How low can you go? The visual detection of 2,4,6-trinitrotoluene and Hg2+ at the sub-zeptomole level is demonstrated. This was achieved using a hybrid material that allowed for the development of a single-particle, single-molecule detection technique, which may be the ultimate in ultra-trace sensitivity with selectivity. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.