Field Enhancement Tuning through Subnanoscale Polymer Encapsulation of Gold–Silver Alloy Plasmon Nanoparticles for SERS Applications

In this work, bimetallic Au–Ag nanoparticles are encapsulated with an ultrathin polymer shell with a high degree of control over the shell thickness up to the subnanometer level, i.e., at a rate of around 0.25 nm per layer, which provides stability without compromising the plasmonic characteristics. The effect of depositing polymer layers was observed in the red shift in the surface plasmon resonance peak wavelength of the Au–Ag alloy nanoparticles and proven numerically using a finite element method. The effect of polymer encapsulation on the field enhancement property of the Au–Ag alloy plasmon nanoparticles was investigated using surface enhanced Raman spectroscopy (SERS), and the variation in the enhancement factor is in line with the well-established distance decay of field enhancement in SERS. The near-electric field simulations also supported the distance decay of field enhancement where the localization of fields by the nanoscale polymer shell can be controlled on the order of 10–100 times in the hot spots using a dimer model. By controlling the thickness of the encapsulating polymer shell at the subnano scale, this work demonstrates that the field enhancement can be tuned using the simple wet chemical colloidal layer-by-layer encapsulation technique, which has an important scope in plasmonic sensing applications.