Turning to trees for sustainable photoluminescence

Photoluminescent materials are essential for numerous modern technologies—displays, solar cells, optoelectronic devices and sensors among them. However, most photoluminescent materials in use today rely on toxic metals and non-renewable resources.

A team of Yale researchers led by first author Ho-Yin (Leo) Tse and professors Julie Zimmerman and Paul Anastas has found a way to make photoluminescent solid-state emitters with lignin, a key component of wood and a waste product in paper manufacturing. Safe and renewable, lignin may prove to have a glowing future in photoluminescent technology.

Their study is published in the journal Chem. We spoke with first author Ho-Yin (Leo) Tse, and co-corresponding author Hanno Erythropel about the study.

What would you say is the main takeaway of this work?

Photoluminescent materials can absorb and re-emit light—for instance, a glow-in-the-dark sticker—and these materials are essential for many modern technologies. Here, we developed photoluminescent materials solely based on a waste product—lignin from the pulp-and-paper industry and the amino acid histidine, without requiring heavy metals or halogens such as bromine, which are commonly needed.

You note that photoluminescent materials currently being used are toxic. How so?

Many of today’s display emitters use heavy metals or halogenated frameworks—for example, cadmium-based quantum dots, platinum/iridium complexes, or lanthanide systems—and they’re often made with hazardous reagents and high-temperature steps.

That combination raises three concerns: (1) Consumer risk is generally low in normal use because materials are encapsulated, but damage or improper disposal can create exposure pathways; (2) worker risk comes from handling toxic metal salts, corrosive/irritant chemicals, and volatile solvents during manufacturing; and (3) environmental impacts stem from metal mining/refining, energy-intensive processing, and e-waste at end-of-life.

What’s important to know about lignin, and what prompted you to try it for this application?

To make paper, you remove the cellulose, but the remains are rich in lignin, which is often burned for energy. However, lignin is the most abundant natural material that contains aromatic groups, which are an important input to the chemical industry. Currently, it’s usually obtained from petroleum. So unlocking the potential of lignin is an important step away from a petroleum-based society.

Here, lignin plays an important role in the material’s photoluminescent properties, specifically due to its aromatic character (besides some other features), so demonstrating that a “natural” material combined with another natural material—an amino acid that is present in protein—is a cool demonstration of what natural materials are capable of.

How can you build off this research?

What’s exciting about this is the following: The developed materials are not yet optimized. So when we say the afterglow is only about 300 milliseconds, the hope is that by varying composition and input, by manipulating both “ingredients” so to speak, both photoluminescent properties (fluorescence and phosphorescence) can be improved.

Also, there are different kinds of lignin, so exploring the range of different lignins may also aid in the optimization of the system. In a way, this study can hopefully lay the groundwork and start the momentum to develop other renewably sourced and non-toxic photoluminescent materials.