N-cetyltrimethylammonium Bromide Research Articles

Development of Novel Piperine-Loaded Mesoporous Silica Nanoparticles: Enhanced Drug Delivery and Comprehensive In Vivo Safety Analysis.

Piperine-loaded mesophorous silica nanoparticles (MSNPs) were synthesized by chemical methods from tetraethylorthosilicate (TEOS) as a precursor, N-Cetyl Trimethyl Ammonium Bromide (CTAB) as a surfactant, piperine, distilled water, and sodium hydroxide (NaOH) as a catalyst at 80 °C. After stirring the mixture for 20 to 30 minutes, the synthesized combined substances were washed with ethanol and the surfactant was removed using hydrochloric acid (HCl). The morphological characterization was assessed by High Resolution-Transmission Electron Microscope (HR-TEM), scanning electron microscopy (FE-SEM), FESEM-EDX, infrared Fourier transform spectroscopic (FTIR), X-ray diffractometer (XRD), Dynamic Light Scattering (DLS) and UV-VIS. HR-TEM final report showed the amorphous nature of the prepared NPs. TEM image at 100 nm, showed typical ball-like geometry with an average particle size of 13.05 nm. FE-SEM analysis proved that mesoporous silica nanoparticles loaded with piperine have a spherical shape with various nm ranges starting from 232 to 552 nm. The results of the piperine release test observed 93.70% of the drug (piperine) over 24 hours. The in vivo toxicity analysis of piperine-loaded MSNPs tested using adult zebrafish showed no toxic effect. Our developed piperine-loaded mesophorous silica nanoparticles (MSNPs) are favorable for achieving sustained release, a lower dose frequency, and better therapeutic effects.

Thermal-responsive gated nanocomposite of antibacterial gold nanoclusters

A thermal-responsive antibacterial nanocomposite was developed based on the photodynamic gold nanoclusters (AuNCs). N-cetyltrimethylammonium bromide was applied as a soft template to prepare silica nanoparticles (sSiO2@CTAB-SiO2). After etching, the hollow mesoporous silica nanoparticles (HMSN) were prepared and subsequently surface-grafted with thermal-responsive poly(N-isopropylacrylamide) to construct nanocomposites, named HMSN-PNIPAM. The ultrasmall-sized AuNCs matched well with the pore size of HSMN. The poly(N-isopropylacrylamide) underwent a reversible phase transition at varied temperature, which accompanied the change of apparent volume. It could realize the open/closed switch of HMSN pores for AuNCs loading in and sealed up effectively, thus constructing the AuNCs-loaded nanocomposites, termed as HMSN-AuNC. Under mild heating and light irradiation, the AuNCs in HMSN-AuNC could be released and generate reactive oxygen species, which could then perform the bacteria killing. Therefore, taking advantages of the thermo-responsiveness of poly(N-isopropylacrylamide) and the photodynamic property of AuNCs, the nanocomposite exhibited a "switching" effect on both temperature and light, and its antibacterial performance could be realized in a controlled manner.

N-Cetyltrimethylammonium Bromide-Modified Zeolite Na-A from Waste Fly Ash for Hexavalent Chromium Removal from Industrial Effluent

Chromium ions released into aquatic environments pose major environmental risks, particularly in developing countries. Here, a low-cost N-cetyltrimethylammonium bromide (CTAB)-modified fly ash-based zeolite Na-A (CTAB@FZA) was prepared for the treatment of industrial wastewater contaminated with Cr(VI). CTAB@FZA was evaluated using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), and scanning electron microscopy (SEM), which showed that CTAB intercalation and coating of the modified zeolite were successful. The effects of influencing variables on the removal of Cr(VI) using CTAB@FZA were also evaluated, including pH, initial concentration, time, temperature, and coexisting ions. Fast adsorption equilibrium was observed after less than 10 min, and CTAB@FZA had a maximum adsorption capacity of 108.76 mg/g and was substantially greater than that of pristine FZA following modification. Furthermore, isothermal and kinetic data demonstrated that Cr(VI) adsorbed onto homogeneous surfaces via rate-limiting monolayer Langmuir adsorption, and according to thermodynamic data, the sorption of the targeted pollutant was exothermic and spontaneous. The application of CTAB@FZA to industrial wastewater treatment yielded Cr(VI) concentrations that were below the USEPA standards. Overall, the findings demonstrated that CTAB@FZA is an effective, promising, and economical adsorbent for the treatment of Cr(VI)-polluted water.

Effect of dynamic adsorption layer over colliding bubble on rate of solid surface dewetting in cationic surfactant solutions

Influence of a motion induced dynamic adsorption layer (DAL) at a liquid/air interface on a kinetic of three-phase contact line (TPCL) expansion in solutions of n-cetyltrimethylammonium bromide (CTAB - cationic surfactant) was studied. It was found that, independently of the solid surface hydrophobicity (assessed using determination of the receding contact angle values), the DAL presence influences profoundly the velocity of the TPCL expansion. For the quartz surface, where the contact angle was controlled by a CTAB concentration, the TPCL expansion rate correlateds with the degree of solid surface hydrophobicity. However, for similar values of the contact angle, much higher rate of the TPCL rate expansion was observed for the fully formed DAL at the colliding bubble surface. This effect was the most pronounced for receding contact angle equal to ca. 60–70° (i.e. for higher CTAB bulk concentrations). In the case of the highly hydrophobic poly(tetrafluoroethylene) (PTFE) surface, similar observations were reported, despite the fact that the receding contact angle values were independent of the CTAB bulk concentration. For the PTFE surface, additionally, similar TPCL expansion velocities were reported for pure water and CTAB solutions where the DAL structure was fully formed at the colliding bubble interface, suggesting similar liquid/gas interface’s hydrodynamic boundary conditions.

Influence of surfactants on structural, morphological, optical and antibacterial properties of SnO2 nanoparticles.

Tin oxide (SnO2) nanoparticles were synthesised using various surfactants of different charges (n-cetyl trimethyl ammonium bromide, sodium dodecyl sulphate and TRITON X-100) by the co-precipitation method. The synthesised nanomaterials were characterised using different techniques to study their structural, surface morphological, optical and anti-bacterial activities. X-ray diffraction patterns revealed the formation of a tetragonal rutile structure in pure and surfactants-aided SnO2 nanoparticles and the results show good agreement with JCPDS data [41-1445]. The crystallite size of SnO2 nanoparticles was found to decrease with the addition of surfactants. Scanning electron microscopy images exhibit spherical shape morphology with an average diameter of 30-75 nm for pure and surfactants-aided SnO2 nanoparticles. The band gap energy of the prepared materials was estimated from the UV-visible absorption spectra and a considerable increase in band gap energy was observed in surfactants-aided SnO2 nanoparticles (3.487, 3.57, 3.50 and 3.3 eV). The antibacterial activities of the synthesised nanoparticles were studied against Escherichia coli and Staphylococcus aureus bacteria.

Kinetics of the bubble attachment and quartz flotation in mixed solutions of cationic and non-ionic surface-active substances

The effect of cationic (n-cetyltrimethylammonium bromide − CTAB) and non-ionic (n-octanol) surface-active substances (SAS) on the kinetics of rising bubble attachment to a quartz surface and flotation recovery of quartz particles was investigated. The measured time of the three-phase contact (TPC) formation (tTPC) and the advancing contact angle (sessile drop) values, in pure and mixed CTAB and n-octanol solutions, allowed to evaluate the importance of electrostatic interactions and solid surface hydrophobicity in stability and rupture of the wetting films formed between a quartz surface and a colliding bubble. It was shown that electrostatic interactions inside the wetting film as well as the quartz hydrophobicity was changing with the composition of the mixed solutions. The experiments revealed that, within the concentration range of the SAS used in this study, CTAB modified properties of liquid/gas and liquid/solid interfaces, while non-ionic n-octanol adsorbed primarily at liquid/gas interface hindering its fluidity and, as a result, decreased bubble bouncing time. The correlation between the times of bubble attachment and the flotation recovery of quartz particles was established.

Electrochemical Synthesis of A Oxalate-Linked Copper (II) Metal Organic Frameworks: X-ray Crystallographic Structure and its Magnetic Properies

The copper (II) extended metal organic frameworks with oxalic (ox = oxalate) Na2 [(Cu (ox)2 (H2O) ]and NH4)2 [Cu(ox)2 (H2O)2].H2O have been synthesized using electrochemical route at room temperature and 12.5 V. This is a new method in which copper rod was taken as a working electrode and platinum wire as a reference electrode. The structures were characterized with single crystal X-ray diffraction (SXRD) and also by other supportive techniques like PXRD,TGA/DTA and infrared spectroscopy. MOFs are crystalline solids in which the metal ions and ligands are assembled infinitely resulting in one, two or three-dimensional networks having direct metal-ligand coordination. The presence of Sodium hydroxide, ammonia and N-Cetyl tri methyl ammonium bromide plays important role in the structure of abovementioned MOFs during electrochemical synthesis. The growth of the solids is interpreted in term of self assembly between a reasonable soluble molecules reacting in the supramoleculer reaction along the mechanistic approach proposed by Ramanan and Whittingham during the nucleation of MOFs crystals. In the context of our magneto-structural research with oxalato-bridged copper (II) complexes and in order to obtain novel coordination modes that could enhance the magnetic exchange coupling we have studied the influence of the supporting electrolyte in crystal packing and hence the magnetic properties present in these compounds.

Preparation and evaluation of a thermosensitive liposomal hydrogel for sustained delivery of danofloxacin using mesoporous silica nanoparticles

Background: Sustained release delivery system can reduce the dosage frequency and maintain the therapeutic level of drugs for a longer time. Biodegradable, biocompatible and thermosensitive chitosan-beta-glycerophosphate (C-GP) solutions can solidify at body temperature and maintain their physical integrity for a longer duration. OBJECTIVES: To develop a novel delivery system based on the integration of liposomes in hydrogel using mesoporous silica nanoparticles (MSNs) for sustained release of danofloxacin in farm animals. METHODS: The MSNs were prepared using N-cetyltrimethylammonium bromide and tetraethylortho silica. The liposomes were prepared by thin film hydration method. C-GP solution containing danofloxacin-loaded MSN liposomes underwent different in-vitro tests, including evaluation of the entrapment efficiency, gelation time, morphology, drug release pattern as well as antimicrobial activities against S. aureus and E. coli. RESULTS: The mean pore size of MSNs was 2.8 nm and the mean MSN entrapment efficiency was 45%. Kinetics of danofloxacin release from liposomal hydrogel followed the Higuchi’s model. This formulation was capable of sustaining the danofloxacin release for more than 96 h. The FTIR studies showed that there were no interactions between danofloxacin and hydrogel excipients. Scanning electron microscopy (SEM) showed that the formed gel had a continuous texture, while the swelled gel in the phosphate buffer had a porous structure. Microbiological tests revealed a high antibacterial activity for lipomosal hydrogel of danofloxacin-loaded MSN comparable with danofloxacin solution. CONCLUSIONS: The liposomal hydrogel solidified at body temperature, effectively sustained the release of danofloxacin and showed in vitro antibacterial effects.

Influence of different electrolytes on bubble motion in ionic surfactants solutions

We determined the influence of concentration and type of added inorganic electrolytes on local and terminal velocities of bubbles in solutions of anionic, sodium n-hexadecylsulfate (SHS) and cationic, n-cetyltrimethylammonium bromide (CTAB) surfactants at concentrations below 3×10−6M. We found that addition of inorganic electrolyte strongly affected the bubble motion. Bubble local velocity passed through the maximum and reached terminal value, which for the electrolyte concentration around 10−3M, corresponded to the immobilized surface. Up to that concentration the width of the maximum of the local velocity profile decreased with electrolyte contents. Surprisingly, addition of higher concentration of inorganic electrolyte caused the reverse effect, the increase and broadening of the maximum, which depended on a type of counterions. For CTAB the effect followed the Hofmeister series of ion activity. That reverse effect may be connected with the modified structure of the dynamic adsorption layer in the presence of high excess of various anions or with the micelization enhanced surfactant desorption rate at the rear part of the rising bubble, reducing the Marangoni stress that immobilizes bubble surface.

Improving structural stability of water-dispersed MCM-41 silica nanoparticles through post-synthesis pH aging process

The colloidal and structural stabilities of MCM-41 mesoporous silica nanoparticles (MSNs) are of great importance in order to prepare optimal nanovectors. In this paper, MSNs (approximatively 160 nm in diameter) were synthesized using n-cetyltrimethylammonium bromide as a template and tetraethyl orthosilicate as a silica source under high N2 flow (MSN/N2) to obtain stable dispersions in water. The degradation of the porous nanoparticles was investigated by immersion in water. The morphology and the porous structure were studied by TEM, XRD, N2 sorption, and 29Si MAS NMR and were compared to that of MSNs prepared in ambient air (MSN/air). The volumetric properties of the MSN/N2 after 1 day in water were drastically more decreased than MSN/air (a pore volume decrease of 85 % for MSN/N2 and 59 % for MSN/air) and the 2D-hexagonal porous structure was totally lost. Furthermore, synthesizing MSNs under a high N2 flow leads to a decrease in the synthesis yield (45 % MSN/N2 and 75 % for MSN/air). The lower structural stability of the MSN/N2 is explained by the lower polycondensation degree of the MSN/N2 observed by 29Si MAS NMR (Q4/Q3 = 0.86 for MSN/N2 and 1.61 for MSN/air) and the lower silica molar ratio in the nanomaterials (SiO2/CTA = 3.9 for MSN/N2 7.1 for MSN/air). This allows for enhanced solubilization of silica in water. Four strategies were hence evaluated in order to reinforce the porous structure of the MSNs. Among them, the most efficient route was based on a pH adjustment of the colloidal suspension (pH 7.5) after 2 h of synthesis without any purification and while keeping a N2 static atmosphere (called MSN/N2/7.5). After 1 day in water, the volumetric and structural properties of MSN/N2/7.5 were similar to that obtained for MSN/air. The improvement of the stability arose as a result of the increase in the silica condensation (Q4/Q3 = 1.58) and silica molar ratio in the nanomaterials (SiO2/CTA = 6.8). After the post-treatment, the silica framework condensation is improved while keeping the colloidal stability, thus allowing further functionalization and/or drug loading. Cytotoxicity assays using SW480 cancer cells show a greater improvement in the cell viability.

Synthesis and characterization of porous WO3–SnO2 nanomaterials: An infrared study of adsorbed pyridine and dimethyl methylphosphonate

High surface area porous W/Sn oxide nanomaterials were prepared via water/oil based (W/O) emulsion. Tungstic acid solution was generated by cation exchange of sodium tungstate in acidic Dowex resin. The acid was then mixed with a clear homogeneous aqueous N-cetyl trimethyl ammonium bromide (CTAB) solution followed by a slow addition of 0.2M SnCl4 solution. The mixture was stirred for 24h and then subjected to slow calcination at 500°C. The prepared materials were characterized using SEM-EDX, BET surface area, and sorption of nitrogen and water. Fourier transform infrared spectroscopy (FTIR) was used to characterize the surface acidic properties using pyridine vapor as a probe. The materials were then tested toward the Dimethyl methylphosphonate (DMMP) adsorption at various temperatures using infrared spectroscopy. At elevated temperatures, the desorption of DMMP from WO3 and SnO2 surfaces results in forming methyl phosphonate that strongly bounds on the metal oxide surfaces. In contrast, the FTIR spectra showed that the adsorbed dimethyl methylphosphonate (DMMP) on the mixed W/Sn oxide powders can be molecularly desorbed without any decomposition.

The effect of serum albumin, surfactant and their mixture on the reduction of a cobalt(III) complex by ascorbic acid

Kinetic investigations on the reduction of CoIII-complex by a biologically active antioxidant molecule, ascorbic acid in a buffer medium of pH 9.0 have been performed using spectroscopic techniques. The reaction has been carried out in presence of a protein, bovine serum albumin (BSA), n-cetyltrimethylammonium bromide (CTAB) and their mixtures. The reaction rate changes remarkably on addition of protein. Unlike the complex, experimental results depict extensive binding of detergent and ascorbate anions with BSA. Both native and cooperative binding have been observed for CTAB–BSA interaction. But only native binding at specific binding sites have been observed for ascorbic acid–BSA interaction. The reactant molecules prefer to bind with domain I and III over II of the BSA molecules. Denaturing action of urea releases the protein bound ascorbates into bulk. The experimental findings have been further verified from the conductometric and circular dichroism studies.

Electrospinning preparation and drug delivery properties of Eu3+/Tb3+ doped mesoporous bioactive glass nanofibers

Luminescent Eu3+/Tb3+ doped mesoporous bioactive glass nanofibers (MBGNFs) with average diameter of 100–120nm were fabricated by electrospinning method. Pluronic P123 and N-cetyltrimethylammonium bromide (CTAB) were used as co-surfactants to generate porous structure of the nanofibers. N2 adsorption–desorption measurement reveals that the MBGNF:Eu3+ have a surface area of 188m2g−1, a pore volume of 0.246cm3g−1 and average pore size of 4.17nm, and the MBGNF:Tb3+ have a surface area of 171m2g−1, a pore volume of 0.186cm3g−1 and average pore size of 3.65nm. Photoluminescence measurements reveal that the MBGNF:Eu3+ show strong red emission dominated by the 5D0→7F2 transition of Eu3+ at 614nm with a lifetime of 1.356ms, and MBGNF:Tb3+ show strong green emission dominated by the 5D4→7F5 transition of Tb3+ at 544nm with a lifetime of 1.982ms. The biocompatibility tests on L929 fibroblast cells using MTT assay reveal low cytotoxicity of MBGNF. These luminescent nanofibers show sustained release properties for ibuprofen (IBU) in vitro. The emission intensities of Eu3+ in the drug delivery system vary with the released amount of IBU, thus making the drug release be easily tracked and monitored by the change of the luminescence intensity.

Synthesis and host-guest properties of pillar[6]arenes

A facile method for the synthesis of pillar[6]arenes was developed. A series of pillar[6]arenes were prepared with FeCl3 as catalyst and chloroform as solvent at room temperature in moderate yields (30%–40%). Their host-guest properties with n-cetyltrimethyl ammonium bromide were investigated by 1 HNMR. The results showed that high selectivity in the host-guest relationship became apparent between pillar[6]arenes and pillar[5]arenes based on the different size of the inner cavity.

Mesoporous silica-coated superparamagnetic particles prepared by pseudomorphic transformation and their application in purification of plasmid DNA

Mesoporous silica-coated superparamagnetic particles were prepared via pseudomorphic transformation of pre-made amorphous silica-coated Fe3O4–polymer composite particles using n-cetyltrimethylammonium bromide (CTAB) and 1,3,5-trimethyl benzene (MES) as template. The mesoporous particles presented almost the same size, shape, and magnetic property as the original amorphous particles but an ordered mesoporous shell with wormhole-like pore structure. The pore size of the shells increased from 2.4 to 3.1 and 4.2 nm as the molar ratio of MES/CTAB increased from 0 to 1.0 and 1.5. DNA extraction experiments showed the mesoporous particles were qualified for purification of plasmid DNA from bacterial lysate.

Capacitively coupled contactless conductivity detection as an alternative detection mode in CE for the analysis of kanamycin sulphate and its related substances

A method was developed to determine simultaneously kanamycin, its related substances and sulphate in kanamycin sulphate using capacitively coupled contactless conductivity detection. Kanamycin is an aminoglycoside antibiotic that lacks a strong UV-absorbing chromophore. Due to its physicochemical properties, CE in combination with capacitively coupled contactless conductivity detection was chosen. The separation method uses a BGE composed of 40 mM 2-(N-morpholino)ethanesulphonic acid monohydrate and 40 mM L-histidine, pH 6.35. A 0.6 mM N-cetyltrimethyl ammonium bromide (CTAB) solution was added as electroosmotic flow modifier in a concentration below the critical micellar concentration (CMC). Ammonium acetate 50 mg/L was used as internal standard. In total, 30 kV was applied in reverse polarity on a fused-silica capillary (65/41 cm; 75 μm id). The optimized separation was obtained in less than 6 min with good linearity (R(2)=0.9999) for kanamycin. It shows a good precision expressed as RSD on the relative peak areas equal to 0.3 and 1.1% for intra-day and inter-day precision, respectively. The LOD and LOQ are 0.7 and 2.3 mg/L, respectively. Similarly, for sulphate, a good linearity (R(2)=0.9996) and precision (RSD 0.4 and 0.6% for intra-day and inter-day, respectively) were obtained.

The Effect of CTAB on the Citrate Sol-gel Process for the Synthesis of Sodium Beta-Alumina Nano-Powders

Sodium beta-alumina (SBA) nano-powders were synthesized by the citrate sol-gel process, and the effects of the cationic surfactant n-cetyltrimethylammonium bromide surfactant (CTAB) were investigated. The structure and morphology of the nano-powders were characterized by X-ray diffraction (XRD) and transmission electron microscope (TEM) techniques, respectively. The effects of CTAB on the citrate sol-gel process and the SBA formation were investigated by thermo gravimetric/differential thermal analysis (TG/DTA) and Fourier transform infrared spectroscopy (FTIR). The conductivity of ceramic pellets of SBA was measured by electrochemical impedance spectroscopy (EIS). The results showed that the CTAB inhibited the agglomeration of SBA powders effectively and consequently decreased the crystallization temperature of SBA, about <TEX>$150^{\circ}C$</TEX> lower than that of the sample without CTAB. The measured conductivity of SBA was <TEX>$1.21{\times}10^{-2}S{\cdot}cm^{-1}$</TEX> at <TEX>$300^{\circ}C$</TEX>.

Fabrication and sustained release properties of porous hollow silica nanoparticles

MCM-41-type mesoporous silica nanospheres (MSN) have been prepared using n-cetyltrimethylammonium bromide (CTAB) as a soft template. The pseudo-moire’ rotational pattern inside the MSN results in many interior defects. Hollow mesoporous silica (HMS) spheres were synthesized by solvent extraction of the template from MSN. The morphology and structure of MSN and HMS were studied by TEM, XRD and nitrogen sorption techniques. A model drug, bromocresol green dye, was packed inside different regions of HMS through impregnation at different pressures and temperatures, and the drug release performance of the resulting materials was compared. The results showed that vacuum evaporation of solvents may enhance the controlled release properties.

Interaction of alkyltrimethylammonium bromides with DMPC- d54 and DMPG- d54 monolayers studied by infrared reflection absorption spectroscopy (IRRAS)

Infrared reflection absorption spectroscopy (IRRAS) was used to analyze the adsorption of the cationic surfactants n-dodecyl-, n-tetradecyl-, and n-cetyltrimethylammonium bromide (DTAB, TTAB and CTAB) at the air–water interface and to study their interaction with 1,2-dimyristoyl- d 54- sn-glycero-3-phosphocholine (DMPC- d 54) and 1,2-dimyristoyl- d 54- sn-glycero-3-phosphoglycerol (DMPG- d 54) Langmuir monolayers. From the Gibbs adsorption isotherms of the pure surfactants on 0.1 M NaCl as subphase the value of surface excess concentration Γ and the standard free energy of adsorption Δ G 0 was determined as a function of the interface coverage Θ. The conformational and orientational changes of the surfactant alkyl chains were followed by analyzing the frequencies of the antisymmetric and symmetric methylene stretching vibrational bands ν(CH 2) during adsorption at the air–water interface. The interaction of the surfactants with lipid monolayers was also followed by IRRAS. Deuterated lipids were used to distinguish between hydrocarbon chains of the surfactants and the phospholipids. Thus, information on the conformational changes of the lipid molecules during incorporation of surfactant molecules into the lipid monolayer could be obtained. The absorbance values of the antisymmetric and symmetric methylene stretching vibrational bands (ν(CD 2) and ν(CH 2)) were used for the direct calculation of the surface partition coefficients P of the surfactants. The influence of the surfactant headgroup charge and its alkyl chain length on the incorporation into lipid monolayers is discussed.

The role of surfactant in synthesis of magnetic nanocrystalline powder of NiFe2O4 by sol–gel auto-combustion method

In this work, a new sol–gel auto-combustion method has been performed to synthesize single phase nickel ferrite nanocrystalline powders by using n-cetyltrimethylammonium bromide, as a cationic surfactant. The gels were prepared from ferric and nickel nitrates and citric acid. Ammonia was used as pH adjusting agent as well. The effects of the surfactant on the after combustion calcination process and the reduction of the resulting powder crystallite size which affects the magnetic properties of the material were investigated by XRD and DTA/TGA techniques. The results showed that the ignition of the gels in air have a self-propagating behavior. Addition of surfactant to the starting solution affected the crystallite size of the synthesized powders and their phase constitution. The crystallite size of nickel ferrite powder in sample with surfactant was obtained 31.2 nm. The other important result of this study was production of single phase nickel ferrite directly after combustion while without surfactant the nickel ferrite single phase was obtained only after a post-calcination process at 1000 °C.