Reactive oxygen species (ROS) are an integral part of many anticancer therapies. Fenton-like processes involving reactions of peroxides with transition metal ions are a particularly potent and tunable subset of ROS approaches. Precise on-demand dosing of the Fenton reaction is an area of great interest. Herein, we present a concept of an electrochemical faradaic pixel that produces controlled amounts of ROS via a Fenton-like process. The pixel comprises a cathode and anode, where the cathode reduces dissolved oxygen to hydrogen peroxide. The anode is made of chromium, which is electrochemically corroded to yield chromium ions. Peroxide and chromium interact to form a highly oxidizing mixture of hydroxyl radicals and hexavalent Cr ions. After benchmarking the electrochemical properties of this type of device, we demonstrate how it can be used under in vitro conditions with a cancer cell line. The faradaic Fenton pixel is a general and scalable concept that can be used for on-demand delivery of redox-active products for controlling a physiological outcome.

Bioelectronic devices such as neural stimulation and recording devices require stable low-impedance electrode interfaces. Various forms of nitridated titanium are used in biointerface applications due to robustness and biological inertness. In this work, stoichiometric TiN thin films are fabricated using a dual Kaufman ion-beam source setup, without the necessity of substrate heating. These layers are remarkable compared to established forms of TiN due to high degree of crystallinity and excellent electrical conductivity. How this fabrication method can be extended to produce structured AlN, to yield robust AlN/TiN bilayer micropyramids, is described. These electrodes compare favorably to commercial TiN microelectrodes in the performance metrics important for bioelectronics interfaces: higher conductivity (by an order of magnitude), lower electrochemical impedance, and higher capacitive charge injection with lower faradaicity. These results demonstrate that the Kaufman ion-beam sputtering method can produce competitive nitride ceramics for bioelectronics applications at low deposition temperatures.

Photocatalytic generation of hydrogen peroxide, H2O2, has gained increasing attention in recent years, with applications ranging from solar energy conversion to biophysical research. While semiconducting solid-state materials are normally regarded as the workhorse for photogeneration of H2O2, an intriguing alternative for on-demand H2O2 is the use of photocatalytic organic dyes. Herein we report the use of water-soluble dyes based on perylene diimide molecules which behave as true molecular catalysts for the light-induced conversion of dissolved oxygen to hydrogen peroxide. In particular, we address how to obtain visible-light photocatalysts which are stable with respect to aggregation and photochemical degradation. We report on the factors affecting efficiency and stability, including variable electron donors, oxygen partial pressure, pH, and molecular catalyst structure. The result is a perylene diimide derivative with unprecedented peroxide evolution performance using a broad range of organic donor molecules and operating in a wide pH range.

Increasing the electrochemical surface area of noble metal electrodes is vital for many applications, including catalysis and bioelectronics. Herein, a method is presented for obtaining porous noble metal thin films via reactive magnetron sputtering of noble metal oxides, MOx, followed by their reduction using chemical reducers or electrochemical current. Variation of reduction conditions yields a range of different electrochemical and morphological properties. This method for obtaining porous noble metals is rapid, facile, and compatible with microfabrication processes. The resulting metallic films are porous and have competitively high capacitance and low impedance.

The approach of using polyether, aka glycol, side chains to afford amphiphilicity to conducting polymers has recently emerged as a powerful technique for next-generation materials for bioelectronics and electrochemical devices. Herein we apply this synthetic logic to the archetypical conducting polymer poly(3,4-ethylenedioxythiophene), PEDOT, to generate a glycolated PEDOT analogue, G-PEDOT. We report on the electropolymerization of this material, and its electrochemical properties: including spectroelectrochemistry, electrochemical capacitance, and operation of microelectrodes and electrochemical transistors. While in many respects performing like PEDOT, G-PEDOT has electrochemical switching within lower potentials with complete de-doping at lower potentials, affording transistors with higher on/off ratios than PEDOT, and electrochromic switching within a smaller electrochemical window. Overall, G-PEDOT emerges as a useful, functional alternative to other PEDOT derivatives, and could be a building block in copolymers.

Hydrogen peroxide (H₂O₂) is an important oxidant, used in various fields of industry, such as paper manufacturing, production of polymers, detergents, and cosmetics. Considering that the molecule degrades only to H₂O and O₂, it is regarded as a green chemical. Unfortunately, the incumbent method of H₂O₂ synthesis, based on anthraquinone oxidation, although efficient, is not environmentally friendly, as it requires fossil fuels and significant energy input. Therefore, there are efforts underway to reduce the ecological impact of hydrogen peroxide production. Some of the most promising approaches involve catalytic reduction of O₂ to H₂O₂ in an aqueous environment. This can be coupled with water oxidation. As the required energy could be delivered in different ways, hydrogen peroxide synthesis can be achieved by electrocatalysis, photoelectrocatalysis, or photocatalysis.

This thesis explores the possibility of using organic electronic materials as catalysts for H₂O₂ evolution in oxygenated water solutions. Organic electronics is a field of materials science focused on conducting and semiconducting organic molecules. These materials offer many possible advantages, related to low cost, flexibility, and good optoelectronic properties. Huge progress in the field over the last years led to their commercial applications in e.g. organic light emitting diodes and photovoltaics. Only very recently have organic electronics begun to be considered from the point of view of catalysis.

In the first two papers, we investigate electrocatalytic activity of an organic pigment (PTCDI) and a conducting polymer (PEDOT) towards oxygen reduction to hydrogen peroxide. Both types of catalysts are chemically stable and able to operate in a wide pH range. In paper 3, we demonstrate that H₂O₂-evolving photocathodes can be based on an organic PN heterojunction, giving devices of a record-breaking performance. In the first part of paper 4, the same concept was tested for a naturally-occurring semiconductor, eumelanin, leading to a first report of photoelectrocatalytic properties of this material. In the second part of paper 4, as well as in papers 5 and 6, we explore, respectively, photochemical hydrogen peroxide synthesis with eumelanin, organic semiconductors, and organic dyes. We show that the photostability of catalysts is higher for materials with low-lying HOMO level and it can be increased by an addition of a reducing agent to the reaction system. Our findings prove that already existing organic electronic materials can be successfully applied in H₂O₂ evolution for environmentally friendly chemical synthesis, suggesting their use in harvesting of solar energy and in situ generation of hydrogen peroxide for biomedical applications.

We report ultrathin organic photovoltaic elements optimized to run photofaradaic reactions in biological conditions. We demonstrate concurrent oxygen reduction to hydrogen peroxide and glucose oxidation. The devices are powered by deep-red irradiation in the tissue transparency window. We utilize bilayers of phthalocyanine, acting as the light absorber, and perylene diimide, functioning as both electron-acceptor and the hydrogen peroxide evolution electrocatalyst. These heterojunction bilayers are stable when irradiated in simulated physiological conditions, producing photovoltages sufficient to simultaneously drive cathodic oxygen reduction to H2O2 and anodic oxidation of glucose. We find that optimization of the anode metal is critical for sustained photofaradaic reactivity. Our results demonstrate a robust "wet" thin film photovoltaic with potential for physiological applications where localized electrochemical manipulation is desired, in particular the delivery of reactive oxygen species.

Means of sustainable on-demand hydrogen peroxide production are sought after for numerous industrial, agricultural, and environmental applications. Herein we present the capacity of lignin and lignin sulfonate to behave as photocatalysts that upon irradiation reduce oxygen to hydrogen peroxide. Water-soluble lignin sulfonate acts as a homogeneous photocatalyst in solution, while lignin in thin-film form behaves as a heterogenous photocatalyst. In both cases, the photochemical cycle is closed via the oxidation of electron donors in solution, a process which competes with the autooxidation of lignin. Therefore, lignins can be destructively photooxidized to produce hydrogen peroxide as well as photochemically oxidizing low-oxidation potential species. These findings enable new photochemistry applications with abundant biopolymers and inform the growing body of knowledge on photochemical evolution of hydrogen peroxide.

Organic semiconductors are gaining increasing attention as promising active materials in aqueous electrochemical and photoelectrochemical applications. Controlling the semiconductor/electrolyte interface is a critical aspect. The aim of this work is to increase the electrochemical surface area of an archetypical polymeric semiconductor, poly(3-hexylthiophene), P3HT. Here we use a technique of blending with polystyrene, phase separation, followed by selective removal of polystyrene to obtain various nano/microstructures of P3HT. We evaluate how three-dimensional structuring of P3HT affects electrochemical capacitance, photovoltage generation, and photoelectrochemical currents. The aqueous wettability of the exposed surface is critical, and it can be significantly modified by employing oxygen plasma treatment. Structured and plasma-hydrophilized P3HT samples demonstrate higher photoelectrochemical currents for the oxygen reduction reaction. We find that regardless of structuring and photocurrent performance, the oxygen reduction on P3HT always proceeds to produce H(2)O(2)with 90%+ faradaic efficiency. The results of our efforts are demonstrations of how to tune and to increase both electrochemical capacitive and faradaic behavior of P3HT layers. Our findings indicate some limitations imposed by the P3HT itself, including low photovoltages and photochemical bleaching. Overall, these findings answer several open questions in the field of P3HT photoelectrochemical interfaces, and provide some general guidelines that can be applied to other organic semiconducting materials.

Electrocatalysis for energy‐efficient chemical transformations is a central concept behind sustainable technologies. Numerous efforts focus on synthesizing hydrogen peroxide, a major industrial chemical and potential fuel, using simple and green methods. Electrochemical synthesis of peroxide is a promising route. Herein it is demonstrated that the conducting polymer poly(3,4‐ethylenedioxythiophene), PEDOT, is an efficient and selective heterogeneous catalyst for the direct reduction of oxygen to hydrogen peroxide. While many metallic catalysts are known to generate peroxide, they subsequently catalyze decomposition of peroxide to water. PEDOT electrodes can support continuous generation of high concentrations of peroxide with Faraday efficiency remaining close to 100%. The mechanisms of PEDOT‐catalyzed reduction of O2 to H2O2 using in situ spectroscopic techniques and theoretical calculations, which both corroborate the existence of a chemisorbed reactive intermediate on the polymer chains that kinetically favors the selective reduction reaction to H2O2, are explored. These results offer a viable method for peroxide electrosynthesis and open new possibilities for intrinsic catalytic properties of conducting polymers.

Optical control of the electrophysiology of single cells can be a powerful tool for biomedical research and technology. Here, we report organic electrolytic photocapacitors (OEPCs), devices that function as extracellular capacitive electrodes for stimulating cells. OEPCs consist of transparent conductor layers covered with a donor-acceptor bilayer of organic photoconductors. This device produces an open-circuit voltage in a physiological solution of 330 mV upon illumination using light in a tissue transparency window of 630 to 660 nm. We have performed electrophysiological recordings on Xenopus laevis oocytes, finding rapid (time constants, 50 mu s to 5 ms) photoinduced transient changes in the range of 20 to 110 mV. We measure photoinduced opening of potassium channels, conclusively proving that the OEPC effectively depolarizes the cell membrane. Our results demonstrate that the OEPC can be a versatile nongenetic technique for optical manipulation of electrophysiology and currently represents one of the simplest and most stable and efficient optical stimulation solutions.

Photochemical generation of hydrogen peroxide via oxygen reduction is a critical component of emerging sustainable energy conversion concepts. Light‐absorbing semiconductors as well as electrodes modified with sensitizers typically catalyze oxygen photoreduction to hydrogen peroxide. Here, it is reported that, in contrast to these heterogeneous systems, a homogeneous solution of a metal‐free organic dye can perform the whole catalytic cycle of hydrogen peroxide photoevolution itself. This cycle can proceed with simultaneous oxidation of various organic molecules as electron donors, or even water. In the three water‐soluble dyes that are experimented with, photoevolution of peroxide occurs favorably at neutral to basic pH. The reaction is first order with respect to dye concentration, and evidence implicates a single‐electron reduction pathway with superoxide as an intermediate. Photostability of the dyes over time correlates with increased oxidation potential of the molecule. The finding that hydrogen peroxide can be produced in aqueous solution with single fully organic molecules performing the entire photocatalytic cycle creates a new avenue for the peroxide carbon free energy cycle.

We report that eumelanin, the ubiquitous natural pigment found in most living organisms, is a photocatalytic material. Though the photoconductivity of eumelanin and its photochemical reactions with oxygen have been known for some time, eumelanins have not been regarded as photofaradaic materials. We find that eumelanin shows photocathodic behavior 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. Autooxidation of the eumelanin competes with the 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 extraction of positive charges 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 eumelanins.

Correction for Organic semiconductor perylenetetracarboxylic diimide (PTCDI) electrodes for electrocatalytic reduction of oxygen to hydrogen peroxide by Magdalena Warczak et al., Chem. Commun., 2018, DOI: ; Web: 10.1039/c7cc08471d.

Correction for Organic semiconductor perylenetetracarboxylic diimide (PTCDI) electrodes for electrocatalytic reduction of oxygen to hydrogen peroxide by Magdalena Warczak et al., Chem. Commun., 2018, 54, 1960-1963.

Low-cost semiconductor photocatalysts offer unique possibilities for industrial chemical transformations and energy conversion applications. We report that a range of organic semiconductors are capable of efficient photocatalytic oxygen reduction to H2O2 in aqueous conditions. These semiconductors, in the form of thin films, support a 2-electron/2-proton redox cycle involving photoreduction of dissolved O-2 to H2O2, with the concurrent photooxidation of organic substrates: formate, oxalate, and phenol. Photochemical oxygen reduction is observed in a pH range from 2 to 12. In cases where valence band energy of the semiconductor is energetically high, autoxidation competes with oxidation of the donors, and thus turnover numbers are low. Materials with deeper valence band energies afford higher stability and also oxidation of H2O to O-2. We found increased H2O2 evolution rate for surfactant-stabilized nanoparticles versus planar thin films. These results evidence that photochemical O-2 reduction may be a widespread feature of organic semiconductors, and open potential avenues for organic semiconductors for catalytic applications.

Solar-to-chemical conversion of sunlight into hydrogen peroxide as a chemical fuel is an emerging carbon-free sustainable energy strategy. The process is based on the reduction of dissolved oxygen to hydrogen peroxide. Only limited amounts of photoelectrode materials have been successfully explored for photoelectrochemical production of hydrogen peroxide. Herein we detail approaches to produce robust organic semiconductor photocathodes for peroxide evolution. They are based on evaporated donor-acceptor heterojunctions between phthalocyanine and tetracarboxylic perylenediimide, respectively. These small molecules form nanocrystalline films with good operational stability and high surface area. We discuss critical parameters which allow fabrication of efficient devices. These photocathodes can support continuous generation of high concentrations of peroxide with faradaic efficiency remaining at around 70%. We find that an advantage of the evaporated heterojunctions is that they can be readily vertically stacked to produce tandem cells which produce higher voltages. This feature is desirable for fabricating two-electrode photoelectrochemical cells. Overall, the photocathodes presented here have the highest performance reported to date in terms of photocurrent for peroxide production. These results offer a viable method for peroxide photosynthesis and provide a roadmap of strategies that can be used to produce photoelectrodes with even higher efficiency and productivity.

Hydrogen peroxide is one of the most important industrial chemicals and there is great demand for the production of H2O2 usingmore sustainable and environmentally benign methods. We show electrochemical production of H2O2 by the reduction of O-2, enabled by an organic semiconductor catalyst, N,N-dimethyl perylenetetracarboxylic diimide (PTCDI). We make PTCDI cathodes that are capable of stable and reusable operation in aqueous electrolytes in a pH range of 1-13 with a catalytic figure of merit as high as 26 kg H2O2 per g catalyst per h. These performance and stability open new avenues for organic small molecule semiconductors as electrocatalysts.