May 5, 2016 - Frameworks as a Sensing Platform to Detect Vanillin. A. T. Ezhil ... tunnelâ to accelerate the electron mediator between solvent and electrode ...
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Fabrication of Palladium Nanoparticles on Porous Aromatic Frameworks as a Sensing Platform to Detect Vanillin A. T. Ezhil Vilian,†,∥ Pillaiyar Puthiaraj,‡,∥ Cheol Hwan Kwak,§ Seung-Kyu Hwang,§ Yun Suk Huh,*,§ Wha-Seung Ahn,*,‡ and Young-Kyu Han*,† †
Department of Energy and Materials Engineering, Dongguk University-Seoul, Seoul 04620, Republic of Korea Department of Chemistry and Chemical Engineering, and §Department of Biological Engineering, Biohybrid Systems Research Center (BSRC), Inha University, Incheon 22212, Republic of Korea
‡
S Supporting Information *
ABSTRACT: Here, we report the fabrication of palladium nanoparticles on porous aromatic frameworks (Pd/PAF-6) using a facile chemical approach, which was characterized by various spectro- and electrochemical techniques. The differential pulse voltammetry (DPV) response of Pd/PAF-6 toward the vanillin (VA) sensor shows a linear relationship over concentrations (10− 820 pM) and a low detection limit (2 pM). Pd/PAF-6 also exhibited good anti-interference performance toward 2-fold excess of ascorbic acid, nitrophenol, glutathione, glucose, uric acid, dopamine, ascorbic acid, 4-nitrophenol, glutathione, glucose, uric acid, dopamine, and 100-fold excess of Na+, Mg2+, and K+ during the detection of VA. The developed electrochemical sensor based on Pd/PAF-6 had good reproducibility, as well as high selectivity and stability. The established sensor revealed that Pd/PAF-6 could be used to detect VA in biscuit and ice cream samples with satisfactory results. KEYWORDS: porous aromatic frameworks, electrochemical sensors, vanillin, palladium nanoparticles, electrocatalysis
1. INTRODUCTION In the past few decades, nanoparticles (NPs) have been synthesized and fabricated using different types of porous materials like, porous organic frameworks (POFs), metal− organic frameworks (MOFs), graphene, and carbon nanotubes.1−3 Among these, the construction of nanoscale MOFs and POFs has gained considerable attention to increase the electrochemical sensing ability for biomolecules.4,5 Metal NPs typically exhibit excellent conductivity and catalytic properties in electrochemical applications, as they act as an “electronic tunnel” to accelerate the electron mediator between solvent and electrode surfaces, as well as catalysts to enhance electrochemical performance.6,7 There have been a few recent reports of the use of MOFs as electrocatalytic electrode materials for practical application in the electrochemical field. Mao et al. found that Cu-BTC and Cu-bipy-BTC (BTC = benzene-1,3,5-tricarboxylic acid and bipy = 2,2′-bipyridine) could be used as electrocatalysts with high electrocatalytic activity for oxygen reduction.8 Kumar et al. studied Cu-BTC as an electrocatalyst for the reduction of CO2 with tetrabutylammonium tetrafluoroborate in N,N′-dimethylformamide.9 Hosseini et al. synthesized Au-SH-SiO2 NPs supported with Cu-BTC and used as an electrocatalyst for the oxidation of L-cysteine.10 Zhang et al. showed the electrocatalytic performance of Cu-BTC/macroporous carbon com© 2016 American Chemical Society
posites in the oxidation of ascorbic acid and reduction of hemoglobin.11 Yuan et al. reported [Co(tib)2]·2NO3 MOF (tib =1,3,5-tris(1-imidazolyl)benzene) modified electrode for the amperometric determination of reduced glutathione.12 Wang et al. prepared Cu-BTC/graphite oxide composites and utilized as an electrochemical sensing platform to detect the luteolin.13 Wang et al. described the use of Cu-based 1,4-benzendicarboxylic acid exhibiting the enhanced electrocatalytic detection of bisphenol A.14 Samadi-Maybodi et al. developed Ag-doped ZIF8 NPs for the amperometric detection of H2O2.15 Wang et al. fabricated hierarchical NiO superstructures/Ni(BTC) and used to detect the glucose.16 Yang et al. established the high performance electrochemical sensor of H2O2 using a Co-based 3-(pyridine-3-yloxy)benzene-1,2-dicarboxylate modified electrode.17 Ling et al. reported Zr-based Iron(III) meso5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin chloride and benzoic acid MOF as an electrochemical probe for a DNA sensor.18 Zhou et al. used Cu-BTC/SWCNTs (single-walled carbon nanotubes) for electrochemical oxidation of catechol and hydroquinone.19 These MOF-based electrochemical sensors have several disadvantages, such as a high cost of Received: April 3, 2016 Accepted: May 5, 2016 Published: May 5, 2016 12740
DOI: 10.1021/acsami.6b03942 ACS Appl. Mater. Interfaces 2016, 8, 12740−12747
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Schematic representation for the synthesis of Pd/PAF-6. TEM images of (b) PAF-6 and (c, d) Pd/PAF-6.
allows us to detect VA in food samples with satisfactory results. The distinguished performance of high surface area Pd/PAF-6 improved the adsorption of VA at the electrode/electrolyte interface, which enhances the electrodics of VA at the modified layer.
ligands and hazardous chemicals to synthesize the MOFs and high temperatures.20,21 To overcome these problems, metal NP-supported POFs may be an alternative candidate for electrochemical electrode materials.22 POF materials have attracted many researchers and undergone tremendous growth in many fields of material science and technology because of their high surface area, porosity, thermal stability, and chemical tenability in organic solvents.23,24 Furthermore, their fabrication can be conducted via an easy preparation method at low cost and with adjustable internal surface properties.25,26 Owing to the excellent features of POF materials, they have been utilized in gas storage/separation, sensors, drug delivery, ion exchange, heterogeneous catalysis, supercapacitors, and photodegradation studies for organic compounds.27−30 The high surface area and porosity of POFs make them suitable supporting materials for preparation of the high immobilization of metallic NPs because they are rich in nitrogen contents inside the framework, which offers an opportunity for uniform particle size distribution and helps to immobilize the particles permanently inside their cavities.31 Vanillin (4-hydroxy-3-methoxybenzaldehyde, VA) acts as an indispensable smell additive in many desserts and beverages, including chocolates, cookies, and many other food products.32,33 However, excessive VA intake can lead to headaches, nausea, vomiting, and kidney problems.34 Recently, several methods have been established to determine VA including chromatography, polarography, and fluorescence spectroscopy.35 The determination of VA using these methods has some drawbacks such as being time-consuming, having high cost, and requiring a huge amount of organic solvents.36 Among these strategies, electrochemical sensors offer many advantages, such as their inexpensive instruments, reliability, simple operation, low cost, and higher sensitivity.37,38 Therefore, the facile fabrication of Pd NPs anchored on PAF-6-based VA electrochemical sensor still requires exploration. To the best of our knowledge, no attempts have been made to develop the Pd NPs supported on PAF-6 for the electrochemical sensors of VA. We have developed a straightforward approach for the selective sensing of VA at subnanomolar levels with a wide linear range; this methodology
2. MATERIALS AND METHODS 2.1. Materials. Piperazine, PdCl2, 1,4-dioxane, K2CO3, cyanuric chloride, hydrazine monohydrate, Na2HPO4, and NaH2PO4 were received from Sigma-Aldrich. All other solvents were purchased from Merck and used as received. Phosphate buffer solution (PBS, 0.05 M, with a pH of 7) was chosen as the background electrolyte. 2.2. Characterization. Electrochemical analysis was carried out in CHI 660C (Shanghai Chenhua Instrument, China) using GCE (working electrode), Pt (counter electrode), and Ag/AgCl (reference electrode). The morphology of Pd/PAF-6 was measured by transmission electron microscopy (TEM; JEM-2010F, JEOL). Electrochemical impedance spectroscopy (EIS) was measured on a VERSASTAT4. Fourier transform infrared (FT-IR) spectrum was measured on a FT/IR-6600 (JASCO, Japan) spectrometer. Nitrogen sorption isotherms was performed at BELSORP-Max (BEL, Japan) at 77 K. The surface area and pore size distribution were calculated using the Brunauer−Emmett−Teller (BET) method and nonlocal density functional theory (NLDFT) with the carbon cylindrical pore model. Thermogravimetric analysis (TGA) was measured on a TG 209 F3 tarsus in an atmosphere of Ar with 5 °C min−1. XPS was performed on a K-Alpha instrument, Thermo Scientific. Elemental analysis of Pd/ PAF-6 was performed using a Thermo Scientific elemental analyzer (Flash EA 1112) to determine the C, H, and N contents. Gas chromatography (GC) analysis was conducted using a Varian 1200L single Quadropole GC/MS system with 3800GC. The X-ray diffraction (XRD) analysis of Pd/PAF-6 was collected from a Rigaku D/max-2500. 2.3. Synthesis of PAF-6. The synthesis of PAF-6 was followed by a slightly modified procedure reported previously.39 To a roundbottom flask, 6 mmol of piperazine and 12 mmol of anhydrous K2CO3 were added in 50 mL of anhydrous 1,4-dioxane. The solution was kept at 15 °C, and 4 mmol of cyanuric chloride in 20 mL of anhydrous 1,4dioxane was added dropwise using a dropping funnel, followed by the mixture being heated to 90 °C for 36 h. Finally, the resulting product was collected by filtration and washed with dichloromethane, ethanol, and water several times to completely remove the unreacted starting precursors. The elemental composition of C9H12N6 was calculated 12741
DOI: 10.1021/acsami.6b03942 ACS Appl. Mater. Interfaces 2016, 8, 12740−12747
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a) Powder XRD pattern of PAF-6 and Pd/PAF-6. (b) N2 sorption isotherm of PAF-6. (c) XPS spectra of N 1s core levels of PAF-6 and fresh Pd-PAF-6. (d) Pd metallic 3d core level of fresh Pd/PAF-6. (%): C, 52.95; N, 5.88; H, 41.17. Found (%): C, 53.31; N, 5.68; H 41.01. 2.4. Synthesis of Pd/PAF-6 NPs. The Pd/PAF-6 NPs were prepared as follows: PdCl2 (10 mg) was dissolved in acetone (20 mL) with vigorous stirring for 10 min. Subsequently, we added PAF-6 (200 mg) into the suspension and stirred for 4 h. Next, hydrazine monohydrate (100 μL) was added slowly under stirring condition for 30 min. Finally, the as-prepared pale gray powder of Pd/PAF-6 was obtained using centrifugation, and washed with acetone. The obtained Pd/PAF-6 was dried in oven for 12 h at 70 °C. 2.5. Fabrication of Pd/PAF-6/GCE. Before coating, the GCE was cleaned with alumina slurries of different grades (0.03 and 0.05 μm) to remove adsorbed organic substances and washed successively using ethanol and double distilled water. For Pd/PAF-6, a mass of 2.0 mg of Pd/PAF-6 was dispersed with 5 mL of ethanol under sonication for 30 min. Next, 5 μL of Pd/PAF-6 suspension was coated on the electrode surface and allowed to dry at ambient conditions to obtain the Pd/ PAF-6-modified electrode.
in the 2θ range of 10−35°, indicating the partial crystalline nature of the framework. These FT-IR and PXRD results are match closely with results reported previously.39 In Pd/PAF-6, strong Bragg reflection peaks were observed at 2θ of 40.10°, 46.31°, and 67.72°, which are assigned to the (111), (200), and (220) face-centered cubic plane of Pd NPs, while other remaining peaks were remained intact after the synthesis of Pd NPs. The TGA results showed that the PAF-6 and Pd/PAF-6 samples were stable up to 300 and 350 °C (Figure S2). The porosity parameters of PAF-6 were measured by N2 adsorption isotherm analysis at 77 K (Figure 2b). The surface area was measured from the pressure region of 0.01−0.06 and showed that the BET surface area was 288 m2 g−1, which is significantly higher than the earlier reports.39 The NLDFT pore size distribution of PAF-6 exhibited small microporosity (
