At nanoscale dimensions, tiny particles of gold lose their characteristic glitter and take on bright new colors through a phenomenon known as surface plasmon resonance, the collective oscillation of conduction electrons in the metal nanoparticles. These oscillations scatter visible light at wavelengths different to that of the bulk material, setting off color changes that can be manipulated using the size and local environment of the particles.

Fig. 1: A new synthetic technique enables gold nanoparticles to act as multicolored DNA sensors.©

When gold nanoparticles are clumped together, they take on colors distinctly different to those obtained when the particles are well dispersed in solution. This behavior has attracted the interest of scientists seeking simple and effective ways to detect biomolecules. Grafting substances such as DNA to the surface of gold nanoparticles, for example, can drive particle aggregation and thus a change in color from red to purple or blue. This only occurs, however, if the DNA strands have specific and complementary interactions that induce the aggregation. Although this has proved remarkably successful as a DNA sensing method, materials that feature more diverse color transitions are still needed.

Now, Ji-Young Kim and Jae-Seung Lee from Korea University in Seoul1 have developed a new technique to synthesize DNA–gold nanoparticle clusters with optical properties that can be precisely controlled. The researchers used a molecule called dithiothreitol (DTT), a strong reducing agent, to control the size of their DNA–nanoparticle hybrid clusters. When mixed into a solution of DNA and gold nanoparticles, DTT cross-links the nanoparticles together, while DNA attaches to the outer nanoparticle surface, thus arresting cluster growth. Using this approach they obtained a series of monodisperse materials with colors that could be changed — from red to purple to violet to blue — by altering the ratio of DTT to DNA in the reaction.

“Nanoparticle aggregation and stabilization have each been thoroughly investigated by researchers,” says Lee. “I wanted to find a niche market between them for 'clustering nanoparticles' — where multiple particles are half-aggregated and half-stabilized.”

Kim and Lee found that the multicolored clusters served as excellent DNA detectors, as they undergo thermally reversible color changes in the presence of complementary nucleic acid strands. Furthermore, the detection limits for this multicolored system are up to five times better than those achieved by the current techniques, which utilize a single color shift. According to Lee, this new technology should help power versatile diagnostic applications that can take advantage of multiple color-based signals.