6.1 Electrochemical Sensing
Electrochemical sensing is one of the oldest techniques used dating back to 1950s.
This technique uses electrodes to measure electrical properties to sense materials
present in the sample without damaging the sample system. Typically there are
several types of electrochemical sensor. The three major types include amperomet-
ric, measurement of current, conductometric, measurement of conductive proper-
ties and potentiometric, measurement of potential difference or charge
accumulation across the electrodes. Impedimetric, which measures impedance
change and field-effect, for measurement of current because of the gate potentio-
metric effect are also, used these days [ 107 ].
Amperometric sensors have been typically developed for determination of
biochemical oxygen demand (BOD) since conventional techniques take 5–6
days for the measurement. Tonning et al. proposed an interesting approach for
sensing of Vibrio fischeri bacterium, Pseudokirchneriellasubcapitata freshwater
marine alga and Daphnia magna freshwater crustacean. The fabricated sensor
consisted of several cells and electrodes for sensing and finally the data was
processed using chemometry mathematical methods [ 108 ]. Also, these sensors
are commonly used for detection of glucose. Mu et al. investigated nickel oxide
modified glucose sensors that use carbon electrodes [ 109 ]. These sensors have a
response time of 5 s hence demonstrating excellent sensitivity. One of the recent
interesting findings include thick-film textile based amperometric sensors.
Direct screen printed amperometric sensors incorporated in clothing has been
reported by Yang et al. These sensors were printed on the elastic of clothing and
studied for mechanical stress, bending and stretching [ 110 ]. Specific enzyme
sensors can be developed using this technique which can prove useful for the
purpose of monitoring alcohol consumption, stress or sweat monitoring for
athletes etc.
Microfluidics is commonly used to position cells, but a novel device has been
fabricated to trap the cells directly over the sensing electrodes. A microfluidic trap
device to measure exocytosis from cells targeting them to electrochemical elec-
trodes has been demonstrated by Gao et al. (Fig.2.34)[ 111 ]. The device is
fabricated in a manner such that there is no need for precise pressure control or
handling of fluids on the chip.
Conductometric sensors are used in varied fields to measure the change in
conductivity of a reaction solution caused by microbial activities. They are pre-
ferred over other sensors because of their inexpensive fabrication process, no
requirement of reference electrode, high accuracy and cancellation of interferences.
In the field of gas sensing, a novel and highly sensitive NO 2 gas sensor using
caesium-doped graphene oxide has been reported by Piloto et al. Doping of caesium
to graphene oxide results in the carbon atoms reducing the graphene oxide, dem-
onstrating very low detection limits for NO 2 (90 ppb) [ 112 ]. Latif and Dickert have
2 Microfluidics Overview 73