Silver nanoparticles loading pH responsive hybrid microgels: pH tunable plasmonic coupling demonstrated by surface enhanced Raman scattering

Abstract

The pH responsive hybrid microgels containing homogeneously distributed silver nanoparticles (Ag NPs) were created by in situ reduction of Ag⁺ ions coordinated into the microgels with semi-interpenetrating polymer network structure composed of linear poly(acrylic acid) and crosslinked poly(N-isopropylacrylamide). When the pH value of the aqueous medium lowers from 6.0 to 4.0, the localized surface plasmon resonance (LSPR) band of the entrapped Ag NPs is pronouncedly red-shifted because of the decreased spatial distances between them as a result of shrinkage of the microgels, leading to their plasmonic coupling. The pH tunable plasmonic coupling is demonstrated by pH dependence of the surface enhanced Raman scattering (SERS) signal of 4-mercapto benzoic acid anchored on the Ag NPs surfaces. Differing from static plasmonic coupling modes from nanostructured assembly or array system of noble metals, the here reported plasmonic coupling can be dynamically controlled by environmental pH value. Therefore, the pH responsive hybrid microgels have potential applications in mobile LSPR or SERS microsensors for probing pH values of living tissues or cells.