Vertical Self-Assembly of Gold Nanoworms for Sensitive Surface-Enhanced Raman Spectroscopy-Based Trace Detection

The assembly of anisotropic nanoparticles into closely packed structures results in unique functionalities. Herein, we report a 3D multilayer vertical assembly of worm-shaped Au nanoparticles using a carefully regulated evaporation-induced deposition method. With the evaporation of the nanoworm (NW) suspension, the concentration of NWs in the suspension gradually increases, which generates a balance between electrostatic interactions and entropically driven attraction to produce multilayer vertical assemblies. These assemblies exhibit enhanced surface-enhanced Raman scattering (SERS) signals due to the presence of a high concentration of hotspots. Moreover, in contrast to nanorods, the bent morphology of NWs creates pores in the vertical assembly, enabling the analyte molecule to penetrate the assembly and access hotspots. Picomolar to micromolar concentrations of rhodamine 6G (R6G) were detected with the NW assembly-based SERS substrate with excellent linearity and uniformity of the signal with a spot-to-spot relative standard deviation (RSD) of only 8.9%. The SERS substrates were also employed for trace detection of ammonium nitrate, which is a well-known constituent of homemade explosives. Our method of producing multilayer vertical assemblies of NWs is simple, highly sensitive, stable, and reproducible for making effective SERS sensors for the quantitative detection of analytes.