pubs.acs.org/Langmuir © 2010 American Chemical Society
Supramolecular Microfibrils of o-Phenylenediamine Dimers: Oxidation-Induced Morphology Change and the Spontaneous Formation of Ag Nanoparticle Decorated Nanofibers Jingqi Tian,†,‡ Sen Liu,† and Xuping Sun*,† †
State Key Lab of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China, and ‡Graduate School of the Chinese Academy of Sciences, Beijing 100039, China Received July 30, 2010. Revised Manuscript Received August 31, 2010
The direct mix of aqueous FeCl3 and o-phenylenediamine (OPD) solutions at room temperature leads to supramolecular microfibrils of OPD dimers generated by the oxidation of OPD monomers by FeCl3 (Sun, X.; Hagner, M. Langmuir 2007, 23, 10441). In this Letter, we report on our recent finding that the subsequent treatment of such microfibrils with a AgNO3 aqueous solution transforms them into nanofibers decorated with spherical silver nanoparticles (AgNPs) with sizes in range of 5-20 nm. The possible formation mechanism involved is also discussed. It is interestingly found that as-formed AgNPs exhibit good catalytic activity toward the reduction of H2O2, leading to an enzymeless sensor with a fast amperometric response time of less than 5 s. The linear detection range is estimated to be from 100 μM to 80 mM (r = 0.998), and the detection limit is estimated to be 62 μM at a signal-to-noise ratio of 3.
During the past decades, conducting polymers (CPs) have been the focus of intense research for their metal-like and adjustable conductivity and promising applications in nanodevices1 and chemical sensors.2 Polyaniline (PANI) is one of the most studied CPs due to its chemical stability and relative high conductivity.3 Polymers based on aniline derivatives have also been extensively investigated.4 Among them, poly(phenylenediamine) (PPD) homopolymer is reported to be a highly aromatic polymer containing 2,3-diaminophenazine or quinoraline repeating unit, and exhibits high thermostability.5 PPD is usually prepared by an electrochemical method.6 On the other hand, chemical oxidation polymerization has also been proven to be an effective preparative method.7 Indeed, we8 and other researchers9 have successfully prepared nanobelts and microparticles of poly(o-phenylenediamine) (POPD) via chemical oxidation polymerization of the OPD monomers by HAuCl4 and ammonium persulfate, respectively. In our other report, we have demonstrated the successful preparation of supramolecular microfibrils of OPD dimers by mixing aqueous *To whom correspondence should be addressed. Telephone/Fax: 0086-43185262065. E-mail:
[email protected]. (1) Li, H.; Xie, K.; Pan, Y.; Yao, M.; Xin, C. Synth. Met. 2009, 159, 1386. (2) Forzani, E. S.; Li, X.; Tao, N. Anal. Chem. 2007, 79, 5217. (3) Pei, Q.; Yu, G.; Zhang, C.; Yang, Y.; Heeger, A. G. Science 1995, 269, 1086. (4) Sulimenko, T.; Stejskal, J.; Prokee, J. J. Colloid Interface Sci. 2001, 236, 328. (5) (a) Premasiri, A. H.; Euler, W. B. Macromol. Chem. Phys. 1995, 196, 3655. (b) Cataldo, F. Eur. Polym. J. 1996, 32, 43. (6) Dai, H.; Wu, Q.; Sun, S.; Shiu, K. J. Electroanal. Chem. 1998, 456, 47. (7) Ogura, K.; Shiigi, H.; Nakayama, M.; Fujii, A. J. Electrochem. Soc. 1998, 145, 3351. (8) Sun, X.; Dong, S.; Wang, E. Chem. Commun. 2004, 1182. (9) (a) Li, X.; Ma, X.; Sun, J.; Huang, M. Langmuir 2009, 25, 1675. (b) Huang, M.; Peng, Q.; Li, X. Chem.;Eur. J. 2006, 12, 4341. (c) Han, J.; Liu, Y.; Li, L.; Guo, R. Langmuir 2009, 25, 11054. (10) Sun, X.; Hagner, M. Langmuir 2007, 23, 10441. (11) (a) Klassen, N. V.; Marchington, D.; McGovan, H. C. E. Anal. Chem. 1994, 66, 2921. (b) Chang, M. C. Y.; Pralle, A.; Isacoff, E. Y.; Chang, C. J. J. Am. Chem. Soc. 2004, 126, 15392. (c) Matsubara, C.; Kawamoto, N.; Takamura, K. Analyst 1992, 117, 1781. (d) King, D. W.; Cooper, W. J.; Rusak, S. A.; Peake, B. M.; Kiddle, J. J.; O'Sullivan, D. W.; Melamed, M. L.; Morgan, C. R.; Theberge, S. M. Anal. Chem. 2007, 79, 4169. (e) Jia, J.; Wang, B.; Wu, A.; Cheng, G.; Li, Z.; Dong, S. Anal. Chem. 2002, 74, 2217. (f) Luo, X.; Xu, J.; Zhang, Q.; Yang, G.; Chen, H. Biosens. Bioelectron. 2005, 21, 190. (g) Wang, B.; Zhang, J.; Pan, Z.; Tao, X.; Wang, H. Biosens. Bioelectron. 2009, 24, 1141.
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FeCl3 and OPD solutions at room temperature.10 Although the microfibrils can be shortened by a sonication process, the transformation of such microfibers into smaller nanofibers has been proven to be unsuccessful. On the other hand, H2O2 is of great importance in the fields of chemistry, biology, clinical control, and environmental protection,11 and many detection techniques have been well developed.12 Among them, the electrochemical technique has been proven to be an inexpensive and effective way due to its intrinsic simplicity and high sensitivity and selectivity. Early H2O2 sensors involved the use of the intrinsic selectivity and sensitivity of enzymatic reactions where nanostructures are also employed to immobilize the enzymes and, at the same time, to reduce the possibility of protein denaturing.13 It has been shown that silver nanoparticles (AgNPs) show good catalytic activity toward the reduction of H2O2,13,14 opening the door for designing of enzymeless H2O2 sensors. In this Letter, we report on our recent finding that further treatment of microfibrils of OPD dimers10 with a AgNO3 aqueous solution can transform them into nanofibers, and at the same time spherical AgNPs ranging in size from 5 to 20 nm are spontaneously formed on the nanofibers, leading to AgNP-decorated nanofibers (AgNPs-NFs). The possible formation mechanism involved is discussed, and the influence of experimental parameters on the formation of AgNPs-NFs is also examined. The AgNPs are found to exhibit good catalytic activity toward the reduction of H2O2. An effective enzymeless H2O2 sensor based on such AgNPs-NFs is also constructed. It exhibits a fast amperometric response time of less than 5 s, and it has a linear detection range from 100 μM to 80 mM and a detection limit of 62 μM. (12) (a) Song, Y.; Wang, L.; Ren, C.; Zhu, G.; Li, Z. Sens. Actuators, B 2006, 114, 1001. (b) Guo, C.; Song, Y.; Wei, H.; Li, P.; Wang, L.; Sun, L.; Sun, Y.; Li, Z. Anal. Bioanal. Chem. 2007, 389, 527. (c) Willner, I.; Katz, E. Angew. Chem., Int. Ed. 2000, 39, 1180. (d) Xiao, Y.; Patolsky, F.; Katz, E.; Hainfeld, J.; Willner, I. Science 2003, 299, 1877. (13) Welch, C.; Banks, C.; Simm, A.; Compton, R. Anal. Bioanal. Chem. 2005, 382, 12. (14) Song, Y.; Cui, K.; Wang, L.; Chen, S. Nanotechnology 2009, 20, 105501.
Published on Web 09/14/2010
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Figure 1. (A) Low and (B) high magnification SEM images of the supramolecular microfibrils of the OPD dimers.
All chemicals were purchased from Aladin Ltd. (Shanghai, China) and used as received without further purification. The water used throughout all experiments was purified through a Millipore system. Phosphate buffer saline (PBS) was prepared by mixing stock solutions of NaH2PO4 and Na2HPO4, and a fresh solution of H2O2 was prepared daily. The microfibrils were prepared using our previously reported method with minor modifications.10 In brief, 6 mL of 0.2 M OPD aqueous solution was diluted to 20 mL with H2O and the mixture was maintained at 0-5 °C for 0.5 h before oxidative polymerization. Then 1.8 mL of 2 M FeCl3 aqueous solution with a 3:1 molar ratio of FeCl3 to OPD was added to the above mixture in one portion under stirring to ensure complete mixing. After that, a large quantity of precipitate occurred gradually. The precipitate thus formed was washed with water several times and then dispersed in 4 mL of water for characterization and further use. The AgNPs-NFs were prepared as follows (sample 1): in brief, 15 μL of 0.5 M AgNO3 aqueous solution was mixed with 30 μL of microfibril dispersion under gentle shaking at room temperature over a 0.5 h period. Scanning electron microscopy (SEM) measurements were made on a XL30 ESEM FEG scanning electron microscope at an accelerating voltage of 20 kV. Samples for SEM examination were made by placing a drop of the dispersion on a bare indium tin oxide (ITO) and air-dried at room temperature. Transmission electron microscopy (TEM) measurements were made on a HITACHI H-8100 electron microscope (Hitachi, Tokyo, Japan) with an accelerating voltage of 200 kV. The sample for TEM characterization was prepared by placing a drop of the dispersion on a carbon-coated copper grid and dried at room temperature. Zeta potential measurements were performed on a Nano-ZS Zetasizer ZEN3600 instrument (Malvern Instruments Ltd., U.K.). Analysis of the X-ray photoelectron spectrum (XPS) was performed on an ESCLAB MKII instrument using Mg as the exciting source. Transmission infrared spectra were collected in the transmission mode on a Nicolet 560 Fourier transform infrared (FTIR) spectrometer. Electrochemical measurements are performed with a CHI 660D electrochemical analyzer (CH Instruments, Inc., Shanghai). A conventional three-electrode cell was used, including a pretreated glass carbon electrode as the working electrode, a Ag/AgCl (saturated KCl) electrode as the reference electrode, and platinum foil as the counter electrode. The potentials are measured with a Ag/AgCl electrode as the reference electrode. All the experiments were carried out at ambient temperature. Figure 1 shows typical SEM images of the precipitates thus formed. The low magnification image indicates that the precipitates exclusively consist of a large amount of microfibrils about tens of micrometers in length, as shown in Figure 1A. The high Langmuir 2010, 26(19), 15112–15116
magnification image further reveals that such microfibrils range in diameter from 1 to 1.5 μm and have a smooth surface, as shown in Figure 1B. It should be pointed out that the microfibrils will gradually precipitate from the solution within hours due to their large size. Li et al. have demonstrated that POPD microparticles can effectively adsorb Ag(I) ions because many amino and imino groups located adjacent to each other in the POPD chains provide abundant coordination sites for Ag(I) ions.9a It was found that POPD has the ability to effectively reduce as-absorbed Ag(I) ions to form AgNPs. Such an observation was also verified by the same group in aromatic diamine polymer based systems.15 We also treated the microfibrils with a AgNO3 aqueous solution and obtained a stable dispersion free from flocculation or aggregation for several months. To get further information on the products thus obtained, we examined the dispersion by TEM. Figure 2A shows the low magnification image, indicating the products exclusively consist of nanofibers about tens of nanometers in diameter and below 3 μm in length. Please note that many small black dots are also observed on each nanofiber. A high magnification image further reveals that these dots are nanoparticles with sizes in the range of 5-20 nm and spherical in shape, as shown in Figure 2B. The chemical composition of such nanoparticle-decorated nanofibers was determined by the energydispersed spectrum (EDS), as shown in Figure 2C. The observation of the peaks of C, N, and Ag elements (other peaks originate from the substrate used) demonstrates that the products are generated from microfibrils of OPD dimers and Agþ and the nanocomposites are AgNP-decorated nanofibers. All these observations demonstrate that the treatment of the microfibrils with a AgNO3 solution not only breaks them into nanofibers but also leads to spontaneous formation of spherical AgNPs along the nanofibers produced. To study the structure of the nanofibers, we collected the corresponding FTIR spectrum of the AgNPs-NFs (Supporting Information Figure S1). The bands at 3369 and 3218 cm-1 can be attributed to the characteristic stretching vibration of N-H bonds in -NH and -NH2 groups, respectively. The bands at 1614 and 1516 cm-1 can be assigned to stretching of quinoid and benzenoid rings, and the bands at 1355 and 1232 cm-1 are associated with C-N stretching of quinoid and benzenoid rings, respectively. Additionally, the bands at 1141 and 617 cm-1 can be ascribed to the in-plane and out-of-plane bending vibration of the C-H bonds of 1,2,4-trisubtituted benzene rings, respectively. These data are quite consistent with those of POPD microparticles,9a suggesting that the OPD dimers10 are further (15) Li, X.; Liu, R.; Huang, M. Chem. Mater. 2005, 17, 5411.
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Figure 2. (A) Low and (B) high magnification TEM images and (C) the corresponding EDS of the AgNPs-NFs obtained by treating the supramolecular microfibrils of the OPD dimers with a AgNO3 aqueous solution.
polymerized by Agþ to form POPD nanofibers after the incubation of microfibrils with a AgNO3 aqueous solution. The formation of such AgNP-decorated POPD nanofibers in our present study can also be attributed to that the POPD can effectively adsorb Ag(I) ions via coordination of their amino and imino groups to Agþ and then in situ reduce the as-absorbed Agþ to form AgNPs.9a We further collected the XPS spectrum of the AgNPs-NFs (Supporting Information Figure S2). It was reported that metallic Ag 3d peaks are centered at 373.9 and 367.9 eV16 and Ag(I) exhibits two peaks at 375.8 and 369.6 eV.17 However, the Ag 3d peaks in our present study appear at 374.7 and 368.6 eV, suggesting that there are both metallic Ag and Agþ adsorbed on the POPD nanofibers. The zeta potentials of the microfibrils and the AgNP-decorated nanofibers were measured to be about 4.1 and 8.7 mV, respectively, indicating that the nanofiber has a more positive surface charge density. Such observation may be attributed to the absorption of positively charged Ag(I) ions on the nanofiber surface.9a We may suggest that both its higher surface charge density and small size of the nanofiber contribute to its good stability. Based on above experimental observations, a possible formation process of AgNPs-NFs is proposed. The oxidation of OPD monomers leads to supramolecular assemblies of OPD dimers first.10 In the present system, two opposite processes occur: the disassembly of microfibrils into free OPD dimers and the assembly of free OPD dimers into microfibrils, and the resultant effect depends on the competition of the two opposite processes. Obviously, (16) Pol, V. G.; Srivastava, D. N.; Palchik, O.; Palchik, V.; Slifkin, M. A.; Weiss, A. M.; Gedanken, A. Langmuir 2002, 18, 3352. (17) Sun, S.; Dong, S.; Wang, E. Macromolecules 2004, 37, 7105.
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the assembly process predominates in the dispersion of the microfibrils. When AgNO3 was introduced, a spontaneous redox between free OPD dimers and Agþ occurred. With the elapsed time, the free dimers were gradually consumed, and as a result the disassembly process predominated, leading to the breaking down of the microfibrils. On the other hand, new Ag atoms were gradually generated in this system, and nucleation occurred as the concentration of Ag atoms reached a critical supersaturation, resulting in the formation of nuclei. The nuclei grew to nanoparticles by further addition of silver atoms. Meanwhile, a second assembly process happened at the same time, due to the attachment of Ag nanoparticle onto the assemblies newly formed; however, the final structures were prevented from assembling into a larger size. As a result, AgNP-decorated nanofibers are obtained finally. Scheme 1 presents a schematic diagram to illustrate the AgNPs-NFs mechanism. We also examined the influence of both the relative amount of Agþ and the reaction temperature on the AgNPs-NFs formation. When the amount of Agþ was increased up to 4-fold (sample 2), a large amount of small nanoparticles with diameters ranging from 3 to 6 nm were formed as the main products, as shown in Figure 3A and B. When the amount of Agþ was further increased up to 12-fold (sample 3), smaller nanoparticles were formed and the particle density on the nanofiber increased, as shown in Figure 3C and D. These observations indicate that the increase of Agþ in amount is favorable for the formation of small AgNPs. On the other hand, the reaction temperature has a small effect on the particle size distribution, as is evidenced by the TEM images shown in Figure 3E and F of the products obtained at 60 °C, under otherwise identical conditions used for preparing sample 1 (sample 4). Langmuir 2010, 26(19), 15112–15116
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Figure 3. Low magnification TEM images of the products of sample 2 (A), sample 3 (C), and sample 4 (E). Corresponding high magnification TEM images are shown in (B), (D), and (F), respectively.
To demonstrate the sensing application of the AgNPs-NFs, we constructed an enzymeless H2O2 sensor by immobilization of the composites on a bare glassy carbon electrode (GCE) surface. Figure 4 shows cyclic voltammetry (CV) results of a bare GCE, the POPD microfibril-modified GCE (POPD/GCE), and the AgNPs-NFs-modified GCE (AgNPs-NFs/GCE) in N2 saturated 0.2 M PBS at pH 6.5 in the presence of 1.0 mM H2O2 (scan rate: Langmuir 2010, 26(19), 15112–15116
0.02 V/s). The response of the bare GCE toward the reduction of H2O2 is pretty weak (about 4 μA in intensity). The GCE modified with microfibrils of OPD dimers shows two reversible redox peaks which could be assigned to the electroactive OPD dimers. A remarkable current peak about 25 μA in intensity is observed at -0.48 V for the AgNPs-NFs/GCE. The above observations indicate that the AgNPs contained in the composites exhibit DOI: 10.1021/la103038m
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Figure 4. CV results of a bare GCE, a POPD microfibril-modified GCE (POPD/GCE), and the AgNPs-NFs-modified GCE (AgNPs-NFs/GCE) in N2 saturated 0.2 M PBS at pH 6.5 in the presence of 1.0 mM H2O2 (scan rate: 0.02 V/s).
Figure 5. Typical steady-state response of the AgNPs-NFs/GCE electrode to successive injection of H2O2 into the stirred N2 saturated 0.2 M PBS at pH 6.5. Inset: calibration curve (applied potential: -0.30 V).
Scheme 1. Schematic Diagram to Illustrate the Possible Formation Mechanism of AgNPs-NFs
good catalytic activity toward the reduction of H2O2. Compared to GCE modified by simple electroreduction of Agþ,18 this AgNPs-NFs/GCE exhibits a 15.2% enhancement of peak current and a 15 mV positive shift of the peak potential, indicating that this form of modified electrode is superior for electrocatalysis. Figure 5 shows the typical current-time plot of the AgNPsNFs/GCE in 0.2 M PBS buffer (pH: 6.5) on successive step change of H2O2 concentrations under optimized conditions. When an aliquot of H2O2 was dropped into the stirring PBS solution, the reduction current rose steeply to reach a stable value. The sensor could achieve 95% of the steady state current within 5 s, indicating a fast amperometric response behavior. It is clearly seen that the steps are more horizontal in the region of lower concentration of H2O2 and the noises become higher with increased concentration of H2O2. The inset in Figure 5 shows the calibration curve of the sensor. The linear detection range is estimated to be from 100 μM (18) Cui, K.; Song, Y.; Yao, Y.; Huang, Z.; Wang, L. Electrochem. Commun. 2008, 10, 663.
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to 80 mM (r = 0.998), and the detection limit is estimated to be 62 μM at a signal-to-noise ratio of 3. In summary, by treating the supramolecular microfibrils assembled from OPD dimers with a AgNO3 aqueous solution, we have successfully transformed them into POPD nanofibers. Furthermore, AgNPs are spontaneously formed on the nanofibers, leading to AgNP-decorated nanofibers. Most importantly, the AgNPs are found to exhibit good catalytic activity toward the reduction of H2O2. The construction of an effective enzymeless H2O2 sensor based on such composites is also demonstrated. Our observations are significant for the following two reasons: (1) It is the first demonstration of an oxidation-induced morphology change of microfibrils of OPD dimers and the spontaneous formation of AgNP-decorated nanofibers. (2) It provides us a general methodology for generation of other noble metal nanoparticle decorated nanofibers for applications. Supporting Information Available: FTIR spectrum of the AgNPs-NFs; XPS spectrum of the AgNPs-NFs. This material is available free of charge via the Internet at http://pubs.acs.org.
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