In electrochemical sensing, why do molecules binding cause changes in current? From my limited understanding in electrochemistry, a set of electrodes are biased at a given potential and when there is a chemical reaction(antibody-antigen binding for example), a subsequent change in current occurs. The electron transfer(binding) is measured by the sensing electrode and converted to a voltage using an amplifier.
I am trying to wrap my head around why there is a change in current when binding occurs. Can anyone explain this simply to me?
 A: In very rough terms, in order to deliver an electron, which cause the atom not to be electrically neutral any longer, the atom (ion) will readily bond into a molecule or into a lattice in order to become closer to a neutral state again.
The electron is now delivered to a conducting path and starts moving due to some voltage (electropotential difference). Remember that current is the flow of electrons. We need many of such electron deliveries to take place for current to appear.
A: In electrochemistry, the working electrode provides a voltage, a source/sink of electrons, and a surface for a reaction to occur. The surface may be chemically inert or activated in some way (antigen coating) to create specific reactions. If an antigen coated electrode (along with everything necessary to measure current vs voltage at the working electrode) is placed in a solution of antibody ions then the reaction at the electrode surface could be described as:
$$ Ab^+ + e^ --> AgAb \ \ at \ \ V_{rxn}$$
where $V_{rxn}$ is the potential for an antigen/antibody bond requiring an electron. The change in current is the flow of electrons to bind the antigen/antibody pairs. This description is very simplified. Actual immunosensors have more complex chemistry involved but the electrode as a source and sink of electrons is still true.
