Photon upconversion crystals doped bacterial cellulose composite films as recyclable photonic bioplastics

The photonics industry is rapidly shifting from glass to synthetic thermoplastics due to their cost-effectiveness and superior properties. However, the environmental impact of non-biodegradable plastic waste is a significant concern. This research explores the use of biodegradable biopolymers as a sustainable alternative. While biopolymers have been used in photonics for linear optical responses, their application in nonlinear optics and the challenge of recycling doped chromophores remain largely unaddressed. This study proposes a novel approach using a pre-programmable co-assembly–disassembly method to create photon upconversion crystals doped within a bacterial cellulose-gelatin (BC-G) composite film. This method aims to facilitate the physical recycling of the toxic chromophores at the end-of-life (EoL) and reduce plastic waste, contributing to a circular bioeconomy.

Existing research on biopolymers in photonics primarily focuses on linear optical color responses through light diffraction or reflection. The bio-sustainability of these materials, particularly the recovery of doped toxic photonic chromophores at EoL, poses a significant challenge. Previous work explored recyclable red/far-red to blue TTA-UC bioplastics using a gelatin-TX-100 system, achieving 67% chromophore recovery. However, the toxicity of the surfactant TX-100 at high concentrations necessitates a more sustainable solution. Cellulose has been used as a matrix for TTA-UC chromophores, but this work presents a novel approach using cellulose nanofibers both as a host and a template for TTA-UC chromophore crystallization.

Optically transparent BC-G films were prepared by culturing *Komagataeibacter xylinus* bacteria to produce bacterial cellulose (BC) films. Gelatin type A (G) was then coated onto the BC films to improve transparency by adjusting the refractive index. Nonlinear optically responsive photonic chromophores (DPA-PdTPBP) were crystallized within the BC-G composite films. Characterization included transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), dielectric measurements, UV-vis spectroscopy, steady-state and time-resolved fluorescence spectroscopy, and upconversion measurements. The recycling process involved dissolving the gelatin in hot water (50°C) to expose the TTA-UC crystals, followed by dissolving the crystals in THF (anti-solvent bathing) to separate them from the BC film. The recovered chromophores were then characterized and reused in a fresh BC film.

The BC-G composite film exhibited high optical transparency, improved by the gelatin coating, with a refractive index of n = 1.48 ± 0.25 in the visible region. The composite material also showed good thermo-mechanical properties (T > 300 °C, E' = 4150 MPa) and a high dielectric constant (ε' = 4.85 at 10 Hz), comparable to synthetic plastics. The successful in-situ crystallization of DPA-PdTPBP TTA-UC crystals within the BC-G film was confirmed through microscopy. The material demonstrated efficient upconversion of red light (λex = 633 nm) to blue light (λem = 443 nm), showing a quadratic to linear dependence on excitation power density. The developed recycling process allowed for the recovery of 66 ± 1% of the DPA annihilator. The recovered chromophores were successfully reused to create a new TTA-UC film, demonstrating the recyclability of the system.

This research successfully demonstrates a sustainable and recyclable approach to creating nonlinear optically active photonics bioplastics. The use of BC-G composite film as a bio-based backsheet material offers a viable alternative to synthetic plastics, addressing the significant environmental concerns associated with non-biodegradable waste in the photonics industry. The high recovery rate of the chromophores underscores the efficacy of the physical separation and anti-solvent bathing method. This study contributes significantly to the development of a circular bioeconomy in the photonics sector, aligning with efforts to reduce plastic waste and promote environmentally friendly materials.

This study presents a novel eco-designed approach for producing recyclable, nonlinear optically active bioplastics. The four-step process, including the creation of a high-performance BC-G film, efficient chromophore crystallization, nonlinear optical emission, and high chromophore recovery (66 ± 1%), demonstrates a significant step toward a more sustainable photonics industry. Future research will focus on expanding the range of chromophore-bioplastic combinations and addressing water vapor permeability.

The current study achieved a 66 ± 1% recovery rate of the DPA annihilator; further optimization may improve this yield. While the BC-G film demonstrated suitable thermo-mechanical properties, water vapor permeability remains an area for improvement. The low fluorescence quantum yield of the prepared crystals limited the determination of the TTA-UC quantum yield using absolute methods. Further research is needed to optimize chromophore design for higher quantum yields.