Schematic diagram showing the use of a molecular mask for selective functionalization of a microelectrode array. (a) All microelectrodes are initially protected from functionalization by a protein-inhibiting mPEG monolayer. (b) This molecular mask can be released by electrochemical means. The left microelectrode is addressed, while all neighboring microelectrodes are actively prevented from electrochemical desorption via a second potentiostat. (c) The bare gold microelectrode surface is functionalized with the required peptide aptamer. (d) By repeating this cycle it is possible to independently functionalize multiple microelectrodes with different proteins within a single device. (e) The formation of a protein-protein complex following exposure to a complex biological solution results in a measurable change (phase shift) in φ(ω) (central microelectrode). φ(ω) remains constant for all microelectrodes in which target-specific binding does not occur (left and right microelectrodes), thus enabling unambiguous identification of protein targets within the biological sample.