Lauri M arttila A I 681 A N N A LES U N IV ERSITATIS TU RK U EN SIS ISBN 978-951-29-9095-5 (PRINT) ISBN 978-951-29-9096-2 (PDF) ISSN 0082-7002 (Print) ISSN 2343-3175 (Online) Pa in os al am a O y, Tu rk u, F in la nd 2 02 2 TURUN YLIOPISTON JULKAISUJA – ANNALES UNIVERSITATIS TURKUENSIS SARJA – SER. AI OSA – TOM. 681 | ASTRONOMICA – CHEMICA – PHYSICA – MATHEMATICA | TURKU 2022 OXIDATIVE MULTILAYER FILMS AND EUMELANIN ANALOGS FOR BIOCOMPATIBLE SUPERCAPACITORS Lauri Marttila Lauri Marttila OXIDATIVE MULTILAYER FILMS AND EUMELANIN ANALOGS FOR BIOCOMPATIBLE SUPERCAPACITORS TURUN YLIOPISTON JULKAISUJA – ANNALES UNIVERSITATIS TURKUENSIS SARJA – SER. AI OSA – TOM. 681 | ASTRONOMICA - CHEMICA - PHYSICA – MATHEMATICA | TURKU 2022 University of Turku Faculty of Science Department of Chemistry Chemistry Doctoral Programme in Exact Sciences Supervised by Professor, Jukka Lukkari Department of Chemistry University of Turku Turku, Finland Adjunct Professor, Mikko Salomäki Department of Chemistry University of Turku Turku, Finland University Teacher, Henri Kivelä Department of Chemistry University of Turku Turku, Finland Reviewed by Doctor, Klemen Pirnat Department of Materials Chemistry National Institute of Chemistry Ljubljana, Slovenia Professor, Tomi Laurila Department of Electrical Engineering and Automation Aalto University Espoo, Finland Opponent Associate Professor, Radosław Mrówczyński Faculty of Chemistry Adam Mickiewicz University Poznań, Poland The originality of this publication has been checked in accordance with the University of Turku quality assurance system using the Turnitin OriginalityCheck service. ISBN 978-951-29-9095-5 (PRINT) ISBN 978-951-29-9096-2 (PDF) ISSN 0082-7002 (Print) ISSN 2343-3175 (Online) Painosalama, Turku, Finland 2022 3 UNIVERSITY OF TURKU Faculty of Science Department of Chemistry Chemistry LAURI MARTTILA: Oxidative Multilayer Films and Eumelanin Analogs for Biocompatible Supercapacitors Doctoral Dissertation, 277 pp. Doctoral Programme in Exact Sciences (EXACTUS) December 2022 ABSTRACT The future applications of implantable electronic medical devices set heavy requirements for the materials powering the devices. They need to be biocompatible or even biodegradable and have sufficient electrochemical properties. The primary goal of this work was to provide knowledge on materials that can be potentially used as electrode materials in biocompatible and/or biodegradable supercapacitors. The first material of interest was eumelanin, pigment found e.g. in the human skin. We studied synthetic eumelanin analogs polydopamine (from dopamine) and DHI-melanin (from 5,6-dihydroxyindole (DHI)). Dopamine has multiple oxidation routes and we focused on the critical initial steps in the Raper-Mason pathway, which leads to the DHI-melanin-like compound. We studied the pH dependence of the dopamine oxidation in two cases: (1) oxidation by O2 only, and (2) oxidation in the presence of Ce(IV), Fe(III) or Cu(II) ion. Secondly, we oxidized DHI to DHI-melanin directly on top of the oxidative film and analyzed its chemical composition in detail. As a second topic, we studied the oxidative layer-by-layer (LbL) films composed of polyphosphate (PP), graphene oxide (GO) and Ce(IV)/Ce(III) ions: (PP/Ce/GO/Ce)n. Ce(IV) ions in these films can oxidize monomers from their surrounding solution onto the film structure. In this work, we prepared oxidative films with advanced spin-spray LbL method. Build-up time of the films was much faster compared with the traditional dip-LbL method, the films were smoother, they had a stratified structure, and their thickness could be controlled with high precision. Oxidative films can be used to prepare thin electroactive GO/PEDOT films with ca. 80 % w/w biocompatible conducting polymer PEDOT. These films have promising charge transfer kinetics and ca. 100 F/cm3 capacitance. By using electrochemical impedance and capacitance spectroscopies, we demonstrated that the main contribution to the capacitance comes from redox processes. (PP/Ce/rGO/Ce)15/(DHI-melanin) film had even higher capacitance. However, the results emphasize that DHI-melanin must be mixed with more electronically conductive materials to improve its dynamic properties. 4 TURUN YLIOPISTO Matemaattis-luonnontieteellinen tiedekunta Kemian laitos Kemia LAURI MARTTILA: Hapettavat monikerroskalvot ja eumelaniinianalogit materiaaleina bioyhteensopivissa superkondensaattoreissa Väitöskirja, 277 s. Eksaktien tieteiden tohtoriohjelma (EXACTUS) Joulukuu 2022 TIIVISTELMÄ Bioyhteensopivien tai jopa biohajoavien elektronisten lääketieteellisten laitteiden kehittäminen asettaa korkeat vaatimukset materiaaleille, joista laitteet saavat virtaa. Tämän työn päätavoitteena oli tuottaa tietoa materiaaleista, joita voidaan potentiaalisesti käyttää elektrodimateriaaleina bioyhteensopivissa ja/tai bio- hajoavissa superkondensaattoreissa. Olemme erityisen kiinnostuneita eumelaniinista, jota on pigmenttinä esimerkiksi ihmisen ihossa. Tutkimme synteettisiä eumelaniinianalogeja, polydopamiinia ja DHI-melaniinia, joita valmistetaan dopamiinista ja 5,6-dihydroksi-indolista (DHI). Dopamiinilla on useita hapettumisreittejä ja me keskityimme kriittisiin ensi- askeleisiin Raper-Mason-reaktioreitissä, joka johtaa DHI-melaniinin kaltaiseen yhdisteeseen. Tutkimme dopamiinin hapettumisen pH-riippuvuutta kahdessa tapauksessa: (1) hapettimena vain O2 ja (2) hapettuminen Ce(IV)-, Fe(III)- tai Cu(II)- ionin läsnäollessa. Hapetimme myös DHI:a DHI-melaniiniksi suoraan hapettavan kalvon pinnalle ja analysoimme sen kemiallista koostumusta yksityiskohtaisesti. Tutkimme myös hapettavia kerros-kerrokselta (LbL) kalvoja, jotka koostuvat polyfosfaatista (PP), grafeenioksidista (GO) ja Ce(IV)/Ce(III)-ioneista: (PP/Ce/GO/Ce)n. Kalvoissa olevat Ce(IV)-ionit voivat hapettaa monomeerejä ympäröivästä liuoksesta kalvon rakenteeseen. Väitöskirjatyössäni valmistimme hapettavia kalvoja kehittyneellä ruiskutuspyöritys-LbL-menetelmällä. Näiden kalvojen kerrostusaika on paljon nopeampi verrattuna perinteiseen kastomene- telmään. Lisäksi kalvot ovat paljon tasaisempia, niillä on kerrostunut rakenne ja niiden paksuutta voidaan kontrolloida hyvin tarkasti. Hapettavia kalvoja voidaan käyttää ohuiden, sähkökemiallisesti aktiivisten GO/PEDOT-kalvojen valmistukseen, joissa on noin 80 % w/w bioyhteensopivaa PEDOT-johdepolymeeriä. Näillä kalvoilla on lupaava varauksensiirtokinetiikka ja noin 100 F/cm3 kapasitanssi. Havaitsimme sähkökemiallisesta impedanssi- spektroskopiasta johdetulla kapasitanssispektroskopialla, että suurin osa kalvojen kapasitanssista on peräisin redoksiprosesseista. (PP/Ce/rGO/Ce)15/DHI-melaniini- kalvolla oli vielä suurempi kapasitanssi. Tulokset kuitenkin korostivat, että DHI- melaniinia on seostettava paremmin elektronisesti johtavien materiaalien kanssa sen dynaamisten ominaisuuksien parantamiseksi. 5 Table of Contents Table of Contents ............................................................................ 5 Abbreviations ................................................................................... 7 List of Included Original Publications ........................................... 9 List of Other Related Publications ............................................... 10 1 Introduction ............................................................................. 11 1.1 PEDOT ................................................................................... 12 1.2 Melanin .................................................................................. 14 1.2.1 Family of eumelanin-like materials............................... 15 1.2.2 Structural overview of polydopamine ........................... 15 1.2.3 DHI-melanin ................................................................ 18 1.2.4 Electrochemical properties of eumelanin ..................... 19 1.3 Oxidative multilayer films ........................................................ 20 1.3.1 Oxidative spin-spray-(PP/Ce/GO/Ce)n multilayers ....... 21 1.3.2 Comparison of oxidative multilayers to other techniques ................................................................... 23 2 Results and Discussion .......................................................... 25 2.1 The oxidation of dopamine to polydopamine .......................... 25 2.1.1 Effect of pH on the first steps of dopamine autoxidation ................................................................. 26 2.1.2 Transition-metal-assisted oxidation of dopamine ......... 30 2.2 Oxidative multilayer films ........................................................ 34 2.2.1 The operating principle of oxidative (PP/Ce/GO/Ce)n multilayer films............................................................. 34 2.2.2 Growth of spin-spray (PP/Ce/GO/Ce)n multilayers and advantages of the assembly method over dip-LbL ....... 38 2.2.3 Electrochemical properties of spin-spray (PP/Ce/GO/Ce)n/PEDOT films ..................................... 41 2.2.4 The structure and the capacitance of the spin-spray (PP/Ce/GO/Ce)n/(DHI-melanin) films ........................... 44 3 Summary and outlook ............................................................ 49 3.1 The oxidation of dopamine to polydopamine .......................... 49 3.2 Oxidative multilayer films ........................................................ 50 6 Acknowledgements ....................................................................... 52 List of References ......................................................................... 53 Original Publications ..................................................................... 67 7 Abbreviations (a1/a2/.../ax)n Multilayer with n (a1/a2/.../ax) layers, e.g. (PP/Ce/GO/Ce)n AFM Atomic force microscopy Ag/AgCl Silver/silver chloride reference electrode CV Cyclic voltammetry DA Dopamine DAC Dopaminechrome DAL Leucodopaminechrome DFT Density functional theory DHI 5,6-Dihydroxyindole DHICA 5,6-dihydroxyindole-2-carboxylic acid DHQ Dopaminehydroquinone DQ Dopaminequinone DSQ Dopaminesemiquinone EDLC Electrochemical double-layer capacitor EDOT 3,4-ethylenedioxythiophene EDS Energy dispersive X-ray spectroscopy analysis EIS Electrochemical impedance spectroscopy GO Graphene oxide HP Hydrogen peroxide H2Q Hydroquinone IQ 5,6-Indolequinone LbL Layer-by-layer LPDP Liquid phase depositional polymerization NMR Nuclear magnetic resonance spectroscopy oCVD Oxidative chemical vapor deposition PAH Poly(allylamine hydrochloride) PDCA Pyrrole-2,3-dicarboxylic acid PEDOT Poly(3,4-ethylenedioxythiophene) PP Polyphosphate PSS Polystyrene sulfonate Q Quinone 8 QM Quinone methide rGO Reduced graphene oxide (r)GO Graphene oxide or reduced graphene oxide SEM Scanning electron microscopy SO Superoxide SQ Semiquinone SWLI Scanning white light interferometry UV-Vis Ultraviolet-visible VPP Vapor phase polymerization XPS X-ray photoelectron spectroscopy 9 List of Included Original Publications The experimental part of this thesis is based on the following original publications, which are referred to in the text by their Roman numerals: I Salomäki, M.; Marttila, L.; Kivelä, H.; Ouvinen, T.; Lukkari, J. Effects of pH and Oxidants on the First Steps of Polydopamine Formation: A Thermodynamic Approach. J. Phys. Chem. B, 2018; 24: 6314–6327. https://doi.org/10.1021/acs.jpcb.8b02304 II Salomäki, M.; Marttila, L.; Kivelä, H.; Tupala, M.; Lukkari, J. Oxidative Spin- Spray-Assembled Coordinative Multilayers as Platforms for Capacitive Films. Langmuir, 2020; 24: 6736–6748. https://doi.org/10.1021/acs.langmuir.0c00824 III Marttila, L.; Salomäki, M.; Kivelä, H.; Hassinen, J.; Granroth, S.; Mäkilä, E.; Nyman, J.; Lukkari, J. Effect of Monomers and Deposition Conditions on Capacitive Polymer Films Prepared Using Oxidative Multilayers. Submitted. The original publications have been reproduced with the permission of the copyright holders. Article I (2018): Creative Commons (CC BY) license; Article II (2020): Creative Commons (CC BY) license. Doctoral candidate’s role under the following themes in each publication: Research design I: Practical detailed research design during the laboratory work. II: Suggestions for research design, and practical detailed research design during the laboratory work. III: Participation in research design. Practical/laboratory work I: Most of the experimental work. II: Practically all experimental work. III: All experimental work except NMR, SEM-EDS, and light interferometry. Data processing I: Participation in data processing. II: Most of the data processing. III: Most of the data processing except NMR, kinetic modelling, and light interferometry. Writing of the manuscript I: Participation in writing of the manuscript. II: Coauthoring of the manuscript. III: Principal author. 10 List of Other Related Publications Salomäki, M.; Ouvinen, T.; Marttila, L.; Kivelä, H.; Leiro, J.; Mäkilä, E.; Lukkari, J. Polydopamine Nanoparticles Prepared Using Redox-Active Transition Metals. J. Phys. Chem. B, 2019; 11: 2513–2524. https://doi.org/10.1021/acs.jpcb.8b11994 11 1 Introduction Our everyday lives are full of electronical devices that increase our standard of living. As a downside, 53.6 million tons of electronic waste was produced in 2019 and the amount is projected to grow sharply.1 Less than one-fifth of this waste was documented as properly collected and recycled.1 Electronical devices are typically designed for long-lifetime operation, but recent reviews show that there is a large interest towards biocompatible and even biodegradable electronic materials and devices.2–7 Biodegradable electronics could be used in many special applications such as environmental monitors, consumer electronics and data-secure hardware,2 easing the electronic waste management problem and opening possibilities for the large-scale environmental analysis. Additionally, ideal transient implantable medical devices would be safely absorbed by the body after their intended lifetime, which would decrease the risk of infection and eliminate the need for surgery for device retrieval.2,3 The heart of an electronical device is its power source. However, as it has been reviewed recently,2,4–7 the development of biocompatible and biodegradable power sources is still an emerging technology. The materials development for these devices is not an easy task since they need to be nontoxic, biocompatible, and biodegradable, have appropriate mechanical properties such as sufficient flexibility, and still have high-quality electrochemical performance.5 In this thesis, I will discuss three main topics relevant to the development of active energy storage materials for supercapacitors. First, I will discuss two interesting energy storage materials for the power sources: classical conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) and natural pigment eumelanin and its synthetic analogues (both are hereby referred as eumelanins or melanins for simplicity unless otherwise specified). The conducting polymer PEDOT is not biodegradable, but it is biocompatible.8–10 Its composites have high potential in supercapacitors powering wearable health monitoring devices11–14 and even implantable bioelectronics15. Melanin, on the other hand, is both biocompatible and biodegradable,16,17 and its potential as an energy storage material in both batteries18–22 and supercapacitors23–27 has been demonstrated in the literature. Lauri Marttila 12 The formation reactions of PEDOT and eumelanins are both complex oxidative polymerization processes,28–30 and, as a consequence, properties of them and their composites naturally vary according to the chosen fabrication method. Thus, as a third main topic in the thesis, I introduce the relatively little studied method for the fabrication of electroactive films: oxidative multilayers. These layer-by-layer (LbL) films can oxidize EDOT or 5,6-dihydroxyindole (DHI) monomers from ambient directly onto the multilayer structure (to PEDOT or DHI-melanin respectively) and thus represent a general technology platform for the fabrication of electroactive films [articles II, III]. Polydopamine is chemically similar to DHI-melanin when it is formed by the oxidation of dopamine under suitable conditions.31 Therefore, we also studied the polydopamine formation reaction in detail.[article I] The more specific main aims of this thesis are twofold. • To develop the method of electroactive film fabrication by oxidative films. I studied the fabrication of the oxidative films by spin-spray layer- by-layer assembly method. As a model system for electroactive films, I polymerized PEDOT onto the oxidative films and studied especially the composition and electrochemical properties of the films. • To study the preparation of synthetic eumelanin analogues polydopamine and DHI-melanin. First, I studied the critical first steps in the oxidation of dopamine to 5,6-dihydroxyindole and further to polydopamine. Second, I oxidized DHI to DHI-melanin onto the oxidative films, studied its chemical composition and showed that the film is electroactive with promising electrochemical properties worthy of further studies. Our primary research questions can be formulated as follows: 1) can the oxidative multilayer films be used for the preparation of good quality DHI-melanin films; 2) is DHI-melanin a suitable material for supercapacitors. These topics will be discussed from the chapter 2 onward. First, I will give a brief introduction to the materials of interest: PEDOT and melanin, and to the oxidative multilayer films. 1.1 PEDOT The conducting polymer PEDOT (Scheme 1) is one of the most studied materials in the world.32,33 It is commercially available either as a monomer (EDOT) or polyelectrolyte complex with poly(styrene sulfonate) counterion (PEDOT:PSS). Both of them are water-processable, even though the polymer itself is not water soluble, since PEDOT:PSS is dispersible in water and EDOT has a moderate solubility of 2.1 g/L[34]. Introduction 13 Despite their name, conducting polymers are essentially nonconductive in their neutral state. However, they can be chemically or electrochemically reduced (n-doping) or, more commonly, oxidized (p-doping) to a more conductive form. The high stability of the p-doped state sets PEDOT apart from other conducting polymers.35–39 In fact, pristine PEDOT is in a heavily oxidized state with typical 33 % oxidation level, i.e. one charge per three monomer units.28,40 This oxidized polymer is no longer an insulator. Instead, positively charged quasiparticles (holes) can now move in the conjugated polymer matrix. PEDOT chains with odd or even oxidation levels are called a polaronic or bipolaronic state, respectively.41 As recently reviewed by Chen et al.,37 the conductivity of PEDOT varies highly from ~0 up to 8797 S/cm[42] and depends, e.g., on the structure and counterions in the material. S O O S OO S O O S OO S O O S OO Neutral PEDOT S O O S OO S O O S OO S O O S OO Polaronic state S O O S OO S O O S OO S O O S OO Bipolaronic state Scheme 1. Neutral PEDOT polymer chain and its polaronic and bipolaronic states. Adapted from [43]. PEDOT films can be utilized as pseudocapacitive components in supercapacitors.44 In general, supercapacitors have long cycle life and high power density but low energy density, making them exact opposites to batteries. However, supercapacitors can be further divided into two sub-classes: electrochemical double-layer capacitors (EDLCs) and pseudocapacitors. The former store energy electrostatically within the electrical double layer at the electrode–solution interface.[article II] Pseudocapacitors, on the other hand, store the energy in the oxidation state of the electroactive materials in addition to the interfacial electrostatic double layer.[article II] The pseudocapacitive behavior brings the materials closer to batteries: improving their energy density while decreasing their power density and cycle life. PEDOT is among the three main conducting polymers studied as a supercapacitor electrode materials together with polyaniline and polypyrrole.45 Based on density functional theory (DFT), theoretical volumetric capacitance of Lauri Marttila 14 PEDOT is ≈100 F/cm3.46 Its mass capacitance is ca. 100 F/g[47,48] and ca. 30 F/cm3[49,50] or 330 F/cm3[51] volumetric capacitances have been reported for PEDOT-based materials. However, pure conducting polymers have shown many problems for the application such as relatively poor electrical conductivity, low energy density or poor cyclic stability.45 Scientists are actively trying to overcome these challenges by composite fabrication, and many recent reviews highlight the potential to achieve further technological advances in the area, e.g., by mixing conducting polymers with metal oxides and/or carbon materials.44,52,53 Much work has been done in the field already, but it is a major challenge to optimize the favorable interfacial connections in these multiphase materials.27,44,52 According to the detailed review of PEDOT-based composites as the active materials of supercapacitors by Zhao et al.,44 the mass capacitances of PEDOT/Carbon and PEDOT/MnO2 nanocomposites are in the order of ~200 F/g, but much higher values have been reported as well. In the Chapter 1.3, I introduce still relatively little explored template polymerization method for the conducting polymer/graphene oxide composite film fabrication: oxidative multilayer films [articles II, III]. 1.2 Melanin Melanins have been a hot topic in materials research since the article by Lee et al. in 2007.54 They reported that various substrates can be easily coated with mussel- inspired synthetic eumelanin analog polydopamine simply by immersing the objects in a dilute aqueous solution of dopamine. At its simplest, the process is initiated by the oxidation of dopamine by dissolved oxygen (autoxidation)54,55 that accelerates with increasing pH [article I]. In their review, Ryu, Messersmith and Lee state55 that the range of surfaces that have been successfully coated with polydopamine is nowadays very broad and includes materials with low surface energy like graphene56 that are normally very difficult to coat. As a consequence, polydopamine coating has been called as the first material-independent surface chemistry process that modifies the properties of virtually any surfaces.57 In addition to the simplicity and versability of the method, it is very cost effective (solid dopamine∙HCl from Sigma-Aldrich costs less than 5 € per gram58).55 Finally, polydopamine contains both nucleophilic and electrophilic groups (see Scheme 2 and the structural discussion below) and its rich chemistry allows further chemical modification of the coating30,59 via Michael addition of amines or thiols30,54,60, metal-ion coordination61–63 or carbonization64,65, just to name a few methods (see ref. [30] for an extensive review of the reaction behavior of polydopamine). Introduction 15 1.2.1 Family of eumelanin-like materials There are many eumelanin-like materials besides polydopamine. First of all, eumelanins are abundant in nature and can be found, e.g., in the skin, hair and eyes of animals, and have a central role e.g., as pigments and photoprotectors.66,67 Notably, eumelanin extracted from the cuttlefish Sepia officinalis is commercially available. On the other hand, synthetic eumelanin analogs can be fabricated from various precursors in addition to dopamine. Eumelanin biosynthesis begins with the oxidation of tyrosine to L-DOPA.68,69 Other notable synthetic eumelanin precursor monomers include 5,6-dihydroxyindole (DHI), 5,6-dihydroxyindole-2-carboxylic acid (DHICA), tyramine, epinephrine, and norepinephrine.68 Now, it is important to emphasize that even though all the above-mentioned materials are classified as natural or synthetic eumelanins, they are expected to have structural differences which induce different properties30,68. In addition to the choice of a precursor, the structure and properties of synthetic eumelanins depend on the reaction conditions (e.g. oxidant, pH, reaction time) and post-treatment procedures (e.g. precipitation, centrifugation, lyophilization and storage conditions).70 In particular, the composition of eumelanin films are expected to differ from that of eumelanin particles in suspensions.30,71 The structural variation is no doubt a huge analytical challenge and it may be misleading to compare materials prepared under different conditions30,70 but, on the positive side, it gives a large freedom for materials design72. For completeness, while I focus only on eumelanin and its synthetic analogs in this thesis, I find it necessary to note that four other natural melanin classes have been found66,73. Sulphur containing pheomelanin and neuromelanin are found in the animal kingdom, whereas nitrogen-free allomelanin and pyomelanin are found in plants and microorganisms.66,73 1.2.2 Structural overview of polydopamine The exact elucidation of the chemical composition and structure of natural and synthetic eumelanins may be impossible30 due to their notorious structural complexity30,72,74 and insolubility that makes them difficult to study67. Due to the complexity of the topic, I will only briefly summarize the excellent structural overview of polydopamine given by Liebscher,30 adding details when relevant. (Beyond this thesis, the article by Liebscher30 and citations within, the reader is referred to the articles by d'Ischia et al.72, Alfieri et al.71, Xie et al.73, Lyu et al.75 and citations within for more comprehensive picture.) The simplified structure and formation of polydopamine is best outlined by the reaction mechanism that is very similar to the historical Raper-Mason scheme for the enzymatic oxidation of tyrosine to eumelanin by tyrosinase76–78 (Scheme 2). In Lauri Marttila 16 the initial step, dopamine (DA) oxidizes to dopaminesemiquinone (DSQ), which further reacts to dopaminequinone (DQ).[article I] This is followed by intramolecular cyclization to leucodopaminechrome (DAL), subsequent oxidation to dopaminechrome (DAC) and molecular rearrangement to 5,6-dihydroxyindole (DHI). The final product, polydopamine, is then formed in subsequent oxidation and coupling reactions. Quinone moieties in the polydopamine are redox active and have three possible oxidation states (hydroquinone, semiquinone and quinone), leading to redox disorder.30,79[article I] Further, it has been shown experimentally that polydopamine contains pyrrolecarboxylic acid units that can be explained by the oxidative degradation of indole quinone moieties (Scheme 3).30,31,80 For example, in the autoxidation of dopamine, oxygen is first reduced to superoxide radical O2·– (or perhydroxyl radical, HO2· depending on the pH) and then to hydrogen peroxide in two one-electron steps.[article I] These highly reactive oxygen radicals can attack the material, leading to the oxidative degradation structures.[article I] Scheme 2. Simplified reaction pathway for the oxidation of dopamine (DA) to polydopamine.[article I] N H HO2C HO2C N H H2C HO2C NH HO2C Scheme 3. Pyrrolecarboxylate units in polydopamine that form in the oxidative degradation of the indole quinone moieties. Adapted from [30]. Introduction 17 The final product, polydopamine, can be thought as a mixture of monomers and/or oligomers with different chain lengths that form a supramolecular structure by weak interactions such as hydrogen bonding, π–π interactions, cation–π interactions and quinhydrone-like interaction.30,31,74,80–87 However, Delparastan et al. have recently confirmed the existence of high-molecular-weight polymer chains in the polydopamine films.74 Furthermore, while the Scheme 2 is a well-established description of the initial steps in the dopamine autoxidation,[article I] its actual structure is much more complicated due to the rich reactivity of the monomer units.30,59,71,75 For example, the oxidation pathway of dopamine is complicated by the Schiff base formation reaction between a quinone ring and an amine group, which may lead to the formation of more complicated structures (Scheme 4).75 Overall, the composition of polydopamine can be significantly different to that of DHI-melanin, an eumelanin analog that is prepared by using 5,6-dihydroxyindole as a precursor.30,71,88 However, as noted above, the reaction route can be controlled by altering the preparation conditions like dopamine concentration or oxidizing agent.30,31,74,80 For example, low dopamine concentration increases the probability of intramolecular cyclization and can thus lead to materials resembling DHI-polymers.31 HO HO NH2+O O HN HO HO H N O HN OH OH H2N HO OH N N N O O Scheme 4. Schiff base reaction between dopamine and dopaminechrome and the further product with dopaminechrome, pyrrole and benzazepine moieties that has been proposed to form in the subsequent reactions after the Schiff base formation. Adapted from [75]. To conclude, the application-oriented studies of polydopamine have expanded to nearly all areas of material science.57 However, the studies discussing the underlying chemistry of polydopamine are still lacking,57 which is rather surprising since, as discussed above, the structure and properties of synthetic eumelanins depend on their Lauri Marttila 18 preparation conditions. One particular question was highlighted in the landmark review article by Liu et al. (2014): it has not been investigated in detail why polydopamine deposition can occur under acidic conditions with the assistance of some specific oxidants.89 In the article I, we present a widely applicable thermodynamic model, which clarifies the pH dependence of the critical first steps in the Raper-Mason dopamine oxidation pathway (Scheme 2) in the presence of dissolved oxygen and/or transition metal ions (Ce(IV), Fe(III) or Cu(II)). 1.2.3 DHI-melanin The fabrication of DHI-melanin coatings is less straightforward since DHI is extremely air sensitive.90–92 On the other hand, the oxidation of DHI leads to the much simpler structure compared with polydopamine, and it is mainly composed of indole-based oligomers and their oxidative degradation structures31,71,88,93–95. I am not aware of a thorough investigation of an effect of pH on the deposition of DHI-melanin films but it is known that, in contrary to dopamine,54 the oxidative polymerization of DHI in alkaline pH (e.g. at pH 8.5) does not yield significant films71,88,92. Instead, the DHI-melanin coating deposition requires neutral or acidic pH88,92 but the deposition kinetics have been shown to slow down when pH is decreased from 7 to 592. However, even at pH 7, the self-limiting thickness of the DHI-based coating is only ca. 20 nm, much less than that of polydopamine coating prepared in the typical conditions (ca. 40 nm, 10 mM DA, Tris buffer with pH 8.5).92 The results by Lyu et al. suggest that monomeric DHI-species, low-molecular- weight oligomers and/or small aggregates can adhere to the surfaces but large aggregates no longer cannot.92 Competing and unavoidable aggregation of DHI in the solution phase can thus lead to small self-limited thickness of the DHI-melanin coating.92 In another study, Arzillo et al. have shown that the hydrodynamic radius of DHI-melanin aggregates can grow up to value of ~1200 nm before precipitation and DHI polymers grow principally via monomer-polymer or small oligomer- polymer coupling instead of large oligomer-oligomer interactions.95 Eumelanins are partially soluble in alkaline aqueous solutions like ammonia and can be simply spin coated onto hydrophilic substrates after ultrasonication and centrifugation.96,97 The downside is that high pH can damage the material by causing aromatic ring fission.97,98 This and other above-described challenges in the DHI-melanin coating highlight the importance to study more advanced DHI-melanin film fabrication methods. DHI-melanin has received much less attention compared with polydopamine but various advanced film fabrication procedures like matrix assisted pulsed laser evaporation,99 electrochemical polymerization90,100 and ammonia-induced solid state polymerization101 have been reported in the literature. Our group has previously shown that DHI can be oxidized onto thin oxidative Introduction 19 films102 and in this thesis work, successful fabrication and electroactivity of such films was demonstrated using the developed, about ten times thicker graphene oxide (GO) containing oxidative multilayer films [article III] (see Chapter 1.3). 1.2.4 Electrochemical properties of eumelanin Before moving to the fabrication of oxidative layer-by-layer films, I will briefly introduce the fascinating electrochemical properties of eumelanin materials. Eumelanin is a hybrid electronic-ionic conductor, and its conductivity depends highly on its hydration level.103–105 Electrical properties of hydrated eumelanin films can be described with an interplay between proton migration, electronic transport, and redox processes.104 First of all, eumelanin is redox active and hydroquinones (H2Q) can undergo two one-electron oxidations to first to the semiquinone (SQ) form and further to the quinone form (Q). The equilibrium concentrations of these redox states is defined by the comproportionation equilibrium (Scheme 5).105 It is evident from the Le Châtelier’s principle that the semiquinone concentration increases with pH but hydration perturbs the comproportionation equilibrium in a similar fashion, increasing the semiquinone free radical and proton concentrations.105 Protons can migrate in the material under the external electric field, resulting in ionic conductivity, while electrons can move within the conjugated aromatic matrix or via comproportionation equilibrium and various redox reactions. N HO HO N O O N O O 2 H2O 2 H3O ++ + 2+ Scheme 5. Eumelanin comproportionation equilibrium (adapted from [106]). The electronic conductivity of eumelanin has recently received more attention as Migliaccio et al. reported an unprecedented 318 S/cm conductivity for the mainly electronically conductive DHI-melanin film that had been thermally annealed in vacuum.103 In comparison, Wünsche et al. obtained mainly ionic (largely protonic) ~10–3–10–4 S/cm conductivity for the commercial eumelanin (synthesized by oxidation of tyrosine) at 90% relative humidity.104 The vacuum annealing of the DHI-melanin film increased its conductivity up to over 9 orders of magnitude and this was attributed to the structural reorganization of the material which increased both the delocalization of the aromatic systems and their π-stacking interactions.103 Carboxylic groups and bound water were evicted from the material during the annealing and the conductivity of the film actually decreased if it was exposed to water.103 Finally, eumelanin composites have been fabricated by mixing it with more conductive materials,107–110 representing another potential approach for the device- Lauri Marttila 20 quality material fabrication. In particular, material hybridization can also enhance packing of eumelanin, decreasing the spacing between elementary units and π-π stacking distance.107 As stated above, optimization of the interfacial connections of hybrid materials is critical in the development of supercapacitor electrode materials.27,44,52 This was clearly seen in the recent research by Santato and others.24,27 In the earlier study,24 they prepared a melanin-based supercapacitor electrode with 5.6 mF/cm2 (167 F/g) capacitance at 5 mV/s sweep rate by drop-casting commercial eumelanin on carbon paper. However, the electrode had poor rate response which, together with the relatively low capacitance, was attributed to the low electronic conductivity of eumelanin and high contact resistance between eumelanin and carbon current collector.24,27 However, in their later study, they obtained significantly better capacitance (1355 mF/cm2 or 452 F/g) and rate response values by chemically engineering the eumelanin/carbon paper interface prior to Sepia melanin deposition on the treated carbon paper.27 1.3 Oxidative multilayer films Oxidative multilayer films are a general technology platform for the electrodeless oxidative polymerization of various monomers to functional films.102,111–113[articles II, III] Oxidative films can be fabricated on a wide variety of substrate materials with various layer-by-layer (LbL) self-assembly techniques. The simplest way is the dip-LbL method: the chosen charged substrate is immersed alternatively in the polycation and polyanion solutions, separated by washing steps. A good example of a LbL film is (PSS/PAH)n; a multilayer film of the sodium salt of poly(styrene sulfonate) and poly(allylamine hydrochloride), where the electrostatic attraction between oppositely charged macromolecules acts as a driving force for multilayer buildup.114 Oxidative multilayer films, on the other hand, are based on reversible coordinative bonding between a redox-active transition metal ion, e.g., Ce(IV), and a binder(s), e.g., polyphosphate (PP) and graphene oxide (GO).102,111–113[articles II, III] Freshly prepared oxidative multilayers are simply immersed in the desired monomer solution, e.g., pyrrole, EDOT or DHI, and the monomer polymerizes onto the oxidative film.102,111–113[articles II, III] The amount of oxidant is the limiting factor in the process and its quantity can be controlled by adjusting the film thickness, i.e., the number of layers in the film. Our research group has focused on the study of cerium(IV) containing oxidative films that have high oxidation potential at acidic pH,112[article I] but by using thermodynamic calculations, we have shown Introduction 21 that redox active biometals iron(III) and copper(II) are promising active ions for the oxidative multilayers as well[article I]. Simple dip-LbL polyphosphate/Ce(IV) multilayer films are already suitable for the surface-controlled oxidation102,111,112 but the addition of graphene oxide as a third component in the films enhances the film growth and their mechanical properties113. GO is inexpensive and its water suspension can be readily used as a precursor of oxidative multilayers after ultrasonication (to exfoliate the GO sheets) due to its anionic charge and coordinative properties.113[article II] 1.3.1 Oxidative spin-spray-(PP/Ce/GO/Ce)n multilayers The simple, facile, and versatile dip-LbL film fabrication method is not without its weaknesses. Deposition of the individual layers and the subsequent rinsing of the film is often slow (e.g. >20 min for the GO containing oxidative multilayers113) and the material consumption in the method is high. In addition, Ce(IV)-based dip-LbL oxidative films grow exponentially with the number of deposited layers111,113 due to vertical diffusion115 of cerium ions in and out of the film during its assembly (internal diffusion of PP chains is also probable in the GO containing films)111,113. As a result, the morphology of the dip-LbL (GO/Ce/PP/Ce)n is not well-defined, GO sheets in the films are randomly oriented, the surface roughness of the films is very high and the film thickness cannot be controlled with a nanometer-level precision.113[article II] In order to improve the film properties, we studied the fabrication of (PP/Ce/GO/Ce)n multilayers with a more advanced, automated spin-spray LbL film assembly technique.[articles II, III] The spin-spray LbL method combines many advantages of the spin-LbL and spray-LbL methods116–120[articles II, III] and its potential in the fabrication of battery electrodes has been demonstrated in the literature117,121. First of all, the spray-LbL method, introduced in 2000 by Schlenoff et al.,122 is suitable for a much faster fabrication of homogenous and smooth films compared with the dip-LbL method123. At its simplest, reagents can be sprayed on a vertically oriented substrate using e.g. common air pump spray cans.123 The substrate can be slowly rotated (e.g. 10 RPM) to avoid drip patterns and the method can be scaled up for industrial applications.124 Another popular method, spin-LbL, is likewise suitable for the rapid fabrication of multilayer films with highly ordered internal structure and low surface roughness.125 Typically, the reagents are sequentially dropped onto a rapidly spinning substrate (e.g. 4000 RPM) or alternatively onto a stationary substrate that is then spun. In the method, the reagent adsorption and the structural rearrangement of the film happen simultaneously as centrifugal, air shear, and viscous forces cause Lauri Marttila 22 desorption of weakly bound material from the film.125,126 Overall, as reviewed by Richardson et al.,126 both spin-LbL and spray-LbL methods generally lead to stratified multilayer structure while layers of the dip-LbL films are typically interpenetrated. The spin-spray LbL film assembly can be easily automated and the preparation of the oxidative (PP/Ce/GO/Ce)n multilayer films with the method is illustrated in the Scheme 6. Briefly, the substrate is attached on a horizontally oriented spinner and spun constantly during the entire multilayer film assembly process. The automated spraying unit sprays a small volume of the first reagent for t1 seconds, followed by drying for t2 seconds. Next, the film is rinsed with water for t3 seconds, followed by drying for t4 seconds. The film assembly is continued until the desired number of layers have been deposited. The individual layers are thinner than those prepared with the dip-LbL method and the deposition of an individual (PP/Ce/GO/Ce) tetralayer with the t1 = 3 s, t3 = 10 s, t2 = t4 = 20 s sequence takes less than four minutes.113[article II] The buildup procedure can be further accelerated if desired.117 Additionally, multiple parameters like reagent concentration, the addition of salt to the reagent solutions, substrate spin rate, reagent and rinsing solution volume and temperature are adjustable, allowing nanolevel control over the multilayer deposition.116–118 Notably, the film fabrication with very small reagent quantities is possible, thereby greatly reducing material waste.116,117 Scheme 6. Preparation of the oxidative multilayer using the automated spin-spray layer-by- layer technique and the polymerization of chosen monomer (EDOT or DHI in the figure) onto the oxidative film.[article II] Introduction 23 1.3.2 Comparison of oxidative multilayers to other techniques The oxidative multilayer films can be conveniently compared with the other oxidative polymerization techniques by using the PEDOT film deposition from the EDOT monomer as an example. An excellent overview of the oxidative chemical polymerization of EDOT can be found in the recent review by Gueye et al.36 and will be briefly summarized here, adding details when relevant. First of all, PEDOT can be polymerized simply by adding oxidants in its monomer solution, but the product is insoluble and infusible and therefore difficult to process.36,127 In 1990, however, Bayer AG introduced a landmark synthesis method of PEDOT.36 EDOT is polymerized in an aqueous solution with PSS polyanion using Na2S2O8 as an oxidant. In this case, PSS counter-anions stabilize the polymer and the processable dispersion of PEDOT:PSS is formed. The conductivity of PEDOT:PSS films can be significantly enhanced by a simple chemical treatment: the immersion of the PEDOT:PSS thin film to the H2SO4 solution decreases the PSS content in the material and 100 % sulfuric acid increased the conductivity of the film from 787.99 to 4839.92 S/cm.128 In vapor phase polymerization (VPP), as performed by Winther-Jensen et al., an oxidant, Fe(III)tosylate in butanol, is first mixed with an alkaline inhibitor pyridine to prevent an acid-initiated polymerization of EDOT at very acidic pH (the addition of pyridine raises the pH of the mixture to roughly 2.5).129,130 The mixture is distributed on a substrate, dried, and moved to the VPP chamber that contains an EDOT reservoir and is filled with EDOT vapor.129 The chamber is flushed with N2. The pyridine inhibitor slowly evaporates in the chamber and the polymerization of EDOT starts. The substrate is kept inside the chamber for 1 h and a film with very high (>1000 S/cm) conductivity is obtained. Oxidative chemical vapor deposition (oCVD) is a solventless method where EDOT and Fe(III)Cl3 vapors are introduced to a temperature-controlled substrate in a vacuum chamber.131 The thus obtained PEDOT film is then rinsed with HBr and methanol to remove the residual oxidant and unreacted monomers and films with very high >1000 S/cm conductivity are again obtained. Contrary to the highly specialized oxidative vapor deposition techniques, the liquid phase depositional polymerization (LPDP) method132–135 uses a much simpler approach. Essentially, a pre-treated substrate coated with an oxidant, e.g. Fe(III) p-toluenesulfonate with pyridine additive, is immersed in the EDOT solution in a solvent in which the oxidant is practically insoluble, e.g. n-heptane.134 Films with relatively high conductivity (up to 530 S/cm) have been obtained with the method.133–135 The LPDP method closely resembles the use of oxidative films to the chemical oxidative polymerization. However, the multilayer approach enables one to control the amount of oxidant on the substrate by simply adjusting the number of Lauri Marttila 24 layers in the oxidative film.[article II] In addition, the preparation of oxidative multilayers and the post-assembly polymerization can be done using only aqueous solutions (apart from the substrate pre-treatment) and hazardous organic solvents like n-heptane are not needed. 25 2 Results and Discussion In this chapter, I will summarize the results in the articles I–III. The reader is referred to the original publications and references within for experimental sections, derivation of all the equations and other additional information. 2.1 The oxidation of dopamine to polydopamine As described above, the mussel-inspired polydopamine coating is a simple, cost- effective, and versatile method to modify the properties of virtually any surface.54,55,57 Simple dissolution of dopamine to mildly alkaline aqueous buffer solution results in the polydopamine formation in solution and on various surfaces. Our aim was to clarify the pH-dependence of the oxidation of dopamine both in autoxidation and in the presence of transition-metal ions. We used a thermodynamic analysis of the process together with experimental results. The general models presented in the work are applicable to other catecholamines as well. Due to the complexity of the dopamine oxidation process, we concentrated only on the critical initial steps of the Raper-Mason dopamine oxidation pathway (Scheme 2) • The oxidation of dopamine to dopaminesemiquinone and its further reaction to dopaminequinone. These steps are practically reversible. • The intramolecular cyclization of dopaminequinone to leucodopaminechrome. This step is practically irreversible and can drive the overall reaction forward even in the unfavorable conditions. Secondly, we focused only on the early stages of the oxidation process and therefore complications due to the further reactions of the formed products can be neglected. Finally, we used low 0.1 mM dopamine concentration. Low dopamine concentration (e.g. 0.5 mM) increases the probability of intramolecular cyclization to eventually create DHI, while dopaminequinones in high dopamine concentration (e.g. 10 mM) are more susceptible to other competing pathways such as dimerization.31 Polydopamine from a low 0.5 mM dopamine precursor already resembles the DHI-polymers.31 Lauri Marttila 26 2.1.1 Effect of pH on the first steps of dopamine autoxidation I begin by considering the oxidation of dopamine to dopaminesemiquinone and its further reaction to dopaminequinone. The O2/O2∙−/H2O2 redox system has a crucial role in the initial phase of the autoxidation process. The specialization of all relevant species depend on pH and their formal standard redox potentials are shown in the Figure 1. Figure 1. Formal standard two-electron (solid lines) and one-electron (dashed lines) redox potentials for the dopamine/dopaminesemiquinone/dopaminequinone (black lines) and oxygen/superoxide/hydrogen peroxide (red lines) redox systems as a function of pH. The dominant species involved at the different pH ranges are shown in the figure. Dashed dark cyan lines represent the stability range for water. All data refer to 25 °C.[article I, modified image] The formal two-electron redox potentials of the O2(aq)/H2O2 and dopaminequinone/dopamine redox pairs suggest that the two-electron oxidation of dopamine by O2 is thermodynamically favorable over the whole pH range. However, this pathway can be ruled out since oxygen has a triplet ground state (3Σg) and its two-electron redox reactions with organic species, which are usually in singlet states, are spin-forbidden.136–139 This creates a large activation barrier and makes the reactions slow. Therefore, the first reaction in the autoxidation process is the one- electron oxidation of dopamine by O2. Results and Discussion 27 O2(aq) + DHQ ⇌ SO + DSQ (1) where acronyms DHQ, SO and DSQ are used for dopaminehydroquinone, superoxide and dopaminesemiquinone, respectively, irrespective of their protonation state. Figure 1 shows that the reaction (1) is thermodynamically unfavorable in the entire studied pH-range, although it becomes less so at pH > 5. However, the superoxide and perhydroxyl radicals are strong oxidants and can oxidize both dopamine and dopaminesemiquinone species. The reaction (2) between the superoxide and semiquinone should be the most important pathway to proceed in the initial phase of the autoxidation process since this reaction is thermodynamically highly favorable and SO and DSQ are very close to each other after the initial reaction (1), possibly still within the encounter complex. SO + DSQ ⇌ HP + DQ (2) where HP and DQ are used for hydrogen peroxide and dopaminequinone respectively. The significance of other reaction pathways such as dopamine oxidation by SO and semiquinone disproportionation reaction (3) increase when more SO and DSQ have been generated in the solution, but we limited the discussion of the oxidation process mainly to the initial phases of the process. 2DSQ ⇌ DQ + DHQ (3) The next critical step in the dopamine’s Raper-Mason reaction pathway (Scheme 2) is the practically irreversible intramolecular cyclization of dopaminequinone to leucodopaminechrome. The reaction takes place as a Michael N-addition of the unprotonated amino group to the oxidized catechol ring. We describe the initial steps in dopamine autoxidation by the following simple model (DAL = leucodopaminechrome, DAC = dopaminechrome): DHQ + O2(aq)𝑘𝑘1𝑓𝑓⇌ 𝑘𝑘1𝑏𝑏 DSQ + SO 𝑘𝑘2𝑓𝑓⇌ 𝑘𝑘2𝑏𝑏 HP + DQ𝑘𝑘𝑐𝑐→ DAL 𝑘𝑘3𝑓𝑓⇌ 𝑘𝑘3𝑏𝑏 DAC (4) where k1f, k1b, k2f, and k2b are the average apparent one-electron-forward- and backward-charge-transfer rate constants for the first and second redox steps, respectively, and kc is the apparent first-order rate constant for the ring closure. Using the classical Marcus theory as an approximative model and different levels of other approximations (see article I and references within for details), the formation rate of DQ can be expressed as Lauri Marttila 28 𝑑𝑑[DQ] 𝑑𝑑𝑑𝑑 = 𝑘𝑘1𝑓𝑓𝑘𝑘2𝑓𝑓𝐾𝐾1(𝑘𝑘1𝑓𝑓 + 𝑘𝑘2𝑓𝑓𝐾𝐾1)𝐶𝐶𝑄𝑄𝐶𝐶𝑂𝑂2𝑒𝑒−𝑘𝑘𝑐𝑐𝑡𝑡 = 𝜉𝜉DQ(pH)𝑒𝑒−𝑘𝑘𝑐𝑐𝑡𝑡 (5) and the formation rate of leucodopaminechrome as 𝑑𝑑[DAL] 𝑑𝑑𝑑𝑑 = 𝑘𝑘1𝑓𝑓𝑘𝑘2𝑓𝑓𝐾𝐾1 �𝑘𝑘1𝑓𝑓 + 𝑘𝑘2𝑓𝑓𝐾𝐾1�𝐶𝐶𝑄𝑄𝐶𝐶𝑂𝑂2[1 − 𝑒𝑒−𝑘𝑘𝑐𝑐𝑡𝑡] 𝑑𝑑 ≪ 1/𝑘𝑘𝑐𝑐 ≈ 𝑘𝑘𝑐𝑐𝑘𝑘1𝑓𝑓𝑘𝑘2𝑓𝑓𝐾𝐾1(𝑘𝑘1𝑓𝑓 + 𝑘𝑘2𝑓𝑓𝐾𝐾1)𝐶𝐶𝑄𝑄𝐶𝐶𝑂𝑂2𝑑𝑑 = 𝜉𝜉DAL(pH)𝑑𝑑 (6) where 𝐾𝐾1 = 𝑘𝑘1𝑓𝑓𝑘𝑘1𝑏𝑏. Dopamine and dissolved oxygen concentrations 𝐶𝐶𝑄𝑄 and 𝐶𝐶𝑂𝑂2 are assumed to remain constant during the initial stages of the reaction. The initial DQ-formation rate (ξDQ) and the proportionality factor of the DAL-formation rate at t ∼ 0 (ξDAL) are plotted in Figure 2a at different pHs. Reorganization energies for the studied reactions were not found from the literature, but as seen from Figure 2, its value does not markedly affect to the form of the pH profiles. The calculations predict that the DQ-formation rate is small and constant below ca. pH 5 but increases above it. As a result, the DAL formation rate also accelerates above pH 5 but significant acceleration is observed only above ca. 8–9 pH (see the inset in Figure 2a with curves on a linear scale). We compared the model with two sets of experiments (Figure 2b). First, an acidic dopamine solution was titrated with NaOH and analyzed UV-Vis spectrophotometrically. The titration method is not well-defined in a thermodynamic or kinetic sense, but it is suitable for the observation of intermediate species. Initial absorbance at 300 nm in Figure 2b is attributed to leucodopaminechrome based on the UV-Vis absorbance of leucodopa140,141. Clear absorbance at this wavelength was observed from ca. pH 5 upward and changes below this pH are attributed to local pH transients. The oxidized form of leucodopaminechrome, dopaminechrome, has a wide and distinctive absorption band at ca. 480 nm and another band at 300 nm.142 The reaction of dopaminechrome to 5,6-dihydroxyindole is slow, which leads to its accumulation in the solution.142,143 Rapid growth at 480 nm is not observed until ca. pH 9. Further products eventually obscure the signal of the intermediates as the reaction proceeds. Results and Discussion 29 Figure 2. (a) Calculated pH profiles for the initial formation of dopaminequinone (𝜉𝜉DQ, black lines; initial rate in units of dm3 mol–1 s–1) and leucodopaminechrome (𝜉𝜉DAL, red lines; in units of dm3 mol–1 s–2, actual rate obtained by multiplying by time as long as t ≪ 1/kc), based on the kinetic model in the text. Calculated for 𝐶𝐶𝑄𝑄 = 1 mM, 𝐶𝐶𝑂𝑂2 = 0.25 mM, 25 °C and three different reorganization energies: solid lines, λ = 1.5 eV; dashed lines, λ = 1.8 eV; dotted lines, λ = 1.2 eV. The inset shows the same curves on a linear scale. (b) Evolution of absorbance in the autoxidation of dopamine as a fuction of pH during titration with NaOH or after 20 h of reaction time. (a),(b): [article I]. Lauri Marttila 30 In our second set of experiments, dopamine solutions in a citrate phosphate buffer at different pHs were allowed to react for 20 h and the products were analyzed UV-Vis spectrophotometrically. The effective onset pH for autoxidation was again ca. 5. Overall, the experimental results are well in accordance with the calculated pH profiles for the process. We can conclude that the decisive physicochemical parameters that control the autoxidation of dopamine are (1) the pKa value of the hydroxyl group in the semiquinone radical (ca. 4.7144,145), which is the turning point of the formal potential curve of the DHQ/DSQ pair in Figure 1. (2) The pKa value of the protonated amino group in the dopaminequinone (ca. 9.6146) since cyclization by Michael addition requires that the amino group is in an unprotonated form. At low pHs, the intramolecular cyclization of dopaminequinone is the rate-determining step in the process and the reaction slowly moves toward an equilibrium determined by the first two redox steps. 2.1.2 Transition-metal-assisted oxidation of dopamine It is known that redox-active transition metals like Ce(IV), Fe(III) and Cu(II) can induce dopamine oxidation in weakly acidic solutions.102,147–149 Our aim in this work was to clarify this phenomenon. In a related research, we have shown that polydopamine particles synthesized in the presence of Ce(IV) or Cu(II) ions contain only small amounts of these ions (ca. 1-2 atomic-%) but the iron content in the Fe(III) oxidized dopamine is significant (ca. 20 atomic-%).61 Both alternatives are interesting since metal ions can improve the properties of melanins, e.g. their conductivity106. We studied the pH dependence of dopamine oxidation in the presence of Ce(IV), Fe(III) or Cu(II) using NaOH titration or long reaction times (Figure 3a,b) (NaOH titration with Ce(IV) is reported in the earlier work102). Similarly to the autoxidation reaction, the formation of dopaminequinone was not observed (maximum at 395 nm142) and the initial absorbance changes at 300 and 480 nm are attributed to the formation of leucodopaminechrome and dopaminechrome respectively. The observed pH shift in the reaction onset is large, 2–6 pH units. In order to study the enhanced reaction rate in the presence of transition metal- ions in more detail, we followed the evolution of UV-Vis absorbance in anaerobic conditions at mildly acidic pH 4.5 (Figure 3c). A trace for a dopaminechrome intermediate is observed in the presence of Ce(IV) and Fe(III). With Cu(II), the reaction proceeds when Cl– ions are added to the system, and more rapidly if also oxygen is present. Results and Discussion 31 Figure 3. (a) Evolution of absorbance during titration of dopamine with NaOH in the presence of O2 only or with Fe(III) or Cu(II) (10:1 metal/dopamine ratio). (b) Absorbance changes at 480 nm after 20 h of reaction time in the presence of O2 only or with Ce(IV), Fe(III) or Cu(II) (4:1 metal/dopamine ratio). (c) Evolution of absorbance at 480 nm during the oxidation of dopamine at pH 4.5 in the presence of Ce(IV), Fe(III) (1:1 metal/dopamine ratio) or Cu(II) (6:1 metal/dopamine ratio). (a)–(c): [article I]. In order to justify the observations, we have constructed the Pourbaix diagrams of the metal-ions (Figure 4a, note that their redox properties are strongly dependent on the ambient. This is discussed in the article I). Dopamine is a strongly complexing ligand but assuming reversible complex formation and intracomplex charge transfer, the complexation with an innocent redox-active ligand is thermodynamically identical to the case with no metal complexation. Therefore, the presence of dopamine is not assumed to influence the oxidizing power of the studied redox- active metal ions. Notably, chloride ions enhance the oxidizing power of the Cu(II)/Cu(I) redox pair, in accordance with previous studies144. We attribute this effect to the stabilization of Cu(I) by soluble chloro complexes. Lauri Marttila 32 Figure 4. (a) Calculated formal one-electron potentials of the Ce(IV)/Ce(III), Fe(III)/Fe(II), Cu(II)/Cu(I), and Cu(II)/Cu(I)+Cl– redox systems as a function of pH. Dopamine and oxygen redox systems are also included for comparison. The solid black and green lines are the two-electron formal potentials of the dopamine/dopaminequinone and oxygen/hydrogen peroxide pairs, respectively. Dashed dark cyan lines represent the stability range for water. See article I for concentrations and other experimental details. (b) Conditional equilibrium constants (K’) as a function of pH for redox reactions between oxygen and dopamine species. (a): [article I, modified image]; (b): [article I]. Results and Discussion 33 Figure 4a shows that Ce(IV) and Fe(III) are highly oxidizing only in acidic conditions. Still, the oxidation of dopamine at pH 4.5 in Ce(IV), Fe(III), and Cu(II)+Cl– solutions proceeds at a reasonable rate. In the absence of oxygen, the oxidation of dopamine to dopaminesemiquinone by the metal-ions is the first reaction in the dopamine-oxidation process. The process can continue via multiple pathways. Semiquinones can disproportionate or be oxidized by metal-ions. The presence of dissolved O2 further accelerates the oxidation process. For instance, O2 can oxidize dopaminesemiquinones to dopaminequinone: O2 + DSQ ⇌ SO + DQ (7) The reaction (7) has a low equilibrium constant (Figure 4b) but it is driven to the right since DQ quickly cyclizes and superoxide is a strong oxidant and can easily react further. Finally, it is important to consider whether the transition metals can enhance the dopaminequinone cyclization reaction. The absorbance increase rate at 480 nm (Figure 3c) allows to estimate the dopaminechrome-formation rate coefficient as 2.3 x 10–4 s–1 under anaerobic conditions in the presence of Ce(IV). The kinetic modelling of the dopamine autoxidation yields a value of 2.0 × 10–4 s–1 for the dopaminequinone cyclization rate at pH 4.5. The values are in very good accordance with each other and suggest that the unassisted rate of ring closure is enough to cyclize practically all dopaminequinone formed in the Ce(IV)-enhanced oxidation. The dopamine oxidation reaction is slower in the presence of Fe(III) or Cu(II)+Cl– at pH 4.5 compared to Ce(IV). Therefore, the thermodynamic enhancement of dopamine oxidation by metal-ions is enough to explain the accelerated dopaminechrome formation in the studied reaction conditions. At low pH, the dopaminequinone cyclization will become rate-determining. Lauri Marttila 34 2.2 Oxidative multilayer films In the second part of the thesis, I will present our process of enhancing the oxidative multilayer fabrication. In addition, I will present results of the use of oxidative films for the deposition of thin redox-active and capacitive films. • I will begin by introducing the operating principle of oxidative (PP/Ce/GO/Ce)n multilayer films and the roles of polyphosphate (PP), graphene oxide (GO) and Ce(IV)-ions in the film. • Secondly, I will present the advantages of the spin-spray LbL method in the oxidative film fabrication over the traditional dip-LbL method. • Thirdly, I will discuss the electrochemical properties of the spin-spray (PP/Ce/GO/Ce)n/PEDOT films in detail. • Finally, I will describe the structure of the spin-spray (PP/Ce/GO/Ce)n/(DHI-melanin) films in detail, and briefly introduce their electrochemical properties. Two experimental details should be noted. (1) All electrochemical studies were performed using a three-electrode configuration with Ag/AgCl reference electrode. (2) The composition of the films was studied primarily with X-ray photoelectron spectroscopy (XPS). It is a surface analysis technique that provides information from the top 1–10 nm of the sample. Therefore, the film interior was studied by sputtering the samples with relatively low energy argon gas cluster ion beam. This minimizes the chemical changes of polymeric materials during the sputtering.150,151 2.2.1 The operating principle of oxidative (PP/Ce/GO/Ce)n multilayer films As discussed in the introduction, oxidative (PP/Ce/GO/Ce)n multilayer films are a general technique for the oxidative polymerization of various monomers to functional films.102,111–113 Our group has previously shown that graphene oxide (GO) improves the growth and mechanical properties of the films. Ce(IV) acts as oxidant and the oxidation potential of the Ce(III)/Ce(IV) redox pair increases with decreasing pH (Figure 4a).112,113 Polyphosphate (PP) binds the oxidative multilayers together. However, polyphosphate degrades hydrolytically in aqueous solutions, and in accordance with the literature,152–154 we showed with 31P NMR (nuclear magnetic resonance spectroscopy) that the process accelerates significantly with decreasing pH. Different metal cations may also either enhance or inhibit the degradation.153,154 Results and Discussion 35 1200 1000 800 600 400 200 0 (a) (PP/Ce/GO/Ce)15 Atomic % C 37 14 O 51 61 P 4 7 Ce 9 18 Ce 3d O 1s Ce 4s C 1s C ou nt s / s Binding Energy (eV) P 2s 4 µm 930 920 910 900 890 880 870 Co un ts / s Binding Energy (eV) Unsputtered (PP/Ce/GO/Ce)15 Ce 3d (b) Ce(III) ratio: 52 % Figure 5. (a) XPS survey spectra of the (PP/Ce/GO/Ce)15 film and relative atomic concentrations of surface elements before (black) and after (red) argon gas cluster sputtering for 30 s (4 eV cluster energy per atom). Inset: SEM image of the film. (b) Ce 3d region XPS spectrum of the unsputtered (PP/Ce/GO/Ce)15 film and ten-peak fit to the data. Ce(III) ratio was estimated from the area of the Ce(III) peaks (see article III for details). (a): [article III]; (b): [article III, modified image]. X-ray photoelectron spectra (XPS) show that the oxidative (PP/Ce/GO/Ce)15 film consists of the expected elements (C, O, P and Ce) and has Ce(III)/Ce(IV) ions in the ratio of 1:1 on its surface (Figure 5). Further, the relative atomic concentrations of the sputtered film in Figure 5a reliably represent its bulk composition since nearly the same values were also obtained with energy dispersive X-ray spectroscopy analysis (EDS). The composition of the oxidative film is discussed in detail in article III. 0 10 20 30 40 50 60 70 0 10 20 30 40 50 60 70 80 (PP/Ce/GO/Ce)15/PEDOT C 63 % (14) O 28 % (61) S 8 % P 0.8 % (7) Ce 0.2 % (18) A to m ic p er ce nt (% ) Sputtering time (s) Figure 6. Depth profile of the (PP/Ce/GO/Ce)15/PEDOT film (calculated from XPS survey spectra, argon gas cluster sputtering with 4 eV per atom). The relative atomic concentrations of the sputtered (PP/Ce/GO/Ce)15 film are shown for comparison as open symbols and percentages in parentheses.[article III] Lauri Marttila 36 By the oxidation of EDOT onto the (PP/Ce/GO/Ce)15 multilayer, we demonstrated that phosphates and Ce(III)/Ce(IV) ions in the oxidative film dissolve in the adjacent solution during the polymerization in highly acidic pH (pH 1.5, 17 h, Figure 6 and Figure 8). The most critical step, disappearance of the Ce(IV) from the film, is almost complete e.g. in two hours with (PP/Ce/GO/Ce)15 film (see article II for details). Reduced Ce(III)-ions have low complexing power, which leads to their dissolution from the film.102 As a result, the (PP/Ce/GO/Ce)15/PEDOT film consists of mainly of PEDOT (ca. 80 % w/w) with some graphene oxide sheets (Figure 9) and has very uniform depth profile (Figure 6). 0 200 400 600 800 1000 0 10 20 30 40 50 60 70 80 (a) 40 s: C 67 % O 22 % N 9 % Na 2 % (PP/Ce/GO/Ce)15/(DHI-melanin) C O N Ce P Na A to m ic p er ce nt (% ) Sputtering time (s) 0 200 400 600 800 1000 0 10 20 30 40 50 60 70 80 (b)(PP/Ce/rGO/Ce)15/(DHI-melanin) C O N Ce P Na A to m ic p er ce nt (% ) Sputtering time (s) 0 200 400 600 800 1000 0 10 20 30 40 50 60 70 80 (PP/Ce/GO/Ce)15/(DHI-melanin) C 1s O 1s Ce 3d (PP/Ce/rGO/Ce)15/(DHI-melanin) C 1s O 1s Ce 3d (c) A to m ic p er ce nt (% ) Sputtering time (s) 0 200 400 600 800 1000 0 5 10 15 20 (d)(PP/Ce/GO/Ce)15/(DHI-melanin) P 2s N 1s Na 1s (PP/Ce/rGO/Ce)15/(DHI-melanin) P 2s N 1s Na 1s A to m ic p er ce nt (% ) Sputtering time (s) Figure 7. Depth profile of the (PP/Ce/GO/Ce)15/(DHI-melanin) film (a) before and (b) after the electrochemical reduction of GO [calculated from XPS survey spectra (see article III for details), argon gas cluster sputtering with 4 eV per atom]. (c-d) Comparison of the depth profiles. (a)–(d): [article III]. On the other hand, significant PP hydrolysis did not take place during the DHI-melanin film formation onto the oxidative (PP/Ce/GO/Ce)15 film at mildly acidic pH (2 h under ambient atmosphere, 5,6-dihydroxyindole in ethanol/water solution with pH 4.5 sodium acetate buffer). As shown in Figure 7 and 8, Results and Discussion 37 DHI-melanin forms mainly on the outer surface of the oxidative film. The graphene oxide in the underlying oxidative multilayer can be reduced after the DHI-melanin formation, which significantly enhances the electroactivity of the films (see Figure 15 and its discussion). Furthermore, depth profiles of the (PP/Ce/rGO/Ce)15/(DHI-melanin) film suggest that the reduction does not have a substantial effect on the film composition (Figure 7). Figure 8. The role of polyphosphate hydrolysis during the polymer film formation onto the oxidative film.[article III] In conclusion, polyphosphate hydrolyzes under acidic conditions and the degradation rate increases with decreasing pH. As a consequence, phosphates and cerium-ions can be dissolved from the oxidative film in the adjacent monomer solution during the oxidative polymerization. This was demonstrated by the oxidation of EDOT at pH 1.5 and the as-synthesized GO/PEDOT film has an uniform depth profile (Figure 6). On the other hand, DHI-melanin film formed mainly on the exterior surface of the oxidative film at lightly acidic pH 4.5, leaving the bulk of the oxidative multilayer partially intact (Figure 7a). In general, oxidative multilayer dissolution is expected when the PP hydrolysis rate is much faster than the polymerization rate (Figure 8). Lauri Marttila 38 2.2.2 Growth of spin-spray (PP/Ce/GO/Ce)n multilayers and advantages of the assembly method over dip-LbL 0 10 20 30 40 0 100 200 300 400 500 (PP/Ce/GO/Ce)n (GO/Ce/PP/Ce)n dip-LbL (PP/Ce/GO/Ce)n / PEDOT Th ic kn es s (n m ) Number of (PP/Ce/GO/Ce) strata (a) 0 5 10 15 20 0 10 20 30 40 50 60 R ou gh ne ss rm s (n m ) 0 100 200 300 400 500 0 100 200 300 400 500 spin-spray LbL (PP/Ce/GO/Ce)n dip-LbL (GO/Ce/PP/Ce)n dip-LbL (PP/Ce)n ox id at iv e m ul til ay er th ic kn es s (n m ) build-up time (min) (b) Figure 9. (a) Thickness of spin-spray (PP/Ce/GO/Ce)n (solid black) and (PP/Ce/GO/Ce)n/PEDOT (solid red) films measured by ex situ AFM as a function of the number of tetralayers (n). Lines are linear fits to the data. The inset shows the rms surface roughness of the films. The behavior of dip-LbL (GO/Ce/PP/Ce)n films is shown for comparison (open red circles). (b) Thickness of dip (PP/Ce)n, dip (GO/Ce/PP/Ce)n and spin-spray (PP/Ce/GO/Ce)n films as a function of the time required for the film build-up process. Additional details for (a) and (b) are given in article II. (c) 10 µm x 10 µm ex situ AFM images of the (PP/Ce/GO/Ce)3/PEDOT and (d) (PP/Ce/GO/Ce)15/PEDOT films. (e) SEM image of the (PP/Ce/GO/Ce)15/PEDOT film. The data for the dip-LbL films in Figure 9a,b is from our earlier work.112,113 (a),(b): [article II, modified image]; (c),(d): [article II]; (e): [article III, modified image]. Spin-spray (PP/Ce/GO/Ce)n films grow linearly from the beginning, by ca. 11 nm per tetralayer (Figure 9a), and their build-up time is much faster in comparison with the corresponding dip (GO/Ce/PP/Ce)n films113 (Figure 9b). In contrast, oxidative dip-LbL films grow exponentially in the beginning (Figure 9a),111,113 which is indicative of intrafilm diffusion during the assembly111,115. The linear growth is typical to spin-spray LbL films116,117 and can be attributed to the high shear stress within the thin solution layer and its rapid drying, which shortens the time available for structural rearrangements or diffusion during the assembly process118,117,120. The Results and Discussion 39 thickness of the (PP/Ce/GO/Ce)n/PEDOT films is thinner than that of the original oxidative films (ca. 7 nm per (PP/Ce/GO/Ce) strata) due to the dissolution of the oxidative multilayer during the polymerization (Figure 9a). In contrast, DHI-melanin forms mainly on the outer surface of the oxidative film. (PP/Ce/rGO/Ce)15/(DHI-melanin) film is approximately 270 nm thick, as roughly estimated by custom-made scanning white light interferometer155 (SWLI, Figure 10) (see article III for details). Essentially, SWLI is optical interference type method, which is suitable for the approximation of the dimensions of craters created by argon gas cluster sputtering. The root-mean-square (rms) surface roughness of the films was approximated with ex situ atomic force microscopy (ex situ AFM, from 10 µm x 10 µm images, Figure 9c,d and articles II, III) and the results for the (PP/Ce/GO/Ce)n and (PP/Ce/GO/Ce)n/PEDOT films are shown in the inset of Figure 9a. The rms surface roughness of the spin-spray (PP/Ce/GO/Ce)n and (PP/Ce/GO/Ce)n/PEDOT films levels off from n ≥ 10 and the films are much smoother than the corresponding oxidative dip-LbL films113. Graphene oxide sheets are clearly visible in the ex situ AFM image of the thinnest (PP/Ce/GO/Ce)3/PEDOT film (Figure 9c) but PEDOT obscures the sheets in the thicker films (Figure 9d,e). Finally, the rms surface roughness of the spin-spray (PP/Ce/(r)GO/Ce)15/(DHI-melanin) film was approximately 19 nm, in line with (PP/Ce/GO/Ce)n and (PP/Ce/GO/Ce)n/PEDOT films (see article III for details). Overall, both the linear growth and the surface roughness leveling imply that the spin-spray (PP/Ce/GO/Ce)n multilayers have a stratified structure with more or less distinct layers. Figure 10. 3D scanning white light interferometry image of the (PP/Ce/rGO/Ce)15/(DHI melanin) film. Film thickness was estimated from the deepest part of the crater (black line).[article III] Lauri Marttila 40 0 50 100 150 200 250 -0.6 -0.4 -0.2 0.0 0.2 0.4 dip (GO/Ce/PP/Ce)n 310 nm 900 nm dip (PP/Ce)n 310 nm 900 nm spin-spray (PP/Ce/GO/Ce)n 310 nm (Ce) 900 nm (oxidized PEDOT) ∆ Α bs or ba nc e Thickness of the oxidative film (nm) (b) Figure 11. (a) Polymerization of 10 mM EDOT onto the (PP/Ce/GO/Ce)15 film at pH = 1.5 as a function of time. The UV-Vis spectrum of the oxidative (PP/Ce/GO/Ce)15 film has been subtracted from the spectra. (b) Total absorbance change due to polymerization of PEDOT at 310 nm (black) and 900 nm (red) as a function of the thickness of the oxidative film. The absorbance changes correspond to the loss of cerium and the formation of oxidized PEDOT respectively. Lines are linear fits to the data. The data of dip-LbL films in Figure 11b is from our earlier work.112,113 (a): [article II]; (b): [article II, modified image]. Results and Discussion 41 The amount of Ce(IV) in the (PP/Ce/GO/Ce)n is the limiting factor in the surface polymerization process onto the films. Therefore, we compared the relative amount of cerium in oxidative films by measuring the absorbance changes at 310 nm during the polymerization. Further, absorbance changes at 900 nm are proportional to the amount of as-formed PEDOT. Figure 11 shows that they both depend linearly on the thickness of the oxidative multilayer and the values are practically independent of the oxidative film preparation method. Additionally, kinetics of Ce dissolution and PEDOT formation were similar with spin-spray (PP/Ce/GO/Ce)15 and dip (GO/Ce/PP/Ce)4[113] films of similar thickness (see article II and references within for details). In conclusion, oxidative spin-spray (PP/Ce/GO/Ce)n films are more organized and smoother compared with the corresponding dip-LbL films and their build-up time is shorter. The major and somewhat unexpected result of the work was that the cerium capacity and the oxidation kinetics were nonetheless unaffected by the assembly technique when comparing films of similar thickness. 2.2.3 Electrochemical properties of spin-spray (PP/Ce/GO/Ce)n/PEDOT films The high-resolution XPS spectra of (PP/Ce/GO/Ce)15/PEDOT film are in accordance with that of a PEDOT polymer, and thus they are omitted from this summary (see article III and references within). Further, in situ UV-Vis spectra of the (PP/Ce/(r)GO/Ce)n/PEDOT films were typical for the PEDOT-based material (see article II and references within). Therefore, the results indicate that the structure of the polymer is as expected for a typical PEDOT polymer. (PP/Ce/GO/Ce)n/PEDOT films were studied electrochemically before and after the electrochemical reduction of GO. The reduction of GO is irreversible and can be carried out by cyclic voltammetry (CV) in cathodic potentials. Cyclic voltammograms of the (PP/Ce/(r)GO/Ce)n/PEDOT electrode are nearly rectangular- shaped, which is typical for a capacitive behavior (Figure 12a and articles II, III). The integrated areal capacitance of the films (in the –0.2 V to +0.6 V vs Ag/AgCl potential range) is nearly linearly dependent on the film thickness in the investigated thickness range and the volumetric capacitance reaches promising values of 120-140 F/cm3 for a PEDOT-based material (Figure 12c,d).49–51 The reduction of GO increases the capacitance only slightly, which is understandable because the relative amount of graphene oxide in the films is quite small. Cycling stability of the (PP/Ce/GO/Ce)15/PEDOT film is high, with ca. 80 % of capacitance remaining after 4000 cycles (Figure 12b), which is comparable to many other PEDOT-based films156–159. Lauri Marttila 42 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 (a) 0.36 mF/cm2 0.74 mF/cm2 0.87 mF/cm2 ν (mV/s): GO 10, 100 rGO 10, 100 Cu rr en t d en si ty (m A / c m 2 ) Potential (V) vs Ag/AgCl 1.03 mF/cm2 0.0 0.1 0.2 0.3 0.4 0.8 0.9 1.0 1.1 C d iff (m F/ cm 2 ) Potential (V) rGO GO 0 1000 2000 3000 4000 0.7 0.8 0.9 1.0 C ap ac ita nc e (m F/ cm 2 ) Cycle (PP/Ce/GO/Ce)15/PEDOT 100 mV/s Capacitance Capacitance retention (b) 0.7 0.8 0.9 1.0 C ap ac ita nc e re te nt io n 0 30 60 90 120 150 0.0 0.4 0.8 1.2 1.6 2.0 A re al c ap ac ita nc e (m F/ cm 2 ) Total film thickness (nm) 10 m V / s GO rGO 100 m V / s GO rGO (c) 0 100 200 300 400 500 0.0 0.5 1.0 1.5 GO rGO A re al c ap ac ita nc e (m F/ cm 2 ) Potential scan rate (mV/s) n = 15 (ca. 100 nm) 0 30 60 90 120 150 0 20 40 60 80 100 120 140 vo lu m et ric c ap ac ita nc e (F /c m 3 ) Total film thickness (nm) (d) Figure 12. (a) Cyclic voltammograms of the (PP/Ce/(r)GO/Ce)15/PEDOT electrode in 0.5 M Na2SO4 solution and the integrated capacitance values obtained from the CVs. Inset: the differential capacitance as a function of potential at 100 mV/s sweep rate. (b) Cycling performance of the (PP/Ce/GO/Ce)15/PEDOT electrode at 100 mV/s. (c) Areal and (d) volumetric capacitance of the (PP/Ce/(r)GO/Ce)n/PEDOT film as a function of film thickness. Inset in (c): areal capacitance of the (PP/Ce/(r)GO/Ce)15/PEDOT electrode as a function of scan rate. Additional details for (c) and (d) are given in article II. All measurements have been taken in three-electrode configuration vs Ag/AgCl reference electrode. (a),(b): [article III]; (c),(d): [article II, modified image]. We used electrochemical impedance (EIS) and capacitance spectroscopies to identify the various dynamic processes that contribute to the electrochemical behavior of the (PP/Ce/(r)GO/Ce)15/PEDOT films. The real (C’) and imaginary (C’’) parts of the complex capacitance are obtained from the impedance data according to equations (8), where 𝑍𝑍′ = 𝑍𝑍𝑜𝑜𝑏𝑏𝑜𝑜′ − 𝑅𝑅𝑜𝑜 (𝑅𝑅𝑜𝑜 = solution and electrode resistance).160,161 C’ is related to the film capacitance and C’’ to conductivity and energy loss terms.162 𝐶𝐶′ = − 𝑍𝑍′′2𝜋𝜋𝜋𝜋|𝑍𝑍|2 𝐶𝐶′′ = 𝑍𝑍′2𝜋𝜋𝜋𝜋|𝑍𝑍|2 (8) Results and Discussion 43 0 10 20 30 40 50 60 0 10 20 30 40 50 60 0 10 20 30 0 10 20 30 −0.2 V 0.0 V +0.2 V +0.4 V +0.6 V vs Ag/AgCl GO rGO -Z '' (Ω ) Z' (Ω) (a) -Z '' (k Ω ) Z' (kΩ) -2 -1 0 1 2 3 4 0 20 40 60 80 (b) -p ha se (o ) log (f / Hz) -2 -1 0 1 2 3 4 0.0 0.4 0.8 1.2 1.6 2.0 -2 -1 0 1 2 3 4 0.9 1.0 1.1 1.2 1.3 −0.2 V 0.0 V 0.2 V 0.4 V 0.6 V GO rGO (c) C ' ( m F/ cm 2 ) log (f / Hz) semi-infinite mass transfer interfacial nonfaradaic redox relaxation C ar ea l ( m F/ cm 2 ) logarithm of characteristic voltammetric frequency / Hz -2 -1 0 1 2 3 4 0.0 0.5 1.0 1.5 -0.2 0.0 0.2 0.4 0.6 0 100 200 300 400 500 600 (d) C '' (m F/ cm 2 ) log (f / Hz) GO rGO R el ax at io n tim e τ (m s) Potential (V vs Ag/AgCl) Figure 13. Impedance and complex capacitance plots of the (PP/Ce/(r)GO/Ce)15/PEDOT electrode in 0.5 M Na2SO4 solution at different potentials. All measurements have been taken in three-electrode configuration vs Ag/AgCl reference electrode. (a) High frequency part of the Nyquist impedance plots (the whole frequency range is shown in the inset). (b) Bode plots. Real (c) and imaginary (d) parts of the complex capacitance as a function of frequency. The processes contributing to the film capacitance in different time domains are schematically shown in (c). Inset in (c) shows integrated areal capacitance of the (PP/Ce/rGO/Ce)15/PEDOT film (cf. inset in Figure 12c) as a function of the inverse of the voltammetric timescale to approximately compare voltammetric and impedance results. Inset in (d) shows the relaxation time corresponding to the frequency of local maximum in different potentials. (a),(c),(d): [article III]; (b): [article III, modified image]. Electrochemical impedance and capacitance spectra of the (PP/Ce/(r)GO/Ce)15/PEDOT film are shown in Figure 13 and their interpretation can be summarized as follows (see article III and references within for details). Interfacial capacitance, redox pseudocapacitance and mass transfer effects contribute to the film capacitance at different timescales (Figure 13c). Lauri Marttila 44 • The small interfacial capacitance is effective at high frequencies. Semicircles in the Nyquist plots and humps in the Bode plots at these frequencies are attributed to the interfacial non-faradaic phenomena. • The main contribution to the capacitance at voltammetric timescale163 (𝑅𝑅𝑅𝑅 𝐹𝐹𝐹𝐹 , see inset in Figure 13c) comes from redox processes. Therefore, they cause the ultimate limitation for the power density of the films. The time constant 𝜏𝜏 = 1 𝑓𝑓(𝐶𝐶𝑚𝑚𝑚𝑚𝑚𝑚′′ ) corresponding to the frequency of local maximum of C’’ can be used as a figure of merit for supercapacitors because it is directly related to the power density of the capacitor.160,164–166 Excluding the rGO-containing film at the –0.2 V potential, the average time constant is 90 ms, with a standard deviation of 20 ms, which is very small compared with many other films based on carboneous materials or polythiophenes and their composites.160,165–171 • The phase angle of the (PP/Ce/(r)GO/Ce)15/PEDOT film at low frequencies is near the ideal value of –90° for pseudocapacitive materials.172 The reduction of GO slightly increases the nonideal behavior of the film, which we attribute to the morphological changes in the porous electrode structure.172,173 C’ is of the order of 1 mF/cm2 at low frequencies, in line with the integrated capacitance values. C’ decreases slightly with potential, which is in line with differential capacitance (Figure 12a), but overall, the effects of potential and the reduction of GO are both very small (excluding the lowest -0.2 V potential where the conductivity of the film is low). 2.2.4 The structure and the capacitance of the spin-spray (PP/Ce/GO/Ce)n/(DHI-melanin) films The (PP/Ce/GO/Ce)15/(DHI-melanin) film was sputtered with argon gas cluster ion beam with 4 eV cluster energy per atom to remove adventitious carbon and other impurities from the film surface. Unlike monatomic Ar+ sputtering, the more recently introduced argon cluster sputtering minimizes the chemical changes of polymeric materials during the process.150,151 The as-obtained high resolution XPS spectra from the film are shown in Figure 14. The C 1s spectrum (Figure 14a) can be deconvoluted with seven peaks corresponding to C–C/C=C, C–N, C–O, C=O, COOH, and COO– carbons, and a satellite peak. Such a fit is justified by several reasons. First of all, the binding energies are generally in line with other studies.102,151,174 Secondly, the ratios of carbon functionalities are close to those expected, e.g., close to the DHI3/(pyrrole- 2,3-dicarboxylic acid) tetramer (Table 1)31,71. Finally, there is good accordance Results and Discussion 45 between the relative areas of the carbon–nitrogen and carbon–oxygen peaks in the C 1s, O 1s and N 1s spectra (Table 1). COOH groups form in DHI-melanin by the catechol ring cleavage, and these groups bind Na+ ions to the film from the adjacent monomer solution63. The O 1s spectrum (Figure 14b) can be deconvoluted with four peaks corresponding to C=O and C–O oxygens,102,151,174 Na Auger peak and satellite peak. About half of the DHI-melanin is in the oxidized form according to the XPS spectrum. 298 296 294 292 290 288 286 284 282 280 (a) C ou nt s / s Binding Energy (eV) C-C/C=C C-N C-O C=O O-C=O COO− satellite C1s 544 542 540 538 536 534 532 530 528 526 (b) C ou nt s / s Binding Energy (eV) C=O C-O Na Auger satellite O1s 410 408 406 404 402 400 398 396 394 392 (c) C ou nt s / s Binding Energy (eV) =NR R2NH N1s 0 5 10 15 200 1 2 [= N -] / [ R 2N H ] Total metal atom-% N H HO HO DHI O O NHIQ N HO O QM _ _ (d) Figure 14. (a-c) The C 1s, O 1s and N 1s region XPS spectra of the (PP/Ce/GO/Ce)15/(DHI-melanin) film after the argon gas cluster sputtering (40 s, 4 eV per atom). Inset in (c) shows the dependence of the imine/amine ratio ([=N-] / [R2NH]) on the total metal atom-% (Ce + Na) in the film and linear fit to the data. (d) Structures of 5,6-dihydroxyindole (DHI), its oxidized form 5,6-indolequinone (IQ) and the tautomer of IQ: quinone methide (QM). (a)–(c): [article III]; (d): [article III, modified image]. Based on theoretical calculations175,176 and thermodynamic considerations,177 5,6-dihydroxyindole (DHI), its oxidized form 5,6-indolequinone (IQ), and its tautomeric form quinone methide (QM) are assumed to be the main reduced and Lauri Marttila 46 oxidized forms of the as-studied DHI-melanin (Figure 14d, see article III and references within for details). The N 1s spectrum (Figure 14c) can be deconvoluted with two peaks. They correspond to the pyrrolic nitrogen (R-NH-R) in the indole structure102,151,174 and to the =NR (imine group)88,151,174 in DHI-melanin. Metal-ion binding likely influences the imine formation in DHI-melanin61,175,178,179 and the depth profiling data allows us to test this hypothesis. The observed [=N–]/[R2NH] ratio increases roughly linearly with sodium, cerium and total (Na + Ce) cation concentrations (see inset in Figure 14c and article III), but the imine/amine ratio is nonzero even when no metal is present. We attribute the nonzero value to the QM tautomeric form and the increase in the imine/amine ratio upon metal content to the metal-coordination-induced imine structure formation in DHI. Table 1. The fitted C 1s, O 1s and N 1s region XPS peak parameters of the (PP/Ce/GO/Ce)15/(DHI-melanin) film after the argon gas cluster sputtering (40 s, 4 eV per atom). DHI3PDCA and its oxidized form IQ3PDCA are examples of DHI-melanin oligomer structures.[article III, modified version] N H HN H N H N HO HO HO OH HO HO COOH COOH DHI3PDCA N H HN H N H N O O O O O O COOH COOH IQ3PDCA Peak Assign- ment Binding energy (eV) FWHM (eV) Normalized peak area Atomic conc. (%) Carbon functionalities in DHI3PDCA/ IQ3PDCA1 (%) (1) C 1s C–C/C=C 284.9 1.6 941.5 44.5 46.7 (2) C 1s C–N 286.0 1.6 545.0 25.8 26.7 (3) C 1s C–O 287.3 1.6 206.6 9.8 10.0 (4) C 1s C=O 288.4 1.6 260.0 12.3 10.0 (5) C 1s O–C=O 289.6 1.6 105.8 5.0 6.72 (6) C 1s COO– 291.0 1.6 57.6 2.7 (7) C 1s Satellite 292.5 2.5 (1) O 1s C=O 531.5 1.8 356.2 54.5 (2) O 1s C–O 533.3 1.8 298.0 45.6 (3) O 1s Na Auger 535.4 2.8 (4) O 1s Satellite 538.4 2.8 (1) N 1s R2NH 400.3 1.6 266.1 86.5 (2) N 1s =NR 398.5 1.6 41.5 13.5 1. Calculated values for the tetramer structure with equimolar amount of 5,6-dihydroxyindole (DHI) and 5,6-indolequinone (IQ) redox states. 2. O–C=O/COO– Results and Discussion 47 -0.4 -0.2 0.0 0.2 0.4 -1.0 -0.5 0.0 0.5 1.0 (a) (PP/Ce/rGO/Ce)15/(DHI-melanin), ν (mV/s): 10, 100 (PP/Ce/rGO/Ce)15 100 C ur re nt d en si ty (m A / cm 2 ) Potential (V) vs Ag/AgCl -1.2 -1.0 -0.8 -0.6 -0.4 -500 -400 -300 -200 -100 0 100 C ur re nt (µ A ) Potential (V) vs Ag/AgCl (b)10 mV/s Glass/ITO/(PP/Ce/GO/Ce)15/(DHI-melanin) 0 100 200 300 400 500 0 5 10 15 20 (c)Capacitance ( ) and capacity ( ) of (PP/Ce/rGO/Ce)15/(DHI-melanin) (PP/Ce/rGO/Ce)6/(DHI-melanin) (PP/Ce/rGO/Ce)15/PEDOT (PP/Ce/rGO/Ce)15 Ca pa ci ta nc e (m F/ cm 2 ) Sweep rate (mV/s) 0 2 4 6 8 10 Ca pa ci ty (m C/ cm 2 ) 0 3 6 9 12 15 18 21 Characteristic voltammetric frequency (Hz) Figure 15. (a) The cyclic voltammograms of the (PP/Ce/rGO/Ce)15 (in 0.5 M Na2SO4) and (PP/Ce/rGO/Ce)15/(DHI-melanin) films (in 0.1 M phosphate buffer with 0.5 M Na2SO4, pH = 7.1). (b) The electrochemical reduction of graphene oxide in the (PP/Ce/GO/Ce)15/(DHI-melanin) film. Last two cycles are highlighted. (c) The integrated capacitance and the charge storage capacity of the (PP/Ce/rGO/Ce)15, (PP/Ce/rGO/Ce)15/PEDOT and (PP/Ce/rGO/Ce)6 or 15/(DHI-melanin) films with different sweep rates (first two films: estimated in the intermediate –0.2 V to +0.6 V vs Ag/AgCl potential range, cf. Figure 12c). All measurements have been taken in three-electrode configuration vs Ag/AgCl reference electrode. (a)–(c): [article III]. (PP/Ce/GO/Ce)n/(DHI-melanin) films were reduced electrochemically (Figure 15b) and their cyclic voltammograms are shown in Figure 15a (see article III for additional voltammograms). Preliminary work prior to this thesis suggested that the reduction of GO significantly enhances the capacitance of the films and thus (PP/Ce/GO/Ce)n/(DHI-melanin) films were not studied electrochemically in this work. The cyclic voltammograms in Figure 15a are rather featureless (–0.4 V to +0.3 V potential range) but a clear redox peak pair was observed at ca. –0.85 V during the electrochemical reduction of GO (Figure 15b), attributed to the Ce(III)/Ce(IV)–DHI-melanin complexes180,181. The integrated areal capacitance and capacity of the (PP/Ce/rGO/Ce)n/(DHI-melanin) films are compared to the Lauri Marttila 48 corresponding (PP/Ce/rGO/Ce)15/PEDOT and (PP/Ce/rGO/Ce)15 films in Figure 15c. Of the films based on similar oxidative multilayers, the DHI-melanin-based film displays clearly the highest values, especially at low sweep rates. In addition, the volumetric capacitance of the (PP/Ce/rGO/Ce)15/(DHI-melanin) is approximately 490 F/cm3 at 10 mV/s sweep rate, about four times the value estimated for the corresponding (PP/Ce/rGO/Ce)15/PEDOT film. However, the dynamic properties of the (PP/Ce/rGO/Ce)n/(DHI-melanin) films are not optimal, and typically for melanin-based electrodes,24,182 both the areal capacitance and capacity of the (PP/Ce/rGO/Ce)n/(DHI-melanin) films decrease markedly with the sweep rate. Although the rGO can act as an electron mediator in the (PP/Ce/rGO/Ce)n/(DHI-melanin) films, their structure is not optimal for the target application. More uniform depth profile would be desirable, as a partially intact oxidative multilayer weakens the electrical contact between the ITO surface and the redox-active melanin-type film, thereby increasing the time constant for electrochemical processes. Overall, melanin-based materials have a good potential as materials for biodegradable and biocompatible charge storage. However, their conductivity needs to be increased and a promising approach for future studies is to blend them with electronically conducting materials. 49 3 Summary and outlook The focus points of this thesis were (1) the study of synthetic eumelanin analogs polydopamine and DHI-melanin, (2) the preparation of oxidative multilayer films and their use for the fabrication of electroactive conducting polymer and melanin- type films. 3.1 The oxidation of dopamine to polydopamine First of all, we studied the pH-dependence of the autoxidation of dopamine to polydopamine, a well-known synthetic eumelanin analog. The critical initial steps in the Raper-Mason reaction pathway of dopamine to polydopamine are (1) the one-electron oxidation of dopamine to dopaminesemiquinone and its subsequent reaction to dopaminequinone, and (2) the intramolecular cyclization of dopaminequinone to leucodopaminechrome. The classical method for the polydopamine fabrication is to dissolve dopamine in the basic aqueous solution and allow O2 to act as an oxidant (autoxidation). By using thermodynamic analysis and experimental studies, we were able to show that the leucodopaminechrome formation rate begins to accelerate already above pH 5 but strong acceleration is observed not until at much higher pH 8–9. Leucodopaminechrome reacts further to eventually yield polydopamine. The decisive physicochemical parameters that control the autoxidation of dopamine are the pKa value of the hydroxyl group in the semiquinone radical (ca. 4.7) and the pKa value of the protonated amino group in the dopaminequinone (ca. 9.6). Secondly, we studied the oxidation of dopamine to polydopamine at different pHs in the presence of three redox active transition metal ions: the widely used strong Ce(IV) oxidant and redox active biometals Fe(III) and Cu(II). All these ions lower the onset pH of dopamine oxidation significantly. The oxidation power of Ce(IV) and Fe(III) ions increases with decreasing pH and both can oxidize dopamine to dopaminesemiquinone in the anaerobic conditions at acidic pH. However, the lightly acidic pH (e.g. 4.5) is optimal for the polydopamine synthesis since the intramolecular cyclization of dopaminequinone becomes rate-determining at low pH. Interestingly, our calculations show that Cu(II) is not as strong oxidant as Ce(IV) and Fe(III) in the strongly acidic pH, but in the presence of chloride ions, it is a better Lauri Marttila 50 oxidant from pH 5 upwards (including the physiological pH 7.4). Chloride ions increase the formal potential of Cu(II)/Cu(I) due to the formation of Cu(I)-chloro complexes. 3.2 Oxidative multilayer films As a second topic in the thesis, we studied the oxidative layer-by-layer (LbL) films composed of polyphosphate (PP), graphene oxide (GO) and Ce(IV)/Ce(III) ions: (PP/Ce/GO/Ce)n. They have been shown to be a general platform for the oxidative generation of electroactive conducting polymer and melanin-type films. In this work, we studied especially the production of the oxidative multilayers and their use for the fabrication of thin electroactive films. In essence, the multilayer films with mobile oxidant (e.g. Ce(IV)) and coordinative binder (e.g. PP) can oxidize monomers from the adjacent solution onto the film structure. The addition of graphene oxide in the (PP/Ce)n films significantly improves their growth, which is important for their practical applicability. However, the preparation of thick dip-LbL (GO/Ce/PP/Ce)n films is still slow, their morphology is not well defined and their surface roughness is very high. Therefore, in order to improve the production and properties of the films, we studied the preparation of oxidative films with more advanced, automated spin-spray layer-by-layer assembly method. The spin-spray-LbL (PP/Ce/GO/Ce)n films have a stratified structure, their assembly time is shorter (under a minute per layer), and their surface roughness is low. Finally, their thickness increases linearly with the number of tetralayers (ca. 11 nm/tetralayer) and can thus be precisely controlled. By utilizing a modern X-ray photoelectron spectrometer with an argon gas cluster depth profiling, we were able to study the operating principle of the oxidative multilayers and the structure of the functional films in detail. The coordinative binder polyphosphate has a central role in the operation. It degrades hydrolytically in acidic aqueous solutions and thus the oxidative film can be dissolved in the surrounding solution during the polymer film fabrication. In this way, we were able to fabricate GO/PEDOT film with ca. 80 % w/w PEDOT by using a long polymerization time (17 h) and low pH (1.5). The process can be accelerated significantly if desired, since based on the UV-Vis absorbance the Ce(IV) ions have almost completely disappeared from the film in two hours with (PP/Ce/GO/Ce)15/PEDOT film. On the other hand, as shown by high-resolution XPS analysis and depth profiling, DHI-melanin grows mainly on top of the oxidative film during a 2 h synthesis at pH 4.5. In this case, it is not possible to use arbitrary long reaction times due to competing self-aggregation of DHI-melanin in the solution phase. However, it would be interesting to study if DHI-melanin can be oxidized onto the oxidative films at acidic pH in order to remove the oxidative film from the structure. Graphene oxide Summary and outlook 51 in the DHI-melanin-based films can be reduced to electronically conductive rGO after the polymerization processes, which enhances the electroactivity of the DHI-melanin-based films significantly. The primary goal of the research was to provide knowledge on materials that can be potentially used as electrode materials in biocompatible and/or biodegradable supercapacitors. Therefore, we studied the electrochemical properties of thin (PP/Ce/GO/Ce)n/PEDOT and (PP/Ce/GO/Ce)n/(DHI-melanin) films in a three-electrode configuration. Capacitance and charge transfer kinetics of the PEDOT-based films were both promising. Thin PEDOT-based films reached promising 120-140 F/cm3 capacitance values. By using electrochemical impedance and capacitance spectroscopies, we demonstrated that the main contribution to the capacitance comes from redox processes. Therefore, they ultimately limit the power density of the films. Bioinspired DHI-melanin-based films reached even more promising areal and volumetric capacitances. However, in line with the literature, the dynamic properties of the melanin-based electrodes were not optimal and studied films in fact had an undesired nonuniform depth profile. Both the fabrication methods and the conductivity of melanin-based materials need to be enhanced and the latter can be achieved by blending more electronically conductive material to the films. In conclusion, oxidative multilayers do not appear to be optimal for the preparation of DHI-melanin films but produce high quality PEDOT-based films. However, our results indicate that DHI-melanin is a promising component for biodegradable supercapacitors. New film formation techniques need to be developed in order to achieve its full potential. Notably, chemically similar films can also be obtained by the oxidation of dopamine under suitable conditions. This work provides an excellent starting point for more application-oriented studies of oxidative multilayer films. In the future work, the assembly time of the oxidative films can be most likely improved by optimizing the film assembly parameters (here, the individual layers were assembled with intentionally slow and safe parameters: reagent spraying = 3 s, drying = 20 s, rinsing = 10 s, drying 20 s). It would also be interesting to study the fabrication of oxidative films with a simpler spray-LbL assembly, preferably on a slowly rotating substrate to avoid drip patterns. This method is more suitable for large, industrial scale applications. Finally, a natural extension of the work is to utilize other chemically reactive metal ions in the oxidative films in addition to bioincompatible Ce(IV), like Fe(III) and Cu(II) biometals used in the oxidation of dopamine. Ce(IV) and Fe(III) are both strong oxidants at acidic pH, but Ce(IV) is stronger of these two and thus it was used in this thesis. On the other hand, Cu(II) is better oxidant than Ce(IV) and Fe(III) in neutral and basic media. 52 Acknowledgements The experimental work in this thesis was carried out in the group of Physical Chemistry, Department of Chemistry, University of Turku. The funding from The Emil Aaltonen Foundation; University of Turku, Department of Chemistry; The Finnish Cultural Foundation and Foundation of Finnish Chemistry Congress and travel grants from Doctoral Programme in Physical and Chemical Sciences (PCS) and University of Turku Joint Research Grant Fund are gratefully acknowledged. I thank my pre-examiners Doctor Klemen Pirnat and Professor Tomi Laurila and opponent Associate Professor Radosław Mrówczyński for your time spent with my thesis. I would like to express my deepest gratitude for my supervisors Professor Jukka Lukkari, Adjunct Professor Mikko Salomäki and Doctor Henri Kivelä for all your time, expertise, patience, and support over the years. I want to also thank all co-authors of my articles Ermei Mäkilä, Jarkko Leiro, Johan Nyman, Jukka Hassinen, Matti Tupala, Sari Granroth and Tuomo Ouvinen for your invaluable contribution. I also thank Adefunke Koyejo, Lippo Lassila, Milla Suominen and Sachin Kochrekar for your participation. I am grateful for my Master’s Thesis experimental work supervisor Antti Viinikanoja who helped me along the way. The work would not have been possible without the custom-made instruments by Mauri Nauma or dedicated IT support from Kari Loikas. Finally, I equally thank all my co-workers in the Department of Chemistry for making this work an unforgettable experience. My special thanks to my parents Irene and Erkki for all the support, my brother Aarne for scientific advice and my beloved wife Henna for standing by my side and supporting me through everything. 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Lauri M arttila A I 681 A N N A LES U N IV ERSITATIS TU RK U EN SIS ISBN 978-951-29-9095-5 (PRINT) ISBN 978-951-29-9096-2 (PDF) ISSN 0082-7002 (Print) ISSN 2343-3175 (Online) Pa in os al am a O y, Tu rk u, F in la nd 2 02 2 TURUN YLIOPISTON JULKAISUJA – ANNALES UNIVERSITATIS TURKUENSIS SARJA – SER. AI OSA – TOM. 681 | ASTRONOMICA – CHEMICA – PHYSICA – MATHEMATICA | TURKU 2022 OXIDATIVE MULTILAYER FILMS AND EUMELANIN ANALOGS FOR BIOCOMPATIBLE SUPERCAPACITORS Lauri Marttila