In the current study we present a fluorescence probe for Cr(iii) in aqueous solution employing as-synthesized silver nanoparticles (Ag NPs) without surface labeling by any extrinsic fluorophore. The principle of the method involves aggregation of Ag NPs by Cr(iii) leading to a red-shift of the fluorescence emission peak from 420 to 684 nm for a corresponding excitation wavelength of 340 nm. The UV-visible absorption spectral studies, dynamic light scattering measurements, and scanning electron microscopy (SEM) corroborated the aggregation of the Ag NPs upon interaction with Cr(iii). Under the optimized conditions, a good linear relationship (correlation coefficient r = 0.975) was observed between concentration of Cr(iii) and the fluorescence intensity at 684 nm in the range of 1 × 10 -3 to 2 × 10 -9 M (50 mg L -1 to 100 ng L -1). The reported probe has a limit of detection as low as 2 nM, which, to the best of our knowledge, is the lowest ever reported for the fluorescence detection of Cr(iii). The developed sensor was successfully tested with chromium containing real samples, such as groundwater and fresh water, demonstrating potential application in the field of environmental sensing. © The Royal Society of Chemistry. 2012.