- Research article
- Open Access
Protein adsorption through Chitosan–Alginate membranes for potential applications
- Dennise A. Murguía-Flores†1,
- Jaime Bonilla-Ríos†1,
- Martha R. Canales-Fiscal†1 and
- Antonio Sánchez-Fernández1Email author
© Murguía-Flores et al. 2016
Received: 16 October 2015
Accepted: 31 March 2016
Published: 30 April 2016
Chitosan and Alginate were used as biopolymers to prepare membranes for protein adsorption. The network requires a cross-linker able to form bridges between polymeric chains. Viscopearl-mini® (VM) was used as a support to synthesize them. Six different types of membranes were prepared using the main compounds of the matrix: VM, Chitosan of low and medium molecular weight, and Alginate.
Experiments were carried out to analyze the interactions within the matrix and improvements were found against porous cellulose beads. SEM characterization showed dispersion in the compounds. According to TGA, thermal behaviour remains similar for all compounds. Mechanical tests demonstrate the modulus of the composites increases for all samples, with major impact on materials containing VM. The adsorption capacity results showed that with the removal of globular protein, as the adsorbed amount increased, the adsorption percentage of Myoglobin from Horse Heart (MHH) decreased. Molecular electrostatic potential studies of Chitosan–Alginate have been performed by density functional theory (DFT) and ONIOM calculations (Our own N-layered integrated molecular orbital and molecular mechanics) which model large molecules by defining two or three layers within the structure that are treated at different levels of accuracy, at B3LYP/6-31G(d) and PM6/6-31G(d) level of theory, using PCM (polarizable continuum model) solvation model.
Polymeric materials constitute a fast-growing area within the global economy, confirmed by the continuous and dynamic production of plastics . Because of the limited source of mineral raw materials and environmental protection, new sources of raw materials can be retaken to produce polymers . The Chitosan, Alginate, and Cellulose biopolymers may have the potential to be used as low-cost raw materials since they represent widely available and environmentally friendly resources  that seem attractive for the use, not only in medicine and tissue engineering (TE) , among others. Biodegradable polymers produced from renewable resources represent plastics that may contribute to the enhancement of natural environment protection [4–7]. Porous matrices from biomaterials  are used in the generation of porous matrices which include collagen , gelatin  silk , alginate , and Chitosan . Alginate is a natural linear polysaccharide copolymer produced by brown algae, and bacteria. It is widely used because of its ability to form strong thermo-resistant gels, non-toxicity, biodegradability, high biocompatibility , and widely used in medical applications  such as tissue TE . Cellulose is mostly used in the paper, textile and medical industry . Chitosan has excellent chemical properties such as, adsorption ; due to the reactive number of the available hydroxyl groups, reactive amino groups, and a flexible polymer chain structure [17, 18]. However, used as an adsorbent brings some drawbacks such as low surface area or porosity, high cost, and poor chemical and mechanical properties [19, 20]. Physical or chemical modifications have been studied, such as: copolymerization, grafting, or cross-linking processes [2, 21–24].
The conjunction of different biopolymers is an extremely attractive, inexpensive and advantageous method to obtain new structural adsorbent materials .
Materials such as fly ash, silica gel, zeolites, lignin, seaweed, wool wastes, agricultural wastes, clay materials, and sugar cane bagasse, among others, have been extensively used for protein removal, due to their sorption sites .
Cellulose-based composite hydrogels blended with various biopolymers can create novel materials for special applications [26–32]. The widespread applications of porous materials is not limited as adsorbents for small active molecules. Various polysaccharide hydrogels have been employed for the entrapment of enzymes [33–40]. Furthermore, specific pore structures and tunable morphology allow the construction of affinity probes for various macromolecules . The usage of porous adsorbents for selective and fast separation of phosphorylated proteins and peptides (β-caseine) ; real samples of human serum , and human urine have been captured with Fe3O4 magnetic micro-spheres coated with TiO2-incorporated mesoporous silica [42, 43] have been recently developed.
On the other hand, microspheres favourably affect mechanical properties of polymers such as modulus of elasticity, tensile strength, hardness, and abrasion resistance . These materials could be reused several times; therefore, they become important in terms of their valuable and unique functional properties. Compounds obtained from mechanical recycling of materials can be completely profitable due to lower costs of biodegradable materials and the possibility to avoid a considerable amount of industrial waste .
In the study of adsorbents the determination of adsorption capacity is fundamental. In this case, DFT (density functional theory) calculations represent the most suitable method for investigation involving systems with large molecules such as porphyrins [44–47]. Becke combined with the Lee–Yang–Parr correlation density functional method (B3LYP) is utilized due to highest theoretical and experimental correlation data [48, 49]. Researchers have employed the gradient-corrected DFT (6-31G basis set) on heavy atoms [49, 50].
To our knowledge, the studies focused on Myoglobin from horse heart (MHH) adsorption performance CA-cellulose viscopearls membranes at different temperatures, and evaluating equilibrium, thermodynamic, and kinetic parameters based on temperature of the system, are very limited.
The objective of this study is to determine and compare the adsorption performances of the CA-cellulose viscopearl membranes in the adsorption removal process of MHH from aqueous solutions at different temperatures in view of equilibrium, kinetic, and thermodynamic studies, using both Langmuir equilibrium constant (K L ) and solute distribution coefficient (K d ) . This, in turn, should stimulate research in the field of investigation of such reinforced biomaterials.
The above-mentioned issues inspired authors to undertake research works aimed at comparison of changes in: (a) adsorption process [mean free adsorption Energy (E fe )], kinetic diffusion properties [the intraparticle diffusion coefficient (D p ) and film diffusion coefficient (D f )], and thermodynamic parameters; (b) tensile strength, (c) tensile strain at break, (d) flexural strength, (g) thermal properties [thermogravimetric analysis (TGA)], (h) structural properties of samples [Fourier transform infrared spectroscopy (FT-IR)], and (i) surface free energy (solid-state carbon-13 nuclear magnetic resonance (solid state 13C-NMR) spectroscopy ), and (j) mechanism of interaction, deformation of compounds, and adsorption energies [ONIOM and molecular dynamics (MD)]. The results are offered in the present paper.
Results and discussion
Figures 1 and 2 also show that an increase in initial MHH concentration decreases the adsorbed ratio. This can be attributed to the increase in the number of MHH molecules competing for available binding sites on the CA-cellulose viscopearls membranes. Thus, the available active sites of the CA-cellulose viscopearl membranes become saturated at higher concentration of MHH [53, 54].
It was important to measure the protein adsorption capacity of the material as well as its capacity to retain the adsorbed compound into polymer matrix so that it could be reusable. In order to determine MMH protein desorption of the membrane, a new compound was prepared. From the CA-V-1A compound, which is the one with the highest protein adsorption capacity, the same formulation was used to synthesize compound P-1000 in which a solution of 1000 ppm is added to MHH during preparation. This occurs after incorporating the Alginate solution and allowing the sample to dry (see “Preparation of Chitosan Alginate (CA)-cellulose viscopearl membranes” section).
After the synthesis of compound P-1000, the sample N-P was encoded and subjected to seven rinses with distilled water at room temperature. These experiments for washing the sample were carried out with 10 mL of MHH; the solution passed through a Hirsch funnel containing the samples by applying vacuum pressure. P-1000 samples of 0.5 g were tested with 1000 mg/L of MHH solutions whose concentration corresponds to 1000 ppm.
Adsorption equilibrium and calculation of mean free sorption energy
In this investigation, the most frequently used equations, Langmuir and Freundlich isotherm models, were used to analyze the isotherm data for the purpose of optimizing the design of an adsorption system. It is also an important step to establish the suitable correlation for equilibrium conditions.
The corresponding mean free adsorption Energy (E fe ) was calculated to interpret the mechanism of MHH removal; meanwhile, the intraparticle diffusion coefficient (D p ) and film diffusion coefficient (D f ) were calculated separately to describe the kinetic diffusion process of MHH adsorption. Also, thermodynamic parameters like ΔG 0 , ΔH 0 , and ΔS 0 were respectively calculated using both Langmuir equilibrium constant (K L ) and solute distribution coefficient (K d ), in order to compare the different thermodynamic calculation methods .
This investigation presents a combined study of ONIOM and molecular dynamics (MD) aimed to understand the mechanisms of interaction and deformation of analyzed compounds. Likewise, adsorption analysis is performed considering the most stable structure of the system at geometrical parameters changes and adsorption energies.
Equilibrium data, known as adsorption isotherms, are basic parameters for the design of adsorption systems. In order to calculate the adsorption capacity of Chitosan–Alginate membranes, the experimental data were fitted to the Linearized Langmuir isotherm and Linearized Freundlich isotherm, Eqs. (2) and (3), respectively [61, 62]:
Freundlich and Langmuir isotherm parameter for adsorption capacity (303 K)
Cellulose viscopearls (gr)
LMM 0.42 wt%
First, the sorption takes place at specific homogeneous sites within the adsorbent. Second, no further sorption can take place at that site once a MHH molecule occupies it. Third, the adsorption capacity of the adsorbent is finite. Fourth, the size and shape of all sites are identical and energetically equivalent . The Freundlich model is suitable for a highly heterogeneous surface composed of different classes of adsorption sites. This model has two main assumptions : first, with the increase of surface coverage of adsorbent, the binding strength gradually decreases. Second, the adsorption energies of active sites on the surface of adsorbent are different.
Fitting the data with the Langmuir and Freundlich equations resulted in high correlation coefficients, varying from 0.99 to 1.00. This indicates that the Chitosan–Alginate membrane surfaces are homogeneous and coverage of MHH on the outer surface of samples is a monolayer adsorption [63, 64].
Adsorption kinetics and calculation of activation energy
Figures 1 and 2 (see “Adsorption experiments” section) showed the effects of MHH initial concentration at 303 K on the CA-cellulose viscopearl sample. It can be observed that the variation of initial concentration of adsorption solution (500 and 1000 ppm) affected the rate of adsorption at initial period. This is due to the increase of initial concentration of adsorption solution and the MHH adsorption on each CA-cellulose viscopearl samples which gradually slowed down as concentration of adsorption solution increased; for each experiment the equilibrium was reached after 30 min. Besides the difference of concentration gradient, the interaction forces between solute and adsorbent become stronger than those between the solute and the solvent, leading to the fast adsorption at the initial stage . As time passed, the sorption rate decreased, and temperature variation influencing the final adsorption capacity is not significant at the later equilibrium stage.
Diffusion mechanism study
Three major rate limiting steps involving the kinetic diffusion mechanism are generally cited : (a) film diffusion; (b) intraparticle diffusion; (c) interior surface diffusion; (d) adsorption or ion exchange on the pore surface. The intraparticle diffusion model (Weber–Morris model) is applied to analyze the empirically found functional relationship (qt versus t1/2) .
According to the theory behind Weber–Morris model, the plot of q t versus t1/2 should be linear when adsorption complies with the intraparticle diffusion mechanism and the intraparticle diffusion should be the only rate-determining step if the line passes through the origin. Otherwise, if the plots are multilinear, there are two or more rate-limiting steps involving in the adsorption process .
Freundlich and Langmuir isotherm parameter for adsorption capacity intraparticle diffusion model parameters for the adsorption of MHH on CA-cellulose viscopearls at 1000 ppm of initial concentration of adsorption solution
The average diameter of MHH particle was determined . Then, the values of D p and D f were calculated under the given conditions explained below. R p (m) is the average radius of the adsorbent particles, ε is the film thickness (10−5 m)  and C s and C L are the concentration of adsorbate in solid and liquid phase, respectively. Debnath et al.  assumed that the intraparticle diffusion will be the rate-limiting step if the calculated intraparticle diffusion coefficient (D p ) value is in the range 10−15–10−18 m2 s−1. For the calculated film diffusion coefficient (D f ) value ranging from 10−10 to 10−12 m2 s−1 the rate-limiting step is controlled by film diffusion. In this study, the calculated D p values ranged from 1.81 10−12 to 11.2·10−12 m2 s−1, and the calculated values of D f were found to be in the order of 10−11 m2 s−1.
Intraparticle diffusion coefficient (D p ) and the film diffusion coefficient (D f ) of adsorption process at 303 K at 1000 ppm and for CA-V-1B is Rp/m 1.8 × 10−4, the value for t 1/2/s corresponds to 335.98, D p (m2 s−1) is 2.56·10−12, and D f (m2 s−1) calculated as 3.89 × 10−11.
Measurements were carried out in a thermogravimetric-analyzer (TGA) from TA Instruments (STD Q600, New Castle, DE, USA).
The IR spectra were carried out in an infrared spectrophotometer Thermo Nicolet® model 6700 FTIR and using the attenuated total reflectance complement with diamond crystal. In order to analyze the data obtained, Omnic 7.3 software was used. The spectra were acquired in a range between 4000 and 400 cm−1 with a resolution of 4 cm−1 and 40 scans per analysis. A reference without the sample was registered before each analysis.
IR bands characteristic of cellulose are distinguished: a broad hydrogen-bound O–H str band of the around 3400 cm−1, the C=O stretching band around 1650 cm−1 and the mixed C–O str and O–H str bands in the 1150–1350 cm−1 region, which suggest interactions between the cellulose components. These findings could indicate that Viscopearl-mini® is esterified.
Solid-State 13C NMR spectroscopy is intrinsically a powerful and versatile tool for revealing the internal structure, composition, interface, and componential dynamics of polysaccharides. Therefore, to determine some structural differences related with the molecular mass of Chitosan, the samples CA-V-1A and CA-V-1B were analyzed by solid state 13C-NMR spectroscopy with an 11.7 Tesla Bruker Avance III equipment. Each sample was tested using cross-polarization (CP) and magic-angle spinning (MAS) with a rate of 125 MHz. A 4 mm inner diameter rotor with a spinning rate of 7 kHz was used. All 13C spectra were referenced to glycine (176.03 ppm, carbonyl, 13C).
In order to observe the particles dispersion on different prepared materials, SEM images were taken using a SEM-FEI Nova NanoSEM 200 (Hillsboro, TX, USA) microscope with an acceleration voltage of 10 kV and secondary electron detector under vacuum was used to characterize the morphology of the CA-cellulose viscopearls with protein immerse in the blending of CA-cellulose viscopearls formulation for their comparison. The Energy-dispersive X-ray spectroscopy (EDS) elemental analysis was carried out with an INCA-x-sight.
Scanning electron microscopy (SEM) analyses were conducted on cryofractured CA-cellulose viscopearl samples in order to investigate the dispersion of porous cellulose beads and interfacial features in membranes. This analysis is discarded only for the A-V compound because it was not possible to prepare the film.
In order to observe the effect of MHH protein incorporation, P-250 (Fig. 9c), and P-2000 (Fig. 9d) samples were obtained. Those formulations were subjected to the same preparation as P-1000 (see “Thermal analysis” section). The results explain the difference of an increasing and decreasing MHH concentration.
Pore sizes of CA-cellulose viscopearl membranes
k id1 (mg mL−1 min-1/2)
k id2 (mg mL−1 min-1/2)
k id3 (mg mL−1 min-1/2)
Energy-dispersive X-ray spectroscopy (EDS) analysis results
Pore size (µm)
Calcium was detected in the analyzed zones and the composition of the CA-cellulose viscopearl matrix id referred where only carbon is found. Also, one important matter on doing this type of test was to prove the presence of Calcium in the matrix, which impacts in properties. Furthermore, P sample was characterized with the detection of N which confirms presence of protein during the synthesis. N-P sample was taken after washing the sample for seven times with distilled water; however, no detection of N2 was found which suggests that this step washes the protein completely off the matrix. In general, it can be said that all the samples presented an intercalated dispersion of calcium ions and the presence of nitrogen in the samples as supported by the micrographics already described above.
To compare mechanical properties of samples, tests were performed in an INSTRON 3365 tensile test machine (Norwood, MA, USA) at a strain rate of 6 mm/min in accordance to ASTM 882 . Tensile properties were measured on 27 rectangular specimens with a length of 10 mm, a width of 5 mm and a thickness of 1 mm. Values reported represent average from five measurements and typical stress–strain curves were selected for presentation in the graphs.
Mechanical properties of all membrane samples
Total energy for compounds involved
Max stress [MPa]
Max strain [%]
Young modulus [MJ/m3]
0.544 ± 0.015
7.615 ± 0.581
0.072 ± 0.003
2.587 ± 0.146
1.385 ± 0.138
1.874 ± 0.097
1.176 ± 0.165
4.203 ± 0.857
0.282 ± 0.28
0.544 ± 0.017
1.127 ± 0.016
0.470 ± 0.008
0.436 ± 0.034
52.781 ± 3.044
0.008 ± 0.000
Density functional theory (DFT) calculations were carried out for the chitosan, sodium alginate, calcium chloride and acetic acid. For the analysis of reactivity between the substances involved, the possibility of protonation and electrophilic attack was examined by calculating the molecular electrostatic potential at a B3LYP/6-31G(d) level of theory, considering an initial optimization included at the same level. The molecular electron densities and the molecular electrostatic potential surfaces of chitosan, sodium alginate, calcium chloride and acid acetic were determined from the wave functions using CUBE (file with both binary and ASCII formats, which is often used as an input for other graphical visualization) option implemented in Gaussian 09 and visualized using GaussView 5.0  computational software.
An adsorption analysis took place considering the total energy and structural parameters for compounds isolated and in a system of interaction between them, ONIOM calculations were carried out with aid of the Gaussian 09 software package and 6-31G(d) basis set. Additionally, excitation energies from the lowest double energy state were calculated using PM6/6-31G(d) level of theory.
The molecular electrostatic potential has been performed by DFT and ONIOM calculations at B3LYP/6-31G(d) and PM6/6-31G(d) level of theory using PCM solvation model. The adsorption energies and geometrical parameters of acetic acid, sodium alginate solutions, and cellulose have been studied for ground and excited-state geometry to deduce the influence of various substituents as well as the solvent effect on the deformation of molecules.
An analysis of adsorption energy and structural parameters between an Alginate/Chitosan system and the surface of the cellulose viscopearls was conducted, for which this structure was used by a total of three chains with 12 molecules and the complex Alginate/Chitosan obtained through the analysis of reactivity. A chemical interaction between both compounds does not exist mainly because of treatment with alginate also did not alter viscopearls dimensions .
Bond length of atoms linked in the chemisorption process for configuration 1 and 2
Total energy (Hartrees)
(b) Sodium alginate
(c) Calcium chloride
(d) Acetic acid
Bond length of atoms linked in the chemisorption process for two configurations in isolated systems
Bond length [Å]
Bond Ca 1
Bond Ca 2
Bond Ca 3
Total and adsorption energies for both configuration in chemisorption effect and structure in physisorption effect computed at a PM6/6-31G(d) level of theory
Bond length [Å]
Bond Ca 1
Bond Ca 2
Bond Ca 3
The interaction achieved in the different mixture of substances, shown in Fig. 12 (see “Reactivity” section), results in a relatively stable structure with energy of 1.5118 Hartrees. Chitosan and Alginate tend to form a circular configuration around calcium ions, which come from a dissociation of calcium chloride. The Sodium ion is replaced by a calcium one. This new compound interacts with a cellulose surface resulting in chemisorption and physisorption effects, with a minimum distance of 4.8665 Å between each other in physisorption case (Fig. 14b) (see “Adsorption” section). Comparing the two configurations found in the chemisorption effect, Configuration 2 is more stable due to strong bonds from the calcium ion; the adsorption energy obtained was −0.7791 Hartrees, compared with −0.961 Hartrees from Configuration 1. This last structure had an invasive presence due to a range change for the length of the cellulose bonds between 3 × 10−1 and 3 × 10−6 Å, finding the nearest one at 3 × 10 −1 Å, while on the other side, a length bond change of 1 × 10−4 Å exists in Configuration 2. In accordance to these reasons, Configuration 2 was considered the most probable structure; nevertheless, it depends strongly on the initial position in which the complex Alginate/Chitosan arrives to cellulose surface.
Therefore, computational data could suggest that the mix (blend) of CA-cellulose viscopearls agree with the experimental data of protein adsorption. Since adsorption experiments also prove a favorable mechanism for physisorption.
Cellulose beads (Viscopearl-A) were obtained from Rengo, Japan. Chitosan of low molecular weight (LMW) (viscosity: 20–300 cP), Chitosan medium molecular weight (MMW) (viscosity: 200–800 cP), calcium chloride (reagent plus ≥ 93 %), Acetic acid (pure reagent ≥ 99 %), Myoglobin Protein lyophilized powder from equine heart ≥90 % essentially salt-free, Alginic acid sodium salt from brown algae (medium viscosity). All chemicals used in this study were analytical grade, provided by Sigma Aldrich and used without further purification.
Porous cellulose beads (Viscopearl-mini®)
A certain type of porous cellulose beads were used for this research. Viscopearl-mini® (VP) or porous cellulose beads obtained from Rengo, Japan with high chemical stability, porosity: <0.01 mm, and range size in diameter: 0.4–0.7 mm .
Preparation of Chitosan Alginate (CA)-cellulose viscopearl
The preparation process for CA-cellulose viscopearl membranes was carried out by mixing the matrix components according to the formulations shown in Table 1. All solutions were first prepared at room temperature ~30 °C. Alginate solution was prepared following Masalova et al.  procedure and two types of Chitosan solution were formulated according to Guo et al. , one of them was made from Chitosan of low molecular weight and the other one from medium molecular weight Chitosan.
For each compound, the total blending volume was as much as 6 mL, in which 0.33 or 0.50 gr of Viscopearls-A were added according to each formulation. Then, Alginate solution (previously prepared) was poured in with porous cellulose beads into a petri dish and left overnight. After that, the Chitosan solution was added into the mixture and left for 24 h to dry and to form a thin film which was then stored in a dry environment.
Nomenclature for sample synthesized for each formulation
Total energy (Hartrees)
Adsorption energy (Hartrees)
Chem. configuration 1
Chem. configuration 2
For all six samples, the solution was stirred manually at 30 °C until a homogenous mixture was attained. The amount of Sodium Alginate solution within the polymeric matrix was kept constant at 3.15 mL in the samples preparation. After the reaction was completed, the different samples were left resting for 1 week to get the diluent to evaporate as much as possible. Afterwards, the prepared materials were press-compressed at 100 °C and 15 MPa for 5 min, followed by cooling at room temperature. Finally, samples were shaped into a desired size for further measurements. Codes names for each formulation sample are listed in Table 10.
Batch adsorption studies were conducted to investigate the adsorption behaviour of the CA-cellulose viscopearl membranes. Adsorption experiments were carried out in a 20 mL screw cap tube container with Myoglobin from Horse Heart (MHH) solution containing different CA-cellulose viscopearl samples to study the effects of various contact times (see Table 10).
The different samples were tested using 0.25 g of CA-V-1B, A-V, CA-V-1A, CA-V-2B, C-V-1B and C-A with 1000 mg/L of MHH. To evaluate the effect of initial MHH solution concentration of 500 and 1000 mg/L, different compound samples (CA-V-1B, A-V, CA-V-1A, CA-V-2B, C-V-1B, C-A) were used. All mixtures were agitated manually at 30 °C where contact time varied on a range of 0–30 min. The mixture was then centrifuged and the absorbance of the supernatant was recorded using Shimadzu UV-2500 spectrophotometer (Shimadzu Corp., Kyoto, Japan) using quartz cuvettes with 10 mm path lengths.
Chitosan–Alginate membranes containing porous cellulose beads with a homogenous internal structure, as showed by SEM, were successfully prepared from biopolymer blending between the Chitosan–Alginate.
Different morphologies were obtained depending on the formulation system used to incorporate the cellulose viscopearls in order to build the biopolymer membranes. FTIR spectra analysis turned out to be a reliable characterization technique to verify if the principal components stayed in the matrix. NMR in a solid state characterization also helped to determine, from a molecular perspective, the existence of all compounds in the polymer matrix.
To improve the adsorption capacity and mechanical structure of said biopolymer blendings between the Chitosan–Alginate (matrix), a physical interaction between the components is desirable.
Using computational chemistry optimization of the present molecules, the total energy for each system was computed. The interactions achieved in the blending carried out a final matrix compound owning the most stable energy structure; physisorption being the most suitable mechanism of protein interaction.
Tensile tests showed the increase of the amount of cellulose viscopearls was not proportional to the tensile strength. The lesser the cellulose viscopearls were added, the better was the performance found in membranes. This is confirmed their support role on preserving membranes shape, a behavior not observed in the blank sample (Chitosan–Alginate). Finally, the Chitosan–Alginate membrane could not be used to adsorb the protein by itself as the film is brittle and mechanically unstable. Also the prepared blending with cellulose viscopearls could be handled with a sufficient mechanical strength to endure the addressed manipulations and applicability.
DAMF, MRCF, ASF, JBR contributed in the same way for the successful publication of this article. All authors read and approved the final manuscript.
This research was supported by Antonio Sánchez-Fernández and Jaime Bonilla-Rios. Thanks for sharing your knowledge during the course of this research and providing insight and expertise that greatly assisted this job. Authors want to thank CIQA and the staff working there for their help in characterization of samples. Last but not least we thank the reviewers for their constructive comments and valuable time for this work.
The authors declare that they have no competing interests.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
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