A European research team developed a dye- and DNA programming-based approach for counting molecules present within individual complexes. The approach supports biomedical and life sciences applications, as well as those involving OLEDs, solar cells, and soft matter — such as photovoltaics and liquid crystals. The research team used a method called “DNA origami” to place individual dye molecules at well-defined distances from one another. The researchers arranged the molecules by manipulating and folding the DNA into desired shapes and sizes of a few nanometers. The fluorescence of the individual “origami” molecules is not immediately distinguishable under a light microscope. To overcome this, team members passed light through a semi-transparent mirror, allowing photodetectors positioned on both sides of the mirror to record the light. As a single molecule capable of emitting only a single light particle at a time, only one of the two photodetectors was able to perceive the light. By taking into account the chronological order in which light hits either detector, the scientists successfully determined the exact number of dye molecules present within the origami structure. Using DNA 'origami' in the work, DNA is programmed in such a way that the molecules arrange themselves by folding the DNA as required at intervals of a few nanometers. Courtesy of University of Regensburg/Felix J. Hoffman. Determining the number of dye molecules in the work correlates directly to the programmability of the DNA. An origami structure with a single dye molecule emits a single photon. An origami structure with five dye molecules emits five photons. The researchers reported that the individual dye molecules also interacted with one another accordingly. The dye absorbed energy and, from there, either reemitted the energy in the form of light or passed it on to a neighboring dye molecule. If that neighboring dye was already excited, the two excitations met, destroying the initial, individual excitation. That quality, described as an “annihilation,” the researchers said, is important in the study of molecular optoelectronics; the method enables the study of excitons at the single-particle level. This enables the ability to improve the design of excitonic probes, such as ultrabright fluorescent nanoparticles and materials for optoelectronic devices, the researchers reported in their paper. The researchers also tested with conjugated polymers. The team consisted of Jan Vogelsang (University of Regensburg), Gordon Hedley (University of Glasgow), and Philip Tinnefeld (LMU Munich). The research was published in Nature Communications (www.doi.org/10.1038/s41467-021-21474-z).