A dual-function gold–zinc ferrite nanocomposite sensor for colorimetric detection of phosphates in environmental water

Abstract

Excess phosphorus in water systems causes eutrophication and poses significant ecological risks. In contrast, the bioavailable forms of phosphorus are valuable for crop production but are often limited. Therefore, the detection and identification of phosphorus forms in environmental water samples are important from both remediation and resource recovery perspectives. The design of effective nanocomposites offers a solution by enhancing the selectivity and differentiation of phosphates. Plasmonic gold nanoparticles (AuNPs), with inherently high extinction coefficients and unique optical properties, offer novel avenues for detecting phosphates. Magnetic zinc ferrite (ZnFe₂O₄) nanomaterials have higher binding affinities and are selective for phosphates through electrostatic forces and/or inner-sphere complex formation. Herein, Au-embedded ZnFe₂O₄ nanocomposites (AuZnFe₂O₄) were developed with an ability to detect and distinguish between inorganic and organic phosphates in water. UV-visible absorbance spectroscopy was applied to monitor the surface plasmon resonance (SPR) peak shifts and corresponding color changes of the plasmonic AuZnFe₂O₄ nanocomposite upon phosphate addition. Enhanced selectivity and varying adsorption stability were reflected by distinct SPR peak shifts and visible color changes, enabling differentiation between inorganic and organic phosphates. Density functional theory (DFT) simulations further provided mechanistic insights into the distinct chemical interactions and binding energies on the nanocomposite surface responsible for this differentiation. All organic phosphates revealed limits of detection (LODs) in the µM range, with ethion exhibiting the highest sensitivity (0.16 µM). Successful integration of this nanocomposite sensor into complex environmental water samples highlights its potential for rapid, selective, and colorimetry-based phosphate detection in environmental systems.