Gap-Dependent Plasmon Coupling in Au/AgAu Hybrids for Improved SERS Performance

Nanogap-based Raman substrates with Raman reporters embedded in the nanogap have been demonstrated to possess superior SERS responses and have found potential applications in molecular detection and sensing. Herein, multigap Au/AgAu hybrids with rhodamine B (RhB) molecules embedded in the nanogap are prepared as gap-enhanced Raman substrates to achieve improved SERS performances. Au/AgAu hybrids with tunable gap widths and counts are synthesized by a controlled galvanic replacement process based on Au nanospheres. The strong and adjustable plasmon coupling in the multigap structure endows Au/AgAu hybrids with prominent plasmon absorption and strong electric fields, endowing the hybrids with outstanding SERS responses. Specially, one-gap Au/AgAu hybrids show tunable enhanced SERS signals by changing the gap width due to the adjustable plasmon coupling. One-gap Au/AgAu hybrids with about a 0.8 nm gap width exhibit the highest SERS signal, and the signal is decreased as the gap width increased from 0.8 to 3.8 nm. Moreover, with RhB molecules fixedly embedded in the first nanogap (width ≈ 1.9 nm), the SERS signals of Au/AgAu hybrids are further improved by adding additional nanogaps. In particular, Au/AgAu hybrids with three gaps possess the highest Raman response, owing to a further enhanced electric field caused by multi-interfacial plasmon coupling. Furthermore, the embedded position of RhB in the four-gap structure is adjusted for higher SERS performance. Four-gap Au/AgAu hybrids with RhB molecules embedded in the third nanogap exhibit the best SERS activity due to the further enhanced electric field, which may be caused by the electric field coupling between the second and fourth nanogaps.