Plasmonic External Nanogap Dumbbells for On-Particle Direct Raman Assays

Metal nanostructures with a plasmonic nanogap are useful for surface-enhanced spectroscopy, and many bio-applications like biosensing and bioimaging. The use of these structures as chemical and biological sensing/imaging probes typically requires an ultra-precise synthesis of the targeted nanostructure and skill in complex assay techniques are required. Here, we designed and synthesized a plasmonic nanostructure based on Au nanorod cores with two types of external nanogap particles with different gap positions. Au dual-gap nanodumbbells (AuDGNs) were synthesized via the anisotropic adsorption of polyethyleneimine (PEI) on Au nanorods to facilitate tip-selective Au growths on nanorod tips for forming mushroom-shaped dumbbell-head structures at both tips. The dual gaps (intra-head and inter-head gaps) were formed within a single particle. Au tip-intragap nanodumbbells (AuTIDs) were synthesized based on co-reduction and dealloying process of Ag and Au on the tip of Au nanorods. Two heads with closed intra-head gap were grown on the tips of Au nanorod and inter-head gap exposed to outer environment were formed within a single particle. Importantly, these particles enabled facile on-particle SERS detection assays in a short period of time through the diffusion of target molecules into the gap exposed to outer environment.