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Vol. 8. Num. 1.
Pages 1-1592 (January - March 2019)
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Vol. 8. Num. 1.
Pages 1-1592 (January - March 2019)
Original Article
DOI: 10.1016/j.jmrt.2018.10.001
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Zn–Al-based layered double hydroxides (LDH) active structures for dental restorative materials
Marcela Piassi Bernardoa,b, Caue Ribeiroa,b,
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Corresponding author.
a Department of Chemistry, Federal University of São Carlos, São Carlos, SP 13565-905, Brazil
b National Nanotechnology Laboratory for Agriculture (LNNA), Embrapa Instrumentation, Rua XV de Novembro 1452, São Carlos, SP 13560-970, Brazil
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Table 1. Phosphate concentrations after interaction with [Zn–Al]-LDH calcined at 300°C and 600°C.

The development of smart dental materials able to react to its environment and release remineralizing ions is attractive point of research. The phosphate interaction with Zn–Al layered double hydroxide was evaluated by the reconstruction method, at 300°C and 600°C. In a general way, thermal stable zinc-phosphate compounds are formed with the increase of the phosphate concentration. To assess the potential to act as bioactive dental restorative materials, phosphate-loaded samples were incorporated on photopolymerizable dental resin (Fill Magic, Coltene) at 2.5% and 4% (m/m) evaluating the phosphate release at artificial saliva medium. After 58 days, the materials showed a useful continuous release of phosphate which in conjunct with other mineralizing elements, could contribute to remineralization of dental tissues and protection against caries and other dental health problems.

Memory effect
Dental resin
Phosphate incorporation
Zinc–phosphate compounds
Phosphate release
Tissue remineralization
Full Text

Dental caries is one of the most common clinical diseases in oral health worldwide. Depending on the extent of tooth damage, different approaches can be used to restore the damaged tissue. The most commonly used restorative materials are amalgam, dental porcelain (dental ceramic), gold, glass ionomer, and resin-based composites [1].

Resin composites are one of the important materials developed that can be used as restorative material and keep the appearance and function of the biological tissues [2]. Besides the restorative function, these materials are used on cavity liners and provisional restorations [3]. This composites are formed by three phases, the first one is the resin composed of polymerizable monomers, which after exposure to UV or visible light create a highly cross-linked polymer; the second phase is the filler, important to enhancement of mechanical properties, radiopacity and alteration of thermal expansion behavior; the last phase is the silane coupling agent, which acts as a binder between the coupling polymerizable moieties and the particle surface [4].

The resin composites restorations interact with the collagen fibrils meshwork of the dentin matrix, reached from dental procedures, forming the so-called hybrid layer [5]. However, this hybrid layer may be degraded over time, leading to the failure, which turn in microgaps that are readily penetrated by pathogens, allowing the development of secondary caries and dental plaque biofilm [6–8]. The main aging mechanisms involved in the degradation of resin-bonded interfaces are (a) hydrolysis and leaching of the resin composite that has infiltrated the demineralized dentin matrix, and (b) degradation of the collagen matrix of the hybrid layer [5,9]. On the other hand, the collagen matrix of pathogen-affected dentin is physiologically remineralizable [10]. Phosphate is an important ion in the process of tissue mineralization, which deposition in conjunct with other elements, leads to remineralization of the resin-dentin bond [11]. Therefore, the incorporation of a phosphate releasing material as a component of dental resin is an important technique that can be used to avoid dental health problems [12].

Layered double hydroxides (LDH) known as “anionic clays” due to structural resemblance with the cationic clays has the general formula [M2+1−xM3+x(OH)2]x+[An]x/n·yH2O, where M2+ is a divalent cation such as Mg2+, Ni2+, Zn2+, Cu2+, or Co2+; M3+ is a trivalent metallic cation such as Al3+, Cr3+, Fe3+, or Ga3+; An is a charge-balancing anion; and x is the molar ratio M3+/(M3++M2+), ranging from 0.1 to 0.5 [13]. LDH exhibit high anionic exchange capacity, with affinity for phosphate and other multivalent anions [14]. Thermal treatment confers important physicochemical properties to LDH [15]: (a) a “memory effect” of the hydroxide lattice, which allows different anionic species to be incorporated into the LDH interlamellar spaces; and (b) higher surface area, which increases anion adsorption [13,16]. LDH have been used in different areas, such as the reinforcement of polymers in order to improve their mechanical properties or thermal stability [17–19], reduction of corrosion in steel materials [20], and reinforcement and protection against corrosion of concrete [21]. Additionally, LDH anion exchange property allows using this material class for ions slow/controlled release. In fact, the use of LDH as matrices for phosphate release was studied at different fields [22,23]. However, the effects of LDH as fillers in resin composites for phosphate release have received little attention [24].

Considering that zinc has antimicrobial properties, ability to inhibit the crystallization (acting as anti-calculus agent), ability to increase demineralization resistance [25–28] and given the potential of LDH to act matrix for phosphate release [29] here we demonstrate the potential of LDH, where M2+ is zinc ([Zn–Al]-LDH), for loading with high amounts of phosphate anions by means of the memory effect for eventual application of these materials on dental field, as fillers of dental polymer resins with potential for slow phosphate release.

2Materials and methods2.1Materials

Zinc chloride (ZnCl2), aluminum chloride hexahydrate (AlCl3·6H2O), sodium hydroxide (NaOH), sodium carbonate (Na2CO3), and potassium phosphate monobasic (KH2PO4) were purchased from Synth (Brazil). All reagents were used as received. Decarbonated deionized water (ρ=18.2cm) obtained from a Milli-Q system (Barnstead Nanopure Diamond, Thermo Fisher Scientific Inc., Dubuque, IA, USA) was used in all the experimental procedures.

2.2Synthesis of [Zn–Al]-LDH

[Zn–Al]-LDH with M3+/(M2++M3+) molar ratio (x) of 0.25 was synthesized by the co-precipitation method, with pH control. The synthesis was carried out in an all-glass reactor (capacity of 300mL) attached to a water circulating system in order to accurately control the temperature at 25.0°C (±0.5°C). In a typical reaction, a mixed chloride solution (0.5molL−1) containing Zn2+ and Al3+ cations was gradually injected at a rate of 0.5mLmin−1 into the reactor containing sodium hydroxide solution (1.0molL−1), under vigorous stirring. At the same time, a solution of Na2CO3 (2mmolL−1) was injected at a rate of 0.025mLmin−1 for pH control. Once injection was complete, stirring was kept for a further hour, for precipitate aging. Subsequently, the mixture was centrifuged at 11,200×g for 10min to remove the excess of NaCl and chloride ions. The precipitate was then purified using three washing-centrifugation cycles with 1:1 water-ethanol solution and was resuspended in water for storage in a freezer. Finally, the material was lyophilized under a vacuum of 1.33×10−4bar (Supermodulyo Freeze Dryer, Thermo Fisher Scientific Inc., Kansas City, MO, USA), yielding a white powder.

2.3Phosphate adsorption by structural reconstruction

Phosphate was loaded into the as-synthesized [Zn–Al]-LDH by means of structural reconstruction. Thermal treatment provides important physicochemical properties to LDH: (i) a “memory effect” of the hydroxide lattice, which allows different anionic species to be incorporated into the LDH interlamellar space, (ii) higher surface area, which increases adsorption of anions, and (iii) elimination of the interlayer carbonate (CO32−), which strongly hinders anion exchange processes in LDH [30]. The synthesis product was calcined for 4h at two different temperatures: 300°C ([Zn–Al]c300) and 600°C ([Zn–Al]c600). Portions of 500mg of the calcined samples were added to 250mL of KH2PO4 solution, previously equilibrated at 75°C and adjusted to pH 7.0 using 0.1M NaOH. The mixture was kept under vigorous stirring for 24h, followed by centrifugation at 11,200×g for 10min. The supernatant was used for quantification of the phosphate content at equilibrium, and the precipitate was lyophilized prior to solid-state characterizations. Different molar ratios of PO43− were studied by varying the PO43− concentration from 0.83mM (1:0.125 Al3+/PO43− molar ratio) to 33.10mM (1:5 Al3+/PO43− molar ratio).

The concentration of phosphorus was determined according to a procedure reported elsewhere [31]: 5mL of supernatant was mixed with 2mL of ascorbic acid solution (0.4M), 0.2mL of citric acid solution (0.03M), and 2mL of a reactant consisting of sulfuric acid solution (4.7M), 5.5mL ammonium molybdate (0.08M), and 0.6mL of antimony and potassium tartrate (0.05M). This mixture was then allowed to react for 15min in a water bath at 50°C, forming a phosphoantimonylmolybdenum blue complex. The concentration of the product was determined by UV–vis spectrophotometry, using a PerkinElmer Lambda spectrophotometer operated at a wavelength of 880nm. All the quantifications were done twice.


Powder X-ray diffraction (PXRD) measurements were performed using a Shimadzu XRD 6000 diffractometer, with Ni-filtered Cu Kα radiation (λ=1.5405Å). The diffractograms were acquired in the 2θ range 3–80°, at a scan speed of 2°min−1. Fourier transform infrared (FTIR) analyses were performed using a Bruker spectrometer, with spectral resolution of 2cm−1. Scanning electron microscopy (SEM) analyses employed a JEOL microscope operated at 15kV. Thermal degradation studies were performed using a TGA Q500 thermogravimetric analyzer (TA Instruments, New Castle, DE), under a flow of nitrogen at 60mLmin−1, with heating at 10°Cmin−1 from 25 to 800°C.

2.5Incorporation of the LDH materials in dental resin and evaluation of phosphate release

The calcined materials ([Zn–Al]c300 and [Zn–Al]c600) loaded with 33.1mM PO43− were incorporated in commercial dental resin (Fill Magic, Coltene) at ratios of 2.5 and 4% (m/m). After complete and homogeneous incorporation of the samples, the composites were submitted to UV radiation (40.3Wm−2) for 10min for photo-curing. Artificial saliva with adapted composition from Ken-ichiro [32], containing KHCO3 (15.01gL−1), NaCl (5.85gL−1), MgCl2 (0.14gL−1), citric acid (0.002gL−1), CaCl2 (0.16gL−1), and sodium carboxymethylcellulose (5gL−1) was used as release media for the dental resin. Resin specimens (2.8cm×1.5cm×0.225cm) were placed in contact with 50mL of artificial saliva under constant stirring (150rpm) at 28°C. The concentration of phosphate was analyzed by the same method (at section “Phosphate adsorption by structural reconstruction”) after incubation for 0, 6, 14, 20, 27, 34, 41 and 58 days.

3Results and discussion3.1Synthesis of [Zn–Al]-LDH

[Zn–Al]-LDH was synthesized by the co-precipitation method with pH control. Fig. 1 shows a scanning electron micrograph of the pristine LDH. The as-synthesized material presented an irregular nanostructure and layers without the presence of hexagonal structures, as well as some stacked nanoparticle flakes [33]. The PXRD patterns of the as-synthesized [Zn–Al]-LDH presented sharp and intense peak lines, with rhombohedral 3R symmetry, as expected for LDH material (JCPDS: 48-1023) [34]. The d-spacing calculated using Bragg's law was 0.78nm, in agreement with the presence of chloride ions in the interlayer space (Fig. 2). The pristine [Zn–Al]-LDH was calcined at 300°C (Fig. 2A1) and 600°C (Fig. 2B3). At 300°C, the typical LDH structure was replaced by the metal oxide phase corresponding to ZnO (JCPDS: 36-1451). When the sample was exposed to pure water, the original LDH structure was restored, confirming installation of the so-called “memory effect”, despite the presence of some residual ZnO phase from the calcination step. On the other hand, calcination at 600°C resulted in formation of not only ZnO, but also a spinel phase (ZnAl2O4) (JCPDS: 05-0669), and when placed in contact with water, the sample was not reconstructed. In other words, the calcination temperature was so high that layers of LDH could not be restored. According to Cavani et al. [30], structural reconstruction is only possible when the heating does not cause modification of the crystal morphology or exfoliation of the layered structure. The lamellar microstructure was retained after thermal decomposition of LDH for 300°C. However, the calcination at 600°C lead to the formation of spinel phase, a very stable phase, causing microstructural changes that not allowed the installation of the memory effect.

Fig. 1.

Representative scanning electron micrograph of pristine [Zn–Al]-LDH.

Fig. 2.

PXRD patterns of pristine [Zn–Al]-LDH (A-0 and B-0); the material calcined at 300°C ([Zn–Al]c300) (A-1); the material calcined at 600°C ([Zn–Al]c600) (B-3); [Zn–Al]c300 reconstructed in water (A-2); and [Zn–Al]c600 reconstructed in water (B-4). *: ZnAl2O4; O: ZnO.

3.2Phosphate interaction with [Zn–Al]-LDH

Although the memory effect was not observed for [Zn–Al]c600, the interactions with phosphate were investigated for [Zn–Al]-LDH calcined at both temperatures. The crystalline structures of the samples were assessed by PXRD (Fig. 3). At lower phosphate concentrations, [Zn–Al]c300 keep the basic LDH crystalline structure and ZnO phase after the thermal treatment. In this case, the interaction of phosphate with [Zn–Al]-LDH was probably due to electrostatic attraction between the external layers of the LDH and the negative ions of PO43−[34]. When PO43− concentration was increased, the LDH phase disappeared and was replaced by ZnO and Zn(OH)2 (JCPDS: 74-0094) phases. At 33.10mM, crystalline zinc-phosphate phases were identified. In this case, ZnO was the precursor material for phosphate interaction and the presence of OH groups was identified (Fig. 4-A4). The PO43− ions could exchange with OH groups and complex with Zn2+ on the surface by outer-sphere complexation or electrostatic attraction [35], allowing the formation of zinc-phosphate compounds.

Fig. 3.

Powder X-ray diffraction (PXRD) patterns for phosphate adsorption by structural reconstruction on (A) [Zn–Al]c300 and (B) [Zn–Al]c600. (0): pristine [Zn–Al]-LDH; (1): 0.83mM PO43−; (2): 3.31mM PO43−; (3): 16.55mM PO43−; (4): 33.10mM PO43−. (#): ZnO; (−): Zn2P2O7; (*): ZnAl2O4; (+): ZnH2P2O7; (O): Zn3(PO4)2.

Fig. 4.

FTIR spectra for phosphate adsorption on (A) [Zn–Al]c300 and (B) [Zn–Al]c600. (0): pristine [Zn–Al]-LDH; (1): 0.83mM PO43−; (2): 3.31mM PO43−; (3): 16.55mM PO43−; (4): 33.10mM PO43−.


In a similar way, when [Zn–Al]-LDH calcined at 600°C was exposed to solutions containing different concentrations of phosphate, the predominant crystalline phases were the spinel component (ZnAl2O4) and ZnO. At a phosphate concentration of 33.10mM, the Zn3(PO4)2 phase (JCPDS: 29-1390) was dominant. However, even at lower starting PO43− concentrations, the phosphate interaction with the materials was evidenced by FTIR (Fig. 4B) and phosphate quantification analyses (Table 1). According to Lv et al. [36], phosphate causes dissolution of ZnO particles. Since ZnO was present with phosphate, a mixture of crystalline and amorphous phases of ZnO and zinc phosphate was obtained. Phosphate can react by adsorption and precipitation on solid phase surfaces, leading to a complex mixture of components and structural transformation of ZnO to zinc phosphate. In addition, the formation of amorphous zinc phosphate phases can occur due to complexation between dissolved PO43− and Zn2+.

Table 1.

Phosphate concentrations after interaction with [Zn–Al]-LDH calcined at 300°C and 600°C.

Initial PO43− (mM)  [Zn–Al]c300 (mgPO43−LDHg−1[Zn–Al]c600 (mgPO43−LDHg−1
00.83  16.00  06.50 
01.65  15.50  04.60 
03.31  25.00  14.50 
06.62  31.20  28.20 
11.58  43.40  67.50 
16.55  63.00  53.00 
26.48  78.40  85.00 
33.10  81.80  92.50 

These results can be compared to our previous findings for other LDH structures, showing that at higher PO43− concentrations Ca–Al-based LDH converted to hydroxyapatite, and Mg-Al-based LDH converted to bobierrite [23,37].

Fig. 4A shows the FTIR spectra for the samples, where the interaction with phosphate was confirmed by the presence of the band at 1040cm−1 attributed to the υ3 (P–O) stretching vibration mode [38]. This was an important indication of the phosphate interaction with [Zn–Al]c300, even though the XRD analysis did not reveal any crystalline phosphate phases. Furthermore, the phosphate contents of the samples (Table 1) increased at higher initial phosphate concentrations. Regardless of the mechanism of interaction of phosphate with calcined [Zn–Al]-LDH, at high phosphate concentrations [Zn–Al]c600 was able to incorporate higher amounts of PO43, while better results were obtained for [Zn–Al]c300 at low phosphate concentrations (Table 1). These results are in agreement with the study of Cheng et al. [16], who found that material calcined at 300°C presented better phosphate adsorption at an initial PO43− concentration of 20mgL−1, due to the greater specific surface area of the sample.

The thermal stabilities and compositions of [Zn–Al]-LDH calcined at 300°C and 600°C and loaded with PO43− were investigated using TG/DTG (Fig. 5). The main phases for 0.83-[Zn–Al]c600 were zinc oxides (ZnO and ZnAl2O4), resulting in no significant mass loss. Although the XRD patterns for the materials produced with intermediate phosphate concentrations showed the presence of the same zinc oxides, changes were observed in the mass loss profiles. The main mass losses occurred at around 75°C, 300°C, 500°C, and 600°C, related to the losses of surface water, loosely bound water molecules, and amorphous phosphate compounds leading to the formation of pure ZnO and ZnAl2O4, respectively [39]. At 33.10mM of phosphate, only two small peaks were present, at 208°C and 535°C, attributed to the loss of adsorbed water and phosphate anions, respectively, culminating in oxide formation.

Fig. 5.

Thermogravimetric (TG) and differential thermogravimetric (DTG) curves for samples [Zn–Al]c300 and [Zn–Al]c600 loaded with PO43− by structural reconstruction using different initial PO43− concentrations: (A) 0.83mM – [Zn–Al]c600; (B) 3.31mM – [Zn–Al]c600; (C) 16.55mM – [Zn–Al]c600; (D) 33.10mM – [Zn–Al]c600; (E) 0.83mM – [Zn–Al]c300; (F) 3.31mM – [Zn–Al]c300; (G) 16.55mM – [Zn–Al]c300; (H) 33.10mM – [Zn–Al]c300.


For [Zn–Al]c300, the thermal profiles loaded with phosphate concentrations up to 16.55mM were typical of LDH, with removal of (i) physically adsorbed water at temperatures below 100°C, (ii) interlayer water up to 200°C, (iii) hydroxyl groups from the layers as water vapor at around 300°C, and (iv) anions, with consequent oxide formation, above 400°C [40]. However, at 33.10mM of phosphate, the peak related to removal of PO43− was no longer observed, indicating thermal stability of the compound formed (Zn3(PO4)2).

3.3Phosphate release from modified dental resin.

According to Ferracane [3], the development of “smart” materials (which reacts to its environment to release remineralizing ions) is very attractive for dental restorations. The remineralization may be promoted by the slow release of phosphate ions, followed by the precipitation of new phases, like calcium-phosphate mineral.

The kinetics of phosphate release from the dental resins containing PO43−-loaded [Zn–Al]-LDHc was evaluated by exposing the materials to artificial saliva. The results (Fig. 6) showed that all the samples could release phosphate, although a substantially higher concentration of phosphate in artificial saliva was obtained using the resin containing 2.5% 33.10-[Zn–Al]c300. Even after 58 days, the materials evidence an increased capacity for phosphate release, attesting the potential of these materials to act as slow phosphate release source, specially the system 2.5% 33.10-[Zn–Al]c300. Several studies demonstrated the tissue remineralization occurring around 21–30 days [41,42], which demonstrate the potential of 2.5% 33.10-[Zn–Al]c300 to be useful at dental reparation.

Fig. 6.

Phosphate release profiles for the samples: (A) 2.5% 33.10-[Zn–Al]c600; (B) 4% 33.10-[Zn–Al]c600; (C) 2.5% 33.10-[Zn–Al]c300; (D) 4% 33.10-[Zn–Al]c300.


CPP-ACP (casein phosphopeptide – amorphous calcium phosphate) is one bioactive agent able to release elements that enhance remineralization of enamel and dentin [43]. Zalizniak et al. [44] observed that phosphate ion release in water was not detectable, however at citric acid at the end of two days 17.6nmol/mm2 of phosphate was released. In contrast, 2.5% 33.10-[Zn–Al]c300 showed potential to release phosphate in an extended way, in an artificial saliva medium. Besides that, Srinivasan et al. [45] verified that CPP–ACP and fluoride combination showed higher remineralization potential than only CPP–ACP. That demonstrates the need of supplementary elements, such as Ca and F, to provide full remineralization of enamel, which can be obtained from other sources like dentifrices and chewing gums [24,46]. Caries are caused by the bacterial production of organic acids that dissolve the dental minerals, and it has been found that the mineral formed during remineralization is more resistant to acid than the original dental enamel [47]. Therefore, the dental resin containing 2.5% 33.10-[Zn–Al]c300 acted as source of phosphorus, which is an important element for mineralization and the avoidance of future dental problems.

Restoratives materials that release Ca, PO4, or F ions are relatively weak and cannot be used in large stress-bearing restorations [48]. Supporting Table 1 (Supporting Information) summarizes the results for the three-point flexural test applied to these four nanocomposites and the pristine dental resin. The values for the resin containing 2.5% 33.10-[Zn–Al]c300 were similar to those for the pristine resin, while decreased for the other materials, mainly because high inorganic loadings in brittle polymers are expected to have negative impacts on their mechanical characteristics, due to the heterogeneous distribution of the particles at the polymer matrix [49,50]. In this sense, 2.5% 33.10-[Zn–Al]c300 was able to release phosphate and keep the mechanical properties of dental resin, indicating the potential of LDH to act as matrices for remineralization ions without losses at the mechanical properties.


The phosphate adsorption by [Zn–Al]-LDH was evaluated by the reconstruction method at 300°C and 600°C. The samples calcinated at 600°C suffered change at the microstructure and the memory effect was not observed. Therefore, the interaction with phosphate was mainly with the external surface of the materials. However, the reconstruction was effective for the samples calcinated at 300°C and the interaction with phosphate leads to the formation of new crystalline phases. The incorporation of these materials at dental resin and evaluation of the phosphate release showed that the sample 2.5% 33.10-[Zn–Al]c300 was able to release significant amounts of phosphate into an artificial saliva medium, indicating that modified dental resin could assist in dental remineralization and provide protection against dental problems.

Conflicts of interest

The authors declare no conflicts of interest.


The authors are grateful to CNPq (grant #402287/2013-4 and 458763/2014-4), SISNANO/MCTI, FINEP, SIBRATEC/Nano, Embrapa AgroNano research network for financial support and to CAPES for the related PhD grant.

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Journal of Materials Research and Technology

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