Materials Science and Engineering B 197 (2015) 18–24 Contents lists available at ScienceDirect Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb Luminescence variations in europium-doped silicon-substituted hydroxyapatite nanobiophosphor via three different methods Cao Xuan Thang, Vuong-Hung Pham ∗ Advanced Institute for Science and Technology (AIST), Hanoi University of Science and Technology (HUST), No 01, Dai Co Viet Road, Hanoi, Viet Nam a r t i c l e i n f o Article history: Received 30 October 2014 Received in revised form 15 February 2015 Accepted 25 February 2015 Available online 26 March 2015 Keywords: Nanosilicon Luminescence Hydroxyapatite Europium Nanophosphors Nanobiophosphors a b s t r a c t This paper reports the first attempt for the synthesis of europium-doped Si-substituted hydroxyapatite (HA) nanostructure to achieve strong and stable luminescence of nanobiophosphor, particularly, by addition of different Eu dopants, Si substitutions, and application of optimum annealing temperatures of up to 1000 ◦ C The nanobiophosphor was synthesized by the coprecipitation, microwave, and hydrothermal methods The nanoparticles demonstrated a nanowire to a spindle-like morphology, which was dependent on the method of synthesis The photoluminescence (PL) intensity of the sample increases with the increase in Si substitutions and Eu dopants The luminescent nanoparticles also showed the typical luminescence of Eu3+ centered at 610 nm, which was more efficient for the annealed Eu-doped Si-HA nanoparticles than for the as-synthesized nanoparticles Among the different synthesis methods, the hydrothermal method reveals the best light emission represented by high PL intensity and narrow PL spectra These results suggest the potential application of Eu-doped Si-HA in stable and biocompatible nanophosphors for light emission and nanomedicine © 2015 Elsevier B.V All rights reserved Introduction The development of new materials and synthesis techniques for improving the light emission performances and long-term stability of devices is one of the most active research areas in optoelectronic industries and nanomedicine [1,2] Fundamentally, the light emission ability of materials is strongly affected by their host matrices, activators, defects, and crystallinity of materials [3,4] This encouraged scientists and engineers to explore new methods of designing materials by tailoring the physical and chemical characteristics of materials, for example, by doping with strong and stable light emission materials and designing the suitable host materials for stimulating energy transfer from the host matrix to the activator [5,6] In order to enhance the light emission performance of the device and its lifetime for agriculture products and nanomedicine, it is necessary to combine the advantages of biocompatible properties with luminescence [7,8] As biocompatible materials, silicon (Si) and hydroxyapatite (HA) have received considerable attention as host materials in the design of luminescent materials [9,10] Introduction of Si to HA resulted in substitution of phosphate ion (PO4 3− ) ∗ Corresponding author Tel.: +84 36230435; fax: +84 43 6230 293 E-mail address: vuong.phamhung@hust.edu.vn (V.-H Pham) http://dx.doi.org/10.1016/j.mseb.2015.02.014 0921-5107/© 2015 Elsevier B.V All rights reserved by silicate ion (SiO4 4− ) in the HA crystal structure, and siliconsubstituted HA (Si-HA) has become a subject of great attention in biomedical research [11] Similarly, europium is a suitable activator for doping into calcium-based materials because they exhibit significant advantages compared with available phosphor such as lower toxicities, photostabilities, high thermal and chemical stabilities, high luminescence quantum yield, and sharp emission band [12,13] The microwave and hydrothermal methods, being powerful wet chemistry synthesis methods, have been widely used to synthesize nanostructured particles such as semiconductors and metal oxide for improving surface characteristic, grain size, and crystallinity as well as performance of materials [14,15] These techniques have been introduced to biomedical engineering for the synthesis of nanorod, nanospider-like particles, which demonstrated positive effects on the in vitro biocompatibility of HA [16,17] Although the physicochemical properties of HA and Si-HA are well documented, thus far, only a few papers have reported on the luminescence of europium-doped HA [18,19], and, to the best of our knowledge, no attempts have been made to synthesize Eu-doped Si-HA with a well-crystalline structure by the microwave and hydrothermal methods, which would open up new avenues for designing strong and stable light emission for agriculture and nanomedicine Recently, Eu-doped silicon-substituted Si-HA was synthesized successfully in our laboratory by the coprecipitation method [20] In C.X Thang, V.-H Pham / Materials Science and Engineering B 197 (2015) 18–24 that research, we investigated the effect of thermal annealing on the luminescence of the Eu-doped Si-HA To expand this research, we herein report the first attempt to synthesize Eu-doped Si-HA by means of wet chemistry synthesis methods such as the coprecipitation, microwave, and hydrothermal methods The effect of the silicon substitutions, europium dopants, and annealing temperature as well as the synthesis method on the light emission of Eu-doped Si-HA was closely examined The crystalline structures of the Eu-doped Si-HA were characterized by X-ray diffraction (XRD) The microstructure of the Eu-doped Si-HA was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively Chemical bonding of the pure HA, Si-HA, and Eu-doped Si-HA was determined by infrared (IR) absorption spectroscopy The luminescence was also determined using a photoluminescence (PL) spectrometer Experimental procedure Eu-doped Si-HA was synthesized through a wet chemical synthesis method, as follows: 10.620 g of Ca(NO3 )2 ·4H2 O (99% purity, Aldrich, Saint Louis, MO, USA) and 5.625 g of (NH4 )2 HPO4 (99% purity, Aldrich, Saint Louis, MO, USA) were dissolved in 100 mL of distilled water (DW) and the concentrations of the solutions were fixed in all sets of experiments For studying the effect of Si concentration, a fixed amount of 0.7 g of Eu(NO3 )3 dissolved in 50 mL of DW was mixed with the abovementioned Ca(NO3 )2 ·4H2 O at 25 ◦ C for 60 to form solution A Varying amounts (2, 4, and g) of Si(OCH2 CH3 )4 (tetraethyl orthosilicate, TEOS; 99% purity; Merck, Darmstadt, Germany) were diluted in 50 mL of DW and then mixed with the abovementioned solution of (NH4 )2 HPO4 at 25 ◦ C for 60 to form solution B Finally, solution B was added at an addition rate of 20 mL min−1 into solution A at 80 ◦ C After the addition, the reaction mixture was further stirred for 0.5 h at 80 ◦ C and the pH was adjusted to 11 by using an aqueous ammonia solution (Duc Giang Chemicals, Hanoi, Vietnam) For comparison purposes, un-doped HA was also prepared with that mentioned above without addition of TEOS or Eu(NO3 )3 at 80 ◦ C and pH 11 The sample with different Si substitutions was prepared according to the above procedure with a fixed amount of Eu(NO3 )3 (0.7 g) and varying amounts of TEOS (2, 4, and g), hereafter designated 0.7Eu:2Si-HA, 0.7Eu:4Si-HA, and 0.7Eu:6Si-HA Similarly, the sample with different Eu dopants was prepared with a fixed amount of TEOS (4 g) and varying amounts of Eu(NO3 )3 (0, 0.5, 0.7, and g), hereafter designated 0.5Eu:4Si-HA, 0.7Eu:4Si-HA, and Eu:4Si-HA For microwave synthesis, the abovementioned solutions A and B were mixed, and their pH was maintained at by addition of ammonia solution at a temperature of 25 ◦ C After the reaction mixture was stirred for 0.5 h at a temperature of 25 ◦ C, the mixture was subjected to microwave apparatus (LG MS3840SR; 80 W; LG Microwave; Seoul, Korea) for 30 For hydrothermal synthesis, the reaction solution prepared in the same manner at pH and at a temperature of 25 ◦ C was transferred into a 200-mL Teflon-lined autoclave, and then the autoclave was sealed and maintained at 150 ◦ C for 12 h The resulting precipitates were washed thrice, and then dried at 100 ◦ C for h A fraction of each sample was treated at 1000 ◦ C with the heating rate of 6◦ min−1 for h in air The crystalline structures of the Eu-doped Si-HA were characterized by XRD (D8 Advance, Bruker, Bremen, Germany) The microstructure of the Eu-doped Si-HA was determined using a field emission scanning electron microscope (JSM-6700F, JEOL Ltd., Tokyo, Japan) and a transition electron microscope (JEOL, JEM 1010, JEOL Techniques, Tokyo, Japan), respectively To investigate the chemical bonding of the Eu-doped Si-HA, IR absorption spectra were recorded in the wave-number range from 4000 to 400 cm−1 with a Perkin-Elmer Spectrum BX spectrometer using KBr pellets 19 PL tests were performed to evaluate the optical properties of the Eu-doped Si-HA A NANO LOG spectrofluorometer (Horiba, Edison, NJ, USA) equipped with a 450-W Xe arc lamp and double-excitation monochromators was used The PL spectra were recorded automatically during the measurements Results and discussion 3.1 Phase characterization Fig shows the XRD diagram of the as-synthesized HA and Eu-doped Si-HA prepared by the coprecipitation, microwave and hydrothermal methods and their thermal annealing samples The coprecipitation specimen showed peaks matching the standard patterns of Ca10 (PO4 )6 (OH)2 , calcium HA (PDF 01-084-1998) On the other hand, a mixture of HA and -TCP (PDF 09-0169) was observed in the microwave and hydrothermal specimen (Fig 1(B) and (C)) When thermal annealing was used, all of the three XRD patterns showed a mixture of HA and -TCP with good crystallinity (Fig 1(D)–(F)) This suggests that the phase characteristic of Eudoped Si-HA can be controlled by changing the synthesis method or by applying thermal annealing It is can also be seen that the XRD diagrams obtained for all of the Eu-doped Si-HA samples not reveal the presence of any phases related to silicon and other europium species, suggesting the successful preparation of europium-doped Si-HA Based on specific application, controlling the phase composition of calcium phosphate is of particular interest The mixture of HA and -TCP found on the specimens may suggest a better performance in the fields of optoelectronics and nanomedicine because of the strong luminescence, high resorbability properties for -TCP, and good osteoconductivity for HA [20–22] Fig shows the XRD diagram of the as-synthesized Eu-doped SiHA prepared by coprecipitation with different TEOS and Eu(NO3 )3 concentrations All of the specimens showed a typical pattern of HA (Fig 2(A)–(E)) This suggests that the phase characteristic of HA remains stable in this study on introduction of different TEOS and Eu(NO3 )3 concentrations in the reaction solution Fig XRD patterns of (A) as-synthesized HA and (B–G) 0.7Eu:4Si-HA specimens, (A–D) before and (E–G) after thermal annealing at 1000 ◦ C Preparation methods: (A, B, and E) coprecipitation, (C, F) microwave, (D, G) hydrothermal (*HA and # -TCP) 20 C.X Thang, V.-H Pham / Materials Science and Engineering B 197 (2015) 18–24 Fig XRD patterns of the Eu-doped Si-HA prepared by the coprecipitation method with different TEOS and Eu(NO3 )3 concentrations: (A) 0.7Eu:2Si-HA, (B) 0.7Eu:4SiHA, (C) 0.7Eu:6Si-HA, (D) 0.5Eu:4Si-HA, and (E) Eu:4Si-HA (*HA) 3.2 Scanning electron analysis The microstructural variations in Eu-doped Si-HA were examined by SEM as shown in Fig 3(A)–(F) It can be seen that the Eu-doped Si-HA nanoparticles have a wire shape and their aspect ratios drop when the TEOS concentration increases in the 0.7Eu:2Si-HA, 0.7Eu:4Si-HA, and 0.7Eu:6Si-HA samples, respectively (Fig 3(A)–(C)) The reduction in the aspect ratios of Si-HA with increase in TEOS concentration observed in the present and other studies can be explained in terms of the higher nucleation density during the precipitation process [23] However, the effect of Eu(NO3 )3 concentration on the morphology of Eu-doped Si-HA was much stronger compared to that of the TEOS concentration The sample prepared at low Eu(NO3 )3 concentration (the 0.5Eu:4SiHA sample) consists of well-dispersed nanowires (Fig 3(D)) When the Eu(NO3 )3 concentration increased (the Eu:4Si-HA sample), the synthesized particle assumed a rod-like shape with a particle size of about 30 nm and a length of