DNA strands offer high programmability and plasticity, characteristics that have been used to prepare well-defined one-, two- and three-dimensional architectures at the nanoscale. Now, a team led by Jwa-Min Nam of Seoul National University and Yung Doug Suh of the Korea Research Institute of Chemical Technology1 has taken advantage of these properties to develop new light-sensitive nanoprobes to detect single molecules using surface-enhanced Raman spectroscopy (SERS).

SERS is a surface-sensitive analytical technique that enhances the Raman scattering of molecules adsorbed to metal surfaces. It has proved extremely valuable in the fields of detection and imaging, but its reliability with regards to single-molecule detection has been hampered by ill-defined and poorly reproducible probe synthesis. “Currently, there is no good way of controlling the nano-environment of Raman-active molecules with high precision and structural reproducibility,” say Nam and Suh.

Fig. 1: (a) Schematic illustration of dumbbell-shaped, Raman-active gold–silver nanostructures. (b) Transmission electron microscopy image of a single dumbbell-shaped nanostructure. The dye is positioned at the center of the nanostructure to maximize surface-enhanced Raman scattering.© 2010 J.-M. Nam and Y.D. Suh

To remedy these shortcomings, the researchers designed dumbbell-shaped gold–silver nanostructures with a SERS-active dye at the center (Fig. 1). First, they modified gold nanoparticles with two different sets of DNA sequences, each designed to be complementary to one part of the single DNA strand targeted for detection. One of the DNA sequences grafted onto gold nanoparticles was tethered to the Raman-active dye. Nam, Suh and their colleagues then purified the two different DNA–nanoparticle conjugates thus obtained through magnetic nanoparticle separation. The complementary target DNA strand hybridizes to the two DNA–nanoparticle conjugates, joining them together and allowing dumbbells to be formed in high yield. The researchers finally exposed the nanostructures to a silver salt to grow nanometer-thick shells around the gold nanoparticles.

The team used a ‘nano-Raman’ setup to image individual nanostructures and detect their Raman responses simultaneously. Noting that SERS partly relies on an electromagnetic enhancement mechanism, they controlled the silver deposition to tune the nanometer-size gap between the dye and metal nanoparticles. This unique strategy significantly enhanced the single-molecule sensitivity of the nanostructures towards DNA, allowing reproducible single-DNA detection.

“Our strategy established a way of controlling the nanogap starting from a known, fixed distance of about 10 nm set by a DNA ruler down to less than about 1 nm,” say Suh and Nam.

Single-molecule detection promises to provide insight into the behavior of individual biomolecules in vitro and in vivo. The team is currently working on improving the target quantification aspect and reproducibility of Raman-based assays using their nanostructures. They are also investigating the potential implementation of these nanostructures in fast, quantitative and multiplexed assays to detect infectious diseases in clinical samples.