Sensitivity of gold nanoparticles Second Harmonic scattering to surrounding medium change

Gold nanoparticles are widely used in sensing, notably in colorimetry-based methods, where a color change is associated to the Surface Plasmon Resonance peak shift due to a binding event or an environment modification in the vicinity of the nanoparticles. In this work, we explore the sensitivity of the Second Harmonic response from gold nanoparticles of two different diameters as this response stems principally from the nanoparticle surface at small sizes as opposed to the whole volume of the nanoparticle at larger sizes. The origin of this response is therefore different from that involved in the colorimetric response, the latter being of volume origin. Upon addition of Glycerol to an aqueous solution of nanoparticles, Surface Plasmon Resonance peak shifts are first observed but they cannot fully explain the Second Harmonic intensity changes recorded. Hence, in order to gain further insights into the origin of the changes observed in the experimental data, polarization-resolved Second Harmonic measurements are performed. A mechanism where first a modification of the first hyperpolarizability of the nanoparticles due to the presence of glycerol occurs followed by nanoparticle aggregation is then proposed. The potential alternative use of this nonlinear optical method of Second Harmonic scattering for sensing purposes is then discussed in light of a figure of merit proposed to describe the sensing sensitivity observed.