Oxygen (O2) contributes in part to the cytotoxic response in macrophages. Over the past decade, several probes have been developed for the selective detection of different oxidants in biological systems. Peroxynitrite (ONOO–), the product of the reaction between nitric oxide (●NO) and superoxide (O2●–), is a strong oxidant that can cause biological damage through oxidation and nitration. Typically used boron-based probes for peroxynitrite detection are prone to oxygen-dependent quenching by pO₂ variations and are highly pH sensitive. Therefore, we are developing a group of fluorescent boronate-based probes (Red-BI and its structural analogues Red-BII and Red-BIII, bearing an extended π-conjugated system) derived from xanthene (Scheme 1). These probes will complement and expand the use of conventional boronates, enabling accurate cellular detection of peroxynitrite and offering new insights into the role of oxygen in oxidant production within the cellular microenvironment.