Nanotube–DNA sensor: Mercury with a twist

Selective detection of mercury ions through the pairing of a carbon nanotube and a fluorescence-tagged DNA complex.

Mercury is a highly toxic heavy metal known for its severe adverse effects on human health and the environment. Several analytical methods are available for the detection of mercuric ions (Hg²⁺) — the mercury species most common in nature — but these typically lack selectivity, or involve probes that are difficult to synthesize. Now, Erkang Wang and colleagues from the Chinese Academy of Sciences in Jilin, China, have created a nanosized sensor that detects mercuric ions selectively and with extreme sensitivity, by pairing a carbon nanotube with single-stranded DNA (ssDNA) tagged with a fluorescent group.¹

The researchers chose a commercially available thymine-rich ssDNA conjugated to the fluorescent FAM dye as their sensor material. When mixed with a solution of single-walled carbon nanotubes (SWNTs), the ssDNA wraps itself around the long carbon structures. The presence of Hg²⁺ ions disrupts this binding process by joining the DNA’s thymine base pairs.

“Because mercuric ions can strongly interact with thymine DNA bases, a DNA duplex containing thymine–thymine mismatches shows high selectivity for Hg²⁺,” says Wang. “Therefore, thymine-rich ssDNA can easily form double-stranded DNA in the presence of mercuric ions.”

Fig. 1: Illustration of the fluorescent sensing of mercury ions using carbon nanotube–DNA hybrids.© 2010 L. Zhang, T. Li, B. Li, J. Li, E. Wang

Fluorescence measurements showed that formation of the ssDNA–SWNT complex suppressed emissions from the FAM-tagged ssDNA. In the presence of Hg²⁺ ions, however, the fluorescence intensity increased significantly, indicating that the ssDNA strands unwrapped from the carbon nanotubes to bind with the added ions (Fig. 1). This provided the researchers with a simple method for detecting mercury.

By taking measurements at different ion concentrations, the researchers observed a linear dependence between fluorescence intensity and Hg²⁺ concentration. They also discovered that the sensor could quantitatively detect Hg²⁺ between 0.05 and 8.0 μM and that its detection limit was 14.5 nM — almost half the level at which Hg²⁺ becomes toxic in drinking water.

To evaluate the selectivity of their sensor, the researchers exposed the ssDNA–SWNT complex to other metal ions that impact the environment, and found that the fluorescence enhancement was far better with mercuric ions than any other ions.

The team is currently investigating similar carbon nanotube–DNA systems for the detection of proteins, small molecules and other metal ions. “This approach can be used to detect other metal ions, such as silver ions, as these can specifically bind to the cytosine–cytosine base pairs in a DNA duplex,” says Wang.