Carbon-Nanodot-Based Bicontinuous Particles for FRET-Based pH Sensing

Developing smart materials capable of sensing different pH levels is crucial for a wide range of practical applications. Sensors utilizing the Forster resonance energy transfer (FRET) modulation mechanism allow sensitive, robust, and environment-independent detection of analytes. Traditional ratiometric FRET platforms encompass a combination of small-molecule/macromolecular dyes or heterogeneous nanomaterials. In situ production of a heterogeneous FRET pair in a single scaffold would greatly simplify the fabrication of solid-state ratiometric platforms, providing an efficient, robust, and photostable device. We introduce a distinct approach to one-step synthesis of a porous organic polymer (POP)-based FRET system for monitoring the pH of a solution and vapors. Using hydrothermal condensation polymerization, soluble precursor monomers are heavily cross-linked to produce POPs. Concurrently, complete carbonization leads to the formation of carbon dots (CDs) that are homogeneously distributed in the POP matrix through imine linkages. This unique bicontinuous POP–CD composite (POP-Dot) features energy transfer from CD to the POP, which is pH-sensitive due to imine bond protonation. Besides the instantaneous change in the color and FRET signal in response to pH modulation, the POP-Dot scaffold exhibits excellent reversibility in both aqueous dispersion and solid state. Notably, POP-Dots demonstrated high sensitivity to the dynamic pH change through a natural antagonistic enzyme pair. Altogether, the straightforward synthesis of a FRET system precluding a ratiometric blend of fluorophores enables high reproducibility, photostability, and reversibility, potentially suitable for the fabrication of pH-sensitive devices.