A ciprofloxacin-functionalized NH2-UiO-66 metal-organic framework for pH-switchable dual spectrophotometric detection of As(iii) and Fe(ii): mechanistic insights, DFT validation, and water analysis.

Arsenic(iii) and iron(ii) are two of the most harmful inorganic pollutants that occur naturally in water environments and commercially in industrial settings; therefore, when they both appear simultaneously, there have been long-lasting issues for both human health and aquatic ecosystems. A functionalized metal-organic framework composite known as CIP-UiO-66 has been synthesized through post-synthetic modification of NH2-UiO-66(Zr) with ciprofloxacin (CIP) using glutaraldehyde-mediated Schiff-base condensation. The properties of the newly developed material were investigated using several analytical techniques including powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FT-IR), N2 adsorption-desorption analysis, scanning electron microscopy combined with energy-dispersive X-ray analysis (SEM-EDX), elemental mapping, and two-dimensional correlation spectroscopy (2D-COS FT-IR). The BET specific surface area of CIP-UiO-66 was found to be 906.42 m2 g-1, with a pore size distribution having a maximum centered around 1.17 nm and a total pore volume of 4.15 cm3 g-1. These results suggest that the synthesized material possesses a hierarchical micro-mesoporous structure that provides adequate space for analyte confinement and rapid transport. A dual detection system that responds to changes in pH has been designed as a means of improving the accuracy of spectrophotometric measurements of arsenic(iii) at pH 7.0 and iron(ii) at pH 6.0 on a single composite substrate. The optimum conditions of operation for the dual detection system have been determined, resulting in two linear response regions for each analyte: As(iii) from 0.0175 to 0.10 ppm (R 2 = 0.9989) and Fe(ii) from 0.0106 to 0.08 ppm (R 2 = 0.9993). In addition, low-detection limits have been observed for both analytes: 1.75 × 10-2 ppm (17.5 µg L-1) for As(iii) and 1.06 × 10-2 ppm (10.6 µg L-1) for Fe(ii). Although the LOD for Fe(ii) is significantly lower than the reference value of 300 µg L-1 for Fe(ii) in drinking water, the LOD for As(iii) is higher than the reference value of 10 µg L-1 for As(iii) in drinking water. Both analytes reach signal equilibrium within 10 minutes. Selectivity was tested against twelve different co-existing cations at equivalent molar ratios as interferences; additionally, DFT studies were performed to obtain mechanistic insights into the differing interactions of As(iii) and Fe(ii) with the sensing ligand. A decrease in the HOMO-LUMO energy difference was observed as a result of coordination; with an energy difference of 0.1580 hartree being found in the case of the uncoordinated ligand and 0.1424 hartree in the case of GA-CIP@As3+ and 0.1089 hartree in the case of GA-CIP@Fe2+. A validation of the real sample was performed on tap water and boiler water which showed recovery percentages of 98.13-101.33% (using the developed methodology) and was supported by ICP-OES results indicating that the methodology has both a high level of accuracy and potential to be applied to practical applications. It has been shown that the CIP-UiO-66 sensor can maintain its performance after six regeneration cycles showing that it is a durable, portable, and capable platform for simultaneously monitoring trace metals within complex environmental samples.