Bioinspired Synthesis of Au Nanostructures Templated from Amyloid Î²

Article Cite This: Biomacromolecules XXXX, XXX, XXX−XXX

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Bioinspired Synthesis of Au Nanostructures Templated from Amyloid β Peptide Assembly with Enhanced Catalytic Activity Yonghai Feng,† Huijie Wang,† Jie Zhang,† Yongxiu Song,† Minjia Meng,‡ Jianli Mi,† Hengbo Yin,‡ and Lei Liu*,† †

Institute for Advanced Materials, School of Materials Science and Engineering and ‡School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, China 212013 S Supporting Information *

ABSTRACT: Peptides have been regarded as useful biomolecule templates to control the synthesis of various inorganic nanomaterials in mild conditions. Inspired by this, the easily self-assembled amyloid β (Aβ) peptide was developed as an alternative template to prepare Au nanostructures for the enhanced catalytic activity, for instance, the reduction of 4-nitrophenol. The presence of Aβ peptide assemblies with different structures could direct the nucleation of Au to form different Au nanostructures. Using the Aβ25−35 monomers, nanoribbons, and nanofibrils prepared by the self-assembly in phosphate buffered (PB) solution at 0, 3, and 12 h, respectively, as templates could controllably prepare Au nanospheres, nanoribbons, and nanofibers, while the Aβ25−35 monomers prepared by the self-assembly in water at 0 h could direct the synthesis of Au nanoflowers. The Aβ25−35-templated Au nanostructures had different catalytic activities due to the size and structure effects, which however are significantly enhanced as compared with the template-free Au nanoparticles.

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INTRODUCTION Biomolecule-based routes for inorganic nanoparticle production have attracted much attention because of the mild synthetic conditions (e.g., aqueous mediate, ambient temperature, and neutral pH) and the well control of the structure of nanoparticles with highly specific or multiple functions.1−4 Selfassembling biomolecules such as peptides, proteins, and DNA that provide specific sites where metal ions can be bound have been widely used as the templates for preparing various functional nanomaterials.5,6 From among the biomeolecules, peptide is one of the most advantageous molecules for directing the growth of nanoparticles owing to its unique self-assembly properties and surface recognition capability.4,7 A number of nanoparticles such as metal or bimetal (e.g., Au,8,9 Pd,10 Ag,2 Pt11 and Au@Ag,12 Pd@Au,13 Au@Pt14), metal oxides (e.g., ZnO15 and TiO216), and metal sulfide (e.g., ZnS17) can be facilely synthesized with controllable morphologies including chains, sheets, and spheres. The peptide templated nanomaterials have been found technologically important applications ranging from catalysis9 and electrochemical fuel cells11,12,18 to biosensing,19,20 detection,21 and biomedicine.22 The peptide-templated Au nanomaterials have been receiving a lot of attention due to their facile fabrication, diversiform structure, excellent optical and electrochemical properties, and versatile applications. For instances, the Au nanoparticles produced with different peptide sequences have demonstrated excellent catalytic activity toward 4-nitrophenol reduction.3,23 Multifunctional hybrid Au and peptide spheres have been © XXXX American Chemical Society

fabricated in water using AG4 (NPSSLFRYLPSD) peptide as template and reducing agent, which had great potential in various biomedical and electronic applications.22 A3 (AYYSGAPPMPPF) peptide-capped gold nanoparticles exhibited color change from red to blue in the presence of Hg2+ or As3+ ions, which could be used as the sensitive and selective probes for detecting Hg2+ or As3+ ions.24 The chemical and physical properties of the peptide-templated Au nanomaterials are significantly affected by their size, shape, structure, and the peptide capped on the surface of the Au nanomaterials which can be tuned by the amino acid sequence of peptide,8,23 ratio of Au precursor to peptide,9 and the reaction temperature.22 Moreover, through the modification of the peptide by conjugation of some organic molecules, the production of Au nanoparticle superstructures can be achieved. For example, coupling succinimide-activated dodecanoic acid to the Nterminus of PEPAu (AYSSGAPPMPPF) to generate C12-PEPAu could direct the synthesis of highly ordered Au nanoparticle double helices.25,26 Using a peptide conjugate (C6-AA-PEPAu) could produce sub-100 nm hollow Au nanoparticle superstructures in a direct one-pot reaction.27 These Au nanoparticle superstructures would play important role in the areas of plasmonics, molecular sensing, and nanoscale electronics. Furthermore, by controlling the structures of the peptide selfReceived: January 11, 2018 Revised: April 3, 2018 Published: April 26, 2018 A

DOI: 10.1021/acs.biomac.8b00045 Biomacromolecules XXXX, XXX, XXX−XXX

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Biomacromolecules

Figure 1. Overview of Aβ25−35 aggregation: (a) molecule structure of Aβ25−35; (b, c) AFM images of P3 and P12 Aβ25−35 assemblies; (d) ThT assay of Aβ25−35 aggregates; and (e) CD spectra of Aβ25−35 aggregates.

their catalytic activities in 4-nitrophenol reduction. The Aβ25−35 peptide (GSNKGAIIGLM, the molecule structure is shown in Figure 1a) derived from the full-length Aβ1−42 peptide was selected as a model Aβ peptide because it represents the biologically active region of Aβ peptide.40 Different selfassemblies of Aβ25−35 peptides were obtained by the incubation in phosphate buffer solution (PB) at different times, characterized by AFM, CD, ThT, and FTIR, and used as templates to direct the synthesis of Au nanomaterials. The asprepared Au nanomaterials were characterized by XRD, TEM, XPS, and UV−vis spectra. The work herein provided the insight into the template effect of amyloid peptide assemblies on the Au nanomaterial and the related catalytic activities in 4nitrophenol reduction. The results display that the Aβ25−35 assemblies could control the morphologies of the Au nanomaterials and enhanced their catalytic activities. The PBinduced Aβ 25−35 monomer and Aβ 25−35 amyloid fibril templated-Au nanomaterials presented better catalytic activities compared to others. It will open up a new approach toward constructing functional peptide−inorganic nanomaterials.

assemblies can control the morphologies of the synthesized Au nanomaterials.28−30 For example, using peptide monomer as template can usually form spherical Au nanoparticles,3,23,29 while using peptide nanofiber as template can produce the onedimensional Au nanofibers, which show potential applications in electronics and optics.28,30 Actually, peptide assembling plays a particularly important role in the construction of peptidetemplated Au nanomaterials with unique structures, and it is certainly closely related to their physical and chemical properties. However, there is limited research focusing on the template effect of different peptide assemblies on the Au nanomaterial construction and related chemical properties. Therefore, it is of great significance to search and develop suitable peptides as the candidate for constructing controllable Au nanomaterials via the self-assembly of peptide and exploring the physical and chemical properties of synthesized nanomaterials such as catalysis. Amyloid-β (Aβ) peptide, a kind of pathogenic amyloid peptide composed of 42 amino acids (Aβ1−42), has the capacity of self-assembly into different shapes and structures like nanospheres, nanoribbons, or nanofibers under some specific conditions such as incubation time, temperature, pH, salt, cosolute or cosolvent, and hydrophobic environments.31−39 It is very interesting and significant to take advantage of the selfassembly of Aβ peptide as an alternative template to controllably synthesize Au nanomaterials and evaluate their catalytic activity. Recently, the catalytic reduction of 4nitrophenol over metal nanomaterials is of growing interesting because it is not only a simple route for producing the important pharmaceutical intermediate 4-aminophenol but also can be used as a probe reaction for evaluating the catalytic activity of metal nanomaterials.3,9,10,23 Therefore, in the present work, we explored the controlling synthesis of Au nanomaterials with Aβ peptide as template and

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EXPERIMENTAL SECTION

Materials. Chloroauric acid (HAuCl4·4H2O) and sodium borohydride (NaBH4) were of reagent grade and purchased from Sinopharm Chemical Reagent Co., Ltd. 1,1,1,3,3,3-Hexafluoro-2-propanol (HFIP) was of reagent grade and acquired from Tokyo Chemical Industry. Aβ25−35 (purity >98%) were supplied by Shanghai ABBiochem. pNitrophenol (AR, 98%) was acquired from Shanghai Aladdin Biochemical Technology Co., Ltd. Milli-Q water was used for all solution preparation. Preparation of Aβ25−35 Assemblies. 2 mg of Aβ25−35 peptide monomer powder was first dissolved in 1 mL of HFIP, followed by sonication for 10 s at 40% amplitude for 3 times. Subsequently, the suspension was placed in a thermo-shaker (PHMT, Grant Instruments, England) at 360 rpm at 298 K overnight. Finally, the peptide monomer suspension was stored at 253 K for further use. B


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Biomacromolecules Table 1. Physical and Chemical Properties of Aβ25−35-Templated Au Nanostructures binding energyd

peptide templates a

catalysts

structure

height (nm)

AuP0 AuP3 AuP12 AuW0 Au0

monomer protofibril fibril monomer

5.2 9.3

particle sizes of Au (nm)

absorbance peaks (nm)

Au 4f7/2

Au 4f5/2

conve (%)

TOFf (h−1)

4.5 5.2 6.4 122.6 32.0

515 520 535 550; 890 520

84.4 84.3 84.4 84.4 84.0

87.8 87.9 88.1 87.9 87.7

89.0 68.0 84.1 37.5 19.9

19.8 13.6 19.2 8.4 4.2

b

c

a

The heights of peptide were measured by AFM. bThe Au particle sizes were calculated by TEM. cThe absorption peaks of Au nanostructures were measured by UV−vis spectra. dThe Au 4f of Au nanostructures were measured by XPS spectra. eThe conversions of 4-nitrophenol over different were calculated according to figure at the reaction time of 2 min. fThe TOF (h−1) = conversion of 4-nitrophenol (mol) divided by the amount of Au (mol) and reaction time (0.017 h).

Scheme 1. Illustration of the Preparion of Aβ25−35-Templated Au Nanostructures with Different Aβ25−35 Aggregates as Templates

10 μL of the as-prepared peptide monomer suspension was poured into a 1.5 mL centrifuge tube and sealed with parafilm. Whereafter, the tube was put into a vacuum drying oven (Jinghong Co., Ltd., China) for 1 h at room temperature. After desiccation, 200 μL of PB solution was immediately added into the tube to dissolve the Aβ25−35 monomer with a final concentration of 100 μM. Afterward, the obtained Aβ25−35 PB solution was incubated in a thermo-shaker at 360 rpm at 310 K for certain time. Different PB-induced Aβ25−35 assemblies were obtained after incubation for 0, 3, and 12 h, which were denoted as P0, P3, and P12, respectively. As comparison, 200 μL of Milli-Q water was used to dissolve the Aβ25−35 monomer and incubated in water for 0 h to obtain the water-induced Aβ25−35 assemblies with the final concentration of 100 μM, which was denoted as W0. All the Aβ25−35 assemblies (P0, P3, P12, and W0) were prepared freshly and used for the following Au nanomaterial synthesis and listed in Table 1. Synthesis of Aβ25−35-Templated Au Nanostructures. The Aβ25−35-templated Au nanostructures were prepared by the reduction of Au3+ with the Aβ25−35 assemblies as templates and the NaBH4 as reductant. The preparation procedure can be simply described by Scheme 1. Typically, 200 μL of Aβ25−35 assembly solution (P0, P3, P12, or W0) was mixed with 200 μL of 10 mM HAuCl4 aqueous solution under stirring for 10 min. The mole ratio of gold to peptide (Au/peptide ratio) was 100. Then, 400 μL of 10 mM freshly prepared ice-cold NaBH4 aqueous solution was added into the mixed solution. After reacting for 1 h, the Au nanostructure suspension was obtained. Finally, the obtained Au nanostructure suspension was purified by centrifugation twice (12 000 rpm, 5 min) and redispersed in Milli-Q water to 800 μL. As listed in Table 1, the Aβ25−35-templated Au nanostructures with P0, P3, P12, and W0 as templates were denoted as AuP0, AuP3, AuP12, and AuW0, respectively. The Au nanostructures prepared in water without peptide template were prepared for comparison, which were denoted as Au0 (Table 1). Characterizations of Peptide-Templated Au Nanostructures. Atomic force microscopy (AFM) images were obtained by an atomic

force microscope (MultiMode VIII SPM, Bruker). The samples were prepared through depositing 10 μL of peptide solution onto a cleaned mica surface and air-dried for 10 min, and the residual liquid was removed. The average heights of the samples are measured based on at least 100 particles. Circular dichroism (CD) measurements were performed at room temperature in a spectropolarimeter (JASCO PTC-348W1) with a 0.1 cm quartz cell. The slit width was set at 2 nm, and scan speed was 100 nm min−1. The signal of the PB solution has been subtracted. Measurements were carried out in quartz cell with a sample volume of 300 μL. ThT fluorescence assay was used to measure the peptide secondary structure. The fluorescence spectrum was recorded using a fluorescence spectrophotometer (F-4500; Hitachi) with excitation and emission at 450 and 485 nm with a 0.1 cm quartz cells, respectively. Measurements were carried out in quartz cell with sample volumes of 300 μL. The volume ratio of peptide solution and ThT (1 mM) was 50:50. The secondary structure of peptide was analyzed on a FTIR spectrometer (Thermo Scientific, Marietta, GA) using a smart multiBounce ARK accessory (Termo Nicolet) equipped with a calcium fuoride crystal. Aβ25−35 peptide solution was transferred onto the crystal for analysis. A background spectrum was subtracted from all the samples’ spectra. Spectra were acquired in the 4000 to 400 cm−1 range with a spectral resolution of 4 cm−1 over 64 scans. The XRD data of peptide-templated Au nanoparticles were recorded on a diffractometer (D8 super speed Bruke-AEX Company, Germany) using Cu Kα radiation (k = 1.540 56 Å) with Ni filter, scanning from 20° to 90° at a speed of 5° min−1. Transmission electron microscopy (TEM) images were acquired using a Tecnai 12 microscope operating at an accelerating voltage of 120 kV. The specimens were prepared by drop-casting 20 μL of the Au nanomaterial solution onto a non-carbon-coated copper grid (Beijing Dajikeyi Co., Ltd.) for 5 min, and then the residues were removed. C


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Biomacromolecules

Figure 2. TEM images of Aβ25−35-templated Au nanostructures with different Aβ25−35 aggregates as templates: (a) AuP0, (b) AuP3, (c) AuP12, (d) AuW0, (e) Au0, and HRTEM image of AuP12 (f). The inset images are the corresponding particle size distributions. The average particle sizes of Au nanoparticles were measured by counting at 200 individual nanoparticles. The high-resolution transmission electron microscopy (HRTEM) images were obtained on a microscope (Tecnai G2 F30) operated at an acceleration voltage of 200 kV. Ultraviolet−visible (UV−vis) absorption spectra of the Au nanoparticle suspensions were recorded with a UV-1800PC spectrophotometer. X-ray photoelectron spectra (XPS) of the representative Au nanoparticles were recorded on an ESCALAB 250 spectrometer (PHI5000 Versa Probe) using Al Kα radiation at 1486.6 eV. The resolution of the XPS is ±0.1 eV. The binding energies of Au were calculated with respect to C 1s peak of contaminated carbon at 284.6 eV. Catalytic Test. The reduction of 4-nitrophenol catalyzed by the Au nanomaterials was carried out by following the previously established procedure.3 Typically, 0.08 mL of the as-prepared Au nanoparticles suspension was first diluted with Milli-Q water into 1 mL. Then, the suspension was mixed with 1 mL of 15 mM freshly prepared NaBH4 aqueous solution and then left undisturbed for 5 min. After that, 1 mL of 4-nitrophenol solution with the initial concentrations of 75−300 μM was added into the mixture to initiate the reaction. The reaction was monitored based on the variation of absorbance at 400 nm at an interval of 1 min via a UV-1800PC spectrophotometer, and the reaction was carried on at temperatures of 293−373 K. The products were confirmed by the high performance liquid chromatograph−mass spectrometer (HPLC-MS, Thermo LXQ).

form of monomer, which can hardly be observed by AFM (as shown in Figure S1). After incubating for 3 h, the Aβ25−35 monomers self-assembled into protofibrillar structures (P3) with the height of ca. 5.2 nm (Figure 1b). Further increasing the incubation time for 12 h, bundles of mature Aβ25−35 fibrils (P12) were observed with a height of ca. 9.3 nm (Figure 1c). The size of Aβ25−35 assemblies increased with increasing the incubation time. To further understand the aggregation of Aβ25−35, the secondary structures have been evaluated by means of ThT assay and CD spectroscopy. As shown Figure 1d, the intensities of ThT fluorescence peak (ca. 485 nm) of P0, P3, and P12 were in the order of P0 (164) < P3 (368) < P12 (722), implying that the secondary structure of Aβ25−35 in PB solution could be changed from unordered structure to β-sheet as the incubation time increased due to the Aβ 25−35 fibrillation.42 In the CD spectra (Figure 1e), P0 presents the unordered structure, whereas with the prolonging the incubation time, the feature of CD spectra of Aβ25−35 changed drastically. The random coil structures disappear in the assembly of Aβ25−35 protofibrils (P3) and mature fibrils (P12), and the β-sheet secondary structure (positive peak at 195 nm, and negative peak at 217 nm) is dominant with longer incubation time. When the Aβ25−35 were dissolved in water without incubation (W0), they also presented the form of monomer and the unordered structure (Figure 1d,e), which was similar to the Aβ25−35 dissolved in PB solution (P0). The FTIR spectra (Figure S2) also show the P12 and P3 had ordered β-sheet secondary structure (peak at 1628 and 1625 nm, respectively), while the P0 and W0 were random structures (peak at 1618 and 1617 nm, respectively).43 TEM Analysis of Aβ25−35-Templated Au Nanostructures. The Aβ25−35 -templated Au nanostructures were prepared in a facile two-step process in water, as shown in Scheme 1. Aβ25−35 assemblies with different structures (P0, P3, P12, and W0) were first prepared, which resulted in the

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RESULTS AND DISCUSSION Aggregation Process of Aβ25−35 Peptides. The Aβ peptides can easily self-assemble into different aggregated forms due to the amphipathicity of polypeptide, which depends on the environment conditions.41,42 Thus, it is very important to observe the structures of the Aβ peptide aggregates before use for the synthesis of Au nanostructures. A combination of bulk and surface analyzing techniques was employed to investigate the aggregation of Aβ25−35 (see Figure 1). The Aβ25−35 dissolved in PB solution without incubation (P0) was the D


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Biomacromolecules localized areas of amines. The Au3+ ion solution was then introduced to these Aβ25−35 assemblies, in which the Au3+ ions could be sequestered within the biological scaffold through the complexation of the metal ions to the peptide amines.3,10,44 Metallic Au nanostructures were formed after the NaBH4 reduction characterized by XRD, as shown in Figure S3. The morphologies of the obtained Au nanostructures could be changed by the structures of the Aβ25−35 assemblies (Figure 2), which however might not change in the Au reduction process (Figure S4). The TEM images of the Aβ25−35-templated Au nanostructures are shown in Figure 2. When the Aβ25−35 monomers (P0) were used as the template, the obtained AuP0 nanostructures were spherical nanoparticles (Figure 2a). The AuP0 nanoparticles had narrow particle sized distribution range, and the average particle size was ca. 4.5 nm. When the Aβ25−35 protofibrils (P3) were used as template, short ribbonlike Au nanostructures (AuP3) were obtained with the average particle size of 5.2 nm (Figure 2b). When the Aβ25−35 fibrils (P12) were used as the template, discrete Au nanofibers were obtained, which were comprised by small sized Au nanoparticles with the average particle size of ca. 6.4 nm (Figure 2c). Figure 2e shows random Au nanoparticles (ca. 32.0 nm) with broad particle size distribution range were formed without using Aβ25−35 as template due to no inhibition of Au nucleation. The results indicated the Aβ25−35 assemblies could well control the morphology of the Au nanostructures because the Au3+ ions were previously located on the peptide assemblies (monomers, protofibrils, or fibrils), which directed the nucleation of Au and finally formed the corresponding Au nanostructures after reduction. Figure 2d shows when the W0 monomers were used as the template, individual flowerlike Au nanostructures (AuW0) with the particle size of ca. 122.6 nm were obtained. It can see that the AuW0 nanoflowers were composed of the primary spherical small sized Au nanoparticles (ca. 9.8 nm). It could be due to that the positive charge Aβ25−35 monomers in water (W0) more easily self-assembled into larger oligomers because of the enhanced electrostatic repulsion as the addition of chloroauric acid decreased the pH of the solution, resulting in the formation of aggregated Au nanoflowers (Figure 2d). However, the Aβ25−35 monomers in PB solution (P0) were relatively stable because the pH was hardly changed when adding chloroauric acid, which could well inhibit the nucleation of Au into large particles (Figure 2a). It implied that the mediate could affect the surface property of the Aβ25−35 monomers and thus affect the nucleation of Au on the surface of Aβ25−35 to form different Au nanostrctures. Figure 2f shows the HRTEM image of AuP12 nanostructures. The lattice fringes of the Au nanoparticles were examined to be ca. 0.235 nm, close to the lattice spacings of face-centered cubic structure of Au, revealing that the Au (111) plane was the predominant crystal plane formed on the surfaces of Au nanostructures because the peptide preferred the adsorption on the Au (111) planes.44,45 UV−Vis Spectrum Analysis. The UV−vis spectra of the Aβ25−35-templated Au nanostructures are shown in Figure 3. Figure 3a shows the Au3+/peptide mixed solutions had no obvious absorbance peak in visible region. Upon reduction (Figure 3b), an absorbance peak associated with the Au nanoparticle localized surface plasmon resonance (LSPR) was evident, with a peak position and shape that varied with peptide template. For the spectra of the AuP0, AuP3, and AuP12

Figure 3. UV−vis spectra of (a) Au3+/Aβ peptide mixed aqueous solutions and (b) different Aβ25−35-templated Au nanostructures.

nanostructures, one sharp absorbance peak appears at 515, 520, and 535 nm, respectively. The absorbance peak of AuP12 was slightly red-shifted and broader than those of AuP0 and AuP3 probably due to that using the Aβ25−35 nanofibrils as template led to more aggregated Au nanoparticles than the monomers and the protofibrils, which is consistent with the TEM analysis (Figure 2d−f). Two broad absorbance peaks at 550 and 890 nm were observed in the spectrum of AuW0, suggesting that using the W0 monomers as template favored the formation of larger, more ploydisperse, and/or more aggregated Au particles, resulting in the red-shift and broadening of the absorbance peak,3 which is consistent with the TEM analysis (Figure 2d). For the spectrum of Au0, one absorbance peak at 520 nm was observed. XPS Analysis. The surface composition and chemical state of Aβ25−35-templated Au nanostructures were analyzed by XPS, as shown in Figure 4. Figure 4a−c shows the experimental C 1s,

Figure 4. XPS spectra of (a) C 1s of Aβ25−35 and AuP0 nanostructures, (b) N 1s of Aβ25−35 and AuP0 nanostructures, (c) O 1s of Aβ25−35 and AuP0 nanostructures, and (d) Au 4f of Aβ25−35-templated Au nanostructures.

N 1s, and O 1s spectra at core level of Aβ25−35 and AuP0 nanostructure as reference. In the C 1s spectrum of Aβ25−35 (Figure 4a), the binding energy detected at 284.8, 285.4, 286.3, and 288.2 eV was assigned to the C−C, C−COOH, C−N/C− O, and OC−N/COO− bonding from Aβ25−35 (structure shown in Figure 1a).46 In the N 1s spectrum of Aβ25−35 (Figure 4b), there were two main binding energy peaks observed, namely the dominant peak at 399.9 eV which was the characteristic of amide N−CO linkages and a smaller peak at 401.2 eV which could be attributed to the protonated E


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Biomacromolecules

Scheme 2. Illustration of Reduction of 4-Nitrophenol to 4-Aminophenol over Aβ25−35-Templated Au Nanostructures with NaBH4 as Reductant in Water

hydrogen was first produced by the addition of NaBH4 in the solution. The active hydrogen reacted with the 4-nitrophenol to form the unstable intermediate 4-nitrosophenol and then to form the 4-hydroxyaminophenol by further hydrogenation, which could be fast converted into the final product, 4aminophenol.10,49 The effects of the morphology of Au nanostructure, reaction temperature, and initial 4-nitrosophenol concentration on the reduction of 4-nitrosophenol were evaluated. Figure 5a shows a typical UV−vis analysis of the 4nitrophenol reduction as a function of time catalyzed by

arginine α-NH3+ of the N-terminus.46 The binding energy peaks corresponding to O 1s of the Aβ25−35 were observed at 531.1, 532.2, and 533.2 eV, assigned to COO−, OC−N, and C−O bonding, respectively.46 The expected peaks at the C 1s, N 1s, and O 1s spectra of AuP0 nanostructure were close to those of the Aβ25−35, revealing that the carbon, nitrogen, and oxygen atoms over the AuP0 nanostructure came from the Aβ25−35 capped on the surface of the Au nanostructure. The area proportion of α-NH3+ of N 1s spectrum increased while the area proportion of COO− of O 1s spectrum decreased over the AuP0 nanostructure as compared to those over the Aβ25−35, which might be attributed to the interaction between the peptide and Au surface. Figure 4d shows the Au 4f spectra at core level of the asprepared Au nanostructures. For the Au 4f spectrum of Au0, the binding energy of Au 4f7/2 was observed at 84.0 eV, in agreement with that for bulk gold.47 The binding energies of Au 4f7/2 over AuP0, AuP3, AuP12, and AuW0 nanostructures were observed at 84.5, 84.3, 84.4, and 84.4 eV, respectively (Table 1), which were 0.3−0.5 eV higher than that of the Au0 nanoparticles. It is reported that decreasing the size of nanoparticle can cause the shift to higher binding energy.48 Moreover, the biomolecules such as peptide could interact with the metallic surface via multiple points of contact by noncovalent interaction, generating a biotic/abiotic interface that maintains the colloidal stability of the nanoparticles.44 Therefore, the shift of the binding energy of Au 4f7/2 over the Aβ25−25-templated Au nanostructures could be attributed to the smaller particle size and the peptide capping. Reduction of 4-Nitrophenol over Au Nanostructures. The reduction of 4-nitrophenol over Aβ25−25-templated Au nanostructures using NaBH4 in water was carried out at the reaction temperatures of 293−323 K in combination with the initial 4-nitrophenol concentrations of 25−100 μM. As shown in Scheme 2, the reaction directly occurs on the metallic Au surface employing as the hydrogen transfer catalyst.49 Active

Figure 5. (a) UV−vis spectra of the peak at 400 nm corresponding to 4-nitrophenol decreases over time. (b) Variation of the intensity of peak at 400 nm, conversion of 4-nitrophenol, and selectivity of 4aminophenol over time. (c) Conversions of 4-nitrophenol-catalyzed Aβ25−35-templated Au nanostructures at different times. Reaction conditions: reaction temperature, 293 K; 4-nitrophenol concentration, 50 μM; catalyst, 66.7 μM. F


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Biomacromolecules AuW0 nanostructures at 293 K. As the reaction time prolonged, the 400 nm intensity gradually decreased, while the 310 nm intensity increased, reflecting that the conversion of 4nitrophenol increased (Figure 5b). 4-Aminophenol was the main product detected in the reaction system (Figure 5b). The conversion of 4-nitrophenol over different Au nanostructures at 293 K is shown in Figure 5c. The conversions of 4-nitrophenol over the Au nanostructures increased with the increment of the reaction time. For the AuP0, AuP3, and AuP12 nanostructures, the 4nitrophenol conversions at 2 min and TOFs were 89.0%, 19.8 h−1; 68.0%, 13.6 h−1; and 84.1%, 19.2 h−1, respectively (Figure 5c and Table 1). The catalytic activity was relatively lower for the AuP3 nanoribbons, while the AuP0 nanospheres and AuP12 nanofibers demonstrated higher catalytic activities. The particle sizes of the three nanostructures were similar (Figure 2). The major difference, however, is the threedimensional structure of the nanomaterials, which was likely mediating the process. The AuP0 nanospheres were individually dispersed particles, providing the highest Au surface area of the three structures,10 which resulted in the highest catalytic activity because the quantity of Au surface area in solution significantly impacted the reaction rate. In the case of AuP3 nanoribbons, the nanoribbon structure had lower Au surface area in comparison with AuP0 nanospheres. As the peptides were capped on the surface of Au, the substrates must diffuse through the peptide shell to react with the Au catalysts, which blocks the reaction. Actually, the catalyst structure with the active position exposure to the solution interface would inherently facilitate the reaction.10 The AuP12 nanofibers had a more densely packed framework with small sized Au nanoparticle exposing, which could drive the reaction process at faster rate, providing comparable catalytic activity to AuP0 nanospheres. It is anticipated that the catalytic activities of AuP0, AuP3, and AuP12 nanostructures were at least 1.5 times higher than that over AuW0 (37.5%, 8.4 h−1) and 3 times higher than that over the Au0 (19.9%, 4.2 h−1), probably due to the much smaller particle sizes (see Figure 2 and Table 1). The catalytic activity of AuW0 was also superior to that of Au0, probably due to the rough surface of the AuW0 nanoflowers providing larger Au surface area and the electron transfer effect. To further evaluate the two structural effects, Figure 6a shows the 4-nitrophenol conversion over AuP0 nanospheres linearly decreased as the amount of extra Aβ25−25 in the solution increased, indicating the extra peptide could inhibit the penetration of 4-nitrophenol molecules onto the Au surface, thus reducing the reduction rate. As shown in Figure 6b, as the amount of AuP12 nanofibers increased, the 4-nitrophenol conversion increased linearly, confirming that greater Au surface areas led to a more rapid reaction. The results suggested that both the catalytic surface area and the penetration depth of 4-nitrophenol played important roles in the catalytic activity of the Aβ25−35-templated Au nanostructures. Therefore, increasing the Au surface area (AuP0) or facilitating the interaction with the substrate (AuP12) can significantly enhance the catalytic activity. The effects of reaction temperature and initial 4-nitrophenol concentration on the conversion of 4-nitrophenol are shown in Figures 6c and 6d, respectively. For the Au nanostructures, the conversion of 4-nitrophenol increased with increasing the reaction temperature from 293 to 323 K. As the 4-nitrophenol concentrations increased, the conversion of 4-nitrophenol decreased. The results implied that the reaction temperature

Figure 6. Effects of (a) peptide concentration, (b) catalyst concentration, (c) reaction temperature, and (d) 4-nitrophenol concentration on 4-nitrophenol conversion. Reaction conditions: (a) reaction temperature, 293 K; 4-nitrophenol concentration, 50 μM; catalyst (AuP0 nanoparticles), 66.7 μM; peptide concentration, 0−5.3 μM; (b) reaction temperature, 293 K; 4-nitrophenol concentration, 50 μM; catalyst (AuP12 nanofibrils), 0−133.3 μM; (c) 4-nitrophenol concentration, 50 μM; catalyst, 66.7 μM; reaction temperature, 293− 323 K; (d) reaction temperature, 293 K; catalyst, 66.7 μM; 4nitrophenol concentration, 25−100 μM.

and initial 4-nitrophenol concentration significantly affected the catalytic reduction process. High reaction temperature and low substrate concentration favored the fast conversion of 4nitrophenol to 4-aminophenol. Reaction Kinetics. A power-function type reaction kinetic equation was used to investigate the effect of initial concentration of 4-nitrophenol and reaction temperature on the reaction rate in the catalytic reduction of 4-nitrophenol over the Aβ25−35-templated Au nanostructures, which is useful to compare catalyst performance more rationally. The NaBH4 concentration was kept constant due to excessive NaBH4 in the system. A linear correlation between the catalyst loading and the conversion is observed in Figure 6b, demonstrating the initial reduction rate was only controlled by chemical reaction rather than by mass diffusion.50,51 Thus, herein, the powerfunction type reaction kinetic equation can be expressed as eq 1: r=−

⎛ −E ⎞ dC 0 = A exp⎜ a ⎟C0 n ⎝ RT ⎠ dt

(1)

where n represents the reaction order, C0 represents the initial concentration of 4-nitrophenol at t = 0 min (in mol L−1), r is the initial reaction rate of 4-nitrophonel (in mol L−1 min−1), A represents the pre-exponential factor, Ea represents the activation energy (in kJ mol−1), R represents the ideal gas constant, 8.314 × 10−3 kJ mol−1 K−1, and T is the reaction temperature (in K). Linear eq 2 is obtained by taking the natural logarithm of both sides of eq 1: ⎛ dC ⎞ E ln r = ln⎜ − 0 ⎟ = ln A − a + n ln C0 ⎝ dt ⎠ RT

(2)

To calculate the reaction order (n) and activation energy (Ea) according to eq 2, the initial rates were calculated at 1 min G


Article

Biomacromolecules Table 2. Preexponential Factors (A), Activation Energies (Ea), and Reaction Orders (n) for Aβ25−35-Templated Au Nanostructure Catalystsa

a

catalysts

ln(A)

std errors

A

−Ea/R (10−3 K)

std errors

Ea (kJ mol−1)

n

standard errors

R2

AuP0 AuP3 AuP12 AuA25 Au0

−1.46 0.38 −1.09 2.09 4.76

0.44 0.37 0.47 0.38 0.21

0.20 1.50 0.30 8.10 116.5

−1.37 −2.05 −1.71 −2.69 −3.31

0.10 0.08 0.11 0.09 0.05

11.40 17.00 14.22 22.40 27.53

0.37 0.35 0.29 0.37 0.51

0.03 0.03 0.03 0.03 0.01

0.9872 0.9933 0.9876 0.9964 0.9993

The data were obtained by fitting the reaction parameter via the multiple linear regression method.

Table 3. Comparison of Catalytic Activities of Aβ25−35-Templated AuNPs with Other Stabilizer-Templated AuNPs in 4Nitrophenol Reduction Reaction entry 1

materials AuP12

2 3 4

AuBP2 AuA3 Au/SC

5 6

AuNPs/Tween cf-CA-Au

7 8

Au/PDMAEMA-PS 1.20 wt % Au(0) @ TpPa-1 0.5 wt % Au/PMMA

9

template peptide Aβ25−35

ratio of Au/ template 1/0.01 (mol)

av particle size (nm) 5.2

NaBH4/4-NP/Au (mol/mol/mol) 75/0.75/1

rate constant (k, s−1) −3

12.8 × 10

−3

activation energy (Ea, kJ mol−1) 14.2

ref this work 10 23 54

peptide BP2 peptide A3 sodium citrate (SC) Tween caffeic acid (CA) PDMAEMA-PS TpPa-1

1/0.5 (mol) 1/0.5 (mol) 1/0.6 (mol)

3.3 2.3 16

75/0.75/1 75/0.75/1 42.5/0.67/1

14.8 × 10 8.9 × 10−3 7.7 × 10−3

1/1.3 (mol) 1/2.5 (mol)

6−15 38.6

37.2/1.6/1 2000/20/1

1.5 × 10−3 5.7 × 10−3

55 56

1/18.8 (mol) 1/60 (mass)

4.2 5

28/0.14/1 3524/2.2/1

3.2 × 10−3 5.4 × 10−3

57 58

PMMA

1/200 (mass)

5.5

22500/15/1

7.9 × 10−3

dC 0 ⎛ −11.40 ⎞ 0.37 ⎟C = 0.2 exp⎜ ⎝ RT ⎠ 0 dt

(3)

r=−

dC 0 ⎛ −17.37 ⎞ 0.34 ⎟C = 1.5 exp⎜ ⎝ RT ⎠ 0 dt

(4)

r=−

dC 0 ⎛ −14.22 ⎞ 0.29 ⎟C = 0.3 exp⎜ ⎝ RT ⎠ 0 dt

(5)

r=−

dC 0 ⎛ −22.40 ⎞ 0.37 ⎟C = 8.1 exp⎜ ⎝ RT ⎠ 0 dt

(6)

r=−

dC 0 ⎛ −27.53 ⎞ 0.51 ⎟C = 116.5 exp⎜ ⎝ RT ⎠ 0 dt

(7)

38.0

59

size of Au nanoparticles by using the peptide incubated in PB solution could facilitate the decrease in the activation energies, resulting in faster reaction rate. Considered the reaction order, it implies the ability of the substrate absorption on the catalyst.52,53 The reaction order with respect to 4-nitrophenol concentration over AuW0 nanostructures was 0.37, lower than that over Au0 nanostructures (0.51), indicating that the Aβ25−35-templated Au nanostructures absorbed 4-nitrophenol more strongly than the Au0. The reaction orders over AuP12 were the lowest among the peptide-templated Au nanostructures, indicating the AuP12 nanofibers have the best ability to facilitate the absorption of 4-nitrophenol. In general, we could evaluate the catalytic activity of 4-nitrophenol by amyloid peptide assembly templated-Au nanostructures to be better than the Au nanostructure with template free. The AuP12 nanofibers in all the nanostructures above present the advantage of being catalysts based on the good evaluation of activation energy and reaction order. Moreover, the fiber structure can be recovered more easily. Comparison of Catalytic Activities of Other StabilizerTemplated Au Nanoparticles in 4-Nitrophenol Reduction Reaction. Comparison of catalytic activities of Aβtemplated Au nanostructure (AuP12) with other stabilizertemplated Au nanoparticles in 4-nitrophenol reduction reaction is shown in Table 3. As compared with the Au nanoparticles stabilized by organic surfactants such as sodium citrate,54 Tween,55 and caffeic acid56 (entries 4−6), the peptidetemplated Au nanoparticles (entries 1−3) had much higher rate constants (k), probably due to the smaller sized Au NPs. The polymers supported Au nanoparticles57−59 (entries 7−9) had comparable particle size with the peptide-templated Au nanoparticles, of which however the rate constants were much lower, probably due to the composition of the catalyst and the larger penetration depth of 4-nitrophenol. The amount of peptide required to synthesize highly reactive Au nanoparticles

under different reaction conditions according to the data shown in Figure S6. The values of pre-exponential factors (A), activation energies (Ea), and reaction order (n) over Au nanostructures are calculated by the multiple linear regression method and listed in Table 2. Good correlation coefficients for the power-function type kinetics in the range of 0.9872−0.9993 were obtained, indicating that the kinetic model was appropriate for evaluating the effect of 4-nitrophenol concentration and reaction temperature on the 4-nitrophenol reduction. The kinetics reaction equations over the Au nanostructures are listed as follows: r=−

26.2 20.0

As shown in the kinetics reaction equations, the activation energies (Ea) over the Au nanostructures were in the order of AuP0 (11.40) < AuP12 (14.22) < AuP3 (17.41) < AuW0 (22.40) < Au0 (27.53). The frequency factor (A) follows the same order. Both the orders of activation energies and frequency factor over Au nanostructures (Table 2) were consisted with the order of particle size of Au nanostructures (Table 1). This implied that small sized Au nanoparticles could activate the reaction more easily than the large sized ones such that decreasing the particle H


Biomacromolecules

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is much less than that of organic surfactant and polymer. Moreover, a relatively low ratio of NaBH4/4-NP/Au over peptide-templated Au nanoparticles was required to obtain high catalytic activity. It is shown that the peptide-templated Au nanoparticles had good cost-effectiveness ratio. For the peptidetemplated Au nanoparticles, the ratio of Au/template for AuP12 (entry 1) was 50 times less than that of AuBP2 and AuA3 (entries 1 and 2), which still presented rather high catalytic activity. The result indicated that using the Aβ peptide as template could significantly reduce the cost of Au nanoparticle production with high catalytic activity.

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CONCLUSIONS

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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.biomac.8b00045. AFM image of Aβ25−35 monomers; figures of the effects of reaction temperature and 4-nitrophenol concentration on the catalytic activities of Au nanostructures (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (L.L.). ORCID

Lei Liu: 0000-0002-6265-9412 Notes

The authors declare no competing financial interest.

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REFERENCES

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The Aβ25−35 peptide was used as template to direct the synthesis of Au nanostructures. The morphology of the Aβ25−35-templated Au nanostructures could be tuned by the Aβ25−35 self-assemblies which were prepared by the incubation in PB solution or water. The Au surface area and the penetration depth of substrate within the scaffold played an important role in controlling the catalytic activity in the reduction of 4-nitrophenol to 4-aminophenol. Using the Aβ25−35 as template can decrease the particle size of Au nanoparticles such as AuP0 nanoparticles or expose more Au nanoparticles on surface such as AuP12 nanofibers, which finally could enhance the catalytic activity. The Aβ25−35-templated Au nanostructures had lower activation energy and reaction orders than the template-free Au0. The Aβ peptide is proved to be a novel and useful biotemplate to synthesize Au nanostructures with enhanced catalytic activity, which can be further used to direct the synthesis of novel biotemplated nanomaterials for more applications such as electronics and optics.

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Article

ACKNOWLEDGMENTS

This work was financially supported by the National Natural Science Foundation of China (No. 21573097, 51503087, and 21606112), China Postdoctoral Fundation Committee (No. 2016M600372), Natural Science Foundation of Jiangsu Province (No. BK20160503), Post Doctoral Fund of Jiangsu Province (No. 1601022A), Natural Science Fund for Colleges and Universities in Jiangsu Province (No. 17KJB180001), and Programs of Senior Talent Foundation of Jiangsu University (No. 15JDG137). I


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