Letter pubs.acs.org/journal/ascecg
Synthesis of Porous Pd Nanostructure and Its Application in EnzymeFree Sensor of Hydrogen Peroxide Lin Yao, Yibo Yan, and Jong-Min Lee* School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, 637459 Singapore ABSTRACT: A novel porous Pd nanostructure has been synthesized in an aqueous solution using a soft micelle template from the interacting species of 1,3,5-trimethylbenzene with a surfactant of hexadecyltrimethylammonium bromide for highly sensitive electrochemical detection of hydrogen peroxide (H2O2). The porous Pd nanostructure has been applied for the dynamic detection of H2O2 with changing concentration through the H2O2 electro-reduction process. Remarkably, the low detection limit of 0.25 μM (signal/noise of 2.6), the long linear range from 0.25 to 900 μM (correlation coefficient R = 0.992), and the high sensitivity of 201 μA mM−1 were achieved. KEYWORDS: Palladium, Porous metal nanostructures, Soft micelle template, Sensor, Hydrogen peroxide
T
There are both the hydrophobic domain and hydrophilic domain in a surfactant of CTAB molecule. As the formation of micelle in the aqueous solution, the inside core of the micelle is hydrophobic while the outside surface of micelle is hydrophilic. Thus, the interacting species of TMB preferably stay in the core of the micelle to trigger the formation of special soft micelle template. On the other hand, the Pd precursor with high solubility in the aqueous solution may have an efficient contact with the outside hydrophilic part of the soft micelle template. In this way, it is possible to synthesize the novel porous Pd nanostructure. The porous Pd nanostructures were synthesized by using (NH4)2PdCl4 as a precursor and NaBH4 as a reduction agent in the aqueous solution. Typically, 100 mg of CTAB was dissolved in 20 mL of DI water at room temperature. Then, 1 mL of 1,3,5-trimethylbenzene (TMB) was added into the aqueous solution to trigger the formation of a micelle structure with CTAB. After the mixture was stirred for about 5 min, 1 mL of the aqueous solution of (NH4)2PdCl4 (100 mg) was added. Because of the hydrophilic outer side surface of the micelle structure, the Pd precursor molecules preferentially located on the surface of the soft micelle template. Finally, 1 mL of an aqueous solution containing NaBH4 (30 mg) was added to the above aqueous solution as a reduction agent to synthesize the porous Pd nanostructures at 90 °C for 1 h. From the TEM image, the pore size of the porous Pd nanostructures is about 2 nm (Figure 1). Hundreds of small size Pd nanoparticles are likely to be embedded on the surface
he assembly of surfactant molecule of hexadecyltrimethylammonium bromide (CTAB) behaves well in terms of micelle morphology in aqueous solutions. The hexagonally packed cylindrical phase of CTAB to get the Mobil Composition of Matter No. 41 (MCM-41) is the famous type of mesoporous silica.1 It has been extensively explored for industrial applications such as separation and catalysis.2 A range of microstructures depend on surfactants’ selfassembly in different ways.3 The nanostructures of surfactant micelles have relationship with the architecture of the surfactant molecules, the concentration of the surfactant, the presence of interacting species and their concentrations.4−6 The studies show that the micellar structures will change by the solubilization of small molecules in micelles.7−9 Hedin et al. reported that the CTAB micelles can elongate upon the solubilization of benzene.9 Zhang et al. reported that the CTAB/KBr micellar system can elongate through the solubilization of benzyl alcohol.10 In addition, the amphiphilic phenolic compounds were able to be doped into the micelles to trigger the phase transition from spheres to cylinders and then to vesicles.11 Furthermore, MCM-41 can be converted to MCM-48 and lamellar phases with high temperatures (>100 °C).12 The resulting mesoporous silica can transform from the cylindrical to the vesicular phase by increasing the packing parameter of the surfactant with the fluorocarbon.13 Previously, we studied the cylinder-to-vesicle phase transition of mesoporous silica and the interdependence of the controlling factors.14 1,3,5-trimethylbenzene (TMB) is one of the important controlling factors. Herein, we report a synthesis process for the porous Pd nanostructures upon the soft micelle template from the interacting species of TMB with CTAB surfactant in an aqueous solution (Figure 1a). © 2017 American Chemical Society
Received: October 31, 2016 Revised: January 7, 2017 Published: January 17, 2017 1248
DOI: 10.1021/acssuschemeng.6b02605 ACS Sustainable Chem. Eng. 2017, 5, 1248−1252
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key element for the palladium particles’ performance in catalysis and electro-catalysis. Traditional methods for enlarging specific surface area are dependent upon reducing the particle sizes.15,16 Nonetheless, high porosity further increasing the surface area of palladium particles was scarcely investigated for sensor application. Herein, we developed porous Pd nanostructure for the applications in dynamic sensing of H2O2. For the electrochemical test, a CHI 660D was employed. The working electrode predeposited with porous Pd nanostructure ink uniformly dried on glassy carbon, reference electrode (Ag/AgCl) and the ancillary electrode (platinum wire) were located in a phosphate buffer solution (PBS, pH 7.2) as the electrolyte. The PBS was purged with N2 for 30 min to get rid of dissolved O2 before tests. The catalyst ink consisted of 1 mL of ethanol with 2.5 mg of the catalyst, before adding a Nafion solution (0.5 mL of 0.05 wt %) was ultrasonicated. The glassy carbon electrode was prepared with 5 μL of the ink. The phosphate buffer solution was prepurged for 1 h with nitrogen. Later, H2O2 was added into the electrolyte. During the cell culturing process, human breast cancer cell line (MDA-MB-435) prepared with DMEM (Dulbecco’s modified Eagle’s medium) containing 10 wt % fetal bovine serum and 100 U/mL penicillin−streptomycin was incubated at 37 °C (5% CO2). Then we trypsinized the cells using 0.25 wt % trypsin solution and centrifuged them followed by rinsing and redispersing in PBS (pH = 7.2) to reach a solution of ∼6 × 105 cells/mL. N-Formyl-L-methionyl-L-leucyl-phenylalanine (fMLP, 300 μM in final solution) was employed as the stimulant for H2O2 releasing. For a control group, a PBS solution containing fMLP (300 μM) only in the same pH was used (without the cells). The application for the electrochemical detection of H2O2 was performed using porous Pd nanostructure. Figure 3A demonstrates the CV (cyclic voltammogram) loops of porous Pd nanostructure with a potential range from −0.45 to +0.65 V. With the scan rate increase, the gradually enlarged CV profiles were achieved with the enhanced redox peaks’ potentials and there was a negative shift of reduction peaks along with higher scan rates, broaden the shape of wave. The peak potential shifts according to scan rate are related as follows:29
Figure 1. (a) Illustration of the formation of Pd nanostructures. (b) Low magnification of TEM image of porous Pd nanostructures. (c) High magnification of TEM image of porous Pd nanostructures.
of the lamellar phases of the soft micelle template of CTAB and TMB. The wide-angle X-ray diffraction was conducted to present the XRD pattern of the as-synthesized Pd nanostructures (Figure 2). In the spectrum, the diffraction peaks are observed
Ep = E θ −
R·T ⎡ 1 n·F ·v ⎤ ⎥ ⎢0.780 + ln n·F ⎣ 2 k f ·R·T ⎦
(1)
where Ep denotes peak potential; Eθ denotes standard peak potential; R = universal gas constant 8.314 472(15) J K−1 mol−1; T = temperature in kelvin; n is number of moles of electrons transferred in the cell reaction; F = the Faraday constant, 9.648 533 99(24) × 104 C mol−1; ν = scan rate (±V/ s); kf = the rate constant. Figure 3B shows the correlation between scan rates and peak currents, both of which are linearly correlated with the scan rates (from 5 to 70 mV s−1) due to the surface-controlled and reversible electrochemical redox process on the porous Pd nanostructure.23−25 The porous Pd nanostructure is achievable for the enhancement of the H2O2 electro-reduction and subsequently applicable for the H2O2 electrochemical detection. In Figure 3C, for the CV profiles with gradually enhanced reduction currents corresponding to the gradually increased H2O2 concentrations, the reduction peaks constantly situate at −0.34 V. The crossed CV curves are in agreement with the previous exploration by Skital verified through the mathemat-
Figure 2. XRD spectrum of the as-synthesized Pd nanostructure.
at 2θ values of 40°, 46°, and 68°, which correspond to the crystal facet of (111), (200), and (220) characteristic reflections of face centered cubic (FCC) crystalline Pd (JCPDS, Card no. 05-0681). Noticeably, the palladium-based nanoparticles were broadly utilized in various fields, such as catalysis,15,16 electrocatalysis,17−19 fuel cell electrodes,20 hydrogen storage,21 and gas detection.22 As a noble metal, the sensor performance of palladium-based materials for hydrogen peroxide dynamic detection, which is commonly employed for food/environmental monitoring and detecting H2O2 released by cancer cells,23−25 has been noticeably investigated due to the high electron transfer efficiency.26−28 The effective surface area is the 1249
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Figure 3. (A) Cyclic voltammograms of porous Pd nanostructure at different scan rate in phosphate buffer solutions (pH = 7.2). (B) Plots of scan rate versus redox peak currents with linear fitting. (C) Cyclic voltammograms of porous Pd nanostructure for different concentrations of H2O2 in phosphate buffer solutions (pH = 7.2). (D) Amperometric response when dropwise adding H2O2 into a phosphate buffer solution (holding potential = −0.34 V). (E) Amperometric responses to 100 μM H2O2, 10 μM glucose, 10 μM urea, and 10 μM ascorbic acid. (F) Amperometric responses to blank PBS and MDA-MB-435 cells (∼5 × 105 cells/mL in PBS, pH 7.2) in PBS both with fMLP (300 μM) stimulation.
coefficient R = 0.992), and a remarkable sensitivity of 201 μA mM−1. Such a low detection limit, long linear range, and high sensitivity are superior to the previously reported 3D nanoplate-assembled La2O3 hierarchical microspheres,23 3D meso-porous samarium oxide hydrangea microspheres,24 hydrothermally derived mesoporous hierarchical europium oxide spheres,25 Ag nanowires,31 and Ag nanoparticles supported on graphene.32 The detection limit is lower than the Pd nanoparticle-modified glassy carbon electrode,33 and the Pd supported on helical carbon nanofiber hybrid nanostructures.26 Such performance is endorsed by the high effective surface area and high exposure of surface active sites on the
ical modeling.30 The reduction of H2O2 yielding hydroxide anions were neutralized by a buffer solution: H2O2 + 2e− → 2OH−, OH− + H2PO4− → H2O + HPO42−. Subsequently, the reduction potential of −0.34 V was set as the holding potential for dynamic amperometric response sensing of different concentrations of H2O2, because the cathode reduction signal was far more sensitive than the anode performance. The amperometric i−t plot is shown in Figure 3D. There are two insets in the magnified part, of which the time starts from 0 to 300 s and a linear current−concentration fitted plot. A low detection limit is 0.25 μM with signal/noise of 2.6, performing with a long linear range from 0.25 to 900 μM (correlation 1250
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cetyltrimethylammonium bromide in the presence of potassium bromide. Langmuir 1989, 5, 1225−1229. (6) Wolff, T.; Suck, T. A.; Emming, C. S.; Buenau, G. V. Control of size and shape of micelles, of flow properties, and of pH-values in aqueous CTAB-solutions via photoreactions of solubilizates. Prog. Colloid Polym. Sci. 1987, 73, 18−29. (7) Toernblom, M.; Henriksson, U. Effect of Solubilization of Aliphatic Hydrocarbons on Size and Shape of Rodlike C16TABr Micelles Studied by 2H NMR Relaxation. J. Phys. Chem. B 1997, 101, 6028−6035. (8) Nagarajan, R. Solubilization in aqueous solutions of amphiphiles. Curr. Opin. Colloid Interface Sci. 1996, 1, 391−401. (9) Hedin, N.; Sitnikov, R.; Furo, I.; Henriksson, U.; Regev, O. Shape Changes of C16TABr Micelles on Benzene Solubilization. J. Phys. Chem. B 1999, 103, 9631−9639. (10) Zhang, W. C.; Li, G. Z.; Shen, Q.; Mu, J. H. Effect of benzyl alcohol on the rheological properties of CTAB/KBr micellar systems. Colloids Surf., A 2000, 170, 59−64. (11) Agarwal, V.; Singh, M.; McPherson, G.; John, V.; Bose, A. Microstructure evolution in aqueous solutions of cetyl trimethylammonium bromide (CTAB) and phenol derivatives. Colloids Surf., A 2006, 281, 246−253. (12) Zhao, D.; Goldfarb, D. Synthesis of mesoporous manganosilicates: Mn-MCM-41, Mn-MCM-48 and Mn-MCM-L. J. Chem. Soc., Chem. Commun. 1995, 875−876. (13) Yang, S.; Zhou, X.; Yuan, P.; Yu, M.; Xie, S.; Zou, J.; Lu, G. Q.; Yu, C. Siliceous Nanopods from a Compromised Dual-Templating Approach. Angew. Chem., Int. Ed. 2007, 46, 8579−8582. (14) Yao, L.; Liu, C.; Chong, W.; Wang, H.; Chen, L.; Chen, H. Understanding the Phase Emergence of Mesoporous Silica. Small 2015, 11, 232. (15) Yan, Y. B.; Chen, Y. T.; Jia, X. L.; Yang, Y. H. Palladium nanoparticles supported on organosilane-functionalized carbon nanotube for solvent-free aerobic oxidation of benzyl alcohol. Appl. Catal., B 2014, 156, 385−397. (16) Yan, Y. B.; Dai, Y. H.; Wang, S. C.; Jia, X. L.; Yu, H.; Yang, Y. H. Catalytic applications of alkali-functionalized carbon nanospheres and their supported Pd nanoparticles. Appl. Catal., B 2016, 184, 104−118. (17) Kanega, R.; Hayashi, T.; Yamanaka, I. Pd(NHC) Electrocatalysis for Phosgene-Free Synthesis of Diphenyl Carbonate. ACS Catal. 2013, 3, 389−392. (18) Kwon, G.; Ferguson, G. A.; Heard, C. J.; Tyo, E. C.; Yin, C. R.; DeBartolo, J.; Seifert, S.; Winans, R. E.; Kropf, A. J.; Greeley, J.; Johnston, R. L.; Curtiss, L. A.; Pellin, M. J.; Vajda, S. Size-Dependent Subnanometer Pd Cluster (Pd4, Pd6, and Pd17) Water Oxidation Electrocatalysis. ACS Nano 2013, 7, 5808−5817. (19) Yang, Y. Y.; Ren, J.; Li, Q. X.; Zhou, Z. Y.; Sun, S. G.; Cai, W. B. Electrocatalysis of Ethanol on a Pd Electrode in Alkaline Media: An in Situ Attenuated Total Reflection Surface-Enhanced Infrared Absorption Spectroscopy Study. ACS Catal. 2014, 4, 798−803. (20) Lan, J. H.; Lin, J. C.; Chen, Z. Q.; Yin, G. C. Transformation of 5-Hydroxymethylfurfural (HMF) to Maleic Anhydride by Aerobic Oxidation with Heteropolyacid Catalysts. ACS Catal. 2015, 5, 2035− 2041. (21) Li, G. Q.; Kobayashi, H.; Taylor, J. M.; Ikeda, R.; Kubota, Y.; Kato, K.; Takata, M.; Yamamoto, T.; Toh, S.; Matsumura, S.; Kitagawa, H. Hydrogen storage in Pd nanocrystals covered with a metal−organic framework. Nat. Mater. 2014, 13, 802−806. (22) Kim, S. J.; Choi, S. J.; Jang, J. S.; Kim, N. H.; Hakim, M.; Tuller, H. L.; Kim, I. D. Mesoporous WO3 Nanofibers with ProteinTemplated Nanoscale Catalysts for Detection of Trace Biomarkers in Exhaled Breath. ACS Nano 2016, 10, 5891−5899. (23) Yan, Y. B.; Li, K. X.; Dai, Y. H.; Chen, X. P.; Zhao, J.; Yan, Y.; Huang, J. J.; Yang, Y. H.; Lee, J. M. Controlled Synthesis of 3D Nanoplate-Assembled La2O3 Hierarchical Microspheres for EnzymeFree Detection of Hydrogen Peroxide. Adv. Mater. Interfaces 2016, 3, 1500833. (24) Yan, Y. B.; Li, K. X.; Dai, Y. H.; Chen, X. P.; Zhao, J.; Yang, Y. H.; Lee, J. M. Synthesis of 3D mesoporous samarium oxide hydrangea
porous Pd nanostructure. Stability retains 95.8% for the 100 μM H2O2 sensing after storage for 8 days. Figure 3E exhibits the high selectivity with the antiinterference capability of glucose, urea, and ascorbic acid for the 100 μM H2O2 detection. Such selectivity is accredited to the constant detection holding a potential of −0.34 V specifically for H2O2 sensing. Furthermore, hydrogen peroxide detection is crucial in food/environmental monitoring, and the tracking of biological dynamic processes where H2O2 is released.34 With the stimulation by certain pro-inflammatory, breast cancer cell releases H2O2.35 The MDA-MB-435 (human breast cancer cell line), for example, releases H2O2 when stimulated by N-formyl-L-methionyl-L-leucyl-phenylalanine (fMLP). Such a dynamic process was recorded by our sensor without labeling the technique, with Figure 3F showing the amperometric response results. The falling reduction current was found under stimulation by fMLP. Then the current−time curve reached a plateau after 600 s upon the final diffusion equilibrium. For benchmark, detection in a mixed solution of fMLP and PBS without breast cancer cells presented indiscernible signals. Such performance sheds light upon the promising utility of the porous Pd nanostructure for dynamic sensing of H2O2 molecules released by biological processes. In summary, we have prepared a novel porous Pd nanostructure in an aqueous solution using a soft micelle template from the interacting species of TMB with the surfactant of CTAB. It achieved excellent performance in nonenzymatic dynamic detection of H2O2 with changing concentration involving the low detection limit of 0.25 μM with signal/noise of 2.6. Meanwhile, the long linear range from 0.25 to 900 μM (correlation coefficient R = 0.992), and the remarkable sensitivity of 201 μA mM−1 were attained as well.
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AUTHOR INFORMATION
Corresponding Author
*Jong-Min Lee (
[email protected]). ORCID
Jong-Min Lee: 0000-0001-6300-0866 Author Contributions
L.Y. and Y.Y. contributed equally. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Academic Research Fund (RGT27/13) of Ministry of Education in Singapore.
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REFERENCES
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