Abstract: This invention relates to the detection of molecular interactions between biological molecules. Specifically, the invention relates to electrical detection of interactions such as hybridization between nucleic acids or peptide antigen-antibody interaction using arrays of peptides or oligonucleotides. In particular, the invention relates to an apparatus and methods for detecting nucleic acid hybridization or peptide binding using electronic methods including AC impedance. In some embodiments, no electrochemical or other label moieties are used. In others, electrochemically active labels are used to detect reactions on hydrogel arrays, including genotyping reactions such as the single base extension reaction.