A novel and smart methacrylic acid (MAA) based macroporous material functionalized with thiol groups (−SH) was developed to achieve highly efficient and selective removal of toxic Hg(II) ions from water. With the aim of visualizing the saturation of the active sites of the material in real-time, a porphyrin-based co-monomer was synthesized and included (only 0.25 % w/w) as a part of the co-polymeric structure. The porphyrin ring can capture Hg(II) ions, causing a color change from red to green. However, due to the higher affinity of Hg(II) towards thiol groups, the porphyrin will be able to interact with free Hg(II) only when the adjacent thiols are no longer available and, therefore, close to the complete saturation of −SH sites. The color shift alerts that the material is approaching saturation, hence, a regeneration step is necessary for subsequent adsorption cycles. The typical interconnected macroporous (3–23 µm …