Article pubs.acs.org/JPCC
Polymerization Assisted Reduction Reaction: A Sequential Electron− Proton Transfer Reaction Catalyzed by Gold Nanoparticle Meenakshi Choudhary,† Sudheesh K. Shukla,‡ Rafique Ul Islam,† Michael J. Witcomb,§ Cedric Wahl Holzapfel,† and Kaushik Mallick*,† †
Department of Chemistry, University of Johannesburg, P. O. Box 524, Auckland Park 2006, South Africa Department of Applied Chemistry, University of Johannesburg, P. O. Box 17011, Doornfontein 2028, South Africa § Department of Physics, University of Johannesburg, P. O. Box 524, Auckland Park 2006, South Africa. ‡
S Supporting Information *
ABSTRACT: During the polymerization process of aniline (and the derivatives of aniline), the released proton and electron initiate the reduction of 4-nitrophenolate through a proton coupled electron transfer (PCET) mechanism. In situ formation of gold nanoparticles during a similar polymerization process catalyze the reduction reaction.
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INTRODUCTION Proton coupled electron transfer (PCET) is an important mechanistic route of charge transfer in a variety of biochemical, electrochemical, organic, and inorganic reactions.1 The conversion of energy during photosynthesis 2 and the respiration process3 follows the PCET mechanism. The hydrogen atom abstraction reactions from a variety of phenols4 and flavonoids5 by the 2,2-diphenyl-1-picrylhydrazyl radical (dpph•) follow three distinct and, in some cases, competitive pathways, such as hydrogen atom transfer (HAT), electron− proton transfer (EPT), and proton-transfer−electron-transfer (PT−ET) mechanisms. In the case of ascorbic acid, a bioreducing agent, it is oxidized to an ascorbyl radical and then to dehydroascorbate. On the basis of thermodynamic and kinetic results, ascorbate oxidation by nitrosobenzene appears to occur by EPT.6 A similar EPT mechanism has been observed for the oxidation of ascorbate by 2,2,6,6-tetramethylpiperidinyl1-oxy radical.7 Ascorbyl radicals can also be generated as a result of the oxidation of ascorbate by phenoxyl or nitroxyl radicals through an EPT mechanism.8 Oxidation of 4-Xsubstituted N,N-dimethylanilines (X = OMe, OPh, CH3, H) by a dpph• radical leads to N-demethlyation of the N,Ndimethylaniline. The rate of the reaction correlates with the electron-donating ability of the substituent, and the reaction proceeds by a HAT mechanism through the N−C−H bond to dpph•.9 Integration of spectroscopy and electrochemistry, spectroelectrochemistry, has proven to be extremely useful in the identification of different oxidation states of inorganic © 2013 American Chemical Society
complexes through spectral changes during electrochemical potential sweeps and has been applied to PCET mechanism studies. Spectro-electrochemical results have shown that the oxidation of [(bpy)2 Ru(H2pzbzim)Ru(bpy)2]3+ [bpy = 2,2′bipyridine, pzbzim = pyrazolyl-3,5-bis(benzimidazole)] during which the transformation of RuIIRuII to RuIIIRuIII was found to have proceeded in a two-step process through the PCET mechanism.10 Cobalt(II) porphine has been shown to catalyze the reduction of O2 by ferrocenes. The major reduction product is water, which from the voltammetry current indicates that it is generated by a PCET mechanism.11 Biologically important analytes, cysteine and vitamin C, have been detected by a colorimetric sensing technique using a tailor-made watersoluble polyaniline as a substrate. A color change of the polymer from a blue to a colorless form was accompanied by an electron as well as proton transfer.12 The oxidation and production of hydrogen occurs naturally in the hydrogenase class of enzymes. These biological systems could serve as the key to the design of effective synthetic catalysts because the catalytic center of the enzyme is comprised of iron and/or nickel. The presence of an amine ligand is significantly responsible for the catalytic activity of the [FeFe] hydrogenase enzymes. The amines may assist in the heterolytic cleavage of H2 by facilitating proton-transfer Received: August 23, 2013 Revised: October 4, 2013 Published: October 10, 2013 23009
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transmission electron microscopy (TEM) specimens were prepared by pipetting 2 μL of the deposited material onto lacey carbon coated copper grids. A small portion of the product was used for optical characterization, the remaining fraction being dried under vacuum for X-ray diffraction (XRD) analysis. Characterization. The UV−vis spectra were measured using a Shimadzu UV-1800 UV−vis spectrophotometer with a 1 cm quartz cuvette. IR spectra were collected utilizing a Shimadzu IRAffinity-1 with a resolution of 0.5 cm−1. TEM studies of the nanocomposite were carried out at 197 kV using a Philips CM200 TEM equipped with a LaB6 source. An energy-dispersive X-ray analyzer (EDX) attached to the TEM was used to determine the chemical composition of the samples. X-ray photoelectron spectra (XPS) were collected in a ultrahigh-vacuum chamber attached to a Physical Electronics 560 ESCA/SAM instrument. The XRD patterns were recorded on a Shimadzu XD-3A X-ray diffractometer operating at 20 kV using Cu Kα radiation (k = 0.1542 nm). The measurements were performed over a diffraction angle range of 2θ = 20−70°.
reactions as well as coupling electron- and proton-transfer reactions.13,14 Oxidation−reduction reactions of metal oxides play a key role in emerging energy technologies such as solar, wind, and other environmentally benign processes.15 The interfacial (solid−solution) redox processes are generally described in terms of electron transfer (ET). In dye-sensitized solar cells, the excited state of the dye injects an electron into nanocrystalline metal oxide following a PCET mechanism.16 Tryptophan oxidation has been reported in bimolecular reactions between tryptophan derivatives and RuIII oxidants.17 The study suggests that the reaction follows either an electron-transfer−protontransfer (ET−PT) or a coupled electron−proton-transfer (CEPT) mechanism. Electrochemical oxidation of 1,4-hydroquinone in acetonitrile, an example of PCET, is facilitated by Brønsted base proton acceptors. Since the bases accept protons released by the oxidation of phenolic groups, a redox potential shift was realized via voltammetry.18 In this work, we show that two different oxidation and reduction reactions occur in the same reaction pot in which the oxidation of aniline (polymerization) leads to the reduction of 4-nitrophenolate, an example of a sequential electron−protontransfer (EPT) mechanism. In addition, emphasis is given to the fact that the reduction process is catalyzed in the presence of gold nanoparticles. This work is based on the combined principle of chemical polymerization of aniline, an oxidation process,19 and the in situ polymerization and composite formation (IPCF) technique.20
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RESULTS Figure 1A shows the UV−vis spectra of 4NP in the presence of ANI (10−1 mol dm−3) and APS (10−1 mol dm−3) at different time intervals. From the figure, it can be seen that the complete quenching of the absorbance peak (400 nm) of 4NP took about 160 min. Figure 1B plots the absorption of 4NP (Ct/C0: Ct is the absorption maxima at different time intervals, and C0 is the initial absorption maxima) as a function of time at different concentrations of ANI and APS. The graphs reveal that the quenching of the 4NP spectra depends on the concentration of ANI and APS. When the concentration of ANI and APS was low (10−2 mol dm−3), no significant quenching of the 4NP spectra was observed over a period of 140 min (curve a), whereas, at a 1.0 mol dm−3 concentration for both ANI and APS, the complete quenching of the 4NP absorption spectra occurred within 32 min (curve c). A moderate rate of quenching of the 4NP spectra was observed when both the ANI and APS concentrations were 10−1 mol dm−3, the complete quenching being achieved within a 160 min time frame. Using auric acid at a concentration of 10−1 mol dm−3 in place of APS dramatically enhanced the rate of quenching of the 4NP absorption peak (Figure 1C), the complete quenching occurring in about 2 min. 4-Amino-3-methylphenol (AMP), a derivative of ANI, has also been used in this set of experiments. Figure 2A shows the UV−vis spectra of 4NP in the presence of AMP (10−1 mol dm−3) and APS (10−1 mol dm−3) at different time intervals. From the spectra, it can be seen that complete quenching of 4NP spectra took around 55 min. Using auric acid at a concentration of 10−1 mol dm−3 in place of APS dramatically enhanced the quenching rate of the 4NP absorption spectra, the complete quenching taking some 2 min (Figure 2B). Figure 2C displays the graphs of the absorption (Ct/C0) versus time of 4NP in the presence of AMP and APS (curve a), while curve b is for 4NP in the presence of AMP and HAuCl4. The graphs reveal that quenching of the 4NP absorption spectra was accelerated dramatically when auric acid was used.
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EXPERIMENTAL SECTION Materials. Aniline (ANI), 4-amino-3-methylphenol (AMP), ammonium persulfate (APS), hydrogen tetrachloroaurate(III) hydrate, and 4-nitrophenol were purchased from Sigma-Aldrich. Methanol was obtained from Merck, and ultrapure water (specific resistivity >17 MΩ·cm) was used in this experiment wherever required. Procedure. 4-Nitrophenolate (4NP) was made by adding sodium hydroxide to the water solution of 4-nitrophenol with a final concentration of 10−4 mol dm−3. First Set of Experiments. A 1.5 mL aliquot of 4NP (10−4 mol dm−3) was mixed with 30 μL of aniline in three different concentrations (10−2, 10−1, and 1.0 mol dm−3) in three quartz cuvettes. To the above solutions, three different concentrations (10−2, 10−1, and 1.0 mol dm−3) of 5.0 μL of APS were added to the corresponding concentration solutions, and the progress of the reactions was monitored using a spectrophotometer. In another experiment, 1.5 mL of 4NP (10−4 mol dm−3) was mixed with 30 μL of aniline (10−1 mol dm−3) in a quartz cuvette, and to this solution was added 5.0 μL of HAuCl4 (10−1 mol dm−3). The progress of the subsequent reaction was monitored using a spectrophotometer. Second Set of Experiments. In two different experiments, 1.5 mL of 4NP (10−4 mol dm−3) was mixed with 30 μL of AMP (10−1 mol dm−3) in two quartz cuvettes. A 2.0 μL aliquot of APS (10−1 mol dm−3) was added to one of the cuvettes, while 2.0 μL of HAuCl4 (10−1 mol dm−3) was added to the other. The progress of the two reactions was monitored using a spectrophotometer. Blank Experiment. A 0.045 g amount of AMP was dissolved in methanol in a conical flask, and then 1.5 mL of HAuCl4 (10−2 mol dm−3) was added slowly to it. A colloidal solution of a deep pink color appeared. The solution was allowed then to stand at rest for 5 min. Subsequently,
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DISCUSSION Among the conjugated polymers, polyaniline is one of the most intensively investigated polymers due to its interesting physical 23010
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Figure 2. (A) UV−vis spectra of the 4NP (10−4 mol dm−3) in the presence of AMP (10−1 mol dm−3) and APS (10−1 mol dm−3) at different time intervals. Complete quenching of the absorbance peak (at 400 nm) in the 4NP spectra took 55 min. (B) UV−vis spectra of the (a) 4NP and (b) 4NP in the presence of AMP (10−1 mol dm−3) and HAuCl4 (10−1 mol dm−3) after 2 min. (C) Absorption of the 4NP (Ct/C0, where Ct is the absorption maxima at different time intervals and C0 is the initial absorption maxima) as a function of time for (a) AMP (10−1 mol dm−3) and APS (10−1 mol dm−3) and (b) AMP (10−1 mol dm−3) and HAuCl4 (10−1 mol dm−3).
Figure 1. (A) UV−vis spectra of the 4NP (10−4 mol dm−3) in the presence of ANI (10−1 mol dm−3) and APS (10−1 mol dm−3) at different time intervals. Complete quenching of the absorbance peak (at 400 nm) in the 4NP spectra took 160 min. (B) Absorption of 4NP (Ct/C0, where Ct is the absorption maxima at different time intervals and C0 is the initial absorption maxima) as a function of time and different concentrations of ANI and APS (1:1) [(a) 10−2 mol dm−3, (b) 10−1 mol dm−3, and (c) 1.0 mol dm−3]. (C) UV−vis spectra of the (a) 4NP and (b) 4NP in the presence of ANI (10−1 mol dm−3) and HAuCl4 (10−1 mol dm−3) after 2 min.
prepared by chemical or electrochemical polymerization of aniline. In the chemical synthesis of polyaniline, various oxidizing agents, such as ammonium persulfate, APS,27,28 potassium dichromate,29 and hydrogen peroxide30 etc., generally have been used although APS is more often utilized
properties such as high environmental stability, controllable electrical conductivity, and multiple redox properties associated with unique optical characteristics21 as well as it having a wide range of potential applications.22−26 Polyaniline can be 23011
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and APS concentration was low (10−2 mol dm−3), the rate of quenching of 4NP absorption spectra was slow, in other words, the rate of reduction was retarded (Figure 1B, curve a), whereas, at higher concentrations of ANI and APS (10−1 mol dm−3 and 1.0 mol dm−3), a faster rate of reduction was achieved (Figure 1B, curves b,c). In addition to this, it should be noted that auric acid also acts as an oxidizing agent and is able to polymerize aniline to polyaniline.31−35 During the process of polymerization, the released electron and proton initiate the reduction process of 4NP through a sequential ET− PT mechanism. While each step of polymerization involves the release of two electrons, the residual electrons, which do not take part in the ETPT process, are used to reduce the auric acid to form gold nanoparticles. The in situ generated gold nanoparticles then catalyze the reduction of 4NP (Figure 1C: (a) the absorption spectrum for 4NP and (b) the absorption spectrum for the reduced form of 4NP). The released electrons during the polymerization process play a dual role in the following: (1) the formation of a radical cation on the nitrogen of the nitro group (first step of the nitro-group reduction mechanism, Scheme 2) and (2) reduction of the auric acid to form catalytically active gold nanoparticles.31−35 A broad band which appeared in the range of 425−600 nm (Figure 1C, spectrum b) results from the superposition of the plasmon absorption band for gold nanoparticles around 520−530 nm due to the collective oscillation of conduction electrons in response to optical excitation37 and the polaron−bipolaron transition at 450 nm due to the formation of polyaniline.38 A shoulder-like appearance seen at 320 nm is due to the π−π* transition of the benzenoid rings.38 A similar experiment was done using preformed gold nanoparticles in the presence of 4NP, aniline, and APS in a quartz cuvette (maintaining concentration identical to that in the first set of experiments) to determine the catalytic effect of the preformed gold nanoparticles for the reduction of 4NP. The spectroscopic data (Figure S1, Supporting Information) revealed that no significant catalytic effect was observed. These measurements indicated that once nanoparticle clusters reach a particular size with a suitable redox potential, these metal clusters behave as excellent catalysts owing to their size-dependent properties. A cluster can behave as an efficient electron relay point between the electron donor and the electron acceptor if the cluster potential is intermediate between that of the donor and the acceptor. The mechanism is similar to previously reported work in which the role of growing metal nanoparticles as redox catalysts was explained.39,40 AMP is a derivative of ANI, and its polymerization follows a mechanistic pathway similar to that of polyaniline. A slow quenching of the absorption peak of 4NP was observed in the UV−vis spectra in the presence of AMP and APS (Figure 2A), while, at the same time, a gradual increase of the absorption band in the range of 500−600 nm was also detected, which is due to the transition from a localized benzenoid highest occupied molecular orbital to a quinoid lowest unoccupied molecular orbital, that is, a benzenoid to quinoid excitonic transition (Figure 2A). The position of the excitonic transition peak shifts from 560 to 630 nm depending on various factors, such as the nature of the counterions present, the nature of the solvent, and the chemical structure of the polymer.41 The quenching of the peak at 400 nm reflected the reduction of 4NP as a result of the effect of the sequential electron- and proton-transfer mechanism which is generated during the
as the oxidant. Similarly, syntheses of polyaniline and its derivatives have also been reported using a metal salt as an oxidizing agent. The preparation of gold nanoparticle− polyaniline composite material has been recorded using preformed polyaniline by exploiting the multioxidative states of the polymer.31 Use of hydrogen peroxide, which acts both as an oxidizing and a reducing agent, has been reported to produce a gold−polyaniline composite.32 An in situ synthesis route has been described for the preparation of a gold−poly(ophenylenediamine) nanocomposite material by the interfacial polymerization route.33 A gold−polyaniline composite material has been synthesized using a phase-transfer catalyst resulting in polyaniline nanoballs of a few micrometers in size decorated with gold nanoparticles (10−50 nm).34 A gold−polyaniline composite with a narrow range of size distribution of the gold nanoparticles (7−12 nm) has also been reported by our group using methanol as a solvent.35 The aniline polymerization suggested the following general mechanism (Scheme 1). The initiation process of aniline Scheme 1. Mechanistic Route for the Oxidative Polymerization of Aniline
polymerization involves a loss of two electrons and one proton to form a nitrenium cation (eq 1), which subsequently attacks aniline by an electrophilic aromatic substitution (eq 2). The propagation proceeds in a similar manner by the oxidation of the primary amine end of a growing polymer chain (eq 3) followed by an electrophilic substitution (eq 4). The process has been referred to as reactivation of chain polymerization to highlight the fact that the chain end formed after each addition of aniline must be reactivated to the nitrenium ion by oxidation and proton loss.36 The quenching of the 4NP spectra is due to the gradual reduction of the nitro group (Figure 1A). The reduction mechanism of 4NP involves a single electron transfer (SET) mechanism with the formation of a radical cation followed by protonation (Scheme 2). Figure 1A revealed that when the ANI Scheme 2. Reduction of 4NP: Single Electron Transfer Followed by Proton-Transfer Mechanism
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Figure 3. (A) TEM image of the product and the selected area electron diffraction (SAED) pattern (inset) from the spherical dark spot confirming the “face centered cubic” crystal structure. (B) TEM image from a different area also shows dark spherical spots with diverse size distribution. (C) EDX analysis obtained from the electron beam being focused onto an isolated dark spot confirms that these spots are gold particles. The TEM images confirmed that the gold nanoparticles are in the size range of 2−7 nm and are stabilized by the polymer matrix.
formation of poly(AMP). The use of auric acid as an oxidizing agent expedited the reduction rate of 4NP due to the in situ formation of gold nanoparticles which catalyzed the ET−PT mechanism (Figure 2B, spectrum b, spectrum a being from the unreduced 4NP). A broad absorption band in the range of 500−600 nm resulted from the benzenoid to quinoid excitonic transition. There is no evidence of gold nanoparticle formation in spectrum b, which is probably due to the very small quantity of auric acid (2.0 μL) utilized. The above reactions were performed at the micrometer level, and, as a result, recovery of the polymer for characterization from the reaction mixture was a tedious task. To show the formation and the characterization of gold−poly(AMP) composite, we performed a blank experiment (see Experimental Section). Figure 3A is the TEM image of the resultant product and the selected area electron diffraction (SAED) pattern from the spherical dark spots confirming the “face centered cubic” crystal structure (inset). Figure 3B is another TEM image from a different area also showing dark spherical spots with a diverse size distribution. A typical EDX analysis (Figure 3C) obtained from the electron beam being focused onto an isolated dark spot confirms that these spots are, in fact, gold particles. The TEM images provided evidence that the gold nanoparticles were in the size range of 2−7 nm; tilting studies revealed that they were distributed within and throughout the polymer, having been stabilized by the polymer matrix. Formation of these gold nanoparticles in the metal−polymer composite material was found to be highly face-selective, as shown in the XRD pattern, Figure 4A. The strong (111) Bragg reflection indicates that the gold particles possess a highly oriented crystalline character, the crystallinity being confirmed by the TEM diffraction pattern (Figure 3A, inset). To identify the chemical state of the polymer stabilized gold nanoparticles, Xray photoelectron spectroscopy (XPS) measurements were carried out. The XPS spectrum of the Au 4f region is shown in Figure 4B. The Au 4f7/2 and 4f5/2 spin−orbit coupling gives rise to peaks positioned at 84.0 and 87.6 eV respectively, which correspond to metallic gold.42 The optical properties of the synthesized material were investigated by both UV−vis and Fourier transform infrared (FTIR) techniques. The electronic absorption spectrum of
Figure 4. (A) Strong (111) Bragg reflection, indicating that the gold particles possess a highly oriented crystalline character. (B) X-ray photoelectron spectroscopy (XPS) spectrum of the Au 4f region: Au 4f 7/2 and 4f5/2 spin−orbit couplings give rise to the peaks positioned at 84.0 and 87.6 eV, respectively, and confirm the metallic character of the gold.
polyaniline has been documented in the literature.43−45 In the present study, the UV−visible spectrum of the gold−poly(AMP) composite revealed a broad absorption spectra that covered the whole visible range from 400 to 700 nm with an absorption maximum at 535 nm (Figure 5A, spectrum a), which is probably the superposition of the polaron−bipolaron and benzenoid-to-quinoid excitonic transitions of the polymer, and the plasmon absorption band due to the presence of gold nanoparticles. Gold nanoparticles in the size range of 5−50 nm have been shown previously to exhibit a plasmon absorption band around 520−530 nm as a result of the collective oscillation of conduction electrons in response to optical excitation.46 In addition, the refractive index of the medium also influences the surface plasmon absorption of gold nanoparticles.46 It can thus be concluded that the absorption maxima at 535 nm is due to the presence of gold particles within the polymer (Figure 5, spectrum a). For the sake of comparison, we also synthesized poly(AMP) alone (Figure 5, inset) and found a wide absorption band from 400 to 700 nm with the absorption maxima at 535 nm (Figure 5, spectrum b). It is thus not possible to differentiate the plasmon absorption band responsible for gold nanoparticles in the gold−poly(AMP) sample since the absorption band of the polymer alone is identical to that of poly(AMP). The only difference is that 23013
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secondary aromatic amine, whereas the peaks at 1007 and 1157 cm−1 represent the aromatic C−H in-plane bending modes. A broad band with a peak position at 809 cm−1 is due to an aromatic out-of-plane C−H deformation vibration which is related to the substituted benzene ring. The resultant material also shows bands at 1225 and 1289 cm−1, which result from the C−N stretching vibration. Spectroscopic analysis confirmed that the aniline oxidation product possessed a head-to-tail (-N− Ph−N−Ph-) arrangement rather than a head-to-head (-Ph− NN−Ph-) type.47 All of the above characterization techniques confirmed the formation of a gold−poly(4-amino-3-methylphenol) metal− polymer nanocomposite, this being synthesized utilizing the in situ polymerization and composite formation (IPCF) technique.48−51
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CONCLUSION Aniline polymerization, an oxidation process, involves the release of two electrons and one proton in each step and the transfer of an electron and proton to the 4NP molecule following the PCET mechanism. This process can occur either as consecutive ET and PT reactions or as a conbined EPT reaction step. The EPT mechanism has additional kinetic constraints from the requirements for simultaneous tunnelling of both an electron and a proton. In the present reaction, the reduction of 4NP involved an ETPT mechanism due to the prioritized demand of an electron for the formation of a radical cation on the nitro group (the first step of the reduction of 4NP molecule is a single electron transfer process, Scheme 2) followed by protonation. The in situ synthesized gold nanoparticles catalyzed the reduction of 4NP by acting as a delivery agent of both the electron and the proton.
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Figure 5. (A) UV−visible spectra of (a) the gold−poly(AMP) composite showing a broad absorption band that covers the whole visible range from 400 to 700 nm with an absorption maxima at 535 nm and (b) poly(AMP) alone (inset: camera image of poly(AMP) prepared by APS). The absorption band in the UV region with peak values at 375 and 360 nm are for gold−poly(AMP) and poly(AMP), respectively, and result from the π − π* transition of the benzenoid rings. (B) FTIR spectrum of the gold−poly(AMP) composite material within the spectral range from 1700 to 700 cm−1. The peak at 1635 cm−1 corresponds to the group NQN (where Q represents a quinoid ring), while the N−B−N group (where B represents a benzenoid ring) absorbs at 1506 cm−1.
ASSOCIATED CONTENT
S Supporting Information *
Figure showing UV−vis spectra of preformed gold nanoparticles in the presence of 4NP, aniline, and APS in a quartz cuvette for determining the catalytic effect of the preformed gold nanoparticles for the reduction of 4NP. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
when the gold nanoparticles are present in the polymer, the absorption peak intensity is considerably higher than that for the polymer alone. The other absorption peaks positioned at 360 and 375 nm for poly(AMP) and gold−poly(AMP), respectively, are due to the π−π* transition of the benzenoid rings. The FTIR spectrum of the gold−poly(AMP) was analyzed within the spectral region from 1700 to 700 cm−1. IR analysis of this fingerprint region is particularly useful for examining the resonance modes of the benzenoid and quinoid units, as well as the individual bonds, such as, the out-of-plane C−H and C−N of the substituted polyaniline compound (Figure 5B). In the IR spectra of the resultant compound, the peak at 1635 cm−1 corresponds to the NQN group (where Q represents a quinoid ring), while the N−B−N group (where B represents a benzenoid ring) absorbs at 1506 cm−1 (Figure 5A). The band at 1346 cm−1 is assigned to the C−N stretching mode for the
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ACKNOWLEDGMENTS The authors acknowledge financial support from the Research Committee and the Faculty of Science of the University of Johannesburg.
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REFERENCES
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