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. 

A range of fluorescent probes has been developed to observe biological molecules, 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. 

The technique involves exciting nanoparticles with a near-infrared ultrafast laser, causing the nanoparticles to absorb and then emit photons. One advantage is that infrared light readily passes through biological material without damage. 

Although symmetrical gold nanoparticles exhibited weak photoluminescence when exposed to very short pulses of laser light at a wave-length of 790nm, asymmetric particles exhibited much stronger photoluminescence through field enhancement effects.   

This photoluminescence was produced by nanoparticles ranging from 2.5-125nm diameter, with strongest emissions at 60nm. Nanoparticles emitted light for hours under continuous excitation and this 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