So, near and yet so far: Stable HGNs for Raman

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  • Published: Apr 1, 2015
  • Author: David Bradley
  • Channels: Raman
thumbnail image: So, near and yet so far: Stable HGNs for Raman

Hollow

Researchers have found a way to stabilise hollow gold nanospheres (HGNs) so that their unique combination of properties can be utilised as nanotags more widely in Surface Enhanced Raman Spectroscopy (SERS) for biomedical applications. Credit: Graham et al/Royal Society of Chemistry

Researchers have found a way to stabilise hollow gold nanospheres (HGNs) so that their unique combination of properties can be utilised as nanotags more widely in Surface Enhanced Raman Spectroscopy (SERS) for biomedical applications.

Samantha Moreton, Karen Faulds and Duncan Graham of the University of Strathclyde, Glasgow, Neil Shand of the Defence Science and Technology Laboratory (DSTL) Porton Down, Salisbury, Wiltshire, UK and Matthew Bedics and Michael Detty of the University at Buffalo, The State University of New York, USA, suggest that HGNs with their thin gold shell and hollow interior are unique nanostructures. They exhibit surface plasmon resonance because the valence electrons will oscillate collectively if an electric field is applied. This phenomenon depends sensitively on the size and shape of the nanoparticles as well as their homogeneity and can be followed using extinction spectroscopy, the team reports in the Royal Society of Chemistry journal Nanoscale. Moreover, careful preparation of HGNs allows the SPR to be tuned to near-infrared wavelengths, viz. from 550 to 1320 nanometres.

So NIR and yet so far

Medical researchers are interested in the NIR wavelengths to which HGNs respond because these hollow particles could potentially be used as NIR-controlled drug-delivery agents for highly targeted therapeutics or in photothermal ablation (PTA) where they are delivered to a diseased sight and unhealthy tissue heated in a very localised manner through the effects of NIR irradiation on the nanoparticles. Graham and colleagues are focused on how HGNs might be used in SERS where those tissue-penetrating NIR wavelengths, more specifically 800-1300 nanometres, provides essentially a transparent sensing window, a way to see through flesh and blood.

All hopes of using HGNs in such medical applications might have been dashed because of the nanoparticles' inability to remain intact in physiological salt solutions or in the face of large changes in pH, but for the team's work in which they have now found a relatively straightforward way to stabilise them. Indeed, stomach acid sits at pH 2, but the intestine is at pH 5 to 8 while cancerous tissue usually lies at a pH between 5.4 and 7.4.

Seeking stability

Other researchers have shown that certain polymers can stabilise other types of nanoparticle. For instance, the well-known polyethylene glycol (PEG) will stabilise gold nanoparticles at high salt concentrations. Thioglucose can improve biocompatibility and so boost rates of cellular uptake as well as provided a sweet shell to preclude unwanted aggregation. Silica too, which is optically transparent can be used to coat nanoparticles and provide a surface that can be further functionalised. Unfortunately, the chemical technology for making solid gold particles and using the current approaches to stabilising them does not lend itself to the galvanic reaction approach commonly used to make hollow gold nanoparticles. The team has now circumvented the problem by successfully adding the different stabilising agents - polymer, silica or sugar - as a capping agent at a different stage in the synthesis.

They used extinction spectroscopy and dynamic light scattering to test which of the stabilising agents was most effective, and supported their findings with scanning electron microscopy (SEM). Ultimately, PEG emerged as the most effective agent in stabilising HGNs against salt solutions and wild pH variations. Moreover, they demonstrated SERS detection at 785 and 1064 nanometres and thus showed how HGNs could have a bright future in a range of medical and biomedical applications including the aforementioned drug delivery and SERS imaging.

In a separate paper, Graham's team has shown SERS at 1280 from HGNs. "Nothing else worked at that wavelength anywhere close to the sensitivity we could find with these dyes and the HGNs although large solid Au does work with the dyes but not as well," he told SpectroscopyNOW. "We now want to combine the 1280 capabilities with the PEG coating to look at in vivo assessment of these nanoparticles and their ability to report at depth in tissue." He points out that that additional work will depend on securing the necessary funding for the research.

Related Links

Nanoscale, 2015, 7, 6075-6082: "Functionalisation of hollow gold nanospheres for use as stable, red-shifted SERS nanotags"

Chem Sci, 2015, online: "Extreme red shifted SERS nanotags"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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