MFC/NFC aerogel has water sensitivity, and it should be improved in strength in water before application. Chitosan was investigated as a MFC/NFC aerogel reinforcing agent in this paper. The reinforced aerogel showed slightly tighter structure and very good water stability and mechanical strength. FTIR disclosed the chemical bonds formed between chitosan and cellulose. Nanoparticles of silver (Ag-NPs) were loaded using the reinforced aerogel. The excellent Ag-NP monodistribution on the aerogel was expressed by TEM. Both chitosan-reinforced Ag-NPs loaded MFC aerogel and NFC aerogel and expressed great antibacterial activity, though reinforced MFC aerogel exhibited better properties, like higher BET, lighter density, more Ag-NP loading, and better distribution, than NFC aerogel in this research. Chitosan-reinforced MFC aerogel is a good potential substrate for nanoparticle loading and biocomposite making.
Aerogels are porous materials of interconnected nanostructures made from gels by replacing the liquid by gas, which exhibit unusual properties, such as high porosity and surface area, low density, and low heat conductivity [
MFC/NFC aerogels generally can be prepared by two steps: one is the MFC/NFC suspension generated by a unique method of enzymatic/chemical pretreatment and mechanical fibrillation by a microfluidizer and the other is to produce aerogel by freezing the suspensions and dried in a special way [
Chemical cross-linking affords a method for the preparation of MFC/NFC cellulose aerogels with high mechanical properties [
In the current work, we firstly reported chitosan-reinforced MFC/NFC aerogels and their application. The aerogels showed improved mechanical properties. Using the reinforced aerogel as a template, nanoparticles, silver as an example, were loaded uniformly.
Fully bleached eucalyptus pulp was passed through the microgrinder, 6 passes, at a concentration of 2.31%, to produce MFC suspension. Then some of the MFC suspension was treated by TEMPO before passing the microfluidizer 10 times to produce NFC. The TEMPO oxidation technique was performed as given in the literature with sodium hypochlorite as the terminal oxidant [
Chitosan (MW 10253, viscosity 20 cP) was purchased from Aldrich (degree of deacetylation: ∼85%). Chitosan solutions were prepared by dissolving chitosan (2% by weight) in (1% by volume) aqueous acetic acid solution [
MFC/NFC dispersion with chitosan mixture at different ratios of 100/0, 90/10, 80/20, and 70/30 (MFC to chitosan weight) was mixed to 0.5% by adding DI water, by using a high-shear mixer for 5 min, followed by a magnetic bar stirring for 1 h, and then sonication for another 30 min. Then the mixture was centrifuged to remove water at a speed of 1.2 × 104 rpm for 1 h. A gel was formed. The gel was frozen by liquid nitrogen, then placed on a freeze dryer (Labconco) for at least 48 h. The properties of density, strength, and BET surface area were examined. The density was measured by the volume and weight of each aerogel. The strength was evaluated by its water stability which was conducted in water for stirring at a given time. After Ag-NPs loaded, both the MFC/CH aerogel and NFC/CH aerogel BET surface areas were measured after 4 h degassing at 105°C (Gemini VII Series surface area analyzer, Micromeritics Instrument Corporation).
The in situ loading of silver nanoparticles (Ag-NPs) onto the aerogels was carried out through the reduction of 10 mM AgNO3 solution. Under ambient temperature (∼25°C), AgNO3 solution was absorbed by the porous matrix by soaking or dropwise addition, keeping totally wet for enough time till the aerogels are not absorbing solution anymore. After a certain time of air drying, a partially dehydrated matrix was obtained. It was then immersed in an aqueous solution of NaBH4 (50 mM) for 20 min. The color of the samples turned to yellow or dark brown due to the reduction of Ag + into silver nanoparticles. The composite was rinsed with Milli-Q water three times to remove water-soluble substances and unbound silver particles. Finally, the composite was freeze dried.
SEM sample preparation of the MFC/NFC slurry and aerogels with/without Ag-NPs : MFC/NFC (20 mL, 0.08%) was taken for magnetic stirring for 45 min, followed by 30 s sonication. A silicon plate was used as the MFC/NFC material support. Firstly, the plate was cut into small pieces, which were soaked in 10% NaOH solution for 30 s and then in Milli-Q water. These silicon pieces were dried by nitrogen blowing, followed by UV exposure for 20 min. A tiny drop of the abovementioned MFC/NFC slurry was placed on the surface of a cleaned silicon piece. Overnight air drying was needed before the SEM test. The samples were coated with gold before SEM operation. The porous structures of MFC/NFC aerogels were examined using a field emission scanning electron microscope (FESEM) at an accelerating voltage of 20 kV (a high resolution JEOL 6400F cold field emission SEM). The MFC/NFC aerogels with Ag-NPs were characterized using VPSEM (Variable Pressure Scanning Electron Microscope, Hitachi S3200N) with an energy dispersive X-ray spectrometer).
TEM preparation of MFC/NFC aerogels with Ag-NPs: A JEOL 2000FX transmission electron microscope (TEM) operating at 20.0 kV was utilized to define the Ag-NPs in the MFC/NFC aerogels. A specimen can be prepared by cutting the sample into thin slices using a diamond saw, then cutting 3-mm-diameter disks from the slice, thinning the disk on a grinding wheel, dimpling the thinned disk, and then ion milling it to electron transparency.
FTIR spectroscopy was performed on a PerkinElmer Spectrum (Version 10.03.09). Spectra were obtained after the accumulation of 6 scans which had a resolution of 4 cm−1 over the range of 4,000–650 cm−1.
In order to determine the thermal decomposition temperature of composite aerogels, thermogravimetric analysis (TGA) was used. It was operated on Perkin Elmer TGA Q500 with a heating rate of 10°C/min to 500°C in a nitrogen atmosphere. In order to obtain the amount of Ag-NPs in aerogel, TGA under oxygen in the range of 500–575°C was continued. The isothermal time for 25 min at 575°C was utilized.
The antibacterial activity of the aerogels was tested against Escherichia coli (E. coli), Gram-negative bacteria, using the viable cell-counting method. Briefly, about 100
MFC/NFC aerogels with chitosan addition were examined for their densities and strength. The dry aerogel-retained stability in water is shown in Figure
MFC/NFC aerogel SEM with different chitosan additions. MFC/CH ratio: (a) 100/0, (b) 90/10, (c) 80/20, and (d) 70/30; NFC/CH ratio: (e) 90/10, (f) 80/20, and (g) 70/30.
Data on the density of chitosan- (CH-) added MFC/NFC aerogel.
Aerogel density (g/cm3) | CH0% | CH10% | CH20% | CH30% |
---|---|---|---|---|
MFC/CH | 0.0258 | 0.0397 | 0.0425 | 0.0426 |
NFC/CH | — | 0.0399 | 0.0442 | 0.0451 |
Note: pure MFC aerogel densities were obtained with 0.0258, 0.0505, and 0.0807 g/cm3 from 2%, 5%, and 8% MFC gel concentrations, respectively.
After Ag-NPs loaded, both MFC/CH aerogel and NFC/CH aerogel surface areas were measured after 4 h degassing at 105°C. The results are listed in Table
BET of both MFC/CH aerogel- and NFC/CH aerogel-loaded Ag-NPs.
Aerogel BET m2/g | CH10% Ag loaded | CH20% Ag loaded | CH30% Ag loaded | CH0% Ag loaded | CH30% with no Ag |
---|---|---|---|---|---|
MFC/CH | 13.4 | 11.3 | 8.8 | 13.2 | 9.7 |
NFC/CH | 1.67 | 1.2 | 1.2 | — | — |
Ag-NP content in MFC/NFC aerogels.
Ag-NPs % | CH 0% | CH 10% | CH 20% | CH 30% |
---|---|---|---|---|
MFC aerogel | 2.5 | 3.8 | 3.6 | 3.6 |
NFC aerogel | — | 3.2 | 3 | 3 |
MFC/NFC aerogels with different chitosan addition levels were conducted and scanned using a SEM. Generally, chitosan can cause the aerogel dense structure with the increase in addition level (see Figure
After Ag-NPs loaded, these aerogels exhibited similar SEM morphology to that of Ag-NPs unloaded, in which structures are denser with the increase in chitosan. This denser structure caused the nanoparticles to fill differently. Denser aerogel has relatively tight structure and fewer capillaries, which penetrates less Ag+ and reducing solution, resulting in fewer Ag-NPs generated. Table
In order to investigate how the nanoparticles are located in these aerogels, the MFC/CH and NFC/CH aerogels with Ag-NPs were scanned by using SEM in high magnification. The images are shown in Figure
Ag-NPs in MFC/CH aerogel: (a) 10%CH, (b) 20%CH, and (c) 30%CH; NFC/CH aerogel: (d) 10%CH, (e) 20% CH, and (f) 30% CH.
Ag-NP size and distribution for aerogel are very important. Based on good particle distribution in SEM images shown in Figure
TEM images of MFC/CH aerogel-loaded Ag-NPs. 20% CH (a) and particle size range (b).
The spectra of MFC aerogel with 20% chitosan is shown in Figure
MFC/NFC aerogel FTIR spectra with chitosan 20%. Note: A, MFC aerogel 0% CH; B, MFC aerogel 20% CH; C, NFC aerogel 20% CH.
In view of the importance of thermal stability in many applications of MFC/NFC aerogels, we examined thermal decomposition of composite-loaded Ag-NPs by thermogravimetry (TGA, Perkin Elmer Q500, and heating rate of 10°C/min) in a nitrogen atmosphere under 500°C, as shown in Figure
TGA behavior of MFC-Ag aerogels with different chitosan addition and NFC-Ag aerogels with different chitosan addition. Note: TGA under N2 atmosphere till 500°C and then switched to oxygen to 575°C.
The antibacterial activity of these four samples MFC80/CH20, NFC80/CH20, MFC80/CH20/Ag-NPs, and NFC80/CH20/Ag-NPs was tested against E. coli by using the viable cell-counting method. NFC aerogel without chitosan was compared. The effects of aerogels on the growth of the recombinant bacteria E. coli are shown in Figure
Antibacterial activity of NFC, MFC/chitosan, NFC/chitosan, MFC/chitosan/Ag-NPs, and NFC/chitosan/Ag-NPs against E. coli.
In the current work, we successfully utilized chitosan as MFC/NFC aerogel reinforcement biopolymer. The reinforced aerogels have overcome the water unstability. With more chitosan addition, MFC/NFC aerogels became denser which have tighter structures that are observed in SEM. Chemical reaction occured between chitosan and MFC/NFC. Comparing both MFC/CH and NFC/CH aerogels, MFC/CH aerogel has higher BET surface area and lower density than NFC/CH aerogel. Ag-NPs were in site loaded to chitosan-added MFC/NFC aerogel. The MFC/CH aerogel performed much better than the NFC/CH aerogel, which obtained monodispersed nanoparticles of Ag-NPs. Finally, both MFC/CH-Ag-NP and NFC/CH-Ag-NP aerogels exhibited excellent antibacterial activity. But the better antibacterial performance of both aerogels should be further examined.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare no conflicts of interest.
The authors would like to thank Prof. Orlando J. Rojas and his group for their help during this research and in editing the manuscript. The Special Support Plan for Xijiang Innovation Team Plan 2016, High-Level Talent Cultivation of Guangdong Industry Polytechnic (No. KYRC2018-020), and Guangdong University Youth Innovation Fund Project (No. 2018GKQNCX032) are greatly acknowledged for the support.
The SEM images showing the surface of MFC aerogel with 10% chitosan addition and 20% chitosan addition.