Tubular Hybrids: A Nanoparticle – Molecular Network - Langmuir

Article Cite This: Langmuir 2018, 34, 2464−2470

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Tubular Hybrids: A NanoparticleMolecular Network Priyadarshi Ranjan,†,‡ Sreejith Shankar,†,∥ Ronit Popovitz-Biro,§ Sidney R. Cohen,§ Iddo Pinkas,§ Reshef Tenne,*,‡ Michal Lahav,† and Milko E. van der Boom*,† †

Department of Organic Chemistry, ‡Department of Materials and Interfaces, and §Department of Chemical Research Support, The Weizmann Institute of Science, Rehovot 7610001, Israel

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S Supporting Information *

ABSTRACT: We report here a new methodology for the formation of freestanding nanotubes composed of individual gold nanoparticles (NPs) cross-linked by coordination complexes or porphyrin molecules using WS2 nanotubes (INT-WS2) as a template. Our method consists of three steps: (i) coverage of these robust inorganic materials with monodispersed and dense monolayers of gold NPs, (ii) formation of a molecular AuNP network by exposing these decorated tubes to solutions containing a ruthenium polypyridyl complex or meso-tetra(4-pyridyl)porphyrin, and (iii) removal of the INT-WS2 template with a hydrogen peroxide solution. Nanoindentation of the template-free AuNP tubes with atomic force microscopy indicates a radial elastic modulus of 4 GPa. The template-free molecular AuNP tubes are characterized using scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy, and micro-Raman spectroscopy. The methodology provides a convenient and scalable strategy for the realization of molecular AuNP tubes with a defined length and diameter, depending on the dimensions of the template.

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INTRODUCTION The design and formation of organized and well-defined onedimensional (1D) and two-dimensional (2D) structures based on individual plasmonic metallic particles and functional organic molecules or metal complexes is a great challenge. Materials based on metallic particles have been designed using bottom-up and top-down approaches. However, relatively few hollow structures composed of nanoparticles (NPs) (i.e., spheres and tubes) have been reported.1,2 Rubinstein and coworkers reported an example of the templated synthesis of metallic NP nanotubes.3,4 Their multiwalled nanotubes consist of fused NPs generated using an outer template of aluminum oxide having nanopores functionalized with an aminosilane to allow binding of citrate-capped AuNPs. Removal of the template under basic conditions resulted in elongated nanostructures. In another example, microporous organic nanotubes were used as templates for the formation of particulate Fe2O3-based nanotubes.5,6 The literature reports on diverse nanotubular structures, including tubes composed of biomaterials7−9 and coordination polymers.10−12 However, to the best of our knowledge, tubular architectures consisting of metallic NPs stitched together via molecular bridging moieties are unknown. In this paper, we report on a method for the surface functionalization of intrinsically inert inorganic nanotubes (INTs)13−18-composed of transition metal chalcogenides (WS2). We show here how the chemically inert surfaces of these inorganic materials can be homogeneously coated with uniform monolayers of AuNPs by interfacial lattice matching involving the individual components. The synthetic procedures available for the functionalization of such inorganic materials with NPs are often surface-defect-guided.19 Furthermore, we © 2018 American Chemical Society

demonstrate here the formation of molecular AuNP nanotubes using INTs as an inner template. These single-walled hybrids consist of continuous networks of AuNPs and ruthenium polypyridyl complexes (1) or porphyrin molecules (2). The advantages of the chosen cross-linkers include (i) high extinction coefficients (facilitating the use of optical spectroscopy to follow the process), (ii) stability under the experimental conditions, (iii) strong and stable bonding between the NPs as well as between NPs and INT-WS2, (iv) starting materials that are either easy to synthesize (1) or commercially available (2), and (v) well-studied coordinative interactions with AuNPs. These INT-WS2 templates are first decorated with a single layer of AuNPs and subsequently crosslinked with the molecular component. Although these transition metal dichalcogenide materials are robust, they react with hydrogen peroxide (H2O2). Reaction of our assemblies with H2O2 results in the oxidative removal of the inorganic template, generating freestanding molecular metallic NP nanotubes (Scheme 1). The use of sulfur-terminated molecules was avoided because such compounds may undergo unwanted oxidative side reactions during the removal of the inorganic templates. The strategy described here not only provides a unique and unprecedented methodology to surface functionalize the otherwise inert INT-WS2 but also allows the generation of hollow tubes of molecularly cross-linked, individual AuNPs via a straightforward and efficient threestep protocol. Received: September 6, 2017 Revised: November 27, 2017 Published: January 16, 2018 2464

DOI: 10.1021/acs.langmuir.7b03125 Langmuir 2018, 34, 2464−2470

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Scheme 1. Schematic Rendering of the Structures of the Molecular Cross-Linkers (1 and 2), Decoration of the INT-WS2 with AuNPs (AuNP/INT-WS2), Cross-Linking of the Surface-Bound AuNPs, and Removal of the Inorganic Template (INT-WS2) to Generate Freestanding Ru × AuNP/NT or Por × AuNP/NTa

a

The average interparticle spacing is similar to the dimensions of the cross-linkers (1 and 2)

Figure 1. (A−C) TEM images showing pristine INT-WS2 reacting with aqueous H2O2 (15%) after 1 h. (D) Photographs of the reaction solution showing the pristine INT-WS2 at t = 0 (left) and the complete disappearance of the black tubes after 15 h (right). (E) UV/vis spectra of the reaction mixture recorded at t = 0 (black), t = 5 h (red), t = 10 h (blue), and t = 15 h (green).

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RESULTS AND DISCUSSION

coatings of a single layer of NPs (Figures 2A left and middle, 2B, and 3A). The average interparticle spacing is ∼2 nm. Freshly prepared AuNP/INT-WS2 was used for the second step. The reaction of a dispersion of AuNP/INT-WS2 with [Ru(mbpy-py)3][PF6]2 (1) in dimethylformamide (DMF)/ toluene (1:1; v/v) under ice-cold conditions resulted in the formation of Ru × AuNP/INT-WS2. The isostructural osmium complex [Os(mbpy-py)3][PF6]2 is known to induce aggregation of AuNPs because of the electrostatic interactions and η1coordination of the vinylpyridine moieties to the gold surfaces.20 The SEM and TEM images of Ru × AuNP/INT-WS2 ascertained that the dense and homogeneous AuNP coating remained intact (Figure 2A, right). Figures 2C and 3B show the nanostructures with partially removed INT-WS2 template. The UV/vis spectrum of Ru × AuNP/INT-WS2 drop-casted on quartz slides shows the absorption minima of the AuNPdecorated INTs at λ = 622 nm (A exciton), λ = 518 nm (B exciton), and the characteristic metal-to-ligand charge-transfer (MLCT) band at λ = 492 nm of the ruthenium polypyridyl complex (Figure S1A). The reaction of a dispersion of AuNP/ INT-WS2 with meso-tetra(4-pyridyl)porphyrin (por; 2) also resulted in the incorporation of the molecular component with retention of the dense and uniform AuNP decoration, as shown by SEM and UV/vis spectroscopy (Figures 2 and S1B). The UV/vis spectrum of por × AuNP/INT-WS2 shows the distinct and intense Soret band of porphyrin at λ = 437 nm in addition

To demonstrate the feasibility of the oxidative removal of the INT-WS2 template, we initially carried out a blank reaction between pristine INT-WS2 and a 15% H2O2 solution at room temperature. Transmission electron microscopy (TEM) images showing the progress of the reaction after 1 h are shown in Figure 1A−C. It appears that the ends of the nanotubes are more susceptible to oxidation. Figure 1D shows how the black nanotubes disappear from solution during the course of the reaction. Likewise, time-dependent UV/vis spectroscopy revealed the gradual disappearance of the characteristic bands of the pristine INT-WS2 over 15 h (Figure 1E). The first step of our method to prepare the tubular hybrids is the decoration of the surface of INT-WS2 with tetraoctylammonium bromide (TOAB)-capped AuNPs (Scheme 1). The AuNP/INT-WS2 preparation was carried out in a straightforward procedure, by sonication of the suspension of INT-WS2 in toluene with the preformed AuNPs (ØAuNP = 5 nm). The use of ice-cold solutions resulted in optimal coating of the nanotubes with AuNPs. The process is nearly quantitative in yield with respect to the amounts of INT-WS2 and AuNPs used for the reaction. All solutions and suspensions were cooled in ice before adding or mixing. These AuNP/INT-WS2 hybrid structures were characterized by scanning electron microscopy (SEM) and TEM revealing monodispersed and uniform 2465

DOI: 10.1021/acs.langmuir.7b03125 Langmuir 2018, 34, 2464−2470

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Figure 3. TEM images of (A) AuNP/INT-WS2, (B) Ru × AuNP/ INT-WS2 with the INT-WS2 template being partially etched with H2O2, and (C) Ru × AuNP/NT. The arrows in the magnified images on the right highlight the etching of the inorganic template.

The removal of the INT-WS2 template of the AuNPdecorated tubes to obtain the freestanding tubular hybrids was carried out using H2O2. Ru × AuNP/INT-WS2 and por × AuNP/INT-WS2 were dispersed in an aqueous H2O2 solution (15%) and kept at 4 °C for 24 h. The low temperature ensured the stability of the coordinative binding of AuNPs and the molecular components 1 or 2, thereby only the inorganic template reacted with H2O2. This treatment resulted in the formation of freestanding nanotubes composed of AuNPs held together by the molecular components (Ru × AuNP/NT and por × AuNP/NT). The INT-WS2 template is oxidized and removed by H2O2 from these tubular single-layered structures as confirmed by the disappearance of the features characteristic of the WS2 van der Waals (vdW) layer spacing (Figures 2D and 3C). Energy-dispersive X-ray spectroscopy (EDS) of the assynthesized Ru × AuNP/NTs shows a high gold atomic fraction of 87 at.%, whereas only minor amounts of tungsten (6 at.%) and sulfur (7 at.%) remained (Figure S2). The interparticle distances (∼2.0 nm) are similar to the distance between the N-pyridine moieties of [Ru(mbpy-py)3][PF6]2 (1; 2.3 nm) and meso-tetra(4-pyridyl)porphine (2; 1.5 nm). The shape, size, and crystallinity of AuNPs are not affected by our etching procedure. However, the impact of the etching process on the packing of the NPs cannot be rigorously excluded. The removal of the INT-WS2 template from AuNP/INT-WS2 (where AuNPs are not cross-linked by the molecules) did not result in tubular hybrids, confirming that the molecular component is essential. The reaction process between Ru × AuNP/INT-WS2 or por × AuNP/INT-WS2 and H2O2 was monitored by ex situ Raman spectroscopy, showing the gradual disappearance of the

Figure 2. TEM and SEM images. (A) Left and middle: INT-WS2 decorated with AuNPs. Right: Ru × AuNP/INT-WS2. (B) Left: highresolution TEM (HRTEM) image of AuNP/INT-WS2; middle: HRTEM image of the area marked in yellow on the left image at the interface of the AuNPs and INT-WS2. The white lines indicate the lattice continuation between AuNPs (111) and INT-WS2 (013). Right: image displaying the corresponding fast Fourier transform showing lattice matching (circle). ØAuNP = 5 nm. (C) Ru × AuNP/ INT-WS2 with the INT-WS2 template being partially etched with H2O2, (D) freestanding Ru × AuNP/NT and por × AuNP/NT. For clarity, only the two images of por × AuNP/NT have labels.

to the bands of the AuNP-decorated INTs. For both Ru × AuNP/INT-WS2 and por × AuNP/INT-WS2, the absorption bands are slightly shifted, relative to the individual molecular and inorganic components (Figure S1A,B). Our observations indicate that the binding affinity of the pyridine-based molecules to the AuNP surface is higher than that of the TOAB-capping layer, but these compounds do not strip the AuNPs off the surface of the INT-WS2 template under the applied reaction conditions. Multiple binding of the sulfur groups of INT-WS2 ensures a reasonably stable coating of AuNPs, which can further be chemically functionalized. 2466

DOI: 10.1021/acs.langmuir.7b03125 Langmuir 2018, 34, 2464−2470

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Langmuir characteristic bands at ν = 353 cm−1 (E2g) and ν = 421 cm−1 (A1g) of the INT-WS2 (Figures 4 and S3).

The decoration of INT-WS2 with metallic NPs is a selflimiting reaction; only monolayers of NPs are formed on the surface of the tubes (Figures 2 and 3). Interestingly, the ruthenium polypyridyl complexes of Ru × AuNP/INT-WS2 and Ru × AuNP/NT allow binding of additional amounts of AuNPs to the surfaces of these tubes. The reaction of Ru × AuNP/INT-WS2 and Ru × AuNP/NT with a toluene dispersion of TOAB-capped AuNPs resulted in the formation of walls consisting of densely packed NPs, as shown by SEM imaging (Figures S5 and S6). The polypyridyl complexes can induce aggregation of metallic NPs via electrostatic interactions.20 In our case, η1-coordination of free vinylpyridine moieties can also contribute to the binding of additional AuNPs to the surface. The radial modulus of INT-WS2 nanotubes was previously measured.22,23 These measurements and their analyses are nontrivial because shear between the internal, multiwalled layers plays a significant role in the force balance. The measurements reported here address a different question, namely, the nature of the cohesion within the outer NP shell and the changes occurring when the supporting INT-WS2 is removed. The larger size of these nanotubes allows, when using small indentation depths, analytical contact mechanics analysis. Although this semiquantitative analysis ignores the nonidealities of this system, it is useful for direct comparison of INT-WS2supported and the unsupported tubes and additionally provides a rough approximation of the modulus value. By keeping the indentation depths to 10−15% of the tube thickness and using large-radius probes, a semiquantitative comparison between the supported and unsupported nanotubes is obtained. The topographic image and profiles in Figure 5 show that the removal of the INT results in flattening of the structure, leaving

Figure 4. Raman spectra of (A) Ru × AuNP/INT-WS2, (B) Ru × AuNP/NT, and (C) enlargement of the area marked in (B); Ru × AuNP/NT (black) and [Ru(mbpy-py)3][PF6]2 (green). These measurements were performed on single nanostructures drop-casted on silicon substrates. The Raman signature of the silicon substrate (*) is evident in the spectra.

Interestingly, the resonance Raman bands of [Ru(mbpypy)3][PF6]2 (1) and meso-tetra(4-pyridyl)porphyrin (2) are not clearly observable in the starting material probably because of the depletion of ground state electrons (Figures 4A and S3A). However, these characteristic bands (ν = 1100−1800 cm−1) are apparent in the freestanding Ru × AuNP/NT and por × AuNP/NT structures (Figures 4B,C and S3B), unambiguously demonstrating the presence of [Ru(mbpy-py)3]2+ (1) and meso-tetra(4-pyridyl)porphyrin (2) in these tubular hybrids. The ruthenium complex is unlikely to be affected by the etching process with H2O2. The characteristic MLCT band is clearly visible after dissolving the Cl derivative of the ruthenium complex in an aqueous solution containing H2O2 (30%) for 15 h, as shown in the UV/vis spectrum (Figure S4). The Cl derivative was used instead of the PF6 salt to enhance the solubility in water. Mass spectrometry showed the molecular mass of the ruthenium complex [m/z = 956.29 (M+ − Cl−) and 460.79 (M2+ − 2Cl−)]. Similar results were obtained with the porphyrin derivative. Although this molecular compound is considered to be very stable,21 it is probably oxidized by H2O2 as judged by both UV/vis and mass spectrometry (m/z = 633.36). Nevertheless, it did not affect the formation of the unique freestanding hybrid nanotubes of individual AuNPs.

Figure 5. Peak-force tapping images of Ru × AuNP/INT-WS2 (A) and Ru × AuNP/NT (B). The upper frames show the topographic image with cross-profile section measuring the nanotube dimensions, as indicated by the profile shown below.

a broader and flatter tube with a slightly irregular shape. The height profile of a representative Ru × AuNP/NT is ∼30 nm, which is 3× the estimated combined thickness of two AuNP walls (5 nm each; Figure 5B). The empty tube is robust mechanically, and multiple atomic force microscopy (AFM) scans with loads at least an order of magnitude greater than typical AFM scan forces do not destroy or alter the structures. This would not have been possible if the NPs were not strongly interconnected. The elastic behavior of the template-free Ru × AuNP/NT was studied by nanoindentation with AFM and compared with the properties of Ru × AuNP/INT-WS2. 2467

DOI: 10.1021/acs.langmuir.7b03125 Langmuir 2018, 34, 2464−2470

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HRTEM images were obtained using a Tecnai F30 electron microscope (UT, FEI), operating at an accelerating voltage of 300 kV with a Gatan UltraScan 1000 charged-coupled device (CCD) camera. TEM images were obtained with CM120 (ST, Philips), operating at an accelerating voltage of 120 kV with a Gatan UltraScan 1000 CCD camera. EDS was also performed on CM120 (ST, Philips) equipped with an EDAX Genesis EDS system. TEM images were also obtained with JEM-2100 (HT) TEM (JEOL) with a Gatan UltraScan 1000 CCD camera. SEM images were collected with Ultra 55 FEG (Zeiss). UV/vis spectra were obtained using Varian Cary 100 spectrophotometers (in double-beam transmission mode). Absorption data were obtained by scatter-free absorbance spectra collected on a CLARiTY 1000 spectrophotometer (Bogart, GA, USA). Raman measurements from 50 to 1800 cm−1 were made on LabRAM HR Evolution (HORIBA, France) with a 532 nm laser and 2 mW maximum power on the sample. The instrument is equipped with an 800 mm spectrograph allowing for a very high spectral resolution and low stray light. The pixel resolution is ∼1.8 cm−1 when working with a 600 gr/ mm grating and a 532 nm laser. The sample was illuminated using several microscope objectives, such as 100× objective (MPlanFL N NA 0.9, Olympus, Japan). The LabRAM measured using a 1024 × 256 pixel open electrode front-illuminated CCD camera cooled to −60 °C (Syncerity, HORIBA, USA). The system utilizes an open confocal microscope (Olympus BXFM) with a spatial resolution better than 1 μm. The measurements were obtained with a 532 nm laser focused on individual nanotubes. AFM was performed using a MultiMode AFM with NanoScope V controller (Bruker, Santa Barbara, CA, USA). The measurements were made in the peak-force QNM mode whereby simultaneously with the topographic image, a force−distance curve is measured at each pixel. From these curves, radial modulus at a given indentation depth can be estimated analytically by presuming a cylindrical nanotube geometry.24 The probe used was TAP525 (Bruker) with a nominal force constant of 200 N/m. Sample deformation under the tip was kept to about 3 nm, and the tip radius was 60 ± 20 nm. The large uncertainty in this parameter is due to the fact that under the high loads used, on the order of a μN, the Si tip becomes dulled under scanning and leads to changes in its dimensions. This results in uncertainty of approximately 15% in the modulus values obtained. Reaction of INT-WS2 with H2O2. INT-WS2 (0.4 mg) was dispersed in 15% H2O2 (500 μL). The reaction progress was monitored using an integrating sphere spectrophotometer. After 15 h, the black colored dispersion disappeared and a colorless solution remained. The remnants were analyzed by SEM and TEM. Decoration of INT-WS2 with Gold NPs. INT-WS2 (0.4 mg) was dispersed in toluene (1.0 mL) via sonication for 3 min. Subsequently, the dispersion was ice-cooled for 1 h, and an ice-cold toluene solution (200 μL) of TOAB-capped AuNPs (2.67 mg/mL; ØAuNP = 5 nm) was added. The mixture was sonicated for 1 min and allowed to react for 30 min at 0 °C. This step was repeated three times (600 μL of the AuNP−toluene solution was added in total). Performing this coating step without ice causes the NPs to coalesce. Reaction of AuNP/INT-WS2 with [Ru(mbpy-py)3][PF6]2 (1). a A solution of [Ru(mbpy-py)3][PF6]2 (1; 5.0 mg, 4.0 μmol in 100 μL DMF) was added dropwise to a dispersion of AuNP/ INT-WS2 (0.4 mg) in toluene (3.0 mL) by manually shaking. Subsequently, the reaction mixture was kept at ice-cold conditions for 20 min and occasionally shaken manually. The dispersion was washed with toluene/DMF (3 × 5 mL; 10:1 v/ v); the product was allowed to settle and decanted. The extinction spectra of the remaining dispersion (Ru × AuNP/ INT-WS2) were measured in 3.0 mL toluene/DMF (10:1 v/v). b A dispersion of AuNP/INT-WS2 (0.4 mg, 200 μL water/ toluene 3:1, v/v) was drop-casted on a silicon substrate and dried under a gentle flow of air. Subsequently, an ice-cold solution of [Ru(mbpy-py)3][PF6]2 (1; 5.0 mg, 4.0 μmol; 200 μL DMF/toluene 1:1, v/v) was drop-casted on the silicon wafer and allowed to react for 20 min. Then, the substrate was

The radial modulus of the structures under small deformations can be estimated by AFM nanoindentation (see Experimental Section).24,25 Ru × AuNP/INT-WS2 has a ∼50% larger modulus than that of the etched tube. The reduced stiffness of the Ru × AuNP/NT reflects the diminished mechanical stability after removing the INT-WS2 template even though the mechanical properties of the Ru × AuNP shell itself should not change much when INT-WS2 is stripped away. In any case, the estimated radial modulus for the unetched tube is 6 GPa, about an order of magnitude lower than the elastic modulus of the Au metal. The value for the etched tube is 4 GPa, similar to the modulus value recently measured by AFM for metal−organic frameworks.26

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CONCLUDING REMARKS A new bottom-up approach for the formation of single-layered tubular structures consisting of a continuous network of both metallic NPs and structurally well-defined metal complexes or porphyrins has been introduced. The chemical steps involved in the overall process are efficient and straightforward: (i) surface functionalization of 1D nanomaterials of transition metal chalcogenides by metallic NPs, (ii) cross-linking of the NPs with a molecular component, and (iii) oxidative removal of the 1D template. The use of two very different molecules to generate very similar hybrid materials suggests that such tubular structures can be generated with a wide variety of molecular components and physicochemical properties. The single walls of the tubular structures can be chemically and structurally modified by exposing the tubes to dispersions of gold NPs. The large number of nanoscale and microscale structures reported based on transition metal chalcogenides is diverse and growing,27,28 suggesting that many different freestanding hybrids could be generated using procedures shown in this study. Various MS2-based INTs and inorganic fullerenes (IFs) have been reported since the discovery of WS2-based nanotubes about 25 years ago.13 INT-WS2 can find industrial applications in electrical and optical devices and catalysis. A substantial number of lubrication-based products containing IF−WS2 NPs are already on the market.16−18 These facts imply that the present three-step method is scalable to large quantities with readily accessible templates. Diverse functions of different tubular structures have been extensively demonstrated.29−32 Our study provides an efficient method to decorate inorganic nanostructures with a high-density coating of NPs, irrespective of the defect density on the nanotube surface.33,34 Combining NPs with other functional materials using our method can lead to composites with new properties. Superimposing the morphology of the NPs with the geometry of the nanotubes results in surface-to-volume ratios that can advance the development of new catalytic processes.35 Moreover, the molecular nanoparticle networks introduced here might have interesting properties related to artificial photosynthesis.

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EXPERIMENTAL SECTION

Materials and Methods. INT-WS2 was supplied by NanoMaterials Ltd, Israel. The length and diameter of INT-WS2 are in the range 1−20 μm to 30−150 nm, respectively. The TOAB-capped AuNPs (ØAuNP = 5 nm)36 and [Ru(mbpy-py)3][PF6]2 (1)37 were prepared according to published procedures. NaAuCl4·2H2O was purchased from Alfa Aesar. meso-Tetra(4-pyridyl)porphine (2) was purchased from Strem Chemicals. TOAB was purchased from Sigma-Aldrich (98%) and Chem-Impex International (99.4%) and was used as received. 2468

DOI: 10.1021/acs.langmuir.7b03125 Langmuir 2018, 34, 2464−2470

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Langmuir washed by immersing in acetonitrile (10 mL; 3×) and dried under a flow of air. Reaction of AuNP/INT-WS2 with meso-Tetra(4-pyridyl)porphine (2). A dispersion of AuNP/INT-WS2 (0.4 mg, 200 μL water/toluene 3:1, v/v) was drop-casted on a silicon substrate and dried under a gentle flow of air. Subsequently, an ice-cold solution of meso-tetra(4-pyridyl)porphine (2; 5.0 mg, 8.0 μmol; 200 μL DMF/ CHCl3 1:1, v/v) was drop-casted on the Si wafer and allowed to react for 20 min. Then, the substrate was washed by immersion in CHCl3 (10 mL; 3×) and dried under a flow of air. Formation of Freestanding Ru × AuNP/NT and Por × AuNP/ NT. A silicon wafer with Ru × AuNP/INT-WS2 was immersed in an aqueous solution of H2O2 (15%; 500 μL) and kept at 4 °C for 24 h. Alternatively, Ru × AuNP/INT-WS2 can be added to the aqueous solution of H2O2. The resulting Ru × AuNP/NT was characterized by SEM and TEM. The sample was drop-casted on a quartz slide for spectrophotometer measurement. The sample was drop-casted on a silicon wafer for Raman spectroscopic measurements. A sample was also prepared after 18 h and analyzed by SEM, TEM, and Raman spectroscopy. All samples were dried in high vacuum for 2 days before analysis. Por × AuNP/INT-WS2 on a silicon wafer was immersed in a H2O2 (15%; 500 μL) solution at 4 °C for 24 h. The resulting dispersion was analyzed by SEM and TEM showing the formation of por × AuNP/ NT. All samples were dried in high vacuum for 2 days before analysis. The sample was transferred by drop-casting on a quartz slide for UV/ vis spectroscopy or Si surface for Raman measurements.

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Minerva Center for Supramolecular Chemistry grant no. 434000340000. M.E.v.d.B. is the incumbent of Bruce A. Pearlman Chair in Synthetic Organic Chemistry.

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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.langmuir.7b03125. Extinction spectra, EDS spectrum, Raman spectroscopy images, [Ru(mbpy-py)3][PF6]2 (1) in water and aqueous H2O2, and SEM images (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (R.T.). *E-mail: [email protected] (M.E.v.d.B.). ORCID

Reshef Tenne: 0000-0003-4071-0325 Milko E. van der Boom: 0000-0003-4102-4220 Present Address

∥ National Institute for Interdisciplinary Sciences and Technology (CSIRNIIST), Industrial Estate P. O., Thiruvananthapuram, Kerala 695019, India.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Irving and Cherna Moskowitz Center for Nano and Bio-Nano Imaging grant no. 7208214, the Perlman Family Foundation, the Israel Science Foundation grant no. 265/12, the Kimmel Center for Nanoscale Science grant no. 43535000350000, the German-Israel Foundation (GIF) grant no. 712053, the Irving and Azelle Waltcher Foundations in honor of Prof. M. Levy grant no. 720821, the EU project ITN“MoWSeS” grant no. 317451, and the G. M. J. Schmidt 2469

DOI: 10.1021/acs.langmuir.7b03125 Langmuir 2018, 34, 2464−2470

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Langmuir

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DOI: 10.1021/acs.langmuir.7b03125 Langmuir 2018, 34, 2464−2470