Bilayer magnetic-plasmonic satellite nanoassemblies for SERS detection of tobramycin with exonuclease amplification

Nanoscale assemblies designed for trace analyte detection typically require a complex fabrication process. Here, we prepare magnetic nanoparticle (Fe₃O₄)-gold nanoparticle (AuNP)-gold nanostar (AuNS) bilayer magnetic-plasmonic satellite nanoassemblies (BMPSNs) for ultrasensitive detection of tobramycin (TOB). BMPSNs are constructed through seed-mediated growth and complementary DNA hybridization, combining magnetic separation and surface-enhanced Raman scattering (SERS) activities. AuNP is in situ growth on the surface of Fe₃O₄ to form the monolayer satellite assemblies. Partially complementary double-stranded DNA (DNA1/DNA2) is modified onto the surface of the first layer satellite AuNP. TOB aptamer (Apt) and fully complementary DNA (cDNA) form the duplex DNA. In the presence of TOB, cDNA of TOB Apt is replaced by TOB/TOB Apt, which can hybridize with DNA2 modified on the surface of Fe₃O₄@AuNP-DNA1/DNA2 and further triggers exonuclease III cyclic amplification to obtain Fe₃O₄@AuNP-DNA1. Finally, Fe₃O₄@AuNP-DNA1 can assemble with AuNS@4-MBA-DNA3 through DNA hybridization to form BMPSNs. Thanks to excellent magnetic separation, exonuclease amplification and huge SERS enhancement of multiple hot spots, the limit of detection can achieve as low as 0.44 fg/mL of TOB, which is more sensitive than the previously reported methods. In addition, this method can be applied to TOB detection in actual samples with good recoveries and without interference by other antibiotics. The proposed method can be easily extended to sensitive detection of other targets by replacing the corresponding aptamers, paving a new avenue for food safety and environment monitoring.