Electrochemical SERS for in Situ Monitoring the Redox States of

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Electrochemical SERS for in situ Monitoring the Redox States of PEDOT and Its Potential Application in Oxidant Detection Min-Han Tsai, Yow-Kuan Lin, and Shyh-Chyang Luo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16989 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 19, 2018

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ACS Applied Materials & Interfaces

Electrochemical SERS for in situ Monitoring the Redox States of PEDOT and Its Potential Application in Oxidant Detection Min-Han Tsai,† Yow-Kuan Lin,† and Shyh-Chyang Luo†, ‡,* †Department

of Materials Science and Engineering, National Taiwan University, No. 1, Sec. 4,

Roosevelt Road, Taipei, 10617 Taiwan ‡Advanced

Research Center for Green Materials Science and Technology, National Taiwan

University, Taipei 10617, Taiwan

KEYWO KEYWORDS: electrochemical surface-enhanced Raman scattering; oxidant detection; conducting polymers; redox behavior; spectroelectrochemistry

ACS Paragon Plus Environment

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ABSTRACT. In response to recent developments for applying conducting polymers on various biomedical applications, the development of characterization techniques for evaluating the states of conducting polymers in liquids is beneficial to the applications of these materials. In this study, we propose a platform using electrochemical surface-enhanced Raman scattering (EC-SERS) technology, which allows a direct measurement of the redox states of conducing polymers in liquids. A thiophene-based conducting polymer, hydroxymethyl poly(3,4-ethylenedioxythiophene) or poly(EDOT-OH), was used to demonstrate this concept. Poly(EDOT-OH) films were coated on Au nanoparticle-coated ITO glass as SERS-active substrates. Taking the advantage of Raman enhancement, we are able to in situ and clearly monitor the redox behavior of poly(EDOT-OH) in aqueous solutions. The Raman peak intensity decreases as the poly(EDOT-OH) film is oxidized. Furthermore, we demonstrated our idea to utilize this phenomenon as the sensing mechanism for oxidant detection. The Raman intensity of conducting polymers reduces faster when oxidants exist and we obtain a quantitative analysis for the detection of oxidants. Moreover, the oxidized poly(EDOT-OH) films can be reused for detection of oxidants simply by applying a reduction potential to activate the poly(EDOT-OH) films. The film stability was also confirmed and the detection of two other oxidants, namely ammonium persulfate and iron chloride, were also demonstrated. The results show different SERS spectra of poly(EDOT-OH) films oxidized by using different oxidants. Besides, the oxidized films can be easily recovered simply by applying a cathodic potential, which allows repeating usage and makes it possible for continuous monitoring applications. To the best of our knowledge, this is the first time to apply PEDOT’s Raman feature for detection purpose.


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1. INTRODUCTION The phenomenon of surface-enhanced Raman scattering (SERS) was first observed in 1973 from pyridine adsorbed on a rough silver surface.1 Since then, the development of nanostructured and metallic substrates, which can contribute to an enhanced Raman scattering signal2 has attracted much attention mainly because of their great potential for creating ultrahigh sensitive and diagnostic devices,3-6 especially targeting analytes whose structures are rich in aromatic or unsaturated bonds.7 Some attention has also been drawn to the integration of electrochemical technique with SERS measurement, which is generally called electrochemical surface-enhanced Raman scattering (EC-SERS) during these years.8-9 The EC-SERS is the most complicated SERS system because the enhancement effect comes from not only electromagnetic field enhancement (EM) but also chemical enhancement (CE). The chemical enhancement is especially strong in ECSERS and is attributed to chemisorption interaction and charge transfer between adsorption and substrate.8 The EC-SERS has been recently demonstrated very useful in several applications, such as the monitoring the reaction dynamics,10 the quantification of drugs and their metabolites,11-12 and the evaluation of electrode materials.13-14 With the recent development of nanoscale scanning microscope technique, the EC-SERS is further applied on the nanoscale scanning probe system, which is electrochemical tip-enhanced Raman spectroscopy (EC-TERS), to probe few or single molecule.15 Most of these EC-SERS studies used noble metal as their conductive substrates, the work using organic conductive substrates was so far rarely reported.16 The properties of conducting polymers in aqueous and biological solutions has been intensively studied more recently because of the great potential in various biomedical applications.17 Poly(3,4-ethylenedioxythiophene) or PEDOT, has been one the most promising conducting polymers in this field because of its high


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conductivity, biocompatibility and feasibility to functionalization.18-19 PEDOT has also been applied in various biomedical applications,20 such as electrochemical transistor,21-25 biosensors,2629

neural probes,30-33 and cell capturing and release.34-37 Notably, many studies have shown that the

redox states or doping states of conducting polymers played a key role to the performance of their biomedical applications, including protein and cell adhesions,38-40 as well as their antibacterial or antifouling properties.41-43 Therefore, the development of techniques for in situ monitoring the redox states of conducting polymers will be beneficial for exploring the impact of their redox states on the performance. Several studies focused on in situ Raman spectroscopy on redox cycling of conducting polymers have been published, such as polypyrrole and polythiophene films on Pt electrodes44 and polyazulene on aluminum.45 These work investigated the different electronic states under positive and negative bias, which resulted in different vibrational modes. Some study has shown that the PEDOT also has strong Raman scattering, which is strongly correlated to the doping level in PEDOT.46 In the dedoped state, the Raman spectrum at symmetric Cα=Cβ stretching is enhanced while in the doped state, the Raman spectrum at symmetric Cα=Cβ stretching decreases. This enhancement in this spectrum is resulted from the resonance effect, which is attributed to the conjugated structure in doped state.47 Most studies focused only on the spectroelectrochemical properties of conducting polymers. In this study, we would like to utilize Raman characteristic of conducting polymers as the sensing mechanism to in situ monitor the redox states or doping/dedoping of conducting polymers by integrating a EC-SERS platform as shown in Figure 1. A PEDOT’s derivative, poly(EDOT-OH), was used as the conductive substrates in this ECSERS setup. Poly(EDOT-OH) provides enhanced adhesion to substrates compared to PEDOT,48 which allows long-term experiments in aqueous solutions in the presence of oxidants. Similar to


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PEDOT, the Raman intensity of poly(EDOT-OH) responds to its doping states. We first demonstrated the feasibility of applying this EC-SERS platform for in situ monitoring the redox conversion of poly(EDOT-OH) in aqueous solutions. Moreover, the SERS substrate provides an enhanced effect on Raman signal from not only electromagnetic enhancement effect but also chemical enhancement effect especially for conducting polymers when monitoring the doping/dedoping of poly(EDOT-OH) in aqueous solutions during potential applied. The Raman spectrum assigned to Cα=Cβ decreased when an anodic potential was applied to poly(EDOT-OH). Because the Raman intensity is mainly determined by the redox states of conducting polymers, we suspect that this phenomenon can be potentially used as a new type of detection mechanism for oxidant detection. In the presence of oxidants, the conducting polymers are oxidized, which leads to a change in their redox states and Raman intensity. In this work, we first used hydrogen peroxide (H2O2) as our model molecule. H2O2 is one kind of radical oxygen species, which is important to maintaining the physio. Several methods for H2O2 detection have been developed, such as fluorimetry,49 chemiluminescence50 and electrochemical ways.51-52 In this work, we took advantage of SERS to in situ observe the decease of Raman intensity of poly(EDOT-OH) film after adding H2O2 solution. The concentration of H2O2 in aqueous solution can be estimated by monitoring the intensity change of the Raman spectrum. We successfully demonstrated a quantitative analysis based on this platform, which is usually not an easy task for detection application by applying a SERS-active substrate. We also examined the Raman spectra of poly(EDOT-OH) films oxidized by using ammonium persulfate and iron chloride. The Raman spectra of poly(EDOT-OH) films were different when oxidized by using different oxidants, which indicates the possibility to identify the species of oxidants based on their spectrum. To the best of


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our knowledge, this is the first time to apply PEDOT’s Raman feature for detection purpose and this study demonstrates a potential new application from conducting polymers.

Figure 1. Setup of electrochemical surface-enhanced Raman scattering (EC-SERS) with an auto mapping stage where the laser beam moves 2 µm every 30 s in 6 × 6 µm2 to monitor the bonding state change with time during polymer oxidation. 2. EXPERIMENTAL METHODS 2.1 Chemicals and Reagents ITO-coated glass with sheet resistance 20-30 Ω sq−1 was purchased from Uni-Onward Corp. Hydroxymethyl 3,4-ethylenedioxythiophene (EDOT-OH), hydrogen tetrachloroaurate (HAuCl4), sodium perchlorate (NaClO4), lithium perchlorate (LiClO4), 4-mercaptobenzoic acid (p-MBA), phosphate buffered saline (0.01 M at pH 7.4), anhydrous dichloromethane (DCM), hydrochloric acid (HCl), sodium hydroxide (NaOH), hydrogen peroxide (H2O2) and ammonium persulfate(APS) were purchased from Sigma-Aldrich and used without further purification. Sodium dodecyl sulfate


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(SDS) was purchased from Showa Chemical, tetrabutylammonium perchlorate (TBAP) and iron chloride (FeCl3) was purchased from Alfa Aesar. 2.2 Fabrication of gold nanoparticle (AuNP)/ITO substrates The AuNPs were deposited on ITO substrates directly via an electrochemical seed mediated method with minor modification.53. The deposition solution contained 0.2 mM HAuCl4 and 0.1 M NaClO4. ITO substrates were washed with 30% ethylenediamine before use. After ITO substrates were immersed in the solutions, a constant voltage of -0.8 V (vs Ag/AgCl) was applied for 10 s to form AuNP seeds. Then, the growth of AuNPs was achieved by applying a cyclic potential from +0.3 to -0.04 V (vs. Ag/AgCl) at scan rate of 50 mV s-1. The AuNPs were deposited on the ITO substrates with 400 cycles in general. After undergoing the electrochemical deposition process, the SERS-active substrates were cleaned with pure water and dried with a N2 flow. The substrates were stored in pure water at 4 °C if not immediately used. 2.3 Electropolymerization of PEDOT and poly(EDOT-OH) The electropolymerization was conducted with a potentiostat (PGSTAT, Autolab). A Ag/AgCl (3.0 M NaCl, ALS Co. Ltd) reference electrode was used for aqueous solutions and a Ag/AgNO3 reference electrode (1.0×10-2 M AgNO3 and 0.1 M tetrabutylammonium perchlorate in acetonitrile, ALS Co. Ltd) was used in organic solutions. A Pt wire was used as a counter electrode. Two electropolymerization protocols were adapted for the fabrication of poly(EDOT-OH) at different morphologies.48, 54 For fabricating homogeneous poly(EDOT-OH) film, we prepared an aqueous microemulsion solution by dissolving 10.0 mM EDOT-OH in DI water in the presence of 5.0×102 mM SDS and 1.0×102 mM LiClO4. A cyclic potential of -0.6 V to 1.1 V vs. Ag/AgCl was applied for 3 cycles to synthesize poly(EDOT-OH) films. For fabricating nanostructured poly(EDOT-OH)


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film, we dissolved EDOT-OH monomers in a CH2Cl2 solution at a concentration of 10.0 mM in the presence of 1.0×102 mM tetrabutyl ammonium perchlorate as electrolytes. A constant potential of 1.5 V vs. Ag/AgNO3 was applied for 10 s to synthesize the nanostructured poly(EDOT-OH) films. 2.4 Surface Characterization The surface morphologies of AuNP-coated ITO and poly(EDOT-OH) films were obtained by using a BioAFM Resolve (Bruker, United States) microscope operating with SCANASYST-AIR tips from Bruker (spring constant = 0.4 N m−1, frequency = 70 kHz) under PeakForce Tapping mode. 2.5 Electrochemical SERS Measurement SERS measurement was performed with a microscopic Raman system (MRI, Protrustech Co., Ltd, Taiwan). An air-cooling spectrometer (AvaSpec-ULS2048L) of grating 1200 lines/mm and slit 50 μm was used as a detector. An exciting line of 633 nm was supplied by a diode laser (NovaPro) with a power of 75 mW. To avoid laser spot damage on the polymers, we applied 10% power (c.a. 7.5 mW) in our SERS measurement. A 50× long-working-distance objective length was used to focus the laser spot onto substrate in liquids. An auto mapping stage moved every 2 µm in a 6 × 6 µm2 and collected signal consistently in each point as shown in Figure 1. The poly(EDOTOH)/Au/ITO substrates were fixed on a homemade electrochemical cell and a Pt wire was connected to the substrate to create the working electrode. The buffer was filled in the cell until it covered three electrodes. The in situ electrochemical Raman measurement was performed at a laser power of 10% (c.a. 7.5 mW), and the integration time was 10 s. A cyclic potential -0.5 V to 0.5 V was applied to study the dedoped/doped states of two poly(EDOT-OH) films. For hydrogen


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peroxide detection, we first kept poly(EDOT-OH) film staying in a dedoped state through applying a constant voltage of -0.5 V for 30 s. After adding solutions containing hydrogen peroxide, we monitored the change of SERS spectra and collected the signals at 30, 60, 120, and 180 s, respectively. 3. RESULTS AND DISCUSSION 3.1 Synthesis of poly(EDOT-OH) on gold nanoparticle (AuNP) substrates The surface morphologies of AuNP substrates and poly(EDOT-OH) films were shown in Figure 2. After Au seeds were initially generated on ITO surfaces by applying a constant voltage, AuNPs formed and grew when a cyclic potential was applied as shown in Figure 2a. By following previous studies54-55 to create poly(EDOT-OH) films of different surface morphologies directly on ITOcoated glass, we applied the similar electropolymerization procedures to make both homogeneous and tubular poly(EDOT-OH) films on AuNPs as shown in Figure 2b and 2c, respectively. Figure 2b displayed a homogeneous poly(EDOT-OH) films formed on the AuNPs by applying a cyclic potential in aqueous solutions. On the other hand, the poly(EDOT-OH) electropolymerized in CH2Cl2 solutions by applying a constant voltage formed tubular structures as shown in Figure 2c. Although the substrates used in this study were different from previous work, the surface morphology of poly(EDOT-OH) films were similar. These results indicate that the morphology of electropolymerized poly(EDOT-OH) films were mainly determined by the solvent and electrical pulse instead of substrates used for electropolymerization.


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Figure 2. The AFM images showing the surface morphology of a) AuNPs on ITO-coated glass; b) poly(EDOT-OH) films deposited on AuNPs by applying a cyclic potential in water; c) tubular poly(EDOT-OH) films deposited on AuNPs by applying a constant voltage of 1.5 V vs. Ag/AgNO3 for 10 s in CH2Cl2 and d) tubular poly(EDOT-OH) films illustrated as same scale as a) and b) for comparison. 3.2 in situ and real-time EC-SERS. For conducting in situ spectroelectrochemical Raman scattering, we evaluated both the Raman spectra of PEDOT on flat Au substrate and the SERS spectra of PEDOT on AuNPs in DI water, as shown in Figure S1 in Supporting Information. Under the same integration time and laser power, the peak intensities of the SERS spectra were about 10 times stronger than those of normal Raman


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spectra. With the high sensitivity promoted by SERS, we were able to conduct in situ and realtime experiment to monitor the doping/dedoping process of conducting polymers. Figure 3a and 3b show the real-time SERS spectra of p-MBA and homogenous poly(EDOT-OH) films on AuNPs when a cyclic potential was applied at a ramping rate of 20 mV s-1. Both samples were immersed into an aqueous solution in the presence of 1.0×102 mM LiClO4 as electrolyte for EC-SERS measurement. Compared to the SERS spectra from poly(EDOT-OH) films, the peak intensity of p-MBA did not increase or reduce when the applied potentials changed. A 2D figure of figure 3b in the potential range from -0.5 V to 0.5 V clearly illustrated the intensity of main peak at the different potentials for comparison in Figure 3c. We observed the main peak at 1421 cm-1 and another peak at 1503 cm-1, which represents the symmetric and asymmetric Cα=Cβ stretching band, when the applied potential was at between -0.5 and 0 V. The intensity of the main peak at 1421 cm-1 was greatly enhanced when a cathodic potential was applied, indicating the enhancement is mainly attributed to the neutral state of poly(EDOT-OH) instead of doped state.47 The enhancement of intensity at main peak is due to the resonance Raman effect which is contributed by the conjugated structure of neutral poly(EDOT-OH). The signal of symmetric peak at 1421 cm-1 was gradually reduced and broadened when the applied potential increased. The intensity of asymmetric peak at 1503 cm-1 gradually decreased while a broad asymmetric peak at 1545 cm-1 slightly appeared as the applied potential increased, which means the asymmetric stretching dominates in oxidized poly(EDOT-OH). Besides, the SERS spectra change of nanostructured poly(EDOT-OH) when applying the cyclic potential -0.5 V to 0.5 V was presented in Figure S2 in Supporting Information.


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Figure 3. a) 3D plot of p-MBA showing the same Raman spectra when a cyclic potential is applied between -0.5 and 0.5 V vs. Ag/AgCl compared to b) 3D plot of homogenous poly(EDOT-OH) on AuNPs showing that the SERS spectra changed when a cyclic potential applied at between -0.5 and 0.5 V vs. Ag/AgCl in 100 mM LiClO4 solution. c) 2D plot SERS spectra of poly(EDOT-OH) on AuNPs in a 100 mM LiClO4 solution when applying -0.5 V to 0.5 V vs. Ag/AgCl. 3.3 Detection of Hydrogen Peroxide through Controlling the Doping State of Poly(EDOT-OH) Based on the in situ electrochemical SERS of homogenous poly(EDOT-OH) (see Figure 3), the SERS intensity of poly(EDOT-OH) decreased when poly(EDOT-OH) is shifted from its neutral state to doped state during electrochemical oxidation. Therefore, we could expect a similar Raman change when polymer films were oxidized by using chemical oxidants. In that case, the monitoring Raman spectra can be used as a detection mechanism for oxidant detection. Here we used hydrogen peroxide (H2O2) as our model oxidant for proof-of-concept. The detection procedure was illustrated as shown in Figure 4a. We kept our polymer film at a neutral state by applying a constant -0.5 V for 30 s in 1X PBS solution. The SERS intensity was collected every 30 s, 1, 2 and 3 min after H2O2 (in PBS) was added to the electrochemical cell. The SERS spectra change of homogeneous poly(EDOT-OH) in the presence of 4.0×102 μM H2O2 was shown as in Figure 4b. The SERS spectra at 1421cm-1 assigned to Cα=Cβ symmetric stretching gradually decreased when


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the polymer film was oxidized by H2O2. The intensity of asymmetric Cα=Cβ stretching at 1503 cm-1 decreased and the peak became broadened. The change of Raman spectra was the same as the oxidation under electrical stimulation (See Figure 3). We also monitored the change of SERS spectra in the presence of various concentrations H2O2 from 1.0 μM to 1.0 mM as shown in Figure S3 in Supporting Information. The results showed that when the concentration of H2O2 increased, the peak intensity dropped more quickly. To exactly correlate the Raman intensity with the H2O2 concentration, we followed these steps for quantitative analysis. We normalized SERS spectra by setting the intensity of 1421 cm-1 peak as 1.0 just before the H2O2 added into the buffer (0 s). Therefore, the normalized intensity of SERS spectra followed the equation, Normalized Intensity 𝐼𝑡′ = 𝐼𝑡/𝐼0 Where 𝐼𝑡 is the intensity SERS spectra at 30 s , 1, 2 and 3 minutes. 𝐼0 is the intensity of SERS spectra at 1421 cm-1 when t = 0 s. By doing this, we were able to receive a normalized intensity vs. time plot as shown in Figure 4c. Moreover, we took the normalized intensity obtained at 30 s of different H2O2 concentration to compose the normalized intensity vs. concentration as shown in Figure 4d. When the normalized intensity decreased with the increasing H2O2 concentration, which indicates a faster oxidation of poly(EDOT-OH) films at a higher H2O2 concentration. This result is also consistent with the exponential decay fitting curve of normalized intensity which shown red in Figure 4c and Figure S4 in Supporting Information. The fitting equation 𝐼𝑡′ = exp ( ―𝑡 𝜏) , where τ is the lifetime. A higher concentration of H2O2 resulted in a shorter lifetime, which means the intensity decreases faster as H2O2 concentration increases. Based on our results, this detection method showed a linear range up to about 4.0×102 μM and the detection limit was 10.0 μM. We also studied the H2O2 detection curve by tubular poly(EDOT-OH) film. Although we


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expected a different detection range from tubular poly(EDOT-OH) film, however, the results showed much higher deviation compared to the homogenous poly(EDOT-OH) film. Besides, the tubular poly(EDOT-OH) film easily peeled off after repeating tests, which is not suitable for longterm study as discussed in the next paragraph. Therefore, we mainly used homogeneous poly(EDOT-OH) film as our sensing platform.

Figure 4. a) The procedure of hydrogen peroxide detection. A cathodic potential (-0.5 V vs. Ag/AgCl) was applied to reduce poly(EDOT-OH) before adding H2O2. SERS spectra collected at 0 s, 30 s 1, 2 and 3 min after adding H2O2. b) The SERS spectra of poly(EDOT-OH) obtained in the presence of 4.0×102 μM H2O2 at 0 s, 30 s 1, 2 and 3 min and c) the fitting curve of normalized intensity at 1421 cm-1 peak. d) The normalized intensity at 1421 cm-1 peak obtained at 30 s vs.


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H2O2 concentration. Error bars represent the standard deviation, n = 3. The curve fitting and equation shown in the inset figure. 3.4 Reversibility and Long-term Stability We demonstrated the H2O2 detection could be reused simply by applying a reduction potential on poly(EDOT-OH) films as shown in Figure 5a. After the polymer films were oxidized by H2O2, they were in oxidation or doping states. Therefore, by simply applying an cathodic potential at 0.5 V vs. Ag/AgCl to the substrates for 30 s, poly(EDOT-OH) films could return to their reduced states and were able to be used for the oxidant detection again. Figure 5b shows the representative data presenting the multiple and reusable detection from the same poly(EDOT-OH) films. The red and black line represented 4.0×102 μM and 2.0×102 μM H2O2, respectively. We also examined the long-term stability of our poly(EDOT-OH) substrates for H2O2 detection. The same poly(EDOTOH) film was tested for detecting 2.0×102 μM H2O2 continuously for 16 days. The intensities from day 1, 3, 5, 7, 9 and 16 were stable as shown in Figure 5c. These results indicate a potential application for continuous monitoring from our platform.

Figure 5. a) The scheme of reversible H2O2 detection. After oxidizing by H2O2, the poly(EDOTOH) substrate could be reduced and recovered by applying a cathodic potential; b) The reversible detection for 4.0×102 μM (black) and 2.0×102 μM (red) H2O2; c) The long-term stability test of


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poly(EDOT-OH) substrate in 2.0×102 μM H2O2 detection for continuous 16 days. Error bars represent the standard deviation calculated by repeating the measurement at least 3 times. We also tested the film stability under a high anodic potential and discovered that the film either over-oxidized or degraded when a 1.1 V (vs Ag/AgCl) was applied. During over-oxidation, the SERS spectra and intensity show different behavior from chemical oxidation in Figure 6. In Figure 6a, the SERS spectra of poly(EDOT-OH) film were measured when a cycling potential between -0.5 V and 1.1 V was applied. The spectrum intensity was gradually decreased after each cycle and the intensity was not reversible. The symmetric C=C stretching peak decayed quickly and another two peaks at 1450 cm-1 and 1560 cm-1 appeared after the polymer film was overoxidized as shown in Figure 6b. The SERS spectra of the poly(EDOT-OH) film of over-oxidation were totally different from the spectra when film was oxidized by H2O2. Based on these results, the poly(EDOT-OH) films are generally stable for H2O2 detection. The poly(EDOT-OH) films only over-oxidized when a high anodic potential is applied, which results in different and unrecovered Raman shift.


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Figure 6. a) 3D plot of homogenous poly(EDOT-OH) on AuNPs showing that the SERS spectra changed when a cyclic potential applied at between -0.5 and 1.1 V vs. Ag/AgCl in 1.0×102 mM LiClO4 solution. b) SERS spectra of reduced (blue) and over-oxidized (red) homogenous poly(EDOT-OH) on AuNPs. 3.5 Comparison between Different Oxidants Two other oxidants were introduced for the examination of oxidant detection by using our platform, namely ammonium persulfate (APS) and iron(III) chloride (FeCl3). The SERS spectra of poly(EDOT-OH) films oxidized by 1.0 mM APS, FeCl3 and H2O2 were shown and compared in Figure 7. For Figure 7a, 7b and 7c, the black lines represented SERS spectra before adding oxidants; the red line represented the SERS spectra after oxidation by oxidants for 30 s; the blue line represented the SERS spectra of poly(EDOT-OH) after a cathodic potential was applied for 30 s. In Figure 7a and 7b, after the poly(EDOT-OH) were oxidized by APS and FeCl3, we found the SERS peaks at 1421 cm-1 clearly broadened and shifted to a higher wavenumber at 1430 cm-1. The intensity of asymmetric Cα=Cβ stretching peak around 1503 cm-1 and 1545 cm-1 clearly decreased. The peak at 1503 cm-1 was almost disappeared but the peak at 1545 cm-1 was still observed. Compared to Figure 7a and 7b, the SERS spectra of poly(EDOT-OH) oxidized by H2O2 was different as shown in Figure 7c. Although the intensity of symmetric Cα=Cβ stretching peak at 1421 cm-1 also decreased, the peak position did not shift to higher wavelength. Besides, the intensity of the asymmetric peak became very low. The difference can be observed when three spectra are plotted together as shown in Figure 7d. The SERS spectra all returned to the original state after a cathodic potential was applied to reduce poly(EDOT-OH) films, which indicates the oxidation processes by these oxidants are all reversible.


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Figure 7. a) The change of SERS spectra of poly(EDOT-OH) films oxidized by a) APS, b) FeCl3, and c) H2O2. SERS spectra comparison of before oxidants added (black line), oxidation process for 30 s (red line), and after a cathodic potential (-0.5 V vs. Ag/AgCl) was applied (blue line). d) SERS spectra of poly(EDOT-OH) films oxidized by APS (black line), FeCl3 (red line) and H2O2 (blue line) for comparison. We also examined the detection of different oxidants consecutively using one single substrate to clearly illustrate the potential application of continuous monitoring different oxidants as shown in Figure 8. In Figure 8a, 8d and 8e, the spectra returned to the same state after applying a cathodic potential at -0.5 V vs. Ag/AgCl. In Figure 8b, the substrate was first oxidized by H2O2. The intensity of symmetric Cα=Cβ stretching peak at 1421 cm-1 decreased, but the peak position did not


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clearly shift. In Figure 8c and 8f, the SERS peaks at 1421 cm-1 clearly broadened and shifted to a higher wavenumber at 1430 cm-1 after the films were oxidized by APS and FeCl3, respectively. The intensity of asymmetric Cα=Cβ stretching peak at 1503 cm-1 became very low and the peak at 1545 cm-1 was still observed. The observation was similar to what we obtained by using different substrates for different oxidants detection as shown in Figure 7. The results clearly demonstrate the feasibility of continuous monitoring from this platform.

Figure 8. The change of SERS spectra on the same poly(EDOT-OH) film after continual three redox cycles using three different oxidants, namely H2O2, APS and FeCl3. a), d), and e) a cathodic potential (-0.5 V vs. Ag/AgCl) was applied to reduce the film. SERS spectra of poly(EDOT-OH) films oxidized by b) H2O2, c) APS, and f) FeCl3, respectively.


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4. CONCLUSION In summary, we have successfully demonstrated our electrochemical SERS (EC-SERS) platform as a method that can provide in situ and real-time monitoring of the doping/dedoping conversion of poly(EDOT-OH) and a quantitative measurement in oxidant concentration. SERS-active AuNP substrates were first prepared by applying a seed-mediate electrodepositing method. Polymer films of two different surface morphologies, homogeneous and tubular structures, were electropolymerized directly on AuNP substrates for evaluation. A homemade electrochemical cell was placed on the auto mapping stage integrated with a Raman microscope for the EC-SERS experiment. Taking the advantage of the enhanced Raman sensitivity on AuNP substrates, we were able to engage in in situ and real-time monitoring of the doping states of poly(EDOT-OH) films in an aqueous environment. The intensity of both the symmetric and asymmetric Cα=Cβ stretching peaks decreased and increased periodically when a cyclic potential was applied to poly(EDOTOH) films. We further demonstrated a proof-of-concept study showing a potential application on oxidant detection by utilizing this characteristic. We controlled the poly(EDOT-OH) at dedoped state and real-time monitored the spectra change in the presence of H2O2 solutions of various concentrations. This method provided a detection range up to 4.0×102 μM H2O2 and a detection limit of 10.0 μM. Most importantly, the detection of H2O2 was reusable simply by applying a cathodic potential (-0.5 V vs. Ag/AgCl) on poly(EDOT-OH) film. We also estimated the longterm stability and reversibility of our poly(EDOT-OH) films for H2O2 detection by repeating the detection for longer than 2 weeks. To confirm the idea of oxidant detection, we also examined two other oxidants, APS and FeCl3, by using our platform. It was also the first time to observe that the spectra of oxidized poly(EDOT-OH) films are dependent on the oxidants. The spectra of poly(EDOT-OH) films oxidized by APS and FeCl3 showed slight difference compared to the one


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oxidized by H2O2. Other than that, the spectra can also be easily recovered after a cathodic potential was applied. These characteristics suggest a potential application of continuous monitoring of redox states of conducting polymers and quantitative analysis oxidant concentration from our SERS-active platform. Supplementary content. SERS effect on AuNPs, SERS spectra of nanostructured poly(EDOTOH) in redox conversion under cyclic voltammetry, SERS spectra of homogenous poly(EDOTOH) in H2O2 solution and its fitting curve for all concentration, the thickness of homogenous poly(EDOT-OH) films synthesized by cyclic voltammetry with different cycles.

AUTHOR INFORMATION Corresponding Author *Shyh-Chyang Luo Department of Materials Science and Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei, 10617 Taiwan Email: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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ACKNOWLEDGMENT We gratefully acknowledge the financial support provided by the “Advanced Research Center for Green Materials Science and Technology” from The Featured Area Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (107L9006) and Ministry of Science and Technology of Taiwan under grant MOST 106-2113-M002-017-MY2 and 107-3017-F-002-001. REFERENCES (1) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Raman Spectra of Pyridine Adsorbed at a Silver Electrode. Chem. Phys. Lett. 1974, 26 (2), 163-166, DOI: 10.1016/0009-2614(74)85388-1. (2) Stiles, P. L.; Dieringer, J. A.; Shah, N. C.; Van Duyne, R. P. Surface-Enhanced Raman Spectroscopy.

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Electrochemical SERS for in Situ Monitoring the Redox States of

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