New bis-indolylmethane derivatives as ionophore for development of optical copper sensor and electrochemical alexandrium minutum biosensor

Abstract

Major water pollution and water toxication have been a predominant issue due to industrial waste, and agricultural waste, affecting the health and safety of the community. Indoles reported to have various applications in the field of biology and chemistry. The main objective of this study is to develop an optical sensor to detect Cu(II) and an electrochemical biosensor to detect Alexandrium minutum DNA using synthesised bis-indolylmethane derivative as ionophores. A total of 20 bis-indolylmethane derivatives have been successfully synthesized with percentage yield of 70 to 99% by condensation reaction between substituted benzaldehyde and indole derivatives and characterized using ultraviolet visible spectroscopy (UV-Visible), Fourier transform infrared spectroscopy (FTIR), and nuclear magnetic resonance spectroscopy (NMR). Density functional theory (DFT) methods were conducted to study the energy values (HOMO-LUMO) and molecular electrostatic potential (MEP) visual maps for all bis-indolylmethane derivatives. All bis-indolylmethane derivatives were screened for binding with Cu(II) using colorimetric detection, and Ultraviolet titration. The binding and selectivity study with ten different metal salts reveals that bis-indolylmethane 15 derivative shows strong interaction towards Cu(II) with binding constant of 2.21 × 104 M-1 at 1:1 stoichiometric ratio. Furthermore, all bis-indolylmethane derivatives undergo screening analysis with CT-DNA by using UV- Visible spectroscopy, Differential Pulse Voltammetry (DPV) analysis and DNA UV-Visible titration. Binding studies of bis-indolylmethane with A. minutum DNA was conducted using UV-Visible titrations, fluorescence titrations, viscosity measurements, circular dichroism, and electrochemical studies. Molecular docking was conducted towards bis-indolylmethane 15 and 3D crystal structure of B-DNA (PDB: 1N37). The analysis of the binding and selectivity studies showed that bis-indolylmethane 15 showed strong intercalative binding towards DNA. Hydrogels of Fmoc-L-phenylalanine and Fmoc-L-phenylalanine/bis-indolylmethane was prepared and characterised as immobilisation matrix. A Cu(II) optical sensor was developed based on the immobilization of bis-indolylmethane 15 with hydrogel and analysed using UV-Visible spectroscopy. The sensor exhibited low detection limit values of 5.39 × 10−7 M for bis-indolylmethane sensor and 9.84 × 10−8 M for Fmoc-L-phenylalanine/bis-indolylmethane sensor with a linear range of 1.6 × 10−8 M -1.24 × 10−3 M (R2= 0.99) with fast response time (30 min). An electrochemical biosensor for detection of A. minutum DNA was developed based on the immobilization of bis-indolylmethane 15 with acrylic poly(n-butyl acrylate-co-N-acryloxysuccinimide) [poly(nBA-co-NAS) microspheres physically on the surface of carbon screen printed electrode (CSPE) by impedimetric method and differential pulse voltammogram (DPV) method. Both sensors exhibit low detection limit values of 9.256 × 10-18 μM for impedance technique and 3.33 × 10-17 μM for DPV technique with a linear range of 1.0 × 10-16 - 1.0 × 10-3 M (R2=0.99). Both the sensors also show high selectivity, good repeatability, and reproducibility with % RSD values around 7.5-7.7% (n=7). Overall, bis-indolylmethane compounds have great potential to be developed for detection of Cu(II) and toxin A. minutum contributing to the improvement of detection of Cu(II) and toxins in the production of good water quality and overcoming water pollution.