Nanotechnologies

The Nano-Metamorphoses of Gold

Regarded for thousands of years as the ‘precious’ metal par excellence, gold is of increasing interest to the scientific community on account of its functional properties, irrespective of its symbolic significance. At the nanoscale level, it exhibits physical and chemical properties that researchers and industrialists alike are enraptured by. Cancer treatment, pollution reduction or the miniaturisation of electronic components are what is at stake in this new ‘gold-rush’.

The Adonis Project – At the nanoparticle level, gold can be bio-functionalised, or linked to the antibody’s organic matter, as it has the property of binding itself to certain cells in living tissue. The Adonis Project – At the nanoparticle level, gold can be bio-functionalised, or linked to the antibody’s organic matter, as it has the property of binding itself to certain cells in living tissue.
© Mass Spectrometry Laboratory – University of Liège (BE)
Research on the MINT project aims to design, within the framework of nanometre-scale integrated transistor circuits, the formation of nano-connections made up of conductive gold nanoparticles. To do this, it uses RNA molecular surveillance tools, which make it possible to bring about a suitable, controllable auto-assembly of the nanoparticles (Fig. C). Fig. D shows how this auto-assembly process is obtained within the nanometric space separating two nano-electrodes. Research on the MINT project aims to design, within the framework of nanometre-scale integrated transistor circuits, the formation of nano-connections made up of conductive gold nanoparticles. To do this, it uses RNA molecular surveillance tools, which make it possible to bring about a suitable, controllable auto-assembly of the nanoparticles (Fig. C). Fig. D shows how this auto-assembly process is obtained within the nanometric space separating two nano-electrodes.

Inert and rust-proof (which is why it has been used in medical applications since time immemorial), a good electrical conductor, valued for its chromatic variations and capacity to bond with organic molecules – these are some of the classic attributes of gold. But when you get down to the nanoparticle level, the precious metal reveals capabilities undreamt of until a short time ago. It has been established that, at this scale, it displays an unsuspected catalytic power and even has the ability to become a semiconductor.

In a metallic mass of gold, the passage of an electric current results from the movement of conduction electrons roaming freely in the metal. However, when gold nanoparticles are isolated, their electrons adapt their movements to the finite dimensions and shapes of the nanometric environment, and present extremely interesting electromagnetic properties, paving the way to numerous optoelectronic applications.

Affinity with the living

Gold has been used in medicine because, when in contact with human tissue, it retains its inert and rust-proof qualities. Today, added to this is its newly discovered capacity for ‘biofunctionalisation’, or ability to absorb antibody or antigen proteins into its surface. When injected into the body, gold particles attach themselves to specific sites, responding to the specialist proteins to which they are bonded. If the gold were to attach itself to, say, cancer cells, they could be easily detected and even destroyed by heating the gold particles using infrared radiation (see box).

This capacity for gold particles to attach themselves to organic material is also used in the manufacture of rapid diagnostic kits, serving to detect the presence of toxic agents, allergens or microbes in bodily fluids (blood, saliva). Specifically biofunctionalised to target a particular agent, the gold particles will accumulate when they come into contact with it. Their red colour allows researchers to easily spot them through a series of dedicated tests.

Detection and elimination of toxic gases

In addition to its medical application, the catalytic qualities of gold at the nanoscale make it highly applicable in other sensor applications. Even if the catalytic mechanisms are not always perfectly understood, this property is nevertheless exploited to improve gas sensors. The European consortium Nanogas has found, for example, that by adding gold particles to a tin oxide capable of detecting the presence of carbon monoxide (CO), which is odourless yet fatal, the sensor’s sensitivity is increased by speeding up the transfer of electrons. The sensor gains an electron when brought into contact with a molecule of CO, modifying its electrical conductivity and transmitting a signal.

The catalytic power of the gold nanoparticles is also exploited to transform, and therefore filter, carbon monoxide or certain other toxic gases. In the presence of oxygen, gold is the only metal capable of oxidising CO at ambient temperature to produce CO2, the toxicity of which is negligible. Studies are being conducted into other procedures aimed at reducing nitrogen oxides or oxidising methane. If successful, industrialists hope to capitalise on the findings to improve the performance of catalytic converters or gas mask filters. Gold particles are already being used in the manufacture of anti-odour filters, e.g. in Japan where they are used for freshening toilets.

Tomorrow – nanoelectronics

Another area, and by no means an insignificant one, in which the nanometre is fast becoming the unit of measurement, is electronics. For David Cumming, coordinator of the European MINT project, ‘integrated circuits are going to reach a scale of around 10 nanometres and new manufacturing methods are being studied.’ It was this that gave rise to the idea of using proteins such as DNA or RNA to build electronic circuits on this scale. The manufacture of nano-cables formed from gold particles linked by strands of RNA and the use of RNA networks to act as masks before gold metallisation are just two of the areas of research already underway. It remains to be seen whether or not the electrical conductivity properties of these hybrid materials can depose silicon from its throne.

If, as is believed by Pierre-François Brevet, head of the Gold Nanoparticles Characterisation Methods division of the Gold-Nano Research Group (National Centre for Scientific Research, France), gold particles are ‘inert and do not initiate chemical reactions in the biological environment – which justifies their biomedical use’, are they, nevertheless, harmless? For the researcher, the risk would be more closely linked to the molecules used for biofunctionalisation, a process closely monitored by regulations on new molecules or drugs. Despite gold’s many attributes and advocates, there is currently no guarantee that the accumulation of gold in the body is completely harmless.

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Sound and light against cancer

Adonis project – An optoacoustic excitation process in which biofunctionalised gold nanoparticles are attached to certain target cells generates ultrasounds that reveal pathological cellular configurations. Adonis project – An optoacoustic excitation process in which biofunctionalised gold nanoparticles are attached to certain target cells generates ultrasounds that reveal pathological cellular configurations.
© Institute of Applied Physics – University of Bern (CH)

Whilst being yellow in its solid state, gold turns red, purple and even blue when it is reduced to nanometric particles. This property, acknowledged for centuries by craftsmen using fine gold powders to make the colour of glass change with the light, is used these days to detect cancerous cells. At very precise dimensions (from 5 to 10 nm), gold particles react to infrared laser emissions by reflecting part of the energy in light form, while the rest is converted into heat. After having biofunctionalised them with specialised antibodies to target specific antigens in diseased cells, it is then possible to ‘illuminate’ them, thanks to the infrared light that passes through the biological tissue. They can thereby be detected with the help of magnetic resonance imagery. Nevertheless, in cases such as prostate cancer, ‘these detection techniques still remain imprecise when it comes to determining the state of advancement and progression,’ cautions Robert Lemor, coordinator of the European project Adonis, dedicated to what is one of the most commonly occurring diseases in males.

This year-old European project also uses ultrasound to locate gold nanoparticles. When subjected to certain infrared frequencies, they emit a sound resulting from the expansion or contraction of the material. ‘The coupling of optical and acoustic detectors offers greater accuracy; acoustic methods penetrate the tissue far more deeply,’ explains Robert Lemor.

But detection is simply the first stage, as the consortium also envisages therapeutic procedures. By fine-tuning the light’s wavelength, as well as the size and shape of the nanoparticles, the thermal part of the energy restored relative to the light emission can be increased. While attached to the cancer cell, the particle is heated. At under 60º C, the membrane’s permeability is altered, thereby destroying the cell. This procedure, which is currently undergoing in vitro testing, could be applied to various tumours by identifying good targets for each one to which gold nanoparticles could be attached.



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