The Green Light: artificial light-harvesting systems

The Green Light: self-assembling artificial light-harvesting systems

Two recent articles in the field of nanotechnology highlight progress towards development of  artificial light-harvesting systems that have the potential to mimic the function of chlorophyll in photosynthesis – leading to a new generation of green fuel cells.

In nature, chlorophyll is one of the major pigments responsible for absorbing light energy from sunlight in a process called as photosynthesis – it is also responsible for the green colouration of leaves.  When light energy is absorbed by chlorophyll, electrons are excited and passed down to the primary electron carrier. The primary electron carrier passes the electron down electron transport chain, which ultimately results in the production of chemical energy as ATP.  The chlorophyll pigments  represent one of the most efficient light-harvesting systems in nature.

Current technologies only allow us to harness a miniscule fraction of light energy that falls on earth. One of ways of improving efficiency is to understand how plants arrange their photosynthetic pigments to maximise light absorption and minimise energy loss.

Transfer of electrons in the photosystem. (c)

Two groups have recently reported successes in trying to mimic the spatial arrangement of the chlorophyll pigment. The first group, led by Shana O. Kelley at the University of Toronto  have recently published an article which proposes utilising quantum dots with DNA binding sites to achieve control over the arrangement of light-absorbing pigments.  Quantum dots are nanosized crystals, about the width of 50 atoms, which can be tuned to absorb and emit light at different frequencies.

The scientists used a form a DNA origami, which takes advantage of the specificity of DNA base-pairing to arrange quantum dots in 3-D space at defined distances from each other. One initiated, the system is self-assembling.
Light energy can then be channeled along the array and ultimately harvested – changing the type and arrangement of quantum dots can modify the wavelength and intensity of light generated.

Using quantum dots to funnel light energy. (c)

The second system, developed by Yan Liu and colleagues once again draws on DNA arrays to connect chromophores (light-absorbing pigments).

Using self-assembling DNA systems, chromophore triads, consisting of a primary donor, secondary donor and a final acceptor are brought close to each other to enable electron transfer – much like how light is transmitted in the photosystems of chloroplasts in leaves. This generates efficient cascading energy transfer system with, with the ability to control the distance between the chormophores at nanometer resolution.

Building chormophore triads usind self-assembling DNA. (c)

In the age of fossil fuel dependency, these studies provides  exciting new avenues for carbon-neutral, and even potentially carbon-negative, fuel generation – and one that can be manipulated with great precision.

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