UK scientists have found that quantum dots can improve optical sensors designed for detecting metal ions in water.

By attaching a simple organic receptor to the surface of a quantum dot - also known as a semiconductor nanoparticle - John Callan and colleagues at the Robert Gordon University, Aberdeen, UK, have developed an optical sensor which can simultaneously detect copper and iron in water-based samples.

'The remarkable aspect of the work is that the receptor itself shows no selectivity for any particular metal ion but displays dual selectivity when anchored to the quantum dot surface,' says Callan.

The three-dimensional shape of these dots provides a framework for the organisation of the receptor molecules. The alignment of the binding sites on the receptor - once attached to the quantum dot - complemented the size, shape and electronic properties of copper and iron ions.

Copper and iron are detected by the sample solution changing colour when the metal comes into contact with the sensor. Colourless to green for copper, and colourless to orange for iron. Each metal has a unique ultraviolet-visible spectrum profile which means that they can be detected simultaneously by ultraviolet-visible spectroscopy.  

Callan's sensor is straightforward to prepare and allows results to be obtained in real time. It compares well with other multianalyte sensors which often involve long-winded methods for receptor synthesis and/or data processing. And also has superior optical properties compared with organic dyes currently used in optical sensors. 

A P De Silva an expert in sensors from Queens University, Belfast, UK, says 'this fine work brings multifunctionality to quantum dot sensors for the first time.'

Ziva Whitelock