The hydroxyl radical (HO•) is a highly reactive oxidant, produced via autoxidation in biological systems, in advanced oxidation processes, or in electrocatalysis. If it is leverage for photodegradation, harmful HO• content must be tempered in many devices (energy conversion materials, biomedical implants, nanotechnologies…) to improve their durability. Probing HO• in situ at the earliest stage is key to understand and control the chemical reaction responsible for its formation. Because of HO• short lifetime, one needs diagnosing its production at the nanoscale, or equivalently at single nanoparticle level (NP). In this context, the PIRaNa project will be devoted to the development of a correlative opto-electrochemical multi-microscopies platform allowing to image HO• formation both at individual and within a set of NPs. It will allow breaking the limit of operando detection and quantification of HO• at the ultimate scales using firstly model chemical systems, such as i) Pt NPs, still key elements in fuel cells and ii) TiO2 NPs, used in many applications, from photocatalysis to biomedicine. The project lies on two complementary sensing methodologies: i) local electrochemical probe microscopies (SECPMs), herein the scanning electrochemical microscopy (SECM, allowing to characterize interfacial reactivity in solution using a micro/nanoelectrode) and the scanning electrochemical cell microscopy (SECCM, allowing to confine, by a nanopipette, single NPs within an electrolytic nanodroplet cell) ii) high resolution interferometric optical microscopy (IOM). The operating range of both SECM and IOM will be delved in WP1 and WP2 respectively, while a synergistic combination of SECPMs (SECM, SECCM) and IOM in WP3 and WP4 will highlight the deleterious effect of HO• (i.e. the material nanocorrosion) and the importance of cross-talk in autocatalytic processes. WP1 will achieve the sensing and imaging of local HO• production based on its chemical reaction with a redox probe next to a micro/nanoelectrode tip of a SECM. The electrochemical (EC) current fluctuations caused by this reaction, amplified by the feedback loop provided by the geometrical confinement, allows a highly sensitive, operando, probing of HO• flux generated at Pt or TiO2 NPs (from ensemble to single entities). WP2 is dedicated to HO• sensing by the highly sensitive IOM. The approach is based on the optical monitoring of gaz nanobubbles growth from the degradation of a chemical probe by HO•. The IOM will image and quantify, at high throughput, the HO• production at the single NP level, allowing for the first time to establish a structure/HO• production relationship through computerized data treatment and statistical analyses. In a second step, and thanks to the new implementation of SECCM in the laboratory, the SECPMs, will be combined to IOM. WP3 will use such correlative opto-SECPM microscopies as an incomparable methodology to visualize and apprehend operando NPs nano-corrosion induced by HO•. WP4 will extend the methodology to decipher autocatalytic mechanisms, at the origin of a drastic increase of [HO•], believed to be triggered by NPs cross talk. It will bring an overview of such processes for which the parameters conditioning their activation will be investigated at multiscale ranging from arrays to individual NPs. The PIRaNa project brings together young researchers having complementary skills and scientific interests (SECPMs, IOM, NPs synthesis, etc.) ensuring the development of a unique platform in France, and only mastered by few prestigious groups worldwide. The completion of this project will lead to a multidisciplinary operational methodology adaptable to many environment (various temperature condition and electrolytic media) making it relevant for many other field of research.