Cellulose Nanocrystals (CNCs), as an environmentally friendly, green and inexhaustible material, are widely used in transparent packaging, drug release, water purification etc. However, CNCs have an innate tendency to aggregate, and are difficult to be redispersed especially after drying. In this work, we propose a simple and easy-to-use route to render the CNCs with excellent redispersibility by adding a debonding agent (DA). The redispersed CNCs are named Re/mDA-CNCs. Here m represents the percentage amount of DA and Re is an abbreviation for redispersed. The results show that the Z-average size of Re/0.4DA-CNCs after redispersion is consistent with that of original CNCs, and the size distribution is uniform. The Zeta potential of Re/0.4DA-CNCs is lower than that of the original CNCs. The reason is that the electrostatic repulsion between the hydrophobic groups in DA and the -OH groups on the CNC surface leads to the neutralization of the cationic groups in DA with the -OH groups on the CNC surface, which reduces the amount of H-bonding formed through -OH groups in CNC intramolecular. Furthermore, the FT-IR and XRD results indicate that the successful introduction of DA to the CNC surface shield the formation of H-bonding by -OH groups in CNC intramolecular, leading to the decrease of CNC cornification without changing the basic chemical structure of the CNCs. The Re/0.4DA-CNCs demonstrate potential applications in the field of food packaging, biological medicine, etc.
Research Article
Endowment of Excellent Water Redispersibility for Cellulose Nanocrystals Using the Strategy of Chemical Shielding and Dispersion with Debonding Agent
https://doi.org/10.21203/rs.3.rs-2216194/v1
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cellulose nanocrystals
debonding agent
redispersibility
electrostatic repulsion
neutralization
With the rapid development of economy and the increase of petroleum-based resource consumption and CO2 release, biodegradable cellulose-based materials have attracted the attention of researchers. Nanocelluloses (NCs) are from natural cellulose, which is renewable, green and biodegradable (Sinquefield et al. 2020; Xu et al. 2022). They can be divided into cellulose nanofibrils (CNFs) (Silva et al. 2021), cellulose nanocrystals (CNCs) (Giorgio et al. 2020), hollow-type annular cellulose nanocrystals (HTA-CNCs) (Xu et al. 2022) and bacterial nanocelluloses (BNCs) (Elayaraja et al. 2020) according to the size and morphology. Among them, CNCs are prepared by natural celluloses through chemical methods (sulfuric acid, hydrochloric acid, phosphoric acid, hydrobromic acid, formic acid, oxalic acid, TEMPO oxidation, deep eutectic solvents, ionic liquid, etc.), physical methods (high-pressure homogenization, milling and high-intensity ultrasound treatment, etc.), biological methods (enzymatic hydrolysis), or mechanical methods in cooperation with chemical/biological methods (Gray 2020; Thomas et al. 2020; Wang et al. 2021). However, during the preparation of CNCs, the gel can be formed when the CNC concentration changes (i.e.: ≥ 1% or ≤ 3%), which is proved by their easily flocculation at lower concentrations. CNCs are more difficult to be redispersed once they are completely dried because a large number of hydroxyl groups (-OH) on their surface will form irreversible hydrogen bonding (H-bonding) through “self-assembly”, resulting in the loss of unique nano-effects (small-size effect, surface interface effect, etc.) (Chen et al. 2021; Cheng et al. 2020; Hossain et al. 2021). Their dispersibility and redispersibility directly determine whether their large-scale production, storage, transportation, functional modification and application could be commercialized (Chu et al. 2020; Heise et al. 2021; Trache et al. 2020; Xie and Liu 2021). How to effectively avoid the flocculation of CNCs has attracted extensive attention from academia to industry.
Due to the polyhydroxy structure of CNCs, Jongaroontaprangsee et al. (2018) prepared the CNCs with phosphate groups via the phosphoric acid hydrolysis and demonstrated the formation of phosphate ester groups (-OPO3H2) by substituting the -OH groups at the C2/C3 positions of glucose unit on the surface of CNC. These negatively charged groups generate strong electrostatic repulsion with the remaining -OH groups on the CNC surface to form a stable colloid, which can prevent the formation of H-bonding between -OH groups in adjacent CNC intermolecular and improve the dispersibility and redispersibility of CNCs. Posada et al. (2020) utilized the special chemical structure of tert-butanol to achieve steric hindrance effect and reduce the formation rate of ice-crystals when tert-butanol is added in the freeze-drying process of CNCs, so as to obtain more refined products, which is conducive to improve the redispersion performance. Using strong acids in the preparation of CNCs leads to low yield and the freeze-drying procedure is costly, leaving many challenges to be investigated for large-scale production.
Debonding agents (DAs) are reagents which are typically applied to weaken the bonding force of fibers and are usually used in the preparation of fluff pulp, they can be categorized as cationic and non-ionic (Xu et al. 2021). Among them, cationic debonding agents (c-DAs) are composed of cationic groups (=N+=, -S+-, =P+, etc.) and hydrophobic groups. Xu et al. (Xu et al. 2021) synthesized a series of alkyl quaternary ammonium salts (AQAS) antibacterial-bonding agents by a two-step method. The obtained products can endow antibacterial properties and high softness to paper and control the appropriate burst strength, making it suitable for improving the properties of fluff pulp. The mechanism is that the positively charged groups is combined with the -OH groups on the molecular chain of the fiber surface, which acts as a role for shielding the free -OH groups, weakening the H-bonding and reducing the static friction coefficient between fibers. The hydrophobic groups of the alkyl chain of AQAS antibacterial-bonding agents have a certain hydrophobicity. In addition, it can also be reduced the formation of H-bonding through the reaction of -OH groups on the CNC surface, Du et al. (2022) prepared antibacterial materials by loading silver nanoparticles on the dialdehyde cellulose with in-situ synthesis method, and showed excellent antibacterial properties. Obviously, DAs can play an important role in the modification of the interface in composite materials because their hydrophobic groups are easily rearranged on the fiber surface, which have been widely used in medicine and various industrial fields (He et al. 2018; Tavakolian et al. 2020; Wei et al. 2017). In contrast, for the excellent performance of DAs, the research of DAs on the redispersibility of CNC is rarely reported.
In this study, we reported a simple and efficient method for the preparation of CNCs with excellent water redispersibility. Debonding agents (DAs) were used as a green and non-toxic H-bonding blocking agent and dispersant to prepare redispersible CNCs. The effects of DAs on the redispersibility and dispersion stability of CNCs were investigated by comparing the chemical structure, crystal structure and thermal stability of Re/mDA-CNCs with that of the original CNCs.
2.1 Materials
Bleached bamboo pulp fibers were produced from Chitianhua Co., Ltd. (Guizhou, China). Sulfuric acid (H2SO4, analytical reagent (AR) grade, purity ≥ 98wt%) was purchased from MACKLIN Inc. (Shanghai, China). Debonding agent (DA, ArosurfPA777) was obtained from Rogate Nutrition Food Co., Ltd. All chemicals were used as received without further purification and deionized (DI) water was applied throughout the experiments.
2.2 Methods
2.2.1 Preparation of Cellulose Nanocrystals (CNCs)
CNCs were prepared according to a previous method (Song et al. 2018). 300 ml of 64 wt% H2SO4 was placed in a constant temperature water bath at 45℃ and mechanically stirred. After the H2SO4 was completely preheated, 30.00 g bleached bamboo pulp fibers were uniformly added and acid-hydrolyzed for 25 min when stirring at a rate of 800 r/min. Subsequently, the suspension was cooled down to room temperature and centrifuged at 10,000 rpm for 15 min. The collected supernatant was put into the dialysis bag and then dialyzed against DI water for 3-4 days until the pH of the supernatant does not change. The concentration of CNC suspension was adjusted to about 2.0 wt%.
2.2.2 Preparation of Redispersible Cellulose Nanocrystals (Re-CNCs)
The above prepared CNC suspension (67.00 g) and DA with different content (0.1%, 0.2%, 0.3%, 0.4% and 0.5%, based on oven-dried CNCs) were added into a 100 mL beaker and stirred until no obvious solid powder could be seen. Then, the mixed suspension was ultrasonicated (400 W) at 25℃ for at least 30 min so as to ensure that DA was fully dissolved into CNC suspension. The mixed suspension was transferred to surface dish and dried for 8 h in a vacuum oven (-0.8Mpa) at 50℃ to obtain a solid-state redispersed CNC film, which is named Re/mDA-CNCs. Here m represents the percentage amount of DAs and Re is an abbreviation for redispersed. In order to ensure the rapid dispersion of Re/mDA-CNCs in water, the Re/mDA-CNCs film were slightly ground with agate mortar to obtain the Re/mDA-CNC powder. Finally, 2.00 g Re/mDA-CNC powder was dispersed into 100 mL DI water at 50℃ for subsequent characterization.
2.3 Characterization
2.3.1 Dispersion Stability of CNCs and Re/mDA-CNCs
The average particle size (Z-average), polydispersity index (PDI) and Zeta potential of original CNCs and Re/mDA-CNCs were measured by nano-particle surface potential analyzer (Nano-ZS90, Malvern, England). Additionally, the original CNC suspension and Re/mDA-CNC suspension were allowed to stand for several days to observe the Tyndall effect. Furthermore, the dispersion stability of samples was performed by a stability analyzer (Turbiscan LAb, France). The samples were transferred to the cuvette standing for 80 min at 25°C, and the cuvette height is 40 mm. The change in backscattered light intensity (ΔBS) was measured under shear effect to evaluate the dispersion stability of samples.
2.3.2 Scanning Electron Microscope
The morphology was observed by an S-8100 field emission scanning electron microscope (SEM, VEGA 3 S-8100, HITACHI, Tokyo, Japan) operated at 5 kV, and all samples were sputtered with gold before being observed.
2.3.3 Fourier Transform Infra-Red Spectroscopy
The presence of functional groups in original CNC powder and Re/mDA-CNC powder was determined on a Fourier Transform Infra-Red Spectrometer (FT-IR, Vertex-70, Bruker, Germany). The solid samples were mixed with KBr at the ratio of 1:100 and grounded continuously, and then pressed to obtain tablets for FT-IR analysis. The transmission spectrum was obtained in the wavenumber range of 4,000-500 cm-1 with a resolution of 4 cm-1.
2.3.4 X-ray Diffraction
The crystallinity index (CrI) of samples was measured by an X-ray diffractometer (XRD, D8-ADVANCE, Bruker, Germany). The samples were scanned at 40 kV and 30 mA in a 2θ range between 5° and 60° using Cu-Kα radiation (λ=15.4×10-2 nm) at a scanning rate of 1°/min. The diffraction spectrum was processed by using software analysis program (PeakFit v4.12) in combination with Gaussian profiles to smooth the data and identify the peaks. The crystallinity index (CrI) of a sample was calculated with the following equation (Ling et al. 2019).
Where Acryst is the area under the calculated pattern for cellulose crystalline region, and Aamorph is the area that represents the amorphous content under the calculation mode (Ling et al. 2019).
2.3.5 Thermogravimetric Analysis
Thermal properties of samples were performed by thermogravimetric analysis (TGA, STA449F3 1053M, USA). The samples (2-5 mg) were placed in a nitrogen atmosphere with a platinum sample pan, heating from 50℃ to 600°C at a heating rate of 10 °C/min.
3.1 Characterization of Particle Size and Zeta Potential
Firstly, the Z-average size and PDI of Re/mDA-CNCs when redispersing into DI water are shown in Table 1. The Z-average size and PDI of original CNCs are 110 nm and 0.213, respectively. However, it is observed from Fig. 3a that the original CNC powder via direct vacuum drying cannot be redispersed. The Z-average size and PDI of Re/mDA-CNCs when redispersed in DI water gradually decrease with the increasing of DA amount (Table 1), indicating the improved redispersion properties of Re/mDA-CNCs at higher DA amount. The Z-average size gradually approaches the size of original CNCs, and the particle size gradually become uniform. The Z-average size and PDI of Re/0.4DA-CNCs are 109 nm and 0.213, respectively. This indicates that Re/0.4DA-CNCs can be restored to its original size when redispersed in water. The size of Re/0.5DA-CNCs with higher DA amount do not change, indicating that the cationic groups of DA are attracted to the negative charges on CNC surface by opposite charges attracting, effectively reducing the number of H-bonding formed through -OH groups in CNC intramolecular, the schematic illustration for preparation and mechanism of Re/mDA-CNCs was shown in Fig. 1. And allowing the redispersion of Re/0.4DA-CNCs.
Furthermore, it can be seen from Table 1 that the Zeta potential of original CNCs is -45.9 mV and the Zeta potential of Re/mDA-CNCs gradually decreases with the addition of DA. The reason is due to the fact that DA is a surfactant composed of cationic groups and hydrophobic groups (C6H5- \ R-O-R (R') \ R-NH-R', which is shown in Fig.4). During the dehydration of CNCs, the electrostatic repulsion between the hydrophobic groups on the DA surface and -OH groups on the CNC surface leads to the neutralization of the cationic groups in DA with -OH groups on the surface of CNCs, which provides more opportunities to reduce the number of H-bonding formed through -OH groups in CNC intramolecular. The Zeta potential of Re/0.4DA-CNCs is -46.2 mV when dissolved in water (Table 1), which is slightly lower than that of original CNCs. The reason is that cationic groups in DA are attracted onto -OH groups on the CNC surface, consequently block the formation of H-bonding formed by -OH groups in CNC intramolecular. In addition, the hydrophobic parts in DA also rearrange on CNCs surface and extend outside, which improves the hydrophilicity and interface compatibility of CNCs. Therefore, DA acts as a H-bonding blocker and dispersant in the redispersion process. The Re/0.4DA-CNC powder is redispersed to the original size when redispersed in water without affecting the basic properties of CNCs.
Table 1 Z-average, PDI and Zeta potential of Re/mDA-CNCs
|
Samples |
Z-average (d. nm) |
PDI |
Zeta Potential (mV) |
|
Original CNCs |
111 |
0.213 |
-45.9 |
|
Re/0.1DA-CNCS |
154 |
0.314 |
-50.5 |
|
Re/0.2DA-CNCS |
126 |
0.312 |
-48.7 |
|
Re/0.3DA-CNCS |
119 |
0.285 |
-47.1 |
|
Re/0.4DA-CNCS |
112 |
0.214 |
-46.2 |
|
Re/0.5DA-CNCS |
112 |
0.213 |
-46.1 |
Note: The relative error of Z-average and Zeta potential measurement are ± 1 nm and ± 0.2 mV, respectively.
3.2 Morphology Characteristics of Re/mDA-CNCs
We observed the cross-sectional morphology of Re/mDA-CNC film and the original CNC film. The original CNC film shows a dense layered structure, which indicates that the -OH groups in CNC intramolecular “self-assembly” through H-bonding to form chiral nematic liquid crystal during drying (Fig.2a) (Ates et al. 2020). The self-assembly behavior of CNCs is hindered with the addition of DA, leading to surface disintegration of Re/mDA-CNC film with an increase in DA amount. When the amount of DA is 0.1%, the Re/0.1DA-CNC film still maintains a good layered structure (Fig.2b). The layered structure is gradually broken and the cross section shows convex and concave morphology when 0.2% DA are added (Fig.2c), and the presence of DA is considered to be the main reason for the cracking of layered structure. When the amount of DA is greater than 0.4%, the “self-assembled” layered structure is completely destroyed (Fig.2e), and the CNCs are dispersed randomly throughout the CNC-DA matrix. The reason for the destruction of the “self-assembly” structure is that the electrostatic repulsion between the hydrophobic groups of DA and -OH groups on the CNC surface causes the cationic groups in DA to be attracted to -OH groups on the CNC surface, resulting in the decrease in the number of H-bonding formed between -OH groups in CNC intramolecular due to the reduction in the number of -OH groups, which is consistent with the change in Zetapotential. It can be seen that DA plays a critical role in chemically shielding and dispersing in the redispersion process of Re/mDA-CNCs.
3.3 Dispersion Stability of Re/mDA-CNCs
As shown in Fig. 3a, the samples 1, 2, 3 are the original CNC suspensions, the dried Re/0.4DA-CNC redispersion suspensions and the CNC powder redispersion suspensions, respectively. The dried Re/0.4DA-CNC redispersion suspension had been stored for 7 days and still remained uniform and transparent with no visible aggregations. There is no significant difference from the original CNC suspension (No. 1 in Fig.3a), and the Tyndall effect was still clearly observed after 7 days of storing. In contrast, the redispersed suspension of CNC powder is cloudier and has visible solid particles. The fact that solid particles settle rapidly to the bottom within a short time shows that CNC cannot be redispersed.
The dispersion stability of original CNC suspension and the redispersed Re/0.4DA-CNC suspension is evaluated by Turbiscan technique. Fig. 3b and Fig. 3c are the back-scattering spectra of original CNC suspension and the redispersed Re/0.4DA-CNC suspension measured by the stability analyzer. The lines with different color in Fig.3 represent each measurement time, and the higher the absolute value of the difference in the back-scattering signal (ΔBS), the more pronounced the instability of the system. The presence of a small amount of flocculation in the original CNC suspension is caused by the floating phenomenon, resulting in a very low concentration (0-1.7 mm) of CNCs at the bottom of cuvette. This makes the bottom of cuvette slightly clear, causing the scattered light intensity to become weaker. As the measurement time increasing, the original flocculation of CNC suspension floats on the upper part of cuvette (37.5-40 mm), causing enhanced back-scattering light intensity in the spectrum. In comparison, the back-scattering light intensity of Re/0.4DA-CNC suspension at the bottom of cuvette is stable (Fig. 3c), and the particle size becomes increasingly homogeneous under the effect of shear stress. As time increases, the redispersed Re/0.4DA-CNCs floats to the upper layer, which is responsible for the increased back-scattering light intensity at the top (37.5-40 mm). It can be concluded from the analysis of dispersion stability that Re/0.4DA-CNC suspension shows an excellent stability by adding 0.4% DA, indicating that the flocculation of CNC particles decreases.
Moreover, the hydrophobicity of Re/0.4DA-CNC film is better than that of the origin CNC film. It can be seen from Fig. 3d that the water contact angle of original CNC film is 71.60°, and the water contact angle of Re/0.4DA-CNC film is 72.80° (Fig. 3e). This is due to the interaction between the cationic groups in DA and -OH groups on the CNC surface, reducing the amount of -OH groups, while the hydrophobic groups in DA rearrange on the CNC surface, which causes the increase in CNC hydrophobicity. It is demonstrated that the use of DA is not only beneficial to the preparation of Re/mDA-CNCs but also to the changes in the interfacial compatibility of CNCs and the increase in the water contact angle.
3.4 FT-IR Analysis
The functional groups of original CNC powder and Re/0.4DA-CNC powder were analyzed by using a FT-IR spectrometer, and the main characteristic peaks of cellulose are shown in the spectrum of CNCs in Fig. 4. The absorption peaks locate at 3,365 cm-1 (-OH stretching) (He et al. 2018), 2,891 cm-1 (C-H stretching) (Aguilar-Sanchez et al. 2020), 1,024 cm-1 (-C-O-C asymmetric stretching vibration), 1,324 cm-1 (C-H stretching vibration of cellulose -CH and -CH2) and 1,665 cm-1 (deformation vibration of O-H caused by moisture change in cellulose), respectively (Cheng et al. 2018; Guo et al. 2017; Nigmatullin et al. 2020). The peaks at 1,442 cm-1 and 1,324 cm-1 belong to -CH2 bending vibration and in-plane -CH bending vibration, respectively. Additionally, 1,058 cm-1 are attributed to -C-O stretching vibration, which are consistent with the characteristic peaks in the references (Li et al. 2018b). Compared to the original CNC film, the sharp new absorption peaks of Re/0.4DA-CNC film at 1545 cm-1 and 1738cm-1 are ascribed to the typical stretching vibration of C6H5- and the stretching vibration caused by -C=O (the presence of ester groups or carbonyl groups of -COOH in DA) (Huang et al. 2020; Ly and Mekonnen 2020), respectively. These groups play a key role in regulating the hydrophobicity and interfacial compatibility of CNCs. Moreover, the absorption peak at 746 cm-1 belongs to the bending vibration of -(CH2)n- in long-chain alkyl groups (Li et al. 2018a). The results reveal that the DA is successfully introduced into the CNC surface and does not change the basic chemical structure of the CNCs.
3.5 XRD Analysis
Fig. 5 shows the XRD pattern of original CNC powder and Re/0.4DA-CNC powder. It can be seen that the characteristic peaks of Re/0.4DA-CNC powder have not significant change compared with that of the original CNC powder, which indicates that the Re/0.4DA-CNC powder does not change the crystalline structure of cellulose I. The diffraction peaks of original CNC powder and Re/0.4DA-CNC powder appear at 2θ=15.1°, 16.8°, 22.5° and 34.7°, corresponding to the (1-10), (110), (200) and (004) cellulose crystal planes, respectively (Shang et al. 2019; Velásquez-Cock et al. 2018). The crystallinity of the original CNC powder and Re/0.4DA-CNC powder is calculated to be 78.26% and 78.07%, respectively. The crystallinity of Re/0.4DA-CNC powder is appropriately equal to that of the original CNC powder. It can be inferred that the addition of DA weakens the bonding force between the CNCs and reduces the number of H-bonding formed by -OH groups “self-assembly” in the CNC intramolecular, leading to the reduction in CNC cornification degree. The decrease in water retention value of Re/mDA-CNCs is due to the change in CNC interface compatibility caused by hydrophobic groups in DA.
3.6 TG-DTG Analysis
TG analysis for the original CNC powder and Re/0.4DA-CNC powder is illustrated in Fig.6a. There are some differences in the thermogravimetric curves of two samples. The free water is removed from the samples at the 50-160℃, where a slightly weight loss occurs for all samples. At the vitrification transition stage of 160-250℃, the samples begin to degrade, resulting in a small amount of weight loss. At 260-390℃, the glycosidic bonding of cellulose is severely broken. At 390℃, the weight loss of CNC powder and Re/0.4DA-CNC powder is 71% and 69%, respectively. At this stage, the electrostatic repulsion between hydrophobic groups and -OH groups in Re/0.4DA-CNCs leads to the neutralization of cationic groups and -OH groups in DA-CNC matrix. Thereby, the redispersed Re/0.4DA-CNCs prepared by adding DA can effectively reduce the number of H-bonding formed by the “self-assembly” of -OH within the CNC intramolecular.
Fig. 6b shows the DTG curves of the original CNC powder and Re/0.4DA-CNC powder. The Re/0.4DA-CNC powder has a degradation peak at 270°C, indicating that this stage is the degradation of glycosidic bonding in Re/0.4DA-CNCs. In this case, the thermal decomposition temperature difference between Re/0.4DA-CNC powder and the original CNC powder is within 50°C. The results show that the higher the intermolecular H-bonding content, the higher the thermal decomposition temperature and the better the thermal stability. The decrease in thermal stability of Re/0.4DA-CNCs is due to the decrease in the number of H-bonding formed by -OH groups within CNC intramolecular.
Re/0.4DA-CNCs with excellent redispersibility and no significant change on the basic structure of CNCs were successfully prepared by a simple and easy-to-use physical method. The results show that the Z-average of Re/0.4DA-CNCs when redispersed is consistent with that of original CNCs, and the size distribution is uniform. In addition, the Zeta potential of Re/0.4DA-CNCs is lower than that of the original CNCs. The reason is that the cationic groups in DA and -OH groups on the CNC surface attracted each other, which blocks the formation of H-bonding formed by the -OH groups in CNC intramolecular. The hydrophobic groups in DA also rearrange on the CNC surface, which improves the hydrophilicity and regulate the interface compatibility of CNCs. The stability test results show that the addition of DA improves the stability of Re/0.4DA-CNC redispersion. Furthermore, FT-IR analyses show that DA is successfully introduced onto the CNC surface without causing changing the basic chemical structure of CNCs. XRD results indicate that the crystallinity of Re/0.4DA-CNC powder (78.07%) is appropriately equal to that of the original CNC powder (78.26%). It can be concluded that the addition of DA weakens the binding force of H-bonding between CNCs, leading to the decrease in CNC cornification, and the decrease in water retention value of Re/mDA-CNCs is due to the change in CNC interface compatibility caused by hydrophobic groups in DA. Finally, TG/DTG analysis demonstrate that the decrease in thermal stability of Re/0.4DA-CNCs is due to the decrease in the number of H-bonding formed by -OH groups within CNC intramolecular, which prevents the formation of excessive H-bonding. In conclusion, the Re/0.4DA-CNCs exhibit a good redispersibility and demonstrate potential applications in the field of food packaging, biological medicine, etc.
Acknowledgments
The Project was supported by the National Natural Science Foundation of China (22178186), the Foundation (No. 202105) of Tianjin Key Laboratory of Pulp & Paper (Tianjin University of Science & Technology, P. R. China) and the Shaanxi University of Science and Technology Academic Leader Training Program (2013XSD25).
Funding
This study has no financial interest.
Conflicts of interest/Competing interests
The authors declare no competing financial interest.
Ethical approval
This article does not contain any studies with human participants or animals performed by any of the authors.
Author’s Contributions
All authors contributed as the main contributors of this work. All authors performed the literature search and analysis: all authors had drafted, revised the work and approved the final paper.
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No competing interests reported.