اصلاح سطحی نانوذرات هیدروکسی آپاتیت فلوئورید دوپ شده با منیزیم با استفاده از اسید آمینه های زیست فعال به عنوان عامل جفت کننده برای کاربردهای زیست پزشکی Surface modification of Mg-doped fluoridated hydroxyapatite nanoparticles using bioactive amino acids as the coupling agent for biomedical applications

۱٫ Introduction Hydroxyapatite(HA) bioactive ceramic with the chemical composition of Ca10(PO4)6(OH)2 has been extensively utilized for orthopedic, dental and maxillofacial applications due to its similarity to the mineral phase of bone and tooth. Incorporation of fluorine into the apatite structure to form fluoridated hydroxyapatite [FHA:Ca10(PO4)6OH2
xFx, where x represents the degree of fluoridation] improves the physical and biological properties of HA. At present, the HA partially substituted by fluorine has received considerable attention, particularly for clinical bone growth. Moreover, the presence of Mg ions instead of Ca ions in FHA structure (Mg-FHA) promotes its bioactivity and osteoconductivity characteristics. The Mg-FHA provides superior biocompatibility and biological properties compared to the unmodified FHA [1,2]. Besides, the biodegradability of the synthesized FA or FHA is controlled through the optimized substitution of Mg2þ into the FHA structure [3]. Recently, polymeric matrix composites have come into spotlight in bone tissue engineering applications owing to their superior biological and mechanical properties as compared to ceramics. In addition, the development of polymer/ceramic composites simulates the structure of natural bone since it is composed of HA in collagen matrix [1,4]. The bioactivity properties can be further improved using bioceramic nanoparticles (NPs) since the natural bone is also composed of nanostructured HA [1,5]. However, bioceramic NPs tend to be agglomerated in the polymeric matrix as a result of their high surface area and incompatible surface polarity with polymers. It is well-established that the surface compatibility of ceramics with the polymeric matrix signifi- cantly affects its biological properties such as protein adsorption and subsequent cellular attachment and proliferation on the structures [5,6]. In order to strengthen the interfacial bonding between the mentioned phases, understanding the surface and interfacial chemistry of nanobioceramic in a polymer matrix is very important. In this respect, the surface modifications of the NPs have been performed using the surface-active agent, the coupling agent, fatty acid and ethanol [5,7–۱۱]. Amino acids are known as the cost-effective biomaterials with suitable biodegradability, biocompatibility, nontoxicity and eco-friendly properties, making them ideal candidates for tissue engineering applications. Moreover, the best solvent for amino acids is water as a nontoxic, natural, environmentally friendly and inexpensive solution [12–۱۴]. Due to the presence of OH
groups on the surface of MgFHA NPs, they considerably tend to be agglomerated in a polymeric matrix. In addition, these superficial OH
groups cause the Mg-FHA NPs to be linked to amino acid molecules. As a result of the chemical bonding between superficial OH
groups of Mg-FHA NPs and the amino acid molecules, the surface of Mg-FHA NPs is changed from a hydrophilic state to a hydrophobic one, leading to the uniform distribution of particles in an organic solution. Thus, the amino acids can be considered as appropriate choices for the surface modification of nanobioceramics, especially in a polymeric matrix [12–۱۵]. In this study, the surface of Mg-FHA NPs was chemically modified by different natural bioactive amino acids to improve NPs dispersion in the polymeric matrix. The modified NPs were characterized by Fourier transmission infrared spectroscopy (FT-IR), dispersion stability, X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) as well as thermogravimetry analysis (TGA). ۲٫ Experimental method ۲٫۱٫ Surface modification of Mg-FHA NPs Mg-FHA NPs were prepared using the sol–gel technique according to a previous research by Biomaterials Research Group lab [1]. Briefly, the starting materials were P2O5 (99.9%), Mg (NO3)2 6H2O (99.9%) and Ca(NO3)2 4H2O (99.9%). Appropriate amounts of the mentioned materials with the stoichiometric composition of Ca9.5Mg0.5(PO4)6(OH)F were independently dissolved in absolute ethanol. These solutions were added drop wise to each other to obtain a solution with a (Ca, Mg)/P ratio of 1.67. The final mixture was continuously stirred for ۲۴ h at ambient temperature to form a gel. As-formed gel was aged for 24 h at ambient temperature and dried in an oven at 100 1C in air for another 24 h. The dried gel was sintered with a heating rate of 5 1C/min up to 650 1C for 1 h in a muffle furnace. The natural amino acids, including L-leucine, L-isoleucine, L-methionine, L-phenylalanine, L-tyrosine and L-valine, were utilized without further purification. The aforementioned amino acids with the concentration of 10 wt% were dissolved in 20 mL of distilled water at room temperature. The prepared Mg-FHA NPs were dried at 120 1C for 24 h to remove the adsorbed water. Then, the Mg-FHA NPs (1.0 g) were added into the ethanol solution containing the amino acid, and stirred at room temperature for 24 h. The prepared mixture was placed into an ultrasonic bath for 30 min. Finally, the obtained suspension was filtered and washed by ethanol several times to eliminate unreacted amino acid molecules, and the remaining powder was subsequently dried at 60 1C for 24 h. The schematic diagram of the surface modification process for MgFHA NPs is presented in Fig. 1.