Masato Miyakawa,
Chizuru Shigaraki,
Takashi Nakamura and
Masateru Nishioka
*
National Institute of Advanced Industrial Science and Technology, AIST, 4-2-1, Nigatake, Miyagino-ku, Sendai, 983-8551, Japan. E-mail: m-nishioka@aist.go.jp
First published on 4th October 2021
We developed a method for in situ fabrication of copper nanoparticles inside cotton fibers. Copper nanoparticles can be fabricated mainly in the central part of the fiber by absorbing a raw material solution and by applying microwave heating in a state where the raw material solution is pressed with immiscible liquids. Surface SEM images and cross-sectional EDS mapping for the fabricated fibers clarified that copper nanoparticles fabricated on the cotton surface were suppressed considerably more by the hydrophobic raw material solution than by the hydrophilic raw material solution. These cotton fibers containing copper nanoparticles were found to have antiviral properties against the influenza A virus.
After developing microwave (MW) heating equipment, we considered the use of MW heating characteristics in a wider range of fields.14–18 We specifically examined characteristics of selective heating using MWs. Such characteristics depend on the dielectric constant of the substance. Moreover, we devised a method for selectively installing a functional material inside of porous fibers (Fig. 1).19 For this method, the porous fiber is first immersed in the raw material solution, which permeates the entire fiber. After the fiber is removed, it is immersed in a liquid that is immiscible with the raw material solution and which is pressurized to transfer the raw material solution into the fiber. Finally, while maintaining the pressurized state, a chemical reaction occurs in the raw material solution during MW or other heating. Results suggest that selective heating can be achieved using a substance in the raw material solution that is heated easily by MWs. The functional material can be installed inside the porous fibers. Reaction operations are not limited to MW heating. They might include holding of the raw materials at room temperature. As a model reaction, we fabricated silver-nanoparticle-containing cotton fibers or fibers with zeolite particles inside of porous polytetrafluoroethylene (PTFE) fibers.
Nanomaterials have been studied in various fields, especially in the field of nanomedicines, in diverse applications related to disease diagnosis and treatment such as drug delivery, biosensors, and cancer therapeutic agents.20–24 Copper and copper-oxide nanoparticles, which are typical nanomaterials, have numerous useful properties: they are UV protection and photocatalytic, particularly providing antibacterial and conductivity. Numerous methods for applying them to cotton fibers have been reported.25–39 In addition, because of the antiviral properties of copper and copper-oxide nanoparticles, interest in them is growing as a potential countermeasure against the COVID-19 pandemic.40–42 Nanomaterials and nanomaterial complexes of the future will be applied in various situations of daily life such as personal protective equipment and antiviral products in public places, in addition to medical use as nanomedicines such as drug and vaccine delivery.43–49
For this study, we performed selective installation of copper nanoparticles inside cotton fibers using MW heating in a state of pressurization with immiscible liquids in the raw material solution. No report of the relevant literature has described an example of the selective introduction of copper nanoparticles inside cotton fibers. The fabricated cotton fiber characteristics were compared using raw material solutions of two types: a hydrophilic ethylene glycol (EG) solution and a hydrophobic octanol solution. Structural analyses of copper nanoparticles and cotton fibers were performed using X-ray diffraction (XRD), transmission electron microscopy (TEM), and infrared absorption spectrometry (ATR-FTIR). The distribution of copper components in the fibers was ascertained using scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDS). Antiviral tests for fabricated fibers were also conducted using influenza A virus.
Entry no. | Raw material solution | Fiber weight ratioa | Immiscible liquids | Reaction operation | Reaction conditions | Maximum internal pressure (MPa) |
---|---|---|---|---|---|---|
a Fiber weight ratio after absorption of raw material solution to dry fiber weight. | ||||||
1 | EG, Cu(CH3COO)2, 100 mM | 2.4–2.5 | Dodecane (30 ml) and hexane (10 ml) | MW heating | 220 °C, 5 min | 1.5 |
2 (ref.) | EG, Cu(CH3COO)2, 100 mM | 2.4–2.5 | — | Electric furnace | 220 °C, 30 min | — |
3 | EG, Cu(CH3COO)2, 10 mM | 1.7–1.8 | Dodecane (30 ml) and hexane (10 ml) | MW heating | 220 °C, 5 min | 1.4 |
4 | EG, Cu(NO3)2·3H2O, 100 mM | 2.4–2.5 | Dodecane (30 ml) and hexane (10 ml) | MW heating | 220 °C, 5 min | 1.7 |
5 | 1-Octanol, Cu(C2H5COO)2·H2O, 100 mM | 1.7–1.8 | EG (40 ml) and hexane (3 ml) | MW heating | 220 °C, 5 min | 1.4 |
6 | 1-Octanol, Cu(C5H11COO)2, 100 mM | 1.7–1.8 | EG (40 ml) and hexane (3 ml) | MW heating | 220 °C, 5 min | 1.4 |
7 | 1-Octanol, Cu(C5H11COO)2, 30 mM | 1.7–1.8 | EG (40 ml) and hexane (3 ml) | MW heating | 220 °C, 5 min | 1.4 |
8 | 1-Octanol, Cu(C5H11COO)2, 300 mM | 1.7–1.8 | EG (40 ml) and hexane (3 ml) | MW heating | 220 °C, 5 min | 1.4 |
9 | EG, Cu(CH3COO)2, 100 mM | 2.4–2.5 | Dodecane (30 ml) and hexane (10 ml) | MW heating | 200 °C, 5 min | 1.2 |
To create copper nanoparticles using a hydrophobic raw material solution, 1-octanol solution (Cu(C2H5COO)2·H2O 100 mM or Cu(C5H11COO)2 30, 100, 300 mM) was used as the raw material solution. A small amount of hexane was floated on EG, which was used for pressurization. Other synthesis procedures were the same as those described above.
Antiviral activity value = log(control sample, infection value immediately after inoculation) − log(test sample, infectious value after 2 h of action) |
Antiviral activity value of 2.0 or higher is regarded as effective. A value of 3.0 or higher is regarded as having a sufficient effect.
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Fig. 2 (a) Appearance photograph, (b) XRD patterns, and (c) ATR-FTIR spectra for the fabricated cotton fibers (entry no. 1 and 5). |
Fig. 3 portrays SEM images obtained for the fabricated fiber surface. Copper nanoparticles were observed as white dots on the fiber surface. Regarding the cotton fibers fabricated using the EG raw material solution, it was confirmed that the MW synthesis tended to have fewer copper nanoparticles on the fiber surface than the reference synthesis using an electric furnace (Fig. 3a and b). Therefore, results confirmed that copper nanoparticles tended to be fabricated inside of the cotton fiber when using this MW method. However, even if the copper raw material concentration was reduced to 1/10 and the absorption amount of the raw material solution was halved, or if the copper raw material was changed from Cu(CH3COO)2 to Cu(NO3)2·3H2O, the fabrication of copper nanoparticles on the fiber surface was not suppressed by the EG material solution (Fig. 3c and d). The general strategy for imparting functionality to cotton fibers is to create functional substances strongly inside or outside of them by oxidizing or cationizing the hydroxyl groups of cellulose included in cotton fibers.6 In our method, it was speculated that the tendency of the hydroxyl groups of the fibers to bond with the hydrophilic raw material solution, which is useful in general functionality-imparting methods, presents a barrier: when the raw material solution was pressurized with the immiscible liquid, presumably the raw material solution remained partially on the fiber surface, leading to copper nanoparticle fabrication on the fiber surface. Therefore, using the opposite combination, i.e. a hydrophobic raw material solution, pressure was applied with a hydrophilic liquid. Then MW heating was conducted. In SEM images of fibers fabricated using the two types of hydrophobic raw material solutions shown in Fig. 3e and f, almost no particle fabrication was observed on any fiber surface. Therefore, this idea for suppressing particle fabrication on the fiber surface is probably a general strategy that is independent of the raw material species. Fig. S2† presents a photograph showing the appearance and SEM images of cotton fibers fabricated using Cu(C5H11COO)2 hydrophobic solutions having different copper concentrations. As the copper concentration increased, the red color became darker, indicating increased copper nanoparticle content. From the surface SEM images, copper nanoparticles on the fiber surface were only slightly observed at a copper concentration of 30 mM. In fact, copper nanoparticles on the surface were few even at 300 mM. These results demonstrate that copper nanoparticles can be fabricated inside the fiber over a wide range of copper concentrations.
Furthermore, using SEM-EDS, the tendency to suppress copper nanoparticle formation on the surface was clarified. Fig. 4 presents the composition distribution of the fiber cross-section obtained using SEM-EDS. The copper component was distributed inside the carbon component which reflects the cotton fiber. Moreover, when using the hydrophobic raw material solution, the copper component was better distributed inside the cotton fiber, which shows good agreement with results depicted on SEM surface images presented in Fig. 3. The TEM observation results show that copper nanoparticles of 5–30 nm and ca. 200 nm were included in both MW-fabricated fibers (Fig. 5). Earlier studies examining silver nanoparticles created inside cotton fibers revealed nanoparticles of about 10 nm, which are of equal size to the microfibrillar substructures.12,13 In the MW fabrications described herein, it is presumed that larger copper particles were observed because the particles were present on the fiber surface and on the inner surface of the fiber hollow portion in addition to nanoparticles because of the microfibrillar structure. Regarding the distribution control of the copper component in the fiber cross-section, the elemental profile of Fig. 4 shows that the copper component distribution and strength differed among fibers. The difference occurred because the raw material solution was absorbed and pressurized in the form of a woven fabric. Therefore, the absorption amount of the raw material solution and the pressure amount from the immiscible liquid for each fiber were non-uniform. This fabrication method is applicable to woven fabrics and to yarns or fine fibers.19 Controlling the copper component amount and position might be possible to by fabricating each cotton fiber so that the pressure and the absorbed raw material amount can be controlled.
When fabricating copper nanoparticles inside the cotton fiber, a key point is application of heating conditions that are almost identical to the conditions for synthesizing copper nanoparticles using only the raw material solution for fabrication inside of the cotton fiber. When the cotton fiber absorbed the Cu(CH3COO)2 hydrophilic solution was heated by MW at 200 °C, yellow color that is peculiar to surface plasmon resonance of Cu2O nanoparticles was observed (Fig. S3,† entry no. 9). When an EG solution containing Cu(CH3COO)2 was synthesized using flow-type MW heating in our earlier study, Cu2O nanoparticles were synthesized at 180 °C; copper nanoparticles were synthesized at 210 °C.15 These conditions resemble those in which copper and Cu2O nanoparticles are fabricated inside the cotton fiber. The reaction operation of this in situ fabrication method is not limited to MW heating.19 When using a method other than MW heating, one must first study the conditions under which copper nanoparticles are synthesized using only the raw material solution and apply those conditions to fabrication inside of the cotton fiber.
After a simplified washing test was applied to these fibers, the time variation of the copper component was investigated using ICP-OES (Fig. 6). The appearance after washing revealed a red color, indicating that it contained metallic copper nanoparticles. The decrease of the copper component was slower in the fabricated fiber using the hydrophobic raw material solution than in the hydrophilic raw material solution, indicating good correspondence with the copper distribution of the SEM image surface and elemental mapping in SEM-EDS. The ATR-FTIR spectra indicate that the chemical state of cellulose has changed only slightly. Therefore, the bond between cellulose fibers and copper nanoparticles might be weak. That weakness might be related to reduction in the copper component by this washing test. Early studies have examined nanoparticles with enhanced bonding by chemical treatment of fibers or use of fiber structures to immobilize nanoparticles.12,52 It will probably be necessary to combine these methods to increase the durability of the acquired function in our method further. It is noteworthy that copper nanoparticles on the fiber surface remained even after washing tests (Fig. S4†). In this regard, a reducing functional group (aldehyde group) is known to be present at the end of the molecular chain of cellulose and is known to have a weak reducing ability.53,54 Copper nanoparticles dotted on the fiber surface were observed, probably because the copper ions remaining on the fiber surface reacted with the reducing functional group of the cellulose and chemically bonded thereafter.
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Fig. 6 (a) Photograph of the appearance after washing test and (b) time variation of copper component in the fabricated fibers investigated using ICP-OES (entry no. 1, 5). |
The amounts of copper components leached in the washing test were ca. 200 mg kg−1 of cotton fibres for 120 min and ca. 900 mg kg−1 of cotton fibres for 240 min (entry no. 5). To some extent, copper is necessary for human metabolism.47 However, in excessive amounts, it has adverse effects. In recent years, many reports have described the safety of nanoparticles. It has been pointed out that water containing copper of 30 mg l−1 or more has adverse effects on human health.46 The leached amount of copper must be controlled to limit effects on the human body according to the application. Furthermore, nanoparticle safety requires assessment not only of direct effects on the human body, but also assessments including aquatic organisms and the natural environment.55,56 Reportedly, copper nanoparticles are more toxic when released as particles than when eluted as copper ions.57 Regarding the release mechanism of the copper component of our fabricated fibers, we evaluated neither the ratio of the leaching amount as copper ions nor the elimination amount as copper nanoparticles. However, our method can suppress the adhesion of particles to the fiber surface. Therefore, it has some potential to reduce particle elimination.
Finally, antiviral tests against influenza A virus were conducted for fibers fabricated with included copper nanoparticles (Table 2). Antiviral activity values of the fabricated fibers were found to be higher than the reference value of 3.0, indicating high antiviral properties: results confirmed that the antiviral functionality of the copper nanoparticles was exhibited even in this fabrication method of creating the copper nanoparticles inside of the cotton fiber. The fabricated fibers suppress the formation of copper nanoparticles on the fiber surface. They also have a certain degree of washing durability. Therefore, the fibers are presumed to exhibit antiviral properties while suppressing contact between human skin and copper metal. Moreover, they can be reused while washing repeatedly. These evaluations are left as subjects for future work. The antiviral property of the copper surface by so-called “contact killing” has been reported not only against influenza virus but also against norovirus, monkeypox, vaccinia virus, human immunodeficiency virus (HIV), SARS-CoV, and SARS-CoV-2 virus.46,58–62 It is expected that the copper-containing cotton fiber is applicable to various viruses by imparting the proper amount of copper nanoparticles according to the contact killing characteristics of the respective viruses.
Sample | Antiviral activity value |
---|---|
Cotton–Cu (entry no. 1) | 4.3 |
Cotton–Cu (entry no. 5) | 3.6 |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra04868f |
This journal is © The Royal Society of Chemistry 2021 |