Over the last decade, the field of single-molecule detection has grown rapidly because technical and methodological developments increased sensitivity and gave access to biological processes not observable before. Single molecule detection unveils the short-lived intermediates, the heterogeneous behavior of individual enzymes and stochastic multistep processes like protein folding.1 Currently, single-molecule detection mostly relies on fluorescence. This technique works well when the molecules under investigation must have a sufficiently high photon emission rate. However, a large fraction of target molecules, including many biologically relevant proteins and metal complexes, fluoresce only weakly. This weak emission is due to a low fluorescence quantum yield or low local electric field, rendering these species undetectable by single-molecule spectroscopy.2
Fortunately, the limitation is overcome by localized plasmon resonance. One kind of plasmonic nanostructure, nanoantenna, can increase the intensity of electric field in the vicinity of the antenna, modify the quantum yield of the fluorophore, and help obtain a significant fluorescence enhancement. Gold bowtie nanoantenna could enhance a single molecules’ fluorescence up to a factor of 1340.3 DNA origami-based optical antennas could detect signal only in the presence of specific target nucleic acid without amplification. 4 A polymeric nanoantenna can harvest energy from thousands of donor dyes to a single acceptor and observe single molecule at illumination power that is>10,000-fold lower than typically required in single-molecule measurements.5
1. Holzmeister, P.; Acuna, G. P.; Grohmann, D.; Tinnefeld, P., Breaking the concentration limit of optical single-molecule detection. Chemical Society Reviews 2014, 43 (4), 1014-1028.
2. Khatua, S.; Paulo, P. M.; Yuan, H.; Gupta, A.; Zijlstra, P.; Orrit, M., Resonant plasmonic enhancement of single-molecule fluorescence by individual gold nanorods. ACS nano 2014, 8 (5), 4440-4449.
3. Kinkhabwala, A.; Yu, Z.; Fan, S.; Avlasevich, Y.; Müllen, K.; Moerner, W., Large single-molecule fluorescence enhancements produced by a bowtie nanoantenna. Nature Photonics 2009, 3 (11), 654.
4. Ochmann, S. E.; Vietz, C.; Trofymchuk, K.; Acuna, G. P.; Lalkens, B.; Tinnefeld, P., Optical Nanoantenna for Single Molecule-Based Detection of Zika Virus Nucleic Acids without Molecular Multiplication. Analytical chemistry 2017, 89 (23), 13000-13007.
5. Trofymchuk, K.; Reisch, A.; Didier, P.; Fras, F.; Gilliot, P.; Mely, Y.; Klymchenko, A. S., Giant light-harvesting nanoantenna for single-molecule detection in ambient light. Nature photonics 2017, 11 (10), 657.