Se(IV) was reduced by NaBH4 to form SeH2 gas that reduces Cu(II) to produce Cu2O–Se particles with a strong resonance Rayleigh scattering (RRS) peak at 374 nm. The RRS intensity was linear to the Se(IV) concentration from 0.008 to 1.5 mg L−1, with a detection limit of 3.3 μg L−1.
