Light Scattering Contrast Inversion of Single Metal Nanoparticles Inside a Nanofluidic Channel

Nanofluidics enables the high-precision control of fluid flow, as well as of tiny particles and molecules, at the nanoscale. Combined with optical microscopy and spectroscopy, this has generated an experimental platform for the study of single nanoentities by means of light, often enhanced by nanoscale interference effects. However, in the realm of metal nanoparticles inside nanofluidic systems, contradictory light scattering properties have been reported, and the underlying physics is poorly understood. Here, we systematically investigate Pt and Au nanodisks nanofabricated into nanotrenches in a poly-(methyl methacrylate) matrix to emulate nanofluidic systems and find that their light scattering signature can appear both brighter and darker than the trench alone, or become completely invisible, depending on the specific interplay between trench and nanodisk diameter. An analytical model based on the electrostatic approximation allows us to qualitatively understand the fundamental physics of this effect based on the interference of light scattered by a metal nanodisk and a nanofluidic structure. To corroborate the model and demonstrate an application in a fully functional nanofluidic system, we show that the optical appearance of single metal nanodisks inside a fully enclosed nanofluidic channel inside an SiO₂ matrix can be dynamically tuned from dark to bright to completely invisible by adjusting the refractive index of a liquid inside the channel. We predict that this effect will find application in optical sensing in nanofluidic systems for volumes smaller than the diffraction limit of light.