Chitosan/alginate-based layer-by-layer films with europium ions as anti-adhesive luminescent coatings

Abstract

Layer-by-layer (LbL) thin films based on anionic and cationic polysaccharides offer a versatile approach to obtain anti-adhesive properties. Luminescent films can be used as coatings that display a luminous response to the chemical environment. The use of Eu3+ ions as a spectroscopic probe is valuable to understand the formation and structure of these films. This study aims to investigate the assembly of LbL luminescent films using Eu3+ and the polyelectrolytes chitosan (CHI) and sodium alginate (ALG). Polysaccharide and Eu3+ solutions were used to manufacture the LbL films using the ex-situ method, where the incorporation of Eu3+ occurs after the film has been assembled, denoted as (AC)nEu, and in-situ (ACEu)n methods, which refers to the introduction of Eu³⁺ during film assembly, where A and C represent ALG and CHI bilayer, respectively, and n is the number of bilayers. The coordination environment and luminescent properties, as well as the behavior of the 2-thenoyltrifluoroacetone (tta) ligand, were investigated. The assembly process was characterized using a quartz crystal microbalance (QCM), and an ex-situ method was employed to produce higher photoluminescent films arising from the hypersensitive 5D0→7F2 transition. Furthermore, the tta-containing films displayed longer emission lifetime and reduced surface roughness ascribed to the substitution of water molecules in the coordination sphere. The thin films were further characterized by X-Ray Photoelectron Spectroscopy (XPS). Both survey and high-resolution spectra confirmed the successful incorporation of Eu3+ions in the films, as well as confirmed the physical deposition of the polyelectrolytes onto the solid substrate. Anti-adhesive surface properties were evaluated against two different bacteria, and the results were used to correlate the hydrophilicity and optical properties of the films. The presence of Eu3+ ions appears to disrupt the structure of Pseudomonas aeruginosa, whereas Staphylococcus aureus attaches more readily to surfaces. The methods of building LbL luminescent films presented here provide insights into the photophysical behavior of multifunctional coatings with optical and anti-adhesive properties.