1、Silica Aerogels with Self-Reinforced Microstructure for BioinspiredHydrogelsJinpei Wang,Yu Du,Jin Wang,* Wenbin Gong, Liang Xu, Lifeng Yan, Yezi You, Weibang Lu,and Xuetong Zhang*Cite This: Langmuir 2021, 37, 59235931Read OnlineACCESSMetrics & MoreArticle Recommendations*sSupporting InformationABSTR
2、ACT: Aerogel is a kind of high-performance lightweightopen-porous solids with ultralow density, high specific surface area,and broad application in many emerging fields includingbiotechnology, energy, environment, aerospace, etc. A giantchallenge remains in preventing of the hydrophilic aerogelframe
3、work shrinkage when replacing of solvent with air in itsextremely abundant nanosized pores during its fabrication processin ambient conditions. In this work, started from a linear polymericprecursor with further condensation reaction, superhydrophilicsilica aerogels with self-reinforced microstructu
4、re and the leastvolume shrinkage have been successfully obtained via ambientpressure drying process without use of any additives in thepresence of a low surface tension solvent. The resultingsuperhydrophilic silica aerogels possess specific surface area up to 1065 m2/g, pore volume up to 2.17 cm3/g
5、and density downto 84 mg/cm3, and these values are comparable to those of their counterparts obtained by supercritical CO2drying process.Moreover, as an application demonstration, the bioinspired hydrogels with desirable mechanical flexibility and adhesive performanceat extremely harsh environment (
6、e.g., below 50 C) have been successfully synthesized by mimicking carrier of a functionalbioagent with the resulting superhydrophilic silica aerogel microparticles. Our work has made a significant step forward for futurehigh-performance hydrophilic aerogels with self-enhanced microstructures and the
7、 resulting superhydrophilic aerogels have showngreat potentials in making functional hydrogels with bionic properties.INTRODUCTIONAerogel, a type of highly porous material with extremely highspecific surface area, ultralow density, and excellent thermalinsulation performance, etc. has found broad ap
8、plication in thefields of energy reduction, environmental remediation, drugdelivery, catalysis, and aerospace.15The building blocks of theaerogels are usually connected by weak physical interactionsuch as hydrogen bonding or van der Waals force (e.g.,supramolecular aerogel,6,7graphene aerogel,810and
9、 celluloseaerogel,1113) or chemical bonding (e.g., silica aerogel1418).Both physical interaction and chemical bonding are too weakto resist capillary force occurred during the ambient pressuredrying (APD) process. In order to obtain the ideal aerogels,special drying techniques such as supercritical
10、fluid drying arerequired,2,1922which are expensive, time-consuming, andhardly controlled with harsh conditions. Alternatively, hydro-phobilization,2329adsorption of metal ions,30or surfactant31of the gel solid network and subsequent solvent-exchange withlow surface tension solvent can help to withst
11、and extremelyhigh capillary force during APD. However, how tofundamentally design a network with self-reinforced structurethat could be free from the special drying, hydrophobilization,as well as solvent-exchange, is still a huge challenge.On the other hand, hydrogels, the parent materials ofaerogel
12、s, are a class of highly flexible networks contain largeamount of water up to 90 wt % and have been widely used intissue engineering, biomedicine, sensors, and electric devi-ces.3236Due to their high content of water, hydrogels facesevere problems that they may be frozen at subzerotemperatures and l
13、ose their desirable properties and specialstructures.3741Strategies to synthesize antifreezing hydrogelsare limited to complexes chemical structure design or solvent-exchange directly with antifreeze, in which they becameorganogels rather than hydrogels. Besides, the method is notgeneral and would r
14、esult in gel shrinkage due to the phaseseparation and precipitation of the network.Received:February 17, 2021Revised:April 23, 2021Published: May 3, 2021Articlepubs.acs.org/Langmuir 2021 American Chemical Society5923https:/doi.org/10.1021/acs.langmuir.1c00476Langmuir 2021, 37, 59235931Downloaded via
15、 TSINGHUA UNIV on August 10, 2021 at 22:57:14 (UTC).See https:/pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.To tackle those problems, we have creatively connectedspherical nanoparticles of silica aerogel building blocks withthick neck rather than traditi
16、onal thin one, which resulted in anenhanced connection that is sufficient to withstand APD freefrom both hydrophobilization and solvent-exchange. Thus, aseries of superhydrophilic silica aerogels with extremely highspecific surface area up to 1065 m2/g, pore volume up to 2.17cm3/g and low density do
17、wn to 84 mg/cm3have been directlysynthesized by the APD approach without use of any additives.Then, bioinspired strategy to synthesize antifreezing hydrogelsby releasing antifreezes and absorbing free-water with thesuperhydrophilic silica aerogels are proposed and successfullyrealized in this work.E
18、XPERIMENTAL SECTIONMaterials. Tetraethoxysilane (TEOS, AR), Hexane (AR), hydro-chloric acid (ca. 37 wt %) and ammonia aqueous solution (2528 wt%) were provided by the Sinopharm Chemical Reagent Co., Ltd.(Shanghai, China). Ethanol (99.7%, AR), Shanghai Titan ScientificCo., Ltd. (Shanghai, China). Pol
19、y(vinyl alchohol) (PVA) with aalcoholysis degree of 8789% (mol/mol) (Aladdin, China) wasdissolved in deionized water before use (10 wt %). Glutaraldehyde(ca. 50 wt % in water, ca. 5.6 mol/L) (Aladdin, China) was diluted to10 wt % before use and hydrochloric acid with a content of 36.038.0wt % (Shang
20、hai Lingfeng Chemical Reagent CO., Ltd., China) wasdiluted to 1 mol/L before use. Glycerol (99.0%) was purchased fromSinopharm Chemical Reagent Company. All other solvents andreagents were of analytical grade.Synthesis of the Superhydrophilic Silica Aerogels via APD.Condensed silica solutions (CS) w
21、ere synthesized with the molarratio of TEOS: ethanol: water (HCl concentration: 102mol/L) =1:4:1.2 (1.9). When the molar ratio of water to TEOS is 1.2:1, the CSis indicated as CS or T-TEOS.14,23,25When ratio is 1.9:1, the CS isindicated as ESOM in the manuscript. Take the synthesis of sample 1for ex
22、ample. Five mL ESOM were added into 7 mL Heptane understirring, then after addition of 1 mL NH3H2O (diluted concentratedNH3H2O by ethanol to 2 mol/L), the mixture was stood still and thegel was formed in 5 min. The gel was aged at 80 C for 0.5 h anddried under ambient pressure at 150 C for 2 h. Supe
23、rhydrophilicsilica aerogel was obtaiend for sample 1 as white microparticles. Allthe other samples were synthesized by the similar process.Synthesis of the Bioinspired Hydrogels. The bioinspiredhydrogel was synthesized by a two-step strategy. First of all, the silicaaerogel microparticles were dispe
24、rsed in glycerol, fully stirred toensure that the pores of the silica aerogel were filled with glycerol andthen centrifuged to remove the excessive amount of glycerol. Second,the bioinspired hydrogel was prepared by mixing silica/glycerolcomposite with PVA aqueous solution (10 wt %) and then cross-l
25、inked by glutaraldehyde under acidic condition. Take the synthesis ofbioinspired hydrogel (glycerol:H2O = 2:1) for example, 12.1 g silica/glycerol composite with the weight ratio of 1:8.5 was added into 6 gPVA aqueous solution (10 wt %) by vigorous stirring, and then 300L 1 M HCl was added with vigo
26、rous stirring for 5 min, finally 144L glutaraldehyde aqueous solution (10 wt %) was added into themixture by vigorous stirring and ultrasonication for 5 min in order toScheme 1. Synthesis of Hydrophobic Silica Aerogelaa(a) Traditional route for the synthesis of hydrophobic silica aerogel by APD usin
27、g TEOS as precursor. (b) New strategy for the synthesis ofsuperhydrophilic silica aerogel by APD using TEOS as the precursor. (c) The antifreezing mechanism of insects: some insets will produce andrelease antifreezing protein (AFP) into their tissues, which depresses the freezing point of fluids and
28、 inhibits the growth of ice crystals, while otherinsects will remarkably reduce their free water molecules in tissues (dehydration). (d) Bioinspired synthesis of the antifreezing hydrogel by usingsuperhydrophilic silica aerogel as the antifreeze carrier, in which the antifreezing agent will release
29、into the water phase of the hydrogel and thusreduce its freezing point, which is similar to those happened in some insects. (e) Schematic description of the non-adhesive PVA hydrogel. (f)Proposed adhesive mechanism on PVA hydrogel in the presence of glycerol.Langmuirpubs.acs.org/LangmuirArticlehttps
30、:/doi.org/10.1021/acs.langmuir.1c00476Langmuir 2021, 37, 592359315924remove air bubbles from the mixture, the bioinspired hydrogel wasformed in 30 min and aged for 2 h at 60 C.Characterizations. The hydrophobicity of the aerogels wascharacterized in an optical angle meter system (Dataphysics Inc.OCA
31、20, Germany), the volume of water droplet was 2.5 m. Thespecific surface area of the aerogels was determined by the BrunauerEmmettTeller (BET) method (ASAP 2020, Micromeritics, Nor-cross, GA), based on the amount of N2adsorbed at pressures 0.05 P/P0 0.3. The cumulative pore volume was measured at th
32、e pointP/P0= 0.99. The microstructural study of the aerogels was performedusing field-emission scanning electron microscopy (Quanta 400FEG). The samples were coated with Au nanopowder under currentof 20 mA for 2 min. All mechanical properties of the bioinspiredhydrogels were tested on Electromechani
33、cal Universal TestingMachine (E44.104, MTS Systems (China) CO., Ltd.,) at thetemperature ranging from 25 to 70 C. Test temperature wasregulated by High Precision Temperature Controller (WK650, MTSSystems (China) CO., Ltd.,) and Environmental Chamber (GDX200,temperature ranging from 350 to 70 C, MTS
34、System (China) CO.,Ltd.,). The compressive mechanical properties at different temper-ature were evaluated at a speed of 3 mm/min in which the gel sampleswere prepared into cylindrical shaped specimens (24 and 15 mm indiameter and height, respectively). Elastic moduli of the hydrogelswere measured us
35、ing a DMA 242 E/1/G (NETZSCH, Germany) intension mode (10 Hz) over a temperature range of 100 to 150 Cwith a heating rate of 5.0 C/min and frequency w = 10 rad/s. Inorder to assess the adhesive properties of the bioinspired hydrogels,lap-shear adhesion test (ASTM D3163) was employed by anelectronic
36、universal testing machine (2 kN load cells, Instron 3365Single Column UTS) using the tensile mode for hydrogels adheringto various solid surfaces (e.g., polyethylene terephthalate (PET),polystyrene (PS) at a strain rate of 3 mm/min. Sample preparation:Take the bioinspired hydrogels adhere to PET as
37、an example. First,PET was cut into pieces with an area of 50 10 mm. Second, 300 Lof the hydrogel precursor solution was dropped onto one end of onepiece, the solution was then evenly spread over an area of 30 10 mmusing a slide glass. The bioinspired hydrogel was formed and aged at60 C. Third, the o
38、ther piece of PET was pressed onto the hydrogelside over the first one to form the adhesive joint with a load of 100 gfor 1 h. A single test sample was prepared. In order to obtainstatistically reliable values, at least five samples were tested for eachsolid surface.Simulation. The theoretical calcu
39、lations were performed usingdensity functional theory (DFT) at the B3LYP level as implementedin the Dmol3. The double numerical plus polarization (DNP) basisset was used to describe the valence orbitals of the atoms and theeffective core potentials (ECP) method was adopted for coretreatment. The ene
40、rgy and force convergence criteria were set at105Ha and 0.002 Ha/, respectively. Since H2O, Gly, and PVAmainly interact with each other with H-bonds, the bond strength wasthen calculated following the equation:=+EEEE()/nHbondm1m2m1m2HbondWhere EHbondand nHbondwere the strength and number of H-bond,E
41、m1+m2and Em1/Em2were the total energy of the bonded system andisolated molecule, respectively.The interaction strength of PVAPET and PVAPS wascalculated following the equation:=EEEEtotalm1m2where Etotal, Em1/Em2were total energy of the interaction system andenergy of isolated molecule, respectively.
42、RESULTS AND DISCUSSIONDesign Silica Aerogels with Self-Reinforced Micro-structure for Bioinspired Hydrogels. In most cases,shrinkage of wet-gels during drying is inevitable, even if theywere dried by supercritical liquids, where the capillary forcemay be eliminated yet volume shrinkage ranging from
43、5 to 30%was observed.4245In order to synthesize superhydrophilicsilica aerogels by the APD approach, as above-mentioned, wecould not count on the spring-back effect because an abundantof SiOH groups was not converted to the Si-CH3groups.However, it would be possible to reduce the shrinkage of thewet
44、 gel by strengthening silica network while reduce thesurface tension of the solvent at the same time. As depicted inScheme 1a, the key problem of the traditional silica aerogels isthat their network is formed by spherical nanoparticles withthin-neck connection, hence their structure is weak and thei
45、rpores are easily collapsed during drying. To solve it, polymericsilica precursor, ethoxysiloxane oligomers (ESOM), wassynthesized by hydrolyzing TEOS with 1.9 molar ratio ofwater as shown in Scheme 1b, which resulted in partiallyhydrolyzed TEOS with nearly two SiOH functional groupsper monomeric un
46、it and facilitated the formation of polymericchains by polycondensation. Due to the polymeric structure, itwas possible for the ESOM to form wet gels with thick-neckconnection4648which may contribute to the APD methodtoward superhydrophilic silica aerogels.Scheme 1c shows the natural strategy in the
47、 antifreezingorganism,4952in which antifreezes such as antrifreezingprotein (AFP) or glycerol (Gly) have been produced in cellsand then released into tissues, consequently depressed waterfreezing point and inhibited ice crystallization so that theinsects, fish and freeze-tolerant plants could surviv
48、e in lowtemperature. Inspired by above-mentioned natural organisms,a series of antifreezing hydrogels was designed as illustrated inScheme 1d. The highly porous superhydrophilic silica aerogelmicroparticles are ideal carrier for antifreezes, and they couldbe well dispersed in various sols, after (an
49、d during) theformation of hydrogels, antifreezes would be released into thehydrogel network from the superhydrophilic silica aerogelmicroparticles while free water molecules may be absorbed bythe superhydrophilic silica aerogel microparticles, which havemade antifreezing hydrogels keep their unfroze
50、n state attemperature as low as 70 C. The advantages of thebioinspired method for the antifreezing hydrogels could besummarized as follows: (1) the method is versatile and simple,no complex structure design or solvent-exchange was needed,and the water content in the hydrogels is constant withoutchan
