Colorimetric analysis of Cu2+ ions using a halochromic indicator solution and an electrospun film prepared using Caesalpinia sappan L.

This study presents a sensitive colorimetric detection system for Cu2+ ions utilizing Caesalpinia sappan L. extract, applied both as a standalone indicator solution and as a functional component within electrospun membranes. Upon exposure to Cu2+, both the indicator solution and the electrospun membrane undergo a visible and measurable color change, which is assessed through RGB analysis of digital images and UV-Vis spectroscopy, enabling quick and straightforward detection of Cu2+ in aqueous environments. An electrospun film incorporating cellulose acetate (CA), polyvinylpyrrolidone (PVP), and Caesalpinia sappan L. extract offers enhanced analyte binding, stability, and selectivity, facilitating trace-level detection in the range of WHO-recommended Cu2+ concentration in drinking water. Colorimetric analysis of the heavy metals in the indicator solution and electrospun film was conducted using RGB-based image analysis, with images captured via a smartphone under standardized conditions. Density functional theory calculations, including electrostatic potential mapping and molecular orbital theory analysis, provided insights into the mechanism of interaction between the metal ions and the colorimetric probe by revealing electron-deficient binding regions and favourable s-orbital overlap facilitating charge transfer. Geometry optimization and vibrational analysis confirmed the 6a-O coordination site as the most probable Cu2+ binding interaction, in agreement with the spectral response observed experimentally. This approach provides a portable, eco-friendly, and efficient alternative for environmental monitoring, water quality assessment, and industrial safety, addressing challenges associated with heavy metal contamination in a cost-effective and scalable manner.