Organotypic brain slice viability, cellular populations, and exogene expression profile. A) Lactate dehydrogenase assay used to monitor cell viability. A significant reduction of cell viability in the 1 μm particle treated OTBS was measured using the LDH assay can be observed 5 days following treatment compared to control and 40 nm treated OTBS (n = 5, *p < 0.05). B) PI labelling at 5 days post transfection shows a significant elevation of the number of necrotic cells in the 1 μm treated OTBS compared to 40 nm treated and untreated OTBS (n = 6, *p < 0.05) as seen in representative brain regions. C) dUTP TUNEL assay at 5 days post transfection shows a slight elevation of nick end labelling in the 40 nm treated OTBS while a significant elevation in the number of dUTP positive cells compared to the untreated control can be observed in the 1 μm treated OTBS (n = 6, *p < 0.05). D) anti-NeuN and anti-GFAP immunolabelling at 5 days post transfection was used to quantify neuronal and glial populations respectively. A significant reduction in the number of NeuN positive cells was observed in comparable visual fields of the CA1 region of the hippocampus of corresponding coronal sections for the 1 μm compared to the 40 nm particle treated OTBS (n = 6, *p < 0.05). E) EYFP expression patterns seen in the 40 nm and 1 μm treated OTBS from 5 to 21 days after transfection. A significantly higher percentage of cells remained EYFP positive three weeks after transfection in the 40 nm treated OTBS compared to the 1 μm treated OTBS (n = 6, *p < 0.05) co-labelled with DAPI.