THURSDAY, 3 JUNE 2021When we think of gold, many of us will see a gold ring, or perhaps an expensive item of jewellery. But what if we think about gold in a scientific way? A-Level chemistry tells us that gold has an atomic number of 79, occurs naturally in a pure state. It is soft, malleable, and is inert, which makes it resistant to corrosion. This is probably why we never used it in our chemistry experiments (nothing to do with budgeting!). However, when we use very small quantities of gold (nanoparticles), we discover new, magical properties that make it valuable in applications from environmental science to medicine...
Gold catalytic converters?
If we look at the structure of gold at the nanometer scale, the properties of gold can be manipulated. This was initially discovered by Dr Haruta of the Osaka Government Industrial Research Institute, who showed that smaller gold nanoparticles supported by metal oxides can exhibit a catalytic activity and facilitate the conversion of carbon monoxide to carbon dioxide.
Before Dr Haruta, this catalytic activity had been reported but at low levels. By using smaller gold nanoparticles, Haruta displayed a greater efficiency of catalytic activity. This discovery revealed how nanoparticle size can affect the functional properties of gold.
A golden way of delivering drugs
Gold nanoparticles are now being used as drug delivery systems. Through controlled fabrication, gold nanoparticles sized 1-150 nm (1 nm is one billionth of a metre) can be bound directly to drugs. This binding can be applied to several different metals, however, the key benefit to using gold nanoparticles is that the gold “core” is non-toxic, biocompatible and inert thus will not promote an immune response in the body. Prodrugs are drugs that are inactive when administered and are then metabolised into pharmaceutically active drugs in the body. The conjugation of a prodrug to a gold nanoparticle enables the delivery of a drug to a cell and the drug can then be released to the cells via external stimuli such as UV radiation.
This approach is now becoming applicable in genetic modification. DNA is bound to gold nanoparticles via complementary charge interactions, and UV radiation is used to reverse the charge on the gold nanoparticle, releasing the DNA . This has been used for targeted gene delivery as once the DNA is removed, the gold nanoparticle will have no hazardous side effects within the body and is excreted.
These studies highlight the versatility of gold nanoparticles and how joining other molecules to the surface can allow drug delivery of gold.If we go further, to nanoparticles as small as 30 nm, we can begin to manipulate the electron properties of gold...
Surface plasmon resonance – the science
The use of gold nanoparticles relates to their basic photophysical responses that do not exist in nonmetallic particles. When gold is exposed to light (an oscillating electromagnetic field), the light induces a collective, coherent oscillation of electrons in the gold nanoparticle. This electron oscillation around the nanoparticle surface induces a charge separation across the nanoparticle. The amplitude of the oscillation reaches its maximum at a specific frequency, this is termed the surface plasmon resonance. Surface plasmon resonance induces a strong absorption of the light, which can be measured using a spectrometer.
The surface plasmon resonance is much stronger in gold than other metals due to its electron structure. The surface plasmon resonance intensity and wavelength depends on the factors affecting the electron charge density on the particle surface such as the metal type, particle size, shape, structure, and composition.
Absorption or Scattering
Upon light striking a nanoparticle, there is energy loss. The two contributors of this loss are absorption and scattering. When light is absorbed, the energy is lost to the surroundings and converted to heat. On the other hand, scattering occurs when the energy causes electron fluctuations in the nanoparticle which in turn emits a form of scattered light.
The absorption and scattering of light is largely dependent on the size of the nanoparticles. For the smaller gold nanoparticles, nearly all of the energy loss is contributed by absorption. However, the ratio of light scattering to absorption increases dramatically for larger particles and this sliding scale guides the choice of which gold nanoparticle to use in science. Due to their higher scattering ability, larger nanoparticles are preferred for biomedical imaging, whereas smaller nanoparticles are preferred for photothermal therapy, as light is mainly adsorbed and is efficiently converted to heat for cell and tissue destruction.
Gold nanoparticles for photothermal cancer therapy
Gold nanoparticles can be used in treatment of skin cancer. Cancer cells are cells that are growing uncontrollably and they can be recognised because they display certain cancer-specific markers on their surface. In research laboratories, antibodies recognising these markers can be manufactured and conjugated to small gold nanoparticles. The antibodies will direct the gold nanoparticles specifically to cancer cells in the skin. Upon exposure to a light source, the gold nanoparticles will convert the light to heat, killing the cancer cells. This treatment does not cause harm to normal cells, which is a major problem with other non-targeted cancer treatments, such as chemotherapy.
Gold nanoparticles can also be used as medical imaging aids. For example, in regenerative stem cell therapies, the tracking of injected stem cells within the body is vital in order to understand the location and mechanisms of their therapeutic effects. By labelling stem cells with large gold nanoparticles, light scattered from the particle can be used to track the stem cell within the body.
A golden renewable energy source
Gold nanoparticles have revolutionised renewable energy by increasing the efficiency of producing hydrogen power. Currently, titanium dioxide is used as a catalyst in hydrogen production, requiring illumination with ultraviolet (UV) light. However, this is not efficient due to low levels of natural UV light. Instead, scientists now use gold nanoparticles in the form of tiny stars, which are coated with a semiconductor to give them the capacity to generate hydrogen from water. This is four times more efficient than gold-free methods. The gold nanoparticles can be excited using visible or infrared light from the sun, causes the electrons to oscillate (surface plasmon resonance). The oscillating electrons then break the chemical bonds in water to form hydrogen ions and oxygen. The gold can donate electrons to the hydrogen ions (H+) to produce hydrogen atoms (H), which can be stored until required. Therefore, the gold nanoparticles act as photocatalysts, using sunlight to make chemical reactions occur faster and more readily in renewable energy devices! Furthermore, the ability to store hydrogen means the major drawback for using solar panels (the intermittent availability of a solar source) is overcome.
When we think of gold, we normally think ‘the more of it there is, the more valuable it is’. However, when we consider gold on the nanometre scale, its chemical properties seem much more valuable than its use in jewellery. Gold can be used for drug delivery systems, cancer therapies, stem cell tracking devices or even to solve the issues of inefficient renewable energy sources. Maybe there is some magic in a sprinkling of gold dust?
William Hotham is a 3rd year PhD student in surgery at St. Catharines' College. Artwork by Josh Langfield