We demonstrate an electrical detection technique for protein array which can be simply extended for high performance multifunctional measurements as the readout of the proposed method functions in a vertical direction. Especially, an electrical detection system using scanning tunneling microscopy (STM) was developed for the protein chip. Protein array was fabricated on gold (Au) pattern using an array machine. We utilized immunocomplexes comprising model protein, prostate specific antigen (PSA), corresponding antibody fragments, and gold nanoparticles. We could therefore quantitatively measure PSA concentrations as low as 33 fM (1 pg/mL) using periodogram analysis of the peak frequency. The proposed detection system makes it possible to measure DNA, RNA, and other kinds of antigen, and allows multiple analyses of analytes to be carried out in a nano-scale chip. (C) 2009 Elsevier B. V. All rights reserved.