Principles of Refractive-Index and Optical-Extinction Sensing of Microliter Samples of Nanocolloids ^†

We analyze theoretically and experimentally a new way of sensing the real and imaginary parts of the effective refractive index of nanocolloids by coherent reflectance from a confined film of a liquid sample. The method is suitable for developing compact sensors with high resolution. We perform a theoretical analysis using a recently developed coherent-scattering model for a turbid colloidal film, which considers absorption and scattering by the particles as well as surface effects. The configuration to measure the coherent reflectance of light from the sample consists of an optical prism in contact with a nanocolloid layer of the sample backed with a glass slide. In this configuration, there are two critical angles for light incident from the prism side, one between the prism’s glass and the air outside, and another one between the prism and the nanocolloid sample. The model predicts a high sensitivity of the reflectance to the optical properties of the nanocolloid just before and at the critical angle, to the imaginary and the real part of the effective refractive index, respectively. We also present experimental reflectance measurements of gold and TiO₂ nanocolloids in the proposed configuration and demonstrate the high sensitivity of both measurements. We illustrate the method for chemical sensing by monitoring changes in PH and temperature of nanocolloids.