Nanosensors - Nanotechnologies for Water Environment Applications - page 433

SPR and thin film gold immobilization for a 2-D oligo array. The polydimethylsiloxane
allowed line arrays with narrow 50 μm channels leading to a line density of 80 lines/cm.
With this design, the observed detection limit was 100 femtomole of targeted ssDNA
(Lee et al., 2001). If integrated with the 3 mm long SPREETA 2000, this type of
configuration would permit 24 lines (probe sets). Theoretically, the width of the
channels could be made even smaller thus allowing smaller sample volumes and spot
sizes and improved detection limit. However, it should be noted that the spot size is
typically limited to 10 μm x 10 μm, by the excitation wavelength-dependent, lateral
propagation length of the SPR platform (Haes and Van Duyne, 2002). For robotic
spotting, areas on the gold surface intended for spots are confined using removable
hydrophobic protecting group. Probes are covalently attached with thiol-linker that
binds to an amine monolayer on the gold surface. The protecting group is then removed,
and target samples are hybridized to the chip (Nelson et al., 2002). Thus, SPR allows
portable label-free detection. The throughput is in hundreds instead of thousands as in
microarrays. The sensitivity and specificity are the same as for conventional fluorescent
label-based microarrays.
13.10 Sample Processing
Currently widespread application of micro- and nano-sensors in the water
industry is hampered by sample preparation steps (Lim et al., 2005). Sample preparation
includes concentration of large quantities of water (1 to 100 L) to a few μL so it can be
introduced into the biosensor platform. It is critical to ensure that the sample
concentration process enriches the analyte/target species and excludes the inhibitory
substances (e.g., humic acids, metals, and salts) for biochemical reactions. Membrane
filtration technologies (microfiltration, nanofiltration, reverse osmosis) are commonly
applied for water purification and analyte/target enrichment. Microfiltration and
ultrafiltration processes efficiently concentrate protozoan ( Cryptosporidium and
Giardia ) but they are not very effective for smaller size microorganisms such as bacteria
and viruses. Similarly, continuous flow centrifugation is a promising tool for
concentration of particulates from large quantities of water
( http://www.scientificmethods.com). The centrifuged sample from continuous flow
centrifugation apparatus can then be filtered by any dead end filtration mechanism to
further concentrate the sample. For viruses, filtration process by charged membranes is
more effective (Scott et al., 2002). However performance, cost and reusability of filters,
recovery of concentrated microorganisms are issues that need further research.
Concentration and amplification of microorganisms can be achieved by culture
techniques or immunological techniques, and amplification of nucleic acids can be
achieved by polymerase chain reaction. The use of capture probes (antibodies or nucleic
acids) coated on magnetic beads is a promising technology to enrich microorganisms or
genes. Super-paramagnetic particles of 1 to 5 μm and colloidal beads of 50 to 200 nm,