where B is the sensitivity of the ISFET, ß is the buffer capacity of the sample and E is
the enzymatic sensitivity parameter representing the change in equivalents of H+
or
OH-
in the membrane as a function of the substrate concentration. E is determined by
the enzyme load of the membrane, the enzyme kinetics, the diffusion constants of the
relevant species in the membrane and the ratio in which protons or hydroxyl ions are
produced per mole of substrate. For reactions that produce acidic products the value
of E is positive and reactions that produce OH-
yield a negative value of E. At
constant pH, the value of E can be considered to be constant provided that the
substrate concentration [S] is considerably smaller than Km, the Michaelis-Menten
constant of the enzyme, or when the response of the sensor is limited by diffusion of
the substrate instead of by the enzyme kinetics.
A similar consideration holds for the electrochemical actuator. For small changes in
pH, the steady-state response of the output voltage on a current I through the
generating electrode may also be considered to be linear and is given by