the environment; it integrates a biological component, such as a whole bacterium or
a biological product (e.g., an enzyme or antibody) with an electronic component to
yield a measurable signal [16]. A biosensor system consists of bio-recognition layer
or receptor, transducer, and electronic display system.
Biosensor systems are categorized according to the type of transducer used in the
system: optical, electrochemical, and acoustic. Optical biosensors use fiber-optic
probes that detect change in optical density for a range of wavelengths (~750 nm)
that can be correlated to analyte concentration. Some optical sensors such as surface
plasmon resonance (SPR) sensors use fluorescence to amplify the signal and
increase the sensitivity of the sensor. Electrochemical biosensors use electrodes
that measure the electric current produced by the oxidation or reduction of
electroactive species. Electrodes are made of metals such as platinum, gold, silver
and stainless steel, or carbon-based materials that are inert at the potentials at which
the electrochemical reaction takes place. Acoustic biosensors use membranes made
of chemically interactive material that detect changes in resonant frequency of an
antigen. Acoustic sensors contain a crystal resonator, usually quartz, connected to
an amplifier whose resonant frequency is a function of the properties of the
membrane.
Biosensors are applied in a wide variety of fields such as medical diagnostics,
bioweapons, agricultural and food processing industries, and environmental sciences. The integration of biosensor technology in real-time water quality monitoring is an upcoming technology. As typical applications, uses of biosensors to detect
pathogens and toxins are discussed below.
3.4.1 Pathogen Detection
Technologies for early warning and rapid detection of pathogens are critical for
human health protection. E. coli, a pathogenic microorganism, is often used as an
indicator of fecal contamination in water. Several advanced techniques such as PCR
(polymerase chain reactions) and RNA (ribonucleic acid) probes that identify
E. coli DNA are developed to detect E. coli presence in water. However, these
techniques can take up 1 or 2 days to obtain results. Only a few technologies have
been developed that allow real-time or rapid detection of E. coli. Although there
have been significant advances in biosensor technology during the last decade, there
are still several shortcomings of biosensors. The most widespread problems include
limited capability for real-time and spatial measurements, as well as durability,
specificity, and the cost of the technology. Typical examples from available scientific literature are discussed below.
Radke and Alocilja [17] developed a technique for real-time detection of E. coli
O157:H7 in water. The technique is called nucleic acid sequence-based amplification (NASBA) method. The system consists of a high-density gold microelectrode
array biosensor fabricated from silicon with a 2-μm layer of thermal oxide as an
insulating layer with an active area of 9.6 mm
2 . To create a biological sensing
surface, the sensor surface is functionalized for bacterial detection using heterobifunctional cross-linkers and immobilized polyclonal antibodies. Bacteria in the
Advances in Water Sensor Technologies and Real-Time Water Monitoring
193
a biological product (e.g., an enzyme or antibody) with an electronic component to
yield a measurable signal [16]. A biosensor system consists of bio-recognition layer
or receptor, transducer, and electronic display system.
Biosensor systems are categorized according to the type of transducer used in the
system: optical, electrochemical, and acoustic. Optical biosensors use fiber-optic
probes that detect change in optical density for a range of wavelengths (~750 nm)
that can be correlated to analyte concentration. Some optical sensors such as surface
plasmon resonance (SPR) sensors use fluorescence to amplify the signal and
increase the sensitivity of the sensor. Electrochemical biosensors use electrodes
that measure the electric current produced by the oxidation or reduction of
electroactive species. Electrodes are made of metals such as platinum, gold, silver
and stainless steel, or carbon-based materials that are inert at the potentials at which
the electrochemical reaction takes place. Acoustic biosensors use membranes made
of chemically interactive material that detect changes in resonant frequency of an
antigen. Acoustic sensors contain a crystal resonator, usually quartz, connected to
an amplifier whose resonant frequency is a function of the properties of the
membrane.
Biosensors are applied in a wide variety of fields such as medical diagnostics,
bioweapons, agricultural and food processing industries, and environmental sciences. The integration of biosensor technology in real-time water quality monitoring is an upcoming technology. As typical applications, uses of biosensors to detect
pathogens and toxins are discussed below.
3.4.1 Pathogen Detection
Technologies for early warning and rapid detection of pathogens are critical for
human health protection. E. coli, a pathogenic microorganism, is often used as an
indicator of fecal contamination in water. Several advanced techniques such as PCR
(polymerase chain reactions) and RNA (ribonucleic acid) probes that identify
E. coli DNA are developed to detect E. coli presence in water. However, these
techniques can take up 1 or 2 days to obtain results. Only a few technologies have
been developed that allow real-time or rapid detection of E. coli. Although there
have been significant advances in biosensor technology during the last decade, there
are still several shortcomings of biosensors. The most widespread problems include
limited capability for real-time and spatial measurements, as well as durability,
specificity, and the cost of the technology. Typical examples from available scientific literature are discussed below.
Radke and Alocilja [17] developed a technique for real-time detection of E. coli
O157:H7 in water. The technique is called nucleic acid sequence-based amplification (NASBA) method. The system consists of a high-density gold microelectrode
array biosensor fabricated from silicon with a 2-μm layer of thermal oxide as an
insulating layer with an active area of 9.6 mm
2 . To create a biological sensing
surface, the sensor surface is functionalized for bacterial detection using heterobifunctional cross-linkers and immobilized polyclonal antibodies. Bacteria in the
Advances in Water Sensor Technologies and Real-Time Water Monitoring
193