L-Cysteine/Silver Nitrate/Iodate Anions System: Peculiarities of Supramolecular Gel Formation with and Without Visible-Light Exposure

Abstract

In this study, novel anion photo-responsive supramolecular hydrogels based on cysteine-silver sol (CSS) and iodate anions (IO₃^-) were prepared. The peculiarities of the self-assembly process of gel formation in the dark and under visible-light exposure were studied using a complex of modern physico-chemical methods of analysis, including viscosimetry, UV spectroscopy, dynamic light scattering, electrophoretic light scattering, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. In the dark phase, the formation of weak snot-like gels takes place in a quite narrow IO₃^- ion concentration range. The visible-light exposure of these gels leads to an increase in their viscosity and dramatic change in their color. The morphology of gels alters after light irradiation that is reflected in the formation of a huge number of spherical/elliptical particles and the thickening of the fibers of the gel network. The interaction of CSS with IO₃^- anions has features of a redox process, which leads to the formation of silver iodide/silver oxide nanoparticles inside and on the surface of CSS particles. CSS possesses selectivity only to IO₃^- anions compared to many other inorganic ions relevant for humans and the environment. Thus, the CSS/IO₃^- system is non-trivial and can be considered as a novel low-molecular-weight gelator with photosensitive properties, as another way to produce silver iodide nanoparticles, and as a new approach for IO₃^- ion detection.

Keywords: anion detection; iodate anions; l-cysteine; low-molecular-weight gelators; photosensitivity; self-assembly; silver nitrate; silver-containing nanoparticles; visible light.

Figure 1

(A) Hydrogel formation in the CSS/IO₃⁻ system. (B) The anion (XO₃—ClO₃, BrO₃ or IO₃) content in the systems (Table); photo of CSGXO₃ systems 2, 4, 6, 8, 10, 12, 14, 16, 18, 20 (see Table, Figure 1B) after visible-light exposure for 1 h. CSGXO₃—cysteine silver gels based on XO₃⁻ anions.

Figure 2

Concentration diagrams and viscosity dependence over time for CSS/IO₃⁻-based gels before (a,c) and after (b,d) visible-light exposure for 1 h. Numbers 1, 2, 3, 4, and 5 on graphs (c,d) correspond to samples 8, 10, 12, 14, and 16 (see Table, Figure 1B).

Figure 3

SEM images and EDS of hydrogels before (dark) and after visible-light exposure (light) for 1 h. Images (a–e) correspond to gel systems 8, 10, 12, 14, and 16 (see Table, Figure 1B). EDS data are presented for irradiated samples. Photos are provided for the corresponding irradiated gels.

Figure 4

UV spectra of the systems before (a) and after (b) visible-light exposure: 1, 2, and 3—aqueous solutions of KClO₃, KBrO₃, and KIO₃ respectively; 4—CSS; 5—CSS/ClO₃⁻ and CSS/BrO₃⁻; 6—CSS/IO₃⁻. Data are presented for systems 2, 4, 6, 8, 10, 12, 14, 16, 18, and 20 (Figure 1B). (c) Comparison of UV spectra of the CSS/IO₃⁻ systems before (bold lines) and after (dash lines) light irradiation. (d) Kinetics of UV spectra evolution for the CSS/IO₃⁻ system 20 (Figure 1B) in the dark and under light exposure.

Figure 5

Particle size and zeta potential distributions for CSS/IO₃⁻ systems before (a,c) and after (b,d) visible-light exposure for 1 h. 1, 2, 3, 4, and 5 correspond to systems 8, 10, 12, 14, and 16 (see Table, Figure 1B), respectively.

Figure 6

The XPS survey scan of CSS/IO₃⁻ gel 16 (see Table, Figure 1B) after visible-light exposure for 1 h. The XPS spectrum for the non-irradiated hydrogel 16 remained unchanged.

Figure 7

High-resolution XPS spectra of (a) I 3d, (b) Ag 3d, (c) Ag MNN, (d) S 2p, (e) O 1s, and (f) N 1s for the CSS/IO₃⁻ gel 16 (see Table, Figure 1B) after visible-light exposure for 1 h.

Figure 8

Photos of CSS with various anions: (a) immediately after adding electrolyte; (b) after 24 h in the dark; (c–e) after visible-light exposure of samples (photo (b)) for 20, 40, and 60 min, respectively. (f) the UV spectra of CSS with various anions (samples on the photo (e)) after visible-light exposure for 1 h. The final electrolyte concentration corresponds to the system 16 (see Table, Figure 1B).

Figure 8

Photos of CSS with various anions: (a) immediately after adding electrolyte; (b) after 24 h in the dark; (c–e) after visible-light exposure of samples (photo (b)) for 20, 40, and 60 min, respectively. (f) the UV spectra of CSS with various anions (samples on the photo (e)) after visible-light exposure for 1 h. The final electrolyte concentration corresponds to the system 16 (see Table, Figure 1B).