Detection of human serum albumin on protein array using scanning tunneling microscopy

Electrical detection of biological binding events, such as protein–protein interaction and DNA hybridization, has emerged as an alternative method to conventional colorimetric and fluorescence based methods. In this study, we demonstrate an electrical detection technique of protein array which can be simply extended for multifunctional measurements and detection of biological binding events on micro-scale array. Micro-contact technique was used for the fabrication of protein chip. The fabricated protein array on Au substrate was characterized by fluorescence microscopy and scanning tunneling microscopy (STM). The chip was designed to investigate immunocomplexes comprised of our model protein, human serum albumin (HSA), corresponding antibody fragments, and Au nanoparticle–antibody conjugates. The peak-like pulse obtained by electrical tunneling current between these complexes and the STM tip varies on the surface density of the bound complexes. Using the electrical detection technique based on STM, 100 fg/mL of HSA could be successfully detected by STM. Importantly, the proposed concept of measurement could allow multiple analyses of analytes at nano-scale array, which is difficult to be analyzed by conventional fluorescence based method.