Surface modification of iron oxide nanoparticles with κ-carrageenan/carboxymethyl chitosan for effective adsorption of bovine serum albumin

2.1 Chemicals and reagents

Bovine serum albumin (BSA; MW 67 kDa) was purchased from Sigma Chemical Company. κ-Carrageenan was obtained from Condinson Company (Denmark). N, O-Carboxy methyl chitosan was obtained from Sigma Chemical Company (Germany). The molecular weight and purity were 300,000 Da and 98%, respectively. All other chemicals were analytical grade and used without any purification.

2.2 Preparation of magnetic carboxymethyl chitosan/carrageenan nanoparticles

The magnetic nanocomposites were prepared via in-situ co-precipitation of iron ions in the presence of binary mixture of N,O-carboxymethyl chitosan and κ-carrageenan. The required amounts of initial materials for synthesis of nanocomposites are shown in Table 1. In brief, 1 g of κ-carrageenan was poured in 25 mL of distilled water and the temperature was adjusted at 70 °C. Carboxymethyl chitosan solution was prepared separately by dissolving 1 g of this biopolymer in 25 mL of distilled water. Two solutions were mixed together and the temperature was adjusted at 70 °C. Then, the desired amount of FeSO₄·7H₂O and FeCl₃·6H₂O salts was dissolved in 5 mL of water (the molar ratio of nFe⁺²/nFe⁺³ = 0.625/1) (Zhang et al., 2012). For fabrication of magnetic nanoparticles in the presence of biopolymers, the 3 M NH₃ was slowly dropped into a solution and the dark magnetic nanoparticles were prepared. The pH of solution was adjusted at 10 and allowed to stir at 70 °C for 1 h. The magnetic nanocomposites were washed with distilled water and finally, immersed into crosslinking solution (0.2 M KCl and 0.2 M CaCl₂) and stirred slowly for 60 min. The cross-linked magnetic nanocomposite beads were washed with distilled water and dried at ambient temperature until reaching a constant weight. The non-magnetic beads (CMChitoCar) were synthesized under the same condition as magnetic nanocomposites without the addition of iron ions or treating with ammonia solution. For the adsorption process, the mCMChitoCar nanocomposites were activated by glutaraldehyde. For this purpose, we followed the procedure reported by Z. Wang et al. (2013). Twenty milligrams of magnetic beads was dispersed in 5 mL phosphate buffer solution (PBS, pH 7.4) with 5% glutaraldehyde solution (V/V). The mixtures were then sonicated for 15 min and then left on a shaker with 150 rpm for 60 min. The resultant nanocomposites were washed with PBS to remove any unreacted chemicals and followed for the adsorption of BSA.

2.3 Swelling measurement

The kinetic of water absorbency of prepared nanoparticles was determined in distilled water. Dry nanocomposites (∼0.2 g) were placed in distilled water and kept at a constant temperature, 20 ± 0.5 °C. Swollen nanocomposites were periodically removed and weighted and the water absorbency was measured according to Eq. (1):

2.4 Structural characterization

Dried nanocomposites were coated with a thin layer of gold and imaged in a scanning electron microscopy spectroscopy (SEM; Vega, Tescan). The magnetic properties of the nanocomposites were studied with a vibrating sample magnetometer (VSM, Model 7400, Lakeshore Company, USA). It may be noted that the magnetic nanocomposites in powdered form were used to investigate the magnetic properties of nanocomposites. The un-cross-linked and wet nanocomposites were dispersed in ethanol and transmittance electron microscopy (TEM) micrographs were recorded with a Zeiss EM-900 operating at 80 kV tension. One-dimensional wide angle X-ray diffraction (XRD) patterns were obtained by using a SiemensD-500 X-ray diffractometer with wavelength, ʎ = 1.54 Å (Cu Kα), at a tube voltage of 30 kV, and tube current of 30 mA. The zeta potentials of CMChitoCar and mCMChitoCar were measured at different pH using a Brookhaven Zeta Plus 90 analyzer. Samples were prepared by diluting 20 mg of both particles in 10⁻³ M NaNO₃ solution at different pH’s adjusted with diluted HNO₃ and NaOH. Ultraviolet–visible (UV–vis) absorption spectra were measured on an UV–VIS-NIR spectrophotometer (Shimadzu UV-3600, Japan).

2.5 Batch adsorption studies

All adsorption experiments were examined through a batch method on a shaker with a constant speed at 120 rpm by immersing the 50 mg of adsorbents in 10 mL BSA solution. In the first attempt, the effect of contact time on the adsorption capacities of CMChitoCar and mCMChitoCar nanocomposites was investigated at a pH of 4.7 (isoelectric point of BSA) and an ionic strength of 0.01 M with concentration of 200 mg L⁻¹. The amount of BSA adsorbed was calculated by subtracting the amount found in the supernatant solution after adsorption from the amount of protein present before addition of the adsorbent mCMChitoCar nanocomposites by UV absorption spectrometer at λ_max = 280 nm using Eq. (2):

3.1 Synthesis and characterization

An easy green synthetic method was performed for synthesizing nano-sized MNPs in biopolymer matrix via in-situ co-precipitation of iron ions under alkaline condition (Scheme 1). In this protocol, MNPs were directly synthesized in aqueous media as a colloidal stable system. Firstly, the biopolymers solution was dropped into solutions comprising Fe²⁺/Fe⁺³ ions for formation of beads through ionic crosslinking. The (Fe²⁺/Fe³⁺)-crosslinked beads were kept in ammonia solution to form magnetic Fe₃O₄ nanoparticles in biopolymer matrix. The synthesis procedure consists of the precipitation of ferric and ferrous ions by adding NH₃ to a solution: $${\text{Fe}}^{2+}+2{\text{Fe}}^{3+}+8{\text{OH}}^{-}\to {\text{Fe}}_3{\text{O}}_4+4{\text{H}}_2\text{O}$$

Close

Scheme 1

Schematic illustration of the synthesis of magnetic κ-carrageenan/carboxymethyl chitosan nanocomposites.

Export to PPT

Finally, the generated magnetic nanocomposites were immersed in a saline solution containing KCl and CaCl₂ salts. The crosslinking of biopolymers by K⁺/Ca²⁺ can be achieved via electrostatic interaction of K⁺ cations with sulfate groups (–OSO₃⁻) on k-carrageenan and also Ca²⁺ cations with carboxylate group (–CO₂⁻) on carboxymethyl chitosan (Z. Wang at al., 2013; Morris et al., 1980). None of the materials used in this protocol were toxic. This procedure also shows the superiority over other synthetic strategies including green reaction media, easy purification of the products and high yield (Shundo et al., 2012). The magnetic nanocomposites were characterized by SEM, XRD, TEM, and VSM techniques.

3.2 SEM studies

In order to investigate the effect of magnetic nanoparticles on the morphology of nanocomposites, SEM images were applied. It was found that the quantity of MNPs plays an important role in changing the morphology of the products. The non-magnetic (CMChitoCar) beads showed a smooth and tight surface (Fig. 1a). Interestingly, incorporation of magnetic nanoparticles significantly affected the morphology of nanocomposites. As could be seen from Fig. 1b and c, a rough structure was obtained for magnetic nanocomposites. Obviously, the more the amount of MNPs is, the stronger the rough surface of biopolymers are; therefore, magnetic nanocomposites exhibited more compact network structure which provide higher possibility of interactions with protein molecules and accelerate the adsorption process. However no clear reason was proposed for this phenomenon (Sapir et al., 2012; Z.G. Wang et al., 2013). The rough surface of beads may be assigned to the magnetic nanoparticles which are shaped needle-like (TEM micrographs) and the more the nanoparticles the rougher the surface is obtained.

Close

Figure 1

SEM images of (a) non-magnetic CMChitoCar, (b) mCMChitoCar1, (c) mCMChitoCar2 and (d) XRD patterns of neat magnetic Fe₃O₄, mCMChitoCar1, and mCMChitoCar3 nanocomposites.

Export to PPT

3.3 XRD, VSM and TEM studies

The crystalline structures and phase purities of the synthesized Fe₃O₄ and magnetic hydrogel were studied by XRD patterns. The XRD patterns of neat magnetic Fe₃O₄ nanoparticles, mCMChitoCar1, and mCMChitoCar3 nanocomposites are shown in Fig. 1d. In the spectrum of neat magnetic nanoparticles, the characteristic peaks of Fe₃O₄ were appeared at 2θ about 30.4°, 35.4°, 43.5°, 53.3°, 57.4°, and 63.2° indicating corresponding indices (220), (311), (400), (422), (511), and (440), respectively (Jiang et al., 2004). The results are in consistent with the database in JCPDS file (PDF No. 65-3107) indicating the formation of highly crystalline and pure magnetite nanoparticles with spinel structure (Zhang et al., 2012). The XRD patterns of mCMChitoCar1 and mCMChitoCar3 displayed the same characteristic diffraction peaks as MNPs without any changes in its structure and crystallinity. In fact, the formations of MNPs in the presence of biopolymers occurred without any phase change of magnetite (Cai et al., 2011). Meanwhile, the hysteresis loops of mCMChitoCar nanocomposites were measured by VSM technique between ±9 kOe at 298 K (Fig. 2). The magnetization curves illustrated that both mCMChitoCar1 and mCMChitoCar2 nanocomposites were superparamagnetic with no coercive at room temperature. The saturation magnetization values for mCMChitoCar1 and mCMChitoCar2 nanocomposites were 3.7 and 6.9 emu g⁻¹, respectively, which indicates that the magnetization of the mCMChitoCar nanocomposites can be tuned by altering the weight ratio of MNPs (Kolhatkar et al., 2013). The TEM micrographs of mCMChitoCar1 (Fig. 3a and b) and mCMChitoCar3 (Fig. 3c) were also studied and according to the results, the magnetic Fe₃O₄ exhibited nano-scale particles with size of less than 100 nm. The shape of nanoparticles was obtained rod and relatively spherical in stabilized or aggregated forms when the ratio of κ-carrageenan and carboxymethyl chitosan was identical (mCMChitoCar1, Fig. 3a and b). The difference in the shape of nanoparticles may be attributed to the variation in the pore size of nanocomposites. When the ratio of κ-carrageenan was higher than that of the carboxymethyl chitosan (mCMChitoCar3), the magnetic nanoparticles were in agglomerated form (Fig. 3c). The results indicated that the size and shape of Fe₃O₄ nanoparticles can be affected by the ratio of biopolymers. Size distributions analysis from TEM images of mCMChitocar1 and mCMChitoCar3 with high number of nanoparticles (min. 120 particles) is shown in Fig. 3d and Fig. 3e, respectively. The average diameters of nanoparticles were obtained ∼9 and 70 nm for mCMChitocar1 and mCMChitoCar3, respectively.

Close

Figure 2

The hysteresis loops of mCMChitoCar1 and mCMChitoCa2 nanocomposites versus applied magnetic field.

Export to PPT

Close

Figure 3

TEM images of (a and b) mCMChitoCar1; and (c) TEM image of mCMChitoCar3; size distribution of nanoparticles for (d) mCMChitoCra1 and (e) mCMChitoCar3.

Export to PPT

3.4 Water uptake

The water absorbing capacity of biopolymers is strongly influenced by their chemical composition. It depends on the ratio and nature of ionic and non-ionic functional groups (Omidian et al., 2010). Accordingly, a preliminary study was conducted on the swelling kinetic of nanocomposites. Fig. 4a represents the dynamic swelling behavior of magnetic nanocomposites in distillated water. Initially, the rate of water uptake was sharply increased and then began to level off. Comparing the dynamic swelling of nanocomposites, it is clear that water uptake and rate of swelling of biopolymers were affected by the composition of nanocomposites. Maximum and minimum swelling capacity was obtained for magnetic-free (CMChitoCar) and mCMChitoCar2 beads, respectively. Besides, the swelling capacity of all magnetic nanocomposites was obtained lower than that of the magnetic-free beads. This decrement in water absorbency can be attributed to the presence of Fe₃O₄ nanoparticles. The decrease in the swelling of magnetic nanocomposites arisen from (a) decrease in the ratio of hydrophilic and anionic functional groups and (b) the interaction between magnetic nanoparticles and polymeric chains resulted in more crosslink points and thereby decrease in swelling capacity (Philippova et al., 2011). In the case of magnetic mCMChitoCar3 nanocomposites, the swelling capacity was obtained higher than that of other magnetic nanocomposites. The weight ratio of two biopolymers in this sample was not equal and the content of k-carrageenan component was high. The reason for the high swelling may be due to the high content of k-carrageenan in the nanocomposite which contains sulfate groups. A similar observation has been reported by Muhamad et al. (2011), in which the swelling capacity of biopolymers consisting of carboxymethyl cellulose and k-carrageenan has been enhanced by increasing the content of k-carrageenan. According to the pK_a of sulfate and carboxylate pendants (pK_a of methane sulfonic acid is −2.5, while that of acetic acid is 4.8), the sulfate groups on the k-carrageenan dissociate in aqueous media more readily than the carboxylate groups (Hosseinzadeh et al., 2005). Accordingly, k-carrageenan behaves as a strong polyelectrolyte and the high swelling capacity of nanocomposites with the high content of k-carrageenan is originated from strong electrostatic repulsion between sulfate groups on k-carrageenan backbone (Mistsumata et al., 2003). Yin et al. have been indicated that the swelling kinetics of cross-linked hydrophilic polymers such as biopolymers is compatible with the Schott’s second-order diffusion models (Schott, 1992). This Phenomenon reveals that the swelling of biopolymers is controlled by stress relaxation of network. Thus, the experimental swelling data were analyzed by non-linear second-order swelling kinetic according to Schott’s models (Yin et al., 2002). The second-order swelling kinetic model is described as in Eq. (3), respectively:

Close

Figure 4

(a) Swelling kinetic of nanocomposites in distilled water; (b) water absorbing of nanocomposites versus time according to experimental and second-order swelling kinetic model.

Export to PPT

3.5 Protein adsorption study

3.5.1

3.5.1 Kinetic of adsorption

The adsorption kinetic was investigated to measure the required equilibrium time for adsorption of BSA onto nanocomposites. The adsorption kinetic of 0.02 mg ml⁻¹ BSA at pH = 4.7 was examined as a function of magnetic nanoparticles and weight ratio of biopolymers. Results showed that the adsorption reaches to equilibrium at first 30 min (Fig. 5a). At first step the adsorption process is very fast because the protein molecules occupy the accessible site of the adsorbent rapidly. As the adsorption process progresses, the rate of adsorption decreases and reaching the saturation adsorption in the final step. Furthermore, the magnetic nanocomposites displayed much higher adsorption value than that of non-magnetic one (CMChitoCar) for BSA molecule. The BSA adsorption capacity of CMChitoCar was 41.59 mg g⁻¹. As the amount of magnetic nanoparticles enhanced in the composition of beads, adsorption capacity was found to be 50.15 mg g⁻¹ and 54.71 mg g⁻¹ for mCMChitoCar1 and mCMChitoCar2, respectively. In the present case, this observation may be attributed to this fact that magnetic nanocomposites possess bigger surface area providing more active sites (Badruddoza et al., 2011). In addition, based on SEM images, by incorporating the MNPs in polymer matrix, the surface becomes rougher resulting in more collision between BSA molecules and the active site of adsorbent. To further support this assumption, the UV−vis absorption of pure BSA, BSA-CMChitoCar and BSA-mCMChitoCar3 nanocomposites was studied in the range of 170–400 nm. The spectrum of pure BSA revealed the special absorption peak at 280  nm. The equal amount of CMChitoCar and mCMChitoCar3 was mixed with the same concentration of BSA solutions (0.02 mg ml⁻¹ BSA in acetate buffer) and supernatant solution was taken for analysis in the abovementioned range. As shown in Fig. 5b, the absorption intensity of BSA at about 280 nm was slightly shifted to a lower wavelength of 210 nm after treating with the CMChitoCar. As form CMChitoCar3, the absorption intensity is greatly decreased to 180 nm (Yang et al., 2010; Shen et al., 2014), suggesting that mCMChitoCar3 has a significantly higher adsorption capacity for BSA than that of CMChitoCar beads. Also, by increasing the ratio of κ-carrageenan in polymer matrix (mCMChitoCar3), an enhancement in protein adsorption capacity was obtained with the adsorption capacity of 63.21 mg g⁻¹. Swelling behavior is the main factor in this case (Fig. 4a). To be specific more, anion–anion electrostatic repulsion of anionic sulfate groups on κ-carrageenan (Shen et al., 2014) may be occurred resulting in higher adsorption efficacy.

Close

Figure 5

(a) Adsorption kinetics of BSA onto magnetic nanocomposites (C₀: 200 mg L⁻¹, pH 4.6 and T 25 °C); (b) The UV–vis spectrum of 0.2 mg ml⁻¹ BSA in acetate buffer solution at pH = 4.8 and UV–vis spectrum of supernatant solution of BSA contacted with CMChitoCar and mCMChitoCar3 nanocomposite.

Export to PPT

Pseudo-first-order and pseudo-second-order kinetic models were examined to obtain rate constant and equilibrium adsorption capacity for beads (Ho and McKay, 1998; Ai et al., 2010). Kinetic data were analyzed using the pseudo-first-order equation (Eq. (4)) and pseudo-second-order equation (Eq. (5)) as follows:

(4)

$$q_t=q_e(1-e^{k_{1}t})$$

(5)

$$q_t=\frac{q_e^2{k}_{2}t}{1+q_e{k}_{2}t}$$ where q_e and q_t (mg g⁻¹) are the amounts of adsorbed protein on the nanocomposites at equilibrium and at time t, respectively. k₁ (min⁻¹) presents the rate constant of first-order adsorption and k₂ (g mg⁻¹ min⁻¹) is rate constant of second-order adsorption. The parameters and the correlation coefficients (R²) for the two models are given in Table 3. Taking the adsorption of BSA by mCMChitoCar3 nanocomposites for instance, the R² value of the pseudo-second-order model was 0.999, which was higher than that of pseudo-first-order model (0.995), indicating the best fitting of experimental data by pseudo-second-order kinetic model. Furthermore, the theoretical adsorption capacity (q_e) calculated from the pseudo-second-order model (38.11 mg g⁻¹) was much closer to the experimental one (q_e.exp = 38.91 mg g⁻¹). So, adsorption of BSA by the non-magnetic and magnetic nanocomposites fitted better by the pseudo-second-order kinetic model.

Table 3 Kinetic parameters of pseudo-first-order and pseudo-second-order rate models for BSA adsorption process.

	Pseudo-first-order			Pseudo-second-order			qe.exp
	k1 × 102	qe.cal	R2	k2	qe.cal	R2
	min−1	mg g−1		g mg−1 min−1	mg g−1		mg g−1
CMChitoCar	2.92	34.58	0.994	4.44	39.91	0.999	41.59
mCMChitoCar1	4.72	44.96	0.992	3.72	48.2	0.999	50.12
mCMChitoCar2	5.32	49.19	0.992	5.76	51.75	0.999	54.71
mCMChitoCar3	6.98	56.14	0.993	3.48	59.47	0.999	63.21

3.5.2

3.5.2 Adsorption isotherm

Adsorption isotherm can be considered as an essential necessity to design the adsorption system. The adsorption isotherms of BSA onto mCMChitoCar1 and mCMChitoCar3 nanocomposites were determined and the results are depicted in Fig. 6a and b. As the initial BSA concentration is increased, the adsorption capacity of beads is enhanced and then switches to level off. At higher initial concentration, the adsorption sites reach on saturation state and the adsorption remains constant. The Langmuir and Freundlich isotherm models can be applied to analysis of experimentally equilibrium data from adsorption of protein onto samples at equilibrium time. In the Langmuir model, the adsorption of adsorbate takes place at specific homogeneous sites within the adsorbent and valid for monolayer adsorption onto adsorbents. The expression of the applied Langmuir model is given by Eq. (6) (Ho and McKay, 1998):

(6)

$$q_e=\frac{q_m{K}_L{C}_e}{1+K_L{C}_e}$$ where C_e is the equilibrium protein concentration in the solution (mg L⁻¹), K_L is the Langmuir adsorption constant (L mg⁻¹), and q_m is the theoretical maximum adsorption capacity (mg g⁻¹). The q_m and k_L can be calculated from the slope and intercept of a linear plot of C_e/q_e versus C_e, respectively. Dimensionless constant adsorption parameter R_L as an important characteristic of Langmuir model is expressed as follows:

(7)

$$R_L=\frac{1}{1+K_L{C}_0}$$ where K_L is the Langmuir constant (L mg⁻¹) and C_o is the initial concentration of protein. The R_L indicating type of isotherm is varied: unfavorable for R_L > 1; adsorption is linear condition for R_L = 1 favorable for 0 < R_L < 1; irreversible condition for R_L = 0 (Piccin et al., 2012). In the Freundlich model, the adsorption of adsorbate occurs on a heterogeneous surface by multilayer sorption and the adsorption capacity can increase with an increase in adsorbate concentration (Hameed et al., 2007). Freundlich model is defined as follows:

(8)

$$q_e=k_F{C}_e^{1/n}$$ where k_f is the equilibrium adsorption coefficient (mg⁽¹⁻ⁿ⁾ Lⁿ g⁻¹), k_f is the Freundlich constant representing the adsorption capacity; and n is a constant depicting the adsorption intensity. All the expressions in Langmuir and Freundlich equations for all nanocomposites were calculated according to experimental data and summarized in Table 4. The high correlation coefficient in Langmuir equation (R² > 0.99) shows that Langmuir isotherm exhibits a better fit for experimental data than the Freundlich model. Also, the R_L values of hydrogel beads calculated from Langmuir model were found between 0 and 1 indicating favorable adsorption system.

Close

Figure 6

Adsorption isotherms of BSA onto (a) mCMChitoCar1 and (b) mCMChitoCar3 nanocomposites at initial concentration range of C₀: 200–600 mg L⁻¹, pH = 4.6 and T = 25 °C.

Export to PPT

Table 4 Langmuir and Freundlich constant for the adsorption of BSA by beads.

	Langmuir model			Freundlich model
	qm	KL	R2	N	kf	R2
	mg g−1	L mg−1		g L−1	(mg g−1) (L mg−1)1/n
CMChitoCar	84.26	4.29	0.99	2.13	91.34	0.99
mCMChitoCar1	99.18	10.31	0.99	2.29	102.17	0.92
mCMChitoCar2	103.45	19.87	0.99	2.61	105.73	0.96
mCMChitoCar3	133.26	48.76	0.99	3.11	139.56	0.96

3.5.3

3.5.3 Effect of pH on BSA adsorption

The electrostatic interactions involved in adsorption mechanism may be explained by the results of Zeta potential. The measured Zeta potentials of BSA, MNPs and mCMChitoCar3 at different pHs are shown in Fig. 7a. The Results indicated that the isoelectric point (pI) of magnetic particles is about 6.68 which agrees well with the literature value pI = 6.5 (Peng et al., 2004). On the other hand, as the surface of the particles was modified with biopolymers (mCMChitoCar3), the absolute value of zeta was shifted to 5.85. The effect of pH on the adsorption of BSA on mCMChitoCar3 nanocomposites is shown in Fig. 7b. At pH values higher than 7.4 (7.4–9.2) both BSA and mCMChitoCar3 are negatively charged, so an electrostatic repulsion between them resists the adsorption of protein. At pH values lower than 4.7, (pH 3.3) both BSA and mCMChitoCar3 are positively charged accordingly, and electrostatic repulsion contributes remarkably to the decrement of BSA adsorption. Notably the maximum adsorption of BSA onto nanocomposites was obtained at pH = 4.66 near the isoelectric point of BSA (pI = 4.7) (Peng et al., 2004; Lee et al., 2002). As it has been reported by researchers (Chun and Stroeve, 2002), the main reason for this phenomenon can be explained by this fact that the BSA molecules undergo lowest conformational changes in this situation and additionally the electrostatic repulsion between biopolymers and BSA molecules is minimized which promote adsorption process. At pH 5.2, magnetic particles are positively charged and BSA is negatively charged; therefore, there should be an electrostatic attraction between BSA molecules and nonmagnetic particles promoting the adsorption of BSA onto the surface of the particles. However, the results revealed that less BSA was adsorbed on the surface of nanoparticles. This phenomenon may be attributed to the expansion of the BSA molecule at acidic pH, which causes the decrement in adsorption (Bajpai, 2000). The results are consistent with the research reported by Kono et al., where the maximum adsorption capacity of BSA on chitosan-based hydrogel was at pH 4.5 (Kono et al., 2013).

Close

Figure 7

(a) Zeta potentials of BSA, MNPs and mCMChitoCar3 in 10⁻³ M NaNO₃ at different pHs and (b) effect of pH on the adsorption of BSA onto mCMChitoCar1 (C₀: 200 mg L⁻¹, ionic strength 0.1 M, T = 25 °C).

Export to PPT

3.5.4

3.5.4 Effect of ionic strength on adsorption

In the current study, we attempted to study the effect of ion strength on the BSA adsorption onto CMChitoCar and mCMChitoCar1 beads at different NaCl concentrations ranging from 0.01 to 0.1 M at pH = 4.7 (Fig. 8). The adsorption of BSA onto beads was increased as the salt concentration increased up to 0.05 M and then decreased. By increasing the salt concentration, the surface tension of the solution enhances resulting in interactions between inner hydrophobic part of BSA and biopolymers (Wang et al., 2008). On the other hand it may cause the suppression of the electric double layer of proteins which decrease the electrostatic repulsion between adsorbed protein and ensuring enhancement in adsorption efficiency (Chen et al., 2007). However, at salt concentration above 0.7 M, the adsorption of proteins decreased. This decrement in protein adsorption may be attributed to the screening of anionic sulfate (–OSO₃⁻) groups on κ-carrageenan by Na⁺ cations (Sharma and Agarwal, 2001). Additionally, compared to BSA molecules the Na⁺ ions are smaller in size; therefore, its mobility is higher than that of BSA molecules. In other words, the adsorbent surface is more available for the Na⁺ cations than BSA molecules (Hu et al., 2013). The effect of ion strength on adsorption of BSA on poly(ethylenimine)-modified Sepharose FF has been investigated by Yu et al., and the adsorption capacity of adsorbents for BSA has been reduced by increasing ion strength of media (Yu and Sun, 2013).

Close

Figure 8

Effect of NaCl concentration on the adsorption of BSA onto CMChitoCar and mCMChitoCar1 nanocomposites (C₀: 200 mg L⁻¹, pH: 4.6 and T = 25 °C).

Export to PPT