Optical Environmental Chemosensor based on Morin-doped Self-plasticized Polyacrylate Optode Membrane for Rapid Quantitation of Aluminium Contaminant

Background: The presence of Aluminium(III) (Al3+) ions in natural waters is considered a potential environmental hazard and may become toxic to the biological communities. Chemical sensors enable online and field monitoring, which can be a useful alternative tool for continuous surveillance of aluminium toxic species in natural waters. Objective: This study aimed to develop an optical chemical sensing material based on a photocurable plasticizer-free poly(n-butyl acrylate) (poly(nBA)) membrane doped with a morin bidentate ligand for high-throughput quantitative analysis of Al3+ ion concentration in environmental samples using an optical absorption transducer. Methods: The optode membrane for Al3+ ions was constructed by immobilizing a chromophoric morin reagent into a poly(nBA) membrane precursor via physical adsorption. As the acrylic matrix possesses strong adhesive characteristics, the immobilization matrix can be directly coated on the plastic substrate without the incorporation of a plasticizer via dip-coating and photocuring techniques to form a transparent sensing zone of 3 cm × 1 cm. Results: The chemical sensor based on a morin-doped self-plasticized polyacrylate optode membrane demonstrated a dynamic linear response range of 2-13 ppm Al3+ ions with a calculated Limit Of Detection (LOD) of 1.6 ppm Al3+ ions. No obvious leaching was observed for the immobilized morin reagent during the 45-minute experimental period. The thin acrylic membrane- based optical sensor exhibited a rapid response time of ~20 s. The validation results showed that the performance of the developed membrane-based optical sensor was on par with that of the Atomic Absorption Spectroscopy (AAS) method for the quantitative determination of aluminium levels in river water samples. Discussion: The complexation reaction between the morin chelator and Al3+ ions was attributed to the auxochrome group of the morin molecule, i.e. the hydroxyl functional group that served as the binding site for Al3+ ions, has rendered the carbonyl chromophore to absorb at a longer wavelength with the increment of the morin chromogen's absorption at a higher absorption intensity. The soft, elastic, and thin acrylic membrane enhanced the mass transfer kinetics of Al3+ ions, allowing for excellent diffusion across the self-adhesive membrane, thereby facilitating the complexation reaction to occur. Conclusion: The proposed optical environmental chemosensor offers advantages in terms of low cost, freedom from electrical interference, and the potential for remote sensing. The acrylic membrane provided a large two-dimensional immobilization surface area, thereby promoting the immobilization of a large number of morin receptor molecules, which serve as chelators for Al3+ ions via metal-to-ligand coordination. This increased the diffusion rate and mass transfer of Al3+ ions to the reagent phase.