Controlled synthesis of gold nanorod dimers with end-to-end configurations

End-to-end gold nanorod dimers provide unique plasmonic hotspots with extremely large near-field enhancements in the gaps. Thereby they are beneficial in a wide range of applications, such as enhancing the emissions from ultra-weak emitters. For practical purposes, synthesis of gold nanorod dimers with high yield, especially on the substrates, is essential. Here, we demonstrate two controllable strategies to synthesize gold nanorod dimers based on the self-assembly of gold nanorods, either in bulk solution or on the surface of a glass substrate directly. Both methods can give a high yield of gold nanorod dimers, yet, assembling them directly on the substrate provides more flexibility in controlling the shape and size of each nanorod within the dimer. We also show that these gold nanorod dimers can be used to enhance two-photon-excited fluorescence signals at the single-molecule level.

: Simulated extinction spectra for GNRs of different morphologies. (a) The extinction spectra excited transversely for a single GNR (blue), a side-by-side GNR dimer (orange), and a side-by-side trimer(green). (b) The extinction spectra excited longitudinally for a single GNR (blue), an end-to-end GNR dimer (orange), and an end-to-end trimer (green). Each GNR was modeled as a spherically capped cylinder with the dimensions of 40 nm × 97 nm. The interparticle gap was set as 5 nm. The simulations were performed by using a boundary element method. S1-S3 The dielectric permittivity for gold was taken from Johnson and Christy, S4 and the refractive index of the ambient medium was taken as 1.33.
S-4 1.1 Control experiments.  The spectra were acquired every 10 min for 2 hours.

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Drop-coating deposition of the GNR assemblies Scanning electron micrographs (SEM) were captured by an Apreo SEM (ThermoFisher Scientific). An accelerating voltage of 5 kV and a current of 13 pA were used for the scanning of images at a working distance of 10 mm from the pole piece of the SEM. To prevent charging inside SEM, GNR samples were sputtered with a Pt/Pd target inside a Cressington sputter coater to get a 2 nm thick film of the target on the sample. Figure S5: SEM images of GNR assemblies on a glass slide. The assembly was triggered by adding the dithiolated streptavidin-biotin compounds to the GNR suspension. The assemblies were deposited onto the glass slide by drop-casting. S-7 1.2 Deposition of the GNR assemblies between two cover slides Figure S6: SEM images of GNR assemblies on a glass slide. The assembly was triggered by adding the dithiolated streptavidin-biotin compounds to the GNR suspension. The assemblies were deposited onto the glass slides and dried between two cover slides. The assemblies of different morphologies are highlighted by colored circles: end-to-end GNR dimer: green, single GNR: red, side-by-side GNR dimer: blue, and clusters: orange. Figure S7: SEM images of GNR assemblies on the same glass slide as figure S6. The assemblies of different morphologies are highlighted by colored circles: end-to-end GNR dimer: green, single GNR: red, side-by-side GNR dimer: blue, and clusters: orange.
S-9 Figure S8: SEM images of GNR assemblies on the same glass slide as figure S6. (a-h) These SEM images are of different areas from the same sample. The assemblies of different morphologies are highlighted by colored circles: end-to-end GNR dimer: green, single GNR: red, side-by-side GNR dimer: blue, and clusters: orange.
S-10 Figure S9: SEM images of GNR assemblies on a glass slide. The sample was prepared using the step-wise assembly method. (a-f) These SEM images are of different areas from the same sample. The assemblies of different morphologies are highlighted by colored circles: end-to-end GNR dimer: green, single GNR: red, side-by-side GNR dimer: blue, and clusters: orange.
Figure S10: SEM images from two different samples prepared by using the step-wise assembly method. (a, c, e) Assemblies of different areas from the first sample. (b, d, f) SEM images from the second sample. The assemblies of different morphologies are highlighted by colored circles: end-to-end GNR dimer: green, single GNR: red, side-by-side GNR dimer: blue, and clusters: orange.

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3 Two-photon-excited fluorescence enhancement.  The two-photon-excitation enhancement by a single GNR (orange) and by a GNR dimer (blue) as functions of position of the molecule. In the simulations, the size for the single GNR was set at 40 nm × 115 nm, and the sizes for the GNRs within the dimer were set at 40 nm × 90 nm, both of which give the plasmon resonance of 760 nm.