Dip Coating of Water-Resistant PEDOT:PSS Films Based on Physical Crosslinking

Water-resistant


Introduction
Organic mixed ion-electron conductors (OMIECs) have attracted considerable interest in electronics and material science fields.[1-2]OMIEC materials transport both ions and electrons (holes); therefore, these materials are desirable for transducing ionic and electronic signals, particularly in biological applications.Bioelectronics is an emerging field that promises improved healthcare monitoring, point-of-care testing, and advanced medical care.[3][4][5][6][7] Biosensing employs various sensing components such as bioelectrodes [8][9][10][11] and organic electrochemical transistors (OECTs).[12,13] A remarkable feature of these devices is that they can operate in aqueous environments, providing significant benefits to medical implant applications.Several previous studies have successfully used these devices to monitor heartbeat, brain activity, and electrophysiology in real time.[14][15][16] An important aspect of aqueous operation is the stability of the film, and the dissolution or delamination of OMIEC films must be avoided.Owing to the nature of OMIECs, the ion transport moieties are located in the hydrophilic part basically.For example, in the wellknown mixed conductor poly (3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), the sulfonate (-SO3 − ) groups on the PSS act as ion-transporting moieties, which stabilize the colloidal particles in commercially available PEDOT:PSS water dispersions.[17][18][19] Crosslinkers are commonly used to stabilize these films against dissolution and delamination of the films by a chemical reaction.Various types of crosslinkers have been proposed for PEDOT:PSS, such as (3glycidyloxypropyl)trimethoxysilane (GOPS), [20,21] divinyl sulfone (DVS), [22,23] and polyethylene glycol diglycidyl ether (PEGDE).[24][25][26] To simplify the fabrication process, these crosslinkers are often added to the film-forming inks before processing.We previously https://doi.org/10.26434/chemrxiv-2023-x7qmdORCID: https://orcid.org/0000-0002-6854-2477Content not peer-reviewed by ChemRxiv.License: CC BY-NC 4.0 applied this method to the inkjet printing of water-resistant PEDOT:PSS layers for OECT applications.[27] Although the addition of crosslinkers simplifies the process, it can induce chemical reactions that destabilize the solution, resulting in aggregate formation or phase separation.Consequently, the use of a crosslinker is beneficial for the facile formation of a stable film; however, the stability of the ink must be monitored.
--<<Figure 1>>--An alternative approach is to perform physical crosslinking process after the film has formed by removing the excess PSS from the film.In typical commercial PEDOT:PSS dispersions (Figure 1) such as Clevios PH 1000, the ratio of PSS to PEDOT far exceeds that of the hole dopant to PEDOT, which stabilizes the colloidal particles.The films produced from these dispersions are soluble in water; however, they can be stabilized by soaking in agents such as ionic liquids [28,29] and concentrated H2SO4 [30,31] to remove the excess PSS.In a previous study, we developed a method to obtain self-standing thick PEDOT:PSS films using a gelfilm formation process with ethanol soaking.[32,33] Although this approach produced selfstanding films with micrometer thicknesses, its application in a thin-film regime has not been tested.
This study presents a crosslinker-free fabrication of water-resistant PEDOT:PSS-based films.
To meet the growing demand for bioelectronic applications, we focused on three aspects: (1) films several tens of nanometers in thickness, (2) film formation on non-planer-shaped substrates, and (3) an annealing-free process under ambient conditions.Processes that meet these criteria will enable the facile coating of implantable bioelectronic devices such as https://doi.org/10.26434/chemrxiv-2023-x7qmdORCID: https://orcid.org/0000-0002-6854-2477Content not peer-reviewed by ChemRxiv.License: CC BY-NC 4.0 needle-shaped electrophysiological probes.Therefore, we employed a dip-coating process to coat differently shaped substrates other than plates.Although dip coating with PEDOT:PSS has been previously reported,[34-37] these studies did not examine the water resistance of the resulting films.We expanded our previously reported gel-film formation process [32,33] to dip-coated PEDOT:PSS films.The resulting PEDOT:PSS films were sequentially immersed in ethanol and water, and their properties were examined.Combined with chemical modification of the substrates to ensure good adhesion, the sequential dipping process afforded water resistance to the films.Spectroscopic and microscopic analyses confirmed that the removal of excess PSS in the dip-coated films proceeded in two stages during each immersion process.In addition, three-dimensional (3D) printed objects were coated to demonstrate conductive coatings on non-planar-shaped substrates.This study proposes a facile and generic methodology to obtain mixed-conductor films on digitally printed objects.

Results and Discussion
--<<Figure 2>>--Figure 2(a) shows photographs of the PEDOT:PSS-coated glass substrates after the three sequential immersion processes of dip coating with PEDOT:PSS, soaking in ethanol, and soaking in water.During the surface modification process, the GOPS concentration was varied between 0.1 and 1 vol%.Delamination was observed on samples modified with 0.1-0.5% GOPS; however, the film on samples modified with 1.0% GOPS remained intact.
Nevertheless, the film retained its shape without dissolving during the water-soaking process, indicating that the ethanol-soaking process afforded water resistance to the film itself, although it did not facilitate adhesion to the glass substrate.X-ray reflectometry (XRR) was used to estimate the thickness and density of the GOPS layers on GOPS-modified substrates with low (0.1%) and high (1.0%)GOPS concentrations (Figure 2 that the adhesion energy of rough-monolayer GOPS was lower than that of spin-coated thick GOPS layers, [41] and the observations of the 0.1% solution treated-sample appeared to correspond to this case.The optical image and XRR results indicated that a multilayered GOPS layer was required to stabilize the PEDOT:PSS film during the water-soaking process.
These results showed that the sequential-soaking process afforded water-resistant PEDOT:PSS films without the addition of crosslinkers to the coating ink or using thermal annealing processes.
--<<Figure 3>>--   based films fabricated using various techniques.Notably, the value observed in this study (500 S cm −1 ) is the highest among the reported values for non-annealed films.The high conductivity indicated that PSS elution from the film bulk was dominant during the ethanolsoaking process, which purified the PEDOT-rich domain and increased the conductivity.In contrast, although the water-soaking process did not significantly impact the composition of the film bulk, it did remove the aggregates from the film surface.Based on the microscopic and spectroscopic data, the proposed mechanism for the formation of a water-resistant PEDOT:PSS film is illustrated in Figure 4(e).During the dip-coating step, a PEDOT:PSS film formed on the substrates and was adhered to the substrate by GOPS via the chemical reaction between PSS and GOPS, as mentioned in previous reports.[21] The subsequent ethanol-soaking process removed the excess PSS from the film bulk and the remaining PSS aggregated on the surface of the film, as observed in the AFM images.The elution of excess PSS continued until the PEDOT to PSS ratio was 1:1.5, at which point all PSS that was not bound to the PEDOT had been removed.This was confirmed by the decrease in the absorbance at 224 nm (Figure 4(a)) because a 39% reduction in absorbance corresponds to PEDOT:PSS = 1:1.5;this was based on an initial ratio of Clevios PH1000 PEDOT:PSS = 1:2.5.[50,51] The subsequent water-soaking process removed the aggregated PSS from the film surface and had minimal impact on the bulk, as indicated by the minor changes in the UV-vis spectra.Throughout the process, removing excess PSS from the film bulk enhanced the resistance of the film to water.The moderately slow elution of PSS during ethanol soaking and the anchoring effect of GOPS prevented the dissolution and delamination of the entire film.
--<<Figure 5>>-- To demonstrate its versatility, the developed technique was used to coat the surfaces of nonplaner objects, including 3D-printed objects.The surface was uniformly coated, and resistance measurement with Kelvin clips was used to confirm a high conductivity of RL −1 = 2.4 Ω mm −1 .This confirms that the technique can be applied to non-planar surfaces in lieu of spin-coating and inkjet printing techniques.Figure 5(b) and (c) shows PEDOT:PSS-coated poly(lactic acid) (PLA) and thermoplastic urethane (TPU) objects printed using a commercially available 3D printer.The complicated surfaces were successfully coated with PEDOT:PSS, which imparted conductivity to the surfaces.In addition, the surface of the object can be modified with GOPS using a vapor-phase treatment under reduced pressure.However, GOPS modification of the sample is not mandatory and a stable coating can be obtained using only oxygen plasma treatment, owing to the presumed formation of polar groups on the surface of the PLA.[52] The stability and conductivity of the PEDOT:PSS coating was tested via bending tests on the coated TPU sheet, as shown in  reported for OECT devices on flat substrates, [12,13] a flexible substrate, [53] and a fully digitally printed device.[27] This confirmed that the PEDOT:PSS films proposed here can serve as a channel layer in OECT devices with non-planar surfaces.In addition, the OECT was applied as a neuromorphic device, as shown in Figure 6(d) -(f), using paired-pulse depression (PPD) and adaptation tests.[54,55] In the PPD test, the drain current exhibited a millisecond-long spike that corresponded to the signal-of-interest, followed by a relaxation to the steady-state on the gate voltage input.Note that the spike is from gate current as we previously pointed out in another paper.[55] The depression degree 1 − A2/A1 was estimated based on the amplitudes of the spikes on the first and second pulses A1 and A2 as a function of the pulse interval Δt (Figure 6(e)).The curves exhibited typical PPD behavior, with the degree of depression decreasing as the pulse interval increased.The retention time was estimated by fitting the curves to an exponential function and the time constant for information retention τPPD was determined to be 178 ms for this device.Based on the time constant, an adaptation test was performed using Δt = 100, 200, and 500 ms.

Conclusions
We developed a facile and generic method to fabricate water-resistant PEDOT:PSS films on non-planar surfaces without the use of chemical crosslinking agents.The analysis highlighted two critical points in the proposed process: (1) elution of excess PSS from the film bulk using ethanol soaking to stabilize the film prior to the water-soaking process, and (2) coating non-planar surfaces, including 3D printed objects, with conductive coatings under ambient conditions.Although further studies are required to elucidate the crystal structure and molecular alignment of the materials, these findings can potentially facilitate the efficient prototyping and production of implantable devices, including OECTs, bioelectronic probes, and bioelectrodes, which can advance the emerging field of organic bioelectronic devices.In particular, the combination of this technique with digital printing technology will enable not only advanced facilities but also small-scale digital fabrication workshops to access cuttingedge conductive polymer technology.

Preparation of PEDOT:PSS Ink
The PEDOT:PSS solution was prepared according to a previously reported method.[54,55] PEDOT:PSS (Clevios, pH 1000) and ethylene glycol (EG) (FUJIFILM Wako Pure Chemical Co.) were used as the polar solvents, and dodecylbenzenesulfonic acid (DBSA) (Tokyo Kasei) as the surfactant; the mixture was sonicated for 15 min.

Preparation of Glass Substrates and 3D-Printed Objects
https://doi.org/10.26434/chemrxiv-2023-x7qmdORCID: https://orcid.org/0000-0002-6854-2477Content not peer-reviewed by ChemRxiv.License: CC BY-NC 4.0 Glass, quartz, and silicon substrates were cleaned in an ultrasonic bath with acetone, isopropyl alcohol, and deionized (DI) water, and treated with UV-ozone for 30 min.The substrates were immersed in an anhydrous toluene solution of GOPS (Tokyo Kasei) at concentrations of 0.1-1 vol% and stored overnight in a desiccator.The treated substrates were thoroughly rinsed with toluene and dried under a stream of nitrogen gas.The 3D-printed objects were prepared using commercially available printers (Dreamer and Finder 3; Flashforge) with thermal extruders.PLA and TPU filaments (Flashforge) were used for printing.The printed objects were treated with an oxygen plasma using a plasma etcher (SEDE-MN; Meiwafosis) before coating.

Dip Coating and Sequential Immersion
The substrates were dip coated with the PEDOT:PSS ink at room temperature with a dip coater (ND-0407-S4; SDI).The coating was performed at a drawing speed of 50 μm s −1 and the coated substrates were dried in a desiccator overnight.The coated objects were immersed in ethanol for 3 h and then in water overnight.The final sample was dried under a stream of nitrogen gas.

Analysis
The surface morphology and thickness were observed using an AFM (SPA400; Seiko Instruments) with Si cantilevers (SI-DF20, 15 N m −1 ; Hitachi) in dynamic mode.XRR measurements were performed using an X-ray diffractometer system (SmartLab; Rigaku).

Pipette-Tip OECT Device Fabrication
A polypropylene pipette tip (200 μL; AS ONE Corporation) was masked using masking tape, treated with oxygen plasma, and coated with a dispersion of gold nanoparticles (Dry Cure Au-J, C-INK), followed by thermal treatment at 90 °C to fabricate the source and drain electrodes.

Figure 2 (
Figure 2(c), which indicated that the thickness of the GOPS on the high concentration-

Figure 3 (Figure 4
Figure 3(a-c) shows atomic force microscopy (AFM) topographic images of the

Figure 4 Figure 4 (
Figure 4(d) shows the conductivity σ of the film estimated using the four-terminal

Figure 5 (
a) shows a coated glass bar with a diameter of 5 mm after GOPS modification, similar to the glass slides shown in Figure 2(a).

Figure 5 (
e) shows the resistance change under repeated bending to L/L0 = 0.2.The change in resistance upon bending was less than 5%, indicating the stable coating and adhesion of the PEDOT:PSS layer to the TPU object.These observations confirmed the conductivity and durability of the PEDOT:PSScoated layer.https://doi.org/10.26434/chemrxiv-2023-x7qmdORCID: https://orcid.org/0000-0002-6854-2477Content not peer-reviewed by ChemRxiv.License: CC BY-NC 4.0 --<<Figure 6>>--The dip-coated PEDOT:PSS films were used to fabricate an OECT device on a non-planar surface.Figure 6 (a) shows a photograph of an OECT device fabricated in a polypropylene pipette tip with a source-to-drain gap of ca. 1 mm.The device exhibited depression mode operation in phosphate-buffered saline (PBS) (Figure 6(b) and (c)).Similar results have been

Figure 6
(f)shows the resulting adaptations in the OECT drain current.The drain current response to the pulse train adapted to a steady-state value when Δt ≾ τPPD.This was because the injected cations from the PBS buffer had insufficient time to return to the electrolyte before a new pulse was injected.In contrast, the long interval of Δt = 500 ms was sufficient for the cations https://doi.org/10.26434/chemrxiv-2023-x7qmdORCID: https://orcid.org/0000-0002-6854-2477Content not peer-reviewed by ChemRxiv.License: CC BY-NC 4.0

Table 1 .
Conductivities of PEDOT:PSS materials prepared via various methods.