Highly Monodisperse Stable Gold Nanorod Powder for Optical Sensor.

Gold nanorods (GNRs) as plasmonic metal nanoparticles are valuable for optical applications due to their tunable plasmonic properties. However, conventional colloidal GNRs face significant optical instability during storage, which limits their practical use. In this work, we developed a highly dispersible GNR powder using an octadecyl trimethylammonium bromide (C ₁₈ TAB)-assisted freeze-drying method, preserving the optical and chemical sensing properties of GNRs for over 4 months. Compared with C ₁₆ TAB, C ₁₈ TAB significantly enhances the GNRs dispersibility even at lower concentrations. Our study demonstrates that C ₁₈ TAB forms a sponge-like crystal structure that prevents aggregation during the freeze-drying process. The resulting GNR powder retains its plasmonic features and water dispersibility, achieving near-identical optical properties to those of fresh GNR solutions. This stability enabled creation of a liquid-free colorimetric test kit with a long shelf life. This work marks a significant step forward in the use of GNRs as standard analytical reagents, opening new avenues for real-world applications.