Three-dimensional nanogap substrate for metal-enhanced fluorescence-based ultra-sensitive detection of trace ammonia

High sensitivity and rapid detection of trace ammonia are crucial for environmental monitoring and disease diagnosis. Polydiacetylene (PDA)-based sensors have attracted significant attention due to their fast response; however, weak fluorescence signals under low ammonia concentrations limit their performance. To address this issue, this study further developed a PDA-based ammonia sensing platform based on a three-dimensional (3D) silver nanogap substrate by utilizing the metal-enhanced fluorescence (MEF) effect to enhance the fluorescence response of PDA. Specifically, a nanopillar array with a diameter of 250 nm, a height of 100 nm, and a period of 500 nm was fabricated on a glass slide using ultraviolet nanoimprint lithography. After that, an oxygen plasma treatment was applied to form a nanogap network, and a 75 nm thick silver layer was deposited on the surface via physical vapor deposition (PVD) to create a 3D silver nanogap structure, thus constructing the 3D nanogap substrate. Following this, PDA vesicles were then immobilized on the substrate surface for ammonia sensing. Experimental results demonstrated that, under static sealed-chamber conditions, the 3D nanogap substrate achieved a limit of detection (LOD) of 4.79 ppm for ammonia, calculated using the standard 3σ/k method based on the theoretically estimated ammonia concentration. In addition, the sensor exhibited excellent selectivity toward ammonia compared with volatile organic compounds such as toluene, ethanol, and methanol, demonstrating great potential for trace ammonia detection applications.