The name – golden color comes from the element gold. But is gold always golden? In fact gold isn’t just golden, have you ever heard of black gold? It was the blackest man-made material before vertically aligned carbon nanotubes (vantablack) took the crown and was made from gold nanoparticles.Black Gold

The Color of Gold

Gold is golden but the gold nanoparticles, the tiny particles of gold with a diameter around 10 millionth fraction of a meter may have different colors according to the size of particles. For instance, it can be black as stated above.

Science is insane, isn’t it? A thing that appears so black in nanoscale appears golden in bulk! The color is a mere parameter representing the absorption of light by an object. Meanwhile, the absorption of several materials can be tuned by changing their shape, size and aggregation state at the nano level.

There are seven visible colors; violet, indigo, blue, green, yellow, orange and red. We see a single color because the object reflects particular colors while absorbing others. We see other color different from these seven if the object reflects a combination of different colors. For gold nanoparticles, the color can be a range of colors from red to black.

Not just gold but usually, for materials, the picture is different when we reach the nano-world. The bulk material’s properties are not the same as nanomaterials. Like gold, the two-dimensional sheet of carbon, graphene also behave very much differently from its bulk counterpart, graphite. It is transparent despite graphite being grey or black.

The Black Gold

Something is black if it absorbs all the visible light radiation and so does the black gold. It absorbs almost every incident radiations upon it, from infrared to visible.

Scientists at King Abdullah University of Science and Technology fabricated such black gold nanoparticles with the highest absorption coefficient. It was published in Nature Nanotechnology in 2015 and the black gold registered its name on Guinness book of world record for blackest man-made material.

The black gold was composed of gold nanorods (length about 80 nanometers (nm) and diameter 20 nm) and nanospheres (diameter about 30 nm). These particles were arranged in a delicate shape to ensure that the absorption was maximum. And eventually, they absorbed 98.4% of light in a range of visible and infrared light with maximum absorption 99.7% at wavelength 850 nm. This is the reason why these particles appeared black.

Different colors of Gold Nanoparticles

The black gold is just one form of colored gold nanoparticles. Apart from black color, these nanoparticles can have several colors as red, pink and purple. By tuning the shape and size properly, we can even synthesize colorless, translucent gold nanoparticles. The absorption takes place in the infrared region in this condition.

But, we must distinguish between bulk gold and gold nanoparticles. The bulk gold appears golden, no matter how you cut it. So, the color of gold is golden, not black or red or purple.

What makes Gold nanoparticles colorful?

Different colors of gold nanoparticles can be explained by a phenomenon called surface plasmon resonance. This phenomenon is caused due to the matching of frequency (resonance) of oscillations between the surface electrons of gold nanoparticles and the visible light.

Generally, the electrons’ oscillation frequency is not large enough to match with the frequency of light. But, the surface electron has increased frequency by the interaction of nearby light due to the variation in permittivity at the interface. This resonance phenomenon enables it to absorb particular light and reflect a particular one. As the size increase, the resonant frequency decreases and absorption shifts, resulting in various other colors.

Moreover, gold nanoparticles and bulk gold are different materials. These surface effects are negligible in a bulk material. Bulk gold gets its color from a different electronic transition on contrary to surface plasmon resonance.

Applications of colored gold nanoparticles

Colored gold nanoparticles have various use in fields like sensing, cancer treatment, microscopy, etc. Due to varieties of colors, it can be used as a colorimetric sensor. Their light scattering property is useful in microscopy for imaging. Also, the absorption of infrared radiation can induce heating in these particles. This property show potential in hyperthermia therapy for killing cancerous cells.

Further, Black gold can emit a monochromatic light, absorbing all the multiple wavelengths of light when it is exposed to one. This can be used as a coherent source of radiation.

Ashwin Khadka is a PhD Scholar in Nano Energy and Thermofluid Lab in Korea University, Republic of Korea under Korean Government Scholarship Program. He has a Masters Degree in Physics from Tribhuvan University, Kathmandu, Nepal. He is a science enthusiast, researcher and writer.