Nanotechnology for Detection of Waterborne Pathogens - Waterborne Pathogens: Detection Methods and Applications - page 295

Other examples, based solely on the colorimetric aspects of AuNPs, are the
detection of Salmonella , 46,47 shiga toxin producing E. coli 48 and the cholera
toxin. 49 The latter provided quantitative detection within 10 min.
To the best of our knowledge, the AuNP aggregation approach has not
been attempted for the detection of waterborne protozoa. For viruses, the first
paper to demonstrate virus detection using this approach was published by
Lee et al. in early 2013, although the technique has been previously applied
to detection of viral nucleic acids, e.g. with the ability to perform single mis-
match detection or direct detection without amplification. 50 Lee et al. stressed
the need for an excess of AuNP to allow for sufficient binding to occur to
achieve AuNPs in close enough proximity to trigger the colorimetric change.
With the colorimetric detection approach, silver can be applied for sig-
nal enhancement. The AuNPs act as nucleation sites for the silver. Metallic
silver has a highly intense black color, easily visible by eye. This has been
exploited for Salmonella detection to reach an LOD of 5 cfu mL −1 (see Ref.
51 ) and has also been applied for the amplification of Giardia detection. 52
So far, the colorimetric assay approach has been little applied for the
detection of waterborne pathogens, but offers a promising and relatively
simple detection approach. This is especially advantageous for low-income
field testing settings, as no microscope is required for the eye to detect colo-
rimetric change. As the authors of the paper reporting the first detection
of a whole virus via this technique points out an excess of AuNPs, and is
essential for the method to work. This is therefore, most likely, best suited
for the detection of viruses at lower numbers. Work over the next few years
will be needed to extend the technique to a wider range of pathogens, and
appropriate surface functionalization will be key to achieving this goal.
As was discussed in Chapter 5, surface enhanced Raman scattering uses
the surface plasmons on roughened metallic surfaces to enhance the Raman
effect. 3 SERS has been applied to the detection of waterborne viruses, 53
bacteria 54,55 and protozoa. 56 However, one of the main challenges of SERS
detection is to obtain multiplex detection with high specificity, due to a
high degree of similarity between SERS spectra, and the variability of SERS
spectra with species, viability, and even age of the sample. The technique is
evidently highly sensitive with the potential to offer a wealth of information
if accurate discrimination between all these possible states can be realized.
In some cases nanostructures are fabricated on the Raman substrate and
in others nanoparticles are employed. For example, AgNPs have been used to
distinguish between Listeria species, 57 and AuNPs integrated with a Raman
reporter have been employed as pathogen specific SERS probes in a study