The electrode was prepared in two simple steps (Fig. 1) i.e. (1) PEDOT:PSS was electrodeposited on a platinum commercial electrode from an aqueous solution of 10 mM EDOT and 2.5 mg·ml− 1 NaPSS at 1 V vs. Ag/AgClsat with deposition time limited to charge of 1 mC (Fig. 2a); The selected conditions were the state-of-the-art parameters for PEDOT:PSS for the application of different electrode applications20–22; (2) Second step included synthesis and deposition of hydrogel layer on the top of the PEDOT:PSS.
The hydrogel precursor solution was a composition of 97.2 mg N-isopropylacrylamide, used as the main monomer (NIPAAm), 2.8 mg N,N′-methylenebisacrylamide (BIS-AAm) as a cross-linker in the proportion 35:1, dissolved in deionized water (90% wt.). The selected monomer and cross-linking agent lead to the formation of a biocompatible hydrogel, which has recently been used in many applications, including bioelectrodes23,24. In the end, 5 mg of photoinitiator 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone was added to the solution to trigger the hydrogel polymerization reaction upon UV irradiation. Ready hydrogel precursor solution in a volume of 2.5 ul was drop cast on the top of the PEDOT:PSS film and cross-linked by UV irradiation for 2 minutes in an ice bath. As a result, PEDOT:PSS-Hyd coating system was fabricated. The composition of the hydrogel solution was optimized to obtain a hydrogel layer with Young’s modulus of ~ 10–20 kPa 25.
The scanning electrode microscopy (SEM) with energy dispersive X-ray (EDX) mapping analysis proved the uniform distribution of the PEDOT layer at the surface of the platinum (Fig. 2b-e). The thickness of the film was determined to be ~ 280 nm (+/- 30 nm). A highly porous hydrogel layer with a thickness of ~ 350 um was obtained on the top of the PEDOT:PSS electrode (Fig. 3).
The electrical interface parameters such as interfacial impedance, potential window, double-layer capacitance (Cdl), charge storage capacity (CSC), and potential transient (stimulation) performance of the PEDOT:PSS, hydrogel (Hyd) and PEDOT:PSS-Hyd electrodes were determined electrochemically in an aqueous solution of phosphate-buffered saline (PBS). Electrochemical impedance spectroscopy (EIS) spectra of the electrodes are presented in Fig. 4a.
Coated the electrode with PEDOT:PSS and a combination of the polymer with hydrogel resulted in a significant drop of the impedance, especially for the frequency range between 1 kHz and 0.1 Hz. The latter was related to the lower double-layer capacitance (Cdl), and thus the much higher surface area of the PEDOT-coated electrode compared to bare or hydrogel-coated platinum (Fig. 5). The CV curves recorded within the narrow potential range revealed a pure capacitive behavior of the electrodes, thus allowing to determination of the Cdl (Fig. 5a). Modification of the platinum with PEDOT or PEDOT-Hyd increased the electroactive surface area of the electrode around ~ 16 times compared to the bare or hydrogel-based substrate (Fig. 5b). This change influenced |Z|, especially in the high-frequency range. In theory, the recorded |Z| at high frequency i.e. 1000 − 100 Hz is mainly related to the solution resistance measured between working (WE) and counter electrode (CE) 26. Any change in electrode geometry such as the increase in porosity, surface area, etc. will influence |Z| at high frequency due to the change in the distance between WE and CE 27. Since the electrode modified with the polymer or polymer-hydrogel revealed higher Cdl compared to the rest samples, it significantly reduced the |Z| at this frequency (Fig. 4).
Further analysis of the EIS results showed that modification of the metallic electrode only with pure hydrogel resulted in a higher impedance of the electrode, especially observed for |Z| at 10 Hz (Fig. 4b). It indicates that the hydrogel layer blocks the charge transfer resistance between the platinum electrode and the solution, thus decreasing the electroactive surface area of the electrode. Interestingly the fact that combination of PEDOT:PSS with hydrogel layer exhibited the same interfacial |Z| compared to PEDOT:PSS alone.
The potential window of the electrode is another important parameter if the application is bioelectrodes. It should be specified firstly for further reliable evaluation of CSC and potential transient of the electrodes. The potential window corresponds to the safe potential range within which the electrode works efficiently avoiding the reactions of hydrogen and oxygen evolution. For this purpose, the CV of the electrodes was performed firstly within the wide potential range i.e. between − 1.2 to 1.2 V vs. Ag/AgClsat to assess the possible undergoing reactions (Fig. 6a). In general, similar redox behavior was obtained for all of the studied samples. In the case of bare platinum, higher intensity peaks of platinum oxide and hydrogen formation were determined at ~ 0.4 V and − 0.7 V vs. Ag/AgClsat, respectively. Platinum-hydrogen desorption was observed at ~-0.8 V vs. Ag/AgClsat, which is all in agreement with the literature 28. Typical CV peak of PEDOT:PSS oxidation could be observed at around ~ 1.1 V Ag/AgClsat for both PEDOT:PSS and PEDOT:PSS-Hyd electrodes 29. The potential shift of the beginning of water oxidation and reduction was observed between samples with and without PEDOT:PSS layer. Therefore, based on the CV analysis, the potential window was determined to be -0.8 V to 1.1 V vs. Ag/AgClsat for bare platinum and hydrogel-platinum electrodes, and − 0.8 V to 1 V Ag/AgClsat for PEDOT:PSS and PEDOT:PSS-Hydrogel. Coating the bare metal with the polymer resulted in a slight reduction of the potential window, due to the appearance of the polymer oxidation peak, and as a result shift of the water oxidation peak to lower potentials. The potential window was independent of the presence of an additional hydrogel layer on the top of the PEDOT:PSS compared to the polymer film alone.
CSC is the parameter, that revealed the ability to store the charge by the electrodes and was determined based on the CVs performed within the determined potential windows for two different scan rates (Fig. 6b and 6c). The evaluation of CSC at least two different rates is desirable if the application is bioelectrode such as a neural electrode, which can stimulate the neurons at different rates 30. CSC was determined to be the lowest for bare platinum electrodes, which is in agreement with the literature10 (Fig. 6d). A slight increase of CSC could be observed for platinum modified with pure hydrogel (Hyd). Metallic electrode coated with PEDOT:PSS exhibited significantly higher CSC of 480 uC·cm− 2 / +/- 8 uC·cm− 2 (50 mV·s− 1) and 1550 uC·cm− 2/ +/-40 uC·cm− 2 (200 mV·s− 1) compared to Pt and Hyd electrodes. Moreover, PEDOT:PSS combined with hydrogel revealed virtually the same CSC as PEDOT:PSS alone. The higher ability to store a charge for the polymer-based electrode is due to their much higher electroactive surface area (porous structure) compared to the substrate or Hyd alone (Fig. 5).
Additionally, voltage transient was evaluated for each of the studied electrodes (Fig. 7). The latter is especially required to assess in the case of the use of such electrodes for the application of stimulation.
Figure 7. Potential response of Hyd (a), PEDOT:PSS (b), and PEDOT:PSS-Hyd (c) to the biphasic current pulse of 0.5–12 mA. Comparison of potential response for all the studied sample to the biphasic pulse current of +/- 3 mA (d).
For this purpose, a symmetric charge-balance biphasic current pulse with 1 ms pulse width was applied to the electrodes in PBS solution. The current injection limit is defined as a quantity of current, which polarizes the electrode interface to the potential corresponding to the potential window determined from CV. In this work, the current injection limit was determined to be 3 mA for Pt and Hyd (Fig. 7a), and 12 mA for PEDOT:PSS (Fig. 7b) and PEDOT:PSS-Hyd (Fig. 7c). Therefore, when applying a current pulse of 3 mA to the electrodes, PEDOT:PSS and PEDOT:PSS-Hyd revealed the same and much smaller voltage rise across the interface compared to the bare or hydrogel-coated platinum (Fig. 7d). The phenomenon was related to the much lower interfacial impedance (Fig. 4), higher surface area (Fig. 5), and CSC (Fig. 6) of these electrodes compared to the Pt or Hyd.
The preliminary electrochemical stability of the electrodes was also evaluated. Figure 8 presents changes in |Z| (a) and CSC (b) before and after 200 cycles of CV carried out within the determined potential windows for each electrode. In each case, the stability of the electrode decreased. However, the change of the parameters before and after 200 cycles was lower for PEDOT:PSS-Hyd compared to PEDOT:PSS alone. CSC decreased by around ~ 1.5 times and ~ 1.3 times for PEDOT:PSS and PEDOT:PSS-Hyd, respectively. However, |Z| increased by around ~ 1.6 times and ~ 1 times for PEDOT:PSS and PEDOT:PSS-Hyd, respectively. The slower degradation of PEDOT:PSS-Hyd electrode can be due to the hydrogel layer, which partially limits the direct contact of the polymer with the solution, and thus it inhibits the process of conducting polymer overoxidation.