Ultralight hybrid silica aerogels derived from supramolecular hydrogels self-assembled from insoluble nano building blocks

Abstract

Supramolecular hydrogels are a type of hydrogel cross-linked by non-chemical bonds and they have been widely applied in the field of smart systems, sensors, tissue engineering, and controlled drug delivery. Most supramolecular hydrogels are formed by soluble molecules, polymers, and metal ions. In this work, supramolecular hydrogels self-assembled between two insoluble nano building blocks (ISNBBs), graphene oxide (GO) and amino-functionalized silica nanoparticles (SiO₂–NH₂), have been discovered and synthesized. The gelation conditions of the two ISNBBs have been investigated. A step further, ultralight hybrid silica aerogels are obtained by supercritical drying of the physical hydrogels. No visible volume shrinkage is observed, due to the fact that the hydrogel networks are formed by rigid ISNBBs. Thus the hybrid aerogels possess ultralow density (down to 7.5 mg cm⁻³), high specific surface areas (178.6 m² g⁻¹), and extremely high porosity (99.6%). The present work shows an alternative strategy to design and synthesize supramolecular hydrogels and aerogels using predetermined building blocks, together with designable morphology and physical properties for the target aerogels.

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Introduction

Supramolecular hydrogels (or physical hydrogels) are a type of network cross-linked by non-covalent interactions such as hydrogen bonding,¹ hydrophobic interactions,² van der Waals interactions,³ electrostatic interactions,⁴ and π–π interactions.⁵ Due to their non-covalent nature, the supramolecular hydrogels are normally reversible soft materials and are considered to be potential materials to construct smart systems, such as stimuli-responsiveness triggered by pH, temperature, electronic field, magnetic field, and light. Thus, they have been widely investigated as biomaterials for use in tissue engineering, controlled drug delivery, and tissue repairs.^6–12

Aerogels are highly porous materials that are generally synthesized by a two-step process, sol–gel transition (to obtain a porous network) and specific dying process (e.g. supercritical liquid drying, to remove liquid in gels while keep the volume and microstructure unchanged or with limited changes).^13–25 Aerogels exhibit various attractive properties such as extremely low density, low thermal conductivity, high specific surface areas, high porosity, and high pore volume. Thus, they have found potential applications in the fields of aerospace, catalysis, filtration, thermal insulation, electronics, sensors, water purification, tissue engineering, and drug delivery. Recently, significant progress in aerogels have been achieved. For example, eco-friendly cellulose aerogels that were capable for oil absorption and oil/water separation,^26–28 anisotropic cellulose nanofibril composite sponges that were able to be used as senor and electromagnetic interference shielding,²⁹ fibrous polyimide sponges and aerogels with superior mechanical and thermal properties,^30,31 anisotropic nanocellulose aerogels that were suitable for fast liquid transportation,³² ultrablack aerogels for solar steam generation,^33,34 etc. have been prepared. Reviews on the comprehensive developments and applications of aerogels are also available.^{13,14,35–37} Aerogels and hydrogels could be considered as the same porous network in different stages, that hydrogels are filled with water in their pores while aerogels are filled with air in their pores. Therefore, there are plenty of similarities between hydrogels and aerogels, such as the same porous structure, and the same potential applications as catalyst support, water purification absorbent, and drug delivery system. Nevertheless, differences can be observed between these two materials. For instance, hydrogels are reversible and stimuli-responsive, but the stimuli-responsiveness in aerogels are rare because the movements of molecules are restricted in the solid state.^10,11

On the other hand, supramolecular hydrogels are normally formed by soluble precursors,^38–41 which are soluble in the molecular level when they form the hydrogel networks. Due to the weak connection of supramolecular hydrogels, their corresponding aerogels cannot be obtained because there are no preformed network to support their volumes and structures. Recently graphene oxide (GO) has been recognized as an attractive two dimensional building blocks for supramolecular hydrogels, which are insoluble dispersed nano plates and various GO-based supramolecular hydrogels have been prepared by different secondary interactions including hydrogen bonding, π–π stacking, electrostatic interaction, and coordination.^42–45 Cross-linkers been used to prepare GO based supramolecular hydrogels including poly(vinyl alcohol) (PVA), poly(ethylene oxide) (PEO), hydroxypropylcellulose (HPC), poly(vinyl pyrrolidone) (PVP), polyethylenimine (PEI), cetyltrimethyl ammonium bromide (CTAB), tetramethylammonium chloride (TMAC), and melamine. The cross-linkers are all soluble components so that they can efficiently form non-covalent interactions with GOs. Moreover, GOs can be self-assembled between themselves via π–π stacking through reduction, and their corresponding hydrogels can be successfully transferred to GO aerogels, which showed potential applications in energy storage and water purification.^46–49 Hybrid GO based hydrogels and aerogels incorporated with other insoluble nano building blocks (ISNBBs), such as MoS₂, carbon nanotubes (CNTs), and boron nitride (BN),^50–54 have been synthesized and they showed improved performances. Nevertheless, these ISNBBs did not act as cross-linkers for the GO to form supramolecular hydrogels, they acted as fillers that were embed in the GO matrix.

In this work, amino group (–NH₂) functionalized silica nanoparticles (SiO₂–NH₂) are synthesized and found to form hybrid supramolecular hydrogels with GO. The SiO₂–NH₂ nanoparticles played as cross-linkers and hydrogen bonds were formed between SiO₂–NH₂ and GO as illustrated in Fig. 1. Hybrid supramolecular hydrogels can be formed in a wide range of concentration. The supramolecular hydrogels were formed solely by ISNBBs, which formed interconnected network throughout the hydrogels. Herein, the gelation behaviour, parameter, and the corresponding hybrid aerogels will the systematically investigated.

Fig. 1 (a) Schematic description of the synthesis of SiO₂–NH₂; (b) GO–SiO₂–NH₂ hybrid supramolecular hydrogels; and (c) the corresponding GO–SiO₂–NH₂ hybrid aerogels.

Experimental

Materials

Graphite powder was purchased from Qingdao Tianheda Graphite Co., Ltd, Qingdao, China (purity 99.8%). 3-Aminopropyltriethoxysilane (APTES) (AR, purity 99%) and tetraethoxysilane (TEOS) (GR, purity 98%) were purchased from Aladdin Industrial Corporation, Shanghai, China. GO were prepared according to literatures by a modified Hummers method^42,55,56 and GO aqueous solution were diluted to various concentrations. Other reagents are of analytical purity and used as received.

Synthesis of SiO₂–NH₂ nanoparticles

SiO₂–NH₂ nanoparticles were synthesized through a revised Stöber method^57,58 as follows: to a mixture of 125 ml distilled water and 100 μl ammonium hydroxide, 10 ml TEOS was added under stilling for 2 h. Then, 1.5 ml APTES was added. SiO₂–NH₂ nanoparticles were formed as white precipitate and dried in vacuum. SiO₂–OH nanoparticles were synthesized via a Stöber method without the addition of APTES for comparison.

Synthesis of GO–SiO₂ supramolecular hydrogels and aerogels

GO–SiO₂ supramolecular hydrogels were synthesized by mixing GO aqueous solution and SiO₂–NH₂ nanoparticle aqueous solution, take the GO5–SiO₂15 for example: 1 ml fleshly prepared SiO₂–NH₂ aqueous solution (15 mg ml⁻¹) was mixed with 1 ml GO aqueous solution (5 mg ml⁻¹) and stirred for 5 min. Supramolecular hydrogels were formed in 2 h. The supramolecular hydrogels were named as GOm–SiO₂n, where m and n indicate the concentration of GO and SiO₂ nanoparticles, respectively. The supramolecular hydrogels were converted to aerogels by solvent-exchange with ethanol and supercritical CO₂ drying.

Characterization

Fourier transform infrared (FT-IR) spectra are recorded on a Nicolet iN10 spectrometer (Thermo Scientific, USA) in the reflection mode. The morphologies are observed on a field-emission scanning electron microscopy (FESEM, Quanta 400 FEG, USA), the samples are coated with Au nanoparticles at a current of 20 mA for 2 min in advance. TEM measurement was carried out on a Tecnai G2 F20 S-TWIN instrument. N₂ adsorption–desorption isotherms are performed on Micromeritics (ASAP 2020, USA). The specific surface area of the samples are determined by the Brunauer–Emmett–Teller (BET) method based on the amount of N₂ adsorbed at pressures 0.05 < P/P₀ < 0.3. The pore volumes are measured at the point P/P₀ = 0.99. The pore size distribution and the average pore diameters are analyzed by the Barrett–Joyner–Halenda (BJH) method. Densities of the aerogels are calculated by weighting the samples and measuring the volumes.

Results and discussion

GO based hydrogels have attractive increasing interests and various cross-linkers can be used. Nevertheless, challenges remain for the self-assembly of GO with ISNBBs, while network formed by IsNBBs (such as metal nano spheres) show great potential for functional materials.⁵⁹ On the other hand, SiO₂ nanoparticles have found wide applications in catalyst, energy, environment, and biomedicine.^60–63 Since there are plenty of –OH groups on the surfaces of SiO₂ nanoparticles prepared by Stöber method, it could be interesting to see if GO and SiO₂ nanoparticles could be self-assembled into hydrogels, as it was happened between GO and PVA. However, hydrogels could not be formed between GO and SiO₂ nanoparticles. Interestingly, we found that –NH₂ functionalized SiO₂ nanoparticles could self-assembly with GO and form supramolecular hydrogels.

The synthetic approach of SiO₂–NH₂ nanoparticles is illustrated in Fig. 1a, which is a revised Stöber method that a diluted NH₃H₂O aqueous solution is used, then TEOS is added under stirring and there are no observable changes. After the addition of APTES, white precipitates were formed. APTES acted as both co-monomers to functionalize the silica nanoparticles with –NH₂ groups and as basic catalyst to promote the hydrolysis and condensation of TEOS.^64,65 As depicted in Fig. 1b, supramolecular hydrogels were formed after mixed GO and SiO₂–NH₂ nanoparticles aqueous solution. The only differences of the silica nanoparticles were the replacement of –OH with –NH₂, which may form stronger hydrogen bonds between GO and result in the formation of supramolecular hydrogels. After solvent-exchange with ethanol, hybrid aerogels (Fig. 1c) were synthesized by supercritical CO₂ drying.

Fig. 2 shows the morphology and the N₂ absorption–desorption isotherm of the SiO₂–NH₂ nanoparticles. As can be seen from the SEM image, SiO₂–NH₂ nanoparticles were spherical particles with diameters lower than 50 nm. The TEM image of the SiO₂–NH₂ nanoparticles shown in Fig. S1, ESI† further indicated that the diameter of them ranged from 20–40 nm. The specific surfaces area (SSA) of the SiO₂–NH₂ nanoparticles was determined to be 163 m² g⁻¹. The small size and large SSA value of the SiO₂–NH₂ nanoparticles may facilitate the formation of supramolecular hydrogels with GO, which significantly increased the contact areas between the two ISNBBs.

Fig. 2 (a) SEM image of SiO₂–NH₂ nanoparticles; (b) nitrogen absorption–desorption isotherm of the SiO₂–NH₂ nanoparticles.

The gelation behaviour between GO and SiO₂–NH₂ nanoparticles are systematically investigated, and the results are presented in Fig. 3. GO were synthesized according to our previous work by a modified Hummers method,^55,56 the size of graphite powder was 30–40 μm. The SEM image of the GO is shown in Fig. S2, ESI.† They were diluted into difference concentrations as shown in Fig. 3. It can be seen that supramolecular hydrogels can be formed with a wide range of GO concentration from 2 mg ml⁻¹ to 10 mg ml⁻¹, while the concentration of SiO₂–NH₂ nanoparticles ranged from 10 mg ml⁻¹ to 50 mg ml⁻¹ (see Fig. S3, ESI† for the photo images of the hydrogels). When the concentration of the IsNBBs were located in the light blue region, supramolecular hydrogels would possibly be formed. It should be pointed out that when the concentration of SiO₂–NH₂ nanoparticles reached to 50 mg ml⁻¹, supramolecular hydrogels did not formed when the concentration of GO was 10 mg ml⁻¹. The dispersion of GO and SiO₂–NH₂ nanoparticles was difficult to homogenously mixed. When the concentration of GO was down to 1 mg ml⁻¹, hydrogels could also not be formed, instead precipitates were obtained when the concentration of SiO₂–NH₂ nanoparticles was 15 mg ml⁻¹. The results indicated that diluted GO dispersion (e.g. 1 mg ml⁻¹) cannot form continuous network throughout the mixture solution. Nevertheless, GO can still interconnected with SiO₂–NH₂ nanoparticles to form precipitates.

Fig. 3 Gelation conditions for the SiO₂–GO hybrid supramolecular hydrogels: black solid circle (●) indicated that supramolecular hydrogels were formed; red solid circle () indicated that hydrogel cannot be formed; blue circle () indicated that precipitates were formed; the light blue region suggest that supramolecular hydrogel may be formed.

Since the supramolecular hydrogels were formed solely by ISNBBs, there must be phase separated network that could persist its porous structure to obtain the corresponding aerogels. Therefore, the supramolecular hydrogels were dried by supercritical CO₂, and GO–SiO₂ hybrid aerogels were successfully obtained (Fig. S4, ESI†). The density of the aerogels was down to 7.5 mg ml⁻¹ for the GO5–SiO₂10 aerogels, which was equal to the theoretical density. Considering the negligible volume shrinkage of the aerogels, it could be concluded that there were no loss of GO and SiO₂–NH₂ nanoparticles during the solvent-exchange and supercritical liquid drying process. The nearly 100% yield, which is a superior aspect for aerogel prepared by ISNBBs. In contrast, GO aerogels that were cross-linked by π–π stacking via reduction of GO,^55,56 or cross-linked by soluble polymers such as PVA, PEO, and PVP,^46–49 were remarkably shrunk during both the solvent-exchange and drying processes.

According to the equation:¹¹

Porosity = 1 − ρ_b/ρ_s

Where ρ_b is the bulk density of the aerogel, and ρ_s is the density of the skeleton of the aerogel. The porosity of the hybrid aerogels was calculated to be 99.6%.

The FTIR spectra of GO, SiO₂–NH₂ nanoparticles, and the hybrid aerogels are presented in Fig. 4. Peaks located at 2982 cm⁻¹ could be clearly observed for the SiO₂–NH₂ nanoparticles, which can be ascribed to the C–H vibration from APTES groups. The broad and strong peak located at 3100–3400 cm⁻¹ suggested the presence of –NH₂. The vibration peaks of Si–O–Si and Si–C can be observed at 1100 and 840 cm⁻¹, respectively. The vibration peaks of C=O stretching vibration in GO of the aerogel located at 1644 cm⁻¹ (black arrow) showed a shift from 1720 cm⁻¹ (red arrow), which indicated the formation of hydrogen bonds between GO and the SiO₂–NH₂ nanoparticles that resulted in the formation of supramolecular hydrogels.⁶⁶ In addition, the supramolecular hydrogel could be transferred into sol when temperature elevated up to 90 °C (Fig. S5a, ESI†), possibly due to the broken of hydrogen bonds between GO and SiO₂–NH₂. Besides, SiO₂–OH nanoparticles without –NH₂ groups were synthesized, and they could not form hydrogels with SiO₂–NH₂ (Fig. S5b, EIS†) and GO (Fig. S5c†). Those results indicated that the hydrogen bonds in the supramolecular hydrogels may be formed between –NH₂ (in SiO₂–NH₂) and –COOH groups (in GO).

Fig. 4 FT-IR spectra of (a) GO; (b) GO5–SiO₂10 hybrid aerogels; (c) GO5–SiO₂15 hybrid aerogels; (d) GO5–SiO₂20 hybrid aerogels; (e) SiO₂–NH₂ nanoparticles.

The microstructure of the GO–SiO₂ hybrid aerogels is investigated by SEM. Fig. 5 shows the SEM image of GO5–SiO₂15 aerogels. Pore size up to 1 μm could be observed, and the pores were homogenously distributed. From the SEM image with larger magnification times, SiO₂–NH₂ nanoparticles can be clearly identified and were found to be dispersed on the GO sheets without remarkable aggregation. The results confirmed that GO were gelled cross-linked by SiO₂–NH₂ nanoparticles, as illustrated in Fig. 1b. Combine with the highly porous structure of the aerogel and hydrogels, the hybrid aerogels might be ideal candidate for tissue engineering and drug delivery matrix.

Fig. 5 SEM images of the GO5–SiO₂15 hybrid aerogels, blue arrows pointed to the SiO₂–NH₂ nanoparticles.

The N₂ adsorption–desorption isotherms and the pore size distribution curves of the hybrid aerogels are shown in Fig. 6. All the aerogels exhibited hysteresis loops, which were due to the typical features of mesoporous materials (type IV isotherms). The BET surface areas of the samples were 178, 120, 150 m² g⁻¹ for the GO5–SiO₂10, GO5–SiO₂15, and GO5–SiO₂20 hybrid aerogels, respectively. All the aerogels possessed high pore volume ranged from 0.92 to 1.24 cm³ g⁻¹. Fig. 6b indicated that the aerogels also possessed mesopores, with average diameter of 43, 37, and 36 nm for GO5–SiO₂10, GO5–SiO₂15, and GO5–SiO₂20 hybrid aerogels, respectively. The SEM and BET results indicated that the hybrid aerogels possessed hierarchically pores ranged from tens of nanometers to several micrometers.

Fig. 6 (a) Nitrogen absorption–desorption isotherm of the hybrid aerogels; (b) pore distribution of the hybrid aerogels.

Conclusions

In conclusions, the gelation between GO and SiO₂–NH₂ nanoparticles, which are ISNBBs, have been observed. They can form supramolecular hydrogels in a wide range of concentration from 2 mg ml⁻¹ to 10 mg ml⁻¹ for GO and 10 mg ml⁻¹ to 50 mg ml⁻¹ for SiO₂–NH₂ nanoparticles. The framework of the supramolecular hydrogels were build up with GO and silica nanoparticles, where silica nanoparticles were dispersed on the GO nanosheet and acted as cross-linkers through the formation of hydrogen bonds between –NH₂ and –COOH groups in SiO₂–NH₂ and GO, respectively. Interestingly, the hybrid hydrogels could be successfully transferred into hybrid aerogels without observable volume shrinkage. The aerogels exhibited extremely low density of 7.5 mg ml⁻¹, large pore volume of 1.24 cm³ g⁻¹, high porosity of 99.6%, and high BET SSA ranged from 120–178 m² g⁻¹. In addition, the aerogels possessed hierarchically porous structures from tens of nanometers to several micrometers, combined with the excellent biocompatibility of GO and SiO₂, the stimuli responsive behaviours, these supramolecular hydrogels and aerogels may find useful application in biomedicine, such as drug carriers and tissue engineering. The results also indicated that hydrogels and aerogels could be synthesised from ISNBBS with predetermined structure and properties.

Author contributions

Z. Liu and L. Liu completed the experiment and analyzed the data. Z. Zhong contributed to formal analysis. J. Wang designed and supervised the project. J. Xi supervised the project. Z. Liu wrote the original draft, and all the author reviewed and edited the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (91963124, 51773225, 81671161).

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Footnote

† Electronic supplementary information (ESI) available: SEM image of GO, photo image of supramolecular hydrogels, etc. See DOI: 10.1039/d1ra00418b

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