Metal Cation Triggered Peptide Hydrogels and Their Application in Food Freshness Monitoring and Dye Adsorption

Abstract

Metal-ligand interactions have emerged as an important tool to trigger and modulate self-assembly, and to tune the properties of the final supramolecular materials. Herein, we report the metal-cation induced self-assembly of a pyrene-peptide conjugate to form hydrogels. The peptide has been rationally designed to favor the formation of β-sheet 1D assemblies and metal coordination through the Glu side chains. We studied in detail the self-assembly process in the presence of H⁺, Li⁺, Na⁺, K⁺, Ca²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Co²⁺, Fe³⁺, and Cr³⁺ and found that the morphology and mechanical properties of the hydrogels are ion-dependent. Moreover, thanks to the presence of the metal, new applications could be explored. Cu²⁺ metallogels could be used for amine sensing and meat freshness monitoring, while Zn²⁺ metallogels showed good selectivity for cationic dye adsorption and separation.

Keywords: biogenic amines; dye adsorption; food spoilage; hydrogel; metallogel; organic amines; peptide; self-assembly; sensing.

Figure 1

Structure of the pyrene–peptide conjugate 1.

Figure 2

(a) UV-Vis absorption and (b) emission spectra of the pH-triggered gel (red line) and the aqueous solutions of 1 at 0.5% (solid black line) and 0.005% concentration (black dotted line); (c) ATR-FTIR, (d) CD spectrum, (e) TEM images and (f) frequency sweep experiment of the HCl triggered gel at 0.5% concentration, G′ is indicated with filled circles and G″ with open circles. ATR-IR and TEM were recorded using dried samples.

Figure 3

(a) Normalized UV-Vis absorption and (b) non-normalized emission spectra (λ_exc = 352 nm) of the monovalent cation triggered gels; (c) Normalized UV-Vis absorption and (d) non-normalized emission spectra (λ_exc = 352 nm) of the divalent and trivalent cation triggered gels. The data of the aqueous solutions of 1 at 0.5% (solid black line) and 0.005% concentration (black dotted line) have been included for comparison.

Figure 4

Transmission electron microscopy (TEM) images of the cation induced metallogels. Images were recorded in dry samples without the use of stain.

Figure 5

Frequency sweep experiments of metallogels triggered by (a) monovalent cations and (b) divalent and trivalent metal cations. G′ values are indicated with filled circles while G″ values are indicated with open circles.

Figure 6

Images of the 1-Cu²⁺ gel (a) as prepared, (b) after exposure to ammonia vapors, and (c) after exposure to HCl vapors, (d) colorimetric responses of the 1-Cu²⁺ gel when exposed to chicken meat stored at 25 °C (left) and 4 °C (right).

Figure 7

(a) Image of cuvettes containing 1-Zn²⁺ xerogels upon dye adsorption, from left to right methylene blue mixed with methyl orange, methylene blue, and methyl orange. Dye absorption abilities of the 1-Zn²⁺ xerogel: (b) Evolution over time of the residual concentration (%) of methylene blue (668 nm), (c) Evolution over time of the residual concentration (%) of dye in a 1:1 mixture of methylene blue (668 nm, blue dots) and methyl orange (463 nm, orange dots) (d) Evolution over time of the residual concentration (%) of methyl orange (463 nm).