Release, Transport and Toxicity of Engineered Nanoparticles - Reviews of Environmental Contamination and Toxicology - page 23

CeO 2 nanoparticles(7nm)wereusedtoelicit E. coli cytotoxicity in the concentra-
tionrangeof0-730mg/L.Resultsobtainedviasorptionisotherms,TEMmicroscopy
and XAS (X-Ray absorption spectroscopy) revealed that the nanoparticles were
embedded in the bacterial membrane, and their presence produced further oxidative
stress(Table 5 )(Thilletal. 2006 ).
Antibacterial activity of QDs on B. subtilis , E. coli and P. aeruginosa was
observed only after the organisms were weathered in acidic (pH 2) or alkaline
(pH12)environments(Mahendraetal. 2008 ). Such weathering results in release of
freecadmium(Cd)andselenium(Se)ionsduetodegradationofsurfacecoatings
(Table 5 ). However, under moderate alkaline and acidic conditions, QDs are not
bactericidal,evenatconcentrationsfrom10mg/Lto1g/L.QDseffectshavebeen
analyzedusingUV-visiblespectrophotometryandviableplatecounts.Thetoxicity
was reduced in the presence of humic acid, protein and other organic ligands that
limitthebioavailabilityofmetalions(Mahendraetal. 2008 ).
It is worthy to note that the studies described above were conducted on different
bacterial species, different types of ENPs, and under varying conditions (e.g.,
medium composition, differential interaction time, dose and reaction medium).
These differential test conditions greatly influenced how the nanoparticles and bac-
teriainteracted.Forinstance,thehydrophobiccarbon-basednanoparticlesattacked
DNA, lipid fractions of cell membranes and other lipid components of the cell. But,
in the natural soil systems, carbon-based NPs do not elicit toxicity. This may be
because there is a physical interaction of the carbon-based nanoparticles with soil
particles, to render them non-available for biological interaction. On the other hand
metal oxide nanoparticles have been shown to induce ROS and oxidative stress-
mediated toxicity by adsorbing to cell membranes or being absorbed through cell
membranes. Once on or in the cell, such NPs can traverse ion channels and replace
cations at their sites of action. Also, NPs interact with compounds containing free
thiols, carboxylates, phosphates, hydroxyls, nitrates and amines present in the cel-
lular constituents. Therefore, more effort must be expended in future research to
betterunderstandthenatureofENPpropertieslikesize,shape,chemicalandcata-
lytic activity and agglomeration behavior.

5.1.2 Algae

SiO 2 NPs having diameters of 10-20 nm were found to be toxic to water algae,
Scenedesmus obliquus (Table 5 ). S. obliquus exposed to SiO 2 NPs at moderate to
high concentrations for 96 h displayed decreased chlorophyll content. The authors
assumed that the toxicity of these NPs to algal cells resulted from sorption through
cell surfaces (Wei et al. 2010 ). In another study, Miao et al. ( 2010 ) noted that
nanosilver was internalized and accumulated in the freshwater alga Ochromonas
danica ,andsuchuptakewasthecausativetoxicmechanism(Table 5 ).
Palomares et al. ( 2011 ) studied the toxicity of CeO 2 nanoparticles on a self-
luminescent cyanobacterial recombinant strain of Anabaena (CPB4337)andonthe
green alga Pseudokirchneriella subcapitata . Results indicated that these NPs caused