One-Step Synthesis of Quadrilateral-Shaped Silver Nanoplates with

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Article Cite This: ACS Omega 2018, 3, 6841−6848

One-Step Synthesis of Quadrilateral-Shaped Silver Nanoplates with Lamellar Structures Tuned by Amylopectin Derivatives Bo Pang,† Robert Köhler,‡ Vladimir Roddatis,§ Huan Liu,† Xiaohui Wang,∥ Wolfgang Viöl,‡ and Kai Zhang*,† †

Wood Technology and Wood Chemistry, Georg-August-University of Goettingen, Büsgenweg 4, 37077 Göttingen, Germany Laboratory of Laser and Plasma Technologies, University of Applied Sciences and Arts Hildesheim/Holzminden/Goettingen, Von-Ossietzky-Str. 99, 37085 Göttingen, Germany § Institute of Materials Physics, Georg-August-University of Goettingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany ∥ State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, China 510640

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‡

S Supporting Information *

ABSTRACT: Polymers or small molecules with functional groups were always employed to synthesize two-dimensional (2D) silver nanostructures, but the polysaccharides and derivatives have rarely been used for their preparation, let alone of uniform quadrilateral shapes. Herein, amylopectin derivatives containing concentrated carboxyl groups were first used for the synthesis of uniform 2D quadrilateral silver nanoplates (QAgNPs) with lamellar structure. As a native hyperbranched polysaccharide, amylopectin was esterified with 10-undecenoyl chloride and then modified via thiol− ene click chemistry to introduce high amount and high density of carboxyl groups. Then, QAgNPs were synthesized via UV photoreduction in the presence of the resultant amylopectin 11-((3-carboxyl)ethylthio)undecanoate (APUE3-MPA) in water−tetrahydrofuran binary system. QAgNPs showed novel uniform quadrilateral shapes with lamellar structure, as verified by their wide-angle X-ray scattering patterns. The average interlayer distance was around 1.3 nm, whereas the average edge lengths of QAgNPs varied between 0.29 ± 0.07 and 1.09 ± 0.25 μm. The concentration of APUE3-MPA and the amount of water in the reaction system strongly affected the shapes of QAgNPs. Thus, the reaction system and the arrangement of numerous carboxyl groups were the key factors for the formation of lamellar-structured QAgNPs.

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INTRODUCTION Nanosilver with various shapes including nanoparticles, nanowires, nanobelts, and nanoplates has been extensively investigated for the past decades because of their wide potential applications in many fields, such as catalysis, optoelectronic devices, and chemical and biological sensing.1−8 Among these silver nanostructures, two-dimensional (2D) silver nanostructures have attracted particular attention, motivated by their unique and interesting optical properties.9−11 For instance, the extinction peak of silver nanospheres with the diameter of 40 nm is located at about 400 nm. Only small shifts of the peak could be observed via changing the size of the silver nanospheres.12 In contrast, silver nanoplates could display resonance in the near-infrared range (NIR),13 which endows them with the ability of being used in optical communication, photodynamic therapy, and biological imaging because of the deeper penetration depth of the NIR light.14 Moreover, the resonance wavelength could be tuned in a wide range by changing the shape and size of the silver nanoplates. Various methods have been developed for synthesizing 2D © 2018 American Chemical Society

silver nanostructures. Generally, various synthetic polymers, such as polyacrylamide (PAM), polyacrylic acid, polyvinylpyrrolidone, or small molecules with functional groups (carboxyl, amino, and hydroxyl groups) were used for this purpose because they can selectively adsorb on certain crystal facets or be utilized as templates during the synthesis process. These properties can promote the anisotropic growth, thus facilitating the formation of 2D silver nanostructures.10,15,16 Citrate was once recognized as the key component for producing of silver nanoplates until it was reported to be replaced by many other di- and tricarboxylate compounds with two or three carboxylate groups.17 The presence of high density of carboxylate or amino groups seems to be advantageous for the synthesis of 2D silver nanostructures. Xie et al. systematically studied the synthesis of silver nanostructures using peptides with different ratios of Received: April 27, 2018 Accepted: June 14, 2018 Published: June 25, 2018 6841

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Figure 1. (a) Schematic illustration for the synthesis of QAgNPs stabilized by AUPE3-MPA. (b) Chemical structure of APUE3-MPA. (c) FTIR spectra of amylopectin, APUE3 and APUE3-MPA. (d) Thermogravimetric curves of amylopectin, APUE3 and APUE3-MPA.

carboxylic groups to per peptide unit.18 Peptides without a carboxyl group resulted in irregularly shaped and spherical Ag nanocrystals, whereas higher yield of 2D Ag nanoplates was obtained using peptides with more carboxyl groups per unit. The selective adsorption of carboxyl groups to certain crystallographic planes and the moderation of the Ag+ reduction kinetics via the interaction with carboxyl groups jointly facilitate the formation of silver nanoplates. Xiong and co-workers reported that PAM could assist the formation of silver nanoplates through mediating the reduction rate via the complex formed between Ag+ and amino groups of PAM.15 Other carboxyl or amino groups containing polymers, for instance, surface-carboxylated polystyrene spheres were also used as templates to synthesize silver nanodisks.19 In addition, 2D silver nanostructures have also been prepared via the selfassembly of silver nanoparticles. For example, silver nanoparticles capped with various alkanethiols have been reported for the formation of 2D superlattice via a self-assembly process.20,21 Thiolate-protected silver nanocrystals were also reported as the assembly components to fabricate 2D silver nanostructures. Jia et al. investigated the synthesis of selfassembled thiolated silver nanocrystals with lamellar structures.22 The results showed that the intriguing photophysical properties of the self-assembled silver nanocrystals are closely related to their lamellar superstructures. Li and Wang also reported the self-assembly of p-aminothiophenol (PATP)capped silver nanocrystals into lamellar-structured nanoleaves and proposed their potential applications as “molecular junctions”.23 Over the past decade, much effort has been devoted to the synthesis of metal nanostructures via the assistance of natural polymers, such as cellulose, starch, chitin, alginate, and their derivatives, owing to their biodegradability, sustainability, and versatile modifications.24−29 However, zero-dimensional nanoparticles were always obtained in these previous studies. Employing native polymers to assist the preparation of 2D silver nanostructures was rarely reported. Until recently, Tufenkji and co-workers reported the synthesis of silver nanostructures in various shapes by irradiating the aqueous solution of AgNO3 with suspended cellulose nanocrystals (CNCs) using UV light.30 The results showed that the carboxyl content of CNC had a great effect on the shapes of the obtained silver nanostructures. Triangular silver nanoplates were obtained with CNC containing low contents of carboxyl

groups. Higher contents of carboxyl groups resulted in a mixture of hexagonal nanosheets and flowerlike/dendric structures. Although this work showed the possibility of forming 2D silver nanostructures with the assistance of CNC, silver nanoplates with uniform shapes could not be obtained. Until now, to our best knowledge, native polysaccharides, such as starch, cellulose and chitin as well as their derivatives have never been reported to be used for the synthesis of uniform 2D silver nanoplates, let alone of lamellar structures. Compared with cellulose and other native polymers, amylopectin is a native polysaccharide with a hyperbranched structure and many branched chains.31,32 Because of the presence of numerous hydroxyl groups within a much more compact space in comparison with linear polymers or most other polysacchrides, a wide variety of functional groups in a high density can be introduced onto its backbone.33 Herein, amylopectin derivatives, namely amylopectin 11((3-carboxyl)ethylthio)undecanoate−3-mercaptopropionic acid (APUE3-MPA) was obtained after the esterification of amylopectin with 10-undecenoyl chloride and further modification with MPA via thiol−ene click chemistry. Then, quadrilateral silver nanoplates (QAgNPs) were synthesized via the UV-induced photoreduction of AgNO3 in the presence of APUE3-MPA, which acted as both stabilizer and mediator to control the shape of the silver nanoplates. Finally, the structure and morphology of the obtained silver nanoplates was systematically analyzed.

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RESULTS AND DISCUSSION Synthesis and Characterization of APUE3-MPA. The synthesis of APUE 3 -MPA and APUE 3 -MPA-stabilized QAgNPs is shown schematically in Figure 1a. APUE3-MPA with a high density of carboxyl groups were obtained via the total esterification of amylopectin with 10-undecenoyl chloride and then modified with MPA via the UV-initiated thiol−ene click reaction (Figure 1b). Compared with the Fouriertansform infrared spectroscopy (FTIR) spectrum of amylopectin (Figure 1c), APUE3 showed the new FTIR peaks at 1745 cm−1 ascribed to CO stretching vibration and at 1644 cm−1 because of vibrations of vinyl groups, while the band at 3292 cm−1 ascribed to vibrations of hydroxyl groups disappeared. Thus, all hydroxyl groups were totally substituted by 10-undecenoyl groups.34 According to elemental analysis, the average degree of substitution (DS) ascribed to 106842

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Figure 2. TEM images of silver nanoplates prepared with 1 mL of 0.02 M aqueous AgNO3 solution and 10 mL of THF solutions of APUE3-MPA with various concentrations: (a) 0.1; (b) 0.5; (c) 1; (d) 2; (e) 4 mg/mL. (f) Control sample prepared with 1 mL of AgNO3 (0.02 M) and 10 mL of THF solution of APUE3 instead of APUE3-MPA (2 mg/mL). Reaction time: 2 h.

In comparison, control experiment using 10 mL of APUE3 solution (2 mg/mL) and 1 mL of aqueous AgNO3 solution (0.02 M) only resulted in silver nanospheres with the sizes of several dozens of nanometers (Figure 2f). This result is also in accordance with the general formation of zero-dimensional silver nanostructures in an aqueous reaction system, such as using other native polymers (sodium alginate and starch) or synthetic polymers without or without enough carboxyl groups.18,24,37−39 These results indicate that the existence of a high-density carboxyl groups in APUE3-MPA facilitated the formation of QAgNPs. Moreover, the sizes of the resultant QAgNPs were greatly affected by the concentration of APUE3MPA, which allows the adjustment of the sizes of QAgNPs. Furthermore, the state of APUE3-MPA in the mixture exhibited a great effect on the formation of the silver nanostructures. The addition of water (as a poor solvent) in 1 mL of aqueous AgNO3 solution to THF solutions of APUE3MPA (Figure S2a) generally lead to white aggregation of APUE3-MPA in the mixture. This aggregation should in fact be unfavorable for the formation of silver nanostructures with any uniform regular shape. In contrast, regular quadrilateral shape was obtained as shown above using APUE3-MPA as the polymeric matrix, in particular, if its amount was sufficient. As the silver nitrate aqueous solution was added, the hydrophilic carboxyl groups were rearranged to expose to the water phase and provided nucleation sites for the formation of silver nanostructures. The dispersion became light yellow during the proceeding reaction (Figure S2b), which indicated the formation of the silver nanostructures. At low concentrations, APUE3-MPA (0.1 mg/mL) could not provide enough carboxyl groups for the formation of platelike silver nanostructures. As the concentration of APUE3-MPA increased up to a certain extent (0.5 mg/mL or more), uniform quadrilateral-shaped silver nanoplates were obtained (Figure 2b−e). This is predominantly because of the presence of a high density of carboxyl groups which are exposed to the water phase during the aggregation of APUE3-MPA chains.40,41 The aggregated APUE3-MPA should have provided sufficient carboxyl groups as nucleation sites for the formation of silver nanoplates. Considering the amphiphilic property of the as-prepared APUE3-MPA and the water−THF binary reaction system, the amount of added water strongly affected the formation of QAgNPs. The effects of a few critical parameters including the

undecenoyl groups was 3, which was in accordance with the results obtained from FTIR. The slight decrease of the signal intensity attributed to the vibrations of alkene CC at 1644 cm−1. The emergence of a new broad band between 3160 and 3400 cm−1 because of O−H stretching vibrations of the carboxyl groups shown in the FTIR spectrum of APUE3-MPA indicated a partial modification of CC bonds with MPA. The average DS ascribed to MPA groups was determined to be 0.99 using elemental analysis. The thermal properties of amylopectin, APUE3, and APUE3MPA were characterized by thermogravimetric analysis (TGA) (Figure 1d). After the esterification, the degradation onset temperature increased from 300 °C (amylopectin) to about 377 °C (APUE3). The enhanced thermal stability of APUE3 is ascribed to the total substitution of the hydroxyl groups by more stable alkane chains.35 In comparison, APUE3-MPA showed a lower degradation onset temperature (349 °C) than that of APUE3, which is ascribed to introduced carboxyl groups. The carboxyl groups can decarboxylate on heating, thus lower the thermal stability of APUE3-MPA.36 Synthesis of QAgNPs Stabilized by APUE3-MPA. As mentioned above, the formation of 2D silver nanoplates can be facilitated by carboxyl groups of a high density. Thus, APUE3MPA with carboxyl groups of a high density were further employed to synthesize silver nanoplates. By using 10 mL of tetrahydrofuran (THF) solution of APUE3-MPA with various concentrations and 1 mL of aqueous AgNO3 solution (0.02 M), QAgNPs were primarily obtained according to their transmission electron microscopy (TEM) images (Figure 2). Using the APUE3-MPA/THF solution with a low concentration of 0.1 mg/mL, only aggregated silver nanoparticles were formed (Figure 2a). When the concentration of APUE3-MPA was increased to 0.5 mg/mL (Figure 2b), silver nanoplates with the regular quadrilateral shape were observed. By increasing the concentration of APUE3-MPA solutions to 1 mg/mL (Figure 2c), 2 mg/mL (Figure 2d) and 4 mg/mL (Figure 2e), silver nanoplates with similar quadrilateral shapes but distinct sizes were generated. The average edge lengths (Figure S1) of silver nanoplates shown in Figure 2b−e were 0.29 ± 0.07, 0.60 ± 0.09, 0.73 ± 0.13 and 0.70 ± 0.14 μm, respectively. Furthermore, although the QAgNPs shown in Figure 2c,d have similar average edge lengths, solutions with higher concentrations of APUE3-MPA resulted in broader size distributions. 6843

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from the two spin−orbit split S 2p3/2 and S 2p1/2 spectra features at a binding energy 162.0 and 163.1 eV with an fwhm of 1.14 and 1.52, respectively. The presence of these signals indicates the interaction between sulfur and carbon (C−S− C).47 Two peaks ascribed to Ag 3d were identified at 368.3 and 376.4 eV both with a fwhm of 1.3 (Figure 4c). These peaks are attributed to the two spin−orbit coupling of Ag 3d into Ag 3d5/2 and Ag 3d3/2. On the basis of the peak position of Ag 3d5/2 at 368.3 eV and Ag 3d3/2 at 376.4 eV, it can be concluded that elemental silver [Ag(0)] was present in the samples.48 To monitor the formation process of QAgNPs, diverse QAgNPs were prepared with 10 mL of THF solution of APUE3-MPA (2 mg/mL) and 1 mL of aqueous AgNO3 solution (0.02 M), whereas the reaction times were varied between 20 s and 5 h. The samples were immediately processed and measured on TEM, as shown for the reaction times of 20 s, 40 s, 1 min, 5 min, 10 min, 60 min, 2 h, and 5 h (Figure 5). It can be seen from Figure 5a that silver nanoparticles with sizes of several nanometers were formed already at the beginning (20 s) of the reaction. Embryonic platelike nanostructures were constructed, when the reaction was prolonged to 40 s (Figure 5b). Nanoplates were already present after 1 min reaction, indicating a fast formation process of potential silver nanoplates. When the time increased to 5 min, more silver nanoplates with regular quadrilateral shape were generated. This indicates a fast reduction of silver ions (at least fivefolds) occurred in our reaction system in comparison with previous work, for example, the CNCs-assisted photoreduction synthesis of anisotropic silver nanoparticles.30 Furthermore, with further increase of the reaction time, no obvious change of the shapes was observed. In contrast, the average lateral length of QAgNPs varied depending on the reaction time. According to the statistical result (Figure S3), the average lateral lengths of the silver nanoplates obtained for 5, 10, and 60 min were 1.09 ± 0.25, 1.02 ± 0.19, and 1.03 ± 0.19 μm, respectively, whereas longer reaction time resulted in a slight decrease in average lateral edge length (0.73 ± 0.13 and 0.79 ± 0.14 μm for silver nanoplates prepared for 2 and 5 h, respectively). When AgNO3 solution was added into the THF solution of APUE3-MPA, silver ions would complex with the carboxyl groups of APUE3-MPA. At the same time, APUE3-MPA binding with Ag+ formed complexes within the water−THF binary system. The aggregation of APUE3-MPA induced by adding water would make more carboxyl groups expose to water phase and interact with Ag+. The numerous nucleation sites facilitate the anisotropic growth of QAgNPs. After 1 min UV irradiation, QAgNPs with obvious quadrilateral shapes already formed. The strategy of immobilizing silver ions with carboxyl groups containing materials for further formation of

reaction time and the amount of water will be discussed further below. The WAXD pattern of QAgNPs prepared with 10 mL of APUE3-MPA/THF solution (2 mg/mL) and 1 mL of aqueous AgNO3 solution (0.02 M) for 2 h indicates the presence of lamellar structures inside the nanoplates, as revealed by 10 significant periodic diffraction peaks between 6.6 and 70° (Figure 3a). The average interlayer distance (Table S1) was

Figure 3. (a) Wide-angle X-ray scattering (WAXD) pattern of APUE3-MPA-stabilized QAgNPs prepared with 10 mL of APUE3MPA/THF solution with the concentration of 2 mg/mL and 1 mL of aqueous AgNO3 solution (0.02 M). Reaction time was 2 h. (b) The average interlayer spacing calculated according to Bragg’s law.

calculated to be 1.342 ± 0.002 nm (Figure 3b). The formation of the lamellar-structured silver nanoleaves has been reported via a self-assembly method.23 The presynthesized silver nanoparticles were first etched by PATP into silver nanoclusters (Ag25). Then, the formed Ag25−PATP−Ag25 complexes self-assembled into lamellar-structured 2D silver nanoleaves driven by the π−π stacking force and the strong dipole− dipole interaction between the neighboring benzene rings of PATP. In contrary, lamellar-structured silver nanoplates have never been synthesized with the assistance of polysaccharides and their derivatives. The chemical surface analysis was performed by using X-ray photoelectron spectroscopy (XPS). Figure 4a−c shows the detailed spectra of C 1s, S 2p, and Ag 3d of QAgNPs prepared in 2 h with 10 mL of APUE3-MPA/THF solution (2 mg/mL) and 1 mL of aqueous AgNO3 solution (0.02 M). For the identification of the chemical states, the spectra were shifted to the main peak of C 1s at 284.6 eV.42 The C 1s peaks (Figure 4a) show the presence of four components. The main peak corresponds to CC and C−C bonds, with a full width at half maximum (fwhm) of 1.3. The peak at 285.7 eV (fwhm of 1.5) indicates the presence of C−O and C−S bonds.43,44 The fourth peak at 288.9 eV with a fwhm of 1.4 can be assigned to the carboxyl group (O−CO).45 The peak at 286.9 eV with a fwhm of 1.5 can be interpreted as the carbon of the carbonyl groups.46 The sulfur 2p (Figure 4b) exhibits a doublet arising

Figure 4. Representative XPS high-resolution spectra of (a) oxygen, (b) sulfur, and (c) silver. 6844

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Figure 5. TEM images of QAgNPs prepared with 10 mL of THF solution of APUE3-MPA (2 mg/mL) and 1 mL of aqueous solution of AgNO3 (0.02 M) after distinct reaction times: (a) 20 s; (b) 40 s; (c) 1 min; (d) 5 min; (e) 10 min; (f) 60 min; (g) 2 h; and (h) 5 h. Scale bars: 200 nm in (a//b) as well as 1 μm in (c−h).

Figure 6. TEM images of the samples prepared with 10 mL of THF solution of APUE3-MPA (2 mg/mL) and the solution of constant amount of AgNO3 dissolved in water−THF mixtures with various volume ratios. The solutions of AgNO3 in water−THF mixtures were denoted as AgNO3X/Y: (a) 1 mL of AgNO3-1/9 (0.02 M), (b) 1 mL of AgNO3-5/5 (0.02 M), and (c) 2 mL of aqueous solution of AgNO3 (0.01 M, only water as solvent for AgNO3). (d−f) WAXD patterns of the samples (a−c), respectively.

(0.02 M), 1 mL of AgNO3-5/5 (0.02 M), or 2 mL of aqueous AgNO3 solution (0.01 M) had more irregular shapes. This difference is ascribed to the diverse amounts of water used in the reaction systems. Water not only induced the aggregation of APUE3-MPA to certain extent but also facilitated the dispersion of the Ag+ and the arrangement of carboxyl groups via the interaction with water molecules. With less water (1 mL of AgNO3-1/9 and 1 mL of AgNO3-5/5) in the reaction system, a worse dispersion of Ag+ and less carboxyl groups exposing to water would be expected. Whereas using 2 mL of AgNO 3 aqueous solution (0.01 M), the APUE 3-MPA aggregation would be more. Thus, an inappropriate amount of water is more likely to result in silver nanoplates with irregular shapes (Figure 6a−c). In addition, the amount of water also affected the sizes of the resultant silver nanostructures. The QAgNPs (Figures 2d and 5) have the largest average sizes, whereas the silver nanostructures prepared with 1 mL of AgNO3-1/9 (Figure 6a) have the smallest average sizes of about several dozens of nanometers. In contrast to the big difference of shapes, these silver nanostructures all have similar lamellar structure according to their WAXD patterns (Figures 3a and 6d−f). However, less periodic peaks were detected for irregular silver nanostructures (Figure 6d−f) compared with regular QAgNPs (Figure 3a). The average interlayer distance (Tables S2−S4) for the silver nanostructures shown in Figure 6a−c was calculated to be 1.343 ± 0.004, 1.342 ± 0.004, and 1.340 ± 0.003 nm,

silver nanoparticles have been employed by many other groups.49−51 In addition, it was reported that UV photoreduction of Ag+ become easier when Ag+ are bonded to carboxyl groups.52 Thus, the fast reduction of Ag+ happened because of the existence of large number of carboxyl groups on the backbone of APUE3-MPA. Obviously, uniform QAgNPs formed within shorter time in comparison with the similar photoreduction method previously reported.30,53 As mentioned above, the role of water in the formation of QAgNPs needs to be taken into account because of its influence on the aggregation of APUE3-MPA, the arrangement of carboxyl groups, and the dispersion of Ag+. To investigate the effect of added water, diverse mixtures of water and THF with volume ratios of 1/9 and 5/5 were used as the cosolvent to prepare AgNO3 solution. The corresponding solutions were denoted as AgNO3-1/9 and AgNO3-5/5. These solutions and the aqueous solution of AgNO3 were further used for the synthesis of silver nanoplates, while keeping the amount of AgNO3 in each reaction system constant. By using 10 mL of APUE3-MPA (2 mg/mL) and 1 mL of AgNO3-1/9 (0.02 M), 1 mL of AgNO3-5/5 (0.02 M) or 2 mL of aqueous AgNO3 solution (0.01 M) for the preparation of silver nanostructures with diverse shapes and sizes were obtained (Figure 6a−c). Interestingly, compared with the silver nanoplates (Figures 2d and 5) prepared with 1 mL of AgNO3 aqueous solutions (0.02 M) and 10 mL of APUE3-MPA (2 mg/mL), the silver nanoplates (Figure 6a−c) resulted from 1 mL of AgNO3-1/9 6845

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repeated for at least four times. Finally, the obtained APUE3 was dissolved in THF and stocked for further use. Elemental analysis found in wt %: C, 71.01; H, 9.75. Synthesis of Amylopectin 11-((3-carboxyl)ethylthio)undecanoate (APUE3-MPA) via UV-Initiated Thiol−Ene Reaction of APUE3 with MPA. APUE3-MPA was synthesized via a UV-initiated thiol−ene click reaction without any other additives. For the reaction, a certain amount of MPA (3 mol per mol CC double bonds of APUE3) was added into the APUE3 solution in THF (20 mg/mL). Then, the mixture was irradiated with UV light (320−400 nm with the intensity of ∼100 mW/cm2) for 3 h at room temperature. After the reaction, the APUE3-MPA solution in THF was dialyzed in THF for 1 week with refreshment of THF every 24 h. Elemental analysis found in wt %: C, 72.25; S, 3.36. Synthesis of APUE3-MPA-Stabilized Silver Nanoplates. Silver nanoplates were synthesized using APUE3MPA as stabilizer via UV irradiation, which has proven to be a straightforward method for synthesizing silver nanostructures.57 Typically, 1 mL of aqueous AgNO3 solution (0.02 M) was added drop by drop within 1 min using a pipette (Eppendorf, volume 100−1000 μL) to 10 mL of the asprepared APUE3-MPA/THF solutions (0.1, 0.5, 1, 2, and 4 mg/mL) under stirring. Then, the mixtures were exposed to UV light with stirring for 2 h. To observe the formation process of the silver nanoplates, samples were also prepared with 1 mL of aqueous AgNO3 solution (0.02 M) and 10 mL of APUE3-MPA/THF (2 mg/mL) solution for different reaction times (1, 5, 10, 60 min, 2, and 5 h). In addition, to investigate the effect of water on the reaction in (water−THF) binary system, samples were also prepared with 10 mL of APUE3MPA/THF solution (2 mg/mL) and a constant amount of AgNO3 dissolved in water−THF mixtures with various volume ratios (water−THF), which were denoted as AgNO3-X/Y: (a) 1 mL of AgNO3-1/9 (0.02 M), (b) 1 mL of AgNO3-5/5 (0.02 M), and (b) 2 mL of aqueous solution of AgNO3 (0.01 M, only water as solvent for AgNO3), reaction time of 2 h. All of the experiments were conducted with UV light of 320−400 nm at the intensity of about 100 mW/cm2. Characterization. Fourier-Transform Infrared Spectroscopy. APUE3 and APUE3-MPA were precipitated by adding five times the volume of ethanol into APUE3/THF and APUE3-MPA/THF solution. The precipitates were collected via centrifugation at 12 000 rpm for 10 min. The obtained solid APUE3 and APUE3-MPA were dried in a vacuum oven at 80 °C overnight prior to the FTIR measurement. FTIR measurements were conducted on a BRUKER ALPHA FTIR spectrometer (Bruker, Germany) equipped with a versatile high throughput ZnSe attenuated total reflection crystal. Spectra were recorded between 4000 and 400 cm−1 with a resolution of 4 cm−1 and accumulated 24 scans. Elemental Analysis. The contents of carbon, hydrogen, and sulfur were determined with an Elemental Analyser 4.1 vario EL III (Elementar, Germany). The total DS was calculated according to the reference previously reported.27 Thermogravimetric Analysis. TGA was performed under nitrogen flow of 20 mL/min using a Netzsch TG209 instrument. Samples of about 10 mg in crucibles were treated from 20 to 700 °C at a heating rate of 20 °C/min. Transmission Electron Microscopy. TEM observation was performed under 120 kV using a CM 12 TEM (Philips, Netherland). Obtained QAgNP dispersions stabilized by APUE3-MPA were diluted fivefold with THF, before a drop

respectively. Thus, the volume of water present in the reaction system has a great effect on the shapes of final silver nanostructures, but it only slightly influences the formation of lamellar structures. In particular, it should be noted that the mechanism of the reaction in a binary system resulting in polymeric aggregates was more complicated than using singlephase homogeneous reaction conditions. Instead, the parallelogram shapes of obtained QAgNPs are easily formed and can even be tuned by changing the amount of water in the reaction system. Thus, we presented herein a novel strategy for preparing a novel group of QAgNPs with lamellar structures using derivatized native amylopectin as substrates. Such large QAgNPs are of great interest for diverse potential applications, such as catalysis, antibacterial surface, and sensing.2,54−56 It should be noted that the aggregation formed within the binary system greatly hinders the exploration of further potential applications. Several methods including washing via solvent exchange, ligand exchange, and acid hydrolysis were not successful to remove the aggregation of APUE3-MPA, whereas further study is required to effectively separate QAgNPs from polymer matrix.

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CONCLUSIONS

In summary, we synthesized a carboxyl group-rich polymer (APUE3-MPA) derived from amylopectin, a kind of natural hyperbranched polysaccharide. APUE3-MPA with a high density of carboxyl groups was utilized to synthesize silver nanostructures in a binary (water and THF) system. Unlike many other silver nanostructures formed with the assistance of small molecules, synthetic polymers or polysaccharides, 2D silver nanoplates synthesized in the presence of APUE3-MPA exhibited regular quadrilateral shapes. Obtained QAgNPs has lamellar structure with an average interlayer spacing of around 1.3 nm, as demonstrated by their WAXD patterns with up to 10 periodic peaks. In addition, the APUE3-MPA-stabilized QAgNPs have a lateral size ranging from about 100 nm to more than 1 μm. Furthermore, we found that the volume of water strongly affected the shapes of the resultant silver nanoplates. The binary (water−THF) reaction systems and the presence of carboxyl groups in a high density were the key factors determining the structures and shapes of QAgNPs.

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EXPERIMENTAL SECTION Materials. Amylopectin from maize and 10-undecenoyl chloride were purchased from Sigma-Aldrich. Silver nitrate (ACS reagent, ≥99.0%) and MPA (≥99.0%) were obtained from Merck KGaA. All other chemicals of analytical grade were received from VWR (Darmstadt, Germany) and used without further purification. Ultrapure water was used for all experiments. Synthesis of Amylopectin 10-Undecenoyl Ester with a DS of 3 (APUE3). Prior to the typical synthesis of APUE3, amylopectin (2 g) was dried in a vacuum oven at 80 °C overnight. Then, amylopectin was suspended in 60 mL of pyridine in a three-neck flask. 10-undecenoyl chloride (16.8 mL) was dropped into the pyridine suspension of amylopectin under stirring, and the temperature was raised to 100 °C. After 4 h reaction, the reaction mixture was poured into 300 mL of ethanol. The precipitate was collected by centrifugation and redissolved into THF after removing the solvent. To remove the impurities, the precipitation−redissolution process was 6846

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of each diluted QAgNPs dispersion was placed onto the copper grid. After that, the sample was dried at ambient temperature. Wide-Angle X-ray Diffraction. WAXD measurements were conducted on a Bruker D8 X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å) in the range of 4°−80° at a scanning rate of 2°/min with the scan interval of 0.05°. X-ray Photoelectron Spectroscopy. The XPS measurements were performed on a PHI 5000 Versa Probe II (ULVAC-PHI, Chigasaki, Japan) with a monochromatic Al Kα radiation with a spot size of 200 μm and a photon energy of 1486.6 eV. Detailed spectra were captured at a pass energy of 46.95 eV and a step size of 0.1 eV for carbon (C 1s), silver (Ag 3d), and sulfur (S 2p).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b00833. Particle size distributions, photo images of the reaction system before and after the reaction, and calculated interlayer distances (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +49551394505. ORCID

Xiaohui Wang: 0000-0002-3769-2325 Kai Zhang: 0000-0002-5783-946X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS B.P. and H.L. appreciate financial support from China Scholarship Council (CSC) for their PhD grants. K.Z. thanks Fonds der Chemischen Industrie (FCI) for the financial support and Georg-August-University of Goettingen for the start-up funding. R.K. and W.V. thanks the German Research Foundation (DFG) for the provision of the XPS and the German Federal Ministry of Education and Research (BMBF) with financial support. We thank Dr. Yuansu Luo from the I Physical Institute, Georg-August-University of Goettingen, for the support by X-ray diffraction analysis.

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REFERENCES

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DOI: 10.1021/acsomega.8b00833 ACS Omega 2018, 3, 6841−6848

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DOI: 10.1021/acsomega.8b00833 ACS Omega 2018, 3, 6841−6848