An important issue for interpreting in vitro nanomaterial testing is quantifying the dose delivered to target cells. Considerations include the concentration added to the culture, the proportion of the applied dose that interacts with the target cells, and the amount that is eventually absorbed by the target cells. Rapid and efficient techniques are needed to determine delivered doses. Previously, we demonstrated that TiO2 and silver nanoparticles (AgNP) were absorbed by cells in a dose dependent manner between 1 μg/ml and 30 μg/ml and were detected in cells by light scatter using a flow cytometer. Here, we compare four potential indices of the dose of AgNP to cells, including: inductively coupled plasma - mass spectrometry (ICP-MS); flow cytometry side scatter (SSC); and amount of silver deposited to the cell layer as estimated with both an integrated Volumetric Centrifugation Method - In Vitro Sedimentation, Diffusion and Dosimetry Model (VCM-ISDD) and a Distorted Grid (DG) model. A retinal pigment epithelial cell line was exposed to 20 nm or 75 nm citrate-coated AgNP for 24 hr. The relationships between particle sizes and internalized doses varied according to the dose metric. Twenty-four hours after exposure, the cell layer contained a greater mass of silver when treated with 75 nm AgNP than with 20 nm AgNP. When the dose was expressed as the number of particles or as the total surface area of absorbed particles, however, the reverse was true; the dose to the cells was higher after exposure to 20 than 75 nm AgNP. Flow cytometry SSC increased with dose for both sizes of AgNP, and was correlated with Ag in cells measured by ICP-MS. The rate of SSC increase was greater for 75 than for 20 nm AgNP, suggesting it could be used as an indicator of cellular dose after accounting for particle size and composition. Silver was detected by ICP-MS in re-suspended supernates of the isolated cell layer suggested that not all the silver deposited to the cell layer was absorbed by the cells. Both the VCM-ISDD and DG models estimated the proportion of Ag deposited to the cellular layer, which in both cases was greater than the amount of silver in the cells measured by ICP-MS. Modeled deposition more closely compared to the total Ag deposition by ICP-MS, i.e. mass of silver in the cells plus the resuspended, unabsorbed Ag from the cell layer. ICP-MS indicated the mass of silver in cells from AgNP treatment, but not whether the Ag was in the form of particles or dissolved ions. Deposition models predicted the amount of AgNP deposited to the cell layer, but not cellular uptake. Flow cytometry SSC was correlated to cellular uptake of particle-form AgNP and could be calibrated against ICP-MS to indicate mass of cellular uptake. Therefore, a combination of approaches may be required to accurately understand cellular dosimetry of in vitro nanotoxicology experiments. In summary, cellular dosimetry is an important consideration for nanotoxicology experiments, and not necessarily related to the applied dose.
Keywords: Flow Cytometry; ICP-MS; Nanoparticles; Particle Deposition Models; Silver.