Both enzymes produce a red solution upon NADPH dependent reduction of SeO3 2-, indicating that both enzymes produce the red allotrope of Se -which is amorphous, as compared to the crystalline black allotrope. The red solutions were drop cast on carboncoated transmission electron microscopy (TEM) grids and examined by TEM (Figure ). This showed that SeR produced approximately spherical SeNPs of diameter 14.5 ± 7.4 nm and 11.2 ± 5.5 nm at 5 mM, and 50 mM SeO3 2-respectively. In contrast GRLMR produced spherical SeNPs of diameter 26.8 ± 7.2 nm and 24.1 ± 7.4 nm at 5 mM and 50 mM SeO3 2-respectively. SeNP aggregation was more commonly observed with SeR but was seen with both SeR and GRLMR synthesized particles at 50 mM SeO3 2-. This trend in size can also be observed by dynamic light scattering measurements (DLS). The particle sizes determined by DLS are significantly larger than those observed by TEM. We attribute this larger particle size to the inclusion of nanoparticle-associated enzymes in the DLS diameter measurement.