Toxic heavy metal detection: Portable sensitivity

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  • Published: Aug 1, 2014
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: Toxic heavy metal detection: Portable sensitivity

The mercury problem

ultrasensitive, cost-competitive, and portable optical sensing system for detecting ionic mercury in environmental water. This analytical system combines structurally engineered and chemically modified nanoporous anodic alumina rugate filters (NAA-RFs) with reflection spectroscopy (RfS) Image credit: American Chemical Society/Kumeria et al

Reflection spectroscopy can be used to detect mercury contamination of water down to a concentration of 200 parts per billion, according to Australian researchers working with colleagues in Spain. Their low cost, portable system will facilitate point of analysis testing for environmental samples.

Tushar Kumeria, Mohammad Mahbubur Rahman, Abel Santos and Dusan Losic of the School of Chemical Engineering, at The University of Adelaide and Josep Ferre ́-Borrull and Lluís Marsal of the Department of Electrical Engineering at the University of Rovira and Virgili, in Tarragona, Spain, have outlined their innovative optical sensing system in the American Chemical Society's journal Applied Materials and Interfaces.

"Mercury has been accumulating in the natural environment since the start of industrialisation and there are worldwide concerns about potential human health and environmental effects, explains project leader Santos, who is an Australian Research Council Postdoctoral Fellow at Adelaide."Recently, these concerns have seen the introduction of a global convention aimed at controlling, monitoring and reducing mercury pollution at a world scale." Mercury occurs naturally in mineral deposits around the globe as cinnabar (mercuric sulfide) but is toxic in soluble forms such as mercuric chloride or methylmercury and ethylmercury), through inhalation of its vapour, or when ingested through contaminated seafood. Mercury can cause harm to the immune and nervous systems as well as leading to genetic mutations in the unborn foetus. It can also interfere with motion coordination, touch, taste, and sight in adults. Nevertheless, it is an important and crucial element for many technological applications still widely used in lighting, in analogue gauges of various sorts, such as thermometers and blood pressure monitors as well as in a wide range of electronic devices. It is also used in gold extraction and incidentally released into the atmosphere during the burning of coal. The World Health Organisation talks of a "tolerable intake" being 1.6 micrograms per kilogram of mother's body weight per week for methylmercury to protect an inborn foetus from neurotoxicity and deformities.

Mercury solution

"There are current systems capable of monitoring mercury at trace levels, but they are huge machines that can't be easily moved, are very expensive and complicated to use and require comprehensive training. Samples also require chemical treatment before analysis," explains Santos. "Our system is very cost-competitive, only as big as a mobile phone and easy to use. With very basic training, someone could take it to a river or lake and obtain a mercury reading on the spot."

While much of the experimental work on this project was carried out by doctoral candidate Kumeria, the team as a whole has engineered a nanoporous material - nanoporous anodic aluminium oxide - to construct a particular type of filter with a rugate, wrinkled structure. The surface of the filter was modified with 3-(mercaptopropyl)-trimethoxysilane (MPTMS) to add thiol groups that make it selective for mercury ions in aqueous solutions flowing through the pores. The addition of a portable optical system using reflection spectroscopy ultraviolet-visible-infrared reflection in narrow bands - then provides a reading for how much mercury is caught on the filter for a given flow rate and thus provides a quantification of the mercury concentration in the solution.

Testing times for mercury

The team's proof of principle tests on samples from the River Torrens show that the device can detect mercury ions in a sample at concentrations as low as 200 parts per billion (1 micromolar concentration, the working range is 1 to 100 micromolar) even in if there are many other metal ions - calcium, chromium, cobalt, copper, iron, lead, magnesium, silver - or substances in the solution. The team is, of course, now working to optimise the optical signals generated to push this lower threshold down.

"The promising sensing performance of this system along with its cost-competiveness and portability make it an excellent potential alternative to current analytical techniques," explains Santos. "This technique could provide the basis for future point-of-analysis systems for monitoring water quality on site and may help implement better monitoring processes around the world."

"As far as the next step is concerned, we are carrying out new experiments and trying new approaches aimed at enhancing the sensitivity of our system and making it more selective towards different heavy metal ions," Santos told SpectroscopyNOW. "We are also testing different surface chemistries which are expected to provide more sensitivity and chemical selectivity to our system."

Related Links

ACS Appl Mater Interfaces 2014, online: "Nanoporous Anodic Alumina Rugate Filters for Sensing of Ionic Mercury: Toward Environmental Point-of-Analysis Systems"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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