Sustainable PVP-Capped Silver Nanoparticles as a Free-Standing Nanozyme Sensor for Visual and Spectrophotometric Detection of Hg²⁺ in Water Samples: A Green Analytical Method

In the proposed method, microwave-assist heating and AgNO3/trisodium citrate were used to create the polyvinylpyrrolidone-capped silver nanoparticles (PVP-AgNPs) sensor. This sensor had a peroxidase-like activity that could catalytically oxidize <i>O</i>-phenylenediamine (OPD, colourless) into 2,3-diaminophenazine (ox-OPD, greenish-yellow colour) in the presence of H₂O₂, otherwise, in the presence of Hg²⁺, this pass has been effectively inhibited. The degree of colour fading was directly correlated with Hg²⁺ concentration. These results indicated the selectivity of Hg²⁺ ions toward PVP-AgNPs after establishing the PVP-AgNPs/OPD/H₂O₂ system. This selectivity was proved by the negative results obtained from other <i>mon</i>-, <i>di</i>-, and <i>tri</i>valent ions such as Na⁺, K⁺, Ca²⁺, Mg²⁺, Ba²⁺, Co²⁺, Ni²⁺, Cd²⁺, and Cr³⁺, instead of Hg²⁺. Consequently, a reliable, selective, and eco-effective spectrophotometric approach was designed for the detection of Hg²⁺ in various types of water samples. LOD was extended to lower than 0.1 µM, and a fading in the obtained colour was shown by the naked eye at a concentration higher than 1.5 µM of Hg²⁺. The elemental details for preparing the used PVP-AgNPs, such as particle size, morphology, polydispersity index (PdI), and their UV-visible spectrum, were identified by SEM technique, TEM, UV-visible spectrophotometer, and zeta-sizer device. Thus, the peroxidase mimicking the activity of OPD/H₂O₂ was confirmed by a fluorescence technique. The greenness profile of this work was confirmed after applying a reported assessment tool.