Chemical Sensors Chapter 8
Introduction • Chemical sensors are very different • Sensing is usually based on sampling • Sample is allowed to react in some fashion with elements of the sensor • Usually an electric output is produced • Transduction can be multi-stage and complex • In some sensors, a complete analysis of the substance occurs • In others a direct output occurs simply due to the presence of the substance.
Introduction • Chemical sensing is quite common • Used in industry for process control and for monitoring, including monitoring for safety. • Important role in environmental protection • Tracking of hazardous materials • Tracking natural and man made occurrences • pollution, • waterways infestation • migration of species • weather prediction and tracking.
Introduction • In sciences and in medicine - sampling of substances such as oxygen, blood, alcohol • In the food industry for monitoring food safety • Military has been using chemical sensors at least since WWI to track chemical agents used in chemical warfare • Around the home and for hobbies (CO detection, smoke alarms, pH meters)
Classification • Direct and indirect output sensors • Direct sensor: the chemical reaction or the presence of a chemical produces a measured electrical output. • Example: the capacitive moisture sensor – the capacitance of a capacitor is directly proportional to the amount of water present between its plates.
Classification • Indirect (also called complex) sensor relies on a secondary, indirect reading of the sensed stimulus. • Example: optical smoke detector. An optical sensor such as a photoresistor is illuminated by a source and establishes a background reading. • Smoke is “sampled” by allowing it to flow between the source and sensor and alter the light intensity, its velocity, its phase or some other measurable property. • Some chemical sensors are much more complex than this and may involve more transduction steps. In fact, some may be viewed as complete instruments or processes.
Approach • Avoid a rigid classification • Concentrate on chemical sensors that are most important from a practical point of view while • Try to cover most principles involved • Steer clear of most chemical reactions and the formulas associated with them, • Replace these by physical explanations that convey the process and explain the results without the need for analytic chemistry.
Approach • Will start with the class of electrochemical sensors. • Includes those sensors that convert a chemical quantity directly into an electrical reading and follows the definition above for direct sensors. • The second group studied are those sensors that generate heat and the heat is the sensed quantity. • These sensors, just like the thermo-optical sensors in chapter 4 are indirect sensors as are the optical chemical sensors. • Following these are some of the most common sensors such as pH and gas sensors. • Humidity and moisture sensors are included here even though their sensing is not truly chemical but because the sensing methods and materials relate to chemical sensors.
Electrochemical sensors • Expected to exhibit changes in resistance (conductivity) or changes in capacitance (permittivity) due to substances or reactions. • These may carry different names. • Potentiometric sensors do not involve current –measurement of capacitance and voltage. • Amperimetric sensors rely on measuring current • Conductimetric sensors rely on measurement of conductivity (resistance).
Electrochemical sensors • These are different names for the same properties since voltage, current and resistance are related by Ohm’s law. • Electrochemical sensors include a large number of sensing methods, all based on the broad area of electrochemistry. Many common sensors including fuel cells, surface conductivity sensors, enzyme electrodes, oxidation sensors and humidity sensors belong to this category.
Metal-oxide sensors • Rely on a very well known property of metal oxides at elevated temperature to change their surface potential, and therefore their conductivity in the presence of various reducible gases such as ethyl alcohol, methane and many other gases, sometimes selectively sometimes not. • Metal oxides that can used are oxides of tin (SnO2), zinc (ZnO), iron (Fe2O2), zirconium (ZrO2), titanium (TiO2) and Wolfram (WO3). • These are semiconductor materials and may be either p or n type (with preference to n –type).
Metal-oxide sensors • Fabrication is relatively simple • May be based on silicon processes or other thin or thick film technologies. • The basic principle is that when an oxide is held at elevated temperatures, the surrounding gases react with the oxygen in the oxide causing changes in the resistivity of the material. • The essential components are the high temperature, the oxide and the reaction in the oxide
Metal-oxide sensors • Typical sensor: CO sensor shown in Figure 8.1a. • Consists of a heater and a thin layer of SnO2
Metal-oxide sensors • Construction: • A silicon layer is first created to serve as temporary support for the structure. • Above it an SiO2 layer is thermally grown. • This layer can withstand high temperatures. • On this a layer of gold is sputtered and etched to form a long meandering wire. • The wire serves as the heating element by driving it with a sufficiently high current. • A second layer of SiO2 is deposited.
Metal-oxide sensors • Then the SnO2 oxide is sputtered on top and patterned with grooves on top to increase its active surface. • The original silicon material is etched away to decrease the heat capacity of the sensor. • The sensing area can be quite small – 1-1.5 mm2. • The device is heated to 300 C to operate but, because the size is very small and the heat capacity small as well, the power needed is typically small, perhaps of the order of 100 mW.
Metal-oxide sensors - operation • Conductivity of the oxide can be written as: • 0 is the conductivity of the tin oxide at 300C, without CO present • P is the concentration of the CO gas in ppm (parts per million), • k is a sensitivity coefficient (determined experimentally for various oxides) • m is an experimental value - about 0.5 for tin oxide.
Metal-oxide sensors - operation • Conductivity increases with increase in concentration as shown in Figure 8.1b. • Resistance is proportional to the inverse of conductivity so that it may be written as • a is a constant defined by the material and construction and • a an experimental quantity for the gas. • P is the concentration.
Metal-oxide sensors - operation • The response is exponential (linear on the log scale) • A transfer function of the type shown in Figure 8.1b must be defined for each gas and each type of oxide. • SiO2 based sensors as well as ZnO sensors can also be used to sense CO2, touluene, benzene, ether, ethyl alcohol and propane with excellent sensitivity (1-50ppm).
Metal-oxide sensors - Variations • A variation of the structure above is shown in Figure 8.2. • It consists of an SnO2 layer on a ferrite substrate. • The heater here is provided by a thick layer of RuO2, fed through two gold contacts. • The resistance of the very thin SnO2 (less than about 0.5 m) is measured between two gold contacts. • This sensor, which operates as described previously is sensitive to ethanol and carbon monoxide
Metal-oxide sensors - notes • The reaction is with oxygen • Any reducible gas (a gas that reacts with oxygen) will be detected. • Lack of selectivity - common problem in metal oxide sensors. To overcome it, • Select temperatures at which the required gas reacts • The particular gas may be filtered. • These sensors are used in many applications form CO and CO2 detectors to oxygen sensors in automobiles.
Metal-oxide sensors - notes • Example: oxygen sensors in automobiles use a TiO2 sensor built as above in which resistance increases in proportion to the concentration of oxygen. • This is commonly used in other application such as oxygen in water (for pollution control purposes). • The process can also be used to determine the amount of available organic material in water by first evaporating the water and then oxygenating the residue to determine how much oxygen is consumed using an oxygen sensor. • The amount of oxygen is then an indication of the amount of organic material in the sample.
Solid elecrolyte sensor • Another important type of sensor is the solid electrolyte sensor • Has found significant commercial application • Most often used in oxygen sensors, including those in automobiles. • Principle: a galvanic cell (battery cell) is built which produces an emf across two electrodes based on the oxygen concentrations at the two electrodes under constant temperature and pressures.
Solid elecrolyte sensor • A solid electrolyte capable of operating at high temperatures is used • Usually made of zirconium dioxide (ZrO2) and Calcium oxide (CaO) in a roughly 90% 10% ratio • It has high oxygen ion conductivity at elevated temperatures (above 500C). • The electrolite is made of sintered ZrO2/ CaO powder which makes it into a ceramic material. • The inner and outer electrodes are made of platinum which act as catalysts and absorb oxygen. The structure is shown in Figure 8.3 for an exhaust oxygen sensor in a car engine.
Solid electrolyte sensor - operation • The potential across the electrodes is • R is the gas constant (=8.314 J/K/mol), • T is the temperature (K) • F is the Faraday constant (=96487 C/mol). • P1 is the concentration of oxygen in the exhaust, • P2the concentration of oxygen in the atmosphere, both heated to the same temperature.
Solid electrolyte sensor - use • Used to adjust the fuel ratio at the most efficient rate at which pollutants (NOx and CO) are converted into nitrogen (N2), carbon dioxide (CO2) and water (H2O), all of which are natural constituents in the atmosphere and hence considered non-pollutants • Usually fuel is enriched to achieve full combustion of pollutants • A PASSIVE SENSOR!
Solid electrolyte sensor - use as an active sensor • Many engines operate in a much leaner mode (for better fuel efficiency), • The solid electrolyte sensor is not sufficiently sensitive (the amount of oxygen in the exhaust is high and the reading of the electrolytic cell is insufficient). • The solid electrolyte sensor is modified to act as a passive sensor
Solid electrolyte sensor - use as a passive sensor • A solid electrolyte between two platinum electrodes as shown in Figure 8.4. are used, but: • A potential is applied to the cell. • This arrangement forces (pumps) oxygen across the electrolyte and a current is produced proportional to the oxygen concentration in the exhaust. • The current is then a measure of the oxygen concentration in the exhaust • This sensor is called a diffusion oxygen sensor or the diffusion-controlled limiting current oxygen sensor. • Operates similar to charging a battery
Oxygen sensor for molten metal • Important in oxygen sensing in production of steel and other molten materials • The quality of the final product is a direct result of the oxygen in the process. The sensor is shown in Figure 8.5. • The molybdenum needle keeps the device from melting when inserted in the molten steel. • A potential difference is developed across the cell (between the molybdenum and the outer layer). • The voltage is measured between the inner electrode and outer layer through an iron electrode dipped into the molten steel. • The voltage developed is directly proportional to the oxygen concentration in the molten steel.
The MOS chemical sensor • Use of the basic MOSFET structure commonly used in electronics, as a chemical sensor. • The basic idea: the classical MOSFET transistor in which the gate serves as the sensing surface. • Advantage: a very simple and sensitive device is obtained which controls the current through the MOSFET. • The interfacing of such a device is simple and there are fewer problems (such as heating, temperature sensing, etc.) to overcome.
MOS chemical sensors • Example, by simply replacing the metal gate in Figure 8.6 with palladium, the MOSFET becomes a hydrogen sensor
MOS chemical sensors • Palladium absorbs hydrogen and its potential changes accordingly. • Sensitivity is down to about 1 ppm. • Similar structures can sense gases such as H2S and NH3. • Palladium mosfets (Pd-gate MOSFET) can also be used to measure oxygen in water, relying on the fact that the absorption efficiency of oxygen goes down in proportion to the amount of oxygen present. • We shall say much more about the MOSFET sensor in the subsequent section on PH sensing since these have been very successful in this capacity.
Potentiometric sensors • A large subset of electrochemical sensors • Principle: electric potential develops at the surface of a solid material immersed in solution containing ions that exchange at the surface. • The potential is proportional to the number or density of ions in the solution. • A potential difference between the surface of the solid and the solution occurs because of charge separation at the surface.
Potentiometric sensors • The contact potential, analogous to that used to set up a voltaic cell cannot be measured directly. • If a second electrode is provided, an electrochemical cell is setup and the potential across the two electrodes is directly measurable. • To ensure that the potential is measured accurately, and therefore that the ion concentration is properly represented by the potential, it is critical that the current drawn by the measuring instrument is as small as possible (any current is a load on the cell and therefore reduces the measured potential).
Potentiometric sensors • For a sensor of this type to be useful, the potential generated must be ion specific – that is, the electrodes must be able to distinguish between solutions. • These are called ion-specific electrodes or membranes. • The four types of membranes are: • Glass membranes, selective for H+, Na+ and NH4+ and similar ions.
Potentiometric sensors • Polymer-immobilized membranes: In this type of membrane, an ion-selective agent is immobilized (trapped) in a polymer matrix. A typical polymer is PVC • Gel-immobilized enzyme membranes: the surface reaction is between an ion specific enzyme which in turn is either bonded onto a solid surface or immobilized into a matrix - mostly for biomedical applications • Soluble inorganic salt membranes: either crystalline or powdered salts pressed into a solid are used. Typical salts are LaF3 or mixtures of salt such as Ag2S and AgCl. These electrodes are selective to F, S and Cl and similar ions.
Glass membrane sensors • By far the oldest of the ion-selective electrodes, • Used for pH sensing from the mid-1930’s and is as common as ever. • The electrode is a glass made with the addition of sodium (Na2O) and aluminum oxide (Al2O3), • Made into a very thin tube-like membrane. • This results in a high resistance membrane which nevertheless allows transfer of ions across it. • The basic method of pH sensing is shown in Figure 8.7a.
pH sensor • Consists of the glass membrane electrode on the left and a reference electrode on the right. • The reference electrode is typically an Ag/AgCl electrode in a KCl aqueous solution or a saturated Calomel electrode (Hg/Hg2Cl2 in a KCl solution). • The reference electrode is normally incorporated into the test electrode so that the user only has to deal with a single probe as shown in Figure 8.7b. • The sensor is used by first immersing the electrode into a conditioning solution of Hcl (0.1.mol/liter) and then immersing it into the solution to be tested. The electric output is calibrated in pH. • A sensor of this type responds to pH from 1 to 14.
Glass membrane sensors • Modifications of the basic configuration, both in terms of the reference electrode (filling) as well as the constituents of the glass membrane lead to sensitivity to other types of ions as well as to sensors capable of sensing dissolved gas in solutions, particularly ammonia but also CO2, SO2, HF, H2S and HCN
Soluble inorganic salt membrane sensors • Based on soluble inorganic salts which undergo ion-exchange interaction in water and generate the required potential at the interface. • Typical salts are the lanthanum fluoride (LaF3) and silver sulfide (Ag2S). • The membrane may be either • a singe crystal membrane, • a sintered disk made of powdered salt • a polymer matrix embedding the powdered salt • each has its own application and properties
Soluble inorganic salt membrane sensors • The structure of a commercial sensor used to sense fluoride concentration in water is shown next • The sensing membrane, made in the form of a thin disk grown as a single crystal. • The reference electrode is created in the internal solution (in the case: NaF/NaCl at 0.1 mol/liter). • The sensor shown can detect concentrations of fluoride in water between 0.1 and 2000 mg/l. • This sensor is commonly used to monitor fluoride in drinking water (about 1mg/l).
Soluble inorganic salt membrane sensors • Membranes may be made of other materials such as silver sulfide. • The latter is easily made into thin sintered disks from powdered material and may be used in lieu of the single crystal. • Other compounds may be added to affect the properties of the membrane and hence sensitivities to other ions. • This leads to selective sensors sensitive to ions of chlorine, cadmium, lead and copper and are often used to sense for dissolved heavy metals in water.
Polymeric salt membranes • Polymeric membranes are made by use of a polymeric binder for the powdered salt • About 50% salt and 50% binding material. • The common binding materials are PVC, polyethylene and silicon rubber. • In terms of performance these membranes are quite similar to sintered disks.