Introduction

Because of their special properties and low cost, considerable attention has been given to synthetic organic–inorganic hybrid materials, which are inspired by biominerals.1, 2, 3, 4, 5, 6 The main inorganic mineral produced in natural organisms is CaCO3; organic–inorganic hybrid materials containing CaCO3 are of great interest for industrial and technological applications. To develop methods for preparation of new CaCO3 polymer hybrid materials, a fundamental understanding of binding kinetics and mechanisms of crystallization in organic polymer–Ca2+ complexes are required.7, 8, 9, 10, 11 Among various methods for preparation of CaCO3 polymer hybrid materials, carbonate-controlled addition is a way to control the mineralization of CaCO3 by simply changing the incubation time of the polymer–Ca2+ complexation process in aqueous solutions before addition of CO32− ions.12 We showed that the carbonate-controlled addition method using poly(acrylic acid) (PAA) resulted in the formation of stable amorphous calcium carbonate composite particles.12 The resulting amorphous calcium carbonate composite particles were monodispersed spheres, and the average particle size increased from 0.18±0.04 to 0.55±0.12 μm with an increase in the incubation time of the PAA–CaCl2 aqueous solution from 3 min to 24 h. The interaction and the reaction kinetics of PAA with Ca2+ were important for controlling the particle size. We recently reported that size-controlled vaterite particles were obtained by the carbonate-controlled addition method with the G0.5 poly(amidoamine) (PAMAM) dendrimer containing carboxylate groups at the surface.13 The minimum average particle size was 0.69±0.07 μm for particles synthesized with a 24 h complexation of the dendrimer-Ca2+ before the addition of CO22− ions.

In this study, we applied a carbonate-controlled addition method using a carboxylate-terminated hyperbranched PAMAM for preparation of CaCO3 particles (Figure 1). The properties of the resulting particles were compared with those of the CaCO3 particles using the G0.5 PAMAM dendrimer, which is a polymer particle composite that was previously reported.13 Although hyperbranched polymers have several practical advantages over the dendrimers, few studies using hyperbranched polymers for preparing CaCO3 particles have been reported.14, 15, 16 Here, we obtained size-controlled vaterite particles using carboxylate-terminated hyperbranched PAMAM by performing a carbonate-controlled addition. We also reported surface coating of the vaterite composite particles with different polymers and gold nanoparticles.

Figure 1
figure 1

Schematic illustration of the carbonate-controlled addition method. A full color version of this figure is available at Polymer Journal online.

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Experimental Procedure

Materials

Unless stated otherwise, all reagents and chemicals were obtained from commercial sources and used without further purification. PAAs (Mw: 5000 and 250 000) were purchased from WAKO Pure Chemical Industries, Ltd. (Osaka, Japan). Poly(arylamine) (PAAM) (Mw: 15 000) was purchased from Nittobo Medical Co., Ltd. (Tokyo, Japan). Poly(sodium 4-styrenesulfonate) (Mw: 70 000) and poly(diallyldimethylammonium chloride) (Mw: 100 000∼200 000) were purchased from Sigma-Aldrich Chemical Co. (St Louis, MO, USA). Methyl acrylate was distilled under reduced pressure and stored under nitrogen before use. Tris(2-di(methyl acrylate)-aminoethyl)amine was synthesized according to the method of Dvornic.17

Measurements

X-ray diffraction (XRD) was recorded on a Smart Lab (Rigaku, Akishima, Japan) with CuKα radiation (r=1.5406 Å) in θ/2θ mode at room temperature. The 2θ scans were collected at 0.01° intervals, and the scan speed was 2° (2θ) min. Fourier transform infrared (FT-IR) spectra were recorded with a FT-IR 4100 (JASCO, Tokyo, Japan) using a KBr pellet method. The morphology of CaCO3 particles was observed by scanning electron microscopy (SEM) using an EV-8800 (KEYENCE, Osaka, Japan) and field emission SEM (FE-SEM) using a JSM-7600F (JEOL Ltd, Tokyo, Japan). Elemental analysis of the particles was performed by energy-dispersive X-ray spectrometry (EDX) using an X-max (OXFORD Instruments, Oxfordshire, UK). Thermogravimetric analysis (TGA) was conducted using on a TGA-2950 (TA-Instruments, New Castle, PA, USA) at a heating rate of 10 °C per min in air. Ultraviolet-visible spectroscopy was performed using a V-670 (JASCO, Tokyo, Japan). Nuclear magnetic resonance (NMR) was performed with a PDX-300 (300 MHz or 400 MHz) (Bruker Biospin GmbH, Rheinstetten, Germany). The molecular weight was measured by gel permeation chromatography with a TOSOH 8020 (TOSOH, Tokyo, Japan) using a TSK-gel a-M column calibrated with standard poly(styrene) using dimethylformamide solution containing 10 mM LiBr as an eluent.

Methylester-terminated hyperbranched PAMAM (HYPAM-OCH3)

Amino-terminated hyperbranched PAMAM denoted as HYPAM, which is chemically analogous to the 4th generation of the PAMAM dendrimers, was synthesized according to the method of Pérignon.18 Tris(2 aminoethyl)amine (1.6 g, 11 mmol) was mixed with tris(2-di(methyl acrylate)-aminoethyl)amine (0.70 g, 1.1 mmol). The solution was stirred under nitrogen at 75 °C for 2 days. After the resulting dispersion was dissolved in CH2Cl2 (5 ml) and poured into THF (200 ml) at 0 °C, HYPAM was obtained as a yellow gum. The HYPAM yield was 1.5 g. The 1H and 13C NMR data for HYPAM were in agreement with data in the literature.18 A methanol solution (15 ml) of HYPAM (1.5 g) was mixed with methyl acrylate (18 ml). The solution was stirred under nitrogen at 40 °C for 5 days. After the reaction mixture was poured into 200 ml of diethylether at 0 °C, methylester-terminated hyperbranched PAMAM (HYPAM-OCH 3) was obtained as a yellow gum. The HYPAM-OCH 3 yield was 2.0 g.

1H NMR (D2O, 300 MHz): 2.44(br s, -CH2CO-), 2.57–2.59(br, NCH2CH2N), 2.68(br s, CONHCH2CH2N), 2.84(br s, NCH2CH2CO), 3.33(br s, -CH2NHCO-), 3.71(s, -OCH3), 1H NMR (CDCl3, 300 MHz): 2.25(br s, -CH2CO-), 2.35–2.45(br, NCH2CH2N), 2.53(m, -CONHCH2CH2N), 2.75–2.80(m, NCH2CH2CO), 3.25(br s, -CH2NHCO-), 3.66(s, -OCH3); 13C NMR (CDCl3, 300 MHz): 32.5(CH2CO), 33.2(CH2CO), 37.5(CH2NHCO), 49.7(NCH2CH2CO), 51.9(-CONHCH2CH2N), 52.6(COOCH3), 59.8(NCH2CH2N), 160.1(COO), 173.0(CONH).

Carboxylate-terminated hyperbranched PAMAM (HYPAM-ONa)

To a methanol solution (10 ml) of HYPAM-OCH 3 (0.92 g) a 1 M NaOH aqueous solution (20 ml) was added. After the reaction mixture was refluxed for 2 h, the solvent was removed under reduced pressure. The residues were purified by size exclusion chromatography (LH-20) using a methanol eluent to obtain sodium carboxylate-terminated hyperbranched PAMAM (HYPAM-ONa) as a yellow powder. The HYPAM-ONa yield was 1.2 g.

1H NMR (D2O, 300 MHz): 2.43(br s, -CH2CO-), 2.72(br s, NCH2CH2N), 2.82(br s, -CH2NHCO-), 13C NMR (D2O, 300 MHz): 35.4(CH2CO), 37.0(CH2NHCO), 39.2(CH2NH2), 51.7(NCH2), 52.8(NCH2), 53.8(NCH2), 177.5(CONH), 184.1(COO).

Determining the amount of unmodified primary amine in HYPAM-ONa

A 2,4,6-trinitrobenzenesulfonic acid (TNBS) method19 was performed to determine the amount of unmodified primary amine in HYPAM-ONa. To a 0.1 M sodium tetraborate aqueous solution of HYPAM-ONa, 0.03 M TNBS aqueous solution was added. After stirring at room temperature for 1 h, the reaction solution was measured by UV–vis analysis at 420 nm. A standard curve for measuring the sample was obtained using tris(2-di(methyl)-aminoethyl)amine.

Precipitation of CaCO3

Standard preparation of CaCO3 precipitate was carried out as follows. First, a stock aqueous solution of HYPAM-ONa (60 mg in 45 ml of distilled water) was prepared in distilled water, and the pH was adjusted to 11 using a dilute aqueous solution of NaOH. Then, 2.5 ml of a 0.1 M CaCl2 aqueous solution (adjusted to pH 8.5 with aqueous NH3) was added dropwise at a rate of 1 ml per min to 45 ml aqueous HYPAM-ONa solution under gentle stirring at 30 °C under nitrogen. After mixing the reaction solution for durations ranging from 3 min to 72 h, 2.5 ml of a 0.1 M (NH4)2CO3 aqueous solution (adjusted to pH 10.0 with aqueous NH3) was added dropwise at a rate of 1 ml per min into the reaction solution under nitrogen. This solution was stored at 30 °C for 1 day with gentle stirring. The precipitated CaCO3 products were collected using a 0.2 μm-pore-sized membrane filter, washed with water several times, and then dried at room temperature under reduced pressure.

Preparation of CaCO3 modified with gold nanoparticles stabilized with HYPAM-ONa

A total of 5 ml aqueous HAuCl4 solution (2.4 × 10−3 mmol) was added to 5 ml of an aqueous HYPAM-ONa solution (6.5 mg, pH 11) under gentle stirring. As soon as the solution was mixed uniformly, 3 ml of an aqueous NaBH4 solution (6.0 × 10−3 mmol) was added to the mixed solution. A stock solution of gold nanoparticles stabilized with HYPAM-ONa was then obtained. The CaCO3 particles (10 mg) obtained by the complexation process for 72 h were dispersed into distilled water (10 ml), and 0.2 ml of the stock solution of the gold nanoparticles stabilized with HYPAM-ONa was added into the dispersion of CaCO3 particles every 15 min for a total of eight times, and the dispersion was gently stirred. The dispersion was stored at room temperature for 1 day. The precipitated red-colored products were collected using a 0.2-μm-pore-sized membrane filter, washed with water several times, and then dried at room temperature under reduced pressure to obtain red-colored CaCO3 particles (6.6 mg). The red-colored CaCO3 particles (6.0 mg) were dispersed into methanol (5 ml). To this dispersion, 1 ml of an aqueous HAuCl4 solution (2.4 × 10−3 mmol) was added with gentle stirring. Next, 1 ml of an aqueous HCHO solution (1.2 × 10−3 mmol) was immediately added into the dispersion. The dispersion was stored at room temperature for 1 day. The resulting dark purple-colored CaCO3 products were collected using a 0.2-μm-pore-sized membrane filter, washed with methanol several times, and dried at room temperature under reduced pressure to obtain purple-colored CaCO3 particles (4.4 mg).

Results and Discussion

Synthesis of sodium carboxylate-terminated hyperbranched PAMAM

The synthesis of a methyl ester-terminated hyperbranched polymer (HYPAM-OCH 3), with a structure similar to that of the PAMAM dendrimer, was based on a method described by Pérignon.18 After the polymerization of excess amounts of tris(2-aminoethyl)amine with tris(2-di(methyl acrylate)-aminoethyl)amine, a Michael reaction was carried out with methyl acrylate to obtain HYPAM-OCH 3 (Scheme 1). When the molar ratio of (tris(2-aminoethyl)amine)/(tris(2-di(methyl acrylate)-aminoethyl)amine) was 10, the weight-average molecular weight (Mw) and number-average molecular weight (Mn) of HYPAM-OCH 3 were 12 300 and 6500, respectively, based on poly(styrene) standards. The molecular weight of HYPAM-OCH 3 is close to that of the G4 amino-terminated PAMAM dendrimer (Mw=14 215). The sodium carboxylate-terminated HYPAM (HYPAM-ONa) was quantitatively obtained by treatment of HYPAM-OCH 3 with NaOH. The 1H and 13C NMR analysis of HYPAM-ONa indicated complete deprotection. According to the TNBS method19 for HYPAM-ONa, the unreacted primary amine in HYPAM-ONa was 6.8 × 10−4 mol%. This supports the 1H and 13C NMR data for HYPAM-ONa.

Effect of the incubation time of the HYPAM-ONa-CaCl2 solution on the formation of calcium carbonate particles

A CaCl2 aqueous solution was added to an aqueous solution of HYPAM-ONa and stirred at 30 °C. Before the addition of an aqueous (NH4)2CO3, the solution was not turbid, even after incubation for 72 h. After the aqueous (NH4)2CO3 solution was added to the reaction mixture at different time periods (from 3 min to 72 h), the solution became turbid. The reaction mixtures were stored at 30 °C for 1 day, and the products were collected using a 0.2-μm-pore-sized membrane filter. The crystal CaCO3 phases of the obtained products were characterized by FT-IR analysis. All the products obtained from the incubation time of 3 min, 1 h, 24 h and 72 h showed two characteristic bands at 877 and 745 cm−1 by FT-IR, indicating vaterite formation (Figure 2). The crystalline phases of the CaCO3 products were also confirmed by XRD analysis (Figure 3). SEM observations showed that all the obtained particles were spherical (Figure 4). The average particle size decreased from 1.3±0.2 to 0.36±0.18 μm with an increase in the incubation time of the HYPAM-ONa-CaCl2 solution from 3 min to 72 h (Table 1). These characteristics were similar to the trends observed in G0.5 PAMAM dendrimers that were previously reported.13

Figure 2
figure 2

FT-IR spectra of the HYPAM-ONa-CaCO3 particles by the carbonate-controlled addition method at different incubation times of (a) 3 min, (b) 1 h, (c) 24 h, and (d) 72 h; These correspond to sample 1, 2, 3 and 4 in Table 1, respectively.

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Figure 3
figure 3

Comparison of XRD patterns of the HYPAM-ONa-CaCO3 particles by the carbonate-controlled addition method at different incubation times of (a) 3 min, (b) 1 h, (c) 24 h, and (d) 72 h; These correspond to sample 1, 2, 3 and 4 in Table 1, respectively. The characteristic of XRD patterns for vaterite (d-spacing/2θ peak: 3.58 Å/24.9°, 3.3 Å/27°, 2.73 Å/32.8°, corresponding to hkl: 110, 111, 112, respectively).

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Figure 4
figure 4

SEM images of the HYPAM-ONa-CaCO3 particles by the addition of (NH4)2CO3 after incubating the HYPAM-ONa-CaCl2 solution for (a) 3 min, (b) 1 h, (c) 24 h, and (d) 72 h; These correspond to sample 1, 2, 3 and 4 in Table 1, respectively.

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Table 1 Effect of the incubation time of the HYPAM-ONa-CaCl2 solution on mineralization of CaCO3a
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The composition of the vaterite particles was estimated by TGA (Figure 5). The results are summarized in Table 1. The TGA analysis showed that the content of H2O in the particles decreased from 6.9 to 4.6 wt% as the incubation time of the HYPAM-ONa-CaCl2 solution increased. The contents of the organic parts in all of the products were approximately 8 wt%.

Figure 5
figure 5

TGA thermographs of the HYPAM-ONa-CaCO3 particles synthesized by a carbonate-controlled addition method at different incubation times of (a) 3 min, (b) 1 h, (c) 24 h, and (d) 72 h; These correspond to sample 1, 2, 3 and 4 in Table 1, respectively.

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Vaterite is the most thermodynamically unstable form among the three crystal structures of CaCO3. It is well-known that vaterite transforms into thermodynamically stable calcite within few days in an aqueous solution at room temperature. However, none of the vaterite particles transformed to stable calcite when the particles were stirred in distilled water for more than 3 days at room temperature. These results indicate that the surface of the vaterite particles was stabilized by HYPAM-ONa, which prevented phase transformation. These characteristics were similar to trends observed in G0.5 PAMAM dendrimers, as previously reported.13

Surface properties of the calcium carbonate particles

The dispersibility of the CaCO3 composite particles was estimated by turbidity measurements. The dispersibility of an aqueous solution of the HYPAM-ONa-CaCO3 particles was compared with that of the previously prepared G0.5 PAMAM dendrimer-CaCO3 particles.13 The size of the HYPAM-ONa-CaCO3 particles and the G0.5 PAMAM dendrimer-CaCO3 particles used here were 0.61±0.28 μm and 0.85±0.16 μm, respectively. Both the HYPAM-ONa-CaCO3 particles and the G0.5 PAMAM dendrimer-CaCO3 particles were obtained by the carbonate-controlled addition method based on a complexation time of 24 h. Figure 6 shows that the dispersibility of the HYPAM-ONa-CaCO3 particles in water was improved compared with the G0.5 PAMAM dendrimer-CaCO3 particles. The TGA analysis of the CaCO3 particles indicated that the HYPAM-ONa and G0.5 PAMAM dendrimer content of the CaCO3 particles was 8 wt% and 4 wt%, respectively. The higher dispersibility of the HYPAM-ONa-CaCO3 particles might be due to the higher polymer content in the particles compared with G0.5 PAMAM dendrimer-CaCO3 particles. It should be noted that the dispersibility of the HYPAM-ONa-CaCO3 particles in methanol were significantly improved compared with the G0.5 PAMAM dendrimer-CaCO3 particles.

Figure 6
figure 6

The turbidity of the aqueous dispersions (4 ml) of the HYPAM-ONa-CaCO3 particles (1.0 mg) (a) and the G0.5 PAMAM dendrimer-CaCO3 particles (1.0 mg) (b) measurements at 500 nm. The size of the HYPAM-ONa-CaCO3 particles and the G0.5 PAMAM dendrimer-CaCO3 particles were 0.61±0.28 μm and 0.85±0.16 μm, respectively, both of which were obtained by a carbonate-controlled addition method with 24 h complexation.

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To further understand the surface properties of the HYPAM-ONa-CaCO3 particles, we studied interactions of the CaCO3 particles with different types of commercially available ionic polymers. The HYPAM-ONa-CaCO3 particles (1.0 mg) obtained by the complexation time of 3 min were dispersed in water (20 ml), and 2 ml of an aqueous solution containing an ionic polymer (1 mg ml–1) was added. After the dispersion was stored overnight at room temperature, the precipitates were filtered with a 0.2-μm-pore-sized membrane. An aqueous solution of poly(sodium 4-styrenesulfonate) (Mw: 70 000) (pH 7), an anionic polymer, was added to the dispersion of the CaCO3 particles, causing no change to the dispersion of the CaCO3 particles. Addition of an aqueous solution of PAA (Mw: 250 000), after adjusting to pH 7, also resulted in no change of particle dispersion. These results indicate that the anionic moieties in the polymers were only slightly interacting with the protonated tertiary amine units in HYPAM-ONa. Conversely, addition of an aqueous solution of poly(diallyldimethyl ammonium chloride) (PDDA) (Mw: 100 000∼200 000) (pH 7) as a cationic polymer resulted in transformation of the polymorphic CaCO3 particles to thermodynamically stable calcite. Because the morphology of the calcite, observed by SEM, was a typical rhombohedral crystal, PDDA might strip HYPAM-ONa from the CaCO3 particles as a result of strong ionic interaction between the anionic parts of HYPAM-ONa and the cationic groups in PDDA. Addition of an aqueous solution of PAAM (Mw: 15 000), with adjustment of the pH to 7, caused the particles to dissolve and form a homogeneous solution. Because PAAM is regarded as a cationic polymer at pH 7, PAAM might also strip HYPAM-ONa from the CaCO3 particles as a result of strong ionic interactions between the anionic parts of HYPAM-ONa and the cationic groups in PAAM.

The effects of PAA and PAAM at pH 10 on the dispersion of the HYPAM-ONa-CaCO3 particles were significantly different from those at pH 7. When an aqueous solution of PAA (Mw: 250 000) (adjusted to pH 10 with aqueous NaOH) was added to the dispersion of the CaCO3 particles, the particles were immediately aggregated to form precipitates. The polymorphs of the resulting precipitates were still vaterite, as determined by FT-IR analysis. Addition of an aqueous solution of a lower molecular weight PAA (Mw: 5000) (adjusted to pH 10 with aqueous NaOH) to the dispersion of the CaCO3 particles also immediately caused sedimentation. After a 24 h incubation, the precipitates dissolved to form a homogeneous solution. These results suggest that the interaction between PAA and Ca2+ was stronger than the interaction between HYPAM-ONa and Ca2+. Because the mobility of the lower molecular weight PAA might be greater than that of the higher molecular weight PAA, dissolution of the CaCO3 particles by the former is faster than that of the later. In the case of PAAM (Mw: 15 000) (adjusted to pH 10) addition to the dispersion of CaCO3 particles, the particles were gradually precipitated in the dispersion within several hours. The SEM analysis of the precipitates showed aggregates of the CaCO3 particles. On the basis of FT-IR analysis, the polymorph of the particles was found to be vaterite. These results indicate that hydrogen interaction between the amide groups of HYPAM-ONa and the amino groups PAAM likely contributes to the formation of CaCO3 particle aggregates.

We studied the effect of PAAM addition (aqueous solutions of different concentrations) to the CaCO3 particles to coat the particles with PAAM. An aqueous solution (2 ml) containing 0.1 mg ml–1, 1 mg ml–1, or 10 mg ml–1 of PAAM was added to the aqueous dispersion of 1.0 mg of HYPAM-ONa-CaCO3 particles prepared by a complexation duration of 3 min. After the dispersion was stored overnight, the particles were filtered with a 0.2-μm-pore-sized membrane. SEM analysis showed that significant aggregation of the CaCO3 particles occurred with PAAM (0.1 mg ml–1) (Figure 7a). However, based on SEM analysis, addition of 10 mg ml–1 of PAAM, resulted in no particle aggregation (Figure 7c). On the basis of FT-IR analysis, the polymorphs for all of the particles were found to be vaterite. The TGA analysis of the particles obtained by the addition of 10 mg ml–1 of PAAM showed two clear weight losses at 350 and 450 °C (Figure 8). The former and latter species are derived from decompositions of HYPAM-ONa and PAAM, respectively. The contents of both polymers in the particles, as estimated by the TGA analysis, are summarized in Table 2. The particles obtained by the addition of 10 mg ml–1 of PAAM consisted of 6.3 wt% of HYPAM-ONa and 23 wt% of PAAM. These results suggest that surface coating of the HYPAM-ONa-CaCO3 particles with PAAM was successfully achieved by controlling the concentration of PAAM in aqueous solutions. The particles obtained by the addition of 0.1 and 1 mg ml–1 of PAAM consisted of 10.3 wt% and 11.1 % of PAAM, respectively, and no detectable weight loss was observed at 450 °C.

Figure 7
figure 7

SEM images of the CaCO3 particles coated by PAAM using aqueous solutions containing (a) 0.1, (b) 1 and (c) 10 mg/ ml–1 of PAAM; These correspond to samples 1, 2, and 3 in Table 2, respectively.

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Figure 8
figure 8

TGA thermographs of the CaCO3 particles coated by PAAM using aqueous solutions containing (a) 0.1, (b) 1 and (c) 10 mg ml–1 of PAAM; These correspond to samples 1, 2 and 3 in Table 2, respectively.

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Table 2 Analysis of TGA measurements of the obtained CaCO3 particles coating by PAAM
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Surface modification of calcium carbonate particles by gold nanoparticles

Gold nanoparticles stabilized with HYPAM-ONa served as a precursor solution for surface modification of the CaCO3 particles and were prepared by treating an aqueous solution containing HYPAM-ONa and HAuCl4 (adjusted pH 11 with aqueous NaOH) with NaBH4. The molar ratios of [HAuCl4]/[HYPAM-ONa] and [NaBH4]/[Au] were 6.2 and 2.2, respectively.20 The precursor solution was gradually added to a dispersion of CaCO3 particles obtained by the 72 h complexation. After the dispersion was stirred overnight at room temperature, the resulting products were isolated as red-colored precursor CaCO3 particles, of which vaterite was the primary crystalline phase, as determined by FT-IR analysis. These results indicate that no transformation of the crystalline phase occurred during this process.

To a methanol dispersion of the red-colored precursor CaCO3 particles, aqueous HAuCl4 and formaldehyde solutions were added. The color of the dispersion immediately changed from red to dark-blue. After the dispersion was stored overnight at room temperature, the resulting products were isolated as dark purple-colored CaCO3 particles. The polymorph of the CaCO3 particles was still vaterite, as determined by FT-IR analysis, indicating that no transformation of the crystalline phase had occurred. The solid-state UV–vis absorption spectra of the red-colored precursor CaCO3 particles and the dark purple-colored CaCO3 particles are shown in Figure 9. The surface plasmon bands of the red-colored precursor particles and the final CaCO3 particles were observed at 570 nm–790 nm, respectively. A red-shifted surface plasmon band is a consequence of increased overlap of the dipole resonances between neighboring gold nanoparticles on the surface of the dark purple-colored CaCO3 particles compared with that of the red-colored precursor CaCO3 particles. According to the TGA analysis of the final dark purple-colored CaCO3 particles, the gold quantity was determined as 9 wt%.

Figure 9
figure 9

Solid-state UV–vis absorption spectra of (a) the red-colored precursor CaCO3 particles and (b) the final dark purple-colored CaCO3 particles after addition of HAuCl4 and HCHO.

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The FE-SEM image of the resulting red-colored precursor particles shows that nanoparticles that are 8.8±0.8 nm in diameter cover the surface of the CaCO3 composite particles (Figure 10b). Conversely, the pristine HYPAM-ONa-CaCO3 particles have a rather smooth surface composed of 35±4 nm particles, based on the FE-SEM image (Figure 10a). Vaterite particles are usually obtained as a spherical shaped poly crystal, which is constructed of 25–35 nm nano-crystallites.21,22 The 8.8±0.8 nm nanoparticles observed on the surface of the red-colored particles might be assigned to the gold nanoparticle precursors. The composition of the red-colored precursor particles was further probed by an EDX analysis. From the distribution of Au and Ca in the red-colored precursor particles measured by the EDX analysis at an electron-accelerating voltage of 5 kV, Au and Ca were uniformly distributed on the particle (Figure 11a). However, in the same area of the EDX image at an electron-accelerating voltage of 15 kV, only Ca signals were detected (Figure 11b). These results indicate that Au was mainly distributed on the surface of the red-colored precursor particles. The FE-SEM images showed that the dark purple-colored particles have a rather smooth surface (Figure 10c). The EDX analysis also indicates that Au was distributed on the surface of the final dark purple-colored CaCO3 particles (Figures 11c and ). On the basis of the results from the TGA analysis, the calculated thickness of the gold nanoparticle layer on the surface of the CaCO3 particles is less than several nanometers.

Figure 10
figure 10

FE-SEM images of (a) the pristine HYPAM-ONa-CaCO3 particles, (b) the red-colored precursor CaCO3 particles, and (c) the final dark purple-colored CaCO3 particles after addition of HAuCl4 and HCHO. The samples were vapor-deposited with Pt and measured at 15 kV.

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Figure 11
figure 11

FE-SEM images and EDX of the red-colored precursor CaCO3 particles measured at 5 kV (a) and 15 kV (b), and the final dark purple-colored CaCO3 particles after addition of HAuCl4 and HCHO measured at 5 kV (c) and 15 kV (d). All samples were vapor-deposited with carbon.

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Conclusions

In this study, the size-controlled CaCO3 particles with HYPAM-ONa were synthesized using a carbonate-controlled addition method. The average sizes of the CaCO3 particles decreased from 1.3±0.2 μm to 0.36±0.18 μm with an increase in the incubation time of the HYPAM-ONa-CaCl2 solution from 3 min to 72 h. All of the CaCO3 particles obtained were composed of vaterite. The dispersibility of the HYPAM-ONa-CaCO3 particles in distilled water and methanol was improved compared with those of the G0.5 PAMAM dendrimer-CaCO3 particles, as previously prepared.13 We also found that surface coating of the HYPAM-ONa-CaCO3 particles with PAAM was successfully achieved by controlling the concentration of PAAM in aqueous solutions. Hydrogen interactions between the amide groups of HYPAM-ONa and the amino groups PAAM likely contribute to the formation of CaCO3 particle aggregates. Gold nanoparticle-coated CaCO3 composite particles were also obtained adding an aqueous solution of gold nanoparticles stabilized with HYPAM-ONa to the HYPAM-ONa-CaCO3 particles and subsequently adding HAuCl4 and formaldehyde. The surface plasmon band of the gold nanoparticle-coated CaCO3 particles appeared at 790 nm. The present composite particles can serve as components of a new class of nanomaterials that are capable of controlling radiation in the near infrared spectral regions.

scheme 1

Synthesis of HYPAM-ONa.