where [H + ]S is the surface proton concentration, which depends on the bulk proton concentration ;. In our model, we can consider AAs immobilized from the Carboxylic (C-) or Amine (N-) terminals by cancelling the corresponding affinity (or dissociation) constants. For this work, we have used the standard pK values in the literature of the molecules in a liquid. The interaction with different sensor substrates can modify these pK values, which could be later considered phenomenologically to refine the model .