11 July 2005: Lighting up time for lead

A chemosensor that selectively lights up when it complexes lead is being developed by Korean chemists.

The sensor is based on the ubiquitous fluorescent label Rhodamine B. It enhances its red-fluorescent output 100-fold in the non-aqueous solvent acetonitrile when it takes up (chelates) lead, report researchers led by Juyoung Yoon at the Ewha Womans University, Seoul.

Lead accounts for 20mg/kg of the earth’s crust. Of the three million tonnes of lead that is mined, smelted, and globally consumed in lead-based products every year, 126 000 tonnes is emitted into the environment, according to a WHO report. 

There are plenty of opportunities for lead to enter the bio-sphere. It is used in batteries, cables, pigments, fuel additives, solder, water distribution pipes and sealing the seams of food-storage cans. The lead concentration in water varies according to hardness and pH. At pH 5.4, hard water contains on average about 30?g/L lead, while in soft water lead concentrations can rise to over 500?g/L. Symptoms of lead toxicity (which are worse in children) include renal problems, psychological and neurological disturbances. In fact, anaemia has been observed in children with lead body-burdens as low as 1.92 ?mol/L. So, a sensor capable of detecting lead at such low concentrations would be timely.

The new lead chemosensor uses an interesting facet of Rhodamine B’s chemical structure. At the 9-position, the carbon atom exists in a spiro conformation that interrupts the conjugation pathway between the two aromatic rings. The Korean team reacted the basic Rhodamine B structure with a complex pyridine-substituted amine called dipicolylamine, to make the new chemosensor molecule. 

This can bind metal ions. However, only with lead ions does the chelation cause the spiro carbon at the 9-position to snap open, allowing conjugation between the two aromatic rings. It is this that causes the dye to fluoresce (chelation-enhanced fluorescence or Chef) when the molecule is irradiated with 510nm wavelength light.

The new sensor is also sensitive; and can pick up lead concentrations as low as 0.1 ?mol/L in acetonitrile. 

’Why this molecule should be so uniquely sensitive to lead is still a mystery, especially as the dipicolylamine ligand has previously been used to sense zinc,’ said AP Da Silva from Queen’s University Belfast. ’The Korean team will need to study this system further to see if it’s amazing selectivity for lead transfers into aqueous solutions where it is most needed,’ he added. Lionel Milgrom