Dr. Marc Madou, UCI, Winter 2012 Class V Potentiometric and
Amperometric Sensors (I) Electrochemistry MAE-295
Slide 2
Table of content Potentiometric Sensors Amperometric Sensors
Nanosensors as electrochemical sensors [Potentiometric and
Amperometric Sensors (II)]
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Potentiometric Sensors Potentiometric techniques are the most
widely used electroanalytical method: Direct potentiometry pH and
ions (pH sensors and ion selective probes) Indirect potentiometry:
Enzyme sensors, Gas sensors Miniaturization of Potentiometric
Sensors
Slide 4
Direct Potentiometric Sensors Best know example is the pH
sensor. Combination electrodes (indicator+reference) for
convenience (tube within a tube) pH sensing component of the
indicator electrode is the glass bulb, which is a thin glass
membrane ~ 0.03 0.1 mm thick When immersed, H + ions from the
solution enter the Si-O lattice structure of the glass membrane in
exchange for Na + Inner tube: pH indicator electrode (pH sensing
membrane, Ag/AgCl reference electrode and HCl Outer tube: reference
electrode (Ag/AgCl) and salt bridge (KCl)
Slide 5
Direct Potentiometric Sensors A traditional pH measurement with
a glass electrode is the best known potentiometric ion selective
electrode (ISE) (e.g. a thin glass layer with this composition 22%
Na 2 O, 6% CaO, 72% SiO 2 ) There is no change in the inner
solution and there is no actual contact between inner and outer
solution for any potentiometric probe or sensor How to construct a
combination electrode?
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Direct Potentiometric pH Sensors The glass bulb creates an
electric boundary potential across the membrane w.r.t. the internal
Ag/AgCl reference electrode. This is called the Donnan potential:
Where a H+ = activity of H + (= concentration in very dilute
solutions). Slope factor (2.303RT/F) is temperature dependent, pH
meter must be adjusted for changes in temperature All modern pH
meters record potential (mV) and transform the voltage caused by H
+ into pH units Standard buffers (4.0, 7.0, 10.0) are used for
calibration Automatically recognize standard buffers and adjust for
temperature
Slide 7
Electrochemical Methods Applications in Environmental Analysis
Direct Potentiometric pH Sensors
Slide 8
Direct Potentiometric Sensors Measurement of Ions by Ion
Selective Electrodes (ISEs) Uses direct potentiometry to measure
ion concentration Membrane responds selectively to a given ion mV
reading between sensing and reference electrode
Slide 9
Direct Potentiometric Sensors There are many other types of
potentiometric ion sensors or ISEs. The so-called Donnan potential
is established on both sides of any ion selective membrane-the
potential on one side is kept constant through the internal
reference solution while the other side is determined by the
analyte solution For other ions than protons (cations and anions)
other membranes are available (see e.g. LaF 3 for F - and a wide
variety of polymeric membranes
Slide 10
Direct Potentiometric Sensors An ion selective polymeric
membrane is often made by mixing an ionophore (e.g. valinomycin, a
natural occuring antibiotic) with PVC and a plasticizer (to make
the rigid plastic more flexible) In these types of ISEs one
sometimes does not use an internal reference solution at all or one
incorporates a hydrogel to replace the aqueous solution. This makes
the electrode easier to handle and store. Especially with no
internal reference electrode drift tends to be larger! The
polymeric ISEs lend themselves well to miniaturization and cost
reduction (it is much more difficult to miniaturize a glass pH
electrode)
Slide 11
Indirect Potentiometric Sensors: Enzyme Base Potentiometric
Sensor A potentiometric urea sensor may consist of two pH sensors
one with the enzyme coated on its surface and one without (the
reference electrode) The electrode with the urease will sense a
local pH change The pH difference bewteen the two electrodes is
proportional to the urea concentration As an example two IrO x
electrodes may be used
Indirect Potentiometric Sensors: Carbon dioxide sensor (MEMS
version) A pH, CO 2 and oxygen electrochemical sensor array for
in-vivo blood measurements was made using MEMS techniques The pH
and CO 2 sensors are potentiometric and the oxygen sensor is
amperometric (see further in this class) The pH sensor is an ISE
based on a pH sensitive polymer membrane. The CO 2 sensor is based
on an IrOx pH sensor and a Ag/AgCl reference electrode..
Slide 15
Miniaturization of Potentiometric Sensors By making ISEs planar
(e.g. on a polyimide sheet) many sensors can be made in parallel
(i.e. batch fabnrication). From 3D structures to 2D ! Mass
production can make them very small (e.g. 2 by 3 mm), cheap
(perhaps disposable), reproducible and even electronics might be
integrated (see below under ISFETs)
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Miniaturziation of Potentiometric Sensors Potentiometric
sensors have been made the size of a transistor in ISFETs
(almost).
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Amperometric Sensors Our first example of an amperometric
sensors involves a "Fuel cell" oxygen sensors consisting of a
diffusion barrier, a sensing electrode (cathode) made of a noble
metal such as gold or silver, and a working electrode made of a
metal such as lead or zinc immersed in a basic electrolyt (such as
a solution of potassium hydroxide). Oxygen diffusing into the
sensor is reduced to hydroxyl ions at the cathode: O 2 + 2H 2 O +
4e- -------- 4 OH- Hydroxyl ions in turn oxidize the lead (or zinc)
anode: 2Pb + 4OH- ------------- 2PbO + 2H 2 O + 4e- 2Pb + O 2
----------------- 2PbO Fuel cell oxygen sensors are current
generators. The amount of current generated is proportional to the
amount of oxygen consumed (Faraday's Law).
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Amperometric Sensors A second example of an amperometric
sensors is a simple (first generation) glucose sensor. This sensor
is based on the enzyme Glucose Oxidase (GO). Enzymes are
high-molecular weight biocatalysts (proteins) that increase the
rate of numerous reactions critical to life itself Enzyme
electrodes are devices in which the analyte is either a substrate
(also called reactant) or a product of the enzyme reaction,
detected potentiometrically or amperometrically Here we consider an
amperometric glucose sensor where the substrate (glucose) diffuses
through a membrane to the enzyme layer where glucose is converted
and H 2 O 2 is produced and electrochemically detected.
Slide 19
Amperometric Sensors Amperometric glucose sensor based on
peroxide oxidation, The lateau of the limiting current is
proportional to the peroxide concentration which in turn is
proportional to glucose - - - typical 0.6 to 0.8 V vs Ag cathode
Glucose oxidase is an oxidase type enzyme, urease is a hydrolytic
type enzyme. Other sensors can be constructed based on those
enzymes. - i l Anodic Cathodic +i -i + + 0.6 V
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Amperometric Sensors Measurement of Dissolved Oxygen e.g.
Polarographic Clark cell
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Amperometric Sensors Measurement of Dissolved Oxygen e.g.
Polarographic Clark cell O 2 + 2H 2 O + 4e - 4OH - (O 2 reduced at
gold cathode) 4Ag(s) + 4Cl - (aq) 4AgCl(s) + 4e - (oxidation of
silver at anode) Membrane is susceptible to degradation, must be
replaced if it dries out Calibrated in air (O 2 ), air saturated
water (aerated water) or by Winkler method
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Amperometric Sensors Measurement of Dissolved Oxygen Calibrate
the probe (in air) Place the probe below the surface of the water
Set the meter to measure temperature and allow the temperature
reading to stabilize Switch the meter to 'dissolved oxygen For
saline waters, measure electrical conductivity level or use
correction feature Re-test water to obtain a field replicate result
NOTE: The probe needs to be gently stirred to aid water movement
across the membrane