A method and apparatus with the sensitivity to detect and identify single target molecules through the localization of dual, fluorescently labeled probe molecules. This can be accomplished through specific attachment of the taget to a surface or in a two-dimensional (2D) flowing fluid sheet having approximate dimensions of 0.5 .mu.m.times.100 .mu.m.times.100 .mu.m. A device using these methods would have 10.sup.3 10.sup.4 greater throughput than previous one-dimensional (1D) micro-stream devices having 1 .mu.m.sup.3 interrogation volumes and would for the first time allow immuno- and DNA assays at ultra-low (femtomolar) concentrations to be performed in short time periods (.about.10 minutes). The use of novel labels (such as metal or semiconductor nanoparticles) may be incorporated to further extend the sensitivity possibly into the attomolar range.