Methylmercury marker

Detecting methylmercury usually involves complex sample preparation and a sophisticated analytical procedure. Now, a European team has developed a novel approach to detecting this hazardous substance much more quickly and easily using a new type of fluorescent material.

A team headed by Ramón Martínez-Máñez at the Polytechnic University of Valencia (Spain) and Knut Rurack at the BAM Federal Institute of Materials Research and Testing (Berlin, Germany) has now developed a rapid test for the detection of the most toxic form of the heavy metal mercury, the organometallic compound, methylmercury. Their approach could eventually be developed into a test strip system for this contaminant.

Mercury enters the environment through rain forest slash and burn, the combustion of coal, chlorine and cement production, and more marginally from broken mercury thermometers and other devices, including fluorescent light bulbs. It is readily biomethylated to the highly toxic form. Scientists have been aware of mercury's toxicity for many years, but increasing levels in the environment and its accumulation in fish species, such as tuna, shark and swordfish are cause for increased concern. Methylmercury concentrations in various food sources frequently exceed the maximum safety levels recommended by the US Environmental Protection Agency (EPA) and the World Health Organization (WHO) for human consumption; 0.1 and 0.23 mg per kilogram of body weight.

"Methylmercury exposure in adults has been linked to cardiovascular diseases, autoimmune effects, hearing impairment, blindness, and death," the researchers explain, "In a number of cases, mercury intoxication is related to the consumption of fish."

The team points out that there are various laboratory-based analytical methods for the determination of methylmercury in biological samples, including gas chromatography with electron capture detection (ECD), inductively coupled plasma mass spectrometry (ICP-MS), and high performance liquid chromatography (HPLC) with elemental or ICP-MS detection. As with all such analytical techniques, they are not portable.

In order to develop a field test for methylmercury, various research teams have turned to optical, electrochemical, and gravimetric approaches. Many of these, however, suffer from the drawback of requiring a biological component for their sensing capacity. Non-biological sensing of this kind has been developed for naked mercury ions, but not for the toxic methylmercury, until now.

Martínez-Máñez and colleagues were well aware that mercury compounds generally have a strong affinity for sulfur and have now developed an organically capped mesoporous inorganic material that selectively detects methylmercury ions through signal amplification. The process is non-biological but mimics the kind of gated pores involved in cellular sensing processes in nature.

To build their methylmercury sensor, the team loaded an inorganic support material, a calcined MCM-41 mesoporous solid with a dye compound, safranine O. They then capped the system with 2,4-bis(4-dialkylaminophenyl)-3-hydroxy-4-alkylsulfanylcyclobut-2-enone (APC) groups.

The sulfur-containing APC groups provide an anchor point for methylmercury ions while the bulky organic part of the APCs blocks the openings to the mineral's pores, keeping the dye inside. When a sample containing methylmercury comes into contact with the material, the methylmercury rapidly latches on to the sulfur-containing groups, splits off the caps, and releases the dye.

The team points out that aside from relative simplicity, one of the main advantages of this approach is the amplification effect. Only a relative few methylmercury ions are needed to cause a flood of dye molecules to be released. This, they explain, lets them reveal even trace concentrations of methylmercury by a simple colour change, even in complex biological samples. Another important advantage is that the test, when employed in a sufficiently lipophilic environment, the test is highly selective for methylmercury. Non-methylated mercury ions or water-soluble inorganic mercury compounds are not detected as they do not have the right chemistry to approach the hybrid sensor material and latch on to the sulfur and uncap the pores.

The dye-loaded material could perhaps quite simply be incorporated into a strip test for the instantaneous detection of methylmercury contamination in the field or in foodstuffs.