Fluorinated two-dimensional nanomaterials (2DFs), with tunable band structures and unique optoelectronic properties, meet the requirements of a wide range of applications and enable device miniaturization down to atomically thin platforms. However, the synthesis of structurally defined 2DFs remains a significant challenge due to the absence of straightforward, mild-condition synthetic protocols. In this work, we report a gram-scale, room-temperature synthesis of 2DFs, yielding thin sheets (1–2 nm) with lateral dimensions extending several micrometers. Nucleophilic substitution of fluorine atoms of perfluoro monomers with alkynyl groups generated reactive intermediates that underwent in-situ [2+2+2] cyclotrimerization. Experimental and computational analyses revealed a porous 2D structure featuring fluorine atoms in a flipped conformation at the center of each pore. The resulting 2DFs exhibited broad optical absorption spanning the visible and near-infrared (NIR) regions (Eg ≈ 2.4 and 3.2 eV) along with strong fluorescence emission (quantum yields up to 66%) in the 300–550 nm range. Notably, this fluorescence was strongly and selectively quenched by low concentrations of per- and polyfluoroalkyl substances (PFAS) in aqueous media, enabling rapid, sensitive, and cost-effective detection of PFAS at subpicomolar concentrations in drinking water. This work establishes a method for fabricating functional two-dimensional (2D) materials to address the urgent environmental monitoring challenge of PFAS. It offers a fast, cost-effective, and straightforward method for monitoring this challenging class of environmental contaminants.