Synthesis of calcium carbonate microspheres via inert gas bubbling for orthopedic applications

Calcium carbonate (CaCO3) microspheres consisting of vaterite polymorph have been widely used in biomedical applications. Specifically, vaterite microspheres having hollow cores showed significant potential in drug de- livery, however the spontaneous
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  Contents lists available at ScienceDirect Ceramics International  journal homepage: www.elsevier.com/locate/ceramint Synthesis of calcium carbonate microspheres via inert gas bubbling fororthopedic applications Ça ğ atay M. Oral a , Arda Çal ış kan b , Ya ğ mur Göçtü a , Derya Kapusuz c , Batur Ercan a,d,e, ∗ a  Metallurgical and Materials Engineering Department, Middle East Technical University, Ankara, 06800, Turkey  b Chemical Engineering Department, Middle East Technical University, Ankara, 06800, Turkey  c  Metallurgical and Materials Engineering Department, Gaziantep University, Gaziantep, 27310, Turkey  d  Biomedical Engineering Program, Middle East Technical University, Ankara, 06800, Turkey  e  Biomaten, Metu Center of Excellence in Biomaterials and Tissue Engineering, Ankara, 06800, Turkey  A R T I C L E I N F O  Keywords: Calcium carbonatePolymorphMicrosphereOrthopedics A B S T R A C T Calcium carbonate (CaCO 3 ) microspheres consisting of vaterite polymorph have been widely used in biomedicalapplications. Speci 󿬁 cally, vaterite microspheres having hollow cores showed signi 󿬁 cant potential in drug de-livery, however the spontaneous transformation of vaterite to other polymorphs in aqueous environments re-duced its controlled  in vivo  release capability. In this work, calcite and aragonite microspheres having hollow/porous inner cores were synthesized -for the  󿬁 rst time-using sodium dodecyl sulfate (SDS) stabilized nitrogen(N 2 ) bubbles as CaCO 3  template in ethylene glycol (EG) solution and water as the precipitation medium. Resultsdemonstrated that porous aragonite microspheres could be synthesized via N 2  gas incorporation, yet for thesynthesis of hollow calcite microspheres, N 2  bubbles had be stabilized with SDS to be utilized as CaCO 3  tem-plates. The synthesized aragonite and calcite microspheres were found to be stable up to 5 days in Dulbecco'sModi 󿬁 ed Eagle's Medium (DMEM), and thus would not allow polymorphic transformation in aqueous en-vironments, while promoting proliferation of human bone cells (hFOB) up to 5 days of culture. These  󿬁 ndings-for the  󿬁 rst time-identi 󿬁 ed a viable synthesis route for hollow/porous calcite and aragonite microspheres andindicated their promising use in orthopedic applications. 1. Introduction Calcium carbonate (CaCO 3 ) is one of the most abundant minerals onearth. It naturally occurs in skeletons of various marine creatures, in-cluding seashells and otoliths [1]. CaCO 3  particles can be fabricatedsynthetically to have di ff  erent morphological and polymorphic char-acteristics for industrial applications [2]. Speci 󿬁 cally, there are threeanhydrous polymorphs of CaCO 3 , namely vaterite (hexagonal), arago-nite (orthorhombic) and calcite (rhombohedral) [3]. The stability of these polymorphs in aqueous environments di ff  er, where vaterite is themost soluble polymorph of CaCO 3 , followed by aragonite [4]. Calcite isthe least soluble (most stable) polymorph of CaCO 3  in aqueous en-vironments, and therefore vaterite and aragonite tend to transform tocalcite spontaneously in aqueous environments via dissolution-re-crystallization reactions [5]. The spontaneous polymorphic transfor-mation of CaCO 3  particles makes it challenging to control their stabilityand require detailed investigations on the in 󿬂 uence of several processparameters on CaCO 3  precipitation [6]. Various studies showed thattemperature and the use of additives were the most e ff  ective para-meters on the obtained polymorph and morphology of CaCO 3  particles[6]. For instance, Chen et al. [7] synthesized phase-pure rhombohedral calcite particles at 90°C, while incorporation of ethylene glycol (EG) upto 50% (without altering synthesis temperature) induced crystallizationof almost phase-pure ellipsoidal vaterite particles. Yan et al. [8] syn-thesized rod-like aragonite particles at 60°C without incorporating anysurfactant or EG into the aqueous precursor solution, however  󿬂 ower-like vaterite particles were crystallized by addition of only PluronicF127 and sodium dodecyl sulfate (SDS) inside the precursor solution.Among various CaCO 3  morphologies, CaCO 3  spheres exhibited sig-ni 󿬁 cant potential for biomedical applications, and thus their synthesisroutes captured the attention of various research groups [6]. Geng et al.[9] synthesized vaterite and calcite spheres via incorporation of poly-styrene sulfonate (PSS) as an additive using gas-liquid di ff  usionmethod. Ahmed et al. [4] utilized micro-emulsion method, where ce-tyltrimethylammonium bromide (CTAB) and 1-butanol were used assurfactants to promote precipitation of vaterite spheres. On the other https://doi.org/10.1016/j.ceramint.2019.10.066Received 10 August 2019; Received in revised form 7 October 2019; Accepted 7 October 2019 ∗ Corresponding author. Metallurgical and Materials Engineering Department, Middle East Technical University, Ankara, 06800, Turkey.  E-mail address:  baercan@metu.edu.tr (B. Ercan). Ceramics International xxx (xxxx) xxx–xxx0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Please cite this article as: Çağatay M. Oral, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.066  hand, Zhang et al. [10] used simple mixing approach where water/ethanol solution in the presence of PSS was mixed with precursors toobtain vaterite spheres.Aside from particle synthesis studies, the use of CaCO 3  spheres indental, orthopedic and drug delivery applications were also reported[6]. For instance, CaCO 3  spheres could transform to hydroxyapatitecompounds, the naturally occurring component of bone and dentin,upon interacting with simulated body  󿬂 uid (SBF)  in vitro  and revealedenhanced bioactivity for bone-tissue engineering [11]. Similarly, Liet al. [12] observed that CaCO 3 /casein spheres could enhance deposi-tion of hydroxyapatite compounds in SBF solution. In another study,Zhong et al. [13] used CaCO 3  spheres as a secondary phase within so-dium alginate to obtain a degradable composite structure. Upon injec-tion of this composite into subcutaneous tissue of mice, improved tissue 󿬁 ber penetration and vessel ingrowth were observed at 3 months of implantation [13]. Once CaCO 3  spheres were synthesized to havehollow cores, they provided distinct advantages,  i.e.  high speci 󿬁 c sur-face area, low bulk density and high permeability, over the con-ventionally-used ceramic based microparticles [14]. For instance, Weiet al. [15] used hollow vaterite spheres to deliver anticancer drugdoxorubicin (DOX) to liver carcinoma cells. Almost no toxic e ff  ect wasobserved without DOX, yet death of cancer cells soared upon in-corporation of DOX into the hollow particles [15]. In addition, CaCO 3 spheres with hollow cores could allow encapsulation of biomolecules, i.e . bovine serum albumin and DNA, for sustained release in body  󿬂 uids[16].Due to the potential use of hollow CaCO 3  spheres in biological ap-plications, sound synthesis methods to advance the properties of hollowCaCO 3  spheres are required. Several studies investigated fabrication of hollow CaCO 3  spheres by synergistically coupling the use of additiveswith a suitable temperature adjustment during particle synthesis. Chenet al. [17] obtained hollow vaterite spheres via incorporation of ureainto water/ethanol mixture and holding the solution temperature at90°C for 24h. Yan et al. [18] showed that hollow calcite spheres couldbe fabricated using an aqueous mixture of Pluronic F127 and SDS at30°C. Water-in-oil-in-water method was also utilized to precipitatehollow vaterite spheres at 30°C, where Tween 85 was used as non-anionic surfactant in the presence of n-hexane [19]. Cumulatively, all of these studies point to either high temperatures or high concentration of surfactants/additives to crystallize CaCO 3  spheres having hollow cores.Until recently, the synthesized hollow CaCO 3  particles were mostlyvaterite, the most soluble anhydrous polymorph of CaCO 3 . As statedpreviously, vaterite particles dissolve in aqueous conditions and spon-taneously transform to more stable polymorphs. This polymorphictransformation induces dramatic changes in CaCO 3  particle size andmorphology, and lead to premature loss of the hollow core inside theparticle before completing its intended use. Precipitation of more stableCaCO 3  polymorphs, while maintaining their spherical and hollowcharacteristics, could be a potential remedy for the aforementionedproblem. For this purpose, nitrogen (N 2 ) bubbles stabilized by SDS wereused -for the  󿬁 rst time-in this study as templates for CaCO 3  precipita-tion. 2. Experimental methods  2.1. Materials Calcium acetate (Ca(CH 3 CO 2 ) 2 ), sodium bicarbonate (NaHCO 3 ),sodium dodecyl sulfate (SDS; CH 3 (CH 2 ) 11 OSO 3 Na), ethylene glycol (EG;(CH 2 OH) 2 ) and ethanol (C 2 H 5 OH) were purchased from Sigma Aldrich(St. Louis, Missouri) and used as-received. Nitrogen (N 2 ;  ≥ 99%) wasobtained from Linde (Kocaeli, Turkey). Ultrapure water prepared with aMillipore Milli-Q puri 󿬁 cation system (Burlington, Massachusetts) wasused in all experiments.  2.2. Particle synthesis CaCO 3  particles were precipitated in an aqueous solution usingpreviously established protocols [20,21]. All experiments were per- formed at two di ff  erent temperatures (25°C and 90°C) using 0% and80% EG concentrations. Aqueous calcium acetate (0.3M) and sodiumbicarbonate (0.9M) precursors were prepared separately and EG wasadded into these solutions to obtain a total volume of 25mL for eachsolution. Afterwards, 0.576g SDS was added into the sodium bicarbo-nate solution. While holding the temperature constant at 25°C or 90°C,N 2  bubbles were introduced (6L/min) into the sodium bicarbonatesolution, followed by incorporation of calcium acetate solution. After15min of N 2  bubbling, the solution was transferred to a fresh tube andaged for 4h at the designated temperature. To collect the precipitatedCaCO 3  particles, the aged solution was centrifuged, followed bywashing with ethanol and ultrapure water, respectively. The synthe-sized powder was dried at 50°C.To di ff  erentiate the e ff  ects of N 2  and SDS addition, 4 di ff  erent sets of CaCO 3  particles were synthesized using the identical protocol withminor changes. For the  󿬁 rst set, neither SDS nor N 2  bubbling was in-troduced (referred as  ‘ No Addition ’ ); for the second set, only SDS wasintroduced (referred as  ‘ SDS ’ ), for the third set, only N 2  bubbling wasperformed (referred as  ‘ N 2  Bubble ’ ) and for the last set, both SDS and N 2 bubbling were introduced (referred as  ‘ SDS and N 2  Bubble ’ . For theexperiments without N 2  bubbling, solutions were mixed under mag-netic agitation at 500rpm.  2.3. Scanning Electron Microscopy (SEM) FEI Nova Nano SEM 430 microscope (Brno, Czech Republic) wasused to characterize CaCO 3  particle morphologies. 20kV acceleratingvoltage was chosen to image the surfaces. Prior to SEM characteriza-tion, Quorum SC7640 high-resolution sputter coater (East Sussex,United Kingdom) was used to coat a thin gold layer on CaCO 3  particlesto create a conductive electrical path.  2.4. X-Ray Di  ff  raction (XRD) CaCO 3  polymorphs were identi 󿬁 ed using Rigaku D/Max-2200 X-raydi ff  ractometer (Tokyo, Japan) with monochromatic Cu K α  radiation( λ =1.54Å) at 40kV. Di ff  raction angles (2 θ ) from 20° to 60° werescanned at a scanning rate of 2°/min. Rietveld re 󿬁 nement analysis wasperformed on the di ff  raction patterns of powders using GSAS-II soft-ware [22]. In addition, average crystallite size of CaCO 3  powders werecalculated using Scherrer equation (d=k× λ / β ×cos θ ) where d re-presented average crystallite size, k was the shape factor,  λ  was thewavelength of the X-ray,  β  was the broadening at half maximum in-tensity and  θ  was the di ff  raction angle. For crystallite size calculations,the most intense peaks of the XRD scans ((104) plane at 29.4° for calciteparticles and (111) plane at 26.2° for aragonite particles) were used.  2.5. Fourier Transform Infrared Spectroscopy (FTIR) CaCO 3  particles were scanned in 4000-400cm − 1 range with 4cm -1 resolution using PerkinElmer 400 spectrometer (Waltham,Massachusetts) using attenuated total re 󿬂 ection (ATR) con 󿬁 guration.Background spectra were subtracted from the obtained re 󿬂 ectance dataand average of 4 spectra was reported for each sample.  2.6. Transmission Electron Microscopy (TEM) Calcite and aragonite microspheres were prepared for TEM char-acterization by dispersing them ultrasonically in 2-propanol. 1 μ l of thissolution was dropped onto holey carbon coated copper grid and driedfor 5min. Analysis were completed using bright- 󿬁 eld, high-resolution(HR) and selected area electron di ff  raction (SAED) modes of FEI Tecnai Ç.M. Oral, et al. Ceramics International xxx (xxxx) xxx–xxx  2  G2 F30 TEM (Hillsboro, Oregon) at 200kV accelerating voltage.  2.7. Gas adsorption (Branuer-Emmett-Teller (BET)) Pore size and morphology of the CaCO 3  particles were investigatedby N 2  gas adsorption using Quantachrome Autosorb 6B gas sorptionanalyzer (Boynton Beach, Florida). Prior to measurements, particleswere degassed at 70°C for 6h. Pore morphology, size and distributionof the particles were calculated based on the Barrett-Joyner-Halenda(BJH) model.  2.8. Particle size analysis Size distributions of CaCO 3  particles were analyzed using MalvernMastersizer 2000 particle size analyzer (Malvern, United Kingdom).Prior to the measurements, particles were dispersed in ultrapure waterfor 10min using an ultrasonicator. Average of 3 measurements wasreported for each sample.  2.9. Cell viability assay  Human bone cells (hFOB, ATCC CRL-11372) were used to evaluatethe e ff  ect of CaCO 3  particles on cell density and viability. hFOB cellswere seeded onto 96-well plate at a density of 30,000cells/cm 2 andincubated using Dulbecco's Modi 󿬁 ed Eagle's Medium (DMEM, SigmaAldrich) supplemented with 10% fetal bovine serum, 1% penicillin-streptomycin and 1%  L -glutamine for 24h in a humidi 󿬁 ed incubator(5% CO 2 , 37°C). Prior to cell culture experiments, the particles weresterilized with 70% ethanol and UV radiation. Afterwards, CaCO 3 particles were dispersed in DMEM at di ff  erent concentrations (0.01, 0.1and 1mg/mL) and hFOB cells were exposed to these media up to 5days. hFOB cells cultured using the same conditions without in-corporation of CaCO 3  particles were used as control. At 2nd and 4thdays of culture, media were replaced with freshly prepared ones con-taining the designated concentration of CaCO 3  particles. At 1, 3 and 5days of culture, media were aspirated and cells were washed with 1xPBS. Sterile 125 μ l 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetra-zolium bromide (MTT, Sigma Aldrich) was added onto each well at1mg/mL concentration to assess cellular viability. After incubating 4hat 37°C, insoluble formazan crystals were dissolved with the addition of 125 μ l isopropyl alcohol containing 0.77% HCl. Finally, absorbance of each well was measured at 570nm using Thermo Scienti 󿬁 c MultiskanGo microplate spectrophotometer (Waltham, Massachusetts). The ab-sorbance values of blank samples without cells were subtracted fromthe obtained absorbance values. Cell densities were calculated bycomparing the absorbance values to a standard curve constructed at thebeginning of each trial.  2.10. Statistical analysis All experiments were performed in triplicate and the results wererepresented as mean ± SD. Statistical analyses were performed withone-way analysis of variance (ANOVA) using Tukey's post-hoc test withsigni 󿬁 cance based on  p  < 0.05. 3. Results and discussion 3.1. In  󿬂 uence of synthesis parameters on CaCO 3  polymorph andmorphology  Investigation of the CaCO 3  particles using SEM (Figs. 1 and 2) andXRD (Fig. S1) highlighted drastic di ff  erences upon SDS incorporationand N 2  bubbling under di ff  erent synthesis conditions. In Fig. 1, themorphologies of the synthesized CaCO 3  particles were shown as perchanges in temperature, precursor solution and template type. Whenwater was used as a precursor solution, the lack of N 2  bubbles and SDSled to the formation of characteristic CaCO 3  polymorphs andmorphologies. Due to stability of calcite at ambient conditions, rhom-bohedral calcite particles (Fig. 1a) were obtained at 25°C [23]. How- ever, with an increase in temperature, crab-like particles (Fig. 1e)consisting of predominantly aragonite, the kinetically favorable poly-morph of CaCO 3  at high temperatures, were crystallized [23]. WhenSDS was incorporated into the precursor solutions, calcite particleshaving platelet morphology (Fig. 1b) crystallized at 25°C. Though mostof the synthesized particles had platelet morphology, microsphericalaggregates consisting of calcite platelets were also present. Formationof calcite microspheres via attachment of thin platelets was in-line withprevious  󿬁 ndings in literature, where Shen et al. [24] used solutionmixing method and utilized poly(vinylpyrrolidone) as a water-solublepolymer and SDS as an anionic surfactant at pH 7.0 to obtain CaCO 3 microspheres consisting of calcite platelets. When synthesis tempera-ture increased to 90°C, calcite particles having platelet morphology(Fig. 1f), rather than the kinetically favorable aragonite polymorph,were obtained. The reason for this situation could be the electrostaticinteraction between the negatively charged SDS surfactant and theCa 2+ cations. In fact, it was observed that the Ca 2+ concentrationdrastically increased around SDS due to their mutual electrostatic at-traction [25]. Afterwards, the Ca 2+ cations surrounding the SDS ag-gregates could further attracted CO 32 − anions present inside the pre-cursor solution [25]. We could speculate that the SDS aggregates couldact as heterogeneous nucleation sites and decrease the activation en-ergy for the nucleation of stable calcite particles.When N 2  bubbling was incorporated into the precursor solutions,rhombohedral calcite particles (Fig. 1c and g) were obtained in-dependent of the synthesis temperature. There was no change in CaCO 3 particle morphology or polymorph upon N 2  bubbling (Fig. 1c) com-pared to the particles synthesized without N 2  incorporation (Fig. 1a),and thus N 2  bubbling alone was identi 󿬁 ed to be ine ff  ective at 25°C inaqueous media for crystallization of CaCO 3 . Upon increasing thesynthesis temperature to 90°C in the presence of N 2  bubbles (Fig. 1g),aragonite polymorph (26.3wt%) as well as calcite polymorph (73.7wt%) crystallized. Crystallization of calcite could be explained with theintroduction of N 2  gas at room temperature into the precursor solution.Since the temperature surrounding the N 2  bubbles locally decreased,the synthesis conditions around the N 2  bubbles were suitable forspontaneous rhombohedral calcite nucleation, as it was typically ob-served at 25°C. Due to the stable nature of calcite under ambientconditions, its transformation to aragonite was not favored [23].However, during the gas bubbling step where local temperatures didnot  󿬂 uctuate and throughout the 4-h aging step at 90°C, the synthesisconditions favored aragonite crystallization (Fig. 1g and Table S1). When the e ff  ects of N 2  and SDS were synergistically coupled, particlemorphology was considerably altered. Similar to the particles obtainedupon SDS incorporation (Fig. 1b and f), calcite particles having plateletmorphology (Fig. 1d and h) were obtained independent of the synthesistemperature. However, almost all of the platelet particles rearrangedthemselves to possess a microspherical morphology at 25°C, whilecalcite microspheres were not present at 90°C. Calcite microsphereshaving constituent particles with a platelet morphology (Fig. 1d) wasreferred as  ‘ SC-P ’  and their higher magni 󿬁 cation image was displayed inFig. 2a.Similar experiments were also performed using precursor solutionscontaining 80% EG to identify the e ff  ects of EG on CaCO 3  particleproperties upon the use of N 2  bubbling and SDS incorporation. Whenneither N 2  bubbles nor SDS were introduced during particle synthesis,CaCO 3  particles consisted of ellipsoidal vaterite (Fig. 1i) and mostly 󿬂 ower-like aragonite (Fig. 1m) at 25°C and 90°C, respectively. OnceSDS was incorporated into the system, image analysis revealed64% ± 14% and 20% ± 4% increase in average size (maximumlength) of vaterite (Fig. 1 j) and aragonite (Fig. 1n) particles, respec- tively. Similar results were also obtained by Qiao et al. [26] whereincrease in SDS content was correlated with an increase in average size Ç.M. Oral, et al. Ceramics International xxx (xxxx) xxx–xxx  3  of calcite microspheres. In this study,  󿬂 ower-like aragonite particleswere referred as  ‘ FL-A ’  and highlighted in Fig. 1n and 2c. The poly- morphic and morphological changes observed for CaCO 3  particles toobtain ellipsoidal vaterite and  󿬂 ower-like aragonite particles as perchanges with EG concentration were consistent with previous  󿬁 ndingsin literature [21]. For example, Flaten et al. [27] increased vaterite content of the synthesized CaCO 3  particles from 0% to 65% by in-corporating 75% (mono)EG into the precursor solutions. Andreassenet al. [28] controlled (mono)EG concentration of the precursor solu-tions to increase supersaturation to alter particle morphology fromdumbbell-like to spherulite. Similarly, increased branching of the ara-gonite polymorph obtained at 80% EG (Fig. 1m) compared to its EG-free counterpart (Fig. 1e) could be the result of improved nucleationrate in the growth front with an increase in supersaturation upon the EGincorporation [29]. Having this said, EG was a non-aqueous solventmiscible in water at all proportions and had high cohesive energy forCa 2+ ions of the precursor solutions [30]. Therefore, a local increase insupersaturation was anticipated upon EG incorporation into aqueousenvironments [30,31]. In addition to the increase in supersaturation due to the decrease in the solubility of CaCO 3  polymorphs with an in-crease in the EG concentration, the lifetime of vaterite polymorph wasalso enhanced in the presence of EG, where the polymorph transfor-mation to calcite was inhibited [32].Similar to the results obtained for crystallization in water (0% EG),N 2  bubbling was not e ff  ective to alter particle polymorph andmorphology at 25°C (Fig. 1k) compared to particles synthesizedwithout N 2  bubbling (Fig. 1i). However, once solution temperatureincreased to 90°C, particle morphology completely changed upon N 2 bubbling and urchin-like aragonite (Fig. 1o) particles, referred as  ‘ UL-A ’ , were obtained. Similar to SC-P particles (Fig. 1d), UL-A particlesformed by the rearrangement of smaller particles in a needle-likemorphology (Fig. 2d) rather than a platelet morphology as in the casefor SC-P. Once N 2  bubbles and SDS were both incorporated into theprecursor solutions containing 80% EG, calcite microspheres (Fig. 1l)were obtained at 25°C. They were similar to SC-P particles synthesizedat 80% EG (Fig. 1d), yet the constituent particles of the microspherealtered from platelet to irregular morphology due to presence of EG. Inthis study, calcite microspheres having irregular constituent particlemorphologies were referred as  ‘ SC-I ’  and highlighted at a higher mag-ni 󿬁 cation in Fig. 2b. 3.2. Properties of CaCO 3  microspheres Since CaCO 3  microspheres exhibited promising characteristics forbiomedical applications, the particles possessing spherical morphologyin this study (SC-P (Fig. 1d), SC-I (Fig. 1l) and UL-A (Fig. 1o)), along with FL-A particles (Fig. 1n) as a non-spherical control group werefurther investigated. The percentages of each polymorph for theseCaCO 3  particles were calculated from their XRD spectra (Fig. 3a) usingRietveld re 󿬁 nement method. As shown in Table 1, calcite or aragonite Fig. 1.  SEM micrographs of CaCO 3  particles prepared using (a – d) 0% EG (25°C), (e – h) 0% EG (90°C), (i – l) 80% EG (25°C) and (m – p) 80% EG (90°C) precursorsolutions. (a, e, i, m) have neither SDS nor N 2  bubbling, (b, f, j, n) have SDS addition only, (c, g, k, o) have N 2  bubbling only and (d, h, l, p) have both SDS and N 2 bubbling. Platelet shaped spherical calcite (SC-P), irregular shaped spherical calcite (SC-I),  󿬂 ower-like aragonite (FL-A) and urchin-like aragonite (UL-A) particles arehighlighted in the  󿬁 gure. Insets show higher magni 󿬁 cation micrographs of the particles. Scale bars are 5 μ m and 1 μ m for lower and higher magni 󿬁 cation micro-graphs, respectively. Ç.M. Oral, et al. Ceramics International xxx (xxxx) xxx–xxx  4  polymorph constituted more than 98% of each investigated sample,making them almost phase-pure particles. Crystallite size of eachsample was also calculated using the XRD spectra, where UL-A particlesexhibited the smallest crystallite size (16.9nm), followed by FL-Aparticles (26.6nm). On the other hand, larger crystallite sizes, 38.8nmand 30.1nm, were present for SC-P and SC-I particles, respectively. Thepolymorphic analysis obtained by SEM and XRD was further supportedwith the FTIR spectra of the particles (Fig. 3b). Calcite and aragonitepolymorphs had characteristic CO 32 − absorption bands within thehighlighted region (900-680cm -1 ). Absorption bands of calcite (at 876, Fig. 2.  Surface morphologies of a) SC-P, b) SC-I, c) UL-A and d) FL-A particles. Scale bars are 1 μ m. Fig. 3.  a) XRD and b) FTIR spectra of SC-P, SC-I, UL-A and FL-A particles. Red lines are JCPDS references for aragonite (#005-0453) and calcite (#005 – 0586).  ‘ C ’  and ‘ A ’  denote peaks corresponding to calcite and aragonite polymorphs, respectively. (For interpretation of the references to colour in this  󿬁 gure legend, the reader isreferred to the Web version of this article.) Table 1 Physical properties of SC-P, SC-I, UL-A and FL-A particles. SampleNameCaCO 3  Polymorph (wt%) CrystalliteSize (nm)AverageParticleSize ( μ m)MultipointSurface Area(m 2 /g)Calcite AragoniteSC-P 100 - 38.8 4.4 24.4SC-I 98.7 1.3 30.1 4.7 28.2UL-A 0.7 99.3 16.9 3.9 28.8FL-A 1.6 98.4 26.6 6.8 4.7 Ç.M. Oral, et al. Ceramics International xxx (xxxx) xxx–xxx  5
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