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Materials

Detecting RNA Based On Color

Diagnostics: With fluorescent polymers, a paperlike device detects clinically relevant nucleic acids

by Sarah Webb
January 11, 2013

SPOTTED
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Credit: Anal. Chem.
In a new paperlike device, an orange color reveals the presence of target RNA in a sample dotted on the sheet, while purple indicates lack of the desired RNA sequence. The upper spots serve as a guide, while the lower spots indicate the colors achieved with experimental samples.
Photo of device with four spots.
Credit: Anal. Chem.
In a new paperlike device, an orange color reveals the presence of target RNA in a sample dotted on the sheet, while purple indicates lack of the desired RNA sequence. The upper spots serve as a guide, while the lower spots indicate the colors achieved with experimental samples.

In remote locations without sophisticated laboratory equipment and trained technicians, people need a simple, cheap way to screen for diseases. Now researchers describe a simple, paperlike device whose color reveals the presence or absence of a short RNA sequence (Anal. Chem., DOI: 10.1021/ac3034008). The technology could lead to easy-to-use diagnostic tests for infectious diseases and a variety of other ailments, says Bo Liedberg of Nanyang Technological University, in Singapore.

Liedberg and colleagues Umit Hakan Yildiz and Palaniappan Alagappan wanted to develop a device that relied on a simple chemical interaction to produce a color that they could detect with the naked eye or by analyzing a photograph. So they looked at polythiophene polymers. These positively charged polymers produce a color in solution as they wrap around a duplex of negatively charged nucleic acids.

The researchers distributed the polythiophene within a polymer: They produced flexible “paper” from a synthetic polymer, polyvinylidene fluoride, by spreading a solution of the material on clean glass slides and peeling off sheets of it. They then infused those sheets with the fluorescent polythiophene and incubated them with a synthesized nucleic acid sequence complementary to the nucleic acid they were interested in.

To test whether the sheets could detect a specific target sequence, they decided to look at a short RNA sequence, mir21, which other researchers have linked to lung cancer. For the nucleic acid sequence complementary to mir21, the team used a polymer of peptide nucleic acids, a more stable and uncharged synthetic cousin of RNA linked by amide bonds. They then spotted droplets of test solutions of RNA on the sheets and monitored the device’s color. In the presence of mir21 RNA, the paper remained orange. But in the presence of noncomplementary RNAs, even those with a single base mismatch, the sheet changed color from orange to purple.

“I’m very, very impressed by the simplicity,” says Mario Leclerc of Laval University, in Quebec. The researchers have provided a tool that might be useful for testing for a range of molecules beyond RNA, he adds. He points out, however, that this initial work used purified RNA samples, rather than blood or serum. The Nanyang Technological University researchers are looking at ways to incorporate a simple filtration process based on molecular size into their devices, to remove unwanted large molecules from biological samples. The researchers are working with a company to develop a test for dengue fever, a major health threat across Southeast Asia.

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