I. Basic Considerations & Definitions 1. Definitions: EE 5344 Introduction to MEMS CHAPTER 7 Biochemical Sensors Chemical/ Biological quantity Biochemical Microsensors Electrical Signal Ex: Chemical species present in mixtures (hydrogen, CO, CO 2, NO, SO 2, NH 3 etc. in air and water) 2. Classification of biochemical microsensors: We classify them according to the sensing principal. The reaction: sensing material X M k f kb chemical species X. M k f forward reaction constant k b backward reaction constant Classification: All of them are modulating. Principle/class Affected Parameter Typical Sensor conductometric resistance/conductance tin oxide gas sensor potentiometric voltage/e.m.f. ion selective FET for ph capacitive capacitive/charge polymetric humidity sensor amperometric current electrochemical cell calorimetric heat/temperature pellistor gas sensor gravimetric mass piezoelectric or SAW sensors optical path length/absorption infra-red detector for methone gas resonant frequency surface plasmon fluorescent intensity fibre-optic 1 / Biochemical Sensors
Selectivity: reaction to more than one measurand. This is often the problem. Preseparation of chemical species might be required. One of the techniques used to separate chemical species is chromatography (gas or liquid). Gas Chromatography: Gas chromatography is a method for separating the components of a solution and measuring their relative quantities. It is a useful technique for chemicals that do not decompose at high temperatures and when a very small quantity of sample (micrograms) is available. The use of gas chromatography is limited by the decomposition temperature of the components of the mixture and the composition of the column. Most columns cannot withstand temperatures greater than 250-350 C. In gas chromatography, a sample is rapidly heated and vaporized at the injection port. The sample is transported through the column by a mobile phase consisting of an inert gas. Sample components are separated based on their boiling points and relative affinity for the stationary phase, which is most often a viscous liquid (wax) within the column. The higher a component s affinity for the stationary phase, the slower it comes off the column. The components are then detected and represented as peaks on a chromatogram. Filters and Sieves: *To achieve selectivity molecular shape recognition might also be used. For this purpose biological sensors come into play. Sensor measurand Electrical signal Filter (eliminates materials that our sensor may be sensitive to) receptor sites Different kinds of chemical sensors. Go over Figure 9.1. 2 / Biochemical Sensors
II. Chemoresistors The resistance of the chemically sensitive layer changes with the amount of absorbed chemical. V I Gas Active material Electrodes Example: metal oxides, organic crystals,conducting polymers Si SiO 2 Inert substrate, AlO, SiO 2 or Si 1. Metal oxide (inorganic) gas sensors: gas absorption ZnO TiO 2 In 2 O 3 SnO 2 Semiconducting material most commonly used large change in electrical resistivity Several commercial tin oxide gas sensors exist. (m/2)o 2 + [vacant site] + e- k1 (O m - ) site X + (O m - ) site k2 (XO m ) site + e- X is the measurand. For example, CH 4 produces CO 2 and water. Since the number of electrons is increased, conductivity increases. = nq n [X] r 3 / Biochemical Sensors
conductance However, the increase in the electronic concentration alone cannot account for the large percentage changes observed in the conductivity. Additional change must come from the mobility. = nq n + nqn The gas diffuses into the granular structure of SnO 2 and changes the way electron move from grain to grain. Therefore, the mobility of the electrons change. Figure 9.6 Microsensors improve the power consumption characteristics of metal oxide gas sensors and increase the sensitivity. Tin oxide microsensors are used today also for gas analysis.. 2. Organic Gas Sensors: They have better selectivity They are more sensitive to pollutants and reactive gases. They can be processed better than inorganic oxides. They can be chemically modified to achieve specificity thereby relieving the burden on the initial chemical separation step. 2 types of organic materials are considered Organic crystals Conducting polymer films Organic crystals: Figure 9.8 Phthalocyanine PbPc H 2 Pc The main disadvantage of biochemical sensors is the recovery time. Also they operate at relatively low temperatures, below 150 C. Concentration NO 2 Conducting Polymers: such as pyrrole 4 / Biochemical Sensors
Chemocapacitors Permable gold electrode Electrode Polymer C Glass Substrate Humidity sensor Voltage R.H. (relative humidity) C pf Here the sensing element is the dielectric constant of the film (for example, polyphenylacetylene) between the electrodes. *These films are sensitive to CO, CO 2, N 2 and CH 4 *They are sensitive to temperature, humidity and operating frequency. *The capacitance changes in pf. III. Chemodiodes Inorganic Schottky diodes: A metal semiconductor diode is used. The work function of the metal is modified by the presence of the chemical species. Therefore the barrier at the metal-semiconductor is modified. This changes the current, which is measured. Examples: Palladium and Platinum as metals; and TiO 2 and ZnO as semiconductors. 5 / Biochemical Sensors
Tunneling Schottky diodes are also used where there is a thin oxide layer (20 50 Å) between the semiconductor and the metal. Then the capacitance vs voltage characteristics are measured. Organic Schottky diodes: Polymer-metal Schottky diodes are used. Again, the work function of the metal is modified with the presence of the chemical species, thus changing the barrier between the polymer and metal. The change in the diode equation is, then, an indicative of the chemical that is detected. (Go over Figure 9.12) IV. Chemotransistors: MOSFET: The gate of the MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is made of a chemical sensitive metal: Palladium (Pd): for Hydrogen Iridium (Ir) and Platinum (Pt): for NH 3 and H 2 S and ethanol The absorption of the chemical species by the metal changes the work function of the metal and therefore the threshold voltage of the transistor. Go over Figure 9.13. Reversibility and stability can be a problem. ISFET This is a gateless FET (Ion Selective Field Effect Transistor). Go over Figure 9.15. ISFET is operated as ph sensor/threshold voltage shift V T depends on the concentration of hydrogen in the electrolytes. V T SiO 2 More linear response obtained by using Al 2 O 3 as the gate oxide. Al 2 O 3 ph 6 / Biochemical Sensors
V. Thermal Chemical Microsensors 1. Thermistors They are used to measure the heat released or absorbed during a chemical reaction. The heat causes a change in the temperature, which modifies the resistance of the thermistor. Pt/In Leads Bridge circuit to capture the change in resistance. The resistance changes with temperature. band gap of the ceramic oxide semiconductor. BaO/SrO r 1 R dr dt k Eg B T 2 Glass Shield Small size improves the thermal film constant! 2. Pellistor gas sensors It captures the heat released on the oxidation of a combustible gas. It is similar to metal oxide chemorsistors except the temperature is measured instead of the conductance. 3. Thermocouple gas sensors We can always measure the heat released in a chemical reaction using a thermocouple. 7 / Biochemical Sensors