1. Introduction

Inorganic minerals have been widely used in drug delivery systems for biomedical applications [1]. Calcium carbonate (CaCO₃), an inorganic biomineral, has been used as an antacid agent. It can be orally administered as a tablet, chewable tablet, capsule, or liquid. Furthermore, it has been used for the controlled and sustained delivery of chemical drugs [2,3,4], photosensitizers [5], and proteins [6,7] because of its biocompatibility and slow biodegradation [8]. CaCO₃ is stable at physiological pH, but can be dissociated under acidic conditions [9,10]. Owing to their pH sensitivity, CaCO₃-based delivery systems concentrate drugs into targeted cancer tissues within the acidic tumor microenvironment (TME) [4,5,10]. In addition, these systems react with protons (H⁺) to neutralize the acid [11,12]. CaCO₃ can generate carbon dioxide (CO₂) in the acidic TME, and this gas-generating property has extended its application as an ultrasound contrast agent for cancer imaging [5,10]. These previous studies have revealed the pH sensitivity, CO₂ gas generation, and antacid properties of CaCO₃ materials.

Polyphenol tannic acid (TA) is composed of five digalloyl ester groups that have been linked to a central glucose molecule, and exhibits various biological properties, such as antibacterial, anti-inflammatory, antioxidant, and anticancer activities [13,14]. Previous studies have revealed the TA-mediated scavenging of free radicals, leading to the inhibition of lipid oxidation and radical-mediated DNA damage [13,15,16]. Given their ability to undergo multiple interactions with various biomacromolecules (i.e., nucleic acids, peptides, proteins, and polysaccharides) through electrostatic and hydrogen bonding and/or hydrophobic interactions [17,18,19,20,21], TA–macromolecular complexes can be easily produced and used as surface modifiers on organic and inorganic substrates [22,23]. Thus, the TA-modified polymeric hydrogels and scaffolds greatly enhance anti-inflammatory effects in vitro or protect cells under reactive oxygen species (ROS) environments [23,24]. Moreover, as TA can coordinate with metal ions, it can be used to synthesize inorganic Ag and Au nanomaterials [25]. Our group recently prepared oxygen-generating calcium peroxide using TA through coordination between the catechol moieties of TA and calcium ions [26].

Based on these previous reports, we propose that the introduction of TA into CaCO₃ materials can endow them with anti-inflammatory and antioxidant functions. As CaCO₃ materials exert no anti-inflammatory or antioxidant functions, herein we fabricated tannylated CaCO₃ (termed as TA-CaCO₃) materials via the coordination of TA to calcium (Ca²⁺) ions and further nucleation of CaCO₃ using carbonate ions (CO₃²⁻). We performed their physicochemical characterization and evaluated their antacid and antioxidant effects using colorimetric methods. In addition, we validated their in vitro antioxidant and anti-inflammatory properties in chondrocytes under inflammatory and ROS conditions.

2. Preparation of TA-CaCO₃ Materials

TA-CaCO₃ materials were synthesized by combining equimolar amounts of Ca²⁺ and CO₃²⁻ at different concentrations with a fixed molar concentration of TA under constant stirring, as depicted in Figure 1. Given the tendency of TA to coordinate with metal ions [25,27,28], TA interacts with Ca²⁺ ions and facilitates the nucleation of CaCO₃ to form TA-CaCO₃ nanoparticles, which then agglomerate into microparticles due to the interactions between TA-CaCO₃ nanoparticles.

Figure 1. Schematic illustration of the synthesis of TA-CaCO₃ materials. TA was sequentially reacted with CaCl₂ for 2 h and Na₂CO₃ for an additional 24 h in pure water (pH = 7.0).

3. Characterization of TA-CaCO₃ Materials

The morphologies of the synthesized TA-CaCO₃ materials were examined using scanning electron microscopy (SEM). As shown in Figure S1, at 25 molar ratios of calcium chloride (CaCl₂)/sodium carbonate (Na₂CO₃), aggregates comprising small TA-CaCO₃ particles were predominantly observed, and very small amounts of micron-sized spherical TA-CaCO₃ particles were formed. More spherical TA-CaCO₃ microparticles were gradually observed as the molar ratio of CaCl₂/Na₂CO₃ was increased to 75. Interestingly, although spherical TA-CaCO₃ microparticles were still observed when the molar ratios of TA and CaCl₂/Na₂CO₃ were 100 or 150, more irregular and broken TA-CaCO₃ particles were detected. These results suggested that small TA-CaCO₃ particles were formed at lower molar ratios of CaCl₂/Na₂CO₃ and that spherical and broken TA-CaCO₃ particles were more common at higher molar ratios of TA and CaCl₂/Na₂CO₃. Based on SEM images, 1:75 TA-CaCO₃ particles were selected for further experiments because they produced more spherical TA-CaCO₃ microparticles than other TA-CaCO₃ particles.

Next, we investigated the particle size of the 1:75 TA-CaCO₃ materials and found it to range from approximately 3 to 6 μm (Figure 2a and Figure S1). Interestingly, the magnified SEM images revealed the presence of small particles on the surface of the 1:75 TA-CaCO₃ materials. These individual small particles ranged from 17 to 41 nm in size, and were approximately 26.18 ± 4.6 nm in diameter and spherical in shape (Figure 2b). SEM images revealed that the micron-sized TA-CaCO₃ materials consisted of small TA-CaCO₃ nanoparticles, probably owing to agglomeration following interactions between individual small nanoparticles.

Figure 2. (a) The representative SEM images and magnified surface of 1:75 TA-CaCO₃ materials. The magnified SEM image revealed that the micron-sized TA-CaCO₃ materials comprised small TA-CaCO₃ nanoparticles. (b) Particle size distribution of the small TA-CaCO₃ nanoparticles of 1:75 TA-CaCO₃. (c) EDS mapping of elemental carbon (C; white), oxygen (O; green), and calcium (Ca; red). Scale bar: 1 μm. (d) EDS spectrum of 1:75 TA-CaCO₃.

The preparation of TA-CaCO₃ materials was confirmed using an SEM coupled with energy-dispersive (SEM-EDS) X-ray spectroscopy, inductively coupled plasma optical emission spectrometry (ICP-OES), Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD) patterns, and X-ray photoelectron spectroscopy (XPS). The EDS mapping and spectrum data revealed the presence of carbon, oxygen, and calcium on the surface of TA-CaCO₃ materials (Figure 2c,d). Consistent with a previous report [29], the EDS spectrum showed the peaks of Kα (0.277 eV) corresponding to carbon, Kα (0.523 eV) corresponding to oxygen, and Kα (3.691 eV) and Lα (0.341 eV) corresponding to calcium. ICP-OES analysis showed that 0.1 mg of 1:75 TA-CaCO₃ contained 18.6 μg of TA and 81.4 μg of CaCO₃. The FT-IR spectrum of commercial CaCO₃ showed the characteristic vibrations of carbonate ions (at 1805, 1410, 1090, and 874 cm⁻¹) (Figure 3), as previously reported [30,31]. The preparation of TA-CaCO₃ materials was confirmed by the presence of the main asymmetric vibrations at 1460 and 1410 cm⁻¹. However, the symmetric vibration at 725 cm⁻¹ disappeared, implying that TA-CaCO₃ has an amorphous structure. Meanwhile, TA-CaCO₃ showed characteristic peaks at 3300–3600 cm⁻¹ (O-H stretching), 1445 cm⁻¹ (C-C stretching of benzene ring and methylene; C-O stretching of phenolic), and 755 cm⁻¹ (C-H torsion of benzene ring) [32], indicative of the presence of TA on the material.

Figure 3. FT-IR spectra of commercial CaCO₃ and TA-CaCO₃.

Next, commercial CaCO₃ and synthesized TA-CaCO₃ crystal phases were identified via XRD analysis (Figure 4a). CaCO₃ exhibited the characteristic peaks, such as the plane of the calcite at 29.3° (104), and the calcite crystal faces at 23.02° (012), 35.9° (110), 39.4° (113), and 43.1° (202), respectively. Meanwhile, the diffraction peaks of TA-CaCO₃ were observed at 24.8° (110), 27.08° (112), 32.7° (114), 43.8° (300), 49.1° (304), and 50.08° (118), respectively, and these diffraction peaks were consistent with the vaterite crystal faces [33,34]. Based on XRD data, we think that the prepared TA-CaCO₃ materials are the vaterite form of calcium carbonate.

Figure 4. XRD and XPS analyses of commercial CaCO₃ and TA-CaCO₃ materials. (a) XRD spectra of CaCO₃ and TA-CaCO₃. (b) Wide scan XPS spectra recorded from CaCO₃ and TA-CaCO₃.

XPS data revealed that CaCO₃ and TA-CaCO₃ showed Ca, O, and C signals (Figure 4b). The binding energy peaks of these two materials appeared O1s at 531 eV, Ca2s at 441 eV, two Ca2p at 351 and 347.2 eV, and two C1s peaks at 289.3 eV (CO₃ in the CaCO₃ surface) and 284.6 eV (adventitious carbon peak), respectively. These data demonstrated the successful synthesis of TA-CaCO₃, and the synthesized TA-CaCO₃ materials are vaterite calcium carbonate.

4. Antacid Effects of TA-CaCO₃

CaCO₃ exists as a stable crystalline solid at physiological pH, but can be dissociated into ionic species at or below weakly acidic pH [5,9]. Under acidic pH, CaCO₃ neutralizes acids by reacting with the proton (H⁺) [11,12], and it has been used as an acid neutralizer [35].

To verify the antacid effects of TA-CaCO₃ materials, commercial CaCO₃ and TA-CaCO₃ were dispersed in phosphate-buffered saline (PBS; physiological pH = 7.4) and simulated gastric fluid (SGF, pH 1.5) containing bromothymol blue (BTB). The color and absorption changes of BTB were monitored before and after the reaction, because BTB is a useful acid/base indicator to distinguish the acidity, neutrality, and alkalinity of an aqueous solution [36,37]. As shown in Figure 5a and Figure S2, the aqueous BTB solution without commercial CaCO₃ and TA-CaCO₃ exhibited a blue color at pH = 7.4 and turned to a yellowish color in the presence of SGF (pH = 1.5). CaCO₃ and TA-CaCO₃ showed a deep blue color of BTB at pH = 7.4, indicating the slight pH increases of the solution following the degradation of CaCO₃ and TA-CaCO₃, even at pH = 7.4. In contrast, the yellowish BTB solution at pH = 1.5 turned to a bluish green color after the reaction with CaCO₃ and TA-CaCO₃, indicating that the pH of the solution increased to a nearly neutral pH (approximately 7). Consistent with these color changes, the λ_max shift of BTB occurred from 615 nm at pH = 7.4 to 433 nm under SGF (pH = 1.5) (Figure 5b). However, the absorptions of both CaCO₃ and TA-CaCO₃ groups increased at 615 nm, while the λ_max of BTB at pH = 1.5 was blue-shifted from 433 nm to 403 nm, implying the pH increases of the solutions after reacting with two kinds of CaCO₃ materials. Meanwhile, TA-CaCO₃ showed significantly higher absorbance below 400 nm at both pH = 1.5 and pH = 7.4, indicating the presence of TA in the solutions. Furthermore, TA-CaCO₃ had a lower absorbance than commercial CaCO₃ at 615 nm. This is attributed to the fact that TA-CaCO₃ contained a lower amount of CaCO₃ than commercial CaCO₃.

Figure 5. Antacid effects of TA-CaCO₃ using the colorimetric bromothymol blue (BTB) method. (a) Color changes and (b) UV/vis spectra of BTB under PBS (pH = 7.4) and simulated gastric fluid (SGF, pH = 1.5) after treatment with commercial CaCO₃ and 1:75 TA-CaCO₃. Green-colored arrows indicate a pH increase.

5. Antioxidant Effects of TA-CaCO₃

The antioxidant effects of TA-CaCO₃ were determined using the stable free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) [38]. As shown in Figure 6, the DPPH solution showed a deep violet color, with an absorbance at approximately 520 nm, owing to the delocalization of the spare electron over the molecule [38]. CaCO₃-treated DPPH solution also had a deep violet color and an absorption band at around 520 nm, indicating that CaCO₃ had no antioxidant effect. In contrast, TA-CaCO₃-treated DPPH solution turned yellowish in color and lost its absorbance at 520 nm. This loss of violet color might be attributed to the presence of TA molecules within TA-CaCO₃ because TA molecules have an effective radical-scavenging activity, as its hydroxyl groups easily reduce the free radicals of DPPH [39,40]. These data imply that TA-CaCO₃ materials have antioxidant properties and effective ROS-scavenging activity.

Figure 6. Antioxidant effects of TA-CaCO₃ determined using the colorimetric DPPH method. Changes in the color and UV/vis spectra of DPPH after treatment with commercial CaCO₃ and 1:75 TA-CaCO₃. Inset: Photos of the DPPH solution treated with commercial CaCO₃ and 1:75 TA-CaCO₃.

6. Conclusions

In the present study, we prepared TA-CaCO₃ materials by reacting TA with CaCl₂ and Na₂CO₃, which led to the interaction between TA and Ca²⁺ ions, followed by nucleation of CaCO₃. Micron-sized 1:75 TA-CaCO₃ materials (ranging from 3 to 6 μm) comprised small nanoparticles in a size range of 17–41 nm. TA-CaCO₃ materials could effectively neutralize the SGF solution and scavenge free radicals. In addition, these particles significantly suppressed the mRNA expression of pro-inflammatory cytokines and mediators and scavenged intracellular ROS in cells. Their anti-inflammatory and antioxidant activities protected chondrocytes from ROS. These results suggest that TA-CaCO₃ materials have excellent antacid, antioxidant, and anti-inflammatory properties. Importantly, TA molecules can undergo multiple interactions with nucleic acids, peptides, proteins, and polysaccharides. Furthermore, due to the molecular adsorption of CaCO₃ materials, CaCO₃-based materials can improve the incorporation efficacy of drugs. Thus, using TA-CaCO₃ materials, we will develop dual drug delivery systems that can ferry both a chemical drug and protein drug, and then apply them to treat inflammatory cells or diseases.

This entry is adapted from the peer-reviewed paper 10.3390/ijms22094614