Simultaneous measurements of microplastic 3D morphology and naphthalene adsorption based on digital holographic microscopy

The accumulation of microplastics (MPs) in the body through biological cycles leads to persistent health risks owing to their degradation resistance. Enhanced toxicity arises from co-occurrence with polycyclic aromatic hydrocarbons (PAHs) that adsorb onto MPs in aquatic environments. Elucidation of MP-PAH adsorption mechanisms requires precise, simultaneous, in situ measurements of two critical parameters, i.e., the dynamic morphologies of MP particles and PAH concentrations. However, there is no comprehensive technology available that can simultaneously measure changes in both of these physical and chemical parameters. This report describes a digital holographic microscopy-based method for simultaneously measuring the 3D MP morphologies and PAH concentration changes in real time. A simple digital off-axis holographic microscopic system is used in combination with a phase difference method to measure the refractive index of the solution to determine the PAH concentration. The measurement results of the phase difference method are consistent with those obtained using a fluorescence spectrophotometer, and the signal-to-noise ratio is 38% higher. The developed technique enables real-time measurements of the PAH concentration changes to derive their adsorption kinetics, thermodynamics, and isothermal models, as verified by adsorption experiments involving various MPs in naphthalene solutions. This work presents a simple and purely optical method for simultaneous physical-chemical characterization in aqueous solutions, thus advancing measurement capabilities for analyzing environmental interfacial processes. The proposed methodology demonstrates broad applicability for co-monitoring morphological and chemical parameters in liquid-phase systems, which expands the scope of optical measurement techniques in biological and environmental sciences.