Structurally Colored Thin Films of Ca²⁺-Cross-Linked Alginate

Abstract

Alginate, or alginic acid, is an unbranched binary copolymer of (1-->4)-linked beta-D-mannuronic acid and alpha-L-guluronic acid. Alginate readily forms binding interactions with a variety of divalent metal ions, such as calcium. This binding has been used to cross-link bulk alginates for a wide variety of applications, particularly in areas of tissue engineering, medical devices, and wound-healing dressings. A new method is identified here for producing Ca2+-cross-linked thin films of sodium alginate, using an aerosolized spray of CaCl2 solution. These thin films exhibit structural color that varies with film thickness. It is demonstrated that this structural color is highly reproducible and can also be tuned to produce a wide range of colored films. The noted ability of alginates to bind metal ions is used in combination with the structural coloration afforded by the thin film structure as a basis for color-based optical sensing of metal ions in aqueous solutions. Changes in film thickness, refractive index, and reflectivity in response to metal ions have been measured and reported. For certain ions such as Cr(III) and Cr(VI), changes in film thickness are the predominate factors in shifting the reflected film color. In the case of other ions such as Pb(II), a change in film refractive index plays a significant role in the reflectance properties of films.

Hierarchical alginate biopolymer papers produced via lanthanide ion coordination

“…As calcium ions bind fundamentally to G residues, the process involves preferentially G blocks of the alginate according to "egg-box" model. [31][32][33] On the other hand, the ions exchange between Ca 2+ ions and Na + ions in the alginate-based hydrogel was confirmed by the previous reports. 34,35 As for the Nd 3+ ions, there are eight-coordinated or more coordinated bonds in the compound.…”

Colours and Metallic Sheen in Beetle Shells — A Biomimetic Search for Material Structuring Principles Causing Light Interference

“…Cathell [113] describes how colour-based metal detectors can be made using very thin layers of alginate biopolymer. Alginate can be produced from algea and are applied to silicon wafers using spincoating.…”

Nanoscience and nanotechnologies for biobased materials, packaging and food applications: New opportunities and concerns

“…Nanostructured biopolymer surfaces and thin films are also functional materials per se, and the fine control of their architectures can afford amazing functional properties. For instance, thanks to their nanometric dimensions lower than the wavelength of light, nanostructured surfaces and thin films can display optical properties such antireflectivity or structural colors (Cathell and Schauer 2007;Cerclier, Guyomard-Lack et al 2011;Azzam, Moreau et al 2014;Azzam, Moreau et al 2015). Finally, as biopolymers are biological molecules, they can be expected to possess characteristic features such as specific recognition, sensitivity to stimuli or susceptibility to enzymes.…”

“…These structurally colored films were achieved from spin-coated or dip-coated polyelectrolyte/CNC multilayer films (Cranston and Gray 2006;Podsiadlo, Sui et al 2007;Jean, Dubreuil et al 2008;Moreau, Beury et al 2012). Alginate films fabricated by spin-coating were then exposed to aerosolized CaCl2 solution to enable layer crosslinking (Cathell and Schauer 2007). All these works showed that homogenous film surfaces of various nanometer-scale thicknesses can be produced with a wide variety of reflected colors simply by varying deposition-procedure parameters (e.g.…”