Firefly-inspired bipolar information indication system actuated by white light

The ability to encode and indicate information in materials is crucial across various applications. Optical encoding, leveraging easily readable and adjustable optical properties, is particularly attractive. However, most existing optical encoding materials can only display one type of information—either static or dynamic—due to mutual interference between different optical states. This limitation hinders the development of multifunctional information systems. This research seeks to overcome this limitation by creating a system capable of simultaneously indicating both unchanging and changing information. Inspiration is drawn from the firefly's unique visual signaling: a constant reflection color during the day and a varying emission color at night. This dual-mode functionality is achieved through a combination of a reflective layer and a photogenic layer within the firefly's lantern. The present study emulates this biological system to develop a non-interfering bipolar information indication system. The significance of such a system is substantial: it allows for the simultaneous display of static information (such as product identification or authentication) and dynamic information (such as degradation state or exposure history). Such capabilities are highly relevant for applications requiring real-time monitoring and verification, especially in areas like pharmaceuticals, where rapid assessment of medication integrity and potency is crucial. The development of a robust and easily scalable production method is also a key focus to enable widespread practical application.

Existing optical encoding materials predominantly utilize either structurally colored materials (like photonic crystals) or luminescent materials. Photonic crystals, with their color arising from periodic nanostructures, offer excellent stability and anti-counterfeiting properties, ideal for indicating unchanging information. However, indicating dynamic information using photonic crystals presents a challenge. Incorporating luminescent materials can enhance information capacity, but traditional luminescent materials typically exhibit stable light intensity, limiting their use for dynamic information indication. Photochromic materials, which change color upon light exposure, offer a potential solution for dynamic encoding, but they often interfere with the indication of unchanging information. Therefore, a significant challenge remains in developing a system that can reliably and simultaneously display both static and dynamic information without interference. The unique dual-layer structure and signaling mechanism observed in fireflies presented a compelling biological model to overcome these limitations. This study draws inspiration from the firefly lantern structure, separating the static and dynamic information signaling pathways to create a non-interfering dual-mode system.

The researchers designed a bipolar information indication system by combining a stable photonic crystal structure with an unstable photochemical afterglow material. The process begins with the synthesis of core-interlayer-shell (CIS) colloidal particles, which serve as the building blocks for the photonic crystal structure. These particles are produced through a stepwise emulsion polymerization process. The key to the dynamic information encoding lies in the photochemical afterglow material, composed of three key components: a photosensitizer (PdOEP), a consumption unit (CU), and an emitter (e.g., europium complex, BODIPY, or perylene). The photochemical process involves white light irradiation of the photosensitizer generating singlet oxygen, which reacts with the consumption unit to produce an unstable intermediate. This intermediate subsequently decomposes and transfers energy to the emitter, resulting in persistent luminescence, with intensity directly correlated to the prior light exposure. The shear-induced ordering technique (SIOT) is utilized to assemble the CIS particles and the photochemical afterglow material into a well-ordered photonic crystal film. This high-throughput technique allows for rapid and large-scale production of the films. The resulting bipolar system features two operational modes: when white light is on, the photonic crystal structure selectively reflects certain wavelengths, generating a stable reflection color which encodes the unchanging information; when white light is off, the afterglow material emits light, and the intensity of this emission, determined by the degree of previous white light exposure, represents the changing information. Various characterization techniques, including USAXS (ultra-small angle X-ray scattering), TEM (transmission electron microscopy), DLS (dynamic light scattering), UV-Vis spectroscopy, fluorescence spectroscopy, and HPLC (high-performance liquid chromatography) were employed to confirm the structural and optical properties of the materials and the system’s overall performance. The effect of CU concentration on afterglow intensity and the stability of the system in darkness were also evaluated.

The researchers successfully fabricated a firefly-inspired bipolar information indication system exhibiting both unchanging reflection color under white light and changing afterglow intensity in darkness. The shear-induced ordering technique (SIOT) proved highly efficient, producing large-area photonic crystal films in less than 10 seconds. The system demonstrated high color tunability; both reflection color and afterglow emission color could be independently adjusted by altering the composition of the CIS particles and the emitter in the unstable unit, respectively. The afterglow intensity was directly correlated with the degree of white light exposure, demonstrating the reliable encoding of dynamic information. The consumption unit (CU) was identified as crucial for afterglow generation; its concentration directly influenced the afterglow intensity and the system's sensitivity to light exposure. A higher CU concentration resulted in higher afterglow intensity and increased resistance to photobleaching. The system’s stability in the dark was also confirmed, with minimal change in afterglow intensity over several days and months. As a proof-of-concept, the researchers applied this system to create a quality control label for mecobalamin, a photosensitive medicine. The label exhibited a stable reflection color (indicating unchanging information like the drug name) and a variable afterglow intensity (reflecting the level of light exposure and consequently the remaining mecobalamin content). The label provided a semi-quantitative assessment of mecobalamin degradation, allowing for rapid and convenient efficacy determination at home. The HPLC analysis corroborated the label's ability to accurately reflect the mecobalamin degradation, demonstrating its suitability as a rapid home-based test. The label testing method significantly reduced the time needed compared to traditional HPLC methods (2 minutes vs. at least 1 hour).

The findings directly address the research question of creating a non-interfering bipolar information indication system capable of simultaneously displaying static and dynamic information. The success in achieving this using a bio-inspired design demonstrates the potential of emulating natural systems in materials science and engineering. The high color tunability, scalability, and efficiency of the SIOT method make the system practical for various applications beyond the demonstrated quality control label for photosensitive medicine. The system’s utility extends to areas such as anti-counterfeiting, smart packaging, and other applications requiring both static identification and dynamic monitoring. The development of a rapid, low-cost method for determining medicine efficacy at home is a significant advancement in healthcare accessibility. The ability to visually assess medicine integrity is particularly important for photosensitive drugs, where light exposure can drastically alter their effectiveness. Future research can focus on optimizing the photochemical afterglow material to improve the brightness, lifetime, and sensitivity of the system, potentially enhancing information capacity.

This study successfully developed a bio-inspired bipolar information indication system, capable of simultaneously displaying both static and dynamic information without interference. The system utilizes a combination of stable photonic crystals and an unstable photochemical afterglow material, effectively utilizing white light in both physical and chemical ways. The use of SIOT enabled a scalable production method, creating a platform for practical applications. The system’s performance as a quality control label for photosensitive medicine demonstrates its real-world potential. Future research directions include optimizing the photochemical afterglow material for enhanced performance and exploring new applications of this versatile information indication system.

The current study focuses on mecobalamin as a model photosensitive drug. Further investigation is needed to determine the system's applicability to a wider range of photosensitive medicines. While the system provides a semi-quantitative assessment of drug degradation, further refinement is needed to achieve greater quantitative accuracy. The long-term stability of the label under various environmental conditions (temperature, humidity) also requires further evaluation.