This study presents a holistic and sustainable, non-covalent method for fabricating a heterogeneous optical sensor for metal detection in water. The sensor consists of a biodegradable polylactic acid (PLA) thin film non-covalently functionalized with a porphyrin molecule. Meso-tetra (4-hydroxyphenyl) porphyrin has been chosen for its excellent optical properties and well-established metal chelation capability, which makes it an ideal candidate for optical sensing. Upon exposure to mercury ions solution, the sensor exhibits evident and typical variations in the Soret and in the Q bands, enabling effective detection. The heterogeneous configuration avoids porphyrin solubility issues, allowing direct sensor platform immersion in water samples. This simple approach overcomes the limitations of cost, time consumption, and the use of not sustainable functionalization methods to link active sites to the PLA. As a preliminary investigation, this work demonstrates the potential of the proposed system for water quality monitoring, paving the way for further optimization and application in environmental sensing.