(181d) Droplet-Based Approach for Scalable Fabrication of Highly Reproducible Uniform Ring Arrays of Ag Nanostructures for Surface-Enhanced Raman Scattering

The droplet-based biphasic reaction is an efficient strategy for the fabrication of surface-bound nanostructures. Herein, we developed a process of fabricating ordered micro-ring arrays of silver (Ag) nanostructures for reproducible detection by surface-enhanced Raman spectroscopy (SERS), utilizing the surface nanodroplets generated on a micro-patterned hydrophobic substrate via solvent exchange. Our process consisted of the generation of surface nanodroplet arrays of vitamin E (VE), followed by a biphasic chemical reaction between the droplets of VE and the continuous flow of silver nitrate (AgNO₃) precursor solution in the presence of alkaline medium. The process parameters such as droplet volume, precursor concentration, pH of the reaction medium, reaction time, and flow rate during the formation and reaction of the droplet arrays were tuned properly to achieve maximum surface coverage, maintaining the uniformity of Ag nanostructures throughout the substrate. By scaling up the process parameters and the size of the microchamber, we were able to produce a SERS substrate with a surface area of > 60 cm² in a single run. Such a large area could be sufficient for analyzing more than a thousand samples. We demonstrated the repeatability of SERS measurements using Ag nanostructures by analyzing three environmental (rhodamine 6G, chlorpyrifos, triclosan) dissolved in river and tap water, a biological compound (adenine), and a psychoactive drug (tetrahydrocannabinol) dissolved in human saliva. 2D mapping of SERS intensities was also performed for both small and large-scale substrates by collecting data from more than 100 locations on each substrate. We also miniaturized the whole fabrication process from droplet formation to analyte detection in to a tiny 3D printed microchamber of 1 cm x 1 cm for one-time use, reducing the consumption of reactants. Our work demonstrated droplet-based biphasic reaction as a simple approach suitable for scalable fabrication of SERS substrate using one-use 3D printed microchambers of desired surface area. The technique may help to eliminate the requirement for sophisticated equipment for the fabrication of SERS active substrate.