Correlative Microscopy Study of Phytoaccumulation of Widely Distributed Iron Oxide Colloidal Particles in Sprouts Exposed to Effluents

This study employs a correlative microscopy approach to investigate the uptake, translocation, and phytoaccumulation of widely distributed citric acid-stabilized iron oxide colloidal particles in red radish ( Raphanus sativus ) sprouts. By integrating light and electron microscopy, this method provides a comprehensive, time-resolved analysis of nanoparticles accumulation within plant roots and their biosorption within a dynamically formed biofilm surrounding the root surface. Red radish sprouts were selected as an ideal model due to their rapid germination, well-developed fibrous root system, and high sensitivity to environmental changes, making them suitable for studying nanoparticle-plant interactions. To assess the role of microbial activity in nanoparticle transformation and bioavailability, a carbon nanodot fluorescence quenching technique was employed as a sensitive sensor-based approach to detect and monitor dissolved iron ions in colloidal suspensions. Results confirmed that no free iron ions were released into the effluent, indicating that uptake occurred exclusively in nanoparticulate form. The light microscopic images demonstrated that the biofilm matrix around the roots acted as a retention and filtration system, selectively adsorbing and concentrating colloidal particles near the root surface. Additionally, cross-sectional root tissue analysis using scanning electron microscopy (SEM) revealed that the smallest nanoparticles were able to penetrate plant tissues and undergo intracellular phytoaccumulation. Further energy-dispersive X-ray spectroscopy (EDS) mapping showed an exponential-like time-dependent increase in iron accumulation within root tissues, suggesting a pattern primarily driven by passive diffusion along concentration gradients. These findings provide novel insights into the mechanisms governing nanoparticle uptake in plants, emphasizing the role of plant-microbe interactions in modulating colloidal bioavailability. By elucidating the dynamics of nanoparticle accumulation and retention in plant-microbe systems, this study advances our understanding of phytoremediation processes and offers valuable implications for sustainable environmental remediation and precision agriculture applications.