Nanosensors - Nanotechnologies for Water Environment Applications - page 416

specific examples of nano-materials/nano-sensors focusing primarily for water industry
are described. Specific examples illustrating the strengths of micro-and nano-technology
for microbial detection are highlighted. A comparison is made between whole cell and
nucleic acid based detection schemes for waterborne pathogens. A brief summary about
sample processing and a novel DNA concentration scheme that could be used to
eliminate substrates inhibitory to PCR is discussed. Selected studies related to market
trends are also summarized.
13.2 Characteristics of Sensors for the Water Industry
A number of parameters must be considered during the development of any
sensor for the water industry. These include specificity, sensitivity, speed, cost, sample
throughput, and target multiplexing (Figure 13.1a). Most micro and nano-scale sensors
attempt to optimize the assay for speed and cost followed by sensitivity and specificity.
Increasing target and sample throughput are often not the goals for these sensors,
although exceptions do exist. The ability to discriminate between closely related
organisms/nucleic acid sequences is critical in screening assays, because of their widely
differing characteristics. Sensitivity or limit of detection is the smallest amount of a
species of interest that can produce a measurable output signal. The cell based detection
systems represents the limit of detection in the units of colony forming units for bacteria
and plaque forming units for viruses.
A lower limit of detection for a given agent helps minimize the risk from its
exposure. The time required to complete an assay is also important because detection
should be rapid enough to allow time to warn the consumer or take preventive action
before the water reaches the consumer. Approaches that eliminate the need for sample
concentration and/or processing and target labeling e.g., nanocantilevers (Carrascosa et
al., 2006), surface plasmon resonance (Chinowsky et al., 2007), and nanoparticles/
nanowires (Jianrong et al., 2004) when combined with real time concentration
techniques may be able to play a key role in continuous monitoring of water for selected
pathogens. In order the make the sensors cost effective, integration of complex
fabrication steps, batch fabrication, and use of cheaper materials are necessary. Micro
and nano-scale sensors are employed to detect pathogenic microorganisms in both
clinical and environmental (water industry) samples. However, with respect to sample
processing, background causing cross hybridization, and sample volume, are two
different problems (Figure 13.1b). Environmental samples usually have high volume,
low amount of targets, and high amount of background while clinical samples have just
the opposite. Processing of environmental samples is complex because of the need to
concentrate small number of targets (1 - 10,000 cells) from a large volume of water (1 -
100 liters) and the presence of high background. This complexity in dealing with
environmental samples limits the widespread use of sensors in the water industry.