Aggregation, Dissolution, and Stability of Quantum Dots in Marine Environments: Importance of Extracellular Polymeric Substances

There is an increasing concern that a considerable fraction of engineered nanoparticles (ENs), including quantum dots (QDs), will eventually find their way into the marine environment and have negative impacts on plankton. As ENs enter the ocean, they will encounter extracellular polymeric substances (EPS) from microbial sources before directly interacting with plankton cells. In this study, EPS harvested from four phytoplankton species, <i>Amphora</i> sp., <i>Dunaliella tertiolecta</i>, <i>Phaeocystis globosa</i>, and <i>Thalassiosira pseudonana,</i> were examined for potential interactions with CdSe nonfunctionalized and functionalized (carboxyl- and amine-) QDs in artificial seawater. Our results show that EPS do not reduce the solubility of QDs but rather decrease their stability. The degradation rate of QDs was positively correlated to the protein composition of EPS (defined by the ratio of protein/carbohydrate). Two approaches showed significant inhibition to the degradation of carboxyl-functionalized QDs: (1) the presence of an antioxidant, such as <i>N</i>-acetyl cysteine, and (2) absence of light. Owing to the complexity in evaluating integrated effects of QDs intrinsic properties and the external environmental factors that control the stability of QDs, conclusions must be based on a careful consideration of all these factors when attempting to evaluate the bioavailability of QDs and other ENs in the marine environments

Chemical analysis of EPS.

<p>*Zhange et al., (2008) <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0021865#pone.0021865-Zhang1" target="_blank">[40]</a>.</p

EPS Assembly analysis.

<p>EPS Assembly analysis.</p

Assembly kinetics of EPS monitored with DLS.

<p>(A) Assembly kinetics of EPS of <i>Amphora sp.</i> (B) Assembly kinetics of EPS of <i>Ankistrodesmus angustus</i> (C) Assembly kinetics of EPS of <i>Phaeodactylum tricornutum</i> EPS assembly in Ca²⁺-free ASW (black) was monitored to investigate assembly kinetics with decreased divalent ion availability. Different concentrations of ENs (polystyrene nanoparticles): 0 (red), 10 (green) and 100 ppb (blue), were added to investigate the effect of ENs on EPS microgel formation.</p

Fluorescence images of EPS and ENs-induced EPS microgels.

<p>Nile Red was used to determine the microgel morphology. Green fluorescent signals indicated the fluorescent ENs. From the overlay images, results showed that the ENs incorporated within EPS matrixes. Scale bar is 10 µm.</p

ESEM images of EPS microgel.

<p>(A) <i>Amphora sp.</i> (Scale Bar = 4 µm) (B) <i>Ankistrodesmus angustus</i> (Scale Bar = 5 µm) (C) <i>Phaeodactylum tricornutum</i> (Scale Bar = 5 µm).</p