Anne
Skogberg‡
*a,
Sanna
Siljander‡
*b,
Antti-Juhana
Mäki
a,
Mari
Honkanen
c,
Alexander
Efimov
d,
Markus
Hannula
a,
Panu
Lahtinen
e,
Sampo
Tuukkanen
a,
Tomas
Björkqvist
b and
Pasi
Kallio
a
aBioMediTech Institute and Faculty of Medicine and Health Technology (MET), Tampere University, Korkeakoulunkatu 3, 33720 Tampere, Finland. E-mail: anne.skogberg@tuni.fi
bAutomation Technology and Mechanical Engineering, Faculty of Engineering and Natural Sciences, Tampere University, Korkeakoulunkatu 6, 33720 Tampere, Finland. E-mail: sanna.siljander@tuni.fi
cTampere Microscopy Center, Tampere University, Korkeakoulunkatu 3, 33720 Tampere, Finland
dChemistry, Faculty of Engineering and Natural Sciences, Tampere University, Korkeakoulunkatu 8, 33720 Tampere, Finland
eVTT Technical Research Center of Finland, Tietotie 4E, 02150 Espoo, Finland
First published on 30th November 2021
In this study, a nanocellulose-based material showing anisotopic conductivity is introduced. The material has up to 1000 times higher conductivity along the dry-line boundary direction than along the radial direction. In addition to the material itself, the method to produce the material is novel and is based on the alignment of cationic cellulose nanofibers (c-CNFs) along the dry-line boundary of an evaporating droplet composed of c-CNFs in two forms and conductive multi-walled carbon nanotubes (MWCNTs). On the one hand, c-CNFs are used as a dispersant of MWCNTs, and on the other hand they are used as an additional suspension element to create the desired anisotropy. When the suspended c-CNF is left out, and the nanocomposite film is manufactured using the high energy sonicated c-CNF/MWCNT dispersion only, conductive anisotropy is not present but evenly conducting nanocomposite films are obtained. Therefore, we suggest that suspending additional c-CNFs in the c-CNF/MWCNT dispersion results in nanocomposite films with anisotropic conductivity. This is a new way to obtain nanocomposite films with substantial anisotropic conductivity.
Cellulose nanofibers (CNFs) are an excellent candidate as a primary ingredient in such composites since they provide an eco-friendly and low-cost material alternative1 with the potential to align and offer anisotropic properties for the resulting composite material.2 CNF-based electrically conducting materials have been produced by coating, blending or doping CNFs with conductive nanoparticles such as carbon nanotubes (CNTs). CNFs act as a matrix and dispersant of the conductive CNT component.5–7 These nanomaterials together provide a combination of their superior properties such as the film forming potential,8 favourable mechanical and chemical properties, colloidal stability, cytocompatibility9–15 and biocompatibility of CNFs,9–11 and high mechanical strength, stiffness, and electrical and thermal conductivity of CNTs.16
Chemically added charged groups influence the physicochemical properties of CNFs17 and can improve their processability and performance.18 Treatment with cations can be performed to render cellulose nanofibers cationic by introducing positive charges on their backbone. Cationic CNFs (c-CNFs) can be used in both high-end applications as nanocomposites and high volume applications,19 and are gaining increasing attention as a potential novel nanomaterial with tailored properties in more specialized applications.20,21 Cationic CNFs have the capability to align along the evaporating droplet boundary line by self-assembly mechanisms.22 By dispersing carbon nanotubes with c-CNF, there is potential to form aligned structures also in electrically conductive composites. None of the previous studies have used c-CNFs in the dispersion of CNTs. In the current study, the use of cationic CNFs as a dispersing and stabilization agent is reported for the first time and their alignment is studied in nanocomposite films.
Superior CNF properties are often challenging to transfer into optimal macroscale performance. In plants, CNFs are naturally aligned into highly hierarchical structures. However, despite several attempts and extensive research, the realignment of disintegrated and fibrillated CNFs remains challenging.2 While the alignment of CNFs alone is challenging as such, it is extremely challenging to align CNFs in composites.2 Another challenge is dispersing CNTs because of their self-aggregation tendency especially in aqueous media,23 which on the other hand is often a prerequisite for hydrophilic CNFs.24 While CNT blending in aqueous media is poor due to strong self-aggregation, sonication treatments have been introduced to obtain homogeneous dispersions.6,25
Several research groups have reported the ability of CNFs to disperse CNTs in aqueous media using sonication and act as a stabilization agent in the CNF–CNT dispersion; furthermore, a stable dispersion can be obtained without the use of surfactants, which have been commonly used.7,26–28 Chemical modifications of at least one of the nanomaterial components by the introduction of charged groups may enhance the dispersing effect. In addition, repulsive forces may prevent the re-agglomeration of the particles in the dispersion state resulting in a more stable dispersion.1 It has been shown, e.g., that anionic CNFs enhance the dispersion of CNTs.29
Sonication is one of the methods used for attaining the dispersion of nanoparticles in aqueous media. It is based on inertial cavitation where imploding cavities, which are known to exist at the boundaries of materials, generate intensive streams of molecules.30 In sonication, a high energy density is introduced to the CNT–CNT interface, which is high enough to cause initial separation of the nanotubes by breaking non-covalent forces between tubes. However, the used amount of energy should be optimized such that only non-covalent forces are overcome and the carbon nanotube backbone remains intact. An overly high sonication energy can cause carbon bond breakage inside individual nanotubes, which deteriorates their ballistically conducting nature. The dispersion quality and functionalization obtained in sonication depend on the used sonication energy, medium, surfactant type, and surfactant amount.6,25,31
In this study, we present a method to produce self-assembled nanocomposite films with adjustable anisotropic thermal and electrical conductivity. The method uses sonochemical treatment and c-CNFs to disperse multiwall carbon nanotubes (MWCNTs). The resulting dispersion is assembled with the help of additional, suspended c-CNFs, which are based on the previously reported c-CNF alignment along an evaporating boundary line.22 We also show how the alteration of the composition and pre-treatment of the two nanomaterials enable the fabrication of nanocomposite films with either isotropic or anisotropic properties. Controlled nanomaterial assembly commonly requires more time, high energy consumption, and expensive proprietary technology.32 The method described in this article is simple, quick, efficient, and safe compared to current attempts to control the assembly of nanomaterials. Such novel nanocomposites could be used as a cell guidance material with cells that benefit from the mechanical improvements offered by MWCNTs or in applications in which anisotropic heat or current conductivity is essential. CNF-based electrically conductive composites have many advantages although many challenges remain, e.g. related to electrical connectivity. In our previous study, we have also shown that MWCNTs can be effectively dispersed with unmodified CNFs with the help of surfactants and form highly conductive nanocomposite films with a relatively low MWCNT concentration.6 In the present article, we combine the advantages of these observed phenomena to create a totally new methodological concept: c-CNFs are used as an aid to disperse MWCNTs homogeneously to form a stable dispersion, and additional c-CNFs in the dispersion undergo interfacial self-assembly with the high energy sonicated (hes-) hes-c-CNF/MWCNT dispersion, resulting in a novel nanocomposite film with anisotropic properties.
The control films show relatively constant resistance throughout the film, as illustrated in Fig. 3. In contrast to the control films, the self-assembled films S1–S3 show evident anisotropy. The resistance increases along the centre line towards the centre (measurements A and C, Fig. 3), while along the circle it essentially remains constant, measured from both circles 1 and 3 (measurements D and E, respectively, Fig. 3).
In measurement A, the measurement probes are moved along the centre line, maintaining a constant probe distance, depicting an increase in resistance when moving towards the centre, and a subsequent decrease in resistance when moving from the centre towards the edge of the film. The resistance increase is apparent in each measurement towards the centre, although the increase is larger closer to the centre.
Measurement B demonstrates an increase in resistance along the different circular zones, that is, circles when zones are located closer to the centre. Thus, the resistance continually increases even though the probe distance decreases. A difference in the isotropic control film is evident, where the resistance decreases with reduced probe distances, as is expected for an evenly conducting film. Measurement B includes radial line locations that correspond to the location at 180° in measurement D and E, further confirming lower resistances along a circular zone than along the centre line.
In measurement C, one probe remains in location 1 in all measurements, while the other probe is moved along the centre line, that is, the probe distance increases in the subsequent measurements. Measurement C again confirms the difference between the control and assembled films. The resistance in the control film increases slightly with an increasing probe distance, while in the self-assembled films S1–S3, the resistance increases significantly until the centre, with a subsequent decrease after the centre towards the opposite edge of the film. This is in accordance with the Hypothesis, which assumes the current passes through the conducting zones, that is, along the circles in the self-assembled S1–S3 films – instead of the shortest route across the centre, as in the control films.
The resistance measured along circle 1 and along circle 3 in measurements D and E, respectively, is consistent with the Hypothesis. In the control film, the resistance in circle 1 is slightly higher than that in circle 3. This is due to the longer probe distance in circle 1 than in circle 3. Thus, the result is compatible for a uniformly conducting film. Opposite to the control film, the self-assembled films (S1–S3) have a lower resistance in circle 1 than in circle 3, even though the probe distance decreases in circle 3. The result is in accordance with measurement B (Fig. 3), which demonstrated the increase in the resistance along the different circles when zones are located closer to the centre. Opposite results between the control films and films S1–S3 further confirm the Hypothesis of an anisotropic assembly in S1–S3. The increase in resistance of all the measured films (control, S1–S3) when moving along circles 1 or 3 results from the increasing probe distance.
The demonstrated anisotropy is stronger in S2 than in S1 because of a higher concentration of suspended free c-CNFs in S2 than in S1. The concentration of suspended free c-CNFs is equal in S2 (supernatant) and S3 (supernatant sonicated 625 kJ g−1). Even though the concentration of suspended c-CNFs is equal in S2 and S3, the anisotropy is stronger in S3. Suspended c-CNFs are more fibrillated in S3 compared to S2, thus the nanofibrils in S3 are expected to be smaller in diameter.
When comparing resistances on approximately the same distance along circle 1 and along the radial line, that is, probe locations (measurement D) and 1–4 (measurement C), respectively – the resistance is approximately 10-fold, 100-fold, and 1000-fold higher in the radial line compared to the resistance along circle 1 for S1, S2, and S3, respectively. Comparing the length of the radial line from the edge until the centre of the film – that is, locations 1–5 (measurement C) – and the corresponding distance on circle 1 – that is, locations
(measurement D) – the difference in the resistance along the centre line (from location 1 to location 5) is even higher, approaching 100-fold and 1000-fold in S1 and S2, respectively. In S3, the resistance in the corresponding locations remains the same as between locations 1 and 4, that is, approximately 1000-fold. In all self-assembled films S1–S3, the highest resistance is found in the middle of the films and is approximately 0.5 MΩ for S1 in locations 4–5, while approaching 10 MΩ and 100 MΩ in S2 and S3, respectively. Composites were subjected to an electric field for at least 11 min as the measurements were repeated three times for each film, to demonstrate the stability of the films. In addition, three parallel films were fabricated and measured in order to show the repeatability of the film manufacture. Parallel and repeat resistance measurement results are summarized in ESI Fig. S1† and the results indicating the stability of the films in the electric field are presented in ESI Fig. S1 and S2.†
In addition to the CV value, a mean orientation angle of each image location was determined in order to show the orientation direction along the evaporating boundary line. While CV only describes the degree of orientation – that is, isotropy or anisotropy – the mean orientation can be used to show if the orientation direction is according to our hypothesis; that is, along the evaporating dry-line boundary, and thus along the imaginary circles. For this purpose, the films were scanned on a specific surface location, and the scanning location (Fig. 4c) was compared with the mean orientation angle (Fig. 4e) of the analyzed images (Fig. 4a and b as an example) and the expected orientation direction according to our hypothesis (Fig. 1). The mean orientation is presented only for assembled films, as the control films have no expected orientation according to the analyzed CV values. However, an example of the mean orientation angle for the control sample is shown in Fig. 4b for the scanning location shown in Fig. 4d.
The CV analysis (Fig. 4h) confirmed the expected higher degree of orientation of the assembled films compared to the control films, as discussed below. The degree of anisotropy is greater for assembled films than for control films with more isotropic CV values. As the film also contains isotropic MWCNT arrangement (Fig. S4a, e and i†), CV values are not expected to be close to zero. This is because only the added c-CNFs are expected to align along the dry-line boundary during evaporation. Thus, the average CV values (n = 4) of the assembled films S1, S2, and S3 are 0.65–0.76 (Fig. 4h), compared to the previously determined 0.27,22 of highly anisotropic pure c-CNF films. The average circular variance (n = 4) of the control films is 0.96.
In summary, CV values show a higher degree of anisotropy for assembled films than control films do, while the mean orientation angles in specific locations show orientation along the circle. This indicates orientation along the dry-line boundary during the evaporation of the liquid.
When analyzing the low vacuum SEM surface scans (one example in Fig. S5†), it is evident that the hes-c-CNF fibrillates during sonication, as the larger fiber bundles that can be seen in S1 and S2 (Fig. S5a and b†) are missing in S3 (Fig. S5c†), in which the suspended component was hes-c-CNF. The decreased size of hes-c-CNF compared to c-CNFs is due to the high energy sonication treatment of the former. The sonication energy used in suspended hes-c-CNF in S3 is the same as that used to prepare the hes-c-CNF/MWCNT dispersion. The degree of orientation is significantly higher in the case of S3 (manufactured using hes-c-CNF + hes-c-CNF/MWCNT) compared to the control film (manufactured using hes-c-CNF/MWCNT). This confirms that the alignment is not dependent on the suspended c-CNF size, but the addition of the suspended c-CNFs.
![]() | ||
Fig. 5 SEM edge view of the intact assembled (a) and control (b) nanocomposite film. Surface scan with visible MWCNTs is presented in (c). More SEM images are presented in ESI file Fig. S4.† Scale bar sizes are as follows: 200 nm (a and b), and 1 μm (c). |
According to the Hypothesis, we expect free c-CNF alignment on the evaporating boundary line; thus, the film edge should be relatively smooth, as demonstrated in Fig. 5a. In addition, the outmost film edge is slightly thickened in the case of the self-assembled films, which is consistent with our previous observation of droplet boundary line pinning in the beginning of evaporation22 and accumulation of aligned fibers on the boundary edge. Therefore, a slight ‘coffee ring’ effect is expected in the very beginning of the evaporation, after which the evaporation progresses evenly as the nanosized fibers of the suspension align parallel to the boundary line. The droplet evaporates by shrinking towards the centre and allows more time for the assembly of c-CNFs in the beginning of droplet evaporation, that is, further from the droplet centre.22 In the previous study, we investigated the droplet evaporation of free c-CNFs in more detail, and it is known that evaporation closer to the centre is faster due to the smaller droplet volume. This likely influences the film thickness, composition, and anisotropic properties. More structural characterization is presented in ESI file Sections 1.3 and 1.4.†
The dispersive effect of the c-CNFs on MWCNTs is visible in the AFM scan of the film surface (Fig. 6a), as well as of the dilute dispersion (Fig. 6b), with no visible MWCNT aggregates present. The dilute dispersion presents larger molecules, that is, MWCNTs. Hes-c-CNF in the dispersion (Fig. 6b) is smaller than c-CNFs in the supernatant (Fig. 6c) due to a higher degree of sonication. This is observed also from surface scans (ESI Fig. S5†), which confirms the presence of larger nanocellulose fibers in S1 and S2 films fabricated using supernatant c-CNFs, while S3 was fabricated using 625 kJ g−1 sonicated hes-c-CNF in which larger fibers are not detected. The TEM images show the c-CNF matrix rigidly attached to individual MWCNTs (Fig. 6d and e) on a torn film boundary. For comparison, Fig. 6f shows the TEM image of pristine multi-wall carbon nanotubes. Similar to SEM imaging, the challenge of TEM is to distinguish individual cellulose nanofibrils from a densely packed matrix. (S)TEM (secondary electron imaging) was used to distinguish individual c-CNFs on top of MWCNTs (Fig. 6h), from a torn film boundary (Fig. 6g). Untreated MWCNTs were examined using STEM (Fig. 6i) for comparison.
Based on the findings of SEM and AFM examination, we can confirm that c-CNFs work as a dispersing/stabilizing agent for MWCNTs. The present study is the first to demonstrate the successful use of c-CNFs to disperse MWCNTs. However, further optimization for sonication energy per dry mass and materials’ concentrations should be done.
The intensity and the area of the signal peak at 2.1 ppm results from –CH3 groups of the functionalized hydroxypropyltrimethylammonium side chain and remain relatively unchanged in c-CNF samples (Fig. S7a–c†), indicating that the functional group remains intact and relatively stable throughout the sonication treatments. Untreated c-CNFs and c-CNFs sonicated with 625 kJ g−1 provide the same signals. Thus, the effect of sonication on the chemical structure of c-CNF is assumed negligible. Evaluating the interaction between c-CNFs and MWCNTs during sonication is not straightforward from 1H NMR spectra (Fig. S7d†) as the analyte is merged and signals are broad and overlapping. However, when compared to the signals obtained from c-CNF alone, we can draw some conclusions on the organization of c-CNFs in the sample: the functional group is expected to point out from the entangled hes-c-CNF-MWCNT nanostructure. As the peaks in 5 ppm to 3.8 ppm are weak and broadened, the corresponding protons are not solvated, indicating strong interaction between those areas of c-CNFs with MWCNTs. The formed nanostructure is tightly packed, which prevents water from penetrating between the interaction sites. In summary, as the trimethyl protons of the cationic substituent remain unchanged in all samples, we can assume it is facing outward from MWCNTs, while the less solvated area refers to cellulose ring protons, in this case one of the H3–H5, which would be the location of the stronger interaction with MWCNTs. The result is presented in more detail in ESI 1.5.†
The NMR result is in accordance with the suggested interaction where hes-c-CNF is covering MWCNTs (Fig. 1f and 6h). The result also provides valuable information regarding the orientation of c-CNFs relative to MWCNTs, which we cannot obtain from other characterization methods. Thus, we can make assumptions regarding the dispersion chemistry that the free c-CNF encounters when suspended with the dispersion. According to NMR, there is no covalent chemical modification of the dispersion components during rather heavy sonication treatment, as also supported by the ATR-FTIR results (ESI Fig. S8†).
We propose that additional free c-CNFs are accountable for the self-assembly of c-CNF-MWCNT nanocomposite films; and this conclusion is consistent with our previous studies on the droplet evaporation of c-CNFs.22
Fig. 1 is an oversimplification of the hypothesized structure of the film in which the nanocomponents are not in scale. The diameter and length of c-CNFs have been determined using AFM and they are on average 5(−15) nm and less than 2 μm, respectively.35 According to the MWCNT manufacturer, the diameter and length of MWCNTs are on average approximately 9.5 nm and 1.5 μm, respectively. Thus, they are similar to those of untreated c-CNFs. Sonication treatment during dispersion preparation is expected to significantly fibrillate and cut the length of hes-c-CNF. AFM images of the dispersion components (Fig. 6b) and the untreated c-CNFs (Fig. 6c) are consistent with this hypothesis.
The amphiphilic nature of the nanocellulose chain affects the solubility and the surface energy, which also affects the dispersibility, the hydrophobic interactions and the aggregation tendency.19,36 During sonication, applied energy affects intramolecular and intermolecular bonds, causing a different degree of fibrillation in the cellulose structure. The increasing surface area and polar/non-polar nature of cellulose aid the dispersion of hydrophobic MWCNTs in an aqueous environment. The high surface-to-length ratio of MWCNTs and the lack of functional groups determines that the chemistry of carbon nanotubes is dominated by dispersion type interactions. These extended π-conjugations, such as π–π and cation–π, enable non-covalent interactions with various substrates. Evidently, cellulose and CNTs exhibit favorable interactions toward each other so that auto aggregation of both nanoparticles does not occur, and they form a strong entangled hybrid nanoparticle dispersion, where the cationic functional groups of c-CNFs point outwards from the hybrid structure, resulting in a stable dispersion. Findings in the SEM and NMR studies endorse that nanocellulose and carbon nanotubes have established robust connections at the molecular level and c-CNFs can be used to disperse MWCNTs.
The alignment mechanism of individual c-CNFs along the evaporating droplet boundary line is based on the theory introduced by Mashkour et al.,32 and it is based on the capillary force gradient and surface tension torque (STT) near a dry-line boundary layer. The dry-line boundary layer is the air–suspension–substrate interface, which is also referred to as a triple line.
We suggest, that in the beginning of the assembly process of the assembled films S1–S3, c-CNF fibers align along the boundary line due to pinning of one fiber end and alignment of the fiber parallel to the dry boundary line due to surface tension torques and capillary forces.22,32 Initiation of the alignment of an individual c-CNF involves surface tension torques, and the propagation of the fibre alignment, when the fibre is bent closer to the boundary line due to capillary forces, according to the alignment process described in the study of Mashkour et al.32 A previous study showing alignment of c-CNFs suggested that the same alignment theory applies to c-CNF during droplet evaporation. The interaction between dispersed MWCNTs and c-CNFs, as well as between the additional free c-CNFs and the hes-c-CNF-MWCNT dispersion component, is assumed to be relatively strong. The c-CNFs in the hes-c-CNF-MWCNT dispersion can further interact with free c-CNFs, which is hypothesized to initiate the assembly, further dragging the c-CNF-MWCNT into circular zones during droplet evaporation. Even though hes-c-CNF is not free to align after dispersing MWCNTs, the excess of cationic groups on the cluster surfaces allows stronger capillary forces to take place during drying when interaction with free aligning c-CNFs can take place. Therefore, free aligning c-CNFs traps along the hes-c-CNF-MWCNT entangled dispersion and an assembled structure is formed. This results in aligned free c-CNFs during droplet evaporation and assembly of the hes-c-CNT-MWCNT entangled dispersion, resulting in alternating aligned c-CNFs and more entangled hes-c-CNF-MWCNTs. Sonication pre-treatment of suspended free c-CNFs increased the resistance, as indicated by the higher resistance of film S3 compared to that of film S2. This is likely due to the smaller size of the c-CNF in S3, which is a result of sonication. Thus, smaller c-CNFs seem to reduce conductive pathways more than larger c-CNFs in S2. In other words, the smaller c-CNFs of S3 may cover MWCNTs better, and therefore cause lower conductivity. It is worth highlighting, that the degree of orientation is significantly higher in the case of S3 (manufactured using hes-c-CNF + hes-c-CNF/MWCNT) compared to the control film (manufactured using hes-c-CNF/MWCNT). This confirms that the alignment is not dependent on the suspended c-CNF size, but the addition of the suspended c-CNFs. The dispersion consists of hes-c-CNF-MWCNT clusters in which cationic groups point outwards. Therefore, the added c-CNFs can interact with these cationic groups and initiate the self-assembly at the evaporating boundary line. The added c-CNFs are expected to initiate the assembly process as described and result in alternating aligned c-CNFs and more entangled hes-c-CNF-MWCNT, shown in Fig. 1g. The anisotropic alignment is detected from the surface topography at a larger scale with image analysis, while the control films are more isotropic. Although individual aligned c-CNFs are not seen in SEM images, the detected larger scale anisotropy indicates tightly packed and aggregated aligned c-CNFs, similar to our previous study.22 Anisotropy of the assembled films is a result of the added c-CNFs, while MWCNTs do not show alignment in the assembled films. When evaporation has proceeded close to the end towards the centre of the film, the evaporation rate is faster due to lower volume remaining, and there is no time for assembly along the liquid boundary line in the centre part of the film.
In the present study, the purpose of the resistance measurement was to indicate the difference in the electrical properties between the control film and the self-assembled films. Furthermore, the resistance measurement was used to characterize the level of anisotropic conductivity in the assembled films. Here, two-point measurement was used since it shows the conductivity exactly between two distinct points. Even though the usage of four-point measurement rules out the possible contact resistances between the metallic probes and the CNT network, it would require homogeneous surface conductivity and, is therefore not a suitable method for characterizing anisotropic films that have circle-shaped conducting pathways.
The distinct two-probe resistance measurement points along the centre line and along the circles were repeatably defined using a designed measurement position template (see Fig. 8b). We noticed that along the circles (that is, the measurement locations in Fig. 8, measurements B, D, and E), the positive and negative probe locations could be switched without affecting the measured resistance. However, switching of the positive and negative probe locations along the centre line (that is, measurement locations in Fig. 8, measurement A and C) showed a significant difference on the measured resistance. This was further studied with IV-curves, which confirmed anisotropy along the centre line (that is, nonlinear IV-curves) in films S1–S3, while along the circular path in S1–S3 the curves were linear. Control films had linear IV-curves in both measured directions. As shown by the IR-images and resistance measurements, the conductivity of the assembled films is different along the radial direction compared to the circular zone direction. This is due to the aligned c-CNFs along the circle, while c-CNFs are packed next to each other along the neighbouring circles. Towards the centre, more and more CNFs are packed between the conducting MWCNT components, blocking the conductive pathways, and increasing resistance towards the centre. While this paper presents that the electrical properties are different along the zone and towards the centre due to c-CNF-driven assembly, the electrical properties along the radial direction should be studied further. Our observation indicates a resistive component along the zone and a resistance coupled to a capacitive component along the radial line.
MWCNTs are often a better option for biomedical applications than SWCNTs due to more standardized methods of chemical functionalization and lower cytotoxicity.37 The c-CNF cover on the MWCNT surface could provide even better biocompatibility while still providing the benefits of MWCNTs, for example, in biomedical applications such as multifunctional antimicrobial drugs, drug delivery vehicles, functional surfaces for cell growth and adhesion, and new therapies for diseases.38 Furthermore, the c-CNF cover on the MWCNT surface could also provide safety for the user in terms of handling the materials. Thus, we expect that the presented cover of c-CNFs on the MWCNT surface resulting from sonochemical treatments is beneficial for the future use of MWCNTs. However, the cytocompatibility and health effects should be studied further.
Anisotropic materials are important functional materials in many fields. For instance, anisotropy in conductivity has been achieved in certain synthetic polymer systems.39,40 However, it has been more difficult to create similar materials using biologically relevant matrices.39 The size scale of c-CNFs in an anisotropic assembled substrate surface22 is optimal for diverse biomedical applications that require protein adhesion. Previously, we have shown cell adhesion and growth on c-CNF–CNT-coated cellulose mesh substrates in our study,41 in which the coating was prepared from the dispersion reported in the materials and methods section of the current paper. In addition, c-CNFs, either alone or as coating, have been shown to support cell growth, charge mediated adsorption, and cell alignment.22,42
Directionally conductive, engineered tissues have applications in a variety of fields, including stem cell biology, cardiac and neural tissue engineering, and biosensor development.39 Models to mimic native tissues, such as anisotropic myocardial fiber architecture,43 would benefit from adjustable platforms/substrates with both anisotropic and conductive properties, suggesting potential application areas for in vitro cell, tissue and disease models (organ on a chip, body on a chip, disease on a chip). In such in vitro platforms, electrical conductivity could serve to stimulate cell growth, differentiation, or drastically intentional damage to the cell, in the case of disease models or photodynamic therapies.41 In addition, compartmentalized heating can be achieved with anisotropic films, as demonstrated with IR-imaging, and is adjustable by tuning film components. The demonstrated formation of assembled anisotropic conductivity is not limited to a specific substrate – that is, a circular film – nor to a specific application, but rather has potential in a variety of fields.
Multiwall carbon nanotubes (MWCNTs, Nanocyl 7000, Nanocyl SA., Sambreville, Belgium) were purchased from Nanocyl Inc. and the product was used in the state received. The MWCNTs are produced using catalytic chemical vapor deposition (CCVD).
The control hes-c-CNF/MWCNT samples were prepared by sonicating the c-CNF supernatant and the MWCNTs to form a homogeneous stable dispersion. The control samples contained the c-CNF supernatant with a concentration 0.05 w% of MWCNTs in aqueous medium. The total dry mass for the control sample was 0.16 g. The dispersion samples were sonicated using a tip horn sonicator Q700 (QSonica LLC, Newton, CT, USA) in 100 mL glass beakers. The amplitude of the sonication vibration was kept constant. The power output remained between 50 W and 60 W for every sonication. The sonication system included a water bath to keep samples cool during the sonication such that the temperature did not rise above 30 °C. The water bath was cooled by circulating cooling glycerol through a chiller (PerkinElmer C6 Chiller, PerkinElmer Inc., Waltham, MA, USA). Samples were sonicated using a same amount of energy per dry mass, respectively 625 kJ g−1. The sonication energy was chosen based on our previous studies.5,6 The resulting hes-c-CNF/MWCNT dispersion was used to prepare control nanocomposite films (Fig. 7).
![]() | ||
Fig. 7 The preparation of the dispersion and suspension, and the subsequent manufacture of the control isotropic and assembled anisotropic films using the dispersion and suspension, respectively. |
The hes-c-CNF/MWCNT dispersion was further suspended (Fig. 7) with varying volume ratios of additional c-CNFs for the preparation of suspension samples S1 (37.5% v/v), S2 (20% v/v) and S3 (20% v/v). The nanocellulose used in S1 and S2 suspension is the 0.15 w% c-CNF supernatant described previously, while the nanocellulose in the S3 suspension was further sonicated with 625 kJ g−1, and is thus hes-c-CNF. Thus, S1 and S2 differ in volume ratios of the dispersion and added c-CNFs, while S2 and S3 differ in pretreatment of the added c-CNFs and hes-c-CNF, respectively, while the volume ratio is constant.
NMR spectra were measured on a 500 MHz JEOL JNM-ECZ 500R spectrometer. Samples (approx. 50 mg) were packed into 3.2 mm diameter zircônia rotors with KelF caps as a tick suspension in D2O. The semi-solid state FG-MAS 1H spectra were recorded at room temperature, with the high-resolution field gradient FG-MAS probe at a spinning rate of 5 kHz. A water suppression pulse sequence was applied during the measurements.
Footnotes |
† Electronic supplementary information (ESI) available: Additional experimental details and results. See DOI: 10.1039/d1nr06937c |
‡ These authors contributed equally. |
This journal is © The Royal Society of Chemistry 2022 |