The Formation Process of Hydroxyapatite Nanoparticles by Electrolysis and Their Physical Characteristics

The electrolysis method for synthesizing hydroxyapatite nanoparticles (NPs) has the advantage of controlling the particle size by adjusting the potential and current used. This study aims to study the electrolysis of hydroxyapatite NPs formation and its characteristics. The solution contained Na₂H₂EDTA.2H₂O, KH₂PO₄, and CaCl₂, with an EDTA/PO₄^3-/Ca²⁺ concentration of 0.2/0.2/0.2 M. The electrolytic potential are 4, 5, and 6 volts for 6 hours. The carbon electrode spacing used is 2 cm. The precipitate formed is filtered with a vacuum jet ejector. Retentate was washed with demineralized water and dried in an oven at 105^oC. The synthesis of pure hydroxyapatite by electrolysis was successfully carried out at a potential of 5 volts. The OH^- ion, which comes from the H₂O reduction process at the cathode, is essential in the formation of brushite, which then forms hydroxyapatite. The hydroxyapatite, synthesized at a potential of 4 volts, had the smallest particle size (442.4 nm) with the largest particle surface area (417.22 m²/gram).

Bioceramics; Electrolysis; Hydroxyapatite; Nanoparticles

Hydroxyapatite (HA) particles are a biomaterial having a chemical formula of Ca₁₀(PO₄)₆(OH)₂. HA has good biocompatibility and bioactivity properties. The structure of HA approximates the structure possessed by bones and teeth (Pietrzykowska et al., 2021; Tomozawa and Hiromoto, 2011; Suchanek and Yoshimura, 1998). HA can bind directly to tissue and stimulate tissue growth. Therefore, HA has the potential to be applied in the biomedical field, especially for bone and dental applications (Zhou and Lee, 2011).

HA belongs to the bioceramic type. In the medical world, ceramic materials are divided into two groups, namely bioinert ceramics and bioactive ceramics. Bioinert ceramics have no effect and interact with body tissues (Panda, Biswas, and Paul, 2021). Meanwhile, bioactive ceramics can bind to living bone tissue, such as HA and calcium phosphate. HA can be used in a variety of biomedical applications, including matrices for drug release control (Kamitakahara, Imai, and Ioku, 2013), scaffolds for new bone formation (Rezwan et al., 2006), and fillers and coatings for repairing osseous damage (Zhou and Lee, 2011; Banerjee,  Bandyopadhyay, and Bose, 2007). In the biomedical field, nano-sized HA particles have better bioactivity than micro-particles (Dorozhkin, 2010).

Various methods have been developed to synthesize HA NPs, including precipitation, hydrothermal, mechano-chemical, flame spray, and electrolysis (Co?rdova-Udaeta et al., 2021; Lin et al., 2017; Martins et al., 2008; Fathi and Hanifi, 2007; Chang and Tanaka, 2002). Djosic et al. (2009) have succeeded in synthesizing monetite nanoparticles electrochemically and transforming them into hydroxyapatite by immersion in NaOH solution. However, no research has been conducted on the conditions affecting the electrochemical process to produce hydroxyapatite directly (one step) (Djosic et al., 2009). Electrolysis is a method that offers an easy and straightforward process (Corona-Gomez,  Chen, and Yang, 2016). The particle diameter can be controlled by adjusting the voltage or current during electrolysis (Djosic et al., 2009). These results were obtained from tests with relatively high solution pH and current density. Theoretically, the higher the pH and current density will accelerate the particle formation reaction and encourage particle agglomeration (Kim, Kim, and Hirasawa, 2002). If the pH and current density are relatively high, the pH and current density no longer affect the particle size but are more influenced by the number of reactants available and the synthesis time. At relatively low current densities, these two parameters may have an effect (Montero et al., 2006). Therefore, this research focuses on studying the operating conditions, namely the potential for synthesizing hydroxyapatite NPs. 

2.1. Materials and Instrumentations

   The raw materials used in this study include Na₂H₂EDTA.2H₂O (Merck), KH₂PO₄ (Merck), CaCl₂.7H₂O (Merck), and commercial hydroxyapatite (Merck). While the instrumentation used includes a DC power supply (GPD X303S, GW Instek), Particle Size Analyzer (Cilas 1190), Surface Area Analyzer (Quantachrome NOVA 1200), and Powder X-ray Diffraction (PANalytical X’Pert3 Powder).  

2.2.  Synthesis of Hydroxyapatite NPs Powder

3.1. Synthesis of Hydroxyapatite NPs

    Figure 2 shows the synthesized white powder's X-ray diffraction pattern (XRD). The synthesized white powder shows a tendency to form hydroxyapatite NPs which is consistent with the peak position at an angle of 2with JCPDS 03-0747, hexagonal, a = b = 9.4302, c = 6.88911 Å, space group P63/m. Referring to the reaction, the formation of hydroxyapatite NPs occurs reversibly; the synthesis of hydroxyapatite NPs requires an optimal reaction equilibrium (Nur et al., 2014). If the electrolysis voltage during the hydroxyapatite NPs formation process is set to the correct value, then the reaction tends to lead to the formation of hydroxyapatite NPs.

    The white powder synthesized from bulk solution at potential 4 Volt has a mixture composition of brushite and hydroxyapatite NPs. The lack of energy used resulted in converting the brushite to hydroxyapatite NPs. The white powder synthesized from bulk solution at a potential of 5 Volts has a pure hydroxyapatite NPs composition. Meanwhile, the white powder which was synthesized from bulk solution at a potential of 6 volts had a dominant composition of hydroxyapatite NPs with a small amount of brushite. This is because the reaction to form hydroxyapatite from brushite is a back-and-forth reaction (Tripathi & Basu, 2012). If the hydroxyapatite NPs still contain brushite, it will reduce their bioactivity when applied medically (Dorozhkin, 2010). The hydroxyapatite formed also has a tendency with the amorphous phase.

Figure 2 The X-ray diffraction pattern of the synthesized white powder by electrolysis

3.2. Reaction Mechanism for the Formation of Hydroxyapatite NPs

After the reduction of water at the cathode producing OH^- ions. The produced OH^- ions induce an acid-base reaction to form PO₄^3- and HPO₄^2- with the following reactions 2 and 3,

                        

This reaction was expected to affect the composition of calcium phosphate ultimately determining whether the apatite phase is in the form of HA or brushite (Montero et al., 2006). This reaction is greatly influenced by the OH- ions' formation rate. If the OH^- ion formation rate is slower than the minimum required amount, HPO₄^2- becomes stable and tends to form brushite (CaHPO₄.2H₂O). When the rate of OH^- formation is faster than the minimum required amount of OH^-, the PO₄^3- becomes more stable than HPO₄^2-, which encourages the formation of HA (Montero et al., 2006). Calcium phosphate precipitates are formed with all the reactants needed for the formation of Ca-P bonds (Ca²⁺, PO₄^3-, HPO₄^2-, and OH^-) with the following reaction 4,

Figure 3 Schematic diagram of the reaction to the electrolytic formation of hydroxyapatite

Figure 4 Photo of color change with time of electrolysis of hydroxyapatite NPs by electrolysis

3.3. Distribution of Hydroxyapatite Nanoparticles Diameter

Figure 5 Size distribution of as-synthesized HA NPs by electrolysis and commercial HA

Figure 6 Schematic diagram of the behavior of hydrogen gas formed on the particle size of hydroxyapatite

H₂ gas in the bulk solution causes an outward pushing force through the cracks of the hydroxyapatite NPs, causing the electrolytic synthesized particles to have a smaller size. The white powder synthesized by electrolysis at a potential of 6 Volt has a more heterogeneous particle distribution than the particles synthesized at 4 and 5 Volts. Meanwhile, the distribution of commercial hydroxyapatite particles tends to have a large distribution range. This is due to particle agglomeration, which causes a larger range of particle size distribution.

3.4. The Surface Area of the Synthesized Particles

The surface area of the HA NPs showed that the hydroxyapatite particles synthesized at the 4 Volt potential had a smaller surface area than those synthesized at the 5 Volt and 6 Volt potentials, which had almost the same surface area. The particles synthesized with low potential have better porosity than those synthesized at high potential. This is because the pores structure of  HA NPs synthesized at the high potential has a tendency to collapse, so that the pore area tends to be small (Hong et al., 2013). Whereas commercial hydroxyapatite particles have a smaller surface area than synthetic hydroxyapatite at a potential of 4 Volts and greater than synthetic hydroxyapatite at 5 Volts and 6 Volts.

Table 1 The surface area of the synthesized particles by electrolysis and commercial hydroxyapatite

Synthesis of pure hydroxyapatite (100% hydroxyapatite) by electrolysis was successfully carried out at a potential of 5 volts. The OH^- ion, which comes from the H₂O reduction process at the cathode, plays a critical role in the formation of brushite forming hydroxyapatite. The hydroxyapatite, synthesized at a potential of 4 volts, had the smallest particle size (442.4 nm) with the largest particle surface area (417.22 m²/gram). Further research needs to examine the effect of the synthesis time of HA NPs, which is longer than 6 hours with a voltage of 4 Volts to determine the most effective time variable in the synthesis of hydroxyapatite.

    This work was supported by HPP (HIBAH PENELITI PEMULA) 2020 through grant Number 436.74/UN10.C10/PN/2020.

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