A scanning electrochemical microscopy (SECM) protein detection methodology has been developed based on the tagging of free cysteines and other nucleophiles in proteins and peptides by benzoquinone. The tagged proteins are detected by the mediated reduction of benzoquinone with a redox species produced electrochemically at the SECM tip. After careful optimization, a sensitivity in the low ng mm(-2) range was reached for bovine serum albumin. One of the major advantages of the present technique is that the selectivity of the protein tagging can be tuned by changing the pH of the reaction media. Depending on the requirements, cysteine selective or general detection can therefore be achieved with a high sensitivity. As a proof of concept, this technique was applied to the detection of protein spots and to the imaging of human fingerprints and further compared to the actual SECM state-of-art approach. (C) 2009 Elsevier B.V. All rights reserved.