Interfacial Rigidification-Driven Fluorescence Enhancement of Novel Ultra-Fine Polyamide-Derived Carbon Quantum Dots for Quantitative Detection of Polyamide Microplastics

The widespread environmental presence of polyamide microplastics (PAMPs) demands rapid, selective, and mechanism-driven detection strategies. Herein, we report a “turn-on” fluorescent sensing platform based on ultra-fine polyamide-derived carbon quantum dots (PACQDs; 1.53 ± 0.72 nm), in which signal amplification is governed by interfacial rigidification. PACQDs adsorb spontaneously onto amide-rich PAMP surfaces, with molecular dynamics simulations revealing favorable adsorption energetics (−39.81 ± 2.05 kcal·mol−1) dominated by long-range electrostatic interactions and reinforced by multivalent hydrogen bonding. The fluorescence response reaches equilibrium within 10 min, follows pseudo-second-order kinetics (R2 = 0.99), and exhibits Langmuir-type saturation (R2 = 1.000), indicating a rapid and saturable PACQD–PAMP interaction. The sensor exhibits high polymer selectivity, showing strong fluorescence enhancement for PAMPs, weaker responses to polypropylene microplastics, and fluorescence quenching with polyethylene terephthalate microplastics. Interfacial binding induces restriction of intramolecular motion, evidenced by a positive shift in the Amide I band and suppression of low-frequency dipole fluctuations in simulations. This rigidification suppresses non-radiative decay, producing concentration-dependent fluorescence enhancement (R2 = 0.92). The platform achieves limits of detection/quantification of 0.06/0.14 g·L−1 and accurate quantification in real water samples (recoveries of 103–107%), highlighting excellent matrix tolerance. This work establishes interfacial rigidification as a powerful design principle for selective polymer sensing.