In this work, we use dark-field optical microscopy (DFM) and hyper spectral imaging (HSI) to study the interactions of single Ag and Au nanoparticles (NPs) with Hg 2+ in real time, at room temperature (25 °C). NPs were immobilized on glass substrates using 3-aminopropyltrimethoxysilane (APTMS) as the anchoring agent. Red, green and blue colors were assigned to the particles in hyper spectral images on the basis of their relative scattering intensities at 640, 550 and 460 nm, where the particles showed maxima in their scattering spectra. While Ag NPs showed all the colors, Au NPs were mostly red and rarely green in DFM images. The scattering spectra of Ag NPs were more blue shifted (with an average shift of 46 nm in the case of red particles) in a given time compared to Au NPs, after passing over Hg 2+ solution and these shifts increased with time. Depending on the extent of blue shift, the colors of the particles got modified. Red particles appeared more reactive than green and blue, as revealed from the larger extent of shifts and their time dependence. The greater reactivity of red particles is attributed to their anisotropic nature possessing reactive tips, edges and more surface area due to their large size. The effect of quality of water on the scattering spectrum was checked by passing over deionized (DI) and tap waters separately, which showed that the effect is minimal compared to the presence of Hg 2+, when data at a given time, flow rate and temperature were compared. Solution phase interactions of NPs with Hg 2+ were also performed for comparison. These were characterized by UV-vis absorption spectroscopy, transmission electron microscopy (TEM) and energy dispersive analysis of X-rays (EDAX). Solution phase experiments showed citrate-induced aggregation of Ag NPs and partial reduction of Hg 2+ to Hg 0 upon exposure to Hg 2+. Immobilized particles cannot aggregate and they show only reduction. © 2012 The Royal Society of Chemistry.