Carbon-Nanodot-Based\nBicontinuous Particles for FRET-Based\npH Sensing

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