Professor Xing Bengang (seated) and his team (from left) Huang Ximin, Liu Songhan and Szeto Mun Wai, Dominic, in their laboratory. Photo credit: M. Fadly.
As the world’s population increases over the coming decades, the demand for food is projected to increase as much as 120 percent by 2050. For Singapore, a small country with limited land and natural resources, increasing the efficiency of its domestic food production is an increasingly urgent concern.
The efficiency of food production, whether in Singapore or elsewhere, depends fundamentally on the effectiveness of photosynthesis, the biological process by which green plants and microorganisms convert sunlight into chemical energy. Practically all the energy in the food we eat originates from photosynthesis, either directly (e.g., by eating green plants) or indirectly (e.g., eating animals that ate plants).
Photosynthesis, as it has evolved in Nature, is far from perfect. One of its biggest limitations comes from the fact that the biological pigments used in photosynthesis absorb green and ultraviolet light poorly. As a result, they end up wasting over 50% of the energy available in sunlight.
Now, a team of chemists from Nanyang Technological University, Singapore (NTU Singapore) have devised a practical way to improve the efficiency of photosynthesis. The new method uses phosphorescent nanoparticles mixed with photosynthetic microorganisms, and has the potential to improve the industrial production of foods or biofuels. It does not require the use of genetic engineering to alter how the microorganisms perform photosynthesis.
Phosphors are chemicals that absorb light at one frequency and re-emit it at a different (usually lower) frequency. An everyday technology in which they are encountered is white LEDs (light emitting diodes), which use phosphor coatings to convert the harsh blue light generated by an electrical diode into “broad-spectrum” white light.
Professor Bengang Xing, a faculty member at NTU’s School of Physical and Mathematical Sciences, had the idea of using phosphors to enhance photosynthesis by “recycling” the light not absorbed by photosynthetic pigments. The phosphor can re-emit the light at frequencies more readily absorbed by the pigments, thereby increasing the overall proportion of the sunlight energy captured for photosynthesis.
The Xing group performed experiments with cyanobacteria to show that their photosynthetic activity can be enhanced using specially-designed phosphors (powders in the bottles in the foreground). Photo credit: M. Fadly.
However, the chemicals most commonly used as phosphors are not suitable, as they do not convert between the frequencies of light most relevant for aiding photosynthesis. To overcome this problem, Prof. Xing and his team developed a special phosphor based on a kind of nanoparticle called a “core-shell structure”, designed to simultaneously convert ultraviolet light to blue light and green light to red. Using spectrometry, they verified that the new “nanophosphors” convert light frequencies that are poorly absorbed by photosynthetic pigments into frequencies that are readily absorbed.
Next, the researchers needed a way to bring the nanophosphors into contact with photosynthetic cells. To accomplish this, they synthesized a three-dimensional nano-scaffold – a porous material in which photosynthetic cells can grow while surrounded on all sides by nanophosphor particles.
“The combination of nanophosphor and nano-scaffold serves as an integrated solar power management platform,” says Prof. Xing. “By fine-tuning both components, we can optimise the efficiency of photosynthesis taking place inside.”
To prove that their scheme works, Prof. Xing’s team grew photosynthetic microalgae using the porous nano-scaffold as a culture medium. The samples containing nanophosphors grew five times the amount of microalgae over the duration of the experiment, indicating that their natural photosynthetic processes were greatly enhanced by the nanophosphors, as predicted.
The newly developed nanophosphors have another intriguing advantage. After absorbing ultraviolet or green light, they do not re-emit instantaneously, but over a period of 2 to 3 hours afterwards. This “afterglow” can drive photosynthesis even after the supply of sunlight is cut off. This could be useful in industrial applications, by allowing photosynthesis to continue during cloudy or rainy weather.
Next, the team aims to explore the use of the nanophosphor platform for performing large-scale culturing of cyanobacteria, microorganisms that can be used as inputs to food production and other industries. If these efforts are successful, it may allow food, biofuel, and other products created from photosynthesis to be produced even in places like Singapore, which are unsuitable for large-scale conventional agriculture.
Close-up view of the nanophosphors developed by the Xing group, which exhibit strong fluorescence under ultraviolet light. Photo credit: M. Fadly.
About the author:
Xing Bengang is a Professor in the Division of Chemistry and Biological Chemistry at the School of Physical and Mathematical Sciences and the School of Chemical & Biomedical Engineering, Nanyang Technological University, Singapore.