To pursue a conceptually different method of identification of AAs, we considered the physicochemical properties of proteins. AAs have different isoelectric properties, swapping charges as a function of pH in a unique way. Following the acid titration's pH of a solution of AAs can provide their fingerprints, which is a classical textbook experiment . However, the identification using this method requires an equivalent number of AA molecules to the titrated acid, which makes it impractical for the typical samples where the concentration of proteins is usually low. Our approach to transduce the amphoteric properties of AA is to use FETs because they can be sensitive to charge changes even with very few AAs approaching single-molecule using graphene-FET . Therefore, here, we will calculate the surface potential and capacitance properties of AAs and small peptides as a function of pH. Our approach does not require the calibration of the absolute surface potential of the FET sensor because it relies on the detection or the proton affinity (pK) of each radical of the AA which produces singularity points on the surface potential when pH is modulated. In our model, we consider a covalent immobilization of AAs by one of the Carboxyl-or Amino-terminals that eliminates the affinity point corresponding to the new bond with the solid phase surface . To provide a proof of concept of our approach and to demonstrate the AA fingerprints, we can first disregard the modifications of proton affinities that occur with the interaction of the substrate, which later can be determined phenomenologically.