Co-Solvent-aided Opto-Thermophoretic Printing of Gold Nanorod Assemblies

The controlled deposition of plasmonic nanoparticles on solid surfaces is critical for the fabrication of medical diagnostics, devices and sensors. Despite recent advances in harnessing thermal gradients to achieve controlled deposition of nanoparticles at a laser spot, the deposition of anisotropic gold nanoparticles via opto-thermophoresis could not be achieved in aqueous solution. Herein, the use of ethanol as a co-solvent was shown to enable rapid (<10 s) and effective deposition of gold nanorods in the presence of polyvinylpyrrolidone on unmodified glass substrates. The influence of key parameters, including polymer concentration and molecular weight, co-solvent content, nanoparticle concentration, and laser power, were studied. While ethanol content and polymer size had the largest impact on the assembly structure, all had an impact on the assembly growth rate. Polymer size was found to significantly impact the degree of nanorod assembly found in the pattern, which led to a surface-enhanced Raman scattering enhancement factor up to 10^8. In summary, this approach utilizes thermal gradients to direct the formation of highly localized plasmonic assemblies without the need of functionalization of the substrate. This simple, fast, and adaptable approach is a promising platform for the scalable patterning of plasmonic nanostructures.