Sono-Synthesis of Nano-Hydroxyapatite

Hydroxyapatite (HA or HAp) is a highly frequented bioactive ceramic for medical purposes due to its similar structure to bone material. The ultrasonically assisted synthesis (sono-synthesis) of hydroxyapatite is a successful technique to produce nanostructured HAp at highest quality standards. The ultrasonic route allows to produce nano-crystalline HAp as well as modified particles, e.g. core-shell nanospheres, and composites.

sintesi sonora

An ultrasonic treatment of the slurry results in very fine particles with even distribution so that more nucleation sites for precipitation are created.
HAp particles synthesized under ultrasonication show a decreased level of agglomeration. The lower tendency to agglomeration of ultrasonically synthesized HAp was confirmed e.g. by FESEM (Field Emission Scanning Electron Microscopy) analysis of Poinern et al. (2009).

Ultrasound assists and promotes chemical reactions by ultrasonic cavitation and its physical effects that directly influence particle morphology during the growth phase. The main benefits of ultrasonication resulting the preparation of superfine reaction mixtures are

• 1) increased reaction speed,
• 2) decreased processing time
• 3) an overall improvement in the efficient use of energy.

Poinern et al. (2011) developed a wet-chemical route that uses calcium nitrate tetrahydrate (Ca[NO3]2 · 4H2O) and potassium dihydrogen phosphate (KH2PO4) as main reactants. For control of the pH value during the synthesis, ammonium hydroxide (NH4OH) was added.
The ultrasound processor was an UP50H (50 W, 30 kHz, MS7 Sonotrode w/ 7 mm diameter) from Hielscher Ultrasonics.

Ultrasonically reduced and dispersed calcium-hydroxyapatite

Steps of nano-HAP synthesis:

A 40 mL solution of 0.32M Ca(NO₃)₂ · 4H₂O was prepared in a small beaker. The solution pH was then adjusted to 9.0 with approximately 2.5mL NH₄OH. The solution was sonicated with the UP50H al 100% di ampiezza per 1 ora.
At the end of the first hour a 60 mL solution of 0.19M [KH₂OP₄] was then slowly added dropwise into the first solution while undergoing a second hour of ultrasonic irradiation. During the mixing process, the pH value was checked and maintained at 9 while the Ca/P ratio was maintained at 1.67. The solution was then filtered using centrifugation (~2000 g), after which the resultant white precipitate was proportioned into a number of samples for heat treatment.
The presence of ultrasound in the synthesis procedure prior to the thermal treatment has a significant influence in forming the initial nano-HAP particle precursors. This is due to the particle size being related to nucleation and the growth pattern of the material, which in turn is related to the degree of super saturation within the liquid phase.
In addition, both the particle size and its morphology can be directly influenced during this synthesis process. The effect of increasing the ultrasound power from 0 to 50W showed that it was possible to decrease the particle size prior to thermal treatment.
The increasing ultrasound power used to irradiate the liquid indicated that greater numbers of bubbles/cavitations were being produced. This in turn produced more nucleation sites and as a result the particles formed around these sites are smaller. Furthermore, particles exposed to longer periods of ultrasonic irradiation show less agglomeration. Subsequent FESEM data has confirmed the reduced particle agglomeration when ultrasound is used during the synthesis process.
Nano-HAp particles in the nanometer size range and spherical morphology were produced using a wet chemical precipitation technique in the presence of ultrasound. It was found that the crystalline structure and morphology of the resulting nano-HAP powders was dependent upon the power of the ultrasonic irradiation source and the subsequent thermal treatment used. It was evident that the presence of ultrasound in the synthesis process promoted the chemical reactions and physical effects that subsequently produced the ultrafine nano- HAp powders after thermal treatment.

HAp Synthesis via Ultrasonic Sol-Gel Route

Ultrasonically assisted sol-gel route for the synthesis of nanostructured HAp particles:
Material:
– reactants: Calcium nitrate Ca(NO₃)₂, di-ammonium hydrogen phosphate (NH₄)₂HPO₄, Sodium hydroxyd NaOH ;
– 25 ml test tube

1. Dissolve Ca(NO₃)₂ and (NH₄)₂HPO₄ in distilled water (molar ratio calcium to phosphorous: 1.67)
2. Add some NaOH to the solution to keep its pH around 10.
3. Ultrasonic treatment with an UP100H (sonotrodo MS10, ampiezza 100%)

• The hydrothermal syntheses were conducted at 150°C for 24 h in an electric oven.
• After the reaction, crystalline HAp can be harvested by centrifugation and washing with deionized water.
• Analysis of the obtained HAp nanopowder by microscopy (SEM, TEM,) and/ or spectroscopy (FT-IR). The synthesized HAp nanoparticles show high crystallinity. Different morphology can be observed depending on the sonication time. Longer sonication can lead to uniform HAp nanorods with a high aspect ratio and ultra-high crystallinity. [cp. Manafi et al. 2008]

Modification of HAp

A causa della sua fragilità, l'applicazione dell'HAp puro è limitata. Nella ricerca sui materiali, sono stati compiuti molti sforzi per modificare l'HAp con i polimeri, dal momento che l'osso naturale è un composito costituito principalmente da cristalli di HAp di dimensioni nanometriche, simili ad aghi (rappresenta circa il 65% dell'osso). La modifica dell'HAp assistita dagli ultrasuoni e la sintesi di compositi con caratteristiche migliori del materiale offrono molteplici possibilità (si vedano alcuni esempi di seguito).

Practical Examples:

Silver Coated HAp

Ignatev and co-workers (2013) developed a biosynthetic method where silver nanoparticles (AgNp) were deposited on HAp to obtain a HAp coating with antibacterial properties and to decrease the cytotoxic effect. For the deagglomeration of the silver nanoparticles and for their sedimentation on the hydroxyapatite, a Hielscher UP400S was used.

A setup of magnetic stirrer and ultrasonicator UP400S was used for the silver-coated Hap preparation [Ignatev et al 2013]

Our powerful ultrasonic devices are reliable tools to treat particles in the sub micron- and nano-sized range. Whether you want to synthesize, disperse or functionalize particles in small tubes for research purpose or you need to treat high volumes of nano-powder slurries for commercial production – Hielscher offers the suitable ultrasonicator for your requirements!