Dye-free blue fluorescence enables easy viewing for interface sciences

Fluorescent markers are extremely useful in science as tools to track molecules or processes as they carry out their unique activities, revealing unknown facts along the way. However, physically introducing fluorescent markers into targets can result in strong background signals, and even when chemically bound, the target’s hydrophobicity may increase, making the process far from straightforward. Moreover, fluorescent markers are often affected by the properties of the solvent in which they operate. To address these challenges, researchers have developed a method to track the behavior of cellulose nanofibers (CNFs) by conjugating water-compatible fluorescent amino acids to the CNFs. As a result, observers can now microscopically visualize CNFs by following the blue fluorescence emitted from them.

Researchers published their results in Carbohydrate Polymer Technologies and Applications in June 2025.

Cellulose nanofibers (CNFs) are a more eco-friendly alternative to conventional polymers, typically comprised of plastics, and are instead made from cellulose, a structural material found in plant cell walls. Researchers attached fluorescent amino acid acridon-2-yl-alanine, or Acd, to a CNF to produce a blue fluorescent CNF that retains the original structure and dispersibility of the materials it is acting upon and is known as Acd-CNF.  The importance of this lies in its ingenuity: conventional methods often contain hydrophobic components that can alter the thixotropic nature of some materials. Thixotropy refers to a property seen in some fluids or gels where, at rest, the material is viscous and thick but upon movement becomes more fluid (less viscous). These characteristics are essential when viewing and studying the ability of two different substances to be mixed, and can broaden the usability of CNFs.

“We aimed to address the challenge of visualizing the interfacial behavior and distribution of cellulose nanofibers in aqueous systems, particularly at oil-water interfaces, without relying on external dyes,” said Izuru Kawamura, lead researcher, author and professor at YOKOHAMA National University.

The covalent bond, or bond between two atoms by sharing electrons, is a strong bond between two molecules. Acd-CNF takes advantage of this bond to increase its stability and visibility when viewing without the added “junk” that conventional dye-based methods might leave or introduce into a system. The importance of unobstructed viewing cannot be understated when attempting to understand the way substances interact with each other, as even subtle disruptions can leave the observer with biased data.

Interface science is concerned with the interactions of physical and chemical phenomena occurring at the boundary of two differing phases of matter. Acd-CNF retains the original properties of the material it acts on while being easily visible upon microscopic observation, opening up opportunities for various fields of study. Results showed that even when the viscosity of a material was increased by 10, Acd-CNF still retained original properties of the material. This can be attributed to its increased capacity for hydration (mitigating the hydrophobic tendencies of the conventional method) and a sturdy network of cellulose nanofibers.

Researchers would like to take this work further to explore the use of Acd-CNF in other systems, such as emulsified food products and cosmetics, and study the effects various conditions have on the product’s behaviors. Additionally, the novelty of functional fluorescent nanomaterials made out of cellulose can allow for ecofriendly nanomaterials to be put into widespread use a variety of fields and applications.

Yuto Ito, Daisuke Sato, Azusa Kikuchi and Izuru Kawamura of the Graduate School of Engineering Science at YOKOHAMA National University with Noriko Kanai of the Graduate School of Environment and Information Sciences at YOKOHAMA National University contributed to this research.

Japan Society for the Promotion of Science, the JST CREST and JST COI-NEXT program contributed to this research.

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YOKOHAMA National University (YNU) is a leading research university dedicated to academic excellence and global collaboration. Its faculties and research institutes lead efforts in pioneering new academic fields, advancing research in artificial intelligence, robotics, quantum information, semiconductor innovation, energy, biotechnology, ecosystems, and smart city development. Through interdisciplinary research and international partnerships, YNU drives innovation and contributes to global societal advancement.