American linden leaves

‘Glow in the dark wood’ inspires phosphorescent material

Image credit: Dreamstime

Scientists from the University of Bath and North East Forestry University, China, have developed a sustainable phosphorescent material with potential applications ranging from medical imaging to glow in the dark dyes and paints. The material is inspired by the natural phosphorescence of wood.

Fluorescent materials are found in nature, including in some jellyfish, corals and tree frogs. Visible fluorescence occurs when these materials absorb energy with a short wavelength, such as UV frequencies, and emit the energy as visible light. Unlike room-temperature phosphorescence (RTP), fluorescence stops immediately and stopping when the light source is switched off.

The new study investigated the natural RTP of lignin, a group of polymers that give wood its rigid structure. It found that basswood (also known as American linden) naturally and weakly phosphoresces, releasing light for a few milliseconds, due to lignin being trapped within a 3D matrix of cellulose.

This inspired the research team to mimic the phosphorescent properties by crosslinking lignin within a 3D polymer network; they were able to cause it to glow visibly for around one second.

Tweaking the cavity sizes within the polymer network and varying the drying times of the polymer allowed them to tune the duration of the glowing.

“All lignin glows weakly, but most of the light energy is lost by vibration of movement of the lignin molecules, meaning it isn’t clearly visible to the naked eye,” said Professor Tony James from the University of Bath’s Centre for Sustainable Circular Technologies. “We’ve found that immobilising the lignin in an acrylic polymer means more energy is emitted as light; in other words, the less it rattles about, the more it glows.

“Most current phosphorescent materials are either toxic or difficult to prepare, so we wanted to develop a new material that overcome these limitations. Although there is room for improvement, our new material shows great potential for making a more stable, sustainable, biodegradable non-toxic phosphorescent material that could be used in a range of applications.”

In a demonstration for their new material, the scientists coated cotton threads with a dye using the material, and used them for luminescent embroidery. They suggest that this could have use in the identification and anti-counterfeiting protection of luxury textiles and handbags. Other applications could span medical imaging, optical sensing and glowing paints.

Professor Zhijun Chen from Northeast Forestry University’s wood-engineering centre commented: “It is indeed an unexpected and interesting discovery. We think this work will not only provide a new option for sustainable afterglow materials but is also a new route for the value-added utilisation of lignin, which is the main naturally occurring aromatic polymer [used in the pulping industry].”

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