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In situ Electrochemical Impedance Spectroscopy of Electrostatically Driven Selective Gold Nanoparticle Adsorption on Block Copolymer Lamellae Tom Wagner,†,⊥ Jaroslav Lazar,‡,⊥ Uwe Schnakenberg,*,‡ and Alexander Böker*,§ †

DWI - Leibniz Institute for Interactive Materials, Forckenbeckstraße 50, 52056 Aachen, Germany Institute of Materials in Electrical Engineering 1, RWTH Aachen University, Sommerfeldstraße 24, 52074 Aachen, Germany § Chair for Polymer Materials and Polymer Technology, University of Potsdam & Fraunhofer Institute for Applied Polymer Research (IAP), Geiselbergstraße 69, 14476 Potsdam-Golm, Germany ‡

S Supporting Information *

ABSTRACT: Electrostatic attraction between charged nanoparticles and oppositely charged nanopatterned polymeric films enables tailored structuring of functional nanoscopic surfaces. The bottom-up fabrication of organic/inorganic composites for example bears promising potential toward cheap fabrication of catalysts, optical sensors, and the manufacture of miniaturized electric circuitry. However, only little is known about the time-dependent adsorption behavior and the electronic or ionic charge transfer in the film bulk and at interfaces during nanoparticle assembly via electrostatic interactions. In situ electrochemical impedance spectroscopy (EIS) in combination with a microfluidic system for fast and reproducible liquid delivery was thus applied to monitor the selective deposition of negatively charged gold nanoparticles on top of positively charged poly(2-vinylpyridinium) (qP2VP) domains of phase separated lamellar poly(styrene)-block-poly(2-vinylpyridinium) (PS-b-qP2VP) diblock copolymer thin films. The acquired impedance data delivered information with respect to interfacial charge alteration, ionic diffusion, and the charge dependent nanoparticle adsorption kinetics, considering this yet unexplored system. We demonstrate that the selective adsorption of negatively charged gold nanoparticles (AuNPs) on positively charged qP2VP domains of lamellar PS-b-qP2VP thin films can indeed be tracked by EIS. Moreover, we show that the nanoparticle adsorption kinetics and the nanoparticle packing density are functions of the charge density in the qP2VP domains. KEYWORDS: impedance spectroscopy, block copolymers, nanoparticles, electrostatics, adsorption kinetics

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INTRODUCTION Charged polymers referred to as polyelectrolytes (PEs) represent key-elements in sophisticated “organo-electronic” devices, such as in fuel cells,1 organic light-emitting diodes (smart phone displays), and transparent flexible electrodecoating materials.2,3 Apart, PEs are currently a topic of intense exploration considering implant coatings and controlled drug release from biocompatible PE complexes.4,5 An intriguing potential of PEs is thus their ability to be organized on arbitrarily shaped surfaces rendering these electrostatically charged and thereby enabling selective attachment of oppositely charged building blocks. Many similar applications often require precisely adjusted electronic and ionic conductivity and/or diffusivity in bulk and at interfaces which is a function of the structural composition of the coating material. In this respect, electrochemical impedance spectroscopy (EIS) represents a powerful tool to correlate such charge transfer dynamics with the material morphology, as reported in studies about conductive polymers,6−9 PE-multilayers,10,11 polymers for corrosion protection and membranes.12−14 Moreover, in situ lectin binding dynamics on multivalent glycopolymer brushes © 2016 American Chemical Society

grafted on interdigitated gold electrodes was investigated in an identical system to the one used in this study, showing possibility of scanning different kinetic EIS signal behavior from different layers close to the electrode surface.15 A-B type diblock copolymer films gained significant attention, since vapor-, shear-, or temperature-induced phase separation gives access to a vast variety of highly ordered repetitive nanopatterns on their surface.16 Most common surface features of phase separated diblock copolymer thin films are alternating lamellar stripes and hexagonally ordered dots whose formation depends on the block copolymer’s degree of polymerization N, the interaction parameter between the blocks χ, and the volume fraction f of the respective block.17 Diblock copolymer films are well suited to be utilized as guiding nanotemplates for selective nanoparticle (NP) placement, especially when site-specific interactions are present in either one or both of the polymer domains. Recently, several Received: June 24, 2016 Accepted: September 22, 2016 Published: September 22, 2016 27282

DOI: 10.1021/acsami.6b07708 ACS Appl. Mater. Interfaces 2016, 8, 27282−27290

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Chemical formula of a PS-b-P2VP diblock copolymer. (b) Schematic representation of macroscopic block−chain arrangement in upstanding (hf ≈ 50 nm) lamellar PS-b-P2VP diblock copolymer thin films on an EIS chip electrode. (c) Scanning force microscopy (SFM) height image from a phase separated lamellar PS-b-P2VP thin film covering EIS chip electrodes (z-range: ± 7 nm). (d) Structural formula of two crosslinked and simultaneously quaternized pyridine moieties indicated through dumbbell shaped black spacers in (e). (e) Schematic representation of macroscopic chain structure in quaternized lamellar PS-b-qP2VP diblock copolymer thin films with positively charged qP2VP blocks. (f) SFM height image from a quaternized lamellar PS-b-qP2VP thin film covering EIS chip electrodes (z-range: ± 10 nm). The bright protruding domains in (c) and (f) correspond to the (q)P2VP domains as can be checked from refs 24 and 25.

look in more detail to the morphologies of the neutral PS-bP2VP lamellae and the charged PS-b-qP2VP lamellae. Electrode coverage on EIS chip electrodes with diblock copolymer films is achieved through spin-coating a solution of PS-b-P2VP (2 wt%) in dimethylformamide (DMF). The subsequent exposure to vapors of toluene causes the individual polymer blocks to phase separate into ordered lamellar domains, consisting of alternating domains of polystyrene (PS) and poly(2-vinylpyridine) (P2VP) (Figure 1b) yielding a striped surface structure.33 In the present study, the lamellar PS-b-P2VP films exhibit an interdomain spacing d of about 58 nm and a film thickness hf of approximately 50 nm. The scheme in Figure 1b illustrates the macroscopic polymer chain configuration in vertically oriented diblock copolymer lamellae on the gold electrodes. The polymer chains in the different blocks are slightly elongated as a consequence of the blocks chemical incompatibility and the resulting A-B block interface minimization.17 Here, it is important to note that in contrast to highly porous morphologies as present in membranes or layer-by-layer PEmultilayers, the phase separated PS-b-P2VP block copolymer forms dense films. Thus, they shield and/or insulate the substrates on which they are prepared.32 Electrostatic charging of the P2VP domains in lamellar PS-b-P2VP films is achieved through exposure of the latter to the vapors of symmetric dihalides that penetrate into the film and connect neighboring pyridine moieties in the P2VP blocks via quaternization at the nitrogen34 (Figure 1d,e). Note that the true portion of crosslinks cannot be directly measured and that pyridine moieties can also be singly alkylated without any cross connection to a neighboring pyridine unit. For this reason we mostly refer to “quaternization” instead of “crosslinking”. Such nitrogen quaternization goes along with the formation of positive charges converting the P2VP domains into positively charged polyelectrolytes (referred to as qP2VP domains). During this procedure, the preservation of the lamellar surface structure is indispensable to ensure site-selective AuNP adsorption (Figure 1f). Ishizu and co-workers were among the first to deal with similar systems, reporting an increased tendency for the charged films to swell in water as a consequence of the films ionic character.34 It should be pointed

techniques to control NP deposition have been reported, including specific wetting,18,19 coordinative binding,20 or galvanic displacement.21 Block copolymers with a nitrogen containing block can be quaternized which converts the same into an electrostatically charged PE and as such permits oppositely charged nano-objects to be adsorbed selectively. As for PS-b-qP2VP block copolymer thin films like used here, Lee et al. as well as Oded et al. showed this method to operate successfully with hexagonally arranged micelles and periodic lamellae, respectively.22−25 So far, only few articles were reported dealing with EIS measurements of A-B type diblock copolymers. Balsara and coworkers for instance, address the electronic and ionic conductivity of lithium salt containing poly(ethylene oxide)based block copolymers.26−29 Gosh et al. carried out similar experiments,30 whereas Soboleva et al. investigated the directional dependence of proton conductivity with respect to the diblock copolymer morphology.31 In these studies, however, the diblock copolymers either contain salts to foster ionic conductivity, or fully converted polyelectrolyte blocks and/or electronically conducting blocks while using the diblock copolymer in the form of pressed pellets being clamped between two electrodes for measurement. In contrast to this, we investigate a nanoscaled, charged lamellar PS-b-qP2VP film, as it was reported just recently by Gupta Chandaluri et al.32 Moreover, our main focus is dedicated to use EIS primarily for tracking the time dependent electrostatic AuNP adsorption at the qP2VP|electrolyte interface (as for interface notations, the term “electrolyte” is used as a synonym for “AuNP solution”) rather than concentrating on ionic and electronic conductivity or diffusion in the film bulk. We demonstrate that the selective adsorption of negatively charged AuNPs on positively charged qP2VP domains of lamellar PS-b-qP2VP thin films can indeed be tracked by EIS. Moreover, we show that the nanoparticle adsorption kinetics and the nanoparticle packing density are functions of the charge density in the qP2VP domains.

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RESULTS AND DISCUSSION In order to assign the acquired impedance data to the processes occurring at the interfaces or in the film bulk, it is necessary to 27283


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ACS Applied Materials & Interfaces out that the remaining iodide counterions localized in the qP2VP domains (cleaved from dihalides during quaternization) give rise to an additive contribution to the attractive forces during AuNP assembly as a consequence of entropy driven counterion release.35 These iodide anions, however, are incorporated inside the qP2VP domains which retards their mobility compared to free ions. Moreover, the high content of charge carriers present in the AuNP electrolyte mitigates strong counterion release through charge screening. The latter mentioned circumstances and the rather low charging of the qP2VP blocks led to the assumption that the electrostatic interactions prevail over the entropic interactions caused through counterion release.36 PS-b-qP2VP films with different degrees of quaternization were prepared (Figure 2) in order to monitor the effect that

Figure 3. Time-dependent evolution of the Nyquist plots during in situ EIS measurements of lamellar PS-b-qP2VP films subjected to an H2O/ AuNP solution/H2O injection sequence. θquaternization: (a) 30%, (b) 45%. Insets in (a) and (b) show the high frequency areas.

Figure 2. XPS spectra of N 1s regions from 30% (a) and 45% (b) quaternized lamellar PS-b-qP2VP thin films on EIS chip electrodes. Gaussian fits for N 1s signals (399 eV) are marked by a magenta line and N+1s signals (402 eV) by a blue line.

Calculated from XPS spectra shown in Figure 2. bat-% = atom percentage.

frequency area accounting for the interfacial double layer capacitance.31 For each of the charged films a constant increase of the imaginary part of impedance (Zim) along the first watering and the subsequent AuNP solution injection was found. This represents a clear indication of the capacitive nature of the ionic films.37 Moreover, the increase of Zim goes along with a decrease of the phase angle (θphase) in a range of −20° to approximately −80° at low frequencies (Figure S1), which further substantiates a capacitive behavior of this system.7 These results likewise indicate a loss of interfacial capacitance at the film|electrolyte boundary upon the initial watering and the AuNP solution treatment,37 which correlates with the charge balancing between AuNPs and the qP2VP domains during particle adsorption. During final rinsing, Millipore water removes remaining ions and weakly bound AuNPs that influences the electrical double layer and therefore the low frequency region. This loss of charge carriers goes hand in hand with an increase of the solution resistance RS and the charge transfer resistance Rct of the polymer film (Figure 4) that causes

EIS spectra between 100 kHz and 10 Hz were continuously recorded during the sequential application of water, AuNP solution, and water rinsing to understand and electrically describe the selective AuNP adsorption on PS-b-qP2VP lamellae. Figure 3a and b show the corresponding Nyquist plots recorded at different times during the injection sequence for different strong charged PS-b-qP2VP films. Here, the shape of the obtained curves comprises minor semicircles in the high frequency region associated with bulk resistance and capacitance elements in parallel,10 followed by the low

Figure 4. Simplified Randle’s circuit for an electrode|film|electrolyte stack using a constant phase element to model the PS-b-qP2VP| electrolyte interface.

qP2VP domains of different charge density evoke during the AuNP adsorption process. The differences in the extent of quaternization were achieved by variable exposure times of the block copolymer films to dihalide vapors. The application of a Gaussian fit to the subsequently acquired X-ray photoelectron spectroscopy (XPS) spectra of the N 1s regions enabled to determine the degrees of quaternization (θquaternization) as depicted in the rightmost column of Table 1. Table 1. Contents of Trivalent Nitrogen and Quaternized Nitrogen in Differently Quaternized Lamellar PS-b-qP2VP Films on EIS Chip Electrodesa spectrum

N 1s [at-%]

quaternized N+1s [at-%]b

a b

70 55

30 45

a

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ACS Applied Materials & Interfaces the real part of impedance (Zreal) to grow again (see eq 1). The insets in Figure 3a and b represent the Nyquist plot close ups of the semicircles standing for the properties of the 30% and 40% quaternized films during the water flushes (1st watering and rinsing). Semicircles were not observed during AuNP application, since the real part of the signal decreased while the imaginary part increased strongly. 30% quaternized films exhibited a steady increase of the semicircle during the first watering period which must be related to structural alterations upon swelling of the film. During rinsing, the semicircles strongly diminish, indicating a lower resistive barrier for charge carriers (increased diffusion) through weakly cross-linked 30% quaternized films. In contrast, semicircles of the 45% quaternized films remain more or less unchanged between the end of the first and second watering steps, illustrating the good stability of these stronger cross-linked films. The former conclusions concerning the capacitive behavior of the films and the film|electrolyte interfacial capacitance can additionally be clarified by modeling the electrode|film| electrolyte system with a simplified Randle’s circuit. Figure 4 illustrates the equivalent circuit, assigning individual circuit elements to each part of the system. A corresponding mathematical formulation to determine the impedance ZR‑CPE of the above-mentioned circuit is given by eq 1.38,39 Z R − CPE = R S + − i·

Z(CPE) =

R ct 2

1 + ω ·Z(CPE)2 ·R ct2 ω·Z(CPE)·R ct2

1 + ω 2 ·Z(CPE)2 ·R ct2

1 (iω)n ·Q

(1)

(2) Figure 5. (a) n in eq 2 with respect to measuring time of a charged PSb-qP2VP film. The closer n approximates the value 1 the more capacitive the CPE element is. (b) Monitoring of the capacitance Q (in microcoulombs) with respect to measuring time. In both diagrams the vertical dashed bars indicate the time points of the respective fluid injections.

In this equation, RS denotes the solution resistance, ω the angular frequency of the applied alternating voltage, and Rct the charge transfer resistance, respectively. The real part of the impedance Zreal is represented by the first and the second term of eq 1, whereas the third term accounts for the imaginary part of the impedance Zim. The absence of vertical lines in the Nyquist plots implies that the lamellar PS-b-qP2VP films do not correspond to ideal capacitors. This is due to energetic and structural inhomogeneities of the film, stemming from unevenly distributed charges, macroscopic grain formation, phase separation, and swelling during the exposure to the AuNP solution that allows for ion diffusion. Such inhomogeneities, as well as irregularities on the gold electrodes, suggest the application of a constant phase element (CPE) to model the film|electrolyte interfacial capacitance according to eq 2.38−42,30 Here, Q accounts for capacitance and n to the extent, up to which a CPE resembles to an ideal capacitor, which can be estimated on the basis of the acquired impedance data. When n is plotted versus the measurement time a quick approximation to 1 is observed along the early course of the measurement (Figure 5a). This result indicates that the CPE, represented by Z(CPE) in eq 1 and eq 2, is mainly of capacitive nature. Moreover, the loss of interfacial capacitance at the film| electrolyte boundary defined by Q was fitted to a proposed model (Figure 5b) that confirms the former observed gain of Zim in the Nyquist plots. The introduced electrical model shows the possibility to determinate the main properties of the copolymer film and the bulk electrolyte along the injection sequence. The adsorption of

the AuNPs takes place at the surface of the copolymer and therefore influences mainly the parameters Z(CPE) and Rct visible at the low frequencies and depending mainly on the electrical double layer. High level of Rct in nonfaradaic EIS systems hinders exact fitting. Therefore, impedance amplitude was considered as more exact and stable parameter for the measurement evaluation. For reliable interpretation of the gathered in situ EIS spectra we referred to EIS data recorded at an excitation frequency of 10 Hz for the following experiments which is sensitive to changes of the electrical double layer at the crossing between the electrolyte and the PS-b-qP2VP film covered electrode.15 Additionally, this frequency provided the highest signalintensity changes toward adhesion of AuNPs (Figure S2) while being reproducible and delivering comprehensive results. The synthesized citrate stabilized AuNP solution was filtered and diluted with Millipore water in a ratio of 1:2 prior to use. This dilution was found to be well suited to follow the changes in impedance (|Z|) at the PS-b-qP2VP|electrolyte interface. In order to warrant that the later observed changes in impedance can be clearly attributed to the presence of charges in the PS-bqP2VP films, a subsidiary EIS measurement of a nonquaternized lamellar PS-b-P2VP film was carried out before 27285


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Figure 6. (a−c) In situ EIS spectra (|Z| vs time) collected during H2O/AuNP solution/H2O injection sequence across neutral PS-b-P2VP lamellae (a) and charged PS-b-qP2VP lamellae at a frequency of 10 Hz. θquaternization: (a) 0%, (b) 30%, (c) 45%. Dotted black lines in (a−c) represent the time points of the respective fluid injections. (d−f) Corresponding SEM images of EIS chips taken after EIS measurements and water rinsing showing AuNPs as white dots. The bright striations in (d) and (e) represent the P2VP and qP2VP domains, respectively.

the initial watering step, it can be concluded that the uncharged PS-b-P2VP lamellae behave like an insulator. As can be seen from the in situ measurements of the charged films (Figures 6b,c), the initially attained impedance values upon the first watering reside about 800 kΩ below that of uncharged PS-b-P2VP lamellae (2nd column Table 2). Since water is known to swell ionized P2VP,34 this strong drop of impedance can be attributed to the mobilization of iodide counterions residing in the quaternized qP2VP domains, which, in combination with the positively charged pyridinium ions, form a conductive medium in the presence of water. The |Z| vs time plots of the charged films further reveal that the attained impedance values at the first watering plateaus decreases from lower charged to stronger charged films (2nd column Table 2). This finding substantiates the assumption that the impedance is

(Figure 6a). Additional measurements of completely bare EIS chips in turn, allowed for extrapolating the conductive properties of the uncharged PS-b-P2VP lamellae (Figure S2). The in situ EIS signal over time acquired from electrodes covered with a lamellar neutral PS-b-P2VP thin film is depicted in Figure 6a. It appears that, in the course of initial Millipore water flooding, the onset of impedance adopts values of 3 orders of magnitude higher (920 kΩ) than those found for bare EIS chips (6 kΩ) (Figure S3). As was found for noncoated EIS chips, a sharp drop of impedance appears upon AuNP solution injection. For such uncharged PS-b-P2VP films SEM imaging revealed that negatively charged AuNPs only rarely adsorbed on the lamellar surface (Figure 6d). This is in agreement with recent results observed for similar films on conventional silicon wafers.25 For reasons of the large impedance measured during 27286


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ACS Applied Materials & Interfaces Table 2. Summary of Impedance Values Reached Upon Levelling (Plateaus) of the Impedance after the Individual Fluid Injections for Each Samplea θquaternization [%]

|Z| 1st watering plateau [kΩ]

|Z| AuNP flow plateau [kΩ]

|Z| rinsing plateau [kΩ]

tbalancing [min]

0 30 45

920 141 110

600 153 170

910 156 172

20 60

a

tbalancing = time required until charge balancing between AuNPs and pyridinium ions was attained (corresponds to the time gap between AuNP solution injection and the subsequently reached AuNP flow plateau in the |Z| vs time plots of Figure 6). The expression “plateau” denominates the levelling of impedance values along the time axis in the |Z| vs time plots.

decreasing with higher qP2VP domain charging (increased ionic conductivity). Upon AuNP injection, we found that in contrast to neutral PS-b-P2VP films, the charged PS-b-qP2VP films exhibit a steep growth of impedance when brought into contact with the AuNP solution (Figures 6b,c). Since the lower frequency range reflects the interfacial impedance close to the gold electrodes,6,15 the observed jump of impedance can be attributed to the selective attachment of the negatively charged AuNPs on top of the positively charged qP2VP domains. This conclusion is supported by SEM images taken after EIS measurements, revealing selective AuNP deposition on top of the qP2VP domains (Figure 6e,f). The decrease in conductivity is thus likely a cause of the charge balancing between the negatively charged citrate shell of the AuNPs and the positively charged pyridinium ions of the qP2VP blocks. A final rinsing step with Millipore water was applied to check if the selectively assembled metal nanoparticles remain adsorbed on the qP2VP domains, and thereby assess whether the attractive electrostatic forces prevent the AuNPs from desorption. For the 30% and 45% quaternized films a short impedance jump occurred upon rinsing followed by a quick leveling back to the beforehand reached values attained at the AuNP flow plateau, as can be seen when comparing the third and fourth column in Table 2. We interpreted this behavior to the circumstance that a major part of the adsorbed AuNPs stuck to the qP2VP domains as confirmed by corresponding SEM imaging visible in Figure 6e,f. Moreover, a significant difference in the AuNP packing density can be observed between 30% and 45% quaternized qP2VP domains, proving an increased AuNP adsorption with higher becoming θquaternization. Similar results gained from resembling systems were reported by Cant et al.,43 Lau et al.,44 and Shustak et al.45 Note here, that EIS measurements revealed each individual block copolymer film to require somewhat different durations to reach impedance equilibration upon the different fluid injections. However, impedance balancing is crucial to serve as reference point for following fluid injections and to ensure full charge neutralization at the film|electrolyte interface during AuNP solution injection as well as to observe significant distinctions in particle packing density after measurement. Performing fluid injections at strictly identical time points is thus not possible. In order to determine the charge dependent adsorption kinetics, the acquired in situ EIS signals over time at 10 Hz, as shown in Figure 6, were normalized to a stable level of water flush before AuNP solution injection and plotted in Figure 7. A simple Langmuir approach can be used to describe adsorption of electrostatically driven attraction for particles and

Figure 7. Normalized in situ EIS spectra revealing the charge dependent adsorption kinetics during the assembly of negatively charged AuNPs on positively charged qP2VP domains of lamellar PSb-qP2VP thin films (see Table 3 for fit parameters).

molecules in microfluidic system.46,47 Therefore, Langmuir-like function |Z|/|Z0|= Zc*exp(−t/τ) + ZSAT was chosen for fitting.47 Here, Zc expresses the relative impedance change, ZSAT the saturation relative impedance, and τ the exponential time constant, respectively. The fitting results in Table 3 clearly Table 3. Time Constants τ and Saturation Levels ZSAT Determined for the Impedance Growth during AuNP Injection for the Differently Strong Quaternized PS-bqP2VP Films θquaternization [%]

τ [min]

ZSAT

30 45

7.5 31.0

1.08 1.66

show that for strongly quaternized films, the time constant τ exceeds the one for weaker quaternized films by a factor of 4.1. This result suggests that the increased amount of positively charged pyridinium ions in the 45% quaternized films demand for an expanded neutralization period which is coupled with an increased adsorption of negatively charged AuNPs. Complementary to this, the function which was used for fitting the data in Figure 7 allows to quantify the differences between the maximum relative amplitude change (|Z|/|Z|0), which can be assigned to a measure of charge needed to stabilize the surface and therefore likewise correlates with the amount of adsorbed AuNPs. The latter conclusion seems obvious when comparing the strongly increased AuNP packing density found for the 45% quaternized films (Figure 6f) with the rather lower packing density appearing for 30% quaternized films (Figure 6e). Finally, the determined time constants and ZSAT values showed to increase with the inherent qP2VP charge density being in agreement with the aforementioned arguments. The propagation of the impedance values shown in Figure 7 is also partially indicating the fashion in which the AuNPs accumulate on the qP2VP domains. For the 30% quaternized films, an initial steep growth of impedance is followed by a flattened leveling of impedance occurring at the later stage of AuNP solution injection. When in this case the AuNP adsorption behavior is addressed, one can assume that the qP2VP 27287


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glass desiccator accompanied by two vessels of which one contains 300 μL 1,4-diiodobutane (DIB) and another one 300 μL 1,3diiodopropane (DIP) (purchased from Sigma-Aldrich). After evacuation and a short initial heating (to create the dihalide vapors) the reaction setup was stored at room temperature. The required duration of the vapor treatment is strongly related to environmental conditions and the physical properties of the diblock copolymer films. Measurement Setup and Procedure. All EIS measurements were carried out in a two electrode mode setup with lithographically fabricated interdigitated gold sputtered electrodes (100 pairs, spacing 10 μm, width 10 μm, thickness: 150 nm; sputtering tool: Nordiko NS 2550, dc power of 250 W, pressure of 4.2 Pa, argon flow of 55 sccm) on Si/SiOx substrate incorporated in a microfluidic channel (25 μm height, 2.5 mm width) made out of SU-8 photoresist with polydimethylsiloxane (PDMS) cover. The microfluidic system was connected to a syringe pump (LA-100, HLL Landgraf Laborsysteme, Langenhagen, DE) by Teflon tubes. The flow rate was adjusted to 0.5 mL/hour. Three electrode pairs in one channel assured triplet reproduction of a single measurement. A custom-made relay multiplexer was used to switch between the electrodes and to connect the electrodes to a combination of potentiostat (263 A, Princeton Applied Research, Oak Ridge, USA) and impedance/gain analyzer (SI 1260, Ametek, Farnborough, UK). The frequency range of all EIS measurements was set from 100 kHz to 10 Hz, AC amplitude was set to VAC = 10 mV versus open circuit and DC amplitude to VDC = 0 versus open circuit. Temperature was kept at 22 °C for all measurements. No commonly used ferri/ferrocyanide was used as a redox couple, since strong corrosion in the presence of gold electrodes occurs.50 Zview (Scribner software, Farnborough, UK) was used for fitting the impedance data to Randle’s equivalent circuit. The lamellar PS-b-qP2VP films were prepared in prior of PDMS cover fixation of the microfluidic channel. Subsequently, in situ impedance spectroscopy of the PS-b-qP2VP films during the consecutive injection of different fluids was performed. In each experiment, a portion of Millipore-water was pumped through the microfluidic channel until the impedance reaches a constant level. This serves as a reference value to compare with the impedance behavior that occurs upon subsequent injection of an aqueous AuNP solution. A final rinsing step of Millipore water finished the measurements.

neutralization through AuNP adsorption mainly takes place during the just mentioned initial impedance gain, followed by impedance saturation with no further AuNP attachment. In contrast, the course of impedance for 45% quaternized films is curved consistently. From this we deduce a more steady and constant sequestration of AuNPs along the entire AuNP injection sequence.

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CONCLUSION Continuous electrochemical impedance spectroscopy was successfully applied for the in situ monitoring of electrostatically driven selective AuNP adsorption on charged polymeric substrates. The excitation frequency of 10 Hz was chosen since the EIS signal at 10 Hz corresponds to the electrical double layer at the border between the electrode-copolymer film and the electrolyte and therefore physically (spaciously) correlates with the AuNPs adsorption. The Coulombic interactions between particles and surfaces on the nanoscale can be described by Langmuir mechanism. This methodology gives access toward comprehensive physical deciphering of the kinetics of the binding processes. In this view we found that the adsorption kinetics among differently charged PS-b-qP2VP films revealed that the extent of nanoparticle packing can be influenced by the qP2VP charge density and that high charge density not necessarily means rapid adsorption of AuNPs. Thus, we observed that the electrostatic AuNP adsorption on less strong charged polymer films is characterized by a quick AuNP sequestration (qP2VP neutralization) in the early stage of AuNP solution injection. This finding indicates that, in this case, less time is required for qP2VP neutralization, whereas for stronger charged films, a much longer period of AuNP solution exposure is needed for neutralization, going along with consistent AuNP adsorption. The ability to control the nanoparticle density has great potential for tuning the conducting path dimensions during the manufacture of nanoscaled electric circuitry. Moreover, the presented methodology can likely be applied to analyze many similar systems dealing with selective nanoparticle adsorption on even planes via electrostatic interactions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b07708. Phase angle evolution during in situ EIS measurements, frequency dependent Bode-plots, in situ EIS measurement of bare EIS chip electrodes, and characterization of the used AuNPs (PDF)

EXPERIMENTAL SECTION

AuNPs for Selective Adsorption on qP2VP Domains. Trisodium citrate trihydrate, sodium borohydride, and gold(III) chloride hydrate were purchased from Sigma-Aldrich. AuNPs were synthesized according to well-established reduction methodology.48 The citrate-ligands act as stabilizers inducing a negative ζ-potential (≈ −60 mV) on AuNPs in aqueous solution. Their average size was determined to range around 12 nm with a size distribution of 5−50 nm as determined from transmission electron microscopy (TEM) and dynamic light scattering (DLS), respectively (Figure S4). The average size of the AuNPs thus represents only a fraction of the qP2VP domain-width (ca. 40 nm), which is a prerequisite for concise selective AuNP deposition. The AuNP-solution is further triggered to reside in a slightly basic pH-regime of ca. 8−9, which hinders citrate to be protonated, and thus remain negatively charged. Preparation of Lamellar PS-b-qP2VP Films. PS-b-P2VP was synthesized using living anionic polymerization as described elsewhere.49 The molecular weight of the diblock copolymer (Mw) amounts to 99 kg/mol with a polydispersity index (PDI) of 1.05 and a styrene fraction of 56% (S56V4499). The diblock copolymer films were fabricated by adding 50 μL of PS-b-P2VP solution (2 wt% in DMF) on top of an EIS chip and subsequent spin-coating with 2000 rpm. Lamellar morphology of the films was generated by solvent vapor annealing in a custom-made glass chamber using toluene vapors. For P2VP quaternization, the phase-separated films were placed in a small

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AUTHOR INFORMATION

Corresponding Authors

*[email protected]. *[email protected]. Author Contributions ⊥

T.W. and J.L. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Tom Wagner acknowledges the Fonds National de la Recherche Luxembourg (FNR) for funding (Grant Nr.: 3983022). SEM measurement time was provided by the Center for Chemical Polymer Technology (CPT) of the DWI Leibniz Institute for Interactive Materials, which was supported 27288


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by the EU and the Federal State of North Rhine-Westphalia, Germany, (Grant Nr.: EFRE 30 00 883 02).

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