Heavy Metal Ion Sensing Using Ultrathin Langmuir–Schaefer Film of Tetraphenylporphyrin Molecule

Abstract

The exciting properties of porphyrin molecules can be employed for several applications like sensing, catalysis, photovoltaics and energy related fields. The assemblies of the molecules at interfaces can give rise to some unique physicochemical properties which can enhance the device performance. In this article, we report our studies on heavy metal ion sensing in aqueous medium using ultrathin film of tetraphenylporphyrin (TPP) molecules. The TPP molecules were synthesized and used for forming Langmuir monolayer at the air-water interface. The monolayer was transferred onto solid substrates by Langmuir-Schaefer (LS) method at different target surface pressure of deposition ( $\pi _{\text {T}}$ ). The morphological analysis of the films indicated supramolecular assembly of the molecules in the LS film deposited at $\pi _{\text {T}}={30}$ mN/m. The LS films of TPP molecules were employed for sensing heavy metal cations viz. Pb2+, Hg2+, Co2+ and Cd2+ from the aqueous medium by measuring piezoresponse from a quartz crystal microbalance. The sensing performance was found to be the best with LS film deposited at $\pi _{\text {T}}={30}$ mN/m. The sensitivity towards Pb2+ ion is found to be the highest. The sensing of the heavy metal cations using randomly oriented TPP molecules in spin coated thin film was found to be much inferior as compared to that of LS films of the molecule. The enhanced sensing performance by the LS film of TPP molecules may be attributed to the supramolecular assembly of the molecules. We report that the interference of the sensing measurement for the recognition of Cd2+ and Pb2+ ions is least and thus these species can be detected selectively with less errors. The cation species recognition features obtained from scanning electron microscope images and Raman spectroscopy were found to be remarkably different and therefore these cationic heavy metal species can be selectively detected using the LS film of TPP.