Researchers in Singapore have stumbled across a simple but as yet unexplained way to make silver nanoparticles luminescent. The finding is potentially important because luminescence is often used to track the movement and location of nanoparticles – within cells, for example. The team, from the National University of Singapore, says that their new method can provide a rapid way simultaneously to synthesise and purify highly luminescent silver nanoparticles. One expert, however, cautions against the possibility of artefact in the absence of a clear explanation for the luminescence.
Jianping Xie and colleagues coated ultrasmall silver nanoparticle clusters – of less than 2 nm – with glutathione, a common technique for protecting certain metallic nanoparticles. They then passed the particles through a desalting column of Sephadex G-25 to purify the mixture, another standard procedure. The researchers were intrigued to notice that as the nanoparticle solution passed down the column its colour changed from deep brown to reddish brown, and that under UV illumination the purified particles emerging from the column showed intense red luminescence – which the unpurified mixture did not.
We were very surprised, as this was an unexpected observation
‘We were very surprised, as this was an unexpected observation,’ says Xie. It remains unclear why the particles spontaneously become luminescent. ‘The original silver nanoclusters were polydisperse, unstable and non-luminescent, and the nanoclusters after travelling through the desalting column were monodisperse, stable and highly luminescent,’ says Xie. It seems that the removal of small ions, such as OH- and Na+, and excess glutathione ligands could have played a role, producing pure nanoclusters of a uniform size of around 1.5nm diameter. But the precise mechanisms remain unclear. ‘The luminescence properties of silver nanoclusters could be affected by the core size, oxidation states of the metals, and ligands,’ suggests Xie.
Interestingly, the researchers also passed the raw nanoclusters through an ultrafiltration membrane, which again resulted in a luminescent filtrate, suggesting that the removal of extraneous material is key.
Xie adds, ‘Our study provides a new strategy for the concurrent synthesis and purification of highly luminescent metal nanoclusters via a common physical separation unit, which could be further advanced in a continuous mode for large-scale production of luminescent metal nanoclusters.’
Experts in metallic nanoparticles have taken mixed views on the work. Joseph Tracy of North Carolina State University in the US says that the work ‘highlights how processing can improve the luminescence of ultrasmall nanoparticles,’ and that the work might open a new way to produce the class of optically active nanoparticles called quantum dots. Whereas, Mathias Brust of the University of Liverpool in the UK remains somewhat tentative. ‘This is a very intriguing report,’ he says. ‘However, without knowing what causes the fluorescence, I would remain cautious regarding the possibility of artefacts.’
Xie says, however, that ‘a number of experiments have been carried out to confirm the luminescence was from the silver nanoclusters.’