Recent advances in the aqueous applications of PEDOT

Water is ubiquitous in life – from making up the majority of the Earth's surface (by area) to over half of the human body (by weight). It stands to reason that materials are likely to contact water at some point during their lifetime. In the specific case of sensors however, there is a need to consider materials that display stable function while immersed in aqueous applications. This mini-review will discuss the most recent advances (2018 to 2021) in the application of the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) in aqueous environments. At its heart, the use of PEDOT in aqueous applications relies on nanoscale understanding and/or nanoengineered structures and properties. This enables their use in water-based settings such as within the human body or buried in agricultural soils.


Introduction
There is currently a global effort to fabricate and utilise emerging nanomaterials, nanocomposites, and nanostructured materials with desirable characteristics for use in aqueous environments, such as biological uids, aquatic ecosystems, and agricultural soil. [1][2][3][4][5] The biocompatibility and sustainability aspects of the desired nanomaterials have been widely investigated with the aim of addressing concerns about how they behave in the mentioned environments. [6][7][8][9] Subsequent to this, understanding and controlling the nanostructure and nanoscale dynamics of materials is also important to the devices integrating these materials. It is essential for the materials to be both nontoxic and robust, because in most water-based settings living organisms are found. At the intersection of these requirements is the search for materials that are biocompatible, stable, and functional (where function is dependent on the device operation). Conducting polymers (CPs) are one class of materials that have shown good compatibility with living organisms, good environmental stability, and excellent electrical function. [10][11][12][13][14] Among all CPs, poly(3,4-ethylene dioxythiophene) (PEDOT) stands out as the prototypical CP displaying a range of desirable properties (relatively high electrical conductivity, high ambient stability, biocompatibility, tunable optical properties, etc.). [15][16][17] The polymeric structure of PEDOT allows for electrostatic interaction with ions in the surrounding environment, making PEDOT an appropriate active material for sensor developments and controlled release drug delivery systems. [18][19][20] Several recent studies have focused on understanding the interaction of PEDOT (doped with tosylate anions) with ions in water from a fundamental perspective. Delavari et al. 21 computationally and experimentally studied the electrochemically driven ion exchange process in water. They showed that the PEDOT thickness increased upon repeated electrochemical cycling, indicative of water intake (facilitated by the hydration shell of the ions). This aligns well with recent work by Sethumadhavan et al. who experimentally observed the roll of water in the hydration shell around ions in the electrochemical reactions with PEDOT. 22 They showed the classication of ions as structure-breaking or structure-making in water to be important for understanding how CPs interacts with ions in water. 23 How these mechanisms on an atomic/molecular level impact on the deployment of PEDOT-based devices in water applications is of growing importance/interest. Therefore, this mini-review will present a brief overview of the recent key literature demonstrating the use of PEDOT in water-based applications. Focus will be placed on the nanoscale aspects of deploying PEDOT in vitro and in vivo for bioelectronics. Furthermore, this article also reviews the recent literature on the emerging applications of PEDOT in the environment (in the equivalent "environmentalfriendly electronics"). For recent analysis of CPs for antifouling applications, which is an important consideration for their use in water, readers should refer to other reviews. [24][25][26] Similarly, the fundamental properties of PEDOT have also been reviewed elsewhere. 15,27,28

PEDOT in biology
PEDOT-based nanolms/nanoparticles/nanocomposites represent a viable way to interface electronic devices with biological matter in vitro and in vivo. [29][30][31][32][33][34][35][36][37][38][39][40] Routinely studied variants of PEDOT, such as those doped with PSS or tosylate, are promising materials for biological applications. Mokhtar et al. 37 have investigated the stability of various doped PEDOT in biological uid, i.e. articial interstitial uid (aISF), and demonstrated that PEDOT shows higher electrochemical stability in biological environments when it is doped with Tos or it is co-doped with Tos/PSS, as compared to doped with just PSS. Guzzo et al. 41 have recently investigated the in vitro cytotoxicity of PEDOT:Naon, and they have demonstrated that this form of PEDOT is not cytotoxic when it is prepared via water dispersion and an aqueous formulation, suggesting an alternative bioelectronic and neuroelectronic material for long-term applications such as chronic neural recording and stimulation sessions.
Despite the underlying biological performance of PEDOT, in some instances it may be necessary to modify it in some manner to improve its performance for biological applications. For instance, ethylene glycol (EG) based additives are seen as benecial to improve the biocompatibility of the desired biological materials including PEDOT. 42,43 Stříteský et al. 44 used EG to modify PEDOT:poly(styrenesulfonate) (PEDOT:PSS) for biocompatibility testing of electroactive polymer inks for printed bioelectronics. The nanoscale CP layers (30 AE 5 nm) treated with EG served as a platform upon which to seed murine cardiomyocytes derived from embryonic stem cells. Along similar lines, Cellot et al. 45 investigated the nanoscale morphology of PEDOT:PSS doped with different amount of EG (0.1-3%) for electrodes in neural applications. On a material level, for increasing EG content the nanoscale roughness (from AFM images) increased leading to an increase in exposed surface area up to 3% for 3% EG addition. They utilised the EG loaded PEDOT:PSS with hippocampal cultures to observe seeding and long-term proliferation of neuronal and glial cells, as highlighted in Fig. 1.
Poly(ethylene glycol)diglycidyl ether (PEGDE) is a derivative of poly(ethylene glycol) (PEG), which has been widely used for crosslinking of potential materials for biomedical applications. [46][47][48][49] The research of Solazzo et al. 50 introduced PEGDE to crosslink PEDOT:PSS for bio-applications. On a molecular level, the crosslinking occurs via the PEGDE epoxy ring interacting with the sulfonic groups of the PSS. Using CH310 mouse embryonic broblasts, they observed greater degrees of cell spreading on the crosslinked samples compared to the controls, i.e. PEDOT:PSSglycidoxy propyltrimethoxysilane (GOPS); noting that pristine PEDOT:PSS could not be used as a control due to dissolution in the cell culture media. While the crosslinked PEDOT:PSS was quite hydrophilic (water contact angle < 20 ) this alone couldn't explain the good biocompatibility. The introduction of the PEG moiety into the structure on the nanoscale was hypothesised as a key to improved biocompatibility.
In a recent review, Wang et al. discussed in detail the development of electrospun nanobers using CPs for biosensors, neural electrodes, electrodes for stimulated tissue regeneration, and controlled drug delivery. 51 This highlights that considering nanostructures of CPs such as PEDOT is benecial for biological applications. Zhang et al. 52 fabricated electrochemical polymerised PEDOT:PF 6 with intertwined nanobers. These nanostructured PEDOT materials displayed desirable electrical properties (Â150 higher charge storage capacity, Â800 lower impedance, compared to the unmodied electrode) with appropriate in vitro biocompatibility and nontoxicity. This was demonstrated by assessing viability from culturing with PC12 cells, through to adhesion and differentiation of the same PC12 cells. Ultimately the nanostructured PEDOT led to formation of a network of neurites that were also longer and larger in number. 52 An alternative method to achieve desirable nanostructure was presented by Richardson-Burns et al. 53 with electropolymerised PEDOT:PSS around neurons which were subsequently removed using enzymatic and mechanical disruption. The creation of PEDOT:PSS with nanoscale cellshaped holes and imprints showed good performance when re-seeded with SY5Y cells (which showed preference for adhering to regions where the neurons once were).
Opposed to nano-templating the PEDOT, Saunier et al. 54 incorporated carbon nanobers (CNFs) within PEDOT as microelectrodes for neuronal therapies. The PEDOT:CNF electrodes were used in electrochemical sensing of neurotransmitters (dopamine and serotonin). Furthermore, they showed in vitro non-cytotoxicity with SH-SY5Y cell populations having a viability percentage of >99%. In the work of Kumar et al. 55 uoro hydroxyapatite (FHA) nanoparticles were incorporated within PEDOT for use as coatings on implants. The introduction of the FHA nanoparticles led to subtle increases in the microscale roughness of the PEDOT with signicant increase in the water contact angle and the mechanical hardness. These properties combined yielded a coating where the in vitro studies showed good adhesion of MG63 cells (human osteosarcoma cells) and increased levels of proliferation across 7 days of incubation. In implant scenarios, the antibacterial properties are equally importantwith the PEDOT:FHA having signicantly lower attachment and proliferation of both Gramnegative and -positive bacteria over a 24 h period, compared to the uncoated implant.
Rather than using PEDOT as the host matrix or major component of a composite/coating system, nanoparticles for embedding are also interesting for bioelectronics. Huang et al. 56 have incorporated PEDOT nanoparticles into chitin hydrogels to improve sciatic nerve regeneration, by providing a desired physicochemical scaffold to promote the nerve cell proliferation. Chemical oxidative polymerisation was used to create PEDOT:persulfate nanoparticles of 200-300 nm in diameter. The porous structure of the hydrogel containing PEDOT nanoparticles has remarkably supported the enhanced in vitro RSC-96 cell adhesion. Once incorporated into the chitin hydrogel matrix and formed into a scaffold, the authors showed the benet of the PEDOT nanoparticles through scaffold implantation in rat models. The in vivo assessment revealed that aer 20 weeks post-surgery the chitin:PEDOT hydrogel had positively facilitated the sciatic nerve regeneration (Fig. 2).
Beyond presenting a favourable interface for desirable biological matter and/or an electrically conducting surface, PEDOT can also be repeatedly electrochemically doped and undoped in biological environments. Such behaviour makes PEDOT (and other CPs) promising materials for controlled/triggered drug delivery. [57][58][59][60] Despite this promise, there are a relatively small number of research articles published in recent years on this topic. Yasin et al. 61 employed the templating approach to form inverse opal structures of PEDOT:Tos using polystyrene nanospheres as a sacricial support. The resultant 3D PEDOT material was used for loading and release of the model anionic drug dexamethasone phosphate (DexP À ), which is used to treat an inactive/underactive adrenal gland or certain immune disorders and skin problems, asthma or arthritis. Through the creation of the 3D structured PEDOT the available surface area increased by 2.9 times above the unstructured form. Subsequently the passive loading of the DexP À into the templated PEDOT was signicantly higher (almost three times higher) and the lms were more responsive to triggered drug release. Krukiewicz et al. 62 incorporated DexP À into electropolymerised PEDOT for enhancing neural growth. The porous and rough PEDOT structure lead to efficient triggered release of the DexP À which had a positive effect on neurite growth. Woeppel et al. 63 load sulfonate modied silica nanoparticles with electropolymerised PEDOT, and subsequently use these as reservoirs for two bioactive compounds (doxorubicin and melatonin). The PEDOT acted as an electrochemically responsive material that could load and release drugs, while the nanoparticle form factor allowed for delivery within a biological environment.
These studies combined highlight how PEDOT's good electrochemical properties can be combined with nanostructures or nanostructuring to yield benets for biological systemsfrom biosensing to cell growth to drug delivery.

PEDOT in the environment
Water makes up a signicant proportion of the world in which we live. These water environments are oen critical to our existence, for example the agricultural soil where much of our food originates. It stands to reason that materials such as PEDOT have similar utility in these applications as they do in biology. This section briey introduces the recent research using PEDOT in the environmentnamely in plants, in soil, and in relation to contamination (and treatment) in water bodies.  (Fig. 3). Expanding on this, Kim et al. 66 used vapour deposition to tattoo nanolms of PEDOT onto the leaves of Vitis vinifera L. to monitor ozone oxidative damage. Impedance spectroscopy was used to interrogate the PEDOT electrode and make a statement about the ozone oxidative damage suffered by the leaves, as oxidative damage in plants changes the high-frequency impedance above 10 4 Hz. Conversely, bioristors, organic electrochemical transistors for in vivo monitoring of key plant physiology parameters, were fabricated for insertion within the trunk of tomato vines 67 and olive trees 68 to assess changing function of the tree. The bioristors are comprised of commercial textile threads functionalised with EG treated PEDOT:PSS. These studies focused on measuring the bioristors resistance as a function of time and found strong correlation with plant transpiration, and postulate how mineral accumulation within leaves may be monitored. These hold promise for PEDOT, as a ora-compatible material, to enable live monitoring of growing plants.
In a similar manner, PEDOT may be used for live monitoring of agricultural soilsnamely water and nutrients within the soil. Recently, nanocomposite lms of PEDOT:PSS doped with a few wt% of titania nanoparticles (2-10 wt%) were used to determine the moisture content within model soils (montmorillonite and kaolinite). 69 The resistance of the nanocomposite PEDOT:PSS displayed a linear response to changing soil moisture content, with a dependency on the specic soil type itself.
Typically contained within the soil water, nitrate (NO 3 À ) is a key macronutrient in chemical fertilisers critical to plant growth and reproduction in modern agriculture, with overuse leading to contamination of groundwater and waterways, resulting in serious health, environmental and economic damage. 70 Rudd et al. 71,72 have explored the use of vapour deposited PEDOT:Tos for sensing of NO 3 À in soil water. Interestingly the PEDOT nanolms showed strong selectivity for NO 3 À in the concentration range (1 to 100 ppm) typically used in agriculture for healthy plant growth. This was determined by measuring the electrical and optical property changes of the PEDOT nanolms. Shahnia et al. 73 leveraged this research to combine vapour deposited PEDOT nanolms onto the end of optical bres to detect NO 3 À in water with a view to expanding the sensing range to NO 3 À concentrations in aquatic ecosystems.
In some scenarios, ions in water present as an issue and dene as undesired contaminants that need monitoring and/or removal. One ion that is related to overuse of fertiliser is nitrite (NO 2 À ). Pang et al. 74 functionalised PEDOT:PSS with silver nanoparticles to form a sensor for NO 2 À . In this study, the enhanced surface area from the nanoscale roughness combined with the specic surface chemistry was hypothesised to be the origins of the excellent sensing performance. Another class of chemicals that can contaminate water bodies are associated with fungicides and pesticides. In the work of Gao et al. 75  , with detection of the Mancozeb in the mM concentration range. These studies highlight that (nano)composites employing PEDOT, with their resultant nanoroughness and electrochemical properties, are useful in monitoring contaminants in water.
Not only can chemicals be monitored, in some cases PEDOT can be used to assist in the removal and/or degradation of these chemicals. The removal of undesirable chemical components from contaminated water is important for producing water that is safe for a range of purposes, such as drinking, medical, and pharmaceutical applications. Methyl orange (MO) is one of the very common water-soluble azo dyes used in applications such as pH indicators, paper manufacturing, printings, food and pharmaceutical industries, 78 causing shin eczema or intestinal cancer when is in contact with the skin or enters in the digestive system. da Silva et al. 77 modied electrospun polymer bres with PEDOT nanolayers to create a composite membrane for removal of MO from aqueous environments. They demonstrated the selectivity of the PVDF/PEDOT membrane to anionic dyes by exposing the membrane to a mixture of cationic and anionic dyes including MO (Fig. 4). This study showed that a shorter exposure time was required for the PEDOT modied membrane to interact with and absorb the MO and other anionic dyes (relative to cationic dyes) with at least 20 cycles of reusability, and operability over a wide pH range of 3-10. Pharmaceuticals are another example of undesirable chemicals found in water. For example, Metformin is a widely prescribed antidiabetic drug that can be found in reasonable concentrations in wastewater streams. Kumar et al. 79 fabricated PEDOT powders that were used under UV light irradiation to photocatalytically degrade Metformin. These PEDOT (doped with Cl) powders were porous with an average size of 51 nm, leading to an active area of 1 m 2 g À1 , aer use in the photocatalytic process. Like other studies discussed here, real water samples (secondary wastewater effluent) were spiked with the chemical of interest and the performance of PEDOT determined. Again, PEDOT showed good performance under these simulated conditions for degrading the model pharmaceutical drug.

Conclusions and perspectives
As discussed, nanoparticles, nanocomposites and nanolms of PEDOT have gained recognition to be utilised in aqueous environments such as biology, agriculture and so forth. In particular, the fabrication versatility of PEDOT to form nanoscale materials combined with its electrochemical and biocompatibility properties in water-based environments make this polymer exciting for new applications such as biosensing. For example, PEDOT can be directly synthesised in the presence of living cells to bridge the biological signals and electronic processing systems, minimising the degree of foreign-body reaction of tissues. In addition, the improvement of reversible doping and de-doping phenomena in PEDOT has enhanced the design of nano-drug delivery systems with smart stimuliresponsive nanoplatforms. Furthermore, PEDOT possesses the promise of practical applications in a wide range of agricultural and environmental initiatives from chemical sensing to dye removal.
The outlook for future research should take into consideration pathways to commercial devices. This entails the scalability of the fabrication process to yield reproducible PEDOT nanomaterials at scale. Further to this is investigation of the long-term stability of PEDOT in water environments. Not only from a functional stability perspective but also from a safety, toxicity, and contamination perspective. This would allow for PEDOT to bridge the gap from lab-based research to deployment in biologically and/or environmentally relevant devices.

Conflicts of interest
There are no conicts to declare.