Migliaccio, Ludovico (2018) Eumelanin exploitation in bioelectronics: ionic-electronic charge transport in eumelanin organic thin films. [Tesi di dottorato]


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Item Type: Tesi di dottorato
Lingua: English
Title: Eumelanin exploitation in bioelectronics: ionic-electronic charge transport in eumelanin organic thin films
Migliaccio, Ludovicoludovico.migliaccio@unina.it
Date: 12 December 2018
Number of Pages: 162
Institution: Università degli Studi di Napoli Federico II
Department: Scienze Chimiche
Dottorato: Scienze chimiche
Ciclo di dottorato: 31
Coordinatore del Corso di dottorato:
Paduano, Luigilpaduano@unina.it
Pezzella, AlessandroUNSPECIFIED
Date: 12 December 2018
Number of Pages: 162
Uncontrolled Keywords: Eumelanin; Organic (bio)-electronics; Charge transport; Photo(electro)catalysis; Vacuum thermal treatments; conductivity
Settori scientifico-disciplinari del MIUR: Area 03 - Scienze chimiche > CHIM/05 - Scienza e tecnologia dei materiali polimerici
Area 03 - Scienze chimiche > CHIM/06 - Chimica organica
Area 02 - Scienze fisiche > FIS/03 - Fisica della materia
Area 09 - Ingegneria industriale e dell'informazione > ING-IND/12 - Misure meccaniche e termiche
Area 09 - Ingegneria industriale e dell'informazione > ING-IND/22 - Scienza e tecnologia dei materiali
Area 09 - Ingegneria industriale e dell'informazione > ING-INF/01 - Elettronica
Date Deposited: 19 Jan 2019 16:22
Last Modified: 16 Jun 2020 10:21
URI: http://www.fedoa.unina.it/id/eprint/12696


Eumelanin, the mammal pigment originating via the oxidative polymerization of 5,6-dihydroxyindole (DHI) and/or 5,6-dihydroxyindole-2 carboxylic acid (DHICA), for more than forty years have been the focus of uncommon interest in biomedical research due to their central relevance to skin and eye photoprotection. During the last two decades, several peculiar properties of eumelanin-inspired polymers have attracted the attention of the materials science community too, following the realization of device quality thin films through controlled polymerization of DHI and derivatives. The properties of these materials include: a) broadband optical absorption in the UV-visible range; b) efficient UV-dissipative mechanisms; c) photoconductivity in the solid state; d) electronic-ionic hybrid conduction properties. Anyway, despite a burst of interest in the use of synthetic eumelanin for organic electronics and bioelectronics, the implementation of competitive eumelanin based technology has so far been hindered by several drawbacks. These ones include: complete insolubility in all solvents, preventing the development of standardized and reproducible synthetic procedures, low conductivity, and the lack of a solid conceptual frame of structure–property–function relationships. In particular, the high molecular heterogeneity of the eumelanin, whose impact on their electrical performances has not been still systematically assessed, generates several critical consequences like, for example, the lack of well-defined HOMO–LUMO gaps. This PhD project aims to identify Structure Property Function relationships in order to propose a mechanistic model describing the ionic-electronic charge transport in eumelanin thin films and finally fabricate eumelanin based devices. In Chapter 1, is described an overview related to the physico-chemical properties of Eumelanin and the different methods used to obtain it , and this chapter will present also a description related to the recent explosion of interest for Eumelanin applications in bio-electronics and organic-electronics fields that will be one of the main focus with which this thesis will deal. In Chapter 2, to increase the eumelanin conductivity performances, hybrid materials were designed, adding eumelanin to conductive materials (in this case, PEDOT:PSS (poly(3,4-ethylenedioxythiophene):polystyrene sulfonate)). Resulting material properties have been studied through spectroscopic characterizations, and by means of X-rays, SEM, UV-VIS, contact angle, profilometry, and electrical measurements on different devices architectures. Thin films are ideal candidates to be studied, because they are easily accessible to chemical and morphological characterizations and potentially susceptible to device applications. The first step in the PhD study has involved the design and characterization of hybrids made from blends of Clevios™ PEDOT:PSS PH1000, a conductive polymer, and melanin pigments. The aim of this blend is to have a material which is conductive or satisfactorily conductive as the PEDOT:PSS and, at the same time, resistant to the possible conditions under which the PEDOT: PSS is unstable, for example, exposure to water and other degrading agents such as oxygen. The melanin is able to ensure factors such as stability to degrading agents, but also adhesion to the substrate, which does not make the PEDOT:PSS. The designed and prepared blends differ in the variable DHI / PEDOT:PSS ratio; the solvent used for the solubilization of the DHI was found to be the propan-2-ol. The blends were prepared in the presence and in the absence of a secondary dopant. A primary dopant differs from a secondary dopant (better defined as “additive”) since the former has a reversible effect, while the effect of the secondary doping is permanent and remains even when the additive is removed. Here, the agent of the secondary doping was dimethylsulfoxide (DMSO). Capitalizing on a recently developed protocol to prepare high quality eumelanin coatings, herein it is reported the design and the integration of standard commercial PEDOT:PSS with eumelanin pigment (see chapter 2). During this study, were prepared different blends with different DHI/PEDOT ratios in order to understand and define an electrical conductivity trend, with eumelanin content increase in the chemical system. The study of electrical properties of these blends was mandatory in order to define and satisfy a good blend able to work as polymeric anode in an Organic light emitting diode device (OLED) that is one of the ideas of application of this material for organic-electronics and bio-electronics purposes. The blend used is the one with DHI/PEDOT ratio = 0.4 w/w. This blend has allowed to get water stable transparent thin films, capable to operate as electrodes for organic devices, complementing the PEDOT:PSS electronic conductivity with the peculiar eumelanin properties, including adhesion, water stability and ionic-electronic conductivity. As a proof of concept, an unprecedented ITO (Indium tin oxide)-free organic light emitting diode (OLED) implementing an eumelanin-PEDOT layer as the anode was fabricated and characterized. To the best of our knowledge, this is the first evidences of an OLED device based on an anode layer integrating eumelanin. To prepare the mixture of eumelanin and PEDOT:PSS (Eu-PH), a protocol involving in situ eumelanin generation was designed. This approach allowed not only to circumvent the actual insolubility of eumelanin in any solvent, but also to gain over the PEDOT molecular organization. The target was to obtain a material with good conductivity (not less than 300 S*cm-1) and a work function at least comparable with the ITO (whose commonly reported around 4.4÷4.7 eV) or larger for an efficient hole injection, to be effectively used as the anodic material in organic electronic devices. The Eu-PH thin films were thus obtained by exposing the DHI-PEDOT:PSS films, obtained via spin-coating onto glass substrates, for 1 h to air-equilibrated gaseous ammonia (AISSP (Ammonia induced solid state polymerization) protocol)), from an ammonia solution (28% in water) inside a sealed chamber at 1 atm pressure and at controlled temperature (25°C÷ 40°C). For the OLEDs, a hole injection layer (HIL), CLEVIOS PEDOT:PSS PVP Al 4083, used as-is, was deposited via dip-coating (thickness 90 nm). Final steps of the OLEDs fabrication were carried out under inert atmosphere. All the substrates were loaded into a glove box system (N2 atmosphere, O2 < 1 ppm, H2O < 1 ppm), and transferred into an in-line evaporation chamber. The active area of the devices was 7 x 10-6 m2. The final OLED architecture includes: the glass substrate; the anode (ITO; or CLEVIOS PH 1000; or Eu-PH); the HIL; the HTL; the ETL-EML and the cathode. Because a more in-depth investigation is required to fully address the eumelanin-PEDOT integration, the effect of the eumelanin on conductivity of the blends was thus related to structural features observed in Wide Angle X-ray Scattering (WAXS) and X-ray diffraction (XRD) patterns of films with different eumelanin content and fixed PEDOT:PSS ratio. Analysis of free-standing films in transmission geometry (WAXS) allowed to get rid of substrate scattering contribution, and mainly probe crystallographic directions parallel to the film plane. On the other hand, reflection geometry (XRD, GIWAXS) allowed probing structural order in both directions, in and out of the film plane, and can be used for the as-prepared films, laying on glass substrates, to better relate structural and electrical properties. The effect of the eumelanin integration within PEDOT:PSS layers was investigated in terms of the changes in the hierarchical structure of the PEDOT:PSS films. The results of the X-ray scattering characterization clearly demonstrate that the presence of the eumelanin affects the PEDOT component of the blend, inducing an overall increase of the crystalline order at low eumelanin contents. This effect is associated with a smaller distance between the PEDOT chains. Moreover, when eumelanin percent content increases, a less steep decay of the blend conductivity was observed, than the one expected on the basis of the data reported on the conductivity of PEDOT ternary mixtures. At the same time, the introduction of the eumelanin gives noteworthy properties to the Eu-PH blend, including a strong adhesion on inorganic substrates, and water stability, which open to an efficient exploitation of the Eu-PH for conductive coatings within biointerfaces for application in bioelectronics. As said, higher values of the eumelanin electrical conductivity are needed for applications in organic electronics, thus several studies explored the integration of the pigment with conductive materials. But, these approaches actually modify the chemistry of the layers. Other approaches also exploited severe modifications of the eumelanin-like materials to gain a graphene-like material, as for example by pyrolitic treatment of polydopamine under hydrogen atmosphere. The mechanism of charge transport in eumelanin is still not fully clear, but several evidences are concurring to sustain a hybrid ionic-electronic behaviour, where the electronic contribution depends on the presence, extent and the redox properties of the delocalized aromatic systems, while the ionic part is largely dictated by the hydration level of the pigment (i.e. humidity in the measuring environment). Basing on the concurring evidences disclosing the correlation between the chemical physical properties of the eumelanins and the polyindole -system staking, as well as the packing of the molecular constituents within the pigment, we speculated about the modulation of the electronic conductivity, by acting on the polyindole packing in eumelanin thin films. Here (Chapter 3), preparation and characterization of eumelanin thin films showing conductivity up to 318 S/cm are discussed. Highly conductive films were prepared via the oxidative polymerization of films of 5,6-dihydroxyindole (DHI), the ultimate monomer precursor in the formation pathways of natural and synthetic eumelanins, then treated under high vacuum thermal annealing. We name the obtained material as High Vacuum Annealed Eumelanin, HVAE. During the 3rd year of the PhD course, I have had a research visiting period in the Laboratory of Organic Electronics of the University of Linköping in Norrköping under the supervision of Dr. Eric Daniel Głowacki. During this time, I worked on a project concerning the photo(electrochemical) properties of the eumelanin. We report (see Chapter 4) that eumelanin is a photo-catalytic material. Though photoconductivity of eumelanin and its photochemical reactions with oxygen have been known for some time, eumelanins have not been regarded as photofaradaic materials. We found that eumelanin shows photocathodic behaviour for both the oxygen reduction reaction and the hydrogen evolution reaction. Eumelanin films irradiated in aqueous solutions at pH 2 or 7 with simulated solar light, photochemically reduce oxygen to hydrogen peroxide with accompanying oxidation of sacrificial oxalate, formate, or phenol. Auto-oxidation of the eumelanin competes with oxidation of donors. Deposition of thin films on electrodes yields photoelectrodes with higher photocatalytic stability, compared with the case of pure photocatalysis, implicating the successful transport and extraction of holes from the eumelanin layer. These results open up new potential applications for eumelanin as a photocatalytically-active biomaterial, and inform the growing fundamental body of knowledge about the physical chemistry of the eumelanin. From all of this work, as result of this finding, we demonstrate new potentialities of Eumelanin in organic-electronics and bio-electronics field, developing new methods and new recipes either to increase electrical conductivity in order to exploit this material for several applications and purposes either to use it as photocatalytically-active material. Although a conclusive picture about the conductor vs semiconductor behavior of the eumelanins and insights about the mobility of charge carriers will require further investigations, results related to annealed thin films of Eumelanin, here reported, radically modify the actual picture of the eumelanin charge transport properties, reversing the paradigm according to which eumelanin conductivity increases with the water content of the pigment. Indeed, when eumelanin molecular constituents are rearranged in conductive layers, the contribution of electronic current is demonstrated to be largely preeminent with respect to the ionic one, allowing to get unprecedented conductivity and to consider the mammalian pigment as an actual conductor.

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