Reactive oxygen species (ROS) are an integral part of our lives. They perform essential functions in our biology and are, due to their high oxidative power, key molecules in advanced oxidation processes in industrial applications such as treatment of wastewater. Likewise, they play a crucial role as active intermediates in chemodynamic or photodynamic therapy of cancer. ROS, such as hydrogen peroxide (H₂O₂) or hydroxyl radicals (OH·), are derived from molecular oxygen (O₂), and represent relatively more reactive oxidants with respect to parent O₂.

At the cellular level, ROS are produced primarily in mitochondria and among others take part in cell signaling and maintaining redox homeostasis. However, when exceeding a certain threshold, ROS can lead to oxidative stress and consequently to cardiovascular and neurodegenerative diseases, as well as cell death. Yet, the ability of modulating the generation of ROS externally gives the possibility to treat certain diseases as well. Advanced oxidation processes such as photoinduced generation of ROS and Fenton-like processes consisting of reactions of H₂O₂ with transition metal ions are particularly potent and tunable approaches of ROS generation.

The aim of this thesis is to expand the range of applications for advanced oxidation processes, and it includes device and electrode fabrication and characterization for local ROS generation and delivery.

In paper 1, we exploited naturally-sourced lignins, which share critical structural features with known photocatalysts, to photochemically reduce O₂ to H₂O₂ with simultaneous degradation of the biopolymer, when irradiated with UV light. By adding electron donors, the autoxidation of the lignins was reduced and H₂O₂ generation partially increased. By showing the possibility to destructively photooxidize lignins to produce H₂O₂ we contribute to new developments of valorization of lignins and engineering solutions.

In paper 2, we created an electro-Fenton device which electrochemically generates H₂O₂ and simultaneously dissolves chromium. Chromium ions and H₂O₂ are cytotoxic in their own right, but also can react with each other to form highly oxidizing hydroxyl radicals. We demonstrated the ability of these electrochemically-generated species to induce cell death in a metastatic human skin cancer cell line. In paper 3 we spun the concept of paper 2 further and demonstrated biphasic electro-Fenton reaction on a single stainless steel electrode and showed the generation of hydroxyl radicals with autoluminescense measurements in situ. In paper 4, we investigated the anodic contributions of electrodes in physiological conditions in regards of ROS formation to shed light onto possible electrochemically induced processes contributing to the disintegration of cells by current treatment. Studying the application of oxygen reduction reaction, electro-fenton processes, and oxidation processes in physiological environment leads to a better understanding of electrochemical processes and could further simplified, cheap and locally applied direct current tissue ablation in anti-tumor treatment with decreased systemic side effects.In summary this thesis elaborates various novel methods of on-demand ROS generation. Paper 1 is a photoinduced process, while papers 2-4 introduce various methods of electrochemical ROS generation via combinations of cathodic (oxygen reduction reactions) and anodic (metal corrosion, direct water oxidation) processes.

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.

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.