Functional regulation of polymer aerogels by graphene doping and peptide nanofiber-induced biomineralization as sustainable adsorbents of contaminants

In this work, we synthesized functional hybrid aerogels with tailored functions for waterpurification application. It shows a few significances in environmental science. 1) Green synthesis of hybrid aerogels as adsorbents of contaminants was reported. 2) 2D graphene nanosheets were used to regulate the porous structure of polymer aerogels,and peptide-medicated biomineralization process was utilized to regulate the function ofaerogels, which enhanced greatly the adsorption capacity of hybrid aerogels towards heavymetal ions and organic dyes. 3) The synthesized hybrid aerogels exhibited better sustainability than other similar adsorbentsvia the Ranked Efficiency Products (REP) analysis.

Functional regulation of polymer aerogels by graphene doping andpeptide nanofiber-induced biomineralization as sustainableadsorbents of contaminants Guozheng Yang,a Peng He,a Danzhu Zhu,a Keming Wan,a Hao Kong,a Xin Luan,a Long Fang,a* YanWang,a* and Gang Wei a* As a green sustainable inorganic material, hydroxyapatite (HA) has potential application in water purification. Aerogelmaterials with 3D structure have great advantages in the field of adsorption due to their low density and large specificsurface area. In this work, peptide nanofibers (PNFs) with motif-designed peptide sequence are synthesized via molecularself-assembly, which are then bound to the surface of graphene oxide (GO) nanosheets through π-π conjugation andprovided active sites for nucleation and growth of HA crystals during the subsequent biomineralization process. The hybridaerogel cross-linked between polyvinyl alcohol (PVA) and GO has a regular pore structure, and the mineralization processpromotes functional regulation of aerogels. The mineralized PVA/PNF/GO-HA hybrid aerogels show good adsorptionperformance for Pb2+, and the saturated adsorption capacity can reach 113.88 mg/g when Pb2+ is 400 mg/L (pH=5, 25°C). Atthe same time, the adsorption effect of PVA/PNF/GO-HA hybrid aerogel on MB is also obvious, and the adsorption capacityreaches 234.85 mg/g when the methylene blue (MB) concentration is 500 mg/L (pH=7, 25°C). In addition, the hybrid aerogelhas better stability in water, which is convenient for recovery after adsorption. Finally, the fabricated hybrid aerogels areevaluated for the sustainability using the Ranked Efficiency Product (REP) method. This study facilitates the design andsynthesis of nanohybrid aerogels through biomimetic synthesis with highly sustainable applications in water purification. 

1.Introduction With the development of modern technology, more and morepollutants are discharged into the natural environment as industrialwastewater, which seriously impacts ecological environment andhuman survival. The unsupervised discharge of industrial,agricultural, and biomedical effluents induces the primarygeneration of wastewaters with heavy metal ions and organicsynthetic dyes. At present, the purification treatment of these twomain pollutants has become a research hotspot.1, 2 Varioustechniques have been developed for the removal of pollutants fromwater, such as electrocoagulation,3 extraction,4 electrodialysis,5, 6 photocatalysis,7, 8 adsorption,9, 10 and others. In recent years, therehave also been many reviews summarizing the regulation andapplication of various materials in environmental processing.11-14 Among these techniques, the adsorption is considered to be themost promising treatment method due to its advantages of simpleoperation, low cost, and low energy consumption, as well as broadapplications.Hydroxyapatite (Ca10(OH)2(PO4)6, HA), an inorganic materialpresented in the hard tissues of bones and teeth, with its maincomponent of HA crystals combining with a well-arranged matrix oftype I collagen,15 has great application potential in wastewatertreatment due to its good adsorption performance, availability, andgreen economy. The development of HA and its composites has also attracted much attention. For example, Jing et al.16 constructed a"molecular cage" in an alkaline solution of glucose, and thensuccessfully synthesized ultra-small HA with a diameter of 7 nm,which showed high adsorption performance for mixed heavy metalions. Fang et al.17 proposed a simple chemical precipitation methodby adding zole during the synthesis of HA to control the crystallinityand morphology of HA, thereby microspherical HA crystals wereobtained. Similarly, Shi et al.18 developed a porous chitosan/HAcomposite membrane using chitosan as a carrier, which exhibitedhigh adsorption capacity and dynamic dye removal efficiency for azodyes.At present, there are many methods for synthesizing HAnanoparticles, such as the solid-state synthesis,19 mechanochemicalsynthesis,20 chemical precipitation,21 hydrolysis method,22 andothers, but these methods are complicated and expensive, and onlya few are satisfactory in terms of economy or performance. As agreen and simple synthesis method, biomimetic synthesis has hugeadvantages. In 1990, KoKubo et al.23 first proposed a cell-freesimulated body fluid (SBF) that could be used to precipitate HA onscaffolds. SBF is a metastable solution containing calcium ions andphosphate ions supersaturated with respect to apatite forbiomimetic growth of HA on templates under mild conditions. Tocontrol the nucleation and growth of HA crystals, functional organicmolecules with ordered functional groups are generally used asreaction templates for HA adsorption and self-assembly. Sufficientgroups can enrich Ca2+ from SBF species and generate localsupersaturation, followed by directional growth to form HAcrystals.24, 25 In recent years, studies have shown that collagenmolecules (such as type I collagen) and non-collagenmacromolecules (such as acid glycoproteins) in tissues are involved in controlling the nucleation and growth of HA crystals.26 In addition,some other natural or synthetic biomolecular matrix can also be usedas templates for HA crystal growth, such as the silk fibers,27 chitosanscaffolds,28 self-assembled peptide amphiphilic nanofibers,29 andothers. Wei et al.30 prepared amyloid fibrin fibrils at pH=2.0 in theabsence of thrombin and performed biomimetic mineralization inSBF solution, and they finally formed spherical carbonated HAcrystals. In a further study,15 protein hybrid nanofibers and proteinheterocyclic nanofibers were successfully cross-linked and thenbiomimetic mineralization was performed to obtain HA-loadedcarbon nanotube (CNT)-protein nanohybrid fiber films. The selfassembled protein nanofibers have exhibited great advantages in thepreparation of HA-based materials due to their controllablestructure, degradability, and high biocompatibility.Graphene oxide (GO), as an amphiphilic scaffold with highlyhydrophobic basal planes and hydrophilic edges, possessesnumerous excellent properties, such as large specific surface area,high mechanical strength, and a large number of functional groups,and active sites.31-33 More importantly, GO, as a two-dimensional(2D) nanosheet, can serve as a carrier and template for biomimeticgrowth and synthesis of nanomaterials with 3D structures, which isundoubtedly an excellent choice for adsorption materials for sewagetreatment. Wang et al.34 reported a 3D graphene aerogel crosslinked with thiourea, which could be used to adsorb dyes and heavymetal ions in water. By cross-linking cellulose and GO, Yakout et al.35 obtained composite films with adsorption capacities of 46.39 and186.48 mg/g for Cu2+ and Pb2+, respectively.

In previous reports, nanofibers formed by self-assembly ofsequence-designed peptide molecules were used as scaffolds forbiomimetic growth of HA, and GO was used as a carrier to loadpeptide nanofibers (PNFs) and biomimetic mineralized HA crystals inSBF.36 Inspired by this, we propose the design of a novel peptidemolecule (the amino acid sequence and structural model are shownin Fig. 1) in which the YWYAF sequence has the ability to bind to thecarbon surface through sequence-specific material recognition. TheKIIIIK peptide motif is a typical β-sheet amino acid sequence, andshort peptides of this similar sequence were found in a previousstudy to tend to form nanofibrous structures through selfassembly.37 We observe the self-assembly process of PNFs in thesystem of trifluoroacetic acid (TFA) and trifluoroethanol (TFE) andsuccessfully incubate short PNFs with a certain length. Then the selfassembled PNFs are combined with GO and hybridized with polyvinylalcohol (PVA). The abundant hydroxyl groups on PVA can formhydrogen bonds with the abundant hydroxyl and carboxyl groups onGO, thereby enhancing the mechanical properties of the hybridaerogel.38 The mixed suspension is freeze-dried to obtain a completeaerogel, which is then soaked in SBF to mediate biomimetic growthof HA nanocrystals (Fig. 1). This PNF-doped GO-PVA aerogel has a uniform and regular pore structure and exhibits good adsorption oforganic dyes. In addition, the PVA/PNF/GO-HA hybrid aerogelobtained by biomimetic mineralization exhibits specific adsorptioneffect on Pb2+, which has potential application prospects in sewagetreatment and environmental remediation.

2. Materials and methods 2.1 Materials and reagents GO suspension (concentration 10 mg/g) was purchased from theHangzhou GaoXi Technology (Hangzhou, China). KIIIIKYWYAFsequence peptide was obtained from the SynPeptide BiotechnologyCo., Ltd. (Nanjing, China). Trifluoroacetic acid (TFA) (99%),trifluoroethanol (TFE) (99.5%), and PVA type 1799 (degree ofalcoholysis 98%-99%) were purchased from the Macklin Company(Shanghai, China). NaHCO3 (99.8%), MgCl2 (99.5%), CaCl2 (96%),K2HPO4, and tris(hydroxymethyl)aminomethane hydrochloride (Tris)were purchased from the Ron's reagent. NaSO4 (99%), NaCl (99.5%),and KCl (99.5%) were purchased from the Sinopharm ChemicalReagent Co., Ltd. 2.2 Peptide self-assembly to form PNFs KIIIIKYWYAF sequence polypeptide powder of 5 mg was firstdissolved into a mixed solution containing 9 mL of 0.1% TFA and 1 mLof TFE, and then stirred to form a homogeneous peptide solution.Followed by the peptide solution was placed in a water bath at 47°Cfor self-assembly of peptide monomers. Finally, a certain timeinterval of the solution was taken for atomic force microscopy (AFM)characterization. 2.3 Preparation of PVA/PNF/GO hybrid aerogels PNF solution of 8 mL was mixed with GO suspension of 60 mg, andafter stirring for 2 h, PVA aqueous solutions of differentconcentrations was add to the mixed solution, in which theconcentration of GO was 3 mg/mL, and the concentration ratio ofPVA and GO was 2:1, 1 :1, 1:3, and 1:5, respectively. Afterwards, themixed suspension was stirred and reacted at room temperature for6 hours, followed by poured into a plastic petri dish and placed in arefrigerator (-18°C) for 12 hours after standing for 10 minutes.Finally, the solution was freeze-dried in a vacuum freeze-dryingmachine for 24 h to obtain PVA/PNF/GO hybrid aerogels withdifferent ratios. 2.4 Biomimetic mineralization of PVA/PNF/GO-HA hybrid aerogels Biomimetic mineralization of the as-prepared PVA/PNF/GO hybridaerogels was performed according to previous reports. Briefly, thePVA/PNF/GO hybrid aerogels were soaked in 30 mL of 1.5x SBFsolution and used for bone-like mineral growth in a 37°C water bath(pH=7.4), and with the SBF solution being changed every other day.After five days, the samples were moved and washed with ultrapurewater to remove adsorbed salts, and then dried in a vacuum freezedryer for further characterization. The final ion concentration of1.5xSBF is 1.5 times higher than that of the human plasma and theSBF, as shown in Table 1.

2.5 Batch adsorption experiments The adsorption properties of PVA/PNF/GO and PVA/PNF/GO-HAhybrid aerogels for organic dyes and heavy metal ions wereinvestigated by batch equilibrium experiments. Pb2+ and Sb3+ wereselected as heavy metal ions, while methylene blue (MB) wasselected as organic dyes. In addition, two parallel samples, namelyPVA aerogel and PVA/GO aerogel, were selected for comparativestudies. Batch adsorption experiments were performed by addingquantitative sorbent and 10 ml of heavy metal ion solution or organicdye solution with different initial concentrations into a 20 mL glassbottle. Considering the effect of the initial concentration of metalions/organic dyes, the amount of adsorbent was 2 g/L. To eliminatethe effect of HA on the adsorption of Pb2+ by HA, the initial pH of Pb2+ was set to 5. The glass vial was placed in a shaker at 25°C and 150r/min. After a period of reaction, the solution was separated byfiltration through a 0.45 µm filter. The residual concentration oforganic dyes was analyzed by UV-vis spectrophotometry, and theresidual concentration of heavy metal particles was detected byinductively coupled plasma atomic emission spectrometry. Theremoval efficiency of heavy metal ions and organic dyes and theadsorption capacity of adsorbents for metal ions were calculated bythe following formula:(1)