Ratiometric Screening of Polystyrene Nanoplastics via Conformation-Adaptive π−π Stacking on 2D Metal−Organic Frameworks

Rapid and accurate detection of polystyrene (PS) micro-/nanoplastics (MNPs) remains a considerable analytical challenge. In this work, a ratiometric fluorescence sensing system based on a two-dimensional metal−organic framework nanosheet (Tb-TCPP) was constructed for the selective recognition of PS MNPs via aromatic π−π stacking interactions. In the free-standing state, the TCPP porphyrin linkers within Tb-TCPP undergo vigorous intramolecular rotation and out-of-plane vibration. Most excited-state energy dissipates through non-radiative pathways, leading to weak fluorescence with a fluorescence lifetime of only 2.56 ns. After Tb-TCPP combines PS particles through π−π stacking and van der Waals forces, the PS forces TCPP linkers into a highly compact conformation, which greatly restricts intramolecular rotation and vibration of the linkers. As a result, the non-radiative transition rate decreases substantially, and the fluorescence lifetime increases to 10.82 ns. Density functional theory (DFT) calculations reveal a binding energy of −11.06 kcal/mol between TCPP and the PS monomer, verifying the thermodynamically spontaneous nature of interfacial binding between Tb-TCPP and PS. Furthermore, by taking advantage of the property that benzoic acid can coordinate with the terbium metal center and form π−π interactions with TCPP, the optical response mechanism of Tb-TCPP to PS was further simulated and analyzed. This confirms that the sensing mechanism originates primarily from the suppression of non-radiative transitions within Tb-TCPP. Notably, Tb-TCPP exhibits a size-sieving effect toward PS as the 50 nm PS possesses larger total specific surface area and higher particle abundance, exposing more aromatic sites. This study provides a reliable analytical strategy for fast and accurate analysis of PS MNPs and offers new insights for the rational design of high-performance optical sensing materials.