deposition of amorphous calcium carbonate 110

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Deposition of Amorphous Calcium CarbonateHemispheres on Substrates

Xurong Xu, Joong Tark Han, and Kilwon Cho*

Department of Chemical Engineering, Polymer Research Institute, Pohang University ofScience and Technology, Pohang 790-784, Korea

Received November 30, 2004. In Final Form: March 31, 2005

The amorphous calcium carbonate (ACC) hemispheres were deposited on the mica and poly-(diallyldimethylammonium chloride) modified surface. The form of the ACC deposit on the substrates canbe controlled by modifying the substrate surface, the introduction of additives, or both. It demonstratedthat substrates (insoluble matrix) and additives (soluble macromolecules) have significant influence onthe crystallization of CaCO3.

Introduction

In biomineralization, organisms control the nucleationof inorganic crystals at a specific site and their growthinto complex and intricate hierarchical structures. Struc-turesthathave a structuralfunction, such as bones,teeth,and shells, usually have superior properties compared tothose of conventional man-made materials with similarcomponents.1 Calcium carbonate (CaCO3) is one of themost abundant biominerals, and extensive research intothebiomimetic synthesis of CaCO3-based biominerals hasbeen carried out. CaCO3 crystals with a variety of complexshapes have been prepared by using many differentadditives.2-6 Various functional templates, includinglangmuir monolayers7 and self-assembled monolayers,8

have been used to induce crystallizationof CaCO3.CaCO3thin films have also been produced using the cooperationbetween insolublematrixes and soluble macromolecules.9

It is thought, however, thatamorphouscalcium carbonate(ACC), a metastable form of calcium carbonate, plays animportant role in the biomineralization and crystallizationof CaCO3.8e,10-15 The rich variety of CaCO3 structures in

nature might be due to the amorphous character of ACC,which is easily molded into many different shapes. Herewe describe the deposition of ACC hemispheres on micaand the surfaces modified with a positively chargedpolyelectrolyte,poly(diallyldimethylammoniumchloride)(PDADMAC) and how control of the deposited ACC formscan be obtained by modifying the surface characteristics

of these substrates.

Experimental Section

The silicon wafers were cleaned by immersion in freshlypreparedpiranhasolution (concentrated H2SO4/H2O2)7:3,w/w)and heated for 1 h at100 C, thenrinsedthoroughly with distilledwater. Newly cleaved mica was produced by peel-off usingadhesivetape. Silicon wafers andmica modifiedwith PDADMAC(Mw 1 105 to 1 106, from Aldrich) and poly(allylaminehydrochloride) (PAH;Mw 70 000, fromAldrich)were obtained byimmersing clean silicon wafers and mica in 1 wt % solutions ofeitherPDADMACand PAHfor 30 minand subsequently rinsingthoroughly with distilled water.

A vial containing a 50 mM CaCl2 solution was placed in adesiccator along with a dish containing ammonium carbonatepowder. The substrate was inverted and placed on top of the

CaCl2 solution.13ACC was deposited onto the substrate by slowdiffusion of theCO2 producedby decomposition of theammoniumcarbonateat roomtemperature.The depositiontime was 50 min.

Afterdeposition,the substratewas rinsed with ethanoland driedusing nitrogen gas.The substratewas immediately placed undera vacuum until ready for observation with a scanning electronmicroscope (Hitachi S-4200 field emission scanning electronmicroscope). The electron diffraction pattern was obtained byusing a transmission electron microscope(JEOL1200EX). Whenpoly(acrylic acid) (PAA, Mw 2000, from Aldrich) was used as anadditive, PAA was mixed with a 50 mM CaCl 2 solution. Theotherstepsforthe useof this additivewerethe same asdescribedabove.

Results and Discussion

In a previous paper,13 we have already reported thepreparation of large-area and continuous ACC films onclean silicon wafers in both the presence and the absenceof PAA inhibitor under mild conditions. Amorphouscharacter of the as-deposited film has been confirmed by

X-ray, IR, and optical microscope. Here we used severaldifferent substrates such as mica and polyelectrolytemodified silicon wafer and mica. When mica is used asthe substrate in the absence of PAA, the properties of theresulting deposits are dramatically different. As shownin Figure 1a, manyhomogeneousspheres appear to have

* Towhom correspondenceshouldbe addressed.E-mail: [email protected]. Fax: (+82)54-279-8269.

(1) Currey, J. D. Proc. R. Soc. London, Ser. B 1977, 196, 443.(2) Walsh, D.; Mann, S. Nature 1995, 377, 320.(3) Walsh, D.; Lebeau, B.; Mann, S. Adv. Mater. 1999, 11, 324.(4) Colfen, H.; Qi, L. M. Chem.sEur. J. 2001, 7, 106.(5) Colfen, H. Curr. Opin. Colloid Interface Sci. 2003, 8, 23.(6) Meldrum, F. C. Int. Mater. Rev. 2003, 48, 187.(7) (a) Mann, S.; Heywood, B. R.; Rajam, S.; Birchall, J. D. Nature

1988, 334, 692. (b) Heywood, B. R.; Mann, S. Adv. Mater. 1994, 6, 9.(c) Heywood, B. R.; Rajam, S.; Mann, S. J. Chem. Soc., Faraday Trans.1991, 87, 735. (d) Xu, G. F.; Yao, N.; Aksay, I. A.; Groves, J. T. J. Am.Chem. Soc. 1998, 120, 11977.

(8) (a)Kuther, J.;Seshadri,R.; Knoll,W.; Tremel,W.J. Mater.Chem.1998, 8, 641. (b) Aizenberg, J.; Black, A. J.; Whitesides, G. M. Nature1999, 398, 495. (c) Aizenberg, J.; Black, A. J.; Whitesides, G. M. J. Am.Chem.Soc. 1999, 121, 4500.(d) Han, Y.J.; Aizenberg, J.Angew. Chem.,

Int. Ed. 2003, 42, 3668. (e) Aizenberg, J.; Muller, D. A.; Grazul, J. L.;

Hamann, D. R. Science 2003, 299, 1205.(9) (a) Zhang, S. K.; Gonsalves, K. E. Langmuir 1998, 14, 6761.(b) Kato, T.; Suzuki, T.; Amamiya, T.; Irie, T.; Komiyama, M.; Yui, H.

Supramol. Sci., 1998, 5, 411. (c) Kato, T.; Sugawara, A.; Hosoda, N.Adv. Mater. 2002, 14, 869.(d) Hosoda, N.; Kato, T. Chem. Mater. 2001,13, 688. (e) Sugawara, A.; Ishii, T.; Kato, T. Angew. Chem., Int. Ed.2003, 42, 5299.

(10) Addadi, L.; Raz, S.; Weiner, S. Adv. Mater. 2003, 15, 959.(11) Becker, A.; Bismayer, U.; Epple, M.; Fabritius, H.; Hasse, B.;

Shi, J. M.; Ziegler, A. Dalton Trans. 2003, 551.(12) Weiner,S.; Levi-Kalisman,Y.; Raz,S. Connect.Tissue Res. 2003,

44 (Suppl. 1), 214.(13) Xu, X. R.; Han, J. T.; Cho, K. Chem. Mater. 2004, 16, 1740.(14) (a) Gower, L. B.; Odom, D. J. J. Cryst. Growth 2000, 210, 719.

(b) Olszta, M. J.; Odom, D. J.; Douglas, E. P.; Gower, L. B. Connect.Tissue Res. 2003, 44 (Suppl. 1), 326. (15) Faatz, M.; GrOhn, F.; Wegner, G. Adv. Mater. 2004, 16, 996.

10.1021/la047069v CCC: $30.25 xxxx American Chemical SocietyPAGE EST: 3.5Published on Web 00/00/0000


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been deposited on the mica substrate. It was found frommicrophotographs taken at high magnification with thestage tilted at 45 (Figure 1b) that these spheres are infact hemispheres. The amorphous character of the hemi-spheresis confirmed from theelectron diffraction pattern(Figure 1b inset). ACC is unstable in humid air, in whichit transforms into anhydrous crystal phases. When usingcrossed-polarized optical microscopy (see SupportingInformation), the hemispheres are dark before crystal-lization but bright after crystallization (ACC will trans-form into crystalline forms in humid air). Scanningelectron microscopy (SEM) microphotographs were usedto investigate the apparent differences between thesurfaces of the hemispheresbeforeand aftercrystallization(kept in humid air to crystallize), which also indicate theamorphous character of the hemispheres. Prior to crys-tallization, thesurfacesof thehemispheres havea smooth,glassy appearance (Figure 1b), but the surfaces of thecrystallized hemispheres are heterogeneous and rough.Many nanoparticlescan also clearlybe seen on thesurfacesof the hemispheres (Figure 1c). Transformation of ACCinto a crystalline form involves the release of watermolecules included in ACC.8e Theporesand rough surfaceobserved in the present work are possibly related to therelease of water during the transformation of ACC.

These images of hemispheres on mica are very similarto those of theclassic contact angle phenomena that occurwhen a liquid droplet contacts with a solid in air. If theliquid molecules at a liquid-solid interface are more

strongly attracted to each other than to the molecules ofthe solid surface (i.e., thecohesiveforces arestrongerthanthe adhesive forces), then the liquid beads up and doesnot wet the solid surface. On the other hand, if themolecules of the liquid have a stronger attraction to themolecules of the solid surface than to each other (i.e., theadhesive forces are stronger than the cohesive forces),then wetting the surface occurs. In practice, a contactangle usually formson the solid/liquid interfaceaccordingto the balance between the three interface tensionsinvolved, those at the solid-gas interface, the liquid-gasinterface, and the solid-liquid interface. The appearanceof the ACC hemispheres on mica is very similar to thephenomena often found in static contact angle measure-ments. The contact angle of ACC on mica is about 90,

which was estimated from the SEM image in Figure 1dby Adobe Photoshop 6.0. However, continuous ACC filmsare formed on clean silicon wafers under the sameexperimental conditions. TheseACC films have a smooth,homogeneous appearance in the SEM image (not shownhere),which issimilarto the images ofACC films onsiliconwafer surfaces in the presence of PAA (Figure 3a,d).

The surfaces of mica and clean silicon wafers arenegatively charged and hydrophilic in water. However,the reasons for this charge on the two substrates are

different. The surface of a clean silicon wafer (coated witha silicon dioxide layer) is negatively charged in water dueto thepresenceof deprotonatedsilanolgroups. Thesurfaceof newly cleaved mica is inherently negatively charged inwater due to the dissociation of the potassium cationsfrom the mica surface. It is well-known that potassiumions in mica are easily replaced with other ions such asH+, Mg2+, Ca2+, and so forth when mica is treated withdifferent solutions.16,17 The ion exchange process is quickand efficient. This ion exchange can modify the frictionandadhesion propertiesof mica surfaces.16A smallamountof net positive charge is accumulated on mica surfacesin calcium dichloride (CaCl2) solution (concentration>20 mM), andits surfacesare slightly morehydrophobic.17

It has been shown that the contact angle of ultrapure

water on mica changes from 7( 1to16( 2 after contactofmicawitha50mMCaCl2 solution for10 min. In contrast,forsiliconwafers thereis no difference in the contact anglebefore and after contact with CaCl2 solution. Therefore,we speculate that ion exchange between potassium ionsand calcium ions modifies the mica surface and lowerstheinteraction betweenACC and themica surface, whichresults in the formation of ACC hemispheres on the micasurface.

To study the effect of surface characteristics on thedeposition of ACC in more detail, two different positivelycharged polyelectrolytes, PDADMAC and PAH,which areoften usedin thepreparation of layer-by-layer polyelectro-lyte mutilayers, were employed to modify the surface ofmica and silicon wafers by electrostatic assembly. Thestructures of the polyelectrolytes are shown in Figure 2.The pH of the solution changes from 7 to 9 during thedeposition of ACC. The surfaces modified by the poly-electrolytes are both positively charged in solution. ThePDADMAC modified surface with its numerous quater-nary ammonium cations is highly positively charged insolution. The PAH modified surface has only a fewpositively charged amino groups in this pH range. It wasfound that deposited ACC takes on the form of hemi-spheres on PDADMAC modified surfaces (Figure 2a,b),which is similar to those found on mica. In contrast, an

ACC film with dense and partial coalescence hemispheresforms on the PAH modified surface. This result showsthat the interaction between ACC and the PAH modifiedsurface is somewhat stronger than that between ACC andthe PDADMAC modified surface. The same behavior isfound on the polyelectrolyte modified mica surfaces. Thisdemonstrates that the positive charge of the surface isresponsible for the formation of the ACC hemispheres.We can find that ACC hemispheres are formed on themica surface, but on the PAH modified mica surface an

ACC filmwill be obtained. Accordingly, on the silicon wafersurface an ACC film is formed, but ACC hemispheres willbe obtained on PDADMAC modified silicon wafer. Thus,we can conclude that the surface characteristics ofsubstrates exert important effects on the deposition ofCaCO3, especially the shape of the deposits.

(16) Xu, L.; Salmeron, M. Langmuir 1998, 14, 2187.(17) Dunstan, D. E. Langmuir 1992, 8, 740.

Figure 1. SEM microphotographs of hemispheres depositedon micabefore crystallization: (a) low magnification; (b) highermagnification with the stage tilted at 45, the inset shows theelectron diffraction pattern of thehemisphere by TEM; (c)aftercrystallization; and (d) side view for the measurement of thecontact angle.

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The structure of ACC has previously been determinedusing several kinds of analysis methods.10-15,18-21 Thermo-gravimetric analysis has been used to show that ACCcontains some water.15,18,19 The short-range order ofbiogenic ACC has been determined using extended X-rayabsorptionfine structure.10-12 Electron microscopeimagesof ACC generally show that ACC is composed ofspheres.11,18ACC prefers a spherical shape in solution tominimize its contact area with solution.19 The initialcomposition of ACC in solution has not been determined

due to thelack of available characterization techniques.20However, Gower et al.14 has foundthata polymer-inducedliquid-precursor process occurs and suggested that poly-merinducesthe liquid-liquidphase separation of dropletsof a mineralprecursor. A recent paper postulated a liquid-liquid phase segregation, and ACC particles are formedfrom a loss of water of the highly concentrated solutionby the gelation process. 15 It can be speculated that ACCprefers a spherical droplet shape and contains a lot ofwater in solution.

Our results can be rationalized by considering ACC asliquidlike colloids, which contain many water molecules.When excess calcium chloride is mixed with sodiumcarbonate, the zeta potential of CaCO3 precipitates ispositive because calcium cations are adsorbed on thesurface of precipitates.22 Even for equimolar solutions,the zeta potential of CaCO3 is still small positive duringfirst 30 min.22 Here a method of slow diffusion of carbondioxide (CO2) into CaCl2 solution is used, and the excesscalcium ions exist in CaCl2 solution. On one hand, ACC,which spontaneously formed first in CaCl2 solution,preferentially adsorbs calcium ions to form stable posi-

tively charged ACC colloids with lots of water. On theother hand, ACC colloids have a tendency to aggregate,

coalesce, dehydrate, and solidify. The nature of the ACCdeposit on thesubstrate depends on therelativestrengthsof the cohesiveforceof ACCon each otherand the adhesiveforce between ACC andthe substrate. Theadhesiveforcesbetween mica surfaces and PDADMAC modified surfaceswith positively charged ACC colloids are weakenedbecause of the electrostatic repulsive interaction, so ACChemispheres are deposited on these surfaces. There is astrongerinteraction between thenegatively charged cleansilicon wafer and the positively charged ACC colloids,which results in the formation of a continuous ACC film.The PAH modified surface has only a few positivelycharged andmanyfree amino groups. Thus,because thereis not much repulsion between ACC and the substrate, an

ACC film also is obtained on it.

When PAA is employed as an additive in the depositionprocess, the situation changes dramatically: continuous

ACC films are deposited on all the substrates despite thevariations in the surfaces charges (Figure 3). PAA is well-known as an anti-scaling additive, so it has the effects ofinhibiting and dispersing CaCO3 precipitates. When PAAis introduced into the solution, it is easily adsorbed ontothe surfaces of the ACC colloids and changes the zetapotential of CaCO3,23 which results in inhibition of thecrystallization and aggregation of ACC and a decrease inthe cohesive forces of ACC. The adhesive force between

ACC and the substrates is then stronger than the cohesiveforce, so ACC tends to spread over the surface, whichresults in a continuous ACC film on the substrate. Thisconfirms again that the form of the ACC deposit on asubstrate is dependent on the balance between thecohesive and the adhesive forces.

From the above results, we conclude that the surfacecharacteristics significantly influence the deposition of

ACC on the substrate. A schematic diagram of thedeposition of ACC on varioussubstrates is shown in Figure4. ACC hemispheres form on positively charged micasurfaces and PDADMAC modified surfaces. A continuous

ACC film is deposited on negatively charged clean siliconwafers. On the weakly positively charged PAH modifiedsurface, a film is also produced. The variety of deposited

ACC forms reflects the differences in the adhesive forces

(18) Brecevic, Lj.; Nielsen, A. E. J. Cryst. Growth 1989, 98, 504.(19) Raz, S.; Hamilton, P. C.; Wilt, F. H.; Weiner, S. Addadi, L. Adv.

Funct. Mater. 2003, 13, 480.(20) Rieger, J.; Thieme, J.; Schmidt, C. Langmuir 2000, 16, 8300.(21) Pontoni, D.; Bolze, J.; Dingenouts, N.; Narayanan, T.; Ballauff,

M. J. Phys. Chem. 2003, 107, 5123.(22) Chibowski,E.; Hotysz, L.; Szczes, A. ColloidsSurf., A 2003,222,

41. (23) Jada, A.; Verraes, A. Colloids Surf., A 2003, 219, 7.

Figure 2. SEM microphotographs of ACC deposited on thesurfaces of silicon wafer modified with polyelectrolytes, deposi-tion time 50 min: (a) the PDADMAC modified surface; (b) parta at higher magnification; (c) the PAH modified surface;(d) part c at higher magnification.

Figure 3. SEM microphotographs of ACC deposited on allkinds of substrates in the presence of PAA: (a) bare siliconwafer; (b) mica; (c) PDADMAC modified silicon wafer; (d) PAHmodified silicon wafer.

Letters Langmuir C


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between ACC and the various substrates, which derivefromthe different surfacecharacteristicsof thesubstrates.

Our study has shown that ACC hemispheres aredepositedon bothmica and PDADMAC modified surfaces.Further, theabove discussionindicatesthat ACC hassomeliquid character and exhibits behavior similar to that ofliquid droplets. Therefore, we suggest that ACC in highlysupersaturated solutions spontaneously forms liquidlikecolloids with an open structure in solution, tends tocoalesce, and deforms into hemispheres or forms continu-

ous films when deposited on substrates. Many differentstructuresof CaCO3 biominerals in organisms may derivefrom thecharacterof liquidlike ACC colloids.Also insolublematrixand soluble macromoleculesexert important effectson the formation of CaCO3 biominerals.

In summary, we have demonstrated that substrates(insoluble matrix) and additives(soluble macromolecules)have significant influence on the crystallization of CaCO3and suggest that ACC plays an important role in thecrystallization and biominerlization of CaCO3. The form

of the ACC deposit on substrates can be controlled bymodifying the substrate surface, the introduction ofadditives, or both.

Acknowledgment. This workwas supported by Grant04K1501-01310 from Center for Nanostructured Materi-als Technology under the 21st Century Frontier R&DPrograms of the Ministry of Science and Technology, theNational Research Laboratory Program; R&D Programfor Fusion Strategy of Advanced Technologies of the

Ministry of Science and Technology of Korea, AdvanceEnvironmental Biotechnology Research Center; and theBrain Korea 21 Program of the Ministry of Education ofKorea.

Supporting Information Available: Optical micro-photographs of partial crystallized hemispheres on mica undercrossed-polarized light in reflective mode. This material isavailable free of charge via the Internet at http://pubs.acs.org.

LA047069V

Figure 4. Schematic diagram showing the deposition of ACC on all kinds of substrates in the absence or presence of PAA insolution.

D Langmuir PAGE EST: 3.5 Letters

deposition of amorphous calcium carbonate 110

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