In situ modulation of gold nanorod's surface charge drives the growth of end-to-end assemblies from dimers to large networks that enhance single-molecule fluorescence by 10 000-fold

End-to-end assemblies of anisotropic plasmonic nanostructures with small nanogaps are of great interest as they create strong hot spots for enhancing weak fluorescence and/or scattering of molecules. Here we report the growth of dithiol-linked end-to-end assemblies of gold nanorods from dimers to large networks containing thousands of individual nanorods, directed by in situ tuning of nanorod's surface charge. Surface charge was lowered to initiate the aggregation process but was subsequently increased to achieve slow tip-specific growth over seven days to form end-to-end networks of nanorods, which were stable in solution for over one month. Furthermore, we showed that these assemblies contained strong plasmonic hot spots which enhanced the fluorescence signal of a weak emitter by 104-fold. This enhancement is approximately 10-fold larger than that obtained using a single gold nanorod and is comparable to the largest enhancement obtained using more expensive lithographically made in-plane antenna arrays.