**Anti-Corrosive Properties of an E**ff**ective Guar Gum Grafted 2-Acrylamido-2-Methylpropanesulfonic Acid (GG-AMPS) Coating on Copper in a 3.5% NaCl Solution**

#### **Ambrish Singh 1,2,\*, Mingxing Liu 1,2, Ekemini Ituen 1,2 and Yuanhua Lin 1,2,\***


Received: 23 December 2019; Accepted: 24 February 2020; Published: 5 March 2020

**Abstract:** Guar gum grafted 2-acrylamido-2-methylpropanesulfonic acid (GG-AMPS) was synthesized using guar gum and AMPS as the base ingredients. The corrosion inhibition of copper was studied using weight loss, electrochemical, and surface characterization methods in a 3.5% sodium chloride (NaCl) solution. Studies including weight loss were done at different acid concentrations, different inhibitor concentrations, different temperatures, and different immersion times. The weight loss studies showed the good performance of GG-AMPS at a 600 mg/L concentration. This concentration was further used as the optimum concentration for all of the studies. The efficiency decreased with the rise in temperature and at higher concentrations of acidic media. However, the efficiency of the inhibition increased with the additional immersion time. Electrochemical methods including impedance and polarization were employed to calculate the inhibition efficiency. Both of the techniques exhibited a good inhibition by GG-APMS at a 600 mg/L concentration. Surface studies were conducted using scanning electrochemical microscopy (SECM), scanning electron microscopy (SEM), and atomic force microscopy (AFM) methods. The surface studies showed smooth surfaces in the presence of GG-AMPS and rough surfaces in its absence. The adsorption type of GG-AMPS on the surface of the copper followed the Langmuir adsorption model.

**Keywords:** corrosion; guar gum; coatings; electrochemical impedance spectroscopy (EIS); SECM; AFM

#### **1. Introduction**

Copper is used worldwide because of its good conductivity, availability, and large tenders. Almost all companies use copper in one way or another, because of its varied applications. Copper is an appropriate material for electrodes, wires, joints, and couplings [1]. An NaCl solution can cause severe corrosion in metals and alloys [2]. It can cause pitting corrosion under the coatings by forming blisters. These pits are difficult to detect under the coatings, and can cause fatal accidents and a shutdown of the systems. Known also as rock salt or common salt, sodium chloride is abundantly found in nature and mostly in sea water. Marine corrosion is very common and causes billions of dollars of losses globally. So, there is always a need to find solutions to marine corrosion.

The application of corrosion inhibitors is currently the most cost-efficient means of dealing with copper corrosion [3]. The corrosion inhibitors used at this stage are mainly organic corrosion inhibitors, because organic molecules may contain O, N, and S, as well as other atoms with unshared pair electrons and π-bonds [3]. Such organic molecules can be empty orbitals, which interact with copper surfaces [3]. The d-orbital acts to form a protective film to achieve corrosion protection. Glue extracted from natural

plants is one of them. Gum contains a large amount of O atoms, and has received a lot of attention and has been widely studied. At the same time, the use of gums as corrosion inhibitors can effectively improve the utilization rate of natural plants. Chemical compounds or inhibitors are used to mitigate corrosion as one of the common available methods. These compounds are inorganic and organic in nature. They are rich in carbon, oxygen, nitrogen, sulfur, benzene rings, and double bonds in their molecular structures [4]. In the recent decade, the corrosion fraternity has been concerned with finding green and environment-friendly inhibitors [5]. Plant extracts, as potential corrosion inhibitors, have been used by several authors [6–10]. The shortcoming of using plant extracts is that they tend to develop fungi/bacteria on their surfaces after some time, and this affects their inhibition efficiency. So, the search for green polymers or biopolymers has been sought worldwide. Several polymers have been studied with heteroatoms in their structures [11–15]. The multifunctional groups in the polymers can easily adsorb or attach to any metal surface, thereby protecting them from corrosion. Polymers may prove to be better than the low molecular inhibitors used in the acidization process. Biocompatible compounds have also been used as inhibitors because of their cost effectiveness, ease of availability, and low-cost machines used. Our motivation was to develop a compound with all of the qualities of a polymer, in addition to being non-toxic and environmentally benign. Thus, guar gum grafted 2-acrylamido-2-methylpropanesulfonic acid (GG-AMPS) was synthesized as a green compound to cope with environmental regulations, and to be used effectively in high concentrations.

A survey of the literature reveals that no work has been done using GG-AMPS as a corrosion inhibitor in an NaCl solution. GG-AMPS has a nitrogen atom, oxygen atom, and sulfur atom in its molecular structure, which provide a good adsorption approach, leading to good bonding and complex grouping on the metal surface. The presence of hydroxyl groups at different points makes it a potential inhibitor that can share electrons and take part in good bond formation. This paper elucidates the inhibition effect of GG-AMPS for copper in a 3.5% NaCl media. The mitigation properties of GG-AMPS were conducted using static weight-loss methods and electrochemical methods. In the meantime, the surfaces of copper were examined by scanning electrochemical microscopy (SECM), scanning electron microscopy (SEM), and atomic force microscopy (AFM).

#### **2. Experimental**

#### *2.1. Copper Samples*

In all of the experiments, such as for the weight loss, the electrochemical and surface morphology of the pure copper samples were utilized. Each of the samples employed for weight loss were cut into rectangle coupons. Prior to the experiments, the surfaces were abraded with silicon sheets of grades 300 to 1200, cleaned with acetone and distilled water, and vacuum-dried. The dimensions of the copper samples employed for the weight loss tests were 2.0 cm × 2.5 cm × 0.2 cm.

#### *2.2. Corrosive Medium*

The inhibitor coatings were concentrated in the range of 100 to 600 mg/L for all of the studies. A 3.5% sodium chloride solution was used as the corrosive medium. It was prepared using pure NaCl and double-distilled water. A freshly prepared solution was used for each of the experiments. The different concentrations of inhibitor solutions were prepared by adding a calculated amount of inhibitor into the corrosive medium.

#### *2.3. Synthesis of GG-AMPS*

The synthesis of GG-AMPS was conducted according to the previous reference [16–18]. One gram of guar gum was slowly dissolved in 100 mL of distilled water. Then, 0.2 g of potassium persulfate was added to the guar gum solution, and the reaction continued for 1 h in a water bath at 70 ◦C. After that, 2 g of 2-acrylamide-2-methyl-1-propane sulfonic acid (AMPS) and 0.2 g of N, N'-methylenebisacrylamide were added to the above solution, and the reaction continued for 3 h at the

same temperature (70 ◦C). The whole reaction process was carried out in a nitrogen atmosphere. After the solution was cooled, the excess acetone was added to the solution so as to separate the desired product. The precipitates were filtered and dried in vacuum at 50 ◦C for 24 h to a constant weight. The product was guar gum grafted with 2-acrylamide-2-methyl-1-propane sulfonic acid (GG-AMPS). The compound obtained was further characterized by infrared (IR) spectroscopy. The plan of the synthesis and molecular structure of the inhibitor coating is shown in Figure 1.

2-acrylamide-2-methyl-1-propane sulfonic acid (AMPS)

**Figure 1.** Molecular structure and synthesis scheme of guar gum grafted 2-acrylamido-2 methylpropanesulfonic (GG-AMPS). M = AMPS.

#### *2.4. Infrared Spectroscopy*

IR spectroscopy was conducted using the Nicolette 6700 infrared spectrometer of Thermo Electric Company Inc. from the USA (West Chester, PA). The infrared spectrum of the GG-AMPS is shown in Figure 2.

Figure 2 shows the IR spectrum of the guar gum and GG-AMPS. In Figure 2, 3461 cm−<sup>1</sup> indicates the tensile vibration of O–H in the guar gum. In addition, the weak peak near 2926 cm−<sup>1</sup> is the C–H, and the peak at 1638 cm−<sup>1</sup> is the vibration peak of the six-membered rings. In Figure 2, the small peaks at 1552 and 1300 cm−<sup>1</sup> are the bending vibrations of N–H in the AMPS amide group, and the wide peak at 3408 cm−<sup>1</sup> is the overlap of the N–H stretching band and O–H stretching band, which resulted in a certain movement of the wide peak at 3461 cm−<sup>1</sup> in the guar gum. The peak at 1657 cm−<sup>1</sup> and its adjacent peak are the result of the overlapping of the C=O vibration and –CONH– vibration in –CO2H. The peaks at 1375 and 1458 cm−<sup>1</sup> are the C–H bending vibrations in –CH3, –CH2, and –CH. The peaks at 1220 and 1042 cm−<sup>1</sup> are characteristic peaks of a sulfonic group. Figure 2 reveals that AMPS was successfully grafted with guar gum.

**Figure 2.** IR spectrum of GG-AMPS.

#### *2.5. Weight Loss*

The duration of all of the weight loss tests was determined following the ASTM G31-2004 standard [19]. The duration of the experiments selected for all of the tests was 24 h. The copper coupons were cleaned with water and then rinsed with Clarke's solution for 5 min. The coupons were then exposed to vacuum drying. The obtained weight loss values were used to calculate the corrosion rate of the metal in the corrosive media. Each of the tests were performed in triplicate and the mean values were reported. The corrosion rates were calculated using the following equation:

$$\mathbb{C}\_{\mathbb{R}}(mm/y) = \frac{87.6W}{atD} \tag{1}$$

where *W* is the average weight loss of copper specimens (mg), *a* is total area of copper specimen, *t* is the immersion time (h), and *<sup>D</sup>* is the density of copper in (g·cm<sup>−</sup>3).

#### *2.6. Electrochemical Analysis*

A Gamry potentiostat workstation (Gamry, Warminster, PA, USA) was utilized for the electrochemical tests. The potentiostat was connected to a cell assembly, which consisted of a reference electrode, counter electrode, and working electrode. Prior to the start of the experiments, the working electrode (copper) was exposed to the 3.5% NaCl solution for 30 min, so as to keep the potential (*E*corr) stable.

The range of the frequency selected was from 100 kHz to 10 mHz, at an amplitude of 10 mV per decade for all of the electrochemical impedance tests. The evaluation of the inhibition efficiency was done using the following equation:

$$
\eta \%= \left(1 - \frac{R\_{\rm ct}}{R\_{\rm ct(i)}}\right) \times 100\tag{2}
$$

where *R*ct and *R*ct(i) are the charge transfer resistances without and with GG-AMPS, respectively.

The potentiodynamic polarization tests were conducted in the limit of −250 to 250 mV, with a 0.167 mV/s scan rate. The following equation was used to determine the efficiency of the coating:

$$\eta\% = \left(1 - \frac{i\_{\text{corr}(i)}}{i\_{\text{corr}}}\right) \times 100\tag{3}$$

where *i*corr and *i*corr(i) are the corrosion current densities without and with GG-AMPS, respectively.

#### *2.7. Scanning Electrochemical Microscopy (SECM)*

The SECM tests were performed using a Princeton workstation equipped with Versa scan software (3000, Versa, TX, USA). The microprobe was made of a silver (Ag)/Pt wire inside a glass tube with a diameter of 10 μm. The probe vibrated over the metal surface at an average distance of 100 μm, and the scanned area was 20 μm × 20 μm.

#### *2.8. Scanning Electron Microscopy (SEM)*

The scanning electron microscopy (SEM) was done to detect the changes in the external area of the metal. The SEM was conducted using a Tescan machine (S800, Tescan, Shanghai, China) equipped with a Zeiss lens (Zeiss, Shanghai, China). The samples were washed with a sodium bicarbonate solution in order to remove the corrosion products, followed by distilled water prior to surface exposure.

#### *2.9. Atomic Force Microscopy (AFM)*

The AFM experiments were conducted using a Dimension Icon Brock instrument (HPI, Bruker, Karlsruhe), made in Germany, for all of the copper coupons. Once the tests were completed, the images obtained were sent to Nanoscope analysis software (2.0), version v1.40r1, so as to obtain the 3D figures. The average roughness and the peak roughness were further confirmed using the linear fitting of the 2D figures.

#### **3. Results and Discussion**

#### *3.1. Weight Loss Experiment*

#### 3.1.1. Effect of Concentration, Time, and Temperature

The effect of the GG-AMPS concentrations on the protective covering of the copper surfaces is portrayed in the form of a concentration vs. inhibition efficiency graph (Figure 3a). From the figure, it is evident that the mitigation activity of GG-AMPS rose with the increase in concentration, and attained values of 95% at 600 mg/L. This increase in mitigation ability was due to the adsorption of the GG-AMPS molecules onto the wide area of the copper surface. Figure 3b displays the increase in inhibition efficiency with immersion time, for up to 12 h. This discovery points to the molecular structure of GG-AMPS having a big effect on the values of inhibition efficiency. Figure 3c depicts the influence of temperature on the inhibition efficiency of GG-AMPS. The efficiency was found to decrease with an increase in temperature. This may have been the result of the desorption of the coating from the copper surface. So, for this coating to be used at high temperatures, the concentration of the coating should be increased. Figure 3d displays the influence of the inhibition efficiency with the NaCl concentration. The efficiency was seen to decrease with the increase in NaCl concentration. This may be due to the NaCl solution penetrating the coating on the copper surface, causing pitting corrosion. In this study, the inhibitor molecules had π-electrons in the benzene ring and non-bonding electrons on the heteroatoms, such as oxygen, sulfur, and nitrogen, which assist the molecules in their adsorption onto the copper surface [20].

The adsorption of the coating on any metal surface also depends on the number of electron-donating functional groups attached to the structure. A greater number of electron-donating groups can help with better adsorption and bond formation, which can lead to a better mitigation of corrosion. GG-AMPS consists of OH, SO3H, and NH groups, which may be a reason for its good mitigation abilities in the corrosion of copper in sodium chloride media.

**Figure 3.** (**a**) Variation of inhibition efficiency (%) with inhibitor concentration. (**b**) Variation of inhibition efficiency (%) with immersion time. (**c**) Variation of inhibition efficiency (%) with temperature. (**d**) Variation of inhibition efficiency (%) with NaCl concentration.

#### 3.1.2. Adsorption Isotherm of Inhibitor on Copper

The adsorption of molecules on the metal surface can be better explained using Langmuir, Frumkin, Flory Huggins, and Temkin isotherms. The obtained experimental data can be fit using the equations of these isotherms. The best fit gives a linear slope with regression coefficient values approaching unity. The fitted experimental values showed that Langmuir was the best out of all of the equations (Figure 4). The following equation was used to determine the fitted results of the isotherm [21]:

$$\frac{C\_{\text{inh}}}{\Theta} = \frac{1}{K\_{\text{ads}}} + C\_{\text{inh}} \tag{4}$$

where *C*inh is the GG-AMPS concentration (mg/L), and θ and *K*ads represent the surface coverage and adsorption constant, respectively.

**Figure 4.** Langmuir adsorption isotherm plots using the values of weight loss, electrochemical impedance spectroscopy (EIS), and polarization.

#### *3.2. Electrochemical Tests*

#### 3.2.1. Electrochemical Impedance Spectroscopy (EIS) Studies

The behavior of the copper electrode was investigated by electrochemical impedance tests. The impedance nature of the metal is represented in the form of Nyquist graphs (Figure 5a). As reported in Figure 5a, at a higher frequency, a capacitive loop is seen, which contains a straight line, tending to be a semicircle. The semicircle is normally the result of the capacitance of double-layer and charge-transfer resistance [22]. The presence of two time constants can be seen in the Nyquist and bode figures. The presence of two time constants may have been because of the roughness and inhomogeneity of the metal surface after corrosion. In addition, the diameter capacitance with GG-AMPS was bigger in comparison with the blank. Meanwhile, the diameter got bigger with higher concentrations of GG-AMPS. This was because the GG-AMPS molecules that were adsorbed on the copper surface formed a barrier and enhanced the corrosion resistance properties [23–25].

**Figure 5.** (**a**) Nyquist plots at different concentrations of GG-AMPS. (**b**) Bode, phase-angle plots at different concentrations of GG-AMPS. (**c**) Equivalent circuit used to fit and analyze the data.

In the bode plots (Figure 5b), the slope values tended to increase in the presence of GG-AMPS, rather than in its absence. This indicates the inhibition action of GG-AMPS on the copper surfaces. In the phase angle plots (Figure 5b), at the intermediate frequency, the pinnacle of the phase angle increased as the GG-AMPS concentration increased. The highest peak was observed at 43.2◦ for copper at a 600 mg/L inhibitor concentration. This was due to the corrosion mitigation on the copper surfaces by the GG-AMPS protective shield, which that isolated the copper from the corrosive media [26]. The binding of the GG-AMPS molecules with the metal surface was quite strong and stable, which finally enhanced its corrosion resistance quality. For the impedance data calculation, the circuit used is shown in Figure 5c. The circuit used was drawn using the model editor in Echem analyst. It contained charge-transfer resistance (*R*ct), a solution resistor (*R*s), film resistance (*R*f), and two constant phase elements (CPEs). The CPEs were included in the circuit for the perfect fitting of the Nyquist curves, as they balance the deviation of surface roughness, disruption, imperfectness, impurity, and adsorption [27–30].

The CPE impedance is given below:

$$Z\_{\rm CPE} = \mathcal{Y}\_{\rm o}^{-1} \left( i\omega \right)^{-n} \tag{5}$$

where *Y*0, ω, *i*, and *n* are the constant, angular frequency, an imaginary number, and an empirical exponent, respectively.

Table 1 shows the impedance values of the fitted curves. It can be seen that the *R*ct and *Y*<sup>0</sup> parameters at all of the concentrations of GG-AMPS display a reverse pattern. This process is credited to the GG-AMPS molecules adsorbing onto the copper surface, which finally enhances the copper corrosion resistance attributes [31]. The value of *R*ct at 600 mg/L for GG-AMPS is 905 kΩ·cm2. Thus, GG-AMPS provides a good resistance to corrosive solutions. The rise in *n* values with the increase in GG-AMPS concentration was due to their adsorption and finally enhancement of their homogeneity [32]. Thus, with a decent number of heteroatom functional groups, the corrosion inhibition property increased.

**Table 1.** Electrochemical impedance parameters in the absence and presence of different concentrations of GG-AMPS at 308 K.


X<sup>2</sup> refers to Chi square.

#### 3.2.2. Potentiodynamic Polarization Tests

The polarization plots of copper in NaCl solutions without and with different concentrations of GG-AMPS are represented in Figure 6. Several essential electrochemical factors, like the corrosion current density (*i*corr), corrosion potential (*E*corr), cathodic Tafel slope (βc), anodic Tafel slope (βc), and inhibition efficiency (η%), are tabulated in Table 2. The analysis of Table 2 suggests that the corrosion current density shifted from 98.7 μA/cm<sup>2</sup> (3.5% NaCl) to 4.9 μA/cm2 (600 mg/L GG-AMPS), and this represents that the GG-AMPS coating was effective for the mitigation of corrosion. The value of maximal efficiency as obtained was 95% at 600 mg/L. As can be seen, after the inclusion of GG-AMPS in the corrosive media, both the anodic and cathodic current density were reduced. The addition of GG-AMPS in higher concentrations caused more H<sup>+</sup> ion reduction in the system than the anodic dissolution process. As is evident in Figure 6, the collateral cathodic slopes suggest the conversion of H<sup>+</sup> to H2 was not varied. Similarly, with higher concentrations of GG-AMPS, the β<sup>c</sup> values were shifted, suggesting that GG-AMPS affected the kinetics of H2 evolution. This may be accredited to the diffusion or the shield phenomenon [33]. Likewise, the values of β<sup>a</sup> also underwent a modification with the increase in the GG-AMPS concentration, suggesting that the designed coating primarily underwent adsorption all over the copper surface. This phenomenon shows the mitigation of the corrosion reaction by obstructing the activated centers, without modifying the mechanism of the anodic process [34].

**Figure 6.** Potentiodynamic polarization curves in the absence and presence of different concentrations of GG-AMPS.

**Table 2.** Electrochemical polarization parameters in the absence and presence of different concentrations of GG-AMPS at 308 K.


In addition, the polarization curves showed that the addition of GG-AMPS mitigated both the cathodic and anodic processes. Therefore, the coating can be categorized into mixed forms. Nevertheless, the variations in the *E*corr values in the presence of the coating were towards the anodic route, as compared with those without the coating, indicating that GG-AMPS is predominantly cathodic.

#### *3.3. Scanning Electrochemical Microscopy (SECM)*

Scanning electrochemical microscopy is very useful for detecting localized corrosion on the metal surface. Figure 7a,b shows the 2D and 3D pictures of the *x*-axis for copper without GG-AMPS in a seawater solution. A very high current was observed in the 2D maps and 3D structures (Figure 7a,b). This can be endorsed by the straight connection of the probe with the copper surface. As the probe was moved at a certain visible corroded part on the metal surface observed through the camera, the corrosion profile was detected. However, for the copper surface with the GG-AMPS film on it, the 2D and 3D maps showed a lower current (Figure 7c,d). A lower current was observed in the presence of the GG-AMPS film, which may have been due to the GG-AMPS coating on the copper surface forming a hydrophobic film that repelled the corrosive solution. This phenomenon indicated that the GG-AMPS film protected the copper surface well from corrosion in the NaCl solution.

**Figure 7.** Scanning Electrochemical Microscopy (SECM) images of (**a**) 2D and (**b**) 3D copper surface in 3.5% NaCl solution and (**c**) 2D and (**d**) 3D copper surface coated with GG-AMPS in 3.5% NaCl solution.

#### *3.4. Scanning Electron Microscopy (SEM)*

The copper coupons with 600 mg/L GG-AMPS and 3.5% NaCl were exposed to SEM, as depicted in Figure 8. The surface morphology of the copper without GG-AMPS was very rough with several corrosion products, because of the rampant dissolution and deterioration of the metal (Figure 8a). Nevertheless, the addition of the GG-AMPS showed a smooth copper surface (Figure 8a). However, the surface showed a porous coating with abraded lines visible through them. This phenomenon suggested that, with GG-AMPS, the rate of corrosion was decreased due to the GG-AMPS coating having formed a conserving film over the copper surface.

**Figure 8.** SEM images of a (**a**) copper surface in a 3.5% NaCl solution, and a (**b**) copper surface coated with GG-AMPS in a 3.5% NaCl solution.

#### *3.5. Atomic Force Microscope (AFM)*

The 3D micro-structural pictures of the copper surface are displayed in Figure 9. The clear indication of the terrible deterioration of the copper surface without coating due to corrosion can be seen in Figure 9a,b. Nevertheless, the roughness of the copper surface was comparatively decreased with the addition of GG-AMPS (Figure 9c,d). This anti-corrosive nature of the coating is surely because of the good binding with the metal surface.

**Figure 9.** Optical image (**a**) copper surface in a 3.5% NaCl solution (**b**) copper surface coated with GG-AMPS in a 3.5% NaCl solution; AFM images (**c**) copper surface in a 3.5% NaCl solution (**d**) copper surface coated with GG-AMPS in a 3.5% NaCl solution.

#### **4. Conclusions**

The tested GG-AMPS coating is good for copper in a 3.5% NaCl solution. The experimental investigations suggest that the number of heteroatoms present in the GG-AMPS coating helps with the good binding to the copper surfaces, thereby reducing the corrosion rate. The SECM suggests that the inductive effect is because of the GG-AMPS coating. SEM and AFM suggest the potential corrosion mitigation of GG-AMPS coatings on copper surfaces in a 3.5% NaCl solution.

**Author Contributions:** Conceptualization, A.S. and M.L.; methodology, M.L. and E.I.; software, A.S.; validation, A.S. and Y.L.; formal analysis and investigation, A.S. and M.L.; resources, data curation and writing—original draft preparation, Y.L., A.S., M.L. and E.I.; writing—review and editing, visualization and supervision, A.S.; project administration and funding acquisition, A.S., Y.L. and E.I. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors are thankful to the Sichuan 1000 Talent Fund; the financial assistance provided by the Youth Scientific and Innovation Research Team for Advanced Surface Functional Materials, Southwest Petroleum University (No. 2018CXTD06); and the open fund project (No. X151517KCL42).

**Acknowledgments:** Authors would like to acknowledge the help provided by Mumtaz Ahmad Quraishi and Kashif Rahmani Ansari, KFUPM, Saudi Arabia. Authors also extend gratitude to Xu Xihua for her help in the experimental procedures.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## *Article* **Polyethylene Glycol (PEG) Modified Porous Ca5(PO4)2SiO4 Bioceramics: Structural, Morphologic and Bioactivity Analysis**

#### **Pawan Kumar 1,\*, Meenu Saini 1,\*, Vinod Kumar 2, Brijnandan S. Dehiya 1, Anil Sindhu 3, H. Fouad 4,5,\*, Naushad Ahmad 6, Amer Mahmood <sup>7</sup> and Mohamed Hashem <sup>8</sup>**


Received: 10 March 2020; Accepted: 29 May 2020; Published: 31 May 2020

**Abstract:** Bioceramics are class of biomaterials that are specially developed for application in tissue engineering and regenerative medicines. Sol-gel method used for producing bioactive and reactive bioceramic materials more than those synthesized by traditional methods. In the present research study, the effect of polyethylene glycol (PEG) on Ca5(PO4)2SiO4 (CPS) bioceramics was investigated. The addition of 5% and 10% PEG significantly affected the porosity and bioactivity of sol-gel derived Ca5(PO4)2SiO4. The morphology and physicochemical properties of pure and modified materials were evaluated using scanning electron microscopy (SEM), X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and Fourier-transform infrared spectroscopy (FTIR), respectively. The effect of PEG on the surface area and porosity of Ca5(PO4)2SiO4 was measured by Brunauer–Emmett–Teller (BET). The results obtained from XRD and FTIR studies confirmed the interactions between PEG and CPS. Due to the high concentration of PEG, the CPS-3 sample showed the largest-sized particle with an average of 200.53 μm. The porous structure of CPS-2 and CPS-3 revealed that they have a better ability to generate an appetite layer on the surface of the sample when immersed in simulated body fluid (SBF) for seven days. The generation of appetite layer showed the bioactive nature of CPS which makes it a suitable material for hard tissue engineering applications. The results have shown that the PEG-modified porous CPS could be a more effective material for drug delivery, implant coatings and other tissue engineering applications. The aim of this research work is to fabricate SBF treated and porous polyethylene glycol-modified Ca5(PO4)2SiO4 material. SBF treatment and porosity of material can provide a very useful target for bioactivity and drug delivery applications in the future.

**Keywords:** calcium phosphate silicate; PEG; bioceramics; sol-gel preparation; hard tissue engineering

#### **1. Introduction**

Ca5(PO4)2SiO4 bio-ceramic is a fully loaded compound consisting of Ca, P and Si elements [1]. The calcium–phosphate–silicate (CPS) ceramics are considered as biocompatible materials which is exclusively utilized as implantable materials for bone defects repairing [2]. Silicon-based bioceramics permit to form a functional silanol group with Si–O–H connectivity on the material surface [3]. The silanol group induces the formation of bone-like apatite by attracting Ca2<sup>+</sup> and PO4 <sup>3</sup><sup>−</sup> through an ion-exchange process in an artificial solution similar to blood plasma [4,5]. This apatite is broadly utilized biomaterial to repair and reconstruct hard tissue defects [6,7]. Thus, researches on silicon-containing bioceramics have received a considerable attention from the biomaterials scientists as such biomaterials are considered as bone substitute materials [8,9]. However, various drawbacks such as less mechanism properties, low chemical stability and reduced bioactivity, the clinical applications of CPS bioceramics are limited [10,11]. It was researched that for a reliable bone fixation, the porosity of implant material should be over 70% so that the body fluids can penetrate easily for the better bone growth [12,13]. In order to get a bioactive and porous material, sol-gel derived polyethylene glycol-modified CPS was prepared in which polyethylene glycol (PEG) was used to modify the phase of Ca5(PO4)2SiO4 via crosslinking of PEG diacrylate chain with PO4 <sup>3</sup><sup>−</sup> and Ca2<sup>+</sup> probably by the OH¯ exchange reactions [14,15]. PEG has been widely accepted as a phase change material, which can be altered through its congruent melting behavior and low vapor pressure [16]. Low molecular weight polyethylene glycol (PEG) is a highly non-immunogenic, non-toxic, biocompatible and biodegradable polymer [17,18]. It possesses a straight poly-ether diol chain that has hydroxyl groups and also shows covalent binding with proteins, phospholipids, functional groups, fluorescent probes, etc [19]. PEG will be a promising agent that may enhance the biocompatibility of Ca5(PO4)2SiO4 by generating porosity in the material. The use of larger PEG chains resulted in more agglomerated hollow particles [20]. Sol-gel process is a flexible and favorable method due to its low-temperature, high purity, easy doping and cost effective approach to prepare various nanocomposites and other nanobiomaterials [21,22]. The reaction time may affect the particle size and stability in the sol-gel process [23]. During the reaction monomers converted into colloidal solution (sol) and then into gel or integrated network of discrete particles [24,25]. Due to bioactive properties, the silica-based glass networks are prepared in various shapes, sizes and hence applied for variety of biomedical applications [26]. Sol-gel synthesized bioceramics are highly biocompatible with controlled degradation rate and effortlessly metabolized in the body [27,28].

In the current research work, we synthesized a pure phase of Ca5(PO4)2SiO4 and PEG-modified porous Ca5(PO4)2SiO4 bioceramic materials through the sol-gel method and investigated various properties. The highly bioactive and porous PEG-modified CPS can be utilized for tissue engineering and drug delivery.

#### **2. Materials and Methods**

#### *2.1. Synthesis of Porous Calcium Phosphate Silicate*

Sol-gel synthesis was convenient because it permits direct fabrication of bioceramics with different configurations. The synthesis of PEG-modified Ca5(PO4)2SiO4 was carried out as follows: 12.17 mL tetraethyl orthosilicate (TEOS, ≥98% Sigma Aldrich, Saint Louis, MO, USA), 150 mL ethanol, 0.80 g P2O5 (≥99.9% Sigma Aldrich) and 6.68 g Ca(NO3)2·4H2O (≥99.9% Sigma Aldrich) was mixed stepwise to get 680 mL homogenous mixture. To make composite, 5% and 10% *w*/*v* of PEG 400 were dispersed in 100 mL distilled water and mix with the above mentioned homogenous mixture. In acidic conditions, TEOS completely hydrolyzed and obtained Si(OH)4 which slow down the condensation rate [22]. Add ammonia (25%) solution to make pH 11 of the mixture which boosts the rate of the gelation process, ideal for the formation of smaller aggregates. TEOS works as a principal network forming agent during

gelation. The change in synthesis conditions or parameters such as pH, temperature and additives affect the silica-based glass networks which produce various shapes, sizes and formats products [26]. The different steps used for the synthesis of PEG-modified Ca5(PO4)2SiO4 are shown through the schematic diagram, see Figure 1. Naming of the samples was done on the basis of concentration variation of additive, i.e., PEG in Ca5(PO4)2SiO4; for pure Ca5(PO4)2SiO4 without PEG was assigned to CPS-1. Similarly, for PEG 5% ND 10% by weight in Ca5(PO4)2SiO4 was assigned CPS-2 and CPS-3, respectively. Further, these three samples were used for different characterization.

**Figure 1.** Synthesis of porous calcium phosphate silicate.

#### *2.2. Characterizations of Porous Calcium Phosphate Silicate*

The phase identification of the synthesized materials was investigated using XRD (Rigaku Ultima IV, Tokyo, Japan). The measurement was taken at 45 kV voltage and 40 mA anodic current. XRD patterns were acquired at a diffraction angle from 15◦ to 60◦. The compositional information of the prepared samples was investigated by FTIR (Perkin Elmer Frontier FTIR, MA, USA). First, the obtained powder was mixed with KBr in an appropriate ratio and then after applying pressure. The mixture was converted into pellets. For investigations of obtained spectra, the background spectra were calibrated with KBr. The morphology and elemental composition of the samples was examined using SEM (JEOL, JSM 6100, Akishima, Tokyo, Japan) and by energy dispersive spectroscopy (EDS) attached with same, respectively. On sputter was then to sputter coat the samples with a palladium layer. After 30 nm palladium coating, observations were done at an accelerating voltage of 20 kV and 10 Pa. The powerful size of the pore was figured as the mean distances across of sample pores. The nanoparticles synthesis confirmed through TEM (TECNAI 200 kV, Hillsboro, OR, USA) at SAIF in AIIMS, New Delhi. To provide contrast under magnification, nanoparticles were suspended in water (1 mg/mL), placed on copper grids of 0.037-mm size and then stained with a 2 g/100 mL uranyl acetate aqueous stain. Before viewing under 50,000 to 120,000 times magnification, surplus liquid on Mesh was wiped off with filter study and the grid was allowed to air dry. Observations were performed at 80 kV. Brunauer–Emmett–Teller (BET) (BELSORP mini II, Osaka, Japan) technique was used to measure the porosity and surface area of the prepared materials. Tris-HCl-buffered synthetic body fluid (SBF) was used to check bioactivity of the sample after 7 days at 37 ◦C in an incubator. The thermal stability of the prepared material was evaluated by the thermogravimetric analysis (TGA; Perkin Elmer STA 6000, Waltham, MA, USA) operated under nitrogen flow in the temperature range from 50 to 800 ◦C at 10 ◦C/min. Using TGA, by inducing heat to the sample, the chemical reactions and physical changes due to dehydration, decomposition and oxidation can be evaluated.

#### **3. Results and Discussion**

#### *3.1. Physiochemical Analysis of Calcium Phosphate Silicate Materials*

The sol-gel derived white powdered samples of CPS and PEG-modified CPS were heated at 450 ◦C to get phase or structural transformation. In the XRD spectra, the distinct peaks of the pure phase of CPS-1 were identified and matched with the standard database card number 00-901-1950 and PDF 40-0393 [29].

The XRD patterns of the PEG-modified CPS bioceramics showed modification in peaks [30]. The addition of 5% and 10% PEG in CPS make some changes in the sample, generate and demolish several phases or peaks in CPS-2 and CPS-3 (Figure 2A). The heat treatment (450 ◦C) to CPS-2 (5% PEG) and CPS-3 (10% PEG) removed the precursor residues of PEG, limiting the densification of material [31]. During the heat treatment, PEG started to decompose within a temperature range of around 250–300 ◦C; this can completely remove PEG from the sample [32,33]. The removal/degradation of PEG may generate a porous structure due to structural rearrangement by various chemical reactions, based on several treatment temperature and duration. The sintered sample of CPS-2 (5% PEG) and CPS-3 (10% PEG) also confirmed the presence of wollastonite [PDF 50–0905].

**Figure 2.** Typical (**A**) XRD pattern and (**B**) FTIR spectra of porous calcium phosphate silicate.

FTIR (Figure 2B) spectrum data revealed O–H stretching vibrations observed at 3500, 3560 and 3678 cm−<sup>1</sup> [34,35] and O–H deformation at 765 and 769 cm<sup>−</sup>1. The band occurred at 2860 cm−<sup>1</sup> due to the existence of the C–H group stretching in CPS-3 as a result of the presence of PEG [36]. The intense bands within 450–510 cm−<sup>1</sup> correspond to Si–O–, P–O, PO4 <sup>3</sup><sup>−</sup> and SiO4 <sup>4</sup>−, while the absorption bands at 1060 cm−<sup>1</sup> assigned to the vibration of the Si–O–Si [30,34]. The functional group SiO4 <sup>4</sup><sup>−</sup> was also recognized by nearly at 612 and 878 cm−<sup>1</sup> [30]. The absorption bands at 820 and 878 cm−<sup>1</sup> represent Si–CH3 and SiO4 <sup>4</sup><sup>−</sup> functional groups. The band between 1000–1200 cm−<sup>1</sup> is associated with the Si–O– stretching, while the band at 930 cm−<sup>1</sup> corresponds to Si–O– with one non-bridging oxygen [34]. The appearance of a medium stretching vibration at 1320 cm−<sup>1</sup> was because of the C=O group. The pure PEG sample showed bands at 992, 1315, 1417 and 1560 cm−<sup>1</sup> while PEG-modified sample showed bands at 1320 and 1365 cm–1, denoting CH3 and C–O stretching vibrations.

#### *3.2. Morphologic Characterizations of Calcium Phosphate Silicate Materials*

The high-resolution 2 D TEM images revealed the particle size estimation of PEG-modified CPS-1 (Figure 3A,B), CPS-2 (Figure 3C,D) and CPS-3 (Figure 3E,F) samples. Figure 3A shows a 506-nm-sized crystalline particle of CPS-1. CPS-2 (Figure 3C) revealed 265.4-nm-sized irregular particles while CPS-3 has not revealed any proper shape. Crystallite size is calculated using the Scherrer formula from XRD patterns for all synthesized samples. The calculated crystal size of CPS-1, CPS-2 and CPS-3 is 24, 18 and 15 nm, respectively. These calculated results are quite different from TEM results. Because the crystallite size determined using Scherrer formula from XRD patterns provided an average value of the bulk sample since the diffraction occurs from a considerable volume of the sample. Apart from that in TEM, we found the crystallite size from a very local area that may not be the representative size of the bulk sample. Removal of PEG at high temperatures during heat treatment may lead to a highly ordered porous structure, as observed in CPS-2 and CPS-3 (Figure 3). At the 200-nm-scale, all the samples showed significant structural differences; CPS-1 (0% PEG) looked like a dense material, while CPS-3 (10% PEG) displayed a better porous structure than the CPS-2 (5% PEG) sample.

**Figure 3.** Typical transmission electron microscopy (TEM) analysis of Ca5(PO4)2SiO4 (CPS)-1 (**A**,**B**), CPS-2 (**C**,**D**) and CPS-3 (**E**,**F**).

The increment of PEG concentration (more than 10% *w*/*v*) led to the densification of materials, which reduced the porosity of the CPS. The SEM results revealed the irregular micro size crystalline particles of CPS-1 (Figure 4A) and CPS-2 (Figure 4B) revealed amorphous particles. The SEM image of CPS-3 showed a porous microstructure, observed after the removal of PEG after heat treatment, see Figure 4C. The morphology of this specimen significantly display that will be beneficial for future applications, for instance tissue engineering and drug loading. The optimized favorite sample CPS-3 was used investigated through EDS analysis (Figure 4D) which showed the relative concentration of the Si, Ca, P and C elements in the synthesized sample. The results obtained from EDS analysis depend on several factors, to name a few, sample topography, beam parameters, field noises (electronic and

external fields), acquisition settings, detector type and atomic number of the elements. From particle size distribution histogram (Figure 4E), it is concluded that particle size of CPS-1 is to be under 10 μm, for CPS-2 it is estimated to be under 100 μm, while for CPS-3, it observed to be under 160 μm due to agglomeration of particles.

**Figure 4.** Typical SEM analysis of CPS-1 (**A**), CPS-2 (**B**), CPS-3 (**C**), energy dispersive spectroscopy (EDS) spectrum of CPS-3 (**D**) and particle size distribution histogram for CPS-1, CPS-2, CPS-3 (**E**).

#### *3.3. Porosity Measurements for Calcium Phosphate Silicate Materials*

The Brunauer, Emmett and Teller (BET) method is a promising technique to determine the surface area through physical adsorption–desorption isotherm analysis [37]. The porosity of a material depends upon the concentration and type or nature of the template used during the fabrication process [38]. The reparative bone formation and inflammatory response are also influenced by the morphologies of the materials [39,40]. From BET analysis (Figure 5A,B), CPS-3 revealed a surface area of 30.6672 m2·g<sup>−</sup>1, pore volume of 0.9722 cm3·g−<sup>1</sup> and pore diameter of 4.58 nm, while CPS-2 revealed a surface area of 27.9840 m2·g−1, pore volume of 0.6243 cm3·g−<sup>1</sup> and pore diameter of 2.84 nm. The addition of PEG affects the microstructure as well as the porosity of the CPS. The BET result revealed that the

CPS-3 (10% PEG) has shown better porosity that will be a key factor in application part. At 450 ◦C, the decomposition of PEG generated the porous network. The bio factors include genes or cells, proteins and nutrients can easily exchange through the porous structure of materials. Porosity and pore size possess high impact on the application of the material [3,41].

**Figure 5.** Adsorption–desorption isotherm and Barrett, Joyner, and Halenda (BJH) plot of CPS-3 (**A**) and CPS-2 (**B**).

#### *3.4. Bioactivity Analysis*

SBF provides the same environment as blood plasma, where surface dissolution starts mineralization through the slow release of ions in the solvent [33]. When materials were immersed in the SBF solution, a set of reactions such as ion exchange, precipitation and dissolution occurred for the apatite formation [42]. The SBF treatment generated two new groups, CO3 <sup>2</sup><sup>−</sup> and PO4 <sup>3</sup>−, at 1420 and 1480 cm<sup>−</sup>1, which are due to the absorbance of CO2 (Figure 6D). The generation of new compounds are directed by both the immersion parameters (immersion time, temperature and pH, temperature) and surface characteristics of the materials [43]. The presence of CO3 <sup>2</sup><sup>−</sup> and PO4 <sup>3</sup><sup>−</sup> groups are associated with apatite formation on the surface of the sample after a seven-day immersion [30,44]. Further, CO3 <sup>2</sup><sup>−</sup> and PO4 <sup>3</sup><sup>−</sup> ions contribute to nucleation and subsequent surface mineralization that leads to actual apatite formation [45,46]. The in vitro apatite-forming ability of material often successfully predicts the actual bioactivity of biomaterials [47]. The bands at 1653 and 1470 cm−<sup>1</sup> denoted CO3 2−, while peaks at 1314 and 2790 cm−<sup>1</sup> were attributed to C–H group. The peaks appearing at 561 and 880 cm−<sup>1</sup> are due to bending mode of O–P–O and variable symmetry of HPO4 <sup>3</sup>−, respectively [48]. The band at 1025 cm−<sup>1</sup> was assigned to the presence of the PO4 <sup>3</sup><sup>−</sup> group [49,50]. The surface of the material released Ca2<sup>+</sup>, HPO4 <sup>2</sup><sup>−</sup> and PO4 <sup>3</sup><sup>−</sup> ions and absorbed calcium and phosphate ions from SBF. The incorporation of other electrolytes, such as CO3 <sup>2</sup><sup>−</sup> and Mg2<sup>+</sup> ions, started to generate the apatite layer [51]. The mineralization behavior of CPS-1, CPS-2 and CPS-3 was shown in Figure 6A–C and also confirmed through FTIR results, as shown in Figure 6D.

**Figure 6.** SEM analysis of synthetic body fluid (SBF)-treated CPS-1 (**A**), CPS-2 (**B**), CPS-3 (**C**) and FTIR analysis (**D**).

#### *3.5. Thermal Gravimetric Analysis*

The physical and morphologic transformations of the PEG-modified CPS samples analyzed through thermal gravimetric analysis. It is well known that the chemical structures are altered by heat treatment as it leads to the thermal decomposition of the materials. TGA results presented in Figure 7, weight loss in CPS-1 and CPS-2 divided into three main steps (S-1, S-2 and S-3): removal of –OH groups, polymeric phase (PEG) and burnout of the CPS mass (Ca, P and Si) while CPS-3 revealed weight loss in two stages. In all the samples, the initial weight loss was confirmed because of the release of absorbed moisture contents. CPS-2 and CPS-3 showed more weight loss than CPS-1 that was because of the PEG thermal degradation within a temperature range of around 250–300 ◦C. CPS-3 fabricated with 10% PEG but thermal decomposition of the organic groups, generated porous microstructure and microporous materials showed large specific surface that support more degradation at high temperature.

**Figure 7.** Thermos-gravimetric analysis (TGA) analysis of CPS-1, CPS-2 and CPS-3.

#### **4. Conclusions**

In summary, the effect of PEG on various properties of CPS was investigated. The use of PEG improves the morphology, physiology and bioactivity of CPS. Porosity and bioactivity of sol-gel-derived samples were greatly influenced by varying the concentration of PEG. The heat treatment at 450 ◦C plays an important role in the phase modification and porosity generation. This could be attributed, as the concentration of PEG increased, the densification and agglomeration in particles were observed. The formation of the apatite layer on the surface of SBF treated CPS exposed mineralization. PEG-modified Ca5(PO4)2SiO4 demonstrated better in vitro bioactivity than pure CPS, by tempting bone-like apatite in the artificial salt solution SBF. PEG-modified Ca5(PO4)2SiO4 bioceramic (CPS-3) is different from those in conventional material and may be a promising material for implant coatings, drug loading and bone regeneration applications. Future works should determine the optimum concentration for controlled porosity and their applications in soft as well as hard tissue engineering.

**Author Contributions:** Conception and design of the experiments, P.K., M.S., V.K., B.S.D., A.S., H.F., N.A., A.M. and M.H. implementation of the experiments, analysis of the data, the contribution of the analysis tools and writing the study. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors would like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for funding this research group (No. RGP-1435-052).

**Acknowledgments:** The authors would like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University, Kingdom of Saudi Arabia for support.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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