Sensing Copper(II) Ions with Hyper Rayleigh Scattering from Gold Nanoparticles

Abstract

Incoherent Second Harmonic Generation (SHG) from gold nanoparticles, also known as hyper-Rayleigh scattering (HRS), is proposed as a sensing method for copper(II) ions. As opposed to colorimetry-based methods relying on the shift of the localized surface plasmon resonance with the copper(II) concentration, which effectively scales with the nanoparticle volume due to the origin of the absorption phenomenon, SHG relies on the surface origin of the response for sufficiently small nanoparticles. As a result, differences can be expected that could be potentially turned into advantages such as improved Limit of Detection and shorter detection response time. The present study demonstrates that the SHG light scattered from aqueous suspensions of gold nanoparticles in the presence of copper(II) ions is indeed sensitive to the copper(II) ion concentration changes. A first approach based on intensity changes shows that there is a competition between the formation of corona-like structures centered around the gold nanoparticles due to the ionic interaction between copper(II) ions and the negatively charged citrate-coated nanoparticles on one side and, on the other side, aggregation of nanoparticles due to charge screening as the copper(II) bromide concentration increases. The former process dominates at low copper(II) concentrations, whereas aggregation takes over above 1 mM copper(II) concentrations. A figure of merit is thus designed in order to provide a quantitative assessment of the sensing performance. In a further analysis, a polarization resolved study of the SHG light scattered from the gold nanoparticles allows the determination of other figures of merit. The first one based on the depolarization ratio seems appropriate, as it is based on the surface origin of the SHG response from gold nanoparticles, whereas the second one, based on the retardation parameter, should not perform better than those derived from colorimetry methods.