Photo-Crosslinked Coumarin-Containing Bis-Urea Amphiphile Hydrogels

Abstract

The design of photo-responsive supramolecular hydrogels based on coumarin dimerization and de-dimerization is described. The photo-responsive coumarin unit is chemically incorporated into an oligo(ethylene glycol) (OEG) bis-urea amphiphile that is capable of co-assembling with non-functionalized OEG amphiphile, to form supramolecular fibers. UV light with two different wavelengths (365 nm and 254 nm) is employed to induce a photo-reversible dimerization and de-dimerization process of coumarin units, respectively. The co-assembled solutions could be photo-crosslinked to induce a sol-to-gel transition through dimerization of coumarin with 365 nm UV light, and de-dimerization occurs with 254 nm UV light, to provide a weaker gel. In this system, the mechanical strength of supramolecular hydrogels can be tuned using the irradiation time, providing precise control of gelation in a supramolecular hydrogelator.

Keywords: bis-urea amphiphile; coumarin; photo-crosslinking; supramolecular hydrogels.

Scheme 1

(a) Chemical structures of OEG bis-urea amphiphile (P8-10 OMe) and coumarin functionalized OEG amphiphile as photo-crosslinker (Cou-crosslinker). (b) Dimerization and de-dimerization of coumarin groups under UV light. (c) Schematic illustration of photo-crosslinked supramolecular fiber networks.

Scheme 2

Synthetic procedure of Cou-OEG300.

Figure 1

The model reaction of Cou-OEG300 under UV light. (a) Optical image of 10 mg/mL turbid Cou-OEG300 aqueous solution and UV-induced dimerization of coumarin units in micelles. (b) Reversibility study of UV-induced dimerization monitored by ¹H NMR.2.2. Synthesis of a Coumarin-Functionalized Photo-Crosslinker.

Scheme 3

Synthesis procedure of the coumarin-based crosslinker (Cou-crosslinker).

Figure 2

¹H NMR spectra of Cou-COOH (a), HO-functionalized OEG bis-urea amphiphile (M8-10 OH) (b), and coumarin based crosslinker (Cou-crosslinker) (c) in d₆-DMSO.

Figure 3

Time-dependent UV-Vis absorption spectra of 10 mg/mL OEG bis-urea amphiphile solution containing 10% Cou-crosslinker under UV light: (a) UV light 365 nm (Insert: absorbance changes at 320 nm recorded as a function of irradiation time) and (b) UV light 254 nm. The red arrows indicate change in absorbance change with time.

Figure 4

MALDI TOF MS results of the mixed solution containing 10% Cou-crosslinker before irradiation (a) and after irradiation with UV light with a wavelength of 365 nm for 10 min (b) and 30 min (c) and irradiation with UV light with a wavelength of 254 nm for 30 min (d).

Figure 5

DLS ((a), 1.0 mM) and SAXS ((b), 10 mg/mL) measurements of OEG bis-urea amphiphile aqueous solution containing 10% Cou-crosslinker before and after UV irradiation (365 nm, 30 min).

Figure 6

Representative cryo-TEM images of diluted mixed aqueous solution (1.0 mM) before irradiation (a) and after UV light irradiation (365 nm, 30 min) (b).

Figure 7

Rheological measurements of UV-induced crosslinked hydrogels. (a) Optical images of the mixed solution of 30 mg/mL of OEG bis-urea amphiphile with 10% Cou-crosslinker before and after UV irradiation. (b) Frequency sweeps of the mixed solution before and after UV light irradiation of 365 nm and 254 nm at a fixed oscillation strain of 1.0%. (c) Time-dependence of in situ measurements of the photo-crosslinked hydrogel by turning on or off UV light (365 nm, 7.2 mW/cm²). The measurement temperature was set at 20 °C.