Plastic electrode decorated with polyhedral anion tetrabutylammonium octamolybdate [N(C₄H₉)₄]₄ Mo₈O₂₆ for nM phosphate electrochemical detection

Inorganic phosphorous (as phosphate (PO₄³⁻), is one of the essential nutrients for all living forms, either terrestrial or marine. In oligotrophic seawaters, this macronutrient is limited (10⁻⁹ M) and its ratio with other elements (nitrogen or carbon) is denoting the health state of the marine environment; a small variation of its concentration can produce eutrophication. The gold standard method used for PO₄³⁻ detection is based on colorimetric detection of phosphomolybdate. The colored complex is obtained by mixing water-soluble molybdenum salts (Mo(VI)) and reducing agents in acid media, along with the sample containing PO₄³⁻. Moreover, the kinetic of complex formation is slow, about 1 h is generally required for color to develop, exposing the assay to the drawbacks of interferences as those from silica. The detection is preferably performed in a controlled environment (i.e. in a laboratory) because several chemicals and steps of preparations are required as well as the optical instrumentation is not intended for in-field use. Electrochemical sensors offer portability and simplicity making them a practical option for on-site detection applications. To gain an analytical alternative in measuring low quantities of PO₄³⁻ (10⁻⁹ M), and overcome some of the drawbacks of the classical approaches, we optimised a new easy way to produce a plastic electrode decorated with an alkyl Mo-polyoxometalate (Mo₈O₂₆⁴⁻), that is soluble in organic solvents. This tetra-butyl-ammonium octamolybdate powder, [N (C4H9)4]4 Mo8O26, purposely synthetized was identified with FTIR, Raman, MS methods, and the electroactivity and reactivity with PO₄³⁻ was confirmed in solution with cyclic voltammetry (CV). When the Mo-decorated electrode was in contact with PO₄³⁻, an electroactive phosphomolybdate aggregate formed at the electrode surface that was electrochemically detectable with square wave voltammetry (SWV). A remarkably low detection limit of 6.1 nM, to PO₄³⁻, as well as good stability and selectivity were obtained also in real samples. In fact, PO₄³⁻ was measured in saline simulated and real seawater samples at nM concentrations in less than 5 min. The present investigation provides a new alternative to the current standard colorimetric methods to detect low phosphate concentrations, showing the potential to be used for monitoring nutrients in oligotrophic seawater.