Environmental pollution caused by heavy metals such as mercury, arsenic, copper, lead, and chromium lead to major health issues in humans and animals, even at very low concentrations. It enters the living system either through the ingestion of contaminated food and drinking water. Hexavalent chromium is one of the most widely used heavy metals that are hazardous to health. Chromium ions are highly soluble, mobile, and accumulate in the living systems. The maximum contaminant level (MCL) of hexavalent chromium in drinking water as recommended by WHO and the United States Environmental Protection Agency (EPA) are 0.05 mgL-1 and 0.1 mgL-1, respectively. Therefore, the detection of highly toxic contaminants in ultra-dilute concentration is a major challenge in clean water technology. While conventional ICP-MS, AES or AAS based chromium detection methods are highly sensitive, the electrochemical and spectrophotometric techniques are highly promising towards the realization of a portable chromium sensor. The spectrophotometric and electrochemical methods use a wide variety of chemoreceptors for chromium detection which includes noble metal nanoparticles, carbon nanomaterials, and composites such as metal-organic frameworks and metal oxides. For optical sensing schemes, in particular, the intrinsic optical absorption (yellow color) of the chromate ion can be exploited for a direct measurement. In the present work, we have designed and developed a metal-organic framework(MOF) coated optical fiber for colorimetric detection of chromium ions from water. Zeolite imidazole frameworks -67 (ZIF-67), a cobalt-based MOF is utilized as a matrix for trapping chromium ions. The highly sensitive U-bent silica fiber optic sensor (FOS) detect the chromium ion characteristic wavelength at 395 nm by means of evanescent wave absorption. The sensor response to chromium ion concentrations ranging from 1 ppb to 100 ppm in deionized and tap water was obtained. The limit of detection (LOD) found to be 1 ppb (19.2 nM). The selectivity of the ZIF-67 coated FOS with potential interfering heavy metal ions and the stability of the sensor probes over a month were validated. The usefulness of the sensor response to real samples was evaluated and the results were verified with that of the gold-standard ICP-MS.
Given these promising results, FOS will be proposed for detection other heavy metal ions including copper and lead.