One stop quantum dot testing

Researchers at Indiana University have shown how to instantaneously identify tens of thousands of genes by using tiny semiconductor crystals that glow in ultraviolet light.

The technique, which employs quantum dots, is said to work like a bar code with each colour and intensity combination corresponding to an individual gene.

The researchers predict that up to 40,000 genes or proteins could be studied in as little as 10 minutes, which is an improvement on biochip technology that can take up to 24 hours to identify a group of genes.

Researchers have tried for years to use quantum dots as glowing labels for genes, proteins, and other biological molecules.

Quantum dots are reported to promise faster, more flexible, less costly tests for on-the-spot biological analysis or patient diagnosis. But they have been difficult to collect and manipulate with enough precision to be useful.

‘We solved all of the technical problems,’ said biomedical engineer Shuming Nie, Ph.D., of Indiana University, who led the research. ‘The idea is very simple and straightforward, but I think we’re the first ones to make it work.’

Quantum dots display an array of colours. Each dot is made from semiconductor crystals of cadmium selenide encased in a zinc sulphide shell as small as one nanometer in diameter. In ultraviolet light, each dot is said to radiate a brilliant colour.

Nie’s group found a way to capture the quantum dots in specific quantities and in a wide range of colours and various intensities.

Using six colours, each with 10 intensity levels, it would be possible to code for one million genes. But the group said that for accurate detection without any spectral overlap, a reasonable range would be 10,000 to 40,000 different codes.

To capture the quantum dots; they made porous microbeads of polystyrene and seeded these with the zinc sulphide-capped cadmium selenide nanocrystals. They made both the beads and the quantum dots water repellent, which encouraged the quantum dots to move into the pores.

‘If they are both water repellent, they will like each other,’ Nie said. ‘Just like water and oil don’t mix: water likes water and oil likes oil.’ Once the quantum dots infiltrated the pores, the researchers sealed the pores.

To demonstrate the use of these quantum dots in DNA analysis, the researchers prepared microbeads of three colours, or spectral wavelengths, and attached them to strips of genetic material.

Each colour corresponded with a specific DNA sequence and was used as probes to seek out complementary pieces of genetic material in a DNA mixture.

Among the advantages of the quantum dot system is its flexibility. Scientists wishing to add a new gene code to the test simply have to mix a new batch of beads, which takes half an hour.

Nie said the technology has not been licensed, but several companies are engaged in similar research, including Quantum Dot Corporation, which is developing a number of biological uses for quantum dots.

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