Spontaneous Deposition of Porphyrin-Based Layers on Polylysinated Substrates: Role of the Central Metal on Layer Structural and Sensing Properties

Noncovalent porphyrin deposition onto polylysinated surfaces proceeds by obeying the self-assembly rules experienced in aqueous solution. In particular, the porphyrins ability to stack in a face-to-face arrangement, leading to H-aggregate formation, strongly depends on the central metal coordination, even during deposition on surfaces. This aspect is important to control the porphyrin layer density and the related optical properties and reactivity to external stimuli, thus pointing to intriguing sensing applications. In this perspective, we demonstrate the ability of porphyrin-based layers, assembled on polylysinated Quartz Micro Balance (QMB) resonators, to act as humidity sensors, whose sorption behavior depends on the presence of the central metal into the porphyrin cavity. The obtained sensors are fast responsive, reversible and stable over a tested period of 6 months. The straightforwardness, robustness and performance repeatability of the presented functionalization procedure, associated with low cost and green chemistry figure of merits, pave the way to the development of porphyrin-based sensitive layers for humidity sensing applications.