Peptide Cross-linkers: Immobilization of Platinum Nanoparticles

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Peptide Cross-linkers: Immobilization of Platinum Nanoparticles Highly Dispersed on Graphene Oxide Nanosheets with Enhanced Photocatalytic Activities Tsukasa Mizutaru,† Galina Marzun,‡ Sebastian Kohsakowski,‡ Stephan Barcikowski,‡ Dachao Hong,§ Hiroaki Kotani,§ Takahiko Kojima,§ Takahiro Kondo,†,¶,∥ Junji Nakamura,†,¶,∥ and Yohei Yamamoto*,†,¶,∥ †

Division of Materials Science, §Division of Chemistry, ¶Tsukuba Research Center for Interdisciplinary Materials Science (TIMS), and ∥Center for Integrated Research in Fundamental Science and Technology (CiRfSE), Faculty of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan ‡ Technical Chemistry I, Center for Nanointegration Duisburg-Essen (CENIDE), and NanoEnergieTechnikZentrum (NETZ), University of Duisburg-Essen, 7 Universitätstraße, 45141 Essen, Germany S Supporting Information *

ABSTRACT: For exerting potential catalytic and photocatalytic activities of metal nanoparticles (MNPs), immobilization of MNPs on a support medium in highly dispersed state is desired. In this Research Article, we demonstrated that surfactant-free platinum nanoparticles (PtNPs) were efficiently immobilized on graphene oxide (GO) nanosheets in a highly dispersed state by utilizing oligopeptide β-sheets as a cross-linker. The fluorenyl-substituted peptides were designed to form β-sheets, where metal-binding thiol groups and protonated and positively charged amino groups are integrated on the opposite sides of the surface of a β-sheet, which efficiently bridge PtNPs and GO nanosheet. In comparison to PtNP/GO composite without the peptide linker, the PtNP/peptide/GO ternary complex exhibited excellent photocatalytic dye degradation activity via electron transfer from GO to PtNP and simultaneous hole transfer from oxidized GO to the dye. Furthermore, the ternary complex showed photoinduced hydrogen evolution upon visible light irradiation using a hole scavenger. This research provides a new methodology for the development of photocatalytic materials by a bottom-up strategy on the basis of self-assembling features of biomolecules. KEYWORDS: platinum nanoparticle, graphene oxide, peptide, β-sheet, photocatalysis

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INTRODUCTION

Herein, we demonstrate that molecularly engineered oligopeptides can immobilize PtNPs on graphene oxide (GO) nanosheets in a highly dispersed state. Peptides are known to play important roles for electron and proton transfer:11−13 For example, properly designed assembling structures, as well as the amino acid sequences, control the efficiency and selectivity as observed in enzymatic reactions. The designed oligopeptide used in the current study contains three functional sites; a thiol group for metal binding, an amino group that can bear a positive charge upon protonation, and a fluorenyl methoxycarbonyl (Fmoc) group that promotes βsheet formation.14 On the other hand, GO shows a tendency to highly disperse in water due to the charged substituents, which is hardly observable for rGO. In addition, GO exhibits a wide photoabsorption band, which provides an attractive advantage

Metal nanoparticles (MNPs) show unusual catalytic properties in comparison with bulk metals because of their unique electronic states on the surface.1,2 In particular, tremendous attentions have been focused on platinum nanoparticles (PtNPs) for their practical applications to advanced electrochemical catalysis in water splitting and fuel cells.3,4 To fully utilize the potent catalytic properties, MNPs are required to be immobilized homogeneously on a support material. As the support media, porous materials with a large surface area, such as titanium oxide, silica, and carbon black have been used.1,5,6 Recently, much attention has been given to the utilization of two-dimensional (2D) materials such as graphene and reduced graphene oxide (rGO), which possess both large surface area and high electrical conductivity.7−10 However, agglomeration of MNPs on the support materials seriously suppresses the catalytic activity.1,7 Accordingly, methodologies to immobilize MNPs on 2D nanomaterials in a highly dispersed state are required. © XXXX American Chemical Society

Received: December 29, 2016 Accepted: March 2, 2017 Published: March 2, 2017 A

DOI: 10.1021/acsami.6b16765 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

peptides (0.5 mM, 0.2 mL) into an aqueous solution of GO (1.0 mg mL−1, 0.2 mL), the resultant mixtures maintained its dispersed state (Figure S2a−c). In contrast, the mixtures of GO and the dicationic peptides gave harsh aggregates (Figure S 2d−f). These results indicate that derivatives with two protonated groups bridge negatively charged substituents of GO, leading to the aggregation (Figure S2h). On the other hand, derivatives with one protonated group are assumed to interact electrostatically with GO, while preserving the dispersed state (Figure S2g). In fact, ζ-potential of GO in a 1/1 mixture of MeOH/water cosolvent was largely shifted from the negative side (−19.3 mV) to the positive side (+16.1 mV) by complexation with Fmoc-VKVVC 1 (Figure S3a and Table S1). An aqueous dispersion of the surfactant-free PtNPs (92 μg mL−1, 5.6 mL) was added to the aqueous dispersion of 1/GO complex (4.4 mL, final concentrations: [1] = 5 μg mL−1, [GO] = 100 μg mL−1). TEM micrographs of the air-dried suspension of the resulting mixture showed that PtNPs are immobilized on GO nanosheets in a remarkably dispersed state (Figure 2a and

as a support material for photocatalysis. As a result of the hierarchical formation of the PtNP/peptide/GO ternary complex (Figure 1), the complex showed efficient photo-

Figure 1. Molecular structure of Fmoc-VKVVC 1 (a) and hierarchical assembly of 1 to form β-sheet (b), 1/GO complex (c), and PtNP/1/ GO complex (d).

catalytic degradation of dyes and further exhibited hydrogen evolution. This research provides a new bottom-up methodology to immobilize MNPs in a highly dispersed state on support materials by utilizing self-assembling features of biomolecules toward the development of efficient photocatalytic materials.

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RESULTS AND DISCUSSION The GO nanosheets were synthesized by a modified Hummers method (see Methods).15,16 Transmission electron microscopy (TEM) allowed us to confirm that the width of the GO nanosheets were several micrometers (Figure S1a). The surfactant-free PtNPs were prepared by pulsed laser ablation of a Pt target immersed in water and subsequent laser fragmentation in phosphate buffer (see Methods).17,18 The size of PtNPs was 2−20 nm by TEM with the number-average diameter (dav) of 11 nm (Figure S1b). Peptides were synthesized by an Fmoc solid phase synthesis method (see Methods and Scheme S1).14,19,20 We have recently reported that Fmoc-VKVVC (1) (V, valine; K, lysine; C, cysteine; Figure 1a) self-assembles to form an antiparallel βsheet in MeOH, where metal-binding thiol groups of cysteine are integrated on one side, while amino groups of lysine are assembled on the other side of the β-sheet (Figures 1b and S1c); the resultant β-sheets showed strong tendency to redisperse agglomerated MNPs.14 Notably, we found later that, when an aqueous dispersion of PtNPs was mixed with MeOH solution of β-sheet of 1, PtNPs with less than ∼5 nm diameter selectively adsorbed on the β-sheet, while those with larger sizes (>5 nm) were agglomerated (Figure S 1d−f). At first, interaction of peptides with GO in MeOH/water cosolvent was investigated. The amino group of lysine tends to be protonated in neutral water to have a positive charge, while GO in water is negatively charged due to the deprotonated carboxyl groups under neutral conditions. Here, six peptide derivatives were prepared, which tend to be protonated in water; three of them are monocationic (Fmoc-VKVVC, FmocVHVVC, and Fmoc-K, H; histidine), and the other three are dicationic (NH2-VKVVC, NH2-VHVVC, and NH2-K, Figure S2). Upon addition of MeOH solutions of the monocationic

Figure 2. TEM micrographs of PtNP/1/GO complex (a, b) and PtNP/GO composite without 1 (c, d). Inset in panel b shows histogram of the size distribution of PtNPs on GO.

b). The average diameter of the PtNPs on GO is ∼5 nm, which is smaller than the as-produced PtNPs (Figure S1b). The dispersion state of the PtNPs on the GO nanosheets with 1 is quite contrastive with that of a simple mixture of PtNPs and GO without 1, where PtNPs are heavily agglomerated on the surface of GO nanosheets (Figure 2c and d). Because the surface of PtNPs is negatively charged (ζ-potential = − 17.3 mV, Figure S3b),17,18 PtNPs are electrostatically repulsive with GO. On the other hand, in the case of 1/GO complex, the GO is covered by the β-sheets of 1, and PtNPs are immobilized on GO via the thiol group of 1. As a result of the formation of the PtNP/1/GO complex, the ζ-potential shows a positive value of +16.8 mV, and no peak in the negative regime was observed (Figure S3b). Because of the positive charge, the PtNP/1/GO complex was highly dispersed in MeOH/water cosolvent. Note B


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alone, 1 alone, and PtNP/1 (Figures 3b, pink, light blue, and blue, respectively, and S4 e, f, and g). Therefore, the photocatalytic activity was 1.8-fold enhanced by utilizing the β-sheet of 1 as a cross-linker between PtNP and GO. Next, the effect of the peptide sequence and its secondary structure on the photocatalytic activities was investigated. A peptide without the Fmoc group at the N-terminus (NH2− VKVVC 2) hardly forms a β-sheet structure (Figure S5).14 Using 2 as a cross-linker of PtNP and GO, the percentage of photodegradation of RhB (ϕPD) after 1 h UV-irradiation was 31.4% (Figures 3c, S6, and S7b), which was smaller than that using 1 (Figure S7a). Due to the dicationic character of peptide 2, 2/GO was highly agglomerated in water (Figure S2d), and thus PtNPs were not well-dispersed on the surface of GO (Figure S8a), resulting in the low degradation efficiency. When lysine in 1 was substituted by histidine (Fmoc-VHVVC, 3), ϕPD with PtNP/3/GO dropped to 28.5% (Figures 3c, S6, and S7c). Because the isoelectric point of histidine is 7.59, which is smaller than that of lysine (9.74), the degree of protonation of histidine in 3 in neutral water might not be sufficient to electrostatically interact with GO in comparison with that of lysine in 1. In the case where the sequence or the length of the peptide was modified to Fmoc-KVVVC (4) and Fmoc-KVVC (5), ϕPD were also smaller (ϕPD = 27.1 and 23.5%, respectively) than that using 1 (Figures 3c, S6, and S7d and e), possibly because these peptides hardly form β-sheet (Figure S4).20 Furthermore, when cysteine in 1 was substituted by glutamic acid (E) without metal-binding ability (Fmoc-VKVVE, 6), ϕPD dropped to 19.6% (Figures 3c, S6, and S7f), which was in a similar level with that using PtNP/GO composite without peptides (ϕPD = 18.9%, Figure 3b, orange). On the basis of the relationship of the dye degradation properties and peptide structure (Table S2), we conclude that Fmoc-VKVVC (1) affords the best photocatalytic dye degradation performance by the formation of β-sheet, which leads to the separation and integration of the functional groups (charged amino groups and metal binding thiol groups) on the opposite surface of the βsheet (Figure 1b). In contrast, peptide without forming the βsheet structure (2−5, Figure S5) or without metal-binding site (6) does not function well as cross-linkers between PtNPs and GO (Figure S8), thus showing the moderate photocatalytic properties in comparison with peptide 1 as a cross-linker. We further investigated how the amount of the added peptide 1 affects the photodegradation activity of PtNP/1/GO assembly. When the concentration of 1 ([1]) was varied from 0.5 to 50 μg mL−1, the degree of the photoinduced dye degradation exhibited the clear maximum in the range of 5−10 μg mL−1 (Figure 3d). In this concentration range, the weight ratio of 1 to GO is 0.001−0.002, and the molar ratio of 1 to the number of Pt atom at the surface of PtNPs is 0.03−0.06. When the amount of the peptide is too small, the number of immobilized PtNPs on GO is also small, which lowers the photodegradation activity. On the other hand, when the amount of the added peptide is too large, the excess peptide covers the surface of PtNPs, leading to the suppression of the degradation efficiency. To clarify the process of the photoinduced dye degradation, the wavelength dependency was studied. Upon photoirradiation for 1 h with an air mass 1.5G solar simulator (450 W) through 520 nm long-pass filter, ϕPD by PtNP/1/GO was only 9.1%, while that without the long-pass filter was 21.8% (Figure S9). Considering that the wavelengths of absorption maxima of RhB and 1 are 550 and 260 nm, respectively, while the

that GO nanosheets without the peptide were rather wrinkled (Figure 2c), while those changed to an unfolded and extended geometry by complexation with peptides (Figure 2a), possibly because the monolayer of stiff β-sheet covers the surface of GO. The PtNP/1/GO complex exhibited significant activities in photodegradation of organic dyes. Upon 1 h irradiation with UV light (λ = 365 nm, 450 mW, Figure 3a) to an aqueous

Figure 3. (a) UV−vis absorption spectra of RhB (pink), 1 (blue), GO (black), PtNP (gray), 1/GO (green), and PtNP/1/GO (red) in MeOH/water cosolvent (1/1 v/v). (b) Time course of absorbance at 550 nm, normalized on the basis of the absorbance at 0 min, during photoirradiation (365 nm UV light, 450 mW) to aqueous solution of RhB (41.8 mM, black) and those containing 1 (light blue), PtNP (pink), PtNP/1 (blue), GO (purple), PtNP/GO (orange), 1/GO (green), and PtNP/1/GO (red). (c) ϕPD of RhB with PtNP/X/GO (X = 1−6) after 60 min of photoirradiation (365 nm UV light, 450 mW). (d) A plot of absorbance at 550 nm after 60 min of photoirradiation (Xe lamp, 450 W), normalized by the initial absorbance, versus concentration of 1 (bottom axis) and the molar ratio of 1 to Pt atom at the surface of PtNPs (top axis) in PtNP/1/ GO.

solution of rhodamine B (RhB, 41.8 mM) containing PtNP/1/ GO complex (0.1 mg mL−1), the absorbance at 550 nm due to RhB was decreased by 35.7% (Figures 3b, red, and S4a). In contrast, as small as ∼20% of the dye degradations were observed for the solutions including GO alone, 1/GO complex, and PtNP/GO composite without 1 (Figures 3b, purple, green, and orange, respectively, and S4b, c, and d), and only 2−8% of the dye was degraded in the case of the solution with PtNP C


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causes inhibition of H2 evolution. Concomitantly, a certain amount of O2 was generated upon photoirradiation, possibly because of the reduction of GO, where some epoxy groups were deoxygenated to release O2.25,26 We further found that cyclic voltammetry measurement of PtNP/1/GO on Pt electrode showed a peak due to the redox couple of Pt2+/Pt0 at +0.1 V vs SCE, which was +0.15 V higher than that of PtNP/GO without 1 (−0.05 V vs SCE, Figure S12). Furthermore, the reduction onset of H2 evolution for PtNP/1/GO was observed at −0.65 V vs SCE, which also positively shifted in comparison with that of PtNP/GO without 1 (Figure S12). These results clearly indicate that peptide linker plays an important role for the better reductive photocatalytic properties of PtNPs in the composites than other composites without the peptide linker.

absorption band of GO covers both UV and visible regions (Figure 3a), the photoexcitation of GO should be the major factor for the degradation of RhB. This is also supported by the photoexcitation study with UV-LED (365 nm) as shown in Figure 3, where the systems containing GO displayed much higher ϕPD than that without GO. As a possible mechanism of the photodegradation of RhB, photogenerated excitons in GO are split into electrons and holes. The excited electrons transfer from GO to PtNPs and efficiently reduce O2 to H2O, preventing the recombination of electrons and holes in GO.21 Increasing the concentration of O2 accelerated the photodegradation of RhB (Figure S10), indicating that electron transfer (ET) from PtNP to O2 as an electron acceptor is essential for the degradation, while decomposition of RhB was much slower under photoirradiation in air in the absence of the catalytic components (Figure 3b). On the other hand, photogenerated holes in GO are transferred to RhB, and the generated RhB•+ is further oxidized and finally decomposes (Figure 4a).22−24

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CONCLUSION In summary, properly designed oligopeptide β-sheets act as cross-linkers between surfactant-free PtNPs and GO nanosheets, where PtNPs are immobilized on GO in a highly dispersed state. The resultant PtNP/peptide/GO complexes exhibit significant dye degradation activity under photoillumination, which induces efficient electron transfer from GO to PtNP and simultaneous hole transfer from oxidized GO to the dye. The ternary complex further shows hydrogen evolution by adding hole scavengers. Utilization of selfassembled functional peptides is a new strategy to immobilize metal nanoparticles in a highly dispersed manner on electronically active carbon nanomaterials. The experimental results also support the theoretical predictions that peptide works as a pathway for efficient electron transfer between redox-active species.11−13

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METHODS

Materials and Measurements. Unless otherwise noted, reagents, resin, and solvents were used as received from Watanabe Chemical Industries, Ltd. [Fmoc-SAL resin, Fmoc-Val-OH, Fmoc-Cys(Trt)-OH, Fmoc-His(Trt)-OH, 2-(1H-benzotriazole-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HBTU), 1-hydroxybenzotriazole (HOBT)], Kanto Chemical Co., Inc. [CH2Cl2, Et2O, DMF, MeOH], Wako Pure Chemical Industries, Ltd. [piperidine], and Tokyo Chemical Industry Co., Ltd. [Fmoc-Lys(Boc)-OH, FmocGlu(OtBu)-OH·H2O, N,N-diisopropylethylamine (DIPEA), Et3SiH, CF3CO2H]. Preparative reversed phase high-performance liquid chromatography (HPLC) was carried out on a Japan Analytical Industry model LC-9210 II NEXT recycling preparative HPLC equipped with a JAIGEL-ODS-AP-A column, using MeOH as an eluent at a flow rate of 1.0 mL min−1. Matrix-assisted laser desorption ionization time-offlight mass spectrometry (MALDI-TOF MS) was performed on an AB SCIEX model TOF/TOF(TM) 5800 system spectrometer using acyano-4-hydroxycinnamic acid (CHCA) as a matrix. Transmission electron microscopy (TEM) observations of PtNPs were carried out on a FEI Tecnai model F20 transmission electron microscope operating the accelerating voltage at 200 kV, equipped with a Philips model CM12 CCD camera. TEM observations of PtNP/FmocVKVVC (1)/GO complex, PtNP/GO, and PtNP/1 composites were conducted on a Hitachi High-technology model H7650 transmission electron microscope operating the accelerating voltage at 80 kV. For sample preparation, the colloidal dispersions were dropped on a carbon-coated copper grid and dried in air. For elemental mapping analysis, scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDS) measurements were carried out using a JEOL model JEM-ARM200F transmission electron microscope operating the accelerating voltage at 200 kV. ζ-Potential measurements were performed with a Malvern model Nano ZS Zetasizer using a

Figure 4. (a, b) Schematic representations of the pathways of photogenerated hole and electron for dye degradation (a) and H2 evolution (b). (c) Bar graph of generated H2 (red) and O2 (blue) upon photoirradiation (Xe lamp, λ = 385−740 nm) to an aqueous solution of Na2S2O4 alone (blank) and that containing PtNP/1, 1/ GO, PtNP/GO, or PtNP/1/GO. H2 was generated only from a system containing PtNP/1/GO.

This mechanism is also supported by experiments using Na2S2O4 as a hole scavenger instead of RhB (Figure 4b): Photoirradiation of an aqueous dispersion of PtNP/1/GO complex and Na2S2O4 under Ar resulted in the H2 evolution, together with the generation of O2 (Figures 4c and S11). In this case, photoinduced ET from GO to PtNP leads to the reduction of H2O to generate H2. In contrast, other combinations such as PtNP/1 and PtNP/GO showed no H2 evolution. PtNP/1 complex hardly shows absorption bands in the visible wavelength region, which is the primary reason why PtNP/1 does not show photoinduced H2 evolution. As for the composite of PtNP/GO, PtNPs are not immobilized on the surface of GO but rather dispersed in water, which causes very low efficiency of photoinduced ET from GO to PtNP that D


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ACS Applied Materials & Interfaces disposable capillary cell (750 μL). Samples were dispersed in a mixture of water/MeOH (1/1 v/v). Analytical disc centrifugation (ADC) measurements were conducted using a DC 24000 from CPS Instruments. A volume of 0.1 mL of the sample was analyzed with a saccharose gradient and an external standard (polyvinyl chloride particles with 0.237 μm diameter), while the disc was operated at 24 000 rpm. Electronic absorption spectra were measured using Thermo Scientific model Evolution 201 or JASCO model V-570 spectrophotometers with the wavelength range of 200−800 nm. For measurements, a quartz cuvette with 10 mm path length was used. Circular dichroism (CD) spectra in MeOH (0.5 mM) were recorded at 25 °C with a JASCO model J-1000 spectropolarimeter using a 1 mm path length SQ-grade quartz cell. Cyclic voltammetry (CV) was performed on an ALS/[H] CH Instruments model 1202 electrochemical analyzer using platinum working and counter electrodes and an Ag/AgCl reference electrode. An aqueous solution containing NaCl (0.3 M) was used as a supporting electrolyte, and the electrochemical potential was determined with respect to standard calomel electrode (SCE). For measurements, the working electrode was coated with PtNP/1/GO or PtNP/GO composites and air-died. Synthesis of Fmoc- and NH2-Peptides. Peptides are synthesized with a typical Fmoc-solid phase peptide synthesis technique with the following three protocols: (i) Fmoc deprotection, (ii) loading of Fmocamino acid monomer, and (iii) cleavage f rom the resin, as reported previously.14,19,20,27 i. Fmoc Deprotection. Typically, piperidine (2 mL) was added to a dimethylformamide (DMF, 8 mL) suspension of Fmoc-SAL resin (0.55 mmol g−1, 0.5 g, Watanabe Chemical Co.), and the mixture was stirred for 20 min at 25 °C. The suspension was filtered to obtain the deprotected resin, which was washed ten times with DMF (3 mL, 1 min stirring). ii. Loading of Fmoc-Amino Acid Monomer. Typically, to the resin subjected to protocol i were added Fmoc-amino acid monomer (Fmoc-Lys(Boc)-OH, Fmoc-Cys(Trt)-OH, Fmoc-Val-OH, FmocGlu(OtBu)-OH·H2O, or Fmoc-His(Trt)-OH, 1.1 mmol), HBTU (375 mg, 0.99 mmol), HOBT (148 mg, 1.1 mmol), DMF (10 mL), and DIPEA (385 μL, 2.2 mmol) in this order, and the mixture was stirred for 1 h. The suspension was filtered to obtain the resin, which was washed ten times with DMF. Procedures i and ii were repeated for preparation of resin bearing Fmoc-peptides. For the preparation of NH2-peptide, procedure i was repeated one more time. iii. Cleavage from the Resin. Typically, the resin bearing the Rpeptide (R = Fmoc or NH2) was washed three times with Et2O (3 mL, 5 min stirring) and dried under vacuum. To the resin was added a mixture of CF3CO2H (TFA, 2.85 mL), Et3SiH (0.075 mL), and H2O (0.075 mL), and the mixture was stirred for 2 h at 25 °C. The mixed suspension including the resin was decanted to obtain a supernatant solution of the peptide. The resin was washed by decantation alternately with MeOH (3 mL, 3 min stirring) and CH2Cl2 (3 mL, 3 min stirring) three times. The combined solution containing peptide was evaporated to dryness, and the residue was purified by reprecipitation from MeOH/Et2O at least three times, affording desired R-peptide as white powdery substance in 20−40% yield (Scheme S1). The products were subjected to recycling preparative HPLC with MeOH/TFA (100/0.1) as an eluent, where the major fraction was collected and evaporated to allow isolation of the corresponding peptide as white solid. The final products were characterized by MALDI-TOF MS spectrometry. Synthesis of PtNPs. PtNPs were produced by pulsed laser ablation of a Pt target (99.95%, AGOSI) in Milli-Q water (resistivity of 18.2 MΩ cm at 25 °C) with a homemade flow-through chamber (liquid layer 5 mm) made of aluminum.28−31 For production of the particles, a Nd:YAG nanosecond laser (Rofin-Sinar RS-Marker 100D, pulse duration 40 ns) was used at a fundamental wavelength of 1064 nm, a repetition rate of 5 kHz, and a pulse energy of 6.4 mJ. The laser beam was focused with an F-theta lens of 63 mm focal length and was guided in a spiral pattern on the target by coupling the laser beam with a scanner system (Scanlab). The concentration of Pt colloid (94.3 μg mL−1 corresponding to a productivity of 0.54 mg h−1) was determined

by weighting the target before and after ablation with a microbalance (Sartorius M235S). To decrease the size of the generated PtNPs, laser fragmentation was conducted using the Pt colloids (92.0 mg L−1) in phosphate buffer (0.1 mM, pH = 7.0) with the laser power of 50 W, repetition rate of 2.0 kHz, and pulse energy of 25 mJ. The evaluation of the size and distribution of the resultant PtNPs were carried out by TEM and analytical disc centrifugation (ADC). dav of the PtNPs was 11 nm by TEM and 5 nm by ADC (Figure S13) Synthesis of GO. The GO nanosheets were prepared by a modified Hummers method from graphite powder (particle size; 45 μm, Wako Pure Chemical Industries).15,16,32 Typically, 3.0 g of graphite powder was added to a mixture of concentrated H2SO4 (95%, 360 mL) and H3PO4 (45 mL). Then, 18.5 g of KMnO4 was slowly added to the solution at 0 °C. The mixture was stirred at 400 rpm for 24 h at 60 °C. The mixture was then cooled to 0 °C, and distilled H2O (390 mL) and H2O2 (30 mL) were slowly added to the mixture. The resultant yellow-brown suspension was centrifuged several times for purification of GO, where HCl, distilled H2O, and C2H5OH were used to wash the sample. The resultant dark-brown suspension was ultrasonicated for 3 h in cooled hexane. The suspension with brightbrown precipitates was filtered by 0.2 mm Omniporemembrane filter. The obtained precipitates were dried under vacuum. The obtained brown powder (0.1 g) was then added to the distilled H2O (30 mL) and ultrasonicated for 5 h to obtain an aqueous dispersion of GO. The band gap of the resultant GO was evaluated by a Tauc plot of the electronic absorption spectroscopy as 4.4 and 3.4 eV (Figure S14).33 The larger value indicates π−π* transition, while the smaller value indicates the band gap at the local area with substituents.34 Preparation of Complexes from PtNP, Peptides, and GO. The complexes of PtNP, peptide, and GO were prepared in accordance with the following procedures. A MeOH solution of peptide (1.0 mg mL−1, 0.005−0.5 mL) and an aqueous dispersion of GO (3.3 mg mL−1, 0.3 mL) were mixed together to form peptide/GO complex. After being evaporated to dryness, the residue was again dispersed in water (4.4 mL). Then, aqueous dispersion of PtNPs (92 mg mL−1, 5.6 mL) was added into the dispersion and stirred to yield a PtNP/peptide/GO complex. From the STEM-EDS mapping measurements, observed nanoparticles were identified as Pt atom (Figure S15). Experimental Procedures of Photoinduced Dye Degradation. Dispersions of PtNPs, peptide, GO, or their complexes were added to an aqueous solution of rhodamin B (RhB, 10 mg mL−1, 10 mL). The resultant mixture was allowed to stand for 1 h in the dark to sufficiently immerse the dye solution with the catalysts. The photoirradiation was conducted using UV-LED (LZ4-40U600; λ = 365 nm, 11 W) from LED Engin in combination with Ledil lenses (FA110911_NIS033U-SS) or SAN-EI model XES-502S AM1.5G solar simulator (450 W). The degree of the degradation was monitored by following the transmittance of the mixed solution at 550 nm. For clarifying the effects of dioxygen, dye degradation experiments after bubbling with dioxygen were carried out under the following setup: A 3 mL of an aqueous dispersion containing GO/1/PtNP (0.1 mg mL−1) and RhB (10 mg mL−1, 41.8 mM) was charged into a quartz cell with 10 mm optical path length and sealed with a rubber septum. Oxygen gas was bubbled for 15 min, and then photoirradiation to the dispersion was carried out with a xenon lamp equipped with a UV-type mirror module (Asahi Spectra, MAX-301, 300 W, λ = 250−385 nm). Experimental Procedures of Photoinduced Hydrogen Evolution. A vial containing an aqueous dispersion (4.0 mL) of PtNP/Fmoc-VKVVC/GO (200 μg/20 μg/4.0 mg) and Na2S2O4 (10 mM) was sealed with a rubber septum. The vial was deaerated by Ar bubbling for 20 min. The reaction was started by the photoirradiation of the solution with a xenon lamp equipped with a VIS-type mirror module (Asahi Spectra, MAX-301, 300 W, λ = 385−740 nm). After 10 min irradiation, 100 μL of Ar gas was injected into the vial, and then the same volume of gas in the headspace of the vial was sampled by a gastight syringe and quantified by a Shimadzu GC-2014 gas chromatograph (GC) [Ar carrier, a packed column with 5 Å molecular sieves (3.0 mm × 3.0 m, 60−80 mesh) at 333 K] equipped with a thermal conductivity detector (TCD). The mole amounts of the generated H2 and O2 were calculated on the basis of calibration curves. E


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(8) Liu, J.; Zhang, Y.; Lu, L.; Wu, G.; Chen, W. Self-Regenerated Solar-Driven Photocatalytic Water-Splitting by Ureaderived Graphitic Carbon Nitride with Platinum Nanoparticles. Chem. Commun. 2012, 48, 8826−8828. (9) Zhang, N.; Yang, M.-Q.; Liu, S.; Sun, Y.; Xu, Y.-J. Waltzing with the Versatile Platform of Graphene to Synthesize Composite Photocatalysts. Chem. Rev. 2015, 115, 10307−10377. (10) Zhang, N.; Xu, Y.-J. The Endeavour to Advance Graphene− Semiconductor Composite-Based Photocatalysis. CrystEngComm 2016, 18, 24−37. (11) Kearley, G. J.; Fillaux, F.; Baron, M.-H.; Bennington, S.; Tomkinson, J. A New Look at Proton Transfer Dynamics Along the Hydrogen Bonds in Amides and Peptides. Science 1994, 264 (5143), 1285−1289. (12) Giese, B.; Graber, M.; Cordes, M. Electron Transfer in Peptides and Proteins. Curr. Opin. Chem. Biol. 2008, 12, 755−759. (13) Tarakeshwar, P.; Palma, G. P.; Fromme, P.; Yarger, J. L.; Mujica, V. Probing the Nature of Charge Transfer at Nano−Bio Interfaces: Peptides on Metal Oxide Nanoparticles. J. Phys. Chem. Lett. 2014, 5, 3555−3559. (14) Mizutaru, T.; Sakuraba, T.; Nakayama, T.; Marzun, G.; Wagener, P.; Rehbock, C.; Barcikowski, S.; Murakami, K.; Fujita, J.; Ishii, N.; Yamamoto, Y. Cysteine-Containing Oligopeptide β-Sheets as Redispersants for Agglomerated Metal Nanoparticles. J. Mater. Chem. A 2015, 3, 17612−17619. (15) Hummers, W. S., Jr.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339. (16) Aikyo, Y.; Kushida, S.; Braam, D.; Kuwabara, J.; Kondo, T.; Kanbara, T.; Nakamura, J.; Lorke, A.; Yamamoto, Y. Enwrapping Conjugated Polymer Microspheres with Graphene Oxide Nanosheets. Chem. Lett. 2016, 45, 1024−1026. (17) Rehbock, C.; Jakobi, J.; Gamrad, L.; van der Meer, S.; Tiedemann, D.; Taylor, U.; Kues, W.; Rath, D.; Barcikowski, S. Current State of Laser Synthesis of Metal and Alloy Nanoparticles as Ligand-Free Reference Materials for Nano-Toxicological Assays. Beilstein J. Nanotechnol. 2014, 5, 1523−1541. (18) Marzun, G.; Nakamura, J.; Zhang, X.; Barcikowski, S.; Wagener, P. Size Control and Supporting of Palladium Nanoparticles Made by Laser Ablation in Saline Solution as a Facile Route to Heterogeneous Catalysts. Appl. Surf. Sci. 2015, 348, 75−84. (19) Behrendt, R.; White, P.; Offer, J. Advances in Fmoc Solid-Phase Peptide Synthesis. J. Pept. Sci. 2016, 22, 4−27. (20) Nakayama, T.; Sakuraba, T.; Tomita, S.; Kaneko, A.; Takai, E.; Shiraki, K.; Tashiro, K.; Ishii, N.; Hasegawa, Y.; Yamada, Y.; Kumai, R.; Yamamoto, Y. Charge-Separated Fmoc-Peptide β-Sheets: SequenceSecondary Structure Relationship for Arranging Charged Side Chains on Both Sides. Asian J. Org. Chem. 2014, 3, 1182−1188. (21) Hamasaki, T.; Kashiwagi, T.; Imada, T.; Nakamichi, N.; Aramaki, S.; Toh, K.; Morisawa, S.; Shimakoshi, H.; Hisaeda, Y.; Shirahata, S. Kinetic Analysis of Superoxide Anion Radical-Scavenging and Hydroxyl Radical-Scavenging Activities of Platinum Nanoparticles. Langmuir 2008, 24, 7354−7364. (22) Chen, C.; Li, X.; Ma, W.; Zhao, J. Effect of Transition Metal Ions on the TiO2-Assisted Photodegradation of Dyes under Visible Irradiation: A Probe for the Interfacial Electron Transfer Process and Reaction Mechanism. J. Phys. Chem. B 2002, 106, 318−324. (23) Wilhelm, P.; Stephan, D. Photodegradation of Rhodamine B in Aqueous Solution via SiO2@TiO2 Nano-Spheres. J. Photochem. Photobiol., A 2007, 185, 19−25. (24) Zhang, Y.; Zhang, N.; Tang, Z.-R.; Xu, Y.-J. Graphene Oxide as a Surfactant and Support for In-Situ Synthesis of Au−Pd Nanoalloys with Improved Visible Light Photocatalytic Activity. J. Phys. Chem. C 2014, 118, 5299−5308. (25) Matsumoto, Y.; Koinuma, M.; Kim, S. Y.; Watanabe, Y.; Taniguchi, T.; Hatakeyama, K.; Tateishi, H.; Ida, S. Simple Photoreduction of Graphene Oxide Nanosheet under Mild Conditions. ACS Appl. Mater. Interfaces 2010, 2, 3461−3466. (26) Iwase, A.; Yoshino, S.; Takayama, T.; Ng, Y. H.; Amal, R.; Kudo, A. Water Splitting and CO2 Reduction under Visible Light Irradiation

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b16765. Materials and measurements, synthesis of peptides, PtNPs, GO, and their complexes, UV−vis spectra, TEM, ADC, ζ-potential, CD, dye degradation, and GC (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tsukasa Mizutaru: 0000-0003-4018-402X Stephan Barcikowski: 0000-0002-9739-7272 Takahiko Kojima: 0000-0001-9941-8375 Yohei Yamamoto: 0000-0002-2166-3730 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Ms. Birgit Gleising from the Team of Interdisciplinary Center for Analytics on the Nanoscale (ICAN) at the University of Duisburg-Essen for the TEM analyses, Dr. Masaki Takeguchi at National Institute for Materials Science (NIMS) for TEM-EDS measurements, and Mr. Amandeep Jindal at Indian Institute of Technology Delhi (IITD) for the support of electrochemical measurements. This work was partly supported by Grant-in-Aid for Scientific Research on Innovative Areas (No. 15H00860 & 15H00861) from JSPS/MEXT Japan, DAAD−University of Tsukuba partnership program, and University of Tsukuba Prestrategic initiative “Ensemble of light with matters and life”. G.M. and S.K. thank the German Ministry of Research and Education (BMBF) for its support within the Young Investigator Competition NanoMatFutur (Innokat, FKZ 03X5523).

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REFERENCES

(1) White, R. J.; Luque, R.; Budarin, V. L.; Clark, J. H.; Macquarrie, D. Supported Metal Nanoparticles on Porous Materials, Methods and applications. Chem. Soc. Rev. 2009, 38, 481−494. (2) Yoshida, H.; Kuwauchi, Y.; Jinschek, J. R.; Sun, K.; Tanaka, S.; Kohyama, M.; Shimada, S.; Haruta, M.; Takeda, S. Visualizing Gas Molecules Interacting with Supported Nanoparticulate Catalysts at Reaction Conditions. Science 2012, 335, 317−319. (3) Wang, C.; Daimon, H.; Onodera, J.; Koda, T.; Sun, S.; Palma, G. L. A General Approach to the Size- and Shape-Controlled Synthesis of Platinum Nanoparticles and Their Catalytic Reduction of Oxygen. Angew. Chem., Int. Ed. 2008, 47, 3588−3591. (4) Tan, T. L.; Wang, L.-L.; Zhang, J.; Johnson, D. D.; Bai, K. Platinum Nanoparticle During Electrochemical Hydrogen Evolution: Adsorbate Distribution, Active Reaction Species, and Size Effect. ACS Catal. 2015, 5, 2376−2383. (5) Tanev, P. T.; Chibwe, M.; Pinnavaia, T. J. Titanium-Containing Mesoporpus Molecular Sieves for Catalytic Oxidation of Aromatic Compounds. Nature 1994, 368, 321−323. (6) Wang, J.; Yin, G.; Shao, Y.; Zhang, S.; Wang, Z.; Gao, Y. Effect of Carbon Black Support Corrosion on the Durability of Pt/C Catalyst. J. Power Sources 2007, 171, 331−339. (7) Yoo, E. J.; Okata, T.; Akita, T.; Kohyama, M.; Nakamura, J.; Honma, I. Enhanced Electrocatalytic Activity of Pt Subnanoclusters on Graphene Nanosheet Surface. Nano Lett. 2009, 9, 2255−2259. F


Research Article

ACS Applied Materials & Interfaces Using Z-Scheme Systems Consisting of Metal Sulfides, CoOx-Loaded BiVO4, and a Reduced Graphene Oxide Electron Mediator. J. Am. Chem. Soc. 2016, 138, 10260−10264. (27) Vairaprakash, P.; Ueki, H.; Tashiro, K.; Yaghi, O. M. Synthesis of Metal−Organic Complex Arrays. J. Am. Chem. Soc. 2011, 133, 759− 761. (28) Barcikowski, S.; Menéndez-Manjón, A.; Chichkov, B.; Brikas, M.; Raciukaitis, G. Generation of Nanoparticle Colloids by Picosecond and Femtosecond Laser Ablations in Liquid Flow. Appl. Phys. Lett. 2007, 91, 083113. (29) Amendola, V.; Meneghetti, M. Laser Ablation Synthesis in Solution and Size Manipulation of Noble Metal Nanoparticles. Phys. Chem. Chem. Phys. 2009, 11, 3805−3821. (30) Rehbock, C.; Merk, V.; Gamrad, L.; Streubel, R.; Barcikowski, S. Size Control of Laser-Fabricated Surfactant-Free Gold Nanoparticles with Highly Diluted Electrolytes and their Subsequent Bioconjugation. Phys. Chem. Chem. Phys. 2013, 15, 3057−3067. (31) Barcikowski, S.; Amendola, V.; Marzun, G.; Rehbock, C.; Reichenberger, S.; Zhang, D.; Göcke, B. Handbook of Laser Synthesis of Colloids; DuE Pubulico: Essen, Germany, 2016. (32) Ding, Y.; Bai, W.; Sun, J.; Wu, Y.; Memon, M. A.; Wang, C.; Liu, C.; Huang, Y.; Geng, J. Cellulose Tailored Anatase TiO2 Nanospindles in Three-Dimensional Graphene Composites for High-Performance Supercapacitors. ACS Appl. Mater. Interfaces 2016, 8, 12165−12175. (33) Tauc, J. Absorption edge and internal electric fields in amorphous semiconductors. Mater. Res. Bull. 1970, 5, 721−730. (34) Liang, H. F.; Smith, C. T. G.; Mills, C. A.; Silva, S. R. P. The band structure of graphene oxide examined using photoluminescence spectroscopy. J. Mater. Chem. C 2015, 3, 12484−12491.

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