BE908 Electrode Theory & Biosensors

Half cell potential

Potential difference between electrode and electrolyte

Drift current

Drift current is the ionic movement under the influence of an electric field, just in the same way that any charged particle moves in an electric field, including electrons inside resistors. Drift current is dependent on the applied field and the ionic conductivity of the ion in solution

Idrift

drift current or current due to ion movement in electric field

Diffusion current

Diffusion currents occur wherever the ionic concentration is not uniform. Usually this is at the face of an electrode where the ion is being reduced or oxidised and converted into its atomic (de-ionised ) form. Ions flow from regions of high concentration into regions of low concentration. The diffusion flow of particles is governed by Fick's first law of diffusion which governs both charged and uncharged particles.

Charge transfer from ions at anode or cathode

charge flows freely across the interface and this can be modelled as a resistor in the circuit

Charge storage at anode or cathode

charge is stored across the interface and this can be modelled as a capacitor in the circuit. This charge behaviour at an electrode is known as a ‘double layer’

Non-polarisable electrodes

Build up of charge doesn't happen

Polarisable electrodes

Can't get exchange of electrode so build up of charge happens

Glucose sensing (1st generation)

non-mediated glucose sensor

Glucose sensing (2nd generation)

mediated glucose sensor

Immunoassay

A substrate on which antibodies are fixed is exposed to a sample containing antigens to be quantified.
The substrate is washed, and exposed to antibodies labelled with coloured / fluorescent / radioactive marker
The substrate is washed. It is now ready to be measured, by reading colour / fluorescence / radioactivity

Immunosensors

Labelled antibodies change substrate properties
(conductivity, reflectivity, …) if label-free: the substrate properties are changed by the antigen itself

Staining

Using fluorescent markers that attach to the chemicals we want to measure / see

Optical Measurements

Spectrophotometer
Dual-path UV-VIS spectrophotometer
Fluorescence
Fluorometer
Staining