Spotting A Single Strand

Common techniques for identifying and measuring DNA or RNA require purification, labeling, or other cumbersome preparations. But a new method pumps samples through a tiny capillary tube and needs no such pretreatment (Anal. Chem., DOI: 10.1021/ac202480w). The ability to analyze samples directly makes the portable device ideal for use in remote locations or in healthcare facilities without the equipment necessary for the standard methods, says Ron Naaman of the Weizmann Institute of Science.

Naaman and colleagues coated the inside of a capillary tube, just 0.3 mm in diameter, with probes that they designed to bind specific single-stranded DNA or RNA molecules. The researchers tested the device by flowing aqueous samples of the biomolecules through the probe-laden tubes. The researchers then elongated the molecules that stuck to the probes by adding either DNA polymerase or reverse transcriptase, depending on whether the probe targeted DNA or RNA, respectively. The enzymes incorporated fluorescently labeled nucleotides into the strand during elongation.

Pictures taken with a fluorescence microscope camera revealed glowing spots on the tube in places where probes had snagged a nucleic acid. The researchers also could detect multiple targets simultaneously by using a tube with one section containing a probe for DNA and another with a probe for RNA. When testing a sample with just one of the nucleic acids, only half the tube glowed, but when the sample contained both, the entire tube lit up.

Using the intensity of the fluorescence, the researchers calculated concentrations and found they could detect even a single molecule, says Naaman. The thinness of the tube is the key to the device’s sensitivity, he explains, because the sample’s biomolecules collide so many times with the surface that the probes are sure to latch onto every one of the targeted nucleic acids.