Photoelectrochemical detection of inorganic mercury in aqueous solutions

[摘要] ENGLISH ABSTRACT: Due to the adverse human health effects associated with mercury (Hg) toxicity, aninnovative method for inorganic mercury (Hg2+) determination in aqueous solutionswas investigated. The method of Hg2+ determination involved the use of a Hg2+selective chemosensor immobilised onto an indium tin oxide (ITO) coated electrodein a photoelectrochemical cell. Upon light activation and in the presence of Hg2+ thefluorescent chemosensor absorbed a photon, and charge separation would occuraccording to the nature of the semiconductive material coated onto the ITO substrate.The charge separators and semiconductors investigated were an n-type carrier TiO2,polyaniline (PANI) and copolymers of polyaniline and 2-bromoaniline (P[A-co-2-BrA]s) with different monomer ratios.The polymer and copolymer coated ITO working electrodes used in the Hg2+ sensitivephotoelectrochemical cell were electrochemically evaluated. The results demonstratedthat the ITO coated with PANI had superior conductive and semiconductiveproperties compared to ITO coated with P[A-co-2-BrA]s. The ITO glass substrateswere coated with TiO2, PANI or P[A-co-2-BrA]s, followed by the fluorescentchemosensor, a rhodamine 6G derivative (RS). The electrodes were subsequentlyphotoelectrochemically evaluated in a photoelectrochemical cell in the presence ofHg2+. The PANI-RS coated electrode behaved as a photocathode in the presence ofHg2+ under illumination. The PANI-RS photoresponse increased with increasing Hg2+concentration in the range 10 to 150 μg L-1, with a limit of detection of 6 μg L-1.ITO coated with TiO2 (ITO/TiO2) followed by a composite of PANI and RS had alinear photoanodic response in the Hg2+ concentration range of 10 to 200 μg L-1 and alimit of detection of 5 μg L-1. ITO and ITO/TiO2 coated with the P[A-co-2-BrA]s andRS had considerably lower photoresponses towards Hg2+ in aqueous solutionscompared to PANI-RS. The photoresponses decreased with increasing 2-bromoanilinecontent.The PANI and P[A-co-2-BrA]s coated ITO and ITO/TiO2 electrodes were then alsocoated with another rhodamine 6G derivative with a thiolactone moiety (RT). ThePANI coated electrode yielded a photocathodic response in the Hg2+ concentration range 0.2 to 5 μg L-1. ITO coated with TiO2 had no photoresponse towards Hg2+ dueto repulsive forces between TiO2 and the RT molecules.The photoresponses of the working photoelectrodes towards Hg2+ were furtherevaluated in a custom photoelectrochemical Hg2+ detector. The photoresponses ofPANI-RS and PANI-RT gave qualitative results for the presence of Hg2+ in aqueoussolutions in concentrations as low as 2 μg L-1.

[发布机构] Stellenbosch University