Controllable Synthesis of SERS-active Magnetic Metal-Organic

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Surfaces, Interfaces, and Applications

Controllable Synthesis of SERS-active Magnetic MetalOrganic Framework-Based Nanocatalysts and Their Application in Photo-Induced Enhanced Catalytic Oxidation Xiaowei Ma, Sisi Wen, Xiangxin Xue, Yue Guo, Jing Jin, Wei Song, and Bing Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03457 • Publication Date (Web): 10 Jul 2018 Downloaded from http://pubs.acs.org on July 10, 2018

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ACS Applied Materials & Interfaces

Controllable Synthesis of SERS-active Magnetic Metal-Organic Framework-Based Nanocatalysts and Their Application in Photo-Induced Enhanced Catalytic Oxidation Xiaowei Ma,† Sisi Wen,† Xiangxin Xue,‡ Yue Guo,† Jing Jin,† Wei Song*,† and Bing Zhao† † State

Key Laboratory of Supramolecular Structure and Materials, Institute of

Theoretical Chemistry, Jilin University, Changchun 130012, P. R. China. *E-mail: [email protected] ‡

Key Laboratory of Preparation and Applications of Environmental Friendly

Materials, Ministry of Education, Jilin Normal University, Changchun, 130103, P. R. China.

Keywords:

metal-organic

framework,

nanocomposite,

peroxidase-like,

surface-enhanced Raman scattering, enhanced catalytic oxidation

ABSTRACT: The fabrication of multifunctional nanocatalysts with surface-enhanced Raman scattering (SERS) activity is of vital importance for monitoring of catalytic courses in situ and studying the reaction mechanisms. Herein, SERS-active magnetic metal-organic framework (MOF)-based nanocatalysts were successfully prepared via a three-step method, including solvothermal reaction, Au seed-induced growth

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process, and low-temperature cycling self-assembly technique. The as-synthesized magnetic MOF-based nanocatalysts not only exhibit outstanding peroxidase-like activity, but also can be applied as a SERS substrate. Exploiting these features, it can be used for monitoring in situ catalytic oxidizing 3,3',5,5'-tetramethylbenzidine (TMB) by H2O2 via a SERS technique, and the concentration of H2O2 was determined. Owing to the intrinsic character of the Fe-based MOF material (MIL-100(Fe)), a novel photo-induced enhanced catalytic oxidation effect was demonstrated, in which the catalytic oxidation of TMB and o-phenylenediamine was accelerated. This study provides a versatile approach for the fabrication of functional MOF-based nanocomposites as a promising SERS substrate with unique photo-induced enhanced peroxidase-like activity for potential applications in ultrasensitive monitoring, biomedical treatment , and environmental evaluation..

1.

INTRODUCTION Surface-enhanced Raman scattering (SERS) is a typical non-destructive

analytical technique that is employed in chemical and biological analyses, and has developed rapidly in the past few decades. SERS provides a highly specific, sensitive, and fingerprint-like spectrum of the vibrational modes of molecules with a large signal enhancement, thereby furnishing intrinsic structural information.1 It has been reported that the detection sensitivity using SERS spectrum has reached single-molecule level.2 The SERS technique has shown promising applications in biology, food security, environmental evaluation, and 2


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medical science. Nevertheless, the SERS effect is dependent on the use of SERS substrates. Thus far, some typical noble metal nanomaterials (e.g., gold, silver, and copper) have been studied as efficient SERS-active substrates due to the electromagnetic (EM) enhancement effect of these species. Recently, certain semiconductors have also been studied as SERS substrates, where charge-transfer between the semiconductors and the target molecules is operative.3 However, the fabrication of multifunctional and highly sensitive SERS substrates remains a formidable challenge. Metal-organic frameworks (MOFs), constructed from metal ions or metal clusters with bridging organic ligands via coordination bonds, are a new variety of hybrid inorganic-organic porous materials.4 Owing to their permanent porosity, large internal surface areas, and active metal sites, MOFs are promising to be applied in catalysis, separation, storage, and molecular recognition.5−9 Recently, MOFs have also been reported to possess certain SERS activity.10 Compared with traditional SERS substrates, MOFs a have large specific surface area and great adsorption ability, and thus can easily capture the target molecules and firmly localize the molecules at the enhancing site. However, the SERS enhancement ability of individual MOFs is not satisfactory. Thus, the combination of MOFs with noble metals is a new approach for improving the signal enhancement ability. For instance, Sugikawa and co-workers prepared a MOF embedding Au nanorods (AuNR-MOFs) using liquid-phase infiltration methods, the SERS activity of AuNR-MOFs has been greatly enhanced, 3



which can be used to detected guest molecules such as N,N-diethylformamide and CHCl3.11 Nevertheless, the shape, size, distribution and composition of the composites is difficult to be accurately controlled via such infiltration methods. Moreover, Jiang and co-workers reported the fabrication of AgNPs/MIL-101(Fe) composites via a depositing approach, which can be applied for ultrasensitive determination of dopamine by SERS technique.12 But in this way, noble metals was coated on the surface of MOFs, which will block some pores of the MOFs and limit the further application of the materials. In addition, Zheng et al. synthesized Au@Ag@ZIF-8 nanoparticles via cladding ZIF-8 on the surface of Au@Ag nanoparticles, which can be used for efficient SERS analysis.13 Nevertheless, the shell thickness of MOFs could not be effectively controlled via such a process. Therefore, a low-temperature cycling self-assembly method is developed to encapsulate noble metals into the MOFs.14

In recent years, much attention have been paid to Fe-based MOFs for catalytic applications. Owing to the existence of Fe3-µ3-oxo clusters and their coordinatively unsaturated Fe(III) active sites, Fe-based MOFs have recently emerged as promising, novel artificial enzyme mimics.15 For instance, Wang et al. successfully prepared an Fe-based MOF material (Fe-MIL-88A) that exhibited noticeable peroxidase-like catalytic activity.16 Moreover, with the introduction of certain reductants under UV/vis irradiation, the catalytic activity was significantly enhanced due to the generation of Fe2+ and autoxidation of the 4


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reductants. This system involved a photo-Fenton-like reaction that improved the rate of generation of ·OH radicals from H2O2. Recently, Zhao and co-workers reported a Fe-MIL-88 based fluorescence enhancement system for the detection of dopamine (DA).17 With the addition of DA, a Fenton-type reaction was formed based on the generation of Fe2+ from Fe-MIL-88, resulting in the significant fluorescence enhancement. Although MOF-based catalysts may exhibit excellent catalytic activity, it is not easy to separate the bare MOFs from the catalytic reaction system. Thus, it is intelligent to prepare magnetically recoverable nanoparticles as promising catalysts for chemical reactions.18 For example, Yang and co-workers synthesized Fe3O4/Cu3(BTC)2 microspheres that were used for the catalytic aerobic oxidation of alcohols and olefins.19 Notably, after five cycles, the catalytic oxidation conversion was above 96%, and solid-liquid separation could be completed within one minute. Herein,

we

prepare

ternary

magnetic

MOF-based

nanocatalysts

(Fe3O4@Au@MIL-100(Fe)) via a three-step reaction involving solvothermal reaction, Au seed-induced growth, and low-temperature cycling self-assembly. The as-prepared magnetic MOF-based nanocatalysts are used as new peroxidase mimics for catalyzing the oxidation of the

3,3,5,5-tetramethylbenzidine (TMB) substrate. The entire

catalytic reaction process is monitored in situ by SERS Spectroscopy. Moreover, owing to the special structure and catalytic properties of the Fe-based MOFs, photo-induced enhanced catalytic oxidation is achieved with the assistance of ascorbic 5



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acid (AA) under photo-irradiation. The catalytic activity towards the oxidation of TMB is greatly enhanced through this system, which is also potentially applicable to other catalytic oxidation systems. As far as we know, this study first report the fabrication of magnetic MOF-based nanocatalysts as SERS substrates for monitoring the enhancement of the photo-induced catalytic activity derived from AA-assistance in the peroxidase-like oxidation reaction.

2.

EXPERIMENTAL SECTION

2.1. Chemicals (3-Aminopropyl)triethoxysilan (APTES), thioglycolic acid (TGA), (BTC),

trimesic acid

Rhodamine 6G (R6G), TMB, ascorbic acid (AA), dopamine (DA),

o-phenylenediamine (OPD), 1,10-phenanthroline, and terephthalic acid were obtained from Sigma-Aldrich (Shanghai, China). Ferric chloride hexahydrate (FeCl3·6H2O) was acquired from Tianjin East China Reagent Works. Ammonia solution, anhydrous sodium acetate (NaAc), dimethyl sulfoxide (DMSO), chloroauric acid and ethylene glycol were purchased from Sinopham Chemical Reagent Co., Ltd. Hydrogen peroxide (H2O2), ethanol, and potassium thiocyanate (KSCN) were bought from Beijing Chemical Works.

2.2. Characterization Transmission electron microscopy (TEM, JEM-2100F) was tested at 200 kV to observe the morphologies of the samples. X-Ray photoelectron spectroscopy (XPS,

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Thermo ESCALAB 250) were acquired employing Al as the excitation source. Nitrogen adsorption-desorption isothermal (Quantachrome Autosorb iQ Station 1 adsorption analyzer) data were obtained at 77 K. The magnetic properties were probed with a quantum design MPMS3 superconducting quantum interference device (SQUID) magnetometer. UV-vis absorption spectra were obtained with a Shimadzu UV-3600 UV-Vis-NIR spectrophotometer. Fluorescence spectra were obtained with a Shimadzu RF-5301PC instrument. SERS detection with R6G and acquisition of Raman spectra for monitoring the catalytic reaction were performed with a LabRAM ARAMIS Smart Raman Spectrometer and the excitation source was the 633 nm line of a HeNe laser.

2.3 Synthesis of the Magnetic MOF-based Nanocatalysts Firstly, Fe3O4 nanoparticles were synthesized via a solvothermal method.20 Briefly, FeCl3·6H2O (3.46 g) was added into ethylene glycol (70 mL) under magnetic stirring. Then NaAc (4.618 g) was quickly added into above solution. After 30 min stirring, the mixture was decanted to a Teflon-lined stainless steel autoclave and heated at 200 ºC for 10 h. The prepared Fe3O4 nanoparticles were separated with a magnet and cleaned with ultrapure water and ethanol, respectively. The product was dried at 60 ºC for 12 h under vacuum. Secondly, Fe3O4@Au composites were synthesized through a in situ seed-mediated growth method. Typically, Fe3O4 particles (45 mg) were dispersed in a solution of APTES (300 mL, 1% wt%) and stirred for 24 h to prepare the aminated Fe3O4 nanoparticles. Small gold seeds with a size of 2−3 nm were also 7



prepared based on the previous method.21 Afterwards, the aminated Fe3O4 nanoparticle was dispersed into the gold seeds suspension and stirred for 5 h. The prepared Fe3O4@Au-seed nanoparticles were separated with a magnet, and washed with water. Subsequently, the as-prepared Fe3O4@Au-seed nanoparticles (50 mg) were dispersed in distilled water (40 mL), and aqueous HAuCl4 (4 mL, 1 wt%) solution and aqueous ammonium solution (12 mL, 10 wt%) were successively added into the above Fe3O4@Au-seed suspension under mechanical stirring. In the following, AA solution (32 mL, 10 mM) was injected to reduce the HAuCl4 on the Au-seeds via an in situ seed-mediated growth approach. After 4.5 h, the final prepared Fe3O4@Au nanoparticles were magnetically separated, cleaned with distilled water and ethanol, and then dried at 60 ºC for 12 h under vacuum. Thirdly, magnetic Fe3O4@Au@MIL-100 (Fe) nanocatalysts were prepared based on a previous report.14 In a typical procedure, Fe3O4@Au nanoparticles (0.10 g) were dispersed into an ethanolic solution of TGA (20 mL, 0.58 mM) under shaking for 24 h to prepare TGA-functionalized Fe3O4@Au nanoparticles. The product was magnetically separated and cleaned with ethanol for several times. Subsequently, the TGA-functionalized Fe3O4@Au nanoparticles (0.10 g) were dispersed in the FeCl3·6H2O ethanolic solution (5 mL, 10 mM) for 15 min and separated by magnetic decantation. The TGA-functionalized Fe3O4@Au nanoparticles were then dispersed in an ethanolic solution of benzenetricarboxylic acid (5 mL, 10 mM) for 30 min at 70 ºC. After separation and washing with ethanol, the procedure was cycled for ten times. 8


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The synthesized magnetic Fe3O4@Au@MIL-100 (Fe) nanocatalysts were cleaned with ethanol and dried under vacuum at 80 ºC.

2.4 SERS Properties of the Magnetic MOF-based Nanocatalysts The R6G was used as a Raman probe to estimate the SERS property of the magnetic MOF-based nanocatalysts. In a typical experiment, the magnetic MOF-based nanocatalysts (5 mg) were dispersed in 1 mL of R6G solution (10-5 to 10-9 M) over the course of 2 h for the SERS measurements. The SERS experiments were performed with a LabRAM ARAMIS Smart Raman Spectrometer and the excitation source was the 633 nm line of a HeNe laser.

2.5 In situ Monitoring of Peroxidase-Like Property of Magnetic MOF-based Nanocatalysts via UV-vis and SERS Spectroscopy In this experiment, a TMB solution (200 µL, 15 mM in DMSO) and the magnetic MOF-based nanocatalyst dispersion (100 µL, 10 mg mL-1) were mixed in acetate buffer (2.67 mL, pH 4.0). Thereafter, H2O2 solution (30 µL, 0.1 M) was injected and the solution was measured by UV-vis spectrum. To determine the concentration of H2O2, a similar procedure was performed by adding different concentrations of H2O2 solution (10-1−10-7 M).

In the typical procedure for SERS monitoring of the above reaction, the TMB solution and the Fe3O4@Au@MIL-100(Fe) magnetic nanoparticle dispersion (100 µL, 10 mg mL-1) was diluted. Then the diluted TMB solution (20 µL, 3 mM) and the 9



catalyst (20 µL, 1 mg mL-1) were mixed together and H2O2 (20 µL, 3 × 10-3 M) was added to monitor the peroxidase-like reaction by SERS spectroscopy. To detect the H2O2 concentration, a similar procedure was performed with different final concentrations of H2O2 (10-3−10-9 M). The SERS measurement was performed at different reaction time under continuous 633 nm laser excitation.

2.6 Photo-Induced Enhanced Catalytic Oxidation with the Assistance of AA Measured by UV-vis and SERS Spectroscopy

To study the photo-induced enhanced catalytic activity with the assistance of AA, the magnetic MOF-based nanocatalyst dispersion (100 µL, 10 mg mL-1) and AA (30 µL, 10-2 M) were mixed in acetate buffer (2.64 mL, pH 4.0) for 10 min. TMB solution (200 µL, 15 mM in DMSO) and H2O2 solution (30 µL, 0.1 M) were then sequentially dropped to the mixture, then photo-irradiated for one minute. UV-vis and SERS spectral measurements were applied to evaluate photo-induced enhanced catalytic oxidation of TMB catalyzed by the magnetic MOF-based nanocatalysts with the assistance of AA. A similar procedure was performed to monitor the photo-induced enhanced catalytic property for oxidizing o-phenylenediamine.

3.

RESULTS AND DISCUSSION

3.1. Synthesis and Characterization of magnetic MOF-based nanocatalysts A schematic for the synthesis of the magnetic MOF-based nanocatalysts is presented in Figure 1. Firstly, the Fe3O4 nanoparticles were synthesized via a 10


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solvothermal reaction. Then, Au nanoparticles were in situ grown on the surface of the Fe3O4 nanoparticles using small gold colloidal nanoparticles as seeds. Thirdly, the magnetic Fe3O4@Au@MIL-100(Fe) nanocatalysts were prepared by cyclic immersion of the TGA-functionalized Fe3O4@Au nanoparticles in ethanolic solutions of FeCl3 and BTC at 70 ºC, separately.

Figure 1. Schematic illustration of the magnetic MOF-based nanocatalysts synthetic process. The structure of the magnetic MOF-based nanocatalyst was characterized by TEM measurement. The corresponding TEM images of Fe3O4, Fe3O4@Au, and the prepared magnetic MOF-based nanocatalysts has been shown in Figure 2. Figure 2a shows that the size of the bare Fe3O4 nanoparticles ranged from 200 11



to 400 nm. After the reduction of HAuCl4 by AA on the surface of the Fe3O4 core, Au nanoparticles with a diameter ranging from 20 to 50 nm can be obviously seen from Figure 2b. Figure 2c shows a representative TEM image of the magnetic MOF-based nanocatalysts. The Fe3O4@Au nanoparticles were successfully decorated by the MOF shell with a thickness of around 20 nm. Figure 2d displays a representative high-resolution TEM (HRTEM) image of the magnetic MOF-based nanocatalysts, revealing their highly crystalline structure. The interplanar spacings shown in the HRTEM image were 0.235 and 0.485 nm, attributing to the (111) plane of face-centered cubic (fcc) Au and the (111) plane of Fe3O4 with an inverse spinel-structure.22,23 Figure 2e shows the energy dispersive X-ray (EDX) spectrum of the magnetic MOF-based nanocatalysts, clearly demonstrating the presence of Fe and Au. Furthermore, Figure S1 shows the corresponding elemental maps of Fe, Au, and C in the magnetic MOF-based nanocatalysts, which was consistent with the previous conclusion. These results demonstrate the successful fabrication of the magnetic MOF-based nanocatalysts.

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Figure 2. Characterization of the prepared magnetic MOF-based nanocatalysts. (a-c) TEM

images

of

(a)

Fe3O4,

(b)

Fe3O4@Au

and

(c)

magnetic

Fe3O4@Au@MIL-100(Fe) nanocatalysts. (d) HRTEM image of magnetic MOF-based nanocatalysts. (e) The corresponding EDX elemental spectrum. (f) XPS survey spectra of the prepared magnetic MOF-based nanocatalysts and MIL-100(Fe) NPs. (g-j) High resolution XPS spectrum of magnetic MOF-based nanocatalysts: (g) Au4f, (h)C1s, (i)O1s, and (j) Fe2p. (k) The magnetic hysteresis loop of Fe3O4 and magnetic MOF-based nanocatalysts. The inset photographs exhibit the quickly solid-liquid separation of the magnetic MOF-based nanocatalysts with the assistance of an external neodymium magnet.

Meanwhile, X-ray photoelectron spectroscopy (XPS) of the prepared magnetic MOF-based nanocatalysts was tested to acquire the further information on their surface chemical composition. The survey spectra of the 13



magnetic MOF-based nanocatalysts and the single MIL-100(Fe) have been shown in Figure 2f, respectively. Compared with the single MIL-100(Fe), the spectrum of magnetic MOF-based nanocatalyst showed an intense peak of Au, which is consistent with the previous conclusion that the Au nanoparticles have successfully decorated onto the Fe3O4 nanoparticles. Figure 2g−j shows the high-resolution XPS profile of Au, C, O, and Fe in magnetic MOF-based nanocatalyst. As illustrated in Figure 2g, two bands could be found at 84.4 and 88.1 eV, which are attributed to the 4f7/2 and 4f5/2 states of Au, respectively,24 demonstrating the formation of Au in the magnetic MOF-based nanocatalysts. The C1s spectrum presented in Figure 2h could be split into several peaks located at 284.6, 285.1, and 289.1 eV. The peaks at 284.6 eV represented C-C/C=C bond of the benzene ring and the peak at 289.1 eV could be attributed to the signal of carboxyl, which are all resulting from the organic ligands in the MIL-100.25 Beyond that, the peak at

285.1 eV is related to carbon on the

surface of the sample.26 This result proves the formation of the carbon skeleton of MIL-100. In addition, the high-resolution XPS spectrum of O exhibited the typical band of the Fe-O and Fe-O-C species at 532.4 eV (Figure 2i).27 In the Fe 2p spectrum (Figure 2j), two representative bands related to the Fe 2p3/2 and Fe 2p1/2 states have been discovered at 712.6 and 726.3 eV.25 The Fe 2p spectrum revealed the formation of Fe3-µ3-oxo clusters from Fe-based MOFs and the presence of Fe3O4. 14


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Figure S2a displayed the relevant nitrogen adsorption-desorption isotherm of the prepared magnetic MOF-based nanocatalysts. It could be seen that the sharp of the curve was between type I (microporous materials) and type IV (mesoporous materials), which was consistent with the calculated pore size distribution (centered at 1.29, 1.54, 1.84, 2.65, and 3.79 nm) (Figure S2b). Moreover, a distinct hysteresis loop could be observed in Figure S2a, indicating a porous structure with a 3D intersection network, which is accordance with the unique construction of MOFs.28 The Brunauer-Emmett-Teller (BET) surface area of the magnetic MOF-based nanocatalyst was approximately 214.8 m2 g-1. The lower BET surface area of the magnetic MOF-based nanocatalyst, compared to the bare MIL-100(Fe), might be ascribed to the low content of MIL-100(Fe). The magnetism of the as-prepared magnetic MOF-based nanocatalysts was quantified from the magnetic hysteresis loop (Figure 2k). The magnetic saturation (MS) value of the Fe3O4 NPs was approximately 40 emu g-1. After coating with the Au and MIL-100(Fe) shell, it decreased to 28 emu g-1. The decrement in the overall MS values indicated that the Fe3O4 NPs was encapsulated with non-magnetic materials, such as Au and MIL-100(Fe), as confirmed by TEM results. Although the magnetism decreased after the formation of the magnetic MOF-based nanocatalysts, the speed of separation the material from the solution was still very fast. As shown in the inset of 15



Figure 2k, the synthesized MNPs could recover quickly from solid-liquid separation (within 10 s) with the assistance of an external 5 cm×5 cm×5 cm neodymium magnet. The excellent magnetism endows the product with quick recoverability and reusability. The SERS property of the magnetic MOF-based nanocatalysts was evaluated using R6G as a probe molecule under continuous 633 nm laser excitation. As shown in Figure 3, compared with the Raman spectra of the magnetic MOF-based nanocatalysts without probe molecules, several dominant peaks at 612, 771, 1125, 1176, 1310, 1362, 1508, 1574, and 1649 cm-1 could be observed in the SERS spectrum when the concentration of R6G was 10-5 M. These spectral bands should be attributed to R6G, as previously reported, and the corresponding assignments have been shown in Table S1.29 The typical bands at 612 and 771 cm-1 were chosen to investigate the variation in the SERS intensity with a change in the concentration of R6G. As shown in the inset of Figure 3, the intensities of the typical bands of R6G decreased with a decrement in the concentration. Nevertheless, the signals could still be detected when the concentration of R6G reduced to 10-9 M, demonstrating the high SERS sensitivity of the magnetic MOF-based nanocatalysts. Hence, this materials can be applied as a SERS substrate to detect various trace molecules and to in situ monitor many catalytic reactions.

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Figure 3. SERS spectra of R6G molecules with different concentrations using magnetic MOF-based nanocatalysts as SERS substrate. The right figure is an enlarged spectrum in the 500-900 cm-1 regions. The inset in Figure 3 shows the relationship between Raman intensity of peaks at 612 and 771 cm-1 and the minus logarithm of R6G concentration.

3.2 Peroxidase-Like Activity of Magnetic MOF-based Nanocatalyst and SERS Monitoring of the Catalytic Reaction The as-prepared magnetic MOF-based nanocatalysts possess not only outstanding SERS activity for the detection of various target molecules, but also efficient peroxidase-like activity for catalytic oxidizing the peroxidase substrate TMB by H2O2. It can be observed in the inset of Figure 4a, the 17



peroxidase substrate TMB was oxidized by H2O2 with the assistance of the magnetic MOF-based nanocatalysts, and the characteristic colorimetric reaction proceeded with the solution color turn to blue. In comparison, significant color change was not obtained in the TMB, TMB+H2O2, and the TMB+magnetic MOF-based nanocatalysts systems. Accordingly, compared with these three systems, typical strong absorption peaks have been shown in the UV-vis absorption spectrum of TMB+H2O2+magnetic MOF-based nanocatalysts system (Figure 4a). As previously reported, the absorption peaks at 370 nm is related to a radical cation (TMB+) and 650nm results from a charge transfer complex (CTC), both due to the oxidation of TMB.30 These results demonstrate that the magnetic MOF-based nanocatalysts exhibit unique peroxidase-like activity.

Figure 4. (a) UV-Vis spectrums of different peroxidase-like catalytic systems (react for 10 min). Inset: final color of different systems. (b) UV-Vis spectrums 18


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of oxTMB molecules catalyzed by (1) Au, (2) Fe3O4, (3) Fe3O4@Au, (4) MIL-100(Fe), and (5-7) magnetic MOF-based nanocatalysts after 5, 10, 15 self-assembly cycle in the presence of H2O2 after 12 min. (c) The histogram of absorbance at 370 and 650 nm with the different catalysts. (d) SERS monitoring of oxidizing TMB molecules by H2O2 catalyzed by magnetic MOF-based nanocatalysts. Inset: The relationship between the time and SERS intensities of peaks at 1611 cm-1. (e) SERS monitoring of oxidizing TMB molecules by H2O2 catalyzed by (1) Au, (2) Fe3O4, (3) Fe3O4@Au, (4) MIL-100(Fe), and (5-7) magnetic MOF-based nanocatalysts after 5, 10, 15 self-assembly cycle after 12 min. (f) The histogram of SERS intensity at 1192, 1337 and 1611 cm-1 with the different catalyst. To demonstrate the superior peroxidase-like activity of the magnetic MOF-based nanocatalysts, we compared the catalytic activity with individual Au, Fe3O4, Fe3O4@Au and MIL-100(Fe) nanoparticles. Figure 4b shows the UV-vis absorption spectrum for the oxidation of TMB with different nanocatalysts. The histogram in Figure 4c demonstrates that the peroxidase-like activity of the magnetic MOF-based nanocatalyst was much higher than the other catalysts, due to the synergistic effect between Fe3O4, Au, and MIL-100(Fe). As we all know, Fe(III) from Fe3O4 and MIL-100(Fe) can promote the generation of ·OH radical, which involve in oxidizing TMB to generate oxidized TMB (oxTMB). Compared with individual Au, Fe3O4 and 19



MIL-100(Fe) nanoparticles, electron density and electron transfer ability of the magnetic MOF-based nanocatalysts have been increased, leading to an enhanced peroxidase-like activity.31 The catalytic activity of the magnetic MOF-based nanocatalysts with different contents of MIL-100(Fe) was compared, indicating that the magnetic MOF-based nanocatalysts synthesized with ten cycling self-assembly process possessed the greatest catalytic property. The increased MOF shell with the increment of the number of self-assembly cycles would increase the number of catalytically active sites. On the other hand, the thicker MOF shell reduces the ability of the reactants to spread through the MOF shell.32 Therefore, the magnetic MOF-based nanocatalyst formed by ten cycling self-assembly process exhibited the greatest catalytic property. In addition, to further compare with other materials and methods, the steady-state kinetic parameters had been determined via controlling one substrate concentration change and keeping another invariability.33 As it can be seen in Figure S3 , the Michaelis-Menten constant (Km) and maximum initial velocity (Vmax) are acquired using a Lineweaver-Burk plots. Table S2 has listed the corresponding information. It is well known that a lower Km represents a better affinity of an enzyme for a substrate and a higher Vmax refers to a higher maximum initial velocity. As shown in Table S2, the Km of the magnetic MOF-based nanocatalysts was much lower than bare MIL-100(Fe) while TMB was the substrate, but higher than bare Fe3O4. Nevertheless, the Km of the 20


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magnetic MOF-based nanocatalysts was 770 times lower than bare Fe3O4 but a little higher than bare MIL-100(Fe) when the substrate changed to H2O2. Meanwhile, the Vmax of the magnetic MOF-based nanocatalysts was much higher than both Fe3O4 and MIL-100(Fe) with both TMB and H2O2 as the substrates. It indicated that the obviously enhanced catalytic ability of the magnetic MOF-based nanocatalysts might result from the synergistic effect between Fe3O4 and MIL-100(Fe). Meanwhile, the Km value has been compared with other materials to further demonstrate the superior peroxidase-like property of the magnetic MOF-based nanocatalyst. As shown in Table S2, the Km of the magnetic MOF-based nanocatalysts with TMB as the substrate was 1.34 times to 28.13 times lower than other materials, and 12.9 times to 473.5 times lower with H2O2 as the substrate. Thus, it is reasonable to believe that the magnetic MOF-based nanocatalysts possess an excellent peroxidase-like efficiency. Due to the high SERS activity and the excellent peroxidase-like property of the magnetic MOF-based nanocatalysts, this system can be applied to in situ SERS monitor the enzyme-catalyzed reaction of TMB. Figure 4d shows the SERS monitoring of oxidizing TMB molecules by H2O2 catalyzed by the magnetic MOF-based nanocatalysts, showing apparent Raman signals. The spectral band at 1192 cm-1 is related to the CH3 bending modes, while 1337 cm-1 is attributed to inter-ring C-C stretching modes and 1611 cm-1 is 21



corresponding to a combination of C-H bending modes and ring stretching.34 These three spectral bands are related to oxTMB and are consistent with previous studies.35,36 The SERS sensitivity of the magnetic MOF-based nanocatalysts synthesized with different numbers of self-assembly cycles was compared with that of the individual Au, Fe3O4, Fe3O4@Au and MIL-100(Fe) nanoparticles. The magnetic MOF-based nanocatalysts synthesized with ten self-assembly cycles exhibited the best SERS intensity (Figure 4e and f). However, the relative intensities of the SERS signals obtained with the different catalysts were not completely consistent with the UV-vis results. In the UV-vis absorption study, the intensity of the absorption on the surface of Fe3O4@Au was lower than MIL-100(Fe), but the result in SERS study was precisely opposite. This phenomenon may due to the higher SERS activity of Fe3O4@Au compared with MIL-100(Fe). In fact, UV-vis absorption spectrum indicates the absorbance of the solution, which is much associated with the target molecules concentration in the solution. However, SERS provides the information about the interface of the substrate, it is not only associated with the target molecules concentration, but also associated with the SERS enhancement of the substrates. In this work, MIL-100(Fe) owned greater catalytic property than Fe3O4@Au. Thus, in the reaction solution, the concentration of oxidized TMB (oxTMB) using MIL-100(Fe) as catalyst was stronger comparing with using Fe3O4@Au as catalyst, thus, the UV-vis absorption intensity of oxTMB on the surface of 22


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Fe3O4@Au was weaker than that of MIL-100(Fe). However, for SERS study, the SERS enhancement of Fe3O4@Au was much stronger than MIL-100(Fe). Therefore, although the concentration of oxTMB was higher while choosing MIL-100(Fe) as catalyst, SERS intensity of oxTMB on the surface of MIL-100(Fe) was lower than that on the surface of Fe3O4@Au. Furthermore, as it can been seen from the inset of Figure 4d, with the passage of time, the SERS intensity of the Raman characteristic lines at 1611 cm-1 rose obviously. After reacting for 12 min, it nearly reached a maximum, revealing that the reaction reached equilibrium in 12 min. Therefore, the SERS spectra of oxTMB after 12 min of reaction with various concentrations of H2O2 has been acquired to study the effect of the H2O2 concentration on this enzyme-catalyzed reaction of TMB. As shown in Figure 5, the Raman characteristic lines at 1192, 1337, and 1611 cm-1 gradually rose with the increment of H2O2 concentration, manifesting the positive relationship between the reaction rate and the H2O2 concentration. Moreover, as shown in the inset of Figure 5, the relationship between the SERS intensity for all three characteristic peaks vs. -lg[H2O2] could be described by two-stage linear equations. In general, according to the Michaelis-Menten equation, enzymic catalytic velocity is related to relevant substrate concentration.37 When the concentration of substrate is low, it is a first-order reaction relative to the substrate. With the substrate concentration increased, the enzymic catalysis transform from first-order reaction to 23



mixed-order reaction. Finally, when the concentration of substrate is high enough, the reaction changes to zero-order reaction. Therefore, two-stage linear equations can be seen because of the different kinetic equations for the enzyme-catalyzed reaction at low and high concentrations, which is similar to the UV-vis absorption data (Figure S4). In the following experiments, SERS analysis was used to determine the concentration of H2O2 using this system. Figure S5 shows the linear portion of the plot of the SERS intensity of the characteristic peak at 1611 cm-1 versus -lg[H2O2], displaying good linearity. Thus, H2O2 with a concentration as low as 10-9 M can be detected via this system.

Figure 5. SERS spectrum of oxTMB catalyzed by the magnetic MOF-based nanocatalysts with changing H2O2 concentration from 10-3 to 10-9 M . 24


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Amplifying spectrum has been placed on the right side. The inset shows the linear portion of the plot of the SERS intensity of the characteristic peak at 1192, 1337, 1611 cm-1 versus -lg[H2O2]. Reusability is of great importance for catalytic applications. It is well known that most nanocatalysts suffer from low separation efficiency in liquid-phase reductions.38 With the introduction of the magnetic Fe3O4 component, the prepared magnetic MOF-based nanocatalysts could recover quickly from a solid-liquid separation (within 10 s) with the assistance of an external neodymium magnet, demonstrating the enhanced separation efficiency. After cleansing, the magnetic MOF-based nanocatalysts could be recycled in another reaction. Figure S6 shows the SERS spectra of this enzymic catalytic reaction for three consecutive reaction cycles. The catalytic performance have not declined significantly (Figure S7). Together, above results demonstrate that this prepared magnetic MOF-based nanocatalyst displays excellent recyclability.

3.3 Photo-Induced Enhanced Peroxidase-Like Activity with the Assistance of Ascorbic Acid

It is well-known that Fe-based MOFs exhibit good catalytic property deriving from the coordinatively unsaturated Fe(III) sites and Fe3-µ3-oxo clusters.15 However, the coordinatively unsaturated Fe(III) sites occupy only a small part of the Fe-based MOF, whereas many coordinatively saturated Fe(III) sites are present. Furthermore, compared with Fe(III) alone, the Fe(II)/Fe(III) 25



Page 26 of 44

system shows stronger catalytic activity due to the Fenton-like reaction. Considering this phenomenon, we established a novel photo-induced enhanced catalytic oxidation reaction employing a peroxidase substrate catalyzed by the magnetic MOFs nanocatalysts with the assistance of AA.

It can be seen that

the blue color of the TMB+H2O2+magnetic MOF-based nanocatalyst system in the presence of AA was much deeper than that in the absence of AA under photo-illumination (inset photograph of Figure 6a). Accordingly, the Raman characteristic lines at 1192, 1337, and 1611 cm-1 in curve A were nearly twice as intense as those in curve B. This demonstrates that enhanced catalytic activity was achieved with the introduction of AA and photo-irradiation. SERS was also applied to in situ monitor the photo-induced enhanced catalytic oxidation reaction. As shown from Figure 6b and d, the characteristic peaks of oxTMB in the photo-induced enhanced catalytic system increased in intensity more rapidly than in the normal catalytic system (Figure 6c), moreover, the reaction has not reached equilibrium as the Raman intensity of the Raman characteristic lines at 1611 cm-1 has not reached the maximum value. This result indicated that the original equilibrium was broken and enhanced catalytic activity was achieved in the new system consisting of TMB, H2O2, magnetic MOF-based nanocatalysts, and AA under photo-irradiation.

26


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Figure 6. a) SERS spectra of (A) TMB+H2O2+magnetic MOF-based nanocatalysts +AA, (B)TMB+H2O2+magnetic MOF-based nanocatalysts, (C)TMB+magnetic MOF-based nanocatalysts, and (D)TMB+H2O2 after mixing for 12 min under light illumination. Inset: final color of different systems. b) The relationship between the time and SERS intensities of peaks at 1611 cm-1 for the photo-induced enhanced catalytic oxidation and normal catalytic 27



oxidation. c) SERS monitoring of oxidizing TMB molecules by H2O2 catalyzed by magnetic MOF-based nanocatalysts in the normal catalytic oxidation system. d) SERS monitoring of oxidizing TMB molecules by H2O2 catalyzed by magnetic MOF-based nanocatalysts in the photo-induced enhanced catalytic oxidation system. e) Schematic illustration of the possible reaction mechanism for the photo-induced enhanced catalytic oxidation. f) UV-Vis absorption spectrum of OPD molecules under different conditions. Inset: final color of different system solution. From the results mentioned above, a conceivable reaction mechanism in this photo-induced enhanced catalytic oxidation was proposed (Figure 6e). According to previous reports, the coordination compound consisting of Fe(III) and organic ligands could be dissociated under photo-irradiation because of ligand-to-metal charge transfer (LMCT), which would generate ligand-free radicals and Fe(II) (Equation 1).39 However, Fe(II) can very facilely transformed to Fe(III). Through this process, some coordinatively saturated Fe(III) sites were transformed into coordinatively unsaturated Fe(III) sites and free Fe(III), which can be detected from the UV-vis spectra in the presence of KSCN. As shown in Figure S8, after photo-irradiation, the typical absorption peak at 410 nm related to the complex of Fe3+ and SCN- appeared. This reveals that the number of coordinatively unsaturated Fe(III) sites increased and free ferric ions were generated after irradiation of the magnetic MOF-based 28


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nanocatalyst suspension with UV-vis light. The catalytic activity was simultaneously enhanced because of the increment in the number of catalytic active coordinatively unsaturated Fe(III) sites. Furthermore, we have also investigated the influence of the solution pH values on releasing free ferric ions. As shown in Figure S8, free Fe3+ could not be readily detected when the suspension was irradiated with light, indicating that the reaction illustrated in Equation 1 may have occurred in acidic solution. Besides, the influence of the irradiation wavelength in the photo-induced enhanced catalytic oxidation system has also been investigated. Figure S9 shows the UV-vis absorption spectrum of oxTMB under the photo-induced enhanced catalytic oxidation system irradiated by UV-vis light with different wavelengths. When the irradiation wavelength was within the ranges of 200−300 and 600−800 nm, there was nearly no enhancement of the catalytic activity. In the wavelength ranges of 300−400 and 500−600 nm, a weak enhancement was obtained. However, in wavelength the range of 400−500 nm, there was an obvious enhancement, indicating that the optimum irradiation wavelength was in the range of 400−500 nm. The optimum irradiation time was also studied. As shown in Figure S10, a further enhancement was observed with increasing irradiation time. However, compared to the enhancement effect under irradiation for 1 min, there was a slow increase as the irradiation time increased from 1 to 12 min, indicating that most coordinatively unsaturated Fe(III) sites 29



Page 30 of 44

and free Fe3+ species was generated within 1 min and the system exhibits great potential for rapid enhancement of the catalytic activity. Although the reaction in Equation 1 corresponds to an increment in the coordinatively

unsaturated

Fe(III)

sites

of

the

magnetic

MOF-based

nanocatalysts, the enhancement was not obvious, and could be further improved by the introduction of a reducing agent such as AA. After UV-vis irradiation, many free ferric ions were generated in this reaction, in addition to the coordinatively unsaturated Fe(III) sites. With the introduction of AA, Fe3+ can be reduced to Fe2+ (see Equation 2). The generation of Fe2+ was substantiated by the UV-vis spectrum by 1,10-phenanthroline. As shown in the inset of Figure

S11a,

an

obvious

red

color

was

observed

after

adding

1,10-phenanthroline to the aforementioned solution. Moreover, the typical strong absorption peak at 510 nm related to the complex of Fe2+ and 1,10-phenanthroline was observed. Thus, the generation of Fe2+ caused a Fenton-like reaction in acidic solution to improve the rate of the production of ·OH radicals from H2O2 (Equation 3).40 Moreover, Fe3+ may have been hydrated to form Fe(H2O)3+, with subsequent decomposition to Fe(OH)2+ and H+ (Equation 4); the generated Fe(OH)2+ can be transformed into Fe2+ and ·OH radicals under UV-vis irradiation (Equation 5), although the rate of this reaction is extremely slow.41 The ·OH radicals generated in these reaction could be substantiated through the ·OH radical scavenger terephthalic acid (TA). In this 30


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process, 2-hydroxyterephthalic acid (2HTA), a highly fluorescent compound, could be produced and detected.42 Figure S12 displays the fluorescence emission spectra of 2-hydroxyterephthalic acid. When AA was introduced into the system, the generation of HTA increased obviously, representing an increment of ·OH radicals, which may enhance the catalytic oxidation. Moreover, controlled experiments were used to further prove the reaction mechanism mentioned above. As shown in Figure S13, weak enhancement was obtained with photo-irradiation or AA individually. When the photo-irradiation and AA were simultaneously applied to the catalytic system, a great enhancement was observed. This is because that only the reactions represented by Equations 1, 4, and 5 occur under photo-irradiation alone, and the increasing catalytically active coordinatively unsaturated Fe(III) sites can only offer a certain enhancement. In the presence of AA alone, the reduction of the intrinsic coordinatively unsaturated Fe(III) sites leading to the Fenton-like reaction is the only process that occurs. However, the intrinsic coordinatively unsaturated Fe(III) sites constitute only a small part of the Fe-based MOF. Therefore, with the simultaneous use of photo-irradiation and AA, AA can reduce the coordinatively unsaturated Fe(III) sites and free Fe3+ generated by irradiation, leading to mutual enhancement effects. FeIII-BTC + hv → [FeIII-BTC]* → FeII + BTC

(1)

Fe3+ + AA → Fe2+ + oxAA

(2) 31



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Fe2+ + H2O2 + H+ → Fe3+ + ·OH + H2O

(3)

Fe3+ + H2O → Fe(H2O)3+ → Fe(OH)3+ + H+

(4)

Fe(OH)2+ + hv → Fe2+ + ·OH

(5)

The effects of the concentration of AA on the photo-induced enhanced catalytic oxidation of TMB were also investigated. Figure S14 displays the UV-vis absorption spectrum of oxTMB molecules in the photo-induced enhanced catalytic oxidation system with different concentrations of AA. The UV-vis absorption at 650 nm rose with increasing the AA concentration from 10-9 to 10-4 M, and reached a maximum value, followed by a rapid decrement with the AA concentration ranging from 10-4 to 10-2 M, ultimately reached zero. Thus, 10-4 M was the optimum concentration of AA in this system. In addition to the reducing of Fe3+, AA can also inhibit the oxidation of TMB;43,44 thus, at low to intermediate concentrations of AA, the reduction of Fe3+ was uncompleted, with the increment of AA concentration in this range, the Fe2+ concentration also showed a significant increasing (Figure S11), thus the promotion of AA was higher than the inhibition effect. However, when the concentration of AA reached a critical point, the reduction of Fe3+ was nearly saturated, the increased concentration of Fe2+ was unapparent. Therefore, the promotion of AA was lower than the inhibition effect and the excess AA will inhibit the oxidation of TMB due to its reduction ability.43 Dopamine was also used as the reducing agent to enhance the catalytic oxidation of TMB. In Figure 32


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S15, the UV-vis absorption peak at 650 nm increased with increasing the added dopamine concentration from 10-9 to 10-5 M, and reached a maximum value, followed by a rapid decrement with the dopamine concentration ranging from 10-5 to 10-2 M, ultimately reached zero. Thus, 10-5 M was the optimum concentration of dopamine in this system. Based on the above results, the photo-induced enhancement by this catalytic oxidation system was operative with different reducing agents, although the optimum concentration differed slightly based on the intrinsic characteristics of the reducing agents. The catalytic oxidation system with photo-induced enhanced activity could also be applied to other peroxidase substrates, such as OPD. As shown in Figure 6f, OPD could be oxidized to 2,3-diaminophenazine in the presence of the magnetic MOF-based nanocatalysts, as the color turn to orange.45 Figure S16 shows the relationship between the time and absorbance intensity of OPD at 450 nm, and Figures S17 and S18 show the UV-vis absorption spectrum for the oxidation of OPD with increasing time in the normal catalytic oxidation system and photo-induced enhanced catalytic oxidation system, respectively. The rate of catalytic oxidation of OPD in the photo-induced enhanced catalytic oxidation system was much faster than the conventional system. The catalytic oxidation of OPD was also monitored using the SERS spectrum. Figure S19 shows a comparison of the SERS spectra of the different reaction systems. Several characteristic peaks of 2,3-diaminophenazine at 610, 1256, 1368, 1463, and 1582 cm-1 were apparent in the SERS spectrum, same 33



with the previous reports.45,46 Furthermore, with the photo-induced enhanced catalytic oxidation system, the SERS intensities of these bands were much stronger than those with the other systems, which is similar with the results mentioned above.

4.

CONCLUSION

In summary, magnetic MOF-based nanocatalysts were successfully prepared as an efficient SERS substrate. Owing to the intrinsic peroxidase-like activity of the magnetic MOF-based nanocatalysts, this system can be applied to in situ monitor the peroxidase-like reaction and sequential determination of the concentration of H2O2 with high sensitivity by the SERS technique. Additionally, a novel technique for the photo-induced enhanced catalytic oxidation of TMB and OPD by the magnetic MOF-based nanocatalysts with the assistance of AA under photo-irradiation was established. This study provides a novel method for enhancing the catalytic activity of Fe-based metal-organic frameworks and introducing SERS technique for in situ monitoring of peroxidase-like catalytic reactions.

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 34


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Characterizations and comparison of experimental results.

AUTHOR INFORMATION Corresponding Author *Wei Song. E-mail: [email protected]

ORCID Wei Song: 0000-0001-9814-419X Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

This work was supported by the research grants from the National Natural Science Foundation of China (21473068, 21327803), the Natural Science Foundation of Jilin Province (20180101295JC), the projects of Jilin Province Department of Education (2016-221); the Development Program of Science and Technology of Jilin Province (20170520134JH).

35



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Controllable Synthesis of SERS-active Magnetic Metal-Organic

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