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

Abstract

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 smallmolecule/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.