Nanotechnology for Detection of Waterborne Pathogens - Waterborne Pathogens: Detection Methods and Applications - page 284

fluorescent dyes, and proteins. Specifically, these materials offer excellent
photostability as well as tunable emission spectra. For a review of and intro-
duction to QDs, see the work by Arya. 13
A QD often consists of a semiconductor core such as cadmium, mixed
with selenium or tellurium, surrounded by a semiconductor outer shell,
often zinc sulphide, and a layer of polymer to stabilize the QD, and allow
for (bio)chemical functionalization. The typical size of the core is 2-10 nm,
with a total diameter of 10-20nm greater than that accounting for the
outer shell and polymer. QDs are commonly prepared using colloidal syn-
thesis although other options are molecular beam epitaxy or electron beam
lithography.
The emission spectra of QDs are tunable from the ultraviolet to the
near infrared region, simply by modulating their size. Simultaneous exci-
tation at a single wavelength of different sized QDs is possible due to the
broad Stokes shift of the QDs thus allowing for simple multiplex detec-
tion. Additionally, QDs are highly bright and photostable, meaning they
avoid the problems of self-absorption, self-fluorescence, and photobleaching
from which traditional fluorophores suffer. Therefore, it is not surprising
that they have found applications within many different areas, including
waterborne pathogen detection.
However, one of the major future challenges is the development and
large-scale production of stable and reproducible QDs with biocompatible
coatings.
9.2.4. Fluorescent polymer nanoparticles
Polymeric nanoparticles encapsulate a fluorophore within a polymeric
coating. Examples include fluorescent silica nanoparticles. These types of
material have been applied for detection using fluorescence spectrometers
or fluorescent reading microtiter plate readers, as well as in flow cytometry.
These nanoparticles are synthesized using a linear or branched polymer
that promotes the encapsulation of a fluorophore within the nanoparticle's
cavity or hydrophobic microdomains. The protection of the fluorophore
by the polymer provides high stability under a range of conditions proving
superior to organic dyes in terms of photostability and versatility. 1
9.2.5. Magnetic nanoparticles
Magnetic NPs (MNPs) are made of compounds comprised of elements
such as iron, nickel, and cobalt, and range in size from 1 to 100 nm. They dis-
play excellent conductivity and are also often used to concentrate samples.