Kinetic Approach to Elucidate Size Controllable Features in

Oct 8, 2014 - Kinetic Approach to Elucidate Size Controllable Features in Nanocomposites of Gold Nanoparticles and Poly(3,4-ethylenedioxythiophene) in...
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Kinetic Approach to Elucidate Size Controllable Features in Nanocomposites of Gold Nanoparticles and Poly(3,4ethylenedioxythiophene) in Aqueous Dispersion Stabilized by Gum Acacia Igor Rocha, Emili Lucht, Izabel C. Riegel-Vidotti, Marcio Vidotti, and Elisa S. Orth* Department of Chemistry, Universidade Federal do Paraná, C.P. 19032, CEP 81531-980, Curitiba, Paraná, Brazil S Supporting Information *

ABSTRACT: Research and development of conductive nanomaterials based on biocompatible matrices has been greatly rising in the past decade since synergistic properties can be achieved by combining metallic nanoparticles and natural/ conductive polymers. Poly(3,4-ethylenedioxythiophene) (PEDOT) is known to be an intrinsically conductive polymer, difficult to handle in aqueous medium. Therefore, in this work, we present a physical-chemical perspective in the development of novel aqueous dispersible nanocomposites of gold nanoparticles (AuNPs) and PEDOT, obtained through a one-pot synthesis, using the biopolymer gum acacia (GA) as stabilizer. A thorough kinetic study was carried out and correlated with microscopy analyses, evidencing that the concentration of GA influences the AuNP size by affecting their nucleation and growth stages. A quantitative detailing using kinetic models is shown, which to the best of our knowledge is the first report relating mechanism and rate constants with size controllable features of the stabilizer. Two distinct kinetic profiles were obtained and related to a critical concentration of GA (1%w/v): (i) above, a characteristic nucleation−growth sigmoidal profile and (ii) below, an unexpected bilogistic profile, accounted to a two-step growth process. Indeed, the bilogistic kinetic model, usual in population growth studies, is presented herein for the first time regarding NP formation. These results incite the targeted design of novel nanomaterials, using kinetic studies as a promising tool to understand the mechanism of the size-controllable features of GA. Overall, we evidence that the nanocomposite characteristics can be optimized rationally. Also, considering the natural occurrence of GA, we contribute to the sustainable development of highly water-dispersible PEDOT-derived nanocomposites.

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gold.9 In contrast, conductive polymers and their composites are not well dispersed in water; therefore, in order to stabilize such compounds in a colloidal aqueous dispersion, different surfactants can be used.10 Given that the majority of the stabilizers possesses high toxicity, the use of natural polymers opens up a field of application for these hybrid substances ranging from the development of materials employed in electrocatalysis to electrochemical biosensors in vivo.11,12 Among the polymers obtained from natural resources, gum acacia (GA) stands out, due to its promising behavior as stabilizers of colloidal dispersions.13 GA is obtained as a natural exudate from some species of Acacia trees, and it is commercially employed as a stabilizer and emulsifier in the food, pharmaceutical, and cosmetic industries. Its complex molecular structure consists of a mixture of branched polysaccharides (containing galactose, rhamnose, uronic acids, and arabinose residues) and protein fractions.13,14 Additionally, GA is considered a residue in tannin industries and, hence, by a

INTRODUCTION The development of nanocomposites has become an important research field due to the possibility of combining the advantages of soft matter with the unique properties of nanoscaled particles, obtaining a new class of versatile materials.1,2 The properties of these nanomaterials are intrinsically related to factors such as shape, size, and overall distribution, thus demanding rational tools for the targeted design of optimized nanocomposites. In this context, physicalchemical studies are promising allies, which furnish a thorough understanding of the overall process involved, one of the apparatuses with the most potential for projecting rationally any system with controlled characteristics. Since the discovery of conductive polymers, a broad research area has been increasing with materials that have high electrochemical activity and easy aqueous handling.3−5 Among these substances, poly(3,4-ethylenedioxythiophene) (PEDOT) is known to be an intrinsically conductive polymer with excellent properties such as reversible doping state, high stability, and low band gap allowing a high conductivity.6 The conductivity can be improved by doping the structure,7,8 for example, using different metals such as silver, platinum, and © 2014 American Chemical Society

Received: August 18, 2014 Revised: October 5, 2014 Published: October 8, 2014 25756

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Scheme 1. Representation of the Proposed Synthesis Reaction to Form the Nanocomposites

Figure 1. Visual aspects for the aqueous dispersions of the nanocomposite PEDOT-AuNPs/GA with varying concentration of GA (0.07−7.00%w/v).

mechanic stirring 19 μL of a 0.1 mol L−1 HAuCl4 solution was added in order to start the reaction. Raman Spectroscopy. The synthesized composites were centrifuged for 30 min, and the powders were dried under vacuum for 24 h before analysis. Raman analyses were made in a Renishaw Spectrophotometer using a 532 nm beam laser. Kinetics. UV−vis spectra were obtained using a HewlettPackard 8453 diode array spectrophotometer. Kinetic profiles (absorbance versus time) were obtained at 540 and 720 nm and fitted using the software OriginPro 8. Electron Microscopy Analysis. The composites were transferred on a 3000 mesh grid for TEM analysis. The samples were dried under vacuum for 24 h before analysis. The electron microscopy was performed on a Jeol JEM 1200EXII Electron Microscope, using the software ImageJ to treat the TEM images. The images made by TEM were taken in various parts of the grid, and more than 800 particles were counted for each sample.

green (environmentally friendly) sustainable perspective could be exploited to develop novel nanomaterials. We present a physical-chemical perspective in the development of novel nanocomposites using gold nanoparticles (AuNPs) inserted in the PEDOT matrix, by a one-pot reaction. In order to obtain stable aqueous dispersions, GA was employed as a stabilizer, which interestingly showed size controllable features for the AuNPs formed. The nanocomposites are referred to here as PEDOT-AuNPs/GA. A thorough kinetic study is presented and correlated with transmission electron microscopy (TEM) analyses. Thus, the role of the stabilizer in the mechanism of the AuNP formation was elucidated. The kinetic approach is innovative and showed for the first time a bilogistic two-step growth profile for NPs, which was adequately fitted. The methodology adopted is a very promising tool in understanding and controlling nanomaterial’s characteristics, still challenging nowadays, and certainly determinant in defining its intrinsic properties, which furnish targeted applications (e.g., nanomedicine).15−17 Overall, we evidence that the nanocomposite characteristics (e.g., NP size) can be optimized rationally.

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RESULTS AND DISCUSSION Synthesis and Characterization of the Nanocomposites. The syntheses of the proposed nanocomposites were performed, based on a modification of the route proposed by Kumar et al.18 Herein, the stabilizer GA is used to obtain the PEDOT-based nanocomposites in aqueous dispersion. Scheme 1 illustrates how the nanocomposites are obtained. In this model, the ions AuCl4− are reduced to form AuNPs concomitantly with the oxidation of EDOT molecules to EDOT•+ radical, which acts as an initiator to propagate the polymerization reaction to form PEDOT. We propose that the AuNPs are strongly incorporated on the PEDOT matrix due to favorable interactions, mainly with sulfur sites.18 The role of the natural polymer, GA, is to guarantee a stable dispersion of the PEDOT-based nanocomposite in aqueous medium. During the ongoing reaction, the product formation was easy to visualize by the change in the color of the dispersions.

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EXPERIMENTAL METHODS Materials. GA from acacia tree, HAuCl4 (99.9+%), and 3,4ethylenedioxythiophene (EDOT) were purchased from SigmaAldrich and used as received. GA composition and molecular properties are presented elsewhere.13 All solutions were prepared using highly purified deionized water, and added salts were of the highest purity available. Synthesis of the PEDOT-AuNP Dispersion. An amount of 2.00 g of commercial GA (used as received) was dispersed under agitation in 10.0 mL of deionized water. Aliquots of this dispersion were diluted in order to obtain concentrations from 0.007 to 7.0%w/v of a 3.0 mL solution (total volume). An amount of 150 μL of a 1.88 × 10−5 mol L−1 EDOT aqueous solution was added to this dispersion, and then, under 25757

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Initially, solutions are yellowish due to AuCl4−, become blue with the formation of PEDOT, and finally turn purple, due to the formation of AuNPs (a combination of the characteristic colors of PEDOT and AuNPs: blue and red, respectively). The influence of GA as a stabilizer was also studied, as will be thoroughly discussed further; nevertheless, it clearly showed a range of colors for varying concentrations of GA, as shown in Figure 1 (given in percentage weight of GA in the solution volume, %w/v,). The noticeable color variation influenced by GA can be associated with AuNP size in the nanocomposite, which is commonly associated with coloration.19 The nanocomposite formation was followed by UV−vis spectroscopy, and representative spectra obtained in different time intervals are given in Figure 2. Results show an absorbance

Figure 3. Raman spectra obtained for PEDOT and PEDOT-AuNPs/ GA (GA 3.50%w/v) using green radiation, λ0 = 514 cm−1. The PEDOT monomeric structure is inserted.

stretching, respectively. The band at 1566 cm−1 has been reported for PEDOT/PSS composites synthesized by the chemical route, attributed to the presence of small oligomers, particularly sensible to the green incident radiation.23 As reported by Gareau22 and Chiu,24 the level of doping (or oxidation) of the polymer can be evaluated mainly by the Cα Cβ (−O) band (1437 cm−1) shifting. Hence, a band at 1414 cm−1 indicates a neutral state and upshifts to 1445 cm−1 at the maximum degree of doping, suggesting that this sample has an average degree of doping. The spectrum for the PEDOT-AuNPs/GA nanocomposite is very similar to PEDOT, presenting the same bands described above, although shifted toward lower frequencies. Interestingly, for the nanocomposite, the band related to C−S−C stretching is suppressed probably due to the favorable interaction between the sulfur site and the gold nanoparticles, due to their strong affinity.25 This strongly evidences the formation of the nanocomposite, as proposed, with the sulfur moieties of PEDOT stabilizing the nanoparticles. Indeed, authors report this interaction as responsible for passivating AuNPs in other systems using sulfur-based stabilizers.19 As commented previously, the shifting of the CαCβ (−O) band (1420 cm−1) is directly related to the doping level of the conducting polymer, indicating that PEDOT is most likely neutral in the nanocomposite. Further, the increased intensity of the Cβ−Cβ stretching signal (1483 cm−1) is also a strong indicator of the low oxidation level of PEDOT in the composite despite the presence of AuNPs. Nevertheless, the displacement of all bands will be further discussed (vide inf ra) since this effect in principle cannot be attributed solely to the doping level of PEDOT. Overall, the results evidence the formation of the nanocomposites PEDOT-AuNPs/GA, with GA playing an essential role in the stability of aqueous dispersion. A reproducible and highly stable AuNP derived nanomaterial was obtained containing PEDOT in a not completely oxidized state. Kinetic Evidence for GA Size Controllable Features: Correlation with TEM Analyses. Kinetic studies, along with TEM analyses, were carried out, varying the concentration of GA in order to elucidate the mechanism of formation for the nanocomposites, as a function of the stabilizer concentration.

Figure 2. UV−vis spectra at different reaction times for the nanocomposite PEDOT-AuNPs/GA formation with 3.50%w/v GA.

increase of the band centered at 540 nm, attributed to the characteristic surface plasmon resonance (SPR) band of AuNPs.19 Additionally, a high absorbance variation is easily noticeable in wavelengths greater than 700 nm corresponding to the π−π* transitions of the doped PEDOT matrix formed.18 It should be noted that all reactions showed similar spectra. Moreover, UV−vis results indicate the formation of AuNPs and PEDOT, hence the proposed nanocomposites. In the absence of GA, the reaction of EDOT and AuCl4− formed a compound that precipitated, and no SPR band of the supernatant was observed, indicating that the nanocomposite was not formed. An attempt to disperse the obtained precipitate in an aqueous medium with GA also failed (even for a high concentration of GA), evidencing the central role of the stabilizer GA during the course of the reaction. In order to elucidate the conformation of PEDOT chains in the studied nanocomposites, Raman spectroscopy was carried out using different radiation wavelengths. It is broadly accepted in the literature that green radiation intensifies the modes of PEDOT reduced form,20,21 while the radiation in the red or near-infrared regions allows a better analysis of its oxidized form.22 Therefore, Figure 3 presents Raman spectra using green radiation, for PEDOT and PEDOT-AuNPs/GA nanocomposites (spectra obtained in the red radiation is given in the Supporting Information). The spectrum for pure PEDOT presents the characteristic bands reported in the literature.22−24 At lower frequencies the bands at 574 and 698 cm−1 are attributed to oxyethylene ring deformation and symmetric C−S−C deformation, respectively. Other important bands found at 1366 and 1508 cm−1 correspond to Cβ−Cβ stretching and asymmetric CαCβ 25758

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Strikingly, results show two distinct behaviors: above and below 1.05%w/v of GA, which is discussed separately in the following. Influence of GA Concentration: Above 1.05%w/v. Kinetic profiles were obtained by UV−vis in 540 and 720 nm, referring to the formation of AuNPs and PEDOT, respectively. Figure 4

Table 1. Kinetic Parameters Obtained for the Formation of the Nanocomposites PEDOT-AuNPs/GA, at Varying Concentration of GA (CGA)a CGA (%m/v) 1.05 1.40 1.75 3.50 5.25 7.00

kAuNPs, 10−6 (s−1) 1.58 1.26 1.14 0.46 0.41 0.38

± ± ± ± ± ±

0.03 0.01 0.04 0.005 0.005 0.003

kPEDOT, 10−4 (s−1) 6.40 3.71 1.66 1.91 1.43 2.18

± ± ± ± ± ±

0.06 0.02 0.02 0.03 0.02 0.04

Obtained from fits presented in Figure 4 (most representative shown only for 1.05, 1.75, 5.25, and 7.0%w/v), using eq 1. The extended form of eq 1 is given in the Supporting Information. The rate constants (kAuNPs and kPEDOT) are not comparable since they regard different kinetic models (sigmoidal and first-order, respectively). Hence, the term time is related to the Avrami exponent for the sigmoidal fit. a

first-order profile was obtained (presented in the Supporting Information) and confirmed the proposed model. Also, the “kPEDOT” values obtained in these experiments are similar to the constants shown in Table 1, in the magnitude of 10−4 s−1; however, no clear correlation with GA concentration was found. Figure 5 illustrates the dependence of “kAuNPs” with the concentration of GA, along with the end time of the reaction

Figure 4. Kinetic profiles obtained for the formation of PEDOTAuNPs/GA, with varying concentration of GA (1.05−7.00%w/v), monitored at 540 nm. Red lines correspond to the fits based on eq 1.

presents the profiles obtained for varying concentration of GA above 1.05%w/v in 540 nm, which shows sigmoidal profiles, typical for NP formation that undergoes first a slow nucleation stage, followed by a more rapid nuclei growth.19,26,27 It should be noted that the characteristic band of PEDOT formation, centered at 720 nm, appears in a broad spectral region, contributing to the kinetic profile, even in 540 nm. Thus, eq 1 was deduced to explain the overall kinetic behavior observed. It assumes a first-order polymer (PEDOT) growth and a characteristic two-step nucleation−growth sigmoidal profile for the AuNP formation, where the species concentration was properly correlated with absorbance (Lambert−Beer Law) n

Abs = A(1 − e−kPEDOTt ) + B(1 − e−kAuNPst )

(1)

“Abs” is the measured absorbance, at 540 nm, at a given time (t); “A” and “B” are constants related to absorbance variation (initial and end points) for each species detected, PEDOT and AuNPs, respectively. The term “kPEDOT” is the first-order rate constant for the formation of PEDOT, while “kAuNPs” is the rate constant for the formation of AuNPs. The Avrami exponent “n” accounts for the sigmoidal growth profile of the nanoparticles.28−31 The fully deduced equation is given in the Supporting Information. Indeed, the proposed eq 1 fitted adequately the experimental results (solid red lines in Figure 4), presenting R2 higher than 0.99 in all cases and therefore corroborating the proposed kinetic model. Analogous results were also obtained at 720 nm (given in the Supporting Information), consistent with the proposed model. Table 1 presents the rate constants for the nanocomposite formation, obtained by fitting data in Figure 4 with eq 1. Results show an inverse proportionality between the values of “kAuNPs” and GA concentrations, indicating that a greater amount of GA slows the AuNP formation. Notwithstanding, “kPEDOT” values showed an overall decrease with higher GA concentration, with no direct proportionality. Although the mechanism for the oxidative formation of conductive polymers is well-known in the literature,32−34 kinetic profiles were also obtained for the formation solely of PEDOT (by reacting EDOT and sodium persulfate, without AuNPs) in order to confirm the first-order growth of PEDOT. Indeed, the expected

Figure 5. Relation between kAuNPs, GA concentration (CGA), and the end time of the reaction (tf), monitored at 540 nm.

(tf). The parameter “tf” corresponds to the time that the reaction takes to conclude, at each condition, and hence is obtained by the end point of the kinetic profile (where absorbance does not vary significantly). Results confirm that lower concentrations of GA lead to higher values of “kAuNPs”, and as a consequence, faster formation of AuNPs (lower tf) is achieved. Another interesting analysis regards the derivatives of the kinetic profile, as discussed by Wang et al.19 The function describing a sigmoidal NP growth profile can be differentiated to obtain two important values, namely, the first and second derivatives, corresponding to the maximum growth rate and to the end of the nucleation time, respectively. The last is proposed herein as the determinant step for the AuNP formation and therefore can correlate with the mechanism and the overall characteristic of the nanocomposites, such as AuNP size. For this reason, all fitted curves were differentiated, as exemplified in Figure 6, along with the values obtained. Interestingly, results show that increasing the stabilizer (GA) concentration leads to higher nucleation time, thus slowing the determinant step. Surely, this behavior affects the AuNP formation, as will be evidenced by TEM images. 25759

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Figure 6. Graph with representative 1st and 2nd derivatives obtained from the fitted kinetic profile, monitored at 540 nm with GA 1.05%w/v. Determined values for the maximum of the 1st and 2nd derivative are given as a function of CGA. *Corresponding TEM images are shown in Figure 7.

Wang et al.19 proposed that a longer nucleation period furnishes more nuclei, thus, larger particles are possible. On the other hand, smaller particles are expected for shorter nucleation times. Therefore, at higher amounts of GA, the formation of larger-sized AuNPs is expected (longer nucleation time) which was confirmed by TEM images. Figure 7 shows TEM images

Influence of GA Concentration: Below 1.05%w/v. As shown so far, all experiments conducted with GA concentration higher than 1.00%w/v presented a kinetic profile that could be well adjusted by eq 1. However, for the experiments performed with CGA lower than 1.00%w/v, a different growth profile was observed, as presented in Figure 8.

Figure 8. Kinetic profiles of the synthesis of PEDOT-AuNPs/GA with CGA lower than 1.00%w/v monitored at 540 nm. Red lines correspond to the fits based on eq 2.

Results show a break in the kinetic curve (CGA < 1.00%w/v), which was attributed to two different growth stages, presumably due to the formation of aggregates. We propose that at low concentrations GA is not able to stabilize the organized growth of AuNPs, resulting in the coalescence of small AuNPs, previously formed. This leads to large NPs, which are thermodynamically more stable. In order to describe the kinetic profiles shown in Figure 8, we fitted the data based on the bilogistic model proposed by Meyer.35 This model describes population growth processes with two different pulses which can be sequential or superposed, and it is based on the assumption that the entities of the second process start to grow with a different rate before the first process finishes or reaches an asymptotic value. Adapting this model to the formation of the PEDOT-AuNPs/GA, we assume the asymptotic value as the absorbance measured in 540 nm referring to the SPR band of the AuNPs. Therefore, this model suggests that first AuNPs nucleate and grow following a sigmoidal growth process as discussed before and that after a time interval these particles reach a size in which they start to aggregate following a different growth profile, as described by eq 2. Moreover, since the formation of PEDOT influences the absorbance variation at 540 nm, we added the first-order equation (kPEDOT) to the bilogistic eq 2, as done in eq 1. To the

Figure 7. TEM images of nanocomposites PEDOT-AuNPs/GA with 1.05 and 3.50%w/v of GA, with respective histograms.

recorded for the nanocomposites with 1.05 and 3.50%w/v of GA, with the corresponding histograms. In these images, the formation of AuNPs is confirmed with notable differences in their characteristics. Calculated average sizes are 6.0 ± 0.4 and 32.6 ± 0.3 nm for 1.05 and 3.50%w/v of GA, respectively. Thereby, higher amounts of GA give larger AuNPs, which can be related to the nucleation time (see Figure 6). Previously we showed that with 1.05 and 3.50%w/v of GA the nucleation times are 135 and 215 s, respectively, corroborating the proposed model: increasing GA gives larger AuNPs. 25760

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stage that lasts longer. The parameter Δt2 is also related to GA concentration, and results evidence that the second growth, attributed to aggregation, is also delayed by higher concentration of GA (up to 1.05%w/v). Analogous data were obtained at 720 nm. According to the bilogistic kinetic model proposed (for CGA < 1.00%w/v), we expect that a longer growth/aggregation stage will consistently lead to AuNPs with distinct sizes and overall characteristics, compared to the classical sigmoidal first-order model (eq 1) proposed for CGA > 1.00%w/v. In order to corroborate this proposal, TEM images for the nanocomposites with CGA = 0.17%w/v were obtained, given in Figure 9. Consistently, results show the presence of different NP size population, and the bimodal distribution shows NP average sizes of 16.4 ± 0.5 and 63.8 ± 8.8 nm that greatly differ from results with CGA > 1.00%w/v. Additionally, the larger aggregates possess various different shapes, attributed to aggregation. Henceforth, the two-step growth accounting for the kinetic profiles, which was fitted with the adapted bilogistic model, agrees with TEM analyses: AuNP aggregate with GA concentration lower than 0.1%w/v. Proposed Model for the Formation of GA-Stabilized PEDOT-AuNP Nanocomposites. The kinetic study along with TEM analyses evidenced the formation of PEDOT-AuNPs/GA nanocomposites, which exhibit different characteristics with varying concentration of the stabilizer GA, as illustrated in Scheme 2. For CGA > 1.00%w/v, a classical two-step mechanism (nucleation−growth) is proposed leading to homogeneous small-sized AuNPs (below 25 nm), with a unimodal size distribution. In this case, with increasing GA concentration (up to 7.00%w/v) the nucleation step becomes slower, and as predicted and also reported in the literature19 larger-sized AuNPs are formed. In contrast, for CGA < 1.00%w/v, a novel bilogistic model is proposed with two different growth stages, assigned to growth and aggregation phenomena. Heterogeneous larger-sized particles were obtained (nearly 100 nm), with a bimodal size distribution, implying that at low concentrations (CGA < 1.00%w/v) GA does not prevent aggregation. Overall, the optimal concentration of stabilizer to obtain nanocomposites with small-sized, homogeneously distributed AuNPs is CGA = 1.05%w/v. The important role of GA in the formation of PEDOTAuNPs/GA is irrefutable since the nanocomposites were obtained solely in the presence of GA. In order to get a deeper insight into the role of GA in stabilizing the as-formed particles, a detailed study was conducted by means of Raman spectroscopy. Figure 10 presents the spectra taken from different nanocomposites maintaining the EDOT/AuCl4− proportion and varying the GA amount. The displacement of all bands for

best of our knowledge, this is the first report for fitting these complex kinetic profiles using an adapted bilogistic model to describe NP growth. C

Abs = A(1 − e−kPEDOTt ) + 1+

⎡ −ln 81(t − tm1) ⎤ e ⎣ Δt1 ⎦

D

+

⎡ −ln 81(t − tm2) ⎤ Δt2 ⎦

1 + e⎣

(2)

In eq 2, the first term corresponds to the first-order growth of PEDOT (parameters were described previously), and the second and third terms are related to the growth process of AuNPs in two different stages. The terms “C” and “D” are the asymptotic absorbance values; “tm” is the characteristic growth time; and “Δt” is the midpoint time of the respective growth processes.36 As shown by the solid red lines in Figure 9, eq 2

Figure 9. TEM images for the nanocomposite PEDOT-AuNPs/GA with 0.17%w/v,GA and respective histogram.

successfully described the observed behavior with values of R2 higher than 0.99 for all curves. The values of “kPEDOT” obtained are in the order of 10−4 s−1 and do not vary significantly with the concentration of GA, accordingly to previous results. The kinetic parameters obtained from fitting with eq 2 are given in Table 2. First, Δt1 showed a direct relation with GA concentration. Indeed, this process represents the characteristic duration for the first growth process which ultimately should be dictated by the stabilizing effect of AuNPs by GA in the nanocomposites. Therefore, increasing GA leads to a growth

Table 2. Kinetic Parameters Obtained by Fitting Data in Figure 8 According to the Bilogistic Model, Equation 2, for the Formation of PEDOT-AuNPs/GA CGA (%w/v) C Δt1 (s) tm1 (s) D Δt2 (s) tm2 (s) kPEDOT (s−1)

0.02 0.07 115.39 113.65 0.18 96.43 390.30 (2.28

± ± ± ± ± ± ±

0.002 39.80 6.75 0.009 8.77 1.70 0.15) × 10−3

0.03 0.13 178.42 159.15 0.13 157.18 517.78 (1.59

± ± ± ± ± ± ±

0.17

0.03 36.57 5.72 0.01 18.71 3.34 0.18) × 10−3 25761

0.18 248.37 231.79 0.09 207.76 651.685 (1.13

± ± ± ± ± ± ±

0.03 28.10 4.87 0.007 19.46 4.02 0.11) × 10−3

0.35 0.30 432.55 468.99 0.08 355.61 1002.15 (3.47

± ± ± ± ± ± ±

0.01 19.66 3.69 0.006 32.73 9.27 0.65) × 10−4

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Scheme 2. Proposed Model for the Formation of the Nanocomposite PEDOT-AuNPs/GA

Figure 10. Raman spectra taken from different PEDOT-AuNPs/GA samples synthesized with CGA: 0.35%w/v, 0.70%w/v, 1.50%w/v, and 2.10%w/v over the region of (A) 600−1700 cm−1 and (B) 900−1300 cm−1.

radiation employed (514 nm), leading to the increase of intensity of the Raman bands. The interaction between GA and PEDOT could be better observed by analyzing the Raman spectra in the frequencies of about 900−1300 cm−1, as shown in Figure 10(B). The neatPEDOT spectrum (Figure 10) shows the bands in 990 and 1096 cm−1 which correspond to the oxyethylene ring and C− O−C deformations, respectively. For the nanocomposites, the first band is shifted to 972 cm−1, and in the C−O−C region, a large band is encountered at lower proportions of GA with a discrete shift. At 1.50%w/v and 2.10%w/v of GA, an intense band at 1136 cm−1 appears which was not found in any other related work in the literature. This band could be associated with a specific vibrational mode caused by the interaction between the OH and COOH groups of GA interacting with the C−O−C sites by means of strong hydrogen bonding. This kind of interaction was previously reported in poly(aniline) composites, by studying the NH band38 but not reported so far for PEDOT-based systems. Finally, the band in 1250 cm−1 is attributed to Cα−Cα inter-ring stretching. For the neat-PEDOT sample, this region is encountered as two bands, which can be explained by the different doping level of the polymeric structure, alternating between C−C and CC bonds, but for the nanocomposites, this band is found as a unique band with

PEDOT-AuNPs/GA in relation to neat-PEDOT was verified as a function of GA content since this effect, in principle, cannot be attributed solely to the doping level of PEDOT (vide supra). In Figure 10(A), it is possible to verify that for all concentrations of GA the band positions remain the same as described in neat-PEDOT (Figure 3). Nevertheless, it was observed that the intensity of the Raman signal was highly enhanced by the amount of incorporated GA in the composite. This effect is indeed intriguing, and some possible explanations are discussed as follows. One possible explanation is related to the enhancement of the electronic effect caused by the conformational changes of PEDOT due to the secondary doping effect from GA. Secondary doping was first described for poly(aniline),37 and in general lines, this effect is caused by the inclusion of large organic molecules within the conducting polymeric matrix changing drastically its conformation from compact to extended coil. By this way, the electric repulsion decreases among the polymeric backbones, enhancing both the crystallinity and conductivity of the polymer. In this proposal, GA would be acting as a structural dopant and allied to the neutral form (as observed with the shifting of the CαCβ (−O) band ∼1437 cm−1), and the resonant effect is enhanced due to the increase of the absorption spectra in the incident 25762

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conformation that allows the formation of NPs with increased surface area. For CGA > 1.00%w/v, the NPs will grow as large as the amount of GA available for their stabilization. In contrast, for CGA < 1.00%w/v, the NPs tend to coalesce, giving rise to large agglomerates. Overall, the optimal concentration of stabilizer to obtain nanocomposites with small-sized, homogeneously distributed AuNPs is CGA = 1.05%w/v. AuNP size can be effectively controlled and the nanocomposite properties optimized, which is surely a benchmark for the rational and targeted design and engineering of novel nanomaterials. The kinetic approach adopted in this study is innovative, comprising the first report using the bilogistic model to describe AuNP formation. Also, kinetic equations were fully deduced, and the formation of PEDOT and AuNPs could be analyzed separately. The presented results should furnish future promising studies on how to avoid aggregation processes and control size distribution of NPs for various complex systems. Namely, the size of a nanomaterial is intrinsically related to its desired properties. The sustainable use of GA as a stabilizer is noteworthy since it is a natural occurring biopolymer that is considered a residue in tannin industries. Thus, what normally would be considered waste can be recycled and used as a stabilizer for obtaining stable nanocomposites. We expect similar behavior of GA with other noble metals and conductive polymers. The nanocomposites have potential electroactivity, foreseeing future applications.

no shifting, corroborating the neutral form of PEDOT, presenting high linearity. It is widely described that molecules with sulfur atoms are those who preferably bind to the AuNPs. Hence, for the nanocomposites, Au should interact strongly with sulfur sites of PEDOT. Considering the above observations related to Raman analysis, it is reasonable to assume that GA interacts with PEDOT-AuNPs through hydrogen bond interactions between the oxyethylene moieties present in PEDOT and multiple functional groups (polysaccharide and protein) of GA. Also, favorable electrostatic attraction will occur since at the pH of the syntheses GA bears negative surface charge (due to the ionization of carboxyl groups) and PEDOT has positive neat charge, due to its oxidation state. The resulting interaction of GA with PEDOT-AuNPs diminishes the repulsion among PEDOT chains, leading to a more extended polymeric conformation that allows the formation of NPs with increased surface area. When enough GA is provided (CGA > 1.00%w/v) the NPs will grow as large as the amount of GA available for their stabilization, in aqueous media. The larger AuNPs induced by higher nucleation time upon increasing concentration of GA, evidenced by TEM and kinetic studies, can be accounted to a more linear polymeric conformation, whereby enabling nucleation sites that are presumably more available and somehow furnishing an extended nucleation step. On the contrary, if the amount of GA is limited (CGA < 1.00%w/v) the only mechanism available for the stabilization of the NPs is to coalesce, giving rise to secondary aggregates (large agglomerates) (Scheme 2). The novelty of our study comprises the proposition of a novel bilogistic model to describe the cases in which the amount of stabilizer is not enough to prevent secondary aggregation (as substantiated with TEM images). We believe the quantitative correlation of kinetic parameters with the stabilizer concentration and NP size (Scheme 2) enriches the knowledge in materials science and furnishes powerful tools for predicting nanocomposite growth.

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ASSOCIATED CONTENT

S Supporting Information *

Complementary Raman spectra in red radiation, kinetic results, and TEM images. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

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*E-mail: [email protected]. Fax: +55 41-33613186. Tel.: +55 41-33613176.

CONCLUSION Highly stable aqueous dispersions of the nanocomposite PEDOT-AuNPs/GA were obtained by a one-pot synthesis and fully characterized by UV−vis and Raman spectroscopy. A thorough kinetic study was carried out and correlated with TEM analyses, evidencing that the formation of PEDOTAuNPs/GA nanocomposites is strongly influenced by the stabilizer (GA) concentration. Interestingly, this behavior was quantified by kinetic parameters, and two different scenarios were observed: (i) CGA > 1.00%w/v, a two-step nucleation− growth mechanism gives homogeneous small-sized AuNPs, which upon increasing GA concentration slows the nucleation step and leads to larger-sized AuNPs; and (ii) CGA < 1.00%w/v, a novel bilogistic model is proposed with two-step growth stages, assigned to aggregation phenomena. This process leads to heterogeneous larger-sized AuNPs, and a lower concentration of GA facilitates aggregation. Herein, the stabilizer effect on the mechanism of AuNP formation was elucidated. We propose that Au bonds strongly with the sulfur sites of PEDOT, which interacts with GA through hydrogen interactions between the oxyethylene moieties present in PEDOT and multiple functional groups of GA. Also, favorable electrostatic attraction should occur between negatively charged GA and PEDOT that has a positive neat charge. Thus, reducing repulsion among PEDOT chains leads to a more extended polymeric

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors acknowledge the financial support from CNPq, CAPES, Fundaçaõ Araucária, and CME (Electronic Microscopy Center-Universidade Federal do Paraná) for microscopy analyses.

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REFERENCES

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