Colorimetric and Spectrophotometric Sensing of Heavy Metal Ions using 5,10,15,20-Tetrakis(4-sulfonatophenyl) porphyrin (TPPS₄) in Aqueous Media: Selectivity, Paper-based Detection, and Field Assessment in Nigerian River Water

Abstract

This study provides a preliminary evaluation of 5,10,15,20-tetrakis(4-sulfonatophenyl) porphyrin (TPPS₄) as a colourimetric and UV–Visible spectrophotometric sensing material for selected heavy metal ions in aqueous media. The response of TPPS₄ was examined towards Fe³⁺, Hg²⁺, Pb²⁺, Cd²⁺, Zn²⁺, Ni²⁺, and Cr⁶⁺ over a concentration range equivalent to 0.05–100 equivalents relative to TPPS₄. Free TPPS₄ in water displayed a characteristic Soret band at 413 nm and Q-bands in the visible region. Among the tested cations, Fe³⁺ produced the most prominent spectral change, including new absorption features at approximately 489–493 nm and near 700 nm. Hg²⁺ induced a bathochromic shift of the Soret band from 413 nm to 445 nm at higher equivalents, with Q-band changes around 586 and 641 nm. Pb²⁺ produced a dual Soret-band pattern at approximately 413 and 466 nm, whereas Cd²⁺ and Zn²⁺ caused smaller spectral shifts. Ni²⁺ and Cr⁶⁺ produced limited spectral changes under the conditions studied. Anions and selected organic compounds produced no notable colourimetric response, indicating preferential interaction of TPPS₄ with coordinating metal cations. TPPS₄-impregnated filter-paper strips also showed visible colour changes after exposure to selected metal ions, particularly Hg²⁺, Fe³⁺, Pb²⁺, and Cd²⁺. Application to water samples from the Niger–Benue confluence, Ganaja River, and borehole water showed no visible false-positive response on the paper strips, while UV–Visible spectra indicated minor matrix-dependent changes. The findings support the potential use of TPPS₄ as a simple preliminary screening material for selected heavy metal ions in aqueous environments.