The smaller size of SeNPs produced by SeR compared to GRLMR can be rationalized by classical nucleation theory. Here, we hypothesize that SeO3 2-is reduced to either Se 2-or Se 0 by the enzymes. Se 2-is rapidly oxidized to Se 0 by dissolved oxygen . The Se 0 species is essentially insoluble in water, so it has a very high propensity to nucleate and grow into SeNPs . In classical nucleation theory, the critical nucleus size describes the size of a particle at which growth becomes favored over dissolution. Particles larger than the critical nucleus size tend to grow while particles smaller than the critical nucleus size tend to shrink. The critical nucleus size depends on how saturated a solution is with a given solute; for a saturated solution, the critical nucleus size becomes zero.