Eco-friendly innovation: three-mode sun-actuated thermotropic devices integrating κ-carrageenan-based film doped with Arundo donax leaf-derived carbon dots and 1-butyl-3-methyl-1H-imidazolium chloride

Abstract

In this study, the morphology, size, and optical properties of carbon dots (CDs) synthesized at various temperatures (250, 300, 350 and 400 °C) are investigated, along with their incorporation into kappa-carrageenan (κ-Cg)-based membranes. A simple, low-cost, and environmentally friendly method is proposed for synthesizing photoluminescent CDs using a single natural source precursor, the Arundo donax leaf. Through a systematic experimental approach we assess the influence of the synthesis conditions on the optical properties of the resulting CDs. The CDs, predominantly spherical with sizes ranging from 3.2 to 4.8 nm, exhibit an essentially graphitic structure. Surface functional groups, such as CO and O–H, enhance their hydrophilicity. The CDs display strong blue-to-green fluorescence in aqueous solutions. When integrated into κ-Cg-based films, the CDs influence the structural and morphological properties of the host matrix. These flexible, scalable, thermally stable, eco-friendly, and mechanically robust films demonstrate quantum yield values comparable to that in aqueous solutions, suggesting that the κ-Cg matrix mantains the CDs' emission properties due to weak interactions, preserving electronic transitions. Additionally we develop a sun-actuated zero-energy dual-mode thermotropic device (TTDs) featuring κ-Cg doped with CDs and 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) thermotropic ionic liquid. At 40 and 60 °C, this TTD operates as a dual-mode system with transmittance variation (ΔT) values of 10/8% and 13/10% at 550/1000 nm, respectively. Upon addition of silver islands the surface plasmon resonance effect (SPRE)-mediated TTD offers a three-mode operation (a bright hot mode (visible and NIR admitted), a semi-bright warm mode (visible and NIR semi-blocked), and a semi-dark semi-cool mode (visible and NIR further semi-blocked) with ΔT values of 42/40% and 58/55% at 40 and 60 °C, respectively), and stable performance over multiple cycles. Overall, our findings highlight the potential of κ-Cg matrices for developing luminescent materials with stable and predictable optical properties for advanced optoelectronics and sensing applications.