Scientists strive for gold nanoparticles

Materials World magazine
,
2 Mar 2016

New research finds that gold nanoparticles give clues to valuable mineral deposits and could identify cancer and Alzheimer’s disease, as Simon Frost reports.

Gold has been one of our most precious commodities for at least 6,000 years, but many of the Earth’s near-surface deposits have already been exploited, and others buried by younger rock sequences. Cue gold nanoparticles, which can indicate the presence of gold deposits, and offer new applications in sensors, targeted drug delivery, gene therapy and cancer treatments. 

Such applications have made gold one of several materials that are becoming eminently useful in nanoparticle (NP) form, and new research into its on-site detection at the parts-per-billion scale aims to allow prospectors to strike gold thanks to geochemical clues in rock samples, soil and even the cells of plants that grow nearby.

Published in Sensors and Actuators B: Chemical in December 2015, Detection of Gold Nanoparticles with Different Sizes Using Absorption and Fluorescence Based Method (visit bit.ly/1QbHSd0) a study carried out at the University of Adelaide, Australia, investigates the use of portable optical methods to overcome the difficulties in detecting gold at such small concentrations in the field. 

Detection at the parts-per-billion scale via atomic emission spectroscopy or parts-per-trillion using inductively coupled plasma mass spectrometry require rock samples to be transported to a laboratory, crushed and subjected to fire assays, aqua regia and cyanide digestion to leach gold from the sample, making them costly and time consuming detection routes. Moreover, portable devices such as X-ray fluorescence-based sensors are not sufficiently sensitive, detecting only at the parts-per-million scale. ‘Thus, there is growing interest in developing a new, easy-to-use and fast method for detection of low concentrations of gold at the site of an exploration drilling rig,’ the paper states.  

Absorption and fluorescence

Led by Dr Agneszka Zuber and sponsored by the Deep Exploration Technologies Cooperative Research Centre, Australia, the researchers compared the effectiveness of absorption and fluorescence analysis methods using both lab-based and handheld spectrometers. They found that the limit of quantification for detecting gold NPs via the fluorescence method was roughly the same using both laboratory and portable equipment – it was possible to detect gold NPs of 5 nm diameter to a sensitivity of 74 ppb, and 1,200 ppb for detecting 50 nm-diameter NPs. 

Using absorption, the opposite relationship between NP size and detection limit was apparent – there was a similar detection level of 71 ppb for 5 nm NPs, but the sensitivity increased to 24.5 ppb for 50 nm NPs. The portable absorption method was around seven times less sensitive than in the lab.

‘The crucial finding towards achievement of a low detection limit with the fluorescence method was the importance of the concentration of the fluorophore,’ Zuber says. Gold NPs in a solution of water with the fluorophore I-BODIPY acted as catalyser for the reaction creating fluorescent H-BODIPY, allowing the researchers to measure the concentration of NPs based on the intensity of the fluorescence. 

It’s not only gold that the methods could benefit, Zuber tells Materials World. ‘Silver NPs also show a surface resonance peak on the absorption spectrum, but at a different wavelength to gold, which allows distinguishing between these metals,’ she says. Silver NPs are already used in antibacterial fabrics and targeted drug delivery, and are being developed for several nanomedical applications. 

‘The chemical design of fluorophores enables the targeting of different elements, such as zinc, aluminium, calcium and cadmium,’ she adds. The next step for the research is to test the methods on rock samples, and the researchers hope to have a field deployable method demonstrated by the end of 2017. 

Porphyry detection

A recent study by the University of Exeter’s Camborne School of Mines (CSM), UK, has similarly used the geochemical signature transferred to identify deposits using minerals from proximate rocks – in this case, porphyry deposits and magmatic rock (visit bit.ly/1Qc0wYC). Such deposits are rare and increasingly difficult to discover, but they provide around 75%, 50% and 20% of world copper, molybdenum and gold, respectively. 

Dr Ben Williamson, Programme Director and Senior Lecturer in Applied Mineralogy at CSM collaborated with Prof Richard Herrington, Head of the Earth Scientists Department at the Natural History Museum, UK, to examine the chemical differences between the magmatic plagioclase from fertile (porphyry-associated) and barren magmatic systems, and found that fertile systems could be identified by an excess of aluminium. It included a case study of a major porphyry discovery in Chile, which helped to corroborate the findings.  

‘This new method will add to the range of tools available to exploration companies to discover new porphyry copper deposits,’ said Williams. ‘Our findings also provide important insights into why some magmas are more likely to produce porphyry copper deposits than others, and add to our understanding of how their parent magmatic rocks evolve.’  

The uses of nanoparticles

Applications of gold NPs are notable in biomedical research – their enhanced absorption and scattering compared with bulk gold make NPs useful in photothermal cancer therapy. Incidentally, this light scattering property was exploited (although not understood) by medieval artisans, who added gold chloride to molten glass to create ruby-red stained glass. 

In 2011, research at the Institute of Biochemistry and Physiology of Plants and Microorganisms, Russia, found in rat models that the localised accumulation of gold NPs administered to a patient could help to quickly diagnose cancers, Alzheimer’s and Parkinson’s disease. 

But functionalising NPs, which can be several nanometres in diameter, presents its own challenges. Chemists at McGill University, Canada, are developing a method to create clusters ranging from a few particles to crystals made of millions of NPs, while controlling their shape. 

Synthetic DNA strands, which are programmed to pair with other strands in certain patterns, can be attached to gold particle surfaces to create a variety of assemblies, but the expense of creating such intricate hybrid nanostructures limits their potential. The McGill researchers are now aiming to lower this cost with the ‘nanoparticle equivalent of a printing press’. 

‘In much the same way that atoms combine to form complex molecules, patterned DNA gold particles can connect to neighbouring particles to form well-defined NP assemblies,’ said lead researcher, Dr Thomas Edwardson. The key to the assembling these is to create a DNA structure with a specific pattern of strands to which gold nanoparticles can adhere. This assembly is then dissolved in distilled water, leaving behind an imprint of the DNA on the restructured NPs. 

‘These encoded gold NPs are unprecedented in their information content. The DNA nanostructures, for their part, can be re-used, much like stamps in an old printing press,’ says senior author Dr Hanadi Sleiman. Their paper, Transfer of molecular recognition information from DNA nanostructures to gold nanoparticles, published in Nature Chemistry (visit bit.ly/1mwMHGh), states, ‘Importantly, the NPs produced exhibit the site-specific addressability of DNA nanostructures.’ The patterns of the strands could be engineered, for example, to target specific proteins to selectively detect or destroy cancer cells. 

The widely established applications of gold NPs include conductive inks, electronic chips, sensors and chemical catalysts, but their potential, it seems, is yet to be fully realised. Increased ease of their use as indicators of gold deposits could accelerate that development.