CaCO3-vaterite-microparticles-for-biomedical-and-personal-care-applications

CaCO 3vaterite microparticles for biomedical and personal care applicationsDaria B.Trushina a ,b ,Tatiana V.Bukreeva c ,d ,Mikhail V.Kovalchuk c ,d ,Maria N.Antipina a ,⁎aInstitute of Materials Research and Engineering,A*STAR,Singapore 117602,Singapore bLomonosov Moscow State University,Faculty of Physics,Moscow 119991,Russia cNational Research Centre “Kurchatov Institute ”,Moscow 123098,Russia dA.V.Shubnikov Institute of Crystallography,Moscow 119333,Russiaa b s t r a c ta r t i c l e i n f o Article history:Received 16January 2014Accepted 21April 2014Available online 14May 2014Keywords:Calcium carbonate (CaCO 3)BiomineralVaterite polymorph Co-precipitation Crystal growthAmong the polymorph modi fications of calcium carbonate,the metastable vaterite is the most practically impor-tant.Vaterite particles are applied in regenerative medicine,drug delivery and a broad range of personal care products.This manuscript scopes to review the mechanism of the calcium carbonate crystal growth highlighting the factors stabilizing the vaterite polymorph in the most cost ef ficient synthesis routine.The size of vaterite par-ticles is a crucial parameter for practical applications.The options for tuning the particle size are also discussed.©2014Elsevier B.V.All rights reserved.1.IntroductionVaterite is a mineral,a polymorph of calcium carbonate (CaCO 3).Vaterite is usually colorless,having spherical shape and porous inner structure.The diameter of a vaterite particle ranges from 0.05to 5μm.Like aragonite,vaterite is a metastable phase of CaCO 3at ambient condi-tions at the surface of the earth.Being less stable than either calcite or aragonite,vaterite has a higher solubility than either of these phases.Therefore,vaterite transforms to calcite or aragonite once it is exposed to water.Temperatures below 60°C facilitate the formation of calcite,and at the higher temperatures,recrystallization to aragonite occurs [1,2].Despite being metastable,vaterite can still be found in nature,for in-stance,in mineral springs.It usually occurs as a minority component of a larger structure or as a result of a pathological process in humans and animals.Thus,vaterite is found in fish otoliths,freshwater pearls,healed scars of some mollusk shells,gallstones and urinary calculi [3].In those circumstances,some impurities (metal ions or organic matter)may stabilize the vaterite and prevent its transformation into calcite or aragonite.Like other polymorphs of calcium carbonate,vaterite rapidly dis-solves at acidic pH,and thus it can undergo degradation both in vivoand in vitro .Biodegradation of vaterite may occur in body fluids or some acidic extracells,or upon the cellular phagocytosis and absorption.Body fluid contains a number of acidic metabolites,such as citrate,lac-tate and acid hydrolysis enzymes,which provide acidic environment for dissolution of the material.After entering cells (mainly macro-phages)by phagocytosis,vaterite particles are split into ions under the effect of cytoplasmic and lysosomal enzymes,and then the degradationproducts,Ca 2+and CO 32−,can be transferred to extracell.Ca2+can then participate in the formation of new tissue without causing organic damage and pathological tissue calci fication [4].Vaterite microparticles can be produced in a number of ways,how-ever,the majority of them is laborious,or requires extreme conditions and special equipment.The main industrial manufacturing method is based on CO 2bubbling through a calcium containing solution,also re-ferred as the Kitano approach [5–8].Among the other popular methods are the double emulsion approach [9],solvothermal growth in auto-clave above 100°C [10],and biomineralization [4,11,12].Synthesis is often carried out in a double jet reactor vessel [13]applying ultrasound [14–16]or magnetic field [14,17,18].The simpli fied method comprises mixing of saturated aqueous stock solutions containing calcium and car-bonate ions.The mixing method is highly cost ef ficient,fast and easy to perform.Moreover,it may be scaled up for industrial production of vaterite particles [19–23].However,the mixing method may result in unwanted recrystallization of vaterite to more stable polymorphs of calcium carbonate.Therefore,a detailed understanding of the CaCO 3crystal growth process is essential for the success in vaterite production.Materials Science and Engineering C 45(2014)644–658⁎Corresponding author.Tel.:+6568741974;fax:+6568720785.E-mail address:antipinam@.sg (M.N.Antipina)./10.1016/j.msec.2014.04.0500928-4931/©2014Elsevier B.V.All rightsreserved.Contents lists available at ScienceDirectMaterials Science and Engineering Cj ou r n a l h o m e p a g e :w ww.e l s e v i e r.c om /l o c a t e /ms e cHere we introduce the current opinions on the vaterite crystal growth and review the conditions promoting the vaterite polymorph of calcium carbonate.Due to its biodegradability,low cost and unique physical and chemical properties,vaterite is highly demanded in bio-medicine and is an essential component of a big variety of personal care products.Besides being actively used in bone implants,abrasives,cleaners and absorbers,vaterite is playing a key role in encapsulation and drug delivery.However,the size of the vaterite particles is often a crucial parameter.Possible ways to in fluence the particle size are also discussed in the manuscript.2.Introduction to the CaCO 3crystal growthThe formation of a new solid phase is initiated through nucleation in supersaturated solution.Solid state precipitates initially in an amorphous sediment of spherical granules with a diameter of 10nm –70nm [5,17,19,20,24,25].Then,the transformation and dissolution –recrystallization processes result in a mixture of calcium carbonate crystalline hydrated forms (hexahydrate and monohydrate)and three anhydrous crystalline polymorphs (vaterite,aragonite and calcite).Vaterite nucleation and growth are of special interest,and the vaterite particle formation mechanism is a subject of on-going debate [25].Two explanations for the formation of primary polycrystalline vaterite spheres exist in the literature.The nano-aggregation concept claims that the spheres are the result of rapid aggregation of initially formed nano-sized crystals [6,16,22,26,27].However,only relatively insoluble substances develop high supersaturations,so that precipitation can also be considered as crystallization of sparingly soluble substances.Thus,the concept of classical spherulitic growth shown schematicallyin Fig.1advocates that a sphere of vaterite with polycrystalline features originates from the formation of a single nucleus followed by branching of this small crystal (often mentioned as ripening of crystallites)[26,28,29].It is also supposed that a final particle can be a result of both crystal-lization processes taking place at a time [30].According to the crystal growth theory,precipitation starts from nu-cleation.The initially formed nuclei grow into crystallites.The crystal growth process occasionally entailed with the formation of secondary nuclei causes signi ficant changes in the initial concentrations of calcium and carbonate ions in solution.The crystal growth itself is a complex process comprising two elementary processes taking place either at some distance from the crystal surface or at the crystal –solution inter-face.These elementary processes are 1)diffusion and/or convection of growth units through the bulk towards the crystal –solution interface,and 2)surface integration processes at the crystal –solution interface.The slowest of these consecutive processes determines the overall crys-tal growth rate.The solid phase thus formed (crystals)undergoes further changes in physical and chemical properties.These secondary changes occur under conditions close to equilibrium signifying the tendency of the system to establish equilibrium.Theoretically,precipitation terminates when the crystals present in the system form one crystal which is in equilibrium with the saturated solution.In practice,precipitation is completed when crystals reach a size that causes sedimentation.The process that leads to equilibrium is called ageing and takes place through several possible ways.Dissolution of small and simultaneous growth of large particles (also referred as Ostwald ripening)and recrystallization are the mechanisms that probably always occur in the early stages of the precipitate formation.Coagulation and agglomeration followed by sintering change the initial dispersion of the system by the formation of more stable large aggregates (Fig.1).The preference for vaterite formation exists when starting ionic ac-tivity productIAP ¼a Ca 2þÃa CO 2−3lies between thermodynamic solubilityproduct for amorphous calcium carbonate K SP (ACC)and the solubility product for vaterite K SP (Vaterite)[26].The logarithmic ion activityproduct of Ca 2+and CO 32−in the suspension at 25°C is shown as a func-tion of time in Fig.2a.Regions I,II and III in Fig.2correspond to unstable stage,metastable stage and stable stage,respectively.Under condition within the region I (unstable stage),calcium car-bonate nanoparticles randomly emerge as an amorphous sediment at all temperatures.Onwards,nanoparticles line up along speci fic crystal-lographic orientations,resulting in a phase change from poorly crystal-line to crystalline.The crystalline CaCO 3begins to form around the time when the IAP curve shows the “in flection point ”(Fig.2a)[19].As the crystals continue to precipitate,the supersaturation status decreases causing the nucleation force to decrease as shown in Fig.2b,and the subsequently formed nanoparticles begin to attach to each other in a crystallographically oriented fashion,which is thermodynamically more stable than random attachment.The transformation of amor-phous calcium carbonate (ACC)is completed just before the steep decrease in the IAP,and all ACC particles disappear forming clusters a few micrometres in diameter.The polymorph composition of precipitated particles in the early metastable stage was presented as a function of temperature in Fig.3.Initial ACC transformed to the combination of vaterite and calcite at low temperatures (14°C to 30°C).Vaterite polymorph occurs predomi-nantly at 30°C –40°C.At intermediate temperatures (40°C to 50°C)the formation of all three polymorphs is observed.Elevating the temper-ature above 60°C stabilizes aragonite polymorph of calcium carbonate.The abundance of a certain modi fication also depends on supersatu-ration [11,13,19,26].The effect of supersaturation has been described by the Ostwald's law of stages:at suf ficiently high supersaturation the most soluble and the least stable form crystallizes first [32].At the be-ginning of the stage II,the system is supersaturated with respect to all polymorphic forms,thus the least stable and higher-energy form of vaterite is the first to crystallize.The crystallization of thevateriteFig.1.Stages and pathways of precipitation processes [15].645D.B.Trushina et al./Materials Science and Engineering C 45(2014)644–658form will proceed until the supersaturation value decreases down to reach the solubility for this form (Fig.4a).At the same time,as the sys-tem is supersaturated with respect to all forms,the crystallization of the second more soluble form (aragonite)begins.Dissolving vaterite con-tributes to further crystallization of the aragonite form until completely transformed [32].Complete dissolution of vaterite induces steep de-crease in the IAP value to Ksp of the next polymorph (aragonite)with subsequent transformation to the most stable calcite [19].Fig.4b pro-vides a range of respective Ksp values for the vaterite,aragonite and cal-cite forms at different temperatures.It can be seen from Fig.4b that the solubility product of all forms decreases at the higher temperatures that is quite uncommon for both organic and inorganic substances.As the vaterite form has the highest solubility among the poly-morphs and precipitates first,it appears to be the most challenging to stabilize its nuclear and suspend the formation of other modi fications.During the metastable stage,all crystals are finally transformed to the most stable modi fication of calcite at all temperatures as a result of water induced dissolution –recrystallization process,Fig.5.The implica-tion of the theory reveals that by controlling the supersaturation and harvesting crystals at an appropriate time,it should be possible to iso-late the certain polymorphic form [32].The stable stage corresponds to particle growth in size without fur-ther changes in morphology.The value of IAP stabilizes at the solubilityproduct of calcite and nuclei grow persistently up to a certain size (Fig.2a)[23].Factors in fluencing the morphology of CaCO 3crystals are those affecting supersaturation.The activity of the calcium and carbonate ions is connected with the ionic strength and temperature.As shown in Fig.6,the time required to reach the metastable stage (T m )of the polymorph mix decreases with increasing temperature.Temperature is an important parameter for bulk crystallizations as it allows to control the product solubility making it possible to obtain only the thermody-namically stable lowest-free-energy form at any given value [32].Tem-perature variations may change the rank order of the thermodynamic stability re flecting the changes in the free energies of the polymorphs.Such a change in terms of solubilities is shown in Fig.7.The change in rank order is characterized by a transition temperature Tc.From these considerations it is clear that the thermodynamic stability of the poly-morphs must be taken into account in studies on crystallization of poly-morphs [32].Changes in the initial pH values have an impact on the ionic strength of the solution,which is related to supersaturation.A higher pH leads to a higher concentration of carbonate ions and a higher supersaturation.When the initial pH value is higher than 7but is lower than 11,the pre-cipitation of calcium carbonate follows the ACC –vaterite pathway de-scribed above [35].The vaterite abundance and its crystal size increase with higher ini-tial concentrations of Ca 2+and CO 32−ions.The change in its dominance with time for the two solutions of different supersaturation values is shown in Fig.8a.In spite of practically equal ion activity of calcium and carbonate in the initial solutions in (A)and (B),the suspension which contains an excess of Ca 2+(A)gives a higher vaterite abundance.Using a computer program,the rate of crystal growth was deter-mined by numerical differentiation and the growth kinetics was found to be parabolic (Fig.8b)[15],which is in good agreement with the ex-perimental data obtained elsewhere [33].Surprisingly,a rather weak dependence of the vaterite growth rate on the ionic strength was found [15].3.Vaterite characterization and properties 3.1.StructureBeing a metastable polymorph,vaterite rarely exists in nature in the pure form,and large single crystals of vaterite are dif ficult for obtaining in-vitro .Because of that,it has been a big challenge to resolve the struc-ture of vaterite for almost acentury.Fig.2.The change in logarithmic ion activity product of calcium and carbonate,log IAP,with time at 25°C (a)(plotted from [19]);the dependence of nucleation rate on supersaturation (b)(plotted from [31]).Fig.3.Plots of abundance of crystalline calcium carbonates at the early metastable stage as a function of temperature (plotted from [19]).646 D.B.Trushina et al./Materials Science and Engineering C 45(2014)644–658The most widely accepted fact is that vaterite has a hexagonal crystal symmetry [10,14,20,29,36,37]and P63/mmc space group [16,25,38],with unit pseudo-cell parameters a ′=4.13Åand c ′=8.49Å.The speci fic gravity measurements reveal that this cell contains two CaCO 3(Z ′=2)[25].Five weak re flections also observed by (X-ray diffraction)XRD initially could not be indexed,and were attributed to the true cell rotated by 30°about the c axis from the main one,with lattice parame-ters a =a 0ffiffiffi33p =7.16Å,c =2c ′=16.98Å,and Z =12[25].The atomic arrangement shown in Fig.9is constructed based on the symmetry el-ements of space group P63/mmc.This model posits 100%occupancy by the calcium and one-third occupancy by the carbonate groups.When the carbon atom was placed in the centre of the prism and the vertical carbonate group was oriented so that each oxygen atom was equidistant from the two nearest calcium atoms,the resultant calci-um –oxygen interatomic distances were found to have the expected values [25].In [38]another structure is suggested.The basic unit cell is essential-ly in agreement with the previous model but the site symmetry of the carbonate groups is different.It was emphasized that additional spectral features observed in diffraction data (diffuse streaks along the c axis and satellite re flections)are attributable to stacking faults perpendicular to the c axis,leading to doubling or tripling of the c lattice parameter,as re flected in Fig.10.The principal debate has arisen on whether the vaterite symmetry is hexagonal or orthorhombic,although recently it was claimed that none of the proposed models is consistent with Raman spectra indicated by the presence of two or more carbonate groups in the asymmetric unit [39].Based on the spectroscopy data,the possibility of at least three structurally independent carbonate groups in the unit cell of vaterite was proposed.A Raman spectrum of vaterite discussed in [39]is characterized by the presence of at least eight relatively broad bands at frequencies corresponding to the external lattice modes whereas the other bands are split into two or three distinct peaks (Fig.11).Vaterite peaks in Raman spectra often have high value of full width at half-maximum in comparison with other polymorphs,which implies that the crystal structure of vaterite is not well ordered.The peak broadening is possibly affected as a result of the stacking faults,layer shifts or syntactic inter-growth irregularities,making it dif ficult to determine the parameters of the crystalstructure.Fig.4.The effect of supersaturation on the crystallization of polymorphs.The circles indicate different supersaturations (a)(plotted from [32]);solubility of the anhydrous crystalline polymorphs at 1bar (b)(plotted from [33]).Fig.5.Schematic depiction of the formation of CaCO 3spherulites and the transformation from amorphous phase to typical calcite [34].647D.B.Trushina et al./Materials Science and Engineering C 45(2014)644–658Different variations of the model were proposed,resulting in the unit cell with P6522or Ama2space group or modi fied orthorhombic unit cell.Based on diffraction tomography and transmission electron microscopic data,two structures with very low symmetries (a mono-clinic unit cell and a triclinic one)were suggested in [40].Taking into account all the diffraction data obtained in the study,the lowest sym-metries were hypothesized.Nevertheless,the most recent report claims that vaterite should be considered as a combination of two different crystal structures that co-exist within a pseudo-single crystal [3].The structure of vaterite crystal is possibly predominantly hexagonal with at least one other coexisting minor crystallographic structure.The minor atomic structure,existing as nanodomains within the major matrix,is still unknown.The coexis-tence of two structures of different symmetries largely explains theamount of contradicting results previously obtained by different research groups all over the world.There are at least three different theoretical models equally well de-scribing the vaterite structure at room temperature.These distinct models,each comprising multiple structures,can explain the disorder of vaterite in terms of different orientations of the carbonate anions,multi-ple stacking sequences of the carbonate layers,and possible chiral forms.Hence,vaterite is not a single “disordered ”structure but should instead be considered as a combination of different forms,and each form can ex-hibit rapid interchange between multiple structures that possess similar average properties.Besides explaining the existence of several vaterite forms it suggests that perhaps not all samples of vaterite might be the same,while still exhibiting similar spectroscopic properties [41].The polycrystalline nature of the vaterite particle compared to monocrystalline calcite is evident from diffraction diagrams recorded from areas within the two particle slices displayed in Fig.12a and b [42].Random orientation of the crystallites within the vaterite particle should produce continuous circles on the diffraction diagram.The points of increased intensity along the circle reveal the preferential alignment of the crystallites.The fact that these diffraction spots are elongated indicates a certain degree of variation in the crystal orienta-tion.Interestingly,however,the broken ring pattern of electron diffrac-tion obtained from a micrometer-scale area exhibited 6-fold symmetry.For example,the diffraction rings of (110)and (114)were composed of six bright and six dark segments (Fig.12c).That means that the crystal-lographic direction of the nanocrystals in the mosaic is not random but roughly arranged in the same orientation.The presence of texture in the vaterite sample supports the concept of spherulitic growth but does not provide suf ficient evidence,since a hypothetical ordered aggregation process could give the same diffraction pattern.High resolution TEM image (Fig.12d)displays the size of the indi-vidual crystallites.The diffraction rings in Fig.12e are more continuous indicating less systematic orientation,emphasizing the variability of the vaterite structure [42].Fig.13reveals an XRD pattern of vaterite power obtained by solvothermal synthesis in the presence of ethylene glycol.The powder XRD exhibits the characteristic re flections of vaterite,which correspond to lattice planes hkl and d-spacings shown in Table 1[16].Diffractograms of vaterite spherulites composed of nanocrystalline sub-domains or grains usually reveal relatively broad peaks of low intensity,which still can be analysed by the Scherrer equation relating the grain size and peak width [20,24].The phase composition can be re fined from X-ray diffractograms by Rietveld analysis [14,29,40].3.2.Shape and morphologyThe particle morphology may be the most uncertain characteristic because it displays a great deal of variety.The main crystal structure of polycrystalline vaterite is hexagonal with typically a spherical crystal habit like the one shown in Fig.14a [5,9,13,17,24,43–47].The spherical form is also called framboid [5]or raspberry [27](translation of ‘la framboise ’from French).A detailed examination of particle surface showed that spheroids are composed of smaller single crystal sub-units (Fig.14b)arranged with some degree of order as mentioned earlier.The most common interior feature is the presence of grooves radiating from the centre of the particle as radial fibre-like (or channel-like)structures (Fig.14c).Fracture cross-section images display nano-grains developed into oriented rods (Fig.14d).The deviation angle of each main bundle of the oriented rods was roughly estimated to be about 60°(Fig.14e).A so-called “sheaf of wheat ”inner structure of spherulites was ob-served in [5,48].One can imagine this structure as a bundle of fibres tied together at the centre,so that the ends fan out.While the number of growing fibres increases their ends close off to form a sphere.The particle growth scenario called “sphere-to-dumbbell-to-sphere ”(Fig.14f)is currently one of the most explored [21,24,29,49].Fig.6.Plot of the time required to reach the metastable stage (T m )as a function of temper-ature (plotted from [19]).Fig.7.Schematic diagram illustrating the solubility relationship between two polymor-phic forms as a function of temperature close to the phase transition temperature Tc (plotted from [32]).648 D.B.Trushina et al./Materials Science and Engineering C 45(2014)644–658Besides the typical spheroidal structure,in vitro precipitation of vaterite often results in a variety of complex shapes:fried-egg shape,different kinds of layered flower-like (Fig.15a,b)or rosette-shaped structures (Fig.15c)united with 6-fold symmetry [24,31,36,50,51].Fig.15d,e,and f represents a typical SEM image and a suggested a for-mation scheme for such hexagonal-shaped vaterite composed of small hexagonal units.Other reported shapes of vaterite [16,35,50,52]are plates and lenses (Fig.16a and b).Plate morphology is probably the characteristic for the intermediate stage of cauli flower-like particle growth,since other petals growing on plane regions can be seen (Fig.16b).However,the development mechanisms,resulting textures,and common features of all different shapes are yet to be explored.Impor-tantly,in minor cases,calcite could also have a spherical habit [42],so the particle analysis must not rely solely on SEM images,but be supple-mented with the methods discovering more exhaustive information about the phase,e.g.,XRD.3.3.PorosityHigh speci fic surface area and porosity of vaterite particles are of particular importance for their practical applications.Brunauer –Emmett –Teller (BET)analysis reveals the speci fic surface area of the vaterite framboids to be 3.2–8.8m 2g −1for different precipitation con-ditions [5,43,53–55].Vaterite particles with the highest surface area24.72m 2g −1were achieved via coprecipitation with inulin [56].Be-sides,the pore size distribution displayed an average pore size of 20–60nm for the particles with a medium diameter of 4–6μm [43].Theo-retical equation evidently indicates that speci fic surface area is subject to exponential decay with increasing particle size [5].3.4.Absorption spectrumFourier transformation infrared spectroscopy (FTIR)is a very useful technique to identify the polymorph composition of CaCO 3particles [21,50,57].The absorption bands of the different crystallographic varie-ties of CaCO 3are listed in Table 2.Pure vaterite exhibits the characteris-tic vibrational bands at 1480,1070,1087,877,848and 745cm −1,where the last three of the listed bands are the most typical and informative [17,18,35,53,58].The selected FTIR spectrum for vaterite samples pre-cipitated in mono-ethylene glycol solution at 40°C (Fig.17)shows representative bands at 1088,874and 744cm −1attributing to the deformation modes of CO 3in the vaterite polymorph [14].The shifts of peak positions in FTIR spectra indicate interactions between calcium ions and chemical groups of additional molecules in the reaction medi-um (additives or impurities)[10,14,16,18].Shift magnitude directly corresponds to the strength of such interactions.3.5.Chemical and physical propertiesOwing to the mass density (2.54g·cm −3),solid vaterite is used in regenerative medicine [5]for doping of implant scaffolds.Besides,calcium carbonate is quite inert and biocompatible.CaCO 3structure has good stability at neutral and alkaline pH but begins to dissolveatFig.8.Plots of vaterite abundance as a function of time in metastable stage as 25°C with initial concentrations of calcium and carbonate (a).Initial concentrations:(A)0.11M CaCl 2–0.10M Na 2CO 3and (B)0.10M CaCl 2–0.11M Na 2CO 3(plotted from [19]).Plots of the square root of the growth rate as a function of the relative supersaturations and various ionic strengths (b)[15].Fig.9.Vertical projection of vaterite showing orientation of carbonate group relative to calcium atoms [25].Fig.10.Structure of the vaterite unit cell [38].649D.B.Trushina et al./Materials Science and Engineering C 45(2014)644–658。