Nanotechnology for Detection of Waterborne Pathogens - Waterborne Pathogens: Detection Methods and Applications - page 294

the QD functionalization procedure to attach antibodies against Cryptospo-
ridium is needed.
QDs offer great photostability, enhanced sensitivity of detection, and
easy simultaneous detection of multiple pathogens. They have been widely
applied to a range of pathogens, detecting mainly nuclei acids but also
whole pathogens and even toxins, though they appear to have been less
applied in the study of waterborne pathogens. Disadvantages that have been
noted are interactions with other compounds, decreasing the sensitivity, or
quenching the fluorescence, 14 which might explain why QDs have not
been applied in complex environmental water samples. However, the main
reason might have to do with their cost, which is higher than traditional
dyes. Concerns have also been raised about their size, which is an order of
magnitude greater than most dyes, and might pose problems for biorecogni-
tion in multiplex assays. 14
Dye-doped silica NPs were employed by Zhao et al. to achieve single
cell E. coli O157:H7 detection in processed beef samples. The samples were
incubated with the NPs, and unbound NPs were removed via centrifuga-
tion. Total detection time was less than 20 min, and the result could be read
out using a spectrofluorometer, or a simple flow cytometer. Each antibody
against the bacteria was bound to an NP containing 1000 dye molecules
inside a silica matrix thus resulting in a 1000 times amplification of fluo-
rescent signal compared to traditional antibody-fluorophore complexes.
Therefore, this method offered single cell resolution without the need for
time-consuming culture steps.
Adenovirus has been detected though the use of europium (III)-chelate
doped NPs to a limit of 5000 virions per milliliter via a sandwich-based
assay. 43 This study, which used patient samples rather than water samples,
reported an 800-fold improvement in detection compared to existing
immunofluorometric assays. We found no reports of fluorescent NPs applied
to the detection of other waterborne viruses or to waterborne protozoa.
AuNPs have been used in a resonance light scattering technique for the
detection of pathogen antigens. The technique was developed by Ray and
co-workers, who exploited the nonlinear optics of AuNPs, and applied hyper-
Rayleigh scattering to detect binding of oligonucleotide probes. 44 Since
double-stranded oligonucleotides have different electrostatic properties the
binding of the target to AuNP conjugated probe leads to dissociation of the
probe from the NP, removing the fluorescence quenching of the probe, and
also initiating aggregation and colorimetric change of the AuNPs. This was
applied by Lin et al. for the detection of E. coli antigen to an LOD of 10 ng. 45