Light‐Controlled Exposure of Cancer Cells to Reactive Oxygen Species Using Organic Semiconductor Thin Films

ABSTRACT Reactive oxygen species (ROS) orchestrate essential signalling pathways, yet bulk dosing with conventional chemical oxidants lacks spatial precision and perturbs the extracellular milieu. Here we exploit organic semiconductor thin films comprising poly(9,9‐dioctylfluorene‐alt‐bithiophene) (F8T2) as solid‐state, reagent‐free photosensitisers that convert dissolved O 2 to superoxide at the solid‐liquid interface under illumination by visible light. Dihydroethidium fluorimetry quantifies a polymer‐loading‐dependent ROS flux where films retain full activity after 15 days of incubation under standard cell culture conditions, demonstrating remarkable operational stability. When functionalised with fibronectin, F8T2 films support confluent MCF7 monolayers that are unharmed in the dark yet undergo rapid, light‐triggered necrosis, reaching half‐maximal viability within 8.8 ± 3.8 min for 61.4 ± 19.4 nm thickness films; fibronectin‐glass controls show no loss of viability. Mechanical scratching of the polymer film creates ROS‐free corridors, demonstrating micron‐scale confinement of cytotoxicity to illuminated polymer regions. Since activation is achieved with commodity white LEDs and requires no soluble reagents, “ROS patches” enable spatiotemporally precise oxidative stress for mechanistic redox studies, antimicrobial surface design, and high‐throughput screening for ROS‐responsive therapeutics.