Abstract
The polymerization of 3,4-dihydroxy-L-phenylalanine leads to a carboxylic acid-rich synthetic melanin-like material (poly-L-DOPA). Synthetic melanin most resembles natural eumelanin in chemical structure. However, its deposition on surfaces leading to colored surfaces by interference is not as easy to accomplish as in the case of the preparation of colored surfaces by dopamine hydrochloride polymerization. This study deals with the preparation of new colored surfaces made from poly-L-DOPA displaying vivid colors by interference. These surfaces were obtained by depositing thin films of poly-L-DOPA on a reflective silicon nitride substrate. A high ionic strength in the polymerization medium was essential to accomplish the coating. The effect of ionic strength on the resulting surfaces was studied via reflectance, Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). The refractive index was determined by ellipsometry, and was nearly constant to 1.8 when λ > 650 nm. In the visible spectral region, the imaginary part of the refractive index becomes relevant. The refractive index in the visible wavelength range (400–600 nm) was in the range 1.7–1.80.
Highlights
Accepted: 30 August 2021Many colors in nature are the result of the interaction of light with structured materials [1]
The surface was smooth and presented a strong reflectance at 400 nm, which was responsible for the violet color that the surface displayed; see Figure S1 (Supplementary Materials), so that any posterior deposit would increase the optical path of the whole set over the silicon substrate, shifting the surface color to longer wavelengths
When DOPA was polymerized in dopamine-like conditions, there was no visual color change, which indicated the absence of poly-L-DOPA deposition
Summary
Accepted: 30 August 2021Many colors in nature are the result of the interaction of light with structured materials [1]. In the case that absorption and interference combine, the color palette becomes more visually pleasant [3] Such is the case of the striking colors that can be observed in some bird feathers due to the structuring of melanin pigments [4,5]. Melanins are natural pigments with interesting optical properties They have a broadband monotonic absorption in the entire UV-visible range in addition to a large refractive index [6]. In this sense, by a clever use of these properties, new biomimetic photonic materials, with variable response at different wavelengths, may be synthetically manufactured [6]
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