Nanosensors - Nanotechnologies for Water Environment Applications - page 420

13.3.3 Types of Transducers
Transducers are electrical/electronic components that convert the signal
associated with a biological detection event into a measurable signal. There are three
types of transducers: i) electrochemical, ii) mechanical, and iii) optical. Sensors are often
classified according to the type of transducer used as described below.
Electrochemical Transducers. Electrochemical sensor measures the change in
current or potential during the biorecognition event (Wang, 2006). It can be classified
according to the observed parameter as amperometric (current), potentiometric
(potential), and impedimetric (impedance). One of the main advantages of
electrochemical sensors is their ability to detect the analytes without labeling. For
example, ultrasensitive detection of cancer genes was demonstrated based on the
catalytic oxidation of guanine base by redox-threading intercalator, without amplifying
the genes of interest (Tansil et al., 2005). Similarly, an electrochemical sandwich
immunoassay was used to detect small quantities of E. coli O157:H7 cells in less than 6
minutes (Muhammad-Tahir and Alocilja, 2003). Electrochemical techniques can be
applied to nucleic acid as well. However the lower specificity of electrochemical
systems is an issue that needs to be addressed (Odenthal and Gooding, 2007).
Mechanical Transducers. Mechanical sensors can sense change of mass/stress
by the bending (static mode) or resonant frequency (dynamic mode) (Carrascosa et al.,
2006). Adsorption of analytes on the sensor surface produces stress that bends the
surface and changes its resonance frequency. The bending and change in resonance
frequency can be monitored by optical, piezoresistive, piezoelectric, capacitance, or
interferometric technique. There are many advantages for using a mechanical sensor
including extreme simplicity as electrodes can be used for sensing changes in frequency
or bending instead of optical lasers and targets do not need to be labeled, thus higher
sensitivity, low assay time, and portability are achieved. Cantilevers based systems have
the potential to detect single target molecule of DNA (Ilic et al., 2004). This was
demonstrated in a recent study where antibody functionalized piezoelectric-excited
cantilevers were used to measure E. coli O157:H7 cells in a 1-liter water sample at 1
cell/ml concentration (Campbell and Mutharasan, 2007). Such techniques have the
greatest potential for online continuous monitoring of selected pathogens and indicators
- a key need for the water industry.
Optical Transducers. Optical sensors can transduce a number of phenomena
like fluorescence, phosphorescence, refraction, dispersion etc. which can be quantified
(Erickson et al., 2008). Optical transduction is the most widely used phenomena for
sensors. Biomolecules to be measured are tagged with an optically detectable compound
(fluorescent dye, quantum dot, gold nanoparticle) and exposed to a light with sufficient
energy. Electrons absorb the light and jump from the valence band to the excited state.