Oxidatively Weathered Quantum Dots: Transformations and Toxicity

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Oxidatively Weathered Quantum Dots
Transformations and Toxicity (NSEC, DMR 0425880)
Joel A. Pedersen, Paige N. Wiecinski, Kevin M.
Metz, Tisha C. King Heiden, Andrew N. Mangham,
Warren Heideman, Richard E. Peterson, Robert J.
Hamers
As production and use of nanomaterials increases,
introduction of engineered nanoparticles into the
environment becomes inevitable. As nanoparticles
enter the environment they have the potential to
be transformed through environmental redox
processes. We developed an in vitro catalytic
model mimicking the extracellular chemistry of
lignolytic fungi and examined the oxidative
stability of poly(ethylene glycol)(PEG)-thiol
coated CdSecore/ZnSshell and poly(maleic
anhydride-alt-octadecane) (PMAO)/PEG wrapped CdSe
quantum dots (QDs). PEGylated QDs were readily
broken down under assay conditions.
Polymer-wrapping appeared to increase CdSe QD
stability against oxidative degradation. We next
examined the degree to which oxidative weathering
altered to toxicity PEGylated QDs to zebrafish
embryos. Oxidatively degraded QDs were more toxic
than as-synthesized QDs, and both were more toxic
than equivalent amounts of CdCl2. Co-exposure of
zebrafish embryos to Cd2 and selenium
nanoparticles recapitulated toxicity observed
upon exposure to weathered CdSecore/ZnSshell QDs.
Simulated oxidative environmental
conditions Methoxyhydroquinone-driven Fentons
reaction
Stability of polymer-wrapped CdSe QDs under
oxidative conditions
Toxicity of weathered PEGylated CdSecore/ZnSshell
QDs
Embryonic zebrafish model
Effect of ligand head group on oxidative stability

• Zebrafish embryos exposed beginning at 4-6 hours
  post-fertilization (hpf)
• Assays conducted in a 96-well plate format (1
  embryo/well).
• Dosing solutions renewed and embryos/larvae
  scored for toxicity and mortality daily for 5
  days.

Acetate buffer
MHQ-Fentons reaction

• Slight blue shift of first exciton peak
  following exposure to MHQ-driven Fentons
  reaction
• Magnitude of shift depends on ligand head group
  
• Stability under oxidative conditions decreases
  from thiols gt carboxylic acid gt phosphonic acid gt
  amine
• Polymer-wrapped CdSe QDs had higher stability
  under oxidative conditions than did PEGylated
  CdSecore/ZnSshell QDs

NH2
Dose-response relationships for weathered and
as-synthesized QDs
SH
PEGylated CdSecore/ZnSshell QD
PMAO/PEG-CdSe QD
Weathered PEGylated QDs show higher lethaliity
(i.e., lower LC50) than as-synthesized QDs. Both
are more toxic than an equivalent amount of CdCl2
Representative micrographs of QD exposed
zebrafish (120 hpf)
Transformations of polymer-wrapped CdSe QDs with
polydecanoic Acid (PDA) head group
Endpoints of toxicity are similar for
as-syntheized and weathered QDs. Endpoints
include altered axial curvatures (aac),
pericardial edema (pe), ocular edema (oe), tail
malformations (tm), and yolk sac malformations
(ysm)
Transformations of PEGylated QDs by MHQ-Fentons
reaction
UV-visible spectra of PMAO/PEG-PDA-CdSe QDs
following exposure to MHQ-Fentons reaction

• Exposure to H2O2 causes blue shift of 1st
  exciton peak
• No additional effect observed with Fentons
  reaction

Synthesis of Se Nanoparticles (SeNPs) Influence
of Se-containing aggregates on toxicity
SEM image of SeNPs
Raman spectra of SeNPs

• Metals analysis indicates dissolution of ZnS
  shell and release of Cd from core

Raman spectra of PDA-CdSe QD following exposure
to MHQ-Fentons reaction
IR spectra of PMAO/PEG-PDA-CdSe QDs
Synthesis produces elemental SeNPs 100-200 nm in
diameter
Dose-response relationships for SeNPs
Oxidation of Se2- to Se0 following exposure to
MHQ-Fentons reaction
Evidence of polymer degradation during
MHQ-Fentons exposure
TEM, electron diffraction and EDX indicate the
formation of amorphous Se-containing aggregates
SeNP co-exposures with CdCl2 recapitulated toxic
endpoints of weathered QD exposures
King Heiden et al., Environ. Sci. Technol. 2009
(43) 1605-1611 Wiecinski et al., in prep
Metz et al. Environ. Sci. Technol. 2009 (43)
1598-1604
Mangham et al., in prep