Stable and Functional Gold Nanorod Composites with a Metal

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Stable and Functional Gold Nanorod Composites with a Metal− Organic Framework Crystalline Shell Kouta Sugikawa,†,‡ Shunjiro Nagata,§ Yuki Furukawa,§ Kenta Kokado,†,§ and Kazuki Sada*,†,§ †

Department of Chemistry, Graduate School of Science, Hokkaido University, Kita 10, Nishi 8, Kita-ku, Sapporo, 060-0810 Japan Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, Fukuoka, 819-0395 Japan § Department of Chemical Sciences and Engineering, Graduate School of Chemical Sciences and Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-ku, Sapporo, 060-8628 Japan ‡

S Supporting Information *

ABSTRACT: Practical and functional surface-enhanced Raman scattering (SERS)-active nanomaterials working in solution require a protecting shell. In this study, we demonstrate the fabrication of gold nanorods coated by metal−organic frameworks of several hundred nanometers in size, which is one kind of crystalline porous materials, as s suspension-based SERS sensor. The composites also showed enough stability and reproducibility for detection of the guest molecules. KEYWORDS: metal−organic framework, porous coordinating polymer, surface enhanced Raman scattering, gold nanorod, hybrid material

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INTRODUCTION The discovery of surface-enhanced Raman scattering (SERS) gave birth to a powerful new analytical tool that has been the focus of extensive research.1−5 In SERS, Raman scattering, normally a very weak effect, was found to be enhanced a millionfold or more when molecules were adsorbed on roughened or patterned metal arrays on the substrate or aggregates between colloidal metal particles. The design of their architectures allowed subattomolar quantities of analyte to be detected by SERS.6,7 Incremental enhancement factors, 20 times greater than individual particles, were observed due to the suppression of nanoparticle aggregation. These studies suggest that SERS sensors based on metal nanoparticle arrays provides a powerful platform for chemical and biological sensing. In addition to the SERS sensors based on metal nanoparticle arrays, suspension-based SERS sensors were also used for various chemical and biological sensing.8−11 Difficulty in suspension-based SERS has been attributed to changes in the electromagnetic or localized surface plasmon resonance (LSPR) properties of the nanoparticles in solution. Slight variations in either the shape or size of a nanostructure will greatly influence the LSPR or the nanoparticles and, as a consequence, their SERS enhancements. Furthermore, nanoparticles easily aggregate and the LSPR of the structures will couple resulting in a new lower energy extinction band which will impact the intensity of a SERS signal. To both maintain consistent electromagnetic properties and prevent aggregation, metal nanoparticles have been protected with stabilizing molecules such as ionic surfactants,12 polymers,4,13−17 and nano or mesoporous silica shell.14,18 In © XXXX American Chemical Society

particular, the use of silica shells offers some advantages including reduction of electromagnetic coupling between metal nanoparticle cores, optical transparency, tenability in the optical properties of metal nanoparticles, versatility in the design of diverse surface morphologies and functionalizations, and improved biocompatibility via surface modification.19,20 However, silica has been used to eliminate SERS enhancements by purposefully blocking the metal substrate to prevent direct interaction between the molecule and metal.21 Because the silica shells are composed of a disordered, microporous structure, the diffusion of molecules toward the metal core is limited10,22 and the development of the SERS sensors with improved selectivity have been difficult and challenging. We conceived the idea that utility of metal−organic framework (MOF), consisting of metal ions and organic ligands linked together by coordination bonds, as a shell of SERS-active nanomaterials. MOF, sometimes referred to porous coordination polymers, possess crystallinity, high surface areas,23 adaptable surface chemistries, pore size,24 and structures25 that make them leading candidates to separate,26−29 capture,30 store,31 deliver,32 transport,33,34 sense35 and catalyze molecules.29,36 Recently, toward integrated their functions, spatial control of MOF crystal growth on substrates, functionalization by chemical modifications, and incorporation of functional materials in MOF have been demonstrated through postimpregnation by chemical vapor deposition,37 oneReceived: August 27, 2012 Revised: June 10, 2013

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dx.doi.org/10.1021/cm302735b | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

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pot synthesis, 3 8 − 4 1 and self-assembled monolayers (SAMs).42−44 We have also succeeded in preparing SERSactive MOF crystals embedding gold nanorods (AuNR) and monitoring transfer of guest molecules into the nanopores in a single piece of MOF crystal by SERS.45 However, the crystals were relatively large in size (μm-mm), and it is difficult to use them as SERS sensors with high selectivity due to inhomogeneous crystallization and complex diffusion of the guest molecules in the nanopores of MOF.46 To explore a suspension-based sensing system, more uniformed and much smaller sized MOF embedding of AuNR has been awaited. In this report, we demonstrate preparation and SERS sensitivity of cubic nanosized MOF-5 crystals with AuNR (AuNR@MOF-5) from zinc ion, terephthalic acid, and AuNR by modified mother solution method45,47 or thin MOF layers on substrate.48−50 The range of the sizes of AuNR@MOF-5 were 400−600 nm, and their sizes were able to be controlled by just changing the concentration of AuNR. Moreover, AuNR was roughly incorporated at the center of the cubic crystals, indicating that AuNR acted as seeds for formation of MOF shell. They were dispersible and stable in organic solvents and demonstrated to be reproducible SERS materials for size selective detection of some pyridine derivatives. In this system, the pyridine derivatives were proved to diffuse through nanopores of MOF-5 shells and interacted with the surface of AuNR encapsulated in MOF-5 nanocrystals.

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crystallization. The supersaturated mother solution was filtered and cooled down to room temperature. Into this MOF-5 mother solution MUA-capped AuNR was added and the mixed solution was incubated at room temperature. The solution became cloudy after about 15 min, suggesting the formation of MOF-5 crystals. Obtained nanocrystal was collected by centrifugation (2000 rpm, 10 min), which is redispersible into organic solvents such as DEF and N,N-dimethylformamide (DMF). SERS Measurements. AuNR@MOF-5 nanocrystal was added into a 1.0 mM pyridine derivative solution (DMF) overnight to include guest molecules into nanopores of MOF-5. For time dependent SERS, SERS spectra were collected every 30−40 s following the addition of AuNR@MOF-5 into 1 mM pyridine derivative solution in DMF. In order to investigate size-selectivity of AuNR@MOF-5, pyridine (Py), 2,6-biphenylpyridine (BPPy), and poly(4-vinylpyridine) (PVPy) were employed as guest molecules. All SERS were taken using the following parameters: excitation wavelength λex = 785 nm, 1 s integration time, power = 2 mW, 2 scans.

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RESULTS AND DISCUSSION Fabrication of Suspension of Nanosized AuNR@MOF5. Nanocrystals of AuNR@MOF-5 were prepared by modified mother solution method using MUA-capped AuNR as seeds. Replacement of CTAB by MUA was carried out by treatment of CTAB-capped AuNR with MUA/ethanol solution under sonication. Introduction of COOH groups were confirmed by vis−NIR absorption spectroscopy and zeta-potential measurements. Into the supersaturated mother solution of MOF-5, MUA-capped AuNR was added and the mixed solution was incubated at room temperature. The solution, red in color, was kept clear for ca. 10 min and became cloudy at ca. 15 min, suggesting the formation of MOF-5 crystal (Figure S1). X-ray powder diffraction (XRPD) pattern of red crystals collected by centrifugation were shown in Figure 1A, in which almost the same diffraction pattern as simulated MOF-5 crystals were observed in addition to the signal characteristic to AuNR (2θ = 38.4°). The incorporation of AuNR in MOF-5 probably caused the relatively low intensity of its XRPD pattern. N2 adsorption measurement revealed that AuNR@MOF-5 possesses 365 m2/ g of BET surface area and the pore with 4.17 Å calculated by

EXPERIMENTAL SECTION

Reagents and Chemicals. N,N-Dimethylformamide, terephtalic acid, Zn(NO3)2·6H2O, pyridine, and 2,6-biphenylpyridine were supplied from Wako Chemicals. N,N-Diethylformamide and poly(4vinylpyridine) and other chemicals were purchased from Tokyo Kasei Chemicals and SIGMA, respectively. General Characterization Methods. 1H NMR spectra were obtained on a JEOL JNM-LA 300 spectrometer. vis−NIR absorption spectra were acquired on a JASCO V-570 spectrometer. Raman and surface-enhanced Raman spectra (SERS) were obtained using a RENISHAW inVia Reflex Raman microscope. Transmission electron microscopy (TEM) images were acquired by using a JEOL JEM-2100 F (acceleration voltage: 200 kV). Samples were prepared on 400 mesh copper grids coated with elastic carbon film. The nanocrystal solution was pipetted onto a grid and promptly drained using filter paper. The grids were allowed to thoroughly dry before imaging. Scanning electron microscopy (SEM) image was obtained by using a JEOL JSM7400F. X-ray powder diffraction (XRPD) patterns were obtained by RIGAKU Smart-Lab Diffractometer with Cu Kα radiation source (40 kV, 30 mA). N2 adsorption measurement was conducted by YUASA IONICS Autosorb 6AG. Preparation of COOH-Terminated AuNR. CTAB-capped AuNR (44 ± 8 × 10 ± 1 nm) was supplied from Dai-Nippon Toryo Co., Ltd. An activation procedure was conducted to replace CTAB with 11mercaptoundecanoic acid (MUA). A tube of 1.0 mL of CTAB-capped AuNR solution was centrifuged at 9000 rpm for 30 min, and the supernatant was disposed to remove the excess of CTAB. The CTABcapped AuNR was redispersed in 1.0 mL of distilled water and centrifuged again. After removal of the supernatant, 0.50 mL of distilled water and 0.050 mL of 20 mM MUA/ethanol solution were added. The mixed solution was kept under constant sonication for 90 min and then incubated for more than 3 h at room temperature. The solution was centrifuged at 9000 rpm for 30 min to collect MUAcapped AuNR. Fabrication of Nanosized MOF-5 Crystals Encapsulating AuNR. We first prepared supersaturated mother solution of MOF-5. Samples of 5.5 mg of phthalic acid (bdc) and 26 mg of Zn(NO3)2·6H2O were thoroughly dissolved in 5 mL of N,Ndiethylformamide (DEF) with ultrasonication. The solution was heated to 90 °C and kept at this temperature until the beginning of

Figure 1. (A) XRPD patterns of AuNR@MOF-5 (red solid line) and simulated MOF-5 crystals (blue solid line). Representative (B) SEM and (C) TEM images of AuNR@MOF-5 nanocrystal. (D) Magnified TEM image of the AuNR@MOF-5 nanocrystal in (C). B



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density functional theory (DFT) method. These results suggest the formation of AuNR composites with MOF-5 crystals. In Figure 1B and C, representative SEM and TEM images, respectively, the formation of uniform nanosized cubic crystals after 5 min incubation is depicted. The composites have cubic structure with 400−500 nm on each side. The size of nanocrystals slightly grew to 500−600 nm by increasing the reaction time to 10 min (Figure S2). All cubic crystals incorporated AuNR roughly at the center of them, in which AuNR was arranged in a random manner (Figure 1D) without side-by-side aggregation which was observed in the aggregated species from DEF solution of MUA-capped AuNR. This result suggested that AuNR in AuNR@MOF-5 had the similar dispersion state to that in the mother solution. This was supported by absorption spectral changes by the addition of MUA-capped AuNR into MOF-5 mother solution. After the addition of MUA-capped AuNR, the mother solution immediately showed a 100 nm red shift, which is characteristic of uncontrollably aggregated or electromagnetically coupled nanoparticles. Even as the incubation time increased to 15 min, the characteristic peak at 990 nm did not shift at all (Figure 2A). This characteristic LSPR band was the same as those both

Figure 3. TEM images of AuNR@MOF-5 along the change of concentration of MUA-capped AuNR in the reaction mixture: [AuNR] = (A) 7.5 × 10−5, (B) 3.8 × 10−5, (C) 2.5 × 10−5, and (D) 1.3 × 10−5 g/mL.

TEM observation (Figure S5). These results clearly show the importance of COOH groups so that MUA-capped AuNR worked as seeds for the formation of AuNR@MOF-5 nanocrystals. Fabrication of AuNR-embedded MOF 5 was successfully demonstrated with several hundred nanometer in size for a suspension-based SERS sensor. SERS Activity of AuNR@MOF-5 for Py. In order to investigate SERS activity of nanocrystals, we incubated AuNR@ MOF-5 in 1.0 mM solution of Py in N,N-dimethylformamide (DMF) overnight. DMF was chosen as the solvent due to insensitivity for integrity of the MOF-5 crystals.27 The guest molecule Py was also chosen as a guest analyte for the following reasons. First, it has a high affinity for the gold core and is often used for the evaluation of SERS.51 Second, the molecular size of Py permits its diffusion through the nanopore of MOF-5 which has the pore size of ∼8 Å.24,27 Finally, Py did not disturb or destroy the structure of MOF-5 at room temperature, which was confirmed by XRPD (Figure S6). As shown in Figure 4A, AuNR@MOF-5 nanocrystals in the 1.0 mM solution of Py in DEF exhibited SERS enhancement at 988 cm−1 and 1035 cm−1 originated from Py. This signal enhancement was attributed to insertion and diffusion of Py in

Figure 2. (A) vis−NIR spectra of MUA-capped AuNR mixed with mother solution of MOF-5. (B) Solid state (red solid line) and redispersion state (in DEF, blue dashed line) absorption spectra of AuNR@MOF-5. Inset: photograph of redispersed AuNR@MOF-5 nanocrystal solution in DEF.

in dried powder state and after redispersion in DEF (Figure 2B). These results clearly suggested that MOF-5 shell was immediately formed and absolutely coated AuNR to prevent their further aggregation. Furthermore, the size of MOF-5 nanocrystals and the concentration of encapsulated AuNR could be easily controlled by just changing the amount of MUA-capped AuNR added into mother solution of MOF-5. The more MUA-capped AuNR was added into the mother solution, the smaller nanocrystals were observed and the more AuNR was incorporated (Figures 3 and S4). When more than 7.5 × 10−5 g/mL of AuNR was added into the mother solution, AuNR could not thoroughly incorporated in MOF-5 nanocrystals as shown in Figures 3A and S4A. From these results, we speculate that carboxylate groups at the surface of MUA-capped AuNR effectively bind Zn2+ from the mother solution and/or surface-exposed Zn2+ sites of the MOF-5 as crystallization nuclei. As references for incorporation of AuNR, CTAB- and PEG-capped AuNR were prepared and mixed with mother solution of MOF-5 in the same manner. In the former case, they immediately aggregated and showed color change from red to purple (Figures S3A and S3B). On the other hand, the latter could redisperse into the mother solution, but no cubic crystals were confirmed from

Figure 4. (A) Raman spectra of Py (black dashed line) and 1.0 mM Py solution (blue solid line), and SERS spectra of 1.0 mM Py solution in the presence of AuNR@MOF-5 (red solid line). (B) Influence of the concentration of AuNR@MOF-5 on SERS spectra of 1.0 mM Py solution ([AuNR] = 7.5 × 10−5 g/mL; black solid line and 1.5 × 10−4 g/mL; red solid line). Experimental conditions are as follows: λex = 785 nm, power = 2 mW, integration time = 10 s, number of scan = 2 times. C



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aggregation and deposition of AuNR from the solution. In contrast, the extinction intensity at 785 nm for AuNR@MOF-5 nanocrystals remained constant after the addition of Py and electromagnetic coupling between the AuNR crystals is prohibited due to protection by the MOF-5 shell. The observed changes in the optical properties of AuNR had large influence on SERS enhancement of Py. In Figure 5B, two important trends were observed. First, the SERS enhancement for both bare AuNR and AuNR@MOF-5 nanocrystals followed the time-dependent absorption spectroscopic data observed in Figure 6A. Because the bare AuNR had constantly changing

the nanopores in the MOF-5 shell and its attachment onto the surface of the AuNR core from the following observations. First, the SERS signal was not detected immediately after AuNR@MOF-5 nanocrystals immersion in the Py solution (Figure 5, red solid circle). Instead, more than 3 min was

Figure 5. Plots of (A) absorbance at 785 nm and (B) SERS intensity at 988 cm−1 of 1 mM Py solution in DMF in the presence of bare AuNR (blue solid squares) and AuNR@MOF-5 nanocrystals (red solid circles).

required before a SERS signal was detected, indicating that Py molecules need time to diffuse through the MOF-5 shell and bind to AuNR. Second, judging from TEM image in Figure 1B,C, the MOF-5 shell had an estimated thickness of 200−300 nm. Since approximately 1 order of magnitude in SERS intensity is lost per nanometer that the target molecules are apart from the enhancing substrate,52−54 the molecules out-side the MOF-5 shell should have no SERS enhancement. Third, the SERS intensity for Py increased when double amount of AuNR@MOF-5 crystals was added to Py solution (Figure 4B). These results suggest that Py molecules are able to diffuse through the MOF-5 shell of AuNR@MOF-5 nanocrystals and subsequently attach to the surface of AuNR. Finally, the direct adsorption of Py to the AuNR in AuNR@MOF-5 nanocrystals was evident by comparing spectral differences between Raman and SERS spectra for Py (Figure 4A). Typical signals characteristic of Py were identified at 988 cm−1 (ring breathing) and 1029 cm−1 (in-plane ring deformation, C−H breathing); however, a spectral difference was also noted. A C−H breathing mode centered at 1029 cm−1 in the normal Raman spectrum shifted to 1035 cm−1 in SERS spectrum with AuNR@MOF-5 nanocrystals. Previously, it was reported that Py derivatives bound metal perpendicularly to the surface thereby preferentially enhancing and shifting the vibration mode.51 This spectral change further supports the hypothesis that Py is binding directly to the gold surface in the AuNR@MOF-5 nanocrystals. Stability and Recyclability of AuNR@MOF-5 on SERS. One limitation of using suspension of metal nanoparticles for SERS sensor system is instability of these nanostructures, that is, deposition or phase separation of nanoparticles by their aggregation. As a result, their optical properties significantly vary, and quantitative detection of target molecules is prevented due to low reliability on electromagnetic properties including all surface-enhanced spectroscopies. Optical properties and SERS enhancement of bare AuNR (CTAB-capped AuNR) and AuNR@MOF-5 nanocrystals over time were compared. Following the addition of 1 mM Py solution in DMF, the solution was shaken briefly, and UV−vis absorption spectra were recorded every couple of minutes. Time dependence of absorption peak at 785 nm (excitation wavelength on SERS) and SERS intensity at 988 cm−1 are shown in Figure 5. As can be seen in Figure 5A, the absorbance of bare AuNR at 785 nm steadily decreases. This response was consistent with

Figure 6. Size-selective SERS detection for Py, BPPy, and PVPy using AuNR@MOF-5. AuNR@MOF-5 nanocrystals was added into 1 mM Py (blue square), BPPy (red circle), and PVPy (green triangle), respectively. SERS intensity at 988, 999, and 992 cm−1 on Py, BPPy, and PVPy, respectively, were plotted against time.

optical properties, the observed SERS signals also fluctuated. On the other hand, AuNR@MOF-5 nanocrystals had stable optical properties, and as a result, SERS intensity was also stable and quantitative. Second, the magnitude of the SERS intensity for the bare AuNR was much larger than that for AuNR@MOF-5 nanocrystals. These results indicated that the stable response of AuNR@MOF-5 was attributed to fixation and protection of AuNR by the MOF-5 shell. Since recyclability is one of the essential factors required for practical SERS active materials,55−57 we investigated recyclability of AuNR@MOF-5 nanocrystals. After the SERS measurement for Py, AuNR@MOF-5 nanocrystals were collected by centrifugation and immersed in fresh DMF more than 6 h to remove Py completely in the nanopores of AuNR@ MOF-5. The removal of Py was confirmed via no detection of Py by SERS. When the cleaned AuNR@MOF-5 nanocrystals were added into 1.0 mM Py solution again, the enhanced Raman intensity assigned to Py was observed as shown in Figure S7. The SERS intensity was maintained at 94% even after 3 times recycle. On the other hand, bare AuNR did not redisperse into DMF solution after centrifugation because of aggregation. Therefore, AuNR@MOF-5 presented good stability and recyclability. Size-Selective SERS Using AuNR@MOF-5. It could be predicted that AuNR@MOF-5 nanocrystals would show selectivity on SERS owing to the nanoscopic pore dimensions and the molecular-level interactions between the migrating molecules and the MOF scaffold.26,27,29 We investigated sizeselectivity of AuNR@MOF-5 nanocrystals on SERS using various sizes of pyridine derivatives, Py, 2,6-biphenylpyridine (BPPy), and poly(vinyl 4-pyridine) (PVPy), as analytes. AuNR@MOF-5 nanocrystals were added into respective 1.0 mM pyridine derivative solution in DMF and their SERS were D



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Global COE Program “Catalysis as the Basis for the Innovation in Materials Science”, Grant-in-Aid for Scientific Research (20651026), and JSPS Research Fellowship for Young Scientists (09J03047).

collected. They showed time-dependent SERS activity for Py and BPPy. The SERS signals assigned as Py were saturated in three to four minutes, while those of BPPy were saturated in five to ten minutes as shown in Figure 6. This delay of the response was more remarkably presented using PVPy under the same concentration for Py. In the case of PVPy, no SERS signal was detected even after 1 h incubation, suggesting that PVPy was not able to diffuse into nanopores of the MOF-5 shell and to interact with AuNR in the core. This was supported by no incorporation and diffusion of PVPy into the nanopores of MOF-5 by immersion of the MOF-5 crystals in the 1 mM solution of PVPy. As a result, this distinct SERS response dependent on the Py derivatives should be attributed to the sieving effect of the MOF-5 crystals, which efficiently discriminates guest molecules by their size.

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CONCLUSION This work presents the fabrication of nanosized MOF crystals encapsulating AuNR, enabling us to detect guest molecules in solution phase as a suspension-based SERS sensor. Specifically, the discovery reported here is that stable and size-selective SERS are achieved in solution-phase using AuNR encapsulated in porous crystalline shell, MOF-5. The AuNR@MOF-5 nanocrystals were synthesized by applying the modified mother solution method of MOF-5, in which MUA-capped AuNR acted as seeds for direct growth of MOF-5. Obtained AuNR@ MOF-5 nanocrystals were stable in some organic solvents and showed recyclable SERS activity for some pyridine derivatives. Detection of analytes using SERS enhancement occurred by diffusion through the MOF-5 shell. Finally, we investigated their size-selectivity on SERS using various sizes of pyridine derivatives. Small molecules such as pyridine and BPPy could diffuse into nanopore of MOF-5 shell and interact with AuNR core. On the other hand, polymeric pyridine derivative, i.e., PVPy, could not and no SERS signals were detected. Future research on MOF-based SERS should focus on tailoring MOF-molecule interaction by changing the structure of the MOF shell around AuNR core, which would enable us to invent different type of SERS-active materials for different purposes. Further refinement of the electromagnetic properties of AuNR core should lead to larger SERS enhancements for target molecules.

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ASSOCIATED CONTENT

S Supporting Information *

Results of TEM, SEM, absorption spectroscopy, and SERS (pdf). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +81-11-7063473. Tel: +81-11-706-3473. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The SEM observation, SERS measurement, SEM and TEM observations, and N2 adsorption mesearment were carried out at the OPEN FACILITY, Hokkaido University Sousei Hall. We appreciate Prof. M. Kato and Assoc. Prof. H.-C. Chang for the XRD measurements. This work was supported by MEXT E



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