Artificial Nacre-Like Gold Nanoparticles–Layered Double Hydroxide

Article pubs.acs.org/IECR

Artificial Nacre-Like Gold Nanoparticles−Layered Double Hydroxide−Poly(vinyl alcohol) Hybrid Film with Multifunctional Properties Yingqi Shu,† Penggang Yin,*,‡ Benliang Liang,‡ Hao Wang,*,† and Lin Guo‡ ‡

Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, Beihang University, Beijing 100124, P. R. China † The College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, P. R. China S Supporting Information *

ABSTRACT: Gold nanoparticles−layered double hydroxide−poly(vinyl alcohol) (Au NPs−LDH−PVA) hybrid films have been successfully prepared for the first time by bottom-up assembly of pretreated Au NPs−LDH nanosheets and subsequent spin-coating of PVA. The cross sections of the as-prepared hybrid films resemble the brick-and-mortar structure of nacre. The tensile strength of the Au NPs−LDH−PVA hybrid film reaches a high value of 122 MPa, which is higher than that of a pure PVA film and surpasses the strength of natural nacre. Furthermore, we demonstrate that Au NPs−LDH−PVA hybrid films possess surface-enhanced Raman scattering and catalytic properties. Therefore, our report on the fabrication of multifunctional Au NPs− LDH−PVA hybrid films not only shows a feasible route to functionalizing artificial nacre-inspired materials but also potentially broadens applications of these nacre-like materials.

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INTRODUCTION Nacre, whose exceptional mechanical properties are superior to many traditional high-performance materials,1,2 owns an optimized hierarchical structure to realize the combination of low weight and remarkable hardness, high strength, and toughness. Its microscale architecture resembles a threedimensional “brick-and-mortar” structure, in which inorganic aragonite layers with submicrometer thickness tightly stacked together by nanoscale biopolymer layers.3 Owing to its delicate hierarchical structure and outstanding mechanical properties, nacre has attracted considerable attention of materials scientists to study and mimic its architecture, aimed at developing analogous nacre-inspired materials.4−11 For example, Kotov and co-workers employed layer-by-layer (LBL) deposition in the design of a montmorillonite clay platelet−poly(vinyl alcohol) (PVA) matrix nacre−mimetic artificial nanocomposite; 5 Bonderer et al. assembled Al2O3 platelets on the air−water interface and developed spin coating into the fabrication of lamellar Al2O3−chitosan hybrid films;6 ice-crystal templates of the microscopic layers were designed to form a brick-andmortar microstructured Al2O3−poly(methyl methacrylate) composite by Deville et al.8 A series of platelet-like inorganic building blocks are selected to assemble with polymers in order to mimic the “brick-andmortar” microstructure of nacre, e.g. nanoclays,12−16 graphene oxide,17−19 Al2O3,6,20 etc. Among them, layered double hydroxides (LDHs) receive considerable attention and have been widely applied in the field of ion-exchange materials,21−23 optical functional materials,24−26 catalyst support,27−29 fireretardant materials,30 etc. PVA is a water-soluble polyhydroxy polymer that has been studied intensively because of its various desirable characteristics, such as biocompatibility, biodegradability, and water solubility.31−33 Hence, LDHs and PVA, as © 2015 American Chemical Society

two of the most favored materials, have been used as essential elements for fabricating novel artificial nacre-like materials. For instance, Han et al.34 fabricated inorganic nanoplateletreinforced polymer films via the alternate LBL assembly of LDH nanoplatelets with PVA, which showed largely enhanced strength and good ductility simultaneously. Previously our group35 had successfully prepared multilayered PVA−LDH films for the first time by the bottom-up LBL assembly of pretreated LDH nanosheets and further spin coating of PVA. The as-prepared PVA−LDH composites were synthesized with a certain LDH aspect ratio that was high enough to carry a significant load but also small enough to allow for fracture under the mode of platelet pull-out,6,36 which led to an even larger tensile strength and strain value at rupture than that of nacre. However, probably because of the lack of a feasible strategy to introduce proper functional components in artificial bionanocomposites, the functional design and fabrication of multifunctional artificial nacre-like PVA−LDH materials are rarely reported. Hence, it is with great significance that multiple functional performances besides mechanical properties in bioinspired PVA−LDH films are achieved for further development in related applications.37 With the purpose of introducing multifunctionalities into nacre-like layered PVA−LDH hybrid films, gold nanoparticles (Au NPs) were previously chosen because of their facile assemblies, fascinating electronic, optical, and chemical properties, and wide applications in traditional functional nanocomposites,38−42 such as advanced electronic sensors,43 nonReceived: Revised: Accepted: Published: 8940

April 22, 2015 August 19, 2015 August 26, 2015 August 26, 2015 DOI: 10.1021/acs.iecr.5b01518 Ind. Eng. Chem. Res. 2015, 54, 8940−8946

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Industrial & Engineering Chemistry Research linear optics,44 biosensors,45 and electrodes for catalytic systems.46 Thus, the decoration of functional Au NPs onto the surface of inorganic LDH nanosheets to fabricate artificial nacre-like polymer−LDH hybrid films with well-defined nanostructures is highly expected. To efficiently deposit Au NPs, LDH nanosheets were previously modified with (3-aminopropyl)triethoxylsilane (APTES) to endow abundant amino groups, which can enhance interaction with Au NPs through coordination bonds.40,47 In particular, hydroxyl and amine groups on the surface of Au NPs−LDH hybrid platelets can easily interact with the oxygen atoms of the PVA backbone to form abundant hydrogen bonding. Thus, multifunctional Au NPs−LDH−PVA hybrid films have been successfully fabricated for the first time by the bottom-up LBL assembly of pretreated Au NPs−LDH nanosheets and subsequent spin coating of PVA. These films exhibit well-defined layered structures, resembling the brickand-mortar structure of nacre. The tensile strength of the Au NPs−LDH−PVA (WAu:WLDH = 0.5:1) hybrid film reaches a high value of 122 MPa, which is higher than that of a pure PVA film and surpasses the strength of natural nacre. Moreover, we demonstrate that these Au NPs−LDH−PVA hybrid films possess surface-enhanced Raman scattering (SERS) and catalytic properties, indicating the efficient functionalization of Au NPs to lamellar hybrid films.

the reaction system was 0.5:1, 1:1, 2:1, and 4:1, respectively. Then the modified LDH−APTES platelets were washed by repeated centrifugation with deionized water and finally redispersed in ethanol. These suspensions of Au NPs−LDH hybrid nanosheets were further used to fabricate hybrid films. Fabrication of Au NPs−LDH−PVA Hybrid Films. A total of 2 g of PVA was dissolved in deionized water (100 mL) under stirring at 90 °C with a concentration of 2 wt %. The fabrication of nacre-like Au NPs−LDH−PVA hybrid films was in accordance with the relevant reference.35 (i) A total of 1 mL of a PVA solution was dropped onto the surface of a 2.4 cm × 2.4 cm glass substrate, and then the substrate was spin-coated at 1000 rpm for 1 min to form a flat organic layer of PVA. The wet PVA layer was dried at 50 °C in an oven for 5 min. (ii) The suspension of Au NPs−LDH hybrid nanosheets was dropped on the air−water surface in a beaker until a new visible monolayer emerged. Then the beaker was sonicated for 15 min in order to form a dense and homogeneous Au NPs−LDH film at the air−water interface. Then the Au NPs−LDH thin film was transferred to the glass substrate with the PVA layer by carefully manually dip coating. The substrate was dried at 50 °C. (iii) A series of alternative deposition operations for the PVA and Au NPs−LHD layers were repeated n times to fabricate Au NPs−LDH−PVA hybrid films (Scheme 1). Typical hybrid films consisted of 20 layers of inorganic platelets.

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EXPERIMENTAL SECTION Synthesis of LDH. The synthesis of CoAl−CO3 LDH was based on previous reports by the hydrolysis method.48 A typical preparation process is described as follows: CoCl2·6H2O, AlCl3· 6H2O, and urea were dissolved in 100 mL of deionized water to give the final concentrations of 10, 5, and 35 mM, respectively. The aqueous mixture was allowed to react in a 100 mL Teflonlined autoclave at 100 °C for 24 h. After cooling to room temperature, the solid products were filtered, subsequently washed several times with deionized water and anhydrous ethanol, and finally air-dried at room temperature. Preparation of Au NPs. Au NPs were prepared by the citrate thermal reduction method.49,50 In a typical procedure, an aqueous solution of HAuCl4 (100 mL, 0.01 wt %) was prepared in a flask and heated to boiling by an oil bath at 110 °C under stirring. Then 1.2 mL of a trisodium citrate solution (1.0 wt %) was added into the flask. A violet-red solution containing monodisperse Au NPs was obtained after further stirring for about 15 min. The solution was cooled through magnetic stirring at room temperature for further use. This resulted in a stable dispersion of Au NPs with an average diameter of around 30 nm. Au NPs−LDH Hybrid Nanosheet Synthesis. A total of 0.2 g of LDH nanoplatelets was surface-modified with (3aminopropyl)triethoxylsilane (APTES), as reported previously.6,35,51 Briefly, prior to surface modification, APTES (10 mL), methanol (25 mL), and deionized water (75 mL) were mixed and stirred for 1 h in order to completely hydrolyze the silane species. Surface modification was then accomplished by adding LDH platelets synthesized as described above into the hydrolyzed APTES solution, followed by ultrasonication for 5 min. After sonication, the suspension was stirred for 30 min at 40 °C. The modified LDH platelets were washed several times by repeated centrifugation with pure ethanol and deionized water. The obtained LDH−APTES nanoplatelets were redispersed in 21, 42, 84, and 168 mL of an Au NP solution via stirring for 12 h, and the mass ratio of Au NPs and LDHs in

Scheme 1. Schematic Illustration of the Synthetic Strategy of Artificial Nacre-like Au NPs−LDH−PVA Hybrid Films

Catalytic Reduction of 4-Nitrophenol (4-NP). The reduction of 4-NP was carried out in a quartz cuvette and monitored using UV−vis spectroscopy at room temperature. In a typical process, the aqueous solutions of 4-NP (0.1 mM) and sodium borohydride (NaBH4, 0.1 M) were freshly prepared. A total of 0.5 mL of an aqueous 4-NP solution was mixed with 2.5 mL of a fresh NaBH4 solution. The color of the 4-NP solution changed from colorless to yellow immediately when a NaBH4 solution was added. Subsequently, a piece of 2.4 cm × 0.8 cm Au NPs−LDH−PVA hybrid film coated ona glass substrate was added, and the mixed solution was immediately transferred into UV−vis measurements. Therefore, the obtained data could be designated as the value for reaction time t = 0. Afterward, the mixed solution was in situ measured continuously (every 2 8941

DOI: 10.1021/acs.iecr.5b01518 Ind. Eng. Chem. Res. 2015, 54, 8940−8946

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Industrial & Engineering Chemistry Research min) by recording optical absorption spectra to obtain successive information about the reaction. SERS Measurement Preparation. Au NPs−LDH−PVA hybrid films with different WAu:WLDH ratios were cut into the same size. Then the small piece of film was immersed into 5 mL of 1 mM 4-mercaptobenzoic acid (4-MBA) in ethanol for several hours. The sample was rinsed with absolute ethanol several times to remove the free 4-MBA molecules and dried at room temperature. The resulting Au NPs−LDH−PVA film modified with 4-MBA was then subjected to Raman measurement. For comparison, the normal Raman spectrum of a 1 mM 4-MBA solution was dropped onto a clean glass and dried at room temperature. Characterization. The morphology and dimensions of the samples were examined with a JEOL JSM-7500 scanning electron microscope, operating at an accelerating voltage of 3 kV. The acceleration voltage was increased to 10 kV for the energy-dispersive X-ray (EDX) measurements. UV−vis absorption spectra were measured using a Shimadzu-3600 spectrophotometer in the range from 300 to 800 nm. The width of the slit is 1.0 nm. Raman scattering spectra were obtained from 900 to 1300 cm−1 by a LabRAM HR800 laser Raman spectroscopy (Horiba Jobin Yvon Co. Ltd., France) with an excitation wavelength of 633 nm. The mechanical properties of hybrid films were measured under tensile mode in a universal mechanical testing machine (Shimadzu AGS-X, Japan). The films were cut into rectangular strips of approximate length 24 mm and width 3 mm. The samples were tested at a speed of 5 mm min−1. The exact cross-sectional widths and thicknesses were carefully determined by scanning electron microscopy (SEM). Tensile property values reported here represented an average of the results for tests run on at least five samples. All of the tests were performed under similar environmental conditions with the relative humidity maintained in the range of ∼20−30% and ambient temperature.

Figure 1. (a) Photographs of a alcoholic suspension of Au NPs−LDH nanoplatelets. (b) UV−vis spectra of an LDH−APTES suspension and a Au NPs−LDH hybrid nanosheets suspension with different Au NPs densities on the LDH nanosheets.

increased with the density of Au NPs because more Au NPs on the nanoplatelets could induce a stronger SPR. However, when a much higher concentration of Au NPs was used, such as WAu:WLDH = 4:1, a new absorbance peak near 650 nm was observed besides the one at 528 nm, which indicated that Au NPs were ready to aggregate with the ratio of WAu:WLDH = 4:1. Therefore, the content of Au NPs decorated on an LDH nanosheet surface was unlikely to increase any more. Further information on the distribution density of Au NPs on LDH nanosheets was obtained by SEM analysis. Figure 2

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RESULTS AND DISCUSSION 1. Structure and Morphology of the Au NPs−LDH− PVA Hybrid Films. Au NPs−LDH functional building blocks were synthesized via attachment of APTES to the surface of LDH nanosheets to endow abundant amino groups, which have a strong capability to adsorb Au NPs efficiently by coordination bonds.40,47 In addition, attachment of slightly hydrophobic amine-terminated silane species to the surfaces of assynthesized LDH platelets also increased its ability to adsorb on the air−water interface. The collected red gels of Au NPs− LDH nanoplatelets were dispersed in an ethanol solution, forming a homogeneous suspension. As shown in Figure 1a, the formation of Au NPs−LDH hybrid building blocks with different contents of Au NPs was indicated by a color change of the solution. The pure LDH solution was pink. With an increase in the distribution density of Au NPs decorated on LDH nanoplatelets, the color of the as-synthesized suspensions of Au NPs−LDH nanoplatelets gradually became darker. For example, when WAu:WLDH equaled 4:1, the suspension turned deep purple. The UV−vis absorption spectra of these Au NPs− LDH hybrid nanosheet suspensions are collected and shown in Figure 1b. Compared with the pure LDH nanoplatelets, all of the suspensions of Au NPs−LDH nanoplatelets exhibited a broad absorption at 528 nm (Figure S1), which was attributed to the surface plasmon resonance (SPR) of Au NPs with diameters of 30 nm on the LDH nanoplatelets.50,52 The absorption intensity of Au NPs−LDH hybrid nanoplatelets

Figure 2. SEM images of Au NPs−LDH nanosheets: (a−d) Au NPs− LDH nanosheets generated in the reaction system of WAu:WLDH = 0.5:1, 1:1, 2:1, and 4:1, respectively.

shows typical SEM micrographs of the as-synthesized Au NPs− LDH hybrid building blocks with different Au NPs densities. Obviously, Au NPs were homogeneously distributed on the surface of LDH nanosheets via an amine−Au coordination interaction without any aggregation. Furthermore, the area beside the Au NPs−LDH hybrid nanosheets in the SEM images was blank, without any individual Au NP appearing, which indicated the Au NPs in the system were all adsorbed on the LDH nanosheets. Notably, by control of the mass ratio of Au NPs in the suspension, the density of Au NPs deposited on 8942

DOI: 10.1021/acs.iecr.5b01518 Ind. Eng. Chem. Res. 2015, 54, 8940−8946

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homogeneously distributed in the hybrid film without aggregation. 2. Multifunctionalities of the Au NPs−LDH−PVA Hybrid Films. The reduction of 4-NP by NaBH4 was selected as a model reaction38,53,54 to study the catalytic performances of multifunctional Au NPs−LDH−PVA hybrid films. Without the as-synthesized hybrid films, no signs of reduction were observed even in a period of 3 days. However, as shown in Figure 5a, an obvious change in the UV−vis spectra had been found upon the addition of a piece of Au NPs−LDH−PVA hybrid film (WAu:WLDH = 4:1). The absorption at 400 nm significantly decreased as the reaction proceeded. Meanwhile, a new peak appeared at 295 nm and gradually increased as the reaction went on, revealing the successful reduction of 4-NP to 4-aminophenol (4-AP). The UV−vis spectra also exhibited an isosbestic point (320 nm) between two absorption bands, which indicated that the catalytic reduction of 4-NP yielded 4AP without any byproducts. Considering that the reductant concentration was much higher than that of 4-NP (CNaBH4/ C4‑NP = 100), pseudo-first-order kinetics could be applied for the evaluation of rate constants. Because the peak at 400 nm was much stronger than that at 295 nm, we decided to measure the concentrations of 4-NP and thus monitor the progress or kinetics of the reaction by recording the absorbance at 400 nm. The ratio of Ct to C0 was measured from the relative intensity of the respective absorbances (At/A0), where Ct and C0 were the concentrations of 4-NP at time t and 0, respectively. Linear relationships between ln(Ct/C0) and the reaction time (t) were obtained from the reduction catalyzed by Au NPs−LDH−PVA hybrid films with different WAu:WLDH ratios (Figure 5b), which well-matched with the first-order reaction kinetics. The rate constant k was calculated as 8.94 × 10−2, 8.26 × 10−2, 5.20 × 10−2, and 3.71 × 10−2 min−1 when Au NPs−LDH−PVA films with varied mass ratios (WAu:WLDH = 4:1, 2:1, 1:1, and 0.5:1, respectively) were employed. These results indicated that an increase in the ratio of Au NPs could improve the catalytic efficiency. The obtained multifunctional Au NPs−LDH−PVA hybrid films showed excellent catalytic performance at room temperature during the reduction of 4-NP. To investigate the reusability, the Au NPs−LDH−PVA hybrid film (WAu:WLDH = 4:1) was immersed in a freshly mixed solution of p-nitrophenol and NaBH4. Figure S3 shows that the hybrid film exhibited a similar catalytic performance without a visible decrease in the conversion efficiency for the same reaction time (18 min) even after 10 cycles. Therefore, this functional layered structure held great promise as a novel gold-based catalyst system for various catalytic reactions. Furthermore, the introduction of catalytic Au NPs into a layer-structured nanocomposite will pave the way for catalytic applications of nacre-like composites. As is well-known, Au NPs have been reported as excellent enhancing substrates for SERS. To investigate the SERS performance of the Au NPs−LDH−PVA hybrid films, 4-MBA was selected as a reporter molecule and adsorbed on the films. Figure 6 shows the SERS spectra of 4-MBA (1 mM) adsorbed on hybrid films with different Au NPs densities and the normal Raman spectrum of 1 mM 4-MBA collected on a glass substrate. It should be noticed that the SERS spectra of 4-MBA adsorbed on Au NPs−LDH−PVA hybrid films were significantly different from the normal Raman spectra of 4-MBA on a glass substrate. The intense peak in the spectra of Au NPs− LDH−PVA hybrid films at about 1078 cm−1 was assigned to the aromatic ring characteristic vibration. The weak ones at

LDH nanosheets could be well manipulated. When the mass ratio of Au NPs to LDH nanosheets reached 4:1, slight aggregations of Au NPs gradually formed. Figure S2 shows that when the mass ratio reached up to 6:1, Au NPs were greatly aggregated and many individual Au NPs appeared outside the LDH nanosheets. Thus, mass ratios of Au NPs to LDHs of 0.5:1, 1:1, 2:1, and 4:1 were employed to fabricate novel and multifunctional nacre-like composites. Because abundant hydrogen bondings could be readily formed between hydroxyl and amine groups, Au NPs−LDH platelets modified with amine groups could be assembled into nacre-like layered materials through multiple cycles of spincoating with PVA. Figure 3a displays typical photographs of Au

Figure 3. (a) Photographs of Au NPs−LDH−PVA nacre-like hybrid film coating on glass substrates. (b−e) Cross-sectional SEM images of Au NPs−LDH−PVA nacre-like hybrid films with different Au NPs densities. (b) WAu:WLDH = 0.5:1 hybrid film. (c) WAu:WLDH = 1:1 hybrid film. (d) WAu:WLDH = 2:1 hybrid film. (e) WAu:WLDH = 4:1 hybrid film.

NPs−LDH−PVA hybrid films, in whichthe color changed with variation of the densities of Au NPs attached to LDH nanosheets. Notably, the higher the content of Au NPs, the darker the hybrid films. The cross section of different Au NPs− LDH−PVA hybrid films on glass substrates was characterized by SEM (Figure 3b−e). The thicknesses of the hybrid films were approximately 15−20 μm. All of the Au NPs−LDH−PVA hybrid films exhibited well-defined nacre-like layered structure, in which Au NPs−LDH hybrid nanosheets were horizontally stacked on the substrates and glued together effectively by PVA adhesive. The high-magnification SEM image in Figure 4a further shows that Au NPs were incorporated in lamellar structures and encapsulated by LDH nanosheets. The full EDX spectrum of the cross section of a WAu:WLDH = 4:1 hybrid film exhibited signals originating from LDHs (cobalt and aluminum) and Au NPs (Figure 4b). The corresponding element mapping (Figure 4c−e) indicated that both LDHs and Au NPs were 8943

DOI: 10.1021/acs.iecr.5b01518 Ind. Eng. Chem. Res. 2015, 54, 8940−8946

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Figure 4. (a) Cross-sectional SEM image of WAu:WLDH = 4:1 hybrid film. (b) EDX analysis element mapping of the Au NPs−LDH−PVA hybrid film (WAu:WLDH = 4:1). The full EDX spectrum with the corresponding elements. (c−e) EDX mapping of the different components: (c and d) cobalt and aluminum for LDH; (e) gold for Au NPs.

NPs−LDH−PVA hybrid films. The results observed above indicated that the Au NPs−LDH−PVA hybrid films could be used as potential SERS substrates. The tensile stress−strain curves of the nacre-like Au NPs− LDH−PVA hybrid films are shown in Figure 7. Compared with

Figure 5. (a) UV−vis spectra of the reduction of 4-NP in an aqueous solution recorded every 2 min using the Au NPs−LDH−PVA hybrid film as a catalyst. (b) Relationship between ln(Ct/C0) and the reaction time (t), wherein the ratios of the 4-NP concentration (Ct at time t) to its initial value C0 (t = 0) were directly given by the relative intensity of the respective absorbance At/A0 and , therefore, the reduction process could be directly reflected by these absorption curves. Figure 7. Tensile stress−strain curves of LDH−PVA and Au NPs− LDH−PVA hybrid films with different concentrations of Au NPs.

the pure LDH−PVA film fabricated by the same method, the ultimate tensile strength and Young’s modulus of these Au NPs−LDH−PVA hybrid films decreased. In the course of decorating Au NPs, part of the locations of the amino groups on the surface of LDH nanoplatelets were occupied with Au NPs via coordination bonds, which eventually decreased the number of hydrogen bonds. Hence, the difference in the interfacial strength strongly influenced the mechanical property of these films. The tensile strength and Young’s modulus of the WAu:WLDH = 0.5:1 hybrid film were 122 MPa and 8.5 GPa, respectively. Obviously, the tensile strength and Young’s modulus of these Au NPs−LDH−PVA hybrid films decreased with an increased amount of Au NPs. The tensile strength of the WAu:WLDH = 4:1 hybrid film was 78 MPa, which was still higher than that of a pure PVA film (Figure S4) and comparable to that of natural nacre.1,16,57 The reasons were as follows: (i) as was reported in our previous work,35 the aspect ratio of LDH nanoplatelets (s = 19−37; critical aspect ratio was 40; s ≲ sc) used in the system determined that Au NPs−LDH−PVA hybrid films would fracture under the platelet pull-out mode; (ii) well-defined nacre-like layered structures, combined with interfacial bondings between Au NPs−LDH nanoplatelets and PVA matrix, led to effective load transfer from the ductile polymer to the rigid inorganic phase. Thus, we

Figure 6. SERS spectra of 1 mM 4-MBA molecules collected on a bare glass substrate and a set of Au NPs−LDH−PVA hybrid films on glass substrates with different Au NPs densities attached on LDH nanoplatelets.

1145 and 1176 cm−1 corresponding to the C−H deformation modes were also observed. It was obvious that the Raman signals of 4-MBA adsorbed on Au NPs−LDH−PVA hybrid films were remarkably enhanced relative to pure 4-MBA in solution. The SERS spectra of these hybrid films were consistent with the previously reported for 4-MBA adsorbed on the surface of other composites.55,56 Furthermore, with the mass ratio of Au NPs to LDH nanoplatelets increasing from 0.5:1 to 4:1, the Raman signals were enhanced accordingly. This was attributed to the increase of active localized surface plasmons or “hot spots” with increasing Au NPs content in Au 8944

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(7) Yao, H. B.; Guan, Y.; Mao, L. B.; Wang, Y.; Wang, X. H.; Tao, D. Q.; Yu, S. H. A designed multiscale hierarchical assembly process to produce artificial nacre-like freestanding hybrid films with tunable optical properties. J. Mater. Chem. 2012, 22, 13005. (8) Deville, S.; Saiz, E.; Nalla, R. K.; Tomsia, A. P. Freezing as a path to build complex composites. Science 2006, 311, 515. (9) Cheng, Q. F.; Jiang, L.; Tang, Z. Y. Bioinspired layered materials with superior mechanical performance. Acc. Chem. Res. 2014, 47, 1256. (10) Shu, Y. Q.; Yin, P. G.; Liang, B. L.; Wang, S. S.; Gao, L. C.; Wang, H.; Guo, L. Layer by layer assembly of heparin/layered double hydroxide completely renewable ultrathin films with enhanced strength and blood compatibility. J. Mater. Chem. 2012, 22, 21667. (11) Wang, Y.; Zhang, D. Bioinspired assembly of layered double hydroxide/carboxymethyl chitosan bionanocomposite hydrogel films. J. Mater. Chem. B 2014, 2, 1024. (12) Walther, A.; Bjurhager, I.; Malho, J. M.; Pere, J.; Ruokolainen, J.; Berglund, L. A.; Ikkala, O. Large-area, lightweight and thick biomimetic composites with superior material properties via fast, economic, and green pathways. Nano Lett. 2010, 10, 2742. (13) Walther, A.; Bjurhager, I.; Malho, J. M.; Ruokolainen, J.; Berglund, L.; Ikkala, O. Supramolecular control of stiffness and strength in lightweight high-performance nacre-mimetic paper with fire-shielding properties. Angew. Chem., Int. Ed. 2010, 49, 6448. (14) Das, P.; Schipmann, S.; Malho, J. M.; Zhu, B. L.; Klemradt, U.; Walther, A. Facile access to large-scale, self-assembled, nacre-inspired, high-performance materials with tunable nanoscale periodicities. ACS Appl. Mater. Interfaces 2013, 5, 3738. (15) Yao, H. B.; Tan, Z. H.; Fang, H. Y.; Yu, S. H. Artificial nacre-like bionanocomposite films from the self-assembly of chitosan-montmorillonite hybrid building blocks. Angew. Chem., Int. Ed. 2010, 49, 10127. (16) Wang, J. F.; Cheng, Q. F.; Lin, L.; Chen, L. F.; Jiang, L. Understanding the relationship of performance with nanofiller content in the biomimetic layered nanocomposites. Nanoscale 2013, 5, 6356. (17) Cheng, Q. F.; Wu, M. X.; Li, M. Z.; Jiang, L.; Tang, Z. Y. Ultratough artificial nacre based on conjugated cross-linked graphene oxide. Angew. Chem., Int. Ed. 2013, 52, 3750. (18) Zhao, Q.; An, Q. F.; Liu, T.; Chen, J. T.; Chen, F.; Lee, K. R.; Gao, C. J. Bio-inspired polyelectrolyte complex/graphene oxide nanocomposite membranes with enhanced tensile strength and ultralow gas permeability. Polym. Chem. 2013, 4, 4298. (19) Li, Y. Q.; Yu, T.; Yang, T. Y.; Zheng, L. X.; Liao, K. Bio-inspired nacre-like composite films based on graphene with superior mechanical, electrical, and biocompatible properties. Adv. Mater. 2012, 24, 3426. (20) Bonderer, L. J.; Feldman, K.; Gauckler, L. J. Platelet-reinforced polymer matrix composites by combined gel-casting and hot-pressing. Part II: Thermoplastic polyurethane matrix composites. Compos. Sci. Technol. 2010, 70, 1966. (21) Khan, A. I.; Ragavan, A.; Fong, B.; Markland, C.; O’Brien, M.; Dunbar, T. G.; Williams, G. R.; O’Hare, D. Recent developments in the use of layered double hydroxides as host materials for the storage and triggered release of functional anions. Ind. Eng. Chem. Res. 2009, 48, 10196. (22) Reinholdt, M. X.; Babu, P. K.; Kirkpatrick, R. J. Preferential adsorption of lower-charge glutamate ions on layered double hydroxides: an NMR investigation. J. Phys. Chem. C 2009, 113, 3378. (23) Guo, Y.; Xiao, Y. P.; Zhang, L. M.; Song, Y. F. Fabrication of (Calcein-ZnS)(n) ordered ultrathin films on the basis of layered double hydroxide and its ethanol sensing behavior. Ind. Eng. Chem. Res. 2012, 51, 8966. (24) Yan, D. P.; Lu, J.; Wei, M.; Qin, S. H.; Chen, L.; Zhang, S. T.; Evans, D. G.; Duan, X. Heterogeneous transparent ultrathin films with tunable-color luminescence based on the assembly of photoactive organic molecules and layered double hydroxides. Adv. Funct. Mater. 2011, 21, 2497. (25) Yan, D. P.; Lu, J.; Wei, M.; Han, J. B.; Ma, J.; Li, F.; Evans, D. G.; Duan, X. Ordered poly(p-phenylene)/layered double hydroxide ultrathin films with blue luminescence by layer-by-layer assembly. Angew. Chem., Int. Ed. 2009, 48, 3073.

believe that these multifunctional nacre-like Au NPs−LDH− PVA hybrid films are valuable for applications in lightweight construction and coatings.

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CONCLUSIONS Through modification of APTES on the surface of LDH nanoplatelets, Au NPs with different densities can be evenly decorated on LDH nanoplatelets by coordination interaction, which offers flexibility from utilizing these Au NPs−LDH hybrid nanoplatelets as inorganic building blocks to fabricating novel and functional nacre-like hybrid materials. In this work, Au NPs−LDH−PVA hybrid films have been successfully prepared for the first time by the bottom-up LBL assembly of pretreated Au NPs−LDH nanoplatelets and subsequent spin coating of PVA. The cross sections of these hybrid films are similar to the brick-and-mortar structure of nacre. Furthermore, we demonstrate that these Au NPs−LDH−PVA hybrid films not only have good mechanical properties but also possess SERS and catalytic properties, which can be potentially used as SERS substrate and catalytic sensors. Therefore, our reported fabrication of a multifunctional Au NPs−LDH−PVA hybrid film not only shows a feasible route to functionalize artificial nacre-inspired materials but also potentially broadens applications of these nacre-like materials.

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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/acs.iecr.5b01518. SEM images Au NPs−LDH and mechanical properties of pure PVA (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel: (+86)-10-823-38-162. E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the National Natural Science Foundation of China (Grant 51272013), the National Basic Research Program of China (Grant 2010CB934700), and the Doctoral Fund of Innovation of Beijing University of Technology.

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REFERENCES

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DOI: 10.1021/acs.iecr.5b01518 Ind. Eng. Chem. Res. 2015, 54, 8940−8946