Kinetic Approach to Elucidate Size Controllable Features inNanocomposites of Gold Nanoparticles and Poly(3,4-ethylenedioxythiophene) in Aqueous Dispersion Stabilized by GumAcaciaIgor Rocha, Emili Lucht, Izabel C. Riegel-Vidotti, Marcio Vidotti, and Elisa S. Orth*

Department of Chemistry, Universidade Federal do Parana, C.P. 19032, CEP 81531-980, Curitiba, Parana, Brazil

*S Supporting Information

ABSTRACT: Research and development of conductive nanomaterials based onbiocompatible matrices has been greatly rising in the past decade since synergisticproperties can be achieved by combining metallic nanoparticles and natural/conductive polymers. Poly(3,4-ethylenedioxythiophene) (PEDOT) is known tobe an intrinsically conductive polymer, difficult to handle in aqueous medium.Therefore, in this work, we present a physical-chemical perspective in thedevelopment of novel aqueous dispersible nanocomposites of gold nanoparticles(AuNPs) and PEDOT, obtained through a one-pot synthesis, using thebiopolymer gum acacia (GA) as stabilizer. A thorough kinetic study was carriedout and correlated with microscopy analyses, evidencing that the concentration ofGA influences the AuNP size by affecting their nucleation and growth stages. Aquantitative detailing using kinetic models is shown, which to the best of ourknowledge is the first report relating mechanism and rate constants with sizecontrollable 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 presentedherein for the first time regarding NP formation. These results incite the targeted design of novel nanomaterials, using kineticstudies as a promising tool to understand the mechanism of the size-controllable features of GA. Overall, we evidence that thenanocomposite characteristics can be optimized rationally. Also, considering the natural occurrence of GA, we contribute to thesustainable development of highly water-dispersible PEDOT-derived nanocomposites.

■ INTRODUCTION

The development of nanocomposites has become an importantresearch field due to the possibility of combining theadvantages of soft matter with the unique properties ofnanoscaled particles, obtaining a new class of versatilematerials.1,2 The properties of these nanomaterials areintrinsically related to factors such as shape, size, and overalldistribution, thus demanding rational tools for the targeteddesign of optimized nanocomposites. In this context, physical-chemical studies are promising allies, which furnish a thoroughunderstanding of the overall process involved, one of theapparatuses with the most potential for projecting rationally anysystem with controlled characteristics.Since the discovery of conductive polymers, a broad research

area has been increasing with materials that have highelectrochemical activity and easy aqueous handling.3−5

Among these substances, poly(3,4-ethylenedioxythiophene)(PEDOT) is known to be an intrinsically conductive polymerwith excellent properties such as reversible doping state, highstability, and low band gap allowing a high conductivity.6 Theconductivity can be improved by doping the structure,7,8 forexample, using different metals such as silver, platinum, and

gold.9 In contrast, conductive polymers and their compositesare not well dispersed in water; therefore, in order to stabilizesuch compounds in a colloidal aqueous dispersion, differentsurfactants can be used.10 Given that the majority of thestabilizers possesses high toxicity, the use of natural polymersopens up a field of application for these hybrid substancesranging from the development of materials employed inelectrocatalysis to electrochemical biosensors in vivo.11,12

Among the polymers obtained from natural resources, gumacacia (GA) stands out, due to its promising behavior asstabilizers of colloidal dispersions.13 GA is obtained as a naturalexudate from some species of Acacia trees, and it iscommercially employed as a stabilizer and emulsifier in thefood, pharmaceutical, and cosmetic industries. Its complexmolecular structure consists of a mixture of branchedpolysaccharides (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

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

Article

pubs.acs.org/JPCC

© 2014 American Chemical Society 25756 dx.doi.org/10.1021/jp508326j | J. Phys. Chem. C 2014, 118, 25756−25764

green (environmentally friendly) sustainable perspective couldbe exploited to develop novel nanomaterials.We present a physical-chemical perspective in the develop-

ment 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 wasemployed as a stabilizer, which interestingly showed sizecontrollable features for the AuNPs formed. The nano-composites are referred to here as PEDOT-AuNPs/GA. Athorough kinetic study is presented and correlated withtransmission electron microscopy (TEM) analyses. Thus, therole of the stabilizer in the mechanism of the AuNP formationwas elucidated. The kinetic approach is innovative and showedfor the first time a bilogistic two-step growth profile for NPs,which was adequately fitted. The methodology adopted is avery promising tool in understanding and controlling nanoma-terial’s characteristics, still challenging nowadays, and certainlydeterminant in defining its intrinsic properties, which furnishtargeted applications (e.g., nanomedicine).15−17 Overall, weevidence that the nanocomposite characteristics (e.g., NP size)can be optimized rationally.

■ EXPERIMENTAL METHODSMaterials. GA from acacia tree, HAuCl4 (99.9+%), and 3,4-

ethylenedioxythiophene (EDOT) were purchased from Sigma-Aldrich and used as received. GA composition and molecularproperties are presented elsewhere.13 All solutions wereprepared using highly purified deionized water, and addedsalts 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 dispersedunder agitation in 10.0 mL of deionized water. Aliquots of thisdispersion were diluted in order to obtain concentrations from0.007 to 7.0%w/v of a 3.0 mL solution (total volume). Anamount of 150 μL of a 1.88 × 10−5 mol L−1 EDOT aqueoussolution was added to this dispersion, and then, under

mechanic stirring 19 μL of a 0.1 mol L−1 HAuCl4 solutionwas added in order to start the reaction.

Raman Spectroscopy. The synthesized composites werecentrifuged for 30 min, and the powders were dried undervacuum for 24 h before analysis. Raman analyses were made ina Renishaw Spectrophotometer using a 532 nm beam laser.

Kinetics. UV−vis spectra were obtained using a Hewlett-Packard 8453 diode array spectrophotometer. Kinetic profiles(absorbance versus time) were obtained at 540 and 720 nm andfitted using the software OriginPro 8.

Electron Microscopy Analysis. The composites weretransferred on a 3000 mesh grid for TEM analysis. The sampleswere dried under vacuum for 24 h before analysis. The electronmicroscopy was performed on a Jeol JEM 1200EXII ElectronMicroscope, using the software ImageJ to treat the TEMimages. The images made by TEM were taken in various partsof the grid, and more than 800 particles were counted for eachsample.

■ RESULTS AND DISCUSSIONSynthesis and Characterization of the Nanocompo-

sites. The syntheses of the proposed nanocomposites wereperformed, based on a modification of the route proposed byKumar et al.18 Herein, the stabilizer GA is used to obtain thePEDOT-based nanocomposites in aqueous dispersion. Scheme1 illustrates how the nanocomposites are obtained. In thismodel, the ions AuCl4

− are reduced to form AuNPsconcomitantly with the oxidation of EDOT molecules toEDOT•+ radical, which acts as an initiator to propagate thepolymerization reaction to form PEDOT. We propose that theAuNPs are strongly incorporated on the PEDOT matrix due tofavorable interactions, mainly with sulfur sites.18 The role of thenatural polymer, GA, is to guarantee a stable dispersion of thePEDOT-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.

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).

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp508326j | J. Phys. Chem. C 2014, 118, 25756−2576425757

Initially, solutions are yellowish due to AuCl4−, become blue

with the formation of PEDOT, and finally turn purple, due tothe formation of AuNPs (a combination of the characteristiccolors of PEDOT and AuNPs: blue and red, respectively). Theinfluence of GA as a stabilizer was also studied, as will bethoroughly discussed further; nevertheless, it clearly showed arange of colors for varying concentrations of GA, as shown inFigure 1 (given in percentage weight of GA in the solutionvolume, %w/v,). The noticeable color variation influenced by GAcan be associated with AuNP size in the nanocomposite, whichis commonly associated with coloration.19

The nanocomposite formation was followed by UV−visspectroscopy, and representative spectra obtained in differenttime intervals are given in Figure 2. Results show an absorbance

increase of the band centered at 540 nm, attributed to thecharacteristic surface plasmon resonance (SPR) band ofAuNPs.19 Additionally, a high absorbance variation is easilynoticeable in wavelengths greater than 700 nm correspondingto 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 andPEDOT, 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 thesupernatant was observed, indicating that the nanocompositewas not formed. An attempt to disperse the obtainedprecipitate in an aqueous medium with GA also failed (evenfor a high concentration of GA), evidencing the central role ofthe stabilizer GA during the course of the reaction.In order to elucidate the conformation of PEDOT chains in

the studied nanocomposites, Raman spectroscopy was carriedout using different radiation wavelengths. It is broadly acceptedin the literature that green radiation intensifies the modes ofPEDOT reduced form,20,21 while the radiation in the red ornear-infrared regions allows a better analysis of its oxidizedform.22 Therefore, Figure 3 presents Raman spectra using greenradiation, for PEDOT and PEDOT-AuNPs/GA nanocompo-sites (spectra obtained in the red radiation is given in theSupporting Information).The spectrum for pure PEDOT presents the characteristic

bands reported in the literature.22−24 At lower frequencies thebands at 574 and 698 cm−1 are attributed to oxyethylene ringdeformation and symmetric C−S−C deformation, respectively.Other important bands found at 1366 and 1508 cm−1

correspond to Cβ−Cβ stretching and asymmetric CαCβ

stretching, respectively. The band at 1566 cm−1 has beenreported for PEDOT/PSS composites synthesized by thechemical route, attributed to the presence of small oligomers,particularly sensible to the green incident radiation.23 Asreported by Gareau22 and Chiu,24 the level of doping (oroxidation) of the polymer can be evaluated mainly by the CαCβ (−O) band (1437 cm−1) shifting. Hence, a band at 1414cm−1 indicates a neutral state and upshifts to 1445 cm−1 at themaximum degree of doping, suggesting that this sample has anaverage degree of doping.The spectrum for the PEDOT-AuNPs/GA nanocomposite is

very similar to PEDOT, presenting the same bands describedabove, although shifted toward lower frequencies. Interestingly,for the nanocomposite, the band related to C−S−C stretchingis suppressed probably due to the favorable interaction betweenthe sulfur site and the gold nanoparticles, due to their strongaffinity.25 This strongly evidences the formation of thenanocomposite, as proposed, with the sulfur moieties ofPEDOT stabilizing the nanoparticles. Indeed, authors reportthis interaction as responsible for passivating AuNPs in othersystems using sulfur-based stabilizers.19 As commentedpreviously, the shifting of the CαCβ (−O) band (1420cm−1) is directly related to the doping level of the conductingpolymer, indicating that PEDOT is most likely neutral in thenanocomposite. Further, the increased intensity of the Cβ−Cβ

stretching signal (1483 cm−1) is also a strong indicator of thelow oxidation level of PEDOT in the composite despite thepresence of AuNPs. Nevertheless, the displacement of all bandswill be further discussed (vide inf ra) since this effect in principlecannot be attributed solely to the doping level of PEDOT.Overall, the results evidence the formation of the nano-

composites PEDOT-AuNPs/GA, with GA playing an essentialrole in the stability of aqueous dispersion. A reproducible andhighly stable AuNP derived nanomaterial was obtainedcontaining PEDOT in a not completely oxidized state.

Kinetic Evidence for GA Size Controllable Features:Correlation with TEM Analyses. Kinetic studies, along withTEM analyses, were carried out, varying the concentration ofGA in order to elucidate the mechanism of formation for thenanocomposites, as a function of the stabilizer concentration.

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

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

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp508326j | J. Phys. Chem. C 2014, 118, 25756−2576425758

Strikingly, results show two distinct behaviors: above and below1.05%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, referringto the formation of AuNPs and PEDOT, respectively. Figure 4

presents the profiles obtained for varying concentration of GAabove 1.05%w/v in 540 nm, which shows sigmoidal profiles,typical for NP formation that undergoes first a slow nucleationstage, followed by a more rapid nuclei growth.19,26,27 It shouldbe 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 1was deduced to explain the overall kinetic behavior observed. Itassumes a first-order polymer (PEDOT) growth and acharacteristic two-step nucleation−growth sigmoidal profilefor the AuNP formation, where the species concentration wasproperly correlated with absorbance (Lambert−Beer Law)

= − + −− −A e B eAbs (1 ) (1 )k t k t nPEDOT AuNPs (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 andAuNPs, respectively. The term “kPEDOT” is the first-order rateconstant for the formation of PEDOT, while “kAuNPs” is the rateconstant for the formation of AuNPs. The Avrami exponent “n”accounts for the sigmoidal growth profile of the nano-particles.28−31 The fully deduced equation is given in theSupporting Information.Indeed, the proposed eq 1 fitted adequately the experimental

results (solid red lines in Figure 4), presenting R2 higher than0.99 in all cases and therefore corroborating the proposedkinetic model. Analogous results were also obtained at 720 nm(given in the Supporting Information), consistent with theproposed 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 greateramount of GA slows the AuNP formation. Notwithstanding,“kPEDOT” values showed an overall decrease with higher GAconcentration, with no direct proportionality. Although themechanism for the oxidative formation of conductive polymersis well-known in the literature,32−34 kinetic profiles were alsoobtained for the formation solely of PEDOT (by reactingEDOT and sodium persulfate, without AuNPs) in order toconfirm the first-order growth of PEDOT. Indeed, the expected

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

concentration of GA, along with the end time of the reaction

(tf). The parameter “tf” corresponds to the time that thereaction takes to conclude, at each condition, and hence isobtained by the end point of the kinetic profile (whereabsorbance does not vary significantly). Results confirm thatlower concentrations of GA lead to higher values of “kAuNPs”,and as a consequence, faster formation of AuNPs (lower tf) isachieved.Another interesting analysis regards the derivatives of the

kinetic profile, as discussed by Wang et al.19 The functiondescribing a sigmoidal NP growth profile can be differentiatedto obtain two important values, namely, the first and secondderivatives, corresponding to the maximum growth rate and tothe end of the nucleation time, respectively. The last isproposed herein as the determinant step for the AuNPformation and therefore can correlate with the mechanismand the overall characteristic of the nanocomposites, such asAuNP 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 thedeterminant step. Surely, this behavior affects the AuNPformation, as will be evidenced by TEM images.

Figure 4. Kinetic profiles obtained for the formation of PEDOT-AuNPs/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.

Table 1. Kinetic Parameters Obtained for the Formation ofthe Nanocomposites PEDOT-AuNPs/GA, at VaryingConcentration of GA (CGA)

a

CGA (%m/v) kAuNPs, 10−6 (s−1) kPEDOT, 10

−4 (s−1)

1.05 1.58 ± 0.03 6.40 ± 0.061.40 1.26 ± 0.01 3.71 ± 0.021.75 1.14 ± 0.04 1.66 ± 0.023.50 0.46 ± 0.005 1.91 ± 0.035.25 0.41 ± 0.005 1.43 ± 0.027.00 0.38 ± 0.003 2.18 ± 0.04

aObtained from fits presented in Figure 4 (most representative shownonly for 1.05, 1.75, 5.25, and 7.0%w/v), using eq 1. The extended formof eq 1 is given in the Supporting Information. The rate constants(kAuNPs and kPEDOT) are not comparable since they regard differentkinetic models (sigmoidal and first-order, respectively). Hence, theterm time is related to the Avrami exponent for the sigmoidal fit.

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

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp508326j | J. Phys. Chem. C 2014, 118, 25756−2576425759

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

recorded for the nanocomposites with 1.05 and 3.50%w/v ofGA, with the corresponding histograms. In these images, theformation of AuNPs is confirmed with notable differences intheir characteristics. Calculated average sizes are 6.0 ± 0.4 and32.6 ± 0.3 nm for 1.05 and 3.50%w/v of GA, respectively.Thereby, higher amounts of GA give larger AuNPs, which canbe related to the nucleation time (see Figure 6). Previously weshowed that with 1.05 and 3.50%w/v of GA the nucleation timesare 135 and 215 s, respectively, corroborating the proposedmodel: increasing GA gives larger AuNPs.

Influence of GA Concentration: Below 1.05%w/v. As shownso far, all experiments conducted with GA concentration higherthan 1.00%w/v presented a kinetic profile that could be welladjusted by eq 1. However, for the experiments performed withCGA lower than 1.00%w/v, a different growth profile wasobserved, as presented in Figure 8.

Results show a break in the kinetic curve (CGA < 1.00%w/v),which was attributed to two different growth stages, presumablydue to the formation of aggregates. We propose that at lowconcentrations GA is not able to stabilize the organized growthof AuNPs, resulting in the coalescence of small AuNPs,previously formed. This leads to large NPs, which arethermodynamically more stable. In order to describe thekinetic profiles shown in Figure 8, we fitted the data based onthe bilogistic model proposed by Meyer.35 This modeldescribes population growth processes with two differentpulses which can be sequential or superposed, and it is basedon the assumption that the entities of the second process startto grow with a different rate before the first process finishes orreaches an asymptotic value. Adapting this model to theformation of the PEDOT-AuNPs/GA, we assume theasymptotic value as the absorbance measured in 540 nmreferring to the SPR band of the AuNPs. Therefore, this modelsuggests that first AuNPs nucleate and grow following asigmoidal growth process as discussed before and that after atime interval these particles reach a size in which they start toaggregate following a different growth profile, as described byeq 2. Moreover, since the formation of PEDOT influences theabsorbance variation at 540 nm, we added the first-orderequation (kPEDOT) to the bilogistic eq 2, as done in eq 1. To the

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 Figure7.

Figure 7. TEM images of nanocomposites PEDOT-AuNPs/GA with1.05 and 3.50%w/v of GA, with respective histograms.

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

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp508326j | J. Phys. Chem. C 2014, 118, 25756−2576425760

best of our knowledge, this is the first report for fitting thesecomplex kinetic profiles using an adapted bilogistic model todescribe NP growth.

= − ++

++

−− −

Δ

− −Δ

⎡⎣ ⎤⎦

⎡⎣ ⎤⎦

A eC

eD

e

Abs (1 )1

1

k tt tt

t tmt

PEDOTln 81( m1)

1

ln 81( 2)2 (2)

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

successfully described the observed behavior with values of R2

higher than 0.99 for all curves. The values of “kPEDOT” obtainedare in the order of 10−4 s−1 and do not vary significantly withthe concentration of GA, accordingly to previous results. Thekinetic parameters obtained from fitting with eq 2 are given inTable 2. First, Δt1 showed a direct relation with GAconcentration. Indeed, this process represents the characteristicduration for the first growth process which ultimately should bedictated by the stabilizing effect of AuNPs by GA in thenanocomposites. Therefore, increasing GA leads to a growth

stage that lasts longer. The parameter Δt2 is also related to GAconcentration, and results evidence that the second growth,attributed to aggregation, is also delayed by higher concen-tration of GA (up to 1.05%w/v). Analogous data were obtainedat 720 nm.According to the bilogistic kinetic model proposed (for CGA

< 1.00%w/v), we expect that a longer growth/aggregation stagewill consistently lead to AuNPs with distinct sizes and overallcharacteristics, compared to the classical sigmoidal first-ordermodel (eq 1) proposed for CGA > 1.00%w/v. In order tocorroborate this proposal, TEM images for the nanocompositeswith CGA = 0.17%w/v were obtained, given in Figure 9.Consistently, results show the presence of different NP sizepopulation, and the bimodal distribution shows NP averagesizes of 16.4 ± 0.5 and 63.8 ± 8.8 nm that greatly differ fromresults with CGA > 1.00%w/v. Additionally, the larger aggregatespossess various different shapes, attributed to aggregation.Henceforth, the two-step growth accounting for the kineticprofiles, which was fitted with the adapted bilogistic model,agrees with TEM analyses: AuNP aggregate with GAconcentration lower than 0.1%w/v.

Proposed Model for the Formation of GA-StabilizedPEDOT-AuNP Nanocomposites. The kinetic study along withTEM analyses evidenced the formation of PEDOT-AuNPs/GAnanocomposites, which exhibit different characteristics withvarying concentration of the stabilizer GA, as illustrated inScheme 2. For CGA > 1.00%w/v, a classical two-step mechanism(nucleation−growth) is proposed leading to homogeneoussmall-sized AuNPs (below 25 nm), with a unimodal sizedistribution. In this case, with increasing GA concentration (upto 7.00%w/v) the nucleation step becomes slower, and aspredicted and also reported in the literature19 larger-sizedAuNPs are formed. In contrast, for CGA < 1.00%w/v, a novelbilogistic model is proposed with two different growth stages,assigned to growth and aggregation phenomena. Heteroge-neous larger-sized particles were obtained (nearly 100 nm),with a bimodal size distribution, implying that at lowconcentrations (CGA < 1.00%w/v) GA does not preventaggregation. Overall, the optimal concentration of stabilizer toobtain nanocomposites with small-sized, homogeneouslydistributed AuNPs is CGA = 1.05%w/v.The important role of GA in the formation of PEDOT-

AuNPs/GA is irrefutable since the nanocomposites wereobtained solely in the presence of GA. In order to get adeeper insight into the role of GA in stabilizing the as-formedparticles, a detailed study was conducted by means of Ramanspectroscopy.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

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

Table 2. Kinetic Parameters Obtained by Fitting Data in Figure 8 According to the Bilogistic Model, Equation 2, for theFormation of PEDOT-AuNPs/GA

CGA (%w/v) 0.02 0.03 0.17 0.35

C 0.07 ± 0.002 0.13 ± 0.03 0.18 ± 0.03 0.30 ± 0.01Δt1 (s) 115.39 ± 39.80 178.42 ± 36.57 248.37 ± 28.10 432.55 ± 19.66tm1 (s) 113.65 ± 6.75 159.15 ± 5.72 231.79 ± 4.87 468.99 ± 3.69D 0.18 ± 0.009 0.13 ± 0.01 0.09 ± 0.007 0.08 ± 0.006Δt2 (s) 96.43 ± 8.77 157.18 ± 18.71 207.76 ± 19.46 355.61 ± 32.73tm2 (s) 390.30 ± 1.70 517.78 ± 3.34 651.685 ± 4.02 1002.15 ± 9.27kPEDOT (s−1) (2.28 ± 0.15) × 10−3 (1.59 ± 0.18) × 10−3 (1.13 ± 0.11) × 10−3 (3.47 ± 0.65) × 10−4

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp508326j | J. Phys. Chem. C 2014, 118, 25756−2576425761

PEDOT-AuNPs/GA in relation to neat-PEDOT was verified asa function of GA content since this effect, in principle, cannotbe 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 asdescribed in neat-PEDOT (Figure 3). Nevertheless, it wasobserved that the intensity of the Raman signal was highlyenhanced by the amount of incorporated GA in the composite.This effect is indeed intriguing, and some possible explanationsare discussed as follows. One possible explanation is related tothe enhancement of the electronic effect caused by theconformational changes of PEDOT due to the secondarydoping effect from GA. Secondary doping was first describedfor poly(aniline),37 and in general lines, this effect is caused bythe inclusion of large organic molecules within the conductingpolymeric matrix changing drastically its conformation fromcompact to extended coil. By this way, the electric repulsiondecreases among the polymeric backbones, enhancing both thecrystallinity and conductivity of the polymer. In this proposal,GA would be acting as a structural dopant and allied to theneutral form (as observed with the shifting of the CαCβ

(−O) band ∼1437 cm−1), and the resonant effect is enhanceddue to the increase of the absorption spectra in the incident

radiation employed (514 nm), leading to the increase ofintensity of the Raman bands.The interaction between GA and PEDOT could be better

observed by analyzing the Raman spectra in the frequencies ofabout 900−1300 cm−1, as shown in Figure 10(B). The neat-PEDOT spectrum (Figure 10) shows the bands in 990 and1096 cm−1 which correspond to the oxyethylene ring and C−O−C deformations, respectively. For the nanocomposites, thefirst band is shifted to 972 cm−1, and in the C−O−C region, alarge band is encountered at lower proportions of GA with adiscrete shift. At 1.50%w/v and 2.10%w/v of GA, an intense bandat 1136 cm−1 appears which was not found in any other relatedwork in the literature. This band could be associated with aspecific vibrational mode caused by the interaction between theOH and COOH groups of GA interacting with the C−O−Csites by means of strong hydrogen bonding. This kind ofinteraction was previously reported in poly(aniline) compo-sites, by studying the NH band38 but not reported so far forPEDOT-based systems. Finally, the band in 1250 cm−1 isattributed to Cα−Cα inter-ring stretching. For the neat-PEDOTsample, this region is encountered as two bands, which can beexplained by the different doping level of the polymericstructure, alternating between C−C and CC bonds, but forthe nanocomposites, this band is found as a unique band with

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 overthe region of (A) 600−1700 cm−1 and (B) 900−1300 cm−1.

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp508326j | J. Phys. Chem. C 2014, 118, 25756−2576425762

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 thenanocomposites, Au should interact strongly with sulfur sites ofPEDOT. Considering the above observations related to Ramananalysis, it is reasonable to assume that GA interacts withPEDOT-AuNPs through hydrogen bond interactions betweenthe oxyethylene moieties present in PEDOT and multiplefunctional groups (polysaccharide and protein) of GA. Also,favorable electrostatic attraction will occur since at the pH ofthe syntheses GA bears negative surface charge (due to theionization of carboxyl groups) and PEDOT has positive neatcharge, due to its oxidation state. The resulting interaction ofGA with PEDOT-AuNPs diminishes the repulsion amongPEDOT chains, leading to a more extended polymericconformation that allows the formation of NPs with increasedsurface area. When enough GA is provided (CGA > 1.00%w/v)the NPs will grow as large as the amount of GA available fortheir stabilization, in aqueous media. The larger AuNPs inducedby higher nucleation time upon increasing concentration of GA,evidenced by TEM and kinetic studies, can be accounted to amore linear polymeric conformation, whereby enablingnucleation sites that are presumably more available andsomehow furnishing an extended nucleation step. On thecontrary, if the amount of GA is limited (CGA < 1.00%w/v) theonly mechanism available for the stabilization of the NPs is tocoalesce, giving rise to secondary aggregates (large agglomer-ates) (Scheme 2).The novelty of our study comprises the proposition of a

novel bilogistic model to describe the cases in which theamount of stabilizer is not enough to prevent secondaryaggregation (as substantiated with TEM images). We believethe quantitative correlation of kinetic parameters with thestabilizer concentration and NP size (Scheme 2) enriches theknowledge in materials science and furnishes powerful tools forpredicting nanocomposite growth.

■ CONCLUSIONHighly stable aqueous dispersions of the nanocompositePEDOT-AuNPs/GA were obtained by a one-pot synthesisand fully characterized by UV−vis and Raman spectroscopy. Athorough kinetic study was carried out and correlated withTEM analyses, evidencing that the formation of PEDOT-AuNPs/GA nanocomposites is strongly influenced by thestabilizer (GA) concentration. Interestingly, this behavior wasquantified by kinetic parameters, and two different scenarioswere 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 nucleationstep and leads to larger-sized AuNPs; and (ii) CGA < 1.00%w/v, anovel bilogistic model is proposed with two-step growth stages,assigned to aggregation phenomena. This process leads toheterogeneous larger-sized AuNPs, and a lower concentrationof GA facilitates aggregation. Herein, the stabilizer effect on themechanism of AuNP formation was elucidated. We proposethat Au bonds strongly with the sulfur sites of PEDOT, whichinteracts with GA through hydrogen interactions between theoxyethylene moieties present in PEDOT and multiple func-tional groups of GA. Also, favorable electrostatic attractionshould occur between negatively charged GA and PEDOT thathas a positive neat charge. Thus, reducing repulsion amongPEDOT chains leads to a more extended polymeric

conformation that allows the formation of NPs with increasedsurface area. For CGA > 1.00%w/v, the NPs will grow as large asthe amount of GA available for their stabilization. In contrast,for CGA < 1.00%w/v, the NPs tend to coalesce, giving rise tolarge agglomerates. Overall, the optimal concentration ofstabilizer to obtain nanocomposites with small-sized, homoge-neously distributed AuNPs is CGA = 1.05%w/v.AuNP size can be effectively controlled and the nano-

composite properties optimized, which is surely a benchmarkfor the rational and targeted design and engineering of novelnanomaterials. The kinetic approach adopted in this study isinnovative, comprising the first report using the bilogisticmodel to describe AuNP formation. Also, kinetic equationswere fully deduced, and the formation of PEDOT and AuNPscould be analyzed separately. The presented results shouldfurnish future promising studies on how to avoid aggregationprocesses and control size distribution of NPs for variouscomplex systems. Namely, the size of a nanomaterial isintrinsically 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 residuein tannin industries. Thus, what normally would be consideredwaste can be recycled and used as a stabilizer for obtainingstable nanocomposites. We expect similar behavior of GA withother noble metals and conductive polymers. The nano-composites have potential electroactivity, foreseeing futureapplications.

■ ASSOCIATED CONTENT*S Supporting InformationComplementary Raman spectra in red radiation, kinetic results,and TEM images. This material is available free of charge viathe Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: elisaorth@ufpr.br. Fax: +55 41-33613186. Tel.: +5541-33613176.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSAuthors acknowledge the financial support from CNPq,CAPES, Fundacao Araucaria, and CME (Electronic MicroscopyCenter-Universidade Federal do Parana) for microscopyanalyses.

■ REFERENCES(1) Rajesh; Ahuja, T.; Kumar, D. Recent Progress in theDevelopment of Nano-Structured Conducting Polymers/Nanocom-posites for Sensor Applications. Sens. Actuators, B 2009, 136, 275−286.(2) Xiao, Y.; Li, C. M. Nanocomposites: From Fabrications toElectrochemical Bioapplications. Electroanalysis 2008, 20, 648−662.(3) Euliss, L. E.; DuPont, J. A.; Gratton, S.; DeSimone, J. ImpartingSize, Shape, and Composition Control of Materials for Nanomedicine.Chem. Soc. Rev. 2006, 35, 1095−1104.(4) Zarras, P.; Anderson, N.; Webber, C.; Irvin, D. J.; Irvin, J. A.;Guenthner, A.; Stenger-Smith, J. D. Progress in Using ConductivePolymers as Corrosion-Inhibiting Coatings. Radiat. Phys. Chem. 2003,68, 387−394.(5) Mortimer, R. J.; Dyer, A. L.; Reynolds, J. R. ElectrochromicOrganic and Polymeric Materials for Display Applications. Displays2006, 27, 2−18.

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp508326j | J. Phys. Chem. C 2014, 118, 25756−2576425763

(6) Maia, D. J.; Paoli, M.-A. D.; Alves, O. L.; Zarbin, A. J. G.; Neves,S. d. Sintese De Polimeros Condutores Em Matrizes So lidasHospedeiras. Quim. Nova 2000, 23, 204−215.(7) Mathiyarasu, J.; Senthilkumar, S.; Phani, K. L. N.; Yegnaraman, V.Pedot-Au Nanocomposite Film for Electrochemical Sensing. Mater.Lett. 2008, 62, 571−573.(8) Crooks, R. M.; Zhao, M.; Sun, L.; Chechik, V.; Yeung, L. K.Dendrimer-Encapsulated Metal Nanoparticles: Synthesis, Character-ization, and Applications to Catalysis. Acc. Chem. Res. 2001, 34, 181−190.(9) Mayer, A. B. R.; Mark, J. E. Poly(2-Hydroxyalkyl Methacrylates)as Stabilizers for Colloidal Noble Metal Nanoparticles. Polymer 2000,41, 1627−1631.(10) Kamyshny, A.; Magdassi, S. Aqueous Dispersions of MetallicNanoparticles: Preparation, Stabilization and Application in Nano-science: Colloidal and Interfacial Aspects. Surfactant Sci. Ser. 2009,147, 747−778.(11) Higgins, T. M.; Moulton, S. E.; Gilmore, K. J.; Wallace, G. G.;Panhuis, M. i. h. Gellan Gum Doped Polypyrrole Neural ProstheticElectrode Coatings. Soft Matter 2011, 7, 4690−4695.(12) Castagnola, E.; Ansaldo, A.; Maggiolini, E.; Angotzi, G. N.;Skrap, M.; Ricci, D.; Fadiga, L. Biologically Compatible NeuralInterface to Safely Couple Nanocoated Electrodes to the Surface of theBrain. ACS Nano 2013, 7, 3887−3895.(13) Grein, A.; Silva, B. C. d.; Wendel, C. F.; Tischer, C. A.;Sierakowski, M. R.; Moura, A. B. D.; Iacomini, M.; Gorin, P. A. J.;Simas-Tosin, F. F.; Riegel-Vidotti, I. C. Structural Characterization andEmulsifying Properties of Polysaccharides of Acacia Mearnsii De WildGum. Carbohydr. Polym. 2013, 92, 312−320.(14) Verbeken, D.; Dierckx, S.; Dewettinck, K. Exudate Gums:Occurrence, Production, and Applications. Appl. Microbiol. Biotechnol.2003, 63, 10−21.(15) Lee, J. S.; Choi, Y.-J.; Wang, S.-J.; Park, H.-H.; Pyun, J. C.Characterization of Au-Metal Nanoparticle-Hybridized Poly (3,4-Ethylenedioxythiophene) Films for Electrochromic Devices. Phys.Status Solidi A 2011, 208, 81−85.(16) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. TheOptical Properties of Metal Nanoparticles: The Influence of Size,Shape, and Dielectric Environment. J. Phys. Chem. B 2003, 107, 668−677.(17) Yan, Y.; Such, G. K.; Johnston, A. P. R.; Best, J. P.; Caruso, F.Engineering Particles for Therapeutic Delivery: Prospects andChallenges. ACS Nano 2012, 6, 3663−3669.(18) Kumar, S. S.; Kumar, C. S.; Mathiyarasu, J.; Phani, K. L.Stabilized Gold Nanoparticles by Reduction Using 3,4-Ethylenediox-ythiophene-Polystyrenesulfonate in Aqueous Solutions: Nanocompo-site Formation, Stability, and Application in Catalysis. Langmuir 2007,23, 3401−3408.(19) Wang, Y.; Zeiri, O.; Neyman, A.; Stellacci, F.; Weinstock, I. A.Nucleation and Island Growth of Alkanethiolate Ligand Domains onGold Nanoparticles. ACS Nano 2012, 6, 629−640.(20) Chiu, W. W.; Travas-Sejdic, J.; Cooney, R. P.; Bowmaker, G. A.Studies of Dopant Effects in Poly(3,4-Ethylenedioxythiophene) UsingRaman Spectroscopy. J. Raman Spectrosc. 2006, 37, 1354−1361.(21) Stavytska-Barba, M.; Kelley, A. M. Surface-Enhanced RamanStudy of the Interaction of Pedot:Pss with Plasmonically ActiveNanoparticles. J. Phys. Chem. C 2010, 114, 6822−6830.(22) Garreau, S.; Duvail, J. L.; Louarn, G. SpectroelectrochemicalStudies of Poly(3,4-Ethylenedioxythiophene) in Aqueous Medium.Synth. Met. 2001, 125, 325−329.(23) Tran-Van, F.; Garreau, S.; Louarn, G.; Froyer, G.; Chevrot, C. AFully Undoped Oligo(3,4-Ethylenedioxythiophene): SpectroscopicProperties. Synth. Met. 2001, 119, 381−382.(24) Chiu, W. W.; Travas-Sejdic, J.; Cooney, R. P.; Bowmaker, G. A.Studies of Dopant Effects in Poly(3,4-Ethylenedioxythiophene) UsingRaman Spectroscopy. J. Raman Spectrosc. 2006, 37, 1354−1361.(25) Daniel, M.-C.; Astruc, D. Gold Nanoparticles: Assembly,Supramolecular Chemistry, Quantum-Size-Related Properties, and

Applications toward Biology, Catalysis, and Nanotechnology. Chem.Rev. 2004, 104, 293−346.(26) Njoki, P. N.; Luo, J.; Kamundi, M. M.; Lim, S.; Zhong, C.-J.Aggregative Growth in the Size-Controlled Growth of MonodispersedGold Nanoparticles. Langmuir 2010, 26, 13622−13629.(27) Richards, V. N.; Rath, N. P.; Buhro, W. E. Pathway from aMolecular Precursor to Silver Nanoparticles: The Prominent Role ofAggregative Growth. Chem. Mater. 2010, 22, 3556−3567.(28) Ruitenberg, G.; Petford-Long, A. K.; Doole, R. C. Determi-nation of the Isothermal Nucleation and Growth Parameters for theCrystallization of Thin Ge2sb2te5 Films. J. Appl. Phys. 2002, 92, 3116−3123.(29) Avrami, M. Granulation, Phase Change and MicrostructureKinetics of Phase Change. Iii. J. Chem. Phys. 1941, 9, 177−184.(30) Avrami, M. Kinetics of Phase Change. Ii Transformation-TimeRelations for Random Distribution of Nuclei. J. Chem. Phys. 1940, 8,212−224.(31) Avrami, M. Kinetics of Phase Change. I General Theory. J.Chem. Phys. 1939, 7, 1103−1112.(32) Chuang, H.-J.; Chen, H.-L.; Huang, B.-H.; Tsai, T.-E.; Huang,P.-L.; Liao, T.-T.; Lin, C.-C. Efficient Zinc Initiators Supported byNno-Tridentate Ketiminate Ligands for Cyclic Esters Polymerization.J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 1185−1196.(33) Matyjaszewski, K.; Patten, T. E.; Xia, J. Controlled/“Living”Radical Polymerization. Kinetics of the Homogeneous Atom TransferRadical Polymerization of Styrene. J. Am. Chem. Soc. 1997, 119, 674−680.(34) Shishov, M. A.; Moshnikov, V. A.; Sapurina, I. Y. Self-Organization of Polyaniline During Oxidative Polymerization:Formation of Granular Structure. Chem. Pap. 2013, 67, 909−918.(35) Meyer, P. S.; Yung, J. W.; Ausubel, J. H. A Primer on LogisticGrowth and Substitution. Technol. Forecast Soc. 1999, 61, 247−271.(36) Meyer, P. S. Bi-Logistic Growth. Technol. Forecast Soc. 1994, 47,89−102.(37) Macdiarmid, A. G.; Epstein, A. J. The Concept of SecondaryDoping as Applied to Polyaniline. Synth. Met. 1994, 65, 103−116.(38) Quintanilha, R. C.; Orth, E. S.; Grein-Iankovski, A.; Riegel-Vidotti, I. C.; Vidotti, M. The Use of Gum Arabic as ‘‘Green’’ Stabilizerof Poly(Aniline) Nanocomposites: A Comprehensive Study ofSpectroscopic, Morphological and Electrochemical Properties. J.Colloid Interface Sci. 2014, 434, 18−27.

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp508326j | J. Phys. Chem. C 2014, 118, 25756−2576425764