Self-Assembled Microporous Peptide-Polysaccharide Aerogels for Oil

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Interface-Rich Materials and Assemblies

Self-assembled microporous peptidepolysaccharide aerogels for oil-water separation Xin Yang, Yanyan Xie, Yuefei Wang, Wei Qi, Renliang Huang, Rongxin Su, and Zhimin He Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01723 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 25, 2018

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Self-assembled microporous peptide-polysaccharide aerogels for oil-water separation Xin Yang,a,‡ Yanyan Xie,a,b,‡ Yuefei Wang,a,d,* Wei Qi,a,d,e,* Renliang Huang,c Rongxin Su,a,d,e and Zhimin Hea

a.

State Key Laboratory of Chemical Engineering, School of Chemical Engineering and

Technology, Tianjin University, Tianjin 300072, P. R. China. b.

Key Laboratory of Industrial Fermentation Microbiology, Ministry of Education, School of

Biotechnology, Tianjin University of Science and Technology, Tianjin (300457), P. R. China c.

Tianjin Key Laboratory of Indoor Air Environmental Quality Control, School of

Environmental Science and Engineering, Tianjin University, Tianjin 300072, P. R. China. d.

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin

300072, P. R. China. e.

Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin 300072,

P. R. China.

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ACS Paragon Plus Environment

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ABSTRACT We report a new kind of peptide-polysaccharide aerogel which was formed by the co-assembly of the fluorenylmethyloxycarbonyl-diphenylalanine (Fmoc-FF) peptide and the polysaccharide konjac glucomannan (KGM). The porosity and hydrophobicity of the hybrid aerogels could be facilely tailored by modifying the mass ratio of Fmoc-FF and KGM. The aerogels with tunable architecture showed good performance for the separation of a wide variety of oil-water mixtures. The results provide an opportunity for the design of peptide materials as a new class of biocompatible absorbents with potential applications in biomedicine and separation.

Keywords: aerogel, dipeptide, konjac glucomannan, self-assembly, oil/water separation

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INTRODUCTION Aerogels are a special kind of gel that are prepared by replacing liquids with air to keep the network structure intact, and commonly obtained by drying organic gels or hydrogels1-4. With the development of preparation technology, aerogels have attracted a lot attention. Supercritical CO2 drying and freeze drying are frequently used methods in general5, 6. Since the disperse medium of aerogels is air and the pore structure is at the micro- or nanoscale, aerogels are among the solid materials with the lowest density7. Owing to the advantages of high porosity, large surface area, low thermal conductivity and low refractive index, aerogels has potential applications in various fields8-13. For example, monolithic silica aerogels could be used as transparent insulation in windows because of its optical quality and low thermal conductivity14. Carbon aerogels could be utilized as electrodes for supercapacitors with long cycle life and enhanced electrochemical performance15. In recent years, aerogels formed by biomacromolecules, such as amyloid aerogels16-18, whey protein aerogels19 or soy protein aerogels20, have attracted great interests, which could be regarded as a new kind of ultralight biomaterials21. Self-assembling peptides have attracted a lot of attention in recent years. Short peptides could be used as fundamental building blocks for bottom-up fabrication. They are easily for chemical modification and have excellent capacity to self-assemble into diverse nanostructures22-25. Owing to their biodegradability and biocompatibility, self-assembling peptides are promising for valid biological applications26-29.

A

well-known

example

is

the

3


diphenylalaine

peptide

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(H-L-Phe-L-Phe-OH, FF), which have been shown to self-assemble into various nanostructures with potential applications in various fields. For example, FF has been reported that it could form nanofibrous hydrogel with lipophilic nanocrystals encapsulated in it30. By introducing reduced graphene oxide decorated by nickel nanoparticles, self-assembled FF peptide microtubes displayed a large radial piezoresponse that suggests a possible design for functional bio-nanostructures31. Moreover, modification at the N-ternimus of FF with different groups such as fluorenylmethyloxycarbonyl, ferroceyl, butoxycarbonyl group, further imparts FF with new functionalities, such as stimuli-responsiveness, chiral amplification and cell culture32-34. Fmoc-FF hydrogel was reported to be used as a 3D cell culture scaffold, which would lead to improved cell study results by better modeling in vivo growth environments35. However, exploring such short peptides to prepare functional aerogels with new applications has not yet been reported. In

this work, we reported a facile strategy

to

prepare microporous

peptide-polysaccharide aerogels by freeze-drying the hydrogels formed by the co-assembly of a simple dipeptide and the polysaccharide Konjac glucomannan (KGM). The peptide consists of a fluorenylmethyloxycarbonyl group linked to the C-terminus of the diphenylalanine peptide (Fmoc-FF), which has been shown to self-assemble into hydrogels composed of 1D nanofibers in water36, 37. While the KGM is a functional polysaccharide with superior hydrophilism and remarkable gelation ability38. The microstructure of the hybrid aerogels could be tailored by simply changing the mass ratio of Fmoc-FF and KGM. The aerogels could be used as 4


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biocompatible absorbents and showed excellent performance for the separation of a wide variety of oil-water mixtures.

EXPERIMENTAL SECTION Materials The lyophilized form of fluorenylmethyloxycarbonyl-diphenylalanine peptide (Fmoc-FF) was purchased from Bachem (Switzerland). Purified Konjac glucomannan (KGM,

92%) was from Shengtemeng Konjac Powder Co. (Sichuan, China). Oil

reagents such as cyclohexane and dichloromethane were from Aladdin Biotechnology Co. (Shanghai, China). Synthesis of hydrogels In a typical procedure, 40 mg Fmoc-FF was dissolved in 10 mL water at pH 10-10.7 by the addition of 0.5 M NaOH. 40 mg KGM powders were dispersed in 10 mL water and stirred for 8 h at 60 °C. Then, 4 mg/mL Fmoc-FF peptide solution and 4 mg/mL KGM solution were mixed with volume ratios ranging from 1:3 to 3:1. The resulting mixture was heated at 60 °C under stirring for about 10 minutes. The pH of the mixture solutions was then adjusted to 7 by adding 0.1 M HCl. The resulting mixture was aged at room temperature for 2 days without disturbance. Then the hybrid hydrogels were obtained. Aerogels preparation The resulting hydrogels were first quickly frozen in liquid nitrogen and then incubated in a lyophilizer and vacuum freeze-dried for 24 hours, leading to the formation of the aerogels. The aerogels were then preserved in a desiccator. 5


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Scanning electron microscopy All the aerogels samples were sputter-coated for 90 seconds with platinum using E1045 Pt-coater (Hitachi High-technologies CO., Japan), and then imaged by a S-4800 field emission scanning electron microscope (SEM, Hitachi High-technologies CO., Japan) at an acceleration voltage of 3 kV. Fourier transform infrared spectroscopy The aerogels samples were deposited on the surface of KBr plate and air-dried at room temperature. The dried samples were then transferred to a desiccator before measurement. Fourier transform infrared spectroscopy (FTIR) spectra of the aerogels were recorded on a Nicolet-560 FTIR spectrometer (Nicolet Co., USA) with 20 scans and a resolution of 4 cm-1. Wide-angle X-ray scattering (WAXS) The WAXS measurements were performed at beamline 1W2A of the Beijing Synchrotron Radiation Facility (BSRF, Beijing, China). The wavelength of the X-rays was 0.154 nm, and the distance from the sample to the detector was set at 146 mm. The peptide solutions were placed in cuvettes with a 1.0 mm path length. Samples were irradiated for 200 or 500 s, and the scattered radiation was detected using a Mar CCD detector. The 1D scattering profiles were obtained by radial integration of the 2D patterns, with scattering from the cuvettes subtracted as the background. Scattering profiles were subsequently plotted on a relative scale as a function of the scattering vector q = (4π/λ) sin(θ/2), where θ is the scattering angle.

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Oil/water separation The dried aerogels samples were used to absorb oils such as cyclohexane and dichloromethane from water, respectively. To have a better illustration, the oils were stained by rathonum red. The weight of the aerogels before (M0) and after the full adsorption of different organic liquids (Msat) were measured on a Sartorius microbalance (Model BP211D) at room temperature. Nitrogen gas adsorption and desorption isotherms Nitrogen gas sorption was carried out on an AUTOSORB-1-C analyzer from Quantachrome Instruments using nitrogen adsorption at liquid-nitrogen temperature (77 K). The samples were first degassed under vacuum at 100 °C for 4 h for preparation. The specific surface areas of aerogels were measured by the Brunauer–Emmett–Teller (BET) equation, and the pore size distributions were derived from the isotherms with the BJH model (for mesoporous). Contact angle measurements In order to characterize the surface wettability, the contact angles of the aerogels were measured using a sessile drop method with a contact angle analyzer (OCA15EC, Dataphysics, Germany). Measurements were performed at room temperature with 5 µL water and cyclohexane. Aerogels were flattened for test.

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RESULTS AND DISCUSSION

Scheme 1. Schematic illustration showing the aerogel preparation and application.

Scheme 1 illustrated the procedure for the fabrication of peptide-polysaccharide hybrid aerogels and its application for the oil/water separation. Fmoc-FF could self-assemble into nanofibers by the π-stacking interactions between fluorenyl groups and phenyl rings36, 39. Then the Fmoc-FF hydrogel could directly form aerogels by quick freeze of liquid nitrogen and vacuum freeze-drying. However, the mechanical strength of the pure peptide aerogel was quite weak. This problem could be overcome by blending the KGM polymer solutions with the peptide solutions40, by which the hybrid hydrogel obtained was composed of nanofibril networks decorated with KGM chains and peptide nanofibers (Scheme 1b). This resulted in more stable and better

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mechanical strength of hydrogels, so that we can acquire aerogels successfully and study its potential applications (Scheme 1c-d).

Figure 1. a-d) Photographs of Fmoc-FF/KGM composite hydrogels with different mass rations: 1:3 Fmoc-FF/KGM hydrogel (a), 1:1 Fmoc-FF/KGM hydrogel (b), 3:1 Fmoc-FF/KGM hydrogel (c) and KGM solution (d). e-h) Photographs of aerogels: 1:3 Fmoc-FF/KGM aerogel (e), 1:1 Fmoc-FF/KGM aerogel (f), 3:1 Fmoc-FF/KGM aerogel (g) and KGM aerogel (h).

Fig. 1 shows the photographic images of the Fmoc-FF/KGM composite hydrogels and aerogels at different mass ratios. It should be noted that the Fmoc-FF hydrogel (Fig. S1) could also transform into aerogel41 after freeze-drying, however the mechanical strength was very weak. In order to enhance the strength of the network structure, we introduce KGM, a neutral polysaccharide with excellent gelling properties42 and has been proved to enhance the mechanical strength of the hydrogel by mixing with Fmoc-FF as additives40. We found that KGM solution could form an aerogel successfully through quickly frozen in liquid nitrogen and freeze-drying as present in Fig. 1d and h. Based on this point, through the addition of KGM with different mass ratios into the peptide solutions, all of the hybrid hydrogels could form white, cotton-like aerogels with relating higher mechanical strength (Fig. 1e-g). KGM made it possible for the hybrid hydrogel to form well shaped aerogels after freeze-drying. However, the mechanical strength of the composite aerogel was still 9


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relatively weak in comparison to inorganic aerogels such as graphene aerogels43, 44. It has been reported that short peptide containing tyrosine could covalently self-assemble into lamella films with an elastic modulus of about 30 GPa45. Covalent bonds such as cysteine disulfide bonds has been reported46 to bring about a 60-fold increase in storage modulus of the peptide hydrogel. Additives such as enzyme47, which could be used as the cross-linking agent, could increase the storage modulus without dramatically changing the architecture of the hydrogel. These methods might provide potential pathways for further improving the mechanical property of the peptide-based aerogels.

Figure 2. Scanning electron microscopy (SEM) images of the aerogels. a-d) SEM images of the Fmoc-FF (a), 1:3 Fmoc-FF/KGM aerogel (b), 1:1 Fmoc-FF/KGM aerogel (c) and 3:1 Fmoc-FF/KGM aerogel. e-f) SEM images of the KGM aerogel.

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We investigated the microstructure of the aerogels using scanning electron microscopy (SEM). As shown in Fig. 2a-d, Fmoc-FF was composed of self-assembled nanofibers with diameter of ~50 nm. While in the composite aerogels, the Fmoc-FF nanofibers interwound with each other resulting in a porous network structure. The micropores observed by SEM were nearly several hundreds of nanometers. We speculated that it was the addition of KGM that enhanced the mechanical strength of the hydrogel, and the resulting network structure was able to maintain after freeze-drying to form well-shaped aerogels. Intriguingly, we also found that with the increase of the peptide mass ratio in the hybrid aerogels, the average pore diameter became smaller. As a contrast, Fig. 2e-f exhibited pore structures of the KGM aerogel, it was composed of structure similar to lamellar sheets and the pores were dozens of micrometers, which were probably caused by the freeze-drying process41, 48, 49. The results indicated that the microstructure and micropore size of the aerogels could be tailored by changing the mass ratio of the peptide and KGM. We explored the specific surface area and mesopore structure through nitrogen gas adsorption and desorption. As shown in Fig. S2d, the specific BET surface area of KGM aerogel was only 30.79 m2/g and the pore size was distributed evenly in the range of 2-50 nm. While the specific surface areas of 1:3, 1:1 and 3:1 Fmoc-FF/KGM aerogels were 42.24 m2/g (Fig. S2a), 47.17 m2/g (Fig. S2b), and 58.42 m2/g (Fig. S2c). Both of the specific BET surface area and total pore volume became larger when the peptide ratio within the hybrid aerogels was increased. The mesoporous measured through nitrogen gas adsorption and desorption also became 11


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smaller with the less additives, this change was in accordance with that of the macropores observed in SEM images. Additionally, the apparent densities (

)

of 1:3, 1:1 and 3:1 Fmoc-FF/KGM aerogels were about 7.3 mg/cm-3, 10.4 mg/cm-3 and 11.5 mg/cm-3, respectively. According to the calculation formula of porosity: φ=1-

, we could infer that the porosity of composite aerogel increased with the

augment of peptide proportion. So that the 3:1 Fmoc-FF/KGM aerogel possessed the highest porosity, pore volume and specific surface area among all the aerogels with different ratios between peptide and polysaccharide.

Figure 3. a) WAXD analysis of Fmoc-FF and 3:1 Fmoc-FF/KGM gels. b) CD analysis of Fmoc-FF gel, 3:1 Fmoc-FF/KGM gel and KGM solution. c) FTIR spectra of Fmoc-FF aerogel, 3:1 Fmoc-FF/KGM aerogel and KGM aerogel.

In situ wide-angle X-ray diffraction (WAXD) and circular dichroism (CD) were used to probe the molecular packing of Fmoc-FF molecules within the hydrogels before and after the incorporation of KGM polymers. The peptide hydrogels before the incorporation of KGM polymers produced scattering peaks at 13.38 and 14.39 nm-1, corresponding to spacings of 4.93 and 4.67 Å50-52, respectively (Fig. 3a and Fig. S3a), which could be attributed to the periodic spacing between peptides within the β-sheet structure39, 53, 54. We also observed a very weak diffraction peak at 18.15 nm-1 in the high q range (Fig. 3a). This value corresponds to a spacing of 3.54 Å, 12


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which might be attributed to the known π-π stacking distances53,

55

between the

aromatic side chains and the Fmoc protecting groups. After the incorporation of KGM, the Fmoc-FF/KGM hybrid hydrogels showed similar diffraction peaks with that of the neat peptide hydrogel (Fig. 3a and Fig. S3b), indicating a similar molecular packing of Fmoc-FF within the hybrid gels. CD analysis revealed that both of the peptide hydrogels and the Fmoc-FF/KGM hybrid gels showed a red-shifted signature band at 221-223 nm in the peptide region (Fig. 3b), which could be attributed to the highly twisted β-sheets56-58. Moreover, two strong, positive Cotton effects at 270 nm and 303 nm was also observed, indicative of the strong π-π stacking interactions39 between the molecules within the self-assembled peptide nanofilaments. To verify the influence of freeze-drying on the secondary structure of the peptide assemblages. Fourier transform infrared (FTIR) spectroscopy was further performed. As shown in Fig. 3c, the KGM aerogels exhibited no obvious adsorption peak in the spectra range of 1500-1800 cm-1. However, for the 3:1 Fmoc-FF/KGM aerogel and Fmoc-FF aerogels, both of them showed a strong adsorption peak centered at 1660 cm-1, which could be attributed to the β-sheet structure formed by the self-assembly of Fmoc-FF peptide39. In the spectra range of 3750-2400 cm-1, the KGM aerogel showed a single O-H stretching vibration at 3407 cm-1. While the spectra of the Fmoc-FF aerogel had two N-H stretching vibrations59, a weaker one at 3407 cm-1 and a sharp at 3303 cm-1. The spectra of Fmoc-FF/KGM aerogel had a broader peak at 3407 cm-1 due to the O-H stretching vibration of KGM and the N-H stretching vibration of Fmoc-FF, and a N-H stretching vibration at 3303 cm-1. The WAXS, CD and FTIR 13


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spectrum results showed that the addition of KGM and freeze-drying process changed little on the peptide molecular arrangement within the Fmoc-FF/KGM aerogels.

Figure 4. Wettability and absorption of organic reagents of the Fmoc-FF/KGM aerogels. a-c) The oil (cyclohexane) contact angle of 1:3 Fmoc-FF/KGM aerogel (a), 1:1 Fmoc-FF/KGM aerogel (b) and 3:1 Fmoc-FF/KGM aerogel (c). d-f) The water contact angle of 1:3 Fmoc-FF/KGM aerogel (d), 1:1 Fmoc-FF/KGM aerogel (e) and 3:1 Fmoc-FF/KGM aerogel (f). g) Diagram of absorption capacity of 3:1, 1:3 and 1:1 Fmoc-FF/KGM aerogels for different organic reagents and water. Weight gain here is defined as the weight ratio of the absorbate to the dried aerogel. h) Absorption of cyclohexane (dyed with rathonum red) on water by 3:1 Fmoc-FF/KGM aerogel. i) Absorption of dichloromethane (dyed with rathonum red) in water by 3:1 Fmoc-FF/KGM aerogel.

The Fmoc-FF/KGM aerogel is a kind of porous biomaterial, which makes it an 14


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ideal candidate for the adsorption of organic pollutants and oils. Aerogels of different ratios exhibited different surface wettability were showed in Fig. 4a-f. We found that all composite aerogels were superoleophilic, and the 3:1 Fmoc-FF/KGM aerogel absorbed the oil droplet completely with a contact angle reaching 0° (Fig. 4c). The 3:1 Fmoc-FF/KGM aerogel was also hydrophobic, of which the water contact angle was about 93.6 °. The results indicated that with the increase of KGM, the hybrid aerogels became more hydrophilic and less oleophilic, which might be attributed to the swelling effect of KGM in water and the oleophilicity of Fmoc-FF. To have a better illustration, the oils were colored with rathonum red. The weight of the aerogels before (M0) and after the full adsorption of different organic liquids (Msat) was measured on a Sartorius microbalance (Model BP211D) at room temperature. The mass adsorption capacity (Mabs) of the aerogels was determined by the equation (1). Mabs = (Msat-M0) / M0

(1)

We explored the absorption efficiency of the hybrid aerogels for different oils and investigate the absorption properties of the aerogels at different mass ratios of Fmoc-FF to KGM. As illustrated in Fig. 4g, nearly all hybrid aerogels could easily absorb oil rather than water, especially the 3:1 Fmoc-FF/KGM aerogel. The 1:3 Fmoc-FF/KGM aerogel performed better to reagents such as dichloromethane and tetraethoxy silicone. While the 3:1 Fmoc-FF/KGM aerogel performed more efficiently to nonpolar n-hexane and n-heptane solvents. The adsorption capacity of 1:1 15


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Fmoc-FF/KGM aerogel for oils was worse than that of the 3:1 Fmoc-FF/KGM aerogel, but better than the 1:3 aerogel. The capacity and efficiency for absorption were influenced by several factors, such as porosity, pore volume and wettability. The 3:1 Fmoc-FF/KGM aerogel was hydrophobic and the most oleophilic, while the others were hydrophilic. Moreover the 3:1 Fmoc-FF/KGM aerogel exhibited higher porosity and pore volume than other ratios, so that it has the best adsorption capacity for oils. As a result, we could control the adsorption capacity, hydrophilicity and oleophilicity of aerogels by changing the ratios between Fmoc-FF and KGM. Moreover, since the 3:1 Fmoc-FF/KGM aerogel displayed a much higher adsorption capacity for oils than that of water (Fig. 4g), we used it for oil/water separation. As shown in Fig. 4h and Supplementary Video S4, cyclohexane dyed with rathonum red was spread on water and the 3:1 Fmoc-FF/KGM aerogel could selectively absorbed the oil successfully. Meanwhile, the 3:1 Fmoc-FF/KGM aerogel could also absorb dichloromethane in water displayed in Fig. 4i. Since the 3:1 Fmoc-FF/KGM aerogel performed well in absorbing oils that were pollutants to the environment, this kind of peptide-polysaccharide aerogel might provide a new method for biocompatible absorbents.

CONCLUSIONS In conclusion, we have successfully synthesized peptide-polysaccharide aerogels using short peptide and polysaccharide as raw materials through molecular self-assembly and freeze-drying. The incorporation of KGM into Fmoc-FF peptide 16


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hydrogels changed greatly on the microstructure of the formed aerogels but has little influence on the molecular arrangement of the peptides. Owing to the oleophilicity of Fmoc-FF and the hydrophilicity of KGM, the 3:1 Fmoc-FF/KGM aerogels could efficiently absorb oil liquids from water. Hence the resulting composite aerogels offer opportunities for the potential applications of peptide materials as a new class of biocompatible absorbents to realize oil/water separation.

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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Photograph of Fmoc-FF hydrogel, nitrogen gas adsorption and desorption isotherms, distributions of pore volumes of different aerogels, 2D X-ray diffraction patterns (PDF), supplementary video for selective absorption (AVI)

AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected]

Authors' Contributions. ‡These authors contributed equally to this work. Notes. The authors declare no competing financial interest. ACKNOWLEDGMENT. This work was supported by the National Natural Science Foundation of China (Nos. 21621004, 21476165, 21606166, and 51773149), the Beiyang Young Scholar of Tianjin University (2012), and the State Key Laboratory of Chemical Engineering (Nos. SKL-ChE-08B01). The authors thank Prof. Zhonghua Wu and Dr. Guang Mo of BSRF for assistance with the X-ray scattering measurement. ABBREVIATIONS. Fmoc-FF: fluorenylmethyloxycarbonyl-diphenylalanine; KGM: Konjac glucomannan

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Self-Assembled Microporous Peptide-Polysaccharide Aerogels for Oil

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