In this article we present the synthesis of oil core silica shell nanocapsules with different shell thicknesses. The surface of the nanocapsules was modified with polyethyleoxide (PEO) and succinic anhydride. Two biomedical tests were then used to study the biocompatibility properties of these nanocapsules with different surface treatments, hemolysis and thromboelastography (TEG). PEO surface modification greatly reduced the damaging interactions of nanocapsules with red blood cells (RBCs) and platelets and attenuated particle size effects. It was found that the blood toxicity of charged particles increased with the acid strength on the surface. Experiments toward the assessment of detoxification of these nanocapsules in model drug overdose concentrations are currently underway.