Opal-like Multicolor Appearance of Self-Assembled Photonic Array

Abstract

Molecular self-assembly of short peptide building blocks leads to the formation of various material architectures that may possess unique physical properties. Recent studies had confirmed the key role of biaromaticity in peptide self-assembly, with the diphenylalanine (FF) structural family as an archetypal model. Another significant direction in the molecular engineering of peptide building blocks is the use of fluorenylmethoxycarbonyl (Fmoc) modification, which promotes the assembly process and may result in nanostructures with distinctive features and macroscopic hydrogel with supramolecular features and nanoscale order. Here, we explored the self-assembly of the protected, noncoded fluorenylmethoxycarbonyl-β,β-diphenyl-Ala-OH (Fmoc-Dip) amino acid. This process results in the formation of elongated needle-like crystals with notable aromatic continuity. By altering the assembly conditions, arrays of spherical particles were formed that exhibit strong light scattering. These arrays display vivid coloration, strongly resembling the appearance of opal gemstones. However, unlike the Rayleigh scattering effect produced by the arrangement of opal, the described optical phenomenon is attributed to Mie scattering. Moreover, by controlling the solution evaporation rate, i.e., the assembly kinetics, we were able to manipulate the resulting coloration. This work demonstrates a bottom-up approach, utilizing self-assembly of a protected amino acid minimal building block, to create arrays of organic, light-scattering colorful surfaces.

Keywords: Fmoc modification; Mie scattering; amino acid self-assembly; biaromatic amino acid; colored surfaces; microspheres; opal-like; self-assembly.

Figure 1

Structure of Fmoc-Dip needle-like crystals. (a) Fmoc-FF and Fmoc-Dip molecular scheme. (b) Phase diagram of 55 assembly conditions and the corresponding structures in solution taken 72 h following sample preparation. Assembly conditions determine the kinetics and morphology: spheres (blue), fibrils (red), and needle-like crystal aggregates (green). The white area represents conditions in which no assemblies were visible. In black are conditions that could not be achieved with the initial stock solution of 10 mg/mL Fmoc-Dip in EtOH. (c) Needle-like crystals propagating from a nucleation site. Scale bar is 100 μm. (d−f) Crystal structure of Fmoc-Dip determined using X-ray scattering. View of the unit cell as determined for single Fmoc-Dip needle-like crystals (d). Crystal packing down the crystallographic c axis (e). Aromatic rings (orange) to display the aromatic continuity within the crystal, as viewed down the a axis (f).

Figure 2

Fmoc-Dip spheres. (a) AFM micrograph of an array of spheres. (b) Bright-field microscopy image of the sphere array at low magnification (×4). Scale bars are 500 μm. (c) STEM image of a single sphere at 0° and 60° tilt. Scale bars are 100 nm. (d) Virtual cross section of the STEM tomogram. Scale bar is 500 nm. (e) 3D image constructed from a tilt image sequence (−72° to 72°, increment 1.5°). (f) Powder XRD spectrum exhibiting a characteristic amorphous peak.

Figure 3

(a) Fluorescence (left) and differential interference contrast (DIC) (right) images of the sphere array. Scale bar is 10 μm. (b) Corresponding emission spectrum. Excitation at 405 nm. (c) Standard total attenuation spectrum of Fmoc−Dip sphere array, taken using a UV−vis spectrophotometer. The probe wavelength was scanned in the range of 200−1000 nm, and the total transmission was measured. (d) Absorption spectrum of a Fmoc-Dip sphere array film, deposited on a quartz coverslip, measured by using an integrating sphere equipped spectrophotometer. The wavelength was scanned in the range of 400−800 nm.

Figure 4

Correlation between sphere diameter and observed coloration. (a,b) Representative SEM micrographs of arrays exhibiting blue (a) and orange coloration (b). Scale bars represent 10 μm. (c) Box plot displays of sphere diameter size distribution obtained by image analysis. The mean value is annotated by a red star.