β-Fe3+O(OH, Cl) Nanospindles in

May 12, 2009 - ... synthesized in the presence of cetyltrimethylammonium bromide and poly(acrylic acid) in aqueous solution using only a one-step proc...
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J. Phys. Chem. C 2009, 113, 9465–9472

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One-Step Synthesis of Au/PEDOT/β-Fe3+O(OH, Cl) Nanospindles in Aqueous Solution Hui Mao, Xiaofeng Lu, Xincai Liu, Jun Tang, Ce Wang, and Wanjin Zhang* Alan G. MacDiarmid Institute, Jilin UniVersity, Changchun 130012, P. R. China ReceiVed: January 12, 2009; ReVised Manuscript ReceiVed: April 17, 2009

A kind of unique noble metal/polymer/inorganic compound nanostructured complex, poly(3,4-ethylenedioxythiophene) (PEDOT)/β-Fe3+O(OH, Cl) nanospindle containing Au nanoparticles (Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindle), was successfully synthesized in the presence of cetyltrimethylammonium bromide and poly(acrylic acid) in aqueous solution using only a one-step process. Their length and width were in the range of 190-280 and 30-50 nm, respectively. Transmission electron microscopy images and electron diffraction patterns of single Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindle indicated the existence of Au nanoparticles in the complex and highly ordered single crystals of beta-akaganeite (β-Fe3+O(OH, Cl)) in the nanospindles, which were further confirmed by X-ray diffraction patterns. Furthermore, on the basis of the results of different experiments, a possible mechanism concerning the formation of Au/PEDOT/βFe3+O(OH, Cl) composite nanospindles was also proposed. During the formation of PEDOT/β-Fe3+O(OH, Cl) nanospindles, the polymerization of 3,4-ethylenedioxythiophene (EDOT) and the hydrolyzation of FeCl3 occurred simultaneously in aqueous solution, and after the addition of HAuCl4 · 4H2O aqueous solution into the reaction system, Au nanoparticles were in situ reduced by unreacted EDOT monomer or PEDOT oligomers to form PEDOT/β-Fe3+O(OH, Cl) nanospindles. If PEDOT/β-Fe3+O(OH, Cl) nanospindles were synthesized first, Au nanoparticles could not be embedded into the nanospindles by postsynthesis method due to the highly ordered crystallinity of beta-akaganeite. This new kind of unique noble metal/polymer/inorganic compound nanostructured complex could act as a good steady electrode material due to its good repeatability of cyclic voltammetry responses for electrocatalytic oxidation of D-ascorbic acid. 1. Introduction Micro/nanostructured materials of conducting polymers, such as polyaniline (PANI), polypyrrole (PPy), polythiophene (PTh), and even their various derivatives, have received tremendous attention during the past 10 years due to their unique optical, electronic, and mechanical properties, which will result in promising applications in many fields, especially in electrical or optoelectronic nanodevices and chemical sensors.1-5 Recently, complex materials composed of conducting polymers and noble metal nanoparticles (such as Au, Pt, Ag, and Pd) have set off much research interest due to the good environmental stability and tunable electrical and optical properties of conducting polymers as well as the unique optical and catalytic properties of metal nanoparticles.6,7 Besides the electrochemical deposition method,8 many chemical methods have been developed for the incorporation of noble metal nanoparticles into conducting polymers, including self-assembly,9,10 phase-transfer method by surfactant,11 and in situ and postsynthesis methods.12 However, most of these methods are often used for the preparation of noble metal nanoparticles/polyaniline or polypyrrole composite micro/ nanostructures, and there is not much work on the synthesis of noble metal nanoparticles/poly(3,4-ethylenedioxythiophene) (PEDOT) composite nanomaterials. Because of its high electric conductivity, moderate band gap, good environmental stability, and high optical transparency,13-16 PEDOT has gradually become a focus of research. Up to now, many different kinds of PEDOT nanostructures have been prepared by several unique methods. For example, surfactantmediated interfacial polymerization17 and dispersion polymer* To whom correspondence should be addressed. Tel./Fax: 86-43185168924. E-mail: [email protected].

ization in alcoholic media18 have been developed for the fabrications of PEDOT nanocapsules, nanoparticles, and vesicles; reverse microemulsion polymerization,19,20 self-assembled micellar soft-template approach,21 V2O5 seeding approach,22 and interfacial polymerization-crystallization23 have been used for the synthesis of PEDOT nanorods, nanotubes, nanofibers, and single-crystal nanoneedles, respectively. But even now, there has been only a few reports related to the fabrication of noble metal/PEDOT composite nanomaterials. By in situ and postsynthesis methods, Manohar et al. made Ag nanoparticles with a diameter of 18-22 nm uniformly distributed along the walls of PEDOT nanotubes which were prepared through reverse microemulsion polymerization.20 Shi et al. successfully obtained Au-PEDOT nanocables by one-step interfacial reaction at room temperature.24 According to our previous work, PEDOT/β-Fe3+O(OH, Cl) nanospindles have been synthesized successfully using FeCl3 · 6H2O as an oxidant in aqueous solution in the presence of cetyltrimethylammonium bromide (CTAB) and poly(acrylic acid) (PAA).25 They have good repeatability of cyclic voltammetry (CV) responses for electrocatalytic oxidation of KI and reduction of KIO3 and can act as a good steady and convenient electrode material for detecting iodic compounds due to their large surface area and good adhesion on a glassy carbon electrode (GCE).26 Herein, we successfully introduced Au nanoparticles into PEDOT/β-Fe3+O(OH, Cl) nanospindles and prepared a kind of unique noble metal/polymer/inorganic nanostructured complex, Au nanoparticles/PEDOT/βFe3+O(OH, Cl) nanospindles (Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindles), through a one-step process in aqueous solution. Transmission electron microscopy (TEM) images, electron diffraction (ED) patterns, and X-ray diffraction (XRD)

10.1021/jp900274b CCC: $40.75  2009 American Chemical Society Published on Web 05/12/2009

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Figure 1. SEM images of (a) Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindles; (b) PEDOT/β-Fe3+O(OH, Cl) nanospindles. Other conditions: [EDOT] ) 0.025 M; [EDOT]/[FeCl3 · 6H2O] ) 1:3; [CTAB] ) 0.0075 M; [PAA] ) 0.02 mg/mL; T ) 50 °C.

Figure 2. TEM images of (a) Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindles (the corresponding size distribution histogram, of Au nanoparticles, inset); (b) PEDOT/β-Fe3+O(OH, Cl) nanospindles; (c) single Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindle (electron diffraction pattern, inset); (d) single PEDOT/β-Fe3+O(OH, Cl) nanospindle (electron diffraction pattern, inset).

patterns proved the existence of Au nanoparticles in the new complex. During the formation of PEDOT/β-Fe3+O(OH, Cl) nanospindles, the polymerization of 3,4-ethylenedioxythiophene (EDOT) and the hydrolyzation of FeCl3 occurred simultaneously in aqueous solution, and after the addition of HAuCl4 · 4H2O aqueous solution into the reaction system, Au nanoparticles were in situ reduced by the unreacted EDOT monomer and EDOT oligomers which could further form PEDOT/β-Fe3+O(OH, Cl) nanospindles. It is the first time that Au nanoparticles were combined with high-crystalline organic/inorganic complex (PEDOT/β-Fe3+O(OH, Cl) nanospindles) to form a new noble metal/polymer/inorganic compound composite nanostructure. They also exhibited good repeatability of CV responses for electrocatalytic oxidation of D-ascorbic acid (D-AA). 2. Experimental Section 2.1. Materials. EDOT (g99.0%) monomer was purchased from Beili Pharm Raw Material (Suzhou) Co., Ltd. and used without further purification. All the other reagents were analytical grade and used without further purification, including FeCl3 · 6H2O (Sinopharm Chemical Reagent Co., Ltd.S, g99.0%), CTAB (Sinopharm Chemical Reagent Co., Ltd.S, g99.0%), PAA (Tianjin Kermel Chemical Reagent Co., Ltd., solid content is 30%), HAuCl4 · 4H2O (Sinopharm Chemical Reagent Co., Ltd.S, content of Au g 48.7%), D-AA (Shenyang No. 3 Chemical Reagent Factory, g96.7%), and ethanol (Tianjin TIANTAI Fine Chemical Co., Ltd., g99.7%).

2.2. Preparation of Au Nanoparticles/PEDOT/β-Fe3+O(OH, Cl) Nanospindles Complex. Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindles were obtained in aqueous solution through a one-step process. In a typical procedure, 1 mmol EDOT and 0.3 mmol CTAB were dispersed in 23 mL of deionized water, and 8 mL of 0.1 mg/mL PAA aqueous solution was added in it. After the above mixture was vigorously stirred for 30 min at 50 °C, 3 mmol FeCl3 · 6H2O dissolved in 8 mL of deionized water was poured into the above mixture system. The reaction was carried out under stirring for 75 min at 50 °C. Then 1 mL of 0.01 g/mL HAuCl4 · 4H2O aqueous solution was added into the reaction system slowly. The reaction was carried out under stirring at 50 °C for another 20 h. The dark blue black flocculent precipitates were collected by centrifugation and then washed and redispersed with water and ethanol several times to remove unreacted chemicals and outgrowths. Finally, the obtained samples were dried in vacuum at 45 °C for 24 h. 2.3. Preparation of Au Nanoparticles/PEDOT/β-Fe3+O(OH, Cl) Nanospindles Complex Modified GCE. After being washed and redispersed into water and ethanol several times, the obtained Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindles were dispersed in ethanol to give 2.20 mg/mL dark blue black suspension. The film was prepared by dropping 6 µL of suspension onto the clean GCE surface and then evaporating the solvent in the environment. The modified GCE was used as the working electrode.

Au/PEDOT/β-Fe3+O(OH, Cl) Composite Nanospindles 2.4. Characterizations and Apparatus. The images of Au/ PEDOT/β-Fe3+O(OH, Cl) composite nanospindles were obtained by scanning electron microscopy (SEM) measurements which were performed on a SHIMADZU SSX-550 microscope. The products were dispersed in ethanol before being dried and then dropped on the surface of a piece of aluminum foil for the measurement of SEM. TEM experiments were performed on a JEM-2000EX electron microscope with an acceleration voltage of 160 kV. XRD patterns were obtained with a Siemens D5005 diffractometer using Cu KR radiation. Analysis of the X-ray photoelectron spectra (XPS) was performed on an ESCLAB MKII using Al as the exciting source. Inductive coupled plasma emission spectroscopy (ICP-AES) was performed on an ICPAES/1000. Fourier transform infrared spectroscopy (FTIR) spectra of KBr powder-pressed pellets were recorded on a BRUKER VECTOR22 Spectrometer. Transmission spectra of Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindles and PEDOT/β-Fe3+O(OH, Cl) nanospindles were recorded on a Shimadzu UV-3101 PC Spectrometer. The electrical conductivity of both nanospindles at room temperature was measured by a four-probe method using a 2182 Nanovoltmeter and 2400 Sourcemeter as the current source. The electrochemical performance of Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindles was investigated by using CV with a CHI660B Electrochemical Station (Shanghai CHENHUA Instrument Co., Ltd.). In a threeelectrode system, a modified GCE, a platinum wire, and a saturated calomel electrode (SCE) were used as the working electrode, the counter electrode, and the reference electrode, respectively. 3. Results and Discussion 3.1. Morphology of Au/PEDOT/β-Fe3+O(OH, Cl) Composite Nanospindles. Figures 1 and 2 give SEM and TEM images of PEDOT/β-Fe3+O(OH, Cl) nanospindles and Au/ PEDOT/β-Fe3+O(OH, Cl) composite nanospindles which were prepared in the presence of CTAB and PAA, respectively. It was obvious that these nanospindles were well-defined and dominant, and other morphologies of the products hardly existed. Parts a and b of Figure 2 show representative TEM images of PEDOT/β-Fe3+O(OH, Cl) nanospindles with and without Au nanoparticles. It was very clear to see that Au nanoparticles were combined with PEDOT/β-Fe3+O(OH, Cl) nanospindles after adding HAuCl4, although the sizes of Au nanoparticles were not very uniform. The corresponding size distribution of Au nanoparticles is shown in Figure 2a (inset), and it is found that it gives an average diameter of about 12.9 nm by calculation. Comparing to PEDOT/β-Fe3+O(OH, Cl) nanospindles, the sizes of Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindles were a little bigger. The lengths and widths of the PEDOT/β-Fe3+O(OH, Cl) nanospindles were in the range of 190-280 and 30-50 nm, respectively, and those of the Au/ PEDOT/β-Fe3+O(OH, Cl) composite nanospindles were in the range of 210-300 and 40-60 nm, respectively. In order to prove the existence of Au nanoparticles in PEDOT/β-Fe3+O(OH, Cl) nanospindles, parts c and d of Figure 2 give TEM images and ED patterns of single PEDOT/β-Fe3+O(OH, Cl) nanospindle with and without Au nanoparticles. The visible atrous points in the single nanospindle were Au nanoparticles (Figure 2c). Because of the perfect combination of PEDOT and betaakaganeite, no points were found in single PEDOT/β-Fe3+O(OH, Cl) nanospindle, and it appeared homogeneous25 (Figure 2d). ED patterns showed the diffraction dots of Au nanoparticles in single Au/PEDOT/β-Fe3+O(OH, Cl) nanospindle and the characteristic diffraction dots of single-crystalline beta-akaganeite in both single PEDOT/β-Fe3+O(OH, Cl) nanospindle and Au/

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Figure 3. XRD scattering patterns of (a) Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindles; (b) PEDOT/β-Fe3+O(OH, Cl) nanospindles.

PEDOT/β-Fe3+O(OH, Cl) nanospindle, which further proved that the complex consisted of Au nanoparticles and PEDOT/ β-Fe3+O(OH, Cl) nanospindles (inset in Figure 2c and d). 3.2. Characterizations of Au/PEDOT/β-Fe3+O(OH, Cl) Composite Nanospindles. The presence of Au nanoparticles in PEDOT/β-Fe3+O(OH, Cl) nanospindles was also confirmed by powder X-ray diffraction patterns, which are shown in Figure 3. PEDOT/β-Fe3+O(OH, Cl) nanospindles indicate a number of diffraction peaks (Figure 3b) which consisted of betaakaganeite (PDF 13-157) as found in our previous work.25 According to those data, beta-akaganeite in the nanospindles complex was attributed to a tetragonal system and body-centered lattice. Comparing curve a with b, four strong peaks appeared with the maximum intensity at 2θ values of 38.33, 44.49, 64.71, and 77.75°, which represented Bragg’s reflections from the (111), (200), (220), and (311) planes of Au, respectively. Although the peak at 2θ ) 64.71° also appeared in curve b, the relative intensity of that in curve a was much stronger than that in curve b. It indicated that the peak which corresponds to the (220) plane of Au and the peak which corresponds to the (541) plane of PEDOT/β-Fe3+O(OH, Cl) nanospindles were overlapped in Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindles. Besides the four strong peaks in curve a, a number of other diffraction peaks appeared in curves a and b, which indicated that the introduction of Au nanoparticles hardly influenced the crystallinity of PEDOT/β-Fe3+O(OH, Cl) nanospindles. The average size of Au nanoparticles has also been calculated by XRD data, which gives an average diameter of about 16.6 nm, which was similar to that from the TEM images. X-ray photoelectron spectra data have been used to characterize the chemical structure of Au/PEDOT/β-Fe3+O(OH, Cl) nanospindles (Figure 4). The elements C, O, S, Fe, and Cl are detected except for Au, though Cl was not very obvious in the survey spectra of Au/PEDOT/β-Fe3+O(OH, Cl) nanospindles (Figure 4i). It might be because Au element did not exist on the surface of Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindles, which pointed out that Au nanoparticles were almost embedded within PEDOT/β-Fe3+O(OH, Cl) nanospindles. According to ICP-AES results, the weight content of Au nanoparticles in Au/PEDOT/β-Fe3+O(OH, Cl) nanospindles was on average 8.95%, which can well confirm the presence of Au element. C1s and O1s core-line spectra of Au/PEDOT/βFe3+O(OH, Cl) composite nanospindles were similar to that of

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Figure 4. XPS spectra of Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindles: (i) survey spectra; (ii) C1s; (iii) O1s; (iv) S2p; (v) Fe2p; (vi) Cl2p.

PEDOT/β-Fe3+O(OH, Cl) nanospindles.25 Three peaks in C1s core-line spectra have been found at 284.87, 286.12, and 287.61 eV in Figure 4ii, which were associated with C-C/C-H, C-S, and C-O, respectively. Two peaks in O1s core-line spectra have been found at 530.61 and 532.56 eV as in Figure 4iii, which

were associated with lattice oxygen (OI) and C-O-C, respectively. It was proved that the part of beta-akaganeite existed in the composite nanospindles. Unlike C1s and O1s spectra, there was little difference in S2p core-line spectra in PEDOT/βFe3+O(OH, Cl) nanospindles with and without Au nanoparticles.

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Figure 5. (i) FTIR spectra and (ii) UV-vis spectra of (a) Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindles; (b) PEDOT/β-Fe3+O(OH, Cl) nanospindles. (They were as-synthesized by the above method and dispersed in ethanol in UV-vis measurement.)

In Figure 4iv, three peaks have been found in S2p core-line spectra of Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindles. The peaks at 164.29 and 166.16 eV were associated with neutral S and cationic S+ in the PEDOT part of the composite nanospindles, respectively. But the peak at 169.44 eV which was associated with cationic S6+ had been detected. It might be due to the strong oxidizability of HAuCl4 · 4H2O, which made S element oxidized further. The chemical structures of Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindles and PEDOT/β-Fe3+O(OH, Cl) nanospindles were also characterized by FTIR and UV-vis absorption spectra. Figure 5i gives typical FTIR spectra of PEDOT/ β-Fe3+O(OH, Cl) nanospindles with and without Au nanoparticles synthesized by the above method in aqueous solution. The vibrational bands at 1526-1475 cm-1 are due to CRdCβ asymmetric stretching. The peaks at 1384 and 1355 cm-1 are assigned to CβsCβ stretching. The absorption at 1089 cm-1 is due to PEDOT skeleton vibration, and the bands at 1205-1054 cm-1 are associated with the stretching in the alkylenedioxy group. The peaks at 982, 842, and 695 cm-1 are due to CsS stretching. The absorption at 1645 cm-1 which is assigned to the doped level of PEDOT is also observed. The bands at 2850-2980 cm-1 are associated with the weak characteristic CH2 stretchings of the dioxyethylene bridge. All of these data are consistent with those reported for PEDOT19,27,28 and indicate the presence of PEDOT in both nanospindles. The presence of Au nanoparticles hardly influenced the vibrational bands of PEDOT/β-Fe3+O(OH, Cl) nanospindles. Figure 5ii presents the UV-vis spectra of Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindles and PEDOT/β-Fe3+O(OH, Cl) nanospindles dispersed in ethanol. Due to the smaller sizes of the two kinds of nanospindles, the absorption peak near 810 nm is observed, which is a little blue-shifted corresponding to the bulk PEDOT, and it is also the feature typically seen in the doped (oxidized) state of PEDOT.29 But the characteristic peak of Au nanoparticles is not observed, which may be due to the embedment of them within the high-crystalline PEDOT/β-Fe3+O(OH, Cl) nanospindles and being covered up by PEDOT in the nanospindles. 3.3. Mechanism of Au/PEDOT/β-Fe3+O(OH, Cl) Composite Nanospindles. In order to investigate the mechanism of the formation of Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindles and the effect of CTAB and PAA, the reaction was

SCHEME 1: Mechanism of the Formation of Au/ PEDOT/β-Fe3+O(OH, Cl) Composite Nanospindles

carried out under the same conditions without adding them into the system. Figure 6 shows TEM images of Au/PEDOT/βFe3+O(OH, Cl) composite nanospindles synthesized without CTAB and PAA. It can be clearly seen that Au nanoparticles were also combined with PEDOT/β-Fe3+O(OH, Cl) nanospindles. Compared with Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindles synthesized in the presence of CTAB and PAA (Figure 2a), many irregular coagula existed among the nanospindles. As known, the formation of PEDOT/β-Fe3+O(OH, Cl) nanospindles was due to the polymerization of EDOT, and the hydrolyzation of FeCl3 occurred simultaneously in aqueous solution,25 so after HAuCl4 · 4H2O was added into the reaction system when the reaction was carried out for 75 min, AuCl4was in situ reduced to Au nanoparticles by the EDOT monomer or the PEDOT oligomer during the growth of PEDOT/βFe3+O(OH, Cl) nanospindles. Along with the increase of reaction time, Au nanoparticles were gradually combined with PEDOT/ β-Fe3+O(OH, Cl) nanospindles. CTAB and PAA not only acted as gelling agents30 but also played a role of soft templates, which made the morphology of nanospindles form better. Because CTAB was a cationic surfactant, there was ionic interaction between CTA+ and AuCl4-, which might cause Au nanoparticles to combine with PEDOT/β-Fe3+O(OH, Cl) nanospindles much better. The scheme of the formation of Au/PEDOT/βFe3+O(OH, Cl) composite nanospindles is shown in Scheme 1.

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Figure 6. TEM images of (a) Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindles and (b) single Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindle which were synthesized without CTAB and PAA.

Figure 7. Cyclic voltammogram of (a) bare GCE and (b) Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindles modified GCE in PBS with D-AA: (i) 0; (ii) 1 mM.

But when we synthesized PEDOT/β-Fe3+O(OH, Cl) nanospindles first and then added HAuCl4 · 4H2O aqueous solution into the system, Au nanoparticles could not be embedded into the nanospindles by this postsynthesis method due to the highly ordered crystallinity of beta-akaganeite. 3.4. Electrical Conductivity of Au/PEDOT/β-Fe3+O(OH, Cl) Composite Nanospindles. Because of the introduction of Au nanoparticles, the electrical conductivity of Au/PEDOT/βFe3+O(OH, Cl) composite nanospindles at room temperature was about 7.89 × 10-3 S/cm using a four-probe method, which was about three times higher than that of PEDOT/β-Fe3+O(OH, Cl) nanospindles (2.70 × 10-3 S/cm). The increase of the electrical conductivity of Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindles was probably because Au nanoparticles serve as a “conducting bridge” between the PEDOT/βFe3+O(OH, Cl) conducting domains, which increases the effective percolation. 3.5. Electrochemical Behaviors and CV Response of D-Ascorbic Acid of Au/PEDOT/β-Fe3+O(OH, Cl) Composite Nanospindles Modified GCE. Figure 7i gives the CV curves of bare GCE and Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindles modified GCE in a phosphate buffer solution (PBS) (pH ) 6.90, 20 °C) with a scanning rate of 50 mV/s. No peaks appeared in the CV curve by bare GCE (Figure 7i-a), and the response current was much weaker than that of the modified GCE (Figure 7i-b). When the CV measurements have been carried out in PBS with 1 mM D-AA, the oxidation potential of D-AA was around at 0.448 V with a peak current at about -3.468 µA (Figure 7ii-a). It was found that the oxidation peak was broad and no peak in the reverse scan was observed, which indicated that the oxidation of D-AA at the

Figure 8. Cyclic voltammogram of Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindles modified GCE in PBS with different concentrations of D-AA: (a) 0; (b) 0.1; (c) 1; (d) 5; (e) 10 mM.

bare GCE was totally irreversible. But for Au/PEDOT/βFe3+O(OH, Cl) composite nanospindles modified GCE, the oxidation peak of D-AA occurs at around 0.028 V, which showed a cathodic shift of 0.42 V compared to the bare GCE, and the peak current was about 6.2 times higher than that of bare GCE (Figure 7ii-b), which indicated the electrocatalytic oxidation of D-AA at the modified GCE. According to Mathiyarasu’s report,31 the large cathodic shift in the oxidation peak potential and the enhanced oxidation current may be due to the following: there are some electrostatic attractions between the surface groups on the modified GCE and ascorbate anions in

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Figure 9. (i) Cyclic voltammogram of Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindles modified GCE at a scanning rate of (a) 10; (b) 20; (c) 30; (d) 40; (e) 50; (f) 60; (g) 70; (h) 80; (i) 90; (j) 100 mV/s in PBS with 1 mM D-AA. (ii) Plot of the square root of scan rate vs peak current.

Figure 10. (i) Amperometric response of Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindles at 0.1 V to successive addition of 10 µL of 100 mM D-AA (0.2 mM) into 5 mL of PBS stirred constantly. (ii) Calibration curve of the current against D-AA concentration.

the solution. Also, according to the results of XPS data, the actual dopants may be either Cl- or [FeCl4]- for the PEDOT part in Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindles, so they could be partly exchanged by ascorbate anions which might lead to accumulation of ascorbate anions at the interface of the modified GCE. Figure 8 presentes CV responses of Au/ PEDOT/β-Fe3+O(OH, Cl) composite nanospindles modified GCE in PBS with different concentrations of D-AA ([D-AA]). When [D-AA] was 0.1 mM, no peaks were found because that too low concentration could not give the CV response of D-AA. 3.6. Effect of Scanning Rate and Amperometric Response of D-Ascorbic Acid on Au/PEDOT/β-Fe3+O(OH, Cl) Composite Nanospindles. Figure 9i shows the CV curves of Au/ PEDOT/β-Fe3+O(OH, Cl) composite nanospindles modified GCE in PBS with 1 mM D-AA at different scan rates. As shown in Figure 9ii, it was found that the nature of the oxidation process in PBS proceeds by diffusion control, as evidenced from plots of the square root versus scanning rate for D-AA. Amperometric response of Au/PEDOT/β-Fe3+O(OH, Cl) composite nanospindles at 0.1 V to successive addition of 10 µL of 100 mM D-AA (0.2 mM) into 5 mL of PBS under constant stirring was recorded, and the current increased rapidly after each addition of D-AA (Figure 10i). The corresponding calibration curve

(Figure 10ii) showed a linear dependence (R2 ) 0.993 69) in the range of [D-AA] from 0.2 to 1.8 mM with a slope of -3.380 21 µA/mM. Amperometric response could also be detected when 2 µL of 100 mM D-AA (0.2 mM) was added successively into 5 mL of PBS, so the detection limit could be as low as 0.04 mM. Their good linear dependence and the low detection limit showed a significant sensitivity of Au/PEDOT/ β-Fe3+O(OH, Cl) composite nanospindles for D-AA detection. 4. Conclusions In summary, high-crystalline PEDOT/β-Fe3+O(OH, Cl) nanospindles containing Au nanoparticles were successfully synthesized in the presence of CTAB and PAA in aqueous solution through a one-step process. The existence of Au nanoparticles in PEDOT/β-Fe3+O(OH, Cl) nanospindles was proven by TEM images and electron diffraction patterns of single Au/PEDOT/ β-Fe3+O(OH, Cl) composite nanospindle. XRD patterns further confirmed that the formation of Au nanoparticles was due to the in situ reduction of HAuCl4 · 4H2O during the growth of PEDOT/β-Fe3+O(OH, Cl) nanospindles, and they were gradually combined with PEDOT/β-Fe3+O(OH, Cl) nanospindles along with the increase of reaction time. It is the first time that Au

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