Preparation, Characterization, and Swelling Behavior of PEGylated Guar Gum @ Ag Nanoparticles

In this study, polyethylene glycol/guar gum @ silver nanoparticles (PEG/ GG@AgNPs) were synthesized by using simple sonication method. The nanoparticles were characterized using Fourier-transform infrared spectroscopy (FTIR) and scanning transmission electron microscopy (STEM). The swelling behaviors of nanoparticles were studied in different pHs (5.5 and 7.4). The experimental results were calculated by Fickian diffusion and Schott kinetic models to understand the swelling mechanism and coefficients of the nanoparticles. The results showed that the linear equation of the Fickian diffusion kinetic model was best fit to explain the water diffusion mechanism of the nanoparticle with high correlation coefficient (R 2 = 0.982 – 0.987). The results confirmed that the swelling degree of nanoparticles were 9.71 g/g at pH 5.5. Also, the results confirmed that PEG/GG@AgNPs can be a good candidate for drug delivery systems in pharmaceutical applications.


Introduction
Nanotechnology focuses on many fundamental disciplines such as physics, chemistry, materials science, and biology [1]. Recently, the synthesis of the nanostructures has gained a great attention due to superior properties (mechanical, optical, thermal, biological, physical, and chemical) as compared to conventional materials [2][3][4][5].These superior properties depend on the size, composition, shape, and origin of nanostructures [6,7]. As a general definition, nanostructures are 1-100 nanometers in size in which case they have a high surface area-to-volume ratios and their reactivities are effected mostly depending on their different shapes such as spherical, conical, spiral, cylindrical, tubular, and hollow [8,9].
Many researchers have reported that iron [10], copper [11], gold [2], and silver [12] were used often for the preparation of stable dispersions of nanoparticles due to their biocompatibility and biodegradability and less reactivity in the biomedical applications. Silver nanoparticles (AgNPs) with desired morphologies, well-known for their antimicrobial activity, are used in both ionic and metallic forms which are incorporated inside the polymer matrix, with excellent biocompatibility [13]. It was known that bare silver nanoparticles were prone to oxidation and tarnishing. Thus, we used PEG/GG polymer blends to improve the dispersion stability and prevent agglomeration of silver nanoparticles in aqueous environment [14].
During the preparation of hydrogels, mostly preferred biopolymers are alginate, chitosan, gum arabic, agar, carrageenans, and guar gum [15][16][17][18][19]. Among these biopolymers, GG is used in the fields of food industry and pharmaceutical and cosmetic applications [20][21][22]. It is not desirable to prepare silver nanoparticles by chemical methods using toxicological chemicals such as reducing agents [23]. For this purpose, we chose a dual biopolymer blend (GG and PEG) which will carry out the green synthesis and improve the distribution. GG is a nonionic water-soluble polysaccharide and consists of galactomannan which has a linear chain of (1-4)β-D-mannopyranosyl units interposed with (1-6)-α-D-galactopyranosyl units distributed as side branches [24]. PEG is a stabilizer effective for the control of size and shape of nanoparticles and also has role on the reduction of silver ions [25].
The key points of the chapter were (i) the green synthesis and (ii) swelling of nanoparticles. The aim of this study was to obtain NPs consisting of PEG/GG and zero valent Ag by using the simple sonication method. We prepared nanoparticles in the presence of silver nitrate in GG/PEG (2:1 mixing ratio) polymer blends. In particular, we performed a green synthesis sonochemical process to reduce Ag ions to form AgNPs by using ultrasonic method without the use of dangerous stabilizing agents such as any reducing agent and surfactant [26]. The breaking of cavitation bubbles under high temperature and pressure and the hydrogen radicals (H*) and hydroxyl radicals (OH*) in the water formed by the ultrasonic effect play an important role in the reduction of Ag ions and the formation of AgNPs [27].
All samples were characterized by using FTIR and STEM techniques. SEM images revealed good compatibility and homogeneous distribution between the PEG/GG matrix and Ag. AgNPs were found to be <500 nm in size. Moreover, we have also demonstrated the swelling behavior of the prepared nanoparticles in finding out the potential of the nanoparticles for drug delivery systems. The swelling uptake (%) of PEG/GG@AgNPs was found to be % 670.5 at pH 5.5.We determined the swelling behavior such as the maximum swelling and gel fraction of samples to interpret the water absorption results. All results showed that one could control the size and the shape of zero valent Ag nanoparticles by polymer blend under the sonication effect.

Preparation of PEG/GG@AgNPs
GG solution (0.5 g GG in 100 mL of deionized water) and PEG (5 mL) were mixed by stirring with a magnetic stirrer at 25°C for 10 min. 5 mL of AgNO 3 (0.1 M) solution was added into GG/PEG solutions and then was sonicated for 15 min. 0.1 M NaOH was added into the solutions until pH 8.4.

Characterization
Samples were ground with KBr powder and analyzed from 4000 to 600 cm À1 with a resolution of 4 cm À1 using eight scans by using a PerkinElmer FTIR emission spectrometer (Spectrum Two). Samples were scanned in the dark-field area with the wet STEM detector by using FEI QUANTA S50 (A copper grid, Ted Pella, support films, carbon type A, 300 meshes was utilized). STEM holder was cooled to 2°C and the pressure was set between 700 and 1300 Pa.

Measurement of the water absorbency
The swelling degree (St, g/g) was calculated from Eq. (2): where S W is the swelling degree per gram dried sample (g g À1 ), W d is the mass of dried samples at time t (g), and W t is the mass of swollen samples at time t (g) [28].

Calculations of the swelling behavior
The swelling behavior was explained by applying different kinetic models such as Fickian diffusion and Schott second-order dynamic model given in Eqs. (2)-(7) [29,30]. To identify the swelling kinetic mechanisms of the prepared samples, the swelling kinetic parameters were evaluated according to models: Fickian diffusion swelling kinetic models: When Eq. (2) was linearized where t is time (min), k is the rate constant (min À1 ), St is the water absorption capacity at time t, and S ∞ is the water absorption capacity at equilibrium. The plots of LnS t =S ∞ versus Ln t were drawn to calculate the parameter (k) of kinetic model and the linear correlation coefficient.
Schott proposed the second-order kinetic model to elucidate the swelling mechanism of the system, and this model was related to the swelling rate at any time and was proportional to the quadratic of the swelling capacity before the equilibrium state [30,31]: When the initial condition was t = 0 and S = 0 where t is time (min), S is the swelling capacity at time t (g g À1 ), A is the reciprocal of initial swelling rate, and B is the reciprocal of S ∞ . The plots of t/W versus t were drawn to calculate the parameters (A and B) of kinetic model and the linear correlation coefficient.
To explain the water diffusion mechanism, fractional solution capacity (F: S t / S ∞ ) less than 0.60 was analyzed using the following equation (Eq. (7)): where t is time (min), Wt is the uptake capacity of the sample at time t, S ∞ is the capacity of the sample at swelling equilibrium (g), k is the gel characteristic constant, and n is the swelling index (n < 0.5 for Fickian diffusion and 0.5 < n < 1 for non-Fickian; n = 1 for relaxation controlled transport and n > 1 for II diffusion) [30]. The Fickian diffusion model had another expression shown as Eq. (8): Diffusion coefficient D (cm 2 /s) was calculated using Eq. (9). For short times, Eq. (9) was used at the early-stage diffusion coefficient of water by where l was the diffusional distance [32,33].

Swelling kinetics
Swelling ability is known to be associated with free hydrophilic groups and surface properties [34]. The sonication method, which is associated with various factors such as ultrasound power, ultrasound frequency, modification time, and temperature, changes the surface properties [35]. In this study, the swelling kinetics was investigated to determine the surface properties of the novel PEG/GG@AgNPs obtained by using sonication method. To analyze the effect of sonication on the swelling kinetic mechanism of the synthesized nanoparticles, swelling kinetic results were observed gravimetrically and performed in pH 5.5 and 7.4 at 25°C.
The equilibrium swelling degrees, Fickian and Schott kinetic models of PEG/ GG@AgNPs were given in Figures 1-3, respectively and the swelling behavior followed a similar behavior due the high surface area.
The O-H groups of the PEG/GG@AgNPs formed hydrogen bonds with water molecules and absorbed water [36]. The equilibrium swelling degrees of PEG/ GG@AgNPs were 9.71 gg À1 (pH 5.5) and 3.82 gg À1 (pH 7.4). According to the experimental swelling results, the swelling degree of the PEG/GG@AgNPs increased within the first 10 minutes and then slowed down until reaching equilibrium after 15 min in two different pH mediums. In this case, the results showed that the nanoparticle had shown greater interest in the water molecules and was proof  that it binds with O▬H groups in the structure which allowed it to swell in a short time in pH 5.5. In order to explain swelling behavior of the nanoparticle, the swelling parameters were calculated by using the Fickian diffusion and Schott's second-order kinetic models. The calculated swelling kinetic parameters and the correlation coefficients (R 2 ) for all the models for the nanoparticles were given in Table 1.
The results showed that the linear equation of the Fickian diffusion kinetic model was best fit to explain the water diffusion mechanism of the nanoparticles with high correlation coefficient (R 2 = 0.982-0.987). According to the Fickian kinetic model, n is known to explain the diffusion mechanism of the solvent. The n values of the nanoparticles were calculated to be in the range of 0.5-1, which was explained by a non-Fickian diffusion behavior of the water transport mechanism [36].

STEM analysis
To investigate the effect of ultrasonic irradiation on the surface properties of the nanoparticle, the STEM images of PEG/GG@AgNPs were presented in Figure 4. It could be clearly seen that the surface of PEG was a homogeneous surface in spherical nanoform with the uniform dispersion of AgNPs. It was concluded that the ultrasonic irradiation may play a role in obtaining homogeneous distribution of the nanoparticles.

FTIR analysis
The FTIR spectrums of pure GG and PEG/GG@AgNPs were given in Figure 5. The FTIR spectrum of pure GG showed peaks at 3499 (▬OH stretching), 2989 (C▬H stretching), and 1030 (▬OH bending) cm À1 . The FTIR spectrum of PEG/GG@AgNPs showed peaks at 2980 (C▬H stretching), 1540 (COOH), 1470 (C▬H stretching), and 1370 (▬C=O) cm À1 . The peaks recorded at 3499 cm À1 of GG expressed in AgNPs indicated that -OH groups were utilized for the stabilization of AgNPs. From the results of the FTIR, we found that reduction and stabilization occurred in AgNPs. Singh et al. observed similar results [42].

Conclusions
In this study, PEG/GG@AgNPs were prepared by using sonochemical method. The swelling ability of nanoparticle was investigated in two different pHs. The  The FTIR spectrums of pure GG and PEG/GG@AgNPs. mechanism of swelling kinetics was explained, and it has been found that the mechanism follows the Fickian diffusion model. In summary, this study was focused on the green, low-cost novel method for producing Ag nanoparticles. The NPs could be served as a promising candidate nanocarrier for drug delivery systems due to its the swelling degrees.