NIMS develops environmentally friendly microbead-type light-emitting material derived from citric acid

The research group of Group Leader Tadaaki Nagao, Postdoctoral Researcher B. K. Barman, JSPS Research Fellow Hiroyuki Yamada, and Researcher Keisuke Watanabe of the International Center for Materials Nanoarchitectonics (MANA) Photonics Nano-Engineering Group; Group Leader Atsushi Goto and Senior Research Fellow Kenjiro Hata of the Solid-State NMR Group Center for Basic Research on Materials; and Principal Engineer Shinobu Ohki and Engineer Kenzo Deguchi of the High Magnetic Field Characterization Unit, Research Network and Facility Services Division at the National Institute for Materials Science (NIMS) has successfully developed an environmentally friendly microbead-type light-emitting material derived primarily from citric acid and other ingredients. Their results were published in Advanced Science.

Many inorganic materials have been used in light-emitting devices to date, such as thin films and nanoparticles of compound semiconductors containing metals or sintered ceramics containing rare-earth elements. However, in a recycling-oriented society, there is a push to develop light-emitting materials that do not use rare-earth elements, whose supply is unstable, or metallic elements with large environmental loads. Research on graphene quantum dots and carbon dots with aromatic ring structures similar to graphite and soot using naturally occurring citric acid and amino acids as main raw materials has made remarkable progress in the past few years.

In response, the research group has developed a new microbead-like material that exhibits strong luminescence intensity even in the solid state while retaining the characteristics of carbon dots, a light-emitting material. When normal colloidal phosphors are dried and solidified, they mutually absorb light emitted from neighboring particles, resulting in self-quenching phenomenon that reduces their luminescence efficiency.

The research team used polymeric amino acid (polylysine), a fermentation product of natural microorganisms, and plant-derived citric acid as the main raw material, added a small amount of p-phenylenediamine to the main raw material, and performed thermal denaturation of polyamino acids. As a result, they succeeded in developing a spherical solid microbead material. In this microbead, structures with fused aromatic rings similar to carbon dots, which absorb light satisfactorily and emit light, are formed in various places. Because these fused aromatic ring structures are dispersed far apart in beads, they do not easily absorb the light emitted by each other. Therefore, these microbeads emit strong light when illuminated. Thus, a light-emitting material primarily that is composed of materials derived from living organisms and plants, does not use rare-earth elements, and does not self-quench even in the solid state has been realized.

The developed microbeads can emit light in various colors depending on the wavelength of the irradiating light. When irradiated with ultraviolet light with a wavelength of 355 nm, it emits blue light in the range of 450-650 nm with a quantum yield efficiency of approximately 50%. When exposed to blue light with a wavelength of 470 nm, it emits yellow light in the range of 550-700 nm. Additionally, when exposed to green light with a wavelength of 532 nm, it emits orange and red light in the range of 550-800 nm. When irradiated with a wavelength of 580 nm or higher, it emits red light in the range of 600-900 nm and near-infrared light with wavelength exceeding 1000 nm. No material has been ever reported to emit not only all visible light (blue, green, yellow, and red) but also near-infrared light of 1000 nm or more at a single-particle level in such a way that its emission can be changed at will.

The emitted light first circulates along the surface of microbeads and then resonates and emits strong light when the optical distance around the outer circumference of beads is an integer multiple of the wavelength of the light. This phenomenon appears as numerous spike-like emission lines in the emission spectrum, and the wavelength, intensity, and width of these lines vary greatly depending on the size and sphericity of beads. This is due to the interference effect of light waves orbiting beads and is called "whispering gallery mode (WGM)." WGM has been observed in the past in glass and plastic beads doped with rare-earth elements and petrochemical dyes.

In this study, the group has shown for the first time that such beads can be synthesized via a simple, single hydrothermal synthesis method using materials derived from living organisms and plants. Because no metal is used in this synthesis, it is expected to be utilized for inexpensive and safe secret inks for everyday applications or marker particles in the medical and biotechnological fields. Furthermore, the WGM emission spectrum of each bead is different, reflecting the individuality of each bead, enabling the identification of individual beads as if they were authentication tags or barcodes. Applications as anti-counterfeiting ink and nonreplicable authentication technology are expected for these beads. In particular, by changing the wavelength of the irradiated light, the WGM spectrum, which can act as an authentication key, also changes. This property of these beads can be utilized as a multiple authentication technology using two or more wavelengths of light. Moreover, they plan to collaborate with the University of Exeter in the U.K. for research into fluorescent probes for cell imaging.

Journal Information
Publication: Advanced Science
Title: Rare-Earth-Metal-Free Solid-State Fluorescent Carbonized-Polymer Microspheres for Unclonable Anti-Counterfeit Whispering-Gallery Emissions from Red to Near-Infrared Wavelengths
DOI: 10.1002/advs.202400693