Barcoding technologies for detecting molecules have become very popular in the last few years thanks to the "Luminex" system. Luminex colour codes microbeads into as many as 100 distinct sets by dying them with fluorescent organic dye molecules and each set of signature beads carries a specific detection reagent (such as oligonucleotide probes, antigens or proteins) on its surface. Molecules to be analysed in a sample bind to these detection reagents and a "reporter" molecule, coupled with another fluorophore, measures the reaction on the bead surface. These interactions are then measured by the Luminex analyser using laser beams.

The technique is very efficient but fluorophore molecules themselves have their limitations because they have broad fluorescence spectra, which makes them difficult to detect. What's more, microbeads doped with multiple organic fluorophores also usually require multiple lasers to excite them. This makes for instruments that are costly and bulky – not exactly ideal for use in countries that do not have the financial resources to buy such equipment.

Quantum dots as fluorophores
Warren Chan and colleagues have now looked at the possibility of using quantum dots instead of fluorophores in the barcodes to detect genetic biomarkers of pathogens like HIV, malaria, hepatitis B and C, and syphilis, all of which are carried in blood. Quantum dots have many advantages over organic fluorophores for making unique optical barcodes. These include the fact that they have a narrower spectral line width and that different light emissions from the dots can be excited with a single wavelength of light.

The researchers made their barcodes using a technique called continuous flow focusing. Here, different colour barcodes can be created by placing different combinations of quantum dots (ones that emit light at 500 nm and ones that emit light at 600 nm, for example) in an organic solvent. As the solvent flows through a nozzle, it can be focused using water at a point in the flow-focusing device.

The process drips out beads with fluorescent properties that mirror the quantum dot ratios in the solvent. For example, if the solution contains three different emitting quantum dots, the microbead will contain three different fluorescence emissions. Once made, the fluorescent dots are functionalized with targeting molecules, such as oligonucleotides, and are ready to use.

Fast detection
For genetic testing, Chan says that the quantum barcodes will be mixed with a patient's gene sample and a secondary target (conjugated to a fluorophore molecule). The patient's gene assembles the quantum dot barcode with the secondary target to form a sandwich complex that can be then detected. "Hundreds of gene fragments can be detected at the same time using this approach and our results show that they can be detected in less than 20 minutes," Chan told nanotechweb.org.

The technique is much faster than previous methods to diagnose infectious diseases, he adds. However, for some diseases, like AIDS, the sensitivity would have to be improved by around 100 times before it becomes reliable. It is already good enough to detect the gene biomarkers for malaria though and could be an excellent alternative to current techniques based on a dipstick format that are not sensitive but just fast.

"Our diagnostic platform enables us to detect diseases at the molecular level in a rapid way," said lead author Supratim Giri. "The approach could also be used detect cancer and cardiovascular disease as well as infectious disease."

The team now plans to improve the sensitivity of its device and test the method on human blood and fluid samples. "Ultimately, we could like to incorporate the barcode into hand held devices for point-of-care testing," revealed Chan.

The work was reported in ACS Nano.