Single-Particle Mid-Infrared Photothermal Imaging Reveals Hidden Heterogeneity in Real-World Micro- and Nanoplastics.

Micro- and nanoplastics are increasingly detected in human tissues, yet quantitative methods capable of resolving their physicochemical heterogeneity and linking such diversity to functional consequences remain limited. Bottled water, a major ingestion route, provides a controlled matrix for investigating real-world nanoplastic contamination. Here, we employ mid-infrared photothermal (MIP) microscopy to achieve single-particle chemical and morphological characterization, revealing substantial heterogeneity at both microstructural and particle scales. Multidimensional spectral analysis of polyethylene terephthalate (PET), the dominant polymer, uncovers pronounced spectral narrowing, indicating enhanced intra-particle uniformity with physicochemical features distinct from laboratory standards. Population-level spectral measurements further show continuous variations in PET crystallinity, demonstrating the nonuniform nature of environmental particles. Morphological profiling resolves discrete size and shape distributions across polymer types, enabling differentiation of contamination sources and highlighting regulatory blind spots in manufacturing. Collectively, these results show that particles of similar size and composition can occupy divergent physicochemical states. Such property-resolved insights are essential for accurate exposure assessment and for guiding predictive models, mitigation strategies, and future standards for micro- and nanoplastic monitoring.