comparing intracellular concentrations of Ag at IC50 values of Ag NPs and AgNO3. The analysis revealed 4.5–9-fold higher intracellular Ag concentrations in the case of Ag NP-exposed cells suggesting that significant fraction of Ag NPs was present in nanoparticulate form.

Figure 6. Association of 10 nm bPEI- (positive surface charge), PEG- (near neutral surface charge) and citrate- (negative surface charge) coated Ag nanoparticles (NPs) with human T-lymphocyte cells according to imaging flow cytometry histograms. SSC — side scatter indicating cell-associated Ag NPs.

Figure 7. The effect of removal of cell surface associated 10 nm bPEI-coated Ag nano-particles (NPs) with chemical etching according to flow cytometry histograms. SSC – side scatter indicating cell-associated Ag NPs.

It has been claimed in previous studies^{77, 78} that flow cytometry analysis provides mainly information about intracellular NPs. Our study revealed that after chemical etching of surface-bound particles SSC signal disappeared. Thus, we suggest that SSC signal mainly originates from surface-associated particles.

ICP-MS analysis showed that although etching removed significant fraction of Ag (as average 85%, 67% and 62–79% of bPEI-, citrate- and PEG-coated particles, respectively), there was still clear difference between intracellular concentrations of Ag NPs with different coatings at the same exposure concentrations (Paper III Figure 5). Decreased cellular uptake with increased particle size was also seen and the result is consistent with previous studies^{58, 60}. As Ag NPs are suggested to be internalized through clathrin or caveolin-mediated endocytosis²⁹ high-affinity binding of cationic particles to cellular membrane explains the increased endocytosis activity.

On the basis of our study we recommend not to use positively charged Ag particles in antimicrobial applications as more intracellularization occurs and particles tend to be more toxic to mammalian cells than negatively and near-neutrally charged particles. Our results indicate that mammalian cells are less susceptible to Ag toxicity than bacteria (IC50 values ~10 times higher; Table 1) whilst it has been shown that the toxic range of Ag NPs to bacteria and mammalian cells may overlap^{12, 15}. Thus, the toxicity of particles towards non-target organisms still needs to be considered when following a safe-by-design principle in product development.

Table 3. Cell-associated (sum of cell surface bound and intracellular) and intracellular Ag in Ag NP and AgNO3 exposed human T- lymphocyte Jurkat cells at different 24 h exposure conditions (Paper III) Particle code Exposure concentration, ng/mL IC50, µg/mL Cell-associated Aga (fg Ag/cell) at IC50

Intracellular Ag b (fg Ag/cell) at IC5010030010003000 10,000 Cell-associated Aga(fg Ag/cell) bPEI-Ag 10 nm12.8±13.026.6±13.968.0±6.1254±75.3 *n.a.3.4±0.4462±47.647.3±4.8 bPEI-Ag 30 nm8.8±3.226.5±17.646.8±35.9 *217±127 *571±248 *6.6±0.6386±33.462.2±4.5 bPEI-Ag 70 nm7.9±2.123.5±15.5 *82.5±44.5 *321±145 *664±348 *14.0±4.2954±28656.9±11.1 PEG-Ag 10 nm0.5±0.10.5±0.44.9±2.021.0±10.560.0±21.611.2±5.293.9±24.939.8±23.7 PEG-Ag 30 nm0.3±0.10.6±0.34.2±1.922.0±18.663.9±35.417.7±4.7124±30.438.4±10.4 PEG-Ag 70 nm0.2±0.10.4±0.33.7±1.615.0±11.540.5±22.444.2±18.2181±74.637.4±10.3 citrate-Ag 10 nm6.2±3.810.4±7.931.4±13.961.4±21.6328±1768.9±4.1218±13155.6±10.8 citrate-Ag 30 nm2.6±0.36.2±2.319.2±9.345.9±22.198.4±24.831.4±9.0303±86.057.4±33.6 citrate-Ag 70 nm4.1±2.613.0±11.119.0±4.846.4±28.291.9±3.542.6±18.0370±15635.0±13.2 AgNO31.1±0.23.0±1.6n.a.n.a.n.a.0.58±0.117.4±1.44.0±2.4 IC50 – half-inhibitory concentration; bPEI – branched polyethylenimine; PEG – polyethylene glycol; a – measured by ICP-MS after the digestion of exposed cells; b – measured by ICP-MS after the digestion of exposed and etched cells; * – significant increase of the flow cytometry side scatter signal; n.a. – not available, could not be measured due to toxicity

5.3. The application of antimicrobial and photocatalytic