Surface coating determines the response of soybean plants to cadmium sulfide quantum dots

Quantum dots (QDs) are used in an array of applications from electronic devices to medicine, leading to environmental release and accumulation in landfills and agricultural soils. However, their effects on plants have been least studied among other engineered nanomaterials. In the current study, soybean seedlings were exposed to 50-200 mg/L cadmium sulfide quantum dots (CdS-QDs) to elucidate their bioaccumulation and biological response in comparison to Cd2+ ions and bulk-CdS. To understand the role of surface coatings on the QD stability, uptake, translocation, subcellular localization and cellular response, CdS-QDs were capped with ligands varying in polarity and surface charge. Commonly used ligands, including trioctylphosphine oxide, polyvinylpyrrolidone, mercaptoacetic acid, and glycine were used to synthesize QD- TOPO, QD-PVP, QD-MAA, and QD-GLY, respectively. In aqueous suspension, QD-MAA were most stable, maintaining their size at 332 nm with least Cd2+ dissolution (9%) after 14d, whereas QD-TOPO formed large aggregates of size 3861 nm with 27% Cd2+ dissolution. Upon exposure to 100 mg/L bare and coated CdS-QDs in vermiculite for 14 d, soybean roots accumulated Cd, ranging from 568 (QD-MAA) to 1010 (QD-PVP) μg/g tissue dry weight (DW); equivalent to bulk-CdS treatment (639 μg/g DW). In the roots, Cd from CdCl2, bulk- CdS, QD-MAA and QD- GLY accumulated primarily in the cell wall (~40-55%) followed by organelles (28~40%), suggesting apoplastic pathway; whereas in QD-TOPO, dissolved Cd2+ ions accumulated in the membranes. The exception was QD-PVP, which sequestered Cd mainly in the organelles (49%) in roots, potentially via symplastic pathway and translocated to shoots, resulting in reduced leaf biomass. Results suggested that peroxidases play the predominant role in quenching the oxidative stress caused by CdS-QD exposure. At the highest CdS-QD treatment (200 mg/L), root lignification allowed the plants to restrict Cd accumulation, except in QD-PVP, where lignification was reduced by 21% leading to higher content in shoots. Increased amino acid content in the leaves was noted as a stress tolerance mechanism by the plants exposed to 200 mg/L QD treatments. This study highlights the significant influence that surface coating exerts on QDs fate and effects in a planted system.