Adsorption of Selenite and Selenate by Metal Oxides Studied with Fluorescent DNA Probes for Analytical Application

Abstract

Selenium is a required micronutrient at low concentrations, while it becomes toxic at high concentrations. Inorganic selenium exists mainly as SeO3 2− (selenite or Se(IV)) and SeO4 2− (selenate or Se(VI)). Currently, few biosensors can effectively measure these selenium species in water. In this work, we study the adsorption of selenite/selenate by various metal oxide nanoparticles including CeO2, CoO, Cr2O3, Fe2O3, Fe3O4, In2O3, Mn2O3, NiO, TiO2, and ZnO. Fluorescently-labeled DNA molecules are used as probes and first adsorbed on these oxides, resulting in quenched fluorescence. Upon addition of selenite, the DNA probes are displaced from the surface with fluorescence recovered for six out of the ten oxides, while the response to selenate was much lower in all the cases. The signaling is optimal below 400 mM NaCl at near neutral pH when Fe3O4 nanoparticles were used. Quantitative studies were performed on three oxides with detection limits for selenite being 0.3 μM (CeO2), 2.0 μM (Fe3O4), and 3.0 μM (Fe2O3), respectively. The system, however, is also responsive to a few other anions such as phosphate and arsenate. Therefore, it can be used as a method for analyzing selenium species when the system is known to be free of such competing anions. This study represents the first effort of designing selenite sensors using DNA as probes, and it may stimulate related work for this important analyte.