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

Incoherent Second Harmonic Generation (SHG) from goldnanoparticles,also known as hyper-Rayleigh scattering (HRS), is proposed as a sensingmethod for copper(II) ions. As opposed to colorimetry-based methodsrelying on the shift of the localized surface plasmon resonance withthe copper(II) concentration, which effectively scales with the nanoparticlevolume due to the origin of the absorption phenomenon, SHG relieson the surface origin of the response for sufficiently small nanoparticles.As a result, differences can be expected that could be potentiallyturned into advantages such as improved Limit of Detection and shorterdetection response time. The present study demonstrates that the SHGlight scattered from aqueous suspensions of gold nanoparticles inthe presence of copper(II) ions is indeed sensitive to the copper(II)ion concentration changes. A first approach based on intensity changesshows that there is a competition between the formation of corona-likestructures centered around the gold nanoparticles due to the ionicinteraction between copper(II) ions and the negatively charged citrate-coatednanoparticles on one side and, on the other side, aggregation of nanoparticlesdue to charge screening as the copper(II) bromide concentration increases.The former process dominates at low copper(II) concentrations, whereasaggregation takes over above 1 mM copper(II) concentrations. A figureof merit is thus designed in order to provide a quantitative assessmentof the sensing performance. In a further analysis, a polarizationresolved study of the SHG light scattered from the gold nanoparticlesallows the determination of other figures of merit. The first onebased on the depolarization ratio seems appropriate, as it is basedon the surface origin of the SHG response from gold nanoparticles,whereas the second one, based on the retardation parameter, shouldnot perform better than those derived from colorimetry methods.