Colour is perceived based on the amount of light absorbed. The colour we see in front of us generally means that the dye used is absorbing all of the colours in the spectrum apart from that particular colour. But structural colour is instead the opposite, and is based on reflection. The iridescent colours produced are made from the microscopic geometry of the material, creating optical properties that reflect light’s wavelength. This phenomenon can be found in the natural world, such as on butterfly wings, in opals or on peacock feathers, where it is an evolutionary development for survival skills such as camouflage, photosynthesis, and mating. But we are now able to replicate these colours using bacteria, which could be used in a variety of applications in the future to create beautiful optical effect colours without using toxic chemicals.
Structural colour uses ordered nano-structures to create colour instead of pigments. These nano-structures interact with light, to alter and ‘reflect’ it, creating a rainbow of colours that change with movement. This type of colour has been found in fossils dating over 500 million years ago, showing that they do not fade. Structural colour has been researched for centuries, with Robert Hooke being one of the first to describe structural colour in his book Micrographia, which discusses the iridescence of the fly. But this natural phenomenon has been inaccessible until recently due to the difficult nature of the organisms that create these vivid colours, meaning the genetics were largely unknown.
Chemists at the University of Cambridge and biotechnology company Hoekmine BV, have created almost metallic structural colour by mutating bacteria. They have become the first to identify the exact genes that are responsible for structural colour. The researchers are able to change the dimensions of bacteria and their ability to move, to change the wavelengths of light they reflect. Colonies of a rod-shaped bacteria called flavobacterium were used in the study, which are usually found in soil and freshwater. After these colonies are modified they are able to produce any colour of the rainbow, and the intensity of each colour can also be changed.
Bacteria are used to produce structural light as they reflect 2D Photonic Crystals when they are organised into colonies. They are easily accessible due to CRISPR gene editing, where parts of the genome are edited by removing, adding or altering sections of the DNA sequence. Bacteria are also cheap to produce. The flavobacterium bacteria holds promise in creating non-toxic pigments on many different commercial products, as they can grow in only 24 hours and can easily be produced in volume. And now that they can be easily modified they can be created in any colour on demand. This discovery means that these structural colours can also be produced cheaply, by growing the bacteria on low-cost biomass such as cellulose based by-products.
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