Measuring quantum dot (QD) concentrations using viruses could lead to improved cancer imaging, according to scientists in Canada. 

Quantum dots have been used in a wide range of biological applications, from tissue imaging to drug delivery. Accurately measuring their concentration is vital for these fields to advance, say Warren Chan and Sawitri Mardyani, at the University of Toronto. The pair used phage display - a method that uses a library of viruses called phages, each with a different peptide exposed on their surface - to measure QDs. 

They coated QDs with one of three different compounds - mercaptoacetic acid (MAA), mercaptoundecanoic acid or bovine serum albumin - and attached them to gelatin-coated substrates. They exposed the QDs to a phage library and some of the surface peptides bound to the QDs. They then washed away the unbound viruses and used an enzyme-linked immunosorbant assay to analyse the bound ones. They found one peptide sequence that bound only to MAA-coated QDs and showed that they could use it to measure the concentrations of MAA-coated QD solutions ranging from 10 nanomolar to one micromolar. 

Absorbance and emission spectroscopy are commonly used to measure QD concentrations. However, these methods do not give accurate results in the presence of trypan blue. This stain is commonly used in cancer studies to colour an organism's dead or damaged cells blue, explains Chan, but its high absorbance renders quantum dots invisible. Chan and Mardyani spiked a solution of MAA-coated QDs with trypan blue and demonstrated that the phage assay could still detect the QD concentration. 

'It is important to develop tools that can quantify whole quantum dots in biological environments,' says Chan, 'because in order to optimise the design of these nanoparticles for diagnostic or therapeutic applications, such as cancer imaging, we need to map out their in vivo behaviour.' 

'I'd like to see the specificity tested in a more complex environment, such as an actual stained cell and then it could be a valuable, useful tool,' comments Paul Barker from the University of Cambridge, UK, who studies peptides interacting with nanoparticles. 

This is something they are exploring, according to Chan: 'We will determine whether immunoassay strategies can be used to measure nanoparticles in cells and tissues and check that the biological environment does not interfere with the binding interactions.' 

Christina Hodkinson 

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