Scholarly article on topic 'Adsorption of albumin by gold nanoparticles: Equilibrium and thermodynamics studies'

Adsorption of albumin by gold nanoparticles: Equilibrium and thermodynamics studies Academic research paper on "Chemical sciences"

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Abstract of research paper on Chemical sciences, author of scientific article — M.S. Maleki, O. Moradi, S. Tahmasebi

Abstract In this research, bovine serum albumin (BSA) experiment was carried out and then the produced BSA was used to investigate the adsorption behavior of BSA from aqueous solutions through UV–Vis spectroscopy. The changes of parameters such as contact time, pH, BSA initial concentration and temperature were tested and investigated by several adsorption experiments. The equilibrium adsorption data were described as well and fitted better by the Freundlich adsorption isotherm than the four linear Langmuir models at all studied temperatures and pHs. Moreover, surface structural change was studied for the presence of BSA and gold nanoparticles (GNPs) before and after the experiment by FT-IR spectroscopy. The maximum adsorption capacity of BSA adsorbed by GNPs was 109.54mg/g and equilibrium constant was 0.0051 calculated by the Langmuir (four type) model at 298K and pH=8.6. The thermodynamic parameters implied that the adsorption processes were spontaneous and exothermic. The kinetic data indicate that the adsorption fits as well as with the pseudo first-order kinetic model.

Academic research paper on topic "Adsorption of albumin by gold nanoparticles: Equilibrium and thermodynamics studies"

Arabian Journal of Chemistry (2013) xxx, xxx-xxx

King Saud University Arabian Journal of Chemistry

www.ksu.edu.sa www.sciencedirect.com

ORIGINAL ARTICLE

Adsorption of albumin by gold nanoparticles: Equilibrium and thermodynamics studies

M.S. Maleki, O. Moradi *, S. Tahmasebi

Department of Chemistry, Shahre-Qods Branch Islamic Azad University, Shahre-Qods, Tehran, Iran Received 11 July 2012; accepted 14 October 2012

KEYWORDS

BSA adsorption; Isotherm;

UV-Vis spectroscopy; Thermodynamic parameters; Kinetic models

Abstract In this research, bovine serum albumin (BSA) experiment was carried out and then the produced BSA was used to investigate the adsorption behavior of BSA from aqueous solutions through UV-Vis spectroscopy. The changes of parameters such as contact time, pH, BSA initial concentration and temperature were tested and investigated by several adsorption experiments. The equilibrium adsorption data were described as well and fitted better by the Freundlich adsorption isotherm than the four linear Langmuir models at all studied temperatures and pHs. Moreover, surface structural change was studied for the presence of BSA and gold nanoparticles (GNPs) before and after the experiment by FT-IR spectroscopy. The maximum adsorption capacity of BSA adsorbed by GNPs was 109.54 mg/g and equilibrium constant was 0.0051 calculated by the Langmuir (four type) model at 298 K and pH = 8.6. The thermodynamic parameters implied that the adsorption processes were spontaneous and exothermic. The kinetic data indicate that the adsorption fits as well as with the pseudo first-order kinetic model.

© 2012 King Saud University. Production and hosting by Elsevier B.V. All rights reserved.

1. Introduction

Bovine serum albumin (BSA) is a globular protein with the approximate shape of a prelate spheroid of dimensions 4 nm x 4 nm x 14 nm (McClellan and Franses, 2003). BSA shows a great conformational adaptability ("soft" protein). The blood plasma protein BSA represents 52-62% of the total plasma protein fraction (Brandes et al., 2006). The most impor-

tant physiological function of serum albumin is maintaining the osmotic pressure and pH of blood, and transporting a wide variety of endogenous and exogenous compounds including fatty acids, metals, amino acids, steroids and drugs (Huang and Kim, 2004). The isoelectric point of BSA is at pH 4.7. This means that in a solution with pH = 6.6, 7.6 and 8.6 BSA is negatively charged, as a whole molecule (Kudelski, 2003). BSA molecules have the ability to bind substances reversibly especially negatively charged substances. For this reason, BSA is able to assume the role of transportation (Hu and Su, 2003). Adsorption of protein on surfaces constitutes an important and very active research field not at least due to its impact on areas such as protein purification (Moradi, 2011a; Brewer et al., 2005), design of food processing equipment, biocompat-ibility (Colombo et al., 2005), and biosensors (Malmsten, 2003). For example, the biocompatibility of an artificial material is closely related to the cellular response when the material

* Corresponding author. Tel./fax: +98 21 4682938. E-mail addresses: moradi.omid@gmail.com, o.moradi@shahryaiu. ac.ir (O. Moradi).

Peer review under responsibility of King Saud University.

1878-5352 © 2012 King Saud University. Production and hosting by Elsevier B.V. All rights reserved. http://dx.doi.Org/10.1016/j.arabjc.2012.10.009

is put into contact with a biological system. The cellular response is in turn influenced by the proteins adsorbed at the implant interface from the bio fluid (Kasemo, 2002). The amount, orientation and the conformational state of the protein are important in the cell-protein mediated response (Kowalczynska et al., 2001). Also, from a more fundamental research perspective the protein adsorption process is of interest due to the complex nature of the system, and ideally one would like to understand in detail how the protein concentration, buffer, pH, ionic strength etc. can influence the protein adsorption. Therefore, numerous studies were conducted in the past decades in order to examine the effect of such experimental conditions on protein adsorption (Hook et al., 1998; Oliva et al., 2003; Sternik et al., 2004; Vroman and Adams, 1969).

For a detailed understanding of the mechanism of protein adsorption on various surfaces and the effect of various parameters, BSA adsorption has been studied extensively. Su et al. (Su et al., 1998) investigated the adsorption of BSA at the hydrophilic silica-water interface using specula neutron reflection and examined the concentration dependence of the surface excess of BSA at pH close to its isoelectric point. The surface excess was found to reach a plateau at a very low bulk protein concentration, suggesting a high affinity of BSA molecules for the oxide surface. Patil et al. (Patil et al.,

2007) investigated the effect of the zeta potential of cerium oxide nanoparticles on the adsorption of BSA and cellular uptake in adenoid carcinoma lung cells. The nanoceria samples showed protein adsorption increase with increasing zeta potential which further confirms that the electrostatic forces are the primary interaction for BSA adsorption. The negative zeta potential for the nanoceria samples was found to be favorable for the nanoparticle uptake in the cells. Zhu et al. (2007) investigated the relationship between protein adsorption and zeta potential of a biphasic calcium phosphate ceramic by polyacrylamide gel electrophoretic methods. The results showed that the zeta potential and the amount of adsorbed BSA were both influenced by pH and ionic strength concentration in the buffers. Lysozyme has higher affinity for biphasic calcium phosphate than BSA and would preferentially bind to the surface. The mechanism can also be explained by the electrostatic interaction, together with the structural stability of protein molecules. Wang et al. (Wang et al., 2008) stated that, however, it is well known that the interaction of proteins with GNPs is highly sensitive to the particles' surface chemistry and the conformational state of the protein (Ding et al.,

2008). In this context, a major challenge remains to investigate the conformational behavior of proteins in a protein-nanopar-ticle complex system, including the denaturation of their tertiary and secondary structures, which are susceptible to occur due to protein adsorption (Roach et al., 2005). Brewer et al. demonstrated the interaction between citrate coated GNPs and BSA proteins (Brewer et al., 2005) and more recently De Paoli Lacerda et al. reported the specific interaction between GNPs and human plasma proteins (De Paoli Lacerda et al., 2010). Moreover, Guo et al. demonstrated the possibility of using GNPs as probes to investigate the conformational change of poly-L-lysine in the range of pH from 6.5 to 11.0 (Guo et al., 2007). In addition, Iosin et al. demonstrated, using spectroscopic techniques, the direct interaction between BSA and GNPs through, assessing the influence of the GNPs surface on the binding of albumin, providing the information concerning the possible protein conformation changes induced

after bio-conjugation (Iosin et al., 2009). However, thermody-namic parameters, such as temperature and pH, can also trigger the disruption of protein conformation which could lead to cancer, diabetes and cardiovascular diseases (Dalle-Donne et al., 2005) and consequently can have a major influence on the nano-bio interfaces. Therefore, to gain a better insight on the nano-bio interaction, it is clearly of great interest to extend our previous studies by integrating the effects of pH and temperature on the nano-bioconjugates. To address this issue McClellan and Franses investigated the interface between GNPs and BSA, as a function of pH and temperature, by employing three different spectroscopic techniques: LSPR, fluorescence and SERS (McClellan and Franses, 2003). The aim of this research was to investigate the effect of contact time, initial concentration, pH and temperature on the adsorption equilibrium and the rate of BSA by GNPs. Moreover, BSA was chosen as a protein experimental by GNPs, because of specific reasons that this particular protein had a very high stability, availability at high purity and its solubility in water. Results from both equilibrium and rate adsorption studies were presented and the adsorption capacities with respect to pH and temperature were obtained for BSA adsorption by GNPs. Also, determination of the appropriate BSA adsorbed by GNPs to obtain the constant parameters of Freundlich and Langmuir models and evaluation of the adsorption rate using various kinetic models was done; to derive the thermody-namic parameters activation energy (Ea), the changes in free energy (AG0), enthalpy (AH0) and entropy (AS0) during the adsorption process.

2. Experimental

2.1. Materials and methods

The GNPs were considered to be a kind of commercial research product in powder form (plasma Chem, Rudower Chaussee 29-D-12489 Berlin), with a particle size of about 20 ± 1 nm. Also, the adsorbent dose of GNPs during the whole experiment has been equal to 0.01 g BSA was purchased from Sigma (catalog number 145-987-324, with purity >99.9%, USA) and was used as received. BSA solution was prepared by dissolving it in phosphate buffer for the study of pH effect (monobasic sodium phosphate and dibasic sodium from Merck Co.) aqueous solution with the concentration of 100, 200, 300 and 400 mg/L prepared by using double distilled, deionized (Milli-Q treated) water and shaken at 250 rpm (from HZQ Company). One milligram GNPs was placed in 1 L of phosphate buffer solution, pH 7.6 (Bruno and Svoronos, 1989), similar to the pH of human blood plasma (Tasman and Ajaeger, 1998), and kept for several hours at different temperatures. To determine if the adsorbent dosage (for BSA with ranging from 100 to 400 mg/L) was equilibrated with the suspension of GNPs as an adsorbent (20 mL GNPs 1 mg/L) at pH 7.6 for a certain time, phosphate buffer aqueous solutions with the concentrations of 100, 200, 300 and 400 mg/mL were prepared by using doubled distilled, deionized (Milli-Q treated) water for all parts of the experiment.

During the experiments the temperature was held 298, 303 and 308 K and the pH was 6.6, 7.6 and 8.6. At the end of equilibrium period, the GNP suspensions were centrifuged at 250 rpm for 2 min, and the supernatant was then filtered

through 0.2 im filter paper (Gelmen Sciences) for later analysis using UV-Vis spectrometer at a wave length of k = 280 nm. Ultrasonic Bath (71020-DTH-E; Model 1510 DTH, 220V; EMS Company) was used to prevent the particles of GNPs to aggregate and form a bulk. The equilibrium BSA concentration determines the amount of BSA adsorbed on GNPs by UV-Vis spectroscopy which was performed twice in each experiment and the experimental results were based on average values. Thermodynamic adsorption experiments were conducted by using a 100 mL pyramid glass bottle containing 1 mg of the adsorbent at all pHs and temperatures (GNPs) and 20 mL of the mentioned BSA solutions with the initial concentration ranging from 100 to 400 mg/L. In order to indicate chemical bonding with GNPs, BSA before and after adsorption FT-IR spectroscopy from Perkin Elmer-E100 Company was used. As previously shown, the adsorption of BSA molecules on the bottle wall is ignorable (Gadh et al., 1999).

2.2. Batch mode adsorption studies

The effects of experimental parameters, such as BSA initial concentration (100-400 mg/L), pH (6.6, 7.6 and 8.6) and temperature (298, 303 and 308 K) on the adsorption amount of various BSAs were studied in a batch mode of operation for the specific period of contact times (0-40 min). In order to determine the effect of each parameter, the other parameters were kept fixed during the experiment. For contact time studies, 20 mL of BSA solution of known initial concentration and a certain pH was taken with a stable quantity of adsorbent (20 mL GNPs) and agitated in a thermostated rotary shaker, with the speed of 250 rpm at 298 K. Also, the quality assurance of the analytical measurements was performed by the researchers. BSA standard solutions of 100, 200, 300, and 400 mg/L ±0.1% were used for the measurement. Calibration curves between 100 and 400 mg/L were prepared and the detection limit was found to be 1 mg/L. The adsorption percentage of each adsorbed BSA was calculated as follows:

Adsorptivity (%) —

Ci — Cf

where, Q and Cf are the initial and final BSA concentrations (after contact to the adsorbent), respectively. The concentration retained in the adsorbent phase (qe, mg/g) was calculated by using the following equation:

(C - Ct)V W

where Ci is the initial BSA concentration and Ct is BSA concentration (mg/L) at any time, V is the volume of solution (L) and W is the mass of the adsorbents (g). The data analysis was carried out using correlation analysis employing the least-square method, and the average relative error (ARE) was calculated by the researchers via using the following equation (Ayrancia and Dumanb, 2010):

ARE(%)^£

100. n

qi,cal qi, exp

where N is the number of data points. Each experiment was conducted in triplicate under identical conditions to confirm the results, and was found reproducible (experimental error within 3%).

3. Results and discussion

3.1. The effect of contact time, initial concentration pH and temperature on adsorption of BSA by GNPs

The GNPs surfaces were adsorbed by the BSA molecule in the solutions with different concentrations (100, 200, 300 and 400 mg/L), pHs (6.6, 7.6 and 8.6) and temperatures (298, 303 and 308 ± 1 K). The amounts of BSA adsorbed by the GNPs as adsorbent surfaces Fig. 1a-c show the percentage of adsorbed BSA by GNPs surfaces as a function of contact time, temperature and initial concentration at pH (pH = 6.6). Also, Fig. 1d-f and g-i represented the percentage of adsorbed BSA by GNPs surfaces as a function of contact time, temperature and initial concentration at pHs of 7.6 and 8.6, respectively. It can be seen that the amount of the BSA molecules adsorbed by GNPs with the increase of time was increased. Also, we can see the amount of BSA adsorbed is at the highest by GNPs at a 20-min period for all conditions (initial concentration, temperature and pH) and with the increase of time the amount of BSA adsorbed remained unchanged with time or better to say reached the equilibrium state. Therefore, the 20-min period was chosen as the optimum contact time for all conditions.

Also, we can see from these Fig. 1(a-i) the amount of BSA adsorbed molecules as a function of the initial concentration of BSA from aqueous solutions and with increasing initial concentration, the amount of BSA adsorbed was increased at all conditions. At this part of the experiment, the following concentrations were chosen for BSA molecules: 100, 200, 300, and 400 mg/L at equilibrium time and all temperatures and pHs. With increasing BSA concentration, the percentage of BSA adsorption increased. Also, more BSA molecules were left unabsorbed in the solution due to saturation of the binding sites which indicates that energetically favorable sites became involved with the increasing of BSA concentration in the aqueous solution (Moradi et al., 2004). BSA molecule adsorption is attributed to different mechanisms of BSA exchange as well as to adsorption. This effect on BSA adsorption can be explained, since at low BSA molecule/adsorbent ratios BSA molecule adsorption involves more energy sites. As BSA molecule/ adsorbent ratio increases, more energy sites are saturated and adsorption begins on fewer energy sites, resulting in low increasing level of BSA molecule adsorption (Lu and Chiu, 2006). Also, we can see from these Fig. 1(a-i), the amount of BSA adsorbed molecules as a function of BSA solution temperature. The percentages of the adsorption experiment were conducted at 298, 303 and 308 K to investigate the effect of temperature, at all times, initial concentration and pHs. The BSA molecular percentage of adsorption yields by adsorbent surfaces decrease with the increase in temperature. Decrease in the amount of equilibrium adsorption of BSA molecules with the rise in temperature may be explained by the fact that the adsorbent sites were less active at higher temperatures. Also, the kinetic energy of BSA molecule decreases at higher temperatures; therefore, the contact between each BSA molecule and the site of adsorbent is not sufficient, leading to decreases in adsorption efficiency. This condition shows that adsorption occurs more physically rather than chemically.

Similar trends have been observed by other researchers for aqueous phase adsorption (Bhattacharya et al., 2008). In addition, the rise of adsorption with temperature may decrease the

time (min)

20 time (min)

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-100 ppm -200 ppm 300 ppm -400 ppm

10 20 30 time (min)

100 ppm 200 ppm 300 ppm 400 ppm

10 20 30 time (min)

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10 20 30 time (min)

10 20 30 time (min)

10 20 30 time (min)

Figure 1 The effect of contact time and initial concentration on BSA adsorption by GNPs surfaces at different pH and temperature (a) T = 298 K, pH:6.6; (b) T = 303 K, pH:6.6; (c) T = 308 K, pH:6.6; (d) T = 298 K, pH:7.6; (e) T = 303 K, pH:7.6; (f) T = 308 K, pH:7.6; (g) T = 298 K, pH:8.6; (h) T = 303 K, pH:8.6; (i) T = 308 K, pH:8.6.

pore of GNPs size, which may affect GNP's adsorption capacity (Bhattacharya et al., 2008). Increases and decreases in adsorption with increasing temperature have also been reported (Sharma, 2001; Sariri and Tighe, 1996). The protein surface is not homogeneously charged and the pH value at which the sum of the proteins' positive and negative electrical charges is zero which is the isoelectric point. By varying the pH of the solution, the charges of the surface and protein can be changed. BSA is usually positively charged below the isoelec-tric point and negatively charged upon the isoelectric point. The electrostatic interaction between proteins and surfaces is an important driving force for the protein adsorption process (Mansch and Chapman, 1996). The isoelectric point of BSA is about pH 4.7 and the solubility of BSA was 6.6, 7.6 and 8.6 similar to the pH of human body (Tasman and Ajaeger, 1998), therefore, BSA has a negative surface charge at experimental pHs. The effect of pH was investigated on the change of BSA solution concentration versus time. Fig. 1(a-i) show the change of BSA adsorbed in solution with time at different

initial concentrations and temperatures (298, 303 and 308 K) for pH values 6.6, 7.6 and 8.6, respectively. It was observed that the adsorption of BSA by GNPs increased for pH values and the highest amount of BSA adsorption was at pH 8.6. The main reason for this behavior of BSA is the competitive adsorption between Hydronium ion (H3O+) and BSA molecules. At low pH values, Hydronium ions are adsorbed more than BSA, since Hydronium ions have a high concentration and more tendency to be adsorbed (Erdema et al., 2009; Alkan and Dogan, 2001). With increasing the pH, Hydronium ion concentration is reduced and results in the BSA molecule get better and more adsorption is seen (Alkan and Dogan, 2001; Xue et al., 2009; Yanhua et al., 2010).

3.2. Kinetic analysis

The kinetic adsorption process of BSA molecules by GNPs surfaces could be well described by the pseudo-second order

and intraparticle diffusion model rate laws and the rate constants increased with a rise in temperature (Li et al., 2006). This could be explained by the fact that increasing temperature results in a rise in the diffusion rate of BSA molecule across the external boundary layer and within the GNPs surface due to the result of decreasing solution viscosity. The kinetic analysis of temperature effect was evaluated at T = 298 K and pH = 7.6 was evaluated and presented at Fig. 2(a-c). The adsorption decreased with the increase of temperature, indicating that the mobility of BSA molecules decreased with the increase of temperature, as did the number of molecules that interact with the active sites at GNP surfaces; moreover, the adsorption was exothermic. In addition, increasing the temperature reduces the viscosity of the solution and decreases the rate of diffusion of BSA molecules. The adsorption is initially (contact time <20 min) rapid, and then slows, perhaps because a small number of vacant surface sites was available for adsorption during the initial stage, and then, the remaining vacant surface sites were difficult to occupy because of the repulsive forces between the BSA molecules on the GNP surfaces and the bulk phase (Mall et al., 2006).

Pseudo first, second and intraparticle diffusion models were applied to test the experimental data and thus elucidate the kinetic adsorption process. The pseudo first-order model can be expressed as:

Hqe - qt) = ln(qe(4)

where qe and qt are the amounts of BSA molecules adsorbed on adsorbents (GNPs surface) at equilibrium and at various times t (mg/g) and k1 are the constant rates of the pseudo first-order model for the adsorption (min-1) (El-Naggar et al., 2010). The values of qe and k can be determined from the intercept and the slope of the linear plot of ln (qe-q) versus t. The pseudo second-order model is given by:

t _ 1 t

qt kql qe

where qe and qt are defined as in the pseudo first-order model; k2 is the constant rate of the pseudo second-order model for adsorption (g/mg min) (Rais et al., 2010). The slope and intercept of the linear plot of t/q against t yielded the values of qe and k2. Furthermore, the initial adsorption rate h (mg/g min) can be determined from 2 h = k2qe2. Since neither the pseudo first-order nor the second-order model can identify the diffusion mechanism, the kinetic results were analyzed by the intraparticle diffusion model to elucidate the diffusion mechanism, the model which is expressed as:

qt = kit1/2 + C

where C is the intercept and ki is the intraparticle diffusion rate constant (mg/g min0.5), which can be evaluated from the slope of the linear plot of qt versus t1/2 (Ozcan et al., 2006a,b). The results of Fig. 2(a-c) are fitted using pseudo first, second order models and intraparticle diffusion model. Table 1a-c presented the coefficients of the pseudo first and second-order adsorption kinetic models and the intraparticle diffusion model at pH = 6.6, 7.6 and 8.6, respectively. The R2 values of the pseudo first and second-order models exceeded, but ARE values of the pseudo first-order model were smaller than those of the pseudo second-order model. Moreover, the q values (qe,cal) calculated from pseudo first-order model were more consistent with the experimental q values (qe,exp) than those calculated from the pseudo second-order model. Hence, this study suggested that the pseudo first-order model better represented the adsorption kinetics. A similar phenomenon has been observed in the adsorption (Chiou et al., 2004; Ozcan et al., 2006a,b).The values of k2, h, qeexp and qecai all increased with the temperature. Ozcan et al. (Ozcan et al., 2006a,b) proposed that the adsorption of ions by natural sepiolite proceeds by physisorption, in which increasing the temperature increases the adsorption rate but reduces the adsorption capacity. However, this study suggested that the thermodynamic analyses were more appropriate for determining whether the adsorption was a physisorption or a chemisorption process, as would be discussed in the following section. Typically, various mechanisms control the adsorption kinetics; mostly limiting the diffusion mechanisms, including external diffusion, boundary layer diffusion and intraparticle diffusion (Guibal et al., 2003). Hence, the intraparticle diffusion model was utilized to determine the rate-limiting step of the adsorption process. If the regression of q versus t1/2 is linear and passes through the origin, then intraparticle diffusion is the sole rate-limiting step (Ozcan and Ozcan, 2005). The regression was linear, but the plot did not pass through the origin, suggesting that adsorption involved intraparticle diffusion, but that was not the only rate-controlling step. Other kinetic models may control the adsorption rate, the finding of which is similar to that made in previous works on adsorption (Ozcan et al., 2006a,b; Ozcan et al., 2005). The k values increased with the temperature (298-308 K), as a result of increasing the mobility of BSA molecules. In addition, the C value varied like the ki values with temperature (Table 1a-c). The values of C are helpful in determining the boundary thickness: a larger C value corresponds to a greater boundary layer diffusion effect (Kannan and Sundaram, 2001). The results of this study demonstrated that increasing the temperature promoted the boundary layer diffusion effect (Ho et al., 2002).

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Time ( min)

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in (mi 0.12 - (m 140

.cr 0.1 ^ y = 0.0052X + 0.0777 90

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Time ( min)

Time05 ( min)0 5

Figure 2 Plots of pseudo first order rates (a), pseudo second order rates (b) and intraparticle diffusion model (c) at pH = 7.6 and T = 298 K.

Table 1 Kinetic parameters for the adsorption of BSA molecule by GNP surfaces. Initial concentration of BSA was 100, 200, 300 and

400 mg/L at pH = 6.6 (Table A) pH = ' 7.6 (Table B) and pH = 8.6 (Table C); contact time 20 min at different temperatures.

Temperature (K) qe,exp (mg/g) k\ (1/min) qcal (mg/g) R2 ARE (%)

Pseudo first-order model

298 239.24 0.2219 240.16 0.9993 0.3

303 232.48 0.2101 234.78 0.9988 0.9

308 229.16 0.2036 229.86 0.9995 0.3

Temperature (K) qe,exp (mg/g) k2 (1/min) qCai (g/mg min-1) R2 ARE (%)

Pseudo second-order model

298 239.24 3.25 x 10-3 244.14 0.9881 2

303 232.48 3.22 x 10-3 237.74 0.9863 2.2

308 229.16 3.20 x 10-3 232.87 0.9868 2.8

Temperature (K) qe,exp (mg/g) k¡ (1/min) qcal (mg/g min0.5) R2 ARE (%)

Intraparticle diffusion model

298 239.24 60.873 209.54 0.9664 14.3

303 232.48 59.392 197.26 0.9620 17.7

308 229.16 57.316 191.39 0.9593 19.7

Temperature (K) qc,exp (mg/g) k1 (1/min) qcal (mg/g) R2 ARE (%)

Pseudo first-order model

298 248.04 0.2346 248.65 0.9991 0.3

303 242.04 0.2234 244.16 0.9995 0.9

308 237.96 0.2138 239.54 0.9996 0.3

Temperature (K) qe,exp (mg/g) k2 (1/min) qcal (g/mg min-1) R2 ARE (%)

Pseudo .second-order model

298 248.04 3.25 x 10-3 250.68 0.9908 1

303 242.04 3.17 x 10-3 244.25 0.9890 0.9

308 237.96 3.09 x 10-3 241.5 0.9881 1.5

Temperature (K) qe,exp (mg/g) k¡ (1/min) qcal (mg g min0.5) R2 ARE (%)

Intraparticle diffusion model

298 248.04 62.075 218.24 0.9691 13.6

303 242.04 60.411 201.35 0.9639 20.2

308 237.96 59.325 190.14 0.9614 25.1

(C) R2

Temperature (K) qc,exp (mg/g) kj (1/min) qcal (mg/g) ARE (%)

Pseudo first-order model

298 257.64 0.2376 259.54 0.9983 0.7

303 251.64 0.2277 253.7 0.9987 0.8

308 246.36 0.2168 249.03 0.9923 1

Temperature (K) qe,exp (mg/g) k2 (1/min) qcal (g/mg min-1) R2 ARE (%)

Pseudo second-order model

298 257.64 3.47 x 10-3 267.03 0.9935 3.6

303 251.64 3.33 x 10-3 256.2 0.9920 1.7

308 246.36 3.25 x 10-3 250.65 0.9930 1.7

Temperature (K) qe,exp (mg/g) kt (1/min) qcal (mg/g min0.5) R2 ARE (%)

Intraparticle diffusion model

298 257.64 63.744 230.36 0.9734 11.5

303 251.64 62.079 212.5 0.9688 18.2

308 246.36 62.155 203.15 0.9721 21.1

3.3. Equilibrium point adsorption 3.3.1. Langmuir isotherm

At room temperature (298 K), BSA molecules adsorbed by the GNPs are in equilibrium with BSA molecule in aqueous solution after a 20-min contact time. The Langmuir model represents one of the first theoretical treatments of non-linear adsorption and suggests that uptake occurs on a homogeneous surface by monolayer adsorption without interaction between adsorbed molecules. In addition, the model assumes uniform

energies of adsorption onto the surface and no transmigration of the adsorbate. Estimation of maximum adsorption capacity corresponding to complete monolayer coverage on the GNPs was calculated by using the Langmuir isotherm model since the saturated monolayer isotherm can be explained by the non-linear equation of Langmuir Eq. (7) (Foo and Hameed, 2010).

QmKLCe

1 + KLCe

Table 2 The four linear forms of the Langmuir isotherm.

Name Linear form Eq. Plot Slope Intercept

Langmuir-1 C _ _j__c q, klq„ ' Qm e (8) Ce/qe versus C, 1/Qm 1/KQm)

Langmuir-2 ¿ — (kLQ^ C + Qm (9) 1/q, versus 1/ Ce 1/(KLQm) 1/Qm

Langmuir-3 qe — Qm — (Kl) C (10) qe versus q,/Ce 1/Kl Qm

Langmuir-4 C- — KLQm — KLq, (11) q,/C, versus q, Kl KLQm

where Ce is the equilibrium concentration (mg/L), qe the amount of metal ion adsorbed (mg/g), Qm a complete monolayer (mg/g) and KL is an adsorption equilibrium constant (L/mg) that is related to the apparent energy of adsorption. Eq. (7) can be linearized into four different forms in Table 2 (Eqs. (8)-(11)), which can give different parameter estimates (Foo and Hameed, 2010).

3.3.2. The Freundlich isotherm

The Freundlich model was chosen to estimate the adsorption intensity of the BSA by GNP's surface based on the adsorption heterogeneous energetic distribution of active sites accompanied by interactions between adsorbed molecules. It can be derived assuming a neperian logarithm decrease in the enthalpy of adsorption with the increase in the fraction of occupied sites through the following non-linear equation (Moradi, 2011b):

qe = KFClJn (12)

where KF (mg/g) stands for adsorption capacity and n for adsorption intensity of BSA molecules on the adsorbent. Eq. (13) can be linearized in neperian logarithm form Eq. (12) and the Freundlich constants can be determined from the linear plot of ln(qe) versus ln(Ce).

lnqe = lnKF + 1 /nlnCe (13)

The four linear Langmuir and linear Freundlich isotherms for the adsorption of the BSA molecule by GNPs are plotted and examination of the correlation coefficients is reported in Tables 3a-c at pHs 6.6, 7.6 and 8.6, respectively. The results showed that correlation coefficients (R2) and ARE for linear

Freundlich are less than four linear Langmuir values. The correlation coefficient obtained for BSA adsorbed ranged between 0.9928 and 0.9985, which indicated that the experimental data fitted well with the Freundlich model. Also, the amount of ARE is obtained from theoretical and experimental results, the amount of ARE is less than other Langmuir isotherms. Also, comparison of experimental adsorption iso-

Table 3b Adsorption isotherm constants for BSA adsorption onto GNPs at pH 7.6.

Isotherm model pH = 7.6

Temperature 298 K 303 K 308 K

0.494 0.497 0.499

114.63 113.24 111.95

0.9954 0.9968 0.9985

95.84 93.45 92.35

0.00495 0.00479 0.00462

0.8997 0.8893 0.8839

81.96 81.30 80.65

0.00526 0.00508 0.00487

0.9784 0.9785 0.9714

102.24 99.86 96.71

0.00482 0.00457 0.00464

0.9781 0.9730 0.9856

108.53 107.93 107.75

0.00470 0.00460 0.00450

0.9781 0.9730 0.9695

Table 3a Adsorption isotherm constants for BSA adsorption onto GNPs at pH 6.6.

Isotherm model pH = 6.6

Temperature 298 K 303 K 308 K

Freundlich 1/n 0.516 0.519 0.523

Kf 102.47 101.77 98.65

R2 0.9955 0.9966 0.9966

Langmuir 1 qm 96.15 95.24 94.34

Kl1 0.00459 0.00443 0.00435

R2 0.8952 0.8749 0.8784

Langmuir 2 qm 81.97 80.01 79.25

KL2 0.00492 0.00484 0.00473

R2 0.9799 0.9793 0.9745

Langmuir 3 qm 103.13 102.28 101.35

KL3 0.00446 0.00430 0.00421

R2 0.9752 0.9662 0.9662

Langmuir 4 qm 110.33 109.46 107.27

KL4 0.00440 0.00420 0.00410

R2 0.9752 0.9662 0.9662

Table 3c Adsorption isotherm constants for BSA adsorption onto GNPs at pH 8.6.

Isotherm model pH = 8.6

Temperature 298 K 303 K 308 K

Freundlich 1/n 0.472 0.475 0.486

Kf 124.36 122.16 119.9

R2 0.9951 0.9967 0.9971

Langmuir 1 qm 95.24 92.59 88.49

Kli 0.00517 0.00497 0.00485

R2 0.8747 0.8906 0.8846

Langmuir 2 qm 80.00 79.92 78.12

KL2 0.00564 0.00554 0.00538

R2 0.9778 0.9735 0.9746

Langmuir 3 qm 96.86 95.47 94.98

KL3 0.00521 0.00505 0.00489

R2 0.9811 0.9715 0.9708

Langmuir 4 qm 109.54 108.65 107.23

KL4 0.0051 0.0049 0.0047

R2 0.9811 0.9751 0.9708

ARTICLE IN PRESS

8 M.S. Maleki et al.

250 -200 -

d 150 -

g. 100 -

50 -0 -

55 75 95 115 135 155 175

Ce (mg/L)

Figure 3 Comparison of experimental results of BSA adsorbed by GNPs surface with adsorption isotherms (pH 7.6; T: 298 K).

therms of BSA with four linear forms of Langmuir model and linear Freundlich is presented in Fig. 3 for BSA adsorption onto GNP's surface at pH = 7.6 and 298 K.

The n values (1.91-2.12) are higher than 1.0, indicating that the BSA molecule is favorably adsorbed by GNPs at all temperatures (298, 303 and 308 K) and pHs (6.6, 7.6 and 8.6). Moreover, the magnitude of KF ranged between 98.65 and 124.36, which indicates a high adsorptive capacity and an easy uptake of BSA molecules from the aqueous solution by GNPs. Freundlich isotherm does not describe the saturation behavior of adsorbents. Regarding the coefficients of Freundlich isotherm, KF decreased with temperature, revealing that adsorption capacity decreased with temperature. Like KF, n decreased with temperature as well. Since all n values obtained from the isotherms exceeded unity, the BSA molecules were favorably adsorbed by GNP surfaces. The highest values of n were 2.12 at 298 K for BSA molecules onto GNPs surfaces, at pH = 8.6. These data indicate favourable adsorption at the lowest temperature. Also, the amount of 1/n less than 1 shows the favourable nature of adsorption of (BSA molecule onto GNP surfaces). Similar results have been reported by several earlier works for Freundlich constant for BSA adsorption by adsorbents (Norde, 1994; Giacomelli et al., 1997). The maximum adsorption capacities for monolayer saturation at 298 K at pH = 8.6 was 109.54 mg/g in GNP's surface. Because lower temperature leads the BSA to find less chance to be adsorbed on the GNPs and increase the adsorption capacity of GNPs, resulting in the enlargement of GNPs. Also some researchers indicated that by decreasing the temperature, the amount of BSA adsorbed is decreased (Iosin et al., 2011). Also, at higher temperatures where denaturation of the protein occurs, the amount of protein adsorption is reduced. Similar results were obtained by Roscoe et al. (Roscoe et al., 1993).

3.4. Adsorption thermodynamic studies

Thermodynamic parameters provide additional in-depth information regarding the inherent energetic changes involved during adsorption. The thermodynamic parameters of BSA molecules are adsorbed by GNP surface, which provide useful information concerning the inherent energetic changes of the adsorption process. The thermodynamics for the adsorption

0.0032 0.00325 0.0033 0.00335 1/T (1/K)

0.0034

Figure 4 ln K0 vs. 1/T plot for the thermodynamic parameters for the percentage of adsorption of BSA by GNPs surfaces, Initial concentration of BSA was 400 mg/L; pH 8.6; (adsorbents dosage, 1 mg/L; contact time, 20 min and T = 298 ± 1 K).

of BSA molecules adsorbed by GNP surfaces was investigated in the range of 298-308 K, and the influence of temperature on the adsorption under the optimized conditions is shown in Fig. 4.

It can be found that there is a little decrease for the equilibrium adsorption capacity with the temperature increase from 298 to 308 K. Thermodynamic constant, K0, for the adsorption reaction at equilibrium can be defined as:

where, as and ae denote activity coefficients of the BSA adsorbed by GNPs as adsorbents and BSA in the equilibrium solution, respectively. qe is the concentration of BSA adsorbed on the surface (mg/g), Ce is the concentration of the considered ion at the equilibrium (mg/L), ys and ye are the activity coefficients of the adsorbed solutes and the solute in the equilibrium solution, respectively. As the concentration of the solute in the solution approaches zero, the activity coefficient, y, approaches unity. Eq. (14) then is written as (Lyklema, 2005):

Kn — ^ =

where, the values of K0 are obtained by plotting ln(qe/Ce) versus qe and extrapolating qe to zero. Its interception with the

0.00324 0.00326 0.00328 0.0033 0.00332 0.00334 0.00336 -1.42

-1.44 --1.46 --1.48 --1.5 ç -1.52-1.54 -1.56 -1.58 -1.6 -1.62

y = 874.4x - 4.368 ^——y=872.6x - 4.375

R2 = 0.997 —---- R2 = 0.993

^^^^^^/ = 833.96x - 4.3006

^^ R2 = 0.995

—•— pH=6.6 -B-pH=7.6 -A-pH=8.6

Figure 5 The plot of ln k1 vs. 1/T for the adsorption of BSA by GNPs surfaces using the Arrhenius equation at different pHs.

vertical axis gives the values of ln K0. The standard Gibbs free energy of adsorption, AG0, is (Lyklema, 2005):

AG0 = -RT ln K0 (16)

RT ln K0 = TAS0 - AH0 (17)

ln K0 = -AH°1+AS! (18)

0 R T R y '

Eq. (17) describes how the equilibrium constant, K0, varies with the absolute temperature, T, for an equilibrium system and Eq. (18) predicts a linear plot of ln K0 versus 1/T for the reversible adsorption of BSA adsorbed by GNP surface adsorbents. Fig. 5 shows the ln K0 vs. 1/T plot for the adsorption of the BSA adsorbed by GNPs surfaces at various temperatures.

Table 4 clearly shows that the value of change of the standard enthalpy (AH0) is negative for the adsorption of BSA adsorbed by GNP surfaces. It is clear that adsorption of BSA molecule by GNPs considered as adsorbents are exothermic, which is supported by the fact that the content of adsorption decreases with temperature. If the heat value of adsorption process range is 40-800 kJ/mol, the adsorption is usually chemisorption, yet values less than 40 kJ/mol refer to a physi-sorption (Levine, 1995).

In this research, the adsorption of BSA adsorbed is an endothermic process. This phenomenon could be due to two processes: the hydration of BSA adsorbed by GNPs is an endothermic process and adsorption of BSA adsorbed by GNPs surfaces is exothermic. It can be that the first process is dominant which leads to an endothermic over-all process (Roscoe et al., 1993). Plot of Eq. (18) shows that the standard enthalpy change was determined to be -8110.4 kJ/mol for the

Table 4 Adsorption thermodynamic parameters for the percentage of BSA molecule adsorption onto GNP surfaces (initial concentration of each BSA molecule was 400 mg/L; pH 7.6 contact time, 20 min).

T (K) AG0 (kJ/mol) AH0 (kJ/mol) AS0 (J/molK)

298 -1196.8 -8110.4 -23.2

303 -1080.8

308 -964.8

BSA molecule adsorbed by GNPs surfaces (Fig. 4). Hence, adsorption of BSA molecules adsorbed by GNP surfaces is a chemical process. The positive value of AS0 is negative and is indicative of decreased randomness at adsorbent-adsorbate interface during the adsorption. The decrease in the adsorption capacity of the adsorbent with temperature is attributable to the enlargement of the pores or activation of the adsorbent surface (Wang and Zhu, 2007). Also, AG° reflects the feasibility of the adsorption and the standard entropy determines the disorderliness of adsorption at solid-liquid interface. The AG0 values were negative at all temperatures of the experiments, verifying that the adsorption of BSA molecule adsorbed by GNP surfaces was spontaneous and thermodynamically favorable. Also, a more negative standard Gibbs energy implies a greater driving force of adsorption, resulting in a lower adsorption capacity. As the temperature increased from 298 to 308 K, Gibbs standard energy got lower negative values. Hence, the amount of the Gibbs standard energy implied that the adsorption affinity of BSA molecule adsorbed by GNPs surfaces was stronger at pH = 8.6 than that on other pH experiments (6.6 and 7.6), because the inclination of BSA molecule for adsorption by GNPs is increased with the increase of pH solution. The pseudo first-order model was identified as the best kinetic model for the adsorption of BSA by GNPs surfaces. Accordingly, the rate constants (k1) of the pseudo first-order model were adopted to calculate the activation energy of the adsorption process using the Arrhenius equation (Moradi and Zare, 2011):

ln k1 = ln A--- (19)

1 RT v ;

where ki, A, Ea, R and T are the rate constants of the pseudo first-order model (Table 1) (g/mg h), the Arrhenius factor, the activation energy (kJ/mol), the gas constant (8.314 J/mol K)

Table 5 The calculation of activation energy (Ea) for the adsorption process using the Arrhenius equation at different pHs.

pH 6.6 7.6 8.6

Ea (kJ/mol) 6.933 7.255 7.270

A 0.01267 0.01258 0.01356

Figure 6 FT-IR spectra of BSA adsorbed by GNPs at pH = 7.6.

and the temperature (K), respectively. The activation energy could be determined from the slope of the plot of ln k1 versus 1/T (Fig. 5).

The activation energy was 6.933, 7.255 and 7.270 kJ/mol at pH 6.6, 7.6 and 8.6, respectively for the adsorption of BSA by GNP surfaces at 298 K (Table 5). Also, the results are indicated with increasing of pH, the amount of Ea is increased and the result to the rate of reaction is decreased. The magnitude of the activation energy yields information on whether the adsorption is mainly physical or chemical. Nollet et al. (Nollet et al., 2003) suggested that the Chemisorption process normally had activation energy of 4-40 kJ/mol. Therefore, AH0, AG0 and Ea all suggested the same fact that the adsorption of BSA by GNP surfaces was a chemisorption process.

3.5. Characteristics of BSA molecule and BSA adsorbed by GNPs

In order to characterize the structure of BSA, presented in Fig. 6, before and after adsorption by GNPs in aqueous solution studied by FT-IR spectroscopy for indicate the chemical bonding between BSAs by GNPs. Fig. 6(a) show several strong bondings of 1690, 3000 and 3450 cm-1 relative to amine (-N-C=O), aliphatic (-C-C-H) functional groups and hydrogen bonding for BSA chemical structure, respectively (Pavia et al. 1996). After adsorbed of BSA by GNPs (Fig. 6b), the hydrogen bonding and aliphatic functional groups is reduced peak intensive and amine functional group is deleted. The result indicated that the new chemical bonding is conformed of bonding between GNP and BSA molecules.

4. Conclusion

GNPs are identified to be an effective adsorbent for the adsorption of BSA molecules from the aqueous medium. The adsorption is highly dependent on various operating

parameters like; contact time, pH, initial BSA concentration and temperature. It has been observed that the adsorption percentage increased with an increase in the contact time and becomes gradual after 20 min. The adsorption percentage of BSA adsorbed by GNPs is maximized at a pH value of 8.6 and decreases with decreasing of the solution pH. The adsorption percentage increased with increasing initial concentration of BSA and pH of solution, also adsorption percentage decreased with increasing of the temperature of the solution. Adsorption kinetics follow pseudo first-order kinetics. Adsorption capacity decreases with temperature. The temperature effect is used to calculate the change in thermodynamics parameters such as AG0, AH0 and AS0, the analysis of these thermodynamic parameters suggests that adsorption is: (a) spontaneous and AG0 is negative, and (b) mainly chem-isorptions because of the high AH0 values. Negative value of AS0 dictates that adsorbed BSA molecules remaining on the GNPs surface are decreased in a random fashion. The equilibrium data are analyzed against four linear Langmuir and linear Freundlich isotherm equations. The result shows that the experimental data are best correlated by linear Freundlich isotherm. The constants of all the isotherms are determined that are the most useful means of predicting adsorption conditions.

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