Mercury levels in water need to be checked carefully as it is a toxic substance that contaminates the food chain. A team at Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Bengaluru, has found an innovative way to develop a sensor that operates using Surface Enhanced Raman Spectroscopy and has high sensitivity (60 X 10-18 M which is 0.01 parts per quadrillion), far better than other methods of detecting mercury in water.
Mercury is a heavy metal that is predominant in the environment. It mixes with the environment due to both natural (e.g. volcanic activity) and anthropogenic (e.g. electrical appliances such as mercury lamps) activity. Studies have shown that industrial effluents can have higher mercury levels than that allowed by the WHO and Indian guidelines. With allowed levels of mercury in drinking water and effluents being in the range of 1–10 microgram per litre, it becomes necessary to develop sensors that can measure mercury levels in water with high sensitivity and selectivity.
The small molecule — histidine conjugated perylene diimide (HPH) —when dissolved in water shows green fluorescence under laser light. When water contaminated with mercury is added to this solution, the fluorescence is absent, and the molecules form a hydrogel. This method can detect only up to 5 nanomolar (0.1 parts per billion) of mercury in water. However, the sensitivity drastically improves with a novel technique developed by T. Govindaraju’s group in collaboration with that of Suresh Bhargava of RMIT, Australia.
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Sensitive detector
The small HPH molecules are organised on gold thin films coated on polystyrene beads. “The small molecule is a bolamphiphile, because it has both hydrophilic (histidine) units on the surface and hydrophobic (perylene) core units,” explains Dr Govindaraju in whose lab the technique was developed. The molecule has two arm-like projections on either side of the core, one of which binds to the gold surface and the other is free pointing outwards. When mercury contaminated water is added to this mixture, the mercury ions bind to the free ends. When subjected to Raman spectroscopy, the response after mercury has bound to the particles is highly enhanced as compared to before the binding of mercury. This gives a measurable optical response. “Our system is capable of detecting attomolar [concentration], it can detect any concentration above this level with very high accuracy,” says Dr Govindaraju. Although the technique has been demonstrated for water, it can come in useful for detecting mercury elsewhere too. “This technique can be used for any other sample, including biofluids or tissue extracts, wherein detection of such low concentration does matter,” he adds.