Article pubs.acs.org/Langmuir
Quartz Crystal Microbalance-Based Evaluation of the Electrochemical Formation of an Aggregated Polypyrrole Particle-Based Layer Deivis Plausinaitis,† Vilma Ratautaite,† Lina Mikoliunaite,† Linas Sinkevicius,† Almira Ramanaviciene,‡ and Arunas Ramanavicius*,† †
Faculty of Chemistry, Department of Physical Chemistry and ‡Faculty of Chemistry, NanoTechnas − Centre of Nanotechnology and Materials Science, Department of Analytical and Environmental Chemistry, Vilnius University, Naugarduko 24, LT-03225 Vilnius, Lithuania ABSTRACT: Electrochemical quartz crystal microbalance (EQCM) was used for the evaluation of conducting polymer polypyrrole (Ppy), which was formed by a sequence of potential pulses on a Au-plated EQCM disc. The Ppy layer was obtained from freshly prepared polymerization solution consisting of pyrrole that was dissolved in phosphate buffer. The main aim of the study was to determine some aspects of the Ppy layer formation process. The polymerization process was estimated by EQCM and chronoamperometry. The Cottrell equation was used for the integration of total charge that was passing through the electrochemical cell during the formation of the Ppybased layer. It was found that the charge of the electrical double layer, which was estimated while applying an Anson plot, is negative. From this observation, it could be assumed that the pyrrole oxidation process could be well described by principles of heterogeneous kinetics. The negative value of the electrical double layer was the result of a charge-transfer restriction. This restriction of charge transfer could occur due to partial blocking of the electrode surface by an aggregated Ppy particle-based layer. Quartz crystal motional resistance (R) was taken into account during this research. Ppy layer formation is represented schematically on the basis of the obtained experimental results and analytical data.
1. INTRODUCTION Conducting polymer polypyrrole (Ppy)-based structures can be formed by chemical and electrochemical methods. Both of these methods have some advantages and disadvantages. Therefore, they are applied for different applications. Chemical polymerization is favorable if colloidal solutions of Ppy-based nanoparticles are required.1,2 Electrochemical polymerization enables the deposition of thin layers of Ppy on electrodes, which could be applied in the design of sensors and biosensors.3−6 These sensors can be studied by various analytical methods, e.g., surface plasmon resonance,7 X-ray photoelectron spectroscopy,8 electrochemical impedance spectroscopy,9 and quartz crystal microbalance.10 For all of these applications, mechanical properties and adhesion of the formed Ppy layer on the electrode surface are very important.11−13 The influence of the electrodeposition potential on the microstructure of the polypyrrole layer was evaluated by electrochemical quartz crystal microbalance (EQCM), and it was concluded that the most dense Ppy films were formed when Ppy was electropolymerized at a relatively low constant potential of +0.55 V to +0.80 V vs SCE.14 In some other research, the understanding of the relationship between the polymerization potential and the formed Ppy chain length was extended, and it was determined that Ppy oligomers consisting of 8−16 units of monomer are obtained at a low current density of 66.7 μA/cm2.15,16 Ppy oligomers consisting of 32−64 © 2015 American Chemical Society
units of monomer are obtained galvanostatically at a higher current density of 366.7 μA/cm2.15,16 Cross-linked Ppy networks are formed if a relatively high electrode potential is applied.15,17 In other research, it was determined that there is a strong correlation between the Ppy layer’s micromorphology and the ion exchange velocity, and this correlation directly depends on the chemical structure of Ppy obtained during polymerization at different current densities.12 A potentialpulse-based polymerization technique was already used in our previous research,3,18,19 but those studies were more focused on the application of the obtained Ppy layer to the analytical purposes studied and the polymerization mechanism was not analyzed. A polymerization technique based on potential pulses is especially efficient in the formation of Ppy layers with entrapped proteins,19,20 single-stranded DNA (ssDNA),4 or low-molecular-weight organic compounds.3 The entrapment of (i) proteins is mostly required for the development of enzymatic biosensors or immunosensors,20 (ii) ssDNA and DNA-aptamers for DNA sensors,4 and (iii) low-molecularweight organic compounds for molecularly imprinted polymerbased sensors.3 The advantage of a potential-pulse-based technique is the possibility to increase the concentration of Received: November 4, 2014 Revised: February 23, 2015 Published: February 23, 2015 3186
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Figure 1. Schematic layout of the potential pulse profile, which was applied for the deposition of the Ppy layer (a, line 1). Change in current during the course of Ppy deposition on the Au-electrode surface (a, line 2). Resonance frequency Δf during the course of Ppy deposition (b). Polymerization of pyrrole performed by potential cycling of the (c) 1st, 2nd, and 15th potential cycles at a potential scan rate 0.1 V/s. Chronoamperograms were registered during the first pulse of 1.1 V vs Ag/AgCl (d): the dashed line represents the chronoamperogram registered in neat PBS, and the solid line represents the chronoamperogram registered in a solution of 50 mM pyrrole in PBS. In all cases, the Ppy deposition was performed in 50 mM pyrrole solution in PBS. from Hewlett-Packard (Germany) and a six-channel two-position valve from Rheodyne (USA). All chemicals were of analytical grade if not stated otherwise. Pyrrole was purchased from Fluka (USA). Phosphate-buffered saline solution (PBS) was prepared by mixing 50 mM NaH2PO4·H2O from Fluka (Germany) and 50 mM Na2HPO4·12 H2O from Roth (Germany) with 100 mM Na2SO4 from Reachim (Russia) until an adjusted pH of 7.0 was reached. The polymerization mixture was prepared in PBS, pH 7.0, with 50 mM pyrrole. 2.2. Electrochemical Formation of a Polypyrrole Layer. Before electrochemical polymerization, the EQCM cell was washed with PBS and the electrode surface was electrochemically cleaned and activated in order to improve the adhesion of the formed polymer to the surface. Electrochemical cleaning was performed in PBS according to the following procedure: the potential was cyclically swept between −0.2 and 1.5 V vs Ag/AgCl at a 100 mV/s sweep rate. Electrochemical cleaning of the electrode was continued until steady cyclic voltammograms (CVs) were observed; steady CVs were observed after approximately 10 potential cycles. The electrochemical polymerization of pyrrole was performed in a polymerization mixture containing 50 mM pyrrole dissolved in PBS. Before the start of polymerization, the resonance frequency Δf and motional resistance R of the EQCM were monitored. Electrochemical polymerization was started only when Δf became stabile: the variation of the signal within 10 min was less than ±1 Hz. The electrochemical formation of the Ppy layer was performed by a sequence of potential pulses of 1.1 V vs Ag/AgCl for 10.0 s and of 0 V vs Ag/AgCl for 1.0 s. In total, 300 potential pulses were applied. 2.3. Dynamic Light Scattering (DLS). Malvern ZetasizerNano ZS (Malvern, Herrenberg, Germany) equipped with a 633 nm He−Ne laser and operating at an angle of 173° was used for dynamic light scattering (DLS) measurements. The data were collected and analyzed by Dispersion Technology Software version 6.01 from Malvern. All measurements were performed at a position of 4.65 mm from the cuvette wall with an automatic attenuator and at a controlled temperature of 25 °C.
pyrrole monomer in the pre-electrode environment during the period when the electrode potential decreases below the potential, which is required for the initiation of pyrrole polymerization.20 Moreover, the potential-pulse-based technique allows significant enrichment of the Ppy layer by entrapped biological compounds (e.g., enzymes, antibodies, ssDNA, or low-molecular-weight organic compounds). In this way, the consumption of expensive biomaterials can be reduced.4,19,20 Therefore, the potential-pulse-based technique seems to be the most attractive for the entrapment of materials, which are used in the design of sensors or biosensors. For this reason, the polymerization mechanism of the potential-pulse-based polymerization technique is especially interesting.20 The main aim of our research was to evaluate some aspects of the potential-pulse-sequence-based electrochemical deposition of Ppy within a flow-through EQCM cell. Most of the attention was focused on the evaluation of polymerization dynamics, and at the same time the motional resistance (R) was analyzed. The motional resistance R data was related to the mechanical properties of the Ppy.
2. EXPERIMENTAL SETUP 2.1. Instrumentation and Reagents. For the evaluation of mass changes, the EQCM from Maxtek (USA) controlled by EQCM data acquisition software from Maxtek (USA) was applied. Electrochemical polymerization was performed with a potentiostat/galvanostat/ZRA reference 600TM from Gamry (USA), which was controlled by Gamry framework software, version 5.30 from Gamry (USA). A 5 MHz goldcoated quartz crystal, model SC-501-1 from Maxtek (USA), was used for EQCM measurements. The geometric area of the working electrode surface was equal to 1.37 cm2, and the active oscillating region was 0.342 cm2. All investigations were performed in a homemade flow-through EQCM cell. The liquids to the EQCM cell were handled by a binary HP 1100 HPLC pump (model G1312A) 3187
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pyrrole monomer due to partial blockage of the surface by formed polypyrrole. To evaluate the rate of electrochemical polymerization, the obtained data were integrated. Integration of the obtained current data was performed for every single potential pulse to estimate the total charge (Q) that passed during a potential pulse. Q was calculated from an integrated form of the Cottrell equation (eq 1)22
3. RESULTS AND DISCUSSION During the electrochemical polymerization of pyrrole, the current (I), the change in resonance frequency (Δf), and the motional resistance (R) of the EQCM were registered and evaluated as analytical signals. Figure 1a represents Δf and I from the beginning of polymerization until 300 s (it is equal to 27 potential pulses). Later (from 28 to 300 pulses, data not shown), the polymerization slowed down, and the speed of changes in Δf decreased to 0.07 Hz/s. It was observed that Δf changed without characteristic jumps, which mostly are related to the unequal formation of Ppy during the potential pulses. A plausible explanation of this effect is related to the electrochemical initiation of cation-radicalbased polymerization, which starts after the formation of cation radicals at high electrode potential and still propagates when the electrode potential is decreased to a low potential level. The polymerization of pyrrole starts during the first potential pulse of 1.1 V vs Ag/AgCl. In this step, the oxidation of the monomer molecules, which were adsorbed on the Au electrode surface before the high-potential pulse was applied, is initiated. Hence, during the initial stages of high-potential pulses, adsorbed cation radicals are formed on the Au electrode surface, and then they initiate the formation of a polymeric backbone, which acts as active sites for polymer elongation and the aggregation of other formed oligomeric/polymeric chains. During the next polymerization stage, when a low potential is applied, the recombination of formed cation radicals is taking place. A similar electrochemical polymerization model was described for thiophene on a Pt electrode.15 A low-potential pulse of 0 V vs Ag/AgCl was used to ensure the recovery of pyrrole concentration at the electrode surface because in some other research it was stated that the recovery of monomer concentration is very important to the formation of dense and concise Ppy layers.21 3.1. Evaluation of Electrochemical Polymerization Data. As observed from the dependence of Δf vs time (Figure 1), the maximal shift, which is equal to 85 Hz, was observed during the first high-potential pulse. The increase in current during the same potential pulse was 5 times higher in comparison to that observed during the second potential pulse (Figure 1a). This effect could be related to properties of the electrode surface. Before the first potential pulse, the electrode surface was electrochemically cleaned in order to improve the adhesion of the polymer. During electrochemical polymerization, the Au-electrode surface was partially blocked, thereby decreasing the polymerization rate. A similar effect was observed in previously published research, where the electrochemical polymerization of pyrrole during a single anodic potential pulse was described.14 In contrast to our study, the referenced experiment used an anodic potential pulse, which lasted for 10 min.14 The results of the referenced experiment further indicated that the polymerization rate in the initial stage is very high but that it slows within the first 100 s as indicated by shifts in both (i) I vs time and (ii) Δf vs time curves. In our case, the decrease in the polymerization rate was observed after the first 20 pulses, which is related to the hindered diffusion of
Q = 2nFACr
Dr t + Q dl + Q ads π
(1)
where the slope of the function calculated using linear eq 1 is related to the concentration of pyrrole (Cr), the number of electrons in the oxidation process (n), the diffusion coefficient of pyrrole (Dr), and the active surface of the electrode (A). The intercept of eq 1 is the sum of the charge of the electrical double layer (Qdl) and charge induced by adsorbed ions (Qads). An Anson plot, which represents the relationship of Q to t1/2 for the 1st, 2nd, 3rd, 10th, and 15th potential pulses, is represented in Figure 2. The relationship of Q to t1/2 was fitted by linear regression, and the parameters of the corresponding linear equations are listed in Table 1.
Figure 2. Relationship of total charge Q to t1/2 integrated according to the integrated form of the Cottrell equation for the 1st, 2nd, 3rd, 10th, and 15th potential pulses registered during the electrochemical polymerization of pyrrole on the Au electrode.
Table 1. Parameters of the Equation for the Relationship Q vs t1/2 Calculated Using the Integrated Form of the Cottrell Equation no. of potential pulse
1st
2nd
3rd
10th
15th
slope, μC/s intercept, μC R2
987.57 −309.35 0.996
419.14 −273.76 0.997
282.85 −219.33 0.992
116.28 −101.54 0.984
88.90 −79.68 0.981
1/2
From the results listed in Table 1, it is obvious that the parameters change significantly from the 1st to the 10th pulse, but after the 10th pulse, the change in the parameters slowed. It is accepted that the parameters became steady after the 15th potential pulse and therefore the parameters of the next potential pulses are not presented. As presented in Table 1, the intercept calculated by the integrated Cottrell equation is negative. From this, it could be assumed that the pyrrole oxidation process is occurring according to principles described by heterogeneous kinetics.23,24 Moreover, eq 1 describes only kinetics of diffusion. 3188
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pyrrole is a constant parameter. According to this assumption, a rapid increase in ti1/2 during the first four potential pulses is related to the decrease of the oxidation rate constant (ko). A plausible explanation for the decrease of k o in the heterogeneous oxidation reaction is based on restricted charge transfer. This restriction is induced by partial blocking of the electrode surface with the formed Ppy layer. 3.2. Evaluation of EQCM Data. The function of the QCM sensor can be described by the Butterworth−van Dyke equivalent circuit.26 In this model, the motional resistance (R) describes the dissipation of mechanical vibration energy.27 This dissipation of energy occurs due to the dispersion of vibrational energy to acoustical waves propagating to the surrounding medium. The relationship of R to the density (ρL) and viscosity (ηL) of the surrounding medium was described by Martin et al.:28
Therefore, in the case of heterogeneous kinetics, the Cottrell equation could be modified as it is presented in eq 225 ⎛2 Q = 2nFACrko⎜ ⎝H
t 1 ⎞ − 2⎟ π H ⎠
(2)
where k o is the charge-transfer rate constant of the heterogeneous oxidation reaction and H was calculated according to eq 3 H=
k kr + o Do Dr
(3)
where kr is the rate of the reduction reaction and Do is the diffusion coefficient of the oxidized form of the pyrrole. The electrochemical polymerization occurs only at a high positive potential (1.1 V). Therefore, the first part of eq 3 can be ignored, and thus it can be assumed that the reduction process rate at such a high electrode potential is equal to 0. Then eq 2 can be rearranged: Q = 2nFACr
Dr t nFACrDr − π ko
R=
πDr 2ko
πρL ηL fs μq ρq
+ Ro (6)
The parameters of the piezoelectric quartz crystal in eq 6 were applied as follows: the shear stiffness (μq) was 2.95 × 1010 Pa, the electromechanical coupling factor (K2) was 7.74 × 10−3, and the density of quartz (ρq) was 2.65 g·cm−3. Other parameters (n, fs, C0, and R0) are characteristic of the particular QCM sensor, which is applied in this research. In this case, these parameters were the following: the harmonic number (n) was equal to 1 because only the first harmonic was evaluated; the resonance frequency of the first harmonic ( fs) was 5.005 MHz; the static capacitance (C0) was 4.04 pF; and the unperturbed (static) sensor motional resistance (R0) was 8.9 Ω. During the electrochemical polymerization of pyrrole, both parameters Δf and R were registered. The registered shift of R in time is represented in Figure 4 as a solid line.
(4)
It was found that the intercept of the linear equation Q = f(t1/2) is negative. This result confirms that eq 4 is more suitable for the evaluation of the kinetics of the pyrrole electrochemical polymerization. The ratio of the first intercept to the slope is equal to ti1/2, which is represented on the x axis of the Anson plot (Figure 2):
ti =
n 8K 2C0
(5) 1/2
Figure 3 presents the shift of ti in time during the application of a large number of potential pulses. In this plot,
Figure 3. Ratio of first intercept ti1/2 vs duration of electrochemical Ppy deposition.
Figure 4. Motional resistance R and thickness of the Ppy layer calculated using the Sauerbray equation and registered Δf, taking into account that the density of Ppy is 1.25 g·cm−3.
only the first 27 potential pulses are represented. This relationship could be divided into several steps: the 1st step, which lasted from 1 until the 5th potential pulse and is characterized by a rapid increase in ti1/2 (Figure 3, the interval from tb to tc); the 2nd step, which starts from the 5th potential pulse and lasts until the 15th potential pulse and is described by an insignificant shift in ti1/2 (Figure 3, the interval from tc to td); and the 3rd step, which is described by a slow decrease in ti1/2 and is valid until the end of the experiment (Figure 3, the interval from td). The Dr value in eq 5 is the pyrrole diffusion coefficient in the buffer solution. We assume that the diffusion coefficient of the
It was found that the most significant shift of R during polymerization was obtained within the time period ranging from 0 until 150 s, which is the framework between the 1st and 15th potential pulse. The increase in R was found to be 8.5 Ω. During the following potential pulses, the shift in R was very low and it decreased by 0.2 Ω (Figure 4, the interval from td) until the end of polymerization. The parameter ρL × ηL, which is a characteristic of the surrounding medium, was calculated using eq 6. The parameter at up to time 0 was found to be 1.13 × 10−2 g2·cm−4·s−1, which 3189
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Figure 5. Most important steps of Ppy electrochemical formation on the Au-based electrode surface.
“island”-based structure, which is deposited on the surface (Figure 5c); during the next phase, the islands of Ppy merge and a monolayer consisting of Ppy particles is formed (Figure 5d); during the other phases of electrochemical formation of the Ppy coating, the multilayered structure, which consists of Ppy particles, is obtained (Figure 5e). According to the data, the initial step of Ppy layer formation can be reasonably visualized as the aggregation of pyrrole-based emulsion nanodrops (Figure 5a). The formation of such pyrrole-based emulsion nanodrops was observed by DLS. It was determined that in the solution used for the electrochemical formation of the Ppy layer, pyrrole emulsion nanodrops of 80 nm diameter dominate. Therefore, when in the flow-through EQCM cell the PBS was replaced by polymerization solution containing 50 mM pyrrole dissolved in PBS, the surface of the Au electrode, which at the same time is an active part of the EQCM sensor, was partially coated with nanodrops of pyrrole emulsion (Figure 5b). After the replacement of PBS with polymerization solution containing 50 mM pyrrole, an increase in Δf by 12 Hz was registered. The Δf of the EQCM sensor increased because of partial replacement of the water, which initially was located on the Au sensor surface, by the pyrrole. The values of Δf increased because water and pyrrole have different viscosities and densities.31 During the electrochemical oxidation of pyrrole, which is adsorbed on the Au-electrode surface, polymerization was initiated (Figure 5c) and therefore the viscosity of the surface layer increased again. This caused an increase in quartz crystal motional resistance (R). A similar effect, which is related to QCM signal dissipation, was observed when liposomes were adsorbed on a QCM sensor.32,33 During next polymerization steps, new pyrrole-based emulsion particles were adsorbed on the positively charged Au surface, and by passing electrical charge, they also became polymerized. Therefore, a further change in quartz crystal motional resistance (R) was observed due to the increased viscosity of structures deposited on the EQCM sensor surface. The increase in R stopped and the parameter reached a steady state only when Ppy nanoparticles covered the whole electrode surface (Figure 5d). It should be noted that the value of ti1/2 (Figure 3) reached its maximum at the same time, which indicates that the heterogeneous oxidation reaction rate constant (k0) is minimized. Any further polymerization phases (Figure 5e) did not change the roughness of the surface
is similar to the value of the same parameter for water (Figure 4, the interval before tb). During the course of electrochemical polymerization, the parameter ρL × ηL has increased by 5 × 10−4 g2·cm−4·s−1 after 150 s. During the evaluation of the QCM results, it was assumed that parameter ρL is a constant. Therefore, the shift in parameter ρL × ηL is dependent only on viscosity changes in the surrounding medium. This assumption was based on the estimated density of Ppy, which could be in the range from 1.01 to 1.45 g·cm−3 depending on doping and polymer-formation conditions.29 These values of ρL are very similar to the parameters of water and most water-based solutions. The dynamic viscosity of the polymer, which could be assumed to be a solid material, can reach a value of 1012 Pa·s, which is much higher than that of the water and/or the most of water-based solutions (10−3 Pa·s). However, according to the data obtained by the QCM sensor in this study, the increase in parameter ρL × ηL is relatively low: approximately 4.5% in comparison to that registered in the initial state before polymerization started. This fact can be explained by some sensing properties of the QCM: under standard conditions, the QCM sensor is sensitive to changes in the surrounding medium at a distance of ∼200 nm.26,30 Therefore, it can be concluded that during the electrochemical polymerization of pyrrole on the QCM sensor surface the thickness of the Ppy layer is lower than the range of ∼200 nm, which limits the QCM sensitivity. The mass of deposited Ppy was calculated using the Sauerbray equation, taking into account that the sensitivity constant of the QCM sensor is equal to 5.6 × 10−2 Hz·ng−1· cm2. The density of the Ppy film (1.25 g·cm−3) was an average value calculated from the literature24 and was used for the calculation of the increase in Ppy layer thickness during electrochemical polymerization (Figure 4, dotted line). 3.3. Evaluation of the Ppy-Layer-Formation Mechanism. As is obvious from Figure 4, the change in R became significantly less after 150 s. We believe that this effect was observed due to the termination of Ppy layer formation. As a consequence, the viscosity change at the sensor surface has also decreased. A possible model for the Ppy electrochemical formation mechanism on the Au electrode, which is part of the QCM sensor, was designed (Figure 5) on the basis of these results. According to this mechanism, the formation of the Ppy layer could be divided into several phases: at the beginning, Ppy does not form a complete layer and could be described as an 3190
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Figure 6. AFM images of the Ppy layer and gold surface before polymerization (a); height scale 140 nm (b); height scale 20 nm, with the dimensions of both (a and b) images being 2 × 2 μm2. Statistical distribution of surface roughness (c) and cross-sectional topography (d) of Ppy layer (black line) and gold surface (gray line). Arrows in pictures a and d indicate the same pinhole, which reaches the Au layer of the electrode.
layer roughness is 37 nm (Figure 6c). The AFM results published by other researchers also revealed that Ppy-based layers are rough and structured.34,35 The average roughness value (RMS) of the Ppy-based layer, which was calculated from AFM investigation (Figure 6a), was 2.61 nm, while the RMS of the clean gold electrode (Figure 6b) was 0.744 nm. After the formation of the Ppy-based layer, the RMS has increased 3.5fold and at the same time the QCM resonance frequency (Δf) has decreased by 289 Hz. Martin et al. have performed comparative research on the QCM resonance frequency (Δf) in water before and after the roughening of the QCM-sensor surface, and it was found that when the surface roughness has increased from 10 to 243 nm, then Δf has decreased by 1000 Hz.28 This decrease in Δf was attributed to the influence of “trapped liquid” on the resonance frequency. The trappedliquid-based model could be applied to explain the decrease in Δf during the formation of a phospholipide-based layer.36 Therefore, we believe that a similar trapped-liquid effect may have a significant influence on the decrease in Δf, which is observed during the formation of the Ppy layer. The model, which is representing the formation of an aggregated particlebased Ppy layer on the surface of the Au electrode of the EQCM sensor, is presented in Figure 7. This model explains why the most significant alteration of Δf is observed during the initial phase of Ppy layer formation: a significantly higher amount of trapped liquid is embedded between dispersed nanoparticles in the Ppy-based structure (Figure 7a) in comparison to the amount of water which is trapped in a
markedly; this assumption is based on the viscosity, which remained constant as indicated by the stabilization of parameter R, which remained almost unchanged after the 15th highpotential pulse. In this way, the thickness of the previously calculated Ppy layer at the beginning of R stabilization conditions can be interpreted as an averaged diameter (h) of the Ppy particles, which are composing the layer formed on the Au-electrode surface. (In Figure 5c, the thickness of the Ppy layer is marked as h.) The Ppy particle diameter h in Figure 4 is in the interval from 12 to 32 nm, taking into account that the Ppy density ranges from 1.01 to 1.45 g·cm−3.29 A comparison of measurement results registered by DLS and those calculated from QCM data at the point when parameter R reaches steadystate conditions shows that the QCM-based measurement provides an approximately 3-fold-lower value. There are several plausible explications for this: when the DLS method is applied, the so-called hydrodynamic particle size is determined, which is much larger than the real diameter of a nanoparticle.1 On the other hand, during polymerization the initial volume of particles, which are forming the emulsion, can be reduced due to a higher density of formed polymer in comparison to that of monomer nanodrops. The surface morphology of the obtained Ppy layer was investigated by AFM. The AFM image of the Au electrode coated with a Ppy layer after 300 high-potential pulses is depicted in Figure 6a. The grainlike structure of the Ppy layer was formed on the electrode surface. A statistical evaluation of the AFM image revealed that the dominant height of the Ppy 3191
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observed during the first phase of the polymerization process. A schematic presentation of the polymerization process consisting of several different phases was suggested on the basis of the experimental and analytical data.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +370 (5) 2336310, +370 (5) 2193115, +370 (5) 2193185. Fax: +370 (5) 233 09 87. E-mail: arunas.
[email protected]. Notes
Figure 7. Model of the QCM sensor during the formation of an aggregated Ppy-particle-based monolayer: (a) at the moment when the very first Ppy particles are formed on the surface and (b) at the moment when the aggregated Ppy-particle-based monolayer is formed. The curvy lines indicate the propagation of mechanical waves to the surrounding medium; the arrows indicate surface motion directions.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was funded by the European Social Fund under the Global Grant measure “Enzymes functionalized by polymers and biorecognition unit for selective treatment of target cells” (NanoZim’s, project no. VP1-3.1-SMM-07-K-02042).
completed Ppy film in which all nanoparticles are touching each other (Figure 7b). Comparing our results with those reported by Martin et al.,28 we have calculated Δf of the QCM sensor when the roughness increased by 2-fold: in our experiment, Δf was 81.6 Hz, whereas in the research by Martin et al.28 it was 41.2 Hz. This difference is based on different principles, which were applied to the formation of roughened structures on a QCM sensor: in our study, Ppy-based polymeric structures were formed occasionally while in the study by Martin et al.28 regular linear structures were formed. Similar results also were obtained by Plausinatis et al. in which regular linear structures were formed and investigated by QCM.37 As reported by Fourati et al.,38 surface features after the deposition of the first molecule on the surface can be amplified during later phases of layer formation, thus leading to increased roughness of the sensor surface. Actually, the formation of polymeric films on substrates is a more complicated phenomenon, and most studies agree on the fact that the first phase of the polymeric film-formation process leads to the appearance of isolated islets on the substrate surface, which rise independently up to a critical thickness, beyond which the islands coalesce and the thin film becomes continuous. The thickness of the formed film mostly depends on the nature of the material and the substrate, the speed of deposition, and temperature.39 It should be noted that doping by counterions, which is a very common issue in the electrochemical formation of conducting-polymer-based layers, was not considered here for the proposed mechanism of aggregated Ppy particle-based layer formation. Moreover, it should also be noted that the oxidation of the formed Ppy layer, which was not considered in our present research, eventually could be evaluated as a separate parameter, which affects the charge that could be accumulated by electrochemical systems.
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REFERENCES
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4. CONCLUSIONS Ppy particle-based layer formation was evaluated by EQCM, and the results of this research illustrated that two parametersthe electrode current and resonance frequency (Δf)are significantly changing during this process. The results obtained during the integration of total charge passing within electrochemical polymerization clarifies that in the course of the polymerization process partial blocking of the electrode surface is observed. Motional resistance R was determined to be the most informative parameter of the system described herein. The most significant shift in R was 3192
DOI: 10.1021/la504340u Langmuir 2015, 31, 3186−3193
Article
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DOI: 10.1021/la504340u Langmuir 2015, 31, 3186−3193
