Dr. Marc Madou

1. BIOMEMS Class II. Electrochemistry Background (I) Winter 2011 Dr. Marc Madou

2. Sensors and interfaces Electrodes (materials) in solutions Metals in solution Semiconductors in solution Solid electrolytes in solution Insulators in solution Mixed conductors in solution Contents

3. Sensors and interfaces • Every chemical sensor is about an interface with the environment, the better one can control the sensor surface the better one can control that interface and thus say something about the environment • We will look into some detail at solid/liquid before we discuss any type of electrochemical sensor in detail • Interfaces are very complex often involving fractals (beach, trees, snow flakes, etc.) rather than smooth transitions, this implies that perfect selectivity will be hard to achieve (too many different binding sites)

4. Electrodes (materials) in solution • Charge carriers in electrode materials: • Metals (e.g. Pt) : electrons • Semiconductors (e.g. n-Si) : electrons and holes • Solid electrolytes (e.g. LaF3 ) : ions • Insulators (e.g. SiO2):no charge carriers • Mixed conductors (e.g. IrOx) : ions and electrons • Solution (e.g. 1 M NaCl in H2O): solvated ions Double layer-(in case of a metal 10-40 µF cm-2) Inner Helmholtz plane (IHP) Outer Helmholtz plane (OHP) Gouy-Chapman layer (GCL)

5. Metals in Solution

6. Metals in Solution

7. Metals in solution 107-10 8 V cm-1 • In order for current to pass the interface Me/solution an electochemical reaction must occur: an Oxidant O (say Fe 3+) gets reduced in a cathodic reaction (on the cathode, also the working electrode in this case (WE)) to become a reductant R (say Fe 2+) • For a complete circuit a counter electrode must also be present in the cell for the reverse or anodic reaction on the anode (Counter electrode (CE)) • Without applied bias the potential drop across the Helmholtz layer on the WE (e.g. a Pt electrode) is determined by the redox species with the largest exchange current density i0,e Metal (working electrode, sensing electrode, detector electrode) Electrolyte Anode also CE (in this case) Cathode also WE (in this case)

8. Metals in solution • The fastest electron-exchange reaction (the rate of electrons going back and forth between redox species and electrode in equilibrium i.e. at zero current) determines the potential of the electrode ---zero external current and no net reaction • Often there are different redox species involved in establishing the equilibrium potential in which case we speak about a mixed potential---zero external current but no net reaction (e.g. corrosion) • A working electrode (e.g. Pt) that changes potential with the redox couple present is called an electrode of the first kind and it is our first sensor we encounter in this course • An electrode that does not change it’s potential with solution composition is an electrode of the second kind i.e. a reference electrode (see below)

9. Metals in solution • The inert species in solution (e.g. NaCl ) also called indifferent electrolyte do not (at zero or low bias) exchange electrons with the Pt electrode but they provide solution conductivity • The inert electrolyte ensures that the electroactive species reaches the electrode by diffusion and not by migration • All redox couples have a known redox potential as measured against a standard reference electrode (e.g. Standard Hydrogen Electrode or SHE) • A first type of sensor measures redox potential of a solution and it consists of a voltmeter, a Pt electrode and a reference electrode

10. Metals in Solution • The two E° values shown refer to "standard" conditions of unit H+ activity (pH=0) and gas pressures of 1 atm. At combinations of pH and E that lie outside the shaded area, the partial pressures of O2 or H2 exceed 1 atm, signifying the decomposition of water. The unity partial pressures are of course arbitrary criteria; in a system open to the atmosphere, water can decompose even at much lower H2 partial pressures, and at oxygen pressures below 0.2 atm. Fortunately, these processes are in most cases quite slow.

11. Semiconductors in solution • In this case most of the potential drop is in the semiconductor instead of in the solution • Transport of charges to and from solution is limited to those redox systems that have states that overlap with the semiconductor bands Electrolyte Semiconductor e.g. TiO2

12. Semiconductors in solution • When the semiconductor is in contact with the solution a band bending results just as in the case of a conductive solution contacting a metal • The flat-band potential (V FB) is that potential one needs to apply to make the bands flat in the semiconductor all the way to the surface (it can be deduced from a capacitance measurement of the interface) • For a semiconductor covered with an oxide (e.g. Si with SiO2 , TiO2) the flat band potential is a function of pH (ionization of the surface OH groups changes with pH) and is often independent of redox systems (depending on their overlap with the semiconductor bands) • This is the second sensor we have encountered in this case the sensor is mainly a pH sensor. Solution V FB

13. Solid electrolytes in solution • No electrons exchange at the surface just ions exchange with the solid often with very high selectively • The fastest ion-exchange reaction determines the potential i.e. i0,i in the case of LaF3 that is F- (also glass for H+) • This is a third type of sensor we encounter here i.e. an ion selective sensor Solution F- Electrolyte Solid electrolyte e.g. LaF3

14. No electron exchange and no ion exchange If it is an oxide insulator it will exhibit pH sensitivity like an oxide semiconductor But how do you measure such a high impedance, the voltmeter will just show an overload ? See later under ISFET !! Insulators in solution Insulator Electrolyte

15. Mixed conductors in solution • Both ions and electrons may exchange at the surface • Depending on the relative magnitude of i o,e vs. i o,i the electrode will be a redox sensor or an ion sensor, for most mixed conductors i o,e >>> i o,i • IrOx may be one of the exceptions we have made this mixed conductor (e, H+) into a very good pH sensor with small redox interference

16. Suggest an array of sensors that could be used for an electronic tongue (five tastes) Make a list of biosensors that have been used in-vivo. How long is the longest that a biosensor has been used in-vivo? Explain why the more selective biosensors are the least reversible (compare in this context an enzyme sensor with an immuno sensor) Draw the equivalent electrical circuit of a metal/electrolyte interface with the electrode at a potential so that a redox reaction occurs Homework