Article pubs.acs.org/crystal
Sonosynthesis of Vaterite-Type Calcium Carbonate Judith A. Juhasz-Bortuzzo,* Barbara Myszka, Raquel Silva, and Aldo R. Boccaccini* Institute of Biomaterials, Department of Materials Science and Engineering, University of Erlangen-Nürnberg, 91058 Erlangen, Germany ABSTRACT: Metastable nanoparticles of vaterite were formed using a simple ultrasound technique. The effects of ultrasound amplitude and duration, as well as solution concentration, were investigated. The produced particles were characterized using standard X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning and transmission electron microscopies (SEM and TEM, respectively). As expected, the ultrasound synthesis process caused the particle size to be smaller than with conventional (magnetic bar) stirring, and allowed for the reaction time to be shortened as the crystallization rate was increased. As shown by XRD, FTIR, and SEM, the ultrasound process also led to the formation of pure vaterite, though the presence of calcite occurred as the ultrasound power was reduced and the pulse time increased. For both conventional and ultrasound techniques, the particle size was reduced with an increase in starting solution concentration. Using this technique allows for reproducible, tailored calcium carbonate particles for controlled drug delivery, as well as for use in composites for soft tissue repair; in particular, when used as the filler during electrospinning and melt electrospinning.
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INTRODUCTION Ultrasound-assisted crystallization or sonochemical crystallization is the process of nuclei formation as a result of a cavitation process which occurs at a faster rate and leads to a reduction in the growth of crystals.1−3 During sonocrystallization, there is an increased number of nuclei, and a change in the primary nucleation growth rate, both of which lead to an effect on particle shape, size, and distribution (uniformity). Sonocrystallization is a facile process for controlling particle size on the nanoscale, as well as for producing a monodispersion of uniform particles. This technique has been used to synthesis calcium carbonate particles by numerous researchers.2,3 Calcium carbonate exists in three anhydrous crystal phases,4 these being calcite, which is the most thermodynamically stable, aragonite, and finally vaterite; both are precursors for calcite.5−8 Also in existence are three forms of amorphous calcium carbonate (disordered, hydrated amorphous calcium carbonate (ACC), less disordered, hydrated ACC and anhydrous ACC).4 The solubility of vaterite is higher than that of either calcite or aragonite and, due to Oswald ripening, is usually the first metastable polymorph to form, acting as the precursor for the other two polymorphs.4,9 Vaterite possesses some excellent preferred features for medical applications such as higher specific surface area, solubility, and dispersion, and smaller specific gravity, when compared with calcite and aragonite.10 In medicine, vaterite has already been used for regenerative medical applications, such as the formation of composite scaffolds, and as the carrier material for drug delivery.11−13 Control of the crystallization process allows for vaterite synthesis in favor of other polymorphs. The process will inevitably play an important role in the properties of the final particles synthesized. The precipitation of vaterite can be © XXXX American Chemical Society
achieved via two routes, namely, the reaction between two salt solutions or a gas−liquid system whereby carbon dioxide gas is bubbled into a calcium hydroxide solution.14−18 Numerous groups have looked into the facilitating factors that preferentially lead to the formation of vaterite in both salt solution reactions and gas−liquid syntheses. These include supersaturation with respect to calcium carbonate leading to a greater number of nuclei19−26 and which allow for the formation of well-defined crystal structures in the nano- and micrometer ranges. The morphology and polymorph-type of calcium carbonate can be controlled by the inclusion of additives, such as magnesium ions, polymers, and proteins, during the synthesis process.5−9,17,18,27−41 The morphology and particle size can also be controlled by other experimental approaches such as the use of ultrasound.42−47 For vaterite, the most common morphology is the spherical shape.14,15,21,48 Many others, such as fried-egg shape, flower-like shape, and hexagonal flake shape, can also be observed.16−18,29,49−51 Varying the ultrasound treatment parameters has been performed by other research groups in order to observe the effect on polymorph type and properties.14−18,42−47,50,52,53 However, the broad range of parameters evaluated in this study, with the specific aim of producing pure and stable vaterite nanoparticles for use as a drug carrier in electrospun and melt electrospun composite fibers have, to the best of the authors’ knowledge, not been evaluated before. Hence, the effect of ultrasound on vaterite formation with controlled particle size is explored, demonstrating the ability to control the particle size Received: October 10, 2016 Revised: February 28, 2017 Published: March 28, 2017 A
DOI: 10.1021/acs.cgd.6b01493 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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of vaterite for its use in medical applications, and offering further insight into how this mineralization system can be controlled.
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EXPERIMENTAL SECTION
Vaterite Synthesis. All chemicals used in this study were analytical grade reagents. First, sodium bicarbonate and calcium acetate monohydrate, both of >99% purity and purchased from SigmaAldrich, were dissolved in deionized water at varying concentrations (0.1, 0.5, and 1.0 M), with a calcium/carbonate molar ratio of 1.0. After the complete dissolution of the chemical substances, the solutions were kept for 1 h at 4 °C. Equivolumetric quantities (10 mL) of these aqueous solutions were then mixed, allowing for the precipitation of a white solid. This mixing was performed with and without ultrasound treatment for various times (from 10 s to 10 min), keeping the processing temperature below 30 °C to encourage maximal vaterite formation.24,54 NB: For the highest amplitude (70%), experiments were not performed for longer periods due to damage caused to the ultrasonic probe and poor temperature control during extended synthesis times. The ultrasound process was performed using a Branson 250 fitted with a 1-cm-diameter titanium horn operating at 20 kHz using 10%, 30%, 50%, and 70% ultrasound amplitude. All testing was performed using a pulsed treatment of 1 s on and 0.2 s off. Post-sonication, the samples were immediately centrifuged at 7000 rpm for 5 min and washed three times with deionized water before being dried at room temperature and under vacuum. Conventional stirring (without ultrasound, on a magnetic plate) was performed using an average speed of 500 rpm, keeping the solution temperatures below 30 °C, as for the ultrasound process. For all experiments, the pH was confirmed as being alkaline (HI-98140, Hannah Instruments) to encourage the formation of vaterite in preference to other polymorphs of calcium carbonate.30,53 Calcium Carbonate Powder Analysis. Several analytical techniques were used to characterize the precipitated powders. X-ray Diffraction. Phase identification and average crystallite size of the resulting powders was carried out using powder X-ray diffraction (XRD, Bruker AXSD8). A Cu Kα source was used over a 2θ range of 20° to 60° and a step size of 0.02° with dwell time of 0.05 s was applied during the analyses. A Rietveld refinement was performed on the powder diffraction patterns to semiquantitatively ascertain the mass percentage of vaterite to calcite present. Fourier Transform IR Spectroscopy. Fourier transmission infrared spectroscopy (ATR-FTIR, Nicolet 6700, Thermo Scientific) was performed on uniaxially pressed powder pellets mixed with KBr. The FTIR analyses were carried out in the 4000−400 cm−1 range with a resolution of 4 cm−1 and with 32 spectral scan repeats for each sample. Scanning and Transmission Electron Spectroscopy. The particle size and morphology were observed using scanning electron microscopy (SEM, Zeiss Auriga) and transmission electron microscopy (TEM, Phillips CM30). Powder samples for SEM were uncoated and observed at a working distance of 3.5 mm and an accelerating voltage of 0.7 kV. The vaterite average particle size was measured by analysis of the SEM images using ImageJ, measuring at least 200−300 particles for each sample. The TEM samples were prepared by dispersing the powders in ethanol and depositing onto a standard holey carbon TEM grid. TEM images were collected using 300 kV with minimized electron current density to avoid ionization.
Figure 1. XRD traces showing the presence of vaterite (*) (major phase) and calcite (■) (minor phase) in the 1.0 M solutions ultrasonically treated at 70% amplitude for 10 s (a), 30 s (b), 1 min (c), and 2 min (d).
Figure 2. XRD traces showing the presence of vaterite (*) (major phase) and calcite (■) (minor phase) in the 1.0 M solutions ultrasonically treated at 10% amplitude for 10 s (a), 30 s (b), 1 min (c), and 2 min (d).
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RESULTS AND DISCUSSION Investigating the effect of ultrasound on vaterite formation involved performing precipitation reactions at different ultrasound amplitudes (10%, 30%, 50%, and 70%) for different lengths of time ranging from 10 s to 10 min. As mentioned previously, experiments were not performed for longer periods at 70% ultrasound amplitude due to damage caused to the ultrasonic probe and poor temperature control during synthesis with extended testing times.
Figure 3. Rietveld refinement for 1.0 M samples showing the percentage calcite and vaterite content when ultrasonically treated at 10% and 70% power amplitude.
The XRD analysis indicated that the calcium carbonate powders consisted predominantly of the vaterite polymorph B
DOI: 10.1021/acs.cgd.6b01493 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 4. FTIR spectra for 0.1 M (a, b) and 1.0 M (c, d) samples (the relevant peaks are discussed in the text).
Figure 5. SEM micrographs of vaterite and calcite particles formed using ultrasound for 1 min (standard magnetic stirring results included for comparison; scale bars = 2 μm). Figure 6. SEM micrographs of particles precipitated using 1 M starting solutions (standard magnetic stirring results included for comparison; scale bars = 2 μm).
(PDF No. 33-0268) with small amounts of calcite (PDF No. 1086-2339) when ultrasound was applied for prolonged times and with higher amplitude. Figure 1 shows the XRD traces for the powders at 70% amplitude and 1.0 M concentration. The traces showed the powders to be mainly vaterite with small amounts of calcite which decreased with prolonged ultrasound treatment. Previously, it has been reported that increasing the addition rate of the sodium stock solution would reduce the preferred formation of vaterite.22 However, in this study and with the use of ultrasound, the fast addition of the sodium solution had no effect on the proportion of vaterite formed.
Decreasing the percentage amplitude (power) led to increased amounts of calcite being present in the XRD traces (Figure 2). The most intense diffraction peaks for calcite, corresponding to the (104), (110), and (113) sets of planes and located at 29.4°, 35.9°, and 39.5°,47,55 decreased, along with the peaks at higher angles attributed to calcite. Concurrently, the diffraction peaks for vaterite with the most intense being located at 24.92°, 26.99°, and 32.78°,55 and related to the (110), (112), and (114) reflections, respectively, indicated the major phase to be C
DOI: 10.1021/acs.cgd.6b01493 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 7. Average particle size in terms of different percentage amplitudes; 10% (a), 30% (b), 50% (c), 70% (d), and at different solution concentrations; 0.1 M (e), 0.5 M (f), 1.0 M (g) over time (mins) (MS = magnetic stirring/without ultrasound).
Table 1. Summary of Key Findings for 0.1 and 1.0 M Stock Solution Samples When Varying the Ultrasound Amplitude (a) and Ultrasound Duration (b) 0.1 M (a) amplitude
10%
70%
(b) duration
10 s
30 s
1 min
2 min
10 s
30 s
1 min
2 min
purity (% vaterite) particle size (μm)
26 0.84 ± 0.11
52 0.74 ± 0.15
75 0.67 ± 0.17
88 0.67 ± 0.19 1.0 M
49 8.05 ± 2.87
78 2.46 ± 0.62
91 0.38 ± 0.10
94 0.40 ± 0.11
(a) amplitude
10%
70%
(b) duration
10 s
30 s
1 min
2 min
10 s
30 s
1 min
2 min
purity (% vaterite) particle size (μm)
55 4.65 ± 1.05
94 3.85 ± 0.96
95 3.60 ± 0.79
98 1.84 ± 0.45
95 1.23 ± 0.51
97 0.45 ± 0.12
100 0.35 ± 0.10
100 0.37 ± 0.11
The findings of the XRD analyses were further supported by FTIR analyses (Figure 4) which allowed the presence of calcite and vaterite to be identified by the stretching and planar deformation of the CO32− bonds in the samples at the vibrational bands of 1420, 874, and 712 cm−1 for calcite and 1070 and 745 cm−1 for vaterite. For the 1.0 M samples at 50% amplitude, there was no significant difference between the traces of samples tested at all time-points and no calcite was detectable for any of the samples. However, for the 0.1 M samples, the presence of calcite was more prominent in all except the 1 and 2 min samples. An increase in ultrasound time, from 10 s to 2 min, led to a decrease in particle size for all concentrations and amplitudes. After this time at 5 and 10 min, no significant change in particle size was noted. However, in the current protocol, as the time increased, it was difficult to maintain the pure vaterite polymorph, particularly at higher ultrasound amplitudes, leading to a transformation to calcite.24,43,54,56,57 As noted by Kirboga et al.,47 it could be possible to better control spherical
Figure 8. SEM micrographs showing the effect of CTAB (a), EG (b), and CTAB+EG (c) on the morphology, surface topography, and particle size of vaterite (scale bars = 1 μm).
vaterite, which increased in intensity with increased sonicator amplitude.30,47 Performing a Rietveld refinement clearly demonstrated the increased presence of calcite with reduced ultrasound time and intensity (Figure 3). D
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average particle size. Though, a bimodal distribution could also be witnessed in some of the higher concentration solutions. The application of an ice bath to control the temperature caused the crystal growth to be slower, resulting in smaller crystal size but with higher efficiency than without temperature control. The ultrasound power, being applied in short pulses, increased reaction efficiency and permitted less heat to be introduced into the system. However, in some instances, changes in vaterite crystal morphology occurred (spherulitic to plate-like structure), as well as the formation of agglomerates which would be undesirable for drug delivery and electrospinning of polymer reinforced composites. The agglomerates were most likely due to the ultrasound pulse effect causing high velocity collisions to take place as the stop−start effect of pulsation caused rapid particle movements to occur within the solution. Using stabilizers such as ethylene glycol (EG) and cetyltrimethylammonium bromide (CTAB) would allow the nucleation and growth of the vaterite particles to be controlled and deter the formation of agglomerates (Figure 8 and refs 57,61), as well as increase surface porosity to be introduced which could further increase their potential for drug release. As Figure 8 shows, with the use of CTAB and EG, the vaterite polymorph is the only phase present after sonosynthesis. However, with CTAB alone, the particles are no longer nanosized (undesirable for electrospinning applications) but are nonagglomerated. With the use of EG and a combined system of CTAB and EG, the particles remain nanosized and demonstrate a high surface porosity (beneficial for drug uptake) as well as a reduced number of agglomerates when compared to the water-only system (Figures 5 and 6). Figure 9 shows the high resolution TEM images of vaterite particles produced after 1 min using 70% amplitude and 1 M starting solutions. The polycrystalline nature of the vaterite particle is indicated in the diffraction in Figure 9c. Since there are discontinuous circles in the diffraction pattern with areas of increased intensity, it can be assumed that the crystallites have symmetry and are preferentially aligned within the vaterite particle, most likely caused by crystallites in the (110) and (114) planes.53 The surfaces of the vaterite particles (Figure 9a,b) can be seen to be textured and irregular, indicating spherulitic growth (Figure 9d).49
Figure 9. TEM images of vaterite particles (a−c) showing the nanocrystalline structure in the same orientation (c). SEM micrograph showing cross section (d).
vaterite formation under ultrasound conditions, by reducing the depth at which the ultrasound probe is inserted into the liquid. The average crystal size was decreased when using ultrasound, which has also been noted by other researchers.32,45,47,58,59 However, the particle size achieved in this study is significantly smaller and occurred within shorter periods of time. The decrease in particle size taking place due to the increased energy being put into the system has an effect on crystallization. The growth of the vaterite particles was noted as being via a spherulitic-type crystal growth and not by aggregation of nanosized precursor crystals, since the inner surface of the vaterite particles could be seen to exhibit a radiating pattern (Figure 8) which has been found to be characteristic of spherulitic crystal growth.48,49 The increase in crystallization and reduction in particle size occurred more abundantly in solutions with higher concentration of calcium and carbonate ions, as also noted by other researchers.50,59,60 Obviously, the volume of liquid and diameter of the container in which the synthesis was performed also played a critical role in the resultant morphology and size of the synthesized vaterite particles. SEM analysis indicated that the vaterite particles had a tendency to form fine-sized, spherical shapes (Figures 5 and 6). Microscopy observations, as well as particle size analyses (Figures 5−7, Table 1) indicated that the lowest average particle size of 354 ± 99 nm could be achieved when using 1 M stock solutions of sodium bicarbonate and calcium acetate monohydrate and applying the highest ultrasound amplitude of 70% for a 1 min pulsed treatment. The results indicated that the nucleation rate of the vaterite crystals increased with higher concentrations, and was unaffected by the rate of addition of sodium to calcium stock solutions. At lower concentrations of stock solution, a reduced ultrasound amplitude was sufficient for the reaction to be completed and for smaller particles to form. However, at higher concentrations and at the same amplitude, the power was insufficient to prevent particle growth, leading to a larger
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CONCLUSION Pulsed sonosynthesis has been successfully used to prepare nanopowders of calcium carbonate. A pure and stable vaterite structure was attained and confirmed by XRD, FTIR, and SEM, which can be used as a nanocarrier for the efficient delivery of drugs or proteins in medicine and as the filler in electrospun biocomposites. With increased ultrasound power and time, the particle size was reduced until the reaction was complete. The present work on vaterite precipitation in various conditions revealed that there is a significant influence of the operating conditions, reagent concentration, and additives on the final particle size distribution and polymorphic composition of the resultant powders. Decreasing temperature, using a short ultrasound pulse duration, and high ultrasound intensity led to a reduction in vaterite crystal size. The parameters were optimized in order to maximize the nanoparticles production yield and to obtain the lowest size of particles to be able to electrospin and melt electrospin nanocomposites for medical applications. The optimal, smallest average particle size was achieved with the 1.0 M stock solutions, and with a 70% ultrasound amplitude for a time of 1 min. The particles E
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produced are candidate bioceramics for a variety of biomedical applications, including use as drug delivery vehicles.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Judith A. Juhasz-Bortuzzo: 0000-0003-1327-0301 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the Marie Curie IntraEuropean Fellowship (FP7-PEOPLE-2012-IEF). The authors would like to thank Ms Samira Tansaz for assistance with FTIR measurements and Ms Anahi ́ Philippart for the TEM images.
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REFERENCES
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