Abstract: The invention provides methods for conducting hybridizations having increased selectivity of hybridization using substrates upon which probe nucleic acids are immobilized. The methods of this invention can be used to increase selectivity in nucleic acid diagnostic devices, such as biosensors and microarrays. The invention provides increased selectivity through control of the substrate surface chemistry and in particular, through control of the density of nucleic acids and other oligomers immobilized on a surface. The invention provides improved signal to noise in hybridization assays via enhanced differences in signal magnitude generated for fully matched target nucleic acid compared to partially matched target nucleic acid prior to signal processing.