Modulation of Calcium Oxalate Crystallization by Colloidal Selenium

Article pubs.acs.org/crystal

Modulation of Calcium Oxalate Crystallization by Colloidal Selenium Nanoparticles−Polyphenol Complex Liyan Chen, Zixia Deng, Cailing Zhong, Yanhui Zhou, and Yan Bai* Chemistry Department, Jinan University, Guangzhou 510632, PR China S Supporting Information *

ABSTRACT: The nanoSe0-polyphenol complexes with a 3:1 polyphenol/nanoSe0 molar ratio were prepared by colloidal selenium nanoparticles (nanoSe0) modified with gallic acid (GA), propyl gallate (PG), and pyrogallic acid (PA), which were spherical with average diameter about 38−77 nm. On this basis, we studied the effect of nanoSe0-polyphenol on the CaC2O4 crystallization and also elaborated the modulation mechanism, which were compared with those for each polyphenol individually. NanoSe0-GA and nanoSe0-PA were easy to induce the formation of calcium oxalate dihydrate (COD) crystals, while nanoSe0-PG induced the formation of quasi-rectangular calcium oxalate monohydrate (COM), multilayered calcium oxalate trihydrate (COT) crystals, and an amount of COD crystals in a dose-dependent fashion by nanoSe0PG. The strong effect of nanoSe0-polyphenol on the formation of COD and COT crystals could be attributed to electrostatic interaction between nanoSe0-polyphenol and CaC2O4 crystals. The results obtained in the polyphenol system were similar to, as well as different from, nanoSe0-polyphenol because the effect of the polyphenol on the CaC2O4 crystallization could result from not only electrostatic interaction between polyphenols and Ca2+ ions, but also hydrogen bonding interaction between the polyphenols and C2O42− groups. All the obtained COD crystals were thermostable even at 70 °C, while COT crystals were temperature dependent. copolymer,13,14 self-assembled monolayer,15 surfactant,16 inorganic ions,17 and so on. Our team chose the colloidal selenium nanoparticles modified with biomolecules as additives for the crystallization of CaC2O4. We have previously reported the synergistic regulatory effects of colloidal selenium nanoparticles (nanoSe0) and some biomolecules on the growth of CaC2O4, including ascorbic acid18 and bovine serum albumin.19 Gallic acid (GA), propyl gallate (PG), and pyrogallic acid (PA), commonly found polyphenols from plants, have the same pyrogallol structure but different side chains (Figure 1). They have been widely used in food, medicine, and other fields because their harmful effects are lower in environment and

1. INTRODUCTION Calcium oxalate (CaC2O4) is a naturally occurring mineral found in many fields and has vital roles in many fields. It has three different hydrate forms, e.g., monoclinic monohydrate (CaC2O4·H2O, COM), tetragonal dihydrate (CaC2O4·2H2O, COD) and triclinic trihydrate (CaC2O4·3H2O, COT). In plants, CaC2O4 crystals are by far the most prevalent and widely distributed minerals deposited throughout the families of higher plants. They appear to play a central role in a variety of important functions depending on their morphology, including tissue calcium regulation,1 defense against herbivory,2 and metal detoxification.3,4 For example, raphide (needleshaped) crystals may serve a dual function of calcium regulation and plant defense, while druse, an aggregation of CaC2O4 crystals in plants, is strictly involved in calcium regulation.1 In industry, calcium oxalate, hydroxyapatite, and organic matter are found to be present in most sugar mill evaporators.5 In addition, CaC2O4 and SiO2 form a hard composite scale that is intractable to conventional cleaning methods.6 In medicine, urolithiasis, of which the main component is CaC2O4,7 is a significant disease of mankind and causes serious health problems. Therefore, the crystallization of CaC2O4 has always been a hot spot of research. The main purpose is to study the growth, inhibition, and dissolution of CaC2O4 crystals. So far, a large number of reports in the literature have recorded the effects of additives on CaC2O4, including amino acid,8 carboxylic acid,9 protein,10 polyelectrolyte,11 polysaccharide,12 © 2016 American Chemical Society

Figure 1. Molecular structures of polyphenols. Received: November 22, 2015 Revised: March 15, 2016 Published: March 25, 2016 2581

DOI: 10.1021/acs.cgd.5b01647 Cryst. Growth Des. 2016, 16, 2581−2589

Crystal Growth & Design

Article

Figure 2. TEM images of nanoSe0-polyphenol: (a) 1 mM nanoSe0-3 mM GA; (b) 1 mM nanoSe0-3 mM PG; (c) 1 mM nanoSe0-3 mM PA. 2.3. Preparation of CaC2O4 Crystals. Supersaturated CaC2O4 subphases were prepared by mixing appropriate amounts of CaCl2, NaCl, and Na2C2O4 solution. Then the supersaturated CaC2O4 subphases were filtered through a 0.22 μm Millipore filter and transferred into 25 mL beakers in which there were small pieces of glass coverslips. After that, different amounts of nanoSe0-polyphenol or the polyphenol were added into the supersaturated CaC2O4 subphases, and the pH was subsequently adjusted to 7.4 by addition of the amount of HCl or NaOH solution. The final concentrations of Ca2+ and C2O42− were 0.40 mM and the final concentration of NaCl was 1.2 mM. Then the subphases were allowed to stand for 3 days. After 3 days, the CaC2O4 crystals grew in the different systems and deposited on the glass coverslips. The small pieces of glass coverslips with the CaC2O4 crystals were removed carefully from the mother liquor and then air-dried for further analyses. The CaC2O4 crystals were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). 2.4. Characterization. The ζ potential, size, and morphology of nanoSe0-polyphenol were measured using a Zetasizer Nano ZS analyzer (Malvern Instruments) and transmission electron microscopy (TEM) (Philips TECNAI-10). UV−vis spectrophotometer (TU1900, Beijing Purkinje General), Fourier transform infrared (FTIR) spectrometer (Nicolet 6700), and differential pulse voltammetry (DY2312 Bipotentiostat) were used to investigate the structure of nanoSe0-polyphenol. Differential pulse voltammetric experiments were carried out in the phosphate buffer solution (0.01 M Na2HPO4/0.01 M KH2PO4, 0.138 M NaCl, and 0.0027 M KCl, pH 7.4) containing 0.20 mM polyphenol and appropriate amounts of nanoSe0 with a glassy carbon electrode as the working electrode, a platinum wire as the counter electrode, and a Ag/AgCl/KCl (sat.) electrode as the reference electrode. The following instrumental parameters were used: pulse amplitude, 50 mV; sample width, 20 ms; pulse width, 50 ms; pulse period, 100 ms; scan rate, 100 mV/s. Morphologies of deposited CaC2O4 were observed by SEM (Philips XL-30), and the CaC2O4 crystals were sputter-coated with gold before SEM analysis. XRD (MSAL XD-2) using Cu Kα radiation at a scan rate of 8° min−1 was used to determine the CaC2O4 crystals. The accelerating voltage and applied current were 36 kV and 20 mA, respectively. The divergence and scattering slit was 1° in the range of 10° < 2θ < 60°. All the experiments including preparation and characterization were conducted at room temperature (25 ± 2 °C) and pH 7.4, unless otherwise specified.

biology. For example, the GA stabilized gold nanoparticles had stronger bactericidal effects at a lower concentration because of a synergistic action with GA.20 PG is a phenolic antioxidant extensively used in the food, cosmetics, and pharmaceutical industries.21 PA is an important multipurpose organic chemical product, particularly in biological applications.22,23 Tea is an important drink in daily life and has become one of the most popular nonalcoholic beverages in the world. It is worth noting here that tea is very rich in polyphenols and green tea has shown an inhibitory effect on urinary stone formation.24 Furthermore, tea extract has been proven to be able to effectively control the formation of calcium oxalate deposit. For example, Li’s group indicated that tea extract mainly induced the formation of COD crystals by hydrogen bonding interaction between polyphenols and CaC2O4 crystals in pH 2.25 In recent years, selenium-rich tea has received considerable attention. Selenium in the selenium-rich tea comes from natural soil selenium or the use of selenium-containing phosphate fertilizers and sewage sludge in agriculture.26 Many studies on the physiological function of selenium-rich green tea have been reported.27,28 In the present study, the complex of colloidal selenium nanoparticles modified with three polyphenols (nanoSe0polyphenol, such as nanoSe0-GA, nanoSe0-PG, and nanoSe0PA) was prepared and used as a novel template for CaC2O4 mineralization. The modulation results in the nanoSe0polyphenol system were also compared to polyphenol individually. It was surprising to obtain CaC2O4 crystals with different phases and morphologies. The nanoSe0-polyphenol modulated CaC2O4 crystallization predominantly through the electrostatic interaction between the nanoSe0-polyphenol and CaC2O4 crystals. While it was mainly hydrogen bonding interaction between the polyphenol and C2O42− groups that polyphenols modulated CaC2O4 crystallization.

2. EXPERIMENTAL SECTION 2.1. Materials. Sodium sulfite (Na2SO3), selenium powder, gallic acid (GA), propyl gallate (PG), pryogallic acid (PA), sodium oxalate (Na2C2O4), anhydrous calcium chloride (CaCl2), and sodium chloride (NaCl) were all of analytical purity. Sodium selenosulfate solution (Na2SeSO3, 0.1 M) was prepared according to the literature.29 All of the chemicals were used as received without any further purification. Double-distilled water was used throughout the experiment. Glass coverslips were thoroughly cleaned by treatment with piranha solution (1 h immersion in freshly prepared 3:1 H2SO4/H2O2) and then sonicated and rinsed with double-distilled water. 2.2. Preparation of NanoSe0-Polyphenol. NanoSe0-polyphenol was prepared by mixing the Na2SeSO3 solution (0.1 mL, 0.1 M) and polyphenol solution (3 mL, 0.01 M) and subsequent dropping 1.0 M HCl until the pH became 4.0. Lastly, the clear reddish solution of nanoSe0-polyphenol was diluted to a final volume of 10 mL and the final pH was adjusted to 7.4.

3. RESULTS AND DISCUSSION 3.1. Characterization of NanoSe0-Polyphenol. The morphology and size of the three complexes were investigated. As shown in Figure 2, the nanoparticles were almost spherical and well-dispersed and their average diameters were 77, 68, and 38 nm for nanoSe0-GA, nanoSe0-PG, and nanoSe0-PA, respectively. The ζ potentials of nanoSe0-polyphenol were −19.11, −24.31, and −24.81 for nanoSe0-GA, nanoSe0-PG, and nanoSe0-PA, respectively. This result indicated that nanoSe0polyphenol was negatively charged at pH 7.4. 2582



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Figure 3. UV−vis absorption spectra for 0.20 mM GA (a), 0.20 mM PG (b), and 0.20 mM PA (c) in the presence of different concentrations of nanoSe0. Curves a−k correspond to (0, 3, 4, 5, 6, 7, 8, 9, 10, and 12) × 10−2 mM nanoSe0 for GA and PG, and (0, 3, 4, 6, 8, 10, 12, 14, 16, and 18) × 10−2 mM nanoSe0 for PA. Plots of A0/(A − A0) vs 1/[nanoSe0] for calculation of binding constant of nanoSe0-GA (d), nanoSe0-PG (e), and nanoSe0-PA (f). Plots of A vs npolyphenol/nnanoSe0 for obtaining the molar ratio of nanoSe0-GA (g), nanoSe0-PG (h), and nanoSe0-PA (i).

Table 1. Binding Constants and Molar Ratios for NanoSe0-Polyphenola UV−vis complex 0

nanoSe -GA nanoSe0-PG nanoSe0-PA a

−1

binding constant Kb/M (3.23 ± 0.4) × 10 (3.01 ± 1.9) × 103 (2.54 ± 0.9) × 103 3

differential pulse voltammetry

molar ratio (npolyphenol/nnanoSe )

binding constant Kb/M−1

molar ratio (npolyphenol/nnanoSe0)

3.20 ± 0.1 2.95 ± 0.1 2.53 ± 0.1

(9.36 ± 0.3) × 10 (2.18 ± 0.1) × 103 (2.95 ± 0.2) × 103

3.35 ± 0.1 2.85 ± 0.1 2.45 ± 0.2

0

3

All values were reported as average ± standard deviation of five parallel tests.

regularly with increasing concentration of nanoSe0, which

UV−vis spectrophotometry is a significant technique to investigate complex formation and study the binding mode of small molecules as well as metal complexes. The UV−vis spectra of the polyphenol in the absence and presence of nanoSe0 provided efficient information about the formation of nanoSe0-polyphenol. As shown in Figure 3, the absorption maximum of polyphenols was at 259, 267, and 319 nm corresponded, respectively, to GA (Figure 3a), PG (Figure 3b), and PA (Figure 3c). The absorption of polyphenol decreased

indicated binding of the polyphenol to nanoSe0 to form the complex.30 The binding constant Kb of nanoSe0-polyphenol was determined from the variation in absorbance before and after the addition of nanoSe0 at the absorption maximum for the polyphenol.31 The Benesi−Hildebrand equation is introduced to calculate the value of Kb 2583



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Figure 4. Plausible structure of nanoSe0-polyphenol (R = H, COOH, COOC3H7).

Figure 5. SEM images of CaC2O4 crystals in various concentrations of nanoSe0-GA and GA: (a) control crystals; (b) 3.3 μM nanoSe0-10 μM GA; (c) 13.3 μM nanoSe0-40 μM GA; (d) 20 μM nanoSe0-60 μM GA; (e) 10 μM GA; (f) 40 μM GA; (g) 60 μM GA.

A0 εG εG 1 = + A − A0 εH − G − εG εH − G − εG Kb[nanoSe 0]

Furthermore, FTIR spectroscopy was carried out to investigate the binding mode of the polyphenol with nanoSe0. FT-IR spectra of the polyphenol and nanoSe0-polyphenol are provided in the Supporting Information (Figure S2). NanoSe0polyphenol exhibited a broad absorption band at 3000−3600 cm−1, which shifted to higher wavenumber, compared with the polyphenol, indicating that the OH group degrees of freedom decrease in the complexes. It suggested that the polyphenol binds with nanoSe0 by a simple adsorption of OH groups to nanoSe0 surfaces.34 Otherwise, the broadness of these bands was indicative of hydrogen bonding in polyphenols.35 The peak at 1620 cm−1 of nanoSe0-polyphenol was assigned to CC stretching. In addition, the bands around 577.4, 578.8, and 578.9 cm−1 were the characteristic absorption of C−O−Se18 for nanoSe0-GA, nanoSe0-PG, and nanoSe0-PA, respectively. Based on the above, the plausible structure of nanoSe0polyphenol could be inferred as in Figure 4. The ionization constants (pK1) of three polyphenols were 4.9, 4.1, and 5.1 for GA, PG, and PA, respectively;36,37 thus, the three polyphenols ionized to form anions in pH 7.4. The three molecules of ionized form of the polyphenol bound to the surface of nanoSe0 to form the nanocomplex stabilized by forming hydrogen bonding between the polyphenols.

(1)

where A0 and A are the absorbance of the polyphenol and nanoSe0-polyphenol, respectively, εG and εH−G are the molar absorption coefficients of the polyphenol and nanoSe0polyphenol, respectively. From plot of A0/(A − A0) to 1/ [nanoSe0] using linear regression analysis (Figure 3d−f), the value of Kb is equal to the ratio of the intercept to the slope and the results are listed in Table 1. In turn, a relationship between absorbance and the molar ratio of polyphenol to nanoSe0 (npolyphenol/nnanoSe0) were shown in Figure 3g−i, in which the molar ratio of complexes was obtained by the intersection points of two straight lines. Further, differential pulse voltammetry is a sensitive and accessible electrochemical method to investigate complex formation. The binding constant and molar ratio were also determined by differential pulse voltammetry.32,33 The differential pulse voltammograms and calculations for the binding constant and the molar ratio of nanoSe0-polyphenol are provided in the Supporting Information (Figure S1). The results obtained from UV−vis and differential pulse voltammetry were consistent and given in Table 1. 2584



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Figure 6. SEM images of CaC2O4 crystals in various concentrations of nanoSe0-PG and PG: (a) 6.7 μM nanoSe0-20 μM PG; (b) 20 μM nanoSe0-60 μM PG; (c) 66.7 μM nanoSe0-200 μM PG; (d) 20 μM PG; (e) 60 μM PG; (f) 200 μM PG.

Figure 7. SEM images of CaC2O4 crystals in various concentrations of nanoSe0-PA and PA: (a) 3.3 μM nanoSe0-10 μM PA; (b) 13.3 μM nanoSe040 μM PA; (c) 33.3 μM nanoSe0-100 μM PA; (d) 10 μM PA; (e) 40 μM PA; (f) 100 μM PA.

nanoSe0-GA. This result agreed with the SEM results in which the morphologies of COD crystals changed from bipyramid crystals to elongated crystals with increasing the concentration of nanoSe0-GA. GA could also induce the formation of COD crystals (Figure 5e) and the growth trend of COD crystals with increasing the concentrations of GA resembled that induced by nanoSe0-GA. However, in comparison with nanoSe0-GA, there were some differences in the morphologies of COD crystals induced by GA. For example, the {101} faces had disappeared and the COD became pencil rod- or flower-shaped in a high concentration of 60 μM GA (Figure 5g), while those were still elongated bipyramid with {101} faces in the corresponding concentration of 20 μM nanoSe0-60 μM GA (Figure 5d). The corresponding XRD patterns (Supporting Information Figure

3.2. Modulation of the Calcium Oxalate Crystallization by NanoSe0-Polyphenol. 3.2.1. Effects of NanoSe0-GA on the Crystallization Behavior of CaC2O4. The control CaC2O4 crystals were plate-like COM penetration twins nucleated from a {010} face (Figure 5a). COD crystals were observed in the presence of nanoSe0-GA. For example, in a low concentration of 3.3 μM nanoSe0-10 μM GA, bipyramidal COD crystals were obtained (Figure 5b). With increasing concentrations of nanoSe0-GA, there was an outgrowth of the COD crystals along the [001] direction, resulting in elongated crystals and the main development of {100} face (Figure 5c,d). The corresponding XRD patterns in the Supporting Information (Figure S3a−c) showed that the {202} face gradually disappear and the {200} face become the main crystal faces of COD crystals with increasing the concentration of 2585



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Figure 8. SEM images of CaC2O4 crystals at various temperatures (a, b, c; 25, 37, 70 °C): 3.3 μM nanoSe0-10 μM GA (a1−c1); 10 μM GA (a2−c2); 3.3 μM nanoSe0-10 μM PA (a3−c3); 10 μM PA (a4−c4); 66.7 μM nanoSe0-200 μM PG (a5−c5); 200 μM PG (a6−c6).

S4) showed similar results with nanoSe0-GA and also agreed with the SEM results. 3.2.2. Effects of NanoSe0-PG on the Crystallization Behavior of CaC2O4. It had been previously documented that increasing the concentration of additives, such as carboxylic acids38 and anionic polyelectrolytes,39 would inhibit the COT phase, resulting in the formation of COD phase. Similar phenomena were also observed in nanoSe0-PG system. Only COM crystals with quasi-rectangular morphologies were found in a low concentration of 6.7 μM nanoSe0-20 μM PG (Figure 6a), indicating that nanoSe0-PG strongly adsorbs on the {100} and {121} faces of COM resulting in a discal habit with wide {100} faces and rounded ends. With increasing the concentration to 20 μM nanoSe0-60 μM PG, irregular COT crystals (Figure 6b) were obtained. As nanoSe0-PG concentration increased to 66.7 μM nanoSe0-200 μM PG, the mixed crystals were obtained (Figure 6c), which consisted of COT with multilayered {100} faces (yellow arrows show) and tetragonal bipyramidal COD (yellow circles show). The COT crystals with multilayered {100} faces were also found in the presence of magnesium and citrate.40 The corresponding XRD pattern in the Supporting Information (Figure S5a) showed that almost all diffraction peaks matched with COT crystals, but they did not match with COD crystals because the amount of COD crystals were too little to be detected. These results suggested that nanoSe0-PG can also induce the formation of bipyramid COD, but the effect is much weaker than nanoSe0GA. In a low concentration of 20 μM PG (Figure 6d), CaC2O4 crystals were similar to those quasi-rectangular COM crystals obtained in the presence of 6.7 μM nanoSe0-20 μM PG. Tabular COT crystals (Figure 6e) and irregular COT crystals with multilayered {100} faces (Figure 6f) were obtained with increasing concentrations to 60 μM PG and 200 μM PG, respectively. However, no COD crystals were found even in a high concentration of 200 μM PG. In addition, there were only diffraction peaks of COT crystals but not diffraction peaks of

COD crystals in the XRD pattern (the Supporting Information Figure S6a), which was consistent with the SEM image in Figure 6f. 3.2.3. Effects of NanoSe0-PA on the Crystallization Behavior of CaC2O4. The crystallization behaviors of CaC2O4 in the presence of nanoSe0-PA and PA were similar to those in the presence of nanoSe0-GA and GA. As shown in Figure 7, COD crystals obtained were irregular in size and the morphologies of the COD crystals were also in dose-dependent by nanoSe0-PA and PA. 3.3. Effects of Temperature on the Crystallization Behavior of CaC2O4 in NanoSe0-Polyphenol Systems. The effects of temperature on three hydrated forms of CaC2O4 crystals had been investigated in nanoSe0-polyphenol and polyphenol systems. As shown in Figure 8, COD crystals exhibited good thermal stability and existed even at 70 °C although the number of COD crystals decreased with temperature increasing from 25 to 70 °C in the presence of nanoSe0 -GA, GA, nanoSe0 -PA, and PA. However, the crystallization of COT crystals was temperature dependent. COT crystals existed at 25 and 37 °C in the presence of nanoSe0-PG and PG (Figure 8a5, b5, a6, b6) and then changed to COM at 70 °C in a high concentration of 200 μM PG (Figure 8c6). Exceptionally, the ellipsoidal crystals with rounded edges considered as COD crystals in the literature41 were formed in a high concentration of 66.7 μM nanoSe0-200 μM PG (Figure 8c5) at 70 °C. Our previous study indicated that nanoSe0 can induce the formation of ellipsoidal COD crystals.18 In addition, the corresponding XRD pattern in the Supporting Information (Figure S5c) indicated the existence of COD and COM crystals among these ellipsoidal crystals. It was possible that these ellipsoidal COD crystals are the transition state from COT to COD. In the presence of 66.7 μM nanoSe0200 μM, COT and COD crystals coexisted at 25 and 37 °C, and then the unstable COT crystals changed to stable COD and COM crystals according to the thermostability of three 2586



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Figure 9. Interaction model of the effects of nanoSe0-polyphenol and polyphenols on the CaC2O4 in the supersaturated CaC2O4 solution (R = H, COOH, COOC3H7). (yellow, Polyphenol; fuschia, NanoSe0-polyphenol; pink, CaC2O4 crystal nucleus; gray, Supersaturated CaC2O4 solution).

With increasing concentration, nanoSe0-PG preferentially inhibited the development of COM nuclei and to enable the formation of COT crystals (Figure 6b). However, a higher concentration of nanoSe0-PG could further inhibit the growth of COT by adsorbing on the COT crystal face, so the COD crystals could grow (Figure 6c).39,44 In addition, the COT seed grows faster than COD;39 thus, a large number of COT crystals were preferentially generated in the presence of nanoSe0-PG. Overall, these findings suggest that nanoSe0-polyphenol can be ranked in decreasing order of inhibition ability for the growth of CaC2O4 as nanoSe0-GA ≈ nanoSe0-PA > nanoSe0PG. 3.4.2. Hydrogen Bonding and Electrostatic Interactions in the Polyphenol System. It has been known that the C2O42− group is a prolific hydrogen bonding acceptor46 and the phenol is an effective hydrogen bonding donor,47 where the phenolic OH group and C2O42− group form a strong hydrogen bonding. In addition, the polyphenols were ionized in pH 7.4 so that the electrostatic attraction also occurred between ionized poyphenols and the Ca2+ ions. As shown in Figure 9, one C2O42− group interacted with two molecules of the polyphenol by hydrogen bonding, meanwhile the poyphenols interacted with Ca2+ ions by electrostatic attraction, resulting in increasing the relative concentration of Ca2+ ions in the microenvironment around the CaC2O4 nucleus. According to the report of Taesung Junga’s team, the excess Ca2+ ions were favored for the formation of COD crystals in crystallization.48 Therefore, GA and PA could induce the formation of COD crystals. However, since there was a steric hindrance between the adjacent PG molecules when PG interacted with C2O42− groups, PG may not be enough to induce the formation of COD crystals. Therefore, although there was also hydrogen bonding interaction between PG and C2O42− groups, COT and COM crystals were obtained, no COD crystals were observed in the presence of PG (Figure 6d−f). From the above discussion, it is clear that these three polyphenols could be ranked in decreasing order of inhibition ability for the growth of CaC2O4 crystals as GA ≈ PA > PG. Besides the above-mentioned electrostatic interaction and hydrogen bonding interaction, the modulation mechanism could also include the encapsulation effect. The formation of COT crystals with surface cracks (Figure 6f) or multilayered {100} faces (Figure 6c) may be caused by the encapsulation of nanoSe0-PG or PG into the CaC2O4 crystal lattice. The encapsulation mechanism was similar to that of our previous study.19

hydrate crystals of CaC2O4, which following a decreasing order as COM > COD > COT. 3.4. Modulation Mechanism of the CaC2O4 Crystallization by nanoSe0-Polyphenol. 3.4.1. Electrostatic Interaction between NanoSe0-Polyphenol and CaC2O4 Crystals. The results from the ζ potential studies indicated that nanoSe0polyphenol exists in anion form with more negative charges than the corresponding polyphenol. As shown in Figure 4 the ionized polyphenol increased the charge of nanoSe0-polyphenol predominantly by forming hydrogen bonding between the polyphenols. The strong effect of nanoSe0-polyphenol on the formation of COD and COT crystals could be explained by the electrostatic interaction between nanoSe0-polyphenol and CaC2O4 crystals as depicted in Figure 9. The results indicated that nanoSe0-polyphenol has a strong adsorption on the {100} faces of COM and COD crystals (Figures 5d, 6a, and 7c), in which the {100} faces were Ca2+rich faces in both COM42 and COD43 crystals and inhibit the crystal growth in [001] directions. Further, the adsorption capability (Γ) of additives on CaC2O4 crystals also affected the growth of three hydrate crystals and the decreasing order of adsorption capability was as follows: Γ (COM) > Γ (COT) > Γ (COD).44 Thus, nanoSe0-polyphenol preferentially inhibited COM crystals growth, followed by COT crystals and to a lesser extent COD crystals. For example, nanoSe0-GA was preferentially adsorbed on the COM and even COT crystals faces, which would be sufficient to inhibit the development of COM and COT nuclei and to enable the formation of COD crystals (Figure 5b−d). The morphology change of COD crystals depending on the concentration of nanoSe0-GA was due to the electrostatic attraction between nanoSe0-GA and the {100} faces, resulting in crystals elongated in the [001] direction and the effect became stronger with the concentration of the nanoSe0-GA increasing. These results indicated that nanoSe0GA is a strong inhibitor for CaC2O4 crystallization. Similarly, nanoSe0-PA also strongly induced the formation of COD crystals (Figure 7a−c). Compared with GA and PA, PG had a propyl ester group in the side chain, which increased hydrophobicity and steric hindrance of nanoSe0-PG, weakening the effect of nanoSe0-PG on the crystallization of CaC2O4. Therefore, nanoSe0-PG induced the formation of COT crystals rather than COD crystals (Figure 6b). More importantly, the crystallization behavior of CaC2O4 was in a dose-dependent fashion by nanoSe0-PG. Only COM crystals with quasirectangular morphologies were found in a low concentration (Figure 6a) because of the strong adsorption of nanoSe0-PG on the {100} faces of COM crystals by electrostatic attraction.45 2587



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4. CONCLUSION The as-prepared complexes, nanoSe0-GA, nanoSe0-PA, and nanoSe0-PG, are spherical with average diameter about 38−77 nm. Three molecules of polyphenol ligate with nanoSe0 (npolyphenol/nnanoSe0 = 3:1) to form the nanocomplex stabilized by hydrogen bonding interaction between polyphenols. Importantly, in the presence of nanoSe0-polyphenol, metastable COD and COT crystals are obtained and the COT and COD crystals are thermostable at 37 °C and even 70 °C, respectively. The results show that nanoSe0-GA and nanoSe0-PA are favored to induce the formation of COD crystals, while nanoSe0-PG preferentially induces the formation of COT crystals with cracks and multilayered {100} faces, and also induces the formation of COD crystals under a higher concentration of nanoSe0-PG. These differences are considered to result from that the electrostatic interaction between nanoSe0-polyphenol and CaC2O4 crystals ranks in decreasing order as nanoSe0-GA ≈ nanoSe0-PA > nanoSe0-PG. In addition, the morphological modulation of COD and COT is in a dose-dependent fashion by nanoSe0-polyphenol. On the other hand, the effect of the polyphenol on the CaC2O4 crystallization mainly resulted from hydrogen bonding interaction between the polyphenols and C2O42− groups. We used the nanoSe0-polyphenol complex as a template system to study CaC2O4 crystal growth. These results may provide fundamental and practical information on the morphology and habit of CaC2O4 crystals for biomineralization.

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ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01647. (1) Differential pulse voltammograms for 0.20 mM GA, 0.20 mM PG, and 0.20 mM PA in the presence of different concentrations of nanoSe0, and the plots of log (1/C nanoSe 0 ) vs log (I polyphenol‑nanoSe 0 /(I polyphenol − Ipolyphenol‑nanoSe0)) and i vs npolyphenol/nnanoSe0 for calculating the binding constant and molar ratio of nanoSe0polyphenol, respectively (Figure S1), (2) FT-IR spectra of polyphenols and nanoSe0-polyphenol (Figure S2), (3) XRD patterns of CaC2O4 crystals at various concentrations of nanoSe0-GA (Figure S3), (4) XRD patterns of CaC2O4 crystals at various concentrations of GA (Figure S4), (5) XRD patterns of CaC2O4 crystals in the presence of 66.7 μM nanoSe0-200 μM PG at various temperatures (Figure S5), (6) XRD patterns of CaC2O4 crystals in the presence of 200 μM PG at various temperatures (Figure S6) (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-15818823611. Notes

The authors declare no competing financial interest.

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REFERENCES

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ACKNOWLEDGMENTS

This work is supported by the National Natural Science Foundation of China (21075053). We are also very grateful for Professor Ouyang, J. M. and co-workers (Chemistry Department of Jinan University), who help us during the experiments. 2588



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