Hydrogel Transformation Reveals a Conventionally Inaccessible Sub-Micrometer Microplastic Fraction in Intact Human Tissues.

Reliable detection of micro- and nanoplastics (MNPs) in human tissues remains analytically challenging due to complex biological matrices, contamination risks, and limited sensitivity of existing spectroscopic and mass-based techniques toward submicrometer particles. Here, we present a histology-compatible analytical approach that enables in situ, volumetric detection and quantification of MNPs in biological tissues with submicrometer sensitivity. The method combines hydrogel-based tissue transformation and optical clearing with fluorescence staining and three-dimensional optical imaging, preserving the spatial architecture of the tissue while removing interfering biological components. This approach effectively minimizes external contamination and enables Nile Red-based identification of MNPs embedded within intact tissue volumes. Analytical performance was validated using spiked biological models and reference polymers, demonstrating reliable retention, detection, and classification of MNPs down to a conservatively defined limit of approximately 0.4 μm. False positives from endogenous hydrophobic structures are suppressed by enzymatic delipidation and are benchmarked using orthogonal fluorescence lifetime signatures against lipid artifacts and plastic reference materials. Applied to human placental tissue, the workflow resolves a particle population dominated by submicrometer MNPs (73% below 1 μm). This size fraction is systematically excluded by filtration-based workflows and, because its cumulative mass is negligible, falls below the detection limits of bulk pyrolysis-based methods. Consequently, the number-dominant MNP fraction in human tissue has remained largely inaccessible to existing analytical approaches. By enabling sensitive, structurally (rather than procedurally) contamination-resistant, and spatially resolved detection of MNPs directly within intact tissues, this approach bridges a critical gap between nanomaterials characterization and exposure biology. It provides a broadly applicable platform for future biomonitoring, exposure assessment, and hypothesis-driven studies on the biological fate of MNPs.