Scientists have attributed the difficulty in achieving single-molecule sensitivity in a tip-enhanced scanning near-field optical microscopy setup to the metal tip's tendency to quench nearby molecular fluorescence. To observe this directly, researchers at Swiss Federal Institute of Technology in Zurich have charted the excited-state lifetimes and fluorescence rates of single dipolar emitters as functions of their position relative to gold-coated Si3N4 tips.The research suggests that single-molecule resolution may be achievable by the selective quenching of individual molecules in small clusters. The results appeared in the Sept. 9 issue of Applied Physics Letters.