March 29, 2006
Naked-eye sensors detect biologically important anions

The term Litmus test is now a phrase in everyday language and derives from a chemistry test where Litmus paper is used to check the pH of a solution. It's probably one of the most recognisable tests from school science classes. Litmus is an example of a "naked-eye colorimetric sensor" – it turns red in the presence of acid and blue in the presence of alkaline.

CSCB researchers, Professor Thorri Gunnlaugsson and Dr Paul Kruger, from the School of Chemistry at Trinity College Dublin, have developed new naked-eye colorimetric anion sensors, which have the potential for checking water quality and detecting the presence of anions in biological samples.

Rivers and lakes can be contaminated by anions such as nitrates and phosphates, which are found in agricultural fertilisers.

"The run off from fertilisers can cause algae blooms which feed on these nitrates and phosphates," explains Kruger. "The aim of a 1991 EU directive is that all EU countries must reduce or prevent water pollution from anions such as nitrates. So it's important to develop a fast method of detecting these pollutants."

"Including these anion sensors in future test kits would give instant and easy-to-interpret results, compared to many standard methods for detecting anions," says Gunnlaugsson. "Testing domestic water for the fluoride anion is just one example of a possible application."

Anions, such as chloride, play a vital role in our bodies and detecting anions in biological samples is another possible application of these sensors. Chloride anions, for example, are key in clearing our lungs of mucus and for people with cystic fibrosis, the channels through which chloride ions move are blocked.

Nature provided the inspiration for the type of chemical sensor the scientists chose. Since anions in biological systems tend to interact using hydrogen bonding, thiourea-based sensors were chosen because they use the same type of bonding to recognise anions. The thiourea-based sensor is yellow in colour and the resulting anion and thiourea complex is purple.

Dr Paul Kruger and Professor Thorri Gunnlaugsson

Dr Paul Kruger holding a sample of the yellow  thiourea-based sensor and Professor Thorri Gunnlaugsson showing the purple colour change when the sensor reacts with acetate

"The first step in the process involves synthesising the thiourea-based sensor in the lab. It's a 3 step process with good yields and we reckon that 1 g of the thiourea compound could probably be used to perform thousands of anion tests," says Gunnlaugsson.

Gunnlaugsson and Kruger have been collaborating for years. Gunnlaugsson heads the supramolecular and medicinal chemistry group at TCD and Kruger leads the inorganic supramolecular group. Key to the success of this research was evaluating the chemistry of the sensor and anion interactions. Techniques such as X-ray crystallography helped explain the colour change and elucidate the structure of the sensor anion complex.

"Our focus now is on investigating developing test kits for pharmaceutical and medicinal purposes, in particular for monitoring electrolytes in critical care analysis," continues Gunnlaugsson.

The big advantage of these types of test kits is that they could really improve the time taken to run even routine tests in hospitals.

"The future is very bright and the possibilities are endless. The potential is there to use these sensors for detecting drugs in biological samples and even for checking the quality of wine," concludes Gunnlaugsson.



Gunnlaugsson, T.; Kruger, P. E.; Jensen, P.; Tierney, J.; Ali, H. D. P.; Hussey, G. M. J. Org. Chem. 2005, 70, 10875 -10878.

Freemantle, M. Chemical & Engineering News 2006, 84 (7), 83-85.


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