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One-Step Synthesis, Characterization, and Properties of Emeraldine Salt Nanofibers Containing Gold Nanoparticles Marcelo M. Nobrega, Vitor L. Martins, Roberto M. Torresi, and Marcia L. A. Temperini* Departamento de Química Fundamental, Instituto de Química da Universidade de São Paulo (USP), C.P. 26077- CEP 05513-970 São Paulo, SP, Brazil S Supporting Information *

ABSTRACT: This paper reports a simple one-step synthesis of gold nanospheres/polyaniline (PANI) nanofiber composites via the modification of the interfacial synthesis of PANI, leading to hybrid materials with emeraldine salt fibers as one of the components. Their morphological and structural characterizations are presented, as well as their electrochemical behaviors. The composites were submitted to heating treatment and UV−visible light radiation, and the products were characterized. Our results showed that the presence of the gold nanoparticles improved the thermal behavior of the composites, increasing the onset temperature. The electroactivity and capacity of the composites were enhanced when compared to PANI−ES, and their behaviors are dependent on the amount of gold nanoparticles in the composites. Submitting to heating treatment and UV−visible light irradiation, the PANI−ES nanofibers presented low dedoping and an extensive formation of cross-linking segments for both treatments, whereas the composites showed a high dedoping process and no crosslinking formation for the heating treatment and high cross-linking formation with no change in the doping level for the UV−visible light irradiation. Our results suggest that the PANI−Au composites may be a promising material for the use in capacitors, solar cells, and other devices.

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INTRODUCTION Polyaniline (PANI) nanostructures have been extensively studied due to its mechanical flexibility, environmental stability, controllable synthesis, high surface area, and simple doping/ dedoping process.1−4 Particularly, the applications of PANI as nanofibers, nanotubes, and nanowires in devices have received growing interest in the last years.5−14 Recently, the synthesis and use of the composites of PANI nanostructures containing metallic nanoparticles are the focus of several research groups.15−22 Metallic gold nanostructures (Au NPs) present fascinating optical properties, chemical stability, and biocompatibility, making them attractive in various fields of science, such as plasmonics, biomedicine, and electronics.23−26 Associating PANI and gold (Au) nanostructures may result in the synergism of their properties, leading to hybrid material that exhibits enhanced catalytic, electric, and optical properties.27,28 Several methods have been developed to synthesize metallic nanostructure−PANI composites; however, these methods describe syntheses of two or more steps. Many composite syntheses reported in the literature lead to the nonconducting forms of PANI, emeraldine base (EB), or pernigraniline base (PB), containing metallic nanoparticles.15,17,18,29,30 Nevertheless, the use of PANI in this kind of device generally requires the conducting form of PANI, the emeraldine salt form. © 2014 American Chemical Society

On the basis of the above considerations, knowing the properties of the composites is of great importance. Understanding the thermal stability of composites is very important for their applications, because the initial step in processing procedures is usually the heating treatments.31−34 It is also important to know its behavior under UV−visible light irradiation, once PANI nanofibers showed a good efficiency in the solar-to-electricity conversion.35 The knowledge concerning the behavior of the PANI−Au composites toward UV−visible light irradiation is also very important, since one of the possible applications of PANI is the use as a hole injection layer or electrodes in organic light-emitting diodes (LEDs) and polymer LEDs. A recent study showed that films of PANI irradiated with a UV light source (output power density of 460 μW/cm2) presented a reduction in the work function and in the electrical conductivity.36 In this paper, we report the facile one-step synthesis of gold nanospheres/polyaniline nanofiber composites via a little modification of the interfacial synthesis of PANI nanofibers.37 The PANI−Au composites, having different amounts of components, were structurally, morphologically, thermally, Received: December 10, 2013 Revised: January 28, 2014 Published: February 4, 2014 4267

dx.doi.org/10.1021/jp4120755 | J. Phys. Chem. C 2014, 118, 4267−4274

The Journal of Physical Chemistry C

Article

alkaline piranha solution NH3/H2O2/H2O (5/1/1 in volume) for 30 min and rinsed with deionized water. The substrates were immediately used after the cleaning. The casting films of PANI−ES and PANI−Au composites were obtained by the deposition of 1 mL of the respective dispersion on clean glass substrates and drying under reduced pressure at room temperature. Heating and UV−Visible Treatments. The heating treatments of the casting films of PANI−ES and PANI−Au composites were performed in an oven under atmospheric air at 150 °C for 30 min. The UV−visible light irradiation treatment of the casting films was performed in an irradiation box, where the samples were placed at a distance of 5 cm from the lamp and irradiated for 15 h. The UV−visible light source was a Hg lamp (Philips − HPL-N) with no glass bulb or filter, which presents strong emission lines at 365, 366, and 391 nm in the UV range and emission lines at 404, 408, 436, 492, 496, 546, 577, 579, and 691 nm in the visible range (the emission spectrum of the UV−visible light source is presented in Figure 1S, Supporting Information). To avoid the sample heating, a cooler was installed in the radiation box and the sample temperature was monitored with a thermometer. Characterization Techniques. UV−vis−NIR electronic absorption spectra of the acidic solution of PANI−ES and PANI−Au composites were recorded using a Shimadzu UVPC3101 scanning spectrometer with a 5.0 mm path length quartz cell. Raman spectra at 632.8 nm excitation radiation (He−Ne laser, Renishaw 7N1753) were obtained in a Renishaw Raman imaging microscope (inVia) with a Leica microscope and a CCD detector, by using a 50× lens (Olympus SM Plan, N.A. 0.55). The spectra were acquired using extended mode, 200− 1800 cm−1, 30 s and 10 accumulations. Raman spectra at 1064 nm excitation radiation (Nd:YAG laser, Coherent Compass 1064−500N) were recorded in a FT-Raman Bruker RFS 100 spectrometer with a liquid nitrogen cooled germanium detector. The spectra were obtained using laser power of 30 mW and 1024 accumulations. SEM images were obtained on a JEOL low vacuum scanning electron microscope (JSM-7401) coupled with an energý dispersive spectrometer (EDX), from Central Analitica do ́ Instituto de Quimica, Universidade de São Paulo (CA-IQUSP), operated with a 5 kV accelerating voltage and a 3 mm working distance. The PANI−Au composites were placed in a silicon chip and fixed on a metallic sample holder with a double-face copper tape. The thermogravimetric analyses (TGA) of the PANI−ES and the composites were performed by heating the samples from 25 to 1000 °C. The TGA curves were recorded on a Netzsch themoanalyser model STA 1500, under synthetic air flow of 50 mL min−1 and a heating rate of 10 °C min−1. The change in frequency (Δf) was monitored by a quartz crystal microbalance with dissipation (ΔD) monitoring (QCMD) (Q-Sense E4). Δf can be related with the change in mass (Δm) in rigid film by the Sauerbrey equation (eq 1):40

and electrochemically characterized. Their heating behavior and UV−visible light response were determined. Our results showed that fibers of the emeraldine salt form are presented in PANI−Au composites. The thermal behavior of the composites was improved and the electroactivity and capacity were enhanced when compared to PANI−ES, and their values depend on the amount of gold nanoparticles in the composites. When submitted to heating treatment and UV−visible light irradiation, PANI−ES fibers presented low dedoping and extensive formation of cross-linking segments, while the composites showed high dedoping and no cross-link formation for heating treatment and no change in the doping level and high cross-linking formation after UV−visible light irradiation. The results presented herein suggest that the PANI−Au composites may be a promising material for the use in capacitors, solar cells, and other devices.

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EXPERIMENTAL SECTION Aniline monomer (C6H5NH2, Merck) was distilled under reduced pressure prior to use. HAuCl4·3H2O (Sigma-Aldrich, >99%), ammonium peroxydisulfate ((NH4)2S2O8, Merck), and HCl (Sigma-Aldrich, 37%) were used as received. Synthesis of Composites of PANI Nanofibers Containing Nanoparticles. PANI−Au composites were prepared following the protocol of the interfacial polymerization of aniline by ammonium peroxydisulfate (APS) described in ref 37 having HAuCl4·3H2O in an acidic aqueous phase. In an interfacial polymerization, two solutions were prepared separately: (i) aniline monomer was dissolved in 10 mL of chloroform, and (ii) ammonium peroxydisulfate (APS) and HAuCl4·3H2O (quantity indicated below) were dissolved in 10 mL of an aqueous solution of 1.0 mol L−1 HCl. The second solution (ii) was slowly added to the first (i), and the system was kept at room temperature for 24 h (Scheme 1). After this Scheme 1. Schematic Illustration of PANI−Au Composite Synthesis in the Modified Interfacial Reaction

period, the aqueous phase containing PANI−Au was centrifuged at 7000 rpm for 10 min and the supernatant was changed by a new aqueous solution of 1.0 mol L−1 HCl; this process was repeated three times. The syntheses were made for three molar ratios of aniline:HAuCl4, 1:5 (denoted as PANI−Au-I), 1:95 (denoted as PANI−Au-II), and 1:240 (denoted as PANI−Au-III). The molar ratio of aniline:APS was kept constant as 4:1 for all syntheses. Preparation of Glass Substrates for Casting Films of PANI−Au Composites. Glass substrates (2.5 cm × 2.5 cm) were cleaned according to procedures described in the literature.38,39 The substrates were immersed in H2SO4/H2O2 solution (7/3 in volume) for 60 min and rinsed extensively with deionized water. Afterward, the substrates were immersed in

⎛ 2f 2 Δfn = −⎜⎜ 2 n ⎝ n μρ

⎞ Δm Δm ⎟⎟ =− ×A A C ⎠

(1)

where Δf is the measured shift in frequency (Hz), A is the active area of the crystal corresponding to the exciting electrode exposed to the working environment, ρ is the quartz density (2.648 g cm−3), μ is the shear modulus (2.947 × 1011 g cm−1 s−2), and f n is the crystal frequency for the n overtone (5 MHz 4268



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the composite diameters. Nevertheless, the diameter range is greater than that obtained for PANI without gold nanoparticles (∼55 nm).7,37,41 The EDX spectrum of the composites (see Figure 2S, Supporting Information) was obtained in order to confirm the presence of gold within the PANI nanofibers. One of the biggest problems to perform the TEM images was the decomposition of the polymers due to the electron beam energy, which difficulties the acquisition of high resolution images that allowed the observation of the Au NP. For one of the composites, our best TEM images obtained were presented in the Supporting Information (see Figure 3S); the TEM image confirms the presence of Au NP in the composites. Thermal Characterization. The thermogravimetric (TGA) measurements were performed to analyze the thermal stabilities of the composites. The TGA curves of PANI−ES in Figure 2 present three major events of mass loss in the ranges

for the fundamental tone). The theoretical mass sensitivity (C) was calculated by using the quartz crystal parameters, and the value was 17.7 ng cm−2 Hz−1. In order to confirm the mass sensitivity value, experiments were run of copper electrodeposition in 0.1 mol L−1 of CuSO4 in 0.5 mol L−1 H2SO4 aqueous solution at six different polarization currents. The C value obtained was 17.9 ± 0.5 ng cm−2 Hz−1, which is in agreement with the theoretical value. A decrease in the resonance frequency indicates that the mass is increased. Changes in ΔDn are related to the viscoelastic properties of the film. The gold quartz crystals (Q-Sense) were cleaned in alkaline piranha solution NH3/H2O2/H2O (1/1/5 in volume) at 70 °C for 5 min and rinsed with deionized water. Afterward, the gold quartz crystals were sonicated for 5 min in deionized water and ethanol, respectively, dried under a nitrogen stream, and kept in an UV/Ozone camera (ProCleaner Plus BioForce) for 15 min. Casting films of PANI−ES, PANI−Au-I, PANI− Au-II, and PANI−Au-III were prepared by the deposition of 100 μL of the corresponding aqueous suspension in the gold quartz crystal and drying overnight. After that, electrochemical characterization was carried out with an Autolab PSTAT 30 (Eco Chemie) using the films as a working electrode (WE), platinum spiral as a counter-electrode (CE), and Ag/AgCl/ NaCl 3 mol L−1 as a reference electrode (RE).

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RESULTS AND DISCUSSION Morphological Characterization. Figure 1 presents SEM images of the PANI−Au-I, PANI−Au-II, and PANI−Au-III

Figure 2. TGA-DTG curves of the PANI−ES and PANI−Au composites. The maximum degradation temperature of the polymer and the residue percentage are marked in the figure.

25−120, 120−300, and 300−750 °C, assigned to the water and adsorbed hydrogen chloride loss, loss of HCl dopant strongly bonded, and the process responsible for the decomposition of the polymer, respectively.34,42−44 According to Figure 2, it is also noticed for PANI−ES that the decomposition of the polymer begins at 370 °C (onset temperature, Tonset), the temperature of maximum decomposition is at ca. 495 °C, and there was 0.1% of residual mass, indicating the total decomposition of the polymer. The TGA curves of the composites presented the same three mass loss events of PANI−ES; however, through this analysis, it is possible to determine the amount of gold in each composite. The masses of gold in each composite in relation to the dry mass of PANI were 13.3% for PANI−Au-I, 46.8% for PANI−Au-II, and 53.4% for PANI−Au-III. The analysis of the onset temperature of the composites showed that the presence of Au NPs in the composites increased the Tonset when compared to PANI−ES; the observed values of Tonset were 460 °C for PANI−Au-I, 480 °C for PANI−Au-II, and 495 °C for PANI−Au-III. This result suggests that the thermal behavior of the composites was improved when compared to that of PANI−ES; this behavior

Figure 1. SEM images of PANI−Au composites prepared by the onestep synthesis. Inset: Diameter distribution of PANI−Au composites considering over 200 measurements.

composites. The SEM images confirm that the one-step synthesis yields a great amount of homogeneous nanofibers of PANI−Au composites. The diameter distribution of the PANI fibers in the composites considering over 200 nanofiber measurements varied from 70 to 120 nm (Figure 1); however, the distribution profile presented high deviation, making it impossible to discuss if there are significant differences between 4269



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was recently assigned to the strong interaction between the PANI chains and the Au NPs.15 Spectroscopic Characterization. The doping level of the PANI−Au-I, PANI−Au-II, and PANI−Au-III composites was monitored by UV−vis−NIR electronic spectroscopy. Figure 3A showed that the three samples presented a broad absorption band at ca. 800 nm extending to the NIR region that is assigned to the delocalized polarons in PANI chains, characteristic of the emeraldine salt form of PANI.39,45,46 This result shows that all the composites presented a high doping level, indicating that unlike some synthesis of PANI with gold nanoparticles whose product is PANI in its dedoped form, PANI−EB,15,17,18,20,30,47 due to the additional steps required to obtain the Au NP, which causes the dedoping of PANI−ES or an excessive oxidation of the polymer, this one-step synthesis in acidic medium leads to PANI−Au composites, presenting the fibers of PANI in the emeraldine salt form (PANI−ES). It is well-known that resonance Raman (RR) is a powerful technique in the study of different oxidation and doping forms of PANI. Using the 632.8 nm exciting wavelength, the bands of the quinoid segments of PANI-EB and the bands of phenazinelike structures (due to cross-linking formation) are enhanced, whereas using the exciting radiation at 1064 nm the bands due to the polaronic segments of PANI−ES present strong resonance conditions. The resonance Raman technique has also been used for identifying chromophoric groups formed after submitting PANI to different treatments, as in the case of heating treatment.10,11,34,45 The resonance Raman spectra of PANI−Au composites for the exciting radiation at 632.8 and 1064 nm are presented in Figure 3B and C, respectively. The PANI−Au spectra for the three samples excited at 632.8 nm radiation (Figure 3B) present the bands at 1166 (βC−H), 1257 (νC−N), 1318− 1340 (νC−N•+), 1515 (βN−H), 1584 (νC−C), and 1623 cm−1 (νC−C), that are associated with the polaronic segments of PANI−ES, showing that the PANI−Au composites are predominantly in the doped form.48−52 The resonance Raman spectra of the composites, excited at 1064 nm radiation (Figure 3C), presented the same bands discussed earlier, as expected. Therefore, the RR results are in accordance with those of UV− vis−NIR (Figure 3A). Electrochemical Characterization. In order to characterize the electrochemical responses of PANI−Au composites and their dependence on the amount of Au NPs in the composites, four films were prepared in the gold surface of a quartz crystal by drop-casting (PANI−ES, PANI−Au-I, PANI−Au-II, and PANI−Au-III). The frequency and dissipation of the gold quartz crystal was measured before and after the casting films deposition. The films presented ΔD smaller than 10% of Δf (see Figure 4S, Supporting Information), which classifies them as acoustically rigid films, which allow the use of the Sauerbrey equation (eq 1) for determining the total mass deposited. The films presented the average mass of 3.5 (±0.6) μg cm−2 (see Figure 5S, Supporting Information), and specific current was determined by normalizing the current/mass of the cyclic voltammetry of each film. Figure 4 shows the cyclic voltammograms of the PANI−Au composites. All composites present a similar profile: two broad peaks one of oxidation and the other of reduction in the electrochemical window studied. These peaks are related to the oxidation of the leucoemeraldine to emeraldine and the reversible redox process in the reverse scan.4 The magnitude of the specific current presented a huge dependence on the Au

Figure 3. (A) UV−vis−NIR electronic absorption spectra of the samples of PANI−Au. (B) Raman spectra of the samples of PANI−Au (λ0 = 632.8 nm). (C) Resonance Raman spectra of the samples of PANI−Au (λ0 = 1064 nm).

NP present in the composites, increasing for the composite with the lowest amount of Au NP (PANI−Au-I) when compared to the PANI fibers without Au NP (PANI−ES). For an intermediate amount of Au NP in the composites (PANI−Au-II), a considerable decay in the specific current was 4270



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Figure 4. Cyclic voltammetry of PANI−Au composites with different Au NP amounts: PANI−ES (black ■), PANI−Au-I (red ●), PANI− Au-II (blue ▲), and PANI−Au-III (green ◆), in 1.0 mol L−1 HCl at 50 mV s−1, using Pt and Ag/AgCl3 mol L−1 NaCl as counter and reference electrodes, respectively. Inset: Capacitance as a function of molar ratio of aniline:HAuCl4 (full symbols), calculated from the cyclic voltammograms, integrating the anodic current and dividing by ΔE: 0.66, 0.52, 0.57, and 0.55 V for PANI−ES, PANI−Au-I, PANI−Au-II, and PANI−Au-III, respectively, and capacitance as a function of molar ratio of aniline:HAuCl4 normalized by the Au NP masses calculated by TGA (open symbols).

Figure 5. Raman spectra of the PANI−ES film and PANI−Au films after heating at 150 °C for 25 min (λ0 = 632.8 nm).

after heating present new bands at 575, 1392, and 1644 cm−1, assigned to the formation of phenazine-like structures,10,11,45,53−60 and no changes are observed in the relative intensities of the bands due to polarons at 1169, 1257, 1340, 1515, 1584, and 1623 cm−1,48−52 indicating that there is no dedoping of the polymer. However, after the heating treatment of the PANI−Au composites films, the Raman spectra present a decrease of the relative intensities of the bands due to the polaronic segments at 1318−1340 cm−1 (Figure 3B) and an increase of the quinoid bands at 1592 (νCC), 1470 (νCN), and 1222 (νCN) cm−1, characteristic of the emeraldine base form of polyaniline, the nonconducting form of PANI,49,50,52 indicating the occurrence of the thermal dedoping of the PANI−Au composites. The bands characteristic of phenazinelike segments were only noticed in the Raman spectrum of PANI−Au-I. These results indicate that, with increasing amount of Au NP, the formation of cross-linking segments is not favored. The mechanism of the formation of cross-linking segments in the emeraldine salt form of polyaniline occurs by the coupling of the protonated nitrogen radical atoms of the polaronic segments to benzene rings, resulting in phenazine-like rings.10,34,45,61−63 Considering this mechanism, we could suggest that in the composites the gold nanoparticles are located preferentially near the nitrogen atoms of the polaronic segments (Scheme 2A),15 preventing the occurrence of crosslinking reactions during heating and favoring the dedoping process in the composites (Scheme 2B). UV−Visible Light Irradiation. The Raman spectra of the PANI−ES film and PANI−Au composites after 15 h of UV− visible light irradiation are presented in Figure 6. After irradiation, the Raman spectra of the PANI−ES film present the bands of the phenazine-like segments at 575, 1395, and 1645 cm−1 with great intensity, indicating extensive formation of cross-linking segments. A weak band at 1469 cm−1 is also observed, assigned to the quinoid segments of PANI-EB,49,50,52 due to the dedoping process. The spectra of the PANI−Au composites also show the characteristic bands of the phenazinelike units. It is important to mention that the relative intensity of the quinoid bands (1469 cm−1) in relation to the polaronic segments (1336 cm−1) is lower in the composites than in PANI−ES, suggesting a lower dedoping process in the

observed. Finally, the composites with the highest concentration of Au NP presented a smaller specific current (PANI− Au-III) among the composites but still higher than PANI−ES. The inset in Figure 4 shows that the capacity (F g−1) of the PANI−Au composites changes with the amount of gold nanoparticles (it was calculated by the integration of the anodic current and dividing by ΔE obtained from voltammograms). The capacity increases drastically, ca. 475 F g−1, for the composite with the lowest amount of Au NP (PANI−Au-I); however, increasing the amount of Au NP in composites, an exponential decay of the capacity of the films is observed. The presence of Au NPs at lower levels increases the composite electroactivity (capacity) due to the improved interconnectivity inside the fibers. The decay of the electroactivity (capacity) with the increase in the amount of Au NP in the composite is related to the addition of more nonelectroactive mass to the composites. As the amount of Au NPs increases, the ratio of electroactive material (PANI) in relation with the nonelectroactive material (Au NP) decreases, resulting in a lower specific current. This effect can be observed in the inset of Figure 4, where the open symbols represent the capacity normalized only by the PANI mass calculated by taking into account that the residual mass obtained with TGA experiments (Figure 2) corresponds to Au NPs. It is clear in this case that the diminution in capacity is not so pronounced as in the case that the total mass was used. Therefore, it is clear that there is an optimal composition where the effect of improving interconnectivity is more important than the amount of Au NPs. The capacity results presented herein indicate that the PANI−Au composites may be a promising material for the use in capacitors, solar cells, and other devices. Heating Treatment. The Raman spectra of the PANI−ES film and PANI−Au films after heating at 150 °C for 25 min are presented in Figure 5. The Raman spectra of the PANI−ES film 4271



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Scheme 2. Schematic Interaction between Gold Nanoparticles and Polyaniline in PANI−Au Composites: (A) Avoiding CrossLinking Formation and (B) Allowing a Thermal Dedoping Process (Adapted from refs 15 and 34)

Scheme 3. Schematic Representation of Cross-Linking Formation in PANI−Au Composites after an UV−Visible Light Irradiation (Adapted from refs 64 and 65)

Figure 6. Raman spectra of the PANI−ES film and PANI−Au films after 15 h of UV irradiation (λ0 = 632.8 nm).

composites. This result suggests that the UV−visible light irradiation induces the formation of cross-linking segments even in the PANI−Au composites with a low change in the doping level. Under irradiation, the formation of cross-linking segments could be explained considering a mechanism for catalysis using metallic nanoparticles proposed in the literature. It was proposed that, under UV−visible light irradiation, the electrons from the Au NPs can be transferred to an electron acceptor, such as O 2 , generating an oxidative species (O 2 •− ); consequently, positive charges, holes, will be left on the Au NPs. In order to neutralize these positive charges, Au NPs will capture electrons from the neighbor molecules. Considering these processes, after the generation of the positive charges in the Au NPs, the neutralization occurs by the removal of electrons of the PANI chains, generating new nitrogen radical cations allowing the cyclization process giving phenazine-like rings as cross-linking segments (Scheme 3).

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The electrochemical characterization indicated that the electroactivity and capacity of the composites were enhanced when compared to PANI−ES and that these properties are dependent on the amount of AuNP in the composites. Raman spectra of the heated composites presented no characteristic bands of cross-linking structures, whereas the dedoping process was observed. Raman spectra of the UV−visible light irradiation presented the maintenance of the doping level and cross-linking formation. These properties were explained considering the

CONCLUSIONS A simple one-step synthesis of nanofibers of PANI−Au composites was demonstrated. UV−vis−NIR and Raman data showed that PANI is in its doped form (PANI−ES). Therefore, this one-step synthesis in acidic medium leads to PANI−Au composites, presenting the fibers of PANI in the emeraldine salt form (PANI−ES). 4272



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interaction of AuNP with the protonated nitrogen radical atoms of PANI chains and the behavior of AuNP under UV−visible radiation. The PANI−Au composites presented an excellent electroactivity and capacity properties and no change in the doping level after UV−visible radiation results, suggesting that the PANI−Au composites may be a promising material for use in capacitors, solar cells, and other devices.

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ASSOCIATED CONTENT

S Supporting Information *

Figures showing an emission spectrum, EDX spectrum, transmission electron microscopy image, plot of the change in frequency and dissipation, and calculated mass using the fifth overtone. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: + 55 11 3091 3890. Fax: + 55 11 3091 3890. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the Brazilian agencies CNPq and FAPESP (grant numbers 2010/18107-8, 09/09209-4, and 09/ 53199-3) for fellowships and financial support. M.L.A.T. and R.M.T. thank CNPq for research fellowships.

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