Autoencoder-Based Detection of Nanoplastics in Biological Matrices via Infrared Hyperspectral Imaging

Mid-infrared hyperspectral imaging is an emerging tool for the qualitative and quantitative analysis of micro- and nanoplastics (MNPs) and for the characterization of biological tissues and complex matrices. Detecting MNPs in biological materials is of particular interest for assessing toxicological and ecological impacts; however, significant spectral overlap between polymer vibrational bands and those of proteins, lipids, and other biological components complicates identification at low MNP levels. In this work, an autoencoder-based anomaly detection approach is employed to learn the spectral characteristics of biological matrix signals from infrared spectra acquired with quantum cascade laser infrared (QCL-IR) microscopy. Residual-based anomaly mapping preserves chemically meaningful spectral features, enabling heatmap visualization of nanoscale plastic (NP) accumulation in two- and three-dimensional cell culture models. Fully connected (FC), convolutional neural network (CNN) and hybrid (CNN-FC) autoencoder architectures were evaluated, with FC and CNN-FC models providing a balance between accurate biological matrix reconstruction and preservation of NP spectral signatures. The method enabled detection of ∼50 nm plastic particles when present as localized accumulations corresponding to approximately 1% (m/m) of the dried biological material, equivalent to surface density on the order of 10³ particles/μm². These results demonstrate that residual anomaly detection can extend hyperspectral imaging to visualize nanoscale plastic accumulations in complex biological media at concentrations approaching the instrumental detection limit.