Agriculture Reference
In-Depth Information
The zeta potential of ZnO nanoparticles with varying pH of the medium was
measured. The zero point charge for ZnO nanoparticles was observed at pH 9.15.
The ZnO and ZnO-TOPO nanoparticles were positively charged between pH 6 and
8 and negatively charged between pH 10 and 12. At acidic pH, partial dissolution of
all ZnO nanoparticles was observed, and zeta potential was negative. After the
addition of ZnO, ZnO-TOPO, and ZnO-Brij-76 nanoparticles (
H
¼
10), a progres-
sive decrease of photosynthetic activity was observed in the first 10 days for both
microorganisms, caused by stress, originated by the addition of the nanoparticles.
For
A. flos
-
aquae
, after 10 days of incubation, the presence of ZnO-TOPO
nanoparticles caused cellular death. For
E. gracilis
, after 10 days of incubation,
the decrease of photosynthetic activity was followed in all cases by cellular death.
A live/dead test was conducted. For
A. flos
-
aquae
, 75 % cell survival was observed
after contact with ZnO and ZnO-Brij-76 nanoparticles, and only 25 % cell survival
was observed after contact with ZnO-TOPO nanoparticles. For
E. gracilis
, the
percentage of cell survival was near 10 % for all cases. For
A. flos
-
aquae
after
contact with ZnO and ZnO-Brij-76 nanoparticles, aggregates of spherical
nanoparticles were observed around the cell wall composed of the polysaccharides,
and after contact with ZnO-TOPO, the cell wall was damaged, and the intracellular
content had leaked out. Internalization of some spherical aggregates of
nanoparticles was also observed. In the case of
E. gracilis
, vesicles filled by
spherical ZnO nanoparticles internalized by endocytosis were observed;
ZnO-TOPO spherical nanoparticles were found in the cytoplasm, and with
ZnO-Brij-76, the cell wall was damaged, and the intracellular content had leaked
out. But in all cases, no nanoparticle agglomeration was observed.
The ability of ZnO to form aggregates and to dissolve was also reported by
Franklin et al. (
2007
), using 30-nm particles. Particle characterization using TEM
and dynamic light scattering techniques showed that particle aggregation is signif-
icant in a freshwater system, resulting in flocks ranging from several hundred
nanometers to several microns. Chemical investigations using equilibrium dialysis
demonstrated rapid dissolution of ZnO nanoparticles in a freshwater medium
(pH 7.6), with saturation solubility in the milligram per liter range, similar to that
of bulk ZnO. They also evaluated the toxicity, using
P. subcapitata
freshwater alga
that revealed comparable toxicity for nanoparticulate ZnO, bulk ZnO, and ZnCl
2
,
with a 72-h IC
50
value near 60
gZnL
1
, attributable solely to dissolved zinc.
Li et al. (
2011a
,
b
) also showed that the dissolved Zn is a key factor for nano-
ZnO toxicity. The authors investigated the effect of five commonly used aqueous
media with various chemical properties on the toxicity of nano-ZnO to
E. coli
,
including ultrapure water, 0.85 % NaCl, phosphate buffered saline (PBS), minimal
Davis (MD), and Luria-Bertani (LB). They concluded that the toxicity of
nano-ZnO is mainly due to the free zinc ions and labile zinc complexes. The
toxicity of nano-ZnO in the five media deceased as follows: ultrapure
water
μ
PBS. The generation of precipitates (Zn
3
(PO
4
)
2
in
PBS) and zinc complexes (of zinc with citrate and amino acids in MD and LB,
respectively) dramatically decreased the concentration of Zn
2+
ions, resulting in
the lower toxicity in these media. Additionally, the isotonic and rich nutrient
NaCl
MD
LB
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