Increase In Citrate Concentration Research Articles

ECL sensor for selective determination of citrate ions as aprostate cancer biomarker using polymer of intrinsic microporosity-1 nanoparticles/nitrogen-doped carbon quantum dots.

Urine citrate analysis is relevant in the screening and monitoring of patients with prostate cancer and calcium nephrolithiasis. A sensitive, fast, easy, and low-maintenance electrochemiluminescence (ECL) method with conductivity detection for the analysis of citrate in urine is developed and validated by employing polymer of intrinsic microporosity-1 nanoparticles/nitrogen-doped carbon quantum dots (nano-PIM-1/N-CQDs). Using optimum conditions, the sensor was applied in ECL experiments in the presence of different concentrations of citrate ions. The ECL signals were quenched gradually by the increasing citrate concentration. The linear range of the relationship between the logarithm of the citrate concentration and ΔECL (ECL of blank - ECL of sample) was obtained between 1.0 × 10-7M and 5.0 × 10-4M. The limit of detection (LOD) was calculated to be 2.2 × 10-8M (S/N = 3). The sensor was successfully applied in real samples such as human serum and patient urine.

Mechanisms and kinetics of citrate-promoted dissolution of a uranyl phosphate mineral

The formation of uranyl phosphate precipitates is a remediation strategy to limit the mobility of uranium in contaminated soils. However, exposure to organic ligands, like the plant exudate citrate, can remobilize the uranium. The purpose of this study is to provide a more thorough comprehension of citrate-promoted dissolution of a uranyl phosphate mineral, chernikovite [(H3O)(UO2)(PO4)·3H2O], by determining the extent of uranium release from chernikovite at a wide range of citrate concentrations. We have quantified the kinetics of dissolution and proposed potential mechanisms of chernikovite dissolution to gain a better understanding of the fate of uranyl phosphate precipitates in the environment. Batch dissolution and continuously stirred tank reactor (CSTR) experiments indicate that increasing citrate concentrations from 0.1 mM to 50 mM increases the concentration of dissolved uranium. However, at citrate concentrations of 10 mM and greater, the effectiveness of the ligand to enhance the dissolution decreases. Density functional theory modeling and Raman spectroscopy indicate a strong interaction between citrate and the uranyl ions at the surface of the uranyl phosphate. The interaction between citrate and uranyl suggests the formation of an alteration layer of uranyl-citrate surface complexes, which could impede uranium release at citrate concentrations of 10 mM or higher. Together, these results indicate that the citrate-promoted dissolution of chernikovite is a fast reaction that is hindered by a combination of surface saturation and a secondary-phase precipitation reaction at higher concentrations of citrate and reaction times greater than or equal to 72 min based on the hydraulic residence time of CSTR experiments. These findings improve our understanding of the stability of uranyl phosphate for use in sequestration of uranium groundwater plumes at contaminated sites.

Simultaneous extraction and recovery of lead using citrate and micro-scale zero-valent iron for decontamination of polluted shooting range soils

Lead (Pb) contaminated soils pose serious risks to human health and the environment. One remediation technique that permanently removes Pb is chemical soil washing. This study investigated a modified chemical soil washing technique whereby Pb extraction in citrate solution and recovery of dissolved Pb by reductive precipitation (cementation) were simultaneously carried out in a single reactor by the addition of micro-sized zero-valent iron (mZVI) to decontaminate Pb-contaminated soil from a shooting range. Pb-loaded mZVI were then recovered by magnetic separation. The effects of mZVI dosage, citrate concentration, pH, and hydrodynamics on Pb removal were evaluated. Under acidic pH, Pb removal increased with increasing citrate concentration. The addition of mZVI suppressed Pb removal from 59.6 to 30% in 0.1 M citrate solution, which was attributed to dissolved Fe ions competing for free citrate ions to form soluble Fe-citrate complexes. Increasing the citrate concentration and shaking speed enhanced Pb extraction from the soil and subsequent recovery via cementation onto mZVI. Pb removal and recovery onto mZVI of up to 99.0% were achieved at citrate concentration of 0.7 M, pH of 5, and shaking speed of 165 strokes/min. The addition of mZVI during chemical soil washing in citrate solution not only extracted and recovered Pb from the contaminated soil but also eliminated the need to wash soil residues obtained after treatment because the bulk of dissolved Pb was sequestered from the solution.

Eco-friendly leaching of gold from a carbonaceous gold concentrate in copper-citrate-thiosulfate solutions

Thiosulfate is considered to be one of the most promising alternative reagents to cyanide due to its non-toxicity and excellent leaching efficiency. However, the use of ammonia as the additive and high reagent consumption have severely limited the development and commercial application of the traditional copper-ammonia-thiosulfate leaching system. In this work, the potential use of an environmentally friendly copper-citrate-thiosulfate leaching system for gold leaching from a refractory carbonaceous gold concentrate has been examined. The experimental results showed that when citrate served as the stabilizer, the copper-citrate-thiosulfate system exhibited similar extraction capability for gold with a significant decrease in thiosulfate consumption compared to the traditional copper-ammonia-thiosulfate system. Thiosulfate consumption increased with Cu2+ and S2O32− concentration, leaching time and temperature, but decreased with citrate concentration and initial pH. The mechanism of the catalysis of gold dissolution in copper-citrate-thiosulfate solutions by the binuclear cupric-citrate complex (Cu2Cit2H−24−) is presented based on the thermodynamic analysis. The stability of the Cu2Cit2H−24− complex decreased with increasing Cu2+ and S2O32− concentration, increased with an increase of citrate concentration. Moreover, the microwave roasting process applied to concentrate facilitated the subsequent gold leaching process probably by exposing encapsulated gold to thiosulfate solutions.

Ligand-promoted reductive cleaning of iron-fouled membranes from submerged membrane bioreactors

Four pathways for cleaning of iron oxide-fouled membranes, namely i) proton (acid)-assisted, ii) ligand-promoted (citric acid), iii) reductive (ascorbic acid), and iv) ligand-promoted reductive (ascorbic acid-citric acid-mediated) were examined and their effectiveness were compared in this study. The cleaning effectiveness under oxic conditions followed the order: proton-assisted < ligand-promoted < reductive < ligand-promoted reductive. Iron oxide dissolution rate initially increased with increase of citrate concentration in the ascorbic acid-citric acid solutions but declined at higher citrate concentrations indicating that an intermediate citrate concentration was required for optimal cleaning. The mechanism of ligand-promoted reductive dissolution mediated by ascorbate and citrate was investigated through studies of dissolution behaviour under both oxic and anoxic conditions with citrate shown to have a mitigating effect on the consumption of oxygen, apparently by reducing the iron catalyzed oxidation of ascorbate. Kinetic modelling showed that the dynamics of dissolution could be reasonably well simulated with the inclusion of a surface > Fe(III)-citrate-Fe(II) ternary complex which facilitates the detachment of surface Fe(II). Use of dual reagents (ascorbic acid and citric acid) under oxic conditions is recommended for the cleaning of iron-fouled membranes in view of the extreme cleaning effectiveness though it is critical that cleaning conditions be carefully optimised.

Citrate-enhanced release of arsenic during pyrite oxidation at circumneutral conditions

The release of arsenic (As) from the oxidation of As-rich pyrite is an important source of the high arsenic in groundwater. As a widespread low-molecular-weight organic acid, citrate plays an important role on the cycling of Fe(II)/Fe(III) through complexation in circumneutral subsurface environments, while the influence of citrate on the release of As from the oxidation of As-rich pyrite is poorly understood. In this study, As was loaded onto pyrite particles under anoxic conditions, and its release was investigated in the presence of 0–1 mM citrate at pH 7.4 under oxic conditions. As-loaded pyrite suspension was prepared by the equilibrium of 2.67 μM As(III) in 10 g/L pyrite under anoxic conditions with the decrease in dissolved As(III) concentration to 1 μM. The suspension was subsequently exposed to air for oxygenation. In the absence of citrate, the oxygenation decreased the partitioning of As in the solution because of the re-adsorption of aqueous As by the in situ generated Fe(III) oxyhydroxides. However, with the increase in citrate concentration from 0.1 to 1 mM, the As partitioned in the solution increased from 0.3 to 2.67 μM. In the presence of 1 mM citrate, the As(III) was almost completely oxidized to As(V) during the oxygenation. The mechanisms of citrate-enhanced release of As were mainly attributed to the ligand exchange of citrate with As for pyrite surface sites, the competitive adsorption of citrate with As on Fe(III) oxyhydroxides and pyrite, and the partitioning of As on the newly formed Fe(III) colloids. This finding presents an overlooked mechanism of the release of pyrite-associated As under oxic and circumneutral conditions.

Isoexergonic Conformations of Surface-Bound Citrate Regulated Bioinspired Apatite Nanocrystal Growth.

The superior biomechanical properties of bone and dentin are dictated, in part, by the unique plate-like morphology of hydroxyapatite (HAP) nanocrysals within a hierarchically assembled collagen matrix. Understanding the mechanism of crystal growth and thus morphology is important to the rational design of bioinspired apatite nanocrystals for orthopedic and dental applications. Citrate has long been proposed to modulate apatite crystal growth, but major questions exist regarding the HAP-bound citrate conformations and the identities of the interacting functional groups and HAP surface sites. Here, we conducted a comprehensive investigation of the mechanism from the angstrom to submicrometer scale by detailed correlation of the results of high-level metadynamics simulations, employing force-fields benchmarked to experiment and density functional theory calculations with the results of high resolution transmission electron microscopy, nuclear magnetic resonance spectroscopy, solution analysis, and thermogravimetric analysis. Crystal morphology changed from needle- to plate-like with increasing citrate concentration. Citrate adsorbed more strongly on the HAP (100) face than on the (001) face, thus resulting in preferential growth in the [001] direction and the plate-like morphology. Two very different bound conformations were obtained, involving interactions of either one or both terminal carboxyl groups with three or five surface calcium ions, respectively, and a hydrogen bond between the citrate hydroxyl and the HAP surface. Remarkably, despite fewer interaction sites in the single bound carboxyl conformation, the structures were isoexergonic, so both exist at equilibrium. Identification of the former conformation is significant because it allows a greater adsorption density than is traditionally assumed and can help explain concentration-dependence of citrate in modulating crystal morphology. These unique results were enabled first by the application of advanced metadynamics, a technique necessary for the accurate simulation of ionic materials but which is rarely employed in the biomaterials and biomineralization fields and second by the detailed correlation of computational, spectroscopic, and analytical results.

Source of cytotoxicity in a colloidal silver nanoparticle suspension

Silver nanoparticles (AgNPs) are increasingly used in a variety of applications because of their potential antimicrobial activity and their plasmonic and conductivity properties. In this study, we investigated the source of cytotoxicity, genotoxicity, and reactive oxygen species (ROS) production on human dermal fibroblast and human lung cancer (A549) cell lines upon exposure to AgNP colloidal suspensions prepared with the simplest and most commonly used Lee–Meisel method with a variety of reaction times and the concentrations of the reducing agent. The AgNPs synthesized with shorter reaction times were more cytotoxic and genotoxic due to the presence of a few nanometer-sized AgNP seeds. The suspensions prepared with an increased citrate concentration were not cytotoxic, but they induced more ROS generation on A549 cells due to the high citrate concentration. The genotoxicity of the suspension decreased significantly at the higher citrate concentrations. The analysis of both transmission electron microscopy images from the dried droplet areas of the colloidal suspensions and toxicity data indicated that the AgNP seeds were the major source of toxicity. The completion of the nucleation step and the formation of larger AgNPs effectively decreased the toxicity.

Citrate Effects on Amorphous Calcium Carbonate (ACC) Structure, Stability, and Crystallization

Understanding the role of citrate in the crystallization kinetics of amorphous calcium carbonate (ACC) is essential to explain the formation mechanisms, stabilities, surface properties, and morphologies of CaCO3 biominerals. It also contributes to deeper insight into fluid–mineral interactions, both in nature and for industrial processes. In this study, ACC formation and its crystallization are monitored in real time as a function of citrate (CIT) concentration in solution. Additionally, synchrotron radiation pair distribution function analyses combined with solid‐state, spectroscopic, and microscopic techniques are used to determine the effect of CIT on ACC structure, composition, and size. Results show an increase in ACC lifetime coupled with an increase in CIT uptake by ACC and slight changes in ACC atomic structure with an increase in CIT concentration. ACC does not form at concentrations ≥ 75% CIT/Ca and vaterite is absent in all cases where CIT is present. These findings can be explained by CIT binding with Ca ions, thereby forming Ca–CIT complexes in solution and decreasing ACC and calcite saturation levels. The formation of CIT‐bearing ACC with calcitic structure and the absence of vaterite formation suggest that these solution complexes form a calcite‐type atomic arrangement while CIT probably also acts as a growth inhibitor.

Cd and Ni transport and accumulation in the halophyte Sesuvium portulacastrum: implication of organic acids in these processes.

The implication of organic acids in Cd and Ni translocation was studied in the halophyte species Sesuvium portulacastrum. Citric, fumaric, malic, and ascorbic acids were separated and quantified by HPLC technique in shoots, roots and xylem saps of plants grown on nutrient solutions added with 50 μM Cd, 100 μM Ni and the combination of 50 μM Cd + 100 μM Ni. Results showed that Cd had no significant impact on biomass production while Ni and the combination of both metals drastically affected plant development. Cadmium and Ni concentrations in tissues and xylem sap were higher in plants subjected to individual metal application than those subjected to the combined effect of Cd and Ni suggesting a possible competition between these metals for absorption. Both metals applied separately or in combination induced an increase in citrate concentration in shoots and xylem sap but a decrease of this concentration in the roots. However, a minor relationship was observed between metal application and fumaric, malic, and ascorbic acids. Both observations suggest the implication of citric acid in Cd, Ni translocation and shoot accumulation in S. portulacastrum. The relatively high accumulation of citric acid in xylem sap and shoot of S. portulacastrum could be involved in metal chelation and thus contributes to heavy metal tolerance in this species.

Struvite crystal growth inhibition by trisodium citrate and the formation of chemical complexes in growth solution

Effect of trisodium citrate on the crystallization of struvite was studied. To evaluate such an effect an experiment of struvite growth from artificial urine was performed. The investigations are related to infectious urinary stones formation. The crystallization process was induced by the addition of aqueous ammonia solution to mimic the bacterial activity. The spectrophotometric results demonstrate that trisodium citrate increases induction time with respect to struvite formation and decreases the growth efficiency of struvite. The inhibitory effect of trisodium citrate on the nucleation and growth of struvite is explained in base of chemical speciation analysis. Such an analysis demonstrates that the inhibitory effect is related with the fact that trisodium citrate binds NH4+ and Mg2+ ions in the range of pH from 7 to 9.5 characteristic for struvite precipitation. The most important is the MgCit− complex whose concentration strongly depends on an increase in pH rather than on an increase in citrate concentrations.

Low concentrations of citrate reduce complement and granulocyte activation in vitro in human blood.

BackgroundThe use of acetate in haemodialysis fluids may induce negative effects in patients including nausea and increased inflammation. Therefore, haemodialysis fluids where acetate is substituted with citrate have recently been developed. In this study, we investigated the biocompatibility of citrate employing concentrations used in haemodialysis.MethodsThe effects of citrate and acetate were investigated in human whole blood in vitro under conditions promoting biomaterial-induced activation. Complement activation was measured as generation of C3a, C5a and the sC5b-9 complex, and granulocyte activation as up-regulation of CD11b expression. For the experimental set-up, a mathematical model was created to calculate the concentrations of acetate and citrate attained during haemodialysis.ResultsCitrate reduced granulocyte activation and did not induce higher complement activation compared with acetate at concentrations attained during haemodialysis. Investigating different citrate concentrations clearly showed that citrate is a potent complement inhibitor already at low concentrations, i.e. 0.25 mM, which is comparable with concentrations detected in the blood of patients during dialysis with citrate-containing fluids. Increased citrate concentration up to 6 mM further reduced the activation of C3a, C5a and sC5b-9, as well as the expression of CD11b.ConclusionsOur results suggest that citrate is a promising substitute for acetate for a more biocompatible dialysis, most likely resulting in less adverse effects for the patients.

Transparent Aggregates of Nanocrystalline Hydroxyapatite

Assemblies of nanoparticles into transparent aggregates have solicited strong research interest in the form of both crystalline or amorphous aggregates of nanoparticles. In the present work, we make short-range ordered several millimeter-sized transparent aggregates of citrate modified calcium phosphate nanoparticles and discuss the mechanism of their formation. Microparticles of hydroxyapatite (HAP) nanocrystals and amorphous calcium phosphate (ACP) were synthesized with citrate as a growth and assembly modifier. Millimeter-sized transparent aggregates of these microparticles were made with 0 to 7.5% citrate/Ca2+. The degree of crystallinity, i.e., the ratio between nanocrystalline HAP and ACP in the microparticles, was determined by Rietveld refinement of powder X-ray diffraction data with an internal standard. It was found to decrease with increasing citrate concentration. Citrate also reduced the nanocrystallite size at low citrate concentrations. Above ∼3% added citrate, the crystallite size did not reduce further. Transparent aggregates were obtained by drying a suspension of particles. The aggregates lacked long-range order and in many cases featured spiral fractures partially propagating through the aggregates. The assembly mechanisms were studied by in situ video imaging, polarized optical microscopy, transmission electron microscopy, and confocal microscopy. The transparent aggregates consisted of polydisperse microparticles. The transparent aggregates form due to evaporation, but sedimentation leads to vertical size segregation with larger microparticles preferentially located at the bottom of the sample.

Enhanced photocatalytic activity of Ag-ZnO hybrid plasmonic nanostructures prepared by a facile wet chemical method.

We report the synthesis of Ag–ZnO hybrid plasmonic nanostructures with enhanced photocatalytic activity by a facile wet-chemical method. The structural, optical, plasmonic and photocatalytic properties of the Ag–ZnO hybrid nanostructures were studied by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), photoluminescence (PL) and UV–visible absorption spectroscopy. The effects of citrate concentration and Ag nanoparticle loading on the photocatalytic activity of Ag–ZnO hybrid nanostructures towards sun-light driven degradation of methylene blue (MB) have been investigated. Increase in citrate concentration has been found to result in the formation of nanodisk-like structures, due to citrate-assisted oriented attachment of ZnO nanoparticles. The decoration of ZnO nanostructures with Ag nanoparticles resulted in a significant enhancement of the photocatalytic degradation efficiency, which has been found to increase with the extent of Ag nanoparticle loading.

Influence of citrate on Chlorella vulgaris for biodiesel production

Abstract The present study was carried out on microalgae Chlorella vulgaris to investigate the effect of citrate metabolite as a nutrient for biodiesel production. The oil obtained from C. vulgaris is a promising fuel substitute and can be used as biodiesel. C. vulgaris was cultured in conway medium at different concentrations of citrate, namely 0.5, 1.0 and 2 g/L were added to the culture medium in different experimental groups. Subsequently, the growth, proximate composition and biomass content of the cultured cells were analyzed. The maximum biomass (1.68±0.044 g/L) in the cultured cells was obtained at a citrate concentration of 2.0 g/L in the culture medium. However, the highest oil yield (28.62%) was obtained at the 1owest citrate concentration. Physico-chemical parameters such as pH, viscosity, density, acid and iodine values of the extracted oil were analyzed. In addition, a freeze-dried sample of the algae was analyzed to ascertain its biochemical composition. The maximum lipid content (37.03±±0.88%) was obtained at the minimum experimental concentration of citrate (0.5 g/L) in the culture media. An increase in citrate concentration in the media resulted in a decrease in total lipid content of the cells, but increased the carbohydrate content up to 29.43±0.72%. Fatty acid profiles of the control and experimental groups were analyzed. The present study was suggested that the presence of lowest concentration of citrate in the culture media can improve the accumulation of lipids in C. vulgaris.

Morphology control of ZnO with citrate: a time and concentration dependent mechanistic insight

The effect of citrate as a crystal habit modifier on the morphology of zinc oxide has been explored in a wide range of citrate/Zn ratio from 0 to 0.67. With an increase in concentration of citrate at the lower range of the ratio, the aspect ratio of the hexagonal prismatic crystals of ZnO decreases, forming flatter disks with a progressive increase of roughness on the hexagonal face. At intermediate ratios, the otherwise smooth rectangular faces also become rough, with progressive formation of porous spherical aggregates. At a high concentration of citrate, solid spherical particles form with very low crystallinity, eventually forming plate-like zinc citrate. Thermal and spectroscopic studies confirm a progressive increase in the adsorbed citrate concentration in the particles. Evidence in support of citrate being adsorbed first on the hexagonal face, followed by the other rectangular faces is presented, and a scheme has been proposed. This adsorbed citrate can be removed by calcination and the morphology remains intact, and so this procedure can be used to make pure ZnO with various morphologies. It was found that the ZnO morphologies synthesized in the presence of citrate show novel photoluminescent and enhanced photocatalytic activities compared with ZnO synthesized in the absence of citrate.

Annealing-induced ultra-efficient NIR-to-VIS upconversion of nano-/micro-scale α and β NaYF4:Er3+,Yb3+ crystals

Nano-/micro-scale α and β NaYF4:Er3+,Yb3+ crystals with ultra-efficient upconversion (UC) of near infrared (NIR)-to-visible (VIS) light were prepared by annealing following citrate-assisted hydrothermal synthesis at 200 °C. Excess fluoride ([F]/[Y + Er + Yb] molar ratio = 9) helped promote the formation of the hexagonal β phase while simultaneous addition of citrate led to crystal shape and size control. The citrate caused the morphology of the β phase crystals to change from irregular elongated microtubes to hexagonal prisms, with particle size decreasing with increasing citrate concentration. However the presence of citrate led to significant deterioration of the upconversion luminescence of the β NaYF4:Er3+,Yb3+ crystals due to surface quenching by adsorbed citrate moieties. We have found that annealing at 700 °C restored the NIR-to-VIS upconversion luminescence (predominantly green) of the citrate-modified β NaYF4:Er3+,Yb3+ crystals and also led to a dramatic enhancement (over several orders of magnitude) not previously reported. According to Raman spectroscopic evidence, annealing led to improved upconversion efficiency via elimination of internal defects and surface hydroxyl/citrate groups. Further, we report on the preparation of unexpectedly strong upconverting (predominantly red) α cubic phase NaYF4:Er3+,Yb3+ nanocrystals.

Photocatalytic Activity and Related Surface Properties of Transparent ZnO Films Prepared by a Low‐temperature Aqueous Route

Transparent ZnO were prepared using solutions with various trisodium citrate concentrations by a spin-spray method at 90°C. The morphological and structural characteristics, as well as photocatalytic activity of the resulting ZnO films were examined with respect to the added trisodium citrate concentration. Photocatalytic activities of the ZnO films were evaluated from photodecomposition of methylene blue (MB) in aqueous solution. With increasing citrate concentrations, the ZnO films came to have higher transmittances in visible region but lower MB decomposition rate. Both high transmittance and high photocatalytic activity were achieved in the ZnO film prepared in the citrate concentration of 0.5 mm. The possible mechanism for the difference in photocatalytic activity by the samples prepared with the various concentrations of citrate was discussed from the viewpoint of film texture, crystal orientation and surface chemical state.

Stabilizing effect of citrate buffer on the photolysis of riboflavin in aqueous solution

In the present investigation the photolysis of riboflavin (RF) in the presence of citrate species at pH 4.0–7.0 has been studied. A specific multicomponent spectrophotometric method has been used to assay RF in the presence of photoproducts during the reactions. The overall first-order rate constants (kobs) for the photolysis of RF range from 0.42 to 1.08×10–2min−1 in the region. The values of kobs have been found to decrease with an increase in citrate concentration indicating an inhibitory effect of these species on the rate of reaction. The second-order rate constants for the interaction of RF with total citrate species causing inhibition range from 1.79 to 5.65×10–3M−1min−1 at pH 4.0–7.0. The logk–pH profiles for the reactions at 0.2–1.0M citrate concentration show a gradual decrease in kobs and the value at 1.0M is more than half compared to that of k0, i.e., in the absence of buffer, at pH 5.0. Divalent citrate ions cause a decrease in RF fluorescence due to the quenching of the excited singlet state resulting in a decrease in the rate of reaction and consequently leading to the stabilization of RF solutions. The greater quenching of fluorescence at pH 4.0 compared to that of 7.0 is in accordance with the greater concentration of divalent citrate ions (99.6%) at that pH. The trivalent citrate ions exert a greater inhibitory effect on the rate of RF photolysis compared to that of the divalent citrate ions probably as a result of excited triplet state quenching. The values of second-order rate constants for the interaction of divalent and trivalent citrate ions are 0.44×10–2 and 1.06×10–3M–1min–1, respectively, indicating that the trivalent ions exert a greater stabilizing effect, compared to the divalent ions, on RF solutions.

Biomimetic Self-Assembling Copolymer−Hydroxyapatite Nanocomposites with the Nanocrystal Size Controlled by Citrate

Citrate binds strongly to the surface of calcium phosphate (apatite) nanocrystals in bone and is thought to prevent crystal thickening. In this work, citrate added as a regulatory element enabled molecular control of the size and stability of hydroxyapatite (HAp) nanocrystals in synthetic nanocomposites, fabricated with self-assembling block copolymer templates. The decrease of the HAp crystal size within the polymer matrix with increasing citrate concentration was documented by solid-state nuclear magnetic resonance (NMR) techniques and wide-angle X-ray diffraction (XRD), while the shapes of HAp nanocrystals were determined by transmission electron microscopy (TEM). Advanced NMR techniques were used to characterize the interfacial species and reveal enhanced interactions between mineral and organic matrix, concomitant with the size effects. The surface-to-volume ratios determined by NMR spectroscopy and long-range 31P{1H} dipolar dephasing show that 2, 10, and 40 mM citrate changes the thicknesses of the HAp crystals from 4 nm without citrate to 2.9, 2.8, and 2.3 nm, respectively. With citrate concentrations comparable to those in body fluids, HAp nanocrystals of sizes and morphologies similar to those in avian and bovine bones have been produced.