Nanotechnology for Detection of Waterborne Pathogens - Waterborne Pathogens: Detection Methods and Applications - page 286

bacteria. Examples solely focused on water treatment, using either filters
modified with nanomaterials 16 or those formed from CNTs, 17 are excluded.
In 2008, Wegmann and colleagues modified the surface of microporous
ceramic filters by dip-coating with zirconia nanopowder, changing the sur-
face charge and increasing the surface area at least six-fold. The outcome
was a large increase in the retention of the virus-like MS bacteriophage
from 75% to over 99.99% (7 log removal). 18
In 2010, Li and co-workers reported the use of nanoalumina fibers in
a disk filter format for the sample processing of viruses. 19 The nanofibers
were spatially constrained onto a microglass surface to create the filter. As
the nanoalumina fibers are electropositive, negatively charged viruses are
attracted to them and captured. A higher recovery rate was reported as com-
pared to the Millipore HAWP filter for adenovirus, assessed by flow cytom-
etry. Additional advantages were that no prefiltration or prior adjustment
of pH or addition of cations was required, and that the filters proved effec-
tive at rapidly processing large volumes of different environmental samples.
These disk filters are also less expensive than the NanoCeram cartridge
filters, made of the same material (see Chapter 4 for more details). However,
when assessing recovery by quantitative real time polymerase chain reaction
(q-PCR), the nanoalumina fibers performed less well than the HAWP filter,
suggesting that inhibitors of the PCR process are also concentrated by this
filter, which is a major disadvantage if the intended detection approach is a
molecular method.
Depletion flocculation of bacteria, using rod-shaped nanoparticles with
a length of 90 nm and a width of 8 nm, was described in 2012 by Sun et al. 20
The depletion of nonadsorbing polymers for the aggregation of bacterial
suspensions has not been well exploited despite the fact that bacteria can
be considered as a dispersion of negatively charged particles surrounded
by nonadsorbing polyelectrolytes, which arise from the bacterially secreted
extracellular polymeric substances. The theory is that colloidal particles
have a surrounding depletion zone into which nonadsorbing polymers can-
not penetrate. When two particles approach each other, the depletion layers
overlap and an imbalance in osmotic pressure, as well as polymer exclusion
from between particles maximizing entropy, drive coagulation of particles
( Fig. 9.3 ). For more details, see Ref. 20 and the references therein.
Aggregation of Escherichia coli bacteria, through depletion interactions in
the presence of nonadsorbing polymer and sodium polystyrene sulphonate,
has only been reported recently. 21 Sun et al. found that using their cellulose
nanocrystals (CNC) flocculation was very effective even at concentrations
of 0.1%. They explain the mechanism as follows, “Both bacteria and CNC