Coated triangular Ag nanoprisms as optical sensors: control of stability and spectral response with a thermo-responsive polymer

Abstract

Silver nanoprisms (AgNPrs) have been coated with end-group functionalized thermo-responsive poly-(N-isopropylacrylamide) (PNIPAM) synthesized via reversible addition–fragmentation chain transfer (RAFT) polymerization. The effect of the density of the polymer coating on the plasmonic properties in the visible range of light are investigated for various local environmental conditions such as temperature, pH and chloride ion concentration employing UV/VIS spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM). It has been found that contrary to uncoated AgNPrs, the polymer modified AgNPrs resist against degradation processes induced by chloride ions. Depending on the surface coverage of the AgNPrs with the polymer, the degradation process can be used to adjust the optical properties of the AgNPrs. The functionalized PNIPAM modified AgNPrs display a thermal responsive behaviour, which can be observed as a reversible shift of the plasmon peak upon heating and cooling above or below the lower critical solution or phase transition temperature of the polymer (LCST). The modified AgNPrs could be employed to sense Cl⁻ in a concentration range promising for the diagnosis of cystic fibrosis. Based on the present results we also present a more general concept for the use of polymer decorated silver nanoprisms as improved sensors for the microenvironment of the particles making use of the local field enhancement, polymer coating, as well as switching of the plasmon response, which enables fine-tuning of plasmon resonances and ultimately dynamic sensing with coated triangular silver nanoprisms.