1 June 2005: Gold glitters even at the nanoscale

Gold is demonstrating a wealth of interesting and unsuspected properties at the nanoscale. Chemists at Boston College, Massachusetts, US, led by John Fourkas, have added to the list with the discovery that gold nanoparticles can act as fluorescent probes to image individual molecules.

Observing the actions of individual biological molecules with the attachment of fluorescent probes has greatly enhanced the understanding of many biological processes. A range of fluorescent probes has been developed, including dye molecules and semiconductor nanoparticles, but all suffer from a number of problems, says Fourkas. The probes often ’give out only a limited amount of light, burn out quickly under continuous observation and are prone to blinking on and off,’ he said.

Fourkas’ team searched for alternatives by investigating gold nanoparticles. They looked at whether the particles could be made to fluoresce using a technique called multiphoton-absorption-induced luminescence (MAIL). The technique involves exciting nanoparticles with a near-infrared ultrafast laser, which causes the nanoparticles to absorb and then emit photons. One of the advantages of this technique is that infrared light readily passes through biological material without harming it.

The team found that although symmetrical gold nanoparticles would exhibit photoluminescence when exposed to very short pulses of laser light (under 100 femtoseconds) at a wavelength of 790nm, it was weak. They then found, however, that asymmetric gold nanoparticles would exhibit much stronger photoluminescence through field enhancement effects. 

This photoluminescence could be produced by nanoparticles ranging from 2.5nm to 125nm in diameter, with strongest emissions at 60nm-diameter. The nanoparticles would emit light for hours under continuous excitation and this light did not blink on and off.

’[These nanoparticles] could allow us to track a single molecule of a drug in a cell or other biological sample,’ predicts Fourkas. Jon Evans