ALD-enabled precision engineering of tunable nanogaps in mace-like morphologies for SERS detection of pesticide in juices

Surface-enhanced Raman spectroscopy has become a vital tool for detecting trace analytes in food safety and environmental monitoring. This study presents a biomimetic architecture inspired by the hierarchical morphology of mace weapons, featuring silver handles (AgH) decorated with gold nanoparticles (AuNPs) separated by precisely tunable nanogaps. The fabrication process leverages atomic layer deposition (ALD) to create nanometer Al2O3 spacer layers, which are subsequently etched to expose high-density nanogaps. This AuNPs-nanogaps-AgH structure exhibits exceptional plasmonic coupling, generating intense electromagnetic hotspots that significantly enhance Raman signals. Theoretical COMSOL simulations confirm that the nanogap regions exhibit enhanced electric fields, with the enhancement increasing as the gap size decreases. Experimental validation using Rhodamine 6G as a probe molecule reveals a detection limit as low as 10−11 M, with excellent linearity and signal uniformity. The substrate's practical applicability was further demonstrated through the detection of thiram at a concentration as low as 0.02 ppm. Moreover, the AuNPs-nanogaps-AgH can effectively detect 2 ppm thiram in orange, lemon, and grapefruit juices. Compared with traditional fabrication techniques such as focused ion beam or electron beam lithography, ALD not only offers superior precision control but also demonstrates relatively low cost and mass production capabilities. These advantages make ALD a highly promising approach for fabricating nanogap structures, enabling ultrasensitive SERS-based detection in real-world applications.