Highly sensitive detection of Cr(VI) in environmental waters by tetra sulfonated azo calix[4]arene functionalized U-bent optical fibre sensor.

Hexavalent chromium (Cr(VI)) is a toxic and carcinogenic contaminant, posing severe health risks to human beings and other organisms. This study developed a novel optical fibre sensor to detect Cr(VI) ions utilising a tetra sulfonated azo calix[4]arene as a receptor. The 5,11,17,23-tetrakis[(p-sulfophenyl)azo]-25,26,27,28-tetrahydroxycalix[4]arene (TSAC) was synthesised from p-tert-butyl calix[4]arene, which was immobilised onto a polyaniline-coated optical fibre. The sensor operates on the principle of evanescent wave absorbance caused by the specific interaction of Cr(VI) with the immobilised receptor on the surface of the U-bent fibre. Interference studies were conducted to assess the selectivity of the sensor in the presence of some major interfering ions, which affirmed the specificity of the sensor for Cr(VI). The sensor demonstrated a limit of detection in the range of 0.48-1ppb, with a linear response range from 1 ppb to 100 ppb (R2=0.95-0.99) in deionised water, tap water, lake water, groundwater, and river water (impacted by discharge from an effluent treatment plant in a steel industry). The synthesised chemosensor displayed excellent selectivity towards Cr(VI) and showed good stability over three weeks. Thus, this sensor is a robust tool for real-time Cr(VI) detection in water.