The low-cost sensor made of a copper-based metal-organic framework performs as well as DNA based sensor, the gold standard for water quality sensors.

Heavy metals are elements with high atomic weights and densities. They play significant roles in various sectors, from manufacturing to agriculture. However, despite their utility, heavy metals also pose significant environmental and health concerns due to their potential toxicity, persistence, and bioaccumulative (ability to accumulate within living organisms) nature.

According to a report by The Energy and Resources Institute (TERI), nearly 718 Indian districts have groundwater contaminated with heavy metals, such as arsenic, cadmium, chromium, and lead. The Ministry of Environment, Forest and Climate Change (MoEF&CC) has also identified 320 locations as having a high probability of contamination with heavy metals. Ingesting these metals can cause serious health problems, including damage to the skin, bones, brain and other organs, especially in children. Efficient detection of these metals in water is crucial for ensuring environmental safety and public health.

In a bid to address heavy metal pollution, researchers from the Indian Institute of Technology, Bombay, and Monash University, Australia, with funding support from the Department of Biotechnology (DBT), Govt. of India, have developed a sensor using a copper-based metal-organic framework (MOF) to detect toxic metals in water more cost-effectively and efficiently.

Metal-organic frameworks (MOFs) are a class of materials characterised by their highly porous structures. At a microscopic level, these frameworks are composed of nodes of metal ions connected by organic compounds, forming a porous network with tunable properties and immense surface area to volume ratio. Due to their unique structure and versatility, MOFs have garnered significant interest in various scientific and industrial applications.

For their study, the team of researchers fabricated an MOF with copper (Cu) forming the metal nodes connected by the organic compound Tetrakis (4-carboxyphenyl) porphyrin, forming copper-tetracarboxyphenylporphyrin, or Cu-TCPP for short. The Cu-TCPP is a two-dimensional (2D) MOF with a paddle-wheel structure. The unique structure also means Cu-TCPP can have more surface area contacting the water and is much more efficient at picking up heavy metal ions than conventional 3D materials. The sensor is able to detect heavy metal ions like lead (Pb), cadmium (Cd) and mercury (Hg) in water samples, even when there are only a few atoms per millilitre present.

“This MOF involves two Cu atoms binding to each carboxyphenyl arm of the TCPP molecule, hence forming the characteristic paddle-wheel structure. This means that other metal ions with similar configurations would be able to replace Cu in the structure and maintain the overall order without causing structural collapse. Other metal ions, especially heavy metal ions, can also accumulate on the MOF lattice,” explains Prashanth Kannan, the first author of the paper and a student at the IIT Bombay-Monash Research Academy, talking about the structure of the Cu-TCPP MOF.

The Cu-TCPP detects heavy metal ions in water in two ways - by substitution, where a metal ion knocks the copper out and replaces it, or by accumulation, where the metal ions just accumulate on the surface. Lead has incomplete p-orbitals, meaning it needs more electrons to be stable. This incompleteness allows the lead to replace the Cu ions in the MOF seamlessly while still allowing the MOF to maintain its structure. Once the lead replaces the copper, MOF’s electronic properties also change, which allows researchers to measure the amount of lead in the water.

Metals like cadmium and mercury, on the other hand, don’t easily substitute with copper ions. Instead of replacing the copper, these metals accumulate at the surface, forming what are known as molecular islands on the surfaces of metals. “When faced with a highly regular periodic lattice arrangement such as the Cu-TCPP MOF, they initially accumulate on the surface of the MOF, then at high concentrations can cause the failure of the MOF structure. By identifying the differences in electrochemical waveform and intensity (during the failure), we are able to accurately estimate nanomolar levels of heavy metals in water,” explains Prashanth.

The researchers tested the sensor on water samples from taps and lakes. It accurately detected the three metals, lead, cadmium and mercury, even when present in trace amounts. The sensor performed well despite being tested with substances that could interfere with the MOF, like alkali metals, debris and other large particles, in the water, indicating its reliability in different conditions. The researchers then also compared their device with the state-of-the-art sensors available in the market and found that it performed comparably, if not better, in most cases. “Our device has the least complexity and comparable sensing limits to the best of the current DNA-based sensors (the gold standard for sensing devices),” remarks Prashanth.

Despite its performance, the sensor does have limitations. After a single use, the MOF structure tends to break down upon prolonged exposure to heavy metals. This means the sensor can only be used once. However, in the particular case of water quality sensors, since the industry standard for low-cost devices is one-time use, reusability is not needed and is not really a limitation of the sensor, according to Prashanth. “The major bottleneck with this type of device lies in material fabrication costs. MOFs are difficult to coat over large areas, but there are ongoing efforts by various research groups worldwide to make manufacturing possible on a large scale,” he adds.

The technology not only holds promise for improving public health but also highlights the potential of science to develop solutions to pressing environmental challenges. Prashanth is already looking ahead to the next challenges. “Currently, there are several topics of major interest worldwide that need materials like MOFs to address them, like detecting Perfluorooctane sulfonic acid (PFOS), perfluoroalkyl substances (PFAS), arsenic and chromium in drinking water and tap water,” he signs off indicating future applications.

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