In Situ Investigation of Electrochemically Mediated Surface-Initiated

Mar 28, 2017 - Electrochemically mediated atom transfer radical polymerization (eATRP) initiates/controls the controlled/living ATRP chain propagation...
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In Situ Investigation of Electrochemically Mediated Surface-Initiated Atom Transfer Radical Polymerization by Electrochemical Surface Plasmon Resonance Daqun Chen and Weihua Hu* Institute for Clean Energy & Advanced Materials, Faculty of Materials and Energy, Southwest University, Chongqing 400715, China S Supporting Information *

ABSTRACT: Electrochemically mediated atom transfer radical polymerization (eATRP) initiates/controls the controlled/ living ATRP chain propagation process by electrochemically generating (regenerating) the activator (lower-oxidation-state metal complex) from deactivator (higher-oxidation-state metal complex). Despite successful demonstrations in both of the homogeneous polymerization and heterogeneous systems (namely, surface-initiated ATRP, SI-ATRP), the eATRP process itself has never been in situ investigated, and important information regarding this process remains unrevealed. In this work, we report the first investigation of the electrochemically mediated SI-ATRP (eSI-ATRP) by rationally combining the electrochemical technique with real-time surface plasmon resonance (SPR). In the experiment, the potential of a SPR gold chip modified by the self-assembled monolayer of the ATRP initiator was controlled to electrochemically reduce the deactivator to activator to initiate the SI-ATRP, and the whole process was simultaneously monitored by SPR with a high time resolution of 0.1 s. It is found that it is feasible to electrochemically trigger/control the SI-ATRP and the polymerization rate is correlated to the potential applied to the gold chip. This work reveals important kinetic information for eSI-ATRP and offers a powerful platform for in situ investigation of such complicated processes.

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termination and confers ATRP the controlled/living properties.3,4 The activator/deactivator (denoted as Cu(I) (Bipy)2 and Cu(II) (Bipy)2, respectively, hereafter for simplicity) is a redox pair and is able to convert to one another via a reversible oneelectron transfer process.5,6 Some chemical methods have been developed to in situ generate/regenerate oxygen-sensitive activators to trigger ATRP by chemically reducing air-stable deactivators with reducing reagents such as ascorbic acid, sugar, elemental copper, tin(II) octanoate, and other radical initiators.7−10 Remarkably, Matyjaszewski and co-workers reported in 2011 that the ATRP could also be initiated by applying an externally electrochemical potential to reversibly generate copper activator via a one-electron reduction of airstable deactivator, called electrochemically mediated ATRP (eATRP).11 The polymerization kinetics is thereby tunable by varying the applied potential in eATRP.12 Various parameters, including the electrochemical techniques used, catalyst concentration, and ligand, were investigated during the eATRP process.13 By using a sacrificial anode, the eATRP setup was simplified without the need of separating the counter

tom transfer radical polymerization (ATRP) is a controlled/living polymerization technique that was established in 1995.1 It proceeds via a concerted atom transfer mechanism. The activator (lower-oxidation-state transition metal complex, e.g., Cu(I) (Bipy)2, Bipy stands for 2,2′bipyridyl) attacks alkyl halide initiator (e.g., Pn-Br) to form deactivator (higher-oxidation-state transition metal complex Br−Cu(II) (Bipy)2) and an active radical (Pn·), which is capable of chain propagation, as shown in Scheme 1.1,2 By Scheme 1. Mechanism of ATRP

building a dynamic and reversible equilibrium between the active radicals (Pn·) and dormant species (Pn-X), ATRP allows for simultaneous growth of each polymeric chain, thereby offering precise control over the polymer molecular weights, dispersity, complex, and architectures.3,4 Moreover, this dynamic equilibrium strongly favors the dormant state (namely, kd ≫ ka), thus resulting in low concentration of propagating species, which consequently suppresses possible bimolecular © XXXX American Chemical Society

Received: January 25, 2017 Accepted: March 28, 2017 Published: March 28, 2017 A

DOI: 10.1021/acs.analchem.7b00316 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry electrode.14 Later, the eATRP was developed successfully in an aqueous system with a set of useful guidelines.12,15 By confining initiators on a solid surface, surface-initiated eATRP (eSIATRP) was also explored to synthesize polymer brush (pattern) on various substrates.16−19 Despite successful demonstrations, important information remains absent regarding the eATRP process itself such as, e.g., the reversibility of the polymerization process upon turn on/off cycles, activating process of the initiators, chain growth rate and stability of active radicals, etc., all of which are essential for fundamental understanding and practical application of eATRP. Previous investigations of homogeneous eATRP only statistically probe the kinetics of the eATRP process because it is impossible to completely and swiftly reduce the deactivator in the bulk solution via electrolysis.5,11 At the same time, the polymerization process was tracked by analyzing the polymer product with ex situ gel permeation chromatography (GPC) at different moments. The time resolution is not high enough to explore the details of eATRP. To investigate eATRP, it is desirable to build a setup enabling (1) rapid deactivator/activator conversion in response to the potential modulation and (2) simultaneous time-resolved monitoring of the polymerization process. In the heterogeneous SI-ATRP system where the ATPR initiator was immobilized on the electrode surface,20 it is indeed feasible to swiftly control the local activator/deactivator ratio on the electrode/solution interface via potential modulation. Unfortunately, in previous research, the grown polymer brush was simply imaged by using ex situ AFM and no real time technique was applied to monitor the eSI-ATRP process.16 Surface plasmon resonance (SPR) is a local refractive indexsensitive optical technique capable of monitoring the changes on metal/solution interface in a real-time and label-free manner.21−24 It has been established as a golden standard to study the biomolecular interaction and adsorption/desorption behaviors of macromolecules.25−28 Noteworthy, SPR generally utilizes a gold film chip to generate surface plasmon wave (SPW), which makes it inherently compatible with electrochemistry for simultaneous electrochemical-SPR investigation.22 Both sensitivity and sampling frequency of SPR are satisfying for most electrochemical processes such as electrochemical polymerization and underpotential deposition.22,29 In this work, we report the first in situ study of eATRP by rationally combining electrochemistry and SPR technology. The experimental EC-SPR setup was shown in Scheme 2. The SPR gold chip was grafted with self-assembled monolayer of ATRP initiator with a thiol terminal group, and it acted as working electrode with potential controlled by a potentiostat. A glassy carbon and Ag/AgCl/saturated KCl electrode were used as counter electrode (CE) and reference electrode (RE), respectively. It is worth noting that both of the counter and reference electrodes were actually connected with the electrolyte solution in the parallel reference chamber of the dualchannel SPR cell to avoid the direct exposure to the working electrode,11 although it was drawn as such for simplicity in Scheme 2. The electrolyte solution was systematically optimized to ensure that the solvent is able to dissolve the metal ion, organic ligand, and supporting electrolyte, as well as the monomer. Most importantly, the refractive index of the final solution should fall in the measurable range of SPR. The optimal electrolyte solution was prepared by dissolving CuBr2, Bipy (1:2 molar ratio) in methanol/H2O (6:1 v/v) containing 1% v/v glycidyl methacrylate (GMA) monomer, and 0.1 M KCl

Scheme 2. Illustration of the EC-SPR Setup for in Situ eSIATRP Investigationa

a

Note that for simplicity the CE, RE, and WE were drawn in the same channel. The charge and halide ions were not shown in the electrochemical reaction equation.

as supporting electrolyte. In the experiments, the electrolyte solution continuously flows in parallel into the testing channel and reference channel with a constant flow rate of 10 μL min−1 and the local activator/deactivator ratio on the gold surface of the testing channel was controlled by the electrochemical potential; thus, the polymerization was controllably triggered and terminated. The growth of polymer brush on the gold surface results in the local refractive index change and arouses SPR signal, which was collected at a temporal resolution of 0.1 S. Both electrochemical and SPR responses obtained reflect in situ transient information on the gold/solution interface, thus offering useful insights into the eSI-ATRP process. The cyclic voltammetry (CV) curve of deoxygenated electrolyte solution containing CuBr2 (1.0 mg mL−1), Bipy (1.4 mg mL−1), 1% v/v GMA, and 0.1 M KCl in methanol/ H2O (6:1 v/v) in the electrochemical SPR setup was recorded at a sweep rate of 50 mV s−1, as shown in Figure 1. It is shown

Figure 1. CV curve and simultaneous SPR response in deoxygenated methanol/H2O (6:1 v/v) solution containing CuBr2 (1.0 mg mL−1), Bipy (1.4 mg mL−1), 1% v/v GMA, and 0.1 M KCl in the EC-SPR setup. Sweep rate: 50 mV s−1.

that the Cu(II) (Bipy)2 demonstrates an evident cathodic peak with a peak potential at −0.1 V corresponding to its reduction to Cu(I) (Bipy)2. On the reverse scanning, the anodic peak was observed at around 0.09 V. The peak separation of about 190 mV for this reversible Cu(II) (Bipy)2/Cu(I) (Bipy)2 redox pair is considerably higher than that observed on a clean Au B

DOI: 10.1021/acs.analchem.7b00316 Anal. Chem. XXXX, XXX, XXX−XXX

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redox reaction on the interface. Therefore, it can be defined as the EC part as it predominantly results from the electrochemical reaction on the interface. After this EC part, the SPR angle starts to increase and maintains an approximately linear increase as long as the potential is held constant. We hypothesize this increase results from the eSI-ATRP process because the electrochemical reduction of deactivator to activator is able to activate the surface-attached initiator and trigger the control/living polymerization process. Therefore, we define this phase as the ATRP part as the signal originates from the SI-ATRP process. The influence of potential was further investigated, as shown in Figure 2. It is clearly shown that the SPR increase rate is closely correlated to the potential applied. In the potential range from 0 to −0.15 V, more negative potential results in faster angle increase, suggesting the acceleration of the SIATRP process. This is in good agreement with the ATRP theory, according to which the local activator/deactivator ratio determines the percentage of active radicals in all the initiators and, in turn, the growth rate of the polymer brush. A more negative potential induces a higher activator/deactivator ratio and faster polymerization. If the potential is further decreased to, e.g., −0.2 or −0.25 V, the increase rate of the SPR angle remains comparable to that for −0.15 V (curve e), implying the ATRP cannot be significantly accelerated by applying a more polarized potential. This may be because the complete concentration polarization has been reached at the electrode surface; i.e., the concentration of deactivator on the gold surface was close to zero, and more negative potential cannot further decrease it. At the same time, more negative potential may cause the risk of serious biradical termination and direct electrochemical deposition of copper.3 For potential higher than 0 V, no linear increase could be obtained except for the initial angle spike (Figure S3), suggesting at these potentials, no detectable ATRP process occurs on the gold surface, possibly because the activator/ deactivator ratio on the surface is too low, although a cathodic current is obtained on the working electrode. According to the ATRP theory, the rate constant of activation ka is much smaller than the rate constant of deactivation kd, which is critical to keep the whole polymerization process under control as the active species is only a small ratio of all the initiators under this circumstance.30 In order to verify that the constant SPR increase is a result of eSI-ATRP, different control experiments were performed. If there is no monomer (curve a in Figure 3) or catalyst (curve b) in the solution, no such increase could be observed. At the same time, if the initiator monolayer on gold surface is displaced by a nonactive thiol monolayer or a clean gold chip is used, the SPR angle does not increase during the ATRP phase although the similar SPR spike appears in the beginning (curves c and d in Figure 3). These control experiments unambiguously confirm that the linearly increased SPR signal originates from the SI-ATRP process. The growth rate of polymer brush under different potentials could be calculated quantitatively by using the SPR response. If one assumes a refractive index of 1.43 for the polymer brush and 1.373 for the solution, the growth rate of polymer under −0.1 V is calculated to be around 0.12 nm/min, which is close to previous calculations.25 By using this EC-SPR platform, the SI-ATRP behaviors of different ATRP monomers were investigated and compared. It is found that the polymerization rate is different for different

electrode in the same solution (Figure S1), which could be assigned to the slight blocking effect of the initiator monolayer on the gold surface. The high IR drop due to the large distance between the RE and the WE (i.e., SPR gold chip) in the ECSPR channel and low concentration of supporting electrolyte in the solution may also contribute to this large peak potential separation. The simultaneous SPR response during the potential scanning was recorded. As shown in Figure 1, on the cathodic scanning, the SPR angle increases with an approximately linear mode for about 30 m° (millidegree). On the reverse scanning, the SPR angle returns to its original value. The control experiment unveils a similar SPR angle shift even in the solution containing no monomer (Figure S2). This suggests that this reversible SPR shift upon potential scanning is caused by the rearrangement of the gold/solution interface including the reconstruction of the electrochemical double layer due to the potential scanning and the reversible conversion between Cu(II) (Bipy)2 and Cu(I) (Bipy)2. No clear SPR angle increase contributed from the eSI-ATRP could be differentiated from the total SPR angle response, possibly because, during the fast potential scanning, the SI-ATRP process cannot last long enough to arouse considerable SPR signal. Therefore, the SPR angle shift in Figure 1 is predominantly caused by the transient double layer charging process and the electrochemical reaction occurring on the gold/solution interface. In order to highlight the SPR signal of the eSI-ATRP polymerization, the potential of the gold chip was stepped from open circuit potential (OCP, ca. 0.15 V) to a certain value and was held constant to maintain a stable Cu(II) (Bipy)2/Cu(I) (Bipy)2 ratio on the electrode surface. In this method, the transient charging current decays rapidly to near zero and a stable Cu(II) (Bipy)2/Cu(I) (Bipy)2 equilibrium is built in a short time on the gold surface, and thus, the disturbance from double layer charging and electrochemical reaction could be efficiently suppressed. When the potential was stepped to −0.06 V, the time-course SPR signal could be defined as two part, as shown as curve c in Figure 2. The first one is maintained for about 10 s and starts with a sharp angle spike, followed by a SPR increase of about 20 m°. The sharp spike could be assigned to the charging current while the 20 m° SPR increase results from the double layer rearrangement and the

Figure 2. Simultaneous SPR response for stepping the potential of gold chip from OCP to 0 (a), −0.03 (b), −0.06 (c), −0.10 (d), and −0.15 V (e). The arrow indicates the moment when the potential was applied. Electrolyte solution: CuBr2 (1.0 mg mL−1), Bipy (1.4 mg mL−1), 1% v/v GMA, and 0.1 M KCl in methanol/H2O (6:1 v/v). C

DOI: 10.1021/acs.analchem.7b00316 Anal. Chem. XXXX, XXX, XXX−XXX

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Fundamental Research Funds for the Central Universities (XDJK2015B014), and Natural Science Foundation Project of CQ CSTC (cstc2016jcyjA0493).

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Figure 3. Simultaneous SPR response for stepping the potential of gold chip from OCP to −0.06 V. (a) No monomer contained in the solution. (b) No catalyst (CuBr2 or Bipy) contained in the solution. (c) The initiator on gold was displaced by 3,3′-dithiodipropionic acid. (d) No initiator on gold. The arrow indicates the moment when the potential is applied.

monomers, even thought the same potential was applied with the same catalyst, as shown in Figure S4. The growth of oligo(ethylene glycol) methacrylate (OEGMA, Mn = 360) is significantly faster than that of 2-hydroxyethyl methacrylate (HEMA), perhaps because each monomer has its intrinsic radical propagation rate.30 This platform also facilitates the investigation on the reversibility of the ATRP process. Our preliminary results disclose that the SI-ATRP process could be reinitiated without an evident decrease in the polymerization rate even after more than 10 on/off cycles (potential step and return to OCP as a cycle, Figure S5). In conclusion, for the first time, we were able to in situ investigate the interesting eSI-ATRP process by rationally combining SPR and the electrochemical method. The results suggest that it is feasible to initiate, control, and terminate the SI-ATRP process via electrochemical modulation. This work also successfully demonstrates the EC-SPR platform as a powerful tool for in situ investigation of electrochemically mediated ATRP and other complicated processes.

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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/acs.analchem.7b00316. CV curve of ATRP catalyst on clean gold electrode and SPR responses for electrochemical modulations in different solutions (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: 0086-23-68254969. ORCID

Weihua Hu: 0000-0001-6278-9551 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from National Natural Science Foundation of China (No. 21273173), D

DOI: 10.1021/acs.analchem.7b00316 Anal. Chem. XXXX, XXX, XXX−XXX