Researchers in the UK compared the growth of plants in greenhouses sprayed with their new spray paint (right) with that of plants in untreated greenhouses (left). [Image: Lambda Agri]
Regions far from the equator receive less light for agriculture and have a shorter growing season than those in equatorial climes. Even greenhouses require supplementation with energy-intensive lighting.
Now, scientists in the UK have developed a spray paint that boosts the amount of usable sunlight coming in through greenhouse windows (Adv. Mater. Technol., doi: 10.1002/admt.202400977). In proof-of-concept trials, basil plants in greenhouses coated with the new material produced 9% more leaf dry weight per plant.
Photosynthesis, sunlight and europium
From a plant’s perspective, not all wavelengths of sunlight are created equal. Ultraviolet photons with wavelengths from 280 to 400 nm have a negligible effect on photosynthesis. On the other hand, plants love light in the range of 400 to 700 nm, particularly on the red and blue ends of the spectrum. A passive mechanism for converting UV photons from sunlight into blue photons would boost the amount of photosynthetically available radiation and increase crop yields without the need for artificial lighting. Scientists call this conversion luminescent downshifting (LDS).
To develop a suitable acrylic-based coating, the research team from the Universities of Bath, Cranfield and Cambridge and a startup company, Lambda Agri, needed a compatible LDS compound. The scientists found luminophores called europium-containing polyoxotitanium cages, which are large, soluble molecules containing trivalent ions of europium.
According to Cambridge chemist Dominic S. Wright, one of the study’s corresponding authors, the presence of the lanthanide element europium is crucial, as it has a variety of f-states that contribute to the luminescence of the molecular cages.
“We originally looked at the literature and were interested in examples which had been reported where there is a high quantum yield [the ratio of the light supplied to the amount down-shifted to lower wavelength in the luminescence process],” Wright says. “We based our studies on this set of compounds by changing the solubilizing groups and [in particular] the antenna ligand that is responsible for light harvesting and energy transfer to the lanthanide f-states during the luminescence process.”
Making downshifting practical
According to Cambridge chemist Dominic S. Wright ... the presence of the lanthanide element europium is crucial, as it has a variety of f-states that contribute to the luminescence of the molecular cages.
The research group tested numerous varieties of the molecular cages to find one that would properly convert UV light to visible wavelengths and remain stable inside an acrylic-based paint. The team found that polypropylene mixtures, for instance, converted much less light after three to five weeks of use, and they also started peeling off the glass samples.
Once team members had concocted an optimized paint mixture, they coated the windows of four greenhouses with the substance, left the windows of four other greenhouses uncoated, and planted basil and strawberry crops in all eight buildings. The basil plants grew bigger leaves under the coated windows, though there was no significant difference in the yield of the strawberry plants. This could be because different agricultural crops have different light responses, and the researchers suggest that the incorporation of different lanthanides into the polyoxotitanium cages would tailor the emission properties of the greenhouse paint to various plants’ needs.
According to Wright, the researchers are aiming for a 10 to 20% increase in photosynthetically available radiation and hope to have a commercial product available within a few years.