Plasmonic Nanoparticle Sensitization of Colorimetric Sensing Dyes*

Sensors based on spectral shaping indicate the detection of analytes by color changes. They are highly sensitive, selective, rugged, reversible, non-toxic, and tout a low-cost and easy fabrication process. As competitive options for environmental sensing, they can detect analytes in a gaseous mixture at concentrations as low as a few parts per million (ppm). However, even this sensitivity can be insufficient, for example, when monitoring highly toxic analytes such as lead, which should not exceed concentrations of 15 ppb in drinking water. Plasmonic nanoparticles added to sensing dyes can increase sensitivity via Mie scattering. Incident light carrying the (nanoparticle) plasmonic frequency drives electrons into oscillations, and at resonance, rapid dipole oscillations generate strong fields which coherently interact with the incident wave to effectively collect much more light than the nanoparticle’s physical size. Thus, absorption by the surrounding dye is enhanced and a portion is scattered elsewhere, thereby increasing the path length of light in the dye. As light is better absorbed, the color is better changed, and the sensitivity is increased. In this presentation we will unpack these ideas applied to colorimetric sensors.