Sodium Borate Research Articles

Polyvinyl Alcohol Hydrogel Reinforcement of Cellulose and Silica Nanoparticles for Wound Healing Application

Hydrogels are smart materials that have the capability to absorb water by over five hundred times their own weight and showed various applications in biomedical field, including drug delivery systems, tissue engineering and wound healing. In this study, the fabrication of polyvinyl alcohol hydrogels reinforced with cellulose and silica nanoparticles was investigated for possible use in wound healing applications. Synthesis was done through chemical cross-linking of polyvinyl alcohol and sodium borate. Results showed that mechanical properties and thermal stability of the reinforced hydrogels were improved compared to the neat hydrogel. The nanosilica-reinforced hydrogels showed antimicrobial activity and improved degree of swelling compared to the neat PVA hydrogels. Sparse growth was observed in the antimicrobial assay with Pseudomonas aeruginosa and Staphylococcus aureus. Cellulose-reinforced hydrogels showed minimal flexibility characteristic compared to the nanosilica-reinforced hydrogel samples. Moreover, it showed no antimicrobial activity against Pseudomonas aerigunosa.

Racemization without deamidation: Effect of racemization conditions on 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate tagged amino acids

The aim of this research study was to provide a more thorough understanding of the underlying mechanism and to broaden the application field of the recently introduced racemization method employing the amino acid derivatization tag 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC, AccQ) for heat-induced stereoisomerization of common amino acids as well as uniformly isotopically labeled [U-13C15N]-amino acids. The influence of different buffer types such as sodium borate buffer and sodium carbonate buffer as well as their pH and molarity on the racemization and deamidation of amino acids were investigated.It was found that a 0.4 M borate buffer with a pH of 8.0 +/− 0.2 was the most suitable derivatization as well as racemization buffer to ensure degradation free racemization of deamidation prone compounds such as glutamine. Hereby essential was the in-solution pH measurement before and after derivatization with AQC as well as after heat-induced racemization. This strategy provided further insight at which pH an actual racemization event was observed and when an unwanted deamidation of glutamine to glutamic acid occurred. In addition also the influence of the presence of oxygen during racemization was studied. In this context it was possible to determine ideal oxidation and racemization conditions for the production of scalemic mixtures of chiral isotopically labeled methionine AQC-DL-[U-13C15N]-Met as well as its oxidation products, AQC-DL-[U-13C15N]-Met-O and AQC-DL-[U-13C15N]-Met-O2. All stereoselective separations were performed on the zwitterionic Chiralpak ZWIX(+) column combined with HPLC–ESI–QTOF–MS analysis in positive ionization mode.

Influence of vanadium addition on the optical and photoluminescence properties of borate glasses and their glass–ceramic derivatives

Nominal B2O3–Na2O–CaO–SrO glasses containing various concentrations of V2O5 ranging from 5 to 40 mol% have been prepared by normal melting and quenching procedure. Data obtained from differential scanning calorimetry (DSC) of the parent glasses were used to prepare glass–ceramic. X-ray diffraction (XRD) patterns of glasses reveal an absence of any identified crystalline phases; while, patterns of the heat-treated glasses showed the crystallization of calcium vanadate (Ca3V2O8), sodium borate (Na2B6O10) and sodium vanadate (NaV3O8) phases. UV–Visible spectra demonstrated characteristic absorption in the UV region centered at ~ 234 and 360 nm that extended to the visible region with increasing V2O5 content. Consequently, the estimated optical parameters such as optical band gap (Eopt), Urbach energy (∆E) and refractive index were dependent on V2O5 content. Photoluminescence spectra were measured before and after crystallization indicating development in the emission spectra after heat treatment. The glasses reveal two sharp emitted peaks at about 466 and 552 nm while the glass–ceramic derivatives depicted an extended emission peak centered at about 700 nm. Based on FTIR data, tetrahedral and trigonal borate groups were identified beside VO4 group when V2O5 was added to the parent glasses. Infrared structural measurements reveal the dual role of V2O5 as glass modifier or former depending on its concentration.

Predicting Composition-Structure Relations in Alkali Borosilicate Glasses Using Statistical Mechanics

Predicting the atomic-scale structure of multicomponent glasses from their composition and thermal history would greatly accelerate the discovery of new engineering and functional glasses. A statistical mechanics-based approach has recently been applied to predict the composition-structure evolution in binary oxide glasses by determining the relative entropic and enthalpic contributions to the bonding preferences. In this work, we first establish the network modifier-former interaction parameters in sodium silicate and sodium borate glasses to predict the structural evolution in sodium borosilicate glasses. Due to the significant fluctuations in the experimentally determined structural speciation in borosilicate glasses, we perform classical molecular dynamics (MD) simulations to establish and validate our structural model. We also show that the statistical mechanical model naturally accounts for the difference in structural speciation from MD simulations and NMR experiments, which in turn arises from the difference in cooling rate and thus thermal history of the glasses. Finally, we demonstrate the predictive capability of the model by accurately accounting for the structural evolution in potassium borosilicate glasses without using any adjustable model parameters. This is possible because all the interaction parameters are already established in the potassium silicate, potassium borate, and sodium borosilicate glasses, respectively.

Determination of oxaliplatin enantiomers at attomolar levels by capillary electrophoresis connected with inductively coupled plasma mass spectrometry

The aim of this study was to develop a method for the separation of oxaliplatin enantiomers at attomolar concentration levels. A combination of capillary electrophoresis and inductively coupled plasma mass spectrometry was chosen due to their unique characteristics, including fast and easy modification of separation selectivity, and significant limits of detection and linearity. In the first step, we optimized conditions for the separation of oxaliplatin enantiomers including background electrolyte composition and concentration, pH, and type and concentration of the chiral selector. Under optimal conditions, sodium borate buffer pH 9.5, ionic strength 40 mmol L−1, with 60 mg mL−1 sulfated β-cyclodextrin, separation was obtained with a resolution of 2.0. This electrolyte system was then used in the ‘in-house’ connection of capillary electrophoresis with inductively coupled plasma mass spectrometer. In this instance, separation lasted for 9.5 min. Calibrations were linear in the range of 0.1–500 μg mL−1 with R2 of 0.9999. LOD and LOQ values were of 64 ng mL−1 and 116 ng mL−1 of oxaliplatin, respectively. This represents detection of 49 fg or 125 attomol of oxaliplatin enantiomers in the capillary electrophoresis injected sample zone. Finally, the method was successfully applied for detection of oxaliplatin enantiomers in spiked urine samples.

Ecofriendly microwave-assisted reaction and extraction of bioactive compounds from hawthorn leaf.

The main active components in hawthorn leaves possess various biological activities such as anti-inflammatory, antioxidant, and hypolipidemic effects. Therefore, it is necessary to develop an effective and reliable extraction method to extract these active compounds from hawthorn leaves. To establish a simple, rapid, and sensitive method for extraction and determination of polyphenolic compounds from hawthorn leaves. In this study, a microwave-assisted reaction and extraction (MARE) combined with ultra-high-performance liquid chromatography with ultraviolet detector method was established to extract and determine the polyphenolic compounds in hawthorn leaves. The solid reagent aqueous solutions were applied as extraction solvents, preventing the use of organic solvents. The target analytes were identified by quadrupole time-of-flight tandem mass spectrometry. Several experimental parameters that can significantly affect the extraction efficiency were evaluated and optimised. The optimal conditions were as follows: 0.1g of sodium carbonate was used as solid reagent, the amount of sodium borate was set at 0.01g, extraction time was 10min, extraction temperature was set at 50°C, pH value was adjusted to 7. The validation experiments demonstrated that the method had high sensitivity with the limits of detection in the range 26.5-37.7ng/mL. The average recoveries ranged from 80.22% to 93.27%. In this work, the proposed MARE method was successfully applied to extract and determine polyphenolic compounds in hawthorn leaf samples. Compared with other reported methods, the present method was faster, greener, and more sensitive.

Hierarchically porous 3D-printed akermanite scaffolds from silicones and engineered fillers

The present investigation is dedicated to the manufacturing of reticulated three-dimensional akermanite scaffolds, developed by direct reaction between silica, from the oxidation of a commercial silicone resin and oxide fillers, forming pastes for direct ink writing. Crack-free scaffolds, with dense and regular struts, were due to the use of CaCO3 (micro) and MgO nano-particles as reactive fillers. An excellent phase purity was obtained, with the help of the liquid phase provided by anhydrous sodium borate (Na2B4O7), upon firing. The structure of the scaffolds, finally, was successfully modified by using Mg(OH)2 and hydrated sodium borate: besides macro-porosity from direct ink writing, the new scaffolds exhibited homogenous ‘spongy’ struts (owing to water vapor release in the heating step), with no crack. Both types of scaffolds (with dense or porous struts) exhibited remarkable strength-to-density ratios.

Triborate Formation Constants and Polyborate Speciation under Hydrothermal Conditions by Raman Spectroscopy using a Titanium/Sapphire Flow Cell

Solvent-corrected reduced isotropic Raman spectra of aqueous boric acid + sodium borate solutions have been obtained from perpendicular and parallel polarization measurements in a novel custom-made titanium flow cell with sapphire windows over the temperature range 25 to 300 °C at 20 MPa using the perchlorate anion, ClO4-, as an internal standard. The reduced isotropic spectra of solutions yielded the first reported quantitative speciation results for polyborate ions in equilibrium with boric acid and borate in high-temperature aqueous solutions above 200 °C. The spectra obtained from solutions at low sodium/boron ratios, 0 < mNaST/ mBST < 0.254, displayed well-defined bands at 880, 747, 615, and 532 cm-1, corresponding to the species B(OH)3 > [B(OH)4]- > [B3O3(OH)4]- ≫ [B5O6(OH)4]-, respectively. The triborate ion, [B3O3(OH)4]-, was found to be the major polyborate species in these boric acid-rich solutions in the range 25 to 300 °C. Thermodynamic formation constants for the triborate species, [B3O3(OH)4]-, calculated from the peak areas, are in agreement with the literature values reported by Mesmer et al at 50, 100, and 200 °C to within the combined experimental uncertainties. At 300 °C, the value for the formation constant, log K31, m b= 2.259 ± 0.060, is larger than the value extrapolated from the results of Mesmer et al. by a factor of ∼3.

Ground Application of Overdoses of Manganese Have a Therapeutic Effect on Sweet Orange Trees Infected with Candidatus Liberibacter asiaticus

There is accumulating evidence that root system collapse is a primary symptom associated with Huanglongbing (HLB)-induced tree decline, especially for commercial sweet orange and grapefruit trees on Swingle and Carrizo rootstocks. Maintaining root health is imperative to keep trees productive in an HLB-endemic environment. Preliminary greenhouse and field studies have shown that HLB-impacted trees had secondary and micronutrient deficiencies that were much greater in the roots than in the leaves, and that treatments containing three-times the recommended dose of manganese (Mn) improved tree health and growth and increased feeder root density in greenhouse trees. These results suggested that trees in an HLB-endemic environment have higher specific micronutrient requirements than those currently recommended. To test this hypothesis, established Vernia sweet orange grafted onto rough lemon rootstock trees were divided into eight supplemental CRF nutrition treatments (including two-times and four-times the recommended doses of Mn and boron) using a randomized complete block design in a commercial grove in St. Cloud, FL. The following supplemental nutrition treatments were used: no extra nutrition (control); Harrell’s–St. Helena mix 0.9 kg per tree; Harrell’s with 32 g of Florikan polycoated sodium borate (PSB) per tree; Harrell’s with 90 g of TigerSul® Mn sulfate (MS) per tree; Harrell’s with 32 g of PSB and 90 g of MS per tree; 180 g of MS per tree; 64 g of PSB per tree; and 180 g of MS plus 64 g of PSB per tree applied every 6 months since Fall 2015. Leaf and soil nutritional analyses were performed in Mar. 2017, Sept. 2017, and May 2018; a quantitative polymerase chain reaction was performed for Candidatus Liberibacter asiaticus (CLas) titer estimation in Nov. 2017. Significantly higher cycle threshold (Ct) values indicating reduced CLas bacterial populations were observed in trees that received the higher doses of Mn, especially those receiving four-times the recommended dosage of Mn (180 g Mn). Many trees exhibited Ct values of 32 or more, indicating a nonactive infection. Fruit yields of these trees were also increased. No significant differences in juice characteristics, canopy volume, and trunk section area were found between control plants and plants treated with 180 g Mn. Soil and leaf nutrients B, K, Mn, and Zn were significantly different among treatments at various times during the study. Our results strongly suggest that overdoses of Mn can suppress CLas bacterial titers in sweet orange trees on rough lemon rootstock, thus providing a therapeutic effect that can help restore tree health and fruit yields. This response was not observed when Mn and B were combined in the overdose, suggesting an antagonistic effect from B on Mn metabolism. When an overdose of Mn is used, biological functions and tree tolerance lost due to nutritional imbalances caused by HLB might be restored. Further studies are needed to elucidate which metabolic pathways are altered by comparing overdosed and conventionally fertilized HLB-impacted trees and to determine if the observed therapeutic effects can be achieved in trees grafted to other important commercial rootstocks.

Characterization of crystalline borates prepared from solution and derived glasses

An aqueous solution route has been developed to prepare calcium sodium borate compositions of the formula xB2O3·(100–x)(0.5CaO·0.45Na2O·0.05P2O5) (36.2 ≤ x ≤ 61.3 mol%). The effect of heat-treatment and melt-quenching on the structure and physical properties was investigated using differential thermal analysis, X-ray diffraction, FTIR spectroscopy, 11B MAS NMR and density. Dried powders were heat treated at temperatures 400–800 °C for 3h. XRD revealed formation of crystalline Na2B4O7·5H2O, Ca5(PO4)3OH, Na3B5O9H2O and CaB3O5(OH) in the samples treated up to 500 °C. Samples treated at higher temperature show the presence of CaNaB5O9, CaNa3B5O10 and CaNaPO4. Quenched solids obtained by melting powders are amorphous. It is deduced from FTIR spectra that CaO and Na2O modify the borate network forming BO4 and BO3 with non-bridging units, which depend on the molar ratio R =(Na2O+CaO)/B2O3 and temperature. The fraction N4 of four coordinated boron atoms changes with R and can be predicted by treating the compositions as if they are mixtures of Na2O–B2O3 and CaO–B2O3 matrices. N4 values have been used to calculate the fraction of all units in the glass structure. The results are confirmed by 11B MAS NMR spectra that show the presence of symmetric and asymmetric trigonal BO3 groups and splitting line shape of [4]B. The latter is assumed being related to different four-coordinated boron environments. Density and molar volume obtained by using mixed matrices concept agree well with the experimental data.

Electrochemical studies on pitting corrosion of tin in sodium borate solutions containing nitrate ions

Purpose The purpose of this paper is to study the electrochemical behavior of Sn electrode in Na2B2O7 solutions in the absence and presence of NaNO3 as a pitting corrosion agent. Design/methodology/approach The electrochemical behavior of Sn electrode was studied by using cyclic voltammetry and potentiodynamic polarization measurements and complemented with scanning electron microscopy examinations. Findings This paper shows that in the absence of NO3 − ions, the anodic polarization of Sn electrode exhibits active/passive transition. Addition of various concentrations of NO3 − anions to the borate solution enhances active anodic dissolution and tends to break down the passive oxide film at a certain pitting potential. The pitting potential, and hence the pitting corrosion resistance, decreases with increasing NO3-ion concentration and temperature but increases with scan rate and repetitive cycling. Addition of CrO42−, WO42− or MoO42− oxyanions to the borate nitrate solution inhibits the pitting corrosion of Sn. Originality/value This is the first study that shows the effect of NO3 − ion as a pitting corrosion agent.

The Use of Enteric Capsules for Releasing a Fragrance over an Extended Period of Time.

A system for releasing a fragrance, citral (CR) over an extended period of time using three types of enteric capsules is reported. The L- and M-type capsules released CR into media with a pH above 6, while the H-type capsule released CR at a pH above 7. The pH of the releasing medium was controlled by sodium borate (SB), i.e., by adding SB-methylcellulose (MC) prepared in different weight ratios (SB-MC 1 : 2, 1 : 1 and 2 : 1) to tablets and by compressing them at different pressures. The tablet containing a large amount of SB and that was pressed at higher pressures permitted the pH of the releasing medium to be changed from 5 to 9, at 4-5 h after the addition of SB to the tablets, while negligible changes were observed for tablets containing low amounts of SB and which were compressed at lower pressures. Reflecting these pH changes, CR was released after different periods of time when SB-MC tablets and capsules containing CR were simultaneously added to the releasing medium. When enteric capsules containing CR and the pH adjusting tablets were simultaneously added to a benzyl acetate (BA) solution, BA was released at a constant rate, while CR was released for different periods of time depending on the type of capsule used. The results suggest that fragrances could be released over different time frames by using enteric capsules and pH adjusting agents, for example, the release of fragrances with sedative effects at night time and with stimulating effects in the morning.

Constant Composition Groups in Glass of Sodium–Borate Systems According to the Data of X-Ray Diffractometry

X-ray spectra of glass of the sodium-borate system are obtained by the method of X-ray diffractometry. Using the proposed processing of these spectra, the smallest elements of the sodium–borate glass structure (constant stoichiometry groups, CSGs), which unambiguously determine their properties, are determined. The method for determining the smallest elements of the glass structure consists of comparing the glass contours with the known contour of a glass of a stoichiometric composition.

Evidence of the coexistence of multivalence cerium oxide nano-particles in a sodium borate glass

A sodium borate glass containing both trivalent cerium oxide and tetravalent cerium oxide states nano particles of 2–5 nm in size was successfully prepared by doping the glass with varying amounts of cerium (IV) oxide with. X-ray absorption spectroscopy measurement was used to determine the coexistence of the multivalent state of cerium oxide in the borate glass. Significant changes in the trivalent and tetravalent cerium oxide concentrations were observed when the glass was melted using different melting parameters and different raw materials. Glass made with borax that contained of 0.05mols of CeO2 and melted at 1100 °C for 3 h had the highest concentration of trivalent cerium oxide. Transmission electron micrographs confirmed the release of trivalent Ce2O3 and tetravalent CeO2 nanoparticles from the glassy matrix. Fourier transform Infrared measurements suggest that the CeO2 in the glass acts as both a glass-former and glass modifier.

Solubilization of palladium in molten mixture of sodium borates and sodium carbonate

Abstract Solubility of palladium in water was enhanced by heating metallic palladium in a molten mixture of sodium borates and sodium carbonate at 1000 °C for 2 h. When 17Na2O − 83B2O3 was used as the sodium borate, 21% of palladium in the obtained product was extracted in water. The solubilization efficiencies differed depending on the chemical composition of the sodium borate. During the heat treatment, oxidation occurred on the palladium surface and the oxidized palladium dissolved in the media.

Formation of Si/SiO2 Luminescent Quantum Dots From Mesoporous Silicon by Sodium Tetraborate/Citric Acid Oxidation Treatment.

We propose a rapid, one-pot method to generate photoluminescent (PL) mesoporous silicon nanoparticles (PSiNPs). Typically, mesoporous silicon (meso-PSi) films, obtained by electrochemical etching of monocrystalline silicon substrates, do not display strong PL because the silicon nanocrystals (nc-Si) in the skeleton are generally too large to display quantum confinement effects. Here we describe an improved approach to form photoluminescent PSiNPs from meso-PSi by partial oxidation in aqueous sodium borate (borax) solutions. The borax solution acts to simultaneously oxidize the nc-Si surface and to partially dissolve the oxide product. This results in reduction of the size of the nc-Si core into the quantum confinement regime, and formation of an insulating silicon dioxide (SiO2) shell. The shell serves to passivate the surface of the silicon nanocrystals more effectively localizing excitons and increasing PL intensity. We show that the oxidation/dissolution process can be terminated by addition of excess citric acid, which changes the pH of the solution from alkaline to acidic. The process is monitored in situ by measurement of the steady-state PL spectrum from the PSiNPs. The measured PL intensity increases by 1.5- to 2-fold upon addition of citric acid, which we attribute to passivation of non-radiative recombination centers in the oxide shell. The measured PL quantum yield of the final product is up to 20%, the PL activation procedure takes <20 min, and the resulting material remains stable in aqueous dispersion for at least 1 day. The proposed phenomenological model explaining the process takes into account both pH changes in the solution and the potential increase in solubility of silicic acid due to interaction with sodium cations.

Conversion of MeOH and Toluene into Styrene and Ethylbenzene Using Composite Catalysts Containing MeOH Dehydrogenation Components

AbstractA new strategy, the coupling of methanol dehydrogenation and side‐chain alkylation of toluene, to realize highly effective conversion of methanol and toluene into styrene and ethylbenzene was proposed. A series of composite catalysts were designed by combining CsX with numerous catalysts reported to be effective for methanol dehydrogenation to formaldehyde. The studies of side‐chain alkylation of toluene with methanol over these composite catalysts indicated that the introduction of methanol dehydrogenation components could universally improve the yield of styrene and ethylbenzene in side‐chain alkylation of toluene with methanol. And the introduction of some methanol dehydrogenation components, i. e. sodium borate (Na2B4O7) and CuO/SiO2, could even double the yield of styrene and ethylbenzene. Furthermore, the influence of the mass ratio of different components and the spatial arrangement of different active sites on composite catalysts was investigated in detail.

Structural and Femtosecond Third-Order Nonlinear Optical Properties of Sodium Borate Oxide Glasses: Effect of Antimony

Structural and optical properties of antimony-containing sodium borate glasses were studied and their ultrafast third-order nonlinear optical (NLO) properties have been evaluated using Z-scan measurements with femtosecond (fs) pulses (∼150 fs, 80 MHz) at 750, 800, and 880 nm wavelengths. Glasses in the (mol %) 20Na2O–(80 – x)B2O3–xSb2O3 (where x = 0, 10, 20, and 30) system have been fabricated via melt quench technique. The structural modifications were analyzed using the Raman and magic angle spinning (MAS)-nuclear magnetic resonance (NMR) (11B MAS-NMR and 23Na MAS-NMR) techniques. The optical absorption spectra revealed that the absorption edge was red-shifted, suggesting the decrease in band gap energy with increase of antimony content in the glasses. Raman scattering results revealed that the boroxol rings are depressed with the incorporation of Sb2O3 for replacing B2O3. 11B MAS-NMR results showed a progressive increase of B4 units at the expense of B3 units. The Raman and 11B MAS-NMR results support the formation of Sb5+ ions due to oxidation of Sb3+ that played the role of charge compensation. 23Na MAS-NMR spectra revealed a decreasing trend in the average of bond lengths of Na–O with increasing Sb2O3 contents. This suggested that sodium changed its role from charge compensator to modifier cation. The antimony-containing glasses demonstrated a reverse saturable absorption in open-aperture Z-scan mode due to two-photon absorption, while closed-aperture Z-scan signatures depicted positive nonlinear refraction due to self-focusing effect. The NLO coefficients were found to increase with Sb2O3 due to the increased nonbridging oxygens and also due to the hyperpolarizability of Sb3+ and Sb5+ ions. The observed NLO data clearly suggest that the investigated glasses are beneficial for optical limiting applications.

Effect of co-doping of lithium on the dosimetric properties of dysprosium-doped sodium borate glass system

The effect of lithium (Li) as a co-dopant on the Thermoluminescence (TL) properties of dysprosium-doped sodium borate (NB: Dy) exposed to two radiation sources (linear accelerator and Gamma-ray from Co-60) is reported in this study. The results showed an enhancement in the TL intensity with the increment of lithium to NB: Dy. Promising dosimetric properties were recorded; glow curve with simple and prominent peak, high signal stability and simple annealing process. Furthermore, the phosphor is reusable and high dose linearity range (up to 103 Gy). The current results propose the using of the new dosimeter in different dosimetric applications.

Separation of cetirizine enantiomers by capillary electrophoresis with a dual selector system based on borate-glucose complexes and sulfated-β-cyclodextrin

The aim of this study was to focus on the reduction of chiral selector concentration, sulfated-β-cyclodextrin, in an attempt decrease the running costs associated with separating cetirizine enantiomers by capillary electrophoresis. The decrease in the concentration of chiral selector was achieved by adding D-glucose to the background electrolyte, which consisted of sodium borate. Optimal separation of cetirizine enantiomers was obtained in the electrolyte containing 500 mmol L−1 borate pH 9.5 with 1.0 mg mL−1 sulfated-β-cyclodextrin, and 1000 mmol L−1D-glucose. This means a 15-fold reduction in the concentration of sulfated-β-cyclodextrin. The mechanism of the separation in this electrolyte was investigated using direct injection mass spectrometry. The electrolyte of borate, D-glucose, and sulfated-β-cyclodextrin forms a dual selector system, in which one selector is represented by the sulfated-β-cyclodextrin and the second selector is represented by the D-glucose-borate complexes.