Detection of ultra-low refractive index variations with colloidal nanoprobes

We implement a simple and fast technique to detect spectral shifts in the light scattering of gold nanoparticles due to refractive index changes of the surrounding medium. Spectral shifts are detected using spectrally balanced photon counting from the scattering spectrum of colloidal gold nanoparticles. We demonstrate the use of this technique for sensing ethanol concentration down to 0.1 M.