Single biomolecules contained in tiny droplets of biological samples can be counted using a clever combination of microfluidics and fluorescence spectroscopy. 

The generation and manipulation of small droplet biological samples is important in many biotechnological applications as it enables fast and efficient biochemical analyses. However, accumulation and containment of biomolecules, such as the DNA or RNA of a single cell, is only useful if they can be seen and counted inside the droplet. 

Existing droplet-based platforms are unable to detect low concentrations of biomolecules without the use of additional amplification techniques,' explains Jeff Wang at Johns Hopkins University, Baltimore, US. He and his team have devised a way of detecting small differences in the content of tiny droplet biological samples without the need for amplification. 

Wang used a retractable valve that constricts part of a microfluidic channel. This stretches the droplet without breaking it and slows its passage through the narrowed channel. The altered droplet shape and extra time allows the trapped biomolecules inside to be detected using fluorescence spectroscopy. Using the technique, the team were able to detect nucleic acids inside droplets. 

Christoph Merten who researches novel in vitro compartmentalization techniques at Louis Pasteur University, Strasbourg, France comments, 'the technology has high potential for single-cell assays, however, tracking individual molecules in entire cell lysates is a challenging concept.' 

'Future work will involve improving throughput in microfluidic platforms for single cell analysis,' says Wang. If this can be achieved then these technologies will remove some of the problems currently associated with the use of mixed cell populations in, for example, stem cell biology, he adds. 

Janet Crombie 

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