Amyloid fibril-UiO-66-NH 2 aerogels for environmental remediation †

A sustainable hybrid aerogel based on b -lactoglobulin amyloid fibril/UiO-66-NH 2 is developed for environmental remediation. The hybrid aerogel’s CO 2 capture and water purification performances were investigated. The hybrid aerogel can achieve CO 2 capture and possesses excellent adsorption capacities for several heavy metals, dyes, and organic solvents.

UiO-66-NH 2 particles on the amyloid network surface.The hybrid aerogels were ultralight and robust, the density was approximately 40 mg cm À3 , and the Young's modulus was approximately E = 2.0 Â 10 À2 MPa (Table S2, ESI †).The compressive strain and stress curve of the hybrid aerogel is shown in Fig. S4 (ESI †).XRD patterns presented in Fig. 2c reveal the purity and crystallinity of the materials.The pattern of amyloid fibrils had no diffraction peaks and exhibited a broad amorphous peak due to b-Lg. 20The pattern of UiO-66-NH 2 was in accordance with previous literature, 21,22 indicating that pure and crystalline UiO-66-NH 2 was successfully synthesized.The pattern contains the characteristic main peaks of UiO-66 at 2y = 7.341 and 8.481. 23Moreover, the presence of the -NH 2 group in UiO-66 did not affect the XRD pattern due to the MOF structure. 24In the pattern of the hybrid aerogel, the same characteristic peaks of UiO-66-NH 2 can be seen, suggesting that the hybridization with amyloid fibrils did not influence the crystal structure of UiO-66-NH 2 .6][27] The thermal stability of the aerogel was evaluated by thermogravimetric analysis (TGA) and shown in Fig. S5 (ESI †).No significant weight change was observed in N 2 air up to 250 1C, indicating a good thermal stability of the hybrid aerogel.The interaction between amyloid fibrils and UiO-66-NH 2 was also investigated using FTIR spectroscopy (Fig. 2d).For UiO-66-NH 2 , the bands at 3475 and 3363 cm À1 can be assigned to N-H symmetric and asymmetric vibrations, respectively. 28A wide and medium absorbance feature at approximately 3300 cm À1 proves the existence of H-bonded hydroxyls.The absorption band at 1570 cm À1 indicates the presence of the reaction of -COOH with Zr. 21The doublet at 1421 and 1387 cm À1 are attributed to the stretching modes of the carboxylic groups in the organic linker. 29The bands at 1337 and 1258 cm À1 are due to C aromatic -N vibration. 30he observed peaks between 600-800 cm À1 represent Zr-O 2 as longitudinal and transverse modes. 31The FTIR spectrum of amyloid fibrils exhibits the amide I region at 1630 cm À1 , reflecting CQO stretching vibration and the amide II region at 1525 cm À1 , due to N-H bending vibration and C-N stretching vibration, and the amide III region at 1230 cm À1 in the protein. 32The broad peak at approximately 3270 cm À1 can be ascribed to the -OH vibration. 33The slight shifts of peaks in the hybrid aerogel pattern can be attributed to the formation of hydrogen bonds between amyloid fibrils and UiO-66-NH 2 at the interface. 24Furthermore, two new peaks in the region between 900-1200 cm À1 appear which are tentatively attributed to the shift of carboxyl stretch of UiO-66-NH 2 , caused by the interaction of amyloid fibrils and UiO-66-NH 2 . 34ig. 3 presents the CO 2 capture performance of hybrid aerogels for different loadings of UiO-66-NH 2 near ambient temperature.The CO 2 uptake of the pure UiO-66-NH 2 and the hybrid aerogels containing 30%, 40%, and 50% UiO-66-NH 2 (weight percentage) were 1.14, 0.11, 0.16, and 0.25 mmol g À1 , respectively.The performance of pure UiO-66-NH 2 was comparable with published data of approximately 1.6 mmol g À1 . 35easurements of pure amyloid aerogel revealed that amyloid  This journal is © The Royal Society of Chemistry 2022 fibrils make no significant contribution to CO 2 capture (Fig. S6, ESI †), in contrast with previous findings on designed amyloids. 36,37Therefore, the hybrid aerogels possessed a larger capacity for CO 2 uptake with higher content of UiO-66-NH 2 .In the hybrid aerogel, while amyloid fibrils act as a robust platform for the materials, UiO-66-NH 2 endowed the materials with binding sites for CO 2 capture.The micropores in UiO-66-NH 2 favored the transport of CO 2 to binding sites and the retention of CO 2 , which is related to physical adsorption.In addition, the interactions of amino groups in UiO-66-NH 2 with CO 2 molecules were responsible for CO 2 capture performance. 14The results demonstrated that the adsorption capacity of the materials was maintained after three consecutive cycles of CO 2 sorption-desorption, confirming the high durability and reversibility of the hybrid aerogels.
Fig. S7 and S8 (ESI †) show the removal performances of the hybrid aerogels for different heavy metals.The mechanism for heavy metal adsorption is mainly attributed to the chemical chelation between amino acids on the amyloid surface and the heavy metal ions. 38The results revealed that the hybrid aerogels had a much higher capacity for Au 3+ (547 mg g À1 ) and Pt 4+ (186 mg g À1 ) than the pure amyloid fibrils aerogel.While the hybrid aerogels demonstrated excellent capacity for Cr 6+ (308 mg g À1 ) and Fe 3+ (226 mg g À1 ), amyloid fibril aerogel showed no capacity for these metals.Additionally, the hybrid aerogels exhibited high removal efficiency for Au 3+ , Fe 3+ , and Pt 4+ .The improved performance of the hybrid aerogel can be ascribed to UiO-66-NH 2 .The adsorption mechanism of UiO-66-NH 2 is expected to be the ion exchange and electrostatic effect of -NH 2 and -COOH groups, combined with the material's nano-porous structure. 39,40To investigate the competitive adsorption, the hybrid aerogel was immersed in a mixture solution containing 10 heavy metals, the results of which are presented in Fig. S9 (ESI †).The data demonstrate that the aerogel had high adsorption efficiency for Au 3+ , Fe 3+ , and Ag + .The efficiency for Pt 4+ was much lower compared with the separately conducted adsorption experiment, which might be because Ag + adsorbed faster than Pt 4+ , and they share the same binding sites.After the adsorption of Ag + , there were probably fewer binding sites available for Pt 4+ .Compared with other type of adsorbents (see Table S3, ESI †), the hybrid aerogels in this work exhibited excellent adsorption capacity for Au 3+ , which could be attributed to the synergies of UiO-66-NH 2 and amyloid fibrils.Fig. S10 (ESI †) shows the fitted binding isotherm of Au 3+ .The hybrid aerogel reduced Au 3+ into Au nanoparticles and pellets when the concentration of Au 3+ in solution was 500 ppm and 5000 ppm, respectively (Fig. 4a).The XRD patterns (Fig. S11, ESI †) revealed the characteristic peaks of Au(0), 41 indicating the redox reaction between the hybrid aerogels and Au 3+ .
The capability of hybrid aerogels for removing organic dyes was also investigated.The results shown in Fig. 4b and c indicate that the removal capacity for Rhodamine B, Crystal violet, Methylene blue, and Malachite green were 54.8, 27.1, 25.2, and 29.6 mg g À1 , respectively.The adsorption can be explained mainly by the hydrophobic interactions between the benzene ring of dyes and the hydrophobic domains of the amyloid fibrils. 38Moreover, Fig. 4d shows the rapid adsorption of n-hexane stained with Oil Red O within 10 s by the hybrid aerogel (ESI † Movie available).The reason for the excellent oil adsorption performance might be that the highly porous structure of the hybrid aerogel provides abundant capillary tunnels that drive the oil to pass through by capillary force rapidly, 42 and the oil remained trapped within interconnected micropores. 43 To evaluate the reusability of the hybrid aerogels, three subsequent adsorption-regeneration experiments were conducted and the results are presented in Fig. 4e-g.The basis adsorption performances, which means 100%, were obtained from fresh aerogels.The changes in adsorption performance are illustrated by light color in each panel.For Crystal violet, the performance for cycle 1, 2, and 3 increased by 16.8%, 2.7%, and 3.6%, respectively.The performance improvement could be explained by introducing new hydrophobic binding sites by methanol treatment. 2The washing could also increase the surface area and roughness of the aerogel by removing other impurities. 44For Fe 3+ and Pt 4+ , the performances decreased after washing, which might be because the regeneration cannot remove all of the pollutants, leading to fewer binding sites available for the next cycle.

Fig. 2
Fig. 2 Characterization of the hybrid aerogel.(a and b) SEM images of the inner structure of the hybrid aerogel.(c) The XRD patterns and (d) FTIR spectra of amyloid fibrils, hybrid aerogels, and UiO-66-NH 2 .

Fig. 4
Fig. 4 Water purification performance.(a) Reduced Au 3+ into Au nanoparticle and metallic gold on top of the aerogel.(b and c) Dye removal performance.(d) Rapid removal of organic solvents from water.(e-g) Changes of adsorption performance after three regeneration cycles.