Gold nanoparticles from plants

In a corner of a laboratory at Massey University’s Institute of Technology and Engineering, plants are growing under bright lights. They look out of place amongst all the high-tech equipment, yet if you could look closely enough inside these plants, you would find something amazing – particles of gold just a few nanometres across.

Phytomining

A few years ago, scientists in New Zealand helped develop a process called phytomining. Plants are grown on gold-rich soil, such as waste from mines. They take up the gold and store it as nanoparticles throughout their tissues. The plants are harvested, and the gold is recovered and accumulated into ‘normal’ gold. Phytomining can also be used to recover other metals.

Gold catalysts

At Massey University, Professor Richard Haverkamp and Dr Aaron Marshall realised that, if gold recovered from plants was kept as nanoparticles, it would be more valuable than normal gold. This is because gold nanoparticles are very active catalysts, helping along a number of chemical reactions, and gold catalysts are expensive to make.

Richard and Aaron received money for their research from an unlikely source – part of their funding was from the United States Air Force. So what could interest the military in plants and gold nanoparticles? The answer is that the work could end up providing portable fuel cells for soldiers.

Unlike batteries, fuel cells need a constant supply of fuel. They need pure hydrogen, and the commonest source of this at the moment is hydrocarbons, such as natural gas. The process also produces carbon monoxide. This is a problem because carbon monoxide ‘poisons’ the platinum catalyst that is used in a fuel cell. It becomes less efficient and eventually stops working.

Gold could replace the platinum as a catalyst in the fuel cells. Not only is it cheaper, but gold is not affected by carbon monoxide.

The military are interested because fuel cells could replace batteries in portable equipment such as radios. It is hard for soldiers to recharge batteries in the field. Fuel cells would be a portable power source that you just pour hydrocarbon fuel (such as methanol) into to keep producing electricity.

Why gold from plants?

Gold nanoparticles can easily be got by mixing chemicals in a beaker. However, there is another problem. In a beaker, the gold nanoparticles tend to clump together and grow in size, and so become less reactive as a catalyst. Surfactants can be added to restrict particle size – these bind to the surfaces of the nanoparticles, surrounding them and stopping them from growing. However, that means using yet more chemicals, so growing plants is a more environmentally friendly way of getting gold nanoparticles.

When Richard and Aaron started their experiments, they hoped that plants would produce nanoparticles of a more reliable size. However, they still got quite a lot of variety, with particles 5–50 nanometres (nm) across.

It was also thought that the gold nanoparticles would be more active catalysts than those obtained chemically, but this has not been the case so far.

How much gold?

Richard and Aaron grow Brassica juncea, also known as Indian mustard, hydroponically (in water and nutrients, without soil), with gold chloride added to the nutrient solution. They use enzymes to break down the plant cells to extract the gold.

They have managed to get the equivalent of 1 gram of gold from 10 kilograms of dried plants. This may sound a lot, but it is still too small to be commercially useful at the moment.

Changing aims and a world first

The results of their early experiments caused Richard and Aaron to change the focus of their work. They found that the amount of gold recovered from plants is small, and there is no proof that the gold nanoparticles are more active catalysts than those got by traditional methods, so they changed the aim to finding out how plants make nanoparticles. This knowledge will help the control of the process in future.

Part of the new direction has involved getting plants to make nanoparticles of other substances. Richard and Aaron were the first in the world to produce alloy nanoparticles (gold-silver-copper) in plants.

Such alloys have potential as catalysts, due to the way catalysts work. Some chemical reactions have several steps, each needing a different catalyst. If scientists can engineer ‘designer catalyst’ mixtures, such as nanoparticle alloys, they will be able to control not just the speed of complex reactions, but also the end products.