Firefly-inspired bipolar information indication system actuated by white light

The long-standing goal of storing unchanging information for extended periods has motivated the use of durable physical media. Beyond static information, there is also a need to indicate changing information over time (e.g., environmental exposure). Optical encoding is attractive due to its readability and tunable properties. Structurally colored materials such as photonic crystals provide stable colors via periodic nanostructures, resist photobleaching, and are difficult to counterfeit, making them ideal for unchanging information. However, integrating indicators for changing information remains challenging. Conventional luminescent materials rely on photophysical processes that yield stable intensities, indicating only unchanging information. Photochromic systems can track changing information by UV-induced isomerization, but UV exposure interferes with other optical states, undermining unchanging information. Inspired by the firefly lantern—which shows stable reflection in daylight and changing emission intensity at night powered by a consumable biochemical substrate—the authors designed a bipolar system coupling a stable photonic crystal reflection layer with a photochemical afterglow unit. The system uses white light in two non-interfering ways: physically (selective reflection for unchanging information) and chemically (afterglow intensity correlated with prior white light exposure for changing information). As a proof of concept, they demonstrate a smart label that simultaneously encodes unchanging drug identity/anti-counterfeiting and tracks the light exposure history of a photosensitive medicine (mecobalamin) to assess efficacy where conventional assays are impractical.

The paper situates the work within optical information encoding and structurally colored photonic materials known for stability and anti-counterfeiting due to their periodic nanostructures. Prior attempts to expand information capacity include integrating luminescent materials; however, photophysical luminescence generally provides stable intensity and thus only unchanging information. Photochromic materials can indicate changing information via UV-dose-dependent color changes, but UV irradiation interferes with other optical states, preventing reliable unchanging information indication. Bioluminescent firefly lanterns inspire decoupling stable reflection from changing emission by using separate layers and a consumable energy source. The authors build on advances in colloidal photonic crystal assembly and prior information-encoding materials to propose a non-interfering dual-mode system actuated by white light.

Design and materials: The bipolar system integrates stable photonic crystal units and unstable photochemical afterglow units within a single film. Stable units are core–interlayer–shell (CIS) colloidal particles (PS@PEA) fabricated by stepwise emulsion polymerization to yield monodisperse particles (PDI < 0.05) capable of forming ordered photonic structures with stable reflection colors. Unstable units comprise a photosensitizer (PdOEP), a consumption unit (CU; oxygen-reactive molecule forming a chemiexcited intermediate), and an emitter (e.g., Eu(TTA)3phen, BODIPY, or perylene). Propylene carbonate serves as a mediating solvent compatible with both units. Assembly via SIOT: CIS particles were blended with the light-responsive solution (typical concentrations: photosensitizer 5 µM, CU 4–12 mM, emitter 2 mM) to form a viscous slurry (~55 wt% CIS). For lab-scale films, the slurry was sandwiched between two glass plates and subjected to shear (rotation-shearing angular velocity 1/3 rad/s, amplitude 1/6 rad, frequency 0.5 Hz) for a few seconds, inducing rapid ordering (<10 s) into photonic crystal films with bright reflection. For large-area production, slurry was sandwiched between PET films and processed on an SIOT machine with continuous bending (60 cycles) at room temperature to obtain ~180 µm thick films. CIS synthesis: Seeds prepared by emulsion polymerization of styrene with BDDA and SDS; core grown by continuous monomer feed; interlayer introduced via EA and AMA; shell formed by EA feed; emulsions freeze-dried to obtain dry CIS particles. Particle size (and thus reflected color: red/gold/green/blue) was tuned by SDS content in the initial step (0.08–0.13 g). Label fabrication: For the demonstration label, three blue grids were made from blue-reflecting CIS blended with europium emitter, PdOEP, and graded CU concentrations (12, 8, 4 mM). A gold gradient rectangle used gold-reflecting CIS with a black mask; red cross and the word “mecobalamin” used red- and green-reflecting CIS, respectively. Characterization: Structural ordering assessed by 2D USAXS (SSRF BL10U1; λ=0.124 nm; Eiger 4M detector; 27.6 m sample-detector distance; 20 s exposure) showing hexagonal close packing with q2/q1=√3 and q3/q1=√4. Particle growth and monodispersity confirmed by TEM and DLS. Optical properties measured via reflectance spectroscopy (Choptics EK2000-Pro), fluorescence/afterglow spectra and decay (Edinburgh FS5/FLS1000; Hamamatsu C14631), and EMCCD imaging (Andor DU897). Afterglow response was tested under white light irradiation (e.g., flashlight 50 mW/cm²) for controlled durations/doses, followed by dark imaging. Thermal stability assessed by storing films at 0–50°C for 1 month. Stability in dark evaluated for up to 3 months. Mechanistic studies: Control systems without CU or without emitter assessed afterglow necessity; weak ~380 nm emission from CU' observed without emitter. 13C NMR verified CU decomposition (carbonyl peaks ~170 and 190 ppm; disappearance of ~110 and 145 ppm). Emitter lifetimes extended from ns–ms to seconds upon CU introduction, indicating chemiexcitation-driven afterglow. Photosensitive medicine test: Mecobalamin photodegradation under white light quantified by HPLC (Waters e2695; C18 column 4.6×150 mm, 5 µm; mobile phase acetonitrile with 0.47 wt% sodium hexane sulfonate and 0.03 mol/L sodium dihydrogen phosphate buffer, 81:19 v/v; flow 0.8 mL/min; detection 266 nm). Peaks at ~6.3 min (mecobalamin) and ~3.4 min (hydroxycobalamin) used to calculate relative remaining content versus light dose. The label with graded CU concentrations provided semi-quantitative phase readouts correlated to remaining mecobalamin content.

• Dual-mode, non-interfering indication: Under white light (on), the photonic crystal outputs stable reflection color (unchanging information). When light is off, a photochemical afterglow indicates changing information via intensity that decreases with prior white-light exposure.
• Rapid, scalable assembly: SIOT forms ordered photonic crystal films in <10 s (lab-scale); mass-production via machine processing (60 bending cycles) yields ~180 µm films.
• Structural ordering: 2D USAXS shows hexagonal close packing with q2/q1=√3 and q3/q1=√4, confirming ordered photonic structures.
• Independent tunability: Reflection color controlled by CIS particle size (red/gold/green/blue). Afterglow emission color tuned by emitter choice (Eu-complex, BODIPY, perylene). CIE coordinates and emission FWHM provided in supplementary data.
• Necessity and role of CU: No afterglow without CU; afterglow present only when CU is included. CU reacts with ¹O₂ to form an unstable intermediate (e.g., 1,2-dioxetane) that decomposes to CU' and transfers energy to emitters. Evidence: spectral controls, weak ~380 nm emission from CU' without emitter, 13C NMR (new carbonyl peaks ~170 and 190 ppm), and lifetime extension from ns–ms to seconds when CU is present.
• Dose-dependent afterglow: Afterglow intensity decreases with increased white light dose (e.g., noticeable drop after 1 min irradiation). Reflection intensity remains stable across doses, indicating no photobleaching of the structural color. Thermal stability of reflection maintained at 0–50°C for 1 month.
• CU concentration tunes sensitivity: Higher CU concentration yields higher initial afterglow and greater tolerance to light dose before extinction. Afterglow imaged and quantified across CU levels (e.g., 4, 8, 12 mM) under a 50 mW/cm² white light source.
• Dark stability: Afterglow capacity stable in darkness; at 12 mM CU, intensity remained nearly unchanged over 7 days and retained a substantial portion after 3 months.
• Application to drug quality: Mecobalamin photodegrades to hydroxycobalamin under white light (HPLC peaks at ~6.3 min and ~3.4 min, respectively). The label’s graded CU grids provide semi-quantitative phases corresponding to remaining mecobalamin content: three lit grids ≈ 80–100%; two grids ≈ 50–80%; one grid ≈ 15–50%; zero grids ≈ 0–15%.
• Practical advantage: The label enables at-home estimation in ~2 minutes versus ≥1 hour for HPLC (sample prep, elution, calculation). Anti-counterfeiting arises from complex, iridescent structural colors.

The study addresses the challenge of simultaneously indicating changing and unchanging information without mutual optical interference. By decoupling the physical reflection (stable photonic crystal structure) from the chemical afterglow (consumable photochemical process), both information types are encoded and actuated by the same white light but read in different states (on vs. off). The stable units guarantee consistent reflection color for persistent identifiers and anti-counterfeiting, while the unstable chemiexcited afterglow encodes exposure history through intensity decay proportional to accumulated light dose. Independent tuning of reflection and emission colors expands the information capacity and customization. Mechanistic evidence substantiates that CU is the consumable energy cache, transforming white-light-generated ¹O₂ into a high-energy intermediate that powers delayed emission; thus, afterglow intensity reliably tracks exposure. The application to mecobalamin demonstrates clinical relevance: graded CU concentrations map to meaningful ranges of remaining drug content, enabling rapid, user-friendly quality control that is otherwise impractical with standard assays at point-of-use. The platform’s high-throughput SIOT manufacturing and environmental stability further support translational potential.

The authors developed a firefly-inspired bipolar information indication system that uses white light both physically (structural reflection) and chemically (photochemical afterglow) to encode unchanging and changing information without interference. SIOT enables rapid and scalable integration of stable CIS photonic crystal particles with a tunable afterglow system comprising a photosensitizer, a consumable energy cache (CU), and diverse emitters. The system offers independently tunable reflection and emission colors, dose-dependent afterglow intensity, thermal and dark stability, and a practical application as a label to monitor photodegradation of photosensitive medicines (e.g., mecobalamin), providing near-instant efficacy estimation at home. Future work may optimize CU molecular structures to fine-tune afterglow brightness and lifetime and design photonic matrices that interact with the afterglow to yield coupled optical effects, further increasing information density and enabling advanced applications.