High-performance o-phenylenediamine carbon dots reinforced by PVA steric hindrance: Achieving ultra-sensitive, wide-range sensing, and digital on-site monitoring of methylene blue

o -PD/PVA carbon dots with a protective “polymer armor” were synthesized. • A broad quantitative range (0.1–300 μM) and low LOD (2.4 nM) were achieved for MB. • Intrinsic emission is enhanced by RIR while apparent signal follows IFE. • A digital smartphone platform (B/G ratio) was built for on-site MB monitoring. • High robustness was validated in diverse waters and benchmarked against HPLC. To address the challenges of monitoring methylene blue (MB) across a broad dynamic range in environmental water and industrial wastewater, yellow-emissive carbon dots (CDs) were synthesized from o -phenylenediamine and polyvinyl alcohol (PVA) via a facile one-step solvothermal approach. By leveraging the steric hindrance provided by the long-chain PVA matrix, non-specific probe aggregation in high-concentration environments was effectively inhibited, thereby broadening the dual-stage quantitative range to 0.1–300 μM. Quantitative kinetic investigations revealed a synergistic mechanism of “coexisting apparent quenching and intrinsic enhancement”: the adsorption of MB molecules suppressed non-radiative transitions through spatial confinement effects (quantified by a decrease in k nr   ), inducing an anomalous extension of the intrinsic fluorescence lifetime and an elevation in photoluminescence quantum yield. Simultaneously, the substantial spectral overlap integral ( J (λ)) between MB and the CDs triggered a potent inner filter effect (IFE), which dominated the sensitive apparent fluorescence response. The resulting sensing platform demonstrated a segmented linear and semi-logarithmic calibration model across the entire measurement range, achieving a limit of detection (LOD) of 2.4 nM. Furthermore, by integrating a smartphone-based colorimetric algorithm, a self-calibrating sensing system utilizing the B/G ratio was established. Crucially, the analytical reliability and matrix universality were rigorously validated across diverse aquatic sources (domestic tap water, river water, lake water, and naturally contaminated aquaculture wastewater), exhibiting exceptional agreement with independent high-performance liquid chromatography (HPLC) benchmarks (recoveries: 94.0%–104.5%, RSD < 5.9%), which establishes a reliable digital strategy for comprehensive environmental surveillance. Download: Download high-res image (387KB) Download: Download full-size image Molecular-level mechanistic roadmap of the o -PD/PVA probe response toward MB.