nanomaterials

Article Structural, Thermal, and Release Properties of Hybrid Materials Based on Layered and Caffeic Acid

Christhy V. Ruiz 1,2,3 , María E. Becerra 1,2,3,4,5 and Oscar Giraldo 1,2,4,*

1 Laboratorio de Materiales Nanoestructurados y Funcionales, Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Colombia- Sede Manizales, Kilometro 9 vía al aeropuerto, La Nubia, 170003 Manizales, Colombia; [email protected] (C.V.R.); [email protected] (M.E.B.) 2 Grupo de Investigación en Procesos Químicos, Catalíticos y Biotecnológicos, Universidad Nacional de Colombia-Sede Manizales, Kilometro 9 vía al aeropuerto, La Nubia, 170003 Manizales, Colombia 3 Departamento de Ingeniería Química, Facultad de Ingeniería y Arquitectura, Universidad Nacional de Colombia-Sede Manizales, Kilometro 9 vía al aeropuerto, La Nubia, 170003 Manizales, Colombia 4 Departamento de Física y Química, Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Colombia-Sede Manizales, Kilometro 9 vía al aeropuerto, La Nubia, 170003 Manizales, Colombia 5 Departamento de Química, Universidad de Caldas, Calle 65 No. 26-10, 17001 Manizales, Colombia * Correspondence: [email protected]

 Received: 10 December 2019; Accepted: 14 January 2020; Published: 17 January 2020 

Abstract: Caffeic acid (CA) molecules were immobilized in a layered inorganic host matrix based on zinc hydroxide structures with different starting interlayer anions, nitrate, and acetate. The chemical composition, structure, thermal stability, morphology, and surface of the host matrices and hybrid compounds were analyzed by X-ray diffraction (XRD), themogravimetric/differencial thermal analysis (TG/DTA), Fourier transform infrarred spectroscopy (FT-IR), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). Additionally, the surface charge of the materials was investigated using zeta potential at pH ~7. The results show an influence of the surface charge on the chemical, interaction, and structure of the resulting hybrid materials as a function of the starting layered structures. An expansion of the basal spacing to 10.20 Å for zinc hydroxide nitrate (ZHN), and a shrinkage to 10.37 Å for zinc hydroxide acetate (ZHA). These results suggest that the CA lies with a tilt angle in the interlayer region of the inorganic host matrix. The immobilization of CA is favored in ZHN, with respect to ZHA, because a single-layered phase was identified. A higher thermal stability at 65 ◦C was observed for ZHN-CA than for ZHA-CA. The evaluation of the release behavior showed a higher percentage of CA released from ZHN than ZHA, and the release mechanism was described by the Elovich model. The hybrid materials show potential characteristics for use as bioactive delivery systems.

Keywords: layered zinc hydroxide; caffeic acid; hybrid material; release systems; immobilization

1. Introduction Layered inorganic materials, such as layered double (LDH) and zinc hydroxy salts (ZHS), are characterized by retaining different chemical species compatible in the interlayer region due to the charge of their layers, thereby achieving the charge balance of the layered structure. This possibility enables the creation of a wide variety of hybrid materials with new applications and functionalities [1]. The differences between LDH and ZHS in terms of the layer structure and bonding confer different physicochemical and interfacial properties to the materials. The materials of the LDH family consist of octahedral layers formed by the partial substitution of divalent metal (M2+) by

Nanomaterials 2020, 10, 163; doi:10.3390/nano10010163 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, 163 2 of 15 trivalent metal ions (M3+), with the interlayer anion balancing the positive charge. In contrast, in the ZHS, one-quarter of the octahedral sites of Zn2+ atoms are empty. Instead, Zn2+ ions are placed in tetrahedral sites at the bottom and top of each empty octahedron [2], with three hydroxyl groups forming the of the tetrahedron, and, on the apex, a water molecule is coordinated. With regard to layered double hydroxides (LDH), a wide range of molecules such as complex ions, vitamins, organic acids, genes, and drugs, amongst others, were extensively investigated [3–7]. However, in terms of ZHS, there is still much to be explored about immobilization studies of functional molecules, such as antioxidants, dyes, and drugs [8,9]. The immobilization process, using zinc hydroxide salts (ZHS) such as zinc hydroxide nitrate (ZHN) and zinc hydroxide acetate (ZHA), takes advantage of the structural characteristics of these layered host materials and their possibility to form covalent binding with different chemical species. The zinc hydroxide salts consist of cationic zinc hydroxy layers and associated anions. The positive charge of the layer can be compensated for with free nitrate ions for ZHN and acetate anions for ZHA in the interlayer region [10,11]. The new materials using ZHN as a host matrix exhibited improved characteristics as in the case of ciprofloxacin intercalated into ZHN, where the intercalated compound showed stronger anti-cancer effects against A549 cancer cells compared to the pure ciprofloxacin molecule [12], or the nanocomposite developed by the intercalation of hippuric acid into ZHN, which presented better anti-microbial properties against drug-resistant bacteria compared to free hippuric acid [13]. Bioactive molecules, of natural origin, attracted considerable attention in the last few years due to their valuable biological and physiological properties [14]. The exchange anion capability, offered by ZHN and ZHA, also allows the incorporation of these types of biomolecules. One such bioactive molecule is caffeic acid (CA), which possesses antioxidant, anticancer, anti-inflammatory, and antiviral properties [15–17]. The anion exchange of caffeic acid in Mg–Al LDH was evaluated by Ito et al.; however, their study was only conducted in order to synthesize a CA–LDH intercalated compound [18]. On the other hand, Choy et al. encapsulated caffeic acid in zinc hydroxide nitrate to evaluate the ultraviolet UVA1 screening ability of the nanohybrid material. However, the thermal and surface characterization was not studied in detail [19]. Therefore, this study aims to incorporate caffeic acid into ZHN and ZHA inorganic host matrices, to determine the effect on the structural, thermal, and surface properties of the hybrid materials obtained from different layered starting materials for the immobilization process of the active molecule, as well as to evaluate the release behavior of the hybrid material for practical cosmetic and pharmaceutical applications.

2. Materials and Methods

Materials and Synthesis of Samples

2.0.1. Synthesis of Layered Zinc Hydroxide Zinc hydroxide nitrate (ZHN) and zinc hydroxide acetate (ZHA) were synthesized using the method reported in previous work [20,21]. For each compound, aqueous solutions with 23.88 g of Zn(NO ) 4H O (Merck 98.9%, St. Louis, MO, USA) and 16.24 g of Zn(CH CO ) (Merck 99.9%, 3 2· 2 3 2 2 St. Louis, MO, USA) were prepared in 60 mL of deionized water. Thereafter, the solutions were mixed in different suspensions, with 7.39 g/60 mL of (ZnO) (Sigma Aldrich 99%–100%, St. Louis, MO, USA) with constant stirring for 24 h. The white slurries were centrifuged, washed three times with deionized water, and dried at room temperature.

2.0.2. Synthesis of Hybrid Material The immobilization of caffeic acid (CA) (Alfa Aesar, 3,4-dihydroxycinnamic acid, predominantly trans, 99%, Ward Hill, MA, USA) was conducted by the dispersion of 5.00 g of ZHN and ZHA, independently, in a suspension of 50.0 mL of deionized water with 1.82 g of CA. The resultant Nanomaterials 2020, 10, 163 3 of 15 suspensions were stirred at room temperature for 24 h. The solid product was recovered by centrifuge and washed three times with deionized water as described above. The product was dried for 72 h at room temperature.

2.0.3. Characterization of Hybrid Materials The characterization of the materials was done by elemental analysis, X-ray diffraction, thermal analysis, Fourier transform-infrared spectroscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy. The chemical composition was estimated from a quantitative combustion technique of carbon, hydrogen, and nitrogen on a Leco TruSpec Micro CHNSO elemental analyzer (St. Joseph, MI, USA), and atomic absorption spectroscopy to determine zinc content on a Thermo Scientific iCE 3000 AA spectrometer (Waltham, MA, USA). The surface charge of the materials was measured using a Malvern Zetasizer Nano ZS instrument (Alcobendas, Madrid, Spain) in 0.1 mol/L NaOH. The XRD patterns were conducted with a Rigaku Miniflex II Diffractometer (Woodlands, TX, USA) with CuKα radiation (1.541 Å) at 30 kV and 15 mA. Thermal analysis was carried out using TGA Q500 (New Castle, DE, USA) from 25 C to 700 C at a heating rate of 10 C min 1 in ◦ ◦ ◦ · − a nitrogen atmosphere. The FTIR spectra were recorded at room temperature for powder samples with a Mid-Infrared FT-IR Nicolet iS5 Thermo Fisher Spectrometer (Waltham, MA, USA) in the range 1 1 1 of 4000 cm− to 400 cm− , with a resolution of 4.0 cm− using pellets prepared with KBr. Scanning electron microscope images were collected on a JEOL JSM-6490LV equipment (North Billerica, MA, USA) with an acceleration voltage of 20 kV. X-ray photoelectron spectroscopy was performed on a PHI 5000 Versa Probe II Scanning XPS Microprobe (Chanhassen, MN, USA) with monochromatic Al Kα radiation. The spectra were obtained at a takeoff angle of 45◦ to the sample normal using a spot diameter of 100 µm. In all samples, the binding energy was referenced using the C 1s peak at 284.5 eV [22]. The high-resolution spectra were used for elemental quantification of the compounds using Multipak data processing software [23]. The schematic representation of the intercalation process in this work was made with CrystalMaker software (Begbroke, OX5, UK) [24].

2.0.4. Controlled Release Studies

The experiments were performed using a buffer solution (pH 7.1) by dissolving NaH2PO4 (0.0534 g) and Na2HPO4 (0.2195 g) in deionized water to make 1000 mL of a solution. The hybrid compounds (0.050 g) were added to 1000 mL of the buffer solution with continuous stirring. Aliquots of 5.0 mL were collected from the reaction at regular intervals. The concentration in each one of the aliquots was measured in triplicate by UV spectroscopy using a lambda UV–visible light spectrophotometer at 19.0 ± 1 ◦C. The maximum absorption wavelength in all solutions of the hybrid materials, in the release study, was 265 nm. Initially, the calibration curve was generated to calculate the amount of intercalated CA using eight aqueous solutions with concentrations ranging from 0.0 to 50.0 mg/L of phosphate buffer solution (pH 7.1) with 50.0 mg/L of CA concentration. The least-square method was employed to fit the equation of the calibration curve of the y = ax + b form, resulting in a = 4.123 10 2, and b = 18.51 10 3 with R2 × − − × − = 0.999, where x is the absorbance and y is the CA solution concentration, expressed in mg/L.

3. Results and Discussion

3.1. Elemental Analysis The elemental analyses of the hybrid materials based on layered zinc hydroxide salts and caffeic acid, as well as those of the inorganic host matrices, are presented in Table1. The calculated formulas were estimated from the stoichiometry comparison of the experimental data and theoretical values of the ideal formula Zn (OH) (CA) 2H O. The absence of the starting interlayer anions of the host 5 8 2· 2 matrices (nitrate and acetate anions), taking into account the calculated formula from the chemical composition, indicates that the guest anion (caffeate) was immobilized into the layered structure to form the hybrid materials. For the hybrid material from zinc hydroxide nitrate, a content of 43.6% of caffeic Nanomaterials 2020, 10, 163 4 of 15

acid was determined, whereas, for zinc hydroxide acetate, the content was 43.1%. These estimated values are close to that represented by the theoretical formula. The difference in the composition of the Nanomaterialshybrid 2020 materials, 10, x FOR as PEER a function REVIEW of the starting host matrices, zinc hydroxide nitrate (ZHN) 4 of and 15 zinc hydroxide acetate (ZHA), given the variation of the content of water, hydroxyl groups, and caffeate compositionanions, of can the be hybrid related materials to the diff aserence a function in the of interaction the starting mode host between matrices, the zinc guest hydroxide anion and nitrate the layers (ZHN)after and displacement zinc hydroxide of the acetate starting (ZHA), anions, given which the ledvariation to a modification of the content of structure of water, for hydroxyl the resulting groups,hybrid and caffeate material. anions, can be related to the difference in the interaction mode between the guest anion and the layers after displacement of the starting anions, which led to a modification of structure for the resulting hybridTable material. 1. Chemical composition and calculated formula of the hybrid materials.

Elemental Content Percentages Observed HybridTable 1. Chemical composition and calculated formula of the hybrid materials. Calculated Formula (Calculated) (wt.%) Material Elemental Content Percentages Observed (Calculated) (wt.%) Hybrid Material Calculated Formula Zn C N H Zn C N H ZHN Zn (OH) (NO ) (CO ) 1.7H O * 54.5 (54.1) 0.4 (0.4) 3.6 (3.7) 1.9 (1.9) ZHN Zn5(OH)5 8(NO38)1.6(CO33)1.60.2∙1.7H23O0.2 * · 54.52 (54.1) 0.4 (0.4) 3.6 (3.7) 1.9 (1.9) ZHA Zn (OH) (C H O ) 3H O 50.0 (49.8) 9.3 (9.1) - 3.2 (3.2) ZHA Zn5(OH)57.5(C2H7.53O2)2.52∙3H3 2O2 2.5· 2 50.0 (49.8) 9.3 (9.1) - 3.2 (3.2) ZHN–CAZHN–CA Zn5(OH)Zn85(C(OH)9H7O84(C)2 9H7O4)2 38.7 (39.8) 38.7 (39.8) 27.5 (26.3) 27.5 (26.3) - - 2.4 (2.7) 2.4 (2.7) ZHA–CAZHA–CA Zn5(OH)Zn 7.5(OH)(C9H7O(C4)2.5∙HH2O ) H O 38.1 (35.5) 38.1 (35.5) 29.3 (25.2) 29.3 (25.2) - - 3.0 (2.9) 3.0 (2.9) 5 7.5 9 7 4 2.5· 2 * Chemical* composition Chemical composition of the zinc of the hydroxide zinc hydroxide nitrate nitrate reported reported in previous in previous work work [21]. [21].

3.2. Interfacial3.2. Interfacial Characterization Characterization The surfaceThe surface charges charges for the for inorganic the inorganic host matrices, host matrices, zinc hydroxide zinc hydroxide nitrate nitrate and zinc and hydroxide zinc hydroxide acetateacetate (ZHN (ZHN and ZHA), and ZHA), as well as as well the ashybrid the hybrid materi materials,als, were studied were studied by measuring by measuring the zeta the potential zeta potential in an aqueousin an aqueous media media of pH of 7.0. pH Zeta 7.0. potentials Zeta potentials of both of inorganic both inorganic host matrices host matrices in the insuspension the suspension were were positive,positive, with an with average an average zeta potential zeta potential given givenby the byinstrument the instrument of 24.2 of± 1.9 24.2 mV1.9 for mV ZHN for and ZHN 38.4 and ± 38.4 ± 1.8 mV 1.8for mVZHA. for A ZHA. zeta potential A zeta potential distribution distribution range from range 23 fromto 53 23mV to for 53 ZHA mV for and ZHA from and 0.2 fromto 43 0.2 to ± mV for43 ZHN mV for can ZHN be estimated can be estimated in Figure in 1. Figure As it was1. As reported it was reported by Xu et byal. Xuand et Mahmoud al. and Mahmoud et al., the et al., zeta potentialthe zeta potentialfor LDH formaterials LDH materials (Mg–Al LDH) (Mg–Al has LDH) a positive has a positivevalue. This value. value This is valuemainly is related mainly to related the structuralto the structural positive positivecharge on charge the electric on the double electric layer double on the layer LDH on surface the LDH [25]. surface In a similar [25]. In case, a similar for layeredcase, forzinc layered hydroxide zinc salts, hydroxide the positive salts,the values positive can be values attributed can be to attributed the structural to the positive structural charge positive givencharge by the Zn given2+ ions by placed the Zn 2in+ ionsthe octahedral placed in theandoctahedral tetrahedral and sites tetrahedral forming the sites layers forming [2]. However, the layers [2]. the interiorHowever, structural the interior charge structural of the interlayer charge ofregi theon interlayeris screened region by the is interlayer screened anions. by the interlayer The charge anions. balanceThe of charge the layers balance with of the the interlayer layers with anions the interlayer is not complete anions isin notthe completelayered structure in the layered since structurethe surface-adsorbedsince the surface-adsorbed anions thermally anions vibrate thermally and temp vibrateorarily and leave temporarily the electric leave double the electric layer double (Stern layer layer)(Stern toward layer) the towardother electric the other layer electric called layer the called diffusion the di layer,ffusion where layer, static where anions static anionscan always can always be be found,found, which which leads leads to the to layered the layered particles particles associated associated with with the the Stern Stern layer layer having having a apositive positive charge charge [25]. [25].

ZHA ZHN ZHN-CA ZHA-CA

Relative Intensity (arb. units)

-100 -80 -60 -40 -20 0 20 40 60 80 100 Zeta potential (mV)

Figure 1. Zeta potential distribution of ZHN, ZHA, ZHN–CA, and ZHA–CA materials.

Figure 1. Zeta potential distribution of ZHN, ZHA, ZHN–CA, and ZHA–CA materials.

Therefore, the difference in the zeta potential value between the layered host matrices can be influenced by the particular characteristics of interaction between the layers and the interlayer anion in the aqueous medium. Therefore, in the case of the ZHN, given its chemical composition from the

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Therefore, the difference in the zeta potential value between the layered host matrices can be influenced by the particular characteristics of interaction between the layers and the interlayer anion in the aqueous medium. Therefore, in the case of the ZHN, given its chemical composition from the 2 elemental analysis, a strong adsorption of the CO3 − and NO3− anions on the surface of the positive layers of the ZHN occurred to form the Stern layer (negative charge), which induced a lower amount of the anions present in the diffusion layer, giving rise to a smaller zeta potential value. A different response was observed for ZHA, in which a more significant zeta potential value was obtained. This response is likely associated with an electrostatic interaction through the hydrogen bonds between the water molecule located on the apex of the tetrahedral units of the layers and the oxygen atom of the acetate anion, which resulted in more acetate anions being found on the diffusion layer of the aqueous medium. Also, the difference in the composition chemical of the hydroxyl layers regarding ZHN can affect the results of the zeta potential value. The values of zeta potential are similar to those for LDH 2 with CO3 − anions reported by Xu et al. [26] and Tran et al. [27]. The values of the zeta potential of the hybrid materials based on caffeic acid and both layered zinc hydroxide host matrices were found to be negative in that ae pH of ~7.0. The average zeta potential for ZHN–CA was 29.0 0.3 mV, and ZHA–CA had a value of 28.7 0.2 mV, with distribution zeta − ± − ± potentials in the range of 9.2 to 43.5 mV and 13.8 to 39.5 mV for each hybrid material, respectively. − − − The negative surface charge for the hybrid material is a fact that demonstrates the immobilization of the organic anion into the layered structure. With the pH in the aqueous medium, the H+ ions of the functional groups of the caffeic acid (carboxylate group) are liberated, which leads to a more negative value of zeta potential [28]. The strong electrostatic attraction between the oxygen atom of the carboxylate group of the caffeate anion and the Zn2+ atoms of the tetrahedral units can be the cause of the zeta potential response. As a consequence of the surface charge of the starting layered host matrix, as well as the resulting chemical composition of the hybrid materials, a change in the values of zeta potential (ZHN–CA and ZHA–CA) was obtained. Based on the results of the zeta potential values, the interfacial behavior indicates suspension stability based on electrostatic interactions of the hybrid materials and the starting host matrices, which have differences owing to the chemical composition and interactions [2,25,29,30].

3.3. Powder X-Ray Diffraction (PXRD) The XRD patterns of the starting inorganic matrices and hybrid materials are shown in Figure2. From now on, the inorganic–organic hybrid materials are referred to as ZHN–CA and ZHA–CA. The X-ray pattern of the inorganic matrix and hybrid materials from ZHN are shown in Figure2a. For the host matrix ZHN, the intense reflection was identified as the phase Zn (OH) (NO ) 2H O (JCDS 72-0627) 5 8 3 2· 2 with a d-value of around 9.67 Å [19,21]. The XRD pattern for ZHN–CA revealed a shift toward lower angles of reflection, which indicates an expansion of the interlayer space of the layered structure. The increase in d-value from 9.67 Å to 10.20 Å revealed the incorporation of a larger molecule than that of the starting anions in the interlayer region. Unlike the results obtained for the ZHN inorganic matrix, the results from the hybrid material with ZHA as the starting material (Figure2b) revealed a reduction of the interlayer space from 13.31 Å [20] to 10.37 Å for ZHA–CA, which is yet to be reported to our knowledge. These results are in accordance with those reported by Choy et al. [19], who recorded a d-spacing of 10.44 Å for intercalated CA in a layered zinc hydroxide nitrate matrix (CA–ZBS). The decomposition of this ZHA–CA hybrid compound was evident, in comparison with the results obtained for hybrid material from ZHN inorganic host matrix, since reflections associated with Zn(OH)2 were observed. Furthermore, the evidence of this mixture of phases in the ZHA–CA hybrid material might justify the differences between the percentages of the elemental content observed and calculated. Particularly, for both starting materials, the predominant arrangement, taking into account the d-value, is that where the organic anions (caffeate) are arranged with a tilt angle close to 70◦ along the z-axis inside the interlayer space of the layered inorganic host matrix. This orientation possibly creates stronger interactions between the carboxylate group, present in the caffeic acid molecule, and the Zn2+ atoms of Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 15

Nanomaterials 2020, 10, 163 6 of 15 the tetrahedral units, which favor the substitution of the water molecules in the apex of tetrahedron of the layersNanomaterials in both 2020 cases, 10, x and FOR result PEER REVIEW in the coordination of the guest anion to the layers [31]. 6 of 15

Figure 2. Powder X-Ray Diffraction patterns of (a) starting material and hybrid materials based on ZHN and (b) inorganic host matrix ZHA, and their intercalated compound with caffeinate anion.

Particularly, for both starting materials, the predominant arrangement, taking into account the d-value, is that where the organic anions (caffeate) are arranged with a tilt angle close to 70° along the z-axis inside the interlayer space of the layered inorganic host matrix. This orientation possibly creates stronger interactions between the carboxylate group, present in the caffeic acid molecule, and 2+ the ZnFigure atoms 2.2. Powder of the X-Raytetrahedral DiDiffractionffraction units, patterns which of favor (a)) startingstarting the substitution materialmaterial andand of hybridhybrid the water materialsmaterials molecules basedbased onon in the apex ZHNof tetrahedron and (b)) inorganicinorganic of the hostlayershost matrixmatrix in both ZHA,ZHA, cases andand and theirtheir result intercalatedintercalated in the compoundcompoundcoordination withwith of cacaffeinate theffeinate guest anion.anion. anion to the layers [31]. TheParticularly, didifferencesfferences for in inboth the the dstarting -valuesd-values materials, of of the the resulting resulting the predominant hybrid hybrid material, material, arrangement, as aas function a function taking of the ofinto inorganicthe account inorganic host the hostmatrix,d-value, matrix, can is that be canrelated where be related tothe the organic to initial the anions chemicalinitial chemical(caffeate) composition compositionare arranged and surface andwith surface chargea tilt angle ofcharge the close starting of tothe 70° starting layered along layeredstructure.the z-axis structure. Theinside more the The significant interlayer more significant surfacespace of charge surfacethe layered of charge the layersinorganic of the into layers host ZHA matrix.into can leadZHA This to can a orientation stronger lead to interactiona strongerpossibly withinteractioncreates the stronger guest with anions, interactionsthe guest giving anions,between rise to thegiving the shrinkage carboxylat rise to of ethe the group, resultingshrinkage present layered of in the the hybrid resultingcaffeic structure, acid layered molecule, ZHA–CA, hybrid and structure,andthe Zn the2+ diatoms ZHA–CA,fference of the in and chemicaltetrahedral the difference composition units, whichin chemical can favor affect compositionthe the substitution degradation can ofaffect of the the waterthe layers degradation molecules for this hybrid ofin thethe layersmaterialapex of for tetrahedron with this respecthybrid of material to the ZHN–CA. layers with in bothrespect cases to ZHN–CA.and result in the coordination of the guest anion to the layersA [31]. schematic representationrepresentation of of the the arrangement arrangement of of the the ca caffeateffeate anions anions in thein the interlayer interlayer space space of the of thehost host matricesThe matrices differences is illustrated is illustrated in the in d-values Figure in Figure3 .of the3. resulting hybrid material, as a function of the inorganic host matrix, can be related to the initial chemical composition and surface charge of the starting layered structure. The more significant surface charge of the layers into ZHA can lead to a stronger interaction with the guest anions, giving rise to the shrinkage of the resulting layered hybrid structure, ZHA–CA, and the difference in chemical composition can affect the degradation of the layers for this hybrid material with respect to ZHN–CA. A schematic representation of the arrangement of the caffeate anions in the interlayer space of the host matrices is illustrated in Figure 3.

Figure 3. Representation of the caffeate anion arrangement in the interlayer space of layered structure Figure 3. Representation of the caffeate anion arrangement in the interlayer space of layered structure based on ZHA and ZHN host matrix. based on ZHA and ZHN host matrix. 3.4. Thermal Analysis (TG/DTA) 3.4. Thermal Analysis (TG/DTA) The TG/DTA curves are shown in Figure4a,b. The thermal decomposition of ZHN–CA occurred The TG/DTA curves are shown in Figure 4a,b. The thermal decomposition of ZHN–CA occurred in three regions with a total mass loss of 34.68%. The first region (I) was identified around 43.71 ◦C in three regions with a total mass loss of 34.68%. The first region (I) was identified around 43.71 °C with a loss of 7.71%. This region corresponds to the removal of physiosorbed water in the interlayer withregion. a loss In theof 7.71%. second This region region (II), corresponds a mass loss to of th 5.23%e removal and 3.63%,of physiosorbed with a sharp water inflection in the interlayer points at region.Figure In the 3. Representation second region of (II),the caffeate a mass anion loss arrangementof 5.23% and in the3.63%, interlayer with spacea sharp of layered inflection structure points at 276.0 and 378.1 ◦C in the derivative analysis, corresponds to the dehydroxylation process of the layers. 276.0based and 378.1on ZHA °C and in ZHNthe derivative host matrix. analysis, corresponds to the dehydroxylation process of the In the third region (III), two peaks were observed, one at 509.3 ◦C with a mass loss of 12.33%, attributed to the quantitative decomposition of organic material, and a weak peak at 578.7 C with a weight loss 3.4. Thermal Analysis (TG/DTA) ◦ of 5.78%, corresponding to the decomposition of the residue of the immobilized organic anions [32]. The TG/DTA curves are shown in Figure 4a,b. The thermal decomposition of ZHN–CA occurred in three regions with a total mass loss of 34.68%. The first region (I) was identified around 43.71 °C with a loss of 7.71%. This region corresponds to the removal of physiosorbed water in the interlayer region. In the second region (II), a mass loss of 5.23% and 3.63%, with a sharp inflection points at 276.0 and 378.1 °C in the derivative analysis, corresponds to the dehydroxylation process of the

Nanomaterials 2020, 10, x FOR PEER REVIEW 7 of 15

the layers. In the third region (III), two peaks were observed, one at 509.3 °C with a mass loss of 12.33%, attributed to the quantitative decomposition of organic material, and a weak peak at 578.7 °C with a weight loss of 5.78%, corresponding to the decomposition of the residue of the immobilized organic anions [32]. For ZHA–CA, the thermal decomposition revealed that the total mass loss value was 37.09% upon heating at 700 °C, which was slightly higher than that of ZHN–CA. In the first region (I), a mass loss of 3.87% is associated with the removal of physiosorbed water molecules [19]. The peaks at 201.5, 210.5, and 280.1 °C, in the derivative analysis, are associated with the beginning of the dehydroxylation process, as well as the decomposition of the organic species, which is evidenced with a mass loss of 16.92%. The third region had a sharp peak with a mass loss of 16.30% at 512.7 °C, which can be associated with the decomposition of the immobilized organic species (CA). From the Nanomaterialsresults, the2020 hybrid, 10, 163 material ZHN–CA showed a thermal stability of 65.5 °C higher than that of the 7 of 15 ZHA–CA, since the start of the dehydroxylation process was delayed.

100 2.0 0.4 Region I Region II Region III (a) Region I Region II Region III (b) 100 512.7°C 7.71% 276.0°C 90 3.87% 5.23% 1.5 0.3 509.3°C 90 210.5°C 43.7°C 201.5°C 10.98% 3.63% 5.94% 80 1.0 0.2

12.33% 80

Weight (%) Weight

Weight (%) Weight 578.7°C 378.1°C 70 0.5 70 41.9°C 280.1°C 16.3% 0.1 5.78% (%/min) Weight Deriv.

Deriv. Weight (%/min) Weight Deriv.

60 0.0 60 0.0 100 200 300 400 500 600 700 100 200 300 400 500 600 700 Temperature (°C) Temperature (°C)

Figure 4. Thermogravimetric analysis (TG/DTA) of the hybrid compounds for (a) ZHN–CA and (b) ZHA–CA.Figure 4. Thermogravimetric analysis (TG/DTA) of the hybrid compounds for (a) ZHN–CA and (b) ZHA–CA. For ZHA–CA, the thermal decomposition revealed that the total mass loss value was 37.09% upon 3.5. FT-IR Spectroscopy heating at 700 ◦C, which was slightly higher than that of ZHN–CA. In the first region (I), a mass loss of 3.87% isThe associated FT-IR spectra with theof the removal host materials of physiosorbed ZHN and ZHA, water and molecules hybrid compounds, [19]. The peaks ZHN– atCA 201.5, and 210.5, ZHA–CA, are shown in Figures 5a,b. For the host matrix, ZHN, a broad absorption band around 3448 and 280.1 ◦C, in the derivative analysis, are associated with the beginning of the dehydroxylation −1 process,cm attributed as well as to thethe O decomposition–H stretching, due of to the the organic presence species, of hydroxyl which groups is evidenced of ZHN and with physically a mass loss adsorbed water, can be observed. The sharp band at 3571 cm−1 can be associated with OH stretching of 16.92%. The third region had a sharp peak with a mass loss of 16.30% at 512.7 C, which can vibrations, non-H-bonded [33–35]. The band at 1638 cm−1 can be attributed to the OH ◦ bending be associatedvibrational withmode the of water decomposition molecules occupying of the immobilized the interlayer organic spacing species [35,36]. (CA). The strong From theband results, the hybridlocated at material 1378 cm ZHN–CA−1 corresponds showed to the stretching a thermal vibration stability of of free 65.5 nitrate◦C higher groups than within that the of interlayer the ZHA–CA, sinceregion, the start whereas of the the dehydroxylation band located at 1022 process cm−1 can was be delayed. associated with the prohibited stretching mode of the nitrate group [36]. The bands at 882 and 832 cm−1 can be assigned to the C–O vibrational mode 3.5.of FT-IR carbonate Spectroscopy anions [37]. The bands around 764 cm−1, 639 cm−1, and 462 cm−1 are attributed to the −1 vibrationsThe FT-IR of spectraZn–O [21]. of theFor the host host materials matrix ZHA, ZHN the and broad ZHA, band and at hybrid3440 cm compounds,, just like in ZHN, ZHN–CA can and be assigned to the O–H stretching vibration mode. The bands around 1560 and 1400 cm−1 are ZHA–CA, are shown in− Figure5a,b. For the host matrix, ZHN, a broad absorption band around associated1 with COO asymmetric and symmetric vibrational modes, respectively. The difference in 3448 cm− attributed to the O–H stretching, due to the presence of hydroxyl groups of ZHN and the νa(COO−) and νs(COO−) (Δνa-s = 160 cm−1) stretching frequency indicates an ionic mode in the 1 physicallycoordination adsorbed of the water, carboxylate can be group observed. to the layered The sharp structure, band according at 3571 cm to− thecan description be associated given withby OH 1 stretchingNara et vibrations, al. [38]. non-H-bonded [33–35]. The band at 1638 cm− can be attributed to the OH bending vibrational mode of water molecules occupying the interlayer spacing [35,36]. The strong band located 1 at 1378 cm− corresponds to the stretching vibration of free nitrate groups within the interlayer region, 1 whereas the band located at 1022 cm− can be associated with the prohibited stretching mode of the nitrate 1 group [36]. The bands at 882 and 832 cm− can be assigned to the C–O vibrational mode of carbonate anions [37]. The bands around 764 cm 1, 639 cm 1, and 462 cm 1 are attributed to the vibrations of − − − 1 Zn–O [21]. For the host matrix ZHA, the broad band at 3440 cm− , just like in ZHN, can be assigned to 1 the O–H stretching vibration mode. The bands around 1560 and 1400 cm− are associated with COO− asymmetric and symmetric vibrational modes, respectively. The difference in the νa(COO−) and νs(COO−) 1 (∆νa-s = 160 cm− ) stretching frequency indicates an ionic mode in the coordination of the carboxylate group to the layered structure, according to the description given by Nara et al. [38]. 1 The bands at 1334 and 1049 cm− can be associated with the vibration modes of the methyl group 1 (CH3) due to deformation and rocking, respectively [39]. The weak band at 831 cm− is attributed to 2 the CO3 − scissoring vibration mode, and the bands at lower wavenumbers are assigned to the Zn–OH vibrations. For both intercalated compounds, the bands associated with the starting anions, located 1 in the interlayer distances (1378 and 1560–1400 cm− ) of the host matrices, were not evident. These changes also demonstrate the immobilization process of the caffeate anions in the interlayer region. 1 For ZHN–CA, the broad band centered at 3440 cm− is associated with the OH vibration mode of the layers and water molecules located in the interlayer space [19], whereas, for ZHA–CA, in this region, 1 the bands at 3431 and 3230 cm− are assigned to the stretching vibration mode of the OH group of the aromatic ring of the caffeic acid molecule [40,41]. These results agree with the thermal analysis, where Nanomaterials 2020, 10, 163 8 of 15 the loss mass due to the water molecules was lower in ZHA–CA than for ZHN–CA in the temperature 1 range 30 ◦C–150 ◦C. The bands observed in the range from 1598 to 1421 cm− are attributed to the 1 stretching vibration mode of the C–C group. The bands at 1643 and 838 cm− are related to the C=O 1 group vibration mode. The band at around 1267 cm− corresponds to the stretching vibration mode of Nanomaterialsthe C–OH group2020, 10 [, 41x FOR,42]. PEER REVIEW 8 of 15

FigureFigure 5.5. FourierFourier transform infrared spectra for: ( (aa)) ZHN ZHN and and ZHN–CA, ZHN–CA, and and (b (b) )ZHA ZHA and and ZHA–CA. ZHA–CA.

1 The bands at in 1334 the range and 1049 from cm 1116−1 can to 796be associated cm− are associated with the vibration with the dimodesfferent of vibration the methyl modes group of (CHthe CH3) due group, to deformation with the higher and wavenumberrocking, respectively due to the [39]. in-plane The weak bending band and at 831 the cm lower−1 is wavenumberattributed to 1 thedue CO to out-of-plane32− scissoring bending vibration vibration mode, and [42]. the The bands bands at at lower 723 and wavenumbers 671 cm− are are assigned assigned to the to aromaticthe Zn– OHring vibrations. out-of-plane For bending both intercalated mode of the compounds, C–C group the and bands in-plane associated bending with mode the of starting the C=O anions, group, 1 locatedrespectively. in the Theinterlayer band arounddistances 592 (1378 cm −andcan 1560–1400 be assigned cm−1 to) of out-of-plane the host matrices, bending were O–H not vibration, evident. 1 Thesewhereas changes the band also at demonstrate 551 cm− corresponds the immobilization to the aromatic process ring of the in-plane caffeate bending anions modein the of interlayer the C–C region.group [ 43For]. ZHN–CA, The vibrations the broad at lower band wavenumbers centered at can3440 be cm attributed−1 is associated to the Zn–O with stretchingthe OH vibration vibration mode [36]. ofThese the resultslayers and are inwater accordance molecules with located the XRD in patternsthe interlayer for the space immobilization [19], whereas, process for ZHA–CA, of caffeate anionsin this region,for both the layered bands inorganic at 3431 and systems, 3230 cm ZHN−1 are and assigned ZHA [to19 the]. stretching vibration mode of the OH group of the aromatic ring of the caffeic acid molecule [40,41]. These results agree with the thermal analysis, where3.6. Scanning the loss Electron mass due Microscopy to the water (SEM) molecules was lower in ZHA–CA than for ZHN–CA in the temperatureThe SEM range images 30 for°C–150 ZHN °C. and The ZHA, bands aswell observed as those in forthe therange hybrid from materials, 1598 to ZHN–CA1421 cm−1 andare attributedZHA–CA, areto the shown stretching in Figure vibration6. ZHN (Figuremode 6ofa) the exhibited C–C group. well-defined The bands crystals at 1643 with theand characteristic 838 cm−1 are relatedplate-like to the morphology C=O group of vibration layered materials. mode. The The band morphology at around of1267 the cm intercalated−1 corresponds compound to the stretching ZHN–CA vibration(Figure6b) mode showed of the agglomerated C–OH group particles [41,42]. with a heterogeneous distribution. For the ZHA host matrix,The a bands non-uniform in the range sheet from and flake-like1116 to 796 structure cm−1 are were associated observed with (Figure the different6c). However, vibration ZHA–CA modes ofexhibited the CH a densegroup, structure with the with higher a smoother wavenumber surface due and to small the particlesin-plane on bending the surface and (Figurethe lower6d). wavenumberThese results showdue to clear out-of-plane differences bending in the vi morphologybration [42]. between The bands the at inorganic 723 and host671 cm matrices−1 are assigned and the tohybrid the aromatic materials, ring which out-of-plane can affect bending the general mode properties of the C–C of thegroup materials. and in-plane bending mode of the C=O group, respectively. The band around 592 cm−1 can be assigned to out-of-plane bending O–H vibration,3.7. X-Ray whereas Photoelectron the band Spectroscopy at 551 cm (XPS)−1 corresponds to the aromatic ring in-plane bending mode of the C–CThe group high-resolution [43]. The vibrations XPS spectra at oflower the chemicalwavenumbers states can of Zn, be C,attributed and O are to shownthe Zn–O in Figure stretching7a,b vibration [36]. These results are in accordance with the XRD patterns for the immobilization process for the inorganic host matrices and hybrid materials. For the ZHN host matrix, the Zn 2p3/2 core oflevel caffeate spectrum anions exhibited for both twolayered contributions inorganic systems, with binding ZHN and energies ZHA at[19]. 1021.4 and 1023.1 eV. These contributions are related to the structural characteristics of the layered host matrix, where the Zn2+ 3.6. Scanning Electron Microscopy (SEM) atoms have tetrahedral and octahedral coordination in the Zn5(OH)8(NO3)2 2H2O structure [14,22]. · 2 The bindingThe SEM energy images values for ZHN for O and 1s were ZHA, 529.9, as well 531.4, as andthose 533.1 for eV,the which hybrid correspond materials, toZHN–CA CO3 −, –OH, and ZHA–CA,and NO3−/ Hare2O, shown respectively. in Figure The presence 6. ZHN of (Figure carbonate 6a) ions exhibited is due to thewell-defined high affinity crystals of the layers with withthe characteristiccompatible chemical plate-like species morphology such as CO of2 [21layered]. Based materials. on the peak The fitting, morphology the intense of peakthe correspondsintercalated compound ZHN–CA (Figure 6b) showed agglomerated particles with a heterogeneous distribution. For the ZHA host matrix, a non-uniform sheet and flake-like structure were observed (Figure 6c). However, ZHA–CA exhibited a dense structure with a smoother surface and small particles on the surface (Figure 6d). These results show clear differences in the morphology between the inorganic host matrices and the hybrid materials, which can affect the general properties of the materials.

Nanomaterials 2020, 10, x FOR PEER REVIEW 9 of 15

3.7. X-Ray Photoelectron Spectroscopy (XPS) The high-resolution XPS spectra of the chemical states of Zn, C, and O are shown in Figure 7a,b for the inorganic host matrices and hybrid materials. For the ZHN host matrix, the Zn 2p3/2 core level spectrum exhibited two contributions with binding energies at 1021.4 and 1023.1 eV. These contributions are related to the structural characteristics of the layered host matrix, where the Zn2+ atoms have tetrahedral and octahedral coordination in the Zn5(OH)8(NO3)2∙2H2O structure [14,22]. The binding energy values for O 1s were 529.9, 531.4, and 533.1 eV, which correspond to CO32−, –OH, Nanomaterials 2020, 10, 163 9 of 15 and NO3−/H2O, respectively. The presence of carbonate ions is due to the high affinity of the layers with compatible chemical species such as CO2 [21]. Based on the peak fitting, the intense peak correspondsto the contribution to the ofcontribution Zn(OH)2, which of Zn(OH) forms2, the which layers. forms In contrast,the layers. the In peak contrast, at higher the bindingpeak at energyhigher bindingis related energy to the is nitrate related anions to the that nitrate act asanions counterions, that act as and counterions, adsorbed water and adsorbed molecules water occupying molecules the occupyinginterlayer spacesthe interlayer of the layered spaces structure. of the layered For the structure. C 1s core-level For the spectrum, C 1s core-level three contributions spectrum, three were 2 contributionsdetected with energieswere detected of 284.7, with 287.4, energies and 289.5 of 284.7 eV, associated, 287.4, and with 289.5 C–H, eV, C =associatedO, and CO with3 −, respectively.C–H, C=O, Theand CO lower32−, respectively. binding energy The value lower is binding assigned energy to adventitious value is assigned carbon, to which adventitious is usually carbon, found which on the is usuallysurface offound most on samples the surface exposed of most to air, samples whereas expose the twod to higherair, whereas binding the energy two higher values binding are associated energy withvalues the are presence associated of the with carbonate the presence . of the carbonate ion.

Figure 6. SEMSEM images images of ( a)) ZHN ZHN host host matrix, ( b) ZHN–CA, ( c) ZHA, and ( d) ZHA–CA.

For thethe intercalated intercalated compound compound ZHN–CA, ZHN–CA, the C 1thes core-level C 1s core-level spectrum showedspectrum four showed contributions four contributionsafter the peak after fitting. the The peak band fitting. at 284.5 The eVband is assignedat 284.5 eV to C–H,is assigned due to to the C–H, aromatic due to ring the ofaromatic the caff ringeate ofanion, the caffeate as well asanion, to the as adventitious well as to the carbon. adventitious The chemical carbon. state The of chemical C 1s at 286.0 state eVof isC attributed1s at 286.0 toeV the is 2+ attributedcarboxylate to group the carboxylate (COO−), which group participated (COO−), which in the participated covalent bonding in the covalent with the bonding Zn tetrahedral with the units Zn2+ tetrahedralof the layers units to form of the the layers hybrid to compound.form the hybrid The compound. band around The 288.4 band eV around corresponds 288.4 eV to thecorresponds phenolic togroup the phenolic (C–OH) confirminggroup (C–OH) the caconfirmingffeic acid molecule,the caffeic andacid the molecule, weak band and atthe a higherweak band binding at a energy,higher binding290.3 eV, energy, is assigned 290.3 to eV, the presenceis assigned of theto the carbonate presence anion of the in the carbonate intercalation anion compound. in the intercalation The O 1s compound.core level for The the O hybrid 1s core compound, level for the ZHN–CA, hybrid compound, exhibited two ZH bandsN–CA, at exhibited 531.9 eV andtwo 533.4 bands eV. at The 531.9 band eV andat the 533.4 lower eV. binding The band energy at the corresponds lower binding to the energy Zn(OH) corresponds2 which forms to tothe the Zn(OH) layers,2 which and the forms band to at the layers,higher bindingand the energyband at can the be higher associated binding with energy the carboxylate can be associated group (COO with−) ofthe the carboxylate organic molecule. group (COOFor the−) hybridof the material,organic twomolecule. chemical For states the hybrid for Zn 2material,p3/2 were identifiedtwo chemical at 1022.2 states and for 1023.4 Zn 2 eV,p3/2 whichwere are related to the different chemical environments of the Zn2+ atom in the octahedral and tetrahedral units of the layered structure. In the case of the ZHA host matrix, the C 1s core-level spectrum showed three bands with binding energy values at 284.8, 286.2, and 288.9 eV. The band at the lower binding energy value is assigned to the C–H associated with the presence of the methyl group of the acetate molecule and the adventitious carbon. The band around 286.2 eV is related to C=O bonds, and the band at the higher binding energy value is also assigned to the presence of the carboxylate group of the acetate anion [22]. These results agree with the FT-IR analysis previously described. The O 1s core-level spectrum exhibited three Nanomaterials 2020, 10, x FOR PEER REVIEW 10 of 15

identified at 1022.2 and 1023.4 eV, which are related to the different chemical environments of the Zn2+ atom in the octahedral and tetrahedral units of the layered structure. In the case of the ZHA host matrix, the C 1s core-level spectrum showed three bands with binding energy values at 284.8, 286.2, and 288.9 eV. The band at the lower binding energy value is assigned to the C–H associated with the presence of the methyl group of the acetate molecule and the Nanomaterials 2020, 10, 163 10 of 15 adventitious carbon. The band around 286.2 eV is related to C=O bonds, and the band at the higher binding energy value is also assigned to the presence of the carboxylate group of the acetate anion [22]. These results agree with the FT-IR analysis previously described. The O 1s core-level spectrum contributions. The band around at 530.2 eV is assigned to the presence of carbon species such as CO2. exhibited three contributions. The band around at 530.2 eV is assigned to the presence of carbon The strong band at 531.6 eV corresponds to the Zn(OH)2 forming the layers, and the band at the higher species such as CO2. The strong band at 531.6 eV corresponds to the Zn(OH)2 forming the layers, and binding energy value (532.6 eV) is related to the carboxylate group (COO ) of the acetate anions. From − − the band at the higher binding energy value (532.6 eV) is related to the carboxylate group2 +(COO ) of the Zn 2p3/2 core level, two peaks were identified, one of them corresponds to Zn with octahedral the acetate anions. From the Zn 2p3/2 core level, two peaks were identified, one of them corresponds coordinationto Zn2+ with octahedral to the hydroxyl coordination groups to atthe 1021.8 hydroxyl eV, groups and the at other1021.8 is eV, a weakand the band other associated is a weak with the 2+ tetrahedralband associated coordination with the tetrahedral of the Zn coordinationatom. of the Zn2+ atom.

Figure 7. High-resolution XPS of chemical state (1) Zn 2p3/2, (2) O 1s, and (3) C 1s for (a) ZHN and Figure 7. High-resolution XPS of chemical state (1) Zn 2p3/2,(2) O 1s, and (3) C 1s for (a) ZHN and ZHN–CA, and (b) ZHA and ZHA–CA. ZHN–CA, and (b) ZHA and ZHA–CA. For the hybrid material ZHA–CA, the C 1s core-level spectrum exhibited only three For the hybrid material ZHA–CA, the C 1s core-level spectrum exhibited only three contributions contributions at 284.8, 286.3, and 289.1 eV. The component related to carbon species at higher binding atenergy 284.8, was 286.3, not found and 289.1like for eV. ZHN–CA. The component However, for related the O 1 tos and carbon Zn 2p species3/2 core-level at higher spectra,binding similar energy wascontributions not found were like identified for ZHN–CA. after theHowever, peak fitting for of this the hybrid O 1s and material. Zn2 Thep3/2 resultscore-level of XPS spectra, are in similar contributionsaccordance with were those identified of XRD and after FT-IR the analysis, peak fitting which of could this hybrid confirm material. the immobilization The results process of XPS are in accordance with those of XRD and FT-IR analysis, which could confirm the immobilization process of the organic molecules in both starting layered structures, since, although the starting materials were different, the hybrid compounds gave rise to a single arrangement of the caffeate anions in both layered structures. The displacement of the binding energy values for the different core levels, C1s, O 1s, and Zn 2p3/2, between the host matrices and the hybrid compounds, is related to the change in chemical environment after the interaction, through H-bonding, between the water molecules of the tetrahedral units of the layers and the carboxylate group of caffeate anions. For both inorganic host matrices, ZHN and ZHA, the binding energy values for the tetrahedral coordinated Zn2+ ions were similar. Nevertheless, the change in the intensity of this contribution can be related to the structural variation of the layered host, since, for ZHA, the amount of water in crystallization may vary, depending on the Nanomaterials 2020, 10, x FOR PEER REVIEW 11 of 15 of the organic molecules in both starting layered structures, since, although the starting materials were different, the hybrid compounds gave rise to a single arrangement of the caffeate anions in both layered structures. The displacement of the binding energy values for the different core levels, C1s, O 1s, and Zn 2p3/2, between the host matrices and the hybrid compounds, is related to the change in chemical environment after the interaction, through H-bonding, between the water molecules of the tetrahedral units of the layers and the carboxylate group of caffeate anions. For both inorganic host 2+ matrices,Nanomaterials ZHN2020 ,and10, 163 ZHA, the binding energy values for the tetrahedral coordinated Zn ions 11were of 15 similar. Nevertheless, the change in the intensity of this contribution can be related to the structural variation of the layered host, since, for ZHA, the amount of water in crystallization may vary, dependingsynthesis method on the [synthesis20], thereby method affecting [20], its thereby availability affecting and the its possibilityavailability of and interaction the possibility with guest of interactionorganic molecules. with guest organic molecules.

3.8.3.8. Controlled Controlled Release Release TheThe CA CA release release profiles profiles of ZHN–CA and ZHA–CAZHA–CA inin pHpH 7.17.1 bubufferffer solution solution are are given given in in Figure Figure8. 8.For For ZHN–CA, ZHN–CA, a fastera faster release release of of CA CA was was reached reached in comparisonin comparison to theto the hybrid hybrid compound compound ZHA–CA. ZHA– CA.The CAThe releasedCA released amount amount was aboutwas about ~80% in~80% 300 in min 300 for min ZHN–CA, for ZHN–CA, whereas whereas only ~66% only was ~66% released was releasedfor ZHA–CA. for ZHA–CA. These results These reveal results di revealfferences difference in the interactionss in the interactions between between the organic the organic anions andanions the andlayers the of layers the two of the host two matrices, host matrices, ZHA and ZHA ZHN. and As ZHN. a result, As a there result, was there a stronger was a stronger coordination coordination bonding bondingbetween between carboxylate carboxylate groups of groups the caff eateof the anions caffeate with anions the ZHA with than the thatZHA with than the that ZHN, with in the concordance ZHN, in concordancewith the analysis with ofthe interfacial analysis characterizationof interfacial charac madeterization in the previous made in section the previous (Section section 3.2). Therefore, (Section 3.2).the releaseTherefore, process the release was slower process and was more slower restricted and more than restricted that for ZHN–AC. than that Forfor ZHN–AC. ZHA–CA, For the releaseZHA– CA,behavior the release achieved behavior an equilibrium achieved an after equilibriu 420 minm withafter ~74%,420 min whereas, with ~74%, for ZHN–CA, whereas, for 480 ZHN–CA, min were 480necessary min were with necessary ~95% release. with The~95% results release. indicate The results that the indicate hybrid that compounds the hybrid have compounds potential usehave as potentialcontrolled use release as controlled systems. release systems.

100 ZHN-CA ZHA-CA 80

60

CA (%) 40

20

0 0 100 200 300 400 500 600 Time (min)

Figure 8. Release profiles for CA from ZHN–CA and ZHA–CA in buffer solutions at pH 7.1. Solid Figurelines are 8. aRelease guide forprofiles the behavior for CA offrom the releasingZHN–CA profile and ZHA–CA and have in no buffer physical solutions significance. at pH The7.1. curvesSolid linescorrespond are a guide to the for average the behavior of three of experimentalthe releasing pr measurementsofile and have for no eachphysical point. significance. The curves 3.9. Kineticcorrespond Analysis to the average of three experimental measurements for each point.

3.9. KineticTo explore Analysis the kinetics of the release process of caffeate anions from the hybrid compounds, four models were evaluated. The release data were fitted by the parabolic, Freundlich, Elovich, and To explore the kinetics of the release process of caffeate anions from the hybrid compounds, four Avrami-Erofe’ev models, and the fitted plots are displayed in Figure9. The equations of these models models were evaluated. The release data were fitted by the parabolic, Freundlich, Elovich, and and the R2 coefficients from the fitting are presented in Table2. It was found that the release data fitted Avrami-Erofe’ev models, and the fitted plots are displayed in Figure 9. The equations of these models well when using the Elovich and Avrami-Erofe’ev models for both hybrid compounds. The model and the R2 coefficients from the fitting are presented in Table 2. It was found that the release data provided better fitting of the data for ZHN–CA with the correlation coefficient being higher than fitted well when using the Elovich and Avrami-Erofe’ev models for both hybrid compounds. The for ZHA–CA. Based on these results, the release processes for the hybrid compounds were mainly governed by the surface diffusion [16,42], and the better release behavior for ZHN–CA can be related

to the chemical composition, structural characteristics, and morphology of this hybrid compound compared to ZHA–CA. Nanomaterials 2020, 10, x FOR PEER REVIEW 12 of 15

model provided better fitting of the data for ZHN–CA with the correlation coefficient being higher than for ZHA–CA. Based on these results, the release processes for the hybrid compounds were mainly governed by the surface diffusion [16,42], and the better release behavior for ZHN–CA can be Nanomaterialsrelated2020 to, 10the, 163 chemical composition, structural characteristics, and morphology of this hybrid12 of 15 compound compared to ZHA–CA.

0.10 1.0 (a) (b)

0.0

0.05 )) ))/t o o /C t /C

t -1.0

(1-(C

0.00 ln(1-(C -2.0

ZHA-CA ZHA-CA ZHN-CA ZHN-CA -3.0 -0.05 0,0 0,1 0,2 0,3 0,4 6543210-1 -0.5 t ln(t) 1.0

1.0 (c) (d) 0.8 )))

o

/C 0.0 t ) 0.6 o /C -1.0 t

0.4

(1-C

ln(-ln(1-(C -2.0 0.2

ZHA-CA ZHA-CA -3.0 ZHN-CA 0.0 ZHN-CA

6543210-1 6543210-1 ln(t) ln(t) Figure 9. Plots of kinetic models to the experimental release of CA anions from ZHN–CA and ZHA– Figure 9. Plots of kinetic models to the experimental release of CA anions from ZHN–CA and ZHA–CA CA hybrid materials: (a) parabolic, (b) Freundlich, (c) Avrami-Erofe’ev, and (d) Elovich models. hybrid materials: (a) parabolic, (b) Freundlich, (c) Avrami-Erofe’ev, and (d) Elovich models.

2 TableTable 2. Equations 2. Equations of kinetic of kinetic models models for for release release analysisanalysis and RR 2coefficientscoefficients of offitted fitted results. results. R2 Model Equation R2 Model Equation ZHN–CA ZHA–CA Parabolic (1 − Ct/C0)/t = kd(t − t0)−0.5 +a ZHN–CA0.9293 0.9139 ZHA–CA 0.5 Parabolic Freundlich(1 Ct/Cln(1)/t = − kCt(/tC0) t= )ln(kd)+ +a aln((t − t0)) 0.92930.8378 0.7837 0.9139 − 0 d − 0 − Freundlich Elovichln(1 Ct/C ) =1ln( − kCt/)C+0 a=ln(( aln((t t t− ))t0)) + b 0.83780.9554 0.9206 0.7837 − 0 d − 0 ElovichAvrami-Erofe’ev1 C t/Cln(−=ln(aln((Ct/Ct 0))t =)) nln+ kbd + nln(t − t0) 0.95540.9754 0.9494 0.9206 − 0 − 0 Avrami-Erofe’ev ln( ln(Ct/C )) = nlnk + nln(t t ) 0.9754 0.9494 C0 represents the amount− of CA0 anions ind ZHN and− ZHA0 at t = 0, Ct is the amount of CA anions in 2 C0 representsZHN and the ZHA amount at time of CA t, and anions kd is in the ZHN rate and of release. ZHA at at,= b,0, andCt isn are the constants. amount of The CA anionsR coefficients in ZHN were and ZHA 2 at timeobtainedt, and kd fromis the three rate of replicas release. ofa ,theb, and responsen are constants.taken for each The R timecoe levelfficients by using were obtainedthe statistical from package three replicas of the responseXLSTAT-2019 taken [43]. for each time level by using the statistical package XLSTAT-2019 [43].

4. Conclusions4. Conclusions CaffeicCaffeic acid acid was was successfully successfully immobilized immobilized into into thethe ZHN and and ZHA ZHA host host matrices. matrices. Variations Variations of of the chemicalthe chemical composition composition of theof the hybrid hybrid materials materials werewere estimated from from the the elemental elemental analysis, analysis, and and the valuesthe values of zeta of zeta potential potential evidenced evidenced di differencesfferences inin thethe surface charge charge of of the the starting starting layered layered host host matrices, which affected the resulting layered structures of the hybrid materials. matrices, which affected the resulting layered structures of the hybrid materials. The basal spacing of ZHN–CA was 10.20 Å and, for ZHA-CA, the d-value was 10.37 Å, indicating that the caffeate anions were arranged, with a tilt angle of 70◦, along the z-axis in the interlayer space of the ZHN and ZHA structures. The strong interaction between the carboxylate groups of the caffeic acid molecule and the Zn2+ of the tetrahedral units produced the substitution of the water molecules occupying the apex of the tetrahedron, giving rise to the arrangement of the CA anions in the interlayer region. The immobilization of caffeate anions in both host matrices was confirmed by FT-IR analysis and XPS analysis, although the differences in chemical environment of the starting layered structures Nanomaterials 2020, 10, 163 13 of 15 defined the superficial characteristics and morphology of the hybrid compounds. The total mass loss during the heat treatment was higher for the intercalation compound ZHN–CA (34.68%) than for ZHA–CA (37.09%) due to the difference in the amount of adsorbed water and the water in crystallization molecules of the host matrices. An enhancement of 65 ◦C of the thermal stability was observed for ZHN–CA due to the retardation of the dehydroxylation process. The release profiles indicated a lower release of CA for ZHA–CA, which is related to the stronger interaction between the layers and the guest anions. The Elovich and Avrami-Erofe’ev models described the release process, through a surface diffusion mechanism, for both hybrid compounds. These results indicate that ZHN–CA and ZHA–CA have potential for use as controlled release formulations of organic molecules, with antioxidant properties.

Author Contributions: Synthesis conceptualization, O.G.; methodology, O.G., C.V.R. and M.E.B.; validation, C.V.R.; formal analysis, C.V.R. and O.G.; resources, O.G. and M.E.B.; investigation and data curation, C.V.R.; writing—original draft preparation, C.V.R.; writing—review and editing, M.E.B. and O.G.; visualization, M.E.B.; supervision, O.G. All authors have read and agree to the published version of the manuscript. Funding: This research was funded by the Fondo de Ciencia, Tecnología, e Innovación del Sistema General de Regalías, the Fondo Nacional de Financiamiento para la Ciencia, la Tecnología y la Innovación “Francisco José de Caldas”, MINCIENCIAS (COLCIENCIAS), Programa Colombia BIO, Gobernación de Boyacá through FP44842-298-2018, and MINCIENCIAS (COLCIENCIAS) as financial support of the doctoral studies of Christhy V. Ruiz through announcement 647-2015. Acknowledgments: The authors would like to thank Enrique Rodriguez Castellón of Málaga University for the XPS analysis and scanning electron microscopy, and the characterization services of the Institute of the Science and Technology of the Polymers (ICTP/CSIC) for the thermal analysis and zeta potential measurements. Conflicts of Interest: The authors declare no conflict of interest.

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