Citrate Anions Research Articles

A Novel "Off-On" Fluorescent Probe Based on Carbon Nitride Nanoribbons for the Detection of Citrate Anion and Live Cell Imaging.

A novel fluorescent “off-on” probe based on carbon nitride (C3N4) nanoribbons was developed for citrate anion (C6H5O73−) detection. The fluorescence of C3N4 nanoribbons can be quenched by Cu2+ and then recovered by the addition of C6H5O73−, because the chelation between C6H5O73− and Cu2+ blocks the electron transfer between Cu2+ and C3N4 nanoribbons. The turn-on fluorescent sensor using this fluorescent “off-on” probe can detect C6H5O73− rapidly and selectively, showing a wide detection linear range (1~400 μM) and a low detection limit (0.78 μM) in aqueous solutions. Importantly, this C3N4 nanoribbon-based “off-on” probe exhibits good biocompatibility and can be used as fluorescent visualizer for exogenous C6H5O73− in HeLa cells.

(Invited) Impacts of Ions on Oxygen Reduction Reaction Kinetics on Platinum and Palladium Surfaces

Fundamental understandings on the impacts induced by anions and cations on oxygen reduction reaction (ORR) are of great interest in designing more efficient catalysts and identifying reasons for discrepancies in activities measured in different protocols. In this paper, the specific adsorption of various anions and cations on platinum and palladium catalysts and their effects on the ORR kinetics were summarized. It was found that ClO4 - had a negligible impact on the ORR activity of Pt/C possibly due to its weak adsorption. Nafion ionomers can significantly poison the bulk Pt surfaces, but its impact on Pt/C is negligible. Self-dissociation of citrate anion was found to occur on both Pt/C and Pd/C.

Preparation of zinc/iron layered double hydroxide intercalated by citrate anion for capturing Lead (II) from aqueous solution

In this investigation, ultrasound irradiation has been proposed as a fast, simple and green method for the synthesis of citrate (Cit) intercalated zinc iron layered double hydroxide (ZnFe/Cit-LDH) through the co-precipitation method. The co-precipitation method created high crystallinity LDH characterized by Fourier transform infrared spectroscopy, X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy and Energy-dispersive X-ray spectroscopy. The synthesized ZnFe/Cit-LDH was applied for the removal of Lead (II) ions. The absorption functions in the batch experiments were studied using the expected parameters for the maximum absorption capacities (qm) such as pH, contact time, and the initial concentration. The obtained qm, corresponding to Langmuir as the best model for the removal of Lead (II) ions, was 94.3mg/g at 30min and the pH6, based on the outside sites accessible on the sorbent surface. The uptake kinetics was also investigated and a good agreement with the experiments was obtained. The results, therefore, revealed that the adsorption kinetics and the adsorption isotherms of Lead (II) ions on ZnFe/Cit-LDH were well-fitted by the pseudo-second-order kinetic model and the Langmuir isotherm model, respectively.

Highly selective and sensitive macrocycle-based dinuclear foldamer for fluorometric and colorimetric sensing of citrate in water

The selective detection of citrate anions is essential for various biological functions in living systems. A quantitative assessment of citrate is required for the diagnosis of various diseases in the human body; however, it is extremely challenging to develop efficient fluorescence and color-detecting molecular probes for sensing citrate in water. Herein, we report a macrocycle-based dinuclear foldamer (1) assembled with eosin Y (EY) that has been studied for anion binding by fluorescence and colorimetric techniques in water at neutral pH. Results from the fluorescence titrations reveal that the 1·EY ensemble strongly binds citrate anions, showing remarkable selectivity over a wide range of inorganic and carboxylate anions. The addition of citrate anions to the 1·EY adduct led to a large fluorescence enhancement, displaying a detectable color change under both visible and UV light in water up to 2 μmol. The biocompatibility of 1·EY as an intracellular carrier in a biological system was evaluated on primary human foreskin fibroblast (HF) cells, showing an excellent cell viability. The strong binding properties of the ensemble allow it to be used as a highly sensitive, detective probe for biologically relevant citrate anions in various applications.

Tunable Self-Assembly of YF3 Nanoparticles by Citrate-Mediated Ionic Bridges.

Ligand-to-surface interactions are critical factors in surface and interface chemistry to control the mechanisms governing nanostructured colloidal suspensions. In particular, molecules containing carboxylate moieties (such as citrate anions) have been extensively investigated to stabilize metal, metal oxide, and metal fluoride nanoparticles. Using YF3 nanoparticles as a model system, we show here the self-assembly of citrate-stabilized nanostructures (supraparticles) with a size tunable by temperature. Results from several experimental techniques and molecular dynamics simulations show that the self-assembly of nanoparticles into supraparticles is due to ionic bridges between different nanoparticles. These interactions were caused by cations (e.g., ammonium) strongly adsorbed onto the nanoparticle surface that also interact strongly with nonbonded citrate anions, creating ionic bridges in solution between nanoparticles. Experimentally, we observe self-assembly of nanoparticles into supraparticles at 25 and 100 °C. Interestingly, at high temperatures (100 °C), this citrate-bridge self-assembly mechanism is more efficient, giving rise to larger supraparticles. At low temperatures (5 °C), this mechanism is not observed, and nanoparticles remain stable. Molecular dynamics simulations show that the free energy of a single citrate bridge between nanoparticles in solution is much larger than the thermal energy and in fact is much larger than typical adsorption free energies of ions on colloids. Summarizing our experiments and simulations, we identify as key aspects of the self-assembly mechanism the requirement of NPs with a surface able to adsorb anions and cations and the presence of multidentate ions in solution. This indicates that this new ion-mediated self-assembly mechanism is not specific of YF3 and citrate anions, as supported by preliminary experimental results in other systems.

Elucidation of hydrated metal ions using flocculation-surface enhanced Raman scattering

Hydrated metal ions such as Na+, Mg2+, and Cu2+ in addition to surface species like citrate, p-mercaptobenzoic acid (PMBA), and halide anions were identified by exploiting coupled localized surface plasmons of flocculated silver nanoparticles (AgNPs). Indeed, SERS spectra provided clear evidence for the formation of citrate complexes of Co2+ and Cu2+ as well as ion pairs of metal ions and PMBA− anions on AgNPs. For the first time, we have detected pronounced Raman bands from hydrated metal ions in flocculates of halide-covered AgNPs assigned to restricted rotation and translation at 600–200 cm−1 and OH stretching bands at 3500–3600 cm−1. Interestingly, the peak frequencies of OH stretching mode from hydrated metal ions are sensitive to metal and halide species, proving the observed water molecules are sandwiched between trapped metal ions and halide ions on adjacent AgNPs while forming a solvent shared ion pair.

Nanospectroscopy of thiacyanine dye molecules adsorbed on silver nanoparticle clusters

The adsorption of thiacyanine dye molecules on citrate-stabilized silver nanoparticle clusters drop-cast onto freshly cleaved mica or highly oriented pyrolytic graphite surfaces is examined using colocalized surface-enhanced Raman spectroscopy and atomic force microscopy. The incidence of dye Raman signatures in photoluminescence hotspots identified around nanoparticle clusters is considered for both citrate- and borate-capped silver nanoparticles and found to be substantially lower in the former case, suggesting that the citrate anions impede the efficient dye adsorption. Rigorous numerical simulations of light scattering on random nanoparticle clusters are used for estimating the electromagnetic enhancement and elucidating the hotspot formation mechanism. The majority of the enhanced Raman signal, estimated to be more than 90%, is found to originate from the nanogaps between adjacent nanoparticles in the cluster, regardless of the cluster size and geometry.

Layered double hydroxide nanoparticles customization by polyelectrolyte adsorption: mechanism and effect on particle aggregation

Adsorption of polyelectrolytes (PEs) is a common strategy for stabilizing nanoparticles and customizing their properties. Although scarcely explored for layered double hydroxide nanoparticles (LDH-NPs), it can be used to increase their functionality in pharmaceutical and biomedical applications, among others. In this work, LDH-NPs intercalated with chloride, ibuprofen and ketoprofen were synthesized and modified with carboxylate containing PEs with different structures and physicochemical properties. The PEs adsorption mechanism on LDH-NPs and its effect on the aggregation of the obtained hybrids were studied by zeta potential and hydrodynamic diameter determinations on chloride intercalated LDH-NPs dispersions in the presence of polyacrylate (PA) at different concentrations. PA adsorption behavior was compared to that of citrate anion to contrast the adsorption mechanism of PEs and anions. Afterwards, the study was extended to other PEs (alginate and carbomer) as well as to drug intercalated LDH-NPs. PA adsorbed at the surface of LDH-NPs with high affinity: at low concentrations, PA was irreversibly adsorbed due to the positive structural charge of LDH-NPs and the flexible and negatively charged structure of PA. This high affinity caused a particle charge inversion, which results in negatively charged LDH-NPs stabilized by both electrostatic interactions and steric hindrance. Due to their structure, alginate and carbomer showed a lower affinity and, consequently, a lower capacity to produce LDH-NPs disaggregation. Finally, PEs adsorption allowed enhancing the disaggregation and modifying the interfacial properties of both ibuprofen and ketoprofen intercalated LDH-NPs producing only a small release of the drug loading.

Hemoglobin becomes electroactive upon interaction with surface-protected Au nanoparticles

In this work, we report on the electrochemical behavior of bioconjugates prepared with gold nanoparticles (AuNP) capped with three different molecular layers (citrate anions, 6-mercaptopurine and ω-mercaptoundecanoic acid) and the protein hemoglobin (Hb). Freshly formed bioconjugates are deposited on a glassy carbon electrode and assayed for electroactivity. A pair of redox peaks with formal potential at −0.37V is obtained, in contrast with the free Hb protein that is inactive on the glassy carbon substrate. The redox response is typical for quasi-reversible processes allowing the determination of the electron transfer rate constant for the three bioconjugates. Additional evidence of the structural integrity of protein upon forming the bioconjugate is obtained by monitoring the electrochemical response of the Hb heme Fe(III)/Fe(II) redox couple as a function of solution pH. Moreover, the Hb forming the protein corona around the AuNPs show good electrocatalytic activity for the reduction of hydrogen peroxide and oxygen. It has been found that only the first layer of Hb surrounding the AuNPs are electroactive, although some part of the second layer also contribute, pointing to the role of the AuNP in the electrochemical response.

Dietary citrate and plasma ionized calcium: Implications for platelet donors.

Platelet donors receive 40 mmol or more of IV citrate anion during donation. When plasma ionized calcium ([Ca2+ ]) falls by ∼20%, half of the donors report symptoms of hypocalcemic toxicity. Citrus juices contain clinically relevant amounts of citrate anion. We asked whether citrus juice can lower [Ca2+ ] thus potentially contributing to hypocalcemic toxicity. Six volunteers were given 20.4 mmol of citrate anion as grapefruit juice or orange juice. Capillary blood obtained by fingerstick was analyzed for [Ca2+ ] using an iSTAT point-of-care blood analyzer. [Ca2+ ] was measured at baseline and then 30, 60, 120, and 180 minutes after drinking juice. Subjects were tested with the alternative juice on a subsequent day. The outcome measure was the percent change in plasma [Ca2+ ] from baseline. [Ca2+ ] fell -2.2% to -11.5% in four of six subjects 30 minutes after drinking grapefruit juice. The effect persisted up to 3 hours. [Ca2+ ] fell -2.1% to -12.2% in four of six subjects 30-60 minutes after drinking orange juice. The effect abated after 2 hours. We could not correlate gender or body surface area to these findings. Citrus juice may lower [Ca2+ ] for 2-3 hours. This could add to the effect of IV citrate infusion during platelet donation, thus worsening the expected fall in [Ca2+ ]. This, in turn, would likely increase the rate and severity of hypocalcemic toxicity. It is prudent to advise platelet donors to avoid high citrate anion beverages, such as citrus juice, for at least 4 hours prior to donation.

Carboxylate Counteranions in Electronic Cigarette Liquids: Influence on Nicotine Emissions.

The wide pH range reported for electronic cigarette (ECIG) liquids indicates that nicotine may be present in one or more chemical forms. The nicotine form affects the bioavailability and delivery of nicotine from inhaled products. Protonated nicotine is normally associated with counteranions in tobacco products. The chemical and physical properties of counteranions may differently influence the nicotine form and emissions in ECIG aerosols. In this study, we examined how these anions influence nicotine emissions and their evaporation behavior and potential decomposition during ECIG operation. ECIG liquid solutions with equal nicotine concentration and pH but different counteranions (formate, acetate, and citrate) were prepared from analytical standards to assess the effect of the counteranion on nicotine partitioning. High performance liquid and gas chromatography methods were developed to determine the counteranions and the two protonated (NicH+) and free base (Nic) forms of nicotine in commercially available and standard solutions of ECIG liquids and aerosols. In commercial samples, acetate and citrate anions were detected. In standard solutions, both formate and acetate ions were found to evaporate intact, but citrate ion decomposed into formic acid and other products. This study also shows that the identity of the counteranion has no effect on total nicotine emission from ECIG in agreement with previous reports on tobacco cigarettes. However, the partitioning of aerosolized nicotine into NicH+ and Nic is anion-dependent even when the parent liquid pH is held constant. These results indicate that the anions found in a given ECIG product may influence the nicotine delivery profile to the user by enriching aerosols with free-base nicotine as in the case of polycarboxylic acids such as citric acid.

Hemoglobin bioconjugates with surface-protected gold nanoparticles in aqueous media: The stability depends on solution pH and protein properties

The identification of the factors that dictate the formation and physicochemical properties of protein-nanomaterial bioconjugates are important to understand their behavior in biological systems. The present work deals with the formation and characterization of bioconjugates made of the protein hemoglobin (Hb) and gold nanoparticles (AuNP) capped with three different molecular layers (citrate anions (c), 6-mercaptopurine (MP) and ω-mercaptoundecanoic acid (MUA)). The main focus is on the behavior of the bioconjugates in aqueous buffered solutions in a wide pH range. The stability of the bioconjugates have been studied by UV–visible spectroscopy by following the changes in the localized surface resonance plasmon band (LSRP), Dynamic light scattering (DLS) and zeta-potential pH titrations. It has been found that they are stable in neutral and alkaline solutions and, at pH lower than the protein isoelectric point, aggregation takes place. Although the surface chemical properties of the AuNPs confer different properties in respect to colloidal stability, once the bioconjugates are formed their properties are dictated by the Hb protein corona. The protein secondary structure, as analyzed by Attenuated total reflectance infrared (ATR-IR) spectroscopy, seems to be maintained under the conditions of colloidal stability but some small changes in protein conformation take place when the bioconjugates aggregate. These findings highlight the importance to keep the protein structure upon interaction with nanomaterials to drive the stability of the bioconjugates.

Transcriptome wide identification and characterization of starch branching enzyme in finger millet.

Starch-branching enzymes (SBEs) are one of the four major enzyme classes involved in starch biosynthesis in plants and play an important role in determining the structure and physical properties of starch granules. Multiple SBEs are involved in starch biosynthesis in plants. Finger millet is calcium rich important serial crop belongs to grass family and the transcriptome data of developing spikes is available on NCBI. In this study it was try to find out the gene sequence of starch branching enzyme and annotate the sequence and submit the sequence for further use. Rice SBE sequence was taken as reference and for characterization of the sequence different in silico tools were used. Four domains were found in the finger millet Starch branching enzyme like alpha amylase catalytic domain from 925 to2172 with E value 0, N-terminal Early set domain from 634 to 915 with E value 1.62 e-42, Alpha amylase, C-terminal all-beta domain from 2224 to 2511 with E value 5.80e-24 and 1,4-alpha-glucan-branching enzyme from 421 to 2517 with E value 0. Major binding interactions with the GLC (alpha-d-glucose), CA (calcium ion), GOL (glycerol), TRS (2-amino-2-hydroxymethylpropane- 1, 3-diol), MG (magnesium ion) and FLC (citrate anion) are fond with different residues. It was found in the phylogenetic study of the finger millet SBE with the 6 species of grass family that two clusters were form A and B. In cluster A, finger millet showed closeness with Oryzasativa and Setariaitalica, Sorghum bicolour and Zea mays while cluster B was formed with Triticumaestivum and Brachypodium distachyon. The nucleotide sequence of Finger millet SBE was submitted to NCBI with the accession no KY648913 and protein structure of SBE of finger millet was also submitted in PMDB with the PMDB id - PM0080938. This research presents a comparative overview of Finger millet SBE and includes their properties, structural and functional characteristics, and recent developments on their post-translational regulation.

A liquid calcium+vitamin D3 supplement is effective prophylaxis against hypocalcemic toxicity during apheresis platelet donation

Hypocalcemic toxicity, because of return of citrate anion to the donor, is the major toxicity of apheresis platelet donation. Oral calcium carbonate, given prophylactically at the start of donation, has shown limited ability to alleviate this toxicity. We examined whether repeated prophylactic doses of calcium carbonate, or of a liquid preparation containing calcium citrate, calcium phosphate, and vitamin D3 , would be more effective at preventing symptoms of hypocalcemic toxicity. Symptoms were reported by 48% of donors who received no prophylaxis and 60% of donors who received 1000 mg of oral calcium carbonate at the start of, and every 20 minutes during, donation (P = 0.711). Only 19.2% of donors who received the liquid preparation (1000 mg calcium, 1000 IU vitamin D3 ) reported symptoms (P = 0.040 versus no prophylaxis, P = 0.039 versus calcium carbonate). This difference was not because of gender, weight, age, or blood volume of the donor. Neither calcium preparation prevented a measurable fall in plasma ionized calcium during donation. We conclude that liquid calcium citrate/calcium phosphate/vitamin D3 provides effective prophylaxis against hypocalcemic toxicity during platelet donation, however it does not prevent a fall in plasma ionized calcium.

A highly sensitive resonance Rayleigh scattering and colorimetric assay for the recognition of propranolol in β-adrenergic blocker.

In this work, a highly sensitive, citrate anion-capped gold nanoparticles (AuNPs)-based assay for the determination of propranolol in real samples with resonance Rayleigh scattering (RRS) and colorimetry was developed. When AuNPs were prepared by the sodium citrate reduction method, citrate anions self-assembled on the surface of AuNPs to form supramolecular complex anions. In BR 4.6 buffer solution, propranolol was positively charged and could bind with AuNPs to form larger aggregates through electrostatic force and hydrophobic effects. This results in remarkable enhancement of the RRS intensity and a color change in the AuNPs solution from red to blue via purple. Thus, a highly sensitive RRS and colorimetric assay the for detection of propranolol was developed with a linear range of 0.2-5.2 and 8-112ng/ml, respectively. In addition, no difference was seen when comparing R-propranolol with S-propranolol, therefore, this method could not be used in the recognition of chiral propranolol. However, upon addition of other β-adrenergic blockers, no phenomenon like that seen with propranolol was observed, meaning that this method can be used for determining the presence of propranolol in a mixture β-adrenergic blockers. Finally, the optimum conditions, factors influencing the reaction, its mechanism and the reasons for enhancement of the RRS were discussed.

Synthesis and study of aggregation kinetics of fluorescence active N-(1-Naphthyl)ethylenediammonium cations functionalized silver nanoparticles for a chemo-sensor probe

N-(1-Naphthly)ethylenediammonium cations (NEDA2+) stabilized silver nanoparticles (NEDA-Ag-NPs) were synthesized and explored their potential use for a chemo-sensor probe. The NEDA2+ cations being fluorescent active make the particles functionalized as well. They physically adsorb (adsorption constant (Kads), 2.65×109M−1) onto the surface of Ag-NPs and make them cationic with zeta potential value of 23±2mV. TEM image shows that particles are indeed spherical in shape. Monitoring SPR band (extinction coefficient (ε), 5.3×108M−1cm−1) the potencies of particles for sensing pH and citrate anions have been explored. The citrate anions interact with the surface adsorbed NEDA2+ cations and form association complex, NEDA2+‐‐‐citrate3‐, leading aggregation which is a kinetic effect. The aggregation kinetics was thus monitored for evaluating the association constant (Kasso) by studying UV–vis and fluorescence spectroscopic tools. The critical coagulation concentrations (CCC) for both the methods are considered to be the detection limit for citrate anions.

Removal of phosphate from aqueous solutions by adsorption onto Ca(OH)2 treated natural clinoptilolite

Phosphorus (P) recovery from wastewater is of great interest especially when the loaded adsorbent can be used in the agriculture as slow-release fertilizer. The application depends on environmental concerns related to the chemical modification of the adsorbent and the release of toxic compounds from the loaded material to the soil or the water during adsorption. The present work focused on the phosphate (PO4-P) removal from aqueous solutions under low P concentrations (0.5–10mg/L) by using Ca(OH)2-pretreated natural zeolite (CaT-Z). As activation agent, Ca(OH)2 presents benefits in terms of pretreatment costs and environmental impact of the applied adsorbent. The pretreatment of natural zeolite (clinoptilolite) with 0.25mol/L Ca(OH)2 led to an increase of P removal from 1.7 to 97.6% at initial P concentration of 10mg/L, pH 7 and 298K. Low residual concentrations of 81–238μg P/L were achieved at 298K rendering CaT-Z a promising sorbent for tertiary wastewater treatment. At 200mg P/L, the adsorption capacity was 7.57mg P/g CaT-Z. The P removal efficiency was pH-independent suggesting a beneficial use of CaT-Z under acidic and alkaline conditions. Adsorption was found to be an endothermic and slow process reaching equilibrium after 120h, whereas the half of the PO4-P was adsorbed in the first 8h. The applied kinetic models showed that both film and intraparticle diffusion contributed to phosphate removal. Phosphate sorption decreased in the presence of the anionic surfactant SDS, Fe2+, HCO3−, acetate and citrate anion. The predominant mechanisms of ligand exchange and Ca-P surface precipitation were confirmed by the IR-ATR and SEM-EDS analyses, respectively.

Insight into the preparation of Fe3O4 nanoparticle pillared layered double hydroxides composite via thermal decomposition and reconstruction

As one of layered double hydroxides (LDH) based heterogeneous catalysts, transition metal oxide pillared LDH composite has attracted considerable research interests as its excellent catalytic efficiency and thermal stability. However, during the preparation especially by reconstruction techniques, abundant interlayer regions of LDH are contaminated by some unexpected anion such as LDH affinitive CO32−, which will result in inaccessible interlayer active site. In this paper, magnetic nanoparticle Fe3O4 pillared LDH composite was synthesized with exchangeable NO3− as the main interlayer anion by decomposition and reconstruction route. The anion exchange, decomposition and reconstruction processes were comprehensively investigated. XRD, TG-DTA-MS, SEM and other methods have been employed to characterize materials. The results show that anion exchange between ferric citrate anion and LDH follows Langmuir isotherm model and pseudo-second-order model. The decomposition and oxidation of anion exchange product (LDH-Fe(C6H4O7)−) can be complete at 550°C for 20min and crystal Fe3O4 with nanostructure is formed between 500 and 600°C. When calcined product (LDO-Fe3O4) is reconstructed by rehydration in 0.015M HNO3-30wt% NaNO3 mixture aqueous solution, CO32− contamination derived from CO2 in air can be averted and lamellar structure is successfully recovered with Fe3O4 and NO3− remaining in the interlayer gallery of LDH.

Effects of surface charge and ligand type of Au nanoparticles on green fluorescent protein-expressing Escherichia coli

The effects of the surface charge and ligand type of three types of Au nanoparticles (NPs), namely anionic polyethylene glycol (PEG)-Au NPs, anionic citrate (Cit)-Au NPs, and cationic branched polyethylenimine (bPEI)-Au NPs, on green fluorescent protein (GFP)-expressing Escherichia coli were evaluated through the combined analysis of optical density (OD) and fluorescence intensity (FI). OD and FI can provide information about cell growth and metabolism of bacteria, respectively. The results demonstrated that PEG- and Cit-Au NPs had no major effects on the OD and FI of GFP-expressing bacteria. However, it was found that Cit-Au NPs may slightly influence cell metabolism at higher concentrations, although it is necessary to perform further in-depth study to clarify this issue. Cationic bPEI-Au NPs showed significant effects on cell density and metabolism of E. coli, with an especially strong effect on metabolism. The combined analysis of OD and FI may be useful for monitoring the effects of a wide range of nanomaterials on microorganisms.

Crystal structure of penta-sodium hydrogen dicitrate from synchrotron X-ray powder diffraction data and DFT comparison.

The crystal structure of penta-sodium hydrogen dicitrate, Na5H(C6H5O7)2, has been solved and refined using synchrotron X-ray powder diffraction data, and optimized using density functional techniques. Each of the two independent citrate anions is joined into a dimer by very strong centrosymmetric O-H⋯O hydrogen bonds, with O⋯O distances of 2.419 and 2.409 Å. Four octa-hedrally coordinated Na+ ions share edges to form open layers parallel to the ab plane. A fifth Na+ ion in trigonal-bipyramidal coordination shares faces with NaO6 octahedra on both sides of these layers.