Smart Hybrid L-DOPA/H2TCPPspm4 Film For Dye-Pollutants Water Remediation

Water pollution, today, represents one of the most severe environmental issues. The rapid global population growth, the use of pesticides and fertilizers, the untreated human and industrial wastewater, have been determining a fast reduction of usable freshwater, which in turn implicates a forthcoming water-scarcity by 2050. Several efforts have been devoted to developing high-efficiency, low-cost, and eco-friendly materials for water remediation [1]. Inspired by adhesive proteins secreted by mussels for attachment to wet surfaces [2], melanin-like polymers have been successfully employed to provide highly resistant and adhesive biomaterials for the deposition of multifunctional films for water-remediation [1,3]. The robust adhesion to surfaces is related to an extensive network of covalent and noncovalent interactions due to phenolic hydroxyl/quinone groups based on DOPA. Indeed, the natural occurrence of melanin arises from enzymatic oxidation, operated by tyrosinase enzyme, of L-3,4-dihydroxyphenylalanine (L-DOPA) which leads to the deposition of melanin polymers [4]. Thus herein, we employ a porphyrin–spermine derivative, namely H2TCPPSpm4, in order to realize a new functionalized DOPA-melanin film. Indeed, by exploiting both i) the ability of H2TCPPSpm4 having primary and protonable amino groups prompt to react with melanin polymers; and ii) the adhesive properties of DOPA-melanins, we report a novel hybrid porphyrin/DOPA-melanin material able to form self-assembled film onto glass substrates through a viable synthesis in aqueous medium and at room conditions. In particular, a small-scale commercial glass substrate -surface area about 3 square centimetres- was dipped in a not-stirred and aerated 0.5mM L-DOPA PBS buffer solution for some days. After twoweek dipping, the glass substrate was coated by a quasi-homogeneous and porous dark melanin-like film also evidenced both from spectroscopic investigations and AFM surface morphology studies. The functionalised substrate was employed to remove methylene blue (MB), a common pollutant, from water revealing a high adsorption rate -more than 90%- in few hours of treatment. Afterwards, the adsorbed MB was either photodegraded by simulated solar irradiation or desorbed, bringing back the quasi-pristine hybrid melanin film. The restored composite substrate was reclaimed as dyeadsorbent showing an exceptional re-usability, for many adsorption cycles. These promising results illustrate the chance to realise a composite biomaterial for water-remediation with multi-purpose advantages: i) low-cost energy, self-assembled and biodegradable material; ii) high efficacy as dyepollutant adsorbent; iii) recyclability; iv) potential scalability for real and practical application overcoming the expensive filtration process based on the most common adsorbent materials. Moreover, to our knowledge, no data concerning composite porphyrin/melanin films have been described so far.